Transformation of carbon-encapsulated metallic Co into ultrafine Co/CoO nanoparticles exposed on N-doped graphitic carbon for high-performance rechargeable zinc-air battery

Abstract

In this work, Co-MOFs microrod structures were firstly fabricated using Co2+ source and trimesic acid (H3BTC) as reaction precursors by a simple solvothermal method, followed by pyrolysis treatment at 900 °C in N2 atmosphere to obtain metallic Co encapsulated into graphitic carbon structure (Co@C) with an average Co particle size of 7.7 ± 0.2 nm excluding large-sized Co particles (>20 nm) and a surface area of 184 g cm−2. Interestingly, ultrafine Co/CoO nanoparticles with an average size of 1.8 ± 0.2 nm anchored on graphitic carbon surface (Co/CoO-C) can be obtained through further acid/alkali rinsing treatment of the as-prepared Co@C using HNO3 and NH3·H2O aqueous solutions respectively, followed by thermal treatment at 900 °C in N2 atmosphere. The formation of Co/CoO-C with a surface area of 247 g cm−2 can be ascribed to the dissolution and reorganization process of carbon-encapsulated metallic Co under acid/alkali rinsing and post-thermal-treatment conditions. As the electrocatalyst, Co/CoO-C exhibits superior bifunctional electrocatalytic activities of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline media, notably better than ORR and OER activities of Co@C. The characterization results show that Co-Nx and N doping can be found in both Co@C and Co/CoO-C due to triethylamine (TEA) as solvent providing N source during Co-MOFs synthesis, which are catalytic active species toward electrocatalytic oxygen reactions. Furthermore, the highly exposed ultrafine Co/CoO on graphitic carbon surface can provide more catalytic active sites for high-performance ORR and OER, while carbon-encapsulated metallic Co in Co@C is incapable of directly contacting the electrolyte (only influencing shell-layer carbon function work) with limitedly improved electrocatalytic performance. The fabricated Co/CoO-C with superior bifunctional ORR and OER activities as air cathode material was assembled into a rechargeable zinc-air battery, exhibiting high power density and long-term stability. Our work provides an approach to transform low catalytic active electrocatalyst to high catalytic active one for renewable energy applications.