Nickel oxide has been a perennial favorite in pseudocapacitor research for good reason. Unlike carbon electrodes, which store charge electrostatically at the surface, nickel-based oxides participate in fast, reversible surface redox reactions in alkaline electrolytes, where hydroxide ions interact with nickel sites to store charge chemically. This faradaic mechanism, the hallmark of pseudocapacitance, can pack far more charge into a given mass of material than pure double-layer storage. The catch is kinetics: pristine nickel oxide is a relatively poor electrical conductor, and its redox reactions can stall when electrons and ions cannot reach active sites quickly enough. Researchers have therefore spent years trying to open up the material’s electronic structure and morphology without sacrificing its intrinsic charge-storage capacity.

The team, led by researchers including Muhammad Shahbaz, Mohsin Saleem, Jung Hyuk Koh, and Kamran Ali, took a dual-element approach. Using a rapid autocombustion synthesis—a self-sustaining exothermic reaction that converts precursor solutions into oxide powders in moments—they fabricated both pristine nickel oxide (NO) and a cobalt- and calcium-co-modified variant (CCNO). The choice of dopants is chemically deliberate. Cobalt, a neighboring transition metal with its own rich redox chemistry, is known to enhance electronic conductivity and contribute additional redox-active sites. Calcium, an alkaline earth metal with no redox activity of its own, acts as a structural modifier, perturbing the oxide lattice and influencing grain growth and morphology. Together, the two elements were intended to tune both the electronic and the microstructural dimensions of charge storage simultaneously.

X-ray diffraction patterns of the synthesized powders indexed to a hexagonal phase assigned to nickel(III) oxide, Ni2O3, according to reference card JCPDS 014-0481. The authors are careful to flag a caveat here: without spectroscopic valence analysis to confirm the nickel oxidation state, this phase assignment remains provisional, and the material is referred to throughout simply as nickel oxide. Such caution matters in a field where the distinction between NiO and Ni2O3 carries real electrochemical consequences, since trivalent nickel sites are associated with enhanced redox activity. The diffraction data nevertheless establish that co-modification did not destroy the parent oxide framework, preserving the crystalline host while subtly altering its internal structure.

Field-emission scanning electron microscopy revealed perhaps the most visually consequential effect of the co-modification. The CCNO powder displayed a connected, coarsened morphology—a network of interlinked particles rather than isolated grains. This architecture is not merely cosmetic. In an electrode, ions from the electrolyte must diffuse through and across the material to reach redox-active sites, and electrons must percolate through the particle network to the current collector. A connected, coarsened structure provides continuous pathways for both species, reducing diffusion lengths and interfacial resistance. The researchers argue that this morphology directly accelerates the redox kinetics, allowing the electrode to sustain its capacitive behavior even at high charge-discharge rates where poorly connected materials typically fail.

To probe the electronic consequences of co-modification, the team turned to optical spectroscopy. Ultraviolet-visible absorption, photoluminescence emission, and Raman spectra collectively revealed structural and electronic variations induced by the cobalt and calcium incorporation. These techniques serve as sensitive fingerprints of how dopants perturb defect populations, band structure, and lattice vibrations—changes that translate directly into altered charge transport behavior. While the paper does not claim a single dominant mechanism, the spectroscopic evidence supports the picture of a modified oxide whose electronic landscape has been reshaped by the dual-element treatment, priming it for faster electrochemical response.

The electrochemical results are where the strategy pays off quantitatively. In a three-electrode configuration, the CCNO electrode blended with activated carbon achieved a maximum specific capacitance of 318 farads per gram at a current density of 2 amperes per gram. Cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge measurements together characterized the electrode’s behavior across timescales, with impedance data revealing reduced charge-transfer resistance consistent with the morphological and electronic modifications. The researchers are transparent about an important limitation: because pristine and singly modified reference electrodes were not synthesized under identical conditions, the improvement of CCNO over pristine nickel oxide cannot be cleanly apportioned between the cobalt-calcium incorporation and the conductive carbon network. Both contributions are discussed explicitly rather than hidden behind a single attribution.

The true test of any electrode material is how it performs in a complete device, where two dissimilar electrodes must operate in tandem across a shared voltage window. The researchers assembled an asymmetric hybrid supercapacitor pairing the CCNO/AC composite as the positive electrode with activated carbon as the negative electrode. Normalized to the combined active mass of both electrodes, the device delivered 160 farads per gram at a scan rate of 10 millivolts per second and 121 farads per gram at 2 amperes per gram. More strikingly, it achieved an energy density of 32.9 watt-hours per kilogram, with power density climbing from approximately 1.6 to 6 kilowatts per kilogram as the current density increased from 2 to 7 amperes per gram. These figures place the device in competitive territory for hybrid supercapacitors, approaching the energy storage regime where batteries begin to matter while retaining the power delivery and cycle life that define supercapacitor technology.

Durability, the quiet killer of promising electrode materials, held up impressively. Over 3000 galvanostatic charge-discharge cycles, the CCNO/AC device retained a coulombic efficiency of approximately 100 percent, meaning that nearly every charge stored during each cycle was recovered on discharge with virtually no parasitic losses. This kind of stability is essential for real-world deployment, where electrodes must withstand thousands of deep cycles without degrading. The structural integrity of the co-modified oxide, combined with the robust carbon network, appears to provide the mechanical and electrochemical resilience needed to sustain repeated redox cycling without capacity fade.

What makes this work notable beyond its headline numbers is its honest accounting of mechanisms and its demonstration that dual-element modification can be a rational design tool rather than trial-and-error alchemy. Cobalt contributes redox activity and electronic conductivity; calcium reshapes morphology and lattice structure; and the activated carbon network provides the conductive scaffold that ties everything together. The synergy among these three elements—engineered oxide, structural modifier, and carbon network—produces an electrode whose ion diffusion pathways, surface redox reactions, and charge transfer kinetics are all simultaneously optimized. As the demand for fast-charging, long-lived energy storage grows across renewable energy integration, electric transportation, and portable electronics, strategies like this one that squeeze more performance from cheap, scalable materials may prove decisive in bringing hybrid supercapacitors from the laboratory bench to the power grid.

Subject of Research: Tailoring redox activity and electrochemical kinetics of nickel oxide through Co and Ca co-modification for high-performance hybrid supercapacitors

Article Title: Tailoring redox activity and electrochemical kinetics of nickel oxide through Co and Ca co-modification for high-performance hybrid supercapacitors

Article References: Shahroz, Shahbaz, M., Saleem, M., Khan, M. Z., Irshad, M., Sharif, S., Koh, J. H., Mureed, S., Ghaffar, A., Shakir, I., & Ali, K. (2026). Tailoring redox activity and electrochemical kinetics of nickel oxide through Co and Ca co-modification for high-performance hybrid supercapacitors. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13810-9

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13810-9

Keywords: Tailoring, redox, activity, electrochemical, kinetics, nickel, oxide, co-modification, high-performance, hybrid, supercapacitors, scientific research