Supercapacitors occupy a unique niche in the energy storage landscape. Unlike batteries, which store energy in slow chemical reactions, supercapacitors store charge at the interface between an electrode and an electrolyte, and in certain materials through rapid, reversible redox reactions known as pseudocapacitance. That mechanism allows them to charge and discharge in seconds, tolerate hundreds of thousands of cycles, and deliver high bursts of power. Their weakness has always been energy density: because conventional carbon-based supercapacitors store charge only at the surface, they hold far less energy per kilogram than batteries. Closing that gap is one of the central challenges in electrochemical energy storage, and it is precisely the problem the new composite is designed to address.

The research team, led by Jasi Akal Sugapriya Sathaiah and Julie Charles of the Department of Physics at Sri Sivasubramaniya Nadar College of Engineering in Kalavakkam, together with Dhakshinamurthy Divya of the Centre for Battery Engineering at Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology in Chennai, built the material through a combination of co-precipitation and polymerization methods. The approach first produces vanadium hexacyanoferrate, a Prussian blue analogue, and then integrates it with conductive polypyrrole and a web of multi-walled carbon nanotubes to form what the researchers call the VHCF-PPY-MWCNT nanocomposite.

The choice of vanadium hexacyanoferrate is chemically deliberate. Prussian blue analogues are open-framework metal-organic coordination compounds in which metal ions are linked by cyanide bridges, creating a lattice riddled with channels through which ions can move. Vanadium hexacyanoferrate offers two redox-active sites, one associated with vanadium and one with the iron-cyanide framework, allowing substantial charge storage through reversible ion insertion. The material has already attracted attention as a cathode for aqueous sodium-ion and zinc-ion batteries, but its modest electrical conductivity has limited its usefulness in high-rate supercapacitor electrodes.

That is where the other two components come in. Polypyrrole is a conducting polymer that wraps around the inorganic particles, improving adhesion, adding pseudocapacitive charge storage of its own and buffering the volume changes that occur as ions shuttle in and out of the crystal framework during charging and discharging. Multi-walled carbon nanotubes, meanwhile, act as a conductive skeleton. Because individual hexacyanoferrate particles are poorly conductive, electrons generated in one region of an electrode must find a path to the current collector, and resistance along that path wastes energy and degrades performance at high charging rates. The nanotubes weave the particles into an interconnected network, creating highways for electrons throughout the electrode and shortening the ion diffusion distances at the same time.

The team characterized the composite extensively before and after electrochemical testing, using analytical techniques to determine its crystal phase, structure, morphology, chemical bonding states and the oxidation states of the elements at its surface. Electrochemical evaluation followed using three standard methods: cyclic voltammetry, which sweeps the electrode potential back and forth to measure charge storage; galvanostatic charge-discharge, which cycles the electrode at constant current; and electrochemical impedance spectroscopy, which probes the resistance and ion transport behavior of the system. All tests were carried out in an alkaline potassium hydroxide electrolyte, an inexpensive and safe aqueous medium that avoids the flammable organic solvents used in many lithium-ion systems.

The results quantified the benefit of adding the nanotube network. The binary VHCF-PPY electrode, lacking nanotubes, achieved a respectable specific capacitance of 821.7 farads per gram at a scan rate of 5 millivolts per second. Introducing the carbon nanotube network raised that figure to 893.9 farads per gram under the same conditions, a gain attributable to the improved electronic pathways and more effective utilization of the active material. The ternary electrode also proved durable, retaining 81.4 percent of its initial capacitance after 5,000 charge-discharge cycles in the three-electrode configuration.

To test the material under realistic operating conditions, the researchers assembled a complete supercapacitor device using the VHCF-PPY-MWCNT composite as the electrode. The device delivered an energy density of 54.7 watt-hours per kilogram at a power density of 750 watts per kilogram when operated at a current density of 1 ampere per gram. For context, energy density determines how much energy a device can store, while power density determines how quickly that energy can be delivered. Aqueous supercapacitors frequently sacrifice one for the other; a device that combines energy density approaching that of some battery-supercapacitor hybrids with the power delivery characteristic of a true supercapacitor represents a meaningful advance.

Durability figures for the assembled device were equally striking. After 10,000 full charge-discharge cycles, the supercapacitor retained 72.9 percent of its initial capacitance and maintained a coulombic efficiency of 98.5 percent, meaning that nearly every unit of charge put into the device during charging was recovered during discharge. Coulombic efficiency is a sensitive indicator of parasitic side reactions; values close to unity over thousands of cycles suggest that the electrode material and the electrolyte interface remain chemically stable, with little energy lost to corrosion, gas evolution or irreversible structural change.

The design philosophy behind the work reflects a broader trend in electrode engineering: rather than seeking a single miracle material, researchers are combining components whose properties complement one another. In this case, the hexacyanoferrate framework provides abundant redox-active sites and open ion channels, polypyrrole contributes additional pseudocapacitance, mechanical flexibility and particle cohesion, and the carbon nanotubes supply the conductive scaffolding that allows all of that stored charge to be extracted quickly. Similar ternary strategies have been explored with cobalt and nickel hexacyanoferrates, graphene and carbon fibers, but the vanadium-based system offers the advantage of dual redox centers within a single, easily synthesized framework.

The synthesis route is also notable for its simplicity. Co-precipitation and polymerization are both low-cost, scalable processes that do not require high temperatures, vacuum systems or exotic precursors, which matters if laboratory results are ever to translate into commercially viable electrodes. The authors acknowledge the use of electrochemical facilities at the Centre for Battery Engineering at Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology in Avadi, Chennai, and infrastructural support from their home institution.

Supercapacitors are expected to play a growing role in applications where batteries struggle: regenerative braking, grid frequency regulation, backup power and smoothing the intermittent output of solar and wind installations. Hybrid systems that pair batteries with supercapacitors can extend battery life by shielding them from high-current pulses, and the economics of such systems improve sharply as supercapacitor energy density rises. Materials like the VHCF-PPY-MWCNT composite, which combine high capacitance, good rate capability and long cycle life in an aqueous electrolyte, could accelerate that shift.

The study, published in Ionics, demonstrates that careful architectural engineering at the nanoscale, in this case weaving a conductive nanotube network through a polymer-coated Prussian blue analogue, can deliver substantial gains in electrochemical performance without changing the underlying storage chemistry. As demand for fast, durable and safe energy storage continues to climb, such multi-component electrode designs are likely to remain at the forefront of the field, bridging the long-standing gap between the speed of a capacitor and the stamina of a battery.

Supercapacitors are valued for their ability to charge and discharge far more rapidly than conventional batteries. They can deliver powerful bursts of energy, tolerate repeated cycling and operate reliably in applications ranging from regenerative braking and portable electronics to grid-support systems and renewable-energy installations. Their major limitation, however, is energy density. Although they can release power quickly, they generally store less energy per unit mass than batteries. This challenge has driven scientists to explore electrode materials that can combine rapid electron transport with abundant electrochemically active sites, allowing supercapacitors to store more charge without sacrificing their characteristic high-power performance.

Ternary transition-metal sulfides have emerged as attractive candidates because they contain multiple metals capable of participating in reversible redox reactions. Nickel, cobalt and iron can each contribute to charge storage, while sulfur-containing compounds often exhibit higher electrical conductivity than their oxide counterparts. Yet producing a high-performance sulfide is not simply a matter of adding more sulfur. During sulfidation, the conversion from a hydroxide precursor to an oxide and then to a sulfide can involve several competing reactions, changes in crystal structure and rearrangements at the nanoscale. If these transformations are not controlled, the resulting material may have poor conductivity, unstable morphology or an unfavorable balance between active and inactive phases.

In the new study, Qing Pang, Hao Wu, Tengfei Wang, Boyu Liu and Hongyu Wang investigated how nickel cobalt iron hydroxide, abbreviated NiCoFe-OH, evolves during controlled sulfidation. The researchers varied the reaction temperature from 35 to 115 degrees Celsius and examined the resulting materials using structural, microscopic and electrochemical analyses. At the lower end of the temperature range, the precursor retained an amorphous hydroxide-like structure. As the temperature increased, it passed through an oxide-dominated state before eventually developing a crystalline sulfide phase. This sequence provided the team with a detailed view of how the electrode’s chemistry and performance changed during the conversion process.

The critical composition appeared at 95 degrees Celsius. The material produced under these conditions, named NCF-S95, contained both a crystalline ternary transition-metal oxide and a crystalline ternary transition-metal sulfide. Rather than forming a simple mixture of large particles, the two phases were integrated within a nanosheet architecture. This morphology creates a large interfacial area, shortening the distance that ions must travel during charging and discharging. It also exposes more electrochemically active regions to the electrolyte, the ion-conducting medium that enables charge storage inside the device.

The performance advantage of NCF-S95 arises from the complementary roles of its two phases. The oxide component can provide mechanical and structural stability, helping the electrode withstand repeated expansion, contraction and redox reactions during cycling. The sulfide component generally offers greater electrical conductivity and strong electrochemical activity, supporting rapid movement of electrons through the electrode. At the boundary between them, electronic states and chemical environments can differ from those in either pure phase. Such a heterojunction may create an efficient pathway for charge transfer, reduce interfacial resistance and improve the movement of electrolyte ions. In effect, the researchers engineered a nanoscale contact zone that allows the oxide and sulfide to compensate for each other’s weaknesses.

Electrochemical measurements confirmed the importance of this intermediate structure. NCF-S95 achieved a specific capacity of 171.52 milliampere-hours per gram at a current density of 2 milliamperes per square centimeter, the highest value among the materials tested in the study. Specific capacity describes how much charge an electrode can store relative to its mass, and the result indicates that the carefully formed oxide–sulfide architecture provided more effective use of the active material. The electrode retained 62.28 percent of its capacity after 10,000 charge–discharge cycles, demonstrating substantial durability despite the intense chemical and structural changes that occur during repeated operation.

To test the material in a more practical configuration, the researchers paired NCF-S95 with activated carbon to assemble an asymmetric supercapacitor. In this design, the two electrodes store charge through different mechanisms, allowing the device to operate across a broader voltage window than a symmetric carbon-based system. The assembled device delivered an energy density of 32.7 watt-hours per kilogram at a power density of 400 watts per kilogram. Energy density indicates how much energy the device can store, while power density describes how quickly that energy can be delivered. The device retained 63.1 percent of its capacity after 7,000 cycles, and two devices connected in series were able to illuminate an LED bulb for 14 minutes, illustrating the potential of the material beyond measurements made on an individual electrode.

The results challenge a common assumption in materials design: that maximizing conversion to the final sulfide must automatically produce the best electrode. Instead, the study shows that intermediate phases can be central to performance. A fully sulfided material may provide conductivity and redox activity, but it can lack the stability or interfacial advantages offered by a carefully preserved oxide component. By tuning temperature, researchers were able to stop the transformation at a point where both phases coexisted in a favorable nanosheet structure. This approach, known as phase engineering, could be adapted to other multimetal electrode systems in which controlled interfaces are used to direct electron and ion transport.

The researchers say that understanding the pathway from hydroxide to oxide and sulfide is essential for designing next-generation supercapacitors rationally rather than relying on trial and error. Their work suggests that temperature-controlled synthesis can serve as a practical tool for adjusting crystal structure, chemical composition and heterojunction density at the same time. Further studies will be needed to determine how the material performs in larger devices, under different electrolyte conditions and at higher industrially relevant mass loadings. Even so, the discovery offers a striking example of how a small change in processing conditions can generate a major difference in energy-storage behavior—and how the most powerful solution may lie not in a single material, but in the precisely engineered boundary between two of them.

Subject of Research: Controlled sulfidation of nickel–cobalt–iron hydroxide for high-performance supercapacitor electrodes

Article Title: Controlled sulfidation of ternary transition metal towards high performance electrode materials for supercapacitors

News Publication Date: 4-Aug-2026

Web References: https://doi.org/10.48130/een-0026-0015

References: Pang Q, Wu H, Wang T, Liu B, Wang H. 2026. “Controlled sulfidation of ternary transition metal towards high performance electrode materials for supercapacitors.” Energy & Environment Nexus 2: e022. DOI: 10.48130/een-0026-0015

Image Credits: Qing Pang, Hao Wu, Tengfei Wang, Boyu Liu & Hongyu Wang

Keywords

Supercapacitors, energy storage, ternary transition-metal sulfides, nickel cobalt iron hydroxide, heterojunctions, phase engineering, electrode materials, electrochemistry, nanosheets, oxide–sulfide interfaces