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 core difficulty is chemical. Aluminum ions in water are strongly hydrated, and the water molecules locked into the Al3+ solvation shell are actually easier to electrochemically reduce than free water. During charging, this triggers vigorous hydrogen evolution, which consumes active aluminum, destabilizes the electrode interface, and wrecks cycling stability. Meanwhile, the bulk water in the electrolyte maintains its high reactivity through an extended, interconnected hydrogen bond network, which narrows the electrochemical stability window of the entire system. Corrosion and surface passivation compound the damage, rapidly thinning the aluminum anode until the cell fails.

Previous attempts to fix this usually tweaked one lever at a time, often through trial-and-error screening of additives. The research team behind the new work, led by scientists supported by Singapore’s A*STAR program, argues that this approach conflates two physically distinct phenomena: the chemistry of the bulk solution and the dynamics of the Al3+ cation at the electrode surface. Suppressing water activity in the bulk helps, but excessive disruption of the solvent matrix degrades ionic conductivity and viscosity. Similarly, modifying the cation’s solvation shell can curb interfacial reactions, but too much modification imposes a heavy desolvation penalty that slows the kinetics of aluminum plating and stripping. The team set out to evaluate these two effects independently and then find the composition where both are simultaneously satisfied.

As model cosolvents, the researchers examined four trialkyl phosphates: trimethyl phosphate (TMP), triethyl phosphate (TEP), tripropyl phosphate (TPP), and tributyl phosphate (TBP). These organophosphorus molecules are inherently non-flammable and feature a strongly electron-rich phosphoryl group that acts as a powerful hydrogen bond acceptor. Density functional theory calculations showed that Al3+ binds more strongly to all four phosphates than to water, with binding energy increasing sublinearly as the alkoxy chains lengthen. Electrolytes were prepared by mixing each cosolvent at volume fractions from 20 to 80 percent into a baseline 1 M aluminum triflate solution, and twelve viable formulations were tested in symmetric aluminum-aluminum cells.

The screening revealed a clear trade-off. Ionic conductivity fell steadily as cosolvent content rose, because every phosphate has a lower dielectric constant and higher viscosity than water; at equal volume fractions, conductivity tracked the dielectric constants, ranking TMP above TEP, TPP, and TBP. At the other extreme, cells with 80 percent cosolvent ran for more than 140 hours but suffered overpotentials above 1.1 volts, a sign of sluggish interfacial kinetics. The sweet spot was 40TMP: a moderate overpotential below 0.3 volts combined with cycling stability beyond 200 hours, and an ionic conductivity of 24.6 mS per centimeter, roughly half the baseline value but still far above other reported organic-cosolvent aluminum electrolytes.

Spectroscopy explained why. Fourier-transform infrared spectra showed the water O-H stretching and bending modes shifting to higher wavenumbers as TMP content increased, evidence that TMP’s phosphoryl oxygen outcompetes water for hydrogen bonding and breaks up the cooperative water-water network. Proton nuclear magnetic resonance displayed a two-stage evolution: an initial downfield shift from strong P=O···H-O interactions up to 40TMP, followed by upfield broadening at higher concentrations as the native hydrogen bond network collapses. Differential scanning calorimetry confirmed the disruption, with freezing points depressed by 5.7 degrees Celsius at 60TMP and no detectable freezing peak at 80TMP. Corrosion current density on aluminum electrodes dropped from 11.5 microamperes per square centimeter in the baseline to 3.1 at 80TMP, and the hydrogen evolution onset shifted negatively by 0.51 volts at the highest TMP loading.

Molecular dynamics simulations and 27Al NMR then dissected the solvation shell itself. In the baseline electrolyte, Al3+ carries an average of 5.42 water molecules in its primary shell. In 40TMP, that number falls to 4.82, with roughly half a TMP molecule entering the inner shell, while anion coordination stays essentially constant. Because coordinated water is more easily reduced than free water, removing even one molecule from the shell meaningfully suppresses interfacial hydrogen evolution. But the simulations also exposed the danger of overcorrection: at 80TMP, 2.37 TMP molecules coordinate to each Al3+, and the strong Al3+-TMP binding raises the desolvation barrier, choking deposition kinetics. The 40TMP composition threads the needle between these competing constraints.

One of the most striking findings is that the optimized electrolyte enables the growth of a genuine solid-electrolyte interphase even in a dilute aqueous system. Cross-sectional electron microscopy and X-ray photoelectron spectroscopy of cycled aluminum foils revealed a chemically graded layer: an organic-rich outer region derived from triflate decomposition products, and an inorganic-rich inner region composed mainly of aluminum fluoride and aluminum sulfide. Time-of-flight secondary ion mass spectrometry mapped these components in three dimensions, confirming the hybrid architecture. Notably, no phosphorus was detected in the interphase, meaning TMP does not directly contribute to SEI chemistry. Instead, by suppressing hydrogen evolution, it prevents gas bubbles from physically disrupting the nascent interphase, allowing a robust, continuous film to form where the baseline electrolyte produces only faint, fragmentary signals.

The electrochemical payoff was substantial. Differential electrochemical mass spectrometry showed dramatically less hydrogen generation during plating and stripping, and symmetric cells ran stably for over 500 hours at 0.1 mAh per square centimeter, compared with failure within 120 hours for the baseline. Full cells pairing the electrolyte with a polyaniline cathode, chosen because it stores charge through nitrogen-centered redox rather than proton-coupled intercalation, delivered an average Coulombic efficiency of 99.0 percent and retained 59 percent of capacity after 250 cycles, whereas baseline cells collapsed within 33 cycles. Adding a tin interfacial layer on the aluminum anode, formed by a simple displacement reaction and acting as underpotential-deposition nucleation sites, pushed performance further: the Sn@Al|40TMP|polyaniline cell retained 70 percent of its capacity after 400 cycles and kept more than 70 percent of its capacity even at 1 A per gram. Combustion tests also showed the 40TMP-wetted separator surviving 30 seconds of direct flame exposure, thanks to TMP’s radical-tracking flame-retardant behavior, at a cost premium of only about 7 percent over the baseline formulation.

Beyond the immediate performance numbers, the study’s real contribution may be conceptual. By explicitly separating bulk solution effects from cation solvation dynamics and then optimizing both together, the researchers provide a transferable design framework rather than a one-off recipe. The work demonstrates that the path to stable aqueous aluminum batteries does not run through maximum suppression of water reactivity, but through a precise balance point where water is tamed enough to stop parasitic reactions, yet the aluminum ion can still shed its solvation shell and move quickly across the interface. If that principle holds across other multivalent chemistries, the humble phosphate molecule may have just moved the entire field a significant step closer to batteries that are safe, cheap, and built to last.

Subject of Research: Electrolyte design for stable aqueous aluminum metal batteries

Article Title: Balancing Bulk Solution Chemistry and Cation Solvation for Stable Aqueous Aluminum Batteries

Article References: Jia, B.-E., Wu, G., Gu, H., Liew, J. J., Chen, D., Song, J., Wang, D.-Y., Hu, E., Choo, H. H., Liu, Y., Hu, Y., Song, J., Tang, A., Xing, Z., Zhu, Q., Yan, C., Lv, C., Ng, M.-F., & Yan, Q. (2026). Balancing Bulk Solution Chemistry and Cation Solvation for Stable Aqueous Aluminum Batteries. Advanced Science, 13(55), Article e76540. https://doi.org/10.1002/advs.76540

Image Credits: AI Generated

DOI: 10.1002/advs.76540

Keywords: aqueous aluminum batteries, electrolyte design, trimethyl phosphate, solvation structure, hydrogen evolution, solid-electrolyte interphase, aluminum metal anode, post-lithium energy storage, hydrogen bond network, polyaniline cathode, tin interfacial layer, corrosion suppression

The research, led by Dr. Zhongqiu Lin together with Associate Professor Toshiki Sugimoto and colleagues, focused on anatase titanium dioxide, one of the most widely studied photocatalytic materials for hydrogen evolution. Although it has been recognized that interactions at the water-catalyst interface are key determinants of photocatalytic performance, systematic experimental studies that explicitly target the structure and reactivity of this interface have remained scarce. The central difficulty is a practical one: probing the molecular structure of interfacial water is challenging under normal circumstances, and it becomes even harder under the conditions where hydrogen is actually being evolved. Compounding the problem, the apparent hydrogen evolution activity measured in an experiment is highly sensitive not only to the surface area of the photocatalyst but also to the amount of water present at the interface, making it difficult to separate genuine differences in reactivity from simple differences in how much water is available to react.

To overcome these obstacles, the team designed a series of experiments using anatase TiO2 photocatalysts with different surface characteristics, allowing them to compare interfaces that interact with water in distinct ways. They combined infrared spectroscopy, which reveals the adsorption states and hydrogen-bonding arrangements of water molecules, with real-time mass spectrometry, which tracks the production of hydrogen gas as it happens. Crucially, the measurements were carried out under precisely controlled hydration conditions ranging from sub-monolayer coverages, where isolated water molecules dot the surface, to several molecular layers of adsorbed water. This control allowed the researchers to examine how water behaves in different interfacial environments while keeping the amount of water explicitly accounted for.

A key methodological advance came from the way the team analyzed their data. By normalizing the measured hydrogen formation rates with respect to both the specific surface area of the photocatalyst and the number of adsorbed water layers, they were able to quantitatively distinguish the intrinsic reactivity of interfacial water from effects that arise simply because different samples hold different amounts of water at their surfaces. This normalization framework meant that when two interfaces showed different hydrogen evolution rates, the difference could be attributed to the molecular structure of the water at those interfaces rather than to trivial differences in surface area or water loading. It is this careful separation of variables that gave the study its power to draw firm conclusions about structure-reactivity relationships.

With this framework in place, the researchers systematically investigated the adsorption state of interfacial water, examining both the strength with which water molecules bind to the TiO2 surface and the mode of adsorption, whether the molecules remain intact or dissociate into hydroxyl groups and protons upon adsorption. The conventional view in photocatalysis has held that strong water-TiO2 interactions should generally be favorable, because strong binding is thought to enhance the trapping of photogenerated charge carriers at the surface, suppress the recombination of electrons and holes, and thereby prolong the lifetimes of the charge carriers that are needed to drive the chemical reactions. Intuitively, longer-lived carriers should mean more opportunities for water molecules to be reduced or oxidized, and hence better catalytic performance.

The experimental results told a different story. Contrary to the conventional expectation, the team found that relatively weaker water-TiO2 interactions were associated with higher reactivity of the interfacial water toward hydrogen evolution. In other words, water molecules that were held less tightly to the surface were, on average, more reactive participants in the photocatalytic reaction than those bound strongly. This observation challenges the intuition that maximizing water-surface binding strength is a reliable design strategy, and it suggests that the factors governing interfacial reactivity are more subtle than charge-carrier dynamics alone.

The explanation, the researchers realized, lies in the fact that interfacial water does not exist as isolated molecules interacting only with the solid surface. Instead, water molecules at the interface also form hydrogen-bond networks with one another, and these networks possess collective structural and dynamical properties of their own. The team therefore turned their attention to how the hydrogen-bonding environment of the interfacial water influences its reactivity. Their analysis revealed that weaker and more flexible hydrogen-bond networks were associated with higher reactivity of the interfacial water. Water held in a rigid, strongly connected network was less reactive, while water embedded in a looser, more pliable network reacted more readily to produce hydrogen.

This finding provides molecular-level insight into what is believed to be the rate-determining step of photocatalytic hydrogen evolution: the initial oxidation of water, which proceeds through proton-coupled charge transfer at the water-TiO2 interface. In such a process, the transfer of a proton is coupled to the movement of electrical charge, and the reaction requires the surrounding molecular environment to reorganize as the reactants transform into products. From the perspective of Marcus theory, the foundational framework for describing electron transfer reactions, the rate of a reaction depends in part on the reorganization energy, that is, the energetic cost of rearranging the molecular environment to accommodate the charge transfer. Greater flexibility and larger fluctuations of the hydrogen-bond network reduce the barriers associated with this molecular reorganization, making it easier for the reaction to proceed. The experimentally observed higher reactivity of more flexible interfacial water is thus consistent with theoretical expectations, and it ties the macroscopic catalytic performance directly to the dynamics of the hydrogen-bond network at the interface.

The implications for photocatalyst design are significant. Because strong water-catalyst interactions have beneficial effects on photogenerated charge carriers, photocatalyst development has traditionally favored hydrophilic interfaces, where water binds strongly to the catalyst surface. The new study reveals, however, that relatively weaker water-TiO2 interactions, which are associated with more flexible hydrogen-bond networks, favor higher reactivity of the interfacial water toward hydrogen evolution. This suggests that the optimal interface is not the one that binds water most tightly, but the one that allows the interfacial water to retain enough structural freedom to undergo the molecular reorganization demanded by the reaction. Surface chemistries, coatings, or morphologies that moderate the strength of water binding while preserving charge-carrier performance could therefore offer a path to more active photocatalysts.

More broadly, the work demonstrates the value of directly characterizing both the adsorption state and the hydrogen-bonding structure of interfacial water and correlating these properties with hydrogen evolution activity under well-controlled conditions. By establishing a quantitative link between the molecular structure of the interface and its catalytic reactivity, the study provides a molecular basis for engineering water-catalyst interfaces to enhance photocatalytic performance. As the field continues to pursue efficient solar-to-chemical energy conversion, the message from the Institute for Molecular Science team is clear: to design better photocatalysts, researchers should look not only at the solid surface itself but also at the delicate, dynamic architecture of the water molecules that sit upon it, and consider giving those molecules a little more room to move.

Subject of Research: Structure and reactivity of interfacial water in photocatalytic hydrogen evolution on anatase TiO2

Article Title: Bridging interfacial water structure and reactivity in photocatalytic hydrogen evolution at TiO₂ interfaces

Article References: Bridging interfacial water structure and reactivity in photocatalytic hydrogen evolution at TiO₂ interfaces. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: photocatalysis, hydrogen evolution, titanium dioxide, interfacial water, hydrogen-bond network, infrared spectroscopy, mass spectrometry, anatase TiO2, solar fuels, water splitting, Marcus theory, charge transfer