Defect-Engineered Hydroxylated Mesoporous Spinel Oxides as Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions

In this work, defect-rich ordered mesoporous spinel oxides, including CoCo₂O₄, NiCo₂O₄, and ZnCo₂O₄, were developed as bifunctional electrocatalysts toward oxygen reduction and evolution reactions (ORR and OER, respectively). The materials are synthesized via nanocasting and modified by chemical treatment with 0.1 M NaBH₄ solution to enhance the defect concentration. The synthesized samples have metal and oxygen divacancies (V_Co + V_O) as the primary defect sites, as indicated by positron annihilation lifetime spectroscopy (PALS). Cation substitution in the spinel structure induces a higher number of oxygen vacancies. The increased number of surface defects and the synergistic effect between two incorporated metals provide a high activity in both the OER and ORR in the case of NiCo₂O₄ and ZnCo₂O₄. Especially, ZnCo₂O₄ exhibits the highest OER/ORR activity. The defect engineering with 0.1 M NaBH₄ solution results in a metal-hydroxylated surface (M-OH) and enhanced the catalytic activity for the post-treated metal oxides in the ORR and OER. This fundamental investigation of the defective structure of the mixed metal oxides offers some useful insights into further development of highly active electrocatalysts through defect engineering methods.