Cation-Modulated Ni/Ni₃N Compound Heterojunctions as Highly Efficient Bifunctional Electrocatalysts for Water Splitting

The exploration and rational design of high-performance, durable, and non-precious-metal bifunctional oxygen electrocatalysts are highly desired for the large-scale application of overall water splitting. Herein, an effective and straightforward coupling approach was developed to fabricate high-performance bifunctional OER/HER electrocatalysts based on core-shell nanostructure comprising a Ni/Ni₃N core and a NiFe(OH)_<i>x</i> shell. The as-prepared Ni/Ni₃N@NiFe(OH)_<i>x</i>-4 catalyst exhibited low overpotentials of 57 and 243 mV at 10 mA cm^-2 for the HER and OER in 1.0 m KOH, respectively, superior to most bifunctional oxygen electrocatalysts reported so far. Compared to the unmodified Ni/Ni₃N, the Ni/Ni₃N@NiFe(OH)_<i>x</i>-4 catalyst exhibited a 43.3-fold increase in mass activity for the OER and an 8.7-fold increase for the HER, as well as a 29.5-fold increase in intrinsic activity for the OER and a 2.6-fold increase for the HER. When employed as both the cathode and the anode of the electrolyzer for the overall water splitting reaction, its voltage was reduced to 1.58 V at 10 mA cm^-2. This surface reconstruction method increased the electrochemically active surface area and enhanced the catalytic activity. Furthermore, in situ Raman spectroscopy revealed that the Fe etching reduced the onset potential for the active phase NiOOH, promoted its formation, and accelerated the reaction kinetics, thereby enhancing the overall electrocatalytic performance of the catalyst.