Chalk, Bismuth and Nanotubes Combine to Smash Supercapacitor Records

Supercapacitors occupy a curious middle ground in the world of energy storage. They charge in seconds, survive hundreds of thousands of cycles, and deliver bursts of power that batteries simply cannot match, yet their Achilles heel has always been how much energy they can pack into a given mass. A team of researchers from Nigde Omer Halisdemir University and Kayseri University in Turkiye now reports a surprisingly humble recipe that pushes that limit upward: a three-part blend of bismuth oxide, precipitated calcium carbonate, and multi-walled carbon nanotubes, held together in a polymer matrix of polyvinylidene fluoride dissolved in dimethylformamide. The work, published in the journal Ionics, shows that the ternary composite outperforms every simpler combination tested, delivering a specific capacitance of 1359.5 farads per gram at a current density of 1 ampere per gram, a figure that places it among the more capable composite electrodes described in recent literature.

The central idea behind the study is division of labor at the nanoscale. Bismuth oxide, in its alpha crystalline phase, is the redox workhorse of the electrode, shuttling charge through fast surface and near-surface reactions that give pseudocapacitors their battery-like storage capacity. Multi-walled carbon nanotubes contribute the opposite virtue: an electrically conductive, graphitic scaffold that lets electrons flow freely through the film, so that no particle of active oxide is ever more than a few nanometers from a current pathway. Precipitated calcium carbonate, a material more often associated with paper fillers and paints than with cutting-edge electrochemistry, plays the role of structural stabilizer, helping to maintain an open, interconnected mesoporous architecture that keeps electrolyte ions moving freely even when the electrode is packed with active material.

Verifying that all three components genuinely coexist, and cooperate, required an extensive characterization campaign. X-ray diffraction confirmed the presence of alpha-phase bismuth oxide, calcite-form calcium carbonate, and graphitic carbon nanotubes within the composite, while Fourier transform infrared spectroscopy added complementary chemical fingerprints of each constituent. Field-emission scanning electron microscopy revealed the morphology of the films, showing an interconnected network in which the different phases are intimately mixed rather than segregated. Brunauer-Emmett-Teller analysis of gas adsorption quantified the textural properties, confirming a mesoporous structure with pores in the size range that allows electrolyte ions to penetrate rapidly and access the interior surfaces of the electrode, a prerequisite for high-rate performance.

The electrochemical results are where the ternary design earns its keep. In cyclic voltammetry and galvanostatic charge-discharge tests, the Bi2O3/PCC/MWCNT composite achieved the highest specific capacitance of all electrodes investigated, reaching 1359.5 farads per gram at 1 ampere per gram. Even when the current density was raised tenfold to 10 amperes per gram, the electrode still delivered 899.6 farads per gram, a rate retention of 66.2 percent. That figure matters because it demonstrates that the composite does not merely store a lot of charge under gentle conditions; it can also release that charge quickly, which is precisely what supercapacitors are asked to do in applications ranging from regenerative braking to grid frequency regulation.

Energy density, the metric that determines how much work a stored charge can actually perform, tells an equally striking story. In a three-electrode half-cell configuration, the composite delivered 196 watt-hours per kilogram at 1 ampere per gram, a value that rivals some lithium-ion battery chemistries and far exceeds the 5 to 10 watt-hours per kilogram typical of commercial carbon-based supercapacitors. Electrochemical impedance spectroscopy helped explain why: the composite exhibited a charge-transfer resistance of just 0.87 ohms, an exceptionally low value indicating that ions and electrons move across the electrode-electrolyte interface with minimal obstruction. The authors attribute this to the cooperative contributions of the redox-active bismuth oxide, the conductive nanotube network, and the structurally stabilizing calcium carbonate phase working in concert.

The fabrication route is as noteworthy as the materials themselves. Rather than casting a conventional slurry onto a metal foil, the team employed an electrospinning-assisted coating approach, drawing the PVDF/DMF solution loaded with the three active components into fine fibers that deposit as a porous, high-surface-area film. Electrospinning has become a favored technique in flexible energy storage because it naturally produces interconnected fibrous networks with tunable porosity, and the group’s earlier work on MXene-PVDF fibers and flower-like MoS2 and MXene electrodes suggested that the method could be adapted to a wider palette of active materials. The present study extends that strategy to a ternary oxide-carbonate-carbon system, demonstrating that the polymer matrix can host chemically dissimilar phases without sacrificing structural integrity.

Long-term durability, however, remains the honest caveat in an otherwise impressive dataset. When the composite was cycled continuously at a demanding 30 amperes per gram for 10,000 cycles, it retained 37 percent of its initial capacitance. That retention exceeded that of the bismuth-oxide-only and nanotube-only electrodes, suggesting that the ternary architecture does buffer some of the degradation mechanisms that plague pure pseudocapacitive materials, such as volume changes during repeated redox reactions. Yet it remained below the retention of the PCC-only electrode, a reminder that high energy density and long cycle life often pull in opposite directions, and that the mechanical and chemical stresses of rapid cycling still take a measurable toll on the composite structure.

The choice of calcium carbonate as the third component is the study’s most unexpected contribution, and arguably its most interesting from a materials-design standpoint. Precipitated calcium carbonate is abundant, inexpensive, and produced at industrial scale, which makes it an attractive structural additive if it can genuinely improve electrode performance. Prior studies have explored carbonate nanoparticles in porous composite electrodes and even built asymmetric supercapacitors around nanostructured calcium carbonate, but incorporating it as a deliberate stabilizing phase within an established bismuth oxide/carbon nanotube system is a new twist. The results suggest that PCC helps preserve the mesoporous network during fabrication and cycling, preventing the collapse or agglomeration that would otherwise choke off ion transport and erode capacitance over time.

Bismuth oxide itself has been enjoying renewed attention in the energy storage community. Compared with heavier investment favorites such as ruthenium oxide or nickel cobaltites, bismuth is relatively inexpensive, environmentally benign, and offers a rich redox chemistry that can be exploited in aqueous electrolytes. Recent years have seen bismuth oxide paired with reduced graphene oxide, hierarchical porous carbon, and polyaniline, each combination attempting to solve the same twin problems of poor intrinsic conductivity and structural degradation. The Turkish team’s contribution to this crowded field is the demonstration that a three-way composite, engineered through electrospinning-assisted coating in a PVDF/DMF matrix, can achieve both high capacitance and favorable rate capability simultaneously, something that binary combinations in the same study could not match.

What comes next is clear from the authors’ own assessment: the ternary architecture provides high specific capacitance and strong rate performance, but further optimization is required to improve long-term cycling durability. Potential strategies, consistent with the broader literature, include refining the ratio of the three components, engineering protective coatings around the bismuth oxide particles, and tuning the electrospinning parameters to produce even more resilient fiber networks. If the durability gap can be closed without sacrificing the remarkable capacitance and low charge-transfer resistance already achieved, the humble trio of bismuth oxide, chalk, and carbon nanotubes could find a genuine place in the next generation of high-power energy storage devices, proving once again that in materials science, the most unglamorous ingredients sometimes deliver the most electrifying results.

Subject of Research: Ternary Bi2O3/PCC/MWCNT composite electrodes in a PVDF/DMF matrix for supercapacitor applications

Article Title: Comparative investigation of the electrochemical performance of Bi₂O₃, PCC, and MWCNT-based composites in a PVDF/DMF matrix for advanced electrode applications

Article References: Unlu, I., Dokan, F. K., Battaloglu, R., & Sahmetlioglu, E. (2026). Comparative investigation of the electrochemical performance of Bi₂O₃, PCC, and MWCNT-based composites in a PVDF/DMF matrix for advanced electrode applications. Ionics. https://doi.org/10.1007/s11581-026-07514-y

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07514-y

Keywords: supercapacitors, bismuth oxide, precipitated calcium carbonate, multi-walled carbon nanotubes, PVDF, electrospinning, ternary composite electrode, specific capacitance, energy density, electrochemical impedance spectroscopy, pseudocapacitance, mesoporous architecture