First-principles insights into site-dependent hydrogen and oxygen evolutions using azo-bridged C3N5 for water splitting

Catalytic water splitting offers a promising route for sustainable hydrogen production, and carbon nitrides have emerged as attractive alternatives to conventional metal-based catalysts. Here, we use density functional theory (DFT) to investigate the structural, electronic, and catalytic properties of the azo-bridged heptazine-based C 3 N 5 2D sheet for the water-splitting reaction. A detailed investigation of catalytic performance across the nitrogen and carbon atoms indicates pronounced site dependence. Gibbs free-energy analysis reveals site-dependent hydrogen evolution reaction (HER) behavior, with a limiting thermodynamic free-energy requirement of nearly 0.42 eV at the C3 site in the gas phase, comparable with other metal-free catalysts. Upon incorporating van der Waals interactions and solvent effects, N3 becomes the most favorable HER site, with a free-energy cost of 0.22 eV. In contrast, the N4 site exhibits the most favorable oxygen evolution reaction (OER) energy profile under an applied potential of 2.15 V. Nevertheless, detailed analysis of the free-energy pathways predicts a high overpotential of 2.08 V, indicating less favorable OER thermodynamics for the C 3 N 5 sheet. These findings provide molecular-level insight into the structure-activity relationships governing the catalytic behavior of C 3 N 5 and offer design principles for optimizing carbon nitride-based catalysts for water splitting.