Concentration Optimization of Localized Cu0 and Cu+ on Cu-Based Electrodes for Improving Electrochemical Generation of Ethanol from Carbon Dioxide

Copper-based electrodes can catalyze electroreduction of CO2 to two-carbon products. However, obtaining a specific product with high efficiency depends on the oxidation state of Cu for the Cu-based materials. In this study, Cu-based electrodes were prepared on fluorinated tin oxide (FTO) using the one-step electrodeposition method. These electrodes were used as efficient electrocatalysts for CO2 reduction to ethanol. The concentration ratio of Cu0 and Cu+ on the electrodes was precisely modulated by adding monoethanolamine (MEA). The results of spectroscopic characterization showed that the concentration ratio of localized Cu+ and Cu0 (Cu+/Cu0) on the Cu-based electrodes was controlled from 1.24/1 to 1.54/1 by regulating the amount of MEA. It was found that the electrode exhibited the best electrochemical efficiency and ethanol production in the CO2 reduction reaction at the optimal concentration ratio Cu+/Cu0 of 1.42/1. The maximum faradaic efficiencies of ethanol and C2 were 48% and 77%, respectively, at the potential of −0.6 V vs. a reversible hydrogen electrode (RHE). Furthermore, the optimal concentration ratio of Cu+/Cu0 achieved the balance between Cu+ and Cu0 with the most favorable free energy for the formation of *CO intermediate. The stable existence of the *CO intermediate significantly contributed to the formation of the C–C bond for ethanol production.


Introduction
An electrochemical CO 2 reduction reaction (CO 2 RR) uses electrical energy to convert CO 2 molecules into hydrocarbon products, which can store the intermediate electricity as chemical energy in the form of fuels [1]. Various products, such as carbon monoxide [2][3][4], formic acid [5,6], methane [7], methanol [8], ethylene [9,10], acetic acid [11], ethanol [12,13], and minor C 3+ products [14], have been acquired from direct and homogeneously catalyzed CO 2 reduction. The selectivity of a specific product of the CO 2 RR is heavily dependent on the electrocatalysts. Generally, Zn [4], Ag [2], and Pd [3] metals are extensively explored to produce CO, while Sn [5], Bi [5], and Pb [6] metals have been studied to generate HCOOH from the CO 2 RR. Other high-value products containing alcohols and olefins are difficult to obtain from the CO 2 RR owing to their multi-electron requirement, which is provided by the surface atomic arrangement of electrocatalysts. A significant barrier to this conversion is the lack of efficient and robust catalysts for CO 2 reduction, particularly the catalysts that can realize high-order products such as ethanol, methanol, or multi-carbon compounds.
Among conventional electrocatalysts, Cu-based electrodes can effectively enhance CO 2 conversion into two-carbon products, such as ethanol and ethylene, owing to their between Cu + and Cu 0 with a stabilized *CO intermediate. The abundant *CO intermediates contributed to the formation of the C-C bond for ethanol production.

Morphology of Cu-Based Electrodes
The Cu-based electrodes were prepared using copper nitrate and MEA as the reaction precursor, and the morphology and valence state agent, respectively, by constant potential electrodeposition at room temperature. In particular, Cu 0.6 and Cu 1.2 electrodes were obtained by adding MEA with a volume ratio of 0.6% and 1.2% to 0.1 mol/L copper nitrate, respectively. For comparison, a Cu-based electrode was fabricated without MEA using the same electrodeposition parameters (denoted as Cu 0 ). Scanning electron microscopy (SEM) images showed that microparticles monodisperse on the electrode in isolation with a size of approximately 2 µm, and Cu 0 consisted of several small nanoparticles with a diameter of 100 nm (Figures 1a and 2c-f). These microparticles were grown on the Cu-based materials formed on the conductive layer of FTO (Figure 2a,b). In comparison, the Cu 0.6 electrode possessed a flat surface densely arranged by smooth spherical particles of size approximately 300 nm, appearing as a uniform film (Figures 1b and 3d-f). The size of the particles increased to approximately 500 nm for the Cu 1.2 electrode, and these particles were decorated by smaller particles with approximately 50 nm diameter (Figures 1c and 3j-l). Clear gaps between grainy particles with exposed substrates of bare FTO were observed (Figure 3a-c,g-i). The selective mapping in the 50 nm range showed that Cu and O elements were widely distributed on the plane for Cu 0.6 , and the distribution of O was mostly localized compared to that of Cu (Figure 1b,d-f). In the precursor solution, Cu 2+ ions were evenly dispersed in the absence of MEA. Under the effect of the electric field, Cu 2+ ions were aligned in the solution and then moved to the cathode. The chemical reduction of considerable Cu 2+ ions improved the anisotropic growth of microparticles. Cu 2+ ions, as a transition metal, are inclined to bond to MEA to form [Cu(MEA) 2 ] 2+ in the presence of MEA in the precursor solution [28]. The UV-vis absorption spectra of precursor solutions also demonstrated that the absorption edge of Cu 2+ ions decreased and then the absorption peak of [Cu(MEA) 2 ] 2+ rose as the addition of MEA increased, which illustrated that the Cu 2+ ions were made more complex by MEA with [Cu(MEA) 2 ] 2+ formation ( Figure 4). Charged [Cu(MEA) 2 ] 2+ moved directionally to the cathode; however, the higher mass of [Cu(MEA) 2 ] 2+ slowed down its movement compared to Cu 2+ , resulting in smaller particles by suppressing grain growth for Cu 0.6 . However, the concentration of Cu 2+ ions was reduced, and all of them were consumed after a large amount of MEA was added. Considerable amounts of [Cu(MEA) 2 ] 2+ delayed the crystallization of Cu-based materials, and insufficient Cu 2+ ions led to cracking on Cu 1.2 . Atomic force microscopy (AFM) characterization further demonstrated that adding a suitable amount of MEA smoothed the surface of the Cu 0.6 electrode so that it became flat, with the lowest surface roughness compared to those of the Cu 0 and Cu 1.2 electrodes ( Figure 5).

The Phase Analysis of Cu-Based Electrodes
The X-ray diffraction (XRD) patterns of Cu-based electrodes clearly showed two peaks at 43.30° and 50.43°, corresponding to the (111) and (200) reflections of Cu (PDF#04-0836), respectively. The two peaks at 36.42° and 42.30° were attributed to the (111) and (200) reflections of Cu2O (PDF#05-0667) (Figure 6a), respectively. The Cu0 electrode exhibited stronger peaks of Cu, while the Cu0.6 and Cu1.2 electrodes exhibited the stronger peaks of Cu2O. To confirm the coexistence of Cu + and Cu on a Cu-based electrode, an anodization experiment was conducted for the Cu0.6 electrode. Two typical oxidation peaks at 0.16 V and 0.07 V vs. RHE were detected (Figure 6b), which were attributed to the oxidation of Cu 0 to Cu 2+ and Cu + to Cu 2+ , respectively [29]

Electrochemical CO2 Reduction for Cu-Based Electrodes
The performance of the CO2RR was evaluated in a 0.1 M KHCO3 saturated solution with potential ranging from −0.5 to −1 V (vs. RHE). Linear sweep voltammetry (LSV) was performed for the Cu0.6 electrode, and it was found that the Cu0.6 electrode exhibited a good activity for the electrochemical reduction of CO2 with a higher current in the CO2saturated electrolyte than in N2-saturated electrolyte (Figure 7). Compared to Cu0, H2 generation was significantly inhibited on the Cu0.6 and Cu1.2 electrodes under all voltages. Moreover, ethanol generation was effectively improved at lower negative voltages, particularly for Cu0.6, which exhibited the most favorable selectivity of ethanol (Figure 8a-c). At the potential of −0.6 V vs. RHE, the FE of Cu0.6 for ethanol production was as high as 48%, which was optimal compared to those of most Cu-based electrodes (Table 1). For the Cu0.6 electrocatalyst, the yield of C1 products (CO and formic acid) under all voltages did

The Phase Analysis of Cu-Based Electrodes
The X-ray diffraction (XRD) patterns of Cu-based electrodes clearly showed two peaks at 43.  (Figure 6b), which were attributed to the oxidation of Cu 0 to Cu 2+ and Cu + to Cu 2+ , respectively [29].

The Phase Analysis of Cu-Based Electrodes
The X-ray diffraction (XRD) patterns of Cu-based electrodes clearly showed two peaks at 43.30° and 50.43°, corresponding to the (111) and (200) reflections of Cu (PDF#04-0836), respectively. The two peaks at 36.42° and 42.30° were attributed to the (111) and (200) reflections of Cu2O (PDF#05-0667) (Figure 6a), respectively. The Cu0 electrode exhibited stronger peaks of Cu, while the Cu0.6 and Cu1.2 electrodes exhibited the stronger peaks of Cu2O. To confirm the coexistence of Cu + and Cu on a Cu-based electrode, an anodization experiment was conducted for the Cu0.6 electrode. Two typical oxidation peaks at 0.16 V and 0.07 V vs. RHE were detected (Figure 6b), which were attributed to the oxidation of Cu 0 to Cu 2+ and Cu + to Cu 2+ , respectively [29]

Electrochemical CO2 Reduction for Cu-Based Electrodes
The performance of the CO2RR was evaluated in a 0.1 M KHCO3 saturated solution with potential ranging from −0.5 to −1 V (vs. RHE). Linear sweep voltammetry (LSV) was performed for the Cu0.6 electrode, and it was found that the Cu0.6 electrode exhibited a good activity for the electrochemical reduction of CO2 with a higher current in the CO2saturated electrolyte than in N2-saturated electrolyte (Figure 7). Compared to Cu0, H2 generation was significantly inhibited on the Cu0.6 and Cu1.2 electrodes under all voltages. Moreover, ethanol generation was effectively improved at lower negative voltages, particularly for Cu0.6, which exhibited the most favorable selectivity of ethanol (Figure 8a-c). At the potential of −0.6 V vs. RHE, the FE of Cu0.6 for ethanol production was as high as 48%, which was optimal compared to those of most Cu-based electrodes ( Table 1). For the Cu0.6 electrocatalyst, the yield of C1 products (CO and formic acid) under all voltages did

Electrochemical CO 2 Reduction for Cu-Based Electrodes
The performance of the CO 2 RR was evaluated in a 0.1 M KHCO 3 saturated solution with potential ranging from −0.5 to −1 V (vs. RHE). Linear sweep voltammetry (LSV) was performed for the Cu 0.6 electrode, and it was found that the Cu 0.6 electrode exhibited a good activity for the electrochemical reduction of CO 2 with a higher current in the CO 2 -saturated electrolyte than in N 2 -saturated electrolyte ( Figure 7). Compared to Cu 0 , H 2 generation was significantly inhibited on the Cu 0.6 and Cu 1.2 electrodes under all voltages. Moreover, ethanol generation was effectively improved at lower negative voltages, particularly for Cu 0.6 , which exhibited the most favorable selectivity of ethanol (Figure 8a-c). At the potential of −0.6 V vs. RHE, the FE of Cu 0.6 for ethanol production was as high as 48%, which was optimal compared to those of most Cu-based electrodes (Table 1). For the Cu 0.6 electrocatalyst, the yield of C 1 products (CO and formic acid) under all voltages did not exceed 15%, and the FE of H 2 was as low as 19%. Furthermore, the largest FE of C 2 products was 77% for the Cu 0.6 electrode at a potential of −0.6 V vs. RHE (Figures 8b and 9a-c), which was attributed to its stronger C-C coupling capability consuming the intermediate *CO. However, the FE of the ethanol and C 2 products of the Cu 1.2 electrocatalyst was lower than those of Cu 0.6 under all voltages, which might be due to the surface structure of the Cu 1.2 electrocatalyst. In addition, the electrocatalytic performance of the Cu 0.3 and Cu 0.9 electrodes also verified that Cu 0.6 is the best electrocatalyst for ethanol production, as shown in Figure 9d,e. The stability of Cu-based electrodes in CO 2 RR is of great significance for practical applications, considering the alkaline electrolyte and interface reliability. Continuous and stable operation of CO 2 RR electrolysis was implemented on the Cu 0.6 electrode for 6 h under the voltage of −0.6 V vs. RHE. Moreover, no obvious changes in pH during the catalytic process ( Figure 10a) was observed. The current remained above −0.5 mA cm −2 with a negligible decrease (Figure 10b-d), and the FE of ethanol was over 40% (Figure 8d) after 6h. The results of the SEM analysis of the Cu 0.6 electrode after the CO 2 RR showed that the Cu-based material was well-maintained during the reaction ( Figure 11a). Moreover, after the CO 2 RR, still two oxidation peaks existed for Cu 0.6 as an anode, which suggested that the Cu 0.6 electrode was stable during the electrocatalytic process ( Figure 11b). Although the current density was slightly lower, it could be significantly improved by atomic rearranging and a composite strategy. not exceed 15%, and the FE of H2 was as low as 19%. Furthermore, the largest FE of C2 products was 77% for the Cu0.6 electrode at a potential of −0.6 V vs. RHE (Figures 8b and 9a-c), which was attributed to its stronger C-C coupling capability consuming the intermediate *CO. However, the FE of the ethanol and C2 products of the Cu1.2 electrocatalyst was lower than those of Cu0.6 under all voltages, which might be due to the surface structure of the Cu1.2 electrocatalyst. In addition, the electrocatalytic performance of the Cu0.3 and Cu0.9 electrodes also verified that Cu0.6 is the best electrocatalyst for ethanol production, as shown in Figure 9d,e. The stability of Cu-based electrodes in CO2RR is of great significance for practical applications, considering the alkaline electrolyte and interface reliability. Continuous and stable operation of CO2RR electrolysis was implemented on the Cu0.6 electrode for 6 h under the voltage of −0.6 V vs. RHE. Moreover, no obvious changes in pH during the catalytic process ( Figure 10a) was observed. The current remained above −0.5 mA cm −2 with a negligible decrease (Figure 10b-d), and the FE of ethanol was over 40% (Figure 8d) after 6h. The results of the SEM analysis of the Cu0.6 electrode after the CO2RR showed that the Cu-based material was well-maintained during the reaction ( Figure 11a). Moreover, after the CO2RR, still two oxidation peaks existed for Cu0.6 as an anode, which suggested that the Cu0.6 electrode was stable during the electrocatalytic process ( Figure 11b). Although the current density was slightly lower, it could be significantly improved by atomic rearranging and a composite strategy.

Electrochemical Characterization of Cu-Based Electrodes
The ultraviolet-visible-near-infrared (UV-vis) spectrometry showed strong absorption of MEA and [Cu(MEA)2] 2+ at 200-320 nm, while no absorption for the Cu0.6 electrode (Figure 12a) was observed in the same wavelength range. This indicated the presence of a Cu-based material and the absence MEA or [Cu(MEA)2] 2+ on the electrode. To study the dynamics of carriers, transient photoluminescence spectroscopy (TRPL) on Cu-based electrocatalysts was performed. Figure 12b shows that the fluorescence lifetime of Cu0.6 was 0.117 ns, which was lower than those of Cu0 (0.182 ns) and Cu1.2 (0.130 ns). The shorter lifetime of the carriers in the Cu0.6 electrode implied that the electrons quickly diffused to the interface, decreasing the non-radiative recombination of free electrons on the Cubased electrodes. At the same time, the electrical impedance test also showed that the impedance of Cu0.6 was the smallest (Figure 12c), indicating that the charge transfer resistance was the lowest in the Cu0.6 electrode. The measurement of electrochemically active specific surface area (ECSA) indicated that the Cu0.6 electrocatalyst was capable of providing higher catalytic activity (Figure 13a-d). At the same time, the kinetic resistance of the catalyst for CO2RR was smaller for the Cu0.6 electrode with a smaller Tafel slope (Figure 12d). To explore the main cause of ethanol production for the Cu-based electrodes, we calcined the Cu0.6 electrode in Ar and air atmosphere (Figure 13e). For the Cu0.6 electrode post-treated in the Ar atmosphere, the obvious generation of ethanol (FE, 32%) and hydrogen (Figure 12d) was observed. However, no ethanol was detected for the Cu0.6 electrode temperature-treated in air, which was in an oxidized state without oxidation peaks (Figure 13f). The surface redox state of Cu was closely related to the CO2RR performance of the Cu-based electrodes.

Electrochemical Characterization of Cu-Based Electrodes
The ultraviolet-visible-near-infrared (UV-vis) spectrometry showed strong absorption of MEA and [Cu(MEA) 2 ] 2+ at 200-320 nm, while no absorption for the Cu 0.6 electrode (Figure 12a) was observed in the same wavelength range. This indicated the presence of a Cu-based material and the absence MEA or [Cu(MEA) 2 ] 2+ on the electrode. To study the dynamics of carriers, transient photoluminescence spectroscopy (TRPL) on Cu-based electrocatalysts was performed. Figure 12b shows that the fluorescence lifetime of Cu 0.6 was 0.117 ns, which was lower than those of Cu 0 (0.182 ns) and Cu 1.2 (0.130 ns). The shorter lifetime of the carriers in the Cu 0.6 electrode implied that the electrons quickly diffused to the interface, decreasing the non-radiative recombination of free electrons on the Cu-based electrodes. At the same time, the electrical impedance test also showed that the impedance of Cu 0.6 was the smallest (Figure 12c), indicating that the charge transfer resistance was the lowest in the Cu 0.6 electrode. The measurement of electrochemically active specific surface area (ECSA) indicated that the Cu 0.6 electrocatalyst was capable of providing higher catalytic activity (Figure 13a-d). At the same time, the kinetic resistance of the catalyst for CO 2 RR was smaller for the Cu 0.6 electrode with a smaller Tafel slope (Figure 12d). To explore the main cause of ethanol production for the Cu-based electrodes, we calcined the Cu 0.6 electrode in Ar and air atmosphere (Figure 13e). For the Cu 0.6 electrode post-treated in the Ar atmosphere, the obvious generation of ethanol (FE, 32%) and hydrogen (Figure 12d) was observed. However, no ethanol was detected for the Cu 0.6 electrode temperature-treated in air, which was in an oxidized state without oxidation peaks (Figure 13f). The surface redox state of Cu was closely related to the CO 2 RR performance of the Cu-based electrodes.

X-ray Photoelectron Spectroscopy of Cu-Based Materials
Fine X-ray photoelectron spectroscopy (XPS) of Cu 2p in a Cu-based electrode was performed to analyze the valence state of Cu on the surface (Figure 14a,b). It is difficult to distinguish between Cu + and Cu 0 from the Cu 2p 1/2 and Cu 2p 3/2 peaks [30]. For Cu 2p 3/2 and O 1s spectra, there was only one peak for each spectral line, which also demonstrated that the individual oxide of Cu 2 O exists in Cu-based material instead of CuO [31]. Further analysis with Cu LMM Auger energy spectrum (AES) demonstrated that Cu + and Cu 0 coexist on the electrodes (Figure 14c), and Cu + ions are dominated at the peak of approximately 569.8 eV compared to 568.0 eV for Cu 0 ions and 565.2 eV for the transition state of the Cu LMM [32]. These corresponding peaks were integrated to determine the ratio between Cu + and Cu 0 ions. For these as-prepared electrodes, the concentration ratio of Cu + and Cu 0 ions increased from 1.24/1 to 1.54/1 with increasing MEA content (Figure 14d-f). The strong interaction between Cu 2+ and MEA affected the reduction of Cu 2+ to Cu + and further to Cu 0 by Cu + [33]. For the Cu 0.6 electrode, the medium concentration ratio was 1.42/1, which showed the predominant CO 2 RR performance. The above AES characterizations were implemented on the flacking Cu-based powders. In addition, the AES of the electrodes was achieved, and it showed only the peak of Cu + without Cu 0 (Figure 15), which could be probably attributed to the localized distribution of Cu 2 O and Cu. Moreover, the electrode surface (0-5 nm depth) was covered by Cu 2 O. The product selectivity could probably be attributed to the synergy between Cu + and Cu 0 ions, which was demonstrated to accelerate CO 2 activation and CO dimerization [26]

X-ray Photoelectron Spectroscopy of Cu-Based Materials
Fine X-ray photoelectron spectroscopy (XPS) of Cu 2p in a Cu-based electrode was performed to analyze the valence state of Cu on the surface (Figure 14a,b). It is difficult to distinguish between Cu + and Cu 0 from the Cu 2p1/2 and Cu 2p3/2 peaks [30]. For Cu 2p3/2 and O 1s spectra, there was only one peak for each spectral line, which also demonstrated that the individual oxide of Cu2O exists in Cu-based material instead of CuO [31]. Further analysis with Cu LMM Auger energy spectrum (AES) demonstrated that Cu + and Cu 0 coexist on the electrodes (Figure 14c), and Cu + ions are dominated at the peak of approximately 569.8 eV compared to 568.0 eV for Cu 0 ions and 565.2 eV for the transition state of the Cu LMM [32]. These corresponding peaks were integrated to determine the ratio between Cu + and Cu 0 ions. For these as-prepared electrodes, the concentration ratio of Cu + and Cu 0 ions increased from 1.24/1 to 1.54/1 with increasing MEA content (Figure 14d-f). The strong interaction between Cu 2+ and MEA affected the reduction of Cu 2+ to Cu + and further to Cu 0 by Cu + [33]. For the Cu0.6 electrode, the medium concentration ratio was 1.42/1, which showed the predominant CO2RR performance. The above AES characterizations were implemented on the flacking Cu-based powders. In addition, the AES of the electrodes was achieved, and it showed only the peak of Cu + without Cu 0 (Figure 15), which could be probably attributed to the localized distribution of Cu2O and Cu. Moreover, the electrode surface (0-5 nm depth) was covered by Cu2O. The product selectivity could probably be attributed to the synergy between Cu + and Cu 0 ions, which was demonstrated to accelerate CO2 activation and CO dimerization [26]

The Formation of *CO Intermadiate for Ethanol Production
The details of the valence state of Cu and its relationship with the *CO intermediate in CO2RR are worth exploring to obtain electrocatalysts with enhanced activity and high selectivity. The stable existence of the initial *CO intermediate played a significant role in C-C coupling, and it could be calculated using density functional theory simulations. The atomic structure models were implemented and *CO intermediates were bonded to the Cu atoms (Figure 16a-c). The degree of stability was represented by the free energy of the *CO intermediate. The free energy of *CO intermediate was −3.03 eV for Cu0.6, which was significantly lower than those for Cu0 (−1.85 eV) and Cu1.2 (−2.20 eV), indicating that the *CO intermediate was more stable on Cu0.6 than on Cu0 and Cu1.2. The stable existence of *CO significantly contributes to C-C coupling by the supply of the reaction precursor [34]. To date, there is no definite reactive process for CO2RR to ethanol. We propose one probable route considering the reported work ( Figure 16d) [19,25,34,35]. The Cu0.6 electrode probably achieves the balance between *CO-Cu + (containing positively charged C) and *CO-Cu 0 (containing negatively charged C) [25]. The electrostatic attraction between these two C atoms contributes to the formation of the C-C bond. In addition to abundant *CO intermediates, adequately monoprotonated *CHO intermediates exist on the electrode, and they are prone to forming *CO-COH intermediates [34]. The CO-COH intermediates then accept multi-protons and electrons to obtain the precursor (*CH2CH2OH) of the final product ethanol [19,35]. The high specific surface area of Cu0.6 provides abundant reaction sites for the CO2RR and an appropriate Cu + /Cu 0 ratio that accumulates reaction intermediates. The high concentration of the *CO intermediates on the Cu 0 -Cu + interface further promotes the C-C dimerization reaction and improves the selectivity of ethanol.

The Formation of *CO Intermadiate for Ethanol Production
The details of the valence state of Cu and its relationship with the *CO intermediate in CO 2 RR are worth exploring to obtain electrocatalysts with enhanced activity and high selectivity. The stable existence of the initial *CO intermediate played a significant role in C-C coupling, and it could be calculated using density functional theory simulations. The atomic structure models were implemented and *CO intermediates were bonded to the Cu atoms (Figure 16a-c). The degree of stability was represented by the free energy of the *CO intermediate. The free energy of *CO intermediate was −3.03 eV for Cu 0.6 , which was significantly lower than those for Cu 0 (−1.85 eV) and Cu 1.2 (−2.20 eV), indicating that the *CO intermediate was more stable on Cu 0.6 than on Cu 0 and Cu 1.2 . The stable existence of *CO significantly contributes to C-C coupling by the supply of the reaction precursor [34]. To date, there is no definite reactive process for CO 2 RR to ethanol. We propose one probable route considering the reported work ( Figure 16d) [19,25,34,35]. The Cu 0.6 electrode probably achieves the balance between *CO-Cu + (containing positively charged C) and *CO-Cu 0 (containing negatively charged C) [25]. The electrostatic attraction between these two C atoms contributes to the formation of the C-C bond. In addition to abundant *CO intermediates, adequately monoprotonated *CHO intermediates exist on the electrode, and they are prone to forming *CO-COH intermediates [34]. The CO-COH intermediates then accept multi-protons and electrons to obtain the precursor (*CH 2 CH 2 OH) of the final product ethanol [19,35]. The high specific surface area of Cu 0.6 provides abundant reaction sites for the CO 2 RR and an appropriate Cu + /Cu 0 ratio that accumulates reaction intermediates. The high concentration of the *CO intermediates on the Cu 0 -Cu + interface further promotes the C-C dimerization reaction and improves the selectivity of ethanol.

Synthesis of Cu-based electrodes on FTO:
The Cu-based electrodes were prepared using a one-step electrochemical deposition method on FTO. The three-electrode cell-Ag/AgCl as the reference electrode, FTO as the working electrode, and counter electrode-was used. Before the deposition, the bared FTO substrates were ultrasonically cleaned with isopropanol, ethanol, and water 6 times and quickly dried with nitrogen gas. Simultaneously, a definite volume of MEA (0, 60, 120, 180, and 240 μL) was dropped into a copper nitrate solution (0.1 mmol, 20 mL) as the deposited electrolyte. Then, the three-electrode cell was operated in the prepared electrolyte at a constant voltage of −0.4 V vs. RHE for 30 s, and five Cu-based electrodes were obtained in five precursor solutions with different volumes of MEA. The as-prepared electrodes were flushed with water three times and finally dried with nitrogen gas.
Characterization: The images were obtained using SEM (jsm-7800F, JEOL, Japan), and the SEM mapping was acquired using an energy-dispersive spectrometer in the same instrument. AFM analysis was conducted using an atomic force microscope (XE-70, Park systems, Korea). The XRD patterns of the samples were collected with a Smartlab (3 KW) X-ray powder diffractometer, and the Cu-K-α radiation wavelength was 0.154178 nm. Moreover, UV-vis spectra were collected using an ultraviolet spectrophotometer (PE Lambda 950, PerkinElmer, U.S.). TRPL analysis was conducted using a time-resolved fluorescence spectrometer (FLS 980, Edinburgh, UK). The XPS data were obtained using a Kalpha X-ray photoelectron spectrometer (PHI5000 Versaprobe, ULVAC-PHI, Japan). The pH of the solution was measured using Sartorius PB-10 (Sartorius, German).
Electrochemical measurements: An electrochemical workstation (CHI660E, Shanghai Chenhua, China) was used for electrochemical measurements. In the CO2RR characterization, the platinum electrode was used as the counter electrode, Ag/AgCl (saturated KCl) as the reference electrode, and the Cu-based electrode as the working electrode to form a

Materials and Methods
Synthesis of Cu-based electrodes on FTO: The Cu-based electrodes were prepared using a one-step electrochemical deposition method on FTO. The three-electrode cell-Ag/AgCl as the reference electrode, FTO as the working electrode, and counter electrode-was used. Before the deposition, the bared FTO substrates were ultrasonically cleaned with isopropanol, ethanol, and water 6 times and quickly dried with nitrogen gas. Simultaneously, a definite volume of MEA (0, 60, 120, 180, and 240 µL) was dropped into a copper nitrate solution (0.1 mmol, 20 mL) as the deposited electrolyte. Then, the three-electrode cell was operated in the prepared electrolyte at a constant voltage of −0.4 V vs. RHE for 30 s, and five Cu-based electrodes were obtained in five precursor solutions with different volumes of MEA. The as-prepared electrodes were flushed with water three times and finally dried with nitrogen gas.
Characterization: The images were obtained using SEM (jsm-7800F, JEOL, Japan), and the SEM mapping was acquired using an energy-dispersive spectrometer in the same instrument. AFM analysis was conducted using an atomic force microscope (XE-70, Park systems, Korea). The XRD patterns of the samples were collected with a Smartlab (3 KW) X-ray powder diffractometer, and the Cu-K-α radiation wavelength was 0.154178 nm. Moreover, UV-vis spectra were collected using an ultraviolet spectrophotometer (PE Lambda 950, PerkinElmer, U.S.). TRPL analysis was conducted using a time-resolved fluorescence spectrometer (FLS 980, Edinburgh, UK). The XPS data were obtained using a K-alpha X-ray photoelectron spectrometer (PHI5000 Versaprobe, ULVAC-PHI, Japan). The pH of the solution was measured using Sartorius PB-10 (Sartorius, German).
Electrochemical measurements: An electrochemical workstation (CHI660E, Shanghai Chenhua, China) was used for electrochemical measurements. In the CO 2 RR characterization, the platinum electrode was used as the counter electrode, Ag/AgCl (saturated KCl) as the reference electrode, and the Cu-based electrode as the working electrode to form a three-electrode system. A proton exchange membrane (Nafion 117, Sigma-Aldrich, German) was inserted in the middle of electrolyte to ensure that only hydrogen ions could pass through the membrane ( Figure 17). Further, 100 mL of 0.1 mmol KHCO 3 solution was added to the reactor as the electrolyte with the remaining 150 mL of headspace volume. Before the reaction, high-purity carbon dioxide (99.99%) gas was vented into the electrolyte for 30 min to reach saturation. three-electrode system. A proton exchange membrane (Nafion 117, Sigma-Aldrich, German) was inserted in the middle of electrolyte to ensure that only hydrogen ions could pass through the membrane ( Figure 17). Further, 100 mL of 0.1 mmol KHCO3 solution was added to the reactor as the electrolyte with the remaining 150 mL of headspace volume. Before the reaction, high-purity carbon dioxide (99.99%) gas was vented into the electrolyte for 30 min to reach saturation. Identification and quantification of gaseous products: Gas chromatography (GC-9860-5C-NJ, Hao Erpu, China) was used to analyze the gas products, with argon (99.99%) as the carrier gas. A series of definite concentrations of gas were injected into the gas chromatograph (GC) to obtain the calibrated concentration of the gas products (H2, CO, CH4, C2H4, C2H6). Carbon-based gases and H2 were detected using a flame ionization detector and a thermal conductivity detector, respectively. Moreover, 1 mL of reactive gas was extracted each time and quickly injected into the GC for analysis.
Identification and quantification of liquid products: All the liquid products were quantified using a nuclear magnetic resonance spectrometer (JNM-ECZ400S/L1, JEOL, Japan). Different concentrations (0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4 mmol/L) of formic acid, methanol, acetic acid, and ethanol were prepared to obtain a correlation between the concentration and the peak intensity of the 1 H spectra. Specifically, 400 μL of the above solution was mixed with 200 μL of deuterated dimethyl sulfoxide (d-DMSO, Adamas, German) nuclear magnetic sample, and presaturation was used for water suppression during the NMR spectrum test. The standard curve was obtained considering the product concentration and NMR spectrum. The product concentration was presented along the abscissa and the NMR peak intensity along the ordinate, with the deuterated peak as consultation ( Figure  18). All the potentials were converted to relative potentials according to the RHE reference value: E(vs RHE) = E vs Ag AgCl + 0.197 V + 0.0592pH V. Identification and quantification of gaseous products: Gas chromatography (GC-9860-5C-NJ, Hao Erpu, China) was used to analyze the gas products, with argon (99.99%) as the carrier gas. A series of definite concentrations of gas were injected into the gas chromatograph (GC) to obtain the calibrated concentration of the gas products (H 2 , CO, CH 4 , C 2 H 4 , C 2 H 6 ). Carbon-based gases and H 2 were detected using a flame ionization detector and a thermal conductivity detector, respectively. Moreover, 1 mL of reactive gas was extracted each time and quickly injected into the GC for analysis.
Identification and quantification of liquid products: All the liquid products were quantified using a nuclear magnetic resonance spectrometer (JNM-ECZ400S/L1, JEOL, Japan). Different concentrations (0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4 mmol/L) of formic acid, methanol, acetic acid, and ethanol were prepared to obtain a correlation between the concentration and the peak intensity of the 1 H spectra. Specifically, 400 µL of the above solution was mixed with 200 µL of deuterated dimethyl sulfoxide (d-DMSO, Adamas, German) nuclear magnetic sample, and presaturation was used for water suppression during the NMR spectrum test. The standard curve was obtained considering the product concentration and NMR spectrum. The product concentration was presented along the abscissa and the NMR peak intensity along the ordinate, with the deuterated peak as consultation ( Figure 18). The FE for the formation of all the products (both gas and liquid products) was calculated as follows: / , where n is the total amount of product (in moles), e is the number of electrons transferred, NA is the Avogadro constant, q is the elementary charge, Q is the charge, I is the current, and t is the running time.
Computational method: We have employed the Vienna ab initio simulation package (VASP) [36,37] to perform all density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) [38] formulation. We have chosen the projected augmented wave (PAW) potentials [39,40] to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10 −6 eV. A geometry optimization was considered convergent when the force change was smaller than 0.03 eV/Å. Grimme's DFT-D3 methodology [41] was used to describe the dispersion interactions. The Brillourin zone was sampled with a gamma-centered grid 2 × 2 × 1 through all the computational process. Periodic boundary conditions were used in all directions and a vacuum layer of 15 Å was used in the z-direction to separate the slabs.
The adsorption energy (Eads) of adsorbate molecule was defined as / , where Emol/surf, Esurf and Emol (g) are the energy of adsorbate molecule adsorbed on the surface, the energy of clean surface, and the energy of isolated molecule in a cubic periodic box, respectively.

Conclusions
Cu-based electrodes were fabricated by a one-step electrodeposition method using MEA as a morphology and valence state agent. Moreover, the electrodes were used for The FE for the formation of all the products (both gas and liquid products) was calculated as follows: FE = n × e × N A × q Q = n × e × N A × q/(I × t), where n is the total amount of product (in moles), e is the number of electrons transferred, N A is the Avogadro constant, q is the elementary charge, Q is the charge, I is the current, and t is the running time.
Computational method: We have employed the Vienna ab initio simulation package (VASP) [36,37] to perform all density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) [38] formulation. We have chosen the projected augmented wave (PAW) potentials [39,40] to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10 −6 eV. A geometry optimization was considered convergent when the force change was smaller than 0.03 eV/Å. Grimme's DFT-D3 methodology [41] was used to describe the dispersion interactions. The Brillourin zone was sampled with a gamma-centered grid 2 × 2 × 1 through all the computational process. Periodic boundary conditions were used in all directions and a vacuum layer of 15 Å was used in the z-direction to separate the slabs.
The adsorption energy (Eads) of adsorbate molecule was defined as Eads = E mol/sur f − E sur f − E mol , where E mol/surf , E surf and E mol (g) are the energy of adsorbate molecule adsorbed on the surface, the energy of clean surface, and the energy of isolated molecule in a cubic periodic box, respectively.

Conclusions
Cu-based electrodes were fabricated by a one-step electrodeposition method using MEA as a morphology and valence state agent. Moreover, the electrodes were used for electrochemical CO 2 RR. The addition of MEA smoothened the surface of the Cu-based electrodes and simultaneously modulated the valence state of the Cu element. The Cu 0.6 electrode had a flat surface and a superior concentration ratio of Cu + and Cu 0 ions, which significantly improved the electrochemical efficiency and ethanol production in the CO 2 RR. For the Cu 0.6 electrode, the maximum FE of ethanol was up to 48% at a potential of −0.6 V vs. RHE, which is also the largest value for single-material catalysts. At the same voltage, the overall C 2 selectivity was above 77%. The flat surface significantly shortened the transfer time of electrons from the electrode to the reactive surface. At the same time, the Cu 0.6 electrode exhibited the optimal Cu + /Cu 0 concentration ratio of 1.42/1 compared to Cu 0 and Cu 1.2 , which can stabilize the *CO intermediates with the lowest free energy. The synergistic effect of Cu + and Cu 0 is advantageous for the C-C coupling formed by the dimerization of carbon intermediates. The possible mechanism is that the C atoms bonding to Cu + and Cu 0 have opposite electrical charges, and the C-C bond is favorably formed under the effect of the electric field, that is, due to the electrostatic attraction. This study provides an efficacious guide to fabricating efficient electrocatalysts, which has potential significance for product selectivity in the CO 2 RR.