Preparation of Yolk–Shell-Structured Co_xFe_1−xP with Enhanced OER Performance

The design and development of low-cost, highly efficient, and stable electrocatalysts to take the place of noble-metal catalysts for the oxygen evolution reaction (OER) remain a significant challenge. Herein, the synthesis of yolk–shell-structured binary transition metal phosphide Co_xFe_1−xP with different Co/Fe ratios by phosphidation of a cobalt ferrite precursor is reported. The as-synthesized Co_xFe_1−xP catalysts were used for the OER. All yolk–shell Co_xFe_1−xP catalysts with different Co/Fe ratios showed much better performance than the corresponding solid catalyst. The formation of Co oxides on the catalyst surface during OER and the optimal Co/Fe ratio were found to be critical to their activity. Among the as-prepared Co_xFe_1−xP catalysts, that with a Co/Fe ratio of 0.47/0.53 (Co_0.47Fe_0.53P) exhibited the best performance. Co_0.47Fe_0.53P has an overpotential of 277 mV at a current density of 10 mA cm⁻², a Tafel slope of 37 mV dec⁻¹, and superior stability in alkaline medium. The outstanding performance is partly ascribed to the transfer of valence electrons from Co to P and Fe. The Co_0.47Fe_0.53P matrix with excellent conductivity and Fe phosphate that is stable on the surface of the catalyst are also helpful for the OER performance. In addition, the yolk–shell structure of Co_0.47Fe_0.53P increases the contact area between electrolyte and catalyst. These characteristics of Co_0.47Fe_0.53P greatly improve its OER performance. This optimized binary transition metal phosphide provides a new approach for the design of nonprecious-metal electrocatalysts.