The oxygen evolution reaction, or OER, is the demanding four-electron process by which water molecules are torn apart at an electrode to release oxygen gas. Because each water molecule must surrender four electrons and four protons in a sequence of energetically unfavorable steps, the reaction imposes a substantial extra voltage, called overpotential, on any electrolyzer. Every additional millivolt of overpotential translates directly into wasted electricity and higher-cost hydrogen. Precious metal oxides such as iridium dioxide perform well but are scarce and expensive, which is why the field has pressed hard on abundant alternatives such as first-row transition metal oxides, particularly the nickel cobalt spinel NiCo2O4, a compound prized for its flexible redox chemistry and its array of accessible nickel and cobalt active sites.

NiCo2O4 has a well-known weakness, however: it is a mediocre electrical conductor. Electrons generated or consumed at its active sites must fight their way through a resistive oxide lattice, slowing reaction kinetics and blunting performance. The new study confronts this bottleneck by interleaving the spinel with Ti3C2Tx MXene, a member of the family of two-dimensional carbides discovered in 2011. MXenes are produced by chemically etching away the atomic layers of a layered ceramic precursor, leaving behind single-atom-thick sheets of titanium carbide terminated with surface groups such as oxygen, hydroxyl and fluorine, the Tx in the formula. These terminations make the sheets hydrophilic and chemically versatile while preserving metallic-like conductivity, a combination that has made MXenes a favorite scaffold for energy materials.

The synthesis was deliberately simple. The researchers employed a hydrothermal route, growing NiCo2O4 in the presence of varying amounts of MXene nanosheets, ranging from none at all to 100 milligrams, yielding a series of composites labeled MXene@NCO-X, where X denotes the MXene loading. Hydrothermal synthesis involves sealed autoclaves in which aqueous precursors are heated well above the boiling point, allowing crystals to nucleate and grow on the dispersed MXene sheets. The resulting composites, examined by structural and surface characterization techniques including X-ray photoelectron spectroscopy, revealed that the spinel did not merely sit on the MXene surface but formed intimate interfacial contact with it, an electronic handshake that proved central to the catalyst’s behavior.

Among the series, one formulation stood out decisively. The composite containing 50 milligrams of MXene, designated MXene@NCO-50, required an overpotential of only 360 millivolts to drive a current density of 10 milliamperes per square centimeter in 1 molar potassium hydroxide electrolyte, a standard benchmark condition for alkaline water splitting. Its Tafel slope, the parameter that describes how quickly current rises with applied voltage and therefore how fast the reaction accelerates, was a lean 65.71 millivolts per decade. Both figures compare favorably with the pristine spinel and with the under- and over-loaded composites, marking an optimum in a landscape where too little MXene leaves the conductivity deficit uncorrected and too much buries or dilutes the spinel’s active sites.

The physical evidence pointed to a clear mechanism. Electrochemical impedance spectroscopy, which dissects the resistive and capacitive elements of an electrode interface, showed that MXene@NCO-50 possessed faster charge transfer kinetics than any of its siblings. Measurements of the double-layer capacitance, a proxy for the electrochemically active surface area, revealed that this sample also exposed the largest effective reaction area. Electron microscopy linked these electrochemical advantages to architecture: the optimized composite grew as well-defined nanorod architectures anchored on the conductive MXene sheets, an arrangement that simultaneously supplies abundant active sites and builds an unobstructed highway for electrons to reach them.

The authors attribute the outstanding activity to a synergistic interaction between the conductive MXene nanosheets and the NiCo2O4 spinel. In essence, the MXene solves the spinel’s conductivity problem while the spinel supplies the catalytic chemistry that the MXene alone lacks. The interfacial electronic interaction between the two phases can modulate the oxidation states and adsorption energies of nickel and cobalt centers, easing the binding of oxygen intermediates along the four-electron pathway. This composite design echoes a broader trend in the literature, where MXene hybrids with cobalt hydroxides, ferrites and other spinels have repeatedly shown that coupling two-dimensional carbides with transition metal oxides produces catalysts greater than the sum of their parts.

Durability, the perennial Achilles heel of non-precious OER catalysts operating in hot, corrosive alkaline media, also held up. The optimized catalyst demonstrated excellent electrochemical stability over prolonged operation, retaining its performance through extended electrolysis. For any envisioned electrolyzer or metal-air battery, such longevity matters as much as the headline overpotential, since catalyst dissolution and phase degradation during long-term current flow are the practical barriers that keep laboratory champions out of commercial stacks.

The significance of the study lies less in any single number than in the design principle it crystallizes: MXene concentration is a tunable dial for OER performance. Rather than discovering a new compound, the team systematically mapped how dosage reshapes morphology, conductivity and activity, and identified the sweet spot where synergy is maximized. As nations scale up green hydrogen production under tightening decarbonization targets, catalysts built from earth-abundant nickel, cobalt and titanium, assembled through a facile hydrothermal process with no exotic equipment, offer a credible path to cheaper electrolysis. The work highlights the promising potential of MXene and spinel oxide nanohybrids as advanced electrocatalysts for sustainable energy applications, and it gives the field a practical recipe: if you want faster oxygen evolution, count your MXene carefully.

Subject of Research: The influence of Ti3C2Tx MXene concentration on the oxygen evolution reaction performance of NiCo2O4 spinel electrocatalysts

Article Title: Tuning the oxygen evolution reaction performance: the influence of MXene concentration in NiCo2O4 catalysts

Article References: Vazhayil, A., C., S. A., Vazhayal, L., Thomas, J., Chicardi, E., Rokhum, S. L., Safra, I., Jeffery, A. A., & Thomas, N. (2026). Tuning the oxygen evolution reaction performance: the influence of MXene concentration in NiCo2O4 catalysts. Ionics. https://doi.org/10.1007/s11581-026-07506-y

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07506-y

Keywords: MXene, NiCo2O4, oxygen evolution reaction, electrocatalysis, water splitting, green hydrogen, spinel oxide, nanocomposite, Tafel slope, charge transfer, renewable energy, hydrothermal synthesis