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Gaikwad, Kiran Sampat.
Development of a solid electrolyte for hydrogen production
h [electronic resource] /
by Kiran Sampat Gaikwad.
[Tampa, Fla.] :
University of South Florida,
Thesis (M.S.E.E.)--University of South Florida, 2004.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
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ABSTRACT: Electrolytic cells convert chemical energy directly into electrical energy cleanly and efficiently. An integral component of a fuel cell and an electrolytic cell is the electrolyte, a material that conducts ions. Liquid electrolytes can be aqueous as in the phosphoric acid and alkaline fuel cells, or molten, as in the molten carbonate fuel cells. A solid electrolyte is preferable because it allows sturdier, more efficient and corrosion resistant systems to be built. The main objective of this work is to develop a solid electrolyte for hydrogen production by electrolysis of hydrogen sulfide. Barium Hydrogen Phosphate, Barium Dihydrogen Phosphate, Cesium Hydrogen Carbonate, and Ammonium Iodide received brief attention but Cesium Hydrogen Sulfate was the primary candidate considered.Initial investigation has verified that Cesium Hydrogen Sulfate undergoes an impressive first-order phase transition at approximately 140°C at which the proton conductivity increases by almost four orders of magnitude. An electrochemical cell was designed and developed by Erik Todd for the production of hydrogen. Hydrogen sulfide can electrolyzed into hydrogen and sulfur in an electrochemical cell. Sulfur is in a low viscosity molten state at a temperature of 150°C. A cell with cesium hydrogen sulfate electrolyte canoperate at this temperature where liquid sulfur and gaseous hydrogen can move out of the cell as they are formed. Consequently, the electrolyte must possess a high conductivity at this temperature to facilitate the migration of hydrogen ions to the negative electrode through the electrolyte. Cesium Hydrogen Sulfate is known to act as an insulator at room temperature and a protonic conductor at 140°C.Hence it comes as an obvious choice as an electrolyte in a hydrogen sulfide electrochemical cell. The structural and chemical properties of Cesium Hydrogen Sulfate were investigated. The CsHSO electrolyte was prepared by the reaction of cesium carbonate and cesium sulfate with sulfuric acid respectively. A punch, die and base were designed and fabricated to 0.5" and 2.0" diameter pellets for that purpose. X-ray diffraction was performed on the 0.5" diameter pellets to identify and characterize the polycrystalline phases of the solid acid electrolyte. Differential Scanning Calorimetry was performed so as to ascertain the phase transition temperature. The temperature at which the phase transition occurs was further confirmed by impedance measurements. A test setup was built in order to perform impedance measurements. An experiment to measure the impedance of a 0.5" diameter pellet of silver iodide was performed in order to test the validity of the setup. An infrared analysis was performed on the prepared sample CsHSO in order to identify the bond environment of the electrolyte. Differential scanning calorimetry was performed with Barium Hydrogen Phosphate, Barium Dihydrogen Phosphate, Cesium Hydrogen Carbonate and Ammonium Iodide to identify their phase transition temperatures. A successful electrolysis of steam experiment was carried out using the CsHSO electrolyte to evaluate its performance. Finally, the CsHSO electrolyte was tested in the hydrogen sulfide electrochemical cell for the production of hydrogen and sulfur.
Adviser: Stefanakos, Elias K.
solid acid electrolytes.
differential scanning calorimetry.
x Electrical Engineering
t USF Electronic Theses and Dissertations.
Development of a Solid Elect rolyte for Hydrogen Production by Kiran Sampat Gaikwad A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in El ectrical Engineering Department of Electrical Engineering College of Engineering University of South Florida Major Professor: Elias K. Stefanakos, Ph.D., P.E. Burton Krakow, Ph.D. Venkat Bhethanabotla, Ph.D. Date of Approval: November 1, 2004 Keywords: solid acid electrolytes, superp rotonic conductivity, impedance measurements, differential scanning calorimetry, x-ra y diffraction, infrared spectroscopy Copyright 2004, Kiran Sampat Gaikwad
ACKNOWLEDGEMENTS First of all I would like to thank Shree Siddhivinayak and my family for giving me the strength and will to complete my work. I am grateful to Dr. Elias K. Stefanakos for being a constant source of motiv ation and showing his faith in me. I would also like to thank Dr. Burton Krakow for his valuable guidance without which it would not be possible to complete my work. I would like to express my sincere gratitude towards Dr. Venkat Bhethanabotla and Dr. Sagues for allowing me to use the facilities in their labs. I am highly indebted to Mr. Sesha Srinivasan for giving his time in achieving the results for my thesis. Finally, I would like to thank Amol Chaudhari, Mahesh Chettiar, Eric Weaver, Eric Todd, Alaa Kababji and Matt Smith for their help and support in my work.
i TABLE OF CONTENTS LIST OF TABLES iii LIST OF FIGURES iv ABSTRACT vi CHAPTER 1 INTRODUCTION 1 1.1 Introduction 1 1.2 Electrolyte 2 1.3 Solid Electrolytes 3 1.4 Solid Acid Electrolytes 3 1.4.1 Properties of Solid Acid Electrolytes 4 1.4.2 Benefits of Solid Acid Electrolytes 5 1.5 Organization of Thesis 5 CHAPTER 2 LITERATURE REVIEW 6 2.1 California Institute of Technology 6 2.2 The University of Texa s at Austin 7 CHAPTER 3 THEORY OF SOLID ACIDS 9 3.1 Structure 9 3.1.1 Atomic Bonding 9 3.1.2 Hydrogen Bonding 10 184.108.40.206 Intra-Hydrogen Bonds 11 220.127.116.11 Inter-Hydrogen Bonds 13 3.2 Coordination 14 3.3 Order-Disorder 14 3.3.1 Dynamic Disorder 15 3.3.2 Intra-Hydrogen Bond Disorder 15 3.3.3 Inter-Hydrogen Bond Disorder 16 3.3.4 Oxy-anion Disorder 16 3.4 Properties 16 3.4.1 Ionic Conductivity 16 3.4.2 Protonic Conductivity 20 18.104.22.168 Atomic Diffusion 20 22.214.171.124 Proton Displacement 20 126.96.36.199 Molecular Reorientation 20 188.8.131.52 Vehicle Mechanism 21 184.108.40.206 Grotthus Mechanism 21
ii 3.5 Phase Transitions 21 3.5.1 General Characterization 22 3.5.2 High Temperature Proton Conduction in Solid Acids 23 CHAPTER 4 DESIGN AND DEVELOPMENT OF SOLID ACID ELECTROLYTE 24 4.1 CsHSO 4 Prepared from Equimolar Amounts of Cs 2 SO 4 and H 2 SO 4 24 4.2 CsHSO 4 Prepared from Cs 2 CO 3 and H 2 SO 4 such that Cs 2 CO 3 : H 2 SO 4 ::1:2 25 4.3 Pellet Preparation 26 4.3.1 For 0.5 Diameter Pellet an d 1 mm Thickness 26 4.3.2 For 2.0 Diameter Pellet an d 1 mm Thickness 27 4.4 Die Design 28 4.4.1 Pellets of 0.5 Diameter 29 4.4.2 Pellets of 2.0 Diameter 31 CHAPTER 5 THEORY OF EXPERIMENTAL MEASUREMENTS 33 5.1 Impedance Measurement 33 5.2 Differential Scanning Calorimetry (DSC) 37 5.3 X-Ray Diffraction 38 5.4 Infrared Spectroscopy 39 CHAPTER 6 EXPERIMENTAL RESULTS 41 6.1 Impedance Measurements Results 41 6.2 Differential Scanning Calorimetry(DSC) Results 46 6.3 X-Ray Diffraction Results 49 6.3.1 Anchor Scan Parameters 49 6.3.2 X-Ray Diffraction Pattern of CsHSO 4 50 6.3.3 Peak List 50 6.3.4 Identified Patterns List 51 6.3.5 Plot of Identified Phases 52 6.4 Infrared Spectroscopy Results 52 CHAPTER 7 SUMMARY AND CONCLUSIONS 54 CHAPTER 8 RECOMMENDATIONS FOR FUTURE WORK 56 REFERENCES 57
iii LIST OF TABLES Table 1.1 Comparison of Electronic and Ionic Conductivity 2 Table 3.1 Correlation Between Hydrogen Bond Types and Bond Character 11 Table 3.2 Effects of Symmetry on Strong, Medium and Weak Hydrogen Bonds 12 Table 3.3 Inter-Hydrogen Bond Networks Exhibited by CsHSO 4 Solid Acid 13 Table 4.1 Amount of Force Required to Pre ss Discs of Different Diameters 28 Table 4.2 Specifications of Die, Punch and Base 29 Table 6.1 Impedance Measurements Results 45 Table 6.2 DSC Results 49 Table 6.3 Peak List for the X-Ra y Diffraction Pattern of CsHSO 4 50 Table 6.4 Identified Patterns List for the X-Ray Diffraction Pattern of CsHSO 4 51
iv LIST OF FIGURES Fig 4.1 0.5 Diameter Pellets of CsHSO 4 27 Fig 4.2 2.0 Diameter Pellets of CsHSO 4 28 Fig 4.3 Design of Punch, Die and Base for Preparing 0.5 Diameter Pellets 29 Fig 4.4 Fabricated Punch, Die and Base fo r Preparing 0.5 Diameter Pellets 30 Fig 4.5 Hydraulic Press Used for Prep aring 0.5 Diameter Pellets 30 Fig 4.6 Design of Punch, Die and Base for Preparing 2.0 Diameter Pellets 31 Fig 4.7 Fabricated Punch, Die and Base fo r Preparing 2.0 Diameter Pellets 31 Fig 4.8 MTS Press Used for Preparing 0.5 Diameter Pellets 32 Fig 5.1 Time Dependent Wave Function 34 Fig 5.2 Nyquist Plot of the AC Impedance of a Material as a Function of Frequency 36 Fig 5.3 Equivalent RC Circuit 36 Fig 5.4 Differential Scanning Calorimetry Setup 37 Fig 6.1 Log[ ] v/s T for 0.5 Dia. Pellet of CsHSO 4 from CS 2 CO 3 with Ag Paste 41 Fig 6.2 Log[ ] v/s T for 0.5 Dia. Pellet of CsHSO 4 from CS 2 CO 3 without Ag Paste 42 Fig 6.3 Log[ ] v/s T for 2.0 Dia. Pellet of CsHSO 4 from CS 2 CO 3 with Ag Paste 42 Fig 6.4 Log[ ] v/s T for 2.0 Dia. Pellet of CsHSO 4 from CS 2 CO 3 without Ag Paste 43 Fig 6.5 Log[ ] v/s T for 0.5 Dia. Pellet of CsHSO 4 from CS 2 SO 4 with Ag Paste 43 Fig 6.6 Log[ ] v/s T for 0.5 Dia. Pellet of CsHSO 4 from CS 2 SO 4 without Ag Paste 44 Fig 6.7 Log[ ] v/s T for 2.0 Dia. Pellet of CsHSO 4 from CS 2 SO 4 with Ag Paste 44
v Fig 6.8 Log[ ] v/s T for 2.0 Dia. Pellet of CsHSO 4 from CS 2 SO 4 without Ag Paste 45 Fig 6.9 DSC Plot of CsHSO 4 46 Fig 6.10 DSC Plot of Barium Hydrogen Phosphate 47 Fig 6.11 DSC Plot of Barium Diydrogen Phosphate 47 Fig 6.12 DSC Plot of Cesium Hydrogen Carbonate 48 Fig 6.13 DSC Plot of Ammonium Iodide 48 Fig 6.14 X-Ray Diffraction Pattern of CsHSO 4 50 Fig 6.15 Plot of Identified Phases 52 Fig 6.16 Infrared Spectroscopy Results of CsHSO 4 made from Cesium Sulfate 52 Fig 6.17 Infrared Spectroscopy Results of CsHSO 4 made from Cesium Carbonate 53
vi DEVELOPMENT OF A SOLID ELECTROLYTE FOR HYDROGEN PRODUCTION Kiran Sampat Gaikwad ABSTRACT Electrolytic cells convert chemical energy di rectly into electrical energy cleanly and efficiently. An integral component of a fuel cell and an elec trolytic cell is the electrolyte, a material that conducts ions. Liquid electrolyte s can be aqueous as in the phosphoric acid and alkaline fuel cells, or molten, as in the molten carbonate fuel cells. A solid electrolyte is preferable because it allows sturdier, more efficient and corrosion resistant systems to be built. The main objective of this work is to develop a solid electrolyte for hydrogen production by electrolysis of hydrogen sulfid e. Barium Hydrogen Phosphate, Barium Dihydrogen Phosphate, Cesium Hydrogen Carbonate, and Ammonium Iodide received brief attention but Cesium Hydrogen Sulfat e was the primary candidate considered. Initial investigation has verifi ed that Cesium Hydrogen Sulf ate undergoes an impressive first-order phase transition at approximately 140C at which the proton conductivity increases by almost four orde rs of magnitude. An electroc hemical cell was designed and developed by Erik Todd for the producti on of hydrogen. Hydrogen sulfide can electrolyzed into hydrogen and sulfur in an electrochemical cell. Sulfur is in a low viscosity molten state at a temperature of 150C. A cell with cesium hydrogen sulfate electrolyte can operate at this temperatur e where liquid sulfur and gaseous hydrogen can
vii move out of the cell as they are formed. Consequently, the electrolyte must possess a high conductivity at this temper ature to facilitate the migr ation of hydrogen ions to the negative electrode through the electrolyte. Cesium Hydrogen Sulfate is known to act as an insulator at room temperature and a prot onic conductor at 140C. Hence it comes as an obvious choice as an electrolyt e in a hydrogen sulfide electroc hemical cell. The structural and chemical properties of Cesium Hydrogen Sulfate were investigated. The CsHSO 4 electrolyte was prepared by the re action of cesium carbonate and cesium sulfate with sulfuric acid respectively. A punch, die and base were designed and fa bricated to make 0.5 and 2.0 diameter pellets for that purpose. X-ray diffraction was performed on the 0.5 diameter pellets to identify and characterize the polycry stalline phases of the solid acid electrolyte. Differential Scanning Calorimetry was perf ormed so as to ascertain the phase transition temperature. The temperature at which the phase transition occurs was further confirmed by impedance measurements. A test setup wa s built in order to perform impedance measurements. An experiment to measure th e impedance of a 0.5 diameter pellet of silver iodide was performed in order to test the validity of the setup. An infrared analysis was performed on the prepared sample of CsHSO 4 in order to identify the bond environmen t of the electrolyte. Differential scanning calorimetry was perf ormed with Barium Hydrogen Phosphate, Barium Dihydrogen Phosphate, Cesium Hydrogen Carbonate and Ammonium Iodide to identify their phase transition temperatures.
viii A successful electrolysis of steam experiment was carried out using the CsHSO 4 electrolyte to evaluate its performance. Finally, the CsHSO 4 electrolyte was tested in th e hydrogen sulfide electrochemical cell for the production of hydrogen and sulfur.
1 CHAPTER 1 INTRODUCTION 1.1 Introduction Hydrogen has the potential to solve major challenges facing America today namely dependence on petroleum imports poor quality of air and greenhouse gas emissions. Currently, the National Aeronautic s and Space Administration (NASA) utilize hydrogen as a fuel for launching shuttles to space from the Kennedy Space Center at Cape Canaveral, Florida. The source of hydr ogen for this purpose comes from natural gas produced in New Orleans and Texas. Each shuttle launch consumes approximately 300,000 pounds of hydrogen which evidently needs to be transported from New Orleans or Texas to Cape Canaveral, Florida. Considering the economics of the task NASA spends a large amount of money in transporta tion of hydrogen from the production site to the launch site. These high costs can be redu ced considerably if a technology to produce hydrogen at or near the launch site can be developed. Moreover hydrogen being a clean and environmentally friendly fuel it can be used in Florida in other applications. In this work the electrolysis of hydroge n sulfide to produce hydrogen and sulfur is considered. Hydrogen sulfide, being a toxic waste emitted from many industrial processes it would be available in abundan ce. Liquid sulfur has a low viscosity at a temperature of 150C. Electroly sis of hydrogen sulfide would facilitate the removal of the sulfur product from the cell. Hence, the need for a soli d electrolyte having a high
2 conductivity at 150C. Cesium Hydrogen Sulf ate undergoes an impressive first-order phase transition at 140-150C at which the pr oton conductivity incr eases by almost four orders of magnitude. The main purpose of this work is to investigat e the use of Cesium Hydrogen Sulfate electrolyte for applicati on in hydrogen sulfide electrochemical cells. Since Barium Hydrogen Phosphate, Barium Dihydrogen Phosphate, Cesium Hydrogen Carbonate, Ammonium Iodide are known to have a similar structure as that of Cesium Hydrogen Sulfate it is therefore important to investigate the properties of these compounds for consideration. 1.2 Electrolyte An electrolyte may be defined as an electr ic conductor in which current is carried by the movement of ions. Most electrolytes are liquids, but some electrolytes are solids.  The following table shows the differe nce between ionic conductivity and electronic conductivity: Table 1.1 Comparison of Electronic and Ionic Conductivit y  Electronic Conductivity Ionic Conductivity Conductivity range 10 S/cm < < 10 5 S/cm Conductivity range 10 -3 S/cm < < 10 S/cm Electrons carry current Ions carry current Conductivity increases as temperature decreases Conductivity decreases as temperature decreases
3 1.3 Solid Electrolytes Solid ionic electrolytes generally ex hibit the following characteristics: A large number of the ions of one species sh ould be mobile in the electrolyte. This requires a large number of empty sites, either vacancies or accessible interstitial sites. Empty sites are required for ions to move through the lattice. The empty and occupied sites should have similar potential energies with a low activation energy barrier for jumping betw een neighboring sites. A high activation energy decreases carrier mobility. Very stable sites (deep potential energy wells) lead to carrier localization. The structure of the electrolyte should have solid framework, preferably 3D, permeated by open channels. The migrating ion lattice should be molten, so that a solid framework of the other ions is needed in order to prevent the entire material from melting. The framework ions (usually anions) shou ld be highly polarizable. Such ions can deform to stabilize transition state geometri es of the migrating ion through covalent interactions. 1.4 Solid Acid Electrolytes Solid acids, or acid salts, like Cesium Hydrogen Sulfate are a class of compounds whose chemistry and properties lie be tween those of a normal acid e.g., H 2 SO 4 and a normal salt e.g., K 2 SO 4 They usually consist of oxyanions, such as SO 4 2, which are linked together by hydrogen bonds. Initial resear ch has revealed that Cesium Hydrogen Sulfate undergoes an impressive first-order phase transition at approximately 140C at
4 which the proton conductivity increases by almost four orders of magnitude. The increase in conductivity may be attribut ed to the local defects in the structure and subsequent protonic transition. At room temperature the Cesium Hydrogen Sulfate has a monoclinic symmetry. Above the phase transition temperature, the symmetry of the compound increases and the oxygen atoms become disordered to contain the higher symmetry. The partial occupancy of the oxygen sites gives a nearly liquid-like na ture to the protons as the earlier static hydrogen bonded system becomes highly dyn amic. In this dynamic system, the SO 4 groups are rearranged with inter-tetrahedra hopping of the proton. This fast reorientation of the tetrahedra in conjunction with proton translations leads to the jump in conductivity across the phase transition and to the s uperprotonic conduction many solid acids exhibit in their high temperature phases. 1.4.1 Properties of Solid Acid Electrolytes Solid acid electrolytes exhibit the following properties: True solid state proton conductors Insulators at room temperature Ionic conductors above a transition temperature usually above 100C Operate with no liquid water Modest catalyst requirements Waste heat is hot enough for use in domestic water heati ng or space conditioning as well as to generate the steam that is electrolyzed Impermeable to fuels and electrode scavengers
5 1.4.2 Benefits of Solid Acid Electrolytes The benefits of solid acid electrolytes are as follows: Sturdy Vibration and corrosion resistant Durable 1.5 Organization of Thesis This thesis is organized in eight ch apters. The second chapter gives a brief description of a few technical papers and th e background work alrea dy carried out on the CsHSO 4 solid acid electrolyte. The third chapter covers the th eoretical aspects of solid acid electrolytes and provides detailed info rmation on the reasons for the superprotonic phase transition of CsHSO 4 solid acids. The development of a solid acid electrolyte is described in chapter four. Chapter five hi ghlights the experiment al setup used for understanding the chemical and structural pr operties of solid elect rolytes. Chapter six contains the results derived fr om the experiments and gives a brief analysis of the results. A summary of the thesis work done and co nclusions are presented in chapter seven. Finally the recommendations for future work are outlined in chapter eight.
6 CHAPTER 2 LITERATURE REVIEW 2.1 California Institute of Technology The Materials Science department at th e California Institute of Technology developed a fuel cell made of a CsHSO 4 electrolyte membrane about 1.5mm thick operating at 150-160C in a H 2 /O 2 configuration. At the transition temperature 50-150C the conductivity of CsHSO 4 increases to a value of 10 -3 to 10 -2 -1 cm -1 The solid acid electrolyte was prepared from an aqueous solution of Cs 2 CO 3 and H 2 SO 4 A layer of the solid acid was sandwiched between two electrocatalysis layers comprised of CsHSO 4 Pt black, carbon black and a vola tile organic in a mass ratio of 6:10:1:1. These layers were then placed betw een two sheets of porous graphite current collectors. The entire assembly was uniaxia lly pressed at 490MPascal, to produce a dense electrolyte membrane of 1-1.5mm in thickne ss with good mechanical contact to the electrocatalyst layers. The CsHSO 4 electrolyte used was a millimeter thick or thicker but for real applications micrometer thin films will be required to reduce th e resistance of the electrolyte layer.
7 2.2 The University of Texas at Austin The Materials Science and Engineering depa rtment at the University of Texas at Austin studied the structural characterization and measured the superionic transitions of CsHSO 4 before and after heating. The solid aci d electrolyte was prepared as follows: Cs 2 SO 4 was first dissolved in a dilute sulfuric acid solution so that the molar ratio of Cs 2 SO 4 :H 2 SO 4 :H 2 O is 1:2:12. After a complete dissolution of Cs 2 SO 4 ethanol was added to the solution to precipitate CsHSO 4 which was then filtered, dried at 100 C overnight, and stored in a vacuum desiccator before further characterization. Structural characterization and the meas urement of superioni c transitions of CsHSO 4 before and after heating were carried out, respectively, by X-ray powder diffraction and a Perkin-Elmer series 7 diffe rential scanning calorimeter (DSC). The DSC experiments were carried out with a pproximately 10 mg of sample in a N 2 atmosphere at a heating rate of 10 C/min. In addition to the research being conducted at the California Institute of Technology and the University of Texas at Aust in there has been c onsiderable additional work done on solid acid compounds and a numb er of papers published explaining the mechanism of proton conductivity in solid ac ids by various methods, as well as the structure of different phases of the solid acid compound CsHSO 4 The mechanism of proton conductivity and the structur e of different phases of CsHSO 4, as mentioned in the technical papers, are summarized below. CsHSO 4 crystals undergo two phase transiti ons III-II at 100C and II-I at 141C. A hydrogenous CsHSO 4 sample grown from an aqueous solution crystallizes in a monoclinic phase III. In phase III the crystals exhibit a high plasticity due to ferroelastic
8 twinning. The crystal structure consists of zigzag chains of hydrogen bonds linking SO 4 tetrahedra. No phase transitions occur when this phase is cooled to liquid helium temperatures. When heated above 100C phase III transforms into phase II which is also monoclinic. It differs from phase II in terms of lattice parame ter, smaller unit cell volume and by the organization of hydrogen bonds. The distance between the nearest protons in different hydrogen bond chains is considerably larger in phase II than in phase III. At a temperature of 141C phase II transforms into phase I which exhibits an extremely high protonic mobility resulting in high prot onic conductivity. Phase I is tetragonal and contrary to the lower te mperature phases, each SO 4 can adopt not one but four crystallographically equivalent orientations. As a result, the number of possible proton positions in the unit cell becomes larger than the number of protons. The hydrogen bonded network becomes dynamically disordered allowing protons to move through the lattice by jumping to vacant positions. Proton ju mps are associated with reorientations of SO 4 tetrahedra. Since the transition is pr imarily driven by the disordering of SO 4 tetrahedra and the proton disorder is a second ary effect the isotope effect on the transition temperature is absent.[5,6,7,8,9]
9 CHAPTER 3 THEORY OF SOLID ACIDS 3.1 Structure There are three important concepts necessa ry to describe the structure of solid acids: atomic bonding, coordination, and order-disorder. Solid acids can be represented by the general chemical formula: M a H b (XO4) c where M is a monovalent or divalent cation, XO 4 is a tetrahedral oxy-anion, and a, b, c are integers. The structure of solid acids is comprised of hydroge n-bonded tetrahedral oxy-anions charge balanced by a host lattice of cations. 3.1.1 Atomic Bonding The theory of atomic bonding is a useful concept for the characterization and prediction of the structure of solids. Solids are held together by cohesive forces, which are the electrostatic interaction between the ne gative charges of electrons and the positive charges of the nuclei. The cohesive forces between atoms may be termed chemical or atomic bonds. Further, chemical bonds are cl assified as ionic or electrostatic bonds, covalent bonds, and metallic bonds.
10 The structure of solid acids is usually dominated by electrosta tic or ionic bonding. When one or more electrons are transferred fr om one atom to another, they form a cation and an anion respectively. An ionic bond is formed when there is an electrostatic attraction between positively a nd negatively charged ions.[10,11,12,13] 3.1.2 Hydrogen Bonding A major contribution in the structure of solid acids is a fourth type of bond, known as the hydrogen bond. The hydrogen bond is defined as an atom of hydrogen attracted by strong forces to two atoms, inst ead of one, so as to act as a bond between them.  While compared to other types of bonds, the hydrogen bond is to a certain extent weak, but it plays an important role in determining the structure and properties of solid acids. The attraction between hydrogen-bonded atoms is mostly due to large ionic forces since the hydrogen atom contai ning only a single 1s electron can form only one covalent bond. In the case of solid aci ds with hydrogen bonds between two oxygen atoms, the two oxygen atoms are labeled as the proton donor oxygen atom (O d ), where the proton lies within the electron density of the oxygen atom and the bond most closely resembles a covalent bond, and the proton acceptor oxygen atom (O a ), which is hydrogen-bonded to O d via largely ionic forces. For solid acid compounds, hydrogen bonds demonstrate unique hydrogen bond geometries within the hydrogen bonds na mely, intra-hydrogen bonds, and interhydrogen bonds.[11,12,13]
11 220.127.116.11 Intra-Hydrogen Bonds Hydrogen bonds between oxygen atoms can be categorized according to their strength, which is related to th e donor oxygen to hydrogen distance (d Od H ), and the donor to acceptor oxygen distance (d Od...Oa ). The strength of a hydrogen bond increases inversely with the covalency of the O d H bond, such that the hydrogen bond strength increases as the hydrogen bond character transitions from bei ng largely ionic to mostly covalent.[11,12,13] Table 3.1 Correlation Between Hydrogen Bond Types and Bond Character  Bond Strength dO d-H (A) d OdOa (A) Character Strong 1.3 to 1.0 2.4 to 2.6 covalent Medium 1.02 to 0.97 2.6 to 2.7 polar covalent Weak Below 1.1 2.7 to 3 ionic Depending on the local symmetry of oxygen atoms participati ng in the hydrogen bond, the hydrogen bonds between oxygen atoms can be further classified into the following types. If both oxygen atoms pa rticipating in a hyd rogen bond occupy crystallographically equivalent positions, th en the bond is symmetric, whereas, if the oxygen atoms occupy crystallographic distinct positions then the bond is asymmetric. Table 3.2 is a schematic representati on of the hydrogen bond potential energies E(r) (or potentials) as a func tion of interatomic distance between two oxygen atoms for strong, medium, and weak symmetric and asymmetric bonds. For symmetric and asymmetric strong hydrogen bonds, at hydrogen bond distances less than 2.4 A, there is no distinction between the donor and acceptor oxygen
atoms and the hydrogen atom sits equidistant between the oxygen atoms in single-well potential. Table 3.2 Effects of Symmetry on Strong, Medium and Weak Hydrogen Bonds  Bond Type Symmetric Asymmetric Strong Medium Weak Not generally observed At medium bond strengths, with a hydrogen bond distance of ~2.6 A, for symmetric and asymmetric bonds the hydrogen atom can reside near either oxygen atom, in one minimum of a double-well potential. At sufficiently high temperatures, thermal 12
oscillations can allow the hydrogen atom to overcome the potential barrier between the minima in the double-well potential, partially occupying each positionthis is known as hydrogen bond disorder. In table 3.2, for medium strength asymmetric hydrogen bonds both single-well potentials, case (1), which are not disordered and double-well potentials, case (2), which are disordered, are possible. At hydrogen bond distances greater than 2.9 A (weak hydrogen bonds), symmetric bonds are not generally observed and for asymmetric bonds the hydrogen atom lies within the minimum of a single-well potential, close to the donor oxygen atom. 18.104.22.168 Inter-Hydrogen Bonds Further than intra-hydrogen bond geometry, the structure of solid acid compounds can also exhibit a wide variety of inter-hydrogen bond geometries, or networks. In general, the distribution of hydrogen bonds can exhibit zero, one, two, and three-dimensional networks depending on the density of hydrogen bonds. The type of hydrogen bond network depends on the ratio of hydrogen to tetrahedral oxy-anions (H/XO 4 ). The CsHSO 4 solid acid has one hydrogen atom per XO 4 and tends to exhibit one-dimensional network in the form of chains. Table 3.3 Inter-Hydrogen Bond Networks Exhibited by CsHSO 4 Solid Acid  Dimensionality Networks H/XO 4 Example 1D Chains 1 CsHSO 4 13
14 3.2 Coordination The coordination of an atom is define d by the number of surrounding nearest neighbor atoms. Coordination plays an importa nt role in the arrangement of atoms in solids, and is influenced principally by the type of bonding and the re lative size of atoms (or ions) in a solid. From simple geometric considerations and assuming a rigid sphere model for ions, the structure of ionic solids can often be inferred from the relative sizes of the constitutive ions. 3.3 OrderDisorder Orderdisorder in solid acid structures is an important feature in identifying the properties of these compounds. The two main types of orderdisorder phenomena observed in solid acids which descri be the arrangement of atoms are structural disorder, in which a single at omic species partially occupies multiple crystallographic positions; and chemical disorder, in which multiple atomic species occupy the same crystallographic position. Structural disorder can be broken into two general categories: Static disorder, when a single atomic species is randoml y distributed over multiple crystallographic positions. Dynamic disorder, resulting from therma lly activated atomic species moving between two or more crystallographic positions. Of the above mentioned categories dynamic disorder is an im portant concept in understanding the properties of solid acid compounds.[11,12,13]
15 3.3.1 Dynamic Disorder Dynamic disorder is responsible for fe rroelectric and superprotonic conductivity properties exhibited by solid acid compounds. The dynamic disorders exhibited in solid acids are as follows: Intra-hydrogen bond disorder, Inter-hydrogen bond disorder, and Oxy-anion disorder. The first of these is responsible for ferroelectric transitions in solid acids, and the second and third, being closely related to each another, are responsible for superprotonic behavior. 3.3.2 Intra-Hydrogen Bond Disorder Intra-hydrogen bond disorder occurs in medium strength symmetric and asymmetric hydrogen bonds, in which there ar e two crystallographic positions separated by a potential barrier for a si ngle hydrogen atom. At lo w temperatures there is insufficient thermal energy for the hydrogen at om to overcome the potential barrier, and the structure becomes ordered with respec t to the intra-hydrogen bondthe hydrogen atom resides in just one crystallographic pos ition. Upon heating, ther mal oscillations of the hydrogen atom become sufficient to overc ome the potential barrier between the two crystallographic positions and the crystallographic structur e becomes disordered with respect to the intra-hydrogen bond. In terms of properties, th is transition from order to disorder in solid acids leads to a ferroel ectric to paraelectri c transition. [11, 13]
16 3.3.3 Inter-Hydrogen Bond Disorder Upon heating some solid acids, th e oxy-anions are set free between crystallographically equivalent positions, wh ile simultaneously breaking and reforming new hydrogen bondsa superprotonic phase transition. The hydrogen bond is thus distributed among several crystallographic positions, i.e., the inter-hydrogen bonding becomes disordered. [11, 13] 3.3.4 Oxy-Anion Disorder Dynamic structural oxy-anion disord er occurs when the oxygen atoms of structural tetrahedral oxy-an ions partially occupy crysta llographically equivalent positions. Due to the strong bonding between the oxygen atoms and the central tetrahedral atom, the overall tetrahedral struct ure is maintained, leading to the release of the tetrahedron between these crystallographi cally equivalent posit ions, and manifesting in several possible orientations of the tetrahedron. [11, 13] 3.4 Properties Electrolytes in general possess high ionic conductivity and little or no electronic conductivity. Therefore, the ionic conductivity a nd specifically prot onic conductivity, is the principle material property of so lid acids investigated here. CsHSO 4 solid acid exhibiting high proton or superprotonic c onductivity is of part icular interest. 3.4.1 Ionic Conductivity A review of the basic underlying princi ples of ionic conduction is given here. Consider an isotropic solid where the material property conductivity is a scalar quantity that relates the current density I to an applied electric field E according to Ohms law I = E (3.1)
17 In an electrolyte with only one type of charge carrier, the current density I for a concentration of charged carriers n each with charge q traveling with an average velocity of v is I = nqv (3.2) The charge carrier mobility is defined as = v (3.3) E can be used to express the conductivity as = nq (3.4) When the concentration of particles n and the electric field E vary along the xdirection, the subsequent flux J (or number of charged carr iers passing through an area per time) of charge carriers is equal to the product of the mean force on the particles F, n number of particles per unit volume, charge q, and their mobility per charge /q J = n F = n ( + qE ) (3.5) q q x where (= G/ n) is the chemical potential of the ch arge carriers. In the absence of an electric field (E = 0) this reduces to J = n ( + qE ) (3.6) q x The flux of particles can also be expresse d in terms of a charge carrier diffusion coefficient D according Ficks first law J = D n (3.7) x Then, from the definition of chemi cal potential in dilute solutions = o + k B T ln n (3.8)
18 where k B is the Boltzmann constant, and T is temperature, and differentiating with respect to x = k B T ( n ) (3.9) x n x Now equating Equations 3.6 and 3.7, and us ing the previous relationship the diffusion coefficient can be related to the mobility of the charge carriers according to the Nernst-Einstein equation, u = qD (3.10) k B T and substituting this into Equation 3.4 gives = nq 2 D (3.11) k B T Assuming uncorrelated motion of the charge carrying species a random walk model can be adopted to describe the diffu sion coefficient, such that D = ao2 (3.12) where is a geometric factor dependi ng on the structure of the solid, a o is the distance the mobile charge carrier jumps betw een vacant crystallographic sites, and is the frequency at which the charge carrier jumps. The jumping of charge carriers between crystallographic sites is a thermally activated process, which is best described by an Arrhenius-type temperature dependence, = o exp ( G a ) (3.13) k B T
19 where o is the attempt frequency and G a is the Gibbs free energy for activation of this process. This leads to a diffusion coefficien t that varies with an Arrhenius behavior, D = a o 2 o exp( G a ) (3.14) k B T = Do exp( G a ) (3.15) k B T where the pre-exponential factor D o = a o 2 o The Gibbs free energy of activation can be expressed in terms of an activation entropy S a and enthalpy H a G a = H a T S a (3.16) Similarly, the activation enthalpy can be e xpressed in terms of an activation energy E a and volume V a H a = E a + P V a (3.17) where the activation volume is often negl ected at ambient pressures. Using these thermodynamic relationships and combining Equations 3.4, 3.10, and 3.14 the Arrhenius relationship for the conductivity of a solid can be written as T = A 0 exp ( H a ) = A 0 exp ( E a P V a ) (3.18) k B T k B T where the pre-exponential factor A 0 is A 0 = D 0 nq 2 exp ( S a ) (3.19) k B k B = a o 2 o nq 2 exp S a (3.20) k B k B Thus, we have derived an expression fo r the ionic conductivity of an isotropic solid in terms of intrinsic material properties as a function of temperature that closely models the bulk behavior of real ionic solids.[14,15]
20 3.4.2 Protonic Conductivity There are five basic mechanisms for proton motion in solids: atomic diffusion, proton-displacement, molecular reorientation, vehicle mechanism, and Grotthus mechanism. 22.214.171.124 Atomic Diffusion This type of proton motion is simply coupled proton-electron diffusion, which is common in materials such as metal hydrides, like Li 3 AlH 6 and Na 3 AlH 6 In such materials, the hydrogen can donate its electron density to the host matrix, accept electron density, or simply remain neutral.[12,13] 126.96.36.199 Proton Displacement This proton motion occurs when a proton hops along a hydrogen bond from one minima of a double-well potential to the ot her. This type of proton motion is quite common in solid acid compounds, and is respon sible for ferroelectric behavior in solid acids, such as KH 2 PO 4 .[12,13] 188.8.131.52 Molecular Reorientation Also referred to as dipole reorientati on, in this process a proton rides piggyback a molecule undergoing a reorientation, rotation, or tumble. This motion was first proposed to describe proton transport in ice, but is also co mmonly observed in solid acid compounds, as well as liquid acids. [12,13]
21 184.108.40.206 Vehicle Mechanism In this mechanism, proton translation is associated with the diffusion of polyatomic species. In this type of prot on motion, a proton rides piggy-back on a mobile molecule, which may carry a positive charge (e.g. NH +4 OH +3 O 2 H +5 O 3 H +7 ), a negative charge (e.g. NH 2 OH ), or be neutral (e.g. NH 3 H 2 O). The vehicle mechanism is, perhaps, the most common type of proton transport mechanism, and is exhibited by many fast-proton conduc tors, such as Nafion, in which H 3 O + ions are transported along sulfonic acid functional groups (SO 3 ), within a polymer host matrix.[12,13] 220.127.116.11 Grotthus Mechanism This mechanism is a cooperative proces s involving both a molecular (dipole) reorientation and proton-displacement. Superprotonic solid acids, such as CsHSO 4 conduct protons via this process. In th ese superprotonic solid acids, the oxyanion rearranges between crystallographically equiva lent positions while carrying a proton with it, then the proton hops along a hydr ogen bond to another oxyanion, followed by another oxyanion reorient ation, and so on.[12,13] 3.5 Phase Transitions Phase transitions are of fundamental importance to this work since the superprotonic behavior in solid acid compounds is associated with a structural phase transition. Here, a general description of phase transitions will be given, as well as a specific description of supe rprotonic phase transitions.
22 3.5.1 General Characterization A phase is a homogeneous solution of matter bounded by a surface so that it is mechanically separated from any other porti on. A phase transition is a transformation of matter induced by a change in a thermodynamic function, such as temperature T, pressure P, volume V, or entropy S, from one phase to another phase that is noticeable from the first. For the purpose of this work phase transitions are identif ied by a discontinuous change in the extensive thermodynamic variab les of a substance, such as volume V entropy S, magnetization M, polarization P, while varying an intensiv e variable, such as pressure P, temperature T, magnetic field B, or electric field E. Specifically, for transitions in which there is a discontinuous change in the entropy through the phase transition while varying the temperature, there will also be a change in enthalpy H or latent heat Q associated with the transition. This sort of phase transition is known as a first order phase transition. If the entropy is continuous, but its first derivative with respect to temperature, or heat capacity Cp is discontinuous, then the phase transition is said to be of second order, and if the en tropy and heat capacity are continuous, and the derivative of the heat capacity Cp/ T is discontinuous, the transi tion is third order, and so on. Cp = T ( S ) p (3.21) T In this work, first order solidsolid phase transitions are of key interest. For these transitions the change in extensive ther modynamic variables such as V and M are negligible compared to the change in S. T = E (3.22) S
23 3.5.2 High Temperature Proton Conduction in Solid Acids At higher temperatures a highly disordered state leads to fast local dynamics of the anion tetrahedra and subsequent proton transition via the Grothhuss mechanism. The physical reorientations of the tetrahedra in these phases suggest that the tetrahedra are rearranging much faster than the protons being transferred. Due to an increase in symmetry across the phase transition there is a disorder on the oxygen sites, which causes the tetrahedra to rotate freely betw een crystallographically identical positions. This nearly free rotation of the tetrahedra creates more crystallographically equivalent proton sites than the protons, resulting in a dynamically disordered hydrogen-bonded network. The combination of fast tetrah edral dynamics and pr oton transitions along hydrogen bonds of a disordered network results in high protonic conductivity. The room temperature phase of CsHSO4 -II is monoclinic, made up of zigzag chains of hydrogen bonded SO4 tetrahedra alternating with zigzag rows of cesium atoms. There are four crystallogra phically distinct oxygens, tw o of which are involved in asymmetric hydrogen bonds. After going into the superprotonic tetragonal phase the oxygens become crystallographically identic al and all oxygens participate in hydrogen bonds. There are two possible orientations of the tetrahedra, resulting in and occupancy of the oxygen and proton sites, respectively whereby the hydrogen bonds connect the oxygens. The method of proton conduction can be summarized as rapid reorientations of the SO4 group forming a dynamically disordered network of hydrogen bonds through which protons can jump from on e tetrahedron to the next. This mechanism of proton transport is responsible for the high conductivity in all s uperprotonic phases of solid acids. [12, 13]
24 CHAPTER 4 DESIGN AND DEVELOPMENT OF SOLID ACID ELECTROLYTE Two procedures were pursued to prepare CsHSO 4 : Reaction of Cs 2 SO 4 and H 2 SO 4 such that the ratio of Cs 2 SO 4 to H 2 SO 4 is 1:1 and, Reaction of Cs 2 CO 3 and H 2 SO 4 such that the ratio of Cs 2 CO 3 to H 2 SO 4 is 1:2 4.1 CsHSO 4 Prepared from Equimolar Amounts of Cs 2 SO 4 and H 2 SO 4 Sulfuric Acid Molecular Weight = 98.08 Normality = 36 Specific Gravity = 1.84 gm/ml Concentration = 98.8 % Cesium Sulfate Molecular Weight = 361.88 Equimolar amounts of cesium sulfate and sulfuric acid are required For every 361.88 gm of Cs 2 SO 4 we need 98.08 gm of H 2 SO 4 Pure H 2 SO 4 = 0.958 x 1.84 = 1.76272 gm/ml For each mil of H 2 SO 4 the amount of Cs 2 SO 4 required = 1.76272 x 361.88 98.08 = 6.5038 gm
25 The following procedure was employed to prepare CsHSO 4 :  First of all, equimolar amounts of Cesium Su lfate and concentrated sulfuric acid are dissolved in a small amount of DI water. The resulting solution is then concentrated under a flow of warm air in an oven at a temperature of 60C until less than half the water has evaporated. Once half the water has evaporated, the solution is then cooled in a chiller at a temperature of 5C. The precipitated crystalline solid is separa ted by filtration and then dried in a vacuum desicator. The powered sample is then dried in an oven at about 105C to ensure that it is completely dry. The sample is then placed in a punch, base and die assembly for pressing in order to prepare pellets of 0.5 and 2.0. 4.2 CsHSO 4 Prepared from Cs 2 CO 3 and H 2 SO 4 such that Cs 2 CO 3 : H 2 SO 4 :: 1:2 The amount of CsHSO 4 required to prepare 0.5 diam eter and 1 mm thick pellet was calculated as follows: Sulfuric Acid Molecular Weight = 98.08 Normality = 36 Specific Gravity = 1.84 gm/ml Concentration = 98.8 %
26 Cesium Carbonate Molecular Weight = 325.82 The ratio of Cs 2 CO 3 to H 2 SO 4 is 1:2 For every 325.82 gm of Cs 2 CO 3 we need 98.08 x 2 = 196.16 gm of H 2 SO 4 Pure H2SO4 = 0.958 x 1.84 = 1.76272 gm/ml For each mil of H 2 SO 4 the amount of Cs 2 CO 3 required = 1.76272 x 325.82 196.16 = 2.9278 gm The procedure adopted for preparing CsHSO 4 from Cs 2 CO 3 and H 2 SO 4 is same as that from Cs 2 SO 4 except that instead of Cs 2 SO 4 Cs 2 CO 3 is to be used. 4.3 Pellet Preparation The amount of CsHSO 4 required to prepare 0.5 and 2.0 diameter pellets with 1 mm thickness was calculated as follows: 4.3.1 For 0.5 Diameter Pellet and 1 mm Thickness A = d 2 4 A = x 0.5 2 4 A = 0.1963 inch 2 Volume = A x thickness of the pellet = 0.1963 x 2.5 2 x 0.1 Volume = 0.1226 cm 3 Density of CsHSO 4 = 3.352 gm/cc
Density = mass Volume 3.352 = mass 0.1226 Mass = 0.4109 gm of CsHSO 4 Fig 4.1 0.5 Diameter Pellets of CsHSO 4 4.3.2 For 2.0 Diameter Pellet and 1 mm Thickness A = d 2 4 A = x 2.0 2 4 A = 3.1415 inch 2 Volume = A x thickness of the pellet = 3.1415 x 2.5 2 x 0.1 Volume = 1.9634 cm 3 Density of CsHSO 4 = 3.352 gm/cc Density = mass Volume 3.352 = mass 1.9634 27
Mass = 6.5813 gm of CsHSO 4 Fig 4.2 2 Diameter Pellet of CsHSO 4 From literature  search it was observed that for 0.5 diameter disc with 1 mm thickness the amount of pressure required to press is 490MPascal. Table 4.1 indicates the amount of force required to press discs of different diameters. Table 4.1 Amount of Force Required to Press Discs of Different Diameters Disc Diameter (inches) Press (tons) 0.6 12 0.8 17 2.0 110 3.5 350 4.4 Die Design The specifications of the die, punch and base for preparing 0.5 and 2.0 diameter pellets are as shown in table 4.2. 28
Table 4.2 Specifications of Die, Punch and Base Material Hardness Die A2 Steel 40-50 Rockwell Punch A2 Steel 40-50 Rockwell Base A2 Steel 40-50 Rockwell 4.4.1 Pellets of 0.5 Diameter The design of the punch, die and base for preparing 0.5 diameter pellets is as shown in the figure 4.3. Fig 4.3 Design of Punch, Die and Base for Preparing 0.5 Diameter Pellets 29
The fabricated punch, die and base for preparing 0.5 diameter pellets is shown in figure 4.4 Fig 4.4 Fabricated Punch, Die and Base for Preparing 0.5 Diameter Pellets The hydraulic press used for preparing 0.5 diameter pellets is as shown in figure 4.5. Fig 4.5 Hydraulic Press Used for Preparing 0.5 Diameter Pellets 30
4.4.2 Pellets of 2.0 Diameter The design of the punch, die and base for preparing 2.0 diameter pellets is as shown in the figure 4.6. Fig 4.6 Design of Punch, Die and Base for Preparing 2.0 Diameter Pellets The fabricated die, punch and base for preparing 2.0 diameter pellets is shown below. Fig 4.7 Fabricated Punch, Die and Base for Preparing 2.0 Diameter Pellets 31
The press used for preparing 2.0 diameter pellets is as shown in figure 4.8. Fig 4.8 MTS Press Used for Preparing 2.0 Diameter Pellet 32
33 CHAPTER 5 THEORY OF EXPERIMENTAL MEASUREMENTS 5.1 Impedance Measurement Theory Protonic conductivity is the principle property of interest in the CsHSO 4 electrolyte. For this, extensive use of alte rnating current (AC) im pedance spectroscopy has been made to characterize the protonic c onductivity of this solid acid compound as a function of temperature at ambient pressure. The advantage of an AC method is that th ere is no net movement of ions, thereby eliminating the need for an ion source. This method is implemented by placing an ionic conducting material under an alternating electric field E, wi th an angular frequency of and amplitude E o which can be described by the complex time (t) dependent wave function E(t) = E o e j t (5.1) The current response I(t) generated by this elec tric field in the mate rial being tested, as shown in Figure 5.1, can be described by a si milar time dependent wave function with some amplitude I o plus a phase shift I(t) = Io e j( t + ) (5.2)
Fig 5.1 Time Dependent Wave Function From Ohms law, E(t) = I(t). Z (5.3) where Z is the complex impedance characterized by a real component Z and an imaginary component Z Z = Z + jZ (5.4) The reciprocal of impedance is admittance Y and is given as Y = 1 = Y + jY (5.5) Z Rewriting Ohms law using admittance Y () = I(t) = I o e j( t + ) = I o (cos+jsin) (5.6) E(t) E o e j t E o When a materials current response is at a frequency at which no phase shift occurs ( = 0), then equation becomes Y (0) = Io = 1 (5.7) Eo R 34 where R is considered as the real resistance of the material under test. As the frequency increases, the materials current response due to mobile charge carriers begins to lag
35 behind the applied electric field by some phase shift This, in turn, leads to a capacitive response from the material under test. This cap acitive response is at a maximum at some characteristic frequency o when the current response is exactly 90 out of phase with the applied electric field, or when = /2. Capacitance C, defined in terms of applied electric field and charge q, C = q(t) (5.8) E(t) can be used to evaluate the imaginary com ponent of the admittance, by substituting in for q(t) into the definition of current, I(t) = d q(t) = C d E(t) (5.9) dt dt Now, substituting in for E(t), using equation 5.1, gives I(t) = j CE(t) (5.10) From Ohms law, and the above result, the im aginary component of the admittance for = /2 is Y ( )= I(t) = 1 + j C = = j C (5.11) 2 E(t) With the real and imaginary components (at = 0, and /2 respectively) of the complex admittance the complete polar form can be written: Y = Y + jY = 1 + j C (5.12) R Similarly, with some rearranging the complex impedance can be written: Z = 1/R j c (5.13) (1/R) 2 +( C) 2 (1/R) 2 +( C) 2
It should be noted that a similar analysis could have been carried out using the impedance, rather than the admittance; however, the result is an expression in which the real component of the impedance increases with frequency, which is not logical. In Figure 5.2, the complex impedance as a function of frequency or Nyquist plot of the equation is presented. Here the apex is defined by a characteristic frequency o in terms of the resistive and capacitive response of the material under test, o = 1 (5.14) RC and the diameter of the semi-circle is given by the real resistance of the material R. These results lead to an equivalent RC circuit shown in Figure 5.3, which is commonly employed in the analysis of AC impedance results.[12,13] Fig 5.2 Nyquist Plot of the AC Impedance of a Material as a Function of Frequency Fig 5.3 Equivalent RC Circuit 36
5.2 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry is a technique used to study the thermal transitions and heat flow of a chemical compound. The compound weighs up to 15 mg and is heated in the apparatus as shown in figure 5.4 Fig 5.4 Differential Scanning Calorimetry Setup In this DSC design, two pans sit on a pair of identically positioned platforms connected to a furnace by a common heat flow path. In one pan the sample is placed. The other one is the reference pan which is left empty. The DSC is interfaced with the computer used to control the furnace. The two pans are heated at a specific rate usually 5/10C per minute. The software installed in the computer ensures consistent heating rate throughout the experiment and also that the two separate pans are heated at the same rate as each other. A graph the temperature and the difference in heat flow between the sample and reference are plotted on the x and y axis respectively. In a "heat flux" DSC, the sample material, encapsulated in a pan, and an empty reference pan sit on a thermoelectric disk surrounded by a furnace. As the temperature of the furnace is changed (usually by heating at a linear rate), heat is transferred to the 37
38 sample and reference through the thermoelectri c disk. The differentia l heat flow to the sample and reference is measured by area th ermocouples using the thermal equivalent of Ohm's Law. q = T R where q = sample heat flow T = temperature difference between sample and difference R = resistance of the thermoelectric disk 5.3 X-Ray diffraction X-Ray diffraction methods were exclusivel y used to identify the phases of the compounds. The diffraction measurements were performed on 0.5 diameter pellets at room temperature. X-Ray diffraction is a versatile, non-destructive analytical technique for identification and quantitative determination of the various crystalline forms known as phases of compounds present in powdered a nd solid samples. Identif ication is achieved by comparing the x-ray diffraction pattern or diffractogram obtained from an unknown sample with an internally recognized data base containing referenc e patterns for more than 70,000 phases. Modern computer contro lled diffractometer sy stems use automatic routines to measure, record and inte rpret the unique diffractograms produced by individual constituents in even highly comp lex mixtures. X-ray diffraction of a sample provides information about the nature of phase s present, the concentration levels of the phases present and the amorphous content of the sample.
39 5.4 Infrared Spectroscopy The vibrational spectrum of CsHSO 4 was measured in flowing nitrogen by infrared spectroscopy. When a beam of electromagnetic radiati on of intensity Io is passed through a substance, it can either be absorbed or transmitted, depending upon its frequency, and the structure of the molecule it encounters. Electromagnetic radiati on is energy and hence when a molecule absorbs radiation it gains energy as it undergoes a quantum transition from one energy state (E initial ) to another (E final ). The frequency of the absorbed radiation is related to the energy of the transition by Planck's law: E final E initial = E = hn = hc/l Thus, if an allowed infrared transition exis ts which is related to the frequency of the incident radiation by Planck's constant, then the radiation can be ab sorbed. Conversely, if the frequency does not satisfy the Planck expression, th en the radiation will be transmitted. There are in general several types of motion that a molecule can posses namely translational, rotational and vibrational mo tion. Each of the vibrational motions of a molecule occurs with a certain frequency, whic h is characteristic of the molecule and of the particular vibration. The energy involved in a particular vibration is characterized by the frequency of the vibration, so that the hi gher the vibrational energy, the larger the frequency of the motion. According to the re sults of quantum mech anics, only certain vibrational energies are allowed to the molecu le (the same may be said of rotational and translational energies), and thus only certa in amplitudes are allowed. Associated with each of the vibrational motions of the molecule, there is a series of energy levels (or vibrational energy states). The molecule may be made to go from one energy level to a
40 higher one by absorption of a quantum of electromagnetic radiation, such that E final -E initial = hn In undergoing such a transition, the mol ecule gains vibrationa l energy, and this is manifested in an increase in the amplit ude of the vibration. The frequency of light required to cause a transition for a particular vibration is equal to the frequency of that vibration, so that we may measure the vi brational frequencies by measuring the frequencies of light which are absorbed by the molecule. Since vibrational motions in molecules often occur at frequenc ies of the order of about 10 14 sec -1 then light of wavelength = c/l = 3 x 10 10 cm/sec/10 14 sec-1 = 3 x 10 -4 cm = 3 microns will be required to cause transitions. As it happens, light of this wavelength lies in the so-called infrared region of the spectrum. IR spectroscopy, then, deals with transitions between vibrational energy levels in molecules, a nd is therefore also called vibrational spectroscopy. An IR spectrum is generally displayed as a plot of the energy of the infrared radiation (expressed either in microns or wave numbers) versus the percent of light transmitted by the compound.
CHAPTER 6 EXPERIMENTAL RESULTS 6.1 Impedance Measurements Results The conductivity of the potential electrolytes was measured by a.c. impedance spectroscopy using a 4294A Precision Impedance Analyzer. Conductivity measurements were taken on 0.5 diameter pellets which were obtained by uni-axially pressing the sample. Silver paint served as the electrode material. Measurements were made at a frequency of 1 MHz with an applied voltage of 0.5 V under ambient atmosphere. -7-6-5-4-3-2-109095100105110115120125130135140145150155160165170175180185190195200205210Temperature (C)log[(-1cm-1)]144C -2.02 142C -5.03 0.5" dia pellet of CsHSO4 prepared from Cs2CO3-with Ag paste Fig 6.1 Log[conductivity] vs Temp for 0.5 Dia Pellet of CsHSO 4 from Cs 2 CO 3 with Ag Paste 41
Fig 6.2 Log[conductivity] vs Temp for 0.5 Dia Pellet of CsHSO 4 from Cs 2 CO 3 without Ag Paste Fig 6.3 Log[conductivity] vs Temp for 2.0 Dia Pellet of CsHSO 4 from Cs 2 CO 3 with Ag Paste 42
Fig 6.4 Log[conductivity] vs Temp for 2.0 Dia Pellet of CsHSO 4 from Cs 2 CO 3 without Ag Paste Fig 6.5 Log[conductivity] vs Temp for 0.5 Dia Pellet of CsHSO 4 from Cs 2 SO 4 with Ag Paste 43
Fig 6.6 Log[conductivity] vs Temp for 0.5 Dia Pellet of CsHSO 4 from Cs 2 CO 3 without Ag Paste Fig 6.7 Log[conductivity] vs Temp for 2.0 Dia Pellet of CsHSO 4 from Cs 2 SO 4 with Ag Paste 44
45 -9-8-7-6-5-4-3-2-100102030405060708090100110120130140150160170Temperature(oCelsius)log[(-1cm1 2.0" dia p ellet of CsHSO 4 p re p ared from 142C, -3140C, -6.36 Fig 6.8 Log[conductivity] vs Temp for 2.0 Dia Pellet of CsHSO 4 from Cs 2 CO 3 without Ag Paste Table 6.1 Impedance Measurements Results CsHSO 4 prepared from Phase Transition Temperature Log[( -1 cm -1 )] 0.5 diameter pellet With Ag paste Cs 2 CO 3 144C -2.02 Without Ag paste Cs 2 CO 3 144C -2.85 2.0 diameter pellet With Ag paste Cs 2 CO 3 142C -2.56 Without Ag paste Cs 2 CO 3 145C -2.95 0.5 diameter pellet With Ag paste Cs 2 SO 4 146C -2.9 Without Ag paste Cs 2 SO 4 148C -2.95 2.0 diameter pellet With Ag paste Cs 2 SO 4 142C -2.5 Without Ag paste Cs 2 SO 4 142C -3
From the table 6.1 it can be concluded that CsHSO 4 undergoes a superprotonic phase transition at a temperature of 140-150C which is in agreement with the technical paper . 6.2 Differential Scanning Calorimetry (DSC) Results The behavior of compounds with increasing temperature was investigated by differential scanning calorimetry. The presence and characterization of phase transitions above room temperature were accomplished by DSC measurements. The compounds response to heating and cooling was examined with a TA Instrument Q Series DSC in a flowing helium atmosphere. The heating and cooling rates were 10C per minute. Fig 6.9 DSC Plot of CsHSO 4 46
Fig 6.10 DSC Plot of Barium Hydrogen Phosphate Fig 6.11 DSC Plot of Barium Dihydrogen Phosphate 47
Fig 6.12 DSC Plot of Cesium Hydrogen Carbonate Fig 6.13 DSC Plot of Ammonium Iodide 48
49 Table 6.2 DSC Results Chemical Compound Phase Transition Temperature Cesium Hydrogen Sulfate 144.88C Barium Hydrogen Phosphate 421.19C Barium Dihydrogen Phosphate 268.34C Cesium Hydrogen Carbonate 215.12C Ammonium Iodide 35.32C, 113.12C, 175.52C From the DSC plot of Cesium Hydrogen Su lfate it can be inferred that the phase transition temperature for this solid acid elect rolyte is 144.88C which is close to what it is in the literature . 6.3 X-Ray Diffraction Results 6.3.1 Anchor Scan Parameters Comment z=7.650 mm Measurement Date / Time 10/26/2004 3:07:14 PM Operator Sesha Raw Data Origin XRD measurement (*.XRDML) Scan Axis Gonio Start Position [Th.] 5.0100 End Position [Th.] 74.9900 Step Size [Th.] 0.0200 Scan Step Time [s] 0.5000 Scan Type Continuous Offset [Th.] 0.0000 Divergence Slit Type Fixed Divergence Slit Size  0.4785 Specimen Length [mm] 10.00 Receiving Slit Size [mm] 0.2500 Measurement Temperature [C] 25.00 Anode Material Cu Generator Settings 45 kV, 40 mA Goniometer Radius [mm] 320.00 Dist. Focus-Diver g. Slit [mm] 91.00 Incident Beam Monochromator No Spinning No
6.3.2 X-Ray Diffraction Pattern of CsHSO 4 The x-ray diffraction pattern of CsHSO 4 is shown in figure 6.13. Position [Theta] 10 20 30 40 50 60 70 Counts 0 200 400 600 800 CsHSO4 pellet for defense.CAF Fig 6.14 X-Ray Diffraction Pattern of CsHSO 4 6.3.3 Peak List The peak list for the x-ray diffraction pattern of CsHSO 4 is shown in table 6.3. Table 6.3 Peak List for the X-Ray Diffraction Pattern of CsHSO 4 Pos. [Th.] Height [cts] FWHM [Th.] d-spacing  Rel. Int. [%] Tip width [Th.] Matched by 23.9849 12.06 0.1181 3.71030 1.72 0.1200 01-082-2214 24.7996 702.65 0.0984 3.59023 100.00 0.1000 01-082-2214 27.2923 563.52 0.0787 3.26772 80.20 0.0800 01-082-2214 29.2677 13.99 0.1181 3.05151 1.99 0.1200 01-082-2214 32.6416 45.82 0.1574 2.74340 6.52 0.1600 01-082-2214 33.2072 127.86 0.0787 2.69796 18.20 0.0800 01-082-2214 35.0055 140.11 0.0480 2.56125 19.94 0.0400 01-082-2214 36.6200 8.75 0.2362 2.45398 1.24 0.2400 01-082-2214 50
51 Table 6.3 Continued 37.5427 532.84 0.1574 2.39576 75.83 0.1600 01-082-2214 38.0610 19.56 0.1181 2.36432 2.78 0.1200 01-082-2214 40.6655 293.38 0.0720 2.21687 41.75 0.0600 01-082-2214 41.2906 76.36 0.0787 2.18655 10.87 0.0800 01-082-2214 43.7619 364.98 0.0720 2.06692 51.94 0.0600 01-082-2214 45.4000 16.16 0.1181 1.99773 2.30 0.1200 01-082-2214 45.8593 15.72 0.0960 1.97715 2.24 0.0800 01-082-2214 48.5805 12.59 0.1574 1.87412 1.79 0.1600 01-082-2214 49.3028 83.84 0.0720 1.84681 11.93 0.0600 01-082-2214 50.8687 73.75 0.0720 1.79358 10.50 0.0600 01-082-2214 54.6267 8.03 0.1920 1.67874 1.14 0.1600 01-082-2214 56.2833 12.54 0.0960 1.63319 1.78 0.0800 01-082-2214 58.5044 61.88 0.0720 1.57637 8.81 0.0600 01-082-2214 58.6616 46.42 0.0720 1.57252 6.61 0.0600 01-082-2214 60.6110 291.63 0.0720 1.52652 41.50 0.0600 01-082-2214 61.4415 21.98 0.0720 1.50786 3.13 0.0600 01-082-2214 61.6638 2.54 0.5760 1.50296 0.36 0.4800 01-082-2214 64.9275 88.28 0.0960 1.43507 12.56 0.0800 01-082-2214 66.3548 114.41 0.0960 1.40762 16.28 0.0800 01-082-2214 69.7184 2.52 0.2880 1.34771 0.36 0.2400 01-082-2214 70.6958 159.80 0.0720 1.33145 22.74 0.0600 01-082-2214 71.1465 6.11 0.0720 1.32412 0.87 0.0600 01-082-2214 73.8739 44.99 0.0960 1.28183 6.40 0.0800 01-082-2214 6.3.4 Identified Pattern List The identified pattern list for the x-ray diffraction pattern of CsHSO 4 is shown in table 6.4 Table 6.4 Identified Patterns List for the X-Ray Diffraction Pattern of CsHSO 4 Visible Ref. Code Score Compound Name Displaceme nt [Th.] Scale Factor Chemical Formula 01-082-214 35 Cesium Hydrogen Sulfate 0.000 0.190 Cs ( H S O4)
6.3.5 Plot of Identified Phases The plot of identified phases for the x-ray diffraction pattern of CsHSO 4 is shown in figure 6.14 Position [2Theta] 10 20 30 40 50 60 70 80 Peak List 01-082-2214 Fig 6.15 Plot of Identified Phases 6.4 Infrared Spectroscopy Results Fig 6.16 Infrared Spectroscopy Results of CsHSO 4 Prepared from Cesium Sulfate 52
Fig 6.17 Infrared Results of CsHSO 4 Prepared from Cesium Carbonate 53
54 CHAPTER 7 SUMMARY AND CONCLUSIONS The main objective of the thesis was to develop a solid electrolyte for producing hydrogen by electrolysis of hydrogen sulfide. Of these potential electrolytes the main focus was on Cesium Hydrogen Sulfate because of its superprotonic phase transition at 140-150C. CsHSO 4 was prepared from two chemical compounds namely cesium sulfate and cesium carbonate. Further a die, punch and a base were designed and fabricated in each of two sizes to prepare 0.5 and 2.0 diameter pellets to study the chemical and structural properties of CsHSO 4 Based on the experimental da ta one can arrive at the following conclusions: CsHSO 4 exhibited a superprotonic phase tran sition at 140-150C with a spectacular change in conductivity from 10 -6 to 10 -2 -1 cm -1 from impedance measurements. Differential Scanning Calorimetry results revealed that the phase transition temperature of CsHSO 4 was between 140C and 145C which is in agreement with the literature . The bond environment of the CsHSO 4 compound was investigated by infrared spectroscopy. The infrared spectrum obtained closely resembles to that in literature .
55 X-Ray diffraction methods were exclusivel y used to identify the phases of the CsHSO 4 compound and the peaks obtained were id entified with the reference peaks available in the library of the x-ray diffractometer. A successful electrolysis of steam at 150C was achieved with the CsHSO 4 electrolyte. Differential Scanning Calorimetry was pe rformed on Barium Hydrogen Phosphate, Barium Dihydrogen Phosphate, Cesium Hydrogen Carbonate, and Ammonium Iodide to identify the thermal transitions of these compounds. The observed properties of the prepared CsHSO 4 suggest a favorable prospect for its use in electrochemical dissociation of hydrogen sulfide.
56 CHAPTER 8 RECOMMENDATIONS FOR FUTURE WORK During the concluding stages of this thesis work there were numerous possibilities for improvement in the results which could not be pursued due to lack of time. There are minor discrepancies in the experimental da ta between the published values and those reported in this thesis work. The discrepancie s can be attributed to a variety of reasons. The highest conductivity va lue achieved in this thesis rese arch is lower than the highest value published. The decomposition/melting temperature obtained by DSC is higher than the one reported in technical papers. Inspit e of these minor discrepancies the results attained through this research are encouraging. The following tasks are recommended as future work for improvisation on the results obtained so far: The recipe adopted for preparing CsHSO 4 electrolyte needs certain modifications and concrete steps need to be taken to prev ent moisture absorption by the electrolyte. The thickness of the electrolytes currently us ed is approximately 1mm. This thickness needs to be decreased further to en sure less electrolyte resistance. The pelletization of the chemical compounds should be carried out in a controlled atmosphere and temperature. In order to ach ieve this vacuum dies may be considered. The x-ray diffraction of the CsHSO 4 was performed at room temperature. The same needs to be done at higher temperatures. More chemical compounds need to be pursued as potential electrolytes.
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