Plant-Based Polymer Electrolyte Boosts Magnesium Supercapacitor Performance

Researchers in India have created a new solid polymer electrolyte that blends a little-known plant-derived biopolymer with polyvinyl alcohol and magnesium chloride, achieving an ionic conductivity high enough to power flexible supercapacitors without a single drop of liquid. The team, led by S. Bakkiyalakshmi and M. Muthuvinayagam of Saveetha Institute of Medical and Technical Sciences, reports in the Journal of Materials Science: Polymers that films made from Cocculus hirsutus, a climbing shrub common in southern India, reach a peak conductivity of 1.17 × 10⁻⁴ S cm⁻¹ — roughly four orders of magnitude above the other formulations tested in the study. Because the films are flexible, flame resistant and free of plasticizers, the work points toward greener, safer energy storage devices that could one day wrap around curved surfaces in wearables, sensors and electric vehicles.

The central problem the researchers tackled is a familiar one in battery science. Liquid electrolytes move ions quickly but bring risks of leakage, chemical reactivity and flammability. Solid polymer electrolytes are safer and mechanically robust, yet they usually conduct ions far less efficiently. To close that gap, materials scientists have been turning to biopolymers — natural, renewable macromolecules that can host salts and shuttle ions — as the backbone of solid electrolytes. Chitosan, cellulose, gelatin and starch have all been explored. Cocculus hirsutus, however, had remained essentially untouched as an ion-conducting host, despite being rich in hydroxyl, carboxyl and ether functional groups that are chemically ideal for coordinating metal ions and forming hydrogen bonds.

The recipe is deceptively simple. Fresh Cocculus hirsutus leaves were collected, cleaned, shade-dried and ground into a fine powder. Half a gram of the powder was dissolved in demineralized water at room temperature, while an equal amount of polyvinyl alcohol — a biodegradable synthetic polymer with a molecular weight of about 115,000 g/mol and 99 percent hydrolysis — was dissolved separately in distilled water at 45 °C. The two solutions were blended, stirred at 40 °C, and then spiked with magnesium chloride in amounts ranging from 0.1 to 0.5 grams. The final mixtures were cast into petri dishes and left to dry at ambient temperature for three to four days, yielding free-standing films roughly 0.087 centimeters thick that could be peeled off and handled like ordinary plastic sheets.

X-ray diffraction revealed why the blend works. Pure crystalline polymers restrict ion movement because ions must hop along rigidly ordered chains. The CH/PVA films, by contrast, showed broad diffraction peaks between 13 and 49 degrees in 2θ, indicating a mostly amorphous structure. Quantitative deconvolution of the XRD patterns showed that the best-performing sample, labeled CHM4 with 0.4 grams of magnesium chloride, had a crystallinity of only about 24 percent. The salt disrupts the alignment of polymer chains and coordinates with oxygen atoms along the backbone, carving out flexible pathways through which magnesium ions can migrate. Interestingly, adding more salt proved counterproductive: the 0.5-gram formulation showed increased crystallinity and reduced conductivity, a signature of ion aggregation and salt clustering that stiffens the polymer network and traps charge carriers.

Fourier transform infrared spectroscopy confirmed that the three components genuinely interact rather than simply coexist. A broad O–H stretching band between 3255 and 3290 cm⁻¹ shifted across the series of formulations, signaling strong hydrogen bonding within the CH/PVA framework modulated by the salt. C–H stretches near 2919–2930 cm⁻¹, C=C vibrations around 1653–1655 cm⁻¹, O–H bending between 1421 and 1433 cm⁻¹, and C–Cl stretches near 817–840 cm⁻¹ all shifted subtly with salt content, mapping out the polymer–salt complexation that underpins ion transport. These molecular fingerprints verified that magnesium ions coordinate with the oxygen-rich sites of both the biopolymer and the synthetic host, dissolving the salt into mobile charge carriers.

Electrochemical impedance spectroscopy quantified just how well the ions move. Sandwiching each film between stainless steel blocking electrodes and sweeping frequencies from 42 Hz to 1 MHz, the team extracted bulk resistance from Nyquist plots. Conductivity climbed steadily as magnesium chloride was added, peaked at the 0.4-gram composition with the minimum bulk resistance, then fell when excess salt induced ion pairing. Conductance spectra displayed the classic three-region behavior of disordered ionic conductors — electrode polarization at low frequencies, a frequency-independent plateau corresponding to DC conductivity, and dispersive behavior at high frequencies that follows Jonscher’s universal power law. Dielectric measurements and electric modulus analysis corroborated the picture, showing strong low-frequency electrode polarization and relaxation features consistent with effective ionic conduction.

The CHM4 film also behaved as expected under temperature variation. An Arrhenius plot of log conductivity against inverse temperature showed a gentle slope, indicating a low activation energy for ion migration — polymer–salt interactions and enhanced segmental motion help ions hop through the amorphous matrix as heat is applied. Differential scanning calorimetry of the optimized film identified a broad endothermic feature between 50 and 85 °C attributed to the release of absorbed moisture, a subtle transition near 222.73 °C linked to increased chain mobility under Mg²⁺ coordination, and a thermal breakdown event near 342.72 °C. That high decomposition temperature, together with the absence of sharp melting peaks, confirms a thermally robust and predominantly amorphous material suitable for demanding solid-state applications.

To prove the electrolyte could actually store energy, the researchers built a symmetric electric double-layer capacitor using two activated carbon electrodes with the CHM4 film in between. Cyclic voltammetry between 0 and 0.7 volts produced rectangular, redox-peak-free curves characteristic of purely capacitive, non-faradaic charge storage. Analysis of the current response revealed a near-perfect log–log linearity with an R² of 0.99 and a slope of 0.388, indicating a roughly balanced mix of diffusion-controlled and surface-capacitive mechanisms — about 57 percent diffusive and 43 percent capacitive at 5 mV/s. Galvanostatic charge–discharge testing delivered the headline number: a specific capacitance of 104.34 F g⁻¹ at a current density of 3 A/g, with nearly symmetric charge–discharge profiles confirming excellent reversibility. When the current was pushed to 10 A/g, the device retained about 40 percent of that capacitance, a respectable rate capability for a fully solid system.

Mechanical and safety tests rounded out the assessment. The film could be bent, folded and rolled without cracking, and its stress–strain curve showed a tensile strength of about 12 MPa at a maximum force of 8.64 N, with a strain at break of 4.39 percent — stiff enough for handling, flexible enough for conformal devices. In flame testing, the film softened, curled and charred slowly rather than igniting explosively, a delayed combustion response the authors attribute to the synergistic interaction between the biopolymer, the PVA matrix and the magnesium salt. In an era when lithium-ion fires dominate headlines, an electrolyte that resists burning while conducting ions is an appealing proposition for consumer electronics and grid storage alike.

The broader significance lies in the sustainability angle. By using a plant-derived biopolymer as the primary host, avoiding plasticizers and fillers that plague many earlier PVA-based systems, and employing an inexpensive, abundant salt, the design offers a lower environmental footprint than conventional electrolytes. Magnesium itself is far more plentiful than lithium, and its divalent charge opens possibilities for high-capacity future batteries. The authors acknowledge that conductivity still trails the best liquid systems and that further optimization is needed, but the demonstration that Cocculus hirsutus — a shrub better known in traditional medicine than in materials labs — can anchor a functional solid electrolyte is a striking reminder that next-generation energy storage may grow on vines as readily as it is synthesized in reactors. The team suggests the CH/PVA/MgCl₂ platform is a promising sustainable candidate for flexible, green supercapacitors and, with further development, magnesium-based solid-state batteries.

Subject of Research: A plant-based Cocculus hirsutus/PVA/MgCl2 solid polymer electrolyte for magnesium-ion-conducting flexible supercapacitors

Article Title: Development of novel cocculus hirsutus/PVA/Mg2+ polymer composite for advanced energy storage devices

Article References: Bakkiyalakshmi, S., Muthuvinayagam, M., Naveen, C., Rajammal, K., & Sivakumar, D. (2026). Development of novel cocculus hirsutus/PVA/Mg2+ polymer composite for advanced energy storage devices. Journal of Materials Science: Polymers, 1(1), Article 8. https://doi.org/10.1007/s44493-026-00008-3

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00008-3

Keywords: Cocculus hirsutus, polyvinyl alcohol, magnesium chloride, solid polymer electrolyte, ionic conductivity, supercapacitor, biopolymer, energy storage, electrochemical impedance spectroscopy, specific capacitance, flexible electronics, green materials