PtP2 nanoparticles on N,P doped carbon through a self-conversion process to core–shell Pt/PtP2 as an efficient and robust ORR catalyst

Abstract

Proton-exchange membrane fuel cells have been reported as one of the most promising substitutes for fossil fuels. However, limited oxygen reduction reaction (ORR) kinetics on the cathode still remains the main bottleneck for commercialization. We report the synthesis of size-controllable monodisperse PtP₂ nanoparticles (NPs) on nitrogen- and phosphorus-doped carbon (NPC) with initial ORR mass and specific activities of 0.466 A mg_Pt⁻¹ and 0.438 mA cm⁻² at 0.9 V in 0.1 M HClO₄ solution, via a combined template and pyrolysis method. The activities increase to 0.724 A mg_Pt⁻¹ and 0.508 mA cm⁻² after 3000 potential cycles and remain stable for a further 20 000 cycles, exhibiting an obvious improvement over the commercial Pt benchmark (0.142 mA cm⁻² and 0.098 A mg_Pt⁻¹ at 0.9 V). The facilitated ORR activity of PtP₂@NPC after incipient cycles of the stability test is due to the self-conversion process from PtP₂ to core–shell Pt/PtP₂ with a thin (≈1 nm) Pt shell and the consequential geometric and strain effects of the Pt skin which contribute to both the robustness and catalytic efficiency of the catalysts. Meanwhile, a four-electron pathway towards the ORR also indicates the high selectivity of our catalyst. Combined computational analysis indicates that the core–shell structure intensifies ORR activity by a more feasible rate-determining step and lower d-band center value.