Mechanistic Implications of Low CO Coverage on Cu in the Electrochemical CO and CO₂ Reduction Reactions

Abstract

Electrochemical CO or CO2 reduction reactions (CO(2)RR), powered by renewable energy, represent one of the promising strategies for upgrading CO2 to valuable products. To design efficient and selective catalysts for the CO(2)RR, a comprehensive mechanistic understanding is necessary, including a comprehensive understanding of the reaction network and the identity of kinetically relevant steps. Surface-adsorbed CO (COad) is the most commonly reported reaction intermediate in the CO(2)RR, and its surface coverage (θCO) and binding energy are proposed to be key to the catalytic performance. Recent experimental evidence sugguests that θCO on Cu electrode at electrochemical conditions is quite low (∼0.05 monolayer), while relatively high θCO is often assumed in literature mechanistic discussion. This Perspective briefly summarizes existing efforts in determining θCO on Cu surfaces, analyzes mechanistic impacts of low θCO on the reaction pathway and catalytic performance, and discusses potential fruitful future directions in advancing our understanding of the Cu-catalyzed CO(2)RR.

Toward Durable CO2 Electroreduction with Cu‐Based Catalysts via Understanding Their Deactivation Modes

“…[ 62b ] The authors found that F‐Cu demonstrated far better C 2+ selectivity in the series, and ascribed it to the specifically adsorbed fluoride facilitating the rate‐determining protonation of *CO. [ 62b ] The authors further performed the same electrolysis under argon, and found that F‐Cu showed the highest activity toward hydrogen evolution reaction (HER), consistent with the expectation that adsorbed fluoride (the most partially negatively charged in the series) best facilitates the proton transfer from water (termed water‐activation in the reference). [ 62b ] The assignment of RDS in this case is the same as the one proposed by Xu and co‐workers from their electrokinetic study, [ 63 ] which is different from the commonly recognized C ─ C coupling.…”

“…[ 73 ] Recent electrokinetic studies by Xu and co‐workers revealed that the surface coverage of *CO is surprisingly low (<0.05 monolayer), and that the reaction order with respect to *CO is not 2, a value that is expected from the postulate that C ─ C coupling is the RDS. [ 63,186 ] The reported self‐poisoning of CO 2 ER on Cu is the deposition of carbon, [ 187 ] which is formed in the methane pathway, Figure 2. [ 23b,187a,b ] Increasing the surface roughness by creating Cu lattice defects is one approach to mitigate the aforementioned carbon deposition, which steer the CO 2 ER toward the C ─ C coupling pathway instead of the methane pathway.…”

Multiscale effects in tandem CO2 electrolysis to C2+ products

“…However, studies have demonstrated that *CO coverage is typically very low on electrochemically active surfaces and it is expected that high monolayer coverages would generally weaken surface binding due to lateral interactions between adjacent *CO species. 22,23 Therefore coverage modifications are likely coupled with additional contributions to tandem catalysis including the local concentration of CO. The presence of an internal CO reservoir has been identified as a contributor to enhanced C 2+ selectivity, which has been experimentally observed on Cu catalysts.…”

Recent development and applications of differential electrochemical mass spectrometry in emerging energy conversion and storage solutions

“…2 H, 13 C, 15 N and 18 O are typical isotopic elements used in studies including water electrolysis, LiBs, CO 2 RR, nitrogen catalysis and photoelectrochemical processes. 98,134,136,159–165,229 The unique labelling method and detailed applications have been discussed in previous sections.…”