Electrolyte cations affect the activity of surface-mediated electrocatalytic reactions; however, understanding the modes of interaction between cations and reaction intermediates remains lacking. We show that larger alkali metal cations (excluding the thickness of the hydration shell) promote the electrochemical CO reduction reaction on polycrystalline Cu surfaces in alkaline electrolytes. Combined reactivity and in situ surface-enhanced spectroscopic investigations show that changes to the interfacial electric field strength cannot solely explain the reactivity trend with cation size, suggesting the presence of a nonelectric field strength component in the cation effect. Spectroscopic investigations with cation chelating agents and organic molecules show that the electric and nonelectric field components of the cation effect could be affected by both cation identity and composition of the electrochemical interface. The interdependent nature of interfacial species indicates that the cation effect should be considered an integral part of the broader effect of composition and structure of the electrochemical interface on electrode-mediated reactions.

Although electrochemical hydrogen evolution and oxidation are arguably the best-understood reactions in electrocatalysis, the anomalous effect of pH on hydrogen reaction kinetics has defied simple explanation for decades. This longstanding puzzle exposes gaps in the fundamental understanding of electrocatalysis by showing that singular adsorption descriptors (e.g., the hydrogen binding energy) cannot describe kinetic effects across electrolytes. In this Perspective, we discuss the strengths and shortcomings of binding energies as HER/HOR activity descriptors across different electrolytes and catalyst surfaces, with a special emphasis on the bifunctional mechanism, and identify several "beyond adsorption" descriptors for chemical dynamics in the double layer, including the potential of zero (free/ total) charge, the binding energy of coadsorbed spectator species, transition state barrier heights, and the solvation strength of electrolyte cations. Recent evidence for and against the importance of these phenomena is assessed in the context of hydrogen electrocatalysis to determine their feasibility to accurately predict catalyst behavior. Finally, we propose paths forward for improving the mechanistic understanding of how specific interactions between the surface and species in solution affect macroscopic rates, which include combining single-crystal voltammetry, electroanalytical chemistry, in-operando spectroscopy, atomic-scale DFT calculations, and molecular "double-layer dopants".

Understanding the pH dependent shift of the oxidation peak of the underpotential deposited hydrogen (H upd )i nc yclic voltammograms on the Pt surface is of significance in terms of both the fundamentals of electrochemistry and the rational design of catalysts for the hydrogen oxidation/evolution reactions (HOR/HER). In this work, we providecompelling evidence that the pH dependent shift in the H upd peak on Pt surfaces is driven by the structure of interfacial water rather than the specific adsorption of cations on the electrode surface.C ombined cyclic voltammetric and surface enhanced spectroscopic investigations using an organic cation and crown-ether chelated alkali metal cations show that specific adsorption of metal and organic cations on the Pt surface at the conditions relevant to the HOR/HER is unlikely. The vibrational band corresponding to strongly bound water is monitored when the electrode potential is varied in the H upd range in both acid and base.

In situ/operando surface enhanced infrared and Raman spectroscopies are widely employed in electrocatalysis research to extract mechanistic information and establish structure-activity relations. However, these two spectroscopic techniques are more frequently employed in isolation than in combination, owing to the assumption that they provide largely overlapping information regarding reaction intermediates. Here we show that surface enhanced infrared and Raman spectroscopies tend to probe different subpopulations of adsorbates on weakly adsorbing surfaces while providing similar information on strongly binding surfaces by conducting both techniques on the same electrode surfaces, i.e., platinum, palladium, gold and oxide-derived copper, in tandem. Complementary density functional theory computations confirm that the infrared and Raman intensities do not necessarily track each other when carbon monoxide is adsorbed on different sites, given the lack of scaling between the derivatives of the dipole moment and the polarizability. Through a comparison of adsorbed carbon monoxide and water adsorption energies, we suggest that differences in the infrared vs. Raman responses amongst metal surfaces could stem from the competitive adsorption of water on weak binding metals. We further determined that only copper sites capable of adsorbing carbon monoxide in an atop configuration visible to the surface enhanced infrared spectroscopy are active in the electrochemical carbon monoxide reduction reaction.

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