In Situ Decoration of Co3O4 on N-Doped Hollow Carbon Sphere as an Effective Bifunctional Oxygen Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions

Transition metal oxide based materials are believed as a sustainable and effective electrocatalysts for oxygen evolution and oxygen reduction reactions (OER/ORR). However, poor conductivity, specific surface area, and tunable crystallography are major bottlenecks. To alleviate such issues, we developed a highly active spinel Co3O4 decorated N-doped hollow carbon sphere (Co3O4/NHCS) by single step decoration using cobalt phthalocyanine (CoPc) precursor for the first time. The introduction of various extents of CoPc significantly affects the sphere wall thickness, defect sites, doping amount of cobalt content, and the concentration of Co3+ and Co2+ sites. The resulting optimum loading of 0.2g of Co3O4/NHCS-0.2, exhibits a highly uniform hollow carbon sphere with a high wall thickness (61nm), better surface area (445 m2 g-1), and moderate defect sites. Notably, an exterior concentration of octahedral Co3+ (35.50%) and tetrahedral Co2+ (26.54%) sites serve as the active centres for OER and ORR, respectively. Remarkably, the obtained material exhibits exceptional OER performance with a potential of 1.80V vs. RHE in 0.1M KOH medium for achieving a current density of 10mAcm-2, outperforming the benchmark RuO2 material. In addition, Co3O4/NHCS-0.2 demonstrates an impressive ORR electrocatalytic activity with onset potential of 0.89V vs. RHE, excellent current density of 5.0mAcm-2, lower Tafel slope of 60mV dec-1 and close to four electron transfer. Moreover, the moderate carrier concentration, flat band potential, higher tetrahedral Co2+ and pyridinic N sites significantly improved the ORR activity and methanol oxidation tolerance ability.