Hierarchical nickel–cobalt phosphide yolk–shell spheres as highly active and stable bifunctional electrocatalysts for overall water splitting

To improve the efficiency of overall water splitting, highly active and stable bifunctional electrocatalysts are highly desirable. Herein, we fabricated mixed Ni-Co phosphides (Ni_1-xCo_x-P) as bifunctional catalysts for overall water splitting. Structural characterizations indicated that the Ni_1-xCo_x-P catalysts (0 > x > 1) exhibited hierarchical yolk-shelled morphologies, with a total diameter of 1-2 μm. Interestingly, the shell was assembled by numerous nanosheets with a thickness less than 10 nm. The electrochemical measurements indicated that the Ni_1-xCo_x-P catalysts (0 < x < 1) showed significantly enhanced OER and HER activities in comparison to the pure Ni-P and Co-P catalysts, and the highest OER and HER activities were achieved as x = 0.31. To drive a current density of 10 mA cm^-2, the Ni_0.69Co_0.31-P catalyst required an overpotential of 266 mV for OER and 96 mV for HER, respectively. The alkaline water electrolyzer with the Ni_0.69Co_0.31-P catalysts as the cathode and anode catalysts required a cell voltage of ca. 1.59 V to achieve a current density of 10 mA cm^-2, which was comparable to the integrated performance of commercial Pt/C and IrO₂. Furthermore, the electrolyzer assembled by the bufictional electrocatalysts showed a more stable performance than one assembled by commercial Pt/C and IrO₂ operated at the simialr current density. The superior activity and long-term stability demonstrate that the hierarchical mixed Ni-Co phosphides have promising potential for application in large-scale water splitting.