Site and Configuration-Dependent Oxygen Evolution on Pristine g-C3N4: First-Principles Insights Into Intrinsic Active-Site Heterogeneity

Abstract

Graphitic carbon nitride (g-C₃N₄) is a promising metal-free electrocatalyst for the oxygen evolution reaction (OER), yet the intrinsic site-dependent reactivity of pristine g-C₃N₄ remains insufficiently characterized. Here, first-principles density functional theory was used to investigate OER at three adsorption sites: the Hollow site, Top C site, and Top N site, considering OH*, O*, and OOH* intermediates. OH* and O* adsorb favorably at all sites, whereas OOH* binds thermodynamically only at Top C (−0.07 eV) and is endothermic at Hollow (+1.83 eV) and Top N (+2.58 eV). Enumeration of all 27 possible site-assignment permutations along the four-step OER pathway using the computational hydrogen electrode (CHE) convention yields nine overpotential groups ranging from 2.18 to 5.80 V. The global minimum overpotential is 2.18 V when both OH* and OOH* occupy Top C, with the first water oxidation step as the potential-determining step at an onset potential of 3.41 V. For every configuration, the four reaction free energies sum to 4.92 eV at U = 0 V, confirming thermodynamic consistency. Bader charge and partial density of states analyses further identify Top C as the most electronically active site, providing a quantitative baseline for rational g-C₃N₄ OER catalyst design.