Frequency dielectric dispersion in the ferroelectric and superionic phases of CsH<sub>2</sub>PO<sub>4</sub>

Баранов, А. І.; Khiznichenko, V. P.; Sandler, V. A.; Shuvalov, L. A.

doi:10.1080/00150198808008840

Cited by 111 publications

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“…In contradiction to these results, Nirsha et al published a study two years later concluding that thermal events at 233 1C and higher in CsH 2 PO 4 are entirely due to decomposition. 14 The matter of a polymorphic phase transition in CsH 2 PO 4 may have remained an obscure point in the field of solid state chemistry were it not for the results of Baranov et al showing a so-called superprotonic transition to occur at 230 1C, 5 precisely the temperature of the reversible, higher temperature transformation first reported by Clark. 10,11 The conductivity was shown to increase by five orders of magnitude at the transition, and apparently reliable data were obtained to temperatures of B250 1C.…”

Section: Phase Transition Behaviormentioning

confidence: 99%

“…In the particular case of proton transport, the dynamic disordering of the hydrogen bond network above the so-called superprotonic transition leads to a dramatic increase in proton conductivity by several orders of magnitude. [1][2][3]5 Subtle changes in the local hydrogen bond geometry similarly give rise to the well-known ferroelectric transition in compounds such as KH 2 PO 4 6 and Cs 3 H(SeO 4 ) 2 . 7 This rich phase behavior has spawned the production of at least 500 papers that broadly address solid acids with stoichiometry MHXO 4 , M 3 H(XO 4 ) 2 , M 2 H(X 0 O 4 ), or some variation thereof, where M = alkali metal or NH 4 ; X = S, Se; and X 0 = P, As.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2007

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The compound CsH 2 PO 4 has emerged as a viable electrolyte for intermediate temperature (200-300 1C) 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[H 2 O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 1C, with the conductivity rising to a value of 2.2 Â 10 À2 S cm À1 at 240 1C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH 2 PO 4 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 CsH 2 PO 4 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 À2. Thus, for fuel cells in which the supported electrolyte membrane was only 25 mm in thickness and in which a peak power density of 415 mW cm À2 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 CsH 2 PO 4 , a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

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Section: Phase Transition Behaviormentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2007

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“…228 uC. [1][2][3][4] The superprotonic phase of CsH 2 PO 4 exhibits proton conductivity greater than 10 22 S cm…”

Section: Introductionmentioning

confidence: 99%

Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C

et al. 2007

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The dehydration behavior of caesium dihydrogen phosphate CsH 2 PO 4 was investigated in the temperature range of 230 uC to 260 uC under high humidity, conditions of particular relevance to the operation of fuel cells based on this electrolyte. The onset temperature of dehydration was determined from changes in ionic conductivity on heating and confirmed by weight change measurements under isothermal conditions. The relationship between the onset temperature of dehydration (T dehy ) and water partial pressure (p H 2 O ) was determined to be log(p H 2 O /atm = 6.11(¡0.82) 2 3.63(¡0.42) 6 1000/(T dehy /K), from which the thermodynamic parameters of the dehydration reaction from CsH 2 PO 4 to CsPO 3 were evaluated. The dehydration pathway was then probed by X-ray powder diffraction analysis of the product phases and by thermogravimetric analysis under slow heating. It was found that, although the equilibrium dehydration product is solid caesium metaphosphate CsPO 3 , the reaction occurs via two overlapping steps: CsH 2 PO 4 A Cs 2 H 2 P 2 O 7 A CsPO 3 , with solid caesium hydrogen pyrophosphate, Cs 2 H 2 P 2 O 7 , appearing as a kinetically favored, transient phase.

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“…2͒. That strategy of the experiment is very good for presenting the thermal decomposition process of CDP as a function of annealing temperature; however, it is insufficient to conclude that the reported transition [3][4][5][6][7][8][9][10][11][12] associated with endothermic anomaly at ca. 231°C does not take place and, moreover, that the high conductivity above 231°C is only a consequence of the dehydration of the crystal surface.…”

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confidence: 99%

“…This feature is accompanied by an increase of the electrical conductivity by three orders of magnitude. 8 Raman spectra of the superionic phase of CDP are characteristic of a plastic phase, implying rapid reorientation of the H 2 PO 4 Ϫ anions on given sites. 10 Reorientation of the O-H groups leads to the breaking up of the old hydrogen bonds and creating of new ones.…”

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confidence: 99%

Comment on “Does the structural superionic phase transition at 231 °C in CsH2PO4 really not exist?” [J. Chem. Phys. 110, 4847 (1999)]

Bronowska

2001

The Journal of Chemical Physics

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Superionic materials have intrigued researchers because they exhibit structural transitions from the paraelectric to the superionic phase which are accompanied by an increase of the electrical conductivity as much as five orders of magnitude. Notwithstanding the number of studies and variety of experiments that have been carried out on superprotonic crystals of the KH 2 PO 4 type, no generally agreed-upon model describing the structural and chemical features that induce superionic transitions exists. Discrepancies between the high-temperature results reported by different groups of scientists have been well summarized in recent papers. 1,2 Lee 1 has discussed the similarities of the phase transitions in the KH 2 PO 4 -type compounds and concluded that the high-temperature phenomena of these compounds are not related to physical changes like structural phase transitions, but related to chemical ones and has suggested that the term ''high-temperature phase transition'' should be replaced by ''onset of partial polymerization at reaction sites distributed on the surface of solids. '' In order to check if the reported high-temperature phase transitions of CsH 2 PO 4 ͑CDP͒ are only related to thermal dehydration, power x-ray measurements were performed by Ortiz et al.2 A fresh powder sample was subsequently heat treated 1 min at 130, 165, 200, 238, and 250°C, and their respective x-ray diffraction patterns taken under dry conditions after cooling at room temperature ͑25°C͒ were plotted ͑see Fig. 3 of Ref. 2͒. That strategy of the experiment is very good for presenting the thermal decomposition process of CDP as a function of annealing temperature; however, it is insufficient to conclude that the reported transition 3-12 associated with endothermic anomaly at ca. 231°C does not take place and, moreover, that the high conductivity above 231°C is only a consequence of the dehydration of the crystal surface.Under normal air conditions the high-temperature transition occurs very close to the region where CDP decomposes by dehydration and these two simultaneous effects considerably complicate the interpretation of the phase relation.Ten years ago the Bond method of precise lattice parameter determination was applied as a sensitive monitor of the structural changes occurring in the CDP crystal.9 Lattice parameters of CDP were calculated on the basis of the measurements of the Bragg angles of the reflections with high angles using the least-square method. The accuracy of the measurements of 2, which was better than 10Љ of arc, allowed the determination of the lattice parameters to an accuracy better than 3ϫ10Ϫ5 . Investigations of the lattice parameters of CDP above room temperature have revealed that up to ca. 137°C all parameters are almost linear. In the vicinity of 149°C a discrepancy of the linearity of the lattice parameters has been noticed, but the space group ( P2 1 /m) of the crystal above room temperature up to ca. 231°C does not change. The investigation of the single-crystal samples of CDP with the Bond diff...

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Frequency dielectric dispersion in the ferroelectric and superionic phases of CsH₂PO₄

Cited by 111 publications

References 5 publications

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

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

Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C

Comment on “Does the structural superionic phase transition at 231 °C in CsH2PO4 really not exist?” [J. Chem. Phys. 110, 4847 (1999)]

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Frequency dielectric dispersion in the ferroelectric and superionic phases of CsH2PO4

Cited by 111 publications

References 5 publications

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

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

Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C

Comment on “Does the structural superionic phase transition at 231 °C in CsH2PO4 really not exist?” [J. Chem. Phys. 110, 4847 (1999)]

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Frequency dielectric dispersion in the ferroelectric and superionic phases of CsH₂PO₄