| Hydrogen fuel cells are a promising energy conversion technology because they are capable of efficiently producing electricity from fuels while avoiding the production of environmental pollution when clean fuels such as hydrogen gas are used. However, Pt and other noble metals that are traditionally used as electrocatalysts for hydrogen fuel cells are scarce and expensive, and is a major factor limiting the commercialization of fuel cells. Therefore, we have searched for new non-noble metal cathode catalysts for the oxygen reduction reaction (ORR) of fuel cells. Three classes of materials have been investigated: the Nb-substituted titanates La2(Ti1-xNb x)2O7 with a 110-perovskite related structure, the Ti-vacancy phases La5Ti4O15 and La 9Ti7O27 with 111-perovskite related structures, and a solid solution of magnesium molybdenum oxynitrides, MgxMo 1-xOyNz, with rock salt related structures.;It was determined that the solubility limit for Nb in La2(Ti 1-xNbx)2O7 occurred for x ∼ 0.1. However, neither this phase nor the Ti-vacancy phases La5Ti 4O15 and La9Ti7O27 showed measureable activity for the ORR, though it is noted that these compounds may be suitable supports for other ORR electrocatalysts due to the conductivity derived from their partially-reduced Ti cations. In contrast, the magnesium molybdenum oxynitrides showed modest ORR activity in acidic conditions, and good activity in basic conditions when prepared on a carbon support (Mg xMo1-xOyNz/C). The mechanisms of these catalysts were investigated through electrochemical techniques, while a detailed structural investigation of MgxMo1-xOyN z compounds was carried out through the co-refinement of X-ray and neutron diffraction data studies of these nanoscale materials. The optimum synthetic temperature for maximum activity was determined for a variety of substitution levels. The best overall activity in both acidic and basic electrolytes was observed for Mg0.25Mo0.75OyNz/C synthesized at 700 °C. |
