Enhancing HER performance nitrogen defects: a comparative DFT study of Fe and Ru single-atom catalysts on graphene.

Exploring high-performance catalysts for the hydrogen evolution reaction (HER) is essential for the development of clean hydrogen energy. Single atom catalysts (SACs) have garnered significant attention due to their maximum atomic efficiency, high catalytic performance and excellent selectivity. In this work, we systematically investigated the HER activity of Ru and Fe SACs on nitrogen-doped graphene using density functional theory (DFT) calculations. Various nitrogen defect configurations (N to 4N) were examined to access their impact on structural stability and catalytic performance. The results indicate that lower N-coordinated moieties, particularly N_pyrrolic and 2N, exhibit superior HER activity, while high N-coordinated moieties (4N) demonstrate greater stability. Volcano plot analysis reveals that catalytic performance is highly sensitive to metal-support interactions, which can be effectively described using binding energy and metal charge state. Ru SACs@N_pyrrolic and Fe SACs@2N achieve the most favorable performance, with additional active sites and low overpotentials of approximately 0.26 V and 0.23 V, respectively. Bader charge analysis further confirms that moderate positive charge states enhance electronic metal-support interactions, optimizing hydrogen adsorption and desorption. These findings highlight the critical role of nitrogen coordination in tuning the electronic and energetic properties of SACs on N doped graphene, providing valuable insights into the rational design of Pt-free SACs for highly efficient HER catalysis.