Citrulline-induced mesoporous CoS/CoO heterojunction nanorods triggering high-efficiency oxygen electrocatalysis in solid-state Zn-air batteries

Interface engineering is recognized as one of the effective strategies to optimize the electrocatalytic behavior of catalysts via triggering surface reconstruction and charge redistribution. However, the deliberate control over rich-phase boundaries in a simple and effective manner is still challenging. Herein, an effective bifunctional oxygen electrocatalyst of mesoporous CoS/CoO heterojunction nanorods (CoS/CoO PNRs) is constructed through two-step topological transformations of Co(CO₃)_0.5OH·0.11H₂O nanorods induced by unique citrulline molecule. The designed CoS/CoO PNRs present multiple advantages of mesoporous rod-like architecture, abundant heterointerfaces, increased oxygen vacancies, as well as dual-phase synergy, which trigger outstanding electrocatalytic performance towards oxygen evolution reaction (OER) with low overpotential (265 mV at 10 mA cm⁻²), low activation energy (E_a = 36.14 kJ mol⁻¹) and robust long-term stability. The CoS/CoO PNRs is also demonstrated to be highly active for the oxygen reduction reaction (ORR) with a positive half-wave potential (0.84 V), making the CoS/CoO PNRs a potential bifunctional oxygen catalyst. As an air-cathode, the CoS/CoO PNRs can enable the solid-state Zn-air battery to achieve a large power density, a fast dynamic response, and long cycle life, outperforming that assembled with commercial Pt/C + RuO₂. Theoretical calculations finally unveil that the interfacial electron transfer from CoS to CoO modulates the electronic structure of CoS/CoO, and subsequently adjusts the binding strength of the intermediates in the OER and ORR. This work opens up a new design strategy for the synthesis of high-efficiency oxygen electrocatalysts to be applied in energy-related electrochemical devices.