A magnetic field can squeeze extra performance out of water-splitting catalysts, but the effect has always been frustratingly unpredictable. Some catalysts respond strongly, others barely at all, and the reasons have remained murky. The work, published in the journal Catal, offers a design principle for a new class of magnetic-field-responsive electrocatalysts.
The oxygen evolution reaction, or OER, is the stubborn half of electrochemical water splitting. While the hydrogen-producing side of the reaction is comparatively cooperative, the anodic OER demands four coupled proton-electron transfer steps and the difficult formation of an oxygen-oxygen bond. This sluggishness limits the practical efficiency of storing intermittent renewable electricity in chemical fuels. Precious iridium and ruthenium oxides catalyze the reaction well but are scarce and expensive, driving the search for cheaper transition-metal alternatives. High-entropy oxides, in which five or more metal cations share a single crystal lattice, have emerged as promising candidates because their compositional complexity generates diverse local coordination environments while configurational entropy stabilizes the overall structure.
The research team, led by Siqi Xie, Bingshuai Fu and colleagues with Ping Xu as senior author, synthesized three compositionally related high-entropy spinel oxides: (FeCrMnCoNi)3O4, (FeCrMnCoZn)3O4 and (FeCrMnZnNi)3O4. All were made by the same sol-gel combustion route and all retained the characteristic spinel crystal structure, as confirmed by X-ray diffraction, Raman spectroscopy and electron microscopy. The samples formed porous architectures of interconnected nanoparticles with nearly identical average sizes of 7.53, 7.63 and 7.66 nanometers, meaning that differences in later performance could not be attributed to particle morphology. Energy-dispersive X-ray mapping and inductively coupled plasma analysis confirmed that the elements were homogeneously distributed at the designed equimolar ratios.
The strategic ingredient was zinc. Zinc ions carry a closed-shell 3d10 electronic configuration, so they contribute no magnetic moment of their own. Instead, their role is to perturb the local coordination environment of the neighboring magnetic cations. X-ray diffraction showed that the zinc-containing samples exhibited larger lattice expansion, consistent with the bigger ionic radius of Zn2+, and Raman spectroscopy revealed that (FeCrMnCoZn)3O4 displayed the largest red shift and broadening of the A1g stretching mode, implying weakened average metal-oxygen bonds and a wider distribution of local bond lengths and angles. X-ray photoelectron spectroscopy showed that zinc incorporation shifted the Co3+-related peaks from 779.30 to 779.73 electronvolts while barely changing the Co2+ fraction, indicating that zinc primarily modifies the coordination environment of cobalt rather than its oxidation state.
The magnetic measurements told a striking story. Saturation magnetization followed the order (FeCrMnCoZn)3O4 at 11.2 emu per gram, then (FeCrMnCoNi)3O4 at 8.47, then (FeCrMnZnNi)3O4 at 7.76, all far below the 80.41 emu per gram of the Fe3O4 reference. Multication solid-solution formation clearly weakens the long-range magnetic ordering of conventional spinel ferrites. Yet the zinc-and-cobalt sample outperformed even the fully magnetic nickel-containing composition, showing that zinc does not simply dilute the magnetic lattice. Effective moments at 300 kelvin reached 8.08, 9.24 and 7.57 Bohr magnetons for the three oxides, corresponding to 3.8, 5.0 and 4.0 unpaired electrons per magnetic ion. The value of 5.0 in (FeCrMnCoZn)3O4 approaches the half-filled 3d5 high-spin limit, pointing to cation redistribution between tetrahedral and octahedral sites and stabilization of higher-spin configurations.
When the researchers applied a 3000 gauss magnetic field during oxygen evolution, the three catalysts responded very differently. Under zero field, the intrinsic OER activity ranked (FeCrMnCoNi)3O4 highest, followed by (FeCrMnCoZn)3O4 and then (FeCrMnZnNi)3O4. But the field-induced overpotential reductions were 6, 9 and 5 millivolts respectively, meaning the zinc-and-cobalt composition gained the most despite not being the most active catalyst to begin with. Normalizing the polarization curves by electrochemically active surface area preserved the same trend, ruling out surface-area differences as the cause. The magnetohydrodynamic effect, which helps detach oxygen bubbles and depends only on field strength, was also excluded because the three catalysts behaved differently under the identical 3000 gauss field.
Instead, the enhancement tracked saturation magnetization and the exchange-coupling-regulated electronic structure. Tafel plots and electrochemical impedance spectra showed reduced slopes and smaller semicircles under the field for all samples, indicating accelerated kinetics and facilitated interfacial charge transfer. The team extended the measurements to 1000, 2000, 6000 and 10000 gauss, testing three additional independently prepared electrodes at each field strength for a total of four measurements per condition. (FeCrMnCoZn)3O4 consistently showed the largest field-induced overpotential decrease, with the enhancement most pronounced at low-to-intermediate fields and weakening at higher fields as the catalyst magnetization approached saturation. A zinc-free (FeCrMnCo)3O4 control, synthesized under identical conditions, showed a smaller lattice shift, a lower saturation magnetization of 9.3 emu per gram, and a weaker field response at 3000 gauss, with the overpotential at 10 milliamperes per square centimeter dropping only from 366 to 359 millivolts.
In situ Raman spectroscopy during OER revealed how the catalysts evolve at work. All three retained their spinel framework throughout the potential range, with no dominant new bands or bulk phase transformation, and post-reaction XRD and Raman confirmed no irreversible structural change. But the degree of local flexibility differed sharply. The A1g mode of (FeCrMnCoZn)3O4 red-shifted from 692.9 to 684.1 per centimeter with its full width at half maximum rising from 42.5 to 51.4, the largest potential-dependent redistribution among the three, while (FeCrMnZnNi)3O4 barely moved. The changes evolved continuously with potential, confirming reversible coordination adjustment rather than a distinct phase transition. Density functional theory calculations using Wannier-based tight-binding models reinforced the picture: zinc incorporation induced a much larger redistribution of cobalt-centered d-orbital energy levels than of nickel-centered ones, matching the experimental trends. Including spin polarization in the free-energy calculations lowered the maximum uphill energy change from 1.89 to 1.69 electronvolts, showing that spin state directly modulates intermediate adsorption energetics.
Post-reaction XPS identified cobalt as the dominant OER-active center. In (FeCrMnCoZn)3O4, the Co3+ fraction rose from 51.50 percent in the pristine catalyst to 60.73 percent and 64.34 percent after OER under zero field and 3000 gauss respectively, a larger increase than for iron or nickel in the corresponding compositions. Partial leaching of chromium and zinc was observed, and the authors note that a thin surface oxyhydroxide or reconstructed layer cannot be fully excluded by the current characterization. Long-term chronopotentiometry at 100 milliamperes per square centimeter under a 3000 gauss field ran continuously for 150 hours with only slight potential increases, and chronoamperometric measurements under periodic field switching showed reversible, recoverable current responses. Together, the results establish that magnetic-field responsiveness arises from the cooperative regulation of local coordination, electronic structure and magnetic exchange interactions, rather than from zinc alone or the size of the intrinsic magnetic moment, giving catalyst designers a concrete lever: engineer flexible local coordination environments, and the magnetic field will do the rest.
Subject of Research: Magnetic-field-enhanced oxygen evolution reaction on high-entropy spinel oxides
Article Title: Enhanced oxygen evolution by magnetic exchange coupling modulation of high‑entropy spinel oxides
Article References: Xie, S., Fu, B., Chen, M., Wang, S., Li, L., Zhang, Y., Zhao, L., & Xu, P. (2026). Enhanced oxygen evolution by magnetic exchange coupling modulation of high‑entropy spinel oxides. Catal, 2(1), Article 21. https://doi.org/10.1007/s44422-026-00034-w
Image Credits: AI Generated
DOI: 10.1007/s44422-026-00034-w
Keywords: oxygen evolution reaction, high-entropy oxides, spinel oxides, magnetic field electrocatalysis, water splitting, zinc incorporation, magnetic exchange coupling, in situ Raman spectroscopy, density functional theory, cobalt active sites, electrocatalysts, Enhanced
