A major impediment to the commercialization of proton exchange membrane (PEM) fuel cells is the low activity of electrocatalysts for the oxygen reduction reaction (ORR). Platinum (Pt) is considered the best cathode catalyst toward four-electron reduction of oxygen to water in acidic environments. It also shows the lowest overpotential and the highest stability. However, Pt remains an expensive metal of low abundance, and it is thus of great importance to find Pt-free alternatives for PEM fuel cells.
University of South Carolina scientists Branko N. Popov, Nalini Subramanian and Hector R. Colon-Mercado have developed a low-cost, easily manufactured carbon-based catalyst having high activity, selectivity, and stability for ORR, according to U.S. Patent U.S. Patent 7618915. The optimized carbon composite catalyst showed no degradation of fuel cell performance even for 350 hours of continuous operation. The manufacturing process results in a highly active metal-free catalyst with support for carbon composite catalysts and the nano-sized transition metal catalyst used for the formation of nitrogen-rich active reaction sites for ORR in proton exchange membrane fuel cells.
They also developed a method of producing composite carbon catalysts. The method consists of providing a carbon precursor comprised of carbon black; oxidizing the carbon precursor; adding nitrogen functional groups to the oxidized carbon precursor; refluxing the oxidized carbon precursor with a non-platinum transitional metal precursor in a solution; and pyrolyzing the solution at a temperature of at least about 500.degree. C.
The pyrolysis in the presence of nitrogen-containing metal compounds followed by post-treatments greatly increased the concentrations of pyridinic and graphitic nitrogen functional groups on the catalyst surface, thereby enhancing the catalytic activity, selectivity and stability. The transition metal was utilized as a catalyst for formation of nitrogen-rich active reaction sites during the high-temperature pyrolysis.
The nitrogen functional groups are provided by polymerizing a nitrogen source in the presence of the oxidized carbon precursor. The nitrogen source comprises melamine, urea, thio-urea, selenourea, or combinations thereof. The metal of the non-platinum transitional metal precursor is chosen from cobalt, iron, chromium, copper, nickel, or vanadium.