Oxygen vacancies rich CoFe-CoFe2O4-x embedded in N-doped hollow carbon sphere as a highly efficient electrocatalyst for zinc-air battery

초록

Electrochemical energy conversion and storage devices such as fuel cells, metal-air batteries play an important role in solving environmental and energy crisis. Especially, rechargeable zinc-air batteries (ZABs) have attracted considerable attention as a sustainable and eco-friendly device. However, the electrochemical efficiency of ZABs has been hindered by sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) to discharge and charge reaction, respectively. Although noble metal-based materials such as platinum and ruthenium oxides are considered as stat-of-the-art electrocatalysts for ORR and OER, practical application has been hampered by high cost, scarcity, and poor electrochemical durability. Thus, the researches for earth-abundant non-noble metal based bifunctional electrocatalysts with high durability, reasonable cost, and high activity for ORR and OER are needed to enhance the performance of ZABs. Transition metal-based spinel oxides (AB2O4, A, B = Ni, Co, Fe, and Cu) are regarded as promising electrocatalysts for ZABs owing to their low cost, high redox property, and superior stability. Furthermore, introduction of oxygen vacancy into spinel type metal oxide can increase the electrocatalytic activity by leading to optimize absorption/desorption energy of oxygen-containing reactant. In this study, N-doped hollow carbon sphere coated with oxygen vacancies rich CoFe/CoFe2O4-x (Vo-CoFe/CoFe2O4-x@NC) was synthesized via a facile polymerization process and a subsequent annealing procedure. Briefly, melamine-formaldehyde resin sphere (MF) was prepared as self-sacrificial template through polymerization of melamine and formaldehyde. And then, MF, dopamine, Co2+, and Fe3+ species were mixed in tris-HCl buffer solution to acquire polydopamine, Co2+, and Fe3+ coated with MF sphere (CoFe/PDA@MF).