Base material electrocatalysts for oxygen cathodes in low temperature acid fuel cells

Abstract

This thesis investigated the potential use of base material electrocatalysts for the oxygen cathode of a low-temperature acid fuel cell. Pt-based oxygen reduction electrocatalysts have so far been the only materials meeting the requirements of electrocatalytic activity, stability in the acidic electrolytes and electronic conductivity. However, Pt is expensive, in limited supply and only electrocatalyses the oxygen reduction reaction at overpotentials of at least 0.1 V. The importance of research into substitutes for Pt-based oxygen reduction electrocatalysts is thus evident. As investigated by potentiostatic Polarisation in 1.5 M H2SO4 at 20 °C and 70 °C in this work, a sputter-deposited Ni/Ta/C coating on a carbon paper support displayed both very good corrosion resistance and good oxygen reduction activity. A sputter- deposited Cr/Ta/C coating on a carbon cloth support and a C^Cö gas diffusion electrode displayed high cathodic oxygen currents and a high mixed potential. This indicates that with development these materials might offer an alternative to Pt-based electrocatalysts. In contrast, sputter-deposited Ag/Ta/C, Ni/Nb and Ni/Zr/C coatings showed good oxygen reduction activity, but their corrosion resistance was inferior to that of the sputter-deposited Ni/Ta/C. The oxygen reduction activity of sputter- deposited Co/Ta/C and Fe/Ta/C coatings was inferior to that of all other sputter- deposited materials. Most Ni/Ta, Ni/Ta/C, Co/Ta, Ni/Nb/C and Ni/W/C materials prepared via mechanical alloying or thermal synthesis corroded at relatively high rates, probably owing to corrosion of the elemental Ni, Co or Fe contained in these materials. Of the electrodes made from commercial base material powders, porous carbon Substrates and PTFE or Nafion binders, TaC and NbC showed relatively high oxygen currents and moderately low corrosion currents and thus promise as oxygen reduction electrocatalysts, while Ta, Ebonex, TaB, TaN, ZrN passivated via growth of a resistive film and ZrB2, TiB2 corroded. The cathodic oxygen currents on sputter-deposited Ni/Ta/C, Co/Ta/C and Fe/Ta/C coatings on a carbon cloth support decreased by 20%, 23% and 30%, respectively when 1 M CH3OH was added to the 1.5 M H2SO4 electrolyte. This indicated that the electrocatalytic activity for the undesired CH3OH oxidation reaction increased in the order Ni/Ta/C < Co/Ta/C < Fe/Ta/C.