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Solid acids as fuel cell electrolytes
Nature
470(19.4.2001)910-913
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 approch 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 approzimately 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.
