Rhombohedral Ordered Intermetallic Nanocatalyst Boosts the Oxygen Reduction Reaction

Abstract

Low platinum-group-metal (PGM) ordered intermetallic catalysts have been considered one of the most promising candidates for catalyzing the oxygen reduction reaction (ORR) in fuel cells, but achieving the desired performances in terms of activity, durability, and cost is still a grand challenge for the fuel cell research field. While the cubic- and tetragonal-ordered (L12 and L10 phases) low-PGM nanocatalysts have been investigated extensively and have shown considerably improved durability and activity toward the ORR, so far, little is known about rhombohedral ordered L11 catalysts. We report the synthesis of an L11-ordered PtCu catalyst for the ORR. We demonstrate that by applying nitrogen (N) doping through a thermal treatment in an NH3 gas, the activity and stability of the N-doped, rhombohedral ordered PtCu catalyst can be further enhanced, and the ORR mass and specific activities achieve nearly 5-fold and 4-fold enhancement in acidic media, respectively, relative to those of commercial Pt/C. In situ synchrotron X-ray absorption and pair-distribution-function measurements reveal that both the formation of the ordered intermetallic structure and N-doping synergistically improve the corrosion resistance of the PtCu catalyst by lowering the Cu diffusivity and introduce a compressive strain effect regulating the adsorption of oxygenated species on the Pt surface, thus accounting for the improved ORR kinetics.