A hierarchical Co₃O₄/CoS microbox heterostructure as a highly efficient bifunctional electrocatalyst for rechargeable Zn-air batteries

Herein, we report the synthesis of a strongly coupled Co3O4/CoS microbox heterostructure, prepared through an annealing treatment with the subsequent hydrothermal sulfidation of a Co-Co Prussian blue analog (PBA) precursor. The unique 3D hierarchical architecture and potential synergies of Co3O4 and CoS provide benefits to the Co3O4/CoS heterostructure for both oxygen reduction and evolution reactions (ORR and OER). It displays comparable ORR catalytic activity (half-wave potential of 0.820 V) to state-of-the-art Pt/C but better durability and methanol tolerance. The Co3O4/CoS electrocatalyst also shows high OER activity with a relatively low overpotential (349 mV at 10 mA cm-2) and small Tafel slope (66.6 mV dec-1), compared to those of commercial RuO2 (366 mV at 10 mA cm-2 and 86.3 mV dec-1, respectively), making it a potential bifunctional electrocatalyst for both the ORR and the OER. Moreover, a rechargeable Zn-air battery with a Co3O4/CoS cathode shows a higher cell voltage (1.51 V), higher power density (168 mW cm-2 at 269 mA cm-2), and better cycling stability (up to 150 cycles) than the same battery with the state-of-the-art Pt/C + RuO2 catalyst. This PBA-based material with a strongly coupled interface between Co3O4 and CoS offers insights into the development of low-cost and highly efficient electrocatalysts for diverse energy-related applications.