Proton-conducting polymer electrolytes are coveted for fuel cells, sensors, supercapacitors and batteries because they are flexible, leak-free and easy to process into thin films. The classic approach has been to soak acid-doped polymers such as poly(ethylene oxide), poly(acrylamide) or poly(vinyl alcohol) with proton sources, which does deliver respectable ambient-temperature conductivity, but at a cost. Acids attack metal electrodes and raise safety concerns, limiting practical devices. An alternative is to dissolve proton-donating salts directly into a polymer host, and among the most effective proton donors are ammonium salts with low lattice energy and bulky anions. The Indian duo chose ammonium thiocyanate, NH4SCN, a salt whose lattice energy of just 605 kilojoules per mole lets it readily fall apart in solution, releasing free ammonium ions that can then hop through a polymer network.

The choice of host polymer was equally deliberate. PVC is better known as pipe and window-frame material than as an ionic conductor, but its mechanical rigidity gives thin films structural stability, and its chlorine atoms carry lone pairs of electrons that help solvate and disperse inorganic salts. The missing ingredient was a way to open up the polymer’s tightly packed structure so ions could move freely. That is where strontium titanate came in. This perovskite ceramic, with particles measuring roughly 70.6 nanometers, brings exceptional thermal and chemical stability along with a large dielectric constant, properties that have made it attractive for high-frequency electrochemical applications. Reviews of the literature showed that PVC-based proton conductors with nanofiller additions had barely been explored, leaving a genuine gap for the team to fill.

Synthesis was disarmingly simple: the solvent casting technique. The researchers dissolved measured amounts of PVC and NH4SCN separately in tetrahydrofuran, stirred the solutions together for three hours to ensure homogeneity, then added varying weight percentages of the nano-sized strontium titanate and stirred again. The viscous mixture was poured into petri dishes and left in a vacuum oven at 60 degrees Celsius for 24 hours, yielding free-standing films ready for testing. All chemicals were used as received, meaning the recipe relies on nothing exotic, a point that matters for anyone hoping to scale the process beyond the laboratory bench.

The electrical results told a clear story. AC impedance spectroscopy, performed between stainless steel electrodes from 1 hertz to 10 megahertz, showed that the pristine PVC–NH4SCN film conducted at 5.233 × 10⁻⁸ S cm⁻¹ at room temperature. Adding 2 weight percent strontium titanate improved matters, but the real jump came at 6 weight percent, where conductivity peaked at 1.457 × 10⁻⁶ S cm⁻¹, nearly two orders of magnitude above the unfilled system. The mechanism is elegant: thiocyanate anions adsorb onto the nanoparticle surfaces, which suppresses ion-pair formation and drives greater dissociation of the ammonium salt into free, mobile ions. Beyond that optimal loading, however, the trend reversed. Excess nanoparticles aggregate into clumps that disrupt continuous ion-transport pathways, and the extra filler raises the mixture’s viscosity, throttling the polymer segmental motion on which ion hopping depends. The lesson is a familiar one in nanocomposite science: more filler is not better filler.

Temperature studies reinforced the picture. Conductivity rose steadily as films were heated, following the classical Arrhenius relationship, and the activation energy for ion migration dropped noticeably when strontium titanate was present. That reduction means ions need less energy to move through the matrix, which the team attributes to the growing amorphous content of the polymer. X-ray diffraction backed this up. Pure PVC showed Bragg peaks at 2θ values of 16.6 and 25.4 degrees, signature of partial crystallinity. When NH4SCN was blended in, the salt’s own sharp peaks vanished entirely, evidence that it had dissolved into the polymer to form a well-integrated complex. Adding strontium titanate then weakened the remaining diffraction intensity, confirming that the nanoparticles disrupt crystalline regions and create free volume, the open space that polymer chains and their hitchhiking ions need to move. Conductance spectra fitted to Jonscher’s universal power law showed a lower exponent for the best-conducting sample, indicating more available hopping sites for the charge carriers.

Differential scanning calorimetry added a thermal twist: the nanofiller behaves like a plasticizer. Pure PVC has a glass transition temperature near 90 degrees Celsius, and incorporating the salt nudged it to 86.66 degrees, with the ammonium ions forming transient cross-links that stiffen the chains. But at 6 weight percent strontium titanate, the glass transition fell to 76.95 degrees, meaning the nanoparticles weaken those constraints and restore chain flexibility, exactly what mobile ions need. The melting temperature of the optimally filled electrolyte was measured at 233 degrees Celsius, hinting at respectable thermal robustness. Dielectric measurements completed the diagnostic work-up: the dielectric constant grew with both filler loading and temperature, and the absence of relaxation peaks indicated that conductivity gains come from an increased density of mobile ions rather than from electrode effects. Wagner’s polarization technique put a number on the ionic dominance, measuring a total ionic transport number of 0.86 for the best film, confirming that charge is carried overwhelmingly by ions rather than stray electrons.

The payoff came when the team assembled an actual primary proton battery in a Teflon jig, sandwiching the 6 weight percent film between a zinc anode and cathodes of lead dioxide or vanadium pentoxide, with hydrated zinc sulfate supplying protons during discharge. At the anode, zinc oxidizes, donating electrons to the external circuit while zinc sulfate and water form; at the cathode, lead dioxide or vanadium pentoxide captures those electrons together with protons to yield metal ions and water. The cell delivered an open-circuit voltage of 1.16 volts that remained stable for up to 73 hours. Under a 1 megaohm load, the voltage initially sagged from 1.16 to 0.92 volts, an effect attributed to activation polarization at the electrode–electrolyte interface, and then settled onto a plateau that persisted for 79 hours, an unusually long discharge window for a lab-scale primary cell.

What makes the study compelling is not any single record-breaking number but the convergence of evidence across half a dozen independent techniques, from impedance spectroscopy and X-ray diffraction to dielectric analysis, thermal measurement and a working device. It demonstrates that an off-the-shelf plastic, a cheap ammonium salt and a well-known perovskite ceramic can be combined into a safe, acid-free, solid proton conductor with genuine battery performance. The durability of the open-circuit voltage over three days suggests the electrolyte–electrode interface is electrochemically quiet, a prerequisite for any practical energy-storage device. Challenges remain, not least that the peak conductivity of about 1.5 microsiemens per centimeter still trails the best acid-based systems, and scaling solvent casting to industrial widths will require engineering work. But the demonstration that strontium titanate simultaneously raises amorphous content, lowers activation energy, softens the polymer and boosts ion dissociation gives materials designers a clear, tunable knob. For a field hunting alternatives to corrosive proton conductors, humble PVC has just made a surprisingly strong case.

Subject of Research: Proton-conducting PVC nanocomposite polymer electrolytes modified with strontium titanate nanoparticles for primary proton battery applications

Article Title: SrTiO3-modified PVC nano composite proton-conducting electrolytes for primary proton battery applications

Article References: Jayanthi, S., & Muthuvinayagam, M. (2025). SrTiO3-modified PVC nano composite proton-conducting electrolytes for primary proton battery applications. Journal of Materials Science: Polymers, 1(1), Article 2. https://doi.org/10.1007/s44493-025-00002-1

Image Credits: AI Generated

DOI: 10.1007/s44493-025-00002-1

Keywords: solid polymer electrolyte, proton battery, PVC, strontium titanate, nanocomposite, ammonium thiocyanate, ionic conductivity, X-ray diffraction, glass transition temperature, dielectric constant, solvent casting, energy storage