The successful deployment of advanced energy-conversion systems depends critically on our understanding of the fundamental interactions of the key adsorbed intermediates (hydrogen *H and hydroxyl *OH) at electrified metal-aqueous electrolyte interfaces. Herein, the effect of alkali metal cations (Li+, Na+, K+ and Cs+) on the non-Nernstian pH shift of the step-related voltammetric peak of the Pt(553) electrode is investigated over a wide pH window (1 to 13) by means of experimental and computational methods. Our results show that the co-adsorbed alkali cations along the step weaken the OH adsorption at the step sites, causing a positive shift of the potential of the step-related peak on Pt(553). Density functional theory calculations explain our observations on the identity and concentration of alkali cations on the non-Nernstian pH shift, and demonstrate that cation-hydroxyl co-adsorption causes the apparent pH dependence of “hydrogen” adsorption in the step sites of platinum electrodes.

The interaction of nitric oxide with metal surfaces has been a traditional model system for (electrochemical) surface science. Moreover, NO is an important intermediate within the currently imbalanced nitrogen cycle. Here, we study the electrochemical reduction of adsorbed NO on Pt(111) and Pt(100) electrodes by means of experimental and computational tools. Using linear sweep voltammetry, we find that the onset potentials on Pt(111) for the reduction of *NO on top and on fcc hollow sites (approximately +0.40 and +0.25 V RHE , respectively) are independent of the surface coverage. On the other hand, *NO adsorbed at a low coverage on Pt(100) is more reactive than a compact, saturated *NO adlayer is, and the reaction kinetics switches from firstto secondorder from high to low coverage. Density functional theory calculations offer an explanation for the experimental observations by suggesting that the stability of the first hydrogenation product (*NHO or *NOH) and thus the reaction mechanism strongly depends on the *NO coverage and the surface facet. Therefore, *NO reduction on platinum exemplifies a reaction in which not only the rate but also the mechanism is sensitive to structure and coverage. These observations hint at the need for a wider scope in materials design methodologies, as facet-and coverage-independent reaction pathways are typically used for materials screening.

Development of reversible and stable catalysts for the electrochemical reduction of CO2 is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO2 reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.

The effect of the alkali-metal cation (Li+, Na+, K+, and Cs+) on the non-Nernstian pH shift of the Pt(554) and Pt(533) step-associated voltammetric peak is elucidated over a wide pH window (1–13), through computation and experiment. In conjunction with our previously reported study on Pt(553), the non-Nernstian pH shift of the step-induced peak is found to be independent of the step density and the step orientation. In our prior work, we explained the sharp peak as due to the exchange between adsorbed hydrogen and hydroxyl along the step and the non-Nernstian shift as a result of the adsorption of an alkali-metal cation and its subsequent weakening of hydroxyl adsorption. Our density functional theory results support this same mechanism on Pt(533) and capture the effect of alkali-metal cation identity and alkali cation coverage well, where increasing electrolyte pH and cation concentration leads to increased cation coverage and a greater weakening effect on hydroxide adsorption. This work paints a consistent picture for the mechanism of these effects, expanding our fundamental understanding of the electrode/electrolyte interface and practical ability to control hydrogen and hydroxyl adsorption thermodynamics via the electrolyte composition, important for improving fuel cell and electrolyzer performance.

Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically considered as an ideal electrolyte for investigating electrocatalytic reactions due to the lack of specific adsorption of the perchlorate anion on several metal electrodes. In this work, cyclic voltammetry and computational methods are combined to investigate the interfacial processes on a Pd monolayer deposited on Pt (111) single crystal electrode in perchloric acid solution. The "hydrogen region" of this Pd ML Pt(111) surface exhibits two voltammetric peaks: the first "hydrogen peak" at 0.246 V RHE actually involves the replacement of hydrogen by hydroxyl, and the second "hydrogen peak" H II at 0.306 V RHE appears to be the replacement of adsorbed hydroxyl by specific perchlorate adsorption. The two peaks merge into a single peak when a more strongly adsorbed anion, such as sulfate, is involved. Our density functional theory calculations qualitatively support the peak assignment and show that anions generally bind more strongly to the Pd ML Pt(111) surface than to Pt(111). Fig. 3 Cyclic voltammogram of Pd ML Pt(111) recorded in (a) 0.1 M HClO 4 , (b) 0.1 M HF, (c) CO 2 saturated 0.1 M HClO 4 solution, and (d) 0.1 M CH 3 SO 3 H. Scan rate: 50 mV s À1 .