Metal-Free Doping Strategies in Two-Dimensional Carbon Nitride C₄N₂ for Enhanced Hydrogen Evolution Catalysis

This study investigates the structural, electronic, and catalytic properties of pristine and doped C₄N₂ nanosheets as potential catalysts for the hydrogen evolution reaction (HER). The pristine C₃₆N₁₈ nanosheets exhibit limited HER activity, primarily due to high positive Gibbs free energies (>2.2 eV). We explored doping it with B, Si, or P atoms at the nitrogen site to enhance catalytic performance. Among these systems, B-doped C₃₆N₁₇ nanosheets exhibit the most promising catalytic activity, with a Gibbs free energy close to zero (≈-0.2 eV), indicating efficient hydrogen adsorption. Band structure, projected density of states (PDOS), charge density, and Bader charge analyses reveal significant changes in the electronic environment due to doping. Although stacking configurations (AA'A^″ and ABC) have a minimal effect on catalytic performance, doping, particularly with B, substantially alters the electronic structure, thus optimizing hydrogen adsorption and facilitating an efficient hydrogen evolution reaction.