Hydrogenation, the addition of hydrogen atoms to molecules, underpins the production of an enormous range of everyday goods, from pharmaceuticals and agrochemicals to plastics and fine chemicals. In conventional industrial practice, the reaction relies on hydrogen gas, usually generated from fossil fuels and handled under pressure with metal catalysts. Electrocatalytic hydrogenation offers a fundamentally different route: instead of using molecular hydrogen, the process generates hydrogen equivalents directly from water at the surface of an electrode, driven by electricity. When that electricity comes from renewable sources, the approach promises a far more sustainable way to carry out reductive chemical transformations, one of the workhorse reaction classes of modern synthesis.

The difficulty has always been the catalyst. Platinum, rhodium, palladium and their relatives are superb hydrogenation catalysts, but they are scarce, costly and geopolitically concentrated. Finding cheaper alternatives that match their activity and, crucially, their selectivity has proved stubbornly hard. A major challenge in electrocatalytic hydrogenation is replacing scarce platinum-group metals with earth-abundant catalysts without sacrificing activity or selectivity, said Mahito Atobe, professor at Yokohama National University’s Faculty of Engineering and a corresponding author of the study. Selectivity matters enormously in this context, because hydrogenation reactions can follow multiple pathways, and unwanted byproducts translate directly into wasted energy, wasted material and costly purification steps.

The Yokohama team, co-led by Naoki Shida, associate professor in the same faculty, turned to cobalt, an abundant and comparatively inexpensive transition metal that has attracted growing interest as a catalytic workhorse for electrochemistry. But rather than simply testing cobalt in one fixed form, the researchers asked a more subtle question: what happens to the oxidation state of cobalt while the catalyst is actually operating, and could the balance between metallic cobalt and cobalt oxide be the real determinant of performance? We wanted to understand how the oxidation state of cobalt changes under operating conditions and whether controlling the balance between metallic cobalt and cobalt oxide could provide an effective catalyst, Shida explained.

To build the catalyst, the researchers started from cobalt sulfate and calcined the material at 750 degrees Celsius, producing cobalt species supported on carbon. They then deployed the catalyst in an anion-exchange membrane electrolyzer, a device in which an electric current drives hydrogenation reactions at the cathode, with water serving as the ultimate hydrogen source. Under ambient electrolysis conditions, the optimized catalyst converted pyridine, a simple nitrogen-containing aromatic ring, into piperidine, its fully saturated counterpart, with a yield exceeding 99 percent. That figure means almost every molecule of pyridine that reacted emerged as the desired product rather than as partially hydrogenated or over-reduced byproducts. Piperidine itself is far from a trivial target: the saturated nitrogen heterocycle is a fundamental building block in synthetic and medicinal chemistry, appearing in the scaffolds of numerous drugs, which makes the reaction a demanding and practically relevant benchmark of catalytic selectivity.

What the team discovered next is the conceptual heart of the paper. The catalytic performance of cobalt is determined not simply by its elemental composition, but by its dynamic oxidation state during electrolysis, Atobe said. During operation, cobalt does not sit still chemically. It shuttles between metallic Co(0) and cobalt oxide, denoted CoOx, as electrons flow and the local chemical environment shifts. The researchers found that catalysts containing too much of either form performed poorly. Purely metallic cobalt and heavily oxidized cobalt each fell short of the activity and selectivity achieved by an intermediate mixture. The optimum lay at a specific ratio of metallic Co to CoOx, a chemical middle ground that neither extreme could match.

To understand why this balance matters, the team combined experimental characterization with in situ X-ray spectroscopy, which allowed them to track the catalyst’s chemical state while it was actually working, and with theoretical calculations of the reaction energetics. The results point to a cooperative mechanism: the coexistence of metallic cobalt and residual cobalt oxide creates a favorable environment for adsorbing pyridine and delivering hydrogen to it. In other words, the two cobalt phases are not competing with each other but complementing each other, with each providing part of the chemical functionality needed to bind the substrate and hydrogenate it cleanly. Maintaining an appropriate balance between metallic Co and residual CoOx enables highly selective hydrogenation, Atobe said. This picture reframes catalyst design in an important way. Instead of treating a catalyst as a static material whose identity is fixed at synthesis, the study shows that the operating state of the catalyst, continuously shaped by the electrochemical conditions, can be the decisive variable.

The generality of the approach was tested across a wide chemical landscape. The catalyst selectively hydrogenated a broad range of nitrogen-containing compounds, including pyridines, quinolines, pyrazines, nitriles and nitroarenes, spanning several important families of substrates in pharmaceutical and agrochemical synthesis. Notably, the cobalt system also suppressed undesired hydrogenation pathways that are commonly observed with rhodium-based catalysts, which are among the most capable but least sustainable options available. In effect, the earth-abundant metal not only matched the precious-metal benchmark on yield but avoided some of its characteristic side reactions, a combination that could make the cobalt platform genuinely attractive for practical synthesis rather than merely a laboratory curiosity.

One practical obstacle remained. During prolonged electrolysis, the catalyst tends to be over-reduced, drifting away from its optimal mixed oxidation state and losing performance over time. The team’s solution was elegantly simple: intermittent electrolysis, in which the current is switched on and off in a controlled pattern, allows the catalyst to recover and maintain the appropriate Co(0)/CoOx balance throughout the reaction. With this strategy, the researchers achieved gram-scale conversion of pyridine to piperidine with an 89 percent yield while keeping the cell voltage stable, demonstrating that the concept can operate at quantities relevant to real synthetic work rather than only in trace-scale demonstrations. The result highlights a broader principle for catalyst design: controlling a catalyst’s chemical state while it operates can be as important as choosing the catalyst itself.

Looking ahead, the team plans to extend the oxidation-state-control strategy to other earth-abundant transition-metal catalysts and to a broader range of synthetically important reactions, potentially opening a general playbook for replacing precious metals across electrochemical manufacturing. Our ultimate goal is to develop scalable electrochemical processes in which catalyst states can be actively controlled under operating conditions, Shida said. This could enable selective chemical manufacturing without relying on scarce precious metals. If that vision is realized, the humble tuning of a metal’s oxidation state, monitored and adjusted in real time by nothing more than the rhythm of an electric current, could become a standard tool of green chemistry, turning one of the most abundant metals on Earth into a precision instrument for building the molecules modern life depends on.

Subject of Research: Oxidation-state control of cobalt electrocatalysts for selective hydrogenation of nitrogen-containing aromatic compounds

Article Title: Fine-tuning cobalt for cleaner chemical transformations

Article References: Fine-tuning cobalt for cleaner chemical transformations. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cobalt catalyst, electrocatalytic hydrogenation, oxidation state, pyridine, piperidine, electrolysis, sustainable chemistry, earth-abundant metals, nitrogen heterocycles, anion-exchange membrane electrolyzer, catalyst selectivity, green chemistry

The core insight concerns a mechanism that electrochemists have courted for a decade: lattice-oxygen redox. In conventional metal oxide and oxyhydroxide catalysts, oxygen atoms in the crystal framework sit passively as spectators; every bond made and broken during water oxidation happens at adsorbed intermediates such as *OH and *O. That is safe but slow, because the adsorption energies of these intermediates are locked together by rigid scaling relationships. Make *OH binding stronger to speed up one step, and *O binding strengthens too, penalizing another. The scaling relations impose a theoretical floor on the overpotential, a tax every conventional catalyst must pay. Lattice-oxygen redox breaks that tax. When framework oxygen atoms themselves participate in the redox chemistry, oxygen-oxygen bonds can form through direct coupling of lattice oxygen, sidestepping the adsorbate scaling conundrum entirely.

The catch is stability, and it has been brutal. Oxidizing lattice oxygen means ripping electrons from the very orbitals that hold the crystal together. Once activated indiscriminately, that oxidation delocalizes across the lattice, holes accumulate on framework oxygen, metal-oxygen bonds soften and break, and the material dissolves or amorphizes. Many catalysts that unlock lattice oxygen in the laboratory die within hours under the punishing polarization of a real electrolyzer. The literature is littered with tantalizing activity reports that collapse under accelerated ageing. The Singapore-led team, whose senior authors include Junmin Xue, Haoyin Zhong, Bin Tian and Xiaopeng Wang, asked a different question: what if lattice oxygen could be activated not all at once, but in a spatially ordered, electronically programmed sequence?

Their answer begins with an unlikely precursor: a composite of chromium(III) oxide and nickel diselenide, Cr2O3/NiSe2. When the electrode is held under anodic polarization in alkaline electrolyte, the reconstruction unfolds in two chemically distinct stages, and the order matters enormously. In the first stage, selenium leaches out of the NiSe2 phase. Losing the chalcogen opens up the nickel 3d band manifold, specifically broadening the antibonding eg band, the orbital set that overlaps with oxygen 2p states. A broader eg band strengthens nickel-oxygen covalency, and stronger covalency does two things at once: it accelerates the deprotonation of *OH intermediates, a known rate-limiting step on nickel oxyhydroxide surfaces, and it begins to tilt the electronic structure toward oxygen participation.

Then comes the second, decisive stage. Chromium begins to leach from the oxide component, and crucially, the chromium departure selectively depletes oxygen atoms that were coordinated to nickel across the composite interface. What remains is a modified nickel oxyhydroxide, SRA-NiOOH, in which oxygen non-bonding states, oxygen 2p orbitals stripped of their bonding partners, are generated not randomly across the lattice but in spatially confined pockets. This is the heart of the strategy. Confined non-bonding states act as local redox reservoirs for lattice oxygen, close enough to the surface to join the catalytic cycle, yet electronically quarantined so that oxidation holes cannot wander freely through the framework. Destructive redox delocalization, the killer mechanism of lattice-oxygen catalysts, is suppressed by construction. The oxygen-redox activity and the lattice stability, usually two ends of a seesaw, are decoupled.

The experimental evidence for this picture is layered across multiple structural and spectroscopic probes. X-ray absorption spectroscopy traces the changes in nickel coordination and oxidation state as reconstruction proceeds, while synchrotron-based measurements carried out at the Singapore Synchrotron Light Source document the emergence of the modified oxyhydroxide phase. Density functional theory calculations, performed with the ASTAR Computational Resource Centre and the National Supercomputing Centre in Singapore, complement the experiments, showing how the broadened eg* band and the confined oxygen non-bonding states reshape the energy landscape of the oxygen evolution mechanism. The team further demonstrates that the approach generalizes: the sequential-reconstruction activation paradigm is not a one-off trick of one composition but a design framework that can be exported to related systems, a claim their fourth figure addresses directly.

The device-level performance is where the work will command the most attention. Anion-exchange-membrane water electrolyzers are the commercial sweet spot for many of these nickel-based catalysts, because AEM technology can use earth-abundant transition metals instead of the iridium that proton-exchange-membrane electrolyzers demand. But AEM electrolyzers still need to run at high current density, above 2 amperes per square centimeter, to reach the cost targets that would make green hydrogen competitive with fossil-derived hydrogen. At 4.1 A cm-2 at 1.8 V in 1 M KOH at 80 degrees Celsius, the SRA-NiOOH anode sits squarely in industrially relevant territory, and the nearly 3,500-hour stability test under accelerated ageing at 60 degrees Celsius suggests the catalyst does not merely sprint; it runs a marathon. Sustained oxygen-redox operation at high current density, the authors argue, is precisely what the sequential electronic reconstruction buys: a durable oxygen-redox window that stays open under demanding polarization.

The conceptual contribution may outlast the specific numbers. For years, the field has treated catalyst reconstruction as something to minimize, a necessary evil on the path to stable operation, or at best a one-shot activation. This work reframes reconstruction as something to choreograph. By choosing a precursor whose constituents leach in a controlled order, selenium first, chromium second, the researchers wrote the electronic structure of the final catalyst as a timed sequence rather than a static composition. Each leaching event performed a specific electronic function, band broadening here, oxygen non-bonding state generation there, and the final NiOOH inherited the benefits of both without the instability that an uncontrolled surface transformation would bring. It is a programming philosophy applied to surfaces, and it echoes a broader shift in electrocatalysis toward operando-informed precatalyst design, in which the working electrode is understood as a dynamic, self-assembling entity rather than a fixed material.

There are, of course, caveats and open questions. The reported metrics come from laboratory-scale AEM cells and accelerated ageing protocols, and the road from such benchmarks to kilowatt-scale stacks involves membrane durability, flow uniformity, and cost questions that no single anode catalyst can answer alone. The precise atomic-scale geography of the confined non-bonding states, and how far the confinement holds as reconstruction continues over thousands of hours, will reward further study. But the bar the paper clears is a meaningful one: it shows that the activity-stability trade-off, long treated as a law of nature for lattice-oxygen catalysis, can be suspended by electronic design. If sequential reconstruction proves as general as the authors suggest, the oxygen evolution bottleneck that has constrained green hydrogen may finally have an engineering-grade exit, built not from a miracle material but from a catalyst taught to rebuild itself in the right order.

Subject of Research: Sequential electronic reconstruction of a Cr2O3/NiSe2 precursor to enable durable lattice-oxygen redox in anion-exchange-membrane water electrolysis

Article Title: Sequential electronic reconstruction enables durable oxygen redox in water electrolysis

Article References: Yu, J., Zhong, H., Zhang, Q., Zhang, X., Ye, Y., Diao, C., Yu, Z. G., Xi, S., Tian, B., Wang, X., & Xue, J. (2026). Sequential electronic reconstruction enables durable oxygen redox in water electrolysis. Nature Sustainability. https://doi.org/10.1038/s41893-026-01940-6

Image Credits: AI Generated

DOI: 10.1038/s41893-026-01940-6

Keywords: water electrolysis, oxygen evolution reaction, lattice oxygen redox, nickel oxyhydroxide, green hydrogen, anion-exchange membrane, electrocatalysis, catalyst reconstruction, eg band, catalyst stability, nickel selenide, Nature Sustainability