The hydrogen evolution reaction (HER) has been crucial for the development of fundamental knowledge on electrocatalysis and electrochemistry, in general. In alkaline media, many key questions concerning pH-dependent structure–activity relations and the underlying activity descriptors remain unclear. While the presence of Ni(OH)₂ deposited on Pt(111) has been shown to highly improve the rate of the HER through the electrode’s bifunctionality, no studies exist on how low coverages of Ni(OH)₂ influence the electrocatalytic behavior of Cu surfaces, which is a low-cost alternative to Pt. Here, we demonstrate that Cu(111) modified with 0.1 and 0.2 monolayers (ML) of Ni(OH)₂ exhibits an unusual non-linear activity trend with increasing coverage. By combining in situ structural investigations with studies on the interfacial water orientation using electrochemical scanning tunneling microscopy and laser-induced temperature jump experiments, we find a correlation between a particular threshold of surface roughness and the decrease in the ordering of the water network at the interface. The highly disordered water ad-layer close to the onset of the HER, which is only present for 0.2 ML of Ni(OH)₂, facilitates the reorganization of the interfacial water molecules to accommodate for charge transfer, thus enhancing the rate of the reaction. These findings strongly suggest a general validity of the interfacial water reorganization as an activity descriptor for the HER in alkaline media.

Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO₂RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo₂C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO₂ activation. The oxides are stable down to potentials as low as −1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo₂C indeed shows CO₂RR activity.

The electrochemical reduction of CO₂ is an important electrolysis reaction that enables the conversion of a waste gas to fuels or value-added chemicals. To make this reaction viable, a profound understanding of central intermediate steps, such as the CO electroreduction, is required. On Cu, the CO reduction reaction (CORR) is intimately linked to the hydrogen evolution reaction (HER) that proceeds via the reduction of water in alkaline or neutral electrolytes. Here, we demonstrate that the interaction of water or more specifically the water reduction kinetics on differently smooth Cu(100) and Cu(111) surfaces during the CORR in alkaline media significantly governs the CORR. On Cu(111), faster HER kinetics and the highest CORR activity are observed, even though HER and CORR onsets are more negative. While on Cu(100) small Cu ad-island clusters form in the cathodic potential range only when CO is present, structural changes appear on a larger length scale on Cu(111) both under CORR conditions and when no CO is present. These differences in the reconstruction characteristics may be attributed to the dominance of either the CORR and its intermediates or the HER on the different Cu surfaces. Therefore, the interfacial water reactivity is considered an essential activity descriptor for the CORR on Cu in alkaline media.

The hydrogen evolution reaction (HER) is a crucial electrochemical process for the proposed hydrogen economy since it has the potential to provide pure hydrogen for fuel cells. Nowadays, hydrogen electroproduction is considerably expensive, so promoting the development of new non-noble catalysts for the cathode of alkaline electrolyzers appears as a suitable way to reduce the costs of this technology. In this sense, a series of tungsten-based carbide materials have been synthesized by the urea-glass route as candidates to improve the HER in alkaline media. Moreover, two different pyridinium-based ionic liquids were employed to modify the surface of the carbide grains and control the amount and nature of their surface species. The main results indicate that the catalyst surface composition is modified in the hybrid materials, which are then distinguished by the appearance of tungsten suboxide structures. This implies the action of ionic liquids as reducing agents. Consequently, differential electrochemical mass spectrometry (DEMS) is used to precisely determine the onset potentials and rate-determining steps (RDS) for the HER in alkaline media. Remarkably, the modified surfaces show high catalytic performance (overpotentials between 45 and 60 mV) and RDS changes from Heyrovsky-Volmer to Heyrovsky as the surface oxide structures get reduced. H₂O molecule reduction is then faster at tungsten suboxide, which allows the formation of the adsorbed hydrogen at the surface, boosting the catalytic activity and the kinetics of the alkaline HER.

A profound understanding of the solid/liquid interface is central in electrochemistry and electrocatalysis, as the interfacial properties ultimately determine the electro‐reactivity of a system. Although numerous electrochemical methods exist to characterize this interface under operating conditions, tools for the in‐situ observation of the surface chemistry, that is, chemical composition and oxidation state, are still scarce, and currently exclusively available at synchrotron facilities. Here, we demonstrate the ability of laboratory‐based near‐ambient pressure X‐ray photoelectron spectroscopy to track changes in oxidation states in‐situ with respect to the applied potential. In this proof‐of‐principle study with polycrystalline gold (Au) as the best‐studied electrochemical standard, we show that during the oxygen evolution reaction (OER), at high OER overpotentials, Au³⁺ governs the interfacial chemistry, while, at lower overpotentials, Au⁺ dominates.