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

Protonic conduction of CsH2PO4 and its composite with silica in dry and humid atmospheres

Proton exchange membranes for fuel cells operated at medium temperatures: Materials and experimental techniques

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.

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

Solid acids as electrolyte materials for proton exchange membrane (PEM) electrolysis: Review

X-ray diffraction, thermogravimetric (TGA), differential scanning calorimetric (DSC), and impedance analysis were used to study the reported high-temperature phase transitions at 107, 149, 230, and 256 °C in crystals of cesium dihydrogen phosphate, CsH2PO4 (CDP). Our results show strong evidence that at all these temperatures, the observed DSC or differential thermal analysis (DTA) endothermic effects appear only as a consequence of a dehydration process starting on the surface of the crystal. Our results thus show that the reported transition at 230 °C is not a polymorphic transition. This means that the monoclinic symmetry, stable at room temperature, with space group P21/m–C2k2, is maintained up to the final decomposition. Moreover, since we have not found any evidence for the existence of a superprotonic high-temperature phase above 230 °C, the high conductivity above 230 °C is thus only a consequence of the dehydration of the crystal surface.

On the high-temperature phase transitions of CsH2PO4: A polymorphic transition? A transition to a superprotonic conducting phase?

On the high-temperature phase transitions of some KDP-family compounds: a structural phase transition? A transition to a bulk-high proton conducting phase?

The sequence of previously found phase transitions in CsHSO4 at around 60, 122, and 141 °C was carefully examined by using thermo-gravimetric analysis (TGA), mass spectroscopy (MS), differential thermal analysis, ac calorimetry, impedance spectroscopy, and x-ray diffraction (XRD). Our results show evidence that at all these transition temperatures, the dehydration processes take place at or very near the surface of the crystal. As a consequence, our results support the phase of CsHSO4 above 141 °C being not a superprotonic-conducting phase but rather a mixture of CsHSO4 and Cs2S2O7 caused by a partial thermal decomposition that includes the break-up of hydrogen bonds and the formation of H2O molecules.

https://iopscience.iop.org/article/10.1088/0953-8984/18/42/003/pdf

E. Ortiz

Papers

On the high-temperature phase transitions of CsH2PO4: A polymorphic transition? A transition to a superprotonic conducting phase?

On the high-temperature phase transitions of some KDP-family compounds: a structural phase transition? A transition to a bulk-high proton conducting phase?

Phase behaviour of the solid proton conductor CsHSO4

On the high-temperature phase transition of RbH2PO4—A polymorphic transition?

On the High Temperature Phase Transitions in Dihydrogen Phosphates