Cesium Dihydrogen Phosphate Research Articles

A Comparative Effect of CsH2PO4/ZrP2O7 Composite Electrolytes for Intermediate Temperature Fuel Cell

Solid acid composite CsH2PO4 (CDP)/ZrP2O7 were synthesized and characterised with different weight ratio of CDP, SDP and ZrO2. The characterisations of the Solid acid composite were observed the structural, thermal stability and conductivity in terms of X-ray diffraction (XRD) analysis, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric analysis, Differential Thermal Analysis, and conductivity measurements. The amount of CDP and ZrP2O7 powder were used for pallet production. The composites powders were to form of pellets of 1mm thickness for conductivity measurement. ZrP2O7 in the composites has a large effect on the conductivity measurement and this material increased the temperature and time dependent conductivity of CDP by up to 4 orders of magnitude. The transition was identified in CDP/ZrP2O7 composites under atmospheric pressure. The thermal stability of the solid acid composites for dehydration, melting, and decomposition was investigated in the temperature range of 50 to 300 °C under environments with atmospheric humidification. Keywords: Composite Electrolytes, Conductivity, Thermal Analysis, Caesium Dihydrogen Phosphate, Pyrophosphates.

Alternative processing routes on CsH2PO4 proton conductors: Cold sintering and ball-milling routes

Cesium dihydrogen phosphate (CsH2PO4) holds great potential as electrolyte for intermediate-temperature fuel cells and electrochemical devices due to its high proton conductivity (≥10−2 S/cm). This study reports on the variation of the microstructure of CsH2PO4 and its effect on its electrical and thermal properties, with special attention to the low-temperature conductivity behavior. To manipulate morphology, the particle size was decreased by wet ball-milling, while variations in grain growth were achieved by cold sintering. Above the superprotonic phase transition, the electrical conductivity was effectively independent of the grain size. Nonetheless, at lower temperatures, a brick layer analysis on the impedance data revealed that the conductivity of the monoclinic phase is governed by the conduction pathways along the parallel grain boundaries due to the humidified atmosphere. Overall, we propose two conduction mechanisms that could explain the grain boundary conductivity of these samples, revealing critical links between sample morphology and low-temperature electrical behavior.

Novel nanocomposite systems based on cesium dihydrogen phosphate: Electrotransport structural, morphological and mechanical characteristics

CsH2PO4 is a proton conductor belonging to the family of acidic salts and exhibits a monoclinic to cubic phase transition at 230 ± 2 °C with the conductivity jump of four orders of magnitude. To change the electrotransport and mechanical characteristics the non-oxide additives, such as detonation nanodiamonds, were firstly used as a dopant for proton nanocomposites based on cesium dihydrogen phosphate. The use of carbon containing materials has made it possible to create a new type of medium-temperature, highly conductive proton electrolytes. Influence of nanodiamonds (NDs) on the electrotransport, structural and thermodynamic properties of (1-x)CsH2PO4-xNDs (x-mole fraction, x = 0–0.99) electrolytes have been firstly investigated in detail by X-ray diffraction method, electrochemical impedance, FTIR-spectroscopy and differential scanning calorimetry. The CsH2PO4 (P21/m) exists in nanocomposites in the dispersed (up to 300 nm) and partially amorphous state resulting in the strong changes of the salt thermodynamic properties with increase of x mole fraction. The heterogeneous doping by non-oxide dopant resulted in the significant increase of the low temperature conductivity of 3.5 orders of magnitude due to interface interaction, while the superionic conductivity of nanocomposites remains close to the individual CsH2PO4 (∼10-2 S/cm) up to x = 0.7 (fvNDs = 11.7 %). The jump in the conductivity during the phase transition decreases significantly and almost disappears. The conductivity of nanocomposites decreases significantly only at x > 0.95 (fvNDs = 51.8 %) due to the percolation effect. The mechanical properties of CsH2PO4-NDs were shown to be much higher than of the individual CsH2PO4 even with small NDs content. Mechanism of nanocomposites formation is associated with the partial binding of CsH2PO4 protons by surface OH– groups of nanodiamonds. The results are important for the establishing and design the next generation membrane of fuel cells and nanocomposite electrodes for medium-temperature electrochemical devices.

(Invited) Understanding the Performance Limits of Solid Acid Fuel Cells with Pt Electrocatalysts

Solid acid fuel cells (SAFCs) confer unique functional benefits over nearby technologies due to the solid state nature and intermediate temperature operability of the electrolyte. Amongst candidate solid acids, the electrolyte of choice in fuel cell applications is almost exclusively cesium dihydrogen phosphate (CDP), CsH2PO4. The material displays excellent proton conductivity (~2 ∗ 10-2 S/cm) at moderate temperatures (228 – 300 °C) with no detectable electronic conductivity. The elevated temperature of operation relative to polymer electrolyte membrane (PEM) fuel cells (< 90 °C), suggests that the kinetics for the oxygen reduction reaction will be elevated, and that SAFCs should provide higher power densities at reduced catalyst loadings. In practice, however, these advantages have not been realized. State-of-the-art SAFCs produce peak power densities of ~200 mW/cm2 with Pt loadings of 1.75 mg/cm2.1 In comparison, PEMFCs have demonstrated peak power densities >1 W/cm2 with Pt loadings of only 0.1 mg/cm2.2 In this work, we begin to unravel the question of why the anticipated thermal activation of reaction kinetics has not resulted in superior SAFC performance. We measure fundamental microstructural and material parameters of SAFC cathodes, and use these to predict fuel cell polarization curves. We find that, while the exchange current density for the oxygen reduction reaction is high in SAFCs, the Tafel slope is steep. Thus, the fundamental steps in the ORR reaction on Pt follow a different and unfavorable pathway under SAFC conditions than PEMFC conditions.We further employ the experimentally informed and validated physical model (see figure) to assess the role of microstructural, operational, and material parameters on SAFC polarization curves. The model suggests that a 10-fold increase in active Pt surface area would be required to approach the DOE fuel cell target of 300 mA/cm2 at 0.8 V (even without accounting for voltage drops through the ~ 50 mm thick electrolyte). In turn, this would require a decrease in CDP particle size in the cathode, where CDP serves as the support onto which Pt nanoparticles are deposited, from 500 to 50 nm, and would be accompanied by an untenable increase in Pt loading to over 30 mg/cm2. We further find that the Pt catalytic activity is strongly inhibited by the presence of steam, required to maintain stability of the electrolyte against dehydration. The humidification requirement moreover increases polarization losses at higher temperatures of operation, at which higher steam levels are required for electrolyte stabilization, overcoming the expected benefits of thermally activated catalysis. These insights motivate prioritization of the development and deployment of alternative catalyst materials and advanced electrolytes with greater thermodynamic stability over attempts to modify electrode microstructure, cell architecture, or operational conditions using today’s material set in order to advance SAFC technology towards commercial realization.1. Papandrew, A. B., Chisholm, C. R. I., Elgammal, R. A., Özer, M. M. & Zecevic, S. K. Advanced Electrodes for Solid Acid Fuel Cells by Platinum Deposition on CsH2PO4. Chemistry of Materials 23, 1659-1667, (2011).2. Zhao, Z. et al. Graphene-nanopocket-encaged PtCo nanocatalysts for highly durable fuel cell operation under demanding ultralow-Pt-loading conditions. Nat Nanotechnol, (2022). Figure 1

High-Conductive CsH2PO4 Membranes with PVDF-Based Polymers Additives.

The study is devoted to one of the important problems of hydrogen energy-the comparative analysis and creation of novel highly conductive and durable medium-temperature proton membranes based on cesium dihydrogen phosphate and fluoropolymers. The proton conductivity, structural characteristics and mechanical properties of (1 - x)CsH2PO4-x fluoropolymer electrolytes (x-mass fraction, x = 0-0.3) have been investigated and analyzed. UPTFE and PVDF-based polymers (F2M, F42, and SKF26) with high thermal stability and mechanical properties have been chosen as polymer additives. The used fluoropolymers are shown to be chemical inert matrices for CsH2PO4. According to the XRD data, a monoclinic CsH2PO4 (P21/m) phase was retained in all of the polymer electrolytes studied. Highly conductive and mechanically strong composite membranes with thicknesses of ~50-100 μm were obtained for the soluble fluoropolymers (F2M, F42, and SKF26). The size and shape of CsH2PO4 particles and their distribution have been shown to significantly affect proton conductivity and the mechanical properties of the membranes. The thin-film polymer systems with uniform distributions of salt particles (up to ~300 nm) were produced via the use of different methods. The best results were achieved via the pretreatment of the suspension in a bead mill. The ability of the membranes to resist plastic deformation increases with the growth of the polymer content in comparison with the pure CsH2PO4, and the values of the mechanical strength characteristics are comparable to the best low-temperature polymer membranes. The proton-conducting membranes (1 - x)CsH2PO4-x fluoropolymer with the optimal combination of the conductivity and mechanical and hydrophobic properties are promising for use in solid acid fuel cells and other medium-temperature electrochemical devices.

Superprotonic conductivity in RbH2-3y(PO4)1-y: a phosphate deficient analog to cubic CsH2PO4 in the (1 - x)RbH2PO4 - xRb2HPO4 system.

In contrast to CsH2PO4 (cesium dihydrogen phosphate, CDP), a material with a well-established superprotonic transition to a high conductivity state at 228 °C, RbH2PO4 (rubidium dihydrogen phosphate, RDP) decomposes upon heating under ambient pressure conditions. Here we find, from study of the (1 - x)RbH2PO4 - xRb2HPO4 system, the remarkable occurrence of cubic, off-stoichiometric RbH2-3y(PO4)1-y, or α-RDP, with a variable Rb : PO4 ratio. Materials were characterized by simultaneous thermal analysis and in situ X-ray powder diffraction performed under high steam partial pressure, from which the phase diagram between RbH2PO4 (x = 0) and Rb5H7(PO4)4 (x = 1/4) was established. The system displays eutectoid behavior, with a eutectoid transition temperature of 242.0 ± 0.5 °C and eutectoid composition of x = 0.190 ± 0.004. Even the end-member Rb5H7(PO4)4 appears to transform to α-RDP, implying y in the chemical formula of 0.2 and a phosphate site vacancy concentration as high as 20%. Charge balance is attained by a decrease in the average number of protons on the remaining phosphate groups. The cubic lattice parameter at x = 0.180, near the eutectoid composition, and at a temperature of 249 °C is 4.7138(2) Å. This value is substantially smaller than the estimated ambient-pressure lattice parameter of stoichiometric RbH2PO4 of 4.837(12) Å, consistent with the proposal of phosphate site vacancies in the former. The superprotonic conductivity of the x = 0.180 material is 6 × 10-3 S cm-1 at 244 °C, a factor of three lower than that of CDP at the same temperature. While the engineering properties of α-RDP do not suggest immediate technological relevance, the discovery of a superprotonic solid acid with a high concentration of phosphate site vacancies opens new avenues for developing proton conducting electrolytes, and in particular, for controlling their transition behavior.

Composite Electrolytes Based on Cesium Dihydrogen Phosphate and Fluropolymers

Composite Electrolytes Based on Cesium Dihydrogen Phosphate and Fluropolymers

Dielectric properties of hydrogen-bonded cesium dihydrogen phosphate-type ferroelectric crystals

Cesium dihydrogen phosphate crystal belongs to the potassium dihydrogen phosphate family of ferroelectrics. It is ferroelectric below 154 K. It shows a large isotope effect like other members of this family. We have modified two-sublattice pseudospin lattice coupled-mode model Hamiltonian by adding phonon anharmonic interactions, extra-spin lattice interactions, and direct spin-spin coupling as well as an electric field. By using modified model Hamiltonian, double-time thermal Green function method and following the Zubarev and Kubo formalism, we have derived theoretically expressions for soft mode frequency, dielectric constant, and loss tangent for cesium dihydrogen phosphate (CDP)-type ferroelectric crystals. By fitting model values of physical parameters, we have calculated thermal variations of normal mode frequency, dielectric constant, and dielectric tangent loss for CDP-crystal. We have compared our theoretical results with the experimental results of others. Our theoretical obtainedresults agree well with the experimental results of others.

Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and poly(vinylidene fluoride-co-hexafluoropropylene)

Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and poly(vinylidene fluoride-co-hexafluoropropylene)

Structural, thermal and electrical properties of composites electrolytes (1\u2212x) CsH2PO4/x ZrO2 (0\u2009\u2264\u2009x\u2009\u2264\u20090.4) for fuel cell with advanced electrode

Composites proton conducting material based on cesium dihydrogen phosphate (CDP) doped with zirconium oxide (1−x) CsH2PO4/x ZrO2 were synthesized with different concentration having in the range such as x = 0.1, 0.2, 0.3 and 0.4 by ball milling method. The prepared solid acid composites were dried at 150 °C for 6 h. Structural and thermal characterization of solid acid composite proton electrolytes were carried out by X-ray diffractometer, Fourier transform infrared spectroscopy, and Raman spectroscopy respectively. Phase transition of the prepared materials was carried out by using differential scanning calorimetry and conductivity was measured by LC Impedance meter in the range 1 Hz to 400 kHz. The ionic conductivity of ZrO2 doped CsH2PO4 (CDP) was increased up to 1.3 × 10–2 S cm−1 at the 280 °C under environment atmospheric humidification which showed high stability as compared to pure CsH2PO4 (CDP). This obtaining result would be useful for establishing and design the next generation fuel cell.

Dielectric Properties of Deuterated Cesium Dihydrogen Phosphate Crystal

Modified two sublattice pseudospin lattice coupled-mode model Hamiltonian has been applied to study the dielectric properties of deuterated cesium dihydrogen phosphate crystal. With the help of model Hamiltonian and Green’s function theory, the theoretical expressions for normal mode frequency, dielectric constant and loss tangent are derived. By fitting the model values of different physical parameters, the thermal dependence of the above quantities has been calculated. Our theoretical obtained results are well compared with experimental data.

Solid Acid Electrochemical Cell for the Production of Hydrogen from Ammonia

Summary Production of high-purity hydrogen by thermal-electrochemical decomposition of ammonia at an intermediate temperature of 250°C is demonstrated. The process is enabled by use of a solid-acid-based electrochemical cell (SAEC) in combination with a bilayered anode, comprising a thermal-cracking catalyst layer and a hydrogen electrooxidation catalyst layer. Cs-promoted Ru on carbon nanotubes (Ru/CNT) serves as the thermal decomposition catalyst, and Pt on carbon black mixed with CsH2PO4 is used to catalyze hydrogen electrooxidation. Cells were operated at 250°C with humidified dilute ammonia supplied to the anode and humidified hydrogen supplied to the counter electrode. A current density of 435 mA/cm2 was achieved at a potential of 0.4 V and ammonia flow rate of 30 sccm. With a demonstrated faradic efficiency for hydrogen production of 100%, the process yields hydrogen at a rate of 1.48 m o l H 2 / g c a t h .

Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and Butvar polymer

The phase composition, electrotransport properties, and spectral characteristics of the (1 − x)CsH2PO4–xButvar system (х = 0–0.2 wt%) were investigated in details. Butvar was proved to be a chemically suitable matrix for CsH2PO4 with a good adhesion to acid salt permitting the formation of flexible membrane. The chemical interaction of organic matrix with acid salt was not observed. A new method of “polymer-CsH2PO4” composites preparation through alcohol solutions of initial components was proposed. The flexible and thin (~ 200 μm) proton membranes with uniform salt distribution were obtained. Composite polymer electrolytes are characterized by high proton conductivity values (~ 5 × 10−3 S/cm) at 240 °C and are stable under conditions of high humidity (PH2O ~ 0.3 atm.).

Hybrid systems based on nanodiamond and cesium dihydrogen phosphate

The first data were obtained on the study of a composite systems based on (1−x)CsH2PO4−x nanodiamonds (ND). It is shown that a matrix of nanodiamonds is suitable for the creation of composites based on CsH2PO4. The phase composition of composite was investigated; the CsH2PO4 structure remains at x = 0–0.5. The particle size of CsH2PO4 and degree of crystallinity change markedly even at x = 0.1, which corresponds to a total volume fraction of nanodiamonds, fv = 0.62%. Due to the interface interaction, salt dispersion, the formation of defects and partial salt amorphization proton conductivity at low temperatures increases with increasing molar fraction of nanodiamonds more than one order of magnitude at x = 0.5; superionic one is close to pure CsH2PO4.

The influence of Cs2HPO4·H2O impurity on the proton conductivity and thermal properties of CsH2PO4

The electrotransport, structural and thermodynamic properties of CsH2PO4 with small amount of Cs2HPO4·2H2O (mole fraction, x = 0.01–0.1) has been investigated by the impedance, IR spectroscopy, X-ray diffraction and differential scanning calorimetric methods. The addition of Cs2HPO4·2H2O up to mole fraction x ≤ 0.03 was shown to increase markedly the low-temperature conductivity and change the thermodynamic characteristics of cesium dihydrogen phosphate. The low-temperature conductivity enhances up to σ150°C ~2 · 10−3 S·cm−1 for x = 0.1 and the activation energy of conductivity decreases to Ea = 0.56 eV. The superionic phase transition becomes more diffusive and almost disappears at x = 0.1. The high-temperature conductivity doesn't change significantly at the doping of CsH2PO4 up to x = 0.1. While the small deviation of proton content in CsH2PO4 shows a weak effect for XRD and IR spectroscopy. According to X-ray diffraction patterns the Cs1+xH2−xPO4 solid solutions were not observed and the formation of (1-2x)CsH2PO4-xCs3(H1.5PO4)2 composites took place. The effect of the deficiency of protons on the conductivity of CsH2PO4 in the low humidity conditions was discovered firstly and is of scientific and practice importance.

Spontaneous formation of nanoparticles on electrospun nanofibres

We report the spontaneous formation of nanoparticles on smooth nanofibres in a single-step electrospinning process, as an inexpensive and scalable method for producing high-surface-area composites. Layers of nanofibres, containing the proton conducting electrolyte, caesium dihydrogen phosphate, are deposited uniformly over large area substrates from clear solutions of the electrolyte mixed with polymers. Under certain conditions, the normally smooth nanofibres develop caesium dihydrogen phosphate nanoparticles in large numbers on their external surface. The nanoparticles appear to originate from the electrolyte within the fibres, which is transported to the outer surface after the fibres are deposited, as evidenced by cross-sectional imaging of the electrospun fibres. The presence of nanoparticles on the fibre surface yields composites with increased surface area of exposed electrolyte, which ultimately enhances electrocatalytic performance. Indeed, solid acid fuel cells fabricated with electrodes from processed nanofibre-nanoparticle composites, produced higher cell voltage as compared to fuel cells fabricated with state-of-the-art electrodes.

Vapor pressure and specific electrical conductivity in the solid and molten H2O-CsH2PO4-CsPO3 system—a novel electrolyte for water electrolysis at ~ 225–400 °C

Cesium dihydrogen phosphate, CsH2PO4 (CDP) was studied for water electrolysis at ~ 225–400 °C. In the presence of sufficient humidity, CDP is structurally disordered and super-protonic conducting with conductivities reaching 0.2–0.25 S cm−1, when determined in suitable H-shaped sealed conductivity cells. Freshly prepared 99.7 ± 0.3% gravimetric pure CDP with correct X-ray diffraction and DSC diagram melted at ~ 345 °C. The vapor pressures, above CDP alone and mixed with 20–50 mol% CsPO3 or 13 mol% H2O, were determined in sealed ampoules up to 355 °C by means of Raman spectroscopy based on internal reference gases. Pressures up to ~ 49 bar were estimated, much higher than previously expected. Conductivities were given as polynomials and plotted in solid and liquid states. Water splitting electrolysis 2H2O → 2H2 + O2 was demonstrated by Raman at ~ 355 °C under a water pressure of ~ 23 bar in a quartz cell with platinum electrodes, showing molten CDP to have significant potential for water electrolysis.

Bottom up synthesis of boron-doped graphene for stable intermediate temperature fuel cell electrodes

The high surface area and electrical conductivity of few-layered graphene would make it an ideal catalyst support and electrode in fuel cells apart from its susceptibility to oxidative corrosion. Here we report the single-step, bottom-up synthesis of oxidation-resistant boron-doped graphene and show its increased stability in the aggressive electrochemical environment of intermediate temperature solid acid fuel cells (SAFCs). Boron was shown to alter the growth mode of single walled carbon nanohorns by laser vaporization to produce high yields of thin (<20 layer) boron-doped (2–3 at.%) crystalline flakes of turbostratic graphene (B-GLF). Alternatively, adding partial pressures of hydrogen less effectively shifted the growth mode of SWCNHs toward planar, turbostratic graphene-like flakes (GLFs). Thermogravimetric analysis indicates that boron-doping enhances the oxidation resistance of few-layer GLFs by >140 °C, with large fractions of multilayered B-GLFs surviving 10 °C/min ramps to 1000 °C. These B-GLFs provided a stable Pt catalyst support and electrode over 40 h operation in SAFCs with cesium dihydrogen phosphate electrolyte operating at 250 °C, as opposed to undoped GLFs or SWCNHs which were nearly completely consumed. The facile synthesis and oxidation-resistant properties of boron-doped GLF appear promising for graphene applications in oxidizing environments.

High Efficiency Anodes for Hydrogen Separation Based on Solid Acid Electrolytes

Abundant, inexpensive, high purity molecular hydrogen as a medium for energy distribution is potentially enabling for adoption of alternative electricity generation schemes. Steam reformation of natural gas remains the most economical way to produce large amounts of hydrogen. In contrast with methane gas, derivatives may exhibit advantages in volumetric energy density, ease of transport and versatility. However, any methane or methane derivative steam reformation process also creates by-products, most notably, CO and CO2 (syngas). Syngas can contain 10-60% 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 contaminants1. Separation to produce 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 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 performance2. Solid acid fuel cells operated using methanol and an integrated steam reformer have also shown similar results3. In the majority of these earlier studies, platinum was used as the catalyst for both the hydrogen reduction and oxidation reactions. 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, but mostly direct CO electrooxidation. In this work, functionally graded anodes are fabricated to balance CO conversion activity with hydrogen oxidation. Although Ru has shown potential as a hydrogen oxidation catalyst, Pt is still superior. A dual-phase Pt and Ru electrode has been fabricated in order to increase the energy efficiency over a single-metal anode. Various carbon supports have be used in order to increase the triple phase boundary leading to drastically reduced metal loadings. These re-engineered anodes are implemented in conjunction with Ni-based cathodes to demonstrate efficient hydrogen separation using ultra low loadings of Pt from syngas-like inputs. We gratefully acknowledge the support of this work by the TN-SCORE (EPS-1004083) and the ARPA-E REBELS project. References 1 National Energy Technology Laboratory. Wabash River Coal Gasification Repowering Project: A DOE Assessment. United States: N. p., 2002. Web. doi:10.2172/790376. 2C. R. I. Chisholm et al., Electrochem. Soc. Interface, 18, 53–59 (2009). 3 T. Uda, D. A. Boysen, C. R. I. Chisholm and S. M. Haile, Electrochem. Solid-State Lett., 2006, 9(6), A261–A264. 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

Acid-base chemistry and proton conductivity of CsHSO4, CsH2PO4 and their mixtures with N-heterocycles

Caesium hydrogen sulfate (CsHSO4) and caesium dihydrogen phosphate (CsH2PO4) are solid acids that undergo superprotonic phase-transitions at about 140 and 230°C, respectively. As a result, the proton conductivity is increased by several orders of magnitude. However, the practical operational temperature range is narrow due to decomposition of the high-conductivity phases. For CsHSO4, it is known that this window can be extended to lower temperatures by addition of carefully selected N-heterocycles. The present work investigates if the same approach can be used to extend the practical operating temperature range of CsH2PO4 as well. Binary mixtures of CsH2PO4 with 1,2,4-triazole, benzimidazole or imidazole were prepared by means of mechanochemical synthesis. Mixtures based on CsHSO4 were prepared as a basis for a comparative discussion. It was found that CsHSO4 formed organic-inorganic salts, while CsH2PO4 formed heterogeneous mixtures with the N-heterocycles due to its weaker acidity. At a N-heterocycle content of 30mol%, enhanced proton conductivity was observed for both solid acids at temperatures below their superprotonic phase transitions.