Computational screening of transition-metal single atom doped C₉N₄ monolayers as efficient electrocatalysts for water splitting

The search for high efficiency and low-cost catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is vital to overall water splitting. In this work, on the basis of first-principles calculations, we screened a series of late transition metal atoms supported on a C₉N₄ monolayer (TM@C₉N₄, where TM represents Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt) as electrocatalysts for both the HER and OER. Our results demonstrate that the TM atoms can be bonded with the nitrogen atoms around the hole to form stable structures, and the bonded TM atoms are stable against diffusion. Co@C₉N₄ exhibits high catalytic activity toward the HER. In particular, the N active sites in the Co@C₉N₄, Ni@C₉N₄, and Pt@C₉N₄ systems demonstrate relatively high performance for the HER. However, Co@C₉N₄ and Pt@C₉N₄ exhibit low OER activities with large overpotentials. Among the ten cases of TM@C₉N₄ considered here, only Ni@C₉N₄ performs as a promising bifunctional electrocatalyst with N and Ni atoms as catalytic active sites for the HER and OER, with a calculated hydrogen adsorption Gibbs free energy (ΔG_H∗) of-0.04 eV and an OER overpotential (η^OER) of 0.31 V. The results demonstrate that TM@C₉N₄ is a promising single-atom catalytic system, which can be used as the non-noble metal bifunctional electrocatalyst for overall water splitting.