Oxygen cathode for fuel cells. ORR activity of mesoporous N-modified carbon doped with non-noble metals

Fuel cells for low temperature applications are the subject of intensive research because they offer promise of reducing consumption of primary fossil fuels and greenhouse gas emissions. A difficulty comes from the kinetic behaviour of cathodic oxygen reduction that requires some form of effective catalytic promotion to proceed fast enough to meet fuel cell power requirements as for instance in the case of automotive transportation. Platinum and Pt-alloys are at present the best catalysts for oxygen reduction in PEM fuel cells. Nevertheless, due to the natural scarcity and cost of platinum, a very interesting challenge is to find an alternative non-precious, though less catalytically performing, material. Among others, nitrogen-modified carbons doped with non-precious transition centres (Fe, Co, etc.) are of interest because of the flexibility by which composition and morphology can be tailored by preparation. In this work we present some results on oxygen reduction by various Pt-free catalysts obtained by carbonization and calcination of nitrogen bases/sugar mixtures in the presence of a metal salt and a silica gel as mesopore templating agent. Catalysts were characterized by physical, chemical and electrochemical methods. Results are given in terms of the influence that sugars, nitrogen bases and metal centres exert on oxygen reduction potential, on the presence of a limiting current and on reaction mechanism. The best materials have been obtained by using fructose, a guanidine derivative and iron acetate. They are characterized by a well-defined limiting current, an onset potential approaching Pt ORR starting potential and a number of exchanged electrons n>3.9.