Seaweed Gel Turned Into Sponge-Like Carbon Powers Supercapacitors for 50,000 Cycles

Agar-agar, the humble gelling agent extracted from red seaweed that has thickened desserts and laboratory cultures for generations, has now been transformed into one of the most capable electrode materials yet reported for water-based supercapacitors. Researchers at the Indian Institute of Petroleum and Energy in Visakhapatnam have shown that a carefully choreographed sequence of carbonization and chemical activation can convert this inexpensive marine polysaccharide into a nitrogen-doped carbon aerogel with an extraordinary specific surface area of roughly 2420.5 square meters per gram. When assembled into a symmetric device filled with a simple, neutral sodium sulfate solution, the material delivered a specific capacitance of 153 farads per gram, an energy density of 35.8 watt-hours per kilogram, and a maximum power density of 23.18 kilowatts per kilogram, all while operating across a cell voltage window of 1.3 volts. The work, published in the journal Ionics, demonstrates how rational pore engineering can coax remarkable performance out of abundant biological feedstocks.

The central challenge the team set out to address is one that has long frustrated designers of biomass-derived carbons. Porous carbons made from plant and algal precursors are attractive because they are cheap, renewable, and naturally rich in the light elements that make carbon such a versatile electrode scaffold. Yet producing a material that simultaneously offers a large accessible surface area, an interconnected network of pores spanning multiple length scales, and effective incorporation of nitrogen heteroatoms has proved elusive. Too often, aggressive activation opens up enormous surface area but destroys the conductive pathways that ions and electrons need, or it generates a chaotic pore landscape in which much of the surface is buried behind bottlenecks that electrolyte ions cannot reach in the microseconds that separate charge and discharge in a high-power device.

The Visakhapatnam group’s solution lies in a dual-function additive that does two jobs at once. Ammonium chloride, a cheap and benign crystalline salt, was blended with the agar precursor before heat treatment. During carbonization, the salt decomposes to release gaseous species that blow through the softening carbon matrix like tiny jets of air through molten glass, carving out an open, graphene-like framework studded with pores. At the same time, the decomposition chemistry supplies nitrogen atoms that substitute into the carbon lattice itself. This in situ approach means that pore formation and nitrogen doping happen in a single, scalable step rather than requiring separate post-treatment stages, an important consideration for any process that might one day leave the laboratory. The nitrogen atoms, particularly in their pyridinic and graphitic configurations, are known to improve the electronic conductivity of the carbon and to contribute additional pseudocapacitance through faradaic interactions with the electrolyte.

After the ammonium chloride-assisted carbonization, the team applied a second, equally critical step: activation with potassium hydroxide. Chemical activation with KOH is the workhorse of the porous carbon field, etching the carbon skeleton and generating the micropores and small mesopores that give supercapacitor electrodes their enormous internal surface area. But the ratio of KOH to carbon, and the temperature at which the activation is carried out, must be tuned with precision. Too little activator, and the surface area remains modest; too much, and the carbon structure collapses into a poorly connected, low-conductivity sponge. The researchers systematically varied both the carbonization and activation temperature and the KOH-to-carbon weight ratio, ultimately arriving at a graphene-like hierarchical architecture in which macropores feed mesopores, which in turn feed the dense mesh of micropores where most of the charge is stored.

This hierarchy is not merely an aesthetic achievement; it is the kinetic heart of the device. Recent work in the field has emphasized that the tortuosity of the pore network, the winding-ness of the paths that ions must follow, can be the dominant factor controlling how fast a supercapacitor can charge. A material with a huge surface area but a tortuous, poorly connected pore system will store plenty of charge if given time but will choke at high current. The interconnected pathways engineered into the agar-derived aerogel allow sulfate and sodium ions to penetrate deep into the electrode rapidly, so that a large fraction of the theoretical surface remains accessible even at demanding rates. That is precisely what the measured power density of 23.18 kilowatts per kilogram reflects: the ability to deliver stored energy quickly, without the voltage losses that plague more convoluted structures.

The choice of electrolyte deserves its own attention. Aqueous electrolytes are the safety and sustainability darlings of the energy storage world; they are non-flammable, inexpensive, and easy to handle compared with the organic solvents used in most commercial devices. Their historical weakness has been the narrow voltage window imposed by the electrolysis of water, which limits energy density because stored energy scales with the square of the voltage. Operating a symmetric carbon-carbon device at 1.3 volts in a neutral 1.5 molar sodium sulfate solution is a meaningful achievement, and it reflects both the stability of the optimized carbon surface and the buffering behavior that neutral salt electrolytes can provide at the electrode interfaces. Every additional tenth of a volt in such a device translates into a disproportionate gain in usable energy.

Durability, the quality that separates laboratory curiosities from deployable technology, is where the new aerogel truly distinguishes itself. The symmetric supercapacitor retained approximately 87 percent of its initial capacitance after 50,000 complete charge-discharge cycles, with coulombic efficiency hovering near 100 percent throughout. Fifty thousand cycles is a punishing test; a lithium-ion battery subjected to equivalent treatment would typically have degraded far more severely. The structural integrity of the hierarchical carbon framework, with its robust graphene-like walls and absence of the dissolution or volume-change problems that afflict many battery electrode chemistries, underpins this endurance. To demonstrate practical relevance, the team showed that the device could power a light-emitting diode, a small but symbolic gesture toward real-world applications such as backup power, portable electronics, and buffers for intermittent renewable generation.

What elevates the study beyond a conventional materials report is its pairing of experiment with two complementary layers of analysis. First, the researchers carried out a kinetic deconvolution of the charge-storage mechanism, separating the contributions of fast, surface-localized capacitive processes from slower, diffusion-controlled ones. This kind of analysis reveals how much of the stored charge is available at high rates and how much depends on ions slowly infiltrating the narrowest pores, providing a quantitative map of where the design succeeds and where further gains might lie. Second, the team employed COMSOL Multiphysics simulations to visualize quantities that are nearly impossible to measure directly: the spatial distribution of current density within the porous electrode, the evolution of the local potential during charging and discharging, and the ion-concentration gradients that build up at the electrode-electrolyte interface. Such modeling, increasingly common in electroanalytical research, turns the black box of a porous electrode into a legible physical system.

The broader significance of the work lies in its demonstration of a complete biomass-to-device pipeline that is sustainable at every stage. The precursor is a renewable seaweed extract; the pore-forming and doping agent is a common, low-toxicity salt; the activation chemistry is standard and scalable; the electrolyte is water with an inexpensive sulfate salt; and the resulting device survives half a century’s worth of daily cycling in laboratory terms. As global demand for grid buffering, regenerative braking, and fast-response energy storage grows, the field of electrochemical capacitors has been searching for electrode materials that combine the power of carbon with energy densities that approach battery territory. This study suggests that the answer may not require exotic precursors or elaborate synthesis at all, but rather a deeper understanding of how to sculpt the invisible architecture of pores inside materials that nature has already provided, and how to read that architecture through the combined lens of electrochemical kinetics and multiphysics simulation.

Subject of Research: Nitrogen-doped porous carbon aerogels derived from agar for aqueous symmetric supercapacitors

Article Title: Pore engineering of agar-derived N-doped carbon aerogels for aqueous symmetric supercapacitors

Article References: Kumari, K., Das, G. S., & Ghosh, S. (2026). Pore engineering of agar-derived N-doped carbon aerogels for aqueous symmetric supercapacitors. Ionics. https://doi.org/10.1007/s11581-026-07564-2

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07564-2

Keywords: supercapacitors, carbon aerogel, agar-agar, biomass-derived carbon, nitrogen doping, KOH activation, hierarchical porosity, aqueous electrolyte, energy storage, COMSOL simulation, sodium sulfate, cycling stability