Role of a Hydroxide Layer on Cu Electrodes in Electrochemical CO₂ Reduction

Abstract

Cu is known as one of the most promising metallic catalysts for conversion of CO2 to hydrocarbons such as methane, ethylene, and ethanol by electrochemical reduction. The oxide-derived Cu (OD-Cu) moiety has been investigated as a candidate for enhancing the activity for CO2 electrochemical reduction to C2+ products. The reduction process is affected by catalytic grain boundary, local pH, residual oxygen atoms, and other features of the catalysts. In order to understand the detailed mechanism, we performed in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (in situ ATR-SEIRAS) measurements for CO2 reduction using several different Cu electrodes whose oxidation states are controlled. The spectroscopic investigations demonstrate that a copper oxide electrode (Cu2O) has low activity against CO2 reduction on the basis of low-level detection of CO as an intermediate of CO2 reduction. On the other hand, other Cu electrodes possessing an OH layer on the Cu surface (Cu­(OH)2/Cu) and metallic Cu exhibit higher CO2 reduction activity with significantly greater detection of produced CO. When the metallic Cu electrode is used, only one peak (2060 cm–1) assignable to CO bound to the atop site of Cu is observed. However, additional peaks are detected in the range of 1900–2100 cm–1 when the Cu­(OH)2/Cu electrode is used. To understand these findings, the adsorption energy of CO on a Cu­(OH)2/Cu electrode and the CO stretching frequency were evaluated by performing DFT calculations. The adsorption energy is enhanced and the CO stretching frequencies are shifted to lower energy in comparison with those using a metallic Cu electrode. These results indicate that it is predominantly favorable to adsorb some CO molecules near the OH moiety of the Cu­(OH)2/Cu electrode and to induce interactions of CO molecules with each other. This observation is consistent with the results of controlled potential electrolysis (CPE), which generates C2+ products as previously reported. When CPE is carried out in D2O solution, residual and/or adsorbed OD– groups on Cu are detected by ATR-SEIRAS and the surface of the Cu­(OH)2/Cu electrode is confirmed to be metallic Cu, as measured by in situ Raman and XPS. From the ATR-SEIRAS experiments when switching from under CO2 to Ar during the electrochemical reduction, the OH layer is suggested to prevent deactivation of the Cu electrode via formation of the CO layer, which is detected as a bridge-bounded form on the metallic Cu electrode. The above findings indicate that the OH layer provides the advantage of attracting CO molecules closer to each other while reducing them to C2+ products without any degradation during electrolysis.

Understanding the electric and nonelectric field components of the cation effect on the electrochemical CO reduction reaction

“…S7). Spectra obtained at a 2 cm −1 resolution show poorer signal to noise ratio, and thus, the 4 cm −1 spectral resolution is used in all experiments, which is consistent with recent studies ( 21 , 27 , 49 , 53 ). According to the GCS double-layer model, the Stark tuning rate reflects the strength of the electric field in the double layer, the change of which causes the shift in vibrational bands ( 54 ).…”

“…This discrepancy could be attributed to the presence of a different counter ion (hydroxide versus bicarbonate) in the electrolyte. Hydroxide-derived oxygen-containing species are known to be present on the copper surface at the CORR conditions ( 30 , 49 ). Adsorbed hydroxide, along with cations at the outer Helmholtz (OHP), has previously been proposed to affect the water dissociation process needed to release protons for the HER ( 50 ).…”

Effect of Electrolyte Composition and Concentration on Pulsed Potential Electrochemical CO2 Reduction

“…We provide additional evidence of long‐term electrode surface stability under pulsed operation using cyclic voltammetry measurements (see Supporting Information Section 4). We attribute the improved stability of p‐eCO 2 R to a revivification of the active sites during the intermittent anodic pulse, which desorbs impurities from the electrode surface [23,31, 38] and prevents CO bridge formation [25] ; both of these are known to deactivate the catalyst in the constant potential case [25,37] . As noted in our previous reports, we rule out mass transport effects (with rotating disk electrode experiments and a computational model), [22] surface oxidation (with in‐situ X‐ray absorption spectroscopy), [25] and surface roughening (with double layer capacitance measurements) [22] as major contributors in the mechanism governing the product distribution in p‐eCO 2 R.…”

“…Although copper‐based electrodes are known to be good eCO 2 R catalysts, [2–3,32] the long‐term stability of these electrodes in aqueous electrolytes presents a persistent challenge with regards to extended operation [10,33–36] . Various explanations have been reported to describe the mechanism and causality of catalyst deactivation including metal impurities in the electrolyte being reduced onto the catalyst surface and eCO 2 R intermediates ( e. g ., CO bridge ) irreversibly binding to the surface [37–38] . Recent reports of adding dopants to copper have helped slow the degrading performance and extend the longevity of the electrode [39–40] .…”

Breaking the Linear Scaling Relationship by Compositional and Structural Crafting of Ternary Cu–Au/Ag Nanoframes for Electrocatalytic Ethylene Production

“…[4] Thus,o ne of the central tasks in catalyst design for CO 2 RR is to break the linear scaling relationship,w hich is both experimentally and theoretically challenging and can only be done by incorporating alternative energy forms or by increasing the complexity of the catalyst. [5] Fort hat many tactics have been explored in constructing copper-based catalysts,i ncluding multimetallic alloying, [6] multivariant compositing, [7] crystalline faceting, [8] nano/meso-structuring, [9] strain engineering, [10] subsurface doping, [11] adsorbate solvating, [12] precatalytic oxidizing, [13] as well as surface reconstruction/tethering. [14] Thec ommon goal is to create am aterial where the atoms are arranged to precisely manipulate each gating mechanistic step of the reaction towards ultimate unity of reduction products.…”