Ligand-based strategies for improving the electrocatalytic performance of Co-N4-C materials for hydrogen evolution reaction: A theoretical insight

Given the imminent environmental challenges and energy crises, advancing the hydrogen evolution reaction (HER) electrocatalysis is imperative for establishing a clean and sustainable energy supply. Here, using density functional theory (DFT) calculations, we report a single cobalt (Co) atom embedded between the N 4 moiety in graphene, and axial ligands (namely, N 4 -C-Co@L), which acts as an effective single-atom catalyst (SAC) for the HER. Axial ligand doping (positive, negative, and aromatic) is crucial for promoting HER performance; therefore, binding energy, formation energy, charge density difference (CDD), full width and half maximum (FWHM), density of states, d-band theory, crystal field theory (CFT), and Fermi level are studied in detail for innovative 2D N 4 -C-Co@L materials. Among the axial ligands studied, Co-N 4 -C@NCS - exhibited excellent stability, low overpotential, and high selectivity, making it a promising candidate for HER, oxygen evolution reaction, and oxygen reduction reaction (OER/ORR). The results indicate that Co-N 4 -C supports itself as an active site for OER and ORR; thus, Co-N 4 -C@NCS - is proposed as a potential multifunctional catalyst for HER/OER/ORR (-0.01 eV/0.56 V/0.44 V), respectively. The study provides a new rational design for high-efficiency HER catalysts and a deeper understanding of HER reactions driven by N 4 -C-Co@L catalysts. • Axial ligands on Co-N₄-C to understand their impact on structural stability. • Modulate the electronic environment of the active site, tuning catalytic reactivity. • Evaluation of d -orbital states of the Co center to determine their role in catalytic activity. • Role of d z ² orbital in catalytic performance and its interaction with adsorbed species. • Bifunctional OER/HER performance, confirming its potential in energy conversion reactions.