Gas-dependent plasma activation of a manganese MOF precatalyst tunes defect accessibility and reconstruction under alkaline HER conditions

Plasma activation is a practical route to introduce defects in MOF-based electrocatalyst precursors, yet how different plasma environments influence defect formation and subsequent oxide evolution remains unclear. Here, manganese (Mn)-MOFs grown on nickel foam (NF) were treated with Ar, N₂, and O₂ glow-discharge plasmas to establish how plasma chemistry governs MOF modification and HER performance. Ar plasma generates a higher population of accessible undercoordinated Mn sites with minimal chemical passivation, which correlates with a more favorable electrochemical reconstruction into a hydrated, layered Mn oxide phase under HER conditions. In contrast, N₂ and O₂ plasmas produce additional chemical alterations such as partial nitriding or oxidative linker damage that limited vacancy accessibility and suppressed oxide formation. Comprehensive characterization (PXRD, Raman, XPS, TGA, EPR, SEM) shows that these gas-dependent pathways correlate with the observed catalytic behaviour, with Ar-treated Mn-MOF/NF achieving the lowest overpotential (219 mV at 10 mA cm⁻²) and smallest charge-transfer resistance. These results provide a practical comparison of plasma treatments for activating Mn-MOF electrocatalyst precursors and highlight Ar plasma as an effective route to generate accessible defect sites for MOF catalysis.