Electrochemical Reduction of CO2 Using Bi-Layer Cu2O Electrodes
Authors: Bugayong Joel , Griffin Gregory L 
Published: February 2020

Unmodified Cu foil electrodes are prepared by etching briefly in 1.0M HCl, followed by ultrasonic cleaning in sequential baths of 2-propanol, acetone and de-ionized H2O. The single-layer Cu2O electrode is prepared by electrodepositing a Cu2O layer onto Cu foil using a mixed CuSO4/ lactic acid bath, adjusted to pH = 9 using NaOH and held at 65̊C for a deposition time of 30 minutes. The bi-layer electrode is prepared by first thermally oxidizing a Cu foil in air at 400̊C for 4 hours, followed by a subsequent 10 minute electrodeposition step.

The CO2 reduction measurements are performed using a two compartment cell with a Nafion membrane separator, a Pt wire counterelectrode, and Ag/AgCl reference electrode. The working electrolyte is aqueous 0.5M KHCO3 that is bubbled continuously with CO2. Product concentrations are determined using gas chromatography. Gas phase products (H2, CO, CH4, and C2H4) are sampled directly from the CO2 purge gas leaving the reactor. Liquid products (mainly CH2H5OH) are measured by taking syringe samples of the electrolyte at 15 minute intervals.

X-ray diffraction measurements confirm that the layers grown either thermally or by electrodeposition consist mainly of Cu2O. After being exposed to CO2 electroreduction conditions, the XRD peaks for Cu2O on the single-layer electrode appear to be completely removed, suggesting the electrode may have become fully reduced to Cu metal. In contrast, the bi-layer electrode still exhibits XRD peaks for Cu2O and CuO following CO2electroreduction, suggesting the inclusion of the thermal oxide layer has increased the electrochemical stability of the bi-layer electrode.

SEM images of the initially deposited single-layer electrode show cubic morphology characteristic of Cu2O crystallites with a relatively large particle dimension (ca. 1μm). Following CO2 electroreduction, the primary morphology remains intact, although many of original crystallites have broken apart, and the particle surfaces show a modest degree of roughening. For the bi-layer electrode, the initial thermal oxide layer shows a more random crystallite morphology with a some what smaller length scale (500 nm). The subsequent electrodeposited Cu2O overlayer reflects the same smaller length scale but with a more sharply define cubic habit. Following CO2electroreduction, the surface of the bilayer sample undergoes restructuring in a manner similar to the single-layer electrode, but with a more sharply defined morphology (i.e., well-resolved cubes with length scale about 100 nm).

The CO2 electroreduction measurements are performed at −1.745 VSCE. As expected, the control experiment using an unmodified Cu foil produces CO and CH4 as the greatest CO2 reduction products (formation rates of 8.3 μmole CO /cm2/hr and 2.8 μmole CH4/cm2/hr, respectively). In addition, the C2H4 formation rate was much lower (0.3 μmole C2H4/cm2/hr), such that the C2H4/CH4 ratio was only 0.1 . By comparison, the single-layer electrode produces CO and C2H4 as the largest CO2 reduction products (formation rates of 4.3 μmole CO/cm2/hr and 2.5 μmole C2H4/cm2/hr, respectively). The CH4 formation rate is reduced to 0.06 μmole CH4/cm2/hr, so that the C2H4/CH4 ratio is reversed to a value of 42. This effect becomes even more pronounced for the bilayer electrode. The CO and C2H4 formation rates increase to 15.6 μmole CO/cm2/hr and 9.1 μmole C2H4/cm2/hr, while the CH4 formation rate decreases slightly, to 0.04 μmole CH4/cm2/hr. The increased CO and C2H4 formation rates may be partially the result of higher electrode surface area, as suggested by a higher current density observed with the bilayer electrode. However, the C2H4/CH4ratio has clearly increased to over 200, indicating that the bilayer electrode provides a higher selectivity for carbon-carbon coupling than for single carbon species hydrogenation.

Our present results support a model in which the Cu2O particles are converted to metallic Cu during CO2 reduction. The morphological evolution of the starting Cu2O nanoparticles during this reduction leads to the formation of more highly dispersed Cu clusters on the surface of the converted particles. The dispersed Cu clusters are expected to contain a higher concentration of more open crystal faces and lower co-ordination surface atoms, which leads to the observation of a higher C2H4/CH4ratio, relative to low-index planar Cu surfaces. Additional work is underway to further characterize these dispersed Cu clusters, and to explore methods to improve their stability during prolonged electrochemical operation.

This material is based upon work supported as part of the Center for Atomic Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001058.