Nano-engineered hierarchical core-shell nickel cobaltite chestnut-like structures were successfully synthesized as a bifunctionally active electrocatalyst for rechargeable metal-air battery applications. Both the morphology and composition of the catalyst were optimized by a facile hydrothermal reaction, resulting in a 10 h reacted sample demonstrating significantly enhanced activity toward both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in 0.1 m KOH. Specifically, the catalyst demonstrated -0.28 and 0.60 V versus SCE (saturated calomel electrode) at the ORR half-wave potential and an OER current density of 10 mA cm , respectively. The resulting ORR/OER potential difference of 0.90 V was the smallest compared to the catalysts synthesized using 2, 6, and 12 h of hydrothermal reaction time. The excellent bifunctional activity of the catalyst is attributed to the nanoscale porous morphology and the spinel nickel cobaltite composition, which improved the active site exposure and transport of reactants and charges during the oxygen reactions.