The hydrogen evolution on platinum is a milestone reaction in electrocatalysis as well as an important reaction towards sustainable energy storage. Remarkably, the pH dependent kinetics of this reaction is not yet fully understood. Here, we present a detailed kinetic study of the hydrogen adsorption and evolution reaction on Pt(111) in a wide pH range. Impedance and Tafel slope measurements show that the hydrogen adsorption and hydrogen evolution are both slow in alkaline media, which is consistent with the observation of a shift in the rate-determining step for H2 evolution.Adding nickel to the Pt(111) surface lowers the barrier for the hydrogen adsorption rate in alkaline solutions and thereby enhances the hydrogen evolution rate. These observations are explained by a new model which highlights the role of the reorganization of interfacial water to accommodate charge transfer through the electric double layer, the energetics of which is controlled by how strongly water interacts with the interfacial field. The new model is supported by laser-induced temperature-jump measurements. Our model sheds new light on the origin of the slow kinetics for the hydrogen evolution reaction in alkaline media.

The electrochemical conversion of carbon dioxide and water into useful products is a major challenge in facilitating a closed carbon cycle. Here we report a cobalt protoporphyrin immobilized on a pyrolytic graphite electrode that reduces carbon dioxide in an aqueous acidic solution at relatively low overpotential (0.5 V), with an efficiency and selectivity comparable to the best porphyrin-based electrocatalyst in the literature. While carbon monoxide is the main reduction product, we also observe methane as by-product. The results of our detailed pH-dependent studies are explained consistently by a mechanism in which carbon dioxide is activated by the cobalt protoporphyrin through the stabilization of a radical intermediate, which acts as Brønsted base. The basic character of this intermediate explains how the carbon dioxide reduction circumvents a concerted proton–electron transfer mechanism, in contrast to hydrogen evolution. Our results and their mechanistic interpretations suggest strategies for designing improved catalysts.

The oxygen evolution reaction (OER) is one of the major bottlenecks hindering the implementation of a global economy based on solar fuels. It is known that Ni-based catalysts exhibit remarkable catalytic activities for the OER in alkaline media. In this joint theoretical-experimental study, we provide a thorough characterization of Ni-based double hydroxides with Cr, Mn, Fe, Co, Cu and Zn at the atomic scale that not only explains the reasons for their high activity but also provides simple design principles for the enhancement of their electrocatalytic properties. Our approach, based on the local symmetry and composition of the active sites, helps rationalize the effect of dopants on the catalytic activity of Ni(OH) 2 . Particularly, NiFe, NiCr and NiMn double hydroxides (DHs) have superior catalytic activity, which reduce the OER potential to reach 0.5 mA cm -2 by 230 mV, 190 mV and 160 mV compared to IrO 2 nanoparticles, the state-of-theart benchmarking catalysts, with 90% Faradaic efficiency for O 2 generation. The active species in NiFe and NiMn DHs are iron and manganese, while in NiCr DH, nickel is the active species.

The electrochemical conversion of carbon dioxide (CO 2 ) into valuable compounds is a promising route toward the valorization of this molecule of high environmental impact. Yet, an industrial process involving CO 2 electroreduction is still far from reality and requires deep and fundamental studies for a further understanding and better development of the process. In this work, we describe in situ spectroelectrochemical studies based on Fourier transform infrared spectroscopy and surface-enhanced Raman spectroscopy of the CO 2 reduction in acetonitrile solutions at copper electrodes. The influence of factors such as the water content and the supporting electrolyte on the reaction products were evaluated and compared to products obtained on metal electrodes other than Cu, such as Pt, Pb, Au, Pd, and Ag. The results show that at Cu electrodes in acetonitrile containing small amounts of water, the main reaction products from CO 2 reduction are carbonate, bicarbonate, and CO. The formation of CO was observed at less-negative potentials than the formation of (bi)carbonates, and the formation of carbonate and bicarbonate species appears to be the result of a reaction with electrochemically generated OH − from water reduction. In general, our experiments show the sensitivity of the CO 2 reduction reaction to the presence of water, even at the residual level.

No abstract

Ag is a promising catalyst for the production of carbon monoxide (CO) via the electrochemical reduction of carbon dioxide (CO 2 ER). Herein, we study the role of the formate (HCOO − ) intermediate *OCHO, aiming to resolve the discrepancy between the theoretical understanding and experimental performance of Ag. We show that the first coupled proton‐electron transfer (CPET) step in the CO pathway competes with the Volmer step for formation of *H, whereas this Volmer step is a prerequisite for the formation of *OCHO. We show that *OCHO should form readily on the Ag surface owing to solvation and favorable binding strength. In situ surface‐enhanced Raman spectroscopy (SERS) experiments give preliminary evidence of the presence of O‐bound bidentate species on polycrystalline Ag during CO 2 ER which we attribute to *OCHO. Lateral adsorbate interactions in the presence of *OCHO have a significant influence on the surface coverage of *H, resulting in the inhibition of HCOO − and H 2 production and a higher selectivity towards CO.

This work discusses the kinetics of the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) on a polycrystalline platinum electrode in acetonitrile, in presence of two different electrolytes. Our findings indicate the sensitivity of the kinetics of these reactions to the presence of small amounts of water. In situ FTIR spectroscopy reveals the ion migration owing to the preferential solvation of protons by residual water, whereas surface‐enhanced Raman spectroscopy confirms that the water leaves the interface under hydrogen oxidation conditions. These observations imply that the kinetic sensitivity of this electrocatalytic reaction towards the preferential solvation processes presents a serious constraint in the establishment of a reference electrode for nonaqueous solvents based on HOR/HER on platinum, and on the comparison of its catalysis in various nonaqueous solvents.

Here we discuss the importance of water in proton solvation for the hydrogen evolution on polycrystalline gold microelectrodes. We perform cyclic voltammetry in acetonitrile electrolyte, in presence and absence of added amounts of water, with the interfacial movement of water and ions monitored by means of in situ Fourier Transform InfraRed (FTIR) spectroscopy. Our results show that trace amounts of water accumulate at the gold-acetonitrile interface. In the absence of protons (from a proton source), this water leaves the interface with more negative potential. In the presence of protons, we observe preferential solvation of protons by water, and water accumulates at the electrode surface under conditions of hydrogen evolution. Tafel plot analysis shows that the hydrogen evolution on gold in acetonitrile presents the same rate-determining step as for polycrystalline platinum, implying that the first electron transfer to yield adsorbed hydrogen is the rate-determining step of the reaction.