Boosting the water splitting activity of cobalt nitride through morphological design: a comparison of the influence of structure on the hydrogen and oxygen evolution reactions

Abstract

To deal with the relatively low HER catalysis efficiencies of existing cobalt nitride (CoN) catalysts, it is highly desirable to design new types of CoN catalysts to realize high-efficiency water splitting. Herein, by using simple hydrothermal and nitridation methods, we realize the synthesis of novel three-dimensional (3D) flower-like CoN (CoN-F), CoN porous spheres (CoN-S), and polyhedral CoN (CoN-P) via utilizing various weak bases. Benefiting from a much larger surface area and a unique 3D flower-like architecture (consisting of 2D porous CoN nanosheets), the CoN-F catalyst shows highly exposed active sites and superior mass/electron transfer abilities. Naturally, the CoN-F catalyst exhibits more outstanding hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance than either CoN-P or CoN-S. In order to achieve a current density of 10 mA cm⁻², 3D flower-like CoN-F needs only low overpotentials of 191.7 mV and 276.2 mV for the HER and OER, respectively. Furthermore, upon using the 3D flower-like CoN-F catalyst as both the anode and cathode material in a two-electrode overall water-splitting cell, the corresponding (−)CoN-F‖CoN-F(+) cell achieves a current density of 10 mA cm⁻² at 1.613 V in 1.0 M KOH, which is just 10 mV larger than a (−)Pt/C‖RuO₂(+) cell (1.592 V). It outperforms most existing non-precious-metal electrocatalysts, demonstrating superior water-splitting efficiency. In addition, the 3D flower-like CoN-F catalyst shows excellent stability during the alkaline HER and OER, and full water splitting. This work successfully opens a new pathway for boosting the HER/OER electrocatalytic abilities of CoN catalysts through structural design, which may promote the efficient utilization of metal nitrides in various energy storage and conversion devices.