Biomass-derived nanostructured carbons have emerged as promising electrode materials for supercapacitors due to their sustainable sourcing, tunable porosity, and cost-effectiveness. These materials, including activated carbon and carbon fibers, are produced through pyrolysis and activation processes that convert organic precursors into highly porous carbon networks. The performance of these materials in supercapacitors is closely tied to their structural properties, which can be controlled through precursor selection, pyrolysis conditions, and activation methods.
The pyrolysis process involves heating biomass under inert conditions to decompose organic matter into carbon-rich solids, gases, and liquids. Temperature, heating rate, and dwell time significantly influence the resulting carbon structure. Low-temperature pyrolysis (below 700°C) typically yields amorphous carbon with limited conductivity, while higher temperatures (800–1000°C) enhance graphitization, improving electrical conductivity. Activation further develops porosity through physical or chemical methods. Physical activation uses oxidizing gases like CO2 or steam at high temperatures, while chemical activation employs agents such as KOH, ZnCl2, or H3PO4 to etch the carbon matrix, creating micropores and mesopores.
Precursor selection plays a crucial role in determining the final carbon structure. Lignocellulosic biomass, such as wood, coconut shells, and agricultural residues, is rich in cellulose and lignin, which contribute to rigid, high-surface-area carbons. In contrast, protein-rich precursors like algae or chitosan yield nitrogen-doped carbons due to their inherent nitrogen content. The choice of precursor also affects heteroatom self-doping, where natural heteroatoms (N, S, P, O) are incorporated into the carbon lattice during pyrolysis. Nitrogen doping, for instance, enhances pseudocapacitance by introducing redox-active sites, while sulfur and phosphorus doping modify electronic properties and wettability.
Hierarchical porosity—a combination of micro-, meso-, and macropores—is critical for optimizing capacitive performance. Micropores (below 2 nm) provide high surface area for charge storage via electric double-layer capacitance (EDLC), while mesopores (2–50 nm) facilitate ion transport, reducing resistance at high charge-discharge rates. Macropores (above 50 nm) act as ion reservoirs, ensuring electrolyte accessibility. Studies show that biomass-derived carbons with balanced hierarchical porosity achieve specific capacitances exceeding 300 F/g in aqueous electrolytes, with excellent rate capability.
Sustainability metrics favor biomass-derived carbons over synthetic counterparts like graphene or carbon nanotubes, which require energy-intensive production processes. Life cycle assessments (LCAs) of waste-derived carbons reveal lower greenhouse gas emissions and energy consumption compared to petroleum-based or chemically synthesized carbons. For example, activated carbon from coconut shells exhibits a carbon footprint up to 50% lower than commercial synthetic carbons. Recent work has focused on valorizing industrial and agricultural waste, such as rice husks, peanut shells, and spent coffee grounds, to produce high-performance electrodes. These efforts align with circular economy principles by converting low-value waste into functional materials.
Recent advancements highlight the potential of waste-derived electrodes in supercapacitors. For instance, carbon derived from banana peels via KOH activation demonstrated a surface area of 1500 m²/g and a capacitance of 250 F/g. Similarly, lignin-based carbon fibers exhibited high mechanical strength and conductivity, making them suitable for flexible supercapacitors. Heteroatom-rich precursors like seaweed have been used to produce self-doped carbons with enhanced pseudocapacitance, achieving capacitances above 350 F/g.
Despite these advantages, challenges remain in scaling up production while maintaining consistency in pore structure and doping levels. Variability in biomass composition necessitates rigorous process control to ensure reproducible material properties. Additionally, the environmental impact of chemical activation agents must be mitigated through greener alternatives or recovery processes.
In summary, biomass-derived nanostructured carbons offer a sustainable pathway for supercapacitor electrodes, combining high performance with environmental benefits. Precursor selection and processing conditions dictate their structural and electrochemical properties, while heteroatom doping enhances functionality. Hierarchical porosity optimization ensures balanced capacitance and rate performance. As research progresses, waste-derived carbons are poised to play a pivotal role in advancing energy storage technologies with reduced ecological footprints.