Oxygen Vacancies Rich CoFe-CoFe₂O_4-x Embedded in N-Doped Hollow Carbon Sphere as a Highly Efficient Electrocatalyst for Zinc-Air Battery

Electrochemical energy conversion and storage devices such as fuel cells, metal-air batteries play an important role in solving environmental and energy crises. 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 a 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 state-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, the introduction of oxygen vacancy into spinel type metal oxide can increase the electrocatalytic activity by leading to optimize absorption/desorption energy of oxygen-containing reactants.<br/>In this study, N-doped hollow carbon sphere coated with oxygen vacancies rich CoFe/CoFe₂O_4-x (V_o-CoFe/CoFe₂O_4-x@NC) was synthesized via a facile polymerization process and a subsequent annealing procedure. Briefly, melamine-formaldehyde resin sphere (MF) was prepared as a 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). As prepared CoFe/PDA@MF annealed in Ar atmosphere at high temperature to obtain hollow structure N-doped carbon coated with CoFe/CoFe₂O₄ (CoFe/CoFe₂O₄@NC). Finally, CoFe/CoFe₂O₄@NC was reduced by 10% H₂/Ar flow at 300 °C for 2 h to induce oxygen vacancy into CoFe₂O₄, denoted V_o-CoFe/CoFe₂O_4-x@NC. The hollow structure not only provides a short ion diffusion distance but also increases exposed active sites owing to the large surface area. Furthermore, heteroatom (such as N, S, and P) doped carbon materials regulate the adsorption energy between oxygen related intermediates and active sites by adjusting the electronic structure, thereby effectively enhancing electrocatalytic performance. As a result, the bifunctional V_o-CoFe/CoFe₂O_4-x@NC catalysts show the better ORR catalytic activity (half-wave potential of 0.858 V and Tafel slope of 56 mV dec^-1) than state-of-the-art Pt/C (half-wave potential of 0.855 V and Tafel slope of 62 mV dec^-1). It also displays outstanding OER kinetics (Tafel slope of 64 mV dec^-1), compared to commercial RuO₂ (Tafel slope of 72 mV dec^-1) in alkaline solution. In rechargeable ZABs, V_o-CoFe/CoFe₂O_4-x@NC has a superior open circuit voltage (OCV) of 1.53 V and power density of 139.5 mW cm^-2 than Pt/C and RuO₂ (OCV of 1.49V, power density of 121.7 mW cm^-2).