Preparation of “Co-Nx Carbon Net” Protected CoFe Alloy on Carbon Nanotubes as an Efficient Bifunctional Electrocatalyst in Zn-Air Batteries

Herein, Co/Fe bimetallic hydroxide nanosheets (Co₃Fe₂ BMHs) were densely deposited on polypyrrole nanotubes (PPy NTs), followed by the successive coating of polydopamine (PDA) and zeolitic imidazolate frameworks (ZIF)-67 to form a composite catalyst precursor. Then, Co₃Fe₂ BMHs, PPy NTs, and ZIF-67/PDA in this precursor were calcined into Co₂Fe alloy nanoparticles, nitrogen-doped carbon NTs (NCNTs), and a Co-N_x activated carbon net, respectively, which constituted a novel composite catalyst. In this composite catalyst, the high-density Co₂Fe alloy nanoparticles are highly dispersed on the NCNT and coated by the Co-N_x activated carbon net. The Co-N_x activated carbon net protects the alloy particles from agglomerating during calcination and from being corroded by the electrolyte. Moreover, the experimental results demonstrated that the calcination temperature and chemical components of the catalyst precursors greatly affect the morphology, structure, composition, and ultimately electrocatalytic activity of the calcined products. The obtained optimum catalyst material exhibited significant electrocatalytic effects on both the oxygen reduction reaction and oxygen evolution reaction with a small ΔE of 0.715 V. The Zn-air battery utilizing this material as the air electrode catalyst showed a power density of 235.5 mW cm^-2, an energy density of 1073.5 Wh kg^-1, and a round-trip efficiency of 62.3% after 1000 cycles, superior to the benchmark battery based on the mixed commercial catalyst of Pt/C and RuO₂. An all-solid-state battery was also assembled to confirm the practical application prospect of the prepared composite material as the air electrode catalyst. More importantly, both experimental data and density functional theory calculations verified that the superior bifunctional catalytic activity was mainly attributed to the synergy between the Co-N_x activated carbon net and Co₂Fe alloy.