The work, published in Frontiers in Batteries and Electrochemistry, centers on a family of materials called nickel cobalt tungstates, written chemically as Ni_xCo_1-xWO₄. Tungstates have attracted growing attention in the supercapacitor community because tungsten can adopt multiple oxidation states and the W-O framework offers relatively high conductivity. But pristine nickel tungstate and cobalt tungstate suffer from compact morphologies, limited surface area, and non-porous structures that hinder electrolyte penetration. The new study tackles these limitations through composition engineering: rather than choosing one metal or the other, the team tuned the cationic ratio to optimize the structural, morphological, and electronic properties of the resulting thin film.

What makes the approach particularly striking is its simplicity. The researchers used a technique called successive ionic layer adsorption and reaction, or SILAR, in which a polished stainless steel substrate is dipped sequentially into a cationic precursor solution containing nickel and cobalt chlorides, rinsed in deionized water, dipped into an anionic precursor containing sodium tungstate, and rinsed again. Each cycle deposits a thin layer of the mixed metal tungstate. After 100 cycles, a uniform, adherent film forms directly on the substrate, with no binder, no conductive additive, and no post-deposition processing. The films changed color from light apple green to purple depending on the nickel-cobalt ratio, a visible sign of the compositional shift.

Direct growth on the conductive substrate is more than a convenience. Because the active material is self-supported, electrons travel efficiently from the substrate into the electroactive film, and strong interfacial adhesion reduces charge-transfer resistance. The absence of a polymeric binder eliminates an extra source of internal resistance that plagues conventional powder electrodes. Meanwhile, the amorphous structure of the films, confirmed by X-ray diffraction showing no distinct diffraction peaks, provides a high density of unsaturated electrochemically active sites, abundant structural defects, and shortened ion diffusion pathways that facilitate rapid redox kinetics.

Microscopy revealed how dramatically the nickel-cobalt ratio reshapes the film morphology. Pristine nickel tungstate formed agglomerated nano-pebbles averaging 340 nanometers. As cobalt content increased, the particles shrank and became more porous, reaching a minimum of 105 nanometers at the 50:50 composition. But the standout electrode, labeled NCW4 with a 25:75 nickel-to-cobalt ratio, formed interconnected nanospherical particles averaging 160 nanometers with a highly porous structure. Pristine cobalt tungstate reverted to compact 320-nanometer particles. The porous nanospherical morphology of NCW4 enables effective electrolyte penetration into the electrode matrix, a critical factor for high-rate charge storage.

Electrochemical testing in a three-electrode configuration using 1 M potassium hydroxide electrolyte confirmed the superiority of the cobalt-rich composition. The NCW4 electrode delivered a specific capacity of 785.72 C g−1 at a current density of 1.1 A g−1, nearly three times higher than pristine nickel tungstate (247.24 C g−1) or cobalt tungstate (186.88 C g−1). Cyclic voltammetry showed distinct redox peaks associated with the Co2+/Co3+, Co3+/Co4+, and Ni2+/Ni3+ couples, and the NCW4 electrode exhibited the largest enclosed voltammetric area and highest current response of the series.

Perhaps the most intriguing finding concerns the charge storage mechanism. Using the Trasatti method and power-law analysis of cyclic voltammetry data, the researchers calculated a b-value for each electrode that reveals whether charge storage is diffusion-controlled (b ≈ 0.5) or surface-controlled (b ≈ 1). The b-values climbed from 0.50 for pristine nickel tungstate to 0.95 for pristine cobalt tungstate, with NCW4 at 0.82. This means increasing cobalt content shifts the charge storage from bulk diffusion toward rapid surface redox reactions. For NCW4, the surface-controlled contribution rose from about 50% at 2 mV s−1 to 69% at 10 mV s−1, explaining its excellent rate capability even at high scan rates.

Impedance spectroscopy added further insight. All electrodes showed comparable series resistance around 0.3 to 0.45 Ω cm−2, but NCW4 exhibited the lowest charge-transfer resistance at 25.72 Ω cm−2, compared to 73.70 Ω cm−2 for pristine nickel tungstate. Double-layer capacitance measurements in a non-faradaic potential window confirmed that NCW4 possesses the highest density of accessible electroactive sites. The combination of optimized composition, favorable morphology, and fast interfacial kinetics explains why this particular ratio outperforms the rest of the series.

Cycling stability is where many pseudocapacitive materials falter, but NCW4 held up remarkably well. After 5,000 charge-discharge cycles at 4.1 A g−1, the electrode retained 77% of its initial capacity with 95% coulombic efficiency. Post-cycling X-ray diffraction, microscopy, and Raman spectroscopy showed that the fundamental tungstate framework remained largely intact, though surface cracks and slight particle agglomeration appeared, consistent with repeated volume changes during faradaic reactions. The capacity fade likely stems from gradual structural relaxation of the amorphous framework, partial loss of active sites, and slight dissolution of transition metal species rather than catastrophic breakdown.

To demonstrate practical viability, the team assembled a two-electrode aqueous hybrid supercapacitor pairing the NCW4 positive electrode with reduced graphene oxide as the negative electrode. The device operated stably at 1.6 V in 1 M KOH, delivering a maximum specific energy of 50.41 Wh kg−1 at a specific power of 1.74 kW kg−1, and retaining 37.03 Wh kg−1 at 4.30 kW kg−1. After 5,000 cycles at 5.6 A g−1, the full cell retained 95.1% of its capacitance with 90% coulombic efficiency. These figures compare favorably with previously reported cobalt tungstate and nickel tungstate devices, suggesting that composition-engineered amorphous nickel cobalt tungstate thin films, grown by a cost-effective binder-free method, could become a serious contender for next-generation supercapacitor electrodes.

Subject of Research: Nickel cobalt tungstate thin film electrodes for high-performance asymmetric supercapacitors

Article Title: Facile synthesis of self-supported amorphous cobalt-nickel tungstate thin films for high performance asymmetric supercapacitor: role of Co and Ni variation

Article References: Mohite, R., Jadhav, G., Jadhav, S., Magdum, V., Pujari, S., Malavekar, D., Patil, U., Kim, J. H., & Pavaskar, P. (2026). Facile synthesis of self-supported amorphous cobalt-nickel tungstate thin films for high performance asymmetric supercapacitor: role of Co and Ni variation. Frontiers in Batteries and Electrochemistry, 5, Article 1903174. https://doi.org/10.3389/fbael.2026.1903174

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1903174

Keywords: supercapacitor, nickel cobalt tungstate, SILAR, pseudocapacitor, reduced graphene oxide, energy storage, thin films, specific energy, Facile, synthesis, self-supported, amorphous