The complexity of the electrocatalytic reduction of CO to CH4 and C2H4 on copper electrodes prevents a straightforward elucidation of the reaction mechanism and the design of new and better catalysts. Although structural and electrolyte effects have been separately studied, there are no reports on structure-sensitive cation effects on the catalyst’s selectivity over a wide potential range. Therefore, we investigated CO reduction on Cu(100), Cu(111), and Cu(polycrystalline) electrodes in 0.1 M alkaline hydroxide electrolytes (LiOH, NaOH, KOH, RbOH, CsOH) between 0 and −1.5 V vs RHE. We used online electrochemical mass spectrometry and high-performance liquid chromatography to determine the product distribution as a function of electrode structure, cation size, and applied potential. First, cation effects are potential dependent, as larger cations increase the selectivity of all electrodes toward ethylene at E > −0.45 V vs RHE, but methane is favored at more negative potentials. Second, cation effects are structure-sensitive, as the onset potential for C2H4 formation depends on the electrode structure and cation size, whereas that for CH4 does not. Fourier Transform infrared spectroscopy (FTIR) and density functional theory help to understand how cations favor ethylene over methane at low overpotentials on Cu(100). The rate-determining step to methane and ethylene formation is CO hydrogenation, which is considerably easier in the presence of alkaline cations for a CO dimer compared to a CO monomer. For Li+ and Na+, the stabilization is such that hydrogenated dimers are observable with FTIR at low overpotentials. Thus, potential-dependent, structure-sensitive cation effects help steer the selectivity toward specific products.

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.

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Electrochemical CO2 reduction is an attractive option for storing renewable electricity and for the sustainable production of valuable chemicals and fuels. In this roadmap, we review recent progress in fundamental understanding, catalyst development, and in engineering and scale-up. We discuss the outstanding challenges towards commercialization of electrochemical CO2 reduction technology: energy efficiencies, selectivities, low current densities, and stability. We highlight the opportunities in establishing rigorous standards for benchmarking performance, advances in in operando characterization, the discovery of new materials towards high value products, the investigation of phenomena across multiple-length scales and the application of data science towards doing so. We hope that this collective perspective sparks new research activities that ultimately bring us a step closer towards establishing a low- or zero-emission carbon cycle.

The first part of this report studies the electrochemical properties of single-crystal platinum electrodes in acetonitrile electrolytes by means of cyclic voltammetry. Potential difference infrared spectroscopy in conjunction with linear voltammetry was used to obtain a molecular-level picture of this interface. The second part of this report studies the hydrogen evolution and the hydrogen oxidation reactions on the three low-index faces of Pt electrodes in acetonitrile electrolytes. The data (CVs and IR spectra) strongly suggest that acetonitrile and CN(-) molecules are adsorbed on single-crystal platinum electrodes in the range of -1.5 to 0.3 V versus Ag/AgCl. Those species block part of the adsorption sites for hydrogen adatoms, and they decompose on the surface in the presence of water. The nature of the cation and the presence of water strongly affect the onset of acetonitrile electrolysis and the kinetics and stability of the adsorbed species on the electrode. Finally, the hydrogen evolution and the hydrogen oxidation reactions on platinum single-crystal surfaces in acetonitrile electrolytes are strongly affected by the surface-energy state of Pt electrodes.