Cation-Modulated\nNi/Ni<sub>3</sub>N Compound Heterojunctions\nas Highly Efficient Bifunctional Electrocatalysts for Water Splitting

The exploration and rational design of high-performance,\ndurable,\nand non-precious-metal bifunctional oxygen electrocatalysts are highly\ndesired for the large-scale application of overall water splitting.\nHerein, an effective and straightforward coupling approach was developed\nto fabricate high-performance bifunctional OER/HER electrocatalysts\nbased on core–shell nanostructure comprising a Ni/Ni<sub>3</sub>N core and a NiFe(OH)<sub><i>x</i></sub> shell. The as-prepared\nNi/Ni<sub>3</sub>N@NiFe(OH)<sub><i>x</i></sub>-4 catalyst\nexhibited low overpotentials of 57 and 243 mV at 10 mA cm<sup>–2</sup> for the HER and OER in 1.0 m KOH, respectively, superior to most\nbifunctional oxygen electrocatalysts reported so far. Compared to\nthe unmodified Ni/Ni<sub>3</sub>N, the Ni/Ni<sub>3</sub>N@NiFe(OH)<sub><i>x</i></sub>-4 catalyst exhibited a 43.3-fold increase\nin mass activity for the OER and an 8.7-fold increase for the HER,\nas well as a 29.5-fold increase in intrinsic activity for the OER\nand a 2.6-fold increase for the HER. When employed as both the cathode\nand the anode of the electrolyzer for the overall water splitting\nreaction, its voltage was reduced to 1.58 V at 10 mA cm<sup>–2</sup>. This surface reconstruction method increased the electrochemically\nactive surface area and enhanced the catalytic activity. Furthermore,\nin situ Raman spectroscopy revealed that the Fe etching reduced the\nonset potential for the active phase NiOOH, promoted its formation,\nand accelerated the reaction kinetics, thereby enhancing the overall\nelectrocatalytic performance of the catalyst.