Phase-mediated robust interfacial electron-coupling over core-shell Co@carbon towards superior overall water splitting

Abstract The development of bifunctional electrocatalysts is an appealing yet challenging task for renewable energy conversion. Herein, taking core-shell structure as an example, we propose a “phase mediation” strategy to boost electronic synergism on hydrogen and oxygen evolution reaction (HER/OER). Through precisely tuning the cobalt cores from face-centered-cubic (fcc) to hexagonal (hcp) structure, the electrocatalytic overpotentials of Co@NC for both HER and OER are reduced by ∼100 mV, and lower than those of most other carbon-based electrocatalysts yet reported. Operando SR-FTIR measurements uncover key intermediates of *H and *O―O bonding to N and C sites of C―N moieties in hcp-Co@NC during the HER and OER processes, respectively, suggesting the adjacent C―N moieties as synergistic active sites. In addition, atomic-level characterizations combined with theoretical calculations reveal robust interfacial metal-ligand coordination and electron injection effect within hcp-Co@NC. This promotes the delocalization of electronic structure of the NC shell, greatly accelerating overall water splitting.