Activated carbon derived from biomass or polymers has emerged as a promising material for capacitive energy storage, particularly in supercapacitor applications. Its high surface area, tunable porosity, and cost-effectiveness make it a competitive alternative to more expensive carbon nanomaterials like graphene and carbon nanotubes. The performance of activated carbon in supercapacitors is closely tied to its synthesis process, pore structure, and surface chemistry, all of which can be optimized for enhanced electrochemical performance.

The production of activated carbon typically involves pyrolysis followed by physical or chemical activation. Pyrolysis is the thermal decomposition of organic precursors—such as coconut shells, wood, or polymers—under inert conditions, yielding a carbon-rich char. The temperature and heating rate during pyrolysis significantly influence the carbon yield and initial porosity. For instance, slow pyrolysis at moderate temperatures (400–700°C) often produces a more ordered carbon structure, while fast pyrolysis can lead to a more disordered but highly porous material. Following pyrolysis, activation further develops the porosity. Physical activation uses oxidizing gases like CO2 or steam at high temperatures (800–1000°C), which selectively burn off carbon atoms to create micropores and mesopores. Chemical activation, on the other hand, employs agents such as KOH, ZnCl2, or H3PO4, which are mixed with the precursor before pyrolysis. Chemical activation generally occurs at lower temperatures (400–800°C) and often results in higher surface areas and more controlled pore size distributions compared to physical activation.

Pore size distribution is a critical factor in determining the electrochemical performance of activated carbon. Micropores (less than 2 nm) contribute significantly to charge storage via electric double-layer capacitance (EDLC) by providing abundant adsorption sites for ions. However, excessively narrow pores can hinder ion accessibility, especially at high charge-discharge rates. Mesopores (2–50 nm) facilitate ion transport, improving rate capability, while macropores (greater than 50 nm) serve as ion-buffering reservoirs. Optimal performance is achieved through a hierarchical pore structure that balances microporosity for high capacitance and mesoporosity for rapid ion diffusion. For example, activated carbons with a bimodal pore distribution—combining micropores and mesopores—have demonstrated capacitances exceeding 200 F/g in aqueous electrolytes while maintaining good rate performance.

Surface chemistry also plays a pivotal role in the capacitive behavior of activated carbon. Oxygen-containing functional groups, such as carboxyl, hydroxyl, and carbonyl groups, can introduce pseudocapacitance through reversible redox reactions, enhancing the overall capacitance. However, excessive surface oxygen can increase the material’s electrical resistance and reduce its stability in prolonged cycling. Nitrogen doping is another strategy to modify surface chemistry, as nitrogen functionalities (e.g., pyridinic, pyrrolic, and quaternary nitrogen) can improve wettability, electronic conductivity, and pseudocapacitive contributions. The balance between porosity and surface functional groups must be carefully tailored to maximize performance without compromising conductivity or cycling stability.

When compared to graphene and carbon nanotubes, activated carbon offers distinct advantages in terms of cost and scalability. Graphene and CNTs exhibit superior electrical conductivity and mechanical properties, but their high production costs and complex synthesis methods limit their practicality for large-scale grid storage. In contrast, activated carbon can be produced from abundant and low-cost precursors using relatively simple pyrolysis and activation processes. Industrial-scale production of activated carbon is well-established, with global capacities exceeding millions of tons annually for applications ranging from water purification to energy storage. While graphene and CNTs may outperform activated carbon in specific metrics (e.g., power density or cycle life), the latter remains the material of choice for cost-sensitive applications where high energy density and moderate power requirements are prioritized.

The performance of activated carbon in supercapacitors is also influenced by the choice of electrolyte. Aqueous electrolytes, such as sulfuric acid or potassium hydroxide, offer high ionic conductivity and low cost but are limited by a narrow voltage window (typically 1 V). Organic electrolytes, like tetraethylammonium tetrafluoroborate in acetonitrile, enable higher operating voltages (up to 2.7 V) but suffer from lower ionic conductivity and higher toxicity. Ionic liquids provide an alternative with wide voltage windows and non-flammability, though their high viscosity and cost remain challenges. The compatibility of activated carbon with these electrolytes depends on its pore structure and surface chemistry, with hydrophobic surfaces generally favoring organic electrolytes and oxygen-rich surfaces performing better in aqueous systems.

Recent advances in activated carbon research have focused on improving its performance through novel precursor selection and advanced activation techniques. For instance, biomass waste materials like lignin, agricultural residues, and even sewage sludge have been explored as sustainable precursors. Polymer-derived carbons, particularly from polyacrylonitrile or phenolic resins, offer precise control over porosity and surface chemistry due to their uniform molecular structures. Additionally, template-assisted methods have been employed to create ordered mesoporous carbons with uniform pore sizes, though these approaches often involve higher costs.

In summary, activated carbon derived from biomass or polymers is a versatile and scalable material for capacitive energy storage. Its performance is governed by a combination of pore structure, surface chemistry, and electrolyte compatibility, all of which can be optimized through careful selection of precursors and activation methods. While graphene and carbon nanotubes offer superior performance in certain aspects, activated carbon remains the most practical choice for large-scale supercapacitor applications due to its low cost, established production methods, and tunable properties. Future research will likely focus on further enhancing its energy density and rate capability while maintaining the cost advantages that make it indispensable for grid-scale storage solutions.