Unified Mechanistic Insights into Hydrogen Evolution and Oxidation from Homogeneous to Heterogeneous Molecular Electrocatalysis

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Abstract
Direct comparisons between homogeneous and heterogeneous electrocatalysis using the same molecular system remain rare, despite their importance for catalyst design and device integration. Here, we investigate the heterogenization of the DuBois–Shaw nickel catalyst [Ni(P Cy ₂N Arg ₂) 2 ] 2 ⁺ (NiCyArg), a well-characterized molecular electrocatalyst for reversible H₂/H⁺ interconversion, and assess whether its established homogeneous mechanistic framework can be extended to surface-confined conditions. The catalyst was successfully immobilized onto edge-plane graphite electrodes via noncovalent interactions. Cyclic voltammetry over a range of pH values under Ar and H₂ demonstrates that the heterogenized catalyst retains its reversibility and bidirectional catalytic activity near the thermodynamic H₂/H⁺ potential, with a pronounced bias toward hydrogen oxidation. Pourbaix analysis reveals redox potentials and p K ₐ values of key intermediates that closely match those obtained under homogeneous conditions, indicating minimal perturbation of the thermodynamic landscape upon immobilization. Kinetic analysis using a simplified ECrECr model shows that the same chemical step remains rate-limiting in both regimes, while internal proton transfer processes are modestly slowed upon surface confinement. Overall, this work provides a quantitative bridge between homogeneous and heterogeneous systems and demonstrates that solution phase mechanistic models can in some conditions be applied to surface immobilised systems.
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