Abstract
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.
Highlights
Solid acid proton conductors, based on tetrahedral oxyanion groups, have received attention as electrolytes in generation fuel cells
Unlike the polymers in more conventional proton exchange membrane fuel cells (PEMFCs), proton conduction in oxyanion solid acids does not rely on the migration of hydronium ions
The temperatures of operation accessible to fuel cells based on solid acids imply that catalysis rates will be enhanced relative to PEMFCs, opening up possibilities for reduction in precious metal loadings or even the elimination of precious metals entirely
Summary
Such as CsHSO4,1 Rb3H(SeO4)[2,2] and (NH4)3H(SO4)[2,3] exhibit anhydrous proton transport with conductivities of the order of 10À3 to 10À2 S cmÀ1 at moderate temperatures (120–300 1C). Wada subsequently confirmed the 230 1C transition by dilatometry measurements of single crystal samples, observing a sharp increase in lattice constants at this temperature.[12] Almost simultaneously, Gupta reported, again on the basis of calorimetry and thermal gravimetric analysis, a polymorphic transition in CsH2PO4 at 235 1C just prior to the maximum in the decomposition process.[13] Again, initiation of the weight loss coincided with the reported polymorphic transition In contradiction to these results, Nirsha et al published a study two years later concluding that thermal events at 233 1C and higher in CsH2PO4 are entirely due to decomposition.[14].
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