Recent progress in post-modified biochar-based material for supercapacitor applications: a review

The escalating demand for efficient and sustainable energy storage solutions has spotlighted post-modified biochar materials as promising candidates for supercapacitor electrodes due to their high power density, rapid charge/discharge rates, and long-term stability. This review provides a comprehensive analysis of recent advancements in the synthesis, activation, and functionalization of biochar for supercapacitor applications. Various biomass sources, including agricultural and industrial wastes, have been pyrolysed or hydrothermally carbonised and further activated using agents such as KOH, NaOH, ZnCl₂, and H₃PO₄, achieving specific surface areas (SSA) as high as 3577 m²/g and pore volumes up to 2.3 cm³/g. The electrochemical performance is significantly enhanced through heteroatom doping (N, O, S, P) and metal oxide composite formation, leading to specific capacitances ranging from 252 F/g to 550 F/g and energy densities up to 45.69 Wh/kg. Further, surface modification improves wettability and electron transport while mesopore/hierarchical structures facilitate ion diffusion. The nitrogen-doped biochar demonstrated a specific capacitance of 420 F g^{− 1} at 1 A g^{− 1}.m, whereas KOH-activated walnut shell-derived biochar exhibited 3577 m²/g SSA and 81% capacitance retention over 5000 cycles. Also, surface oxidation techniques have improved wettability and charge transfer, leading to excellent long-term cycling stability, with capacitance retention above 95% after 10,000 cycles. Owing to increased attention towards eco-friendly, viable, and scalable energy solutions, this article presents a thorough overview of the advanced techniques to treat biochar as supercapacitors. Challenges such as scalability, performance, and cost-effectiveness are presented, and a discussion of the future outlook for integrating biochar for sustainable energy storage is provided.