Superprotonic Transition Research Articles

Implementation of Phase Transitions in Rb3H(SO4)2 under K Substitution

A series of solid acid compounds, representing the large family MmHn(AO4)(m + n)/2·yH2O (where M = K, Rb, Cs, NH4; AO4 = SO4, SeO4, HPO4, HAsO4), is characterized by high values of own proton conductivity, which arises as a result of a phase transition through the formation of a dynamically disordered hydrogen bond network. Such superprotonic phase transitions are observed, however, not for all compounds of the family and Rb3H(SO4)2 is one of them. The occurrence of superprotonic phase transitions has been experimentally demonstrated in the (KxRb1−x)3H(SO4)2 solid solutions through cation substitution. The high-temperature phases are unstable towards decomposition reaction, and their temperature range of existence is about 1–7 °C. The implementation of superprotonic transitions is discussed in terms of hydrogen bond lengths.

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.

Conduction and stability performance of CsH2PO4 with NaH2PO4/ZrO2 for fuel cell

Solid acid composites of CsH2PO4/NaH2PO4/ZrO2 with different weight ratios of CsH2PO4 (CDP), NaH2PO4 (SDP), and ZrO2 were synthesized and characterized. The characterizations of the solid acid composite were carried out for determining structure, thermal stability, and conductivity, utilizing x-ray Diffraction, Fourier Transform Infrared Spectroscopy, Thermogravimetric analysis, Differential Thermal Analysis, and conductivity measurements. The composite powders were in the form of pellets of 4 mm thickness for conductivity measurement. SDP in the composites has a large effect on the conductivity measurement and this material increased the low-temperature conductivity of CDP by up to 1.5 orders of magnitude. The superprotonic transition was identified at temperatures from 220 to 270 °C in CDP/SDP/ZrO2 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.

Structure and properties of Cs7(H4PO4)(H2PO4)8: A new superprotonic solid acid featuring the unusual polycation (H4PO4)+

We report the discovery of a new superprotonic compound, Cs7(H4PO4)(H2PO4)8, or CPP, which forms at elevated temperatures from reaction of CsH2PO4 and CsH5(PO4)2. The structure, solved using high temperature single crystal x-ray diffraction and confirmed by high temperature 31P NMR spectroscopy, crystallizes in space group Pmn and has lattice constant 20.1994(9) Å at 130 °C. The unit cell resembles a 4  4  4 superstructure of superprotonic CsH2PO4, but features an extraordinary chemical moiety, rotationally disordered H4PO4+ cations, which periodically occupy one of every eight cation sites. The influence of this remarkable cation on the structure, thermodynamics, and proton transport properties of the CPP phase is discussed. Notably, CPP forms at a temperature of 90 C, much lower than the superprotonic transition temperature of 228 C of CsH2PO4, and the compound does not appear to have an ordered, low temperature form. Under nominally dry conditions the material is stable against dehydration to ~ 151 C, resulting in a particularly wide region of stability of a superprotonic material in the absence of active humidification. The conductivity of Cs7(H4PO4)(H2PO4)8 is moderate, 5.8  10-4 S cm-1 at 140 C, but appears nevertheless facilitated by polyanion (H2PO4-) group reorientation. Figure 1

Structure and Properties of Cs7(H4PO4)(H2PO4)8: A New Superprotonic Solid Acid Featuring the Unusual Polycation (H4PO4).

We report the discovery of a new superprotonic compound, Cs7(H4PO4)(H2PO4)8, or CPP, which forms at elevated temperatures from the reaction of CsH2PO4 and CsH5(PO4)2. The structure, solved using high-temperature single-crystal X-ray diffraction and confirmed by high-temperature 31P NMR spectroscopy, crystallizes in space group Pm3̅n and has a lattice constant of 20.1994(9) Å at 130 °C. The unit cell resembles a 4 × 4 × 4 superstructure of superprotonic CsH2PO4, but features an extraordinary chemical moiety, rotationally disordered H4PO4+ cations, which periodically occupy one of every eight cation sites. The influence of this remarkable cation on the structure, thermodynamics, and proton transport properties of the CPP phase is discussed. Notably, CPP forms at a temperature of 90 °C, much lower than the superprotonic transition temperature of 228 °C of CsH2PO4, and the compound does not appear to have an ordered, low-temperature form. Under nominally dry conditions, the material is stable against dehydration to ∼151 °C, and this results in a particularly wide region of stability of a superprotonic material in the absence of active humidification. The conductivity of Cs7(H4PO4)(H2PO4)8 is moderate, 5.8 × 10-4 S cm-1 at 140 °C, but appears nevertheless facilitated by polyanion (H2PO4-) group reorientation.

Facile Proton Transport in Ammonium Borosulfate—An Unhumidified Solid Acid Polyelectrolyte for Intermediate Temperatures

High proton conductivity is reported for unhumidified ammonium borosulfate, NH4 [B(SO4 )2 ], a solid acid coordination polymer that contains 1D, hydrogen-bonded NH4 + ···1 ∞ [B(SO4 )4/2 ]- chains. NH4 [B(SO4 )2 ] is thermally stable to 320°C and is amenable to sintering into monolithic, polycrystalline discs at 200°C and about 300MPa of uniaxial pressure. Impedance spectroscopy measurements reveal ionic conductivities for sintered ammonium borosulfate of 0.1mScm-1 at 25°C and up to 10mScm-1 at 180°C in ambient air. No superprotonic transition is observed in the temperature range of 25-180°C. Ab initio molecular dynamics simulations show these high conductivities are aided by free rotation of the NH4 + units and significant gyrational mobility of the SO4 tetrahedra, which, in turn, provide facile pathways for proton locomotion. High conductivities, a wide operational temperature window, and tolerance to both ambient and anhydrous conditions make NH4 [B(SO4 )]2 an attractive candidate electrolyte for intermediate-temperature hydrogen fuel cells that may enable operation at temperatures as high as 300°C without active humidification.

Entropic restrictions control the electric conductance of superprotonic ionic solids.

The crystallographic structure of solid electrolytes and other materials determines the protonic conductivity in devices such as fuel cells, ionic-conductors, and supercapacitors. Experiments show that a rise of the temperature in a narrow interval may lead to a sudden increase of several orders of magnitude of the conductivity of some materials, a process called a superprotonic transition. Here, we use a novel macro-transport theory for irregular domains to show that the change of entropic restrictions associated with solid-solid phase or structural transitions controls the sudden change of the ionic conductivity when the superprotonic transition takes place. Specifically, we deduce a general formula for the temperature dependence on the ionic conductivity that fits remarkably well experimental data of superprotonic transitions in doped cesium phosphates and other materials reported in the literature.

(Invited) Insights into Proton Transport in Superprotonic Solid Acids

Superprotonic solid acid electrolytes, materials with chemical and physical properties intermediate between conventional acids (e.g., H3PO4) and conventional salts (e.g., Cs3PO4) have emerged as attractive candidates for fuel cell and other electrochemical applications. Key characteristics of these materials are tetrahedral oxyanion groups linked by hydrogen bonds, and a polymorphic structural transition to a disordered state at moderate temperatures. Rapid oxyanion reorientation and dynamic disorder of the hydrogen bond network facilitate high proton conductivity in the high temperature phase. Materials exhibiting a superprotonic transition include CsHSO4, Cs3H(SeO4)2, CsH2PO4, and Cs2(HSO4)(H2PO4). Here we review the present state of understanding of proton transport mechanisms and the factors governing the transition behavior as gathered from macroscale measurements of conductivity and thermal properties and from atomistic level studies using nuclear magnetic resonance spectroscopy. We focus in particular on CsH2PO4, the technologically most significant superprotonic solid acid, and its chemical modifications produced by substitution on either the alkali metal or proton site (deuterated materials). Dramatic changes in phase behavior and proton conductivity can be induced by only minor changes in chemistry, suggesting routes for tuning behavior to achieve desired outcomes.

Stability of the superprotonic conduction of (1-x)CsH2PO4/xSiO2 (0 ≤ x ≤ 0.3) composites under dry and humid environments

We have used temperature- and time-resolved electrochemical impedance spectroscopy, x-ray diffraction, and thermal analysis methods to investigate the effect of mixing CsH2PO4 with nano-silica on the superprotonic conduction of this solid acid. We collected data on (1-x)CsH2PO4/xSiO2 (0 ≤ x ≤ 0.3) composites in dry (air) and humid (PH2O ∼ 0.38 atm) environments at temperatures below and above the superprotonic transition of CsH2PO4 (TSP ∼ 234 °C). We first observed that a three-order-of-magnitude proton conductivity jump occurred in the unmixed sample (x = 0) at TSP, even under dry conditions and despite chemical changes (dehydration). We also found that the proton conductivity of the x = 0.1, 0.2 and 0.3 composites measured at T = 260 °C in air is nearly one order of magnitude greater than that of the unmixed phosphate (x = 0). Even more significantly, we found that humid sample environments have no effect on the stability of the proton conductivity of the x = 0.2 composite measured over a 10 h timespan at temperatures above TSP. This is contrary to the behavior of the x = 0 sample, which is known [31] to be stable under humid conditions, but undergoes a three-order-of-magnitude proton conductivity drop in air.

Synthesis, structural study and thermal behaviour of a new compound: trirubidium bis(hydrogen sulphate) dihydrogen arsenate Rb3(HSO4)2.5(H2AsO4)0.5

ABSTRACTMixed crystals Rb3(HSO4)2.5(H2AsO4)0.5 have been prepared by slow evaporation from aqueous solution at room temperature. The crystals were characterized by X-ray single analysis, which revealed that Rb3(HSO4)2.5(H2AsO4)0.5 crystallizes in the space group P with lattice parameters: a = 7.471(3) Å; b = 7.636(1) Å; c = 12.193(2) Å; α = 71.91(1)°; β = 73.04(6)° and γ = 88.77(2)°. In this structure, the ordered S(1)O4 and the disordered S(3)/AsO4 tetrahedra are connected by O–H..O hydrogen bonds, to a zigzag chains running in the b-direction. These chains are, in turn, bonded to one another by disordered hydrogen bridges O–H..H–O, to give a planar structure, with hydrogen-bonded sheets, laying parallel to (1 0 0). Each disordered tetrahedron is linked to a tetrahedron neighbouring S(2)O4 by ordered hydrogen bonds. Broader peaks in IR spectrum of the title material support the assumption of disordered structure. Thermal analysis of the superprotonic transition in Rb3(HSO4)2.5(H2AsO4)0.5 showed that the transformation to the high-temperature phase occurs by one-step process at 404 K. Thermal decomposition of this compound takes place at much higher temperatures, with an onset of approximately 473 K.

Structure, thermal behavior, and dielectric properties of new cesium hydrogen sulfate arsenate: Cs2(HSO4)(H2AsO4)

Synthesis and structural characterization by single-crystal X-ray diffraction method, thermal behavior, and electrical proprieties are given for a new compound with a superprotonic phase transition Cs2(HSO4)(H2AsO4). The title compound crystallizes in the monoclinic system with the P21/n space group. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (10ī). The existence of O–H and (S/As)–O bonds in the structure at room temperature has been confirmed by IR and Raman spectroscopy in the frequency ranges 4000–400 cm−1and 1200–50 cm−1, respectively. Differential scanning calorimetry analysis of the superprotonic transition in Cs2(HSO4)(H2AsO4) showed that the transformation to high temperature phase occurs at 417 K by one-step process. Thermal decomposition of the product takes place at much higher temperatures, with an onset of approximately 534 K. The superprotonic transition was also studied by impedance and modulus spectroscopy techniques. The conductivity in the high temperature phase at 423 K is 1.58 × 10−4 Ω−1 cm−1, and the activation energy for the proton transport is 0.28 eV. The conductivity relaxation parameters associated with the high disorder protonic conduction have been examined from analysis of the M”/M”max spectrum measured in a wide temperature range. Transport properties of this material appear to be due to the proton hopping mechanism.

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.

Advances in solid acid electrolytes for fuel cell applications

Fuel cells represent a class of high-efficiency energy conversion devices that are expected to play a major role in reducing the environmental pollution in hydrogen economy. These cells typically employ polymer membranes as electrolytes for protonic conduction that are operable only under humid conditions. This requirement limits their operating temperature range to 373 K. Solid acids are a relatively new class of electrolytes that allow protonic conduction at temperatures above 373 K and are thereby free from water management issues. Each solid acid has a specific transition temperature beyond which its protonic conductivity increases. Unlike polymer membranes, they do not require hydration for proton conduction and follow Grotthus mechanism which makes them a promising candidate for further research. In the last decade, significant amount of attention has been paid to these compounds and the results have been encouraging. Addition of additives like SiO2 to solid acids has been found to be an effective method to increase their conductivity and lower the superprotonic transition temperature. This work describes the recent developments and the future challenges in the field of solid electrolytes for fuel cell applications.

The thermodynamics and kinetics of the dehydration of CsH2PO4 studied in the presence of SiO2

The thermodynamic and kinetics of the dehydration and hydration of CsH2PO4 is investigated by thermogravimetric, differential scanning calorimetry and X-ray diffraction analysis in the temperature range of 200 to 400°C, water partial pressure range of 0.06 to 0.90atm. SiO2 powder was added to CsH2PO4 to accelerate both dehydration and hydration. When dehydrated in the presence of SiO2, CsH2PO4 has a fine microstructure (~200nm feature size), and no reaction between CsH2PO4 and SiO2 is observed. By making use of the enhanced kinetics afforded by SiO2, the phase boundary between CsH2PO4, CsPO3 and dehydrated liquid was precisely determined. The triple point connecting these phases is located at pH2O = 0.35±0.2atm and T=267.5±1.0°C. The stability of CsH2PO4 and the liquid dehydrate, CsH2(1−x)PO4−x(l), were confirmed by the complete reversal of dehydration to recover these phases in the appropriate temperature and water partial pressure ranges. Rehydration and conversion of CsPO3(s) to CsH2PO4(s) occur over a period of several hours, depending on temperature, water partial pressure, and morphology of the metaphosphate. High pH2O and small particles favor rapid dehydration, whereas the temperature dependence of the rehydration kinetics is non-monotonic, reaching its fastest rate in the vicinity of the superprotonic transition.

High pressure synchrotron x-ray diffraction studies of superprotonic transitions in phosphate solid acids

We have used high-pressure synchrotron x-ray diffraction to investigate the structural modifications that accompany the abrupt, several-order-of-magnitude proton conductivity increase exhibited upon heating by CsH2PO4 (CDP) and RbH2PO4 (RDP). Our temperature-resolved data collected on polycrystalline CDP subjected to a pressure P=1GPa reveal a polymorphic structural transition at T~260°C from this solid acid's room-temperature monoclinic (P21/m) phase to a stable cubic (Pm3m, a=4.88Å) modification. The transition occurs under the same (P, T) conditions as the 1000-fold jump in CDP's proton conductivity, and Rietveld analysis confirms that the high-pressure phase has essentially the same crystal structure as the cubic CDP polymorph previously observed under a saturated water vapor atmosphere. For RDP we find a structural transformation from its room temperature tetragonal (I-42d) phase to an intermediate temperature monoclinic (P21/m) modification, which, remarkably, is isomorphic to its monoclinic CDP counterpart. This suggests that a monoclinic→cubic structural transition, similar to the one observed in CDP, is responsible for RDP's superprotonic behavior. We present the first direct evidence for the existence of a high temperature cubic RDP phase.

Rule of superprotonic phase transition in CsxRb1 − xH2PO4

We have investigated the relation between the superprotonic phase transition and elastic property in CsH2PO4 (CDP) type compounds. It was found that, in CsxRb1−xH2PO4 (x=0–1), the transition temperature Tc decreases in proportion to the square of spontaneous strain component d. From this result, it is indicated that the spontaneous strain plays an important role in the appearance of superprotonic conductivity in CDP type compounds. In addition, by thermodynamic analyses, proton transfer in superprotonic phase is induced by the release of the strain energy. These results indicate that the superprotonic transition temperature in CDP type compounds is determined by the competition between the kinetic energy of proton, the increase of entropy and the released-strain energy.

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...

The hydrogen dynamics of CsH5(PO4)2 studied by means of nuclear magnetic resonance

We have investigated the hydrogen dynamics of cesium pentahydrogen diphosphate,CsH5(PO4)2, by means of nuclear magnetic resonance (NMR) spectroscopy, in order to address thequestion of why there is no superprotonic phase transition in this compound, in contrast toother structurally similar hydrogen-bonded ionic salts, where a superprotonic transition isfrequently found to be present. The analysis of the NMR spectrum and the spin–latticerelaxation rate revealed that the temperature-dependent hydrogen dynamics ofCsH5(PO4)2 involves motional processes (the intra-H-bond jumps and the inter-H-bond jumps atelevated temperatures, as a mechanism of the ionic conductivity) identical to those for theother H-bonded superprotonic salts. The considerably stronger H-bond network inCsH5(PO4)2 prompts the search for a higher superprotonic transition temperature. However, dueto the relatively weak bonding between the planes in the [100] direction of theCsH5(PO4)2 structure by means of the ionic bonding via the cesium atoms and the small number ofH bonds in that direction (where out of five H bonds in the unit cell, four aredirected within the planes and only one is between the planes), the bonds between the planes becomethermally broken and the crystal melts before the H-bond network rearranges viawater release into an open structure typical of the superprotonic phase. Were thecoupling between the planes in the CsH5(PO4)2 somewhat stronger, the superprotonic transition would occur in the same manner as it doesin other structurally related hydrogen-bonded ionic salts.

Examination of the superprotonic transition and dehydration behavior of Cs 0.75Rb 0.25H 2PO 4 by thermogravimetric and differential thermal analyses

The dehydration and the superprotonic transition behavior of Cs 0.75Rb 0.25H 2PO 4 have been investigated via thermogravimetry and differential thermal analysis. To enable measurements under the high water partial pressures required, a high humidity system ( p H 2O = 0.02–0.8 atm) capable of weight change detection at 0.7% resolution and thermal events with 0.01 °C resolution was assembled. It has been found that the superprotonic phase transition temperature of Cs 0.75Rb 0.25H 2PO 4 is 231 °C, slightly higher than that of CsH 2PO 4. In contrast, dehydration of Cs 0.75Rb 0.25H 2PO 4 under p H 2O = 0.35 atm occurs at 265 °C, a somewhat lower temperature than that at which CsH 2PO 4 undergoes dehydration. Analysis of the temporal evolution of both transformations suggests a mechanism of continuous nucleation of the high temperature phase at a constant nucleation rate combined with three-dimensional growth.