The escalating global demand for efficient and sustainable energy storage has cast a spotlight on materials derived from agricultural and industrial waste. A review article, “Recent progress in post-modified biochar-based material for supercapacitor applications: a review,” by Ranjeet Kumar Mishra and colleagues, published in Materials for Renewable and Sustainable Energy, explores the significant advancements in converting biomass-derived biochar into highly effective supercapacitor electrodes. Biochar is emerging as a promising alternative to traditional carbon-based materials due to its sustainability, low cost, and customizable properties. This is particularly relevant as traditional fossil fuels deplete, making the pursuit of cleaner energy sources critical.

Biochar is typically produced from various waste sources, including mulberry, corn stalk, coffee grounds, and food waste, through processes like pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More or hydrothermal carbonization. However, the raw biochar often has a limited surface area, which restricts its utility in high-performance applications like supercapacitors. The key to unlocking its potential lies in post-modification techniques, such as activation and functionalization. Chemical activation, utilizing agents like KOH, NaOH, ZnCl₂​, and H_3​PO_4​, is a highly effective method to dramatically increase the material’s surface area. For example, KOH-activated biochar derived from walnut shells has achieved a specific surface area as high as 3577 m²/g and retained 81% of its capacitance over 5000 cycles. Biochar from flaxseed residue, activated with KOH, showed an even better result with 98.10% capacitance retention after 10,000 cycles. These impressive numbers are critical because high surface area provides more active sites for ions to accumulate, which is the foundation of energy storage in an electrical double-layer capacitor (EDLC).

The electrochemical performance of biochar electrodes is further enhanced by introducing heteroatoms—atoms other than carbon—such as nitrogen (N), oxygen (O), sulfur (S), and phosphorus (P), through a process called heteroatom doping. Nitrogen-doped biochar, for instance, has demonstrated a high specific capacitance of 420 F g⁻¹ at a current density of 1 A g⁻¹. These atoms, especially N and O, can form functional groups that improve the electrode material’s wettability with the electrolyte and introduce pseudo-capacitance through fast, reversible redox reactions on the surface. This pseudo-capacitance mechanism significantly boosts the device’s energy storage capacity, with reported specific capacitances ranging from 252 F/g up to 550 F/g and energy densities reaching 45.69 Wh/kg. Surface oxidation techniques are also utilized to improve wettability and charge transfer, which ultimately contributes to exceptional long-term cycling stability, with some materials maintaining over 95% capacitance retention after 10,000 cycles.

While a large specific surface area is beneficial, the physical structure of the pores is equally important. Many porous carbons contain numerous tiny micro-pores that are too small for large electrolyte ions to pass through efficiently, limiting the material’s electrochemical performance. To solve this, researchers focus on engineering a hierarchical porous structure—a combination of micro-pores (for surface area), meso-pores (for fast ion transport), and macro-pores (for easy ion access). The introduction of mesopores, with diameters typically between 2 and 50 nm, acts as a “highway” for the electrolyte ions, facilitating rapid diffusion and increasing the utilization rate of the inner surface area. Materials like nitrogen-doped porous carbon from egg white, possessing a three-dimensional honeycomb structure, have demonstrated specific capacitances of 335 F/g and 91.70% capacitance retention over 10,000 cycles. This structural optimization ensures both high energy density (from the large surface area) and high power density (from the fast ion transport). The integration of biochar, derived from abundant and sustainable biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More, into supercapacitor technology aligns perfectly with the principles of the circular economy. This research, which shows that biochar can achieve electrochemical performance comparable to or even surpassing commercial alternatives while using waste as a raw material, suggests a transformative direction for sustainable energy storage. Future efforts will continue to focus on improving the scalability, performance under harsh conditions, and overall cost-effectiveness of these biochar-based materials.

Source: Mishra, R. K., Kumar, D. J. P., Chinnam, S., Sankannavar, R., Sharma, A., & Mohanty, K. (2025). Recent progress in post-modified biochar-based material for supercapacitor applications: a review. Materials for Renewable and Sustainable Energy, 14(1), 63.