Biomass-Derived Metal-Free Nanostructured Carbon Electrocatalysts for High-Performance Rechargeable Zinc–Air Batteries

Metal–air batteries, such as zinc–air, are known for their high specific capacity and environmental friendliness. Operational longevity and energy efficiency, however, remain constrained by sluggish reaction kinetics, elevated overpotential, and interfacial instability during charge–discharge cycles. While noble metal catalysts have historically addressed these gaps, strategic resource allocation now prioritizes abundant, commercially reachable, and cost-effective alternatives. Biomass, a sustainable resource, is crucial in the development of metal-free heteroatom-doped biomass carbon nanostructured electrocatalysts and porous air electrodes with excellent performance for such batteries. These novel materials emerge as critical enablers, leveraging inherent heteroatom density, tunable pore architectures, and the potential for transition metal doping and codoping to optimize bifunctional activity. They have also been identified as prospective alternatives for the next generation of bifunctional electrocatalysts for oxygen reduction and evolution reactions. This review provides a comprehensive overview of the potential of metal-free heteroatom-doped biomass carbon nanostructured electrocatalysts for the forthcoming generations of oxygen reduction and evolution processes, as well as bifunctional electrocatalysts and porous electrodes for zinc–air rechargeable batteries. The physicochemical features of these batteries, stabilization techniques for zinc electrodes, reaction processes, and the dynamic evolution of the electrolyte–electrode interface have also been conferred.