Structure- and Potential-Dependent Cation Effects on CO Reduction at Copper Single-Crystal Electrodes

Abstract

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.

Competition between H and CO for Active Sites Governs Copper‐Mediated Electrosynthesis of Hydrocarbon Fuels

“…We invoke methane formation via ar ate-limiting LH coupling of H ads and CO ads ,a sp ostulated computationally, [8] and rate-limiting LH coupling of two H ads species to form hydrogen. [6,8,18,26,28,37] We cannot rule out other species that may form by recombination of Ha nd CO such as COH. We propose that ethylene formation proceeds via electron-mediated LH coupling of two CO ads intermediates with the nearly saturated CO ads population explaining the insensitivity of this reaction to changes in p CO (Figure 2; Supporting Information, Figure S4).…”

Structure‐Sensitivity and Electrolyte Effects in CO2 Electroreduction: From Model Studies to Applications

“…However, the decrease of the length of the terraces (n rows <4) caused a decrease of the C 2 H 4 selectivity against the formation of CH 4 . All these results strongly support that (100) sites are responsible for stabilising adsorbed CO to form the dimer intermediate …”

“…Later, this was confirmed by Perez‐Gallent et al. who investigated the cation effect on the CORR combining FTIR and DFT calculations, concluding that the cation acted as a promoter agent stabilising the reaction intermediates.…”

“…The electrolyte composition (Figure A) plays an important role on both the activity and selectivity of the CO 2 RR, as it can: i) regulate the pH and minimise the HER against CO 2 RR, ii) affect the solubility of CO 2 , iii) stabilise the reaction intermediates …”

Facile Deposition of Cu−SnOx Hybrid Nanostructures on Lightly Boron‐Doped Diamond Electrodes for CO2 Reduction

“…In addition to surface morphology, composition, and substrate effects; solution pH and presence of electrolyte ions (Na + , SO 4 2− , CO 3 2− , HCO 3 − ) also play role in selective CO 2 reduction reaction . After purging CO 2 in 25 mM Na 2 SO 4 aqueous electrolyte, solution pH is changed from 6.8 to 5.…”