Thinner Is Better: Single-Layer NiOOH Nanoparticles Slash the Energy Cost of Splitting Water

Green hydrogen has a stubborn bottleneck, and it sits at the anode. Splitting water into hydrogen and oxygen requires two half-reactions, and the oxygen evolution reaction (OER) is by far the more demanding of the two. While the hydrogen side of electrolysis needs only two electrons per molecule, oxygen evolution demands four sequential hole transfers through a tangle of reaction intermediates, and every extra volt of overpotential is energy wasted as heat. Now a team at Technion-Israel Institute of Technology has used quantum-mechanical simulations to reveal exactly where that wasted energy goes on one of the most promising cheap catalysts, nickel oxyhydroxide, and their findings point to a strikingly simple design rule: make the catalyst thinner, and put the active site in the right place.

The study, published in Discover Electrochemistry by Prince Gollapalli, Santu Biswas, Leeor Meir, Nofar Refaeli and Maytal Caspary Toroker, tackles a question that has lingered because of an experimental catch. The catalysis does not happen on the stable nickel hydroxide phase that is easy to characterize in the lab. Instead, water oxidation occurs on a metastable nickel oxyhydroxide (β-NiOOH) surface that forms only under operating potentials of roughly 1.3 to 1.5 volts versus the reversible hydrogen electrode, when the material sheds protons and nickel transitions from the +2 to the +3 oxidation state. Capturing that fleeting phase at the atomic scale requires demanding in situ experiments, so most studies have settled for characterizing the stable precursor. Density functional theory (DFT) offers a way around the problem, letting researchers build atomistic models of the active phase and watch the reaction unfold electron by electron.

To make the model as realistic as possible, the team anchored it in experiment. Transmission electron microscopy studies of nickel hydroxide consistently reveal hexagonal, plate-like nanoparticles, so the researchers carved a hexagonal nanocrystal out of the unit cell of staggered β-NiOOH, a structural variant predicted to be stable in earlier first-principles work. The cluster measures roughly one nanometer across in width and length, closely matching the approximately 2.5-nanometer particles synthesized and imaged in a combined experimental and theoretical study the team used as a benchmark. They then built two versions of the particle: a three-layer stack about 1.35 nanometers tall, and a single layer just 0.38 nanometers thick. Crucially, the top layer is identical in both, so any difference in catalytic performance could be attributed purely to what lies beneath.

On that shared top surface, the researchers identified three chemically distinct places where the reaction could begin. At the edge of the particle, an undercoordinated nickel atom bonded to only five oxygens offers a natural docking point for water, with no vacancy needed. In the interior, the surface alternates between oxygen-terminated and hydroxyl-terminated triangular sites, and the team created vacancies at each, removing an oxygen atom in one case and an oxygen plus a hydrogen atom in the other, to open adsorption sites. This choice of locations matters because the conventional wisdom in nanocatalysis holds that edges are the hot spots of reactivity. The simulations were set to test whether that wisdom survives on a particle this small.

The answer, at least for thermodynamics, is a resounding no. Running the full four-step OER mechanism, in which water adsorbs and then loses four proton-electron pairs through *OH2, *OH, *O and *OOH intermediates, the team computed Gibbs free energies for every step on every site of both particle sizes. The overpotential, the extra voltage beyond thermodynamic requirement needed to make every step proceed downhill, dropped consistently when the particle was thinned from three layers to one. On the hydroxyl-terminated site, the overpotential fell from 0.78 volts to 0.57 volts; on the oxygen-terminated site, from 0.98 to 0.75 volts; and on the edge site, from 1.62 to 1.11 volts. The single-layer hydroxyl-terminated configuration, at 0.57 volts, matched or beat values reported in the literature for extended surfaces of the same material.

Two mechanisms appear to explain the advantage of going thin. First, shrinking the particle raises the catalytically active surface area per unit volume, a straightforward geometric effect. Second, and more subtly, a single layer has no neighboring layer above it for protons to migrate into or to hydrogen-bond with. Previous work had shown that hydrogen desorption is easier by about half an electron-volt in monolayers precisely because interlayer hydrogen bonding is absent, and that adding protons to β-NiOOH is otherwise spontaneous. Without a second layer to hold protons back, deprotonation steps become easier, and the tallest energy barrier on the reaction coordinate shrinks. The team confirmed this with an implicit solvation model, which raised the best-case overpotential only modestly, by 0.06 to 0.1 volts, leaving the single-layer advantage intact.

The identity of the potential determining step, the slowest rung on the energetic ladder, also depended on where the reaction happened. At the edge site, where the reacting intermediate touches only a single nickel atom, the hardest step was the second deprotonation, converting *OH to *O. At both interior sites, where three nickel atoms surround the adsorbate, the bottleneck shifted to the third deprotonation, from *O to *OOH, and the overpotential dropped by roughly a factor of two. In every configuration, the *O intermediate turned out to be the pivotal species, appearing either as the product or the reactant of the rate-limiting step, underscoring its central role in limiting oxygen evolution on this catalyst.

Because nickel can access multiple oxidation states, the team could use magnetism as a fingerprint of electronic change. Assigning magnetic moments of roughly 1.7, 1.2 and 0.2 Bohr magnetons to nickel in the +2, +3 and +4 states, they found that at the edge site only one nickel atom oxidizes from +3 to +4 during the reaction, while at the oxygen-terminated site none do, and at the hydroxyl-terminated site two of the three supporting nickel atoms make the jump to +4. Robustness checks with different values of the DFT+U correction parameter and with the hybrid HSE06 functional confirmed the assignments. The hydroxyl-terminated site, with its more oxidized nickel atoms, sits closest to the highly oxidized state the material is expected to adopt during real oxygen evolution, marking it as the genuinely active reaction center.

The final piece was dynamics rather than thermodynamics. Each reaction step ejects a proton-electron pair, leaving behind a hole in the valence band that must travel to the active site to drive oxidation. The team propagated quantum wave packets through the electrostatic potential landscapes of each intermediate, using a split-operator algorithm to solve the time-dependent Schrödinger equation on a femtosecond timescale, and measured the cumulative hole flux crossing a barrier near the adsorbate. The results tracked the thermodynamics beautifully: the edge site showed the lowest hole flux and the highest electrostatic barriers, while the hydroxyl-terminated site showed the highest flux, with a cumulative probability of 0.83 for its potential determining reactant compared with just 0.37 at the edge. More nickel atoms sharing the load and a more oxidized surface mean more holes available to do chemistry.

Taken together, the study rewrites a piece of nanocatalyst intuition. On β-NiOOH nanoparticles, the bulk-like hydroxyl-terminated site outperforms the edge, contradicting the general rule that edges are more active, and the thinnest possible particle delivers the lowest overpotential. For engineers designing anode materials for green hydrogen electrolyzers, the prescription is concrete: maximize surface-to-volume ratio by exfoliating the catalyst toward single layers, and engineer hydroxyl-terminated, vacancy-rich surfaces where multiple nickel atoms can participate and change oxidation state. The work also suggests magnetism itself could serve as a control handle, since spin-polarized band structures mean the number of available states at the Fermi level differs for spin-up and spin-down carriers. As the hydrogen economy scales up, such atomistic design rules, extracted from simulations that can finally see the metastable phase where the real chemistry happens, may prove as valuable as any new material discovery.

Subject of Research: Computational study of how nanoparticle size and active site location govern the thermodynamics and hole transport dynamics of β-NiOOH catalysts for the oxygen evolution reaction

Article Title: Effect of nanoparticle size and active site position on the thermodynamics and charge dynamics of NiOOH catalyst for oxygen evolution reaction

Article References: Gollapalli, P., Biswas, S., Meir, L., Refaeli, N., & Caspary Toroker, M. (2026). Effect of nanoparticle size and active site position on the thermodynamics and charge dynamics of NiOOH catalyst for oxygen evolution reaction. Discover Electrochemistry, 3(1), Article 50. https://doi.org/10.1007/s44373-026-00135-0

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00135-0

Keywords: oxygen evolution reaction, NiOOH, water splitting, green hydrogen, density functional theory, electrocatalysis, nanoparticles, overpotential, hole transport, oxidation state, nickel oxyhydroxide, DFT+U