Copper doping tunes d-band center to enhance hydrogen evolution in global minimum Fe clusters on FeN4‒graphene

The creation of efficient and economical electrocatalysts for hydrogen evolution reaction (HER) is crucial for positioning hydrogen as a clean and sustainable energy carrier. Iron (Fe) is a viable candidate, due to its prevalence and affordability. We examine the effects of size and copper (Cu) doping on Fe clusters supported by FeN₄‒graphene substrate (Fe_n‒FeNC, n = 1–6) for the hydrogen evolution process utilizing density functional theory (DFT) simulations. The stable configurations of the Fe_n‒FeNC are determined using a global optimization technique. The results indicate that Fe clusters have positive stability owing to the robust electronic interactions with the substrate, with Fe₄–FeNC identified as the most stable configuration. Nevertheless, pure Fe clusters fail to attain ideal hydrogen adsorption free energy for the HER, thereby constraining their catalytic activity. To resolve this, Cu atoms are incorporated into the Fe₄ clusters, yielding the Fe₃Cu₁–FeNC, Fe₂Cu₂–FeNC, and Fe₁Cu₃–FeNC models. The introduction of Cu efficiently adjusts the hydrogen adsorption strength by altering the d-band center, enabling Fe₂Cu₂–FeNC to attain an optimal distribution of active sites with adsorption free energies |△G_∗H| < 0.1 eV. This study emphasizes the structural and electrical determinants affecting the hydrogen evolution reaction activity of iron clusters on FeN₄‒graphene. This highlights the capability of Cu alloying to improve catalytic performance via electronic modulation.