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Title:
Investigation and evaluation of a bi-polar membrane based seawater concentration cell and its suitability as a low power energy source for energy harvesting/MEMS devices
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Book
Language:
English
Creator:
Merz, Clifford Ronald
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University of South Florida
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Tampa, Fla
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Subjects / Keywords:
Dialytic Battery
Fuel cell
Semi-permeable membrane
Renewable energy
Ocean energy
Dissertations, Academic -- Electrical Engineering -- Doctoral -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Abstract:
ABSTRACT: It has long been known from Thermodynamics and written in technical literature that, in principal, instant energy can be made available when dilute and concentrated solutions are mixed. For example, a river flowing into the sea carries with it a physical-chemical potential energy in its low salt content, some of which should be recoverable. As also known, a naturally occurring, diffusion-driven, spontaneous transport of ions occurs throughout a solution matrix, thru barrier interfaces, or thru ion-selective membranes from the side containing the salts of higher concentration to the compartments containing the more dilute solution to effect the equalization of concentration of the ionic species. Since this ion movement consists, preferentially, of either cations or anions, it leads to a charge separation and potential difference across the membrane, otherwise known as a membrane potential. Eventually, when the concentrations in the compartment are the same, the cell ceases to function. However, if operated as a fuel cell with its respective concentrations continually replenished, equilibrium at a specific value of potential difference is established. To capture the energy of this potentially significant albeit low power energy source, a suitable energy extraction device is required. The focus of this Ph.D. research effort is to address the concept, research and evaluation of a Bi-Polar membrane based seawater concentration cell and its suitability as a low power energy source for Energy Harvesting/MEMS devices (patent pending).
Thesis:
Dissertation (Ph.D.)--University of South Florida, 2008.
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Includes bibliographical references.
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by Clifford Ronald Merz.
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Title from PDF of title page.
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Document formatted into pages; contains 113 pages.
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Includes vita.

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aleph - 002001651
oclc - 319883642
usfldc doi - E14-SFE0002671
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Investigation and Evaluation of a Bi-Polar Membrane Based Seawater Concentration Cell and Its Suitability as a Low Power Energy Source for Energy Harvesting/MEMS Devices by Clifford Ronald Merz A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Electrical Engineering College of Engineering University of South Florida Major Professor: Wilfrido A. Moreno, Ph.D. Marilyn Barger, Ph.D. Kenneth A. Buckle, Ph.D. Stephen M. Lipka, Ph.D. Paris H. Wiley, Ph.D. Date of Approval: October 27, 2008 Keywords: Dialytic Battery, Fuel cell, Se mi-permeable membrane, Renewable energy, Ocean energy, Anomalous Osmosis, Salinity Gradients, Electrochemical testing Copyright 2008 Clifford Ronald Merz

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Acknowledgements I would like to expres s a heart felt thanks to my major-professor Dr. Wilfrido Moreno and all my committee members for th eir continual support and participation during this long adventure. I’m also indebt ed to Professors Luis Garca-Rubio, College of Marine Science, and Alberto A. Sags, Department of Civil and Environmental Engineering, for their guidance, encouragem ent, and generous use of lab space and equipment. Many thanks, as well, to Dr. Robert Carnahan, USF Professor Emeritus, whose exposure to Desalination Science both ig nited and fueled my interest in this exciting technology and for whose unselfish dedication early on in my program made such a difference in this becoming a real ity. Many thanks to: Mr. Tony Greco, USF College of Marine Science, for the SEM/XRay imagining; to Kingsley Lau, USF Civil and Environmental Engineering Ph.D. Candidate whose help and friendship will always be appreciated; and to Vembu Subramania n, USF College of Marine Science, whose friendship and supportive greetings of “Dr. Cliff” during this journey were always appreciated. To my wife and family, whose s acrifices were many as I pursued this full time obsession on a part time basis. And finall y, to my parents, especially my Dad, for without his lifelong encouragement, and stead fast support, none of this would have happened.

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i Table of Contents List of Tables iii List of Figures iv Abstract vi Chapter 1. Introduction 1 1.1 Focus and Direction 1 1.2 Motivation and Purpose 3 1.3 Participants 4 Chapter 2. Required Technical Background and Discussion 5 2.1 Summary of Rule Basics with respect to Cell Reactions and EMF 5 2.2 Electrochemical Cells – Types and Definitions 7 2.3 Dialytic Power Generation 9 2.4 Concentration Cell Basics 9 2.4.1 Concentration Cells with Transference 10 2.5 General Ion-Exchange Membrane Discussion 11 2.5.1 Ion-Exchange Membranes 12 2.5.2 Characterization of Ionic Membranes 15 2.5.3 Diffusion in Ion-Exchange Membranes 18 2.6 Bi-Polar Ion-Exchange Membrane s and Their Uses 20 2.6.1 Bi-Polar Membrane Side Orientation 22 2.6.2 Bi-Polar Membrane Water Splitt ing Discussion 23 2.6.2.1 Bi-Polar Membrane Water Splitt ing EMF Calculation 25 2.7 Electrodialysis 27 2.8 Concentration Polarization Effects in Diffusive Membrane Systems 29 2.9 Osmosis 31 2.9.1 Anomalous Osmosis Basics 32 Chapter 3. Initial Phase I Investigation and Discussion 34 3.1 Technical Background and Discussion 34 3.1.1 General Ion-Exchange Elec tro-Membrane Theory Discussion 34 3.1.2 Relevant Related Work 36 3.1.3 Bi-Polar Membrane Discussion 37 3.2 Test Results and Recommendations 38

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iiChapter 4. Detailed Phase II Test Discussion 42 4.1 Overview and Purpose 42 4.2 Research Objectives 43 4.3 Testing Summary 43 Chapter 5. Detailed Phase II Test Results and Analysis 45 5.1 Specific Test Methodology and Details 45 5.1.1 Synthetic Seawater Solution Discussion 48 5.2 Electrode Discussion 49 5.2.1 Electrode Details 49 5.2.2 Scanning Electron Micr oscopy and X-Ray Results 50 5.2.3 Cyclic Voltammetry Test Results 56 5.2.4 Pourbaix Diagram Discussion 59 5.2.5 Electrode Summary 60 5.3 Bi-Polar Membrane Discussion 61 5.3.1 Membrane Details 61 5.3.2 SEM Test Results and Summary 64 5.3.3 Membrane Summary 69 5.4 Electrochemical Impe dance Spectroscopy Discussion 69 5.4.1 Equivalent Circuit Modeling Discussion 76 5.4.2 ECM Model Result Under External Electrical Loading Discussion 79 5.5 Bi-Polar Membrane Concentration Cell Electrical Loading Discussion 84 5.5.1 Electrical Loading Comparison 84 5.5.2 EIS Comparison during Loading 85 5.6 Solution Pumping Speed Dependency on Cell Output Potential 87 5.7 Bi-Polar Membrane Or ientation Discussion 88 5.8 Bi-Polar Membrane Concentration Ce ll Ion Migration and Osmotic Flow 89 5.9 Design of Experiment Modeling Discussion 92 5.9.1 Test Set-up Discussion 94 5.9.2 DoE Predictive Model Results 95 5.9.3 Measured Test Data vs. Predicted 6 DoF Model Results 103 5.10 Micro Electrical Mechanical Sy stems Suitability Discussion 106 Chapter 6. Summary and Future Research 108 Bibliography 112 About the Author End Page

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iii List of Tables Table 2.1 Relation between Signs of G and E 7 Table 2.2 Typical Seawater Composition 28 Table 5.1 ECM 1:10 MII RT OCV Data 78 Table 5.2 ECM 1:10 MII RT 500 Ohm Ext Load Data 80 Table 5.3 Engineering Design Test Resu lts with 500 Ohm External Loading 94 Table 5.4 Measured Test Data vs Predicted 6 DoF Model Results 104

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iv List of Figures Figure 2.1 Galvanic (a) and Electrolytic (b ) Electrochemical Cells 8 Figure 2.2 Schematic Representations of Typi cal Ion-Exchange Membranes 16 Figure 2.3 Chemical Structures for Nafion a nd Sulfonated Polyethylene CEM 17 Figure 2.4 Typical Electr odialysis Unit Layout 29 Figure 3.1 Standard Bi-Polar Membrane Con centration Test Cell 39 Figure 3.2 Bi-Polar Membrane Concentration Ce ll Proof-of-Concept Test Results 40 Figure 5.1 EIS Testing at USF’s Co rrosion Engineering Laboratory 46 Figure 5.2 Cell Performance Testing at USF’s College of Marine Science 47 Figure 5.3 Used 3M HCL Di p 80 Ag Mesh Electrodes 50 Figure 5.4 New 3M HCL Dip 80 Ag Mesh SEM 51 Figure 5.5 New 3M HCL Dip 80 Ag Mesh X-Ray 51 Figure 5.6 Used Electrode SEM Results fr om Concentrated Side Cell Top 52 Figure 5.7 Used Electrode X-Ray Results from Concentrated Side Cell Top 53 Figure 5.8 Used Electrode SEM Results fr om Concentrated Side Cell Bottom 54 Figure 5.9 Used Electrode X-Ray Results from Concentrated Side Cell Bottom 54 Figure 5.10 Used Electrode SEM Results from Dilute Side Cell Bottom 55 Figure 5.11 Used Electrode X-Ray Resu lts from Dilute Side Cell Bottom 56 Figure 5.12 Waveform Used in CV Testing 57 Figure 5.13 New/Used 80 Mesh El ectrode CV Test Results 58 Figure 5.14 Ag Pourbaix Diagram at 25 C in Chloride Solution 59 Figure 5.15 Used MII BPM-9000 Bi-Polar Membrane, CEM/ Side 62 Figure 5.16 Used MII BPM-9000 Bi-Polar Membrane, AEM/ Side 62 Figure 5.17 Used Fumasep FBM Bi-Polar Membrane, CEM/ Side 63 Figure 5.18 Used Fumasep FBM Bi-Polar Membrane, AEM/ Side 64 Figure 5.19 New MII BPM-9000 SEM Image, CEM Side 64 Figure 5.20 New MII BPM-9000 SEM Image, AEM Side 65 Figure 5.21 New Fumasep FBM SEM Image, CEM Side 65 Figure 5.22 New Fumasep FBM SEM Image, AEM Side 66 Figure 5.23 Used MII BPM-9000 SEM Image, CEM Side 66 Figure 5.24 Used MII BPM-9000 SEM Image, AEM Side 67 Figure 5.25 Used Fumasep FBM SEM Image, CEM Side 67 Figure 5.26 Used Fumasep FBM SEM Image, AEM Side 68 Figure 5.27 Sinusoidal Current Re sponse in a Linear System 73 Figure 5.28 Nyquist Plot with Impedance Vector 75 Figure 5.29 Simple Equivalent Circ uit with One Time Constant 75 Figure 5.30 Bode Plot with One Time Constant 76 Figure 5.31 ECM – 1:10 MII RT OCV 78 Figure 5.32 Nyquist Plot – 1:10 MII RT OCV – Measured vs. Modeled 79

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vFigure 5.33 ECM – 1:10 MII RT 500 Ohm Ext Load 80 Figure 5.34 Nyquist Plot – 1:10 MII RT 500 Ohm Ext Load – Measured vs. Modeled 81 Figure 5.35 Bode Plot – 1:10 MII RT 500 Oh m Ext Load – Measured vs. Modeled 82 Figure 5.36 1:10 MII RT Cell Loading Comparison 85 Figure 5.37 EIS Load Comparison Plot 86 Figure 5.38 1:10 MII 80 Mesh Fluid Pumping Speed Comparison Test 87 Figure 5.39 1:100 MII 80 Mesh Bi-Pol ar Membrane Orientation Test 89 Figure 5.40 Evidence of Positive Anomalous Osmosis 90 Figure 5.41 Ion Migration a nd Osmotic Flow Example 91 Figure 5.42 SAS 6 DoF Predicted Model Results (1 of 6) 97 Figure 5.43 SAS 6 DoF Predicted Model Results (2 of 6) 98 Figure 5.44 SAS 6 DoF Predicted Model Results (3 of 6) 99 Figure 5.45 SAS 6 DoF Predicted Model Results (4 of 6) 100 Figure 5.46 SAS 6 DoF Predicted Model Results (5 of 6) 101 Figure 5.47 SAS 6 DoF Predicted Model Results (6 of 6) 102 Figure 5.48 Measured Test Data vs Predicted 6 DoF Model Results 105 Figure 5.49 Comparison of Energy Sources for Energy Harvesting Applications 106

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vi Investigation and Evaluation Of A Bi-Polar Membrane Based Seawater Concentration Cell and Its Suitability as a Low Power Energy Source for Energy Harvesting/MEMS Devices Clifford Ronald Merz ABSTRACT It has long been known from Thermodynamics and written in technical literature that, in principal, instant energy can be ma de available when dilute and concentrated solutions are mixed. For example, a river fl owing into the sea carri es with it a physicalchemical potential energy in its low salt cont ent, some of which s hould be recoverable. As also known, a naturally o ccurring, diffusion-driven, spon taneous transport of ions occurs throughout a solution matrix, thru barrier interfaces, or thru ion-selective membranes from the side containing th e salts of higher co ncentration to the compartments containing the more dilute solution to effect th e equalization of concentration of the ionic species. Since th is ion movement consists, preferentially, of either cations or anions, it leads to a char ge separation and potential difference across the membrane, otherwise known as a membra ne potential. Eventually, when the concentrations in the compartment are the same the cell ceases to f unction. However, if operated as a fuel cell with its respectiv e concentrations continually replenished, equilibrium at a specific value of potential difference is established.

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viiTo capture the energy of this potentia lly significant albeit low power energy source, a suitable energy extraction device is required. The focus of this Ph.D. research effort is to address the concept, research and evaluation of a Bi-Polar membrane based seawater concentration cell and its suitabili ty as a low power energy source for Energy Harvesting/MEMS devices (patent pending).

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1 Chapter 1 Introduction 1.1 Focus and Direction A concentration cell is an electrochemi cal cell consisting of a semi-permeable ion-exchange membrane between two solutio ns containing the same electrolyte in different concentration. With no extern al voltage applied, a naturally occurring, diffusion-driven (osmotic), spontaneous tr ansport of ions occurs through the ionexchange membrane. Ion-exchange membrane s between solutions act as a barrier across which almost no electrolyte can diffuse. Theo retically, the free energy of the system can be completely converted into electric energy. Since ion movement consists, preferentially, of either cations or anions, it leads to a charge separation known as a potentia l difference across the membrane. Nonreplenishing concentration cells using singl e anion/cation ion conducting semi-permeable membranes have been examined in the past, however, the idea of a replenishing Bi-Polar membrane based concentration cell for instant energy genera tion is unique and innovative, and is the basis of this research dissertation.

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2This renewable instant energy has uses in a wide variety of a pplications and size scales depending upon the source of the suppl ied ionic solutions and the anticipated scale/end use of the system. Some of these applications include, but are not limited to: Micro/Nano capacity direct power generation systems required for energy harvesting and nanotechnology low power devices; low power electronic, communi cation devices (e.g., Pico-Radio, or SCADA); devices operated in the marine environment; Medium scale supplemental power generation (such as dire ct and energy recovery devices used in power generation and desalination plants); Large scale direct commercial power generation systems using naturally occurring sa linity gradient differences such as those found where rivers discharge into the sea; and as a possible supplemental energy source for use in the generation of H+ (protons) via membrane water dissociation (or splitting) technology. One envisioned use, and the focus of this dissertation effort, is to provide energy to low power remote sensor networks or wireless monitoring t echnologies where it is impractical or impossible to provide wired pow er. Successful application will result in a combined energy harvester/generator and stor age system capable of providing the very small amounts of power required to couple with and take advantage of miniaturized, lowpower electronics, wireless standards, and modern t echnologies such as MicroElectroMechanical Systems (MEMS) fabric ated using standardized semiconductor processing techniques.

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31.2 Motivation and Purpose The overarching contribution and technical obj ective of this Ph.D. research is to develop increased technical understanding into the ioni c, environmental, and electrochemical effects on th e generated current and volta ge of a membrane based seawater concentration cell. Knowledge obt ained from this effort will lead to the development of innovative engineering pro cess schemes, designs, and equipment for economic generation, based on the applied con cepts of recovery and reuse of energy released from salinity concentration gradient differences: for both defense and domestic applications. Utilizing modern standardized semiconducto r processing techniques, it is possible to make very small, inexpensive sensors. It is desirable, if not necessary, to provide small, low power systems for these devices. Therefore, considering the near limitless source of ionic solutions, the market and tec hnological impact is enormous. Potential impact of the increased techni cal understanding derived from th is research effort can be practically and immediately applied to the development of such a suitable low power system. The intent of this research effort is to: 1) Develop increased technical understanding into the membrane, ionic, environmental, and electrochemical effects be cause of charge separation e ffects of a Bi-Polar semipermeable membrane based, seawater c oncentration cell (p atent pending),

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42) Examine the feasibility/applicability of this Dialytic based membrane concentration cell as a power source for energy ha rvesting devices (patent pending), 3) Evaluate possible MEMS based a pplications (patent pending). 1.3 Participants Dialytics, Inc. (Dialytics) is a sma ll business technology spin-off entity founded by Clifford R. Merz to undertake developmen t of a University of South Florida (USF) technology he invented and was the focus of this research effort. Dialytics and USF’s Division of Patents and Licensi ng have entered into a tec hnology license agreement for development and ultimate commercialization of this technology. As such, Dialytics is the research sponsor and technology licensee while Mr. Merz is the project principal investigator.

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5 Chapter 2 Required Technical Background and Discussion 2.1 Summary of Rule Basics with respect to Cell Reactions and EMF1 A fuel cell is an electrochemical system which converts the free energy change of an electrochemical reaction into electrical energy. In deali ng with the energy relations of cells, thermodynamic principles find very exte nsive applications. However, the use of these principles is subject to one very importa nt restriction, namely, that the processes to which the principles are applied are reversible It is recalled that the conditions for thermodynamic reversibility of processes are (a ) that the driving a nd opposing forces be only infinitesimally different from each other an d (b) that it should be possible to reverse any change taking place by applying a force in finitesimally greater than the one acting. The net electrical work performed by a reaction yielding an electromotive force (EMF) and supplying a quantity of electricity (q) is qE. The EMF of a battery (or other source) is the maximum potential difference (E). But that can be reduced by the drop across the internal resistance of the source. In the real world, there is always an internal resistance, and in batteries the internal resistance increases with time and usage. So the potential difference is the actual measured voltage while EMF is what you would

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6measure in a resistance-free device. Each equi valent reacting q is e qual to the faraday F, hence for n equivalents reacting, q = nF. The electrical work obtai ned from any reaction supplying nF coulombs of el ectricity at a potential E is: Net electrical work = nFE But any work performed by a cell can be accomplished only at the expense of a decrease in free energy. Further, when the electrical work is a maximum, as when the cell operates reversibly, the decrease in free energy, G, must equal the electrical work done as presented in Equation 1, G = -nFE Equation 1 E is the reversible potential and is derived fr om the free energy change for the reaction. The reversible EMF of any cell is determin ed by the free energy change of the cell and Equation 1 is the “bridge” between thermodyna mics and electrochemistry. Information provided by EMF measurements assist in the evaluation of thermodynamic properties. For any spontaneous reaction at c onstant pressure and temperature G is negative, for any nonspontaneous reaction at constant pressure and temperature G is positive, while for any reaction in equilibrium G = 0. In view of this, it may be deduced that for any spontaneous reaction E will have to be positive, for any nonspontaneous

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7reaction E will have to be negative, while for any reaction in equilibrium E will have to be equal to 0. Summarized in Table 2.1 as follows: Table 2.1 Relation between Signs of G and E ----------------------------------------------------Reaction G E ----------------------------------------------------Spontaneous + Nonspontaneous + Equilibrium 0 0 2.2 Electrochemical Cells – Types and Definitions2 Electrochemical cells in which Faradaic currents are flowing are classified as either galvanic or electrolytic cells. A galvanic cell is one in which reactions occur spontaneously at the electrodes when they are connected externally by a conductor (Figure 2.1a). These cells are often empl oyed in converting chemical energy into electrical energy. Galvanic cells of commercial importance include primary (nonrechargable) cells, secondary (recha rgeable) cells, and fuel cells. An electrolytic cell is one in which reactions are effected by th e imposition of an external voltage greater than the open circuit voltage (OCV) of the cell (Figure 2.1b). These cells are frequently used to carry out desired chemical reacti ons by expending electrical energy. Commercial processes involving electrolytic cells include electrolytic syntheses (e.g., the production

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8of chlorine), electrorefining (e.g., copper) and electroplating (e.g., silv er and gold). The lead-acid storage cell, when being “recharged”, is an electrolytic cell. Figure 2.1 Galvanic (a) and Electro lytic (b) Electrochemical Cells In discussing Electrochemical cells, th e following rules are generally used: 1) Cell reaction is the sum of the single electr ode reactions as they occur in the cell. a) The half-cell, called the anode (positive electrode ), reaction is oxidation (looses electrons becomes more negative) – Attracts anions. b) The half-cell, called the cathode (negative electrode ), reaction is reduction (gains electrons – gets less negative or more positive) – Attracts cations. 2) The total cell EMF is the algebraic sum of the single electrode potentials provided each EMF be affixed with the sign correspond ing to the reaction as it actually takes place at the electrode. 3) A current in which electrons (e-) cross the interface from th e electrode to a species in solution is a cathodic current, while electr on flow from a solution species into the electrode is an anodic current.

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94) In an electrolytic cel l, the cathode is negative with respect to the anode; but in a galvanic cell, the cathode is posi tive with respect to the anode. 2.3 Dialytic Power Generation Because concentration gradient driven systems force ion migration from the high concentration side to the low, they are some times referred to as Reverse Electrodialysis or Dialytic systems. Dialytic Power Gene ration Systems can be operated as a fuel cell (an electrochemical cell in which the chemical energy in a fuel is converted directly into electrical energy) or a battery depending upon if the source of the energy is continually fed to the cell or intern ally stored and consumed3 (patent pending). The relationship between the membrane OCV potential and the standard-state Gibbs free energy can be calculated by the Nernst equation – A ther modynamically derived equation relating the potential of an electrochemical cell to the c oncentration of products and reactants. The applicability of the Nernst equation for a concentration cell under external electrical loading will be determined dur ing this research effort. 2.4 Concentration Cell Basics A concentration cell is an electrochemi cal cell consisting of a porous divider between two solutions containing the same elect rolyte in different concentrations. With no external voltage applie d, a naturally occu rring, diffusion-driven (osmotic), spontaneous transport of ions occurs across th e separating barrier, in this case a semi-

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10permeable ion-exchange membrane, across wh ich almost no electrolyte can diffuse. Theoretically, the free energy of the system can be completely converted into electric energy. Since ion movement consists, preferenti ally, of either cations or anions, it leads to a charge separation across the membrane called a membrane potential. Using reversible electrodes of identic al composition inserted into th e two identical solutions of differing concentrations, the potential di fference can be measured directly. 2.4.1 Concentration Cells with Transference1 Unlike chemical cells where in EMF arises from a chemical reaction, concentration cells depend for their EMF on a tr ansfer of material from one electrode to another due to a concentration differen ce between the two. For example, for a concentration cell containing electrodes made up of the same material s (e.g., Silver [Ag]), solutions containing the same ions but at different concentrations where Cconc > CInterface > Cdilute, and a semi-permeable junction separa ting the two, the electrical current transferring across the membrane can be an alyzed using the following expression: Ag(s) | Seawater (conc) ||AEM NaCl aq. Soln (Interface) ||CEM Seawater (dilute) | Ag(s) Since the same electrode is used in each side of the cell, the EMF of the Galvanic Cell Ecell = ECathode – EAnode = 0.00 VDC.

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11This is a dilution process, sodium cati ons permeate through the Cation Exchange Membrane (CEM) from right to left and chloride ions permeate through the Anion Exchange Membrane (AEM) from left to right; with the anode compartment becoming more concentrated and the cathode comp artment more dilute. Through a suitable electrode system, the chemical potentia l becomes electrical potential. The OCV membrane potential of a Bi-Polar membrane is based on an extension of the Nernst equations of monopolar charged semi-permeable membranes to the case of the CEM in series with an AEM and the di ffusion boundary layers adjacent to the BiPolar membrane. Ionic transport in homogene ously charged membranes such as Bi-Polar ones, which consist of a layered structure of two oppositely charged layers, has received attention because these memb ranes show several interes ting phenomena: specifically permselectivity for mono-valent ions and water splitting4. 2.5 General Ion-Exchange Membrane Discussion The heart of any electro-membrane process is the semi-permeable ion-exchange membrane. The main properties required of ion-exchange membranes for success in technical processes are5: 1) Low electrical resistance. The permeability for the counter-ions under an electrical potential gradient should be high to minimize the membrane IR drop loss.

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122) High permselectivity. It should be highly permeable for counter-ions, but should be impermeable to co-ions, and to non-ionized molecules and solvents. 3) Good mechanical stability. It should be mechanically st rong, to prevent high degrees of swelling or shrinking due to osmotic effects, when transf erred from concentrated to diluted salt solutions and vice vers a, and be dimensionally stable. 4) Good chemical stability. It should be stable over a wide pH-range and in the presence of oxidizing agents. 5) Good operating characteristics. Operation ove r a wide range of current densities and under varying conditions of temperature, current density, pH, pressure, etc. 6) Good water permeability. The stability of a membrane is of para mount importance because of their high cost, membranes are required to operate for periods of several years. A factor in the operation of cells with membrane is th e transport of solvent (e.g., water) which accompanies the transferring ions. In aqueous systems, the transport of water can be significant (e.g., 3-5 water molecules accompany one sodium ion in a chlorine cell). If protons (hydrogen ions) are transferred th en typically, two molecules of water are transferred per ion. 2.5.1 Ion-Exchange Membranes6 The most important feature, which dis tinguishes ion-exchange from isotopicexchange membranes, is the electric coupli ng of the ionic fluxes. Conservation of

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13electroneutrality requires stoichiometric exch ange, i.e., the fluxes (in equivalents) of the exchanging counter ions must be equal in magnitude; otherwise, a net transfer of electric charge would result. The regulating mechanism that enforces the equality of the fluxes is the electric field (diffusion potential) set up by the diffusion process that produces an electric transference of both counter ions in the directi on of diffusion of the slower counter ion; this electric tr ansference is superimposed on the diffusion. The resulting net fluxes of the counter ions are equivalent to one another, while pur ely diffusional fluxes, as a rule, are not. Thus, electroneutrality is preserved. If an ionic electro-membrane is in contact with an ioni c solution, a distribution of ions in the solution will be established as well as a distri bution inside the membrane (Donnan equilibrium). If the membrane ha s a negative fixed charge, ions of opposite charge (positively charged i ons or counter-ions) will be attracted towards the membrane surface while ions of the same charge (negativ ely charged ions or co-ions) are repelled from the membrane surface. Ions with the sa me charge as the fixed ions (co-ions) are excluded and cannot pass through the membrane This effect is known as the Donnan exclusion. Because of the fixed charge, there will be an excess of c ounter ion charge at the interface and a so -called electrical double layer (E DL) is formed. Protons and Hydroxyl ions are not effectively retain ed by a Donnan potential and this allows removing these ions from other ions with the same charge. In a basic solution (pH > 7), an ideal CEM is able to reta in all anions except for hydroxyl ions. Similarly, an ideal AEM retains all cations excep t for protons and a separation can be achieved between protons and other cations.

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14Electric current in an ion-exchange membrane transfers predominantly via counter-ions by diffusion. The simple Nernst equation holds reasonably well within the concentration range of about 10-4 to 10-1 N. Deviations at highe r solution concentrations are caused by co-ion transference, and at lower concentrations by H+ or OH(hydroxyl) ions (stemming from dissociation of H20) that compete with th e electrolyte counter ion ( i.e., the increasing concentration of the co-ion in the ion exchanger causes a decrease in the transport number of the counter-ion). Using the Nernst equation, the maximum reversible OCV (Erev) generated across the membrane of a concentration cell using an ideal permselective membrane as a salt bridge, a 1,1 valence electrolyte, and car efully selected elec trodes of identical composition in both compartments can be calculated using Equation 2 as: Erev = Ecell (RT/nF) ln (a conc/a dil) Equation 2 where a = ionic activities (approx. concentratio ns), n is the number of electrons transferred, and = charge on the active ion. Consid ering an ideal membrane (a = 1.0), monovalent active ion, and a 1:10 activity ratio for the two solu tions (i.e., concentrated solution = 10*dilute) the OCV obtained is: Erev = 0.0 – [ *(8.314 J K-1 mol-1 298K) / 1*96,500 C mol-1] ln (0.1) Erev = +0.059 V (spontaneous)

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15Note that because the same electrode is used in the cell, Ecell is zero in the Nernst Equation (see Section 2.4.1). The membrane potential may be higher if the co-ion is more mobile than the counter-ion and if ther e is little Donnan exclusion of the co-ion 2.5.2 Characterization of Ionic Membranes7 Polyelectrolytes are a speci al class of polymers that contain charged ionic groups, with the properties completely determined by the presence of the ionic groups. Besides the fixed charge present, the properties of solubility, diffusi vity, and pore size distribution affect separation. Charged membranes or ion-exchange membranes are typically employed in electrically driven processes su ch as Electrodialysis (ED) and membrane electrolysis as well as pressure or concen tration driven proce sses such as diffusion dialysis and Donnan dialysis (combina tion of Donnan exclusion and diffusion). Strong attractions exist between counter ions and the membrane fixed charge groups. As mentioned, Polylelectrolytes that contain a fixed negatively charged group fixed to the polymeric chain are called Cation Exchange Membranes (CEM) because they are capable of exchanging positively charged cations. When the fixed charged group is positive, the polyelectrolytic membrane is called an Anion Exchange Membrane (AEM) because it is capable of exchanging negatively charged ions. The counter ions (ions with charge opposite to the fixed charge group), can move freely within the limits of the Coulomb forces and electroneut rality. A schematic represen tation of typical membranes of both types is given in Figure 2.2.

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16 Cation Exchange Membrane (CEM) Anion Exchange Membrane (AEM) CH2 – CH – CH2 – CH CH2 – CH – CH2 – CH | | | | R-A+ R-A+ R+AR+AWhere R = SO3(Sulfonic Acid Group) R = N (CH)3 + (Quaternary COO(Carboxylic Acid Group) Ammonium Salt Group) Figure 2.2 Schematic Representations of Typical Ion-Exchange Membranes In water or other strongly polar solvents polyelectrolytes are ionized. However, unless cross-linked, the polymer potion will swe ll or even become soluble because of its high affinity to water. Even very hydrophobic polymers such as polysulfone can be made water-soluble by introducing a large number of sulfonic groups. A very interesting polymer for preparing ionic membranes is Polyte trafluororethylene. This polymer is very stable with respect to chemicals, pH and temperature. Ionic groups can be introduced into this polymer to yield a very stable polyelectrolyte based on a Teflon matrix. One such polymer obtained on this basis is Nafion shown in Figure 2.3 below.

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17[CF2 – CF2] – CF2 – CF CH2 – CH – CH2 – CH | | | [O – CF2 – CF -] – O – CF2 – SO3 -Na+ SO3 -Na+ SO3 -Na+ | CF3 Nafion Sulfonated Polyethylene Figure 2.3 Chemical Structures for Na fion and Sulfonated Polyethylene CEM Cation selective membranes are usually made of cross-linked poly (styrene-codivinylbenzene) base polymer that has been sulf onated to produce sulfonate groups [-SO3 -] or carboxylate groups [-COO-] attached to the polymer. Anion membranes can be cross-linked poly (sulfone) base polymer containing quatern ary ammonium groups [-N (CH)3 +]. Currently, aliphatic anion membrane s are favored because they have lower electrical resistance8. Homogeneous membranes are coherent unsupported gels. Heterogeneous membranes are prepared by incorporating colloidal ion-exchanger particles into an inert binder. To obtain structural support, a membrane is fabricated by applying the cationand anion-selective polymer to a fabric or wide-mesh plastic tissues material9. Membranes are made in flat sheets and cont ain about 30 to 50% water. Each membrane has a network of molecular-si ze pores that are too small to allow significant water flow and that have electronegative [-SO3 -] or electropositive [-N(CH)3 +] fixed charges10.

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18For electrical neutrality to be main tained, each of the fixed charges on the membrane must be associated with an i on of the opposite charge. The ion can easily move from one fixed charge to another. Thus, the membrane can pass an electrical current in the form of migrating ions. Si nce the fixed-charged groups on the membrane repel like-charged ions, anions canno t enter the anion selective membrane11. Not being perfectively semi permeable, the membranes do not completely reject ions of the same charge. However, their permselectivity is > 90%. 2.5.3 Diffusion in Ion-Exchange Membranes When sodium chloride (NaCl) is dissolve d in water, it is ionized and dissociates into hydrated Na+ (aq) cations and Cl(aq) anions. Strong electrolytes, such as Na+ (aq) and Cl(aq), are so well hydrated th at they are too far apart to interact directly with each other, even in solutions of great ionic streng th. Ions that are so well hydrated that they experience only nonspecific interactions with each other are termed “free ions”. Ions not so well hydrated can come into closer contact In cases where ions are close enough to share some of their primary sa lvation shells, the re sulting strong electros tatic attraction is referred to as ion pairing12. Because ions are hydrated in solution a nd the extent of hydration depends on the charge and size of the ion, di-a nd tri-valent ions move more slowly across the membrane than do monovalent ions. Acco rdingly anions such as Clpass through the electromembranes more readily than H+, Na+, K+, and other cations. Sodi um chloride in water

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19does not diffuse as a single molecule; instea d the sodium ions and chloride ions move freely through the solution. Although the sodium ion diffuses more slowly than the chloride ion, the diffusion of sodium chlo ride can be accurately described by a single average diffusion coefficient. Protons a nd hydroxyl ions have unusually high diffusion constants. The ion-exchange mechanism is primarily a redistribution of the counter ions by diffusion. The co-ion has relatively little e ffect on the kinetics and the rate of ion exchange. The basic equation used to co mpare various membrane processes when transport occurs by diffusion is given in Equation 3 as: Ji = ( i/d)*(ci,1 s – i ci,2 s exp [-Vi (P1-P2)/RT]) Equation 3 Where, Ji = Flux of component I through the membrane; m/s Vi = Molar Volume (m3/mol) i = Permeability coefficient = Di (Diffusional Coefficient) Ki (solubility constant defined as the ratio of activity coefficients) i = K I,2 / K I,1 d = Membrane thickness (m) P1-P2 = Pressure difference across the membrane (N/m2) R = Universal Gas Constant = (J/mole.K) T = Temperature in Degrees Kelvin (C + 273)

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20In concentration cells, li quid phases containing the same solvent are present on both sides of the membrane in the absence of a pressure difference. The pressure terms can therefore be neglected and Equati on 4 developed with the condition of i = 1 as: Ji = ( i/d)*(ci,1 s – i ci,2 s) Equation 4 Equation 4 shows that the concentration cell solute flux is proportional to the concentration difference (as in Reverse Osmosi s). Separation arises from differences in permeability coefficients: these macromolecules have much lower diffusion coefficients and distribution coefficients than low mol ecular weight compounds such as salts. 2.6 Bi-Polar Ion-Exchange Membranes and Their Uses A Bi-Polar membrane consists of a monopolar CEM and monopolar AEM joined together with an intermediate transitional pha se layer between. To explain the transport of ions through a charged membrane, the in teraction between ions and fixed charge groups inside the membrane as we ll as at the interface has to be considered. In Bi-Polar membranes, there are 3 interfaces: (i) the in terface between the concentrated saline solution and the anion-exchange membrane (ii) the interface between the anionexchange membrane and the cation-exchange membrane (intermediate transitional phase layer), and (iii) the interface be tween the cation-exchange membrane and the dilute saline solution. Among these three interfaces, the in termediate transitional phase layer is the most difficult to observe and the concentr ation cannot be measured experimentally.

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21The transport properties of Bi-Polar membranes are quite different from those of monopolar membranes. When an electric field is established across a Bi-Polar membrane the anions and cations contained in the inte rmediate layer migrate through the AEM and CEM in the direction of the el ectric field. Because of the current flow the intermediate layer becomes impoverished in salt and its resistance increases. Two EDL and Donnan potential differences develop between the in termediate layer and the outside and are opposite to the applied field. For a constant outer potential difference, the current density of initial value io reduces to a much smaller value it with the current density below a limiting current ilim 13. In the Bi-Polar membrane’s matrix the concen tration of the counter-i ons is equal to the sum of the concentration of the fixed ions and co-ions. As one approaches the intermediate layer of the Bi-Polar membra ne the concentration of the mobile ions (counterand co-ions) d ecreases until a depleted layer aris es in which the mobile ions have a fundamentally lower concentration than the fixed ions. Although made up of well-defined indivi dual components, once combined the BiPolar membrane acquires unique capabilities and additional uses. These include: 1) a variation in membrane poten tial depending upon which side is in contact with the concentrated solution, which is not the case in monopolar ion-exchange membranes14, and 2) its use in converting water-soluble salts to their correspondi ng acids and bases via the process of water dissociati on, known also as splitting.

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222.6.1 Bi-Polar Membrane Side Orientation The two-monopolar layers of a Bi-Polar memb rane always differ in their fixed ion molarities and in the sign of their charge These differences are the cause of the asymmetrical character of Bi-Polar membranes13. Unlike monopolar ion-exchange membranes6, in Bi-Polar membranes, the membrane facing direction and the intermediate phase condition will alter the direction of the membrane potential charge. The intermediate layer in a Bi-Polar membrane se ems to act as an alteration barrier for the membrane potential according to the membrane facing direction15. If the concentration of the immediate layer is lower than that of the external solutions, the ion-exchange layer which faces the concentrated solution will pl ay the dominant role in determining the whole membrane potential, because the con centration ratio between the intermediate layer and the external concentrated soluti on is much higher than that between the intermediate phase and the external dilute solution. According to evolving literature conventi on, a Bi-Polar membrane is in the (+) orientation when it’s positively charged (anion -active) AEM layer is in contact with the more concentrated solution. Generally, the values of the concen tration polarization Ec, consisting of two Donnan potentials on th e two boundaries membrane-solution, are smaller with monopolar membranes and less posi tive with Bi-Polar membranes in the (+) orientation than the (-) orientation. In the (+) orientation, the overall charge and (and the Ec value) of a Bi-Polar membrane must be less positive than in the other because the

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23contact of the positively char ged layer with the more concentrated solution leads to a screening of the positive charge by the electrolyte and vice versa. 2.6.2 Bi-Polar Membrane Water Splitting Discussion It is well known that salts in solution can be converted unto their corresponding acids and bases by a process called electrodialytic water splitting across a Bi-Polar membrane. In the presence of a potential fi eld, water at the inte rface will dissociate according to the following relation: H20 H+ + OHThis water dissociation and its coupling with ion transport of fer the possibility of using Bi-Polar membranes in a great vari ety of practical applications. The process of electrodialytic water split ting consists of a Bi-Polar membrane arranged between two electrodes. If an el ectrical potential difference is established between the electrodes, charge species are removed from the interface between the two layers. When all the salt ions are re moved from the solution between the two membranes, further transport of electrical charges can be accomplished only by the protons and hydroxide ions available from th e ionization of water. Water so removed from the interface is replenished by water diffusing into the interface16.

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24When no ions are available w ithin this region, further tran sport of electric charge can be accomplished only by H+ and OHions, which are available even in completely desalinated water. At a theoretical potent ial of 0.828 V (see Secti on 2.6.2.1), the water in the AEM dissociates (splits) in to equivalent amounts of H+ and OHions. These ions ideally migrate from the intermediate layer with the H+ ions permeating through the CEM side and the OHions permeating the AEM side. However, H+ and OHions are not very effectively retained by a Donnan potential and co-ion leakage of H+ through the AEM as well as the OHleakage through the CEM can occur7. In addition, the H+ leakage through the AEM will increase with the water content of the membrane. In Bi-Polar membranes both the cationand anion-exchange groups of the membrane polymer adjacent to the interphase la yer can react with the water molecules. It is thought, however, that the water splitting reacti on takes place primarily at the anionic surface17. One theoretical analysis suggests th at water splitting in anion exchange membranes containing quaternary ammonium gr oups is due to the pr esence of tertiary alkyl amino groups in the surface regions that cannot bond with the water molecules. By contrast water splitting is not manifested by cation exchange memb ranes with sulphonic groups if the system is sufficiently clean18. Although the theoretical potential to achieve water dissociation is 0.828 V, the charged species within the intermediate layer are removed at a lesser potentia l and research is ongoing to determine if some H+ and OHion generation may occur at a lesser potential caused by dire ct membrane interactions.

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252.6.2.1 Bi-Polar Membrane Water Splitting EMF Calculation The conventional method for generating H+ and OHions from water utilizes electrolysis. Electroly sis also generates O2 and H2 gas, and the over voltage for this generation consumes about half of the electrical energy of th e process. An alternative to this uses special ion-exchange membrane s developed specifically for splitting water directly into H+ + OHions without generating gases. Membrane water splitting technology is a general purpose unit for c onverting water-soluble salts to their corresponding acids and bases. The proce ss uses Bi-Polar membranes in conjunction with conventional AEM/CEM under the presence of a direct cu rrent driving force for ion separation and rearrangement. The water splitting process is electrodialytic in nature because the process merely involves changing the concentra tion of ions that are alrea dy present in solution. The theoretical energy for concentrating H+ and OHions from their concentration in the interface of the Bi-Polar membrane to the ac id and base concentrations at the outer surfaces of the membrane can be calculated readily. The free energy change in going from the interior of the membrane to the outside is given by: G = -nFE = -RT ln ([(ai H+ ai OH-)/(ao H+ ao OH-)]

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26Where a’s are the activities of the H+ and OHions, superscripts i and o refer to the interface and outer surfaces of the membrane s respectively. For generating one normal ideal product solution it reduces to (since n = 1) to: G = -nFE= -RT ln (ai H+ ai OH-) or E = -RT/nF ln Kw and Kw is the ion-product or diss ociation constant of water. To overcome this potential, a potential E = Ecell = -E must be applied across the membrane. Using the data on free energy for dissociation of water one can calculate the theoretical potential for generating acid a nd base for an ideal, i.e., perfectively permselective, Bi-Polar membrane as 0.828 V at 25C and 0.874 V at 70 C19. The actual potential drop across a Bi-Polar membrane is qu ite close to this being in the range of 0.9 – 1.1 V3. Using standard reduction potentials, Ecell and Kw can be calculated for H20 H+ + OH. At the electrode surfaces the follo wing half-cell reactions occur: Anode (positive/right electrode): 2H+ (aq) + eH2(g) E = 0.000 V Cathode (negative/left electrode): 2H20 (l) + 2eH2(g) + 2OH(aq) E = -0.828 V From which the standard EMF for this reaction can be calculated as:

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27Ecell = ECathode EAnode = -0.828 0 = -0.828 V and the ion-product or Kw calculated below from Ecell to be equal to 0.986 x 10-14 from Kw = exp (nFEcell/RT) = [(1*96,500 C mol-1 -0.828) / (8.314 J K-1 mol-1 298K)] 2.7 Electrodialysis ED is the most common electro-membra ne process used for desalination and concentrating of aqueous solutions. ED depends on the following general principles: 1) Most dissolved in seawater salts are ioni c, being positively (cationic) or negatively (anionic) charged (See Table 2.2), 2) These ions are attracted to elec trodes with an opposite charge, 3) Seawater is nominally 86% NaCl with Na and Cl nearly 100% ion free, 4) When NaCl is dissolved in wate r, it dissociates into hydrated Na+ and Clions. ED uses an electrical pote ntial to move salts selectively through permselective membranes, leaving behind fresh product water. ED is different from other desalinati on membrane processes in that it is electrically driven rather than pressure driven. Thus, in ED, only ions and associated

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28Table 2.2 Typical Seawater Composition Major constituents in surface seawater*Seawater TDS = 35,000 m g /l ( pp m) T yp ical O p en Ocean p H = 8.2 Atomic H y dratio n Ion% Concentration CationWeightNumberChar g eIon Freeg/kgmg/kg (ppm)mol/gmmol/kg (ppm)meq/kg (ppm) CaCO3 meqSodiumNa+ 22.989821989910.78110,7810.4689468.947468.94723447.35 PotassiumK+ 39.09830.6198-990.3993990.010210.20510.205510.25 MagnesiumM g ++ 24.3055.1287-901.2841,2840.052852.829105.6575282.86 CalciumCa++ 40.084.3289-910.41194120.010310.27720.5541027.69 StrontiumSr++ 87.623.720.0079480.00010.0910.1819.06 BoronB+++ 10.8130.004550.00040.4161.24962.44AnionChlorideCl35.4530.9-110019.35319,3530.5459545.878545.87827293.88 SulfateSO4-96.0676-239-542.7122,7120.028228.23056.4602823.01 BicarbonateHCO364.0118-169-800.1261260.00201.9681.96898.42 BromineBr79.9040.9-10.0673670.00080.8420.84242.11 FluorineF18.99841.8-10.001310.00010.0680.0683.42 An Introduction to the Ch emistry of the Sea by M. E. Q. PilsonTotal salt weight (gms)/Kg seawater =35.15 % Monovalent Content =86% % Divalent Content =13%General Notes/Definitions1. A one molar solution is prepared by adding one mole of solute to one liter of solution 2. Based on Boric Acid [B(OH)3] concentration of 0.0257 g/kg at 35 per mil salinity which does not ion pair 3. 1 ppm or 1 mg/kg is the lower limit of the major constitiuents which are mostly conservative 4. meq = milli equivelents = mol/g normalized by ion charge 5. 1 Liter = 1000 grams so meg/kg is also meg/Liter, similarly mmol/kg is also mmol/Liter 6. CaCO3 meg is defined in terms of Alkalinity by mult meg/kg by 50 (the equivelent weight of CaCO3)7. What is the molarity of sodium chloride in seawater? Re member 1000g = 1 liter and a 1 molar solution is prepared by adding 1 mole of solute to 1 liter of solution 10.781+19.353 = 30.134 gms of NaCl in seawater (86% of total salt). Mol wt of NaCl is 22.9898+35.453 = 58.4428 g/mol. Therefore, 30.134/58.4428 = .52 molar 8. What is the molarity of the water in seawater? Reme mber 1000g = 1 liter and a 1 molar solution is prepared by adding 1 mol e of solute to 1 liter of solution 1000 35.15 = 964.85 gms of H2O in seawater. Mol wt of H2O is 1+1+15.99 =18.01528 g/mol. Therefore, 964.85/18.015= 53.56 molar or about 100:1 vs the NaC l water is transferred. Cations, under the influe nce of the negative electrode move through the CEM but are stopped at the AEM interface. Similarly, anions under the influence of the positive electrode move through the AEM but are stopped at the CEM interface. In a typical ED configuration, AEM and CEM’s ar e alternately arranged with a spacer sheet between to form a “cell”. The basic ED un it consists of severa l hundred cell pairs bound together with electrodes on the outside and is referred to as a membrane stack. Pathways in ED units are separated by a cation/anion membrane stacks and direct current provides the motive force for ion migration from th e low concentration side to the higher concentration side. By this arrangement, con centrated and diluted solutions are created in

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29the spaces between the alternating membrane pairs. Figur e 2.4 presents a typical ED unit layout. Figure 2.4 Typical Electrodialysis Unit Layout 2.8 Concentration Polarization Effe cts in Diffusive Membrane Systems In pressure driven processes such as Re verse Osmosis (RO), Microfiltration (MF) and Ultrafiltration (UF), solutes are retained to some extent by the membrane. In this way the solute concentration profile has been established by the convective flow towards

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30the membrane and the diffusive back transport of the solute towards the bulk. In ED units, adjacent fluid pathways are separa ted by a cation/anion membrane stack where applied direct current provides the motive force for ion migration. In these cases, concentration polarization and boundary layer effects can be significant. In ED application, the solu tion compartments and membranes, being in series, must carry the same electrical current. In the solution compartments, both the cations and anions carry the current. In the membrane s, however, only one type of ion (cation or anion) can do this. Therefore, the ions in the membranes must travel at twice the speed that they move in the bulk solution compartments. This causes the concentration of the ions to be depleted on the entrance side of the membrane in comparison with the concentration in the bulk so lution. This concentration polarization requires a higher current to transport the ions. If the curre nt is increased to the critical point ilim – at which the membrane surface on the entrance side is tota lly depleted of ions (cations or anions) – two results follow: 1) An increase in the resistance cons iderably boosts energy consumption, 2) H+ and OHions are transported across the me mbranes, and water is dissociated. The transport of H+ and OHions result in locally high and low pH levels at the membrane surfaces20. The locally high pH on the brine side can cause salts with limited solubility such as CaSO4 and CaCO3 to precipitate19. The limiting current, the current at the critical point, ilim, is directly related to: (1) the con centration of the ions in the bulk

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31solution (the limiting current is smaller for a lower total-dissolved-solids solution), and (2) the thickness of the boundary layer. However, in purely diffusive driven systems such as concentration cells, preferential ion transport occurs through the membrane according to the internally generated driving force with the concentration of solute at the membrane surface dependant upon the flux through the membra ne, membrane retention, the diffusion coefficient of the solute D, and the th ickness of the concentration boundary layer d, i.e ., the region near the membrane in which the concentration of solute varies. Because of current densities generally below ilim, low transport rates, and low solute mass transfer rates, it is frequently assumed that the resi stance to ion transport in concentration cell systems is determined primarily by the memb rane phase with boundary layer resistances neglected5. 2.9 Osmosis Osmosis is the phenomenon of water, solvent, flow through a semi-permeable membrane that blocks the transport of sa lts (solute). With membranes which are nonionic or completely impermeable for the solu te, the solvent flux is from dilute to the concentrated solution and is proportional to the osmotic pres sure difference between the two solutions. This pressure difference occurs whenever a membrane separates a solvent from a solution and is given almost entirely by the total concentrati ons of the dissolved species (ions or molecules) and depends very little on the individual species.

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322.9.1 Anomalous Osmosis Basics21 Solvent diffusion, osmosis, across an i on-exchange membrane is, as a rule, anomalous, i.e., not proportional to the osmotic pressure between the solutions. The pore liquid in the membrane carries a net electric charge and, hence, is affected by the both the pressure gradient and the elec tric potential gradient which arises from ionic diffusion. The effect of the pressure gradient alone always results in Positive Osmosis. The swelling pressure, i.e., the pr essure difference between the ion-exchange membrane and the equal hydrostatic pressure, is higher on the side of the dilute solution (higher free energy) and drives the solvent toward the concentrated solution side. The effect of the pressure may be enhan ced, partly balanced, or even out weighed by the electric field. Strong diffusion potential s arise when the mobility of the counter ion and co-ion differ greatly. If the counter ions are faster, th e resulting electric field – in addition to enforcing equivalence of the ionic fl uxes – drives the electrically charged pore liquid as a whole toward the concentrated solu tion. The effect of the electric field thus adds to that of pressure and produces Anomal ous Positive Osmosis. However, if the coion is faster, the electric field has opposite sign and drives the pore liquid toward the dilute solution. The electric field may be strong er than the pressure. In this case there is Anomalous Negative Osmosis. Here, solven t diffusion provides the energy required for transferring the electrolyte ag ainst its chemical potential gradient. The described anomalous phenomena only apply to the solvent.

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33The transport of solvent (water) throu gh the membrane is a critical factor determining the performance of fuel cells, and the water balance between anolyte and catholyte is important in many applications of electrolysis, determining for example the maximum concentration of product which can be achieved. Beside s the pressure and chemical gradients discussed, water passes th rough the membrane with the ions within their hydration shells, i. e., electro-osmosis.

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34 Chapter 3 Initial Phase I Investigation and Discussion 3.1 Technical Background and Discussion A primary driving force behind the indus trial development of membranes has been desalination for municipal drinking wa ter supplies where ED and RO have been used for water desalting. More frequently, desalination plants are being co-located near power generation stations such that power plant discharge cooling water of elevated temperature and thus increased ion-mobility becomes the desalination plant source water. High TDS RO waste stream concentrate is th en available for additional processing to allow economic recovery and reuse of the energy in the waste stream. Sources of dilute brine include brackish river/estuarine water or treated municipal water (also typically colocated). In order to capture the energy of this potentially significant energy source, a suitable energy extraction device must first be developed. 3.1.1 General Ion-Exchange Electro -Membrane Theory Discussion The heart of any electro-membrane proce ss is the ion-exchange membrane. It usually consists of a polymer film with ionic groups attach ed to the polymer backbone. As mentioned in Chapter 2, if these “fixed charges” contain a negatively (anionic) charged group fixed to the polymeric chai n they are called a CEM because they are

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35capable of exchanging positively charged ca tions. When the fixed charged group is positive (cationic), the membrane is called an AEM because it is capable of exchanging negatively charged anions. In the case of ca tionic fixed charges, the freely moveable counter-ions are anions. The membrane, therefore, exhibits ion-exchange properties for counter-ions, which can permeate the membrane easily and excludes co-ions (of the same charge as the fixed charges) from the passa ge. The “permselectivity” between counterand co-ions can reach values up to 99%. Th e permselectivity decreases with increasing ion concentration of the outsi de solution and decreasing capacity and degree of cross linking of the ion-exchange me mbrane. All electro-membrane processes make use of the above permselectivity of ion-exchange membranes3. An electric field in an electrolyte solu tion produces transferen ce of ions whose transport across an ion-exchange electro-membrane. In a solution of uniform composition under the assumption of electro-neut rality, the transference of an arbitrary ionic species in the direction of the current is proportional to the gradient of the electric potential, the concentration difference, and the electrochemical va lence of the ionic species. It is irrelevant whether the field is generated by an external source (as in ED) or generated internally via concen tration gradient driven diffu sion, since the individual ion has no means of knowing the orig in of the electric field6. Electric current in an ion-exchanger tr ansfers predominantly via counter-ions by diffusion. The co-ion has relatively little e ffect on the kinetics and the rate of ion exchange. The Nernst equation holds reasona bly well within the c oncentration range of

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36about 10-4 to 10-1 N for estimations of OCV. Deviati ons at higher solution concentrations are caused by co-ion transference, and at lower concentrations by H+ (protons) or OH(hydroxyl) ions (stemming from dissociation of H20) that compete with the electrolyte counter ion ( i.e., the increasing concentration of the co-ion in the ion exchanger causes a decrease in the transport number of the c ounter-ion). As discussed in Section 2.5.1, the limiting OCV value of the membrane potenti al (at room temper ature) is 0.059 V per power of 10 activity ratios using a single ideal monopolar membrane, a 1,1 valence electrolyte, and reversible electrodes4. This cell membrane potential may be higher if the co-ion is more mobile than th e counter-ion and if there is li ttle Donnan exclusion of the co-ion6. 3.1.2 Relevant Related Work Although numerous authors have written on the subject of non-replenishing concentration cells, Ohya’s22 test configuration using a sing le pair of anion/cation monopolar semi-permeable membranes separa ted by a center region is particularly relevant. Ohya reported maximum cell OCV values of nominally 0.100V after several hours before dropping off. Ohya theorized that the reason for this OCV build up and drop off might be that as time passed, the center compartment concentration increased with a corresponding decrease in electrical re sistance, resulting in an increase in the observed OCV. However, as the center compartment’s concentration continued to increase, the relative concentration between th e center and either si de decreased with a corresponding decrease in th e cell’s potential. This s hould not be the case in a

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37replenishing Bi-Polar membra ne concentration cell design an d to the author’s knowledge there is no comparable research or technol ogy currently operating on this unique concept. 3.1.3 Bi-Polar Membrane Discussion An extensive literature and membrane eval uation review revealed the presence of Bi-Polar membranes. A Bi-Polar memb rane consists of a monopolar CEM and monopolar AEM joined together with an interm ediate transitional phase layer in between. Although made up of well-defined components, once combined the Bi-Polar membrane acquires unique capabilities and additional uses. These include: 1) an apparent variation in membrane potential depending upon whic h side is in contact with the more concentrated solution, which is not the case in monopolar ion-exchange membranes14; 2) its use in converting water-soluble salts to their corresponding acids and bases via the process of water dissociation (or splitting). Where H+ and OHions, removed from the transitional phase layer, ar e replenished by water tran sported into the membrane2. These additional and unique benefits coupled with Ohya’s findings led me to BiPolar membrane concentration cell testing ra ther than continuing with the typical ED based anion/cation monopolar membrane stack arrangements. As is the case with standard AEM/CEMs, when an electrical fiel d is established across a Bi-Polar membrane the transfer of electrical char ge will be carried preferentia lly by the ions present. However, under the effect of an electric fiel d, charged species are also removed from the transitional phase layer between the two ion-exchange layers.

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38When no ions are available w ithin this region, further tran sport of electric charge can be accomplished only by H+ and OHions, which are available even in completely desalinated water. At a theoretical potent ial of 0.828 V, the water in the transitional phase layer dissociates (splits) into equivalent amounts of H+ and OHions. These ions ideally migrate from the intermediate layer with the H+ ions permeating through the CEM side and the OHions permeating the AEM side. However, H+ and OHions are not very effectively retained by a Donnan potential and co-ion leakage of H+ through the AEM as well as the OHleakage through the CEM can occur7. 3.2 Test Results and Recommendations An initial baseline, bench top, proof-ofconcept laboratory test apparatus was constructed, see Figure 3.1. It consisted of a single concen tration cell of cubic design, a peristaltic pump, and a computer running a data acquisition program The test cell consisted of two symmetrical sections (a conc entrated solution side and a dilute solution side) separated by a single Bi-P olar membrane. Each section consisted of an end plate, electrode, and test chamber. The concentr ated ionic solution si de consisted of an entrained saturated slurry reservoir. 1:10 ionic test solutions were made from dilutions of the saturated concentrate. The dilute solu tion was pumped through the dilute cell side and collected in a 1,000 ml beaker. Measuremen ts of the dilute input/output ionic test solution salinity revealed higher levels of Clin the output solution corresponding to possible CEM co-ion migration (lower an ion permselectivity). Low cation AEM

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39permselectivity may also be present but measurements were not made for specific cations, nor were they made for solution pH, or the presence of multi-valence ions. Figure 3.1 Standard Bi-Polar Memb rane Concentration Test Cell OCV and various loaded voltage measurements were made with a computer running a data acquisition program. Figure 3.2 be low shows a result from one of the test configurations using a Bi-Polar membrane in the (+) orientation. Note the battery like performance including a possible Coup de F ouet effect occurring just after initial discharge. As can be seen in Figure 3.2, the OCV value meas ured was several times that predicted by the Nernst equa tion and more than twice that reported by Ohya, although measurements made under electrical loading were significantly lower in value.

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40 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 130160190112011501 1801210124012701 # of data pointsPotential (mVDC) 500 Plateau = 18.0 mvDCOC Plateau = 218.3100 Plateau = 5.7 OC Plateau = 257.850 Plateau = 2.2 10 Plateau = 0 Initial Phase I RT 1:10 Test Results July 2003 Figure 3.2 Bi-Polar Membrane Concentrat ion Cell Proof-of-Concept Test Results Although small in value, the measured cell membrane potentials were in the realm suitable for low power energy harvesting or MEMS applications. Based on these favorable results, a more thorough testing prog ram using a consistent test configuration subject to changing membrane/electrode parameters and ionic/environmental effects was devised and implemented. Additional research was conducted in order to investigate and evaluate the membrane potential generati on in a Bi-Polar electro-membrane based seawater concentration cell and its suitabili ty as a low power energy source for energy harvesting/MEMS devices. Included in this te st effort was electrochemical testing and modeling required to determine an equivalent cell circuit design impedance for maximum power delivery, via impedance matching, to a co upled electrical device. This phase II

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41detailed test discussion is presented in Chapter 4. Phas e II testing methodology, results and analysis are presented in Chapter 5, fo llowed by a contribution and future research recommendations summary in Chapter 6.

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42 Chapter 4 Detailed Phase II Test Discussion 4.1 Overview and Purpose As discussed in Chapter 3, a phase I pr oof-of-concept Bi-Polar membrane based concentration cell was built and tested with the measured OCV values several times the value predicted by the Nernst equation and more than twice that reported by Ohya, although measurements made under electrical load ing were significantly lower in value. The initial proof-of-concept test apparatus was a single cubic shaped cell, but other geometries are possible depending upon the e nd use. Further, a lthough a single cell was initially tested, it is envisioned that in actual use a plurality of cells will be aligned in series/parallel configurations to generate the desired output power. In addition, a variety of differing cell designs may also be included within the array structure, such as an annular design suitable for inclusion into a high flow rate power plant or desalination plant application, designs in cluding ion exchange resins or conductivity enhancing materials, and designs combin ed with monopolar membranes. The broad use of options permits a wide variety of applications for the instant power generating system, with controls bei ng devised by the types and numbers of cells

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43in the generating array. This dissertation provides documentation detailing the testing results along with analysis and recommendati ons, serving as a baseline for full-scale testing during later phases of this technology development. 4.2 Research Objectives The over arching research goal is to provide a contribu tion to the body of knowledge as well as to suggest one soluti on to the engineering problem of how to extract useful energy from available dilute and concentrated salin e solutions. This exploratory research effort c onsisted of several major areas: 1) Determination of membrane/elect rode processes and kinetics, 2) Equivalent circuit component modeling, 3) System wide cell performance testing, 4) Determination of cell parameter inter-relationships, 5) Examine the feasibility of this Dialytic based membrane concentration cell as a low power energy source for energy harvesting/MEMS devices. 4.3 Testing Summary This Phase II testing effort consisted of an extended testing/monitoring program designed to determine OCV and loaded voltage and current output vice parameters such as: external electri cal loading; temperature; pH, solution concentration, membrane selection and orientation, and electrode surface area. To minimize membrane and

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44electrode fouling effects, synthetic seawat er solutions were used and the results compared. Close inspection of the test se t up, membrane, and electrodes were conducted during the testing period includ ing the use of a Scanning Electron Microscope (SEM) to ascertain membrane and electrode state of h ealth. Detailed ion-transfer across the BiPolar membrane was analyzed by frequent Clion titration of solu tion samples removed from each side of the test cell at va rious times during the test runs. Electrochemical methods such as Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) along with e quivalent circuit mode ling were used to establish electrode processes and kinetic s corresponding to changes in operating parameters. Statistical Design of Experime nt (DoE) methods were used to investigate various parameter interactions Cell performance testing consisted of the standard baseline proof-of-concept cell design opera ted over various test factor variable configurations. MEMS based application cons iderations were then considered in the latter part of the Phase II effort.

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45 Chapter 5 Detailed Phase II Test Results and Analysis 5.1 Specific Test Methodology and Details To examine performance, the Phase I s tandard Bi-Polar membrane concentration test cell was run for various periods of time and loading conditions and the results compared. The standard atmospheric pressure testing effort consisted of a monitoring program designed to determine OCV and load ed cell membrane potential voltage and current vice the following input parameters: 1) Synthetic seawater solution temper ature (5 to 40 C, nominally), 2) Synthetic seawater solu tion concentration differe nces (e.g., 1:10, 1:100), 3) Bi-Polar membrane end use differences (e .g., industrial electrochemical plating vs. water purification ED), 4) Solution pumping speed (Fisher Scientific low flow peristaltic pump 13-876-1), 5) Bi-Polar membrane orientation, 6) Silver (Ag) wire mesh electrode surface area.

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46Room temperature CV and EIS measurem ents were conducted over a frequency range of 10 MHz to 1 mHz using a So lartron SI 1260/1287 Frequency Response Analyzer and supporting test equipment to establish electrode, membrane, and full cell component characterization. This testing occurred at the Un iversity of South Florida’s (USF) Corrosion Engineering Laboratory (Tampa, FL, ENL 111, Dr. Alberto Sags) and the University of Kentucky’s Center of Applied Energy Research (Lexington. KY, Dr. Stephen Lipka). Figure 5.1 shows EI S testing at USF’s Corrosion Engineering Laboratory. Figure 5.1 EIS Testing at USF’s Corrosion Engineering Laboratory Full Cycle temperature and ex ternal electrical loading performance measurements were conducted using a using a hand held data logger (Vernier Software and Technology), computer, and supporting test equi pment to establish cell membrane output

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47potential characterization. This testing occu rred at the University of South Florida’s (USF) College of Marine Science (CMS) (St. Petersburg, FL, KORC 2129, Dr. Luis Garca-Rubio). Figure 5.2 shows pe rformance testing at the USF CMS. Close inspection of the test set up, me mbrane, and electrodes were conducted during the testing period. SEM and X-Ray im aging techniques were used to ascertain membrane and electrode state of health. Bi -Polar membrane from several manufactures were obtained and used duri ng the testing period for comp arison. Specific membrane details came from the manufacturer and literatu re, as available, supplemented by testing. Figure 5.2 Cell Performance Testing at USF’s College of Marine Science

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48 Because of the number of cell compon ents and their coupled parametric interactions, it was desirable to characteri ze each individually and determine their respective interactions using Design of Experiment (DoE) and analysis techniques via Statistical Analysis System’s (SAS) statisti cal analysis package. EIS impedance plots and cell Equivalent Circuit Modeling was anal yzed using Solartron’s ZView2 software package by Scribner Associates, Inc. 5.1.1 Synthetic Seawater Solution Discussion Instant Ocean Synthetic Sea Salt, made by Aquarium Systems, was used throughout this testing effort to provide a su itable medium for a s eawater concentration cell with out the sometime deleterious effects of marine biofouling. Test solutions along with nominal Cltitration and pH measurement values were made as follows: 1) Concentrated Test Solution: 300 grams of Instant Ocean added to enough Deionized (DI) water [Millipore Milli-Q 18.2 MW*cm] to make 1 liter of total solution. pH = 8.1 and Cl= 3.62M (N), 2) 1:10 Test Solution: 100 ml of concentrated test solution added to 900 ml of DI water. pH = 8.8 and Cl= 0.45M (N), 3) 1:100 Test Solution: 100 ml of 1:10 test so lution added to 900 ml of DI water. pH = 9.0 and Cl= 0.05M (N).

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49Although a 1:10 concentration difference exis ts between 1 to 2 and 2 to 3, their Molarities differed by 8:1 and 9:1, respectivel y, which concurs, based on their respective activity coefficients and concentration valu es. Visual examination of the actual test solutions reveal a white, saturated Calcium Carbonate (CaCO3) precipitate present in the concentrated test solution, however, the 1: 10 and 1:100 solutions were clear. 5.2 Electrode Discussion 5.2.1 Electrode Details In order to allow for the sole eval uation of the Bi-Polar membrane in the seawater concentration cell, the electrode materi al was carefully selected so as to only act as a charge collector. The chosen electrodes were made from standa rd silver (Ag) wire mesh with a solid Ag tab soldered for good el ectrical conduction and increased clip test lead attachment longevity. In itial surface treatment include d a 20 minute dip in 3M HCL followed by a deionized water rinse. Electrode overall cross secti on is 3 inches by 3 inches with 2.75 inches by 2.75 in ches in solution contact. Figure 5.3 presents a pair of used 80 Mesh electrodes. Two sizes of mesh were used in the te sting effort, one with 80 threads per inch and one with 40 threads per inch, in orde r to examine any electrode surface area depenendancy. Based on a cylindrical surface area estimation of each wire multiplied by

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50the number of wires, the 40 Mesh electrodes should have 11% more surface area than the 80 mesh electrode. Figure 5.3 Used 3M HCL Dip 80 Ag Mesh Electrodes 5.2.2 Scanning Electron Microscopy and X-Ray Results Figures 5.4 and 5.5 present SEM and X-Ray re sults, respectively, for a section of new 3M HCL dipped 80 Ag wire mesh. In the area X-Rayed, analysis revealed >96% of the total net counts in the X-Ray spectrum we re Ag, as expected, with trace elements ---------3 inch es ----------

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51present of C, Mg, Al, Cl and Si. These results were consistent with those found in a similarly sampled section of new 3M HCL 40 Ag wire mesh. Figure 5.4 New 3M HCL Dip 80 Ag Mesh SEM Figure 5.5 New 3M HCL Dip 80 Ag Mesh X-Ray

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52 Figures 5.6 and 5.7 present SEM and X-Ray results, respectively, for a section of used 3M HCL dipped 80 Ag wire mesh electr ode from the concentrated solution side near the cell top. In the area X-Rayed, analysis revealed >97% of the total net counts in the X-Ray spectrum were Ag as expected with trace elements of O, Mg, Na, and Cl present. These results were consistent with those found in a similarly sampled section of a used 3M HCL dipped 80 Ag electrode from the dilute solution cell side. Figure 5.6 Used Electrode SEM Results from Concentrated Side Cell Top

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53 Figure 5.7 Used Electrode X-Ray Result s from Concentrated Side Cell Top Figures 5.8 and 5.9 present SEM and X-ra y results, respectivel y, for a section of used 3M HCL dipped 80 Ag wire mesh electr ode from the concentrated solution side near the cell bottom. Precipitated Ca crystals are clearly seen form ed on portions of the imaged wire. This concurs with the previ ously noted observation of a white, saturated Calcium Carbonate (CaCO3) precipitate in the 3.6M concentrated test solution. In the shinny metal area X-rayed, analysis revealed >75% of the total net counts in the X-Ray spectrum were Ag with Ca (10 %) and trace elements of O, Mg, Si, S, Na, Al, and Pb. X-Ray analysis on the precipitated coated wire reveal >83% of the net counts in the X-Ray spectrum, primarily Ca with trace elements of O, Sr, S, Cl, Mg, Ag, and Na.

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54 Figure 5.8 Used Electrode SEM Results fr om Concentrated Side Cell Bottom Figure 5.9 Used Electrode X-Ray Results from Concentrated Side Cell Bottom

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55 Figures 5.10 and 5.11 present SEM and XRay results, respectively, for a section of used 3M HCL dipped 80 Ag wire mesh electro de from the dilute solution side near the cell bottom. In the bare wi re area X-Rayed, analysis rev ealed >88% of the total net counts in the X-Ray spectrum were Ag with trace elements of Mg, O, Mg, Na, and Cl present. X-Ray analysis on the precipitate in the upper left corner revealed NaCl (>98%) with trace elements present of O, Mg and Ca. Figure 5.10 Used Electrode SEM Result s from Dilute Side Cell Bottom

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56 Figure 5.11 Used Electrode X-Ray Result s from Dilute Side Cell Bottom 5.2.3 Cyclic Voltammetry Test Results Many electrode reactions can proceed either as oxidation or as reduction, depending upon on the direction of the current flowing through the el ectrode/electrolyte interface, e.g., metal deposition/ dissolution or Reduction/Oxidation (RedOx) reactions. Metal deposition/dissolution is a class of el ectrode reactions invol ving RedOx of a solid metal and its dissolved ion, e.g., where Ag ions can be cathodically reduced to Ag metal, or the Ag metal can be anodically oxidized to Ag ions. Compare with a RedOx reaction where both the oxidized and redu ced species are in solution. Voltammetry is an electrochemical measuring technique used for the determination of chemical reaction rates and their causes, kinetics, along with electrode

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57reaction mechanisms at the electrode surface. CV is a commonly used variation of the technique in which the direction of the worki ng electrode potential is reversed at the end of the first scan all while th e current flowing through the elect rode is measured. CV has the advantage that the product of the electron tran sfer reaction that occurred in the forward scan can be probed again in the reverse scan. The waveform used in this analysis is composed of two isosceles triangles and is presented in Figure 5.12 below23. Operation begins by first holding the initial potential where no electrolysis occurs and hence no fa radaic current flows. As the voltage is scanned in the positive direction any redu ced compound is oxidized at the electrode surface. At (+ve), the scan re duction is reversed and the mate rial that was oxidized in the positive scan is then reduced. Once the voltage reaches (–ve) it is then retuned to the initial value. This operation is then repeated until repeatability is achieved. Figure 5.12 Waveform Used in CV Testing

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58 CV testing was conducted on both new a nd used Ag 80 mesh electrode samples and a summary plot is presented in Figure 5.13. Examination of Figure 5.13 reveals that within the potential ra nge typically encountered in an el ectrically loaded cell condition, the curve shape is flat with no transfer of charge occurring at th e electrode. Supporting the original hypothesis that th e electrodes would not be a co ntributor to th e overall cell potential at the working voltages encountered. Figure 5.13 New/Used 80 Mesh Electrode CV Test Results WE = Ref 1 = Ag 80 Mesh 3M Dip; CE=Graphite Rod; RE= Ref 2 = Ag/AgCl; Electrolyte is 3.5% NaCl Testing Conducted at UK/AER OCV (New) = +0.0098 V OCV (Used) = -0.0056 V 0 Ref,1.2 Ref,-1.2 Ref,0 Ref,1mV/S,x10 Anodic Sweep (Oxidation) Ag -> Ag+1 + 1e(Product Reaction) Cathodic Sweep (Reduction) Ag+1 + 1e-> Ag (Reactant Reaction) -2-1012 -2 -1 0 1E (Volts)I (Amps/cm2)Ag 80 Mesh Cyclic Voltammogram New Vs. UsedNew Ag 80 Mesh 808063MAgcliffaqueous01_Cy10.cor Used Ag 80 Mesh 3m used ds stirr 7-11-06 _cy10_filtered.cor

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595.2.4 Pourbaix Diagram Discussion The Pourbaix diagram distinguishes re gions of active corrosion, passivity, and immunity in terms of pH, abscissa, and RedO x potential, ordinate. Figure 5.14 is a Ag Pourbaix diagram at 25C in chloride solution at 1M concentration Cl, where (a) and (b) correspond to RedOx potentials determined by H and O saturation at standard temperature and pressure (STP)24. Figure 5.14 Ag Pourbaix Diagram at 25C in Chloride Solution

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60 Room temperature performance cell test resu lts of pH and loaded cell potential, E, revealed pH ranges of 6.5 to 9.0 and E ranges of 10 to 20 mV DC. Figure 5.14 shows that at these values reveal that Ag may become an ionic species (Ag Ag+ + e-), or will remains in its metallic form (Ag Ag) as a degenerate form. This is supported by SEM/X-Ray electrode analysis confirming pr imarily the presence of Ag alone in both new and used electrodes. All evidence supports the hypothesis that Ag is in equilibrium with its own ions Ag+ with the Cathodic/Anodic reactions occurring at the same rate in each cell side. 5.2.5 Electrode Summary Analysis of electrochemical, physical, and performance based testing of the Ag mesh electrode confirmed that corrosion effect s were found not to be a contributor to the overall cell potential and: 1) Functioned simply as charge collectors, 2) Confirmed electrode material selection, design, and ruggedne ss for subsequent field testing, 3) Allowed for the sole evaluation of the Bi-Polar membrane performance in a membrane based seawater concentration cell.

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615.3 Bi-Polar Membrane Discussion 5.3.1 Membrane Details Bi-Polar membranes from two manufactur es were used in the testing effort. Details specific to each membrane are provided as follows: 1) Membranes International Inc., USA (MII BPM-9000): 1. Primarily an industrial grade membrane used in the metal plating industry, 2. Test cell dimensions: 4 inches by 4 inches, 2.75 by 2.75 in solution contact, 3. Polymer structure – Gel polystyrene cross linked with divinylbenzene, 4. Thick, stiff, “fabri c-like appearance”, 5. Shipped dry in “open” container, 6. CEM: surface rough and dark color, Functional Group – Sulphonic Acid, 7. AEM: Surface rough and light color, F unctional Group – Quaternary Ammonium, 8. Selected test cell measured and ve ndor provided sample properties: a. Dry Weight = 12.0532 grams b. Wet Weight = 13.7901 grams c. Water Uptake = 14.4% d. Thickness (dry) = 0.955 mm (0.0376 inches) e. Thickness (wet) = 1.082 mm (0.0426 inches) f. Electrical Resistance (Ohm ) <1 (EIS Measured)

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62A used MII Bi-Polar membrane, CEM and AE M side, presented in Figures 5.15 and 5.16. Figure 5.15 Used MII BPM-9000 Bi-Polar Membrane, CEM Side Figure 5.16 Used MII BPM-9000 Bi-Polar Membrane, AEM Side 2) Fumatech, Germany (Fumasep FBM): 1. Primarily used in the wa ter purification industry, 2. Test cell dimensions: 4 inches by 4 inches, 2.75 by 2.75 in solution contact,

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633. Polymer structure – (Kraton, PPO [poly( phenyleneoxide)]) cross linked with PEEK [Poly(etheretherketone)], 4. Thin, flexible, “sandwich wrap like appearance”, 5. Shipped in sealed container surrounded by 1 M NaCl solution, 6. CEM: surface smooth and shinny, Functional Group – Sulphonic Acid, 7. AEM: surface not slippery and opa que, Functional Group – Amines, 8. Selected test cell measured and ve ndor provided sample properties: a. Dry Weight = 1.7963 grams b. Wet Weight = 2.0705 grams c. Water Uptake = 15.3% d. Thickness (dry) = 0.189 mm (0.0074 inches) e. Thickness (wet) = 0.201 mm (0.0079 inches) f. Electrical Resistance < 3 Ohm Used Fumasep FBM membrane, CEM/AEM side, presented in Figures 5.17 and 5.18. Figure 5.17 Used Fumasep FBM Bi-Polar Membrane, CEM Side

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64 Figure 5.18 Used Fumasep FBM Bi-Polar Membrane, AEM Side 5.3.2 SEM Test Results and Summary SEM images of both new and used MII BPM-9000 and Fumasep FBM Bi-Polar membranes were made, see Figures 5.19 to 5.26. Figure 5.19 New MII BPM-9000 SEM Image, CEM Side

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65 Figure 5.20 New MII BPM-9000 SEM Image, AEM Side Figure 5.21 New Fumasep FBM SEM Image, CEM Side

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66 Figure 5.22 New Fumasep FBM SEM Image, AEM Side Figure 5.23 Used MII BPM-9000 SEM Image, CEM Side

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67 Figure 5.24 Used MII BPM-9000 SEM Image, AEM Side Figure 5.25 Used Fumasep FBM SEM Image, CEM Side

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68 Figure 5.26 Used Fumasep FBM SEM Image, AEM Side Analysis of SEM images revealed: 1) The presence of many small holes distri buted unevenly on the membrane surface, 2) The size of the membrane hole is sometimes much smaller than the distance between discrete holes and sometimes much larger than the distance between holes connected together in trench like lines, 3) That additional “openings” and “bumps” were present in the “USED” CEM/AEM membrane surfaces than were present when imaged new.

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695.3.3 Membrane Summary 1) MII BPM-9000 and Fumasep FPM Bi-Polar membranes were selected for testing because even though both were Bi-Polar membranes, they differed in composition, weight, stiffness, thickness, end application use, and cost. 2) Analysis of test data, however, reveal ed similar performance and water take up properties with results from DoE modeli ng showing no membrane manufacturer/end use related main effect interaction. 3) Based on the above findings: 1. The small holes in these charged membra nes could be acting in a voltammetric behavior on a microelectrode or ultra microelectrode scale21. 2. Combined membrane swelling and linkage of the “openings” present in used CEM and AEM membranes aided by the oppositely charged EDL can account for the measured Clco-ion/counter-ion migration ac ross the Bi-Polar membrane from concentrated to dilute sides of the functioning Bi-Polar concentration cell. 5.4. Electrochemical Impedance Spectroscopy Discussion An excellent primer into the basis of EIS can be found in an application note entitled “Basics of Electrochemical Impe dance Spectroscopy” by Gamry Instruments25. A brief EIS summary is included herein.

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70 Ohm’s Law applies to electrical circuits and states that the current through a conductor between two points is directly propor tional to the potential difference (i.e., voltage drop or voltage) across the two points, and inversely proportional to the resistance between them. The mathematical equation that describes this relationship is: E = IR Where E is the potential difference in volts, I is the current in amperes, and R is a circuit parameter called the resistance (measured in ohms also equivalent to volts per ampere). Electrical resistance is the ability of a circuit element to resist the flow of electrical current. Ohm’s Law can be rewritte n in terms of resistance (Equation 5) as the ratio between voltage E and current I. R = E/I Equation 5 The simplest electrical circuit element is the resistor. An ideal resistor has the following simplifying but important properties: 1) Follows Ohm’s Law at all current and voltage levels, 2) Its resistive value is i ndependent of frequency, 3) AC current and voltage signals through a re sistor are in phase with each other.

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71The real world contains additional circuit elements that exhibit much more complex behavior. These elements force the abandonment of the simple concept of resistance. In stead, the more general circui t parameter concept of electrical impedance or simply impedance is used. Electrical im pedance extends the concept of resistance to alternating current (AC) circuits, describi ng not only the relative amplitudes of the voltage and current, but also th e relative phases. Like resistance, impedance is a measure of the ability of a circuit to resist the flow of a sinusoidal AC electrical current. Unlike resistance, impedance is not limited by the simplifying properties listed above. The mathematical representations of indi vidual circuit elements can be converted into phasor notation, and then th e circuit can be solved usin g phasors. In phasor notation, resistance, capacitance, and inductance can be combined together into a single term called “impedance” and like resistance, is meas ured in units of Ohms. The phasor used for impedance is “Z” and Ohm’s law for phasors becomes: E = IZ and it’s important to acknowledge that Ohm’ s law holds true for both the phasor domain as well as the time domain. As mentioned, resistors do not affect the voltage or current, only the magnitude. Therefore, the impedance of a resistor with resistance R in phasor notation is: Z = R 0

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72Capacitors and Inductor circuit elements do, ho wever, affect the voltage or current. A capacitor with a capacitance C has an impedance value of: Z = 1/ C /2 or, in terms of degrees as, Z = 1/ C Illustrating that the current leads the voltage by 90 Conversely, an inductor with an inductance L has an impedance value of: Z = L /2 or, in terms of degrees as, Z = L Illustrating that the current lags the voltage by 90 Electrochemical impedance is measured by applying an AC potential to an electrochemical cell and measuring the curr ent through the cell. Assuming a sinusoidal excitation is applied to a linear system, the re sponse will be an AC current signal with the same frequency. This current signal can be analyzed as a sum of sinusoidal functions, a Fourier series. Electrochemical impedance is normally measured using a small excitation signal (in this case 10 mV) so the cell’s response is ps eudo-liner. In a lin ear (or pseudo-linear system), the current response to a sinusoidal potential will be a sinusoid at the same frequency but shifted in phase ( ) as shown in Figure 5.27.

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73 e (t) i (t) t 0 EoIo Figure 5.27 Sinusoidal Current Response in a Linear System The excitation signal, e(t), expressed as a function of time, has the form: e(t) = Eo sin( t + 0) Where Eo is the amplitude of the signal, and is the angular frequency in radians/sec given by: = 2

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74Where is the frequency in cycles per second (Hz). The excitation signal can also be expressed in phasor notation as: E = Eo 0 In a linear system, the time domain res ponse signal, i(t) is shifted in phase ( ) and has a peak amplitude, Io. i(t) = Io sin( t + ) Similarly, the response signal can also be expressed in phasor notation as: I = Io By Ohm’s law, the impedance of the system is written in Equation 6 as: Z( ) = Eo 0 / Io = Zo Equation 6 The impedance is therefore expressed in terms of a magnitude, Zo, and a phase shift . Equation 6 has both real and imaginary parts. Plotting the real part on the X-axis and the imaginary part on the Y-axis, produces a “Nyqui st Plot” as presented in Figure 5.28. On the Nyquist Plot the impedance can be represen ted as a vector (arrow) of length |Z|. The angle between this vector and the X-axis is the phase angle (= arg Z)). The semi-circle is characteristic of a single “time constant” system.

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75 Figure 5.28 Nyquist Plot with Impedance Vector The Nyquist Plot in Figure 5.28 results fr om the electrical circuit of Figure 5.29. Another popular method is the Bode Plot. The impedance is plotted with log frequency on the X-axis and the impedance and the phase-s hift on the Y-axis. The Bode Plot for the electric circuit of Figure 5.29 is shown in Figure 5.30. Unlike the Nyquist Plot, the Bode Plot does show the frequency information. Figure 5.29 Simple Equivalent Circ uit with One Time Constant = 0 =

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76 Figure 5.30 Bode Plot with One Time Constant 5.4.1 Equivalent Circuit Modeling Discussion EIS data is commonly analyzed by fitti ng it to an equivalent electrical circuit model. To be useful, the elements in th e model should have a basis in the physical electrochemistry of the system. As an exam ple, most models contain a resistor that models the cell’s solution resistance. Solart ron’s ZView2 software package was used for both EIS Impedance and Equiva lent Circuit Modeling (ECM) analysis and display. EIS

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77determined values of the 3.6M//0.45M Solution Resistance (Rs) were approximately equal to: 0.33 Ohms for 3.6M Cland 1.41 Ohms for 0.45M Clsolutions and 0.69 Ohms for the MII membrane resistance (Rm). The Bi-Polar membrane concentration cell was closely modeled as a Randles’ Cell in parallel with an external load as presented in Table 5.1 and Figure 5.31. The exception here being that the Randles Cell idea l capacitor, in parallel with the system Polarization Resistance (Rp), is replaced by a Constant Phase Element (CPE). This was done because “Double Layer Capacitors” in re al electrochemical cells are sometimes modeled as a CPE rather than single capacitive elements. The impedance of the CPE can be expressed as: Z = (1 / Yo)(j )Where, Yo = C = the Capacitance and, = An exponent that equals 1 for an ideal capacitor and <1 for a CPE While several theories (surface roughness, “leaky capacitor”, non-uniform current distribution, etc.) have been proposed to account for the non-ideal behavior of the double layer, is treated here as an empirical consta nt with no physical basis until additional research is conducted and a more gene rally accepted theory put forward.

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78 Rext 2Rs + Rm CPE Rp Table 5.1 ECM 1:10 MII RT OCV Data Figure 5.31 ECM 1:10 MII RT OCV The Nyquist Plot in Figure 5.32 results from the electrical circuit of Figure 5.31. Examination of Figure 5.32 shows a reasona ble good fit between OCV modeled and measured results with the exception being in the extreme low frequency region which is attributed to local environm ental noise contamination. Also evident is the depressed semi-circle, characteristic of a parallel RC circuit element. Element Freedom Value Rext Fixed (X) 1E09 2Rs + Rm Fixed (X) 2.901 CPE-T Fixed (X) 0.0031 CPE-P Fixed (X) 0.764 Rp Fixed (X) 2550

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79Rp 0100020003000 -2000 -1000 0 1000Z'Z''1:10 MII OCV EQUIVALENT CIRCUIT MODELING122306C.Z FitResult Figure 5.32 Nyquist Plot 1:10 MII RT OCV Measured vs. Modeled 5.4.2 ECM Model Result Under External Electrical Loading Discussion ECM model result comparison analysis was conducted using the same developed OCV model with the exception that Rext was changed from near infinity (1E09) Ohms to Rext = 500 Ohms as presented in Table 5.2 and Figure 5.33. No high frequency semicircle was seen in the Nyquist plot, shown in Figure 5.34, nor present was a step in the intermediate frequency area in the Bode diagram, see Figure 5.35, therefore, the Interfacial Capacitance is la rge enough to compare to the systems high Rp value (e.g. metals with very low corrosion rates26) at frequencies low e nough that the overall cell

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80 Rext 2Rs + Rm CPE Rp impedance (Z) Rct (Charge Resistance). Further examination of Figure 5.35 reveals a (-) phase angle which is indicative of a capacitive impedance network. Table 5.2 ECM 1:10 MII RT 500 Ohm Ext Load Data Figure 5.33 ECM 1:10 MII RT 500 Ohm Ext Load Element Freedom Value Rext Free (+) 500 2Rs + Rm Fixed (X) 2.901 CPE-T Fixed (X) 0.0031 CPE-P Fixed (X) 0.764 Rp Fixed (X) 2550

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81 Measured Lab EIS Data Modeled data using OCV Model with Ext Load Changed Predicted Actual 0100200300400500 -400 -300 -200 -100 0 100Z'Z''1:10 MII RT 500 Ohm External Load EIS042007.Z FitResult Figure 5.34 Nyquist Plot 1:10 MII RT 500 Ohm Ext Load Measured vs. Modeled

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82 Figure 5.35 Bode Plot 1:10 MII RT 500 Ohm Ext Load Measured vs. Modeled Diffusion can create impedance called Warburg impedance. On a Nyquist plot, the Warburg impedance appears as a diagonal line with a slope of 45 at very low frequencies. On a bode plot, the Warbur g impedance exhibits a phase shift of 45 No evidence of this low frequency diffusion cont rol was evident in ei ther the Nyquist or Bode plots. This supports the proposed mode l as primarily a parallel combination of CPE Interfacial capacitance and polarization resistance. Th is in conjunction with the corrosion evidence supports belief that Ag is in equilibrium with its own ions Ag+ with Predicted Actual Actual Predicted 10-410-310-210-1100101102103104105106107 10-1100101102103Frequency (Hz)|Z|1:10 MII RT 500 Ohm External Load EIS042007.Z FitResult 10-410-310-210-1100101102103104105106107 -75 -50 -25 0 25 50Frequency (Hz)theta

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83the Cathodic/Anodic reactions occu rring at the same rate in ea ch cell side, as previously discussed in Section 5.2.4. Whenever the potential of an electrode is forced away from its value at open circuit it is referred to as “polarizing” the electrode. When an el ectrode is polarized, it can cause current to flow thr ough electrochemical reactions at the electrode surface. The amount of current is controlled by the kine tics of the reactions and the diffusion of reactants both towards and away from the elect rode. In this case, where RedOx reactions are occurring at the same rate in each cell side, a single ki netically-controlled electrochemical reaction at equilibrium occurs and charge is transferred. This charge transfer reaction has a certain speed depending upon the kind of reaction, the temperature, the concentration of the reaction products a nd the potential. When the polarization depends only on the charge-transfer kinetics the ButlerVolmer equation can be used to estimate the value of Rct at equilibrium as: Rct = RT/nFio Equation 7 Using data presented later in Se ction 5.9, Table 5.3, a MII 80 Mesh 22 C (RT) performance test run under an external load of 500 Ohms resulted in an equilibrium voltage of nominally 15.5 mV. Remembering Ohm’s Law, this results in a value of io (exchange current density) of nominally 3E -5 Amps. Back subbing into Equation 7 results in:

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84Rct = [(8.314 J K-1 mol-1 295K) / (1 96,500 C mol-1 3E-5A)] ~ 850 Ohms Which is approximately double the value of Z’ shown in Figure 5.34. Although close in value it indicates the overall cell potential does not depend solely on the electrode chargetransfer kinetics. A likely contributor is the Donnan excl usion of the re action products under the internally generated concentra tion gradient driving force (Section 2.8). 5.5 Bi-Polar Membrane Concentration Cell Electrical Loading Discussion Cell loading measurements were made on numerous runs using both EIS techniques as well as direct DC monitoring of the cell output potenti al using a digital volt meter (DVM)/data logger/computer storage system. Representative plots of each are presented in the following sections. 5.5.1 Electrical Loading Comparison Figure 5.36 presents a 5-day run cons isting of both OCV and varying load conditions. Of particular im portance revealed is the consistency and repeatability of conditions, starting at OCV (69 mV), with a 1K Ohm load (18.2 mV), then a 500 Ohm load (8.7 mV), then a 10 Ohm load (0.25 mV), followed back to OCV (68 mV) and then back to a 500 Ohm load (8.2 mV). This conf irms a parallel external load connection with the cell, resulting in a halving of cell output with a halving of externally applied load. Revealing a fairly constant cel l output current condition across th e load conditions tested.

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85 1:10 MII Cell Loading Comparision 22 Deg C (RT) April 2008 0 10 20 30 40 50 60 70 80 0123456 Test Duration (days)Cell Membrane Potential (mV) Day 0: E = 68.9 mV OCV, Conc = 3.320M [Cl-], Dilute = 0.371M [Cl-] Day 1: E = 18.2 mV, 1K Ohm, Dilute = 0.383M [Cl-] Day 2: E = 8.7 mV, 500 Ohm, Dilute = 0.396M [Cl-] Day 3: E = 0.3 mV, 10 Ohm, Dilute = 0.395M [Cl-] Day 4: E = 67.3 mV OCV, Dilute = 0.404M [Cl-] Day 5: E = 8.2 mV, 500 Ohm, Dilute = 0.406M [Cl-] Figure 5.36 1:10 MII RT Cell Loading Comparison Agitation of the cell was performed at vari ous times before and after this test and although variations did occur in the OCV c ondition, the affects were small (<5%) under load. With loaded equilibrium quickly reached once the agitation was removed. 5.5.2 EIS Comparison during Loading Figure 5.37 presents various EIS test results for variati ons in concentration ratios and external loading values for a standard 80 Mesh MII test configur ation run in the (+) membrane convention under solution pumpi ng (nominally 430 ml/day). With the exception of OCV conditions, no applicable difference in measured cell impedance was noted with a 10 fold increase in concentr ation differences. While a doubling of the

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861000 Ohm Load; 1:10 Concentration Diff.; E= 19.9 mV 500 Ohm Load; 1:10 Concentration Diff.; E= 8.6 mV 10 Ohm Load; 1:10 Concentration Diff.; E= 0.25 mV 1000 Ohm Load; 1:100 Concentration Diff.; E= 25.6 mV 500 Ohm Load; 1:100 Concentration Diff.; E= 12.6 mV OCV; 1:10 Concentration Diff.; E= 290 mV OCV ; 1:100 Concentration Diff.; E= 302 mV 10 Ohm Load; 1:100 Concentration Diff.; E= 0.25mV OCV 1000 Ohm Ext load 500 Ohm Ext. Load 0100020003000 -2000 -1000 0 1000Z'Z''80Mesh 3M Dip MMI BPM RT EIS Load Comparision041907.Z 050107.Z 042007.Z 050207.Z 042107.Z 050407.Z 122306C.Z 122806C.Z Figure 5.37 EIS Load Comparison Plot applied external load results in an approximate doubling of the cell output membrane potential (parallel connected); a 10 fold increase in solution concentration does not produce a doubling of cell output as predicted by the Nernst Equation. Therefore, test results illustrate that the Bi-Polar membra ne concentration cell ope ration under external loading can not be adequately described by the Nernst Equation27.

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875.6 Solution Pumping Speed Dependency on Cell Output Potential The effects of pumping speed on cell pe rformance were evaluated by examining the measured change in a loaded 80 mesh M II 1:10 room temperature cell operated under differing fluid pumping speeds. Figure 5.38 presents an impedance comparison plot showing variation from low to high sp eed, 0.0036 ml/sec (310 m l/day) to 0.0064 ml/sec (550 ml/day), respectively. Data analysis revealed a minimal change in overall cell impendence under a nominal 500 Ohm load and a resulting negligible 1 mV variation in output voltage. Figure 5.38 1:10 MII 80 Mesh Fluid Pu mping Speed Comparison Test 1:10 Low/Low Speed 0.0036ml/sec 1:10 Low/High Speed 0.0064ml/sec 1:10 Low/Low 1:10 Low/High 0100200300400500 -400 -300 -200 -100 0 100Z'Z''Fluid Pumping Speed Comparison042007.Z 042707.Z

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88 Final system operation consisted of dual peristaltic pumps (one for each cell side), positioned over a reservoir of synthetic seawater of varying concentrations and operating at the same speed (nominally 430 ml/day). This set up was run in both a laboratory grade incubator/oven or refrigerato r/freezer to obtain the temperature affects desired (Figure 5.2). 5.7 Bi-Polar Membrane Orientation Discussion The difference in membrane output potential and how it may vary depending upon which Bi-Polar membrane side (CEM or AE M) is in contact wi th the concentrated and dilute solutions was inves tigated. Figure 5.39 presents an 8-day data plot of a room temperature measurement of 1:100 concentr ation cell membrane potential under the condition of a nominal 500 Ohm external load, 80 mesh electrodes, and a MII membrane. Close examination of Figure 5.39 reveals a si gnificant difference in output membrane potential with membrane orientation as il lustrated by the data plots and corresponding orientation layout schematics. Investigation reveals that for maxi mum potential output, the cell needs to operated in what is typically referred to as the (+) Bi-Polar membrane orientation, that being with the concentrat ed solution in contact with the Bi-Polar membrane AEM side and the Bi-Polar membrane CEM in contact with the dilute solution side. Also illustrated in the orientation layout schematics are the measured electrode polarities, which in the case of the preferred (+) orientation is the same as an electrolytic cell, discussed in Section 2.2.

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89 Figure 5.39 1:100 MII 80 Mesh Bi-Pol ar Membrane Orientation Test 5.8 Bi-Polar Membrane Concentration Cell Ion Migration and Osmotic Flow Ion Migration and Osmotic Flow inves tigations were made on numerous runs. Visual evidence of Positive Anomalous Osmosi s – confirming the migration of solvent, water, moving across the Bi-Polar membrane from the dilute side to the concentrated side – is presented in Figure 5.40. Figure 5.40 cl early shows an accumulation of solvent overflowing from the concentrated side of th e cell in this standa rd (1:10 MII 80 Mesh) example of a room temperature, static (no pumping occurring), open cell configuration. Standard (+) Test Membrane Orientation Reversed (-) Test Membrane Orientation 2/11/08 @ 19:49 2/19/08 @ 17:28 1:100 80 Mesh MII RT 500 External Load 0.9 mV 15.4 mV

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90 Figure 5.40 Evidence of Positive Anomalous Osmosis Figure 5.41 presents a 6-day room temp erature performance measurement of a 1:10 MII cell membrane output potential under OCV and varying external loading in the (+) Membrane Orientation. Close examination of this plot reveals ve ry similar OCV preand post-loading values along with the pr eviously shown halving of cell output corresponding to a halving of ex ternal load. Interesting he re is that although the OCV values are similar, the loaded cell membrane potential values are a bout a half of that previously shown, for similar loading conditions Illustrating a repeatable but variable output potential which will be discussed and qua ntified in detail in the following section, Section 5.9, DoE Modeling Discussion. This va riation was, however, within the 95% Confidence Intervals (CI) computed and pr ovided in the Section 5.9 results section.

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91 Figure 5.41 Ion Migration and Osmotic Flow Example Revealed in Figure 5.41 is the measur ement of the volume in the cell output beaker verses time for both the concentrated and dilute side. As shown, there is an overall increase in the concentrated side volum e and a decrease in the dilute side volume with time, indicating a net transport of solvent (water) across the membrane from dilute to concentrated (Anomalous Positive Osmosis). Shown also are the results of numerous Cltitrations made during the test revealing a net decrease in Clions in the concentrated side and an increase in the dilute side. Interesting that although small in amount there was more loss in dilute volume than in concentrated volume gain. The concentrated side pump had stopped sometime Cc = 4.13M, pH = 7.0 Cd = 0.45M, pH = 8.4 Cc = 3.47M, pH = 7.4 Cd = 0.54M, pH = 8.1 62.4 mV OCV 59 mV OCV 3.6 mV** @ 500 3.6 mV @ 500 7.4 mV @ 1000 3/13/08 @ 17:02 3/19/08 @ 16:26 Vc = Vd = 600 ml Vc = 602 ml Vd = 587 ml Vc = 604 ml Vd = 573 ml Vc = 607 ml** Vd = 557 ml Vc = 609 ml Vd = 538 ml Vc = 611 ml Vd = 527 ml 1:10 80 Mesh MII RT OCV and Load ** Found Concentrated Pump had stopped Restarted 1:10 80 Mesh MII RT OCV and Load

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92during the evening mid way in the test and coul d attribute as well as a slight cell leakage of the AEM side along with some solvent passage through the membrane with the Clions within their hydration shells. pH measur ements made during this test run reveal an increase in the pH of the concentrated solu tion and a decrease in the pH of the dilute solution. Although the measured cell output poten tials were far lower than the theoretical “water splitting” pot ential value of 0.828 (S ection 2.6.2.1), it is stil l possible that the water passing through the AEM dissociates (splits) into equivalent amounts of H+ and OHions within the intermediate layer. These ions could then migrate with the H+ ions permeating through the CEM side and the OHions permeating the AEM side, affecting the pH as seen. Both these areas will be examined in more detail in follow-on research. 5.9 Design of Experiment Modeling Discussion Because the Nernst Equation did no t adequately pred ict the cell output performance under load, a DoE approach was implemented to determine a suitable equation defining the cell loaded output performance. An ev aluation of pertinent factors that might affect cell membrane potential out put performance was conducted and resulted in eleven (11) factor variab les: electrode composition, elec trode surface area, solution concentration, solution composition, solution flow rate, cell temperature, membrane type, membrane orientation, operating pressure, ag itation, and external el ectrical loading. That’s 211 or 2048 runs with a single replicate. Screening expe riments were performed to reduce this number down significantly by syst ematically examining each variable while keeping in mind the focus of this research effo rt – that being to i nvestigate the membrane

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93potential of a seawater concen tration cell and its suitability as a low power energy source for micro and nano devices, such as wireless communication devices. With anticipated sea surface operation, the n eed to test at any pressures other than atmospheric was removed. Seawater solu tion composition variation was minimized through the use of synthetic seawater. Memb rane orientation was examined separately and (+) orientation was used extensively th rough out the DoE test program. Solution flow rate was examined separa tely and found to be negligib le at the anticipated flow rates. External loading effects on cell performance were examined and quantified separately and all DoE testing conducted with an external load of 500 Ohms present. Electrode composition variation issues were removed from c ontributing to the overall cell potential via careful material selection and the use of identical electrodes for both cell sides. Cell agitation was obs erved to have an affect on the cell potential in the OCV condition but the affect was neg ligible when an external elect rical load was present. This effectively reduced the variable count down to four (4), however, because of the number of possible coupled interactions with just four, a fractional, factorial experimental design using SAS statistical DoE analysis package wa s used to determine the significant factors in the concentration cell output pot ential voltage (E1) in terms of: 1) Electrode surface area (ESA), 2) Bi-Polar membrane end use type (MEM), 3) Synthetic seawater soluti on concentration (CONC), 4) Cell operating temperature (TEMP).

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945.9.1 Test Set-up Discussion A 24-1 fractional, factorial design was c hosen for this purpose. Using a 24-1 design, four variables were stud ied at two levels by performing eight experiments (24-1 = 8). The response (E1) is the magnit ude of the concentration cel l output voltage in mVDC operated in the (+) membrane orientation and under an external el ectrical load of 500 Ohms. The design of experiment matrix (Tab le 5.3) shows the measured response along with the two levels of the va riables coded such that a minus one (-1) represents the low level and a plus one (+1) repr esents the high level. Thes e variables and coded levels were chosen based on previous experiments and practical considerations anticipated to be encountered in the field for an opera tional seawater c oncentration cell. Table 5.3 Engineering Design Test Re sults with 500 Ohm External Loading Run ESA MEM CONC TEMP Date Temp C Cell Voltage (mV DC) 1 80 (-1) FUM (-1) 1:100 (-1)L (-1) 10/29/074.6 3.4 2 40 (1) FUM (-1) 1:100 (-1)H (1) 11/03/0738.9 38.3 3 80 (-1) MII (1) 1:100 (-1)H (1) 1/26/08 38.9 23.6 4 40 (1) MII (1) 1:100 (-1)L (-1) 1/23/08 3.6 4.1 5 80 (-1) FUM (-1) 1:10 (1) H (1) 10/31/0740.4 5.6 6 40 (1) FUM (-1) 1:10 (1) L (-1) 10/30/074.5 5.9 7 80 (-1) MII (1) 1:10 (1) L (-1) 9/27/07 3.1 3.6 8 40 (1) MII (1) 1:10 (1) H (1) 10/18/0738.9 26.1

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955.9.2 DoE Predictive Model Results The results of this seven (7) Degree of Freedom (DoF) analysis were used to develop an equation which shows the cell pa rameter inter-relationships between ESA, MEM, CONC, and TEMP. E1 = 13.825 + 4.775*ESA + 0.525*MEM – 3.525*CONC Equation 8 + 9.575*TEMP – 4.025*ESA*MEM + 0.925*ESA*CONC + 4.025*ESA*TEMP Inspection of Equation 8 shows that cell temp erature is the most important independent factor affecting cell output voltage and the interaction between ESA and CONC the least important. In order to obtain Standard Error (SE) and 95% CI data, the lowest contributing 2-way interaction effect was re moved and a 6 DoF analysis re-run with the resulting predictive equation presented in Equation 9. E1 (SE) [ CI] = 13.825 + 4.775*ESA + 0.525*MEM – 3.525*CONC Equation 9 + 9.575*TEMP – 4.025*ESA*MEM + 4.025*ESA*TEMP In equation 8, the values of ESA are either -1 (low, 80 mesh) or +1 (high, 40 mesh); values of MEM are either (low Fumasep) or +1 (high, MII); values of CONC are either -1 (low, 1:100) or +1 (high, 1:10); and values of TEMP are ei ther -1 (low, 5 C) or +1 (high, 40 C). With 80 mesh electrodes, M II membrane, 1:10 Concentration, and Room Temperature (RT), Equation 9 predicts:

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96 E1 = 13.825 + 4.775*(-1) + 0.525*(+1) – 3.525*( +1) + 9.575*(0) – 4.025*(-1)*(+1) + 4.025*(-1)*(0) = 10.1 mV (2.068) [-16.2, 36.36]. This compares favorably to the actual meas ured result of 15.5 mV for this condition. Additional related 6 DoF SAS DoE analysis in formation is presente d in the following 6 figures, Figures 5.42 to 5.47 which comprise the full report.

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97 Figure 5.42 SAS 6 DoF Input Model Data (1 of 6)

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98 Figure 5.43 SAS 6 DoF Input Model Data (2 of 6)

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99 Figure 5.44 SAS 6 DoF Predicted Model Results (3 of 6)

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100 Figure 5.45 SAS 6 DoF Predicted Model Results (4 of 6)

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101 Figure 5.46 SAS 6 DoF Predicted Model Results (5 of 6)

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102 Figure 5.47 SAS 6 DoF Predicted Model Results (6 of 6)

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1035.9.3 Measured Test Data vs. Predicted 6 DoF Model Results Examination of the DoE main effects plot for E1 reveal that the membrane end use type, manufacture, and physical characteris tics made very little difference in the cell output performance. Temperature was the major parameter evaluated, as indicated in both the E1 main effects and Pareto plots. Additional data analysis and testing wa s performed, using the easier to use MII membrane, to quantify the model skill vs. actual loaded measured cell performance vice operational temperature and solution concentra tion gradient. Modele d data input values were computed using Equation 6, and ar e displayed in Table 5.4 along with the corresponding measured cell da ta. All data is presented in Figure 5.48 along with the standard error and 95% CI bars.

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104 Table 5.4 Measured Test Data vs. Predicted 6 DoF Model Results Run ESA MEM CONC Temp C Type Date Cell Voltage (mV DC) 1 80 (-1) MII (1) 1:10 (1) 3.95 Model 8/1/08 4.5 2 80 (-1) MII (1) 1:10 (1) 21.63 Model 8/1/08 10.1 3 80 (-1) MII (1) 1:10 (1) 39.3 Model 8/1/08 15.6 4 80 (-1) MII (1) 1:10 (1) 3.1 Measured9/27/07 3.6 5 80 (-1) MII (1) 1:10 (1) 21.8 Measured2/22/08 15.4 6 80 (-1) MII (1) 1:10 (1) 36.8 Measured2/23/08 24.6 7 80 (-1) MII (1) 1:100 (-1)3.95 Model 8/1/08 11.6 8 80 (-1) MII (1) 1:100 (-1)21.63 Model 8/1/08 17.1 9 80 (-1) MII (1) 1:100 (-1)39.3 Model 8/1/08 22.7 10 80 (-1) MII (1) 1:100 (-1)3 Measured1/31/08 8.4 11 80 (-1) MII (1) 1:100 (-1)22.3 Measured2/15/08 15.4 12 80 (-1) MII (1) 1:100 (-1)38.9 Measured1/26/08 23.6 Analysis of Figure 5.48 reveals that a ten-fold increase in cell temperature (4 to 40 C) for an 80 Mesh electrode resulted in a: 1) Two fold increase in membrane potential ( = 11.1 mV) with a 1:100 concentration difference, slightly less than predicted by Nernst ( = 14.3 mV), 2) Four (4) fold increase in membrane potential ( = 11.1 mV) with a 1:10 concentration difference, slightly more than predicted by Nernst ( = 7.5 mV).

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105 -40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 051015202530354045Cell Temperature (Deg C)Cell Potential (mV) SAS Model Prediction 1:10 (3.6M Clto 0.45M Cl-) Measured 1:10 (3.6M Clto 0.45M Cl-) SAS Model Prediction 1:100 (3.6M Clto 0.048M Cl-) Measured 1:100 (3.6M Clto 0.048M Cl-)(Std Error) [95% CI] (2.447) {-19.5, 42.7} (2.447) {-26.6, 35.6} (2.068) {-19.2, 43.4} (2.068) {-16.2, 36.4} (2.447) {-8.4, 53.8} (2.44) {-15.5, 46.7} Figure 5.48 Measured Test Data vs. Predicted 6 DoF Model Results The predicted loaded cell membrane potential is fair ly linear with a consistent offset between 1:10 and 1:100 c oncentration differences. Meas ured results indicate that although generally matching the predicted values the magnitude of the variation is less and then only at the min/max temperatures – no difference was noted with concentration variations at room temperature with the M II membrane. All measured values under load are significantly lower than the OCV values predicted by via the Nernst equation. Equation 9 predicts a 15% increase in E1 for a 40 mesh electrode over an 80 mesh electrode at a 1:10 concen tration ratio and similarl y a 9% increase at a 1:100 concentration ratio. Recalling that the 40 me sh electrode should ha ve 11% more surface

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106area than the 80 mesh electrode this provides an indication of the DoE model skill while confirming both that an increase in surface area results in a better charge collector and the near-linear relation with concentration ratio. 5.10 Micro Electrical Mechanical Syst ems Suitability Discussion A comparison of energy sources for nano and micro systems and energy harvesting applications is presented in Figure 5.49. Figure 5.49 Comparisons of Energy Sources for Energy Harvesting Applications

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107 Table 5.3 measured cell output test result average resulted in: 1) Average Measured Instantane ous Current = 2.77E-05 Amps (0.028 milli Amps or 28 micro Amps), 2) Average Measured Instantaneous Volta ge = 0.0138 Volts (13.8 mVolts) 3) Average Measured Instanta neous Power = 3.8E-7 Watts (0.38 micro Watts or 382 pico Watts). In Comparisons from Figure 5.49: 1) Meso-scale low cost Pico Radio limits pow er dissipation from picoWatts/cm2 to 100microWatt/cm2 for energy scavenging target, 2) MEMS-device operations falls between picoWatts/cm2 to Watts/cm2, 3) Standard solar cells produce 15 milliWatt/c m2 in bright sun, 1 milliWatt/cm2 when averaged over 24 hours, 6 microWatt/cm2 inside a typically illuminated office, 4) Methanol Micro-Fuel Cell operational targ ets to match Li-ion polymer in cell phone applications are between 150 200 milliWatt/cm2, 5) Traditional PEM based fuel cells are between 300 500 milliWatt/cm3. In Summary: All though the industry tr end is toward lower power devices, 382 picoWatts is Orders of Magnitude below target PEM or Methanol Micr o-Fuel Cell or Liion polymer cell phone batteries. It is, how ever, in the low range of nano/MEMS or Energy Harvesting Devices.

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108 Chapter 6 Summary and Future Research The focus of this Ph.D. research effort was to address the concept, research and evaluation of a Bi-Polar membra ne based seawater concentrati on cell and its sutability as a low power energy source for Energy Harvesting nano/MEMS devices. In support of this, increased technical understanding into the membrane, ionic, environmental, and electrochemical effect s on the generated membrane current and potential of a Bi-Polar membra ne based, seawater concentra tion cell was developed. This was done thru: the use of equivalent circu it modeling to establish membrane, electrode processes and kinetics; an evaluation of pertinent factors effecting cell output performance followed by screening experiment s to reduce the number of factors to a manageable quantity for testing; and a Desi gn of Experiment based fractional factorial design analyses with accompanying performa nce testing to determine a predictive cell output performance optimized model in terms of pertinent input parameters tested. Modeled performance output was then compar ed to actual test measurements with nominal output values compared against ot her types of energy sources for nano and micro systems and energy harvesting applica tion. The final results confirmed that the

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109average cell output value of 380 picoWatts fa lls within the power range required for MEMS or Energy Harvesting Devices. Contributions to the field included the development of an increased technical understanding into a novel fu el cell based design method and apparatus which uses a replenishing concentration differential of ion solutions across a Bi-Polar semi-permeable membrane to effect ion mobility and electr ical power generation. The feasibility and applicability of this Bi-Polar membrane ba sed Dialytic Power Generator was in turn evaulated as a power source for low power sy stems such as energy harvesting devices, where it is impossible or impractable to provide wired or traditional battery power. During the testing process, the following specific contributions were identified: 1) Confirmed the Ag wire mesh electrode func tioned simply as charge collectors and the material selection, design, and ruggedness is suitable for subsequent field testing, 2) Identified membrane swelling and linkage of the “openings” present in used CEM and AEM membranes that can account for the measured Clco-ion/counter-ion migration across the Bi-Polar membrane from the concentrated to dilute sides of the functioning Bi-Polar concentration cell, 3) Determined that for maximum potential output the cell needs to operated in what is typically referred to as the (+) Bi-Polar me mbrane orientation, that being with the

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110concentrated solution in contact with the Bi-Polar membrane AEM side and the BiPolar membrane CEM in contact with the dilute solution side, 4) Determined that Anomalous Positive Os mosis and co-ion/counter-ion migration effects are occurring simultaneousl y across the Bi-Polar membrane, 5) Determined an insensitivity in cell out put performance due to changes in fluid pumping speed across the flow rates tested, 6) Determined that although OCV variations occurred with cell agitation, the affects were small under load with equilibrium quickly reached once agitation is removed, 7) Determined that Bi-Polar membrane c oncentration cell oper ation under external loading can not be described adequately by the Nernst equation, 8) Determined that membrane end use type/manufacture made very little difference and that temperature was the primary driving fact or in terms of the ce ll output potential, 9) The Bi-Polar membrane seawater concentrati on cell under an external electrical load produced a near constant cu rrent output acros s the load conditions tested, 10) The Bi-Polar membrane seawater concentra tion cell under an external electrical load operates as an electrolytic capacitor when operated in a fuel cell configuration,

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111In general, although small in output power, when under external load, the cell was found to produce repeatable results while relatively invariant to concentration variations and overall motion. All good characteristics for a Dialytic based sea wa ter concentration cell power generator. Significant progress was made during this re search effort. Additional research is planned to continue this progess to futher develop the concept into a functioning energy generation system. This research would en tail investigating output performance changes through: changes in cell design and size scalability; differing configuration combinations; and alternate electrode selec tion. In addition, the possi ble existance of Bi-Polar Membrane “water splitting” effects will be examined.

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112 Bibliography 1. Maron, S.H. and Prutton, C.F., “Principal s of Physical Chemistry”, The Macmillan Company, New York, 1962 (fourth printing) – LOC: 58-5133, pp. 544-569. 2. Bard, A.J. and Faulkner, L.R., “Elect rochemical Methods – Fundementals and Applications”, Wiley and Sons, 20001, Second edition. 3. Kemperman, A. J. B., “Handbook on Bi-Polar Membrane Technology”, Twente University Press, The Netherlands, 2000. 4. Higa, M. and Kira, A, “Transport of Ions across Bi-Polar membranes. 1. Theoretical and Experimental Examination of the Memb rane Potential of KCL Solutions”, J. Phys. Chem., Vol. 99, 1995, pp. 5089 5093. 5. Scott, Keith, “Handbook of Industrial Membranes”, 1995, Elsevier Advanced Technology, Oxford, ISBN 1856172333. 6. Helfferich, F., “Ion Exchange”, 1962, McGraw-Hill, New York. 7. Mulder, M., “Basic Principles of Memb rane Technology, second edition”, Kluwer Academic Publishers, 2003. 8. Mattson, M.E. and Lew, M., Desa lination, Vol. 41, 1982, page 1. 9. Solt, G. in “Handbook of Water Purifica tion”, Lorch, W., ed. McGraw-Hill, London, 1981, Chapter 9. 10. Pretz, J., Staude, E., “Reverse Electrodial ysis (RED) with Bi-Polar Membranes, an Energy Storage System”, Berichte der Bunsenges. Phys. Chem., Vol. 102, No. 4, 1998, pp. 676-685. 11. Applegate, L. E., “Membrane Separation Pr ocesses”, J. of Chemical Engineering, June 11, 1984, pp. 64-89. 12. Libes, S., “An Introduction to Marine Biogeochemistry”, John Wiley & Sons, 1992, ISBN 0471509469.

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11313. Kudela, V., Richau, K., Bleha, M., Paul, D., “Orientation effects on Bi-Polar and other asymmetric membranes as observed by concentration potentials”, Separation and Purification Technology, Vol. 22-23, 2001, pp. 655-662. 14. Suendo, V., Eto, R., Osaki, T., Higa, M., Ta nioka, A., “Ionic Envir onmental Effect on the Time-Dependent Characteristics of Memb rane Potential in a Bi-Polar membrane”, Journal of Colloid and Interf ace Science, Vol. 240, 2001, pp. 162-171. 15. Strathmann, H., Krol, J.J., Rapp, H. J., and Eigenberger, G., Journal of Membrane Science, Vol. 125, 1997, pp. 125. 16. Strathmann, H., Bauer, B., Rapp, H., “Better Bi-Polar membranes”, Chemtech, June 1993, pp. 17 – 24. 17. Rubinstein, I., Warshawsky, A., Schechtman, L., Kedem, O., Desalination, Vol. 55, 1984, pp. 55. 18. Simons, R., “The origin and eliminati on of water splitting in ion exchange membranes during water demineralization by electrodialysis”, Desalination, Vol. 28, 1979, pp. 41-42. 19. Mani, K.N., “Electrodialysis water sp litting technology”, Journal of Membrane Science, Vol. 58, 1991, pp 117-138. 20. Vlahakis, J., Jacobsen, W., and Miller, G., Electrotechnology, Vol. 1, Ann Arbor, Michigan, 1978, pp. 15. 21. Wang, H., Yu, Z., Wang, E., “The Transf er of Chloride Ion Across an Anion Exchange Membrane”, Electroanalysis, Vol. 8, No. 8-9, 1996, pp. 821-825. 22. Ohya, H., “Dialytic Battery Convertible Free Energy of Mixing of Seawater and River Water”, The 3rd Pacific Chemical Engineering Congress, Seoul, Korea, May 811, 1983, pp 451-456, Proceedings. 23. Cyclic Voltammetry Primer, wwwbiol.paisley.ac.uk/marco/enzyme_electr ode/chapter1/cyclic_voltammetry1.htm. 24. Uhlig, H., “Uhlig’s Corrosion Handbook”, John Wiley & Sons, 2000, Second edition. 25. EIS Primer Application Note, “Basics of Electrochemical Impedance Spectroscopy”, Gamry Instruments, Warminster, Pa, 2007. 26. Gabrielli, Claude, “Use and Application of Electrochemical Impedance Techniques”, Technical Report Number 24, Sola rtron Analytical, April 1997. 27. Bockris, J., Drazic, D., “Electro-Chemical Science”, Taylor and Francis, Ltd, 1972.

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About the Author Clifford Ronald Merz is currently the Director and Program Engineer for the University of South Florida (USF)/Colle ge of Marine Science's Coastal Ocean Monitoring and Prediction System (COMPS) Pr ogram. He holds a bachelor degree in Ocean Engineering from Florida Atlantic Univ ersity (1984) and is a licensed Professional Engineer in the State of Flor ida. In 2003, he formed Dialyt ics, Inc., a technology start up company specializing in renewabl e energy and water reuse concep ts. He is married to his wife Michelle of 26 years and has five beau tiful children: Zachary, Rachel, Melissa, Eric, and Trent. After obtaining his BS degree, he went b ack to school part-time to refine his professional skills, leading to advanced MS degrees in Ocean (1986), Civil/Water Resources Engineering (1997), and a graduate certificate in Desalination Technology and Engineering (2003). Most recently completi ng hands-on field installation workshops in solar, wind, and biodiesel through the Mi dwest Renewable Energy Association.


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Investigation and evaluation of a bi-polar membrane based seawater concentration cell and its suitability as a low power energy source for energy harvesting/MEMS devices
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Dissertation (Ph.D.)--University of South Florida, 2008.
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ABSTRACT: It has long been known from Thermodynamics and written in technical literature that, in principal, instant energy can be made available when dilute and concentrated solutions are mixed. For example, a river flowing into the sea carries with it a physical-chemical potential energy in its low salt content, some of which should be recoverable. As also known, a naturally occurring, diffusion-driven, spontaneous transport of ions occurs throughout a solution matrix, thru barrier interfaces, or thru ion-selective membranes from the side containing the salts of higher concentration to the compartments containing the more dilute solution to effect the equalization of concentration of the ionic species. Since this ion movement consists, preferentially, of either cations or anions, it leads to a charge separation and potential difference across the membrane, otherwise known as a membrane potential. Eventually, when the concentrations in the compartment are the same, the cell ceases to function. However, if operated as a fuel cell with its respective concentrations continually replenished, equilibrium at a specific value of potential difference is established. To capture the energy of this potentially significant albeit low power energy source, a suitable energy extraction device is required. The focus of this Ph.D. research effort is to address the concept, research and evaluation of a Bi-Polar membrane based seawater concentration cell and its suitability as a low power energy source for Energy Harvesting/MEMS devices (patent pending).
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