Engineering biomass into advanced carbon-based materials for Zn-air batteries
The global energy transition necessitates cost-effective, high-performance electrocatalysts for zinc-air batteries (ZABs). Biomass-derived carbon materials offer a sustainable platform due to their intrinsic heteroatom doping, hierarchical porosity, and tunable electronic properties. This review critically examines the engineering of biomass into advanced carbon-based oxygen electrocatalysts for ZABs, focusing on three material classes: metal-free heteroatom-doped carbons, transition metal-nitrogen-carbon (M-Nx-C) sites, and carbon/transition-metal composites. We analyze synthesis-structure-performance relationships, emphasizing how biomass precursors and processing parameters dictate active site density, pore architecture, and catalytic kinetics for oxygen reduction (ORR) and evolution (OER) reactions. Key experimental benchmarks from recent literature are consolidated, including onset potentials (0.85–0.95 V vs. RHE), half-wave potentials (0.75–0.85 V), and Tafel slopes (60–120 mV dec⁻¹). The construction of air electrodes with optimized three-phase interfaces is discussed, highlighting monolithic and self-standing architectures that mitigate mass transport limitations. Flexible ZAB configurations are evaluated, with areal capacities reaching 5–10 mAh cm⁻² and stable cycling over 100–200 hours. Despite progress, challenges persist in scaling biomass conversion, controlling metal dispersion, and achieving long-term operational stability. This review provides a rigorous framework for translating biomass-derived carbons from laboratory curiosities to industrially viable ZAB components, emphasizing the need for standardized testing protocols and techno-economic assessments.