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 appeal of metal sulfide anodes lies in their chemistry. Compounds containing two electrochemically active metals can undergo multiple redox reactions, and the presence of sulfur helps accommodate some of the volumetric strain that develops when metals alloy with lithium or sodium. Bismuth sulfide, for example, offers a theoretical capacity of about 625 mAh per gram through the formation of Li3Bi or Na3Bi phases. Crucially, bismuth sulfide and antimony sulfide form a complete solid solution across the entire compositional range, meaning researchers can tune the bismuth-to-antimony ratio freely to optimize performance. Previous studies of related compositions, such as Bi0.94Sb1.06S3 nanorod clusters, had already shown improved cycling stability compared with single-metal sulfide anodes, but the fabrication method itself remained a limiting factor in how uniformly the active material adhered to the current collector.

The Indian team’s solution was to eliminate the polymer binder altogether. Conventional electrodes are made by mixing active material with conductive additives and a glue-like binder, then coating the slurry onto metal foil. Binders add dead weight, can impede electron transport, and may fail mechanically over time. Instead, the researchers used electrophoretic deposition, a technique in which charged particles suspended in a liquid migrate toward and deposit onto an oppositely charged electrode under an electric field. By applying a constant DC voltage of around 300 volts in an isopropanol-based bath containing nickel nitrate, poly(acrylic acid), and carbon black, they fabricated complete electrodes on copper foil in roughly three minutes. The method is inherently scalable and produces a film whose adhesion, porosity, and uniformity can be controlled through deposition parameters rather than through binder chemistry.

Synthesis of the active material itself relied on a one-step hydrothermal process. Graphene oxide, prepared by a modified Hummer’s method from graphite powder, was dispersed in water and sonicated to achieve partial exfoliation. Bismuth nitrate and antimony trichloride precursors were dissolved separately and combined, then thioacetamide was added as the sulfur source. The negatively charged graphene oxide sheets electrostatically attracted the positively ionized Bi3+ and Sb3+ species, promoting a uniform distribution of metal ions across the carbon surface. Under hydrothermal conditions of 200 degrees Celsius for 24 hours in a Teflon-lined autoclave, the amorphous bismuth-antimony-sulfur complex nucleated and grew into crystalline nanorods anchored on the reduced graphene oxide sheets. Prolonged treatment favored unidirectional growth along the (011) crystallographic plane, producing the one-dimensional nanorod clusters that give the material its distinctive morphology.

Characterization confirmed the composite’s structure in detail. X-ray diffraction revealed an orthorhombic Bi1.09Sb0.91S3 phase matching the standard reference pattern, with crystallite sizes of approximately 22 nanometers in the composite compared with about 30 nanometers for the pristine sulfide. Raman spectroscopy showed the characteristic D-band and G-band of reduced graphene oxide, and thermogravimetric analysis quantified the composition at roughly 94 weight percent sulfide and 6 weight percent graphene. Electron microscopy showed nanorod clusters with average diameters near 35 to 40 nanometers and lengths around 0.35 micrometers, well anchored to the graphene sheets without phase segregation. Nitrogen adsorption measurements revealed that adding graphene increased the surface area from about 11 square meters per gram to roughly 48 square meters per gram, providing more electrochemically active reaction sites and shorter ion diffusion pathways.

Electrochemical testing in lithium-ion coin cells highlighted the graphene’s contribution. The composite anode delivered an initial discharge capacity of about 1269 mAh per gram, with the substantial first-cycle irreversibility attributed to electrolyte decomposition and solid electrolyte interphase formation, a familiar feature of nanostructured metal sulfide anodes. From the second cycle onward the electrode stabilized, and over 100 cycles at 0.5 amperes per gram it maintained average capacities near 506 mAh per gram, substantially outperforming the pristine sulfide electrode, which managed about 331 mAh per gram. Rate capability testing told a similar story: at 4 amperes per gram the composite still delivered about 128 mAh per gram, more than double the 58 mAh per gram of the bare material, and when the current was lowered back to 0.5 amperes per gram the composite recovered roughly 353 mAh per gram, evidence of excellent structural reversibility.

Sodium-ion performance followed the same pattern. Cyclic voltammetry identified the expected sequence of reactions: conversion of the sulfide into metallic bismuth, antimony, and sodium sulfide, followed by alloying of the metals into Na3Bi and Na3Sb, with the reverse reactions on charge. Over 33 cycles at 0.1 amperes per gram, the composite averaged about 413 mAh per gram with a Coulombic efficiency near 95 percent, compared with roughly 257 mAh per gram for the pristine electrode. Impedance spectroscopy revealed the underlying kinetics: the composite’s charge transfer resistance after cycling measured just 12.5 ohms in lithium cells and 21.7 ohms in sodium cells, dramatically lower than the pristine material’s values, and calculated diffusion coefficients confirmed faster ion transport through the graphene-supported structure. The team noted that sodium-ion gains were more modest than lithium-ion gains because the larger sodium ion is intrinsically slower and more constrained by the crystal lattice.

Perhaps the most compelling evidence came from postmortem analysis using synchrotron X-ray absorption spectroscopy and electron microscopy. Ex situ XANES measurements at the Bi L3-edge and Sb K-edge showed the reduction of trivalent bismuth and antimony to their metallic states during discharge, with irreversible edge shifts consistent with the formation of pseudo-crystalline Li3Bi, Na3Bi, Li3Sb, and Na3Sb phases. Cross-sectional electron microscopy quantified the mechanical toll of cycling: the pristine electrode swelled from about 10.9 micrometers to 29 to 34 micrometers, nearly a threefold increase, while the graphene composite expanded only to about 20.4 micrometers in lithium cells and 18.2 micrometers in sodium cells. The pristine electrode’s nanorods fragmented and agglomerated into dense aggregates riddled with voids, whereas the composite largely retained its nanorod morphology, with the graphene sheets acting as a mechanical buffer that absorbed volume fluctuations and prevented active material from detaching from the copper collector.

The study demonstrates that combining a tunable bimetallic sulfide solid solution with a conductive, flexible graphene scaffold and a fast, binder-free deposition process can produce an anode that works effectively in both lithium and sodium chemistries. Because sodium-ion batteries rely on abundant, inexpensive materials and are viewed as a complement to lithium technology for grid-scale storage, an anode strategy that serves both systems is particularly valuable. The electrophoretic deposition step, completed in minutes at ambient conditions, avoids the energy-intensive drying and calendering complexities of slurry coating while producing electrodes with a practical mass loading of about 1.3 milligrams per square centimeter. As the authors conclude, the Bi1.09Sb0.91S3/reduced graphene oxide composite represents a promising dual-functional anode candidate, and the scalable fabrication route suggests a plausible path from laboratory coin cells toward practical electrode manufacturing.

Subject of Research: Binder-free bismuth-antimony sulfide nanorod anodes on reduced graphene oxide for lithium-ion and sodium-ion battery storage

Article Title: Binder-free Bi₁.₀₉Sb₀.₉₁S₃ nanorod clusters anchored on reduced graphene oxide via electrophoretic deposition for enhanced lithium and sodium-ion storage

Article References: Dashairya, L., Das, D., & Saha, P. (2026). Binder-free Bi₁.₀₉Sb₀.₉₁S₃ nanorod clusters anchored on reduced graphene oxide via electrophoretic deposition for enhanced lithium and sodium-ion storage. Discover Electrochemistry, 3(1), Article 28. https://doi.org/10.1007/s44373-026-00116-3

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00116-3

Keywords: lithium-ion batteries, sodium-ion batteries, bismuth sulfide, antimony sulfide, reduced graphene oxide, electrophoretic deposition, binder-free electrode, anode materials, nanorods, solid solution, XANES, energy storage