Investigating Why Sulfurization Can Greatly Improve Ethanol Selectivity for Carbon Dioxide Electroreduction

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE3 Oct 2022Investigating Why Sulfurization Can Greatly Improve Ethanol Selectivity for Carbon Dioxide Electroreduction Qian-Yu Wang†, Yu-Hang Li†, Yu Zhao†, Yu-Yu Chen, Bi-Jun Geng, Rong-Kai Ye, Qiong Liu, Xiao-Qing Liu, Ye-Xiang Tong, Yue-Jiao Zhang, Jun Cheng, Ping-Ping Fang, Jian-Qiang Hu, Jian-Feng Li and Zhong-Qun Tian Qian-Yu Wang† MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Laboratory of Low-Carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 Department Key Laboratory of Fuel Cell Technology of Guangdong Province, Nanobiological Medicine Center, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640 , Yu-Hang Li† MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Laboratory of Low-Carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Yu Zhao† State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, College of Energy, Xiamen University, Xiamen 361005 , Yu-Yu Chen Department Key Laboratory of Fuel Cell Technology of Guangdong Province, Nanobiological Medicine Center, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640 , Bi-Jun Geng Department Key Laboratory of Fuel Cell Technology of Guangdong Province, Nanobiological Medicine Center, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640 , Rong-Kai Ye Department Key Laboratory of Fuel Cell Technology of Guangdong Province, Nanobiological Medicine Center, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640 , Qiong Liu MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Laboratory of Low-Carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Xiao-Qing Liu MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Laboratory of Low-Carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Ye-Xiang Tong MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Laboratory of Low-Carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Yue-Jiao Zhang State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, College of Energy, Xiamen University, Xiamen 361005 , Jun Cheng State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, College of Energy, Xiamen University, Xiamen 361005 , Ping-Ping Fang *Corresponding authors: E-mail Address: [email protected] E-mail Address: jq[email protected] E-mail Address: [email protected] MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Laboratory of Low-Carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Jian-Qiang Hu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department Key Laboratory of Fuel Cell Technology of Guangdong Province, Nanobiological Medicine Center, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640 , Jian-Feng Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, College of Energy, Xiamen University, Xiamen 361005 and Zhong-Qun Tian State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, College of Energy, Xiamen University, Xiamen 361005 https://doi.org/10.31635/ccschem.021.202101557 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Carbon dioxide (CO2) electroreduction of products using renewable electricity sources is a promising avenue for chemical energy storage that is attracting increasing attention. Considerable effort has been focused on improving the activity and selectivity for CO2 electroreduction of C2+ products, especially for ethanol. However, studies of this process's underlying mechanism are still lacking. Herein, we use operando Raman spectroscopy combined with density functional theory (DFT) calculations to reveal mechanisms that explain why sulfurization can greatly improve ethanol selectivity for CO2 electroreduction. We found that the Faradaic efficiency (FE) of ethanol increased from 15% to 55% when the Ag–Au–Ag nanorods (NRs) were sulfurized to a Au–Ag–S NRs structure, which is an increase of over three times. Operando electrochemical surface-enhanced Raman spectroscopy empowered by 12C/13C isotope exchange finds that intermediates of CO2− and CO on the sulfurized Au–Ag–S NRs can greatly facilitate CO–CO coupling and improve ethanol selectivity. DFT calculations show that the Au–Ag–S NR structure, compared with the Ag–Au–Ag NRs, can lower the energy barrier for CO–CO coupling combined with a proton, greatly facilitating the ethanol formation. Therefore, sulfurization is an effective way to improve ethanol selectivity for CO2 electroreduction that can provide an important strategy to improve the selectivity for other reactions. Download figure Download PowerPoint Introduction Electrochemically converting carbon dioxide (CO2) to liquid fuels is an efficient way to solve the environmental problem of rising levels of atmospheric CO2.1 Considerable effort has been focused on finding efficient catalysts capable of improving the activity and selectivity of CO2 electroreduction to liquid products using renewable electricity, especially for converting CO2 to C2 products, such as ethanol (CH3CH2OH).2 Although various catalysts have been synthesized, the catalytic activity, selectivity, conversion rate, and mechanisms still need systematic studies before CO2 electroreduction can become a viable option for storing renewable electricity.3,4 Selectivity is especially important for CO2 electroreduction; however, the mechanism studies of the selectivity are difficult and lack efficient techniques. Therefore, finding efficient methods to understand the mechanisms and guide high selective electrocatalysts design, while challenging, is urgently needed. Many techniques have been developed to investigate the mechanisms for CO2 electroreduction. X-ray photoelectron spectroscopy (XPS),5 surface-enhanced infrared adsorption spectroscopy,6,7 electrochemical surface-enhanced Raman spectroscopy (EC-SERS),8 and others have been used for the mechanism studies for CO2 electroreduction.9,10 EC-SERS, a powerful technique for molecular fingerprint recognition that is not influenced by water, is a good choice to study the intermediates during CO2 electroreduction and facilitate finding an explanation of the mechanisms. Thus, EC-SERS is an efficient way to study the intermediates during CO2 electroreduction, especially operando EC-SERS, with a borrowing strategy developed by depositing the target catalyst on the surface of the SERS active substrate.11,12 Chernyshova et al.8 have shown the first intermediate of the CO2 conversion to formate on copper is a carboxylate anion *CO2− using operando EC-SERS. Currently copper is a widely used catalyst for CO2 electroduction, and the mechanisms have been widely investigated using the above-mentioned techniques.5,6,11 Although such reactions occur on the copper surface, the selectivity is also involved with oxygen, and oxygen-bearing copper can in fact boost \selectivity.13,14 The question, whether sulfur, which is in the same group as oxygen, can boost selectivity, remains unanswered. Herein, we used operando EC-SERS empowered by 12C/13C isotope exchange to study how the sulfurization of the Ag–Au–Ag nanorods (NRs) varied the intermediates and finally steered the pathway toward ethanol during the CO2 electroreduction. The ethanol Faradaic efficiency (FE) was increased from 15% to 55% after the Ag–Au–Ag NRs were sulfurized to Au–Ag–S NRs for CO2 electroreduction at −0.95 V. Operando EC-SERS combined with 12C/13C isotope exchange measurements found that intermediates, such as CO2− and CO, steered the reaction pathway toward ethanol production, and they could only be observed on the sulfurized Au–Ag–S NRs structure, which facilitated the CO formation and CO–CO coupling, and ultimately improved the ethanol selectivity. Density functional theory (DFT) calculations combined with experimental results revealed that the sulfurized Au–Ag–S NRs could improve the CO–CO coupling and lower the energy barrier for the coupled CO combined with a proton on the surface to form *COCOH. Compared with the Ag–Au–Ag NRs, the Au–Ag–S NRs provided a more feasible pathway with a lower overpotential for the production of ethanol, and thus improved the selectivity and catalytic activity for CO2 electroreduction to ethanol. Experimental Methods Synthesis of Ag–Au–Ag NRs The Ag–Au–Ag NR structure was formed by growing Ag NRs along each side of the Au nanodecahedron core ( Supporting Information Figure S1). In a typical synthesis of Ag–Au–Ag NRs, 420.0 μL of HAuCl4 (48.56 mM) aqueous solution, 640.0 mg cetyltrimethylammonium chloride (CTAC), and 100.0 mg poly(vinyl pyrrolidone) (PVP) were introduced into 10.0 mL water in a 30.0 mL Teflon-lined stainless-steel autoclave and stirred for 10 min at room temperature and ambient conditions. Then 0.8 mL of AgNO3 (100 mM) aqueous solution was added and stirred for another 5 min. The autoclave was then charged with N2 to 1.0 MPa and moved into an oil bath at 225 °C under magnetic stirring. After 24 h, the autoclave was taken out of the oil bath and immersed into cold water to cool it down. The final product was collected by centrifugation at 10,000 rpm for 5 min, and washed for three times with pure water to remove any residual CTAC or PVP. Synthesis of Au–Ag–S NRs 3 mL of the as-prepared Ag–Au–Ag NRs solution was stirred continuously, and 6 μL of 0.025 M Na2S aqueous solution was added to it (the atomic molar ratio of Ag:S was 400:1) for 2 h to form the sulfurized Au–Ag–S NRs. The product was then centrifuged at 10,000 rpm for 15 min and washed three times with pure water before use. Results and Discussion Synthesis and characterization The well-defined and high-yield (> 90%) Au–Ag–S NRs were synthesized through sulfurizing the Ag–Au–Ag NRs with Na2S (Figures 1a and 1b). The transmission electron microscopy (TEM) images are shown in Figure 1. The Ag–Au–Ag NRs were synthesized by a one-step high-pressure hydrothermal method. The surface of Au nanodecahedrons was capped by Ag although it was only slightly capped in the longitudinal direction ( Supporting Information Figure S1). Through this sulfurization reaction, the Ag–Au–Ag NRs were sulfurized to Au–Ag–S NRs (Figures 1b and 1c). The two-fringe lattice spacing of 0.172, 0.204, and 0.203 nm in the Au–Ag–S NRs correspond to Ag2S (220), Ag (200), and Au (200) planes, respectively (Figure 1d). Energy-dispersive X-ray (EDX) mapping shows that the sulfur element was distributed uniformly on the surface of the Ag–Au–Ag NRs (Figures 1d–1g). The surface Ag was sulfurized while Au was not sulfurized ( Supporting Information Figure S2). After sulfurization, the binding energy increased for Ag while it decreased for Au, which means that the Ag lost electrons while the Au gained them ( Supporting Information Figure S2). This indicates that the sulfurization (i.e., the formation of Ag2S nanostructures) occurred only on the outer surface of the Ag–Au–Ag NRs. Figure 1 | TEM images of (a) Ag–Au–Ag NRs, (b) Au–Ag–S NRs, and (c) a single Au–Ag–S NR. (d) High-resolution TEM of Au–Ag–S NRs, (e–g) EDX mapping of Au–Ag–S NRs. Download figure Download PowerPoint The catalytic activity and selectivity of CO2 electroreduction High activity and selectivity of CO2 electroreduction We investigated the catalytic activity of the Ag–Au–Ag and Au–Ag–S NRs for CO2 electroreduction systematically. The cyclic voltammogram (CV) curves are shown in Figure 2a, and the current density in the CO2-saturated 0.5 M KHCO3 solution had higher catalytic activity than that in the Ar-saturated solution, indicating enhanced CO2 electroreduction catalytic activity in the CO2-saturated solution. The catalytic activity for the Au–Ag–S NRs was higher than that of the Ag–Au–Ag NRs when shifting from Ar to CO2-saturated solution, indicating a higher CO2 elecreoduction efficiency. The reduction peak at about −0.2 V is due to oxygen adsorption/desorption on the Ag surface from Ag–Au–Ag NRs (Figure 2a).15 Oxygen adsorption/desorption on Ag occurred on the Au–Ag–S NRs (Figure 2a), indicating both Ag and Ag2S surfaces were exposed to the electrolyte. Linear sweep voltammetry (LSV) curves show that the Au–Ag–S NRs exhibit a 20 mV increase in the onset potential at 5 mA mg−1 compared with the Ag–Au–Ag NRs (Figure 2b). Therefore, the Au–Ag–S NRs exhibited higher electrocatalytic activity toward CO2 electroreduction than the Ag–Au–Ag NRs. Figure 2 | CV and LSV curves of Ag–Au–Ag and Au–Ag–S NRs in a CO2-saturated 0.5 M KHCO3 solution. Download figure Download PowerPoint In addition to enhanced catalytic activity, the high selectivity of the Au–Ag–S NRs as well as different CO2 electroreduction products were further studied. Of the CO2 electroreduction products, the liquid products were the most interesting and were quantified in the liquid phase using 1H NMR. The main liquid products detected by 1H NMR were ethanol, methanol, and formic acid ( Supporting Information Figure S3). The catalytic selectivity was then systematically investigated, and the alcohol FEs for the Ag–Au–Ag and Au–Ag–S NRs at different potentials are shown in Figures 3a and 3b. The highest catalytic activity and selectivity for methanol was achieved on Ag–Au–Ag NRs at −0.85 V while the highest catalytic activity and selectivity for ethanol was achieved on Au–Ag–S NRs at −0.95 V. Generally, the Ag–Au–Ag NRs exhibited a relatively higher methanol FE while the Au–Ag–S NRs exhibited a relatively higher ethanol FE (Figures 3a and 3b). The highest methanol FE of 58% was achieved on the Ag–Au–Ag NRs at −0.95 V while the corresponding ethanol FE was 15%. In comparison, the highest ethanol FE of 55% was achieved on the Au–Ag–S NRs at −0.95 V while the corresponding methanol FE was 40% (Figures 3a and 3b). Therefore, we mainly focused on the products in the liquid phase ( Supporting Information Figure S3). The ethanol FE increased from 15% for the Ag–Au–Ag NRs (Figure 3a) to 55% for the Au–Ag–S NRs (Figure 3b) at −0.95 V, which is greater than a three-fold increase. The Ag–Au–Ag NRs convert CO2 to methanol more efficiently while the Au–Ag–S NRs convert CO2 to ethanol more efficiently. Therefore, the sulfurization can greatly improve the ethanol selectivity for CO2 electroreduction. Figure 3 | Methanol and ethanol FE on (a) Ag–Au–Ag and (b) Au–Ag–S NRs at various applied potentials after CO2 electroreduction reaction for 2 h. Average methanol and ethanol production rates obtained from CO2 reduction on (c) Ag–Au–Ag and (d) Au–Ag–S NRs at different potentials. Download figure Download PowerPoint In addition to their high selectivity, Au–Ag–S NRs also exhibited high catalytic activity and stability. The Ag–Au–Ag NRs exhibited reaction rates of 400 μmol h−1 mg−1 for methanol at −0.85 V (Figure 3c). The Au–Ag–S NRs exhibited reaction rates of 390 μmol h−1 mg−1 for methanol and 250 μmol h−1 mg−1 for ethanol at −0.95 V (Figure 3d), achieving a total alcohol reaction rate of 640 μmol h−1 mg−1. Furthermore, the Au–Ag–S NRs exhibited good chemical stability ( Supporting Information Figure S4), as i-t curves were controlled by the diffusion, and the current density was usually lower than that in the LSV curves (Figure 2b). Therefore, such Au–Ag–S NRs successfully maintained high catalytic activity, selectivity, and stability for CO2 electroreduction to alcohols. What should be noted is that the ethanol FE of the Au–Ag–S NRs at −0.95 V is high compared to the Cu- and Ag-based catalysts in the literature.11,16–20 Normally, Cu is used for CO2 electroreduction, even though its selectivity is relatively low. Many kinds of products are obtained from CO2 electroreduction, and oxidation of Cu is needed to get ethanol. Different from Cu, Ag can achieve high selectivity of nearly 100% for CO and depress hydrogen evolution, which means it can improve selectivity. Despite the challenges of obtaining ethanol on Ag during CO2 electroreduction, we were able to achieve the milestone of achieving the sulfurized Ag which can greatly improve ethanol selectivity for CO2 electroreduction. Similar to the way that the oxidation of Cu improves the selectivity of ethanol, sulfurization is another important way to change the electronic structure of Ag to improve ethanol selectivity for CO2 electroreduction by Ag-based materials. Mechanisms using operando EC-SERS to probe the intermediates In light of our findings, we wanted to investigate why such Au–Ag–S NRs are so efficient in the conversion of CO2 to ethanol. Operando SERS experiments were carried out at different potentials to reveal the reaction intermediates during CO2 electroreduction on the surface of Au–Ag–S NRs (Figure 4a). To date, carbonate adsorption on catalysts during CO2 electroreduction has been poorly understood. We used operando SERS empowered by 13C/12C isotope exchange to prove the intermediates of CO32− and *CO2− (Figure 4a and Supporting Information Figure S5). We first explored the initial potential for CO2 electroreduction at open circuit potential, and found that the 686 and 1061 cm−1 peaks were in the characteristic frequency range of the CO32− vibration of carbonate ( Supporting Information Figure S5).8,21 These vibrations were the centrosymmetric movements from the oxygen atoms with respect to the central carbon atoms of the CO32−, which was negligibly affected by the 13C/12C isotope exchange ( Supporting Information Figure S6).8 The CO32− was detected on the Au–Ag–S NRs in the KHCO3 solution due to chemisorption.8 The CO32− was formed by activation of CO2 on the surface oxygen/OH− site and then maintained the equilibrium between CO32− and *CO2−.8 Figure 4 | Operando SERS spectra of CO2 electroreduction on the surfaces of Au–Ag–S NRs (a) and Ag–Au–Ag NRs (b) at different potentials in 0.5 M KHCO3. Download figure Download PowerPoint The antisymmetrical stretching frequency of *CO2– (νas CO2–) was reported to be at about 1515–1545 cm−1 on Cu.8 Since the frequency of *CO2– can be shifted on different substrates and at different charge densities,22,23 the peak at about 1508 cm−1 was the *CO2–,8,20 which was adsorbed strongly and contained a significantly negative charge on the Au–Ag–S NRs surface. The frequency shifted from 1508 to 1466 cm−1 when the 12C changed into 13C, which further proved the existence of such a νasCO2− vibrational mode ( Supporting Information Figure S6). The weak peaks at about 1330 and 700 cm−1 (the shoulder peak near 686 cm−1) in Figure 4a were associated with symmetric stretching of νasCO2− and in-plane bending *CO2− vibrations, respectively.8 Therefore, the CO2 electroreduction on the Au–Ag–S NRs at the beginning was performed through the *CO2– intermediate.24 The first step for CO2 electroreduction is the activation of the CO2 molecule.8,25 It is commonly believed that the activation and electroreduction of CO2 is difficult, either because the first electron transfer to form the *CO2– radical intermediate needs a very negative redox potential or because CO2 is too stable. But this was not the case here. Electrocatalysts stabilized the *CO2– intermediate by forming a chemical bond between CO2 and the electrocatalysts, leading to a less negative redox potential. With the right electrocatalysts, it is possible to reduce CO2 to *CO2–, COOH, or CO at low overpotential. This electrocatalyst is very important for intermediate formation at the low overpotential. By sulfrization, the *CO2– forming potential on the Au–Ag–S NRs was 0.45 V at the open circute potential (Figure 4a), the overpotential of which is very low. With the decrease of the potential, the *CO2− can be reduced to CO (2085 cm−1) at 0.35 V (Figue 4a), the overpotential of which is also very low, indicating the importantce of the right electrocatalyst after sulfurization. Therefore, the sulfurization of the catalysts can greatly lower the overpotential for CO2 electroreduction to different intermediates and ultimately improve the activity and selectivity. The formation of CO on the Au–Ag–S NRs was proven by the strong peak observed around 2085 cm−1 corresponding to the atop CO (Figure 4a).11,26 The atop CO on the catalysts indicated a high surface coverage of *CO facilitating the CO–CO coupling, which was further electroreduced to C2 products.27,28 After the CO–CO coupling, the peaks coresponding to the C–H streching of methanol or ethanol appeared at about 2700–2900 cm−1 on the Au–Ag–S NRs, indicating such coupled CO are further hydrogenized to alcohols (Figure 4a). Also, more peaks appeared on the Au–Ag–S NRs, indicating the formation of ethanol ( Supporting Information Figure S7). The peaks at about 2700–2900 cm−1 were much stronger on the Au–Ag–S NRs, indcating a much higher catalytic activity. When the potential was decreased to −0.55 V, the peak at 1441 cm−1 occurred, indicating the appearance of the intermediate *CH2 (Figure 4a),29,30 which was ultimately electroreduced to alcohol (Figure 4a and Supporting Information Figure S7). Therefore, the operando EC-SERS has efficiently found the most important intermediate of *CO and *CH2 to form ethanol during CO2 electroreduction on the Au–Ag–S NRs. To explain why the Au–Ag–S NRs exhibited higher ethanol selectivity toward CO2 electroreduction than the Ag–Au–Ag NRs, operando SERS during CO2 electroreduction at different potentials on the Ag–Au–Ag NRs was further investigated to make comparisons. At the beginning, both CO32− and *CO2− were formed on the Au–Ag–S NRs (Figure 4a); however, these two active intermediates did not appear on the Ag–Au–Ag NRs (Figure 4b). The formation of the CO32− and *CO2− can greatly facilitate CO2 electroreduction and improve activity,8 which is why the Au–Ag–S NRs exhibited higher catalytic activity than the Au–Ag–S NRs (Figures 2 and 3). With the decrease of the potential, the peak that appeared at 1757 cm−1 on the Ag–Au–Ag NRs corresponded to formic acid (Figure 4b and Supporting Information Figure S8).31 What should be noted is that atop CO (2085 cm−1) was detected on the Au–Ag–S NRs (Figure 4a), but no obvious CO was detected on the Ag–Au–Ag NRs (Figure 4b).31 The local enrichment of the CO intermediate can suppress the competing hydrogen evolution reaction while enhancing the selectivity toward C2+ products though enhanced CO–CO coupling, which, in most cases, promotes ethanol production.32–36 Therefore, the CO intermediate on the Au–Ag–S NRs forms the CO–CO coupling much more easily than on the Ag–Au–Ag NRs, which ultimately facilitates the ethanol formation. This is why the ethanol selectivity toward the CO2 electroreduction on the Au–Ag–S NRs is much higher than on the Ag–Au–Ag NRs. The operando SERS experiments show that sulfurization can greatly enhance the CO–CO coupling, which will ultimately facilitate the formation of ethanol. Operando EC-SERS empowered by 13C/12C isotope exchange proved that the sulfurization can greatly facilitate the formation of *CO2– and CO, which will transform to the CO dimer and finally improve the ethanol selectivity. Experimental37 and theoretical38 studies have suggested a CO–CO coupling intermediate is the key species in the C2 pathway.25,37 Here, the results show that CO can only be observed on the sulfurized Au–Ag–S NRs, which indicates that sulfurization is the critical condition to obtain CO and then form the CO-based dimer on the Ag–Au–Ag surface. The formation of the CO-based dimers greatly facilitates the formation of ethanol. Recent studies by Koper and coworkers39 show that CO can be reduced to C2 on the Ag surface, indicating that CO is the key species to form C2 products for CO2 electroreduction. Therefore, the CO mechanism explains why the sulfurized Au–Ag–S NRs exhibit higher ethanol selectivity compared with the Ag–Au–Ag NRs. DFT calculations to explain the mechanisms To further screen and determine the intermediates involved in CO2 reduction on the NRs, we calculated the most stable adsorbed molecular configurations for each step by DFT calculations according to Nørskov and co-workers'40 ( Supporting Information Figure The of the NR are proved by EDX and ( Supporting Information Figure The results are shown in Figure while the energy for the intermediates on the Ag–Au–Ag and Au–Ag–S NRs are shown in Figures and the Ag–Au–Ag NRs the potential step from the first *CO2– was from adsorbed CO2 and then *CO2– could with a proton and form in the water solution to form the intermediates in Figure This is also proven by the operando SERS in Figure 4b that *CO2– can be Therefore, the DFT calculations further proved that this step is on the Au–Ag–S NRs than on the Ag–Au–Ag NRs. Figure 5 | (a) energy evolution for CO2 reduction to ethanol on the surface of the Ag–Au–Ag and Au–Ag–S NRs. intermediates on the surface of (b) Ag–Au–Ag and (c) Au–Ag–S NRs. Download figure Download PowerPoint However, with the of sulfur in the Au–Ag–S NRs in Figure the first step was no the and the of strong *CO2– is also shown by the operando SERS in Figure the energy due to from CO–CO coupling combined with a proton in the of adsorbed *COCOH. CO–CO coupling was on the Au–Ag–S NRs because the energy barrier for the CO formation and CO–CO was lower on the Au–Ag–S NRs than on the Ag–Au–Ag NRs (Figure 4a), which, combined with a proton in the of adsorbed further facilitated CO–CO the energy for Au–Ag–S NRs was lower than that of Ag–Au–Ag NRs that the Au–Ag–S NRs could provide a more feasible pathway to ethanol. Therefore, Au, and Ag2S have to the ethanol selectivity, and the possible active be the of Au, and Ag2S in the Au–Ag–S NRs Sulfurization greatly changed the formation pathway of *CO2– and CO, and then further changed the CO–CO coupling and the which facilitated the formation of ethanol. we can that DFT calculations combined with the operando EC-SERS explain very well why the sulfurization improves the selectivity for CO2 to ethanol. Operando Raman spectroscopy combined with DFT calculations revealed why sulfurization can greatly improve ethanol selectivity for CO2 electroreduction. Operando EC-SERS empowered by 13C/12C isotope exchange proved that the intermediates of CO, and *CH2 on the Au–Ag–S NRs after sulfurization during CO2 electroreduction. The formation of atop CO can greatly facilitate the CO–CO coupling to CO which ultimately improves the ethanol selectivity. The sulfurization of Ag–Au–Ag NRs was found to significantly improve the ethanol FE from 15% to 55% at −0.95 V during CO2 electroreduction, which is an increase of over three times. DFT calculations proved that the energy for of the intermediate during CO–CO coupling combined with a proton on the surface of Au–Ag–S NRs was lower than that on the Ag–Au–Ag NRs, that the Au–Ag–S NRs a more feasible pathway for ethanol. Therefore, operando EC-SERS combined with DFT calculations very well why such a sulfurized Au–Ag–S NRs structure exhibited high ethanol selectivity. This mechanism study for catalysts and catalytic with high

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