Superprotonic Transition Research Articles

Synthesis, structural study and thermal behaviour of a new superprotonic compound: Cs2(HSeO4)(H2AsO4)

Crystals of a new compound with a superprotonic phase transition Cs2(HSeO4)(H2AsO4), have been synthesized for the first time by a slow evaporation method at room temperature. The structure was solved by a three-dimensional Patterson function and refined to R1=0.0309 and WR2=0.0482 on the basis of 2062 unique observed reflections using 67 parameters. The structure contains zigzag chains of hydrogen bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron neighbouring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (101¯). Thermal-differential analysis of the superprotonic transition in Cs2(HSeO4)(H2AsO4) showed that the transformation to hightemperature phase occurs at 515 K by one-step process. Thermal decomposition of the product takes place at much higher temperatures, with an onset of approximately 760 K. The superprotonic transition was also studied by impedance and modulus spectroscopy techniques. The conductivity in the high temperature phase at 523 K is 2.91×10−4 Ω−1cm−1, and the activation energy for the proton transport is 0.16 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 in this material appear to be due to proton hopping mechanism.

Phase transformation and hysteresis behavior in Cs 1 − xRb xH 2PO 4

A new theory on the origin of hysteresis in first order phase transformations was evaluated for its applicability to the phase transformation behavior in the Cs 1 − x Rb x H 2PO 4 solid solution system. Specifically, the correlation between λ 2, the middle eigenvalue of the transformation matrix describing the cubic-to-monoclinic superprotonic transition, and the transformation hysteresis was examined. The value of λ 2 was estimated from a combination of room temperature diffraction data obtained for compositions in the solid solution system and high temperature diffraction data obtained for the CsH 2PO 4 end-member. The transformation hysteresis was determined for Cs 1 − x Rb x H 2PO 4 compositions ( x = 0, 0.25, 0.50 and 0.75) by single-frequency electrical impedance measurements. It was found that the transition temperature increases monotonically with increasing Rb content, from 227.6 ± 0.4 °C for the end-member CsH 2PO 4 to 256.1 ± 0.3 °C for Cs 25Rb 75H 2PO 4, as does the hysteresis in the phase transition, from 13.4 °C to 17.4 °C. Analysis of the transformation matrix reveals that, for this system, λ 2 depends only on the b lattice parameter of the paraelectric phase and the a 0 lattice parameter of the cubic phase. The computed values of λ 2, based on extrapolations accounting for chemical contraction with increasing Rb substitution and thermal expansion on heating, were far from 1, ranging from 0.9318 to 0.9354. The observation of λ 2 increasing with Rb content is attributed to the relatively large thermal expansion in the b-axis of the low temperature monoclinic phase in combination with an increase in transition temperature with increasing x. That the hysteresis does not decrease as λ 2 approaches 1, counter to the theoretical expectations, may reflect uncertainties in the method of estimating λ 2 for Rb substituted compositions, or the discovery of a system in which hysteresis is not dominated by considerations of crystallographic compatibility.

Kinetics of the thermal decomposition in CsH2PO4 superprotonic crystal

AbstractThermal stability of CsH2PO4 was investigated in an extended temperature range below and above the superprotonic transition. Single and polycrystalline samples were studied by optical microscopy and measurements of ac conductivity using different electrodes. The kinetics of CsH2PO4 thermal decomposition and the influence of different factors such as quality of the sample and especially its surface, heating rate, type of electrodes and atmosphere were studied in detail. Different stages of this process were identified such as induction time, surface nucleation, intensive growth of the nuclei until decomposition products completely cover the surface of the sample and growth of this layer into bulk of the crystal. Two clearly different high‐conductive states with the conductivities 0.04 S cm–1 and 0.003 S cm–1, which appear during the decomposition process, were found for the first time. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Superprotonic conductivity in MH(PO 3H) (M = Li +, Na +, K +, Rb +, Cs +, NH 4+)

Electrical conductivity studies have been performed of solid acid phosphites MH(PO 3H) with M = Li +, Na +, K +, Rb +, Cs +, NH 4 +. Superprotonic conductivity following a phase transition in the temperature range from 120 to 190 °C is observed for the monoclinic forms, including Na +, K +, Rb +, Cs + and NH 4 + derivatives. At temperatures beyond the superprotonic transition temperature, thermal decomposition occurs in the case of the Na +, Rb +, and NH 4 + salts. No superprotonic phase transition is observed for orthorhombic LiH(PO 3H). The magnitude of the superprotonic conductivity of KH(PO 3H) and CsH(PO 3H), which is known to be stable under hydrogen atmospheres from literature, amounts to 4.2 · 10 − 3 Ω − 1 cm − 1 (at 140 °C) and 3 · 10 − 3 Ω − 1 cm − 1 (at 160 °C), respectively, which values are comparable to that of well known CsHSO 4.

High temperature properties of Rb 3H(SO 4) 2 at ambient pressure: Absence of a polymorphic, superprotonic transition

The high temperature properties of Rb 3H(SO 4) 2 have been studied by calorimetry, impedance spectroscopy and X-ray powder diffraction under moderate humidification. At ~ 205 °C the conductivity of Rb 3H(SO 4) 2 increases sharply, rising from 3.3 × 10 − 5 to 1.9 × 10 − 3 S/cm, suggestive of a polymorphic, superprotonic phase transition. This conductivity anomaly is accompanied by an endothermic thermal event with a heat of transition of ~ 18 kJ/mol. The X-ray powder diffraction pattern of Rb 3H(SO 4) 2 collected at 214 °C, however, shows peaks that can be attributed to Rb 2SO 4 and an unknown solid phase. The results indicate that, rather than a polymorphic transition, the conductivity increase of Rb 3H(SO 4) 2 corresponds to solid state disproportionation, described as Rb 3H(SO 4) 2(s) → Rb 2SO 4(s) + Rb m H n (SO 4) p (s), where the phase of unknown composition is rich in sulfuric acid relative to Rb 3H(SO 4 ) 2. Drop solution calorimetry, carried out using molten sodium molybdate as the solvent, revealed the enthalpy of the alternative reaction Rb 3H(SO 4) 2(s) → Rb 2SO 4(s) + RbHSO 4(s) to be essentially zero (0.9 ± 2.7 kJ/mol), supporting the assertion that the observed transformation involves different product phases. The standard enthalpy of formation of Rb 3H(SO 4) 2 from the elements at 25 °C was found to be − 2602 ± 10 kJ/mol.

Synthesis, X-ray structure and thermal behavior of the new superprotonic conductor Cs 2(HSeO 4)(H 2PO 4)

Investigation of solid solutions of CsHSeO 4–CsH 2PO 4 have led to the discovery of the compound Cs 2(HSeO 4)(H 2PO 4) (denoted CsHSeP). The structural properties of the obtained crystals were characterized by single-crystal X-ray analysis: Cs 2(HSeO 4)(H 2PO 4) crystallizes at room temperature in the space group P2 1/ c with lattice parameters a = 12.256(7) Å, b = 7.883(6) Å, c = 10.140(6) Å, β = 90.32(5)°, Z = 4 and V = 979.6(1) Å 3. In this structure, the SeO 4 and PO 4 tetrahedra are connected by O–H⋯O hydrogen bonds, to a zigzag chains running in the b-direction. These chains are also linked by hydrogen bridges to form layers parallel (1 0 0). The thermal-differential analysis of the superprotonic transition in Cs 2(HSeO 4)(H 2PO 4) showed that the transformation to high-temperature phase occurs at 400 K by one-step process. Thermal decomposition of the product takes place at 471 K. This decomposition occurs in several stages and characterized by weight loss of (CsHSeP). The first two transitions were also studied by X-ray powder diffraction at various temperatures and by impedance and modulus spectroscopy techniques. ac-impedance measurements revealed that, upon heating, the compound undergoes a transformation into a phase of high conductivity at ∼400 K. The activation energy increases from 0.12 to 0.24 eV, while the conductivity jumps from 2.60 × 10 −5 Ω −1 cm −1 at 393 K to 2.77 × 10 −4 Ω −1 cm −1 at 403 K. Information about charge carrier transport mechanism is obtained by comparison of Δ E f with Δ E σ . The activation energies for the (CsHSeP) compound calculated respectively from the modulus and impedance spectra are approximately close, suggesting that transport properties above and below the superprotonic phase transition (400 K) is probably due to H + protons hopping mechanism.

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

The compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature (200-300 degrees C) fuel cells. In order to settle the question of the high temperature behavior of this material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (p[H2O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 degrees C, with the conductivity rising to a value of 2.2 x 10(-2) S cm(-1) at 240 degrees C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH2PO4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH2PO4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm(-2). Thus, for fuel cells in which the supported electrolyte membrane was only 25 microm in thickness and in which a peak power density of 415 mW cm(-2) was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH2PO4, a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

Entropy Evaluation of the Superprotonic Phase of CsHSO4: Pauling's Ice Rules Adjusted for Systems Containing Disordered Hydrogen-Bonded Tetrahedra

The entropy of the superprotonic transition (phase II → phase I) of CsHSO4 is evaluated both experimentally and theoretically. Calorimetric measurements reveal a value of 14.75(22) J mol-1 K-1. Under the assumption that the entropy is entirely configurational, arising from both sulfate group orientational disorder and disorder in the hydrogen-bond network, we evaluated several structural models of CsHSO4 for their consistency with the measured entropy. For a structure in which hydrogen-bond disorder is independent of sulfate-group orientational disorder, simple methods of calculating the number of structural configurations are inadequate. Thus, the configurational entropy of the superprotonic, disordered phase of CsHSO4 is evaluated using an approach similar to that employed by Pauling to describe the residual entropy of ice at 0 K. Analogous to ice and the so-called ice rules, superprotonic CsHSO4 is assumed to obey a set of structural rules. Key among these are that there is only one proton per sulfate tetrahedron and only one proton per hydrogen bond. Defects are argued to make a negligible contribution to the transition entropy. The transition entropy obtained from this model, 14.9 J mol-1 K-1, is in excellent agreement with the measured value. Such a match between theoretical and experimental values suggests that of all published Phase I structures, the structure proposed by Jirak2 more correctly describes the arrangements of the sulfate tetrahedra and protons attached to them. The assumption of a low defect concentration implies that the jump in proton conductivity at the transition is due to an increase in the mobility of charge carriers rather than their concentration.

Proton dynamics in Cs 3(HSO 4) 2(HPO 4) studied by 1H NMR

Proton dynamics in Cs 3(HSO 4) 2(H 2PO 4) has been studied by means of 1H solid-state NMR as well as thermal analyses. The thermal analysis shows an endothermic peak at 408 K, which corresponds to a superprotonic transition. Above the transition temperature a mass loss is observed in a dry atmosphere, which is easily recovered in a conventional dry atmosphere below the transition temperature. The 1H magic-angle-spinning NMR spectra at room temperature show two peaks at 13.5 and 15.8 ppm, and a shoulder at 11.3 ppm from tetramethylsilane, demonstrating a presence of several inequivalent proton sites. Translational diffusion of protons takes place in both a room-temperature phase (RT) and a high-temperature phase (HT). In both phases reorientation of the SO 4/PO 4 tetrahedron limits the rate of the proton transport. The 1H mean residence times are estimated as E a = 33 kJ mol − 1 and τ 0 = 0.97 × 10 − 9 s for phase RT from the second moment analysis and as E a = 20 kJ mol − 1 and τ 0 = 5.0 × 10 − 12 s for phase HT from the analysis of the 1H T 1 results.

Impedance spectroscopy characterization of functionalized alumina membranes

Anodic alumina membranes have been impregnated with a protonic conductor either by immersion or by vacuum permeation of a saturated aqueous solution of CsHSO 4 for different times. Synthetized salt, obtained through the reaction of cesium carbonate with sulphuric acid (in excess), contained a small quantity of Cs 2SO 4. Unmodified membranes consist of amorphous Al 2O 3 with a regular distribution of pores (average diameter: 200 nm) and are stable up to 850 °C. Long impregnation times caused partial dissolution of alumina, with formation of Al(HSO 4) 3 on the front surfaces as well as into pore walls. From the frequency dispersion of the impedance, the “macroscopic conductivity” of membranes after different treatments was derived as a function of temperature, and compared with those relative to both unmodified membranes and the pure protonic conductor. The latter shows a sudden increase in conductivity at T > 140 °C, where a superprotonic transition occurs according to DTA analysis.

High‐Temperature Behavior of CsH2PO4 under Both Ambient and High Pressure Conditions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

High-Temperature Behavior of CsH2PO4 under Both Ambient and High Pressure Conditions

The high-temperature behavior of CsH₂PO₄ has been carefully examined under both ambient and high pressure (1.0 ± 0.2 GPa) conditions. Ambient pressure experiments encompassed thermal analysis, AC impedance spectroscopy, ¹H NMR spectroscopy, and polarized light microscopy. Simultaneous thermogravimetric analysis, differential scanning calorimetry, and evolved gas analysis by mass spectroscopy demonstrated that a structural transition with an enthalpy of 49.0 ± 2.5 J/g occurred at 228 ± 2 °C, just prior to thermal decomposition. The details of the decomposition pathway were highly dependent on sample surface area, however the structural transformation, a superprotonic transition, was not. Polarized light microscopy showed the high temperature phase to be optically isotropic in nature, consistent with earlier suggestions that this phase is cubic. The conductivity of CsH₂PO₄, as revealed by the impedance measurements, exhibited a sharp increase at the transition temperature, from 1.2 × 10⁻⁵ to 9.0 × 10⁻³ Ω¹⁻cm⁻¹, followed by a rapid decline due to dehydration. In addition, chemically adsorbed surface water was shown to increase the conductivity of polycrystalline CsH₂PO₄ over well-dried samples, even at mildly elevated temperatures (>200 °C). At high pressure an apparent irreversible phase transition at 150 °C and a reversible superprotonic phase transition at 260 °C were observed by impedance spectroscopy. At the superprotonic transition, the conductivity increased sharply by ~3 orders of magnitude to 3.5 × 10⁻² Ω¹⁻cm⁻¹ at 275 °C. This high conductivity phase was stable to the highest temperature examined, 375 °C, and exhibited reproducible and highly Arrhenius conduction behavior, with an activation energy for charge transport of 0.35 eV. Upon cooling, CsH₂PO₄ remained in the high-temperature phase to a temperature of 240 °C.

ChemInform Abstract: Synthesis, Structure, and Properties of Compounds in the NaHSO4—CsHSO4 System. Part 2. The Absence of Superprotonic Transitions in Cs2Na(HSO4)3 and CsNa2(HSO4)3.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Structure and thermal behavior of the new superprotonic conductor Cs2(HSO4)(H2PO4).

Ongoing studies of the CsHSO(4)-CsH(2)PO(4) system, aimed at developing novel proton conducting solids, resulted in the new compound Cs(2)(HSO(4))(H(2)PO(4)) (dicesium hydrogensulfate dihydrogenphosphate). Single-crystal X-ray diffraction (performed at room temperature) revealed Cs(2)(HSO(4))(H(2)PO(4)) to crystallize in space group P2(1)/n with lattice parameters a = 7.856 (8), b = 7.732 (7), c = 7.827 (7) Å, and beta = 99.92 (4) degrees. The compound has a unit-cell volume of 468.3 (8) Å(3) and two formula units per cell, giving a calculated density of 3.261 Mg m(-3). Six non-H atoms and two H atoms were located in the asymmetric unit, with SO(4) and PO(4) groups randomly arranged on the single tetrahedral anion site. Refinement using all observed reflections yielded weighted residuals of 0.0890 and 0.0399 based on F(2) and F values, respectively. Anisotropic temperature factors were employed for all six non-H atoms and fixed isotropic temperature factors for the two H atoms. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron in a neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (1;01). Thermal analysis of the superprotonic transition in Cs(2)(HSO(4))(H(2)PO(4)) showed that the transformation to the high-temperature phase occurs by a two-step process. The first is a sharp transition at 334 K and the second a gradual transition from 342 to 378 K. The heat of transformation for the entire process ( approximately 330-382 K) is 44 +/- 2 J g(-1). Thermal decomposition of Cs(2)(HSO(4))(H(2)PO(4)) takes place at much higher temperatures, with an onset of approximately 460 K.

Structure and Vibrational Spectrum ofβ-Cs3(HSO4)2[H2−x(P1−x, Sx)O4] (x∼0.5), a New Superprotonic Conductor, and a Comparison withα-Cs3(HSO4)2(H2PO4)

The structure of the new protonic conductor,β-Cs3(HSO4)2(H2−x(P1−x, Sx)O4) (with a superprotonic transition at 125°C), has been determined by single-crystal X-ray diffraction methods. Structural features of this and the related compound,α-Cs3(HSO4)2(H2PO4), have been further examined by Raman and IR spectroscopy. The new compound crystallizes in space groupC2/c, contains four formula units in the unit cell, and has anxvalue (in the stoichiometry) of approximately 0.5. X-ray intensity data, collected at room temperature, yielded lattice parametersa=20.04(1),b=7.854(5),c=8.954(5) Å, andβ=100.11(2)° and a unit cell volume of 1387.5(14) Å3, implying a density of 3.304 g/cm3. Within the asymmetric unit, 10 nonhydrogen atoms and 3 hydrogen sites were located. Refinement of the X-ray data using anisotropic temperature factors for all 10 of the former yielded residualswR(F2)=0.0952 andR(F)=0.0358. The structure contains alternating zig-zig chains of anion tetrahedra and cesium ions, in a manner similar to that found inα-Cs3(HSO4)2(H2PO4) and in CsHSO4-II. The structure is unique, however, in that P and S occupy the same crystallographic position, and there is, accordingly, a variable H content and a partially occupied hydrogen bond. Broader peaks in the Raman spectrum ofβ-Cs3(HSO4)2(H2−x(P1−xSx)O4) support the assumption of greater structural disorder in this compound than inα-Cs3(HSO4)2(H2PO4).

Pressure effect on phase transitions and protonic conductivity in Rb 3H(SeO 4) 2 and (NH 4) 3H(SO 4) 2

The pressure effect on phase transitions and protonic conductivity in Rb 3H(SeO 4) 2 and (NH 4) 3H(SO 4) 2 was studied at P ≤ 2.0 GPa, 300 K ≤ T ≤ 500 K. The temperature of the superprotonic transition in both crystals decreases with pressure at P ≤ 1.0 GPa. New phases originate in Rb 3H(SeO 4) 2 at P ≥ 1.22 GPa and in (NH 4) 3H(SO 4) 2 at P ≥ 1.02 GPa. The activation volumes for low- and high-conductivity phases were calculated from baric dependencies of protonic conductivity.