Electrochemical Reduction of CO2 Using Bi-Layer Cu2O Electrodes

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 V_SCE. 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.