Laser cladding rapid solidification technique is an effective strategy for manufacturing ultra-high-strength martensitic stainless steels (UHS-MSS). Due to super-saturation solution strengthening of interstitial atoms (IAs), martensitic stainless steels containing IAs exhibit excellent ultra-high strength and toughness and have high tolerance for oxygen impurities. Hence, studying the specific speciation and structural characteristics of IAs is of great significance for guiding laser cladding of ultra-high-strength steels. Herein, we use density functional theory (DFT) computations to analyze the stable occupancies of IAs and their interactions in body-centered cubic iron (BCC Fe). The findings show that single IAs prefer to occupy octahedral sites over tetrahedral sites. Therefore, octahedral sites are selected as the optimal sites for the following double IAs study. For homo IAs, C-C and N-N configurations exhibit greater stability at long-range distances, whereas O-O demonstrate optimal stability at intermediate distances. Crucially, hetero IAs configurations are more stable compared to single IAs and homo IAs, exhibiting a synergistic effect. Especially, the C-O combination shows the highest stability and strongest bonding character. Meanwhile, the dissociation behavior of O indicates that C-O and N-O have higher dissociation temperatures than single O, further verifying the synergistic effect of hetero IAs. This provides a theoretical basis for understanding the interstitial solution strengthening of laser cladding UHS-MSS.

The modification of axial ligands (ALs) can greatly affect the electrocatalytic performance of M-N-C single-atom catalysts (SACs), whereas their potential ligand-active site interaction mechanism is still elusive. Herein, an asymmetric MnN3H2 substrate model with nitrogen vacancies was constructed based on density functional theory, and 21 different types of ALs were introduced to construct MnN3H2-X (X = F, Cl, Br, I, P, S, O, H, OH, OOH, HCO3, HSO4, NO, NO2, NH, NH2, NH3, CH, CH2, CH3, C6H5) SACs. Formation and binding energy calculations were utilized to screen thermodynamically stable structures, followed by calculations of their free energy variations and overpotentials for the OER/ORR. The results show that MnN3H2-NO (0.49 V for OER) and MnN3H2-P (0.38 V for ORR) demonstrate outstanding electrocatalytic performance due to their lower overpotentials, which are conform to the volcano plot analysis. Moreover, MnN3H2-P (1.17 V) and MnN3H2-CH (1.20 V) have favorable bifunctional catalytic properties. The remarkable electrocatalytic activity originates from the synergistic regulation of ALs and Mn, thereby optimizing the charge redistribution and adsorption of intermediates, ultimately improving the subsequent ORR and OER. This study establishes theoretical underpinnings and mentality of designing for the directional devisal of highly active Mn-N-C SAC electrocatalysts.