Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte‐Driven Nanostructuring

Abstract Production of multicarbon products (C2+) from CO2 electroreduction reaction (CO2RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO2RR catalysts that are highly selective for C2+ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO2 into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO3 solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm−2 for C2+ products at −0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C2+ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu+ species, and the adsorbed halides.

regions, and high-energy resolution fluorescence detected (HERFD) XANES. To ensure maximum sensitivity to the sample surface with the XAS technique, the angle of incidence was decreased to 10°, the lowest value allowed by the current cell geometry. As a result, the contribution of deeper sample layers unmodified during nanostructuring could be minimized, as it is illustrated in the HERFD-XANES spectra of the as-prepared Cu_I sample in Figure S5.
The Cu K-edge (8979 eV) TFY-XAS (XANES/EXAFS) spectra were recorded at beamline P65 of PETRA III synchrotron light facility (DESY) in Hamburg (Germany) using a passivated implanted planar silicon (PIPS) detector. A Si(111) double crystal monochromator detuned to 65% of intensity was used for the energy scan. The HERFD-XANES measurements were performed at the Rossendorf beamline BM20 (ROBL) of the European Synchrotron Radiation facility (ESRF), Grenoble (France). The photon energy was scanned by a double-crystal Si(111) monochromator and higher harmonics were rejected by a Ptcoated collimating mirror. Cu K-edge HERFD-XANES spectra were recorded at Cu Kα1 emission line using a spherically bent Ge crystal analyzer. The IFEFFIT software package [1] was used to process and analyze the XAS data. Athena was used for data reduction, background subtraction, self-absorption correction (for TFY spectra), and linear combination analysis (LCA). For the latter, a set of reference spectra was used that was measured in the same configuration of the beamline and crystal analyzer. For each experimental spectrum, a weighting parameter (X), representing a fractional content of the corresponding species and the energy shift (∆E) of the reference spectra were fitted, the results are summarized in Table S5 and examples of LCA are shown in Figure 2D and Figures S5-S7. Various combinations of basis sets were tried, and those resulting in the lowest R-factor were chosen. As an additional figure of merit, the combinations requiring too high-energy shift of one or more components (by more than 5 eV) were not accepted. Extended X-ray absorption fine-structure spectra (EXAFS) were fitted in Artemis using theoretical backscattering amplitudes and phases calculated by the FEFF6 code [2] for face-centered cubic Cu metal and cubic CuI structures [3] . Coordination numbers (CN), interatomic distances (r), Debye-Waller factors (σ 2 ), and energy shift (ΔE0) were the fitting parameters.

Quasi in situ XPS characterization
The quasi in situ XPS measurements were carried out in an ultrahigh-vacuum (UHV) setup equipped with a non-monochromatic Al X-ray source (hν = 1486.6 eV) and a hemispherical electron analyzer (Phoibos100, SPECS GmbH). The Cu 2p3/2 peak corresponding to Cu2O (932.67 eV) was used for energy alignment. The XPS analysis chamber was connected to an in situ electrochemical (EC) cell (SPECS GmbH). An in situ electrochemical (EC) cell (SPECS GmbH) was connected to the XPS analysis chamber.
The potential was controlled with an Autolab potentiostat (PGSTAT 302N). The sample transfer from the EC cell to the XPS UHV chamber was performed under vacuum. For the deconvolution of the Cu LMM Auger spectra, data acquired in our laboratory from a metallic Cu 0 foil (reduced in situ by H2 plasma), commercial CuCl (Alfa Aesar, 99.999%), CuBr (Sigma-Aldrich, 99.995%), CuI (Sigma-Aldrich, 99.995%) powders, and CuO and Cu2O foils from the literature were used as references. The Cu Auger spectra are more sensitive to the presence of Cu + species than the Cu 2p XPS region. In particular, they can help us to distinguish Cu2O from metallic Cu. The analysis of the O 1s spectra is much more challenging because they are dominated by the contribution of adsorbed species not associated with Cu + .

Electrochemical measurements
Electrochemical measurements were carried out in a gas-tight H-cell separated by an anion exchange membrane (Selemion AMV, AGC Inc.) Both, working and counter compartments were filled with 40 ml 0.1 M KHCO3 (Sigma-Aldrich, 99.7%) and purged continuously with CO2 (99.995%, 20 ml min −1 ). 0.1 M KHCO3 solution was prepared by ultrapure water and further pre-purified by Chelex 100 Resin (Bio-Rad).
Prior to the measurement, the electrolyte is bubbled with CO2 for 30 min to remove oxygen in the solution and saturate the solution (pH 6.8). A platinum gauze (MaTecK, 3600 mesh cm −2 ) was used as counter electrode and a leak-free Ag/AgCl electrode (Innovative Instruments) as the reference electrode. The      Figure 1 and Figure S1 in the as-prepared state, after sample immersion in the different electrolytes for 30 min before applying any potential and after The EXAFS data of the as-prepared Cu_Br and Cu_Cl samples show close resemblance to metallic Cu, with an intense backscattering event at 2.25 Å (phase shift uncorrected, Figure S3-A). Their intensity, however, is lower than that of the bulk Cu reference spectrum, indicating a defective structure rich in undercoordinated Cu sites and/or larger structural disorder. EXAFS fitting revealed Cu-Cu coordination numbers (CNs) of 8.3 and 7.5 for Cu_Br and Cu_Cl, correspondingly (Table S4), while CN = 12 is expected for a face-centered Cu structure. The spectrum of the as-prepared Cu_I is significantly less intense, broader and shifted to 2.3 Å (uncorrected). Typically, various neighbors contributing to the firstshell peak can explain such a behavior, and indeed, the spectrum cannot be fitted with a Cu-Cu contribution alone, but requires an addition of Cu-I ( Figure S4-A). After 1 h of CO2RR, all three samples must still have defective surfaces as indicated by the low magnitude of the EXAFS spectra ( Figure S3-B), and the corresponding Cu-Cu coordination numbers obtained (10.5, 8.8, 8.5 for Cu_Br, Cu_Cl and Cu_I respectively). Figure S8. HERFD-XANES spectra of the as prepared Cu_I sample collected at different incidence angles. Bulk Cu and CuI spectra are plotted as reference.            [4] .