Atomic-Level Insight into Ni Clusters Supported on N-Terminated Diamond (111) Surface for HER, OER, and ORR

Single-cluster catalysts (SCCs), an emerging frontier between single-atom catalysts and conventional metal nanoparticle catalysts, have attracted significant attention due to their unique geometric and electronic structures. Herein, we systematically investigate the structural models and the hydrogen-evolution reaction (HER), oxygen-evolution reaction (OER), and oxygen-reduction reaction (ORR) performances for Ni_x clusters (x = 2-9) anchored on a N-terminated diamond (111) surface (Ni_x@ND) through Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO)-based structural prediction and density functional theory (DFT) calculations. The Ni-H bonding strength serves as a critical descriptor for HER activity, where a balanced relative strength of ad-/desorption is beneficial for enhancing it. The Gibbs free energy change (ΔG_*H) of the Ni₇@ND catalyst is -0.25 eV, demonstrating optimal HER performance with an efficiency approaching that of highly efficient Pt-based catalysts. Furthermore, Ni₄@ND and Ni₂@ND catalysts demonstrate superior OER and ORR performance with low overpotentials (η), which are significantly lower than those of single-atom catalyst Ni₁@ND. Volcano curve analysis reveals that the OER performance is maximized at the intermediate adsorption strength of key reaction species (*O, *OH). A pronounced linear correlation is found between η^ORR and adsorption energies of intermediate *OH. Furthermore, size effects in SCCs narrow the band gap and increase the number of active sites, thus promoting the catalytic performance (optimizing charge redistribution and reducing the rate-determining step barrier). This design strategy provides atomic-level insights into highly efficient diamond-based SCCs for HER, OER, and ORR.