C_9N_4 as excellent dual electrocatalyst: A first principles study

We perform first principles calculations to investigate the catalytic behavior of C₉N₄ nanosheet for water splitting. For the pristine C₉N₄, we find that, at different hydrogen coverages, two H atoms adsorbed on the 12-membered ring and one H atom adsorbed on the 9-membered ring show excellent performance of hydrogen evolution reaction (HER). Tensile strain could improve the catalytic ability of C₉N₄ and strain can be practically introduced by building C₉N₄/BiN, and C₉N₄/GaAs heterojunctions. We demonstrate that the HER performance of heterojunctions is indeed improved compared with that of C₉N₄ nanosheet. Anchoring transition metal atoms on C₉N₄ is another strategy to apply strain. It shows that Rh@C₉N₄ exhibits superior HER property with very low Gibbs free energy change of -30 meV. Under tensile strain within ～2%, Rh@C₉N₄ could catalyze HER readily. Moreover, the catalyst Rh@C₉N₄ works well for oxygen evolution reaction (OER) with an overpotential of 0.58 V. Our results suggest that Rh@C₉N₄ is favorable for both HER and OER because of its metallic conductivity, close-zero Gibbs free energy change, and low oneset overpotential. The outstanding performance of C₉N₄ nanosheet could be attributed to the tunable porous structure and electronic structure compatibility.