Screening Surface Structure–Electrochemical Activity Relationships of Copper Electrodes under CO2 Electroreduction Conditions

Understanding how crystallographic orientation influences the electrocatalytic performance of metal catalysts can potentially advance the design of catalysts with improved efficiency. Although single crystal electrodes are typically used for such studies, the one-at-a-time preparation procedure limits the range of secondary crystallographic orientations that can be profiled. This work employs scanning electrochemical cell microscopy (SECCM) together with co-located electron backscatter diffraction (EBSD) as a screening technique to investigate how surface crystallographic orientations on polycrystalline copper (Cu) correlate to activity under CO2 electroreduction conditions. SECCM measures spatially resolved voltammetry on polycrystalline copper covering low overpotentials of CO2 conversion to intermediates, thereby screening the different activity from low-index facets where H2 evolution is dominant to high-index facets where more reaction intermediates are expected. This approach allows the acquisition of 2500 voltammograms on approximately 60 different Cu surface facets identified with EBSD. The results show that the order of activity is (111) < (100) < (110) among the Cu primary orientations. The collection of data over a wide range of secondary orientations leads to the construction of an “electrochemical–crystallographic stereographic triangle” that provides a broad comprehension of the trends among Cu secondary surface facets rarely studied in the literature, [particularly (941) and (741)], and clearly shows that the electroreduction activity scales with the step and kink density of these surfaces. This work also reveals that the electrochemical stripping of the passive layer that is naturally formed on Cu in air is strongly grain-dependent, and the relative ease of stripping on low-index facets follows the order of (100) > (111) > (110). This allows a procedure to be implemented, whereby the oxide is removed (to an electrochemically undetectable level) prior to the kinetic analyses of electroreduction activity. SECCM screening allows for the most active surfaces to be ranked and prompts in-depth follow-up studies.


S2. Electrochemical Movie Captions
Movie S1: Electrochemical (potentiodynamic) movie of voltammetric SECCM measurements on a polycrystalline Cu substrate under CO2 condition for the results discussed in Figure 2 of the main text. The pulse-LSV protocol (as discussed in the Methods section of the main text) was used for the scan acquisition. The movie shows a series of electrochemical frames that correspond to current maps of the Cu substrate as the potential, Esurf, is swept from -0.45 V to -1.05 V vs Ag/AgCl.
Movie S2: Electrochemical (potentiodynamic) movie of voltammetric SECCM measurements on a polycrystalline Cu substrate under Ar for results presented in SI, Section S6. The pulse-LSV protocol (as discussed in the Methods section of the main text) was used for the scan acquisition.
The movie shows a series of electrochemical frames that correspond to current maps of the Cu substrate as the potential, Esurf, is swept from -0.45 V to -1.05 V vs Ag/AgCl. Scan area is 100 µm by 100 µm and consists of 2,500 pixels (2 µm pitch).
Movie S3: Electrochemical (potentiodynamic) movie of voltammetric SECCM measurements on a polycrystalline Cu substrate without pulse treatment (as discussed in Figure 5 of the main text).
LSVs correspond to the electrochemical stripping of native passive Cu(OH)2 layer on Cu. The movie shows a series of electrochemical frames that correspond to current maps of the Cu substrate as the potential, Esurf, is swept from -0.45 V to -1.05 V vs Ag/AgCl. Scan area is 100 µm by 100 µm and consists of 2,500 pixels (2 µm pitch).

S4. hkl detail of grains in the SECCM scan area
Table S1-A: Details of crystal grains in SECCM eCO2RR scan presented in Figure 5, main text. The grains selected as representations of low index facets are prsensented first in ID numbers 1-3, and highlighted, in color red, blue, and green for (100), (111), and (110) respectively.

S5. Estimation of density of broken bonds
The broken bond density on the metal surface is closely affiliated with the surface energy of single crystals and the stepped/kinked nature of the surface. 1,2 The broken bond density per atom (dbb) depends on the crystal orientation, and is geometrically determined for fcc metals as: where h, k, l denotes Miller indices. S11

S6. Results of SECCM scan under Argon
Results of the SECCM scan under Ar are presented Figure S2. The trend of lower current density on the secondary facets within the triangle is clear and different from the observation under CO2 discussed in Figure 3. However, it should be noted that the grains closest to the (100) and (111) poles in the HER scan in Figure S2 are outside the 10 o acceptable range of low-index approximation and possess hkl parameters of stepped surfaces (see highlighted line 1-3 of Table   S1-B), and therefore they cannot be directly compared grain-to-grain to the low-index grains discussed in Figure 2 of the main text. A trend of (110) > (111) > (100) is expected for HER on the primary orientations, but the closest grains to (111) and (110) in Figure S2C are oriented 11.3 and 13.7 degrees away and may explain why the current densities observed on these grains are lower than measurement on the grain that is 6.5 degrees away from (100). Therefore, we have not interpreted them as a trend of HER among the primary orientations. Rather, we have only compared the trend across the entire triangle to deductions of the eCO2RR scan.

S7. XPS results
We investigated the surface composition of as-prepared polycrystalline Cu through ex-situ XPS analysis. XPS measurements were made before and after macroscale electrochemical reduction treatment. Peak fitting of the O1s and Cu 2p1/3 spectra are presented in SI, Section S7, Figures S3 and S4, respectively. Details of the peaks for Cu oxide materials are in SI (Section S7, Tables S2, and S3). In essence, the results confirmed the presence of Cu2O (4.57%), Cu(OH)2 (12.10%), and CuO (49.6%) on the as-prepared Cu. After the electrochemical treatment at -1 V vs Ag/AgCl for 1 hour, the values decreased to 0.36%, 0.80%, and 10.89%, in the same order. The removal of surface oxides by electrochemical treatment was also evident in the Cu 2p spectra (SI, Figure S4 of the SI). However, being ex-situ, the XPS signature associated with the oxide features (especially the Cu2O) was not entirely eliminated due to some reoxidation during sample transfer from electrochemical cell to XPS, for which Cu2O formation is most rapid. 3 S14 Figure S3: Peak fitting of O1s XPS spectra for polycrystalline Cu surface (A) before, and (B) after treatment at -1.0V vs. Ag/AgCl for 1 hour in a three-electrode macroscale setup. (See SI, Section S1).