The in situ study of surface species and structures of oxide-derived copper catalysts for electrochemical CO2 reduction

Oxide-derived copper (OD-Cu) has been discovered to be an effective catalyst for the electroreduction of CO2 to C2+ products. The structure of OD-Cu and its surface species during the reaction process are interesting topics, which have not yet been clearly discussed. Herein, in situ surface-enhanced Raman spectroscopy (SERS), operando X-ray absorption spectroscopy (XAS), and 18O isotope labeling experiments were employed to investigate the surface species and structures of OD-Cu catalysts during CO2 electroreduction. It was found that the OD-Cu catalysts were reduced to metallic Cu(0) in the reaction. CuOx species existed on the catalyst surfaces during the CO2RR, which resulted from the adsorption of preliminary intermediates (such as *CO2 and *OCO−) on Cu instead of on the active sites of the catalyst. It was also found that abundant interfaces can be produced on OD-Cu, which can provide heterogeneous CO adsorption sites (strong binding sites and weak binding sites), leading to outstanding performance for obtaining C2+ products. The Faradaic efficiency (FE) for C2+ products reached as high as 83.8% with a current density of 341.5 mA cm−2 at −0.9 V vs. RHE.


Synthetic procedures for Cu nanorods (Cu-nr):
The Cu-nr was prepared by electroreduction of the CuO nanorods (CuO-nr) for 10 min at -0.9 V vs. RHE. CuO-nr was fabricated through the annealing of Cu(OH) 2 nanorods under N 2 atmosphere. Firstly, the Cu(OH) 2 nanorods were prepared by a literature method. [S1] Typically, 1 g of Cu(NO 3 ) 2 was dissolved in 100 mL distilled water. Then, 30 mL NH 3 ·H 2 O (0.15 M) solution was added to the Cu(NO 3 ) 2 solution under constant stirring at room temperature. A blue precipitate of Cu(OH) 2 was produced when 10 mL of NaOH (1 M) solution (≈2 mL min -1 ) was dropwise added to the above solution to adjust the pH value to 9-10. After 30 min, the blue Cu(OH) 2 precipitate was centrifuged and washed several times with H 2 O to obtain a solid product, which was dried by freeze-drying for 24 h. Secondly, the CuO-nr was prepared by annealing the Cu(OH) 2 nanorods in the N 2 atmosphere at 500 °C for 2 h with a heating rate of 20 °C min -1 . Finally, the Cu-nr was prepared by electroreduction of the CuO nanorods (CuO-nr) for 10 min at -0.9 V vs. RHE, and the electrolyte used was 1M KOH solution.
Synthetic procedures for Cu-nr-OR：The Cu-nr-OR was prepared by electroreduction of the Cu-nr-O for 10 min at -0.9 V vs. RHE. Firstly, the Cu-nr-O was prepared by electrochemical cycling of Cu-nr in 1.0 M KOH solutions. The experiment was performed in multi-potential steps mode. The potential and time for the step 1 was 1.0 V vs. RHE and 2s; the potential and time for the step 2 was 0.4 V vs. RHE and 1s; Then the Cu-nr-O was obtained after 20 cycles. Secondly, the Cu-nr-OR was prepared by electroreduction of the Cu-nr-O for 10 min at -0.9 V vs. RHE, and the electrolytewas 1M KOH solution.
Characterization of the materials. The SEM and TEM characterizations were carried out using a HITACHI S-4800 and JEOL JEM-2100F, equipped with EDS. The operando X-ray adsorption spectroscopy (XAS) measurements were performed using a modified flow cell at the 1W1B, 1W2B beamline at Beijing Synchrotron Radiation Facility (BSRF). In situ Raman measurements were carried out using a Horiba LabRAM HR Evolution Raman microscope in a modified flow cell, which was produced by GaossUnion (Tianjin) Photoelectric Technology Company ( Figure S10). A 785-nm laser was used and signals were recorded using a 20 s integration and by averaging two scans.

Preparation of electrodes.
To construct the cathode electrode, a catalyst slurry that contained 5 mg of obtained catalysts, 1 mL of methanol and 20 µL of Nafion ionomer solution (5 wt% in H 2 O) was first mixed and sonicated for 30 min. Then, the catalyst slurry (0.2 mL) was slowly drop cast onto a PTFE membrane (Fuel Cell Store) under vacuum to achieve a catalyst loading of ~1.0 mg cm -2 . Ni foam was used as anode electrode.
Electrochemical study. Electrochemical studies were conducted in an electrochemical flow cell which including a gas chamber, a cathodic chamber, and an anodic chamber, as reported in our previous work. [S2] An anion exchange membrane (FumasepFAA-3-PK-130) was used to separate the anodic and cathodic chambers, and an Ag/AgCl electrode and Ni foam were used as the reference and counter electrodes, respectively. The electrolysis was conducted using a CHI 660e electrochemical workstation equipped with a high current amplifier CHI 680c.
The measured potentials after iR compensation were rescaled to the RHE by E (versus RHE) = E (versus Ag/AgCl) + 0.209 V+0.0591V/pH×pH. For performance studies, 1 M KOH was used as the electrolyte, and it was circulated through the cathodic and anodic chambers using peristaltic pumps at a rate of 20 mL min -1 . The flow rate of CO 2 gas through the gas chamber was controlled to be 20 sccm using a digital gas flow controller.

EIS study. The EIS measurement was carried out in 1 M KOH solution at an open circuit potential (OCP) with an
amplitude of 5 mV of 10 -2 to 10 6 Hz.
Double-layer capacitance (C dl ) measurements. The electrochemical active surface area is proportional to C dl value. C dl was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammogram (CV). The CV ranged from 0 V to -0.1 V vs. RHE. The C dl was estimated by plotting the Δj (j a -j c ) at -0.05 V vs. RHE against the scan rates, in which the j a and j c are the anodic and cathodic current density, respectively. The scan rates were 20, 30, 50, 80, 100 and 120 mV s -1 .
Pb underpotential deposition: Cu ECSA in catalysts was determined using Pb underpotential deposition. An N 2saturated solution of 100 mM HClO 4 + 1 mM Pb(ClO 4 ) 2 was used as the electrolyte. The cathode was held at -0.081 VRHE for 60 s and then cyclic voltammetry was recorded between -0.5 and 0.2 VRHE at 5 mV s -1 . Pt foil was used as the anode. The electrolyte was not circulated during the cyclic voltammetry measurement. The Cu ECSA in the catalyst is calculated from the charge associated with 2eoxidation of monolayer of Pb adatoms coverage over Cu surface.
Product analysis. The gaseous product of electrochemical experiments was collected using a gas bag and analyzed by gas chromatography (GC, HP 4890D), which was equipped with TCD detectors using argon as the carrier gas. The liquid product was analyzed by 1 H NMR (Bruker Avance III 400 HD spectrometer) in deuteroxide.

Calculations of Faradaic efficiencies of gaseous and liquid products. liquid products:
After electrolysis, a certain amount of internal standard solution was added to the electrolyte as the internal standard. Because the concentration of internal standard was known, the moles of liquid products can be calculated from integral areas and calibration curves. To accurately integerate the products in NMR analysis, two standards located in different regions were used in NMR analysis. The sodium 2, 2-dimethyl-2-silapentane-5sulfonate (DSS) was the reference for n-propanol, ethanol and acetic acid, and the phenol was the reference for formate. 400 μL catholyte after the reaction was mixed with 100 μL 6 mM DSS solution, 100 μL 200 mM phenol and 200 μL D 2 O, and then analyzed by 1H NMR (Bruker Avance III 400 HD spectrometer).
The Faradaic efficiency of liquid product is: (Q: charge (C); F：Faradaic constant (96485 C/mol); n: the number of electrons required to generate the product)

Gaseous products:
From the GC peak areas and calibration curves for the TCD detector, we can obtain the V % of gaseous products .
Since the flow rate of the CO 2 was constant, the moles of gaseous products can be calculated. The Faradaic efficiency of gaseous product is:        In our work, the DSS was used as internal standard, and there was a small amount of impurity in DSS, the signal at 0.6 ppm, 1.7 ppm and 2.9 ppm were assigned to the impurity. Figure S9. A typical GC spectrum of as products after electrolysis over the Cu-nr-OR.

Figure S10
The FE of H 2 for Cu-np, Cu-nr and Cu-nr-OR.     (B) 1 H NMR spectra of the liquid products using 13 CO 2 as gas source over Cu-nr-OR for 60 min.
We can observe that the H signal of the products spilts into two group peaks, and the singal of n-propanol were located at 0.6ppm and 1.05ppm, when using 13 CO 2 as reactant. Due to the signal of impurity of DSS was also located at 0.6 ppm, which was overlapped with the signal of n-propanol, thus the internal standard cannot be used in Figure S15. In addition, we can observe that the intensity of products increased with the reaction time..                 Nanoporous Cu -0.67 62 653 S1 Table S2. Cu ECSA determined by Pb UPD method for different samples.

CuECSA (cm 2 )
Cu-nr-OR 61.2 Cu-nr 57.8 Cu-np 49.2 Table S3. Structural parameters of Cu-nr and Cu-nr-OR at different potentials extracted from the EXAFS fitting.
S 0 2 is the amplitude reduction factor S 0 2 =0.85; CN is the coordination number; R is interatomic distance (the bond length between central atoms and surrounding coordination atoms); σ 2 is Debye-Waller factor (a measure of thermal and static disorder in absorber-scatterer distances); ΔE 0 is edge-energy shift (the difference between the zero kinetic energy value of the sample and that of the theoretical model). R factor is used to value the goodness of the fitting.