The central component of any solid-state battery is the electrolyte, the medium through which charge-carrying ions shuttle between electrodes while electrons are forced around the external circuit. Liquid electrolytes dominate today’s commercial cells, but they bring well-documented problems: flammability, leakage, volatility, and complicated packaging. Solid polymer electrolytes promise a way around these hazards, yet many of the synthetic polymers used to make them, such as poly(vinyl alcohol) and polyacrylonitrile, are derived from petrochemicals and resist degradation. Biopolymer electrolytes offer an alternative route. Natural polysaccharides and plant-derived materials are renewable, inexpensive, and rich in oxygen-containing functional groups that can host mobile ions. Over the past decade, research groups have explored tamarind seed polysaccharide, iota-carrageenan, pectin, gellan gum, cellulose acetate, sodium alginate, agar, and even Moringa oleifera seeds as ion-conducting hosts. The new study adds a familiar garden herb to that growing roster.

The team’s method was deliberately simple. Leaves of Plectranthus amboinicus were processed and blended with ammonium nitrate, NH4NO3, in varying proportions using the solution casting technique, a low-energy process in which a dissolved mixture is poured into a mold and left to dry into a free-standing film. The resulting bio-membranes were then interrogated with a battery of characterization techniques: X-ray diffraction to probe their internal structure, differential scanning calorimetry and thermogravimetric analysis to assess thermal behavior, electrochemical impedance spectroscopy to measure ionic conductivity, linear sweep voltammetry to test electrochemical stability, and ion transport number measurements to confirm which charge carriers were actually moving.

The structural findings explain much of the material’s performance. X-ray diffraction revealed that the films were predominantly amorphous, meaning their molecular chains lacked the long-range ordered packing of a crystal. This matters because ion transport in polymer electrolytes proceeds through flexible, disordered regions: amorphous chains segment and rearrange with thermal motion, opening transient pathways through which small ions can hop. The most amorphous membrane, labeled PALN3, contained 1 gram of Plectranthus amboinicus leaf material complexed with 0.8 percent molecular weight of ammonium nitrate. In crystalline regions, by contrast, polymer chains are locked in place and ions are trapped or blocked. The amorphous character of the best composition thus set the stage for the conductivity results that followed.

And those results were striking. Electrochemical impedance spectroscopy showed that the PALN3 membrane achieved an ionic conductivity of 1.18 times ten to the minus two siemens per centimeter, a figure that places it among the highest values reported for biopolymer electrolyte systems. For context, many undoped biopolymer membranes conduct at rates six or more orders of magnitude lower. The ammonium nitrate salt is the key additive: when ammonium ions dissociate from nitrate counterions within the leaf-derived polymer matrix, they provide mobile proton carriers, while the salt disrupts chain packing and further suppresses crystallinity. The researchers also computed transport parameters using an equivalent circuit formalism developed by A. K. Arof, a widely used analytical approach that separates bulk, grain boundary, and electrode contributions from the impedance response.

Proton conduction itself is a fascinating and unusual branch of ion transport. Unlike lithium or sodium ions, which drag a solvation shell through the electrolyte in a vehicle-like mechanism, protons are far too small to exist freely in a condensed medium. Instead, they typically migrate through hydrogen-bonded networks via the Grotthuss mechanism, essentially relayed from one hydrogen-bonded site to the next, like a bucket brigade passing water along a line. Plant-derived polymers, dense with hydroxyl, ether, and carbonyl groups, are naturally rich in hydrogen-bonding sites, which is precisely why they make such effective proton hosts. Recent work in the literature, including studies published in Communications Chemistry, has focused on directly observing this proton transfer process, and the field increasingly recognizes biological and bioinspired materials as ideal scaffolds for engineering Grotthuss-type conduction.

Before a membrane can serve in a device, it must survive the electrochemical environment. The team’s thermogravimetric analysis established the material’s thermal stability window, while linear sweep voltammetry confirmed that the electrolyte remains electrochemically stable across the operating range of the cell. Ion transport number measurements verified that protons, rather than electrons or other ionic species, dominate the conduction, a crucial check because electronic leakage would cause self-discharge and render the membrane useless as a battery separator. With these criteria satisfied, the researchers selected the highest-conductivity PALN3 membrane as the electrolyte for fabricating a primary proton-conducting battery.

The assembled cell delivered an open-circuit voltage of 1.64 volts, a respectable figure for a proton battery built from a plant-based electrolyte, comparable to the nominal voltages of many conventional primary chemistries. The team then examined the battery under different load conditions to assess how the voltage and current output held up as the cell was asked to do real work. Load testing is where laboratory curiosities either prove their mettle or fall apart, since internal resistance and mass transport limitations typically cause voltage to sag under current draw. While the study reports the device as a primary, non-rechargeable cell, the demonstration completes the full chain from raw leaf to functioning electrochemical device, a pipeline that earlier work from the same research community has applied to Centella asiatica, Peltophorum pterocarpum, and Ocimum sanctus membranes.

The appeal of this approach extends beyond novelty. Plectranthus amboinicus is fast-growing, widely cultivated, and requires no exotic processing infrastructure, and ammonium nitrate is an inexpensive commodity chemical. Solution casting needs no high temperatures or vacuum systems, making the entire fabrication route accessible and low-carbon. At end of life, a biodegradable membrane avoids the persistent waste problems associated with synthetic polymer separators. None of this means herb-based proton batteries will displace lithium-ion in smartphones tomorrow; primary cells, energy density, cycle behavior, and scale-up all remain open questions, and the published abstract does not report long-term cycling data. But as a proof of concept, the work is compelling. It suggests that the next generation of safe, sustainable solid-state energy storage might be grown in a garden rather than refined from a mine, and that the humble oregano pot on a windowsill could one day share a lineage with the battery in your pocket.

Subject of Research: A plant-derived biopolymer electrolyte from Plectranthus amboinicus leaves and ammonium nitrate for proton-conducting solid-state batteries

Article Title: Development of biomaterial plectranthus amboinicus for proton conducting battery

Article References: Kowsalya, S., Prameela, P., Selvasekarapandian, S., & Devi, S. K. (2026). Development of biomaterial plectranthus amboinicus for proton conducting battery. Ionics. https://doi.org/10.1007/s11581-026-07569-x

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07569-x

Keywords: Plectranthus amboinicus, biopolymer electrolyte, proton conduction, ammonium nitrate, solid-state battery, solution casting, ionic conductivity, X-ray diffraction, electrochemical impedance spectroscopy, Grotthuss mechanism, primary battery, sustainable energy materials

The appeal of solid electrolytes in battery research is easy to understand. Conventional batteries rely on liquid electrolytes—flammable organic solvents that carry ions between the electrodes but also introduce risks of leakage, fire, and chemical degradation. Replacing that liquid with a solid film would make batteries safer, more compact, and potentially compatible with flexible and implantable electronics. The catch is that most solid electrolytes, whether ceramic or polymer based, struggle to move ions as freely as liquids do. Magnesium adds another layer of difficulty: the doubly charged Mg2+ ion interacts strongly with its surroundings, making it notoriously sluggish inside solid frameworks. That is precisely why the conductivity numbers reported in this study turn heads.

The team fabricated their membranes using a straightforward solution casting technique, dissolving Centella asiatica material with magnesium chloride hexahydrate and then adding succinonitrile as a plasticizer. Succinonitrile is an intriguing molecule in its own right—it is a plastic crystal, a solid whose molecules rotate freely in a disordered lattice, and this rotational mobility creates pathways through which ions can hop. When blended into a polymer host, it loosens the rigid structure, suppresses crystallinity, and dramatically increases the fraction of amorphous material where ion transport is fastest. The researchers confirmed these structural changes with a battery of analytical techniques, including scanning electron microscopy with energy-dispersive X-ray analysis, elemental mapping, X-ray diffraction, and X-ray photoelectron spectroscopy.

Those characterization methods told a coherent story. Electron microscopy and elemental mapping showed that the magnesium, chlorine, and nitrogen species were distributed uniformly throughout the membrane, with no signs of the salt agglomeration that plagues poorly formulated polymer electrolytes. X-ray diffraction revealed a dominance of amorphous phases after succinonitrile incorporation—a critical feature, because crystalline regions in a polymer act as barriers that force ions to take long, tortuous detours. X-ray photoelectron spectroscopy verified the chemical interactions between the plant polymer, the magnesium salt, and the plasticizer, confirming that the components were genuinely complexed rather than simply mixed together in a fragile physical blend.

Thermal analysis added further support. Differential scanning calorimetry measured a glass transition temperature of 56.8 degrees Celsius for the plasticized membrane, a reduction that signals enhanced chain mobility in the polymer backbone. Thermogravimetric analysis demonstrated robust thermal stability, meaning the membrane will not decompose under the modest operating temperatures a battery is likely to encounter. This combination of low glass transition temperature and good thermal endurance is the sweet spot that electrolyte designers chase: the polymer chains must wiggle enough to let ions pass, yet the material must hold together over time and temperature without softening into uselessness or breaking down chemically.

The electrical measurements are where the study delivers its headline numbers. Using AC impedance spectroscopy, the researchers measured an ionic conductivity of 1.17 times ten to the minus four siemens per centimeter for the unplasticized membrane. After adding succinonitrile, that figure jumped fifteen-fold to 1.76 times ten to the minus three siemens per centimeter—one of the more respectable values reported for magnesium-ion-conducting biopolymer electrolytes. Temperature-dependent dielectric analysis showed that conduction followed the expected thermally activated behavior, consistent with ions hopping between coordinating sites along the flexible polymer chains. Evans and Wagner polarization measurements were used to determine the transference number, confirming that the current is carried overwhelmingly by ions rather than electrons—an essential property for any electrolyte, since electronic leakage would slowly self-discharge the cell.

Electrochemical stability is often the Achilles heel of biopolymer electrolytes, but here too the results were encouraging. Linear sweep voltammetry established a stability window extending up to 2.74 volts, wide enough to support practical magnesium battery chemistries. Cyclic voltammetry demonstrated high reversibility over 150 cycles, indicating that the electrode processes at the membrane interface could proceed repeatedly without severe degradation or passivation. For a material derived from a medicinal plant, sustaining 150 reversible electrochemical cycles is a meaningful benchmark, suggesting the membrane interface remains chemically compatible with the electrode materials over extended operation.

The proof, of course, lies in an actual device. The team assembled primary magnesium-ion batteries using their highest-conductivity plasticized membrane as the electrolyte separator, paired with two different cathode materials. With molybdenum disulfide as the cathode, the cell delivered an open-circuit voltage of 1.96 volts; with manganese dioxide, it delivered 1.91 volts under load testing. Both values are consistent with the thermodynamics of magnesium-based primary cells and demonstrate that the biomaterial electrolyte is not merely a laboratory curiosity that performs well in isolation—it can function as the working heart of a complete electrochemical cell.

The choice of Centella asiatica as the polymer host deserves attention. The plant is rich in polysaccharides and other oxygen-bearing functional groups that can coordinate magnesium ions and provide the hopping sites needed for conduction. It is also abundant, inexpensive, and biodegradable, which aligns with a growing movement in materials science toward sustainable, environmentally benign battery components. Previous work from the same research group and others has explored biopolymer electrolytes derived from cellulose acetate, pectin, gellan gum, agar, tamarind seed polysaccharide, sodium alginate, and even corn silk extract, building a body of evidence that natural polymers can serve as credible hosts for ion-conducting membranes. This study extends that lineage by showing that Centella asiatica, when properly complexed with a magnesium salt and plasticized with succinonitrile, can reach conductivities at the upper end of the biopolymer electrolyte spectrum.

There are, naturally, caveats. These are primary batteries, not rechargeable ones, and magnesium’s tendency to form passivating layers on metal anodes remains a formidable obstacle to rechargeable magnesium technology. The 2.74-volt stability window, while adequate for the demonstrated cells, would need to widen considerably for high-voltage applications. And scaling from laboratory-cast films to industrially manufactured membranes always presents challenges in reproducibility and mechanical consistency. Still, the study offers a compelling demonstration that the path to safer, greener batteries may run through unexpected territory. A plant prized for centuries in traditional healing now helps shuttle charged atoms through a solid film—and in doing so, it lights up a small but promising corner of the future of energy storage.

Subject of Research: Succinonitrile-plasticized Centella asiatica-based solid biomaterial electrolytes for primary magnesium batteries

Article Title: Succinonitrile-plasticized centella asiatica-based solid biomaterial electrolytes for primary magnesium batteries

Article References: Begam, M. M., Chandra, M. L., & Selvasekarapandian, S. (2026). Succinonitrile-plasticized centella asiatica-based solid biomaterial electrolytes for primary magnesium batteries. Ionics. https://doi.org/10.1007/s11581-026-07538-4

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07538-4

Keywords: solid electrolyte, biomaterial, Centella asiatica, magnesium battery, succinonitrile, ionic conductivity, polymer electrolyte, MgCl2, primary battery, biodegradable materials, electrochemistry, energy storage