There is an emerging interest in developing bifunctional oxygen electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as they are key electrochemical reactions that govern the overall performance of emerging rechargeable metal-air batteries. However, such undertaking has been a huge challenge due to the high cost of noble metals (Pt, Ir, and Ru) and their stability when used as catalysts for both the oxygen reactions. Herein, we report CoMn₂O₄ embedded on three-dimensional (3D) hierarchical porous carbon (HPC) derived from waste biomass (corn cobs) as an efficient noble metal-free bifunctional electrocatalyst for both ORR and OER. The hybrid catalyst (CoMn₂O₄/N-3DHPC) is fabricated by solvothermal reaction of as-prepared nitrogen-doped 3D HPC and CoMn₂O₄. The three-step template-free approach in preparing N-3DHPC ensures ample nitrogen doping using melamine as source to improve electronic conductivity of the carbon and formation of three-dimensional interconnected pore network which is favorable to a well-dispersion of CoMn₂O₄ crystals. Thus, the N-3DHPC possesses a high specific surface area and good electrical conductivity as a result of N doping. The same hybrid material also presents high catalytic activity towards OER, rendering them high-performing inexpensive dual-function electrocatalyst. Our result opens up new alternative cathode materials based on earth-abundant, scalable, noble metal-free catalysts for future electrochemical energy storage systems.