Cell voltage versus current density (polarization curves) for several CsHSO4 fuel cells with differing Pt loadings operated under the conditions indicated. Platinum content was varied by changing the thickness of the electrode layer while maintaining the overall composition.

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.

CsH 2 PO 4 preparation. Cesium dihydrogen phosphate powder was synthesized by methanol-induced precipitation from aqueous solutions of stoichiometric quantities CsCO 3 and H 3 PO 4 . The resulting precipitate was dried under vacuum at 100 °C.Thermal analysis. Simultaneous gravimetric analysis and differential scanning calorimetry was performed using a Netzsch Jupiter 449c, equipped with a Balzers AMU 200 mass spectrometer for exhaust gas analysis. Data were collected from 30 to 400 °C

CsH 2 PO 4 preparation. Cesium dihydrogen phosphate powder was synthesized by methanol-induced precipitation from aqueous solutions of stoichiometric quantities CsCO 3 and H 3 PO 4 . The resulting precipitate was dried under vacuum at 100 °C.Thermal analysis. Simultaneous gravimetric analysis and differential scanning calorimetry was performed using a Netzsch Jupiter 449c, equipped with a Balzers AMU 200 mass spectrometer for exhaust gas analysis. Data were collected from 30 to 400 °C

We demonstrate cathodes for solid acid fuel cells fabricated by vapor deposition of platinum from the metalorganic precursor Pt(acac) 2 on the solid acid CsH 2 PO 4 at 210 °C. A network of platinum nanoparticles with diameters of 2-4 nm serves as both the oxygen reduction catalyst and the electronic conductor in the electrode. Electrodes with a platinum content of 1.75 mg/cm 2 are more active for oxygen reduction than previously reported electrodes with a platinum content of 7.5 mg/cm 2 . Electrodes containing <1.75 mg/cm 2 of platinum show significantly reduced catalytic activity and increased ohmic resistance indicative of a highly discontinuous catalytic-electronic platinum network.