Cobalt Sulfide Meets Nickel Selenide: A Tiny Interface That Supercharges Supercapacitors

Supercapacitors promise the best of both worlds in energy storage: they charge in seconds, deliver bursts of power that batteries cannot match, and survive tens of thousands of charge-discharge cycles. Yet their Achilles heel has always been energy density, the amount of energy they can pack per kilogram. A research team led by Imran Khan and Akif Safeen at the University of Poonch Rawalakot, working with collaborators across Pakistan, China, Saudi Arabia, Thailand and Ethiopia, now reports a cleverly engineered electrode material that pushes supercapacitor performance to striking new levels. Writing in the Journal of the Saudi Chemical Society, the researchers describe a cobalt sulfide-nickel selenide heterostructure, a nanoscale marriage of two transition metal compounds whose intimate interface transforms the way the electrode stores and shuttles electrical charge.

The logic behind the design begins with the chemistry of the two building blocks. Cobalt sulfide, or CoS, is a well-studied pseudocapacitive material: in an alkaline electrolyte it undergoes fast, reversible faradaic redox reactions that store charge at the electrode surface. Compared with metal oxides, metal sulfides conduct electricity better because sulfur atoms, with their larger atomic radius and lower electronegativity than oxygen, form weaker metal-sulfur bonds and allow electrons to delocalize more freely. But CoS has a serious flaw. During repeated cycling, its structure swells and contracts, gradually degrading the electrode. Nickel diselenide, NiSe2, brings complementary virtues: selenium conducts electricity even better than sulfur, and NiSe2 displays multivalent redox behavior with resistivity below 10 to the minus 3 ohm-centimeters, making it an exceptionally conductive host. Its weakness is poor mechanical strength. The team reasoned that fusing the two could let each material compensate for the other’s shortcomings.

The synthesis itself is disarmingly simple, a point the authors emphasize as key to commercial viability. CoS microspheres were grown hydrothermally from cobalt chloride and thiourea in two thermal steps, first at 105 degrees Celsius and then at 240 degrees Celsius. NiSe2 nanoparticles were produced by reacting nickel nitrate and selenium powder in hydrazine hydrate inside a sealed autoclave at 180 degrees Celsius for 24 hours. The two powders were then blended in ethanol, sonicated, stirred and oven-dried, a wet chemical route requiring no exotic equipment. X-ray diffraction confirmed that both crystalline phases survived intact in the composite, with sharp diffraction peaks matching standard reference cards and no detectable impurities. Raman spectroscopy told a subtler story: in the composite, the characteristic vibrational peaks of each material merged and broadened, a fingerprint of interfacial strain, lattice disorder and strong phonon scattering at the newly formed heterointerface, direct evidence that the two compounds were genuinely coupled rather than merely mixed.

Electron microscopy revealed the composite’s architecture. Pure CoS forms nest-like microspheres, while NiSe2 appears as smooth, dense cubes. In the hybrid, cube-shaped and spherical nanoparticles disperse uniformly together, creating a textured landscape rich in exposed surfaces. Energy-dispersive X-ray spectroscopy detected only cobalt, sulfur, nickel and selenium, confirming high purity. Perhaps most telling were the gas-adsorption measurements. The BET surface area climbed from 12 square meters per gram for pure CoS to 19 for NiSe2 and 28 for the composite, while pore-size analysis showed a well-developed network of mesopores concentrated between 15 and 35 nanometers. That porous architecture matters enormously in practice: it prevents the nanoparticles from clumping together, multiplies the number of electrochemically active sites, and lets hydroxide ions from the electrolyte flood into the electrode with minimal diffusion resistance.

The electrochemical payoff was dramatic. In a standard three-electrode cell filled with 2 molar potassium hydroxide, cyclic voltammetry showed that the composite electrode produced the largest current response and the biggest enclosed curve area of all three samples, with clear redox peaks arising from cobalt and nickel ions shuttling between the 2-plus and 3-plus oxidation states. Crucially, those peaks barely distorted even at high scan rates, a sign of outstanding reversibility and fast ion transport. Galvanostatic charge-discharge tests delivered the headline number: a specific capacitance of 871.4 farads per gram at 1 ampere per gram, far exceeding the individual components. The discharge curves showed a pronounced plateau at low potentials, characteristic of battery-type intercalation processes running alongside surface capacitive storage, a hybrid mechanism that the impedance data reinforced, with the composite showing the lowest solution and charge-transfer resistances of any sample.

To understand where the stored charge actually resides, the team applied Dunn’s and Trassati’s kinetic analyses to the voltammetry data. The b-values, which reveal whether charge storage is surface-dominated or diffusion-controlled, fell between 0.5 and 1 for the composite and drifted toward 1, indicating that capacitive surface reactions dominate while diffusion still contributes meaningfully. At low scan rates, ions have time to penetrate deep into the nanostructure, so diffusion-controlled storage prevails; at high rates, surface adsorption takes over. The practical implication is that the heterointerface enables both mechanisms simultaneously, letting the electrode harvest charge from its bulk and its surface alike, which is precisely the synergy the designers were aiming for.

The real test came when the researchers assembled a full asymmetric supercapacitor, pairing the CoS-NiSe2 composite as the positive electrode with activated carbon as the negative one. Electrode masses were carefully balanced using the charge-balance equation so that neither electrode limits the other. The device operated stably across a generous 1.6-volt window, free of the oxygen evolution reactions that plague aqueous devices pushed too far. It delivered a specific capacitance of 173 farads per gram at 1 ampere per gram, an energy density of 61.5 watt-hours per kilogram at a power density of 950 watts per kilogram, and sustained output at a maximum power density of 4010 watts per kilogram. For context, those energy figures approach the lower range of lithium-ion batteries while retaining the raw power and cycle life that only supercapacitors offer, and they outperform most previously reported cobalt sulfide and nickel selenide devices.

Durability, the perennial stumbling block, also held up impressively. After 5,000 charge-discharge cycles at a demanding 8 amperes per gram, the device retained 91.4 percent of its initial capacitance, with coulombic efficiency hovering around 94 percent, indicating highly reversible chemistry with few parasitic side reactions. The modest decline the authors attribute to gradual structural collapse and conductivity loss over extended cycling, familiar degradation pathways in pseudocapacitive chalcogenides, but the interfacial coupling appears to buffer the volume changes that normally destroy pure CoS electrodes. Impedance spectroscopy on the full device showed a solution resistance of just 1.7 ohms and charge-transfer resistance of 3.5 ohms, confirming that the conductive network forged at the sulfide-selenide interface carries electrons with minimal loss.

What makes this work resonate beyond the laboratory bench is its economics and its message. The entire synthesis relies on inexpensive, analytical-grade chemicals, water-based hydrothermal chemistry and a simple blending step, with no precious metals, templates or high-vacuum deposition. The authors, whose study was funded by Pakistan’s Higher Education Commission and King Saud University’s research funding program, argue that the strategy offers an economically feasible route to next-generation storage for renewable energy buffering and portable electronics. More broadly, the result underscores a growing theme in materials science: the most valuable real estate on an electrode may be the atomically thin boundary where two imperfect materials meet. By engineering that interface deliberately rather than leaving it to chance, researchers can coax out synergies, faster electron transfer, richer redox activity, and structural resilience, that neither compound achieves alone. As solar and wind power continue their intermittent march onto the grid, storage devices built on such heterointerface engineering may prove essential to keeping the lights on when the sun sets and the wind stalls.

Subject of Research: CoS-NiSe2 heterostructure electrodes for high-performance asymmetric supercapacitors

Article Title: Interfacial coupling and redox engineering in CoS-NiSe2 heterostructures for high-performance asymmetric supercapacitors

Article References: Khan, I., Arif, D., Shah, S. H., Safeen, K., Shah, M. Z. U., Alotaibi, K. M., Din, S. U., Rehan, I., Haider, F., Girma, W. M., & Safeen, A. (2026). Interfacial coupling and redox engineering in CoS-NiSe2 heterostructures for high-performance asymmetric supercapacitors. Journal of Saudi Chemical Society, 30(5), Article 72. https://doi.org/10.1007/s44442-026-00115-4

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00115-4

Keywords: supercapacitors, CoS-NiSe2, heterostructure, pseudocapacitance, energy storage, transition metal chalcogenides, hydrothermal synthesis, specific capacitance, energy density, cycling stability, electrochemistry, nanocomposites