Synthesis and Catalytic Performance of Bimetallic Oxide-Derived CuO–ZnO Electrocatalysts for CO2 Reduction

Steering the selectivity of electrocatalysts toward the desired product is crucial in the electrochemical reduction of CO2. A promising approach is the electronic modification of the catalyst’s active phase. In this work, we report on the electronic modification effects on CuO–ZnO-derived electrocatalysts synthesized via hydrothermal synthesis. Although the synthesis method yields spatially separated ZnO nanorods and distinct CuO particles, strong restructuring and intimate atomic mixing occur under the reaction conditions. This leads to interactions that have a profound effect on the catalytic performance. Specifically, all of the bimetallic electrodes outperformed the monometallic ones (ZnO and CuO) in terms of activity for CO production. Surprisingly, on the other hand, the presence of ZnO suppresses the formation of ethylene on Cu, while the presence of Cu improves CO production of ZnO. In situ X-ray absorption spectroscopy studies revealed that this catalytic effect is due to enhanced reducibility of ZnO by Cu and stabilization of cationic Cu species by the intimate contact with partially reduced ZnO. This suppresses ethylene formation while favoring the production of H2 and CO on Cu. These results show that using mixed metal oxides with different reducibilities is a promising approach to alter the electronic properties of electrocatalysts (via stabilization of cationic species), thereby tuning the electrocatalytic CO2 reduction reaction performance.

. ICP results of the electrodes before and after catalytic testing.No significant change in Cu:Zn ratio is observed, whereas some metal loss has taken place.

Figure S1 .
Figure S1.Custom-made H-type electrochemical cell used for catalytic tests.From left to right: glassy carbon current collector and carbon paper containing the CuO-ZnO catalyst, cathode compartment with Ag/AgCl reference electrode, Fumasep FAA-3-PK-130 anion exchange membrane, anode compartment, and carbon paper and glassy carbon counter electrode.

Figure S2 .
Figure S2.Schematic illustration of flow cell designed for in-situ XAS and high current density measurements.

Figure S3 .
Figure S3.XRD patterns of the Cu1-xZnxO catalysts and ZnO reference pattern.Shaded gray and yellow areas indicate the regions that show the respective ZnO and CuO reflections most clearly.The higher relative intensity of the (002) peak indicates a preferential growth direction of ZnO, likely caused by a structure directing effect of the hexamethylenetetramine (HMT) molecule present in the hydrothermal synthesis solution.

Figure S4 .
Figure S4.Chronoamperometry data of (A) ZnO, (B) Cu0.43Zn0.57O,(C) Cu0.75Zn0.25Oand (D) CuO catalysts at five consecutive applied potentials.Currents are normalized by geometric surface area and metal weight.The indicates lines are an average of three independent measurements, with the shaded areas indicating the standard deviation.

Figure S5 .
Figure S5.Partial current density for (A) CH4 (B) C2H4 production of the five different catalysts tested, normalized by geometric surface area and metal weight.The indicated lines are an average of three independent measurements.

Figure S7 .
Figure S7.Geometric current density of all electrodes at -0.9 V vs RHE, normalized by geometric surface area only.The indicated lines are an average of three independent measurements.

Figure S8 .
Figure S8.Partial current density for (A) H2, (B) CO, (C) CH4 and (D) C2H4 production of the five different catalysts tested, normalized by geometric surface area only.The indicated lines are an average of three independent measurements.

Figure S9 .
Figure S9.Faradaic Efficiency of H2 (A) and C2H4 (B) over time during stability tests of the different catalysts for 15 hours.

Figure S10 .
Figure S10.XRD patterns of the Cu1-xZnxO catalysts after catalysis.No copper-or zinc-related peaks are observed.

Figure S11 .
Figure S11.SEM images of ZnO and CuO electrodes after catalytic testing.Significant restructuring into amorphous structures has taken place compared to the pristine catalysts.

Figure S14 .
Figure S14.XANES Zn K-edge spectra of different Zn-O and ZnCu-O oxides.The solid lines in the figure point out the main contribution due to the cubic structure present in the experimental spectrum corresponding to the last electrochemical treatment of the Cu0.14Zn0.86Ocatalyst at -0.6 V.

Figure S18 .
Figure S18.Potential dependence of in-situ (a) Cu K-edge and (a) Zn K-edge normalized XANES spectra of Cu0.75Zn0.25Oand reference materials in CO2 saturated 0.1 M KHCO3 under CO2RR conditions

Sample Cu:Zn atom% Metal loading on C paper (wt.%) Fresh Spent Fresh Spent
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