Multi-Level Architecture Optimization of MOF-Templated Co-Based Nanoparticles Embedded in Hollow N‑Doped Carbon Polyhedra for Efficient OER and ORR

Emerging clean energy technologies such as regenerative fuel cells and rechargeable metal–air batteries have attracted increasing global interest because of their high efficiency and environmental benignity, but the lack of highly active bifunctional electrocatalysts at low cost for both oxygen reduction and evolution reactions (ORR and OER) greatly hinders their commercial applications. Here, we report the multilevel architecture optimization of Co-based nanoparticles (NPs) embedded in hollow N-doped carbon polyhedra for boosting the ORR and OER, which are fabricated by a two-step pyrolysis–oxidation strategy with a Co-based MOF (ZIF-67) as precursor. The key for this strategy lies in the precise and effective control of the oxidation processes of Co NPs, which enables the synthesis of a series of Co–Co₃O₄-based nanoarchitectures that are embedded in hollow nitrogen-doped carbon polyhedra (HNCP), including core–shell Co/Co₃O₄, yolk@shell Co@Co₃O₄, and hollow Co₃O₄ NPs. Benefiting from its abundant oxygen vacancies and tetrahedral Co²⁺ and the potential synergies of CoO_<i>x</i> species and nitrogen-doped carbon as well as the efficient mass transfer of hollow and yolk–shell structures, the optimal yolk@shell Co₃O₄/HNCP-40 exhibits high activity for the OER with a low overpotential of 333 mV at 10 mA cm^–2 and a small Tafel slope of 69 mV dec^–1, which is better than those of commercial IrO₂ (its overpotential and Tafel slope are 409 mV at 10 mA cm^–2 and 104 mV dec^–1, respectively). Meanwhile, the catalyst also exhibits comparable ORR catalytic activity with a half-wave potential of 0.834 V but better stability and methanol tolerance relative to commercial Pt/C (20 wt %), making it a potential bifunctional electrocatalyst for both the OER and ORR. This MOF-templated strategy for multilevel nanostructures provides insights into the development of highly efficient and low-cost bifunctional electrocatalysts for the OER/ORR