Cesium Dihydrogen Phosphate Research Articles

Metal Phosphide-Based Novel Anodes for Intermediate Temperature Fuel Cells

Metal phosphides are world-widely investigated as highly-active and selective catalysts for hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation. These reactions require catalytic ability for dissociation of H-H bond as well as C-S, C-N, or C-O bond. In addition to the catalytic activity, metal phosphides are known to be thermally and chemically stable and to possess electric conductivity comparable to metals and hardness similar to ceramics [1]. Because of these features, metal phosphides are expected to function as electrodes in electrochemical cells. Recently, metal phosphide based electrodes have been investigated as an electrode for hydrogen evolution reaction in H2O electrolysis. McEnaney et al. carried out cyclic voltammetry of phosphide anodes in sulfuric acid and obtained stable current-voltage curves [2]. This indicates that metal phosphides have potentials to be used as anodes of fuel cells employing acidic electrolytes. So far, application of metal phosphides to a fuel cell anode is quite limited. Chang et al. developed anodes of Pd supported on Ni2P/C for direct formic acid fuel cells [3].The Pd anode with nickel phosphide exhibited improved power generation characteristics when they were compared with the cell without the nickel phosphide. The role of the nickel phosphide seems, however, still unclear in the anode, since the power generation temperature was ca. 30ºC, which is too low for the phosphide-based catalysts to exert catalytic activity.In this study, metal phosphides such as Ni2P, CoP, FeP, WP, and MoP have been investigated as anode catalysts for intermediate temperature fuel cells. Composites composed of CsH2PO4 and SiP2O7, the metal phosphates, and a commercial Pt/C electrode were used as the electrolyte, anode, and cathode, respectively, and power generation characteristics were evaluated at 220ºC as H2-O2 fuel cells. Cesium dihydrogen phosphate, CsH2PO4, was prepared by dissolving stoichiometric quantities of Cs2CO3 and H3PO4 in distilled water and drying overnight at 120 °C. SiP2O7 was prepared from mesoporous SiO2 and H3PO4 as reported previously [4]. As a cathode catalyst Pt/C-loaded carbon paper (Pt loading 1 mg cm-2, ElectroChem Inc.) was used. As an anode catalyst, the phosphide-loaded carbon paper was prepared by filtration of the phosphide dispersed in an ethanol solution. The membrane electrode assembly (MEA) was prepared by uniaxial pressing at 250 MPa for 10 min to form the MEA. In power generation experiments, humidified H2 and O2 were supplied to anode and cathode, respectively, at 50 cm3 min-1. A water vapor concentration of 30 vol% was obtained by bubbling the gas flow through water at 70 °C. Current-voltage characteristics were measured at 220 °C using a potentiostat (Solartron 1287), and AC impedance measurements were carried out at open circuit condition with a frequency-response analyzer (Solartron 1260).Figure 1 summarizes current-voltage characteristics of H2-O2 fuel cells with phosphide anodes at 220°C. The phosphide electrocatalysts were found to function as anodes of intermediate temperature fuel cells. The power generation characteristics are in the following order: MoP > WP > FeP > CoP > Ni2P. Impedance analysis showed that ohmic resistance of the cells with the phosphides except FeP was small and comparable to that of the Pt/C anode. This result indicates that these phosphide anodes possess good conductivity applicable to fuel cell anodes. On the other hand, the non-ohmic resistance of FeP, CoP, and Ni2P was considerably large, which corresponds to the order of the power generation performance. [1] S.T. Oyama, T. Gott, H. Zhao, Y.-K. Lee, Catal. Today 143, 94-107 (2009).[2] J.M. McEnaney, J.C. Crompton, J.F. Callejas, E.J. Popczun, A.J. Biacchi, N.S. Lewis, R.E. Schaak, Chem. Mater. 26, 4826-4831(2014).[3] J. Chang, L. Feng, C.P. Liu, W. Xing, X. Huet, Angew. Chem. Int. Ed. 53, 122-126 (2014).[4] T. Matsui, T. Kukino, R. Kikuchi, K. Eguchi, Electrochem. Solid-State Lett. 8, A256-A258 (2005). Figure 1

Production of Bio-based 1,3-Butadiene by Highly Selective Dehydration of 2,3-Butanediol over SiO2-supported Cesium Dihydrogen Phosphate Catalyst

We investigated the catalyst that promotes the selective dehydration of 2,3-butanediol to 1,3-butadiene. We found that the SiO2-supported CsH2PO4 catalyst showed the highest 1,3-butadiene selectivity (>90%) with the single-bed catalyst system. The present process was successfully applied to bio-based 1,3-butadiene production from 2,3-butanediol produced by the microbial fermentation of biomass resources.

Effect of telluric acid addition on thermal behavior of cesium dihydrogen phosphate

xCsH2PO4·(1-x)Te(OH)6 composites have been synthesized for the different compositions, x = 0.02, 0.04, 0.05, 0.06, 0.08 (labeled composites A-E). The composites were characterized by Scanning Electron Microscopy (SEM), XRay Diffraction (XRD), Thermogravimetry (TG) and Differential Thermal Analysis (DTA). Telluric acid, Te(OH)6, added to cesium dihydrogen phosphate (CsH2PO4) significantly effects the structure, particle size and thermal stability of the composite material. Enhanced thermal stability could lead to use of CsH2PO4 as an electrolyte in low temperature fuel cells.

Systematic Investigation of Carbon Supported Ru Based Chalcogenides As ORR Catalysts and Their Application in Safc

The primary consideration in enabling solid acid fuel cells operating at 250oC such as those using CsH2PO4 (CDP) from reaching commercial reality is based on the cost of the MEA, where noble metal loading is a critical factor. In addition there is the perennial issue of carbon corrosion. The former has been extensively considered in the seminal article by Gasteiger et. al.1 Typical operating conditions for CDP, 250oC, Reformate/Air are 200 mA/cm2, 0.7 V vs RHE. This translates to 120-140 mW/cm2. Considering the typical Pt loading of >3 mg/cm2 for CDP we have a requirement of 11-25 g(Pt)/KW. Contrast this with the current loading of 0.5-1.1 g(Pt)/KW at ≥ 0.65 V vs. RHE for a lower temperature (typically 800C) polymer electrolyte fuel cell, which represents a ten to twenty fold loading difference between the two types of fuel cell technologies. The goal therefore is to significantly lower Pt loading. This presentation critically examines the potential use of chalcogenides as oxygen reduction reaction (ORR) catalyst to either replace or embellish noble metal catalysts. Hence, the overall objectives of this effort and the associated portfolio of catalysts are specifically aimed at lowering the Pt loading requirement per KW as well as ensure durable performance over the lifetime of the fuel cell. While Pt based system is the focal point of ORR catalysis in acidic conditions, a wide range of non-noble group metals (non PGM) are viable options in alkaline condition. This disparity arises due to dissimilar double layer structures in acid and alkaline conditions, which provides a kinetic facility to the underlying electrode surface in alkaline media2. This presentation will demonstrate a detailed investigation of ruthenium based chalcogenide catalyst, which is a promising non PGM catalyst alternative. Mechanistic details showcasing ORR pathways in acid and alkaline electrolyte will be discussed. In addition to electrochemical data, in situ X- ray absorption spectroscopy (XAS) results will be discussed. Taking advantage of the XAS analysis, correlation with the electrochemical data will be drawn. The effect of chalcogen modification on Ru surface and a detailed description of the active site will be provided. Advantages such as controlled carbon content, limited carbon corrosion and resistance to anion poisoning suggest Ru based chalcogenide catalysts as potential cathode catalyst in fuel cells, such as solid acid fuel cell (SAFC). SAFC such as that using cesium dihydrogen phosphate as electrolyte exhibit anhydrous proton transfer (2) thereby enabling simpler humidification and balance of plant required for reformation of most fuels. Actual fuel cell polarization curves will be used to compare with comparable Pt/C cathodes. In conclusion, a comprehensive discussion on ruthenium based chalcogenide catalyst will be made, emphasizing on structural details, ORR mechanistic pathways and SAFC performance. Acknowledgement: The authors gratefully acknowledge the financial support from Arpa- E, US DOE under a grant led by SAF Cell Inc., Pasadena, CA. Use of Stanford Synchrotron Radiation Light Source (SSRL), Stanford Linear Accelerator Center is supported by U.S Dept. of Energy under contract number DE- AC02-76SF00515. Use of beamline 2-2 at SSRL was partially supported by National Synchrotron Light Source II, Brookhaven National Laboratory, under U.S Dept. of Energy under contract number DE-SC0012704.

Solid Acid Reforming Methanol Fuel Cells and Hydrogen Generators

Background: Solid Acid Electrolytes and Solid Acid Fuel Cells Solid acid fuel cells (SAFCs) operate at intermediate temperatures (~250°C), are inherently impermeable to gases, and transport "bare" protons through a solid electrolyte. Solid acids are compounds whose chemistry and properties are intermediate between those of a normal acid, such as H2SO4 or H3PO4, and a normal salt, such as Cs2SO4. The proton conductivity of these materials can reach values higher than 10-2 Ω-1cm-1 when heated to moderately elevated temperatures (150-250°C). In the case of the standard SAFC electrolyte, cesium dihydrogen phosphate (CsH2PO4), the conductivity is 2.5 x10-2 Ω-1cm-1 at 250 °C. To date, solid acid fuel cells (SAFCs) utilizing this electrolyte as thin (10-25 μm) gas tight electrolyte layers have demonstrated peak power densities of over 330 mW/cm2on hydrogen/air with lifetimes of thousands of hours. Recent development efforts at SAFCell have demonstrated SAFC stacks with robustness to thermal cycling, power outputs of over 1 kW, and lifetimes in the thousands of hours. SAFC stacks have also demonstrated very high tolerances to typical anode catalyst poisons such as carbon monoxide (CO), ammonia (NH3), and hydrogen sulfide (H2S): measured tolerances are 20%, 100 ppm, and 200 ppm, respectively, without significant performance decreases. These high impurity tolerances and the intermediate operating temperatures allow SAFC power systems to operate on reformed fuels with simplified, and therefore, less costly reforming and gas clean-up sub-systems. Internally Reforming Methanol Solid Acid Fuel Cells: The application of SAFCs as reforming methanol fuel cells (RMFCs) is particularly compelling due to the nearly ideal operational temperature match between the fuel cell stack (235-275ºC) and standard methanol steam reforming catalysts (250-300ºC). The operational temperature match allows for methanol steam reforming inside the stack to create a solid acid-based RMFC with higher performance and fuel efficiencies than current polymer-based RMFCs and direct methanol fuel cells (DMFCs). In fact, SAFCell has demonstrated single cell performance similar to diluted hydrogen on vaporized methanol/water fuel streams using a solid acid RMFC (255 mW/cm2 versus 237 mW/cm2, respectively). Further results at the stack level show the effectiveness of the solid acid RMFC configuration to run directly on methanol for at least thousands of hours. This system design is so simple that a solid acid RMFC unit is projected to cost nearly half of an equivalent DMFC system, exceeding most of the DOE’s 2015 target values for such a system. Talk will describe the development of a solid acid RMFC stacks and systems for O&NG remote power applications. Stack and system is designed for extended use without maintenance and under harsh environmental conditions expected for remote power applications (e.g, temperatures from -40°C to 45°C), and to run on “field methanol” which can be easily delivered to remote locations in large quantities. Internally Reforming Methanol Solid Acid Electrochemical Hydrogen Generators: The high impurity tolerances of SAFC stacks also enable them be used as electrochemical hydrogen separation (EHS) systems with superior properties to those currently in existence. Preliminary testing has been done in EHS mode to show the tolerance of SAFC stacks to carbon monoxide (CO) in EHS mode, and cyclic voltammetry experiments show that CO can be directly oxidized on SAFC anodes. Moreover, EHS testing on synthetic reformate with a CO water gas shift catalyst incorporated into the anode suggests significant chemical conversion of CO to H2 can be achieved. As fuel cell results described above show little affect on performance of H2S, NH3, and CH4, SAFC stacks should be equally tolerant to these impurities in EHS mode. As such, a solid acid EHS unit should separate almost all hydrogen from “dirty” hydrogen containing gas streams without severe performance penalties. The key difference of a solid acid EHS system is the higher operating temperature (~250°C / 482°F) versus the 90°C to 160°C of alternate EHS technologies. As described above, this higher temperature makes the system highly tolerant impurities and contaminants, and provides better integration with cooling and shifting of the supply gas, increasing hydrogen production. It also provides waste heat of a higher value than lower temperature technologies. Talk will describe the development of solid acid EHS stacks using internally reformed methanol, and other hydrocarbon reformates (e.g., natural gas and propane).

Modifying Electrode Architectures for Solid Acid Electrochemical Hydrogen Separation Devices

Abundant, inexpensive, high purity molecular hydrogen as a medium for energy distribution is potentially enabling for adoption of alternative electricity generation schemes. Steam reforming of natural gas remains the dominant method of producing large amounts of hydrogen. However, this process also creates by-products, most notably, CO and CO2. Separation to ultra-high purity hydrogen from these syngas reformate streams by traditional methods, such as pressure swing absorption, has its disadvantages including long cycle times, contamination and a large equipment footprint. Alternative methods of hydrogen separation, such as electrochemical pumping, are a viable alternative to this separation dilemma due to their relative simplicity and potential efficiency. The solid-state proton conductor cesium dihydrogen phosphate (CsH2PO4 or CDP) has shown potential in electrochemical devices operating on reformed hydrocarbons. Solid acid fuel cells operated using methanol and an integrated steam reformer have shown promising results1. We have also shown that solid acid electrochemical devices are effective for hydrogen separation from reformate streams containing CO. Published work has demonstrated devices based on CDP operating in fuel streams containing up to 20% CO2. Syngas can contain 30-60% carbon monoxide (CO), 25-30% hydrogen (H2), 0-5% methane (CH4), 5-15% carbon dioxide (CO2), plus a lesser or greater amount of water vapor, smaller amounts of the sulfur compounds hydrogen sulfide (H2S), carbonyl sulfide (COS), and finally some ammonia and other trace contaminants3. At these CO concentrations, further oxidative conversion of CO is desirable to boost hydrogen yield. Recently, we demonstrated that Ru4 and Ni5 are viable alternatives to platinum electrocatalysts for hydrogen oxidation and evolution, respectively. In particular, Ru is capable of enhanced hydrogen production in CO-rich input streams. We attributed this result to a synergistic interaction of the water-gas shift (WGS) reaction and CO electrooxidation. In this work, we consider separately the heterogeneous WGS and CO electrooxidation properties of Ru and other supported catalysts for optimizing electrode architectures for hydrogen separation from highly CO-enriched simulated syngas streams. Functionally graded anodes are fabricated to balance CO conversion activity with hydrogen oxidation. These re-engineered anodes are implemented in conjunction with Ni-based cathodes to demonstrate efficient Pt-free hydrogen separation from syngas-like inputs. This work is supported by the National Science Foundation through TN-SCORE (EPS-1004083). References 1 T. Uda, D. A. Boysen, C. R. I. Chisholm and S. M. Haile, Electrochem. Solid-State Lett., 2006, 9(6), A261–A264. 2C. R. I. Chisholm et al., Electrochem. Soc. Interface, 18, 53–59 (2009). 3National Energy Technology Laboratory. Wabash River Coal Gasification Repowering Project: A DOE Assessment. United States: N. p., 2002. Web. doi:10.2172/790376. 4A. B. Papandrew, R. W. Atkinson III, R. R. Unocic, and T. A. Zawodzinski, Jr., J. Mater. Chem. A, 3, 3984 (2015). 5 A. B. Papandrew, T.A. Zawodzinski Jr., J. Power Sources 245 (2014) 171. Figure 1

Engineering Carbon Nanotubes As Oxygen Reduction Catalysts for Solid Acid Fuel Cells

We report on the effect of the chemistry of multi-walled carbon nanotubes (CNTs) on the oxygen reduction reaction (ORR) in solid acid fuel cells (SAFCs) based on the proton conducting electrolyte cesium di-hydrogen phosphate (CDP). CDP undergoes a super-protonic phase transition at ~2300C (10-6 S/cm2-10-2 S/cm2), under humidified gas atmosphere, making it a suitable fuel cell electrolyte. Due to the high cost of using platinum as the state of the art oxygen reduction reaction (ORR) catalyst, significant effort has been devoted to developing non-precious catalysts. In this work we explore the suitability of functionalized carbon nanotubes as ORR catalysts. Composite electrodes of fine micron-sized CDP particles and CNTs were prepared by mechanical milling and applied to either side of a dense membrane of CDP, in turn, placed between carbon paper current collectors. The CNTs were acquiring commercially and included both hollow (standard) and bamboo CNTs. In both cases, in addition to non-functionalized forms of the CNTs, both amine-functionalized and acid-functionalized analogs were evaluated. Electrochemical activity was measured by A.C. impedance spectroscopy under humidified oxygen (pH20 of 0.4 atm) at 2400C. Of the non-functionalized CNTs, the bamboo structure was found to have a higher activity than the hollow structure, with an area specific electrode resistance of 1700 ohm cm2 vs 3500 ohms cm2. Functionalization of the CNTs led to orders of magnitude increase in the ORR activity. In particular, the electrode resistance values dropped to ~350 ohms cm2 for the amine functionalized CNTs and ~40 ohms cm2 for the carboxylic acid functionalized hollow CNTs. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) of the electrodes showed a relationship between the chemistry of the CNTs and the degree of mixing in the CNT/CDP composite, which may contribute to the differences in electrochemical behavior.

Electrochemical Devices Based on Solid Acid Electrolytes for Conversion of Methane Derivatives

A boom in natural gas production in the United States presents opportunities for economically effective use of methane-derived molecules. In contrast with methane gas, derivatives may exhibit advantages in volumetric energy density, ease of transport and versatility. Electrochemical conversion of these species to electricity and other fuels is desirable, but depending on the system in question, catalyst stability, activity, and impurity intolerance, or alternately, the cost and durability of the electrolyte may be impediments to adoption The solid-state proton conductor cesium dihydrogen phosphate (CsH2PO4 or CDP) has shown potential in electrochemical devices operating on fuels such as reformed NG or methanol. CDP undergoes a solid-solid phase transition at 228 °C and an associated increase in proton conductivity of more than three orders of magnitude (8.5 x 10-5 S cm-1 at 223 °C to 2.5 x 10-2 S cm-1 at 250 °C). These systems have been shown to tolerate reformate streams in H2-air cells containing CO, H2S, NH3, CH3OH, C3H8 and CH4 of 20%, 100ppm, 100 ppm, 5%, 3% and 5% respectively, retaining 90-95% of pure H2 performance1. Solid acid fuel cells operated using methanol and an integrated steam reformer have also shown similar results2. Recently, we showed that this material can also be used in an electrochemical hydrogen separation system using similar reformate streams3. In the majority of these earlier studies, platinum was used as the catalyst for both the hydrogen reduction and oxidation reactions. We have also demonstrated that unsupported nickel is an effective hydrogen evolution catalyst in solid acid hydrogen pump devices4. Here, we have synthesized a suite of carbon-supported non-platinum catalysts (Pt, Pd, Ru, Ni and Cu) for implementation in electrodes. We evaluate these materials as hydrogen oxidation and evolution catalysts in CDP-based devices for electricity and fuel production from the methane derivatives methanol and dimethyl ether. This work is supported by the National Science Foundation through TN-SCORE (EPS-1004083). References 1 C. R. I. Chisholm et al., Electrochem. Soc. Interface, 18, 53–59 (2009). 2 T. Uda, D. a. Boysen, C. R. I. Chisholm, and S. M. Haile, Electrochem. Solid-State Lett., 9, A261 (2006). 3 A. B. Papandrew et al., J. Electrochem. Soc., 161, F679–F685 (2014). 4 A. B. Papandrew, T.A. Zawodzinski Jr., J. Power Sources 245 (2014) 171. Figure 1

CsH2PO4/NdPO4 Composites as Proton Conducting Electrolytes for Intermediate Temperature Fuel Cells

Composite proton conducting materials based on cesium dihydrogen phosphate and neodymium phosphate hydrate were prepared and investigated in terms of X-ray diffraction, thermogravimetry, conductivity, stability and fuel cell performance. At 150°C the conductivity was 1.8 × 10−6 S cm−1 for the pristine cesium dihydrogen phosphate and 0.8 × 10−4 S cm−1 for neodymium phosphate hydrate, while that of the composite containing 29 mol% neodymium phosphate and 71 mol% cesium dihydrogen phosphate was 0.4 × 10−2 S cm−1. It was proposed that the interaction between the two components establishes a dynamic hydrogen bonding network enabling efficient proton conduction long before the development of the extensive phase disordering of the superprotonic transition. The presence of thermally stable hydrate water present in neodymium phosphate may also play a role in improving both conductivity and stability of the solid acid. The electromotive force, open circuit voltage and fuel cell performance were measured as demonstration of the material application.

Characterization of Solid Acid Electrolyte CsH5(PO4)2by NMR Spectroscopy

Department of Chemistry, Kyungpook National University, Daegu 702-701, KoreaReceived September 6, 2013, Accepted September 15, 2013Key Words : Solid acid electrolyte, Cesium pentahydrogen diphosphate, Proton dynamics, Cesium dihydro-gen phosphate, Solid-state NMR spectroscopyThe electrolyte is one of the most important constituentsof fuel cells (FCs), and determines the application areas, theperformance, and the operation temperature range. The highproton conductivities of solid acids in specific temperatureranges have made them as excellent candidates for FCelectrolytes.

A new fabrication method of an intermediate temperature proton exchange membrane by the electrospinning of CsH2PO4

A novel method for the fabrication of an intermediate temperature proton conducting composite membrane was developed. Cesium dihydrogen phosphate, CsH2PO4 (CDP), was electrospun to obtain a highly interconnected proton conducting fibre mat. CsH2PO4 was heat treated above its dehydration temperature (Tdehy ∼ 230 °C) in order to induce a partial polymerisation. The partially polymerised material produced a viscous aqueous solution which could be electrospun in the absence of a carrier polymer, thus leading to pure inorganic fibres. The electrospun fibre mats were characterised in terms of composition, structure and morphology by XRD, MAS NMR and SEM and their proton conductivity determined by electrochemical impedance spectroscopy. The electrospun fibres (CDPf) showed a maximum proton conductivity of 8 × 10−3 S cm−1 at 250 °C.

Mechanochemical synthesis of proton conductive composites derived from cesium dihydrogen phosphate and guanine

Solid inorganic–organic composites were synthesized from cesium dihydrogen phosphate (CsH2PO4, CDP) and guanine (C5H5N5O, Gn) to use the solid proton conductive electrolytes. CDP and Gn were mechanochemically milled in dry nitrogen atmosphere to obtain composites with xCDP·(100−x)Gn (x=90–50mol%). The changes in chemical structure, such as tautomerism of Gn and formation of hydrogen bond between CDP and Gn, after solid-state mechanochemical treatment, were confirmed from FT-IR study. The CDP-Gn composite electrolytes showed significantly higher proton conductivity than those of raw substances, particularly in the lower temperature region than the superprotic phase-transition temperature of CDP. The 80CDP·20Gn composite, for example, showed in anhydrous and hydrous proton conductivities from 2.4×10−5 to 8.1×10−4S cm−1, in dry nitrogen atmosphere, and from 1.3×10−4 to 1.3×10−3S cm−1, in humidified nitrogen atmosphere (pH2O=0.6atm) on heating in the temperature range between 80 and 180°C. From the FT-IR and impedance studies, tautomerism of Gn and newly developed hydrogen bonds between CDP and Gn are highly expected in the composites. These structural changes imply that a development of new chemical interactions to form the acid–base composite cluster is essential to improve the proton conductivity.

Effect of surfactants in synthesis of CsH2PO4 as protonic conductive membrane

Cesium dihydrogen phosphate (CDP) powders were synthesized by cetyltrimethylammoniumbromide (CTAB), polyoxyethylene-polyoxypropylene (F-68) and mixture of both surfactants F-68: CTAB with two molar ratios 0.06 and 0.12 as surfactant solutions at room temperature. The synthesized CsH2PO4 is characterized by ICP, XRD, TEM, SEM, FT-IR, BET and IS techniques. Based on the width of the (011) XRD diffraction peak and BET measurement, the average size of nanoparticles was ∼ 10 nm in diameter, while the TEM images indicate smaller size than both techniques. The analysis reveals existence of P and Cs with mole ratio 1.02 ± 0.03 which is compatible to molar ratio CsH2PO4 formula. The experimental results show that the conductivities increase in the order of CDPCTAB > CDP(F-68: CTAB)0.12 > CDP(F-68:CTAB)0.06 > CDPF-68. The sequence of increasing conductivity is in accordance with the ion exchange capacities of the samples that has direct proportional effect on the proton mobility of samples. Indeed CTAB as cationic surfactant shows the highest proton mobility in the as-obtained samples.

Mechanochemically synthesized CsH2PO4–H3PW12O40 composites as proton-conducting electrolytes for fuel cell systems in a dry atmosphere

Cesium dihydrogen phosphate (CsH2PO4, CDP) and dodecaphosphotungstic acid (H3PW12O40·nH2O, WPA·nH2O) were mechanochemically milled to synthesize CDP–WPA composites. The ionic conductivities of these composites were measured by an ac impedance method under anhydrous conditions. Despite the synthesis temperatures being much lower than the dehydration and phase-transition temperatures of CDP under anhydrous conditions, the ionic conductivities of the studied composites increased significantly. The highest ionic conductivity of 6.58×10−4 Scm−1 was achieved for the 95CDP·5WPA composite electrolyte at 170 °C under anhydrous conditions. The ionic conduction was probably induced in the percolated interfacial phase between CDP and WPA. The phenomenon of high ionic conduction differs for the CDP–WPA composite and pure CDP or pure WPA under anhydrous conditions. The newly developed hydrogen interaction between CDP and WPA supports anhydrous proton conduction in the composites.

CsH<sub>2</sub>PO<sub>4</sub>: Electrolyte for Intermediate Temperature Fuel Cells

This review paper discusses the temperature behavior and thermal event of cesium dihydrogen phosphate (CsH2PO4) in both ambient and high pressure atmosphere. A complete transition from the room-temperature to a high-temperature of CsH2PO4 (monoclinic to cubic phase) occurs between 230 to 240 °C, even in the absence of humid conditions and the superprotonic transition precedes the onset of the dehydration/chemical decomposition of the title compound. Decomposition or dehydration can be avoided by either keeping the sample under a H2O-saturated atmosphere, or subjecting the sample to a pressure of 1.0 ± 0.2 GPa.

Elucidating the Time Scale and Geometry of Phosphate and Phosphonate Rotation in Solid Acid Electrolytes Using Multinuclear NMR

The dynamics of the anions in a series of solid acid electrolyte materials have been determined using solid-state 31P CODEX NMR. The time scale and activation energy of the phosphate reorientation were quantified; however, the characterization proved to be nontrivial due to interactions with the quadrupolar 133Cs nuclei. The series of compounds studied included other known proton conductors, where the alkali cation was replaced systematically with Rb+, Tl+, or K+. The nuclear properties of the cations, together with the relative proton conductivities of these electrolyte analogues, allowed for a complete analysis of the interactions (both structure reorientations and spin dynamics) giving rise to the observed CODEX buildup curves. The high-temperature super-protonic transition, and the narrow range of thermal stability, limits the usefulness of cesium dihydrogen phosphate (CDP) in fuel cell applications. With the goal of achieving a broader range of thermal stability and enhanced proton dynamics, the phos...

Surfactant effect on the conductivity behavior of CsH2PO4: Characterization by electrochemical impedance spectroscopy

Cesium dihydrogen phosphate (CDP) nanoparticles were synthesized using the surfactants cetyltrimethyl ammonium bromide (CTAB), polyoxyethylene–polyoxypropylene (F-68) and (F-68:CTAB) with molar ratio 0.06. The samples conductivity such as CDPCTAB, CDPF-68 and CDP(F-68:CTAB)0.06 was studied by impedance spectroscopy in the frequency range 0.01Hz to 1MHz. The Nyquist plots were drawn at different temperatures of 210, 230 and 260°C, which are defined below transition, phase transition and above transition, respectively. The measured conductivities obey the Arrhenius relation. The influence of surfactants on conductivity are more significant at higher temperature due to grain boundary. The conductivity of CDPCTAB increased slightly with increasing temperature to 260°C, whereas the conductivity of other samples decreased with increasing temperature over 230°C. The results indicated that the conductivities increase in the order of CDPCTAB>CDP(F-68:CTAB)0.06>CDPF-68. These are in accordance to the ion exchange capacities of the samples that the surfactant shows a direct influence on the samples proton mobility. It is found that the conductivity of CsH2PO4 is influenced by surfactant type.

メカニカルミリングによるプロトン伝導性固体酸複合体の作製

New proton conductive solid acid composites were prepared by mechanical milling. First, the formation of highly proton conductive cesium-containing sulfate-phosphate double salt and their phase transition are described. Second, a mechanochemical synthesis of ortho-oxosalt-heteropoly acid composites, and the relationship between proton conductivity and hydrogen bonding network in the composites are discussed. Mechanochemical treatment of mixtures of cesium hydrogen sulfate (CsHSO4) and cesium dihydrogen phosphate (CsH2PO4) using a planetary type of ball mill formed Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2. Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2 were transformed to the higher temperature phase of Cs2(HSO4)(H2PO4) by heating at around 100°C. Proton conductivity of the treated compound remarkably increased on heating to 2×10−3 S·cm−1 at around 180°C, whereas no steep decrease was observed on cooling. The high proton conductivity was ascribed to the presence of the high temperature phase of Cs2(HSO4)(H2PO4). Milling of Cs-containing ortho-oxosalts (Cs2SO4, Cs2CO3 or CsHSO4) and phosphotungstic acid (H3PW12O40·6H2O: WPA-6) mixtures obtained partially substituted CsxH3−xPW12O40 composites. Chemical durability and proton conductivity of the resultant composites markedly improved under both humidified and dried conditions. In addition, mechanochemically prepared 90CsHSO4·10WPA-6 (mol%) composite maintained high proton conductivity from room temperature to 180°C. Conductivity of the composite was 3.3×10−3 S·cm−1 at 100°C under dry atmosphere, much higher than those of pure CsHSO4 (4×10−7 S·cm−1) and WPA-6 (1×10−7 S·cm−1). The –O(H)···O hydrogen bonding distance in the CsHSO4–WPA-6 composites was estimated from the 1H-isotropic chemical shift of 1H MAS NMR spectra. Proton conductivities of CsHSO4–WPA-6 composites under dry atmosphere were strongly related to the bond distance.

Electrical and thermodynamic properties of Cs0.97 Rb0.03H2PO4

The transport properties of Cs0.97Rb0.03H2PO4 have been studied using polycrystalline samples and single crystals. The mixed salt is isostructural with cesium dihydrogen phosphate and has slightly smaller unitcell parameters. The cation substitution increases the low-temperature ionic conductivity of the material by about two orders of magnitude but has an insignificant effect on the conductivity of the high-temperature phase. The low-temperature conductivity of single-crystal samples exhibits significant anisotropy, with σa < σb±c. The conductivity of the polycrystalline material is close to σb±c. The substitution reduces the temperature of the superionic phase transition by 20°C and enhances the thermal stability of the high-temperature phase at low humidity (1 mol % H2O).

Thermal analysis of CsH2PO4 nanoparticles using surfactants CTAB and F-68

A study concerned to thermogravimetric analy- sis is performed in cesium dihydrogen phosphate (CsH2PO4) that was synthesized, using cetyltrimethylammonium-bro- mide (CTAB), polyoxyethylene-polyoxypropylene (F-68) and mixture of (F-68:CTAB) with two mole ratio 0.06 and 0.12 as surfactant. The dehydration behavior of particles was studied using thermal gravimetric analysis and differential scanning calorimetric. Subsequently, the experimental results indicated that the first dehydration temperature in the range of 237-239 C upon heating, the second peaks occur at temperature range 290-295 C and overlapping in the ther- mogravimetric events is observed. The mass loss values are obtained in the range of 6.62-6.97 wt% that is less than reported theoretical value 7.8 wt%. These values show well compatibility of reaction CsH2PO4 to Cs2H2P2O7 with 3.92 wt% whereas mass loss value of CsH2PO4 to CsPO3 is less than theoretical value 7.8 wt%. The activation energy of two steps dehydration are calculated using Kissinger equa- tion for the samples synthesized via CTAB and (F-68) with minimum value mass loss 6.62% and maximum value mass loss 6.97%, respectively. The calculation results reveal that the reaction rate in the first step (CsH2PO4 ? Cs2H2P2O7 )i s faster than the second step (CsH2PO4 ? CsPO3). The weight loss values of the samples demonstrate that existence of CTAB can be considered as effective factor which pre- vents more weight loss during the dehydration process.