Copper-based catalysts occupy a special place in carbon dioxide electrochemistry because copper is the only metallic element known to drive the full cascade of proton and electron transfers needed to build multicarbon products, yet this same versatility makes copper notoriously difficult to steer toward a single product. On copper surfaces, carbon dioxide reduction can yield carbon monoxide, methane, ethylene, ethanol, acetate, and formate, with the distribution governed by subtle features of the surface geometry, oxidation state, and local chemical environment. The reference literature cited by the authors reflects this landscape, spanning stepped copper surfaces optimized for ethylene, copper antimony nanowire arrays tuned for carbon monoxide, copper clusters confined within covalent organic frameworks that favor methane, and copper oxide species whose role in the reduction mechanism continues to be debated. Within this crowded field, copper selenide offers a distinct electronic starting point, because the semiconducting character of the chalcogenide and the strong copper-selenium bonding modify the d-band electronic structure that governs how reaction intermediates bind to the surface.

The central mechanistic insight of the new study concerns the direction of charge flow at the junction between palladium nanoparticles and the copper selenide support. When palladium particles are deposited onto the mesoporous copper selenide nanoribbons, electrons shift from copper atoms toward selenium atoms, a redistribution that alters the electronic landscape at the sites where carbon dioxide and its hydrogenated intermediates adsorb. This charge redistribution serves two purposes at once. First, it promotes the hydrogenation steps that convert carbon-containing intermediates toward formate, the pathway that terminates in formic acid. Second, it suppresses the competing hydrogen evolution reaction, which otherwise consumes protons and electrons that would otherwise be available for carbon dioxide conversion. The dual effect is significant because hydrogen evolution is the dominant parasitic process on many carbon dioxide reduction catalysts, particularly at the negative potentials needed to drive the reaction at useful rates.

The choice of palladium as the decorating metal is grounded in a substantial body of prior work. Palladium is unusual among metals in its ability to absorb hydrogen into its lattice, forming palladium hydride, and this hydride phase has been implicated in the hydrogenation chemistry that leads to formate on palladium-containing surfaces. Recent in situ transmission electron microscopy and diffraction studies, cited in the article’s bibliography, have shown that the interconversion between metallic palladium and palladium hydride under electrochemical conditions has a direct impact on carbon dioxide reduction performance, meaning that the active form of palladium in these catalysts is a dynamic, potential-dependent phase rather than a static decoration. The comparative differential electrochemical mass spectrometry studies of palladium, copper, and tin-based electrocatalysts further underscore that each of these metals follows distinct mechanistic routes to products, so combining them in a single heterostructure creates the possibility of merging favorable attributes from each component.

The mesoporous nanoribbon morphology of the copper selenide support deserves attention in its own right. Nanoribbons are one-dimensional structures with high aspect ratios, and when they are made mesoporous, meaning their internal pores fall in the two-to-fifty-nanometer range, they combine the directional electron transport of a nanowire with the high internal surface area of a porous framework. This architecture matters for gas-involved electrocatalysis because carbon dioxide must diffuse to active sites while the gaseous product or competing hydrogen must escape, and mesoporosity facilitates both transport processes. The cation-exchange synthesis route used to prepare copper selenide nanoribbons, previously demonstrated in the context of hydrogen sulfide sensing materials, provides a soft-chemistry pathway to these morphologies without requiring high-temperature processing that would coarsen the pore structure.

The performance metrics reported for the optimal copper selenide-palladium catalyst place it in a competitive position among recently reported formate-producing systems. A Faradaic efficiency of 61.85 percent for formic acid at minus 0.9 volts versus the reversible hydrogen electrode in a flow cell indicates that roughly three-fifths of the electrons passing through the cell are captured in the desired product, with the remainder split among hydrogen evolution and minor carbon-containing byproducts. The partial current density for formic acid production, reaching minus 129.78 milliamperes per square centimeter at minus 1.1 volts versus the reversible hydrogen electrode, is the more industrially relevant figure, because partial current density reflects the product of selectivity and total rate. A catalyst can show excellent selectivity at tiny currents, but only high partial current densities at moderate overpotentials translate into the volumetric productivity that a commercial electrolyzer would require.

The flow cell architecture used in the study is itself an important contextual element. Flow cells, in which the electrolyte is pumped through a cathode compartment separated from the anode by an ion-exchange membrane, and in which gaseous carbon dioxide is often fed directly to the back of a gas diffusion electrode, have become the standard testing platform for carbon dioxide reduction because they alleviate the mass transport limitations that cripple performance in simple H-cells. In an H-cell, the low solubility of carbon dioxide in aqueous electrolytes caps achievable current densities at levels far below industrial targets, whereas flow cells with gaseous carbon dioxide feeding can sustain ampere-level currents, as demonstrated in the bismuth-based formate electrosynthesis work cited by the authors. Testing the copper selenide-palladium catalyst under flow cell conditions therefore gives a more realistic picture of how it would behave in a scaled-up device.

Stability is the metric that most often separates laboratory demonstrations from deployable technology, and the roughly fifteen hours of continuous operation at a constant current density of minus 100 milliamperes per square centimeter reported for this catalyst represents a meaningful but still preliminary benchmark. For context, industrial electrolyzers of any kind are expected to operate for tens of thousands of hours, so a fifteen-hour test serves primarily to demonstrate that the catalyst does not immediately deactivate through palladium agglomeration, selenide dissolution, or carbonaceous poisoning. Copper selenide phases can undergo surface reconstruction under reducing potentials, with metallic copper potentially segregating to the surface, and the palladium hydride formation and decomposition cycles described in the in situ microscopy literature can also induce structural stress. Longer-term studies tracking the catalyst’s morphology and composition after extended operation would be needed to establish which of these degradation pathways, if any, limits the system.

The second application explored in the study, the lithium-carbon dioxide battery, connects the catalyst to a different but conceptually related corner of the carbon utilization landscape. In a lithium-carbon dioxide battery, carbon dioxide is reduced at the cathode during discharge, typically forming lithium carbonate together with carbon, and then the discharge products must be oxidized back to release carbon dioxide during charging. The large gap between discharge and charge voltages, known as overpotential, is the central efficiency problem in these batteries, and it stems from the sluggish kinetics of both directions of the lithium carbonate chemistry. Catalysts that can lower the energy barrier for decomposing lithium carbonate during charge are therefore the key to round-trip efficiency, and the authors’ choice to evaluate the same copper selenide-palladium material as a battery cathode tests whether the hydrogenation-regulation logic developed for electrolysis carries over to the battery context.

The cited work on porous palladium nanoparticles modified on copper foam, which enhanced the adsorption energy of the key lithium carbonate intermediate to enable lithium-carbon dioxide batteries with ultralow overpotential and resistance to operation at zero degrees Celsius, provides a direct precedent for palladium-copper combinations in this battery chemistry. Similarly, prior demonstrations of zinc cathodes generating fuel-gas carbon monoxide in lithium-carbon dioxide cells and of free-standing three-dimensional carbon catalysts show the breadth of cathode materials explored for this battery type. Against this background, the achievement of stable charge-discharge cycling for over 250 hours at a current density of 100 milliamperes per gram with the copper selenide-palladium cathode represents a substantial cycling lifetime, since many lithium-carbon dioxide battery reports fail within tens of hours due to accumulation of insulating discharge products and electrolyte decomposition.

The dual-use strategy embodied in this work, in which one catalyst material serves both an electrosynthesis role and an energy storage role, reflects a broader trend in the energy materials literature toward coupling carbon dioxide conversion with renewable energy storage. Electrolytic formic acid production is an intermittent-consumption technology that can absorb surplus renewable electricity, while lithium-carbon dioxide batteries are a storage technology that chemically fixes carbon dioxide during discharge. A material effective in both roles simplifies the materials supply chain and suggests shared mechanistic principles, namely the ability to adsorb and activate carbon dioxide and to manage carbonate or formate intermediates, across the two devices.

From a mechanistic standpoint, the finding that palladium decoration drives charge transfer from copper to selenium invites comparison with other electronic-structure engineering strategies in the copper carbon dioxide reduction literature, including the copper-titanium subsurface alloying that was shown to tune carbon-carbon coupling toward multicarbon liquid fuels, the amphiphilic cationic surfactants that reorganize interfacial water to favor reduction over hydrogen evolution, and the sulfur-anchored sodium cations that modified bismuth facets for ampere-level formate production. All of these approaches share the goal of reshaping the binding energies of the *OCHO intermediate, widely regarded as the key species on the path to formate, relative to the *H species that leads to hydrogen evolution. The charge redistribution reported here provides a chalcogenide-specific route to the same end, with the selenium sites presumably playing a role in stabilizing oxygen-bound intermediates while the palladium sites contribute hydrogenation capability.

The funding context of the study, through an open project fund for a joint laboratory of spatial intelligent perception and large model application and through the fundamental research funds for central universities, illustrates the institutional support structures behind this line of research in China, where carbon dioxide utilization has been identified as a strategic technology area. The authors’ data availability statement, offering data upon reasonable request, is consistent with standard practice in the materials chemistry community, though the full experimental details underlying the performance claims reside in the subscription-access article.

Looking forward, the copper selenide-palladium system raises several questions that follow naturally from the reported results. The precise role of the selenium anion in mediating the charge transfer and in stabilizing the formate pathway remains to be fully resolved, and isotopic labeling or in situ spectroscopy could clarify whether the palladium hydride phase directly participates in hydrogenating carbon-containing intermediates or acts primarily as an electronic modifier. The selectivity ceiling of just under 62 percent Faradaic efficiency leaves room for improvement, and strategies such as further tuning of palladium loading, controlling the palladium-copper selenide interface geometry, or modifying the electrolyte composition with ionic liquids or deep eutectic solvents, which have been shown to suppress hydrogen evolution, could push the selectivity higher. Extending the stability testing from hours to hundreds or thousands of hours, and coupling the flow cell results with techno-economic analysis of formic acid production costs, would be the logical next steps toward assessing whether this hydrogenation-regulation strategy can graduate from an experimental insight into a component of practical carbon utilization infrastructure.

Subject of Research: Palladium-decorated mesoporous Cu2Se nanoribbons for enhanced electrocatalytic CO2 reduction to HCOOH and high-performance Li-CO2 batteries

Article Title: Palladium-decorated mesoporous Cu2Se nanoribbons for enhanced electrocatalytic CO2 reduction to HCOOH and high-performance Li-CO2 batteries

Article References: Wang, X., Gong, L., Chen, H., Qin, Z., Hu, C., Wang, T., & Hu, F. (2026). Palladium-decorated mesoporous Cu2Se nanoribbons for enhanced electrocatalytic CO2 reduction to HCOOH and high-performance Li-CO2 batteries. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13936-w

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13936-w

Keywords: electrocatalysis, CO2 reduction, formic acid, palladium, copper selenide, nanoribbons, Li-CO2 battery, Faradaic efficiency, hydrogen evolution suppression, energy storage, mesoporous catalysts, renewable electricity

The stakes are high. Electrochemical CO2 reduction promises to convert captured carbon dioxide into valuable fuels and chemicals such as carbon monoxide, methane, ethylene and ethanol, offering a route to decarbonise chemical manufacturing and transport. Copper remains the most promising electrocatalyst for these products because its selectivity can be tuned, and specific facets and ensembles of copper atoms enable the carbon-carbon coupling steps that build multicarbon molecules. Yet decades of mechanistic studies have relied on idealised static surfaces and simplified adsorbate models, and the discrepancies between different in situ spectroscopy studies have persistently frustrated efforts to build a unified picture. Most past experiments were also performed at current densities far below those relevant to deployment, in cells whose stagnant electrolytes and non-uniform electric fields do not resemble a real electrolyser.

The Barcelona team tackled this problem on two fronts. First, they used a three-electrode open flow cell with 1 M potassium bicarbonate electrolyte, which supplies a continuous stream of CO2 and decouples thermodynamic driving forces from mass transport limits. Then they built a custom operando Raman spectroscopy platform integrated directly into a membrane electrode assembly, achieving optical access to the buried cathodic interface through the anion exchange membrane and the iridium oxide-coated titanium anode. This allowed them to record spectra during genuine continuous CO2 electrolysis across a broad range of current densities, all the way to one ampere per square centimetre, while product analysis proceeded in parallel through gas chromatography and high-performance liquid chromatography.

The key to their analysis lies in how water organises itself at an electrified interface. Liquid water is best viewed as a continuously evolving, disordered hydrogen-bond network, but individual molecules can be classified phenomenologically by how many hydrogen bonds they make: tetrahedral water with four hydrogen bonds, trihedral water with three, and isolated free water. These populations shift with potential, current density, surface coverage and local ionic environment, and they can be quantified from the hydrogen-oxygen stretching band of the Raman spectrum. The researchers deconvoluted the stretching envelope into Gaussian components centred near 3,200 and 3,400 reciprocal centimetres for the four- and three-hydrogen-bond waters respectively, and tracked how these populations evolved as current was applied.

What emerged was a strikingly non-monotonic story. At rest, interfacial water sat in a random orientation with a dominant four-hydrogen-bond character. As current density rose toward 0.1 A per square centimetre, the three-hydrogen-bond fraction climbed from about 45 percent to 84 percent, with molecules flipping into an H-down orientation that aligns their hydrogens toward the negatively charged surface. This population of three-hydrogen-bond water turned out to be the primary proton source for CO2 reduction, and its rise coincided with growing selectivity toward multicarbon products. The four-hydrogen-bond fraction fell correspondingly, dipping to a minimum of about 16 percent. Between 0.2 and 0.4 A per square centimetre the picture shifted again, and beyond 0.4 A per square centimetre both water types flipped to H-down configurations as the increasingly strong interfacial electric field took over.

To understand why, the team turned to ab initio molecular dynamics simulations run on copper surfaces with varying coverages of potassium cations and adsorbed species. The simulations revealed a division of labour: adsorbed potassium ions act as hydrogen-bond breakers, dismantling the interfacial water network, whereas adsorbed hydroxide, carbon monoxide and carbonate species act as hydrogen-bond makers, extending and stiffening the network. Increasing the number of potassium cations in the double layer dropped the four-hydrogen-bond water fraction from 0.70 to 0.45, while adding hydroxide at that same cation coverage restored it to 0.73. This explained the experimental oscillation: as the potential grows more negative, anion coverage falls and cation concentration rises, disrupting water order, until the accumulation of polar adsorbates such as hydroxide and carbonate reorders the network once more, making the interfacial layer increasingly ice-like and rigid.

The coupling between water structure and reaction intermediates was mapped with two-dimensional correlated Raman spectroscopy, a technique that compares intensity fluctuations across many spectra to reveal which vibrational modes change together and in what order. The analysis traced a sequential pathway: adsorbed carbon monoxide forms first, shifting from a low-frequency to a high-frequency band; adsorbed hydroxide then appears and stabilises the high-frequency carbon monoxide species; and that intermediate converts into a carbonaceous intermediate implicated in carbon-carbon coupling. A strong positive correlation between the three-hydrogen-bond water signal and the high-frequency carbon monoxide band confirmed that interfacial water physically rearranges to accommodate and stabilise the species needed for multicarbon products.

Then came the gating mechanism itself. Beyond roughly 0.2 A per square centimetre, hydroxide generation falls behind its removal through the anion exchange membrane, carbonate accumulates, and the four-hydrogen-bond water fraction rebounds. This denser, more ordered water layer restricts access of carbon dioxide and carbon species to the interface while providing abundant protons for the competing hydrogen evolution reaction. A control experiment feeding argon instead of CO2 produced nearly 90 percent faradaic efficiency for hydrogen with a similar four-hydrogen-bond water content, confirming that the ordered water layer gates access to the catalyst surface regardless of which reactants are present. Electrochemical performance tracked the spectroscopy closely: carbon dioxide selectivity peaked, multicarbon selectivity reached 45 percent and the hydrogen efficiency bottomed out at 0.2 A per square centimetre, precisely where hydroxide coverage and three-hydrogen-bond water were maximal and carbonate and four-hydrogen-bond water were minimal.

The practical implications are considerable. If the interfacial hydrogen-bond network is a design variable rather than background noise, it can be engineered. The authors suggest that surface functionalisation with hydroxyl, amine, imidazole or sulfonate groups, hydrophobic terminations that modulate water orientation, tailored electrolytes that control cation concentration, and selective water-exchange membranes could all be used to hold the interface in the productive regime. Carbon monoxide reduction studies, which eliminate the carbonate signals that complicate spectroscopy, could refine the picture further, since surface-bound hydrogen donors and intermediates could then be tracked cleanly.

What makes the work resonate beyond electrochemistry is the demonstration that at industrial current densities, the hardest problems may not be about the catalyst at all, but about water management at a scale of a few molecular layers. The same molecules that deliver protons for synthesis can also cement themselves into a barrier that chokes off the reaction. Seeing that gate open and close, molecule by molecule, while the electrolyser runs, gives catalyst and system designers for the first time a molecular target to aim for: not merely a better copper surface, but a better-ordered dance of water above it.

Subject of Research: Interfacial water structure and its role in gating CO2 electroreduction on copper catalysts at high current density

Article Title: Operando resolution of CO2 electrolysis interfaces at application-relevant current density reveals water gating

Article References: Guha, A., Seemakurthi, R. R., Polesso, B., Haldar, P., Ram, R., Pinilla-Sánchez, A., Mondal, I., Kernan, D., López, N., & García de Arquer, F. P. (2026). Operando resolution of CO2 electrolysis interfaces at application-relevant current density reveals water gating. Nature Catalysis. https://doi.org/10.1038/s41929-026-01620-w

Image Credits: AI Generated

DOI: 10.1038/s41929-026-01620-w

Keywords: CO2 reduction, electrocatalysis, copper catalyst, interfacial water, Raman spectroscopy, molecular dynamics, hydrogen evolution, membrane electrode assembly, hydrogen-bond network, multicarbon products, electrolyser, water gating

In a study published in Nature Chemistry, Xunyu Hu, Will Boyd, Feng Wang and colleagues describe a so-called Janus palladium membrane electrode that decouples hydrogen formation from carbon dioxide conversion by physically separating the two processes on opposite faces of a thin palladium membrane. On one side of the electrode, hydrogen is generated from water in the ordinary way. But rather than bubbling off as unwanted gas, the hydrogen atoms permeate through the palladium membrane — a material famous for its ability to absorb and transport hydrogen — and emerge on the other side, where they encounter a completely different chemical environment that faces the CO2-containing electrolyte.

The elegance of the design lies in what happens at that CO2-facing interface. Instead of allowing protons and electrons to combine there, the membrane independently polarizes the rear surface, converting the permeated hydrogen atoms into palladium hydride species, written chemically as Pd–H. These hydride species carry a tunable hydricity — a measure of how strongly the hydrogen atom is delivered as a hydride ion, H-minus, rather than as a proton or a neutral radical. Hydride transfer is a fundamentally different chemistry from the proton-coupled electron transfer that dominates conventional electrocatalysis. In a hydride transfer, the hydrogen atom arrives together with two electrons, ready to insert directly into a carbon dioxide molecule and reduce it to formate in a single, well-defined step.

Because the two faces of the Janus electrode can be controlled independently, the researchers can sustain a directed flux of hydride from water, where the hydrogen originates, to the carbon dioxide waiting on the other side of the membrane. Under only mild cathodic polarization of the CO2-facing interface, this directional hydride delivery drives highly selective formate production. Formate is itself a valuable product — a liquid chemical used in fuel cells, as a hydrogen carrier, and as a feedstock — and it is one of the most commercially attractive targets of CO2 electroreduction.

The performance numbers reported in the study are striking. In aqueous electrolytes, the membrane electrode achieves excellent Faradaic efficiencies, meaning that an unusually large fraction of every electron passing through the circuit ends up stored in formate rather than wasted on hydrogen gas or other byproducts. The turnover frequencies — a measure of how many product molecules each active site can generate per unit time — are also high, indicating that the catalyst is not merely selective but genuinely productive. Isotope labelling experiments, in which deuterium is substituted for hydrogen to trace the origin of the atoms in the product, confirmed that hydride transfer through the palladium membrane is the dominant reaction pathway. In other words, the formate is genuinely being built from hydrogen atoms that traveled across the metal, not from protons reacting directly at the surface.

The membrane architecture also solves a second chronic problem in CO2 electroreduction: catalyst stability and selectivity over long operating periods. On conventional catalysts, adsorbed carbon monoxide — an intermediate in many CO2 reduction pathways and a poison in others — accumulates and degrades performance. In the Janus design, the continuous delivery of hydrogen through the membrane keeps the CO2-facing surface in a hydrogen-rich state, which suppresses the formation of adsorbed CO and allows the electrode to operate stably over extended periods. The steady stream of permeating hydrogen atoms effectively washes the surface chemistry toward formate formation and away from competing carbon monoxide pathways.

Perhaps the most significant demonstration, however, is that the electrode works not only in water but also in fully aprotic electrolytes — solvent systems that contain no protons to donate at all. Conventional proton-coupled electrocatalysis cannot function meaningfully in such environments, because there are no protons to couple with the electrons. The Janus membrane electrode, by contrast, carries its own hydrogen supply across the metal barrier, delivering hydride reactivity to the CO2-facing surface regardless of what solvent surrounds it. This opens the door to running CO2 electroreduction in organic solvents and other nonaqueous media where new selectivities, solubilities, and product distributions might be accessed — possibilities that are simply unreachable within the proton-coupled paradigm.

The broader implication is that heterogeneous hydride chemistry, long the province of molecular catalysts and stoichiometric reagents in homogeneous solution, can now be exercised at a solid electrode under electrochemical control. Hydride transfer is one of the most powerful and selective transformations in chemistry: biological systems use hydride carriers such as NADH to reduce carbon dioxide in photosynthesis and metabolism, and synthetic chemists reach for hydride donors like borohydrides when they want clean, targeted reductions. Bringing that same hydride reactivity to an electrochemical interface — powered by renewable electricity rather than sacrificial chemical reductants — represents a conceptual bridge between electrocatalysis and classical hydride chemistry.

Palladium is uniquely suited to play the central role. The metal dissolves large quantities of hydrogen, forming a palladium hydride phase in which hydrogen atoms occupy interstitial sites in the metal lattice, and it conducts hydrogen across thin foils with remarkable facility. By tuning the electrochemical potential applied to the CO2-facing side, the researchers can adjust the hydricity of the Pd–H species at the surface — effectively dialing in how reactive, how reducing, the delivered hydrogen will be. That tunability matters because different CO2 reduction products require different degrees of reducing power, and a hydride donor whose strength can be adjusted offers a level of control that conventional co-catalysts cannot match.

The work arrives at a moment of intense global interest in carbon utilization technologies. Electrochemical CO2 reduction promises a route to close the carbon cycle, converting emissions from power plants, industrial facilities, or even direct-air capture into fuels and chemicals using renewable electricity. Yet the technology’s economic viability has been persistently undercut by the hydrogen evolution reaction, which steals current, lowers selectivity, and complicates downstream separation of products. A strategy that physically removes proton-coupled electron transfer from the CO2-facing interface — while still drawing the hydrogen ultimately from water — addresses the problem at its mechanistic root rather than merely suppressing it with catalyst additives or electrolyte tricks.

There is also a design philosophy on display here that may prove as influential as the specific result. Rather than engineering a better catalyst composition, the researchers engineered a better reaction architecture: a spatially segregated, two-compartment system in which the generation of one reagent and its delivery to another are handled by different faces of the same device. The Janus membrane is simultaneously an electrode, a hydrogen separator, and a hydride reagent generator. Such multifunctional device-level thinking, the authors suggest, offers a general route to access heterogeneous hydride reactivity beyond proton-mediated electrocatalysis — a toolbox that could extend well beyond CO2 to other reductions that benefit from hydride delivery.

Challenges remain before such electrodes could be scaled. Palladium is scarce and expensive, membrane thickness and hydrogen permeation rates must be optimized for industrial current densities, and the long-term mechanical and chemical stability of the hydride-loaded membrane under continuous operation will need to be proven. Nevertheless, the demonstration that selective, high-efficiency CO2-to-formate conversion can be achieved through directed heterogeneous hydride transfer — in water and in fully aprotic media alike — marks a genuine conceptual advance. It shows that the competing hydrogen evolution reaction, instead of being an enemy to be suppressed, can be domesticated: generated on one side of a metal membrane, converted into tunable hydride reagent, and delivered with precision to the carbon dioxide waiting on the other.

Researchers led by Professor Yousung Jung at Seoul National University have proposed a catalyst-design principle that selectively suppresses hydrogen evolution while preserving the electrochemical reaction that converts nitrogen into ammonia. Their approach relies not on removing protons from the reaction environment, but on controlling how easily those protons can physically reach the electrode surface. The concept could offer a new route toward cleaner ammonia production powered by renewable electricity.

Ammonia is increasingly viewed as a practical carrier for hydrogen energy. It stores a large amount of hydrogen, remains liquid at room temperature under moderate conditions, and is easier to transport than gaseous hydrogen. Today, most ammonia is manufactured through the Haber–Bosch process, which requires high temperatures and pressures and consumes substantial amounts of energy. Because the process is commonly powered by fossil fuels, it also produces significant carbon dioxide emissions.

Electrochemical nitrogen reduction has emerged as a possible alternative. In principle, the process uses nitrogen, water and electricity to produce ammonia, potentially allowing production facilities to operate near wind or solar farms rather than relying on massive centralized chemical plants. The central obstacle, however, is that the electrode preferentially produces hydrogen instead of ammonia. This competing hydrogen evolution reaction consumes electrons and protons that would otherwise contribute to nitrogen reduction, sharply reducing ammonia yields.

Previous strategies have often attempted to control hydrogen evolution by changing the proton concentration or acidity of the electrolyte. That solution creates a fundamental trade-off: protons are needed not only to generate hydrogen but also to convert nitrogen into ammonia. Reducing their availability can therefore suppress both reactions. Jung’s team instead focused on the reaction interface—the narrow region where the electrolyte, catalyst and electrode meet—and asked whether the two reactions could be separated geometrically.

The researchers designed a reaction environment in which proton donors encounter greater steric hindrance as they approach the electrode surface. Steric hindrance occurs when the size and arrangement of molecules make it physically difficult for another molecule to reach a reactive site. In this case, the researchers effectively create a molecular gate around the electrode. Proton donors can still exist in the surrounding solution, but their paths toward the surface become restricted, making the initial step of hydrogen production less favorable.

That initial step is known as the Volmer reaction. During the Volmer reaction, a proton receives an electron and forms an adsorbed hydrogen atom on the electrode. Two such hydrogen atoms can subsequently combine to form molecular hydrogen, or an adsorbed hydrogen atom can react with another proton and electron. By making it harder for proton donors to reach the electrode, the new strategy raises the energy barrier for the Volmer reaction and slows the entire hydrogen evolution pathway.

The geometry of nitrogen reduction is different. According to the researchers, nitrogen molecules protrude outward from the catalyst environment, allowing protons to interact with nitrogen rather than needing to reach the electrode surface directly. This difference means that the same steric barrier that obstructs hydrogen evolution has a much smaller effect on nitrogen reduction. The result is a form of molecular selectivity based on access and positioning rather than solely on chemical composition.

The team used microkinetic modeling to examine how these competing pathways respond to changes in steric hindrance and applied voltage. Microkinetic models describe the rates of individual elementary reactions by tracking intermediates and solving differential equations for the overall reaction network. The simulations indicated that increasing steric hindrance can maintain high Faradaic efficiency across a broad voltage range. Faradaic efficiency measures the fraction of supplied electrical charge that produces the desired product—in this case, ammonia rather than hydrogen.

The findings could address a major limitation in electrochemical ammonia research, where Faradaic efficiencies have often remained near 70 percent or lower. The researchers report that their design principle could raise the value toward nearly 100 percent under modeled conditions, although future catalyst development and experimental validation will be essential. The strategy may also be transferable to other electrochemical systems in which a desired reaction competes with an unwanted one, including carbon dioxide reduction.

The study, published in the Journal of the American Chemical Society, presents the reaction interface itself as a programmable component of catalyst design. Rather than treating the electrolyte as a passive medium, the approach uses molecular shape to control which reactants can reach specific locations. Jung’s team plans to identify highly active catalyst materials that can incorporate this principle and to test the concept across additional reactions. If successful, the work could help bring renewable-powered ammonia production closer to practical use while offering a broader blueprint for controlling chemical reactions at the atomic scale.

Subject of Research: Electrochemical nitrogen reduction and selective suppression of the hydrogen evolution reaction

Article Title: Selective Suppression of Hydrogen Evolution in Electrochemical Nitrogen Reduction through Steric Control of Proton Donors

News Publication Date: July 22

Web References: https://doi.org/10.1021/jacs.6c07080

References: Journal of the American Chemical Society, DOI: 10.1021/jacs.6c07080

Image Credits: Seoul National University College of Engineering

Keywords

Electrochemical ammonia production, nitrogen reduction reaction, hydrogen evolution reaction, steric hindrance, catalyst design, green hydrogen, renewable energy, Faradaic efficiency, microkinetic modeling, Seoul National University