Development of electrocatalysts for glycerol oxidation

Glycerol is a very promising alternative fuel to hydrogen in fuel cells. However, the utilisation of glycerol as a fuel requires a good catalyst, due to the slow kinetics of glycerol electrooxidation. Gold has been identified as a promising catalyst due to its high activity and stability for glycerol electrooxidation – although the overpotentials are higher than on platinum and palladium. Modification of a nano-Au/C catalyst by the addition of MnO2, in an attempt to further improve the activity and lower the overpotential for glycerol oxidation, was therefore first explored. This was followed by investigations into the effects of gold particle size and loading. Finally, the effect of gold particle size on oxidation of gold-catalysed glycerol oxidation intermediates was also briefly explored. Studies into MnO2 addition showed that the pre-deposition of MnO2 yielded catalysts with smaller, more uniform gold particles, and catalysts with MnO2 contents of 5 and 9 wt % had higher mass activities and lower onset- and peak- potentials than Au/C. All the Au/xMnO2/C catalysts were more active than the palladium- and platinum-based catalysts reported in literature, which effectively demonstrated the advantage of using a gold-based catalyst for glycerol oxidation – especially when supported by MnO2 which lowered the overpotential for glycerol oxidation over gold. For the study into gold particle size, small gold particles of average diameter ≤ 4.7 nm had higher gold mass-based activities than medium-sized (14.7 nm) particles and were at least twice as active as catalysts containing large (≥ 43 nm) gold particles. The small gold particles also gave lower glycerol oxidation onset potentials, which was attributed to the predominance of Au(110) planes on those particles. Glycerol oxidation also appeared to proceed further along the oxidation pathway over small gold particles, which was confirmed in preliminary studies into the oxidation of glycerol oxidation intermediates. However, specific activity increased with increasing gold particle size, due mainly to the higher intrinsic activity of the Au(111) plane, which increased relative to Au(110) with increasing gold particle size. The important requirements for fuel cell applications are factors such as high mass activity, low overpotentials and high stability – all of which were met by the catalysts containing small gold particles defined by predominantly Au(110) facets. Investigations into the gold loading effect showed similar mass- and specific- activities for catalysts with 5-20 % gold loading. However, only the catalysts with higher gold loadings (15-20 %) did not deactivate early during CV, indicating that a larger gold surface area is necessary to resist poisoning at high potentials. On the basis of low onset potentials, high mass activity, and stability at low overpotentials, a minimum gold loading of 12.5 % appears to be necessary for a supported gold catalyst with small gold nanoparticles; although even higher loadings may be preferable for a higher power output in a fuel cell. Importantly, the insights gleaned from this study on the fundamental properties required for early activation, activity and stability of the gold catalysts could lead to a more intelligent design of gold-based catalysts in future