Unveiling coordination transformation for dynamically enhanced hydrogen evolution catalysis

Identifying the real active species in the hydrogen evolution reaction (HER) is crucial to understand reaction mechanisms. Herein, we investigate the dynamic structural transformation induced by MXene for cobalt-based electrocatalysts containing various oxyanions (e.g., borate, carbonate, and tungstate) during the HER activation process. Benefiting from the in situ substitution and coordination of oxyanions, the actual active species Co(OH)₂ is gradually formed on the hydrophilic MXene nanosheets and dynamically enhances the HER activity. Specifically, the borate coordinated Co(OH)₂/MXene achieves a small overpotential of only 15 mV@10 mA cm⁻² and robust stability at 500 mA cm⁻² for 200 h, which outperforms the electrocatalytic activity of Pt/C for the HER in alkaline environments. Multiple spectroscopy analysis demonstrates that in situ coordination of borate induces asymmetric active center formation, which triggers the persistent role of electron-deficient Co sites. Theoretical calculations confirmed that the transfer of electrons from Co sites to coordinated oxyanions modulates the electron density of active sites, thereby reducing the energy barrier for water dissociation and hydrogen adsorption. This work inspires the design of novel electrocatalysts and deepens the understanding of the dynamic evolution of the coordination environment of metal sites in electrocatalysis.