Anaerobic digestion (AD) was initially evaluated as a potential preprocessing method for preparing biomass-based carbon electrocatalysts in this study. The AD pretreatment succeeded in the structural depolymerization and nitrogen enrichment of Hybrid Pennisetum, which provided favorable conditions to achieve efficient and homogeneous nitrogen introduction due to microorganism community enrichment and provided a porous structure by degradation of the biodegradable components. The resulted biochar exhibited improved physiochemical properties including higher specific surface areas, nitrogen content and graphitization degree than that obtained from pyrolyzing raw biomass. These improvements were positively correlated with the AD time and showed to have enhanced the performance in oxygen reduction reaction and practical microbial fuel cell applications. Amongst the investigated samples, the obtained biochar pretreated by AD for 15 days exhibited the most excellent performance with an onset potential of 0.17 V (VS. saturated calomel electrode) and the maximal power density of 543.2 mW cm⁻² assembled in microbial fuel cells. This study suggested applying AD as a new biological pretreatment in the preparation of biomass-based electrocatalysts, and provided a unique pathway for fabricating high-performance biochar-based catalysts by structure optimization and N-containing active sites construction via gentle biological method, thereby providing a cost-effective method to fabricate metal-free catalysts for oxygen reduction reaction. Graphical Abstract

The plateau capacity of a hard carbon anode plays a crucial role in achieving the high energy density of sodium-ion batteries. However, the loss of plateau capacity due to polarization at high current densities imposes significant limitations on hard carbon applications. Ultra-micropores could maintain plateau capacity at high current densities but are generally prepared accompanied by micropores and mesopores that deteriorate the Na-ion storage performance. Herein, a hard carbon with ultra-micropores is prepared by a protonation-mediated strategy using an N, P co-doped biomass precursor. The P dopants favor reducing the interaction between N and C so that the confined volatiles of NH₃ could be utilized to create ultra-micropores concentrated at 0.4–0.8 nm. These ultra-micropores enable the hard carbon to deliver a reversible capacity of 386 mA h g⁻¹ at 20 mA g⁻¹, a high plateau capacity of 173 mA h g⁻¹, and an excellent rate capability of 106 mA h g⁻¹ at 2 A g⁻¹. The plateau capacity and rate capability are superior to those of the reported hard carbons. This work provides a new approach for ultra-micropore construction within hard carbons and a new perspective for sodium-ion batteries toward high energy and power densities.