Exploring the catalytic activity of MXenes M_n+1C_nO₂ for hydrogen evolution

Currently, finding the non-precious metal catalyst for hydrogen evolution reaction (HER) is significant and urgent. By performing the first-principles calculations, we studied the structural and electronic properties, as well as the catalytic activity of MXenes M_n+1C_nO₂ (n = 1, 2; M = Sc, Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta and W) toward HER. These 22 MXenes are metallic except for Sc₂CO₂, Ti₂CO₂, Mn₂CO₂, Zr₂CO₂ and Hf₂CO₂ which are semiconducting. By calculating the Gibbs free energy change (ΔG_H) for H adsorption which is the simple but effective descriptor for HER catalytic activity, and analyzing the electrical conductivity, we find that Nb₂CO₂, W₂CO₂, Zr₃C₂O₂ and W₃C₂O₂ are catalytic active for HER at low H coverage. Particularly, Nb₂CO₂ and Zr₃C₂O₂ maintain to be active for catalyzing HER under tensile strain less than 5%. And Nb₃C₂O₂ shows enhanced catalytic activity for HER under 3–5% tensile strain. Additionally, the differential ΔG_H and average ΔG_H were used to evaluate the HER catalytic activity at high H coverage (1/8–6/8). Our calculations show that Nb₂CO₂ is active for HER at H coverage less than 3/8. Plus, V₂CO₂, Ti₃C₂O₂ and Cr₃C₂O₂ demonstrate the catalytic activity for HER at high H coverage.