(Invited) Nano-Structured ORR Catalysts for PEM Fuel Cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are the most attractive power source for zero-emission vehicles owing to their environmental friendliness, high energy efficiency, and high power density. However, their high cost and low durability have prevented PEMFCs from being fully implemented and commercialized, mainly due to the use of platinum as a catalyst to overcome the sluggish kinetics of the oxygen reduction reaction (ORR) in the cathode. To reduce the use of Pt, extensive studies have been conducted and a large variety of ORR catalysts have been developed, such as Pt-M alloys as well as shape-controlled, dealloyed, and structurally ordered catalysts. Although those catalysts exhibited excellent intrinsic activity in aqueous environment, they have been hardly implemented in membrane-electrode assemblies (MEAs) for practical application. In this talk, we report Pt-M intermetallic catalysts and M-doped PtNi octahedral nanoparticles on a carbon support (M-PtNi/C) that exhibit superior catalytic activity and stability in half-cell and MEA tests as an ORR catalyst for PEMFCs. To prevent agglomeration of Pt-M nanoparticles during heat treatment in which to transform the Pt-M alloy into Pt-M intermetallic structure, a facile synthetic process was conducted at low temperatures. Octahedral PtNi nanoparticles are known as one of the best ORR catalyst with its superior activity toward ORR. However, dissolution of Ni from the surface and collapse of the octahedral shape degrade the activity rapidly under the PEMFC operation conditions. To stabilize Ni and the octahedral shape, small amount of dopant was incorporated into the surface of octahedral PtNi. Electrochemical, chemical, and structural stability was thoroughly investigated through a harsh durability test. The effects of intermetallic structure and doping elements on the surface properties of Pt-based nanoparticles were examined by X-ray absorption fine structure (XAFS) analysis and density functional theory (DFT) calculation.