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

The films were made with one of the simplest and cheapest techniques in materials science: sol–gel spin coating. The researchers dissolved zinc acetate dihydrate in 2-methoxyethanol with monoethanolamine as a stabilizer, added nickel acetate or lithium acetate at a 5 percent doping level, stirred the solutions at 60 degrees Celsius, and spun them onto glass at 3000 rpm. Ten coating cycles, each followed by a brief bake at 200 degrees, built up films thick enough to study, and a final anneal at 400 degrees for an hour locked in good crystallinity. For the electrochemical measurements, the same recipe was applied to fluorine-doped tin oxide electrodes. The low-temperature, solution-based route matters because it avoids the expensive vacuum equipment of sputtering or spray pyrolysis, keeping the door open for large-area, low-cost manufacturing.

X-ray diffraction confirmed that every film, doped or not, crystallized in the hexagonal wurtzite structure of zinc oxide, with no impurity phases sneaking in. The main diffraction peaks, corresponding to the (100), (002) and (101) crystal planes, shifted slightly to higher angles in the doped samples — a fingerprint that the foreign atoms had actually taken up zinc sites in the lattice rather than clustering into separate compounds. Using Scherrer’s equation on the dominant (002) peak, the team calculated average crystallite diameters of 23.79 nanometers for pure ZnO, 20.82 nanometers for the Ni-doped film and 22.5 nanometers for the Li-doped film. Rietveld refinement of the diffraction data gave a weighted profile factor of 9.35 percent and a goodness-of-fit of 2.56 for the parent compound, indicating a solid structural model.

Under the scanning electron microscope, all three films revealed a wrinkled surface texture studded with nanograins. The wrinkles, the authors suggest, arise from compressive stress that builds up as the wet film dries and the temperature difference between substrate and material drives shrinkage; the dopant atoms add their own tensile stress as they squeeze into zinc sites. Energy-dispersive X-ray spectroscopy verified the presence of zinc, oxygen and, in the nickel case, the dopant itself. Lithium, with an atomic number of just 3, sits below the detection threshold of EDS, which can only resolve elements heavier than atomic number 5, so its incorporation had to be inferred from the X-ray peak shifts instead.

Optically, the undoped film was the star of transparency, transmitting more than 90 percent of visible light. Doping dimmed that performance — below 80 percent for nickel and below 85 percent for lithium — because impurity atoms scatter light and disrupt crystallinity. Yet doping also delivered something valuable: a slight red shift of the absorption edge and a narrowing of the optical band gap, from 3.33 electronvolts in pure ZnO to 3.30 for Ni-ZnO and 3.315 for Li-ZnO, as extracted from Tauc plots. The team attributes the narrowing to higher carrier concentrations introduced by the dopants, with crystallite size, strain and lattice parameters also playing a role. Band gap engineering of this kind is precisely what supercapacitor designers look for, since a smaller gap can improve charge transfer at the electrode surface.

Deeper optical analysis sharpened the contrast between the two dopants. The lithium-doped film kept its refractive index below 2 across the visible spectrum, while the nickel-doped film pushed above 2, and the extinction coefficient — a measure of optical loss — was correspondingly lower for lithium. Packing density, computed from the refractive index, was highest for the Ni-doped film, and the volume and surface energy loss functions showed that energy dissipation occurred mainly inside the films rather than at their surfaces. Optical conductivity rose steadily with photon energy in all samples as photons excited electrons across the gap. Taken together, the optical data mark the lithium-doped film as the better candidate for transparent optoelectronic devices.

The wettability experiments produced perhaps the most striking result. A plain water droplet beads up on the Ni-doped film at a contact angle of 105 degrees — comfortably hydrophobic — while the Li-doped film sits at 85 degrees and pure ZnO at 68 degrees, both hydrophilic. The rougher nickel surface traps air under the droplet, propping it up. But when the films were bathed in 254-nanometer ultraviolet light, whose photons carry more energy than the band gap, all three surfaces flipped toward superhydrophilicity within 5 to 15 minutes as photoexcited electrons trapped water molecules and spread the droplet. The Ni-doped film transformed fastest, its contact angle falling to just 0.07 of its initial value after 15 minutes. Left in the dark, the surfaces recovered their original wettability over 21 to 24 hours, a reversible switch with obvious appeal for self-cleaning coatings on windows and photovoltaic panels.

Electrically, lithium pulled decisively ahead. Four-point probe measurements gave sheet resistances of 32.24, 29.37 and 27.1 megaohms per square for the undoped, Ni-doped and Li-doped films respectively, translating into conductivities of 0.134, 0.147 and 0.159 siemens per meter. Doping populates the material with interstitial atoms and oxygen vacancies that ionize easily and release charge carriers, and the lithium film’s better crystalline quality appears to have let those carriers move more freely. For anyone designing transparent electrodes or electronic devices on top of these films, that difference in conductivity is not a rounding error — it is a meaningful edge.

The electrochemical tests, run in a three-electrode cell with 1 molar potassium hydroxide, showed that doping converts ZnO from a resistor-like electrode into a genuine capacitor. Cyclic voltammetry at scan rates from 10 to 100 millivolts per second produced nearly ideal rectangular curves centered on the zero-current axis for both doped films — the classic signature of an electric double-layer capacitor, where charge is stored by fast, reversible ion adsorption rather than by chemical redox reactions. The curves stayed symmetrical even at the highest scan rates, and their enclosed area grew with scan rate as more ions became available to build the double layer. The Li-doped film enclosed the largest area, and galvanostatic charge–discharge measurements quantified the advantage: specific capacitances of 50.16 farads per gram for Li-ZnO versus 33.54 farads per gram for Ni-ZnO at a current density of 0.2 milliamperes per gram, dropping to 35.84 and 24.2 farads per gram at 0.4 milliamperes per gram. Both figures beat a recently reported value of 31.94 farads per gram for an undoped ZnO film, and the team links the capacitance gain to the same band gap narrowing seen optically, with dopant-induced defects extending charge and discharge times through improved ionic mobility.

The broader lesson is that neither dopant is simply better; each earns its place depending on the job. Lithium-doped ZnO is the pick for thin-film supercapacitors and conductive transparent layers, thanks to its superior conductivity, optical clarity and charge storage. Nickel-doped ZnO, with its hydrophobic-to-superhydrophilic switch and stronger band gap reduction, suits self-cleaning surfaces and applications where tailored absorption matters. And because lithium is relatively scarce in nature, the authors note that nickel could serve as a viable substitute where its particular strengths suffice. For a material as cheap and versatile as zinc oxide, a drop of the right impurity — just 5 percent by weight, added in a beaker at 60 degrees — turns out to be enough to reshape its electronic, optical and surface chemistry in ways that matter for the next generation of energy storage devices.

Subject of Research: Comparative effects of nickel and lithium doping on the structural, optical, electrical and electrochemical properties of sol–gel zinc oxide thin films for energy storage

Article Title: Comparative analysis of Ni and Li doped ZnO with enhanced structural, optical, and electrochemical performance in energy storage applications

Article References: Baig, F., Zaheer, Z., Khan, R., Althubeiti, K., Al Otaibi, S., Abdullaev, S., Elboughdiri, N., & Khan, A. (2026). Comparative analysis of Ni and Li doped ZnO with enhanced structural, optical, and electrochemical performance in energy storage applications. Journal of Saudi Chemical Society, 30(2), Article 20. https://doi.org/10.1007/s44442-026-00056-y

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00056-y

Keywords: zinc oxide, thin films, doping, nickel, lithium, supercapacitors, sol-gel spin coating, band gap, wettability, electric double-layer capacitor, optical properties, energy storage

Supercapacitors occupy a unique niche on the energy storage landscape, bridging the gap between traditional capacitors and rechargeable batteries. Where batteries excel at storing large amounts of energy for long durations, supercapacitors deliver exceptional power density, ultrafast charge-discharge kinetics, and an operational temperature range spanning from minus 40 to 85 degrees Celsius. Unlike conventional batteries, they can endure millions of charge-discharge cycles with minimal degradation, making them exceptionally reliable for high-frequency applications. On a Ragone plot, which maps the trade-off between energy density and power density, supercapacitors sit firmly in the high-power regime. This makes them ideal for regenerative braking in electric buses and rail transit, frequency regulation in smart grids, short-term buffering in solar and wind installations, and rapid-charging support in smartphones and wearable electronics. Hybrid systems that pair supercapacitors with batteries are already optimizing energy management in electric vehicles, aerospace platforms, and portable medical devices.

The problem with conventional supercapacitor electrodes is their pedigree. Activated carbon, the workhorse material, is typically derived from coal tar, petroleum pitch, or synthetic polymers, tying an ostensibly green technology directly to fossil fuel extraction. Graphene and carbon nanotubes offer superior conductivity and mechanical integrity, but their large-scale synthesis remains energy-intensive and costly. Worse still, the processing and disposal of materials containing fluorine, sulfur, cyanide groups, or heavy metals carry significant environmental burdens. The review argues that advancing sustainable electrode materials is not merely a scientific challenge but an environmental imperative, and this is precisely where biomass-derived carbons enter the picture, offering a renewable, low-cost, and carbon-neutral alternative that can be engineered with remarkable precision.

The chemistry underlying this transformation is elegant. Lignocellulosic biomass, the structural material of plants, is composed of three biopolymers whose thermal behavior dictates the final carbon architecture. Cellulose, a linear polymer of glucose units making up 35 to 50 percent of the material, decomposes between 260 and 350 degrees Celsius, releasing volatile species such as carbon dioxide, carbon monoxide, methane, and water vapor that carve out micropores in an amorphous carbon framework. Hemicellulose, a branched heteropolymer rich in acetyl and hydroxyl groups, breaks down even earlier, between 180 and 260 degrees Celsius, promoting micropore formation and oxygen doping. Lignin, an aromatic biopolymer, decomposes gradually over 200 to 500 degrees Celsius, condensing into graphitic domains that improve electrical conductivity and mechanical stability. The coexistence of these three components yields hierarchically porous carbon networks combining micropores smaller than two nanometers, mesopores between two and fifty nanometers, and macropores larger than fifty nanometers, a structure that facilitates both electrolyte accessibility and rapid ion transport.

Beyond porosity, biomass brings a built-in doping chemistry that conventional precursors cannot match. Algal biomass, rich in proteins and amino acids, introduces nitrogen in pyridinic, pyrrolic, and graphitic configurations, along with sulfur dopants, all of which enhance redox activity and conductivity. Food waste contributes phosphorus species that expand interlayer spacing and improve surface polarity and wettability. Even the inorganic constituents play a role: potassium carbonate and calcium carbonate act as in-situ activation catalysts through carbothermal reduction reactions, while silica species provide rigid templates that define mesoporous channels before being washed away. The numbers emerging from this approach are striking. A nitrogen-doped hierarchical porous carbon reported in the review achieved an ultrahigh specific surface area of 3,142 square meters per gram and an energy density of 88 watt-hours per kilogram with only 9 percent capacitance loss over 10,000 cycles. A nitrogen-phosphorus-sulfur co-doped carbon derived from peanut meal delivered a specific capacitance of 525 farads per gram, retaining 68 percent of that value even at ten times higher current density.

Synthesis routes matter enormously, and the review dissects them with technical rigor. Pyrolysis, heating biomass in an oxygen-free atmosphere at 300 to 1,200 degrees Celsius, promotes graphitization and conductivity at high temperatures while preserving oxygen-containing functional groups that improve wettability at lower temperatures. Hydrothermal carbonization, which treats biomass in water under high pressure, offers a greener, lower-temperature route that retains oxygen functionalities, making it suitable for flexible, binder-free, solid-state devices. In one standout example, researchers combined hydrothermal carbonization with chemical activation to create a 3D-printed electrode from poplar sawdust that mimicked wood’s vascular patterns, achieving a specific surface area of 253 square meters per gram and an areal capacitance of 7.95 farads per square centimeter at a mass loading of 17 milligrams per square centimeter. Activation, whether physical using steam or carbon dioxide, or chemical using potassium hydroxide, zinc chloride, or phosphoric acid, remains the decisive step for unlocking high surface areas, though the review notes that corrosive chemical activation poses environmental concerns of its own.

Some of the most creative work dispenses with external chemicals entirely through self-activation and bio-inspired templating. Platycladus orientalis leaves were converted in a single step into nitrogen-oxygen-sulfur co-doped hierarchical porous carbon, with the plant’s own bioactive elements driving simultaneous carbonization, doping, and pore formation. Transgenic technology has even entered the field: researchers regulated the SlHDA3 gene in tomato stems to modify their cellular structure before carbonization, yielding a material that achieved a specific capacitance of 201.6 farads per gram and an energy density of 11 watt-hours per kilogram in a symmetric device. A spider web-inspired three-dimensional carbon network built on metal-organic framework templates reached 395 farads per gram and successfully powered 19 red LED lights. Meanwhile, hydrogen-bonded organic framework templates converted heavy bio-oil into flower-like carbon architectures delivering 117.5 watt-hours per kilogram in zinc-ion hybrid supercapacitors, and ice-templating produced hollow carbon cages with hierarchical porosity for the same application.

The waste-to-wealth case studies read like a catalog of unlikely hero materials. Cooked chicken bones, activated to an ultrahigh surface area of 2,235.8 square meters per gram, delivered a capacitance of 329.41 farads per gram with 99.8 percent coulombic efficiency and 90.1 percent retention after 20,000 cycles. Chicken fat oil was transformed into multilayered graphitic carbon nano-onions via simple flame pyrolysis, and sulfur-nitrogen doping pushed their capacitance to 261 farads per gram, enabling an asymmetric device with 32.8 watt-hours per kilogram energy density. A dual-precursor strategy combining hydrochar with polymerized bio-tar produced an interconnected mesoporous network with a surface area of 2,714 square meters per gram and 88.6 percent retention after 5,000 cycles. Daylily, an edible herb, yielded nitrogen-phosphorus co-doped carbons with 299.1 farads per gram and 99.6 percent retention after 4,000 cycles. Even human hair has found a role as a conductive scaffold, supporting silver and nickel-doped manganese oxide composites that reached 1,770 farads per gram in a hybrid supercapacitor.

Hybridization with pseudocapacitive materials represents perhaps the most powerful design strategy, bridging the gap between battery-like energy and capacitor-like power. A nickel-cobalt layered double hydroxide heterostructure anchored on pomelo peel-derived porous carbon delivered an extraordinary 3,290 farads per gram, and the assembled asymmetric device achieved 51 watt-hours per kilogram with a peak power density of 16 kilowatts per kilogram. Biomass-derived carbon nanosheets encapsulating iron oxide nanoparticles retained 99.6 percent of their capacitance after 100,000 cycles, with a redox additive doubling the energy density to 32 watt-hours per kilogram. Aloe vera gel served as a natural reducing agent for green synthesis of cobalt oxide nanostructures, eliminating toxic chemicals while achieving 468 farads per gram. These results demonstrate that biomass-derived carbons function not merely as cheap substitutes but as sophisticated conductive scaffolds that buffer structural strain, facilitate charge transport, and enable rapid ion diffusion in next-generation composite electrodes.

Significant hurdles remain before banana peels and bone ash power our grids. Feedstock variability leads to inconsistent carbon yields and electrochemical performance, making process standardization difficult. Chemical activation demands high energy input and generates hazardous by-products, and achieving uniform pore size distribution and precise dopant control across batches is still elusive. Flexible and wearable applications require freestanding films rather than binder-bound powders. The review’s authors propose a clear roadmap: scalable continuous processing integrated into existing agro-industrial supply chains such as rice mills and sugar refineries, green activation methods, standardized performance metrics, life-cycle and techno-economic assessments, and prototype-scale validation. If those pieces fall into place, biomass-derived carbons could deliver a rare double win in the energy transition, simultaneously diverting waste from landfills and storing the renewable electricity that will replace fossil fuels, one charge at a time.

Subject of Research: Biomass-derived carbon materials for sustainable supercapacitor electrodes

Article Title: From biomass to energy storage: sustainable carbon materials for next-generation supercapacitors

Article References: Merum, D., & Kang, M. (2025). From biomass to energy storage: sustainable carbon materials for next-generation supercapacitors. Advances in Industrial and Engineering Chemistry, 1(1), Article 26. https://doi.org/10.1007/s44405-025-00028-7

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00028-7

Keywords: supercapacitors, biomass-derived carbon, activated carbon, heteroatom doping, pyrolysis, hydrothermal carbonization, porous materials, energy storage, waste valorization, circular economy, electric double-layer capacitor, renewable energy