Synergistically Enhanced Oxygen Reduction Electrocatalysis by Subsurface Atoms in Ternary PdCuNi Alloy Catalysts

For Pd-based alloy catalysts, the selection of metallic alloying elements and the construction of composition-gradient surface and subsurface layers are critical in achieving superior electrocatalytic activities in, e.g., the oxygen reduction reaction (ORR). Based on the Pd-containing alloy, highly monodispersed PdCuNi ternary alloy nanocrystals are prepared through a wet-chemical approach, and a solution-based oxidative surface treatment protocol is utilized to activate the surface of the nanocrystals. A drastically enhanced ORR activity can be achieved by removing the surface Ni and Cu atoms through the surface treatment protocol. The treated catalyst demonstrates a mass activity of 0.45 A mg_Pd ⁻¹ in alkaline medium, 5 and 2.4 times those of commercial Pt/C and Pd/C, respectively. The first-principle calculation result suggests the critical roles of the coexistence of Ni and Cu atoms and their synergistic interaction beneath the outmost pure Pd layer in optimizing the oxygen binding energy for ORR. The calculation also suggests that the optimal binding energy of oxygen requires an appropriate Ni/Cu ratio in the subsurface layer. This work demonstrates a class of high-performance Pt-free ternary alloy ORR catalysts and may provide a general guideline for the structural design of Pd-based ternary alloy catalysts.