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

Haile, Sossina M. and Chisholm, Calum R. I. and Sasaki, Kenji and Boysen, Dane A. and Uda, Tetsuya (2007) Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes. Faraday Discussions, 134 . pp. 17-39. ISSN 1359-6640. doi:10.1039/b604311a.

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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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Haile, Sossina M.0000-0002-5293-6252
Additional Information:© Royal Society of Chemistry 2007 Received 24th March 2006, Accepted 4th May 2006. First published on the web 7th August 2006. This work has been supported by the US National Science Foundation, Division of Materials Research and the US Department of Energy through a subcontract via the DOE-funded Cornell Fuel Cell Institute, of Cornell University. Faraday Discussion 134: Atomic Transport and Defect Phenomena in Solids. Faraday Discussion 134: Atomic Transport and Defect Phenomena in Solids. See also: Solid acid proton conductors: From laboratory curiosities to fuel cell electrolytes. Sossina Haile reviews the scientific and technological status of selected solid acids.
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Deposited On:05 Jan 2007
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