The choice of material is deliberate. Bismuth sulfide is an earth-abundant, environmentally benign semiconductor that has attracted growing attention as an electrode material for energy storage. Its layered orthorhombic crystal structure, built from covalently bonded Bi2S3 ribbons held together by weaker van der Waals forces, offers open channels along which ions can migrate, making it naturally suited to intercalation-based charge storage. Previous studies have produced Bi2S3 in the form of nanorods, nanowires, nanoflowers and nanoflakes using solvothermal, chemical bath and spray methods, and several groups have reported respectable supercapacitor performance from composites that pair the sulfide with graphene, carbon nitride or nickel foam. What has been missing, the authors argue, is a systematic understanding of how the electrolyte itself, not just the material, controls the charge storage process.

To build their electrode material, the researchers turned to hydrothermal synthesis, a technique that grows crystals from aqueous precursors inside a sealed, heated vessel. Under the right combination of temperature, time and precursor chemistry, the reaction self-assembles Bi2S3 into petal-like nanopetals, which the team abbreviates BSNP. This flower-like architecture is more than aesthetic. The petals create a high surface-area scaffold with abundant exposed active sites, short diffusion pathways for ions and open voids that accommodate the volume changes that occur as charge is stored and released. Such morphological engineering is a central strategy in modern electrode design, because electrochemical reactions happen only at surfaces and interfaces; the more accessible surface a material presents to the electrolyte, the more charge it can store and the faster it can deliver it.

The heart of the experiment is a comparison across three alkaline electrolytes: 2 M solutions of lithium hydroxide (LiOH), sodium hydroxide (NaOH) and potassium hydroxide (KOH). All three contain the same charge-carrying hydroxide ion, but they differ in the size and hydration behavior of their cations, and those differences ripple through every aspect of electrode kinetics. When the team tested BSNP as the positive electrode in each solution, the results were striking. In 2 M KOH, the nanopetals delivered a gravimetric capacity of 797.9 coulombs per gram at a scan rate of 1 millivolt per second, outperforming the same material in LiOH and NaOH. In other words, simply swapping the electrolyte cation from lithium or sodium to potassium unlocked a substantially larger fraction of the material’s theoretical storage capability.

Why would potassium ions give bismuth sulfide an edge? The answer, the researchers show, lies in the electronic and ionic interplay at the material’s surface. Using DFT calculations, they probed how hydroxide species interact with the Bi2S3 lattice under each electrolyte condition. Orbital hybridization analysis revealed favorable overlap between the electronic states of the surface and the adsorbed species in the KOH case, while Bader charge analysis quantified the electron transfer between them. Charge density difference maps and electron localization function analysis visualized where electrons accumulate and deplete during adsorption, painting a consistent picture: the potassium-containing environment promotes stronger, more favorable electronic coupling at the Bi2S3 surface, which lowers the energetic barriers for the redox reactions that store charge. This computational evidence aligns neatly with the electrochemical data, giving the performance claims a mechanistic foundation rather than leaving them as empirical observations.

Kinetic analysis added another layer of insight. Applying Dunn’s method, a widely used technique that separates the capacitive (surface-controlled) and diffusion-controlled contributions to charge storage from cyclic voltammetry data measured at different scan rates, the team confirmed that diffusion-dominated processes govern the behavior of the BSNP electrode. That finding matters because it places the material firmly in the battery-type category rather than the purely capacitive one. Battery-type electrodes store charge through faradaic reactions, in which ions insert into the crystal lattice and electrons transfer to or from the metal centers, allowing much more charge to be packed per gram than in the electrostatic double-layer storage that characterizes carbon-based supercapacitor electrodes. It is precisely this battery-like positive electrode, paired with a capacitive negative electrode, that defines the supercapattery architecture.

To prove the concept at the device level, the researchers assembled an asymmetric supercapacitor (ASC) in which the BSNP nanopetals served as the positive electrode and activated carbon, a classic high-surface-area capacitive material, served as the negative electrode. The two electrodes were balanced in mass and charge so that each could operate within its stable voltage window, maximizing the overall cell voltage. The finished device achieved a gravimetric capacity of 209.76 coulombs per gram at a current density of 1 ampere per gram and, more importantly for real-world applications, an energy density of 46.61 watt-hours per kilogram. That figure sits well above what conventional symmetric supercapacitors typically deliver and approaches the lower range of lithium-ion batteries, while retaining the rapid charge-discharge capability and long cycle life that batteries struggle to match.

The significance of the electrolyte comparison extends beyond this particular material. Electrolyte engineering is often treated as an afterthought in electrode research, yet the ionic radius, mobility, hydration shell and desolvation energy of the charge carriers fundamentally shape how deeply ions can penetrate an electrode and how quickly redox reactions proceed. Potassium ions, being larger than lithium and sodium ions in their bare form but less strongly hydrated in aqueous solution, can shed their water shell more easily and interact more directly with electrode surfaces. The present study demonstrates that this seemingly subtle difference can translate into hundreds of coulombs per gram of additional capacity, a lesson that applies to the entire family of transition metal and post-transition metal sulfides under investigation for supercapatteries.

There are, of course, hurdles between a laboratory cell and a commercial device. Aqueous supercapatteries are inherently limited to modest cell voltages by water’s electrochemical stability window, and the long-term cycling stability, rate capability at high current densities and mechanical robustness of sulfide electrodes must be validated over thousands of cycles before such devices could power phones or grid buffers. The authors note that their data are available upon reasonable request, inviting the community to build on the results, and the combination of hydrothermal scalability, inexpensive raw materials and water-based processing gives the approach a credible path toward practical manufacturing. The work was supported by the Ongoing Research Funding Program at King Saud University, with additional support from the University of Kerala.

What makes this study a compelling piece of the energy storage puzzle is its methodological completeness. Rather than reporting a capacity number in isolation, the team connected morphology, electrolyte chemistry, electronic structure and device performance into a single coherent narrative, using DFT not as decoration but as a genuine explanatory tool. As the world races to decarbonize electricity grids and electrify transport, hybrid devices that blend the best of batteries and supercapacitors could fill critical niches, from regenerative braking and renewable energy smoothing to portable electronics that charge in minutes. Petal-like bismuth sulfide, coaxed into existence in a simple hydrothermal reactor and tuned by nothing more exotic than the right potassium hydroxide bath, may prove to be one of the quiet building blocks of that future.

Subject of Research: Electrolyte-dependent charge storage in hydrothermally synthesized petal-like Bi2S3 nanostructures for supercapattery electrodes

Article Title: Electrolyte-dependent charge storage in hydrothermally synthesized petal-like Bi2S3 nanostructure: experimental and DFT insights

Article References: Mottammal, D., Thomas, S. A., Cherusseri, J., Arumugam, D., Pallavolu, M. R., Fatehmulla, A., Ramasamy, S., Parasuk, V., & Rajendran, D. N. (2026). Electrolyte-dependent charge storage in hydrothermally synthesized petal-like Bi2S3 nanostructure: experimental and DFT insights. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13692-x

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13692-x

Keywords: bismuth sulfide, supercapattery, asymmetric supercapacitor, hydrothermal synthesis, DFT calculations, potassium hydroxide electrolyte, battery-type electrode, energy density, charge storage kinetics, nanopetals, aqueous electrolytes, energy storage materials

The novelty of the work lies in its discipline. Rather than varying several synthesis parameters at once, the team isolated manganese precursor molarity as a single, independent lever—the variable that controls how supersaturated the solution becomes with Mn2+ ions at the moment of nucleation. Because the resulting powders contain no conductive scaffold or composite additives, the measured electrochemical behavior reflects the intrinsic properties of the β-MnO2 itself. That makes the study a unusually clean probe of a question that has long hovered over pseudocapacitor research: how exactly do structural defects govern whether charge is stored quickly at surfaces or slowly through bulk diffusion?

Electron microscopy revealed that the precursor concentration directly sculpted the material’s architecture. All three samples formed flower-like agglomerates of nanorods, but their mean diameter grew steadily with concentration, from 171 nanometers at 0.05 M to 268 nanometers at 0.20 M. Higher molarity means more Mn2+ supersaturation, faster nucleation and growth, and consequently larger, more developed structures. Morphology, however, turned out to be only half the story. Raman spectroscopy told a subtler tale about what was happening inside the crystal lattice.

The Raman ν2 vibrational mode of the material shifted from 647.1 to 638.5 inverse centimeters as the precursor concentration rose, while the peak broadened from 36.8 to 47.4 inverse centimeters. In the language of vibrational spectroscopy, that combination of shifting and broadening is a fingerprint of increasing oxygen-vacancy-related disorder—missing oxygen atoms in the crystal lattice that disturb the local bonding environment. Oxygen vacancies are not mere imperfections; in manganese dioxide they are known to enhance electronic conductivity and can open additional pathways for cation storage. The Indonesian team had, in effect, tuned the defect density of their electrodes simply by choosing how concentrated the starting solution should be.

To quantify how those defects translated into charge-storage behavior, the researchers turned to cyclic voltammetry and analyzed the current response using the well-established power-law framework, extracting the kinetic exponent known as the b-value at a fixed anodic potential. The b-value is a diagnostic dial: a value of 1.0 indicates ideal capacitive behavior, where current scales linearly with scan rate and charge is stored at surfaces, while 0.5 signals diffusion-controlled processes, where ions must slowly intercalate into the bulk. A b-value of 0.96 for the lowest-concentration sample (Mn-0.05) meant that electrode behaved almost like an ideal capacitor. The intermediate sample (Mn-0.10) dropped to 0.62, deep into diffusion-limited territory, and the highest concentration (Mn-0.20) recovered only partially to 0.74.

That non-monotonic pattern is the study’s most striking result. If morphology alone dictated kinetics, the trend should have been smooth. Instead, the intermediate composition was the most kinetically crippled of the three. The authors’ independent Dunn deconvolution analysis—which separates the current at each potential into surface-capacitive and diffusion-controlled contributions—reproduced the same ordering. At a scan rate of 10 millivolts per second, the capacitive fraction of stored charge was 80 percent for Mn-0.05, only 23 percent for Mn-0.10, and 44 percent for Mn-0.20. Those percentages are consistent with the values of 2b−1 predicted from the power-law exponents, a cross-check that lends confidence to the kinetic picture.

Electrochemical impedance spectroscopy filled in the mechanistic details. Fitting the spectra to an equivalent circuit comprising solution resistance, a charge-transfer resistance in parallel with a double-layer constant phase element, and a low-frequency constant phase element, the team achieved excellent fits with chi-squared values at or below 4.9 × 10−3. The charge-transfer resistance told a dramatic story: it plummeted from 3471 ohms for the intermediate sample and 174 ohms for the lowest concentration down to just 25 ohms for Mn-0.20. Meanwhile, the low-frequency exponent nL approached the Warburg limit of 0.53—the signature of semi-infinite diffusion—only for the highest-concentration material. In other words, the defect-rich, larger-structure Mn-0.20 moved charge across its interface with ease but then throttled it in slow, diffusion-limited storage.

The capacitance rankings added a final layer of nuance. Mn-0.20 delivered the highest specific capacitance, roughly 13 farads per gram, but its impedance was dominated by Warburg diffusion, meaning that capacity comes at the price of sluggish kinetics. Mn-0.05, by contrast, exhibited more ideal capacitive phase behavior, with phase angles near 70 degrees, but at a lower total capacity. Notably, capacitances derived independently from cyclic voltammetry, galvanostatic charge–discharge, and impedance spectroscopy ranked the three samples identically, and the galvanostatic and impedance values agreed to within 1.0 farad per gram—a level of internal consistency that strengthens the reliability of the conclusions.

For the supercapacitor field, the message is that precursor stoichiometry is not a dial for size alone; it is a dial for a coupled morphology–defect–kinetics relationship. Increasing manganese precursor concentration simultaneously enlarges the nanorod agglomerates and enriches the lattice with oxygen-vacancy disorder, and these two effects combine in a non-obvious way to redistribute charge between fast surface reactions and slow bulk diffusion. An electrode designer who wants rapid, high-power response might deliberately choose dilute precursor conditions to favor near-ideal capacitive behavior, while one chasing maximum capacity at moderate rates might accept the Warburg-limited regime of the concentrated recipe.

The work also speaks to a broader movement in electrochemistry toward defect engineering as a design principle. Oxygen vacancies in manganese oxides have been shown in numerous studies to boost conductivity and charge-transfer kinetics, but this study demonstrates that defect density can be controlled through something as mundane as the molarity of the starting salt—no dopants, no post-treatment, no conductive scaffolds required. Because the electrodes here are unsupported powders, the findings provide a baseline understanding of intrinsic material behavior that can later be layered onto composite and scaffold-based architectures. As demand grows for grid buffering, fast-charging electronics, and regenerative energy capture, understanding how a single synthesis variable redistributes the speed and location of charge storage brings the rational design of pseudocapacitive electrodes one concrete step closer.

Subject of Research: Effect of manganese precursor concentration on structural defects and charge-storage kinetics in hydrothermal beta-MnO2 supercapacitor electrodes

Article Title: Correlation between structural defects and charge storage kinetics in manganese oxide-based supercapacitor electrodes: the role of manganese precursor concentration

Article References: Nurfani, E., Mahardhika, L., Khamidy, N. I., Arundina, R. Y., & Marlina, R. (2026). Correlation between structural defects and charge storage kinetics in manganese oxide-based supercapacitor electrodes: the role of manganese precursor concentration. Ionics. https://doi.org/10.1007/s11581-026-07528-6

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07528-6

Keywords: supercapacitors, manganese dioxide, oxygen vacancies, hydrothermal synthesis, pseudocapacitance, charge-storage kinetics, Raman spectroscopy, electrochemical impedance spectroscopy, cyclic voltammetry, Dunn deconvolution, nanorods, defect engineering