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Solid acids as fuel cell electrolytes


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 °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 °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 °C). Here we show that a cell made of a CsHSO4 electrolyte membrane (about 1.5 mm thick) 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. 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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Figure 1: The conductivities of selected solid acids, shown in Arrhenius form.
Figure 2: Cell voltage versus current density (polarization curve) for a CsHSO4 fuel cell.
Figure 3: Conductivity of CsHSO4 as a function of time at 146 °C and under humidified air.
Figure 4: Weight as a function of time of CsHSO4 at 160 °C.

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We thank J. Snyder for assistance with computer automation of the fuel cell test station and S. R. Narayanan for technical discussions. Financial support has been provided by the California Institute of Technology through the Yuen Grubstake entrepreneurial fund.

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Correspondence to Sossina M. Haile.

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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. (JPG 39 kb)

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Haile, S., Boysen, D., Chisholm, C. et al. Solid acids as fuel cell electrolytes. Nature 410, 910–913 (2001).

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