A scalable method for preparing Cu electrocatalysts that convert CO2 into C2+ products

Development of efficient catalysts for selective electroreduction of CO2 to high-value products is essential for the deployment of carbon utilization technologies. Here we present a scalable method for preparing Cu electrocatalysts that favor CO2 conversion to C2+ products with faradaic efficiencies up to 72%. Grazing-incidence X-ray diffraction data confirms that anodic halogenation of electropolished Cu foils in aqueous solutions of KCl, KBr, or KI creates surfaces of CuCl, CuBr, or CuI, respectively. Scanning electron microscopy and energy dispersive X-ray spectroscopy studies show that significant changes to the morphology of Cu occur during anodic halogenation and subsequent oxide-formation and reduction, resulting in catalysts with a high density of defect sites but relatively low roughness. This work shows that efficient conversion of CO2 to C2+ products requires a Cu catalyst with a high density of defect sites that promote adsorption of carbon intermediates and C–C coupling reactions while minimizing roughness.


Supplementary
. GI-XRD data of the Cu_KX samples after immersion in air-saturated KHCO3 and subsequent reduction in CO2-saturated 0.1 M KHCO3 by LSV from its OCP to -1.8 V at a scan rate of 5 mV/s (left) and the same data magnified (right) to reveal the small peak corresponding to Cu2O. These GI-XRD data of reduced catalysts (left) are nearly identical to that of the original electropolished Cu shown in Fig. 1a. In the data corresponding to reduced Cu_KI, the peak from Cu2O (111) appears at 2 theta = 36.6°. This result is consistent with the EDS data (see Fig. 3d and h), which show reduced Cu_KI has a higher content of O relative to Cu_KCl and Cu_KBr. Source data are provided as a Source Data file. For example, in the case of the as-prepared chlorinated Cu (Cu_KCl), in order to convert the atomic composition into the molecular composition, three species are assumed to be present: Cu, Cu2O, and CuCl.
Let x and y represent the molecular composition of Cu2O and CuCl, respectively. Since the total molecular composition should be 100%, the molecular composition of metallic Cu is then 1-x-y. From the assumed molecular composition, the relative number of atoms can be expressed: atomic Cu comes from metallic Cu (1-x-y), Cu2O (2x), and CuCl (y). When considering stoichiometry, Cu2O has two atoms of Cu (2x).

Supplementary Note 2. Characterization of residual halide with XPS and EDS
Both X-ray photoelectron spectroscopy (XPS) and EDS were used to analyze the catalysts for residual halide after anodic halogenation and subsequent reduction ( Supplementary Figures 12 and 13, respectively).
Both techniques are used to profile the surface and subsurface of the catalysts to depths up to 10 nm (XPS) or a few microns (EDS). 2,3 XPS data shown in Supplementary Figure 12 reveals the presence of cuprous halide peaks on the asprepared Cu_KX samples (at binding energy = 199 eV for Cl 2p3/2, 68.7 eV for Br 3d5/2, 619 eV for I 3d5/2).
In contrast, halide peaks are not observed on any of the Cu_KX samples when reduced by LSV from OCP to -1.8 V vs. Ag/AgCl at a scan rate of 5 mV/s. where T is the temperature in Kelvin and S is salinity in parts per thousand.
From the reaction (S1), There is an additional generation term for dissolved CO2 from a constant feed of gaseous CO2: where b is the transfer coefficient (b = k1ad), k1a is the volumetric mass-transfer coefficient (unit: s -1 ) on the liquid side of gas-liquid interface, and d is the fraction of inlet CO2 transferred from the gas phase to the liquid phase. For this work, b = 0.33 s -1 was used, which is the value used in Singh et al.    (111)] in (a) and (b) plotted as a function of incidence angle. The intensity was normalized by the intensity measured at an incidence angle of 5° to eliminate sample-to-sample variations. Because diffraction requires periodic structures, a surface with a high density of defects (i.e., low crystallinity) will exhibit GI-XRD peaks of lower intensity than those of a surface with fewer defects. Thus, the intensity of GI-XRD signal of Cu_KBr decreases relative to that of EP-Cu (highlighted in the dashed red rectangle) due to a high density of defect sites at the surface of Cu_KBr. Also included in (c) is the X-ray penetration depth in Cu plotted as a function of incidence angle. The GI-XRD measurement becomes more surface-sensitive as the incidence angle and corresponding penetration depth decrease (i.e., penetration depth is calculated to be 2025 nm or 123 nm at incidence angle of 5° or 0.5°, respectively). It should be noted that porous or rough surfaces of Cu(I)-halide-derived catalysts may alter refraction at the Cu surface, making quantitative determination of defect density with GI-XRD difficult. Source data are provided as a Source Data file.

Supplementary Note 4. Calculation of the penetration depth as a function of incidence angle
The penetration depth as a function of incidence angle was calculated as: 6 where z1/e is the depth at which the intensity is reduced to 1/e, l is the wavelength of the X-ray, ai is the incidence angle of the X-ray, ac is the critical angle of copper, β is the imaginary part of the refractive index of copper or the extinction coefficient. The refractive index can be expressed by r = 1 -δ + iβ, where δ is the deviation from the real part of the refractive index. Values for δ and β of copper at l = 1.54 Å are 2.405 × 10 -5 and 5.58 × 10 -7 , respectively. 6 The critical angle can be calculated by ac = (2δ) 0.5 , which leads to ac = 6.935 ×10 -3 radians = 0.3974° at l = 1.54 Å. The critical angle calculated at different wavelengths are given in Supplementary   catalyst oxidized for 300 s produced C2H4 with a FE of 35.8 % and C2+ products with a FE of 47.7 %, which are higher than those obtained using electropolished Cu but lower than those using Cu(I)-halide-derived catalysts. These results show the effectiveness of anodic halogenation in producing catalysts that favor C2+ products from the electrochemical CO2 RR. The capacitance of the electropolished Cu was 0.1177 mF/cm 2 . The roughness factor was determined by: