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 oC; 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
250oC). 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
125oC). Here we show that a cell made of a CsHSO4
electrolyte membrane (about 1.5 mm thick) operating at 150-160oC
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.
