Common Glycerol Makes Chitosan Polymer Electrolytes Thousands of Times More Conductive

A humble, cheap molecule found in every pharmacy cabinet may hold one of the keys to greener, safer energy storage. In a study published in Polymer Bulletin, researchers led by Safar Saeed Mohammed of the University of Raparin and Shujahadeen Bakr Aziz of the University of Sulaimani report that adding glycerol to a biodegradable polymer blend based on chitosan and polyvinylpyrrolidone (PVP), doped with sodium nitrate, boosted the direct-current ionic conductivity from 1.02 × 10⁻⁸ S/cm to 2.34 × 10⁻⁴ S/cm — an improvement of more than four orders of magnitude. The films were prepared by the simple, low-cost solution casting method, meaning the recipe could in principle be scaled up without exotic equipment or hazardous solvents.

The significance of the result lies not just in the headline number but in how it was achieved. Solid polymer electrolytes are prized as replacements for flammable liquid electrolytes in batteries and supercapacitors, but their Achilles heel has always been sluggish ion transport. In most polymer hosts, ions can only move through disordered, amorphous regions where chain segments are mobile enough to open and close pathways. Crystalline regions, by contrast, act as roadblocks. The Iraqi and Turkish team therefore set out to maximize the amorphous fraction of their blend, and glycerol proved to be an extraordinarily effective tool for the job.

X-ray diffraction (XRD) provided the first and most striking piece of evidence. The pristine chitosan/PVP/NaNO₃ film, labeled CPNG-0, showed a degree of crystallinity of 28.3 percent. As glycerol content was increased step by step, that figure fell progressively, dropping to just 6.8 percent in the most plasticized sample, CPNG-40. In other words, glycerol did not merely sit between the polymer chains; it systematically dismantled their ordered packing, enlarging the amorphous phase that serves as the highway for sodium ions. This kind of quantitative, composition-resolved crystallinity data is exactly what the field has needed to connect structure to performance.

Fourier-transform infrared spectroscopy (FTIR) filled in the molecular picture. The characteristic functional-group bands of chitosan, PVP, sodium nitrate, and glycerol all shifted and broadened as the formulations changed, which the authors interpret as evidence of complex formation and enhanced polymer–salt interactions. Hydrogen bonding between glycerol’s hydroxyl groups and the polar sites on the two polymers effectively competes with the polymer–polymer and polymer–salt interactions that would otherwise promote crystallization. The result is a more homogeneous, better-connected matrix in which sodium ions are dissociated from the salt lattice and coordinated loosely enough to hop from site to site.

The electrochemical consequences were dramatic. Electrochemical impedance spectroscopy (EIS) showed the bulk resistance of the films collapsing from 295,600 ohms for the unplasticized CPNG-0 to a mere 18 ohms for CPNG-40. The Nyquist plots told the same story visually: the pristine sample displayed a large semicircle, the classic signature of a highly resistive material dominated by charge-transfer and bulk effects, while the glycerol-rich sample produced a spike-dominated response, indicating that the material had transitioned into a regime of highly efficient ion transport in which the blocking-electrode capacitance dominates the low-frequency behavior.

Dielectric measurements added a further layer of insight. At low frequencies, the glycerol-rich samples exhibited significantly enhanced dielectric constant and dielectric loss, which the researchers attribute to increased accumulation of charge carriers at the electrodes — the well-known phenomenon of electrode polarization. A higher dielectric constant matters because it weakens the Coulombic attraction between dissociated cations and anions, allowing more ions to move freely. The relaxation peak frequency shifted from 0.133 kHz to 133.65 kHz, and the corresponding relaxation time plummeted from 1.196 milliseconds to 1.191 microseconds — a thousand-fold acceleration in the characteristic timescale over which dipoles and ions reorganize in response to an applied field.

Beyond conductivity itself, the team extracted the fundamental ion-transport parameters that govern device performance. Ionic diffusivity, charge-carrier density, and ionic mobility all increased with glycerol loading, a combination that is not always achieved simultaneously. Often, adding a plasticizer increases the number of mobile ions but reduces their mobility, or vice versa; here, both quantities rose together, which explains why the conductivity gain was so large. Argand plot analyses reinforced the picture, showing that ionic relaxation occurs more rapidly and that polymer segmental motion improves as glycerol content increases — a direct link between chain dynamics and ion hopping.

What makes the work particularly timely is the choice of materials. Chitosan is derived from chitin, the structural polymer of crustacean shells and one of the most abundant biopolymers on Earth, while PVP is a water-soluble, biocompatible synthetic polymer. Sodium nitrate provides sodium ions from an element that is vastly more abundant and cheaper than lithium. Together, they point toward electrolytes that are flexible, non-toxic, environmentally benign, and potentially compatible with wearable and implantable electronics. The field of biopolymer electrolytes has been growing rapidly, with recent studies exploring methylcellulose, dextran, starch, and carrageenan hosts, and the new results fit into a broader pattern in which glycerol plasticization consistently emerges as a powerful, low-cost strategy for enhancing ion transport.

The physics of why glycerol works so well is worth unpacking. As a small molecule with three hydroxyl groups, glycerol inserts itself between polymer chains, screening the intermolecular hydrogen bonds and dipole interactions that hold crystallites together. It also lowers the glass-transition temperature of the blend, giving chain segments the thermal mobility they need even at room temperature. Because ion motion in polymer electrolytes is coupled to segmental relaxation — ions effectively surf on the moving chains — faster segmental dynamics translate directly into faster ion transport. Glycerol additionally increases the free volume of the matrix and can solvate sodium cations, further promoting salt dissociation. The combination of amorphous phase enlargement, enhanced segmental motion, and improved salt dissociation is what produced the four-order-of-magnitude conductivity leap reported here.

Challenges remain before such films can power real devices. The authors’ measurements establish structural, dielectric, and electrochemical properties, but translating a laboratory film into a battery or supercapacitor requires demonstrations of long-term cycling stability, electrochemical stability windows, and mechanical robustness under repeated flexing. Nevertheless, the numbers reported — a bulk resistance of just 18 ohms and a conductivity approaching the 10⁻⁴ S/cm range often cited as a practical threshold for electrochemical device applications — make this chitosan/PVP/NaNO₃ system one of the most compelling biopolymer electrolytes described to date. If the recipe’s simplicity holds up at scale, the future of flexible, sustainable energy storage may owe as much to a bottle of glycerol as to any high-tech nanomaterial.

Subject of Research: Glycerol-plasticized chitosan/PVP sodium-ion conducting biopolymer electrolytes

Article Title: Amorphous phase enlargement and boosting the ionic conductivity in glycerolized chitosan/PVP-NaNO3 biopolymer electrolytes

Article References: Mohammed, S. S., Aziz, S. B., Rasul, H. H., Dağdelen, F., & KÖK, M. (2026). Amorphous phase enlargement and boosting the ionic conductivity in glycerolized chitosan/PVP-NaNO3 biopolymer electrolytes. Polymer Bulletin, 83(12), Article 654. https://doi.org/10.1007/s00289-026-06702-0

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06702-0

Keywords: polymer electrolyte, chitosan, PVP, glycerol plasticizer, NaNO3, ionic conductivity, XRD, FTIR, electrochemical impedance spectroscopy, dielectric properties, biopolymer, energy storage