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Journal ArticleDOI

Electrical conduction in Na3H (SO4)2 and (NH4)3 H(SO4)2 crystals

01 Jul 1981-Journal of Materials Science (Kluwer Academic Publishers)-Vol. 16, Iss: 7, pp 2011-2016
About: This article is published in Journal of Materials Science.The article was published on 1981-07-01. It has received 13 citation(s) till now. The article focuses on the topic(s): Solid mechanics.

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Topics: Solid mechanics (55%)
Citations
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Journal ArticleDOI
19 Apr 2001-Nature
TL;DR: It is shown that a cell made of a CsHSO4 electrolyte membrane operating at 150–160 °C in a H2/O2 configuration exhibits promising electrochemical performances: open circuit voltages of 1.11 V and current densities of 44 mA cm-2 at short circuit.

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Abstract: Fuel cells are attractive alternatives to combustion engines for electrical power generation because of their very high efficiencies and low pollution levels. Polymer electrolyte membrane fuel cells are generally considered to be the most viable approach for mobile applications. However, these membranes require humid operating conditions, which limit the temperature of operation to less than 100 degrees C; they are also permeable to methanol and hydrogen, which lowers fuel efficiency. Solid, inorganic, acid compounds (or simply, solid acids) such as CsHSO4 and Rb3H(SeO4)2 have been widely studied because of their high proton conductivities and phase-transition behaviour. For fuel-cell applications they offer the advantages of anhydrous proton transport and high-temperature stability (up to 250 degrees C). Until now, however, solid acids have not been considered viable fuel-cell electrolyte alternatives owing to their solubility in water and extreme ductility at raised temperatures (above approximately 125 degrees C). Here we show that a cell made of a CsHSO4 electrolyte membrane (about 1.5 mm thick) operating at 150-160 degrees C in a H2/O2 configuration exhibits promising electrochemical performances: open circuit voltages of 1.11 V and current densities of 44 mA cm-2 at short circuit. Moreover, the solid-acid properties were not affected by exposure to humid atmospheres. Although these initial results show promise for applications, the use of solid acids in fuel cells will require the development of fabrication techniques to reduce electrolyte thickness, and an assessment of possible sulphur reduction following prolonged exposure to hydrogen.

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769 citations


Journal ArticleDOI
TL;DR: The result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness, and a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

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Abstract: The compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature (200–300 °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 °C, with the conductivity rising to a value of 2.2 × 10−2 S cm−1 at 240 °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 μm 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.

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244 citations


18


Journal ArticleDOI
TL;DR: In this review, the potential use of phosphate compounds as electrolytes for intermediate temperature fuel cells is summarized and various examples on ammonium polyphosphate, pyroph phosphate, cesium phosphate and other phosphate based electrolytes are presented and their preparation methods, conduction mechanism and conductivity values are demonstrated.

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Abstract: The electrolytes currently used for proton exchange membrane fuel cells are mainly based on polymers such as Nafion which limits the operation regime of the cell to ~ 80 °C. Solid oxide fuel cells operate at much elevated temperatures compared to proton exchange membrane fuel cells (~1000 °C) and employ oxide electrolytes such as yttrium stabilized zirconia and gadolinium doped ceria. So far an intermediate temperature operation regime (300 °C) has not been widely explored which would open new pathways for novel fuel cell systems. In this review we summarize the potential use of phosphate compounds as electrolytes for intermediate temperature fuel cells. Various examples on ammonium polyphosphate, pyrophosphate, cesium phosphate and other phosphate based electrolytes are presented and their preparation methods, conduction mechanism and conductivity values are demonstrated.

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103 citations


Journal ArticleDOI
01 Jan 2003-Energy & Fuels
Abstract: The chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods. Thermal decomposition of CsHSO_4, a material which has shown promise as an alternative electrolyte for proton exchange membrane (PEM) fuel cells, initiates at ∼175°C under inert conditions. The overall decomposition process can be expressed as 2CsHSO_4 → Cs_2SO_4 + H_2O + SO_3 with Cs_2S_2O_7 appearing as an intermediate byproduct at slow heating rates. Under reducing conditions, chemical decomposition can occur via reaction with hydrogen according to 2CsHSO_4 + 4H_2 → Cs_2SO_4 + 4H_2O + H_2S. In the absence of fuel cell catalysts, this reduction reaction is slow; however, materials such as Pt, Pd, and WC are highly effective in catalyzing the reduction of sulfur and the generation of H_2S. In the case of M_3H(XO_4)_2 compounds, where M = Cs, NH_4, or Rb and X = S or Se, a similar reduction reaction occurs: 2M_3H(XO_4)_2 + 4H_2 → 3M_2XO_4 + 4H_2O + H_2X. In an operational fuel cell based on CsHSO_4, performance degraded with time, presumably as a result of H_2S poisoning of the anode catalyst. The performance loss was recoverable by exposure of the fuel cell to air at 160 °C.

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54 citations


Journal ArticleDOI
L Schwalowsky1, V Vinnichenko2, Andrey N. Baranov2, Ulrich Bismayer1  +2 moreInstitutions (3)
Abstract: The protonic conductivity of has been measured using the complex-admittance method in the frequency range 30 Hz-200 MHz and the temperature interval 290-500 K covering both the ferroelastic and the paraelastic phases. The dc conductivity shows quasi-two-dimensional behaviour and in the trigonal paraelastic phase its values in the (001) plane are typically super-protonic with low activation enthalpy . The temperature dependence of the monoclinic superstructure reflection 120 has been studied using elastic neutron diffraction. It was found that the observed anomalies of the macroscopic and microscopic quantities, such as the morphic birefringence and the high-frequency dielectric constant on the one hand and the diffusion proton dynamics as well as the integrated intensity of the reflection 120 on the other hand, show pronounced differences in their temperature evolution below the ferroelastic phase transition temperature. The neutron scattering results as well as the dielectric measurements indicate precursor effects above . The results are discussed on the basis of a phenomenological two-order-parameter model for the - ferroelastic phase transition. It is argued that properties which originate from the disorder of the oxygen and proton subsystem can be described by irreducible representations of the wave vector at the point and the L point of the Brillouin zone, while properties which originate from displacements of the heavy atoms (the displacement mode) are solely described by the wave vector at the L point of the Brillouin zone of the paraphase.

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33 citations


References
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Journal ArticleDOI
Michael O'Keeffe1, C.T. Perrino1Institutions (1)
Abstract: The electrical conductivity has been measured over a wide range of temperatures and under an equilibrium vapor pressure for pure and sulphate-doped KH2PO4. For pure KH2PO4 the conductivity perpendicular to the c-axis is given by log σ = −0.89−12.65 kcal 2.3 RT T kcal 2.3 RT T > 180°C with very similar results parallel to the c-axis. At 25°C the conductivity of doped samples is proportional to sulphate concentration. The results are analyzed in terms of defects likely to occur in KH2PO4.

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102 citations



Journal ArticleDOI
K. Gesi1Institutions (1)
Abstract: Phase transitions in triammonium hydrogen disulfate (NH4)3H(SO4)2 were studied by means of dielectric measurements and differential thermal analysis. Below room temperature three phase transitions were observed at −8, −132, and −140°C. The dielectric constant along the c*-direction shows breaks at the −8 and −132°C transitions, while it shows a discontinuous change accompanied with a thermal hysteresis at the −140°C transition. In addition to the above-mentioned transitions, a broad maximum of the dielectric constant was found at about −28°C along the c*-direction. However, no thermal anomalies were observed around −28°C. Mit Hilfe dielektrischer Messungen und differentieller Thermoanalyse wurden die Phasenubergange in Triammoniumhydrogendisulfat (NH4)3H(SO4)2 untersucht. Unterhalb der Raumtemperatur wurden drei Phasenubergange bei −8, −132 und −140°C gefunden. Die entlang der c*-Richtung gemessene Dielektrizitatskonstante zeigt bei den −8 und −132°C-Ubergangen stufenformige Anderungen, beim −140°C-Ubergang andert sie sich, begleitet von thermischen Hystereseerscheinungen, diskontinuierlich. Zusatzlich zu den o. g. Ubergangen wurden bei Messung in c*-Richtung ein breites Maximum der Dielektrizitatskonstanten bei etwa −28°C, aber keine thermischen Anomalien gefunden.

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85 citations


Journal ArticleDOI
Abstract: Experiment, plus critical discussion of previous work, suggests the following. Conductivity is primarily due to the migration of L defects (proton vacancies), the enthalpy for L‐defect mobility being 0.53 eV in KDP and 0.46 eV in ADP. Such L defects are produced by impurities, but there is an additional tendency towards incipient decomposition in both crystals. This leads to a high‐temperature conductivity which, in KDP, is associated with the formation of water molecules and the thermal generation of L defects and, in ADP, with the formation of ammonia molecules and the thermal generation of proton vacancies in the ammonium lattice (A defects). The enthalpy of L‐defect formation in KDP is 0.46 eV; that for A‐defect formation in ADP is 0.19 eV. Previously reported overheating effects are spurious. The conductivity is independent of field.

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82 citations


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