Five thiols—different in the length of the side chain and the hydrogen-binding abilities of the non-thiol terminal group—were selected for a systematic evaluation of their effect on CO2 reduction. Regarding the preparation of self-assembled monolayers (SAMs) of the thiols on copper surfaces, a pre-adsorption etching step with glacial acetic acid (i.e. to remove surface oxides) greatly enhanced the quality and reproducibility of the obtained SAMs. The functionalized surfaces were thoroughly characterized prior to the reaction and following electrolysis at different potentials by x-ray photoelectron spectroscopy (XPS). The measurements indicated that the stability of the adsorbed thiol SAMs depended on the electrolysis potential and on the length of the thiol side chain. Nevertheless, the adsorbed thiols were found to typically suppress the total current density without influencing much the product selectivity or, in the case of –OH and –NH2-terminated ligands, to promote the HER, particularly in the case of the latter. Despite this interesting result, attempts to exploit this effect under conditions with decreased proton availability (e.g. CO reduction in 0.1 M KOH) did not result in an enhancement of multicarbon product formation.
Initial work in a zero-gap membrane electrode assembly (MEA)-type electrolyzer revealed that the neutralization of CO2 by OH− ions generated in situ by the eCO2RR leads to CO2 crossover as carbonate through the anion-exchange membrane (AEM) and to a non-stoichiometric decrease of the outlet flow from the reactor. If not accounted for, this effect can lead to large overestimations of the Faradaic efficiencies of the gas eCO2RR products and to an incorrect assessment of catalytic performance at elevated current densities. This problem was found to affect a large fraction of published studies, in which figures of merit—particularly for CO2 reduction to ethylene production under alkaline conditions—can be vastly overstated. Therefore, significant work was aimed at correcting these experimental shortcomings and developing robust protocols for the accurate quantification of catalytic performance at industrially relevant current densities. Based on these findings, awareness of this issue among the eCO2RR community is rapidly growing, leading to the adoption of methodological improvements and better data collection practices.
The zero-gap configuration of the electrolyzer (i.e. in which the catalyst layers are directly in contact with the membrane, with no liquid electrolyte in between) enabled the reduction of ohmic losses but complicated the recovery of liquid products. In light of this, ethylene was selected as the target product for the second part of the project. To this end, we evaluated nanostructured copper catalysts with abundant crystallographic facets over which CO2 reduction to ethylene is favored. Compared to benchmark catalysts, this strategy was successful in achieving higher production rates of ethylene, although selectivity over the parasitic hydrogen evolution reaction (HER) still needs to be improved. These results point toward the need for a better understanding of the factors that promote the HER at high current densities, such as water transport across AEMs, electrode flooding and the occurrence of mass transfer limitations. Nevertheless, this work points toward a viable strategy for enhancing the ethylene productivity of the eCO2RR at industrially relevant reaction rates.
