Advancements in biochar nanocomposites: multifunctional platforms for sustainable energy storage and conversions

The increasing demand for renewable energy sources has intensified the need for sustainable, low-cost and efficient energy storage and conversion materials. Biochar is a carbonaceous solid obtained from biomass pyrolysis. It has recently attracted interest as an electrochemical material due to its tunable porosity, surface chemistry and renewable origin. When combined with nanoscale components, biochar-based nanocomposites (BCNs) exhibit markedly improved electrochemical behaviour arising from synergistic interactions between the porous carbon framework and functional nanomaterials. This review critically examines recent progress in BCNs for energy storage and conversion applications, including supercapacitors, lithium- and sodium-ion batteries, electrocatalysis and photocatalysis. Emphasis is placed on correlating biomass feedstock selection, pyrolysis and activation conditions. The nanocomposite architecture, with dominant charge-storage mechanisms such as electric double-layer capacitance, pseudocapacitive redox processes and ion intercalation, has been discussed. Reported BCNs demonstrate specific capacitances exceeding 400-700 F g^-1, energy densities approaching 90 Wh kg^-1, and long-term cycling stability beyond 10⁴ cycles, depending on composition and electrolyte environment. Beyond performance metrics, this review highlights theoretical study, key limitations related to feedstock variability, conductivity, and scalability. Emerging strategies, including heteroatom doping, hybrid architectures, and AI based data-assisted material optimization have been investigated. By integrating energy storage and conversion within a unified framework, this work provides a structured perspective on the rational design of scalable and sustainable biochar-based nanocomposites for next-generation energy technologies.