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

The central obstacle in electrochemical urea synthesis has never been a lack of possible reactions but a stubborn mismatch of timing. To build the carbon–nitrogen bond at the heart of urea, a catalyst must simultaneously reduce two very different feedstocks: carbon dioxide, which needs to be transformed into a carbon-containing intermediate such as *COOH or carbon monoxide, and nitrate, which must be stripped of oxygen and hydrogenated toward nitrogen-containing species like *NH2. These two reaction cascades proceed at different rates and prefer different potentials, so on most catalysts the intermediates simply never coexist on the surface long enough, or close enough, to couple. Instead, the cell churns out side products, typically ammonia from nitrate reduction or carbon monoxide and hydrogen from competing reactions, and the desired C–N coupling is left as a minor footnote in the product distribution.

The researchers, led by Yongwen Tan of Hunan University with co-first authors Jiao Lan and Zhiwen Chen, attacked this kinetic mismatch with a design philosophy built on what they call dual atomic pairs. Rather than scattering catalytic functions randomly across a surface, they engineered an atomically ordered CuAuZn2 intermetallic compound, a crystalline solid in which copper, gold, and zinc atoms occupy fixed, alternating lattice positions. That order is the crucial ingredient. In an ordinary alloy, atoms mix in a more or less statistical fashion, so the local environment of any given site is unpredictable. In an intermetallic, by contrast, the crystal structure guarantees that specific pairs of neighboring atoms appear at high density and in a repeating, deterministic pattern. The team’s material therefore presents dense arrays of Cu–Zn pairs sitting alongside dense arrays of Au–Zn pairs, each with its own catalytic specialty.

The division of labor is elegant. Spectroscopic and computational analyses described in the paper indicate that the Cu–Zn sites are tuned to drive nitrate reduction efficiently, guiding the six-electron-plus-six-proton transformation all the way to the *NH2 intermediate rather than letting it run to completion as ammonia. Neighboring Au–Zn sites, meanwhile, excel at the first step of carbon dioxide reduction, generating the *COOH intermediate that supplies the carbon partner. Because these two site types are atomically interspersed throughout the ordered lattice, the nitrogen and carbon intermediates are produced in immediate proximity, at matched rates, dramatically raising the probability that they will couple into a C–N bond before drifting away or over-reducing. It is a tandem catalysis concept executed not at the scale of particles or domains but at the scale of individual atoms, enforced by crystallography itself.

The performance numbers reported for the ordered CuAuZn2 electrode are remarkable by the standards of this young field. Under co-reduction of carbon dioxide and nitrate at zero volts versus the reversible hydrogen electrode, a potential so mild it sits near the thermodynamic neutral point for the combined reaction, the catalyst achieved a Faradaic efficiency of 85.5 percent, plus or minus 1.6 percent, meaning that fraction of the electrical current went into making urea rather than waste products. The energy efficiency reached 50.1 percent, plus or minus 1.5 percent, a figure that matters enormously for any future industrial deployment, since electricity is the dominant operating cost of electrosynthesis. For a reaction that stitches together two small molecules through a multi-electron cascade, delivering half of the input energy into the target product is a benchmark few competing systems have approached.

Durability, often the Achilles heel of designer electrocatalysts, received equally serious attention. The team moved beyond the small laboratory beakers typical of early-stage studies and built a membrane electrode assembly, the compact device architecture used in commercial electrolyzers and fuel cells. In that demanding configuration, the ordered intermetallic catalyst operated continuously for 1,000 hours at a current density of 100 milliamperes per square centimeter, sustaining a urea production rate of approximately 8.3 milligrams per hour per square centimeter of electrode. A thousand hours of stable operation at ampere-scale current densities is the kind of stress test that separates laboratory curiosities from technology candidates, and the fact that the atomically ordered structure survived it suggests the intermetallic lattice is genuinely robust under operating conditions rather than a fleeting surface reconstruction.

The structural characterization underpinning these claims drew on a comprehensive toolkit. X-ray absorption spectroscopy, including analysis of the fine structure near the absorption edges of each metal, was used to confirm the local coordination environments of copper, gold, and zinc and to verify that the atoms occupy their ordered lattice sites rather than segregating. Electron microscopy and diffraction methods documented the morphology and crystalline phase of the material, which the authors describe as a nanoporous form of the intermetallic with isolated single-atom alloy characteristics. The nanoporous architecture is not merely cosmetic: it multiplies the electrochemically active surface area and ensures that the high-density dual atomic pairs built into the bulk crystal are actually exposed to the electrolyte where the reaction happens.

Mechanistic experiments and density functional theory calculations, presented in the study’s fifth figure, flesh out the picture of how the C–N bond forms on this surface. The computational work supports the assigned roles of the two site types, showing that the energetics of nitrate-to-NH2 conversion are favorable on Cu–Zn ensembles while CO2-to-COOH conversion is favored on Au–Zn ensembles, and that the coupling step itself benefits from the co-location of the two intermediates. In situ spectroscopy tracked the reaction as it proceeded, providing experimental corroboration for the intermediates identified computationally. Together, the evidence paints a coherent mechanistic narrative in which the ordered lattice functions as a nanoscale reaction vessel with two complementary active sites working in concert, resolving the kinetic mismatch that has plagued the field.

The broader implications extend well beyond urea itself. The authors note that the dual atomic pair concept can be extended to other carbon–nitrogen coupling reactions and, more generally, to the electrosynthesis of high-value amides, a chemical class that includes pharmaceuticals, agrochemicals, and commodity materials. If the design principle holds, it offers a general recipe for multi-intermediate catalysis: choose an ordered intermetallic whose lattice positions place different metal pairs in fixed adjacency, assign each pair a half-reaction, and let crystallographic order guarantee the spatial coordination that random alloys cannot. The team’s earlier work on ordered Cu–Zn intermetallics for nitrite-to-ammonia conversion, also published in Nature Communications, suggests this is a deliberate, evolving research program rather than a one-off discovery.

There are, of course, familiar caveats on the road from bench to plant. Gold is expensive, and any commercial catalyst would need to minimize its loading or find cheaper substitutes that preserve the same electronic structure. Nitrate feedstock, while abundant in agricultural runoff and industrial wastewater, would need reliable supply chains for a distributed manufacturing model, though that very abundance is part of the appeal, since the process could in principle turn a pollutant into fertilizer. Scale-up of membrane electrode assembly devices for this chemistry remains largely unexplored territory. Still, the combination of near-zero overpotential, record-setting efficiency, and thousand-hour stability makes this ordered intermetallic one of the most compelling demonstrations yet that electricity, water, carbon dioxide, and nitrate can be woven directly into the molecules that feed the world, one precisely placed atom pair at a time.

Subject of Research: Atomically ordered CuAuZn2 intermetallic catalysts with dual atomic pairs for electrochemical urea synthesis from carbon dioxide and nitrate

Article Title: Atomically ordered dual atomic pairs in CuAuZn2 intermetallics for efficient urea electrosynthesis

Article References: Atomically ordered dual atomic pairs in CuAuZn2 intermetallics for efficient urea electrosynthesis. (n.d.). https://doi.org/10.1038/s44160-026-01187-1

Image Credits: AI Generated

DOI: 10.1038/s44160-026-01187-1

Keywords: urea electrosynthesis, CuAuZn2 intermetallic, dual atomic pairs, C–N coupling, CO2 reduction, nitrate reduction, electrocatalysis, Faradaic efficiency, membrane electrode assembly, sustainable fertilizer, tandem catalysis, Nature Synthesis

The appeal of alcohol fuels begins with chemistry that hydrogen cannot match in practical terms. While hydrogen packs more energy per kilogram, it must be stored under high pressure as an explosive gas, creating safety and logistics problems that have hampered large-scale fuel cell deployment. Liquid alcohols sidestep this entirely. Methanol, ethanol, ethylene glycol and glycerol can be transported and stored with minimal explosion risk, and their volumetric energy densities of roughly 4,000 to 7,000 watt-hours per litre rival those of gasoline. In a direct alcohol fuel cell, the alcohol is oxidised at the anode in the presence of water, releasing protons and electrons; the electrons travel through an external circuit to do useful work, while the protons migrate through a polymer membrane to the cathode, where they combine with oxygen to form water. The elegance of the design is that water is consumed at the anode and regenerated at the cathode, opening the door to closed, self-regulating systems.

The review distinguishes carefully between passive and active fuel cell architectures, a distinction that shapes every downstream engineering decision. Passive cells are loaded with fuel once and left to run, shedding their waste products by natural diffusion; they are compact, easily automated and well suited to phones, laptops and other portable electronics, but they suffer slow reaction kinetics, fuel crossover and chronic water management problems, with fuel efficiencies of only 10 to 30 percent. Active cells, by contrast, use pumps and blowers to maintain fuel concentration and regulate temperature, achieving efficiencies of 70 to 90 percent and far longer lifespans, at the cost of bulk, complexity and reduced portability. The trade-off mirrors the classic tension in energy technology between convenience and performance, and the review makes clear that neither architecture yet dominates.

At the heart of the performance problem lies the membrane. The industry benchmark, Nafion, conducts protons superbly, with conductivities between 80 and 130 millisiemens per centimetre, but it is expensive and notoriously leaky: between 20 and 40 percent of the methanol fuel can cross from anode to cathode within two hours, where it is oxidised wastefully without generating electricity, poisons the cathode catalyst and drags down the open circuit voltage. Crossover proceeds by two mechanisms, simple diffusion down the concentration gradient and electroosmotic drag, in which methanol hitches a ride with the water molecules that solvate the migrating protons. Crossover rates rise linearly with methanol concentration and fall with increasing current density, which suggests one operational remedy, but the deeper solution lies in membrane redesign.

The membrane engineering literature reviewed here is remarkably inventive. Researchers have doped Nafion with titanium dioxide nanoparticles, graphene oxide and sulfonated graphene oxide, each adding transport selectivity and mechanical strength; TiO2-doped membranes have delivered power densities ranging from 308 milliwatts per square centimetre up to 2.02 watts per square centimetre at elevated temperature and humidity. Cross-linked cellulose-based membranes have cut methanol permeability threefold relative to Nafion, down to 8.2 by 10 to the minus 9 square centimetres per second, and zeolite molecular sieves embedded in the polymer matrix, with pores sized between hydrated protons and methanol molecules, reduced crossover 2.5-fold by acting as physical barriers. Layer-by-layer deposition of chitosan and polyvinyl sulfuric acid on Nafion exploits electrostatic self-assembly to build a bilayer that slows methanol while improving water uptake. The recurring caveat is that every gain in selectivity tends to cost some proton conductivity, so each application demands its own optimisation.

Catalysis presents the other half of the cost-and-performance equation. Platinum is the most active catalyst for methanol oxidation, but it is precious, scarce and vulnerable to poisoning by carbon monoxide, an intermediate that binds to platinum sites far more strongly than oxygen does. The standard fix is alloying with ruthenium, which mitigates poisoning through two complementary effects: an electronic induction that alters platinum’s electron density and weakens CO adsorption, and the provision of extra active sites that activate water to oxidise the adsorbed CO away. The review also documents the mechanical dividends of alloying, since the distorted lattices of bimetallic catalysts resist tensile deformation, and the more reactive partner metal corrodes sacrificially to shield the platinum. For ethanol fuels, platinum-tin alloys show superior CO tolerance and perform well at lower voltages.

The most consequential shift, however, is the migration away from noble metals altogether. Transition metal phosphides, nickel-copper alloys, cobalt-based systems and metal-organic frameworks are emerging as credible anode catalysts, prized for low cost, high surface area and easy modification. On the cathode side, where the sluggish oxygen reduction reaction is the rate-determining step of the whole cell, iron-nitrogen-carbon catalysts have matched platinum’s onset potentials across a wide range of methanol concentrations and survived 40 hours of continuous operation while tolerating chloride, amines and other contaminants. Because the cathode reaction is slower than the anode reaction, catalyst loadings are conventionally set at roughly twice the anodic amount, typically in the range of 2 to 6 milligrams per square centimetre, though optimised nanostructured anodes have achieved 117 milliwatts per square centimetre at just 0.21 milligrams per square centimetre.

Durability is the quieter killer of commercialisation prospects. Membranes degrade mechanically by swelling in excess water or shrinking when dehydrated, and chemically through a Fenton-like mechanism in which hydrogen peroxide, formed at the cathode and as a methanol oxidation byproduct, is activated on catalyst surfaces to generate hydroxyl radicals that indiscriminately attack the polymer backbone. Radical scavengers such as dimethyl sulfoxide, mannitol and n-butanol can be pre-loaded into the cell to mop up these radicals, sacrificing themselves to spare the membrane. Nanofillers help on both fronts, retaining water, reinforcing the polymer against hydrostatic pressure and forcing methanol along tortuous diffusion paths. The review also highlights the membrane electrode assembly itself as a design frontier, with researchers pursuing miniaturised and even flexible, wearable assemblies, and noting that a passive cell’s output depends strongly on its orientation, since gravity and capillary action jointly steer the fuel.

The choice of fuel remains an unresolved compromise. Methanol offers the fastest kinetics and works at low temperatures, achieving efficiencies of 70 to 85 percent, but it is toxic, prone to crossover and produces abundant CO intermediates. Ethanol is renewable, safer and largely immune to crossover because its larger molecule cannot easily penetrate membrane pores, yet its carbon-carbon bond demands high activation energy and its oxidation stalls at acetic acid and acetaldehyde, capping efficiency at around 33 percent. Ethylene glycol and glycerol deliver higher energy densities and negligible crossover but are viscous, kinetically sluggish and demand expensive noble catalysts. No single alcohol satisfies every criterion, and the review argues that the field must ultimately match fuel to application rather than seek a universal winner.

What emerges from this synthesis is a technology that has solved many of its scientific problems piecemeal while still lacking an integrated, affordable package. Nafion remains too costly, platinum remains too scarce, and every clever workaround introduces its own trade-off between conductivity, selectivity, durability and price. Yet the trajectory is unmistakable: noble-free catalysts are closing the activity gap, hybrid membranes are slashing fuel losses, and passive designs are approaching the convenience of the lithium batteries they hope to complement, particularly in remote settings where recharging is impossible. If the remaining engineering paradoxes can be balanced, the liquid-fuelled fuel cell may finally graduate from laboratory curiosity to the power source inside the devices we carry every day.

Subject of Research: Advances in direct alcohol fuel cell membrane engineering, electrocatalysis and commercialization challenges

Article Title: Direct alcohol fuel cells: advances in membrane engineering, catalysis, and commercialization challenges

Article References: Muzenda, C., & Sigwadi, R. (2026). Direct alcohol fuel cells: advances in membrane engineering, catalysis, and commercialization challenges. Results in Chemistry, 31, Article 103856. https://doi.org/10.1016/j.rechem.2026.103856

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103856

Keywords: direct alcohol fuel cells, direct methanol fuel cell, methanol crossover, Nafion membranes, platinum-ruthenium catalysts, oxygen reduction reaction, proton exchange membrane, ethanol fuel cells, membrane electrode assembly, noble-free catalysts, fuel cell commercialization, renewable energy