A carefully controlled chemical transformation has produced a supercapacitor electrode that combines the advantages of metal oxides and metal sulfides, offering a promising route toward faster and more energy-dense storage devices. Researchers from Qinghai University have shown that changing the temperature of a sulfidation process by only a few dozen degrees can determine whether a nickel–cobalt–iron material remains amorphous, becomes an oxide, transforms into a sulfide, or develops a highly beneficial junction between two distinct crystalline phases. Their findings reveal that the most effective electrode was not the material containing the greatest possible amount of sulfur, but an intermediate structure formed under precisely tuned conditions.

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

Tags: amorphous versus crystalline electrode structurescontrolled sulfidation processenergy-dense supercapacitorsfast-charging energy storage devicesimproving supercapacitor energy densitymetal oxide and sulfide hybrid electrodesnickel-cobalt-iron electrode synthesisphase junction engineering in electrodesregenerative braking energy storagesupercapacitor electrode materialstailored chemical transformations for electrode performancetemperature-dependent phase transformation