Hierarchical Ni₂P/Cr₂CT_{: X} (MXene) composites with oxidized surface groups as efficient bifunctional electrocatalysts for overall water splitting

Electrocatalysis has the potential to become a more sustainable approach to generate hydrogen as a clean energy source. Developing effective electrocatalysts for water splitting is central to the area of renewable energy. Herein, we performed density functional calculations to predict the electrocatalytic performance of a Ni₂P surface and Ni₂P/Cr₂CO₂ (MXene) interface as bifunctional electrocatalysts and clarified the mechanisms of the improvement of the electrocatalytic activity of the Ni₂P/Cr₂CO₂ interface. The optimal overpotential of the Ni₂P surface for the oxygen evolution reaction is 0.95 V, and the ideal overpotential of the Ni₂P/Cr₂CO₂ interface for the oxygen evolution reaction is 0.80 V. The Gibbs free energy for the adsorption of atomic hydrogen (ΔG_H∗) on the Ni₂P/Cr₂CO₂ interface can reach -0.09 eV at a suitable hydrogen coverage. The HER processes of both Cr₂CO₂ and the Ni₂P/Cr₂CO₂ interface follow the Volmer-Heyrovsky mechanism with the activation energy barriers (E_a) of 0.76 eV and 0.68 eV, respectively. The Ni₂P/Cr₂CO₂ interface promotes charge transfer from the Ni atom to O atom, which weakens the interaction between Ni and intermediates and therefore improves the catalytic effect of Ni₂P for either the HER or OER. The present results indicate that the hierarchical Ni₂P/Cr₂CT_x composite could be an effective medium for achieving a promising bifunctional electrocatalysis for water splitting.