Alternatives to Pt as the Electrode Catalysts for Polymer Electrolyte Membrane Fuel Cells

Various non-precious metal materials are examined in this study for their possible use in polymer electrolyte membrane fuel cells (PEMFC) to promote both fuel oxidation at the anode and oxygen reduction reaction (ORR) at the cathode. A Pt-Mo alloy is studied in detail and compared to pure Pt as the anode catalyst for its catalytic activity in activating the C-C bond of ethanol and tolerance to C1 species poisoning. It is found that the introduction of surface Mo atoms to Pt enhances upon pure Pt in both of these two aspects. On the reduction electrode side, a class of organometallic complexes known as transition metal nitrogen-doped carbon (TM-N-C) materials was studied as possible substitutes for Pt. It is found that similar to the natural cytochrome c enzyme that inspires their discovery, the TM-N-C materials can achieve ORR activities rather close to that of Pt. Moreover, the 2e- incomplete reduction and 4e- complete reduction of oxygen can be selectively promoted by employing different metal centers due to different electron backdonating capabilities of transition metals. A further study of TM-N-C structures residing between various graphene edge structures showed even higher activities towards O-O bond activation. However, such enhanced O-O cleavage also results in severe O* poisoning. Three N-doped carbon systems without the presence of metal elements were further examined in this study to account for ORR activities recently observed in experiments on these materials. It is found that the catalytic activity of the active carbon site is closely related to the N-doping configurations, with the graphitic-N doping configurations generally leading to better ORR performances.