Zinc-Induced Ordering in L10-Type Platinum-Based Nanoalloys for the Electrocatalytic Oxygen Reduction Reaction

Abstract

Platinum (Pt)-based intermetallic compound (IMC) nanoparticles (NPs) show high activity and durability in the oxygen reduction reaction (ORR) even in the highly acidic environment of polymer electrolyte fuel cells. Although Pt-based IMCs with L1₀ crystal structure have been thoroughly studied, further improvement of ordering degree is required to enhance their ORR catalytic performance. Typical syntheses of L1₀-ordered PtM (M is usually a 3d transition metal) IMC NPs require high-temperature annealing to induce the disorder-order transformation, which leads to unfavorable NP aggregation. Given that L1₀-PtZn has a much lower phase transformation temperature than other L1₀-PtM, Zn was included to allow the phase transformation of L1₀-Pt(M,Zn) NPs to occur at relatively low temperature, thus suppressing aggregation and increasing the average ordering degree of NPs. We fabricated sub-6-nm L1₀-Pt(M,Zn) NPs with a high ordering degree (>80%) through a facile wet chemical route followed by low-temperature annealing. Lattice constant tuning achieved by precise composition control revealed a positive correlation between the strain effect and corresponding ORR activity. Highly ordered L1₀-Pt₅Co₄Zn₁ (formed at 600 °C) and L1₀-Pt₅Ni₄Zn₁ (formed at 550 °C) NPs showed high specific activities of 1.71 and 1.40 mA/cm²_Pt, respectively, which are almost comparable to that of L1₀-PtCo NPs (formed at 650 °C).