Black Gram Skins Turned Into High-Performance Supercapacitor Electrodes

Every year, the global processing of pulses generates mountains of agricultural waste that is usually burned, discarded, or left to decompose. A research team in Tamil Nadu, India, has now shown that one of the humblest of these byproducts, the seed skin of the black gram plant Vigna mungo, can be transformed into a high-performance electrode material for supercapacitors. The study, published in the journal Ionics by A. Oviya Darshni, R. Gopalakrishnan, Srinivasan Anandan, and M. Tamilvanan, demonstrates how a carefully controlled carbonization and activation recipe can convert a waste stream into porous carbon capable of storing electrical charge with impressive efficiency. The work adds a compelling new entry to the growing catalog of biomass-derived materials that could make energy storage cheaper, cleaner, and more sustainable.

Supercapacitors occupy a unique niche in the energy storage landscape. Unlike lithium-ion batteries, which store energy in slow chemical reactions, electric double-layer capacitors, or EDLCs, store charge electrostatically at the interface between an electrode surface and an electrolyte. This mechanism allows them to charge and discharge in seconds, deliver high power density, and survive hundreds of thousands of cycles with minimal degradation. The catch is that their performance depends almost entirely on the properties of the electrode material. An ideal EDLC electrode needs an enormous accessible surface area, good electrical conductivity, and a carefully engineered network of pores that lets electrolyte ions move quickly to and from the charging interface. Activated carbons derived from biomass have long been attractive candidates because they are cheap, abundant, and naturally rich in the kind of disordered, porous architecture that favors ion transport.

The Indian team chose Vigna mungo seed skin, the thin outer coat removed during the milling of black gram, a staple pulse cultivated widely across South Asia. Because India produces black gram on an enormous scale, the skins accumulate as a low-cost, renewable, and essentially untapped carbon precursor. Crucially, using the waste skin does not compete with the pulse’s established food uses, which addresses one of the persistent criticisms of biomass energy materials: that they divert agricultural resources away from food production. The researchers also note that pulses are naturally nitrogen-rich, a feature that has been exploited in other contexts, such as carbon dioxide capture, and that may contribute beneficial surface chemistry in carbon electrodes.

The synthesis route followed a two-stage strategy that has become the gold standard for biomass-derived porous carbons. First, the seed skins were carbonized under a nitrogen atmosphere, a process that pyrolyzes the organic material and leaves behind a carbonaceous skeleton while preventing combustion. Then the resulting char was chemically activated with potassium hydroxide, or KOH, mixed at several different precursor-to-activator ratios. KOH activation is prized in the field because the molten hydroxide and its reduction products aggressively etch the carbon matrix, carving out an interconnected network of micropores and mesopores. Micropores, smaller than two nanometers, provide the vast internal surface area where charge is stored, while mesopores, between two and fifty nanometers, act as ion highways that shuttle electrolyte species rapidly into the interior of the material.

The ratio of carbon to KOH proved to be the decisive variable. When the researchers evaluated the different activated samples, the material designated UAC 1:4 emerged as the clear winner. At this optimized ratio, the activation process generated an ideal balance of pore sizes, a high specific surface area, and enhanced accessibility for electrolyte ions. Too little activator, and the carbon remains dense and poorly connected, limiting both capacitance and rate capability. Too much, and the etching can over-develop the pore network, collapsing structural integrity and wasting surface area in pores that ions cannot efficiently exploit. The 1:4 formulation hit the sweet spot where pore development, ion transport, and charge storage all reinforce one another.

The electrochemical consequences of this optimization were substantial. The UAC 1:4 electrode delivered high specific capacitance, meaning it could store a large amount of charge per gram of active material, while maintaining the rapid charge-discharge kinetics that define a good supercapacitor. The team correlated the structural and surface properties of the material, examined through standard characterization of its texture and porosity, with its electrochemical behavior, tracing a direct line from pore architecture to ion accessibility to charge-storage performance. This kind of structure-property correlation is essential for the rational design of electrode materials, because it transforms electrode fabrication from trial and error into a predictable engineering exercise.

Long-term cycling stability, the third pillar of supercapacitor performance alongside capacitance and rate capability, also depends on the robustness of the porous carbon framework. Because EDLC charge storage is purely physical, involving the adsorption and desorption of ions at the electrode surface rather than the repeated formation and breaking of chemical bonds, well-constructed carbon electrodes can endure enormous numbers of cycles. The interconnected micro-mesoporous network produced by KOH activation supports this durability by distributing mechanical and electrochemical stress across a continuous conductive scaffold, avoiding the isolated pockets and dead-end pores that cause localized degradation in poorly designed materials.

The study situates itself within a vibrant international research effort to mine supercapacitor electrodes from agricultural and food waste. Previous teams have demonstrated high-performance porous carbons from rice husk, corn silk, waste cotton stalk, redgum, pitaya peel, coffee beans, lemon peel, pineapple crown waste, jackfruit seeds, mangosteen peel, banana stem fibers, fallen autumn leaves, and even spent mushroom substrate. Each precursor brings its own characteristic texture and heteroatom content to the final carbon. What distinguishes the new work is the specific demonstration that Vigna mungo seed skin, a waste fraction from one of India’s most important pulse crops, can serve as an EDLC electrode material in its own right, with its properties systematically tuned through activation chemistry. Notably, other researchers have recently explored catalytic graphitization of Vigna mungo biomass for energy storage, suggesting the plant’s waste streams are attracting broad interest as carbon feedstocks.

The broader implications extend beyond a single laboratory result. If pulse-milling waste can be reliably converted into competitive electrode materials, the economics of supercapacitor manufacturing could shift meaningfully, particularly in agricultural economies where the feedstock is essentially free and locally abundant. The process described by the Tamilvanan group requires no exotic reagents beyond KOH, a commodity chemical, and standard thermal processing equipment. Scaling such routes will demand careful attention to energy consumption during carbonization, chemical recovery, and quality control across variable biomass batches, but the fundamental chemistry is well understood and industrially familiar. The authors report that the study received no external financial assistance and was carried out with institutional facilities at K.S. Rangasamy College of Technology, with additional expertise contributed from the International Advanced Research Centre for Powder Metallurgy and New Materials in Hyderabad.

For now, the black gram skin joins a rapidly lengthening list of agricultural residues that have been given a second life as energy storage materials, but the message of the research is bigger than any single precursor. It shows that the key variables in biomass-derived electrodes, namely activation ratio, pore size distribution, surface area, and ion accessibility, can be deliberately engineered rather than left to chance. As demand grows for high-power storage in applications ranging from regenerative braking and grid frequency regulation to portable electronics and hybrid energy systems, electrodes built from farm waste offer a rare combination of performance, sustainability, and low cost. A material once swept away as milling dust may soon help store the pulses of electricity that power the technologies of the future.

Subject of Research: Conversion of Vigna mungo seed skin agricultural waste into KOH-activated porous carbon electrodes for electric double-layer capacitor supercapacitors

Article Title: Conversion of agricultural waste into high performance supercapacitor electrodes: a study on Vigna mungo skin – derived activated carbon

Article References: Conversion of agricultural waste into high performance supercapacitor electrodes: a study on Vigna mungo skin – derived activated carbon. (n.d.). https://doi.org/10.1007/s11581-026-07573-1

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07573-1

Keywords: supercapacitors, activated carbon, Vigna mungo, agricultural waste, KOH activation, electric double-layer capacitor, porous carbon, biomass, energy storage, electrochemistry, black gram, ion transport