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