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 new work arrives at a moment when CO2 electroreduction stands at a crossroads. Laboratory demonstrations of copper-based catalysts converting carbon dioxide into ethylene, ethanol and other multicarbon products have multiplied rapidly over the past decade, but translating those results into industrial electrolyzers capable of gigatonne-scale operation demands catalysts that remain active, selective and stable for thousands of hours. Catalyst reconstruction sits at the heart of that challenge. When a copper catalyst reorganizes itself under operating conditions, its activity can rise or fall, its product selectivity can drift toward desired chemicals or toward wasteful hydrogen evolution, and its lifetime can be cut dramatically short. Until now, the field has lacked a standardized, reproducible methodology for investigating these transformations—leaving individual laboratories to improvise their own approaches, with results that are often difficult to compare or reproduce.

The protocol is organized around what the authors call a ‘reconstruction–understanding–intervention’ workflow, a modular pipeline that guides researchers from the first observation of structural change all the way to deliberate control of the catalyst’s final state. The first stage involves the identification and taxonomy of reconstruction phenomena, classifying the many ways a copper surface can transform: morphological reshaping, chemical state changes such as the reduction of copper oxides to metallic copper or the persistence of transient copper(I) species, and compositional evolution in alloyed or bimetallic systems. By establishing a common vocabulary and systematic identification procedures, the protocol addresses one of the field’s most persistent problems—different groups describing fundamentally different phenomena under the same broad label of ‘reconstruction’.

The second stage tackles the question of what drives these transformations in the first place. Reconstruction is governed by an interplay of electrochemical and environmental factors: applied potential, local pH, the identity and concentration of electrolyte cations and anions, mass transport of CO2 to the surface, and the adsorption of reaction intermediates such as carbon monoxide. The protocol lays out quantitative methods for disentangling these variables, allowing researchers to determine whether a particular restructuring event is triggered by potential cycling, by the accumulation of hydroxyl species, by the migration of alkali metal cations into the interfacial layer, or by some combination of influences. This quantitative grounding is essential, because interventions can only be rationally designed once the governing factors are known.

At the technical core of the protocol lies a battery of complementary in situ and operando characterization techniques, each chosen to illuminate a different aspect of the catalyst’s evolving structure. Operando Raman spectroscopy tracks surface oxides, adsorbed intermediates and the formation of species such as copper carbonyl in real time under working conditions. Infrared spectroscopy, including surface-enhanced variants based on attenuated total reflection, probes the vibrational fingerprints of adsorbed molecules and interfacial water networks. X-ray absorption spectroscopy, typically performed at synchrotron facilities, reveals changes in the oxidation state and local coordination environment of copper atoms deep within the working electrode. Quasi-in situ X-ray photoelectron spectroscopy bridges the gap between fully operando measurements and conventional ex situ analysis: the protocol describes a compact titanium-alloy transfer cell, sized to pass through a glovebox antechamber, that allows electrodes to be interrogated at defined electrochemical states without exposure to air, preserving chemical information that would otherwise be lost.

The methodological rigor extends to the hardware itself. The protocol provides detailed descriptions of electrochemical cell configurations—flow cells, gas diffusion electrode assemblies and spectroscopy-compatible electrolytic cells—because the authors emphasize that the very design of the cell influences how catalysts reconstruct. Extended data accompanying the article specify, for example, the geometry of an operando Raman flow cell built from polyetheretherketone with a titanium flow field and quartz optical window, and the configuration of an operando XAS cell sealed with Kapton film and oriented at 45 degrees to the incident X-ray beam. Standardizing these details means that structural dynamics observed in one laboratory can be meaningfully compared with results from another, a prerequisite for building a reliable, field-wide picture of copper’s behavior under reaction conditions.

With identification and diagnosis in hand, the protocol’s third pillar moves into territory that sets it apart: active intervention. Rather than treating reconstruction as an inevitable degradation process to be tolerated, the authors present three primary strategies for steering it toward desired active states. The first is catalyst structure modulation, in which the starting material—its composition, oxide content, strain and dopant profile—is engineered so that the reconstruction pathway terminates at a favorable configuration. The second is electrochemical operation regulation, including pulsed or intermittent electrolysis protocols that periodically reset or regenerate the catalyst surface. The third is reaction microenvironment management, in which the electrolyte composition, local hydrophobicity, cation distribution and interfacial water structure are tuned to stabilize particular surface states and suppress destructive pathways.

The methodology was validated across representative classes of copper catalysts, demonstrating its breadth. Commercial metallic copper foil, oxide-derived copper—the family of catalysts in which pre-formed oxides reorganize during reaction to create highly active surfaces—and bimetallic copper-based systems denoted Cu–X, where a second metal tunes copper’s electronic structure, all serve as test cases. In each case, the workflow linked specific reconstruction dynamics to catalytic behavior and showed that deliberate control strategies enhanced both performance and stability of CO2 reduction. The validation examples map onto the team’s own published record, including work on oxophilicity-controlled multicarbon alcohol production over Lewis-acid-doped copper, lanthanide-induced tensile-strained copper oxide catalysts, acid-fed lanthanum–copper spheres operating at ampere-level currents, in situ periodic regeneration of catalysts, and self-adaptive catalysts for CO2 electroreduction.

The broader significance of the protocol extends well beyond copper. The authors explicitly frame the workflow as an adaptable framework for investigating dynamic surface evolution in other electrocatalytic reactions, from nitrate reduction to ammonia—where correlated operando microscopy and spectroscopy have similarly revealed restructuring—to carbon monoxide reduction and beyond. As the global push toward carbon neutrality intensifies, the ability to design ‘self-adaptive’ electrocatalysts that respond constructively to their operating environment, rather than degrading under it, is emerging as a defining goal of the field. By providing a reproducible, modular and quantitative path from observation to mechanistic insight to rational control, this protocol gives the community a shared toolkit for reaching that goal. In effect, it transforms one of electrocatalysis’s most stubborn complications into an engineering variable—one that can be measured, modeled and ultimately mastered in the service of converting waste carbon dioxide into the fuels and chemicals of a sustainable economy.

The scientific backdrop to this protocol is a decade of discoveries that progressively dismantled the assumption of a static copper surface. Operando studies have shown that metallic copper can fragment into active nanograins under reaction conditions, while solution-based transient copper(I) species have been identified as mediators of surface reconstruction. Other work has revealed that oxygen trapped within oxide-derived copper can diffuse and persist during catalysis, and that hydroxyl radicals play a decisive role in reoxidizing reduced copper surfaces. Adsorbed hydroxide itself has been described as a double-edged sword, simultaneously promoting carbon–carbon coupling and destabilizing the catalyst. Each of these findings underscores why a single snapshot of a catalyst before or after electrolysis is insufficient: the active state may exist only transiently, sandwiched between structural configurations that are themselves catalytically inert.

The protocol also responds to a subtler problem: the observation itself can depend on how it is made. Recent comparative studies have demonstrated that cell configuration measurably alters how copper reconstructs, since flow geometry, electrolyte layer thickness and local mass transfer shape the interfacial chemical environment. Operando X-ray absorption work has quantified these mass-transfer effects directly, showing that concentration gradients near the electrode influence both the reaction pathway and the structural evolution of the catalyst. By specifying standardized cell geometries and measurement procedures, the protocol helps ensure that apparent differences between catalysts reflect genuine materials behavior rather than apparatus artifacts.

Another theme the protocol consolidates is the mechanistic link between reconstruction and selectivity. Spectroscopic observations of carbon monoxide bridge species forming on dynamically restructured copper, together with reconstruction-dependent coordination descriptors, suggest that the evolving surface geometry directly tunes how intermediates bind and couple. This reframes selectivity in CO2 electrolysis as a property of a moving target. The practical consequence is that stability and performance can no longer be optimized independently; a protocol that treats the catalyst’s trajectory through time as the design object, rather than its initial composition, aligns catalyst development with how these materials actually behave in operating electrolyzers.

Subject of Research: Dynamic reconstruction of copper-based catalysts during electrochemical CO2 reduction and methods for probing and controlling it

Article Title: Probing and controlling Cu catalyst reconstruction during CO2 electroreduction

Article References: Zhang, L., Zheng, C., Xu, L., Feng, J., Jia, S., Wu, L., Song, X., Zhang, M.-D., Wang, R., Zhang, X., Zhao, Z., Sun, X., & Han, B. (2026). Probing and controlling Cu catalyst reconstruction during CO2 electroreduction. Nature Protocols. https://doi.org/10.1038/s41596-026-01430-1

Image Credits: AI Generated

DOI: 10.1038/s41596-026-01430-1

Keywords: CO2 electroreduction, copper catalyst, catalyst reconstruction, electrocatalysis, operando spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, multicarbon products, catalyst stability, oxide-derived copper, carbon neutrality