Insight into Composition and Intermediate Evolutions of Copper-Based Catalysts during Gas-Phase CO₂ Electroreduction to Multicarbon Oxygenates

Abstract

Conversion of CO₂ to valuable chemicals driven by renewable electricity via electrocatalytic reduction processes is of great significance for achieving carbon neutrality. Copper-based materials distinguish themselves from other electrocatalysts for their unique capability to produce multicarbon compounds in CO₂ electroreduction. However, the intrinsic active composition and C–C coupling mechanism of copper-based catalysts are still ambiguous. This is largely due to the absence of appropriate in situ approaches to monitor the complicated processes of CO₂ electroreduction. Here, we adopted operando spectroscopy techniques, including Raman and infrared, to investigate the evolution of compositions and intermediates during gas-phase CO₂ electroreduction on Cu foam, Cu₂O nanowire and CuO nanowire catalysts. Although all the three copper-based catalysts possessed the activity of electroreducing gas-phase CO₂ to multicarbon oxygenates, Cu₂O nanowires showed the much superior performance with a 71.9% Faradaic efficiency of acetaldehyde. Operando Raman spectra manifested that the cuprous oxide remained stable during the whole gas-phase CO₂ electroreduction, and operando diffuse reflectance infrared Fourier transform spectroscopy (DRFITS) results provide direct evidences of key intermediates and their evolutions for producing multicarbon oxygenates, in consistence with the density functional theory calculations.

1. Introduction

Massive emission of CO₂ from fossil fuels consumption caused global warming and serious environment problems. Conversion of CO₂ by sustainable way is an effective strategy to realize carbon neutrality [1,2,3]. Electrochemical CO₂ reduction, driven by renewable energy derived from solar energy, wind energy, etc. is a promising technology for desirable coupling of CO₂ utilization and intermittent renewable energy transition. Although many metals have been studied as electrocatalysts, copper is a unique metal that possesses the capability to electroreduce CO₂ into multicarbon products [4]. Further studies have shown that the activity of copper-based catalysts is influenced by numerous factors such as particle size [5], morphology [6], crystal facet [7], grain boundaries [8] and valence states [9]. For instance, porous copper dendrites facilitate reducing CO₂ to C2+ products [10], and annealed copper is superior to electrodeposited counterpart for converting CO₂ to multicarbon products [9,10]. Mesoporous oxide-derived Cu foam prepared by a template-assisted electrodeposition process showed superior selectivity toward C2 product formation [11]. Copper particles formed by reducing µm-thick Cu₂O layers enhanced CO₂ electroreduction to C2 products [12]. C2+ products on Cu-Cu₂O catalyst derived from electro-synthesized copper complex could be formed with a Faradaic efficiency (FE) of ~80% at −0.4 V vs. RHE (reversible hydrogen electrode) [13]. Size-controlled Cu showed a dramatic increase in catalytic activity and nanoparticles ranging from 2 to 15 nm is facilitated to C–C coupling compared to bulk Cu [5].

Although the aforementioned factors have been widely studied, the CO₂ electroreduction activity of copper and copper oxides remains quite contradictory, due to the instability of copper species in aqueous electroreduction systems [13,14]. Many studies show that the oxide species of copper would be reduced into metallic Cu in the initial stage of CO₂ electroreduction [12,15], benefiting from the intrinsic capability of Cu⁰ to form C2+ products. In contrary, some opinions considered that Cu⁺ is more active than Cu⁰. Lee et al. [16] found that abundant Cu⁺ can strongly bind and preserve reaction intermediates for a longer time on catalyst surface, and then lengthen the carbon chain during CO₂RR. Gong et al. [17] attributed the enhanced activity to the synergistic effects between Cu⁰ and Cu⁺ that the former could activate CO₂ molecule and then promote C–C coupling.

To insight into the intrinsic active composition and C–C coupling mechanism of copper-based catalysts, gas-phase CO₂ electroreduction was applied in this work, which catered for the operando spectroscopy techniques to investigate the evolution of compositions and intermediates during CO₂ reduction. In addition, gas-phase electroreduction CO₂ could provide massive CO₂ feedstock, break its solubility limitation and hinder hydrogen evolution reaction effectively [18].

2. Results

Three distinct copper-based catalysts with Cu, Cu₂O and CuO as dominant components were synthesized by thermal oxidation/reduction processes. Based on their foam and nanowire morphologies, the as-obtained catalysts were denoted as Cu foam, Cu₂O NWs and CuO NWs, respectively. Scanning electron microscopy (SEM) images (Figure S1 in Supplementary Materials) showed the average diameter and length of Cu₂O NWs were 150 nm ± 30 nm and 750 nm ± 100 nm, respectively. The size of nanostructures in Cu₂O NWs was much smaller than that in CuO NWs, implying the more active sites in Cu₂O NWs (vide infra). The high-resolution transmission electron microscope (TEM) image (Figure 1A) showed that the lattice fringe spacing of 2.09 Å is assigned to the Cu (111) plane in Cu foam. As for Cu₂O NWs (Figure 1B), the d-spacings of 2.47 Å and 3.01 Å corresponded to the (111) and (110) planes of Cu₂O, respectively, and 2.09 Å to the Cu (111) plane, indicating that the outer part is Cu₂O and the core part is Cu, in agreement with the previous report [19]. CuO NWs showed the d-spacings of 2.47 Å and 2.32 Å corresponding to Cu₂O (111) and CuO (111) planes, respectively (Figure 1C). Figure 1D displayed the X-ray diffraction (XRD) patterns of Cu foam, Cu2O NWs and CuO NWs catalysts in the range of 30–90° (2θ). The diffraction peaks at 43.3°, 50.4° and 74.5° in Cu foam corresponded to metallic Cu (111), (200) and (220) planes, respectively (JCPDS no. 040836). Besides metallic Cu peaks, three weak peaks at 29.5°, 42.3° and 61.3° in Cu₂O NWs appeared, which were attributed to Cu₂O (111), (200) and (220) planes, respectively (JCPDS no. 050667). CuO NWs possessed both Cu₂O and CuO (JCPDS no. 481548) phases with trace of Cu (111) plane. The XRD results of the catalysts were consistent with the TEM observations. Furthermore, Cu foam showed no Raman peak, while Cu₂O NWs possessed two cuprous oxide peaks at 140 and 210 cm⁻¹, respectively (Figure 1E). Besides the characteristic peaks of Cu₂O, CuO NWs also showed three new peaks at 295, 341, 488 and 628 cm⁻¹, respectively (Figure 1E), which are assigned to cupric oxide species, as previously reported [20,21,22].

To understand surface properties of these Cu-based catalysts, surface element composition and electron structure were analyzed by X-ray photoemission spectroscopy (XPS). Characteristic Cu²⁺ 2p peaks and their strong Cu²⁺ satellite peaks of CuO NWs indicated numerous copper oxide species on the surface (Figure S2). However, the binding energy difference between Cu^I and Cu⁰ peaks is too small (only 0.1 eV) to distinguish them by only Cu 2p XPS spectra [23]. Thus, Auger electron spectroscopies (AES) were applied to further discern the possible Cu^I and Cu⁰ surface species. Cu LMMs spectra (Figure 1F) suggested the Cu foam surface with 79 at% Cu⁰ and 21 at% Cu^I, and the Cu₂O NWs surface with 72 at% Cu^I and 28 at% Cu⁰, the CuO NWs surface with 80 at% Cu^II and 20 at% Cu^I, respectively, implying each dominant Cu⁰, Cu^I and Cu^II surface composition in the catalysts. Furthermore, O 1s spectra of Cu-based catalysts (Figure 1G) exhibited that O-related species in Cu foam and Cu₂O NWs are lattice O atoms (Cu-O-Cu, 530.5 eV) and oxygen vacancy (O_v, 531.7 eV). The surface atomic ratios of O_v for Cu foam, Cu₂O NWs and CuO NWs were 65.3%, 59.6% and 24.8%, respectively. This means that the total numbers of O_v in Cu foam and Cu₂O NWs were much higher than counterparts in CuO NWs due to the small-sized nanostructures. The O_v sites frequently reported in various catalysts implied more coordination-unsaturated Cu sites in Cu foam and Cu₂O NWs, which could stabilize surface-adsorbed intermediate species [23,24]. Chemical structures and phase analysis of Cu-based catalysts are consistent with electron microscopies results.

The electrochemical characterization and performance of gas-phase CO₂ electroreduction over Cu foam, Cu₂O NWs and CuO NWs were conducted in a custom two-compartment cell, and all the experimental details are described in the section of Materials and Methods. The cyclic voltammetry (CV) curves (Figure S3) show the electrochemical behaviors of the catalysts in Ar and CO₂ atmosphere saturated with water vapor. For all copper-based catalysts, significant increment of reduction currents in CO₂ compared to those in Ar in the cell voltage range of −1.3 to −2.0 V, implying the occurrence of CO₂ electroreduction over copper-based catalysts. Furthermore, the electrochemical impedance spectroscopy (EIS) spectra (Figure S4) show that the charge transfer resistances of these catalysts follow the orders of Cu₂O NWs < Cu foam < CuO NWs, indicating the most favorable reduction kinetics in Cu₂O NWs. Figure 2 showed that all catalysts had the capability to produce methanol (CH₃OH) and multicarbon products mainly including acetaldehyde (CH₃CHO), propanal (CH₃CH₂CHO) and acetone (CH₃COCH₃), while ethanol (CH₃CH₂OH) was also formed on Cu foam. Note that no hydrogen was produced over all catalysts, indicating the merit of gas-phase CO₂ electroreduction, i.e., the complete suppression of hydrogen evolution reaction (HER), as previously reported [25]. Furthermore, acetaldehyde was the most favorable product over all catalysts, and the faradaic efficiency (FE) of acetaldehyde over Cu₂O NWs were much higher than those over Cu foam and CuO NWs. Cu₂O NWs achieved 71.9% of FE_CH3CHO, which is 2.3 and 5.8 times that of Cu foam and CuO NWs at cell voltage of −1.4 V, respectively. Moreover, the combined FE of propanal and acetone over Cu₂O NWs was 20.3%, which is 2.4 and 7.1 times that over Cu foam and CuO NWs, which can be attributed to the appropriate chemical composition with more oxygen vacancy sites and small-sized nanostructures. Current densities of gas-phase CO₂ electroreduction over the catalysts increased as the cell voltage negatively shifted from −1.3 to −1.7 V (Figure S5). At −1.3 and −1.4 V, the current densities of these catalysts were similar. Then, Cu foam showed the largest current densities at −1.5, −1.6 and −1.7 V compared to Cu₂O NWs and CuO NWs. However, the current densities of gas-phase CO₂ electroreduction over the catalysts were far less than those of aqueous CO₂ electroreduction [26] due to their high charge transfer resistances (Figure S4). That is the conventional aqueous CO₂ electroreduction showed the excellent ion conductivity, which benefited from the strong electrolyte aqueous solution. While gas-phase CO₂ electroreduction highly depended on the conductivity of proton exchange membrane, resulting in the low current densities [26]. In addition, the total FE of the products over Cu foam and CuO NWs were found to be less than 100% over the entire potential range because of high charge transfer resistance and diffusion resistance in gas-solid interface reaction system [25,27,28]. Therefore, Cu₂O NWs and Cu foam exhibited the preferable activities of gas-phase CO₂ electroreduction to multicarbon oxygenates compared to CuO NWs.

The intrinsic active composition and C–C coupling mechanism of Cu-based catalysts were further investigated by operando Raman and DRIFTS, and the illustration of operando system were shown in Figure S6. Operando Raman spectra (Figure 3A) showed that the characteristic peaks of Cu₂O NWs remained unchanged during gas-phase CO₂ electroreduction, indicating the good active component stability. The investigations in both Cu foam and CuO NWs also indicated that the electroreduction processes of gas-phase CO₂ did not change the initial compositions (Figure S7). Moreover, post-reaction XPS (Figures S2 and S8) and XRD (Figure S9) results verified that the surface and bulk phase compositions of Cu-based catalysts were stable during the gas-phase CO₂ electroreduction. Furthermore, operando DRIFTS was conducted to capture key adsorbed intermediate species during gas-phase CO₂ reduction (Figure 3B). At the initial stage (0 to 50 min), operando DRIFTS spectra of Cu₂O NWs showed no peak. With further increasing reaction time, the characteristic peaks in the range of 1000 to 2000 cm⁻¹ appeared and gradually became intense. The bands at 1992 and 1885 cm⁻¹ could be assigned to the vibrations of adsorbed linear *CO species, i.e., v_*CO(L) and v_M-*CO(L), indicating the formation of the initial CO₂ reduction intermediate [29,30]. *CO was also considered as the main key intermediate of CO₂ electroreduction to methanol, which appeared in gas-phase CO₂ electroreduction over Cu-based catalysts, in consistent with heterogeneous gas-phase methanol synthesis from CO₂ or CO hydrogenation [29]. Moreover, the other important intermediates including C–C bond such as *OCCOD (1587 cm⁻¹) and *OCCDO (1333 cm⁻¹, 1230 cm⁻¹ and 1156 cm⁻¹) were also observed, which indicated the occurrence of C–C coupling, in consistence with the previous reports [29,30,31]. Schouten et al. [31] suggested the hydrogenation of CO to CHO was the key step in the formation of multicarbon products especially acetaldehyde, and *CHO reacting with *CO to form *OCCHO was favorable over *CO dimerization and subsequent reduction to form *OCCOH [32].

Surface-structure models (Figure S10) of Cu (111), Cu₂O (111) and CuO (111) were constructed based on the above oprando Raman and DRIFTS results to further reveal C–C coupling mechanism during gas-phase CO₂ electroreduction. Adsorption energy of CO* and *CHO on Cu (111), Cu₂O (111) and CuO (111) were calculated through Ab Initio Simulation. The relative adsorption energy of CO* on CuO (111) surface (−0.39 eV) was much higher than those on Cu₂O (111) (−1.50 eV) and Cu (111) (−1.82 eV). Weak adsorption of CO* on CuO (111) hindered hydrogenation of CO* to *CHO. Simultaneously, the adsorption energy of *CHO on CuO (111) (−3.47 eV) was much higher than those on Cu₂O (111) (−2.36 eV) and Cu(111) (−1.91 eV), hindering *CHO attacked *CO to form *COCHO. The distinct weak adsorption of *CO but strong adsorption of *CHO on CuO (111) surface have great impact on the corresponding reactivity. DFT calculations indicated CuO (111) was not facilitated with C–C coupling, which was consistent with poor performance of gas-solid CO₂ electroreduction on CuO NWs. In contrast, CO and *CHO species adsorbed moderately on Cu(111), favoring subsequent C−C coupling. Theoretical calculated barrier indeed showed that the reaction CO* + HCO* → COCHO* on the Cu (111) surface was superior to that on the Cu₂O (111) surface. The corresponding barriers were 0.75 eV and 1.07 eV for the CO–CHO coupling on the Cu (111) and Cu₂O (111) surface. Cuprous oxide enhanced CO–CHO coupling to transform CO₂ to multi-carbon products, indicated that Cu₂O NWs performed a better activation to produce acetaldehyde. *OCCDO was the key intermediate of acetaldehyde product, as evidenced by the operando DRIFTS results. However, *CHO species were not detected directly, due to weak adsorption on gas-solid interface [33] Consequently, operando DRIFTS and theoretical calculations demonstrated that Cu⁰ and Cu^I components synergistically facilitated CO₂ reduction to form *OCCDO species for selectively producing acetaldehyde in gas-phase CO₂ electroreduction.

3. Materials and Methods

3.1. Materials

Commercial Cu foam were purchased from Suzhou Taili foam metal Co., Ltd., (Suzhou, China) (purity >99.9%). Nafion 117 proton exchange membrane were purchased from Dupont, (Shanghai, China). Ir-Ti mesh was purchased from Alibaba Group, (Hangzhou, China). Pt wire was purchased from Shanghai Fanyue Electronic Technology Co., Ltd., (Shanghai, China). Nafion 117 membrane was activated with 5% H₂O₂ at 80 °C for 1 h and rinsed with deionized water at 80 °C for 1 h, soaked in 5% sulfuric acid (mass ratio) at 80 °C for 1 h and then rinsed with deionized water at 80 °C for 1 h. And other reagents were used without further purification.

3.2. Catalyst Preparation

Commercial Cu foam was first cleaned with ethanol and deionized water in turns and then dried in vacuum oven at 40 °C overnight. The Cu-based catalysts were prepared according to previous work [19]. Briefly, the pre-treated Cu foam was heated in air (flow rate was 200 mL min⁻¹) at 550 °C for 3 h with a heating rate of 5 °C min⁻¹ to obtain CuO foam. Then these CuO foam was further heated in pure H₂ (purity 99.999%, flow rate was 200 mL min⁻¹) at 400 °C for 3 h with a heating rate of 5 °C min⁻¹. The obtained sample was named as Cu foam and stored in vacuum dry oven for use. In a typical procedure to prepare CuO nanowires (marked as CuO NWs), a layer of CuO nanowires was first formed on the surface of Cu foam by calcining Cu foam at 350 °C for 3 h with a heating rate of 2 °C min⁻¹. Cu₂O NWs was fabricated by reducing CuO NWs at 110 °C for 1 h in pure H₂ with a heating rate of 0.5 °C min⁻¹.

3.3. Catalyst Characterization

The crystalline structure of Cu-based catalysts were determined by X-ray diffraction (XRD, Ultima IV, Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation (λ = 1.5406 Å, 40 kV, 40 mA). The morphologies of Cu-based catalysts were analyzed by scanning electron microscopy (SEM, SUPRA 55, 5 kV). Transmission electron microscopy (TEM) experiments were conducted on a JEM-ARM300F microscope operated at 200 kV. Samples for the TEM measurements were suspended in ethanol and supported on Lacey Support Film. X-ray photoelectron spectroscopy (XPS) was conducted using a Quantum 2000 Scanning ESCA Microprobe Instrument (Thermo Fisher Scientific Inc., Waltham, MA, USA) with a monochromatic Al Kα source (1486.6 eV). The binding energy (BE) scale was calibrated according to the C 1s peak (284.8 eV). The elemental compositions were calculated from the peak area ratios after sensitivity factor correction of each element. XPS spectra were deconvolved by Avantage software (Thermo Fisher Scientific Inc., Waltham, MA, USA).

3.4. Electrochemical Measurements

All of the electrochemical measurements were conducted with a potentiostat (VMP3, Bio-Logic Inc., Seyssinet-Pariset, France) in a two-compartment electrochemical cell with an anion exchange membrane (Nafion 117, Dupont) to separate the cathode chamber and anode chamber. The electrocatalytic experiments were measured in a two-electrode configuration with Ti-Ir mesh as both counter and reference electrode in anode chamber, while the Cu-based electrocatalyst was used as a working electrode placed in cathode chamber. The electrocatalytic experiments were carried out at 80 °C (optimum working temperature for Nafion membrane) in atmospheric pressure with 0.5 mL min⁻¹ CO₂ flow for the cathode and Ar flow (36% water vapor with Ar balance) for the anodic stream, at 80 °C (optimum values for Nafion membrane). The CO₂ reduction was conducted under the potentiostatic mode for 1 h at each potential. Cyclic voltammetry (CV) was performed in the cell voltage range from 0 V to −2.0 V, and the electrochemical impedance spectroscopy measurements were performed at a cell voltage of −1.4 V over a frequency range from 200 kHz to 0.1 Hz.

Since all the electrocatalytic experiments were carried out at 80 °C, exhaust gas line connected with GC chromatogram was held at 90 °C to guarantee all products in gas phase and would be injected into GC chromatogram, a double channel gas chromatograph (GC-2014, SHIMADZU Co., Ltd., Kyoto, Japan) equipped with TDX-01 and GDX-502 consecutive columns and a Hayesep C column, along with thermal conductivity and flame ionization detectors, respectively. A typical GC run was initiated every 30 min. H₂, methane and CO were detected by TDX-01 and GDX-502 columns, while methanol, acetaldehyde, acetone, methyl formate, methyl acetate, ethanol, 2-propanol and n-propanol were detected by Hayesep C column. The faradaic efficiencies of all the products were calculated by using the concentrations (C_product) detected by the GC as follows:

$$FE\%=\frac{C_{product\times {10}^{-6}\times v_{CO2}\times {10}^{-3}\times t\times \mathrm{96,485}\times \alpha }}{24.04\times Q}$$

where C_product is the concentration of the gas-phase products, v_CO2 is the flow rate of CO₂, α is the number of electrons transferred from CO₂ to products, t is the reaction time, Q (A∙s) is the total quantity of electric charge.

3.5. Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

In situ DRIFTS were obtained with a Bruker vertex70 FTIR instrument through a homemade cell with a KBr crystal as infrared transmission window (cutoff energy of 400 cm⁻¹), aperture setting is 8 nm, and scanner velocity is 5 kHz. Operando DRIFTS H-cell reactor and operando DRIFTS system of gas-phase CO₂ electroreduction were illustrated in Figure S6A,B. To ensure the quality of the obtained DRFITS, feed gas (1% CO₂) was cooled down with dry ice to remove trace water and D₂O replaced H₂O as proton source to reduce impacts on spectra. Each infrared absorption spectrum was acquired by 1000 scans at a resolution of 2 cm⁻¹. According to results of gas-phase CO₂ electroreduction, Cu₂O NWs was regarded as the better catalyst compared with the other catalysts of Cu foam and CuO NWs. Cu₂O NWs was set an example to understand mechanism of gas-phase CO₂ electroreduction over Cu-based catalysts with in situ DRIFTS instrument. Argon (20 sccm) with D₂O (pumped in with syringe) passed in anode chamber.

3.6. Operando Raman Spectroscopy

Operando Raman spectra were collected using i-Raman Pro spectrometer (BWTS 475-532H, Newark, NJ, USA) with a working distance of 5 mm and laser core center of 100 μm. A home-made spectroelectrochemical reactor (Figure S6B) was same with operando DRIFTS cell, and differently, silicon plate replaced KBr plate was selected as measurement window of Raman. Ir-Ti mesh and Cu-based catalysts were used as counter electrode and working electrode, respectively. Working conditions were the same with that of CO₂ electroreduction reaction. The acquisition time for each spectrum was 30 s, and the spectra were collected continuously for 60 min at −1.4 V (vs. Ir-Ti mesh). Raman spectra of Cu-based catalysts during gas-phase CO₂ electroreduction were showed in Figure S6, indicated the good stability of Cu foam, Cu₂O NWs and CuO NWs during gas-phase CO₂ electroreduction process.

3.7. Density Functional Theory (DFT) Calculations

DFT calculations were performed using the Vienna Ab Initio Simulation Package (VASP) code [34,35]. In an attempt to gain the origin of CO₂ electroreduction into C2+ species on three types of Cu and copper oxides surfaces, namely, the Cu (111), Cu₂O (111) and CuO (111) surfaces, a DFT study of acetaldehyde formation was carried out in the present simulated electrochemical interfaces. To treat the exchange–correlation interactions and solve the proton–electron interactions in a periodic boundary system, we chose Perdew–Burke–Ernzerhof (PBE) functionals [36] and the projector-augmented wave (PAW) method [37]. According to previous reports [33,38], surface-structure models of 4-layer Cu (111), Cu₂O (111) and CuO (111) were constructed as Figure S10. The vacuum thickness between graphene layers was set as 13 Å to avoid interlayer interactions. A Monkhorst–Pack grids [8] with 3 × 3 × 1, 2 × 1 × 1 and 2 × 2 × 1 k-point sampling was used for Cu (111), Cu₂O (111) and CuO (111) surface. The energies and the forces were converged to within 10−4 eV per atom, 0.03 eV Å⁻¹, respectively. Continually we also examined the H adsorption energy (Figure S11), CO adsorption energy (Figure S12), HCO adsorption energy (Figure S13), adsorption energy of CO dimerization (COCHO) (Figure S14) and reaction energies (Figure S15) of CO* + HCO* → *COCHO on Cu (111), Cu₂O (111) and CuO (111) surface.

4. Conclusions

In this work, copper-based catalysts with different Cu valance states were prepared by a simple thermal oxidation/reduction method. The composition evolution and enhanced C–C coupling induced by the synergetic effects of Cu⁰, Cu^I and their shape and size were investigated via operando spectroscopies during gas-phase CO₂ electroreduction. Operando Raman revealed Cu₂O and CuO could be relatively stable during gas-phase CO₂ electroreduction. Operando DRIFTS provided the direct evidence of key intermediates to understand mechanism of gas-phase CO₂ electroreduction to produce multicarbon oxygenates. By virtue of theoretical calculations, reaction mechanisms of CO₂ electroreduction on Cu-based catalysts were further manifested that *OCCHO species were the transition intermediate for C–C coupling to multicarbon oxygenates. This work demonstrates using operando spectroscopy characterization techniques to understand possible active compositions and reaction mechanisms of gas-phase CO₂ electroreduction.