Cobalt–Nickel Boride Nanoflowers Made in 20 Seconds Boost Oxygen Evolution Catalysis

Chemists have synthesized a crystalline cobalt–nickel boride catalyst in just twenty seconds, using molten boron oxide as both a reaction medium and a template that sculpts the product into nanoflowers. The material, CoNiB₂, was reported in the journal Catal by a team led by Ruifeng Qi, Junqi Liu and Qingsong Huang, who showed that it drives the oxygen evolution reaction — the sluggish half of water electrolysis that limits cheap hydrogen production — with an overpotential of 271 millivolts at a current density of 10 milliamperes per square centimeter in alkaline media. That figure undercuts the precious-metal benchmark ruthenium dioxide, which needed 337 millivolts at the same loading in the same tests, and it was achieved with a Tafel slope of only 67 millivolts per decade, the lowest among the catalysts compared in the study.

The appeal of the work lies as much in the manufacturing route as in the electrochemistry. The new method, which the authors call liquid-phase soaking technology, starts from ordinary raw oxides: cobalt monoxide, nickel monoxide and boron powder are ball-milled together in a CoO:NiO:B molar ratio of 1:1:4, with an equal mass of boron oxide added, and pressed into a dense disc two centimeters across under 15 megapascals of pressure.

That disc is then placed on graphite paper, which acts as the heating source, inside a vacuum magnetic levitation induction furnace set to a coil current of 25 amperes and a chamber pressure of 5 kilopascals. Graphite paper heats the powder compact layer by layer from the contact surface upward. When the temperature rises, the boron oxide melts into a liquid phase at roughly 450 degrees Celsius, and the transition-metal oxides react with boron dissolved in that melt through a redox reaction that converts them into CoNiB₂. Once the oxide reactants are exhausted, the surrounding molten boron oxide physically isolates the newly formed crystals, preventing them from growing further. Rapid cooling then locks well-dispersed crystallites into place, and washing with anhydrous ethanol removes the boron oxide, followed by an acid wash to strip out unreacted metal.

Because the entire reaction lasts only twenty seconds, the liquid soaking window is extraordinarily short, and the compact even retains its disc shape afterward. The authors argue that this brevity is what produces the distinctive morphology: scanning electron microscopy reveals a stacked, flower-like architecture rich in folds and edges, with transmission electron microscopy confirming edge-rich substructures. High-resolution imaging shows interplanar spacings of 0.248 and 0.214 nanometers, corresponding to the (300) and (222) planes of CoNiB₂, and selected-area electron diffraction rings identify the product as polycrystalline. Energy-dispersive X-ray mapping confirms that cobalt, nickel and boron are distributed uniformly across the nanoflowers.

Structural analysis pinned down an unexpected crystal chemistry. X-ray diffraction patterns of the reference powders matched known orthorhombic NiB and CoB phases, but the bimetallic product refined to a simple cubic structure in the Pm-3m space group, with unit cell parameters a = b = c = 7.426 Å and Rietveld fitting factors of Rp = 9.17% and Rwp = 10.1%. Because cobalt and nickel atoms have nearly identical radii, the refinement could not distinguish their positions; they occupy mixed sites in the model. X-ray photoelectron spectroscopy showed that the Co 2p and Ni 2p peaks associated with metal–boron bonding shift to lower binding energies in CoNiB₂ relative to the monometallic borides, indicating an enhanced electron cloud around both metal atoms — an electronic change the team links to the improved catalytic performance.

The electrochemical tests were run in a standard three-electrode configuration with argon-degassed 1 M potassium hydroxide, an Hg/HgO reference electrode and a graphite rod counter electrode. On 3-millimeter glassy carbon electrodes, loaded at roughly 0.085 milligrams per square centimeter, the overpotentials required for 10 milliamperes per square centimeter were 421 millivolts for NiB, 281 for CoB, 337 for RuO₂ and 271 for CoNiB₂, all referenced to the reversible hydrogen electrode with iR compensation and a 5 millivolt-per-second scan rate. The Tafel slopes followed the same ordering: 230, 132, 76 and 67 millivolts per decade respectively. The authors report that the bimetallic boride also outperforms most recently reported bimetallic borides such as FeCoB and NiFeB, as well as widely used catalysts like nickel–iron layered double hydroxide, based on a comparison table in the supporting information.

Impedance and capacitance measurements point to why the mixed-metal compound wins. Charge transfer resistance decreased progressively from NiB to CoB to CoNiB₂, and the double-layer capacitance used to estimate the effective electrochemical surface area jumped from 0.58 millifarads per square centimeter for NiB and 1.29 for CoB to 14.95 for CoNiB₂ — roughly a tenfold increase over the monometallic borides that signals a far larger population of accessible active sites. The team attributes this to a synergistic effect of incorporating a second metal, which modifies the electronic structure and enhances charge transfer.

Stability, often the weak point for boride catalysts, held up well under demanding conditions. In chronopotentiometry on carbon paper at a high current density of 100 milliamperes per square centimeter, CoNiB₂ ran continuously for 25 hours; the overpotential rose gradually by 29.8 millivolts over that period, but the authors characterize the operation as stable, and a longer 100-hour test likewise showed no significant performance degradation. Post-reaction X-ray photoelectron spectra revealed a distinct change in the cobalt oxidation state, consistent with sustained adsorption of hydroxide and oxyhydroxide groups. The team concludes that the true active species is a surface layer of CoOOH or NiOOH formed in situ on the boride core in the alkaline environment — a surface reconstruction long suspected for transition-metal borides and now directly documented for this material.

Density functional theory calculations supplied a theoretical rationale for the bimetallic advantage. Both CoB and CoNiB₂ show metallic character near the Fermi level, but CoNiB₂ has a slightly higher electron density of states there, suggesting greater carrier mobility. More strikingly, the computed energy barrier at the rate-determining step of the oxygen evolution reaction drops from 1.98 electronvolts for CoB to 0.42 electronvolts for CoNiB₂. The authors connect this lowering to the magnetic fields associated with nickel and cobalt, which they propose modulate the adsorption of reaction intermediates; they are careful to note that the calculation establishes the activity advantage only in theory.

The broader significance is a route to nanoscale borides with well-defined morphologies, bulk crystalline phases and exposed bulk surfaces, synthesized in seconds from cheap raw oxides rather than pre-made precursors. Developing efficient, stable oxygen evolution catalysts from non-precious metals matters because the best current catalysts, based on ruthenium and iridium oxides, are economically unattractive due to scarcity and cost, and the sluggish kinetics of the oxygen evolution reaction remain a primary obstacle for hydrogen production by water electrolysis. The team suggests the liquid-phase soaking strategy can be extended to other boride families, though the reported performance figures come from laboratory three-electrode tests at small electrode loadings, and scaling the induction-heating synthesis to industrial electrodes remains untested. For now, the study offers a striking demonstration that a twenty-second soak in molten boron oxide can turn simple oxides into a competitive alkaline water-splitting catalyst.

Subject of Research: Rapid synthesis of cobalt–nickel boride nanoflower electrocatalysts for the oxygen evolution reaction

Article Title: Green construction of CoNiB₂ nanoflowers towards OER catalyst by interactive coupling the raw oxides via liquid soaking for a few seconds

Article References: Qi, R., Liu, J., Qiao, X., Zhang, T., Liu, Q., Gao, F., & Huang, Q. (2026). Green construction of CoNiB₂ nanoflowers towards OER catalyst by interactive coupling the raw oxides via liquid soaking for a few seconds. Catal, 2(1), Article 3. https://doi.org/10.1007/s44422-026-00017-x

Image Credits: AI Generated

DOI: 10.1007/s44422-026-00017-x

Keywords: CoNiB2, oxygen evolution reaction, electrocatalysis, transition metal borides, water splitting, hydrogen production, liquid-phase soaking, molten boron oxide, nanoflowers, DFT calculations, alkaline media, stability testing