Abstract Temperature affects both the thermodynamics of intermediate adsorption and the kinetics of elementary reactions. Despite its extensive study in thermocatalysis, temperature effect is typically overlooked in electrocatalysis. This study investigates how electrolyte temperature influences CO2 electroreduction over Cu catalysts. Theoretical calculations reveal the significant impact of temperature on *CO and *H intermediate adsorption thermodynamics, water microenvironment at the electrode surface, and the electron density and covalent property of the C–O bond in the *CH–COH intermediate, crucial for the reaction pathways. The theoretical calculations are strongly verified by experimental results over different Cu catalysts. Faradaic efficiency (FE) toward multicarbon (C2+) products is favored at low temperatures. Cu nanorod electrode could achieve a FEC2+ value of 90.1% with a current density of ~ 400 mA cm− 2 at − 3°C. FEC2H4 and FEC2H5OH show opposite trends with decreasing temperature. The FEC2H4/FEC2H5OH ratio can decrease from 1.86 at 40°C to 0.98 at − 3°C. Introduction Electrochemical CO2 reduction reaction (CO2RR) into high-value products stands as one of the most promising strategies for mitigating CO2 emissions through the utilization of renewable electricity1–2. CO2RR is a complex process involving multiple reaction pathways that harvest a diverse array of chemical products3–4. However, the simultaneous occurrence of various CO2RR routes alongside the hydrogen evolution reaction (HER) can diminish the selectivity toward desired products5–8. The adsorption behavior of carbonous intermediates and the intricate water microenvironment at the electrode surface are pivotal factors for influencing these reaction pathways, thereby dictating the distribution of products9–12. By far, researchers have developed a wide range of electrode materials and electrolytes tailored to finely control intermediate adsorption and the water microenvironment on the electrode surface13–16. These advancements hold significant promise for steering the CO2RR pathway toward desired product with enhanced efficiency and selectivity. The adsorption or dispersion of intermediates, as well as the water microenvironment, are significantly influenced by temperature since they are thermodynamically controlled17–19. For instance, both C2H4 and C2H5OH share the same precursor *CH–COH, leading to their simultaneous production20–23. The kinetics of their distinct reduction pathways can be influenced by temperature, offering a feasible means to control the ratio of C2H4 to C2H5OH. Hence, adjusting the temperature of the electrolyte to regulate both thermodynamic and kinetics processes emerges as a potent method for steering the CO2RR pathway. Consequently, a comprehensive investigation into the relationship between performance and temperature is crucial, providing invaluable insights and guiding significance for optimizing CO2RR performance4, 24. CO2RR experiments are typically conducted at room temperature, which can vary, for example from − 3°C to 40°C, depending on seasons and regions. The environmental temperature, typically indicated by the electrolyte temperature, can significantly influence the performance of CO2RR, yet it is often ignored in CO2RR studies25–28. In this study, we systematically investigated the impact of temperature on CO2RR performance. We initiated our study with theoretical calculations, including density functional theory (DFT) and molecular dynamics (MD) simulations, to explore the impact of temperature on intermediate adsorption and kinetics of elementary reactions in CO2RR. Subsequently, Cu catalysts were synthesized and employed for CO2RR at various temperatures. The theoretical findings aligned well with experimental observations, indicating that lower temperatures favor C2+ production and promote the formation of C2H5OH over C2H4. For instance, a Faradaic efficiency toward multicarbon products (FEC2+) of 90.1% was achieved with a current density of ~ 400 mA cm− 2 at − 1.3 V vs RHE over a Cu nanorod (Cu-NR) electrode at − 3°C. Moreover, the FEC2H4/FEC2H5OH ratio decreases gradually from 1.86 to 0.98 in 1 M KOH as the temperature decreases from 40°C to − 3°C. Further characterizations, including in situ surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), in situ Raman spectroscopy and electrochemical analysis, provide a comprehensive understanding of the temperature effect on CO2RR performance.
The addition of salts in solution of sorbitol-based surfactant (SGCTB) containing open-chain sugar as headgroup and two hydrophobic tails, enhances intermolecular hydrogen bonding in the hydrophilic polyhydroxy spacer, promoting the gelation.
Abstract Tuning the selectivity of CO 2 electroreduction reaction (CO 2 RR) solely by changing electrolyte is a very attractive topic. In this study, we conducted CO 2 RR in different aqueous electrolytes over bulk metal electrodes. It was discovered that controlled CO 2 RR could be achieved by modulating cations in the electrochemical double layer. Specifically, ionic liquid cations in the electrolyte significantly inhibits the hydrogen evolution reaction (HER), while yielding high Faraday efficiencies toward CO (FE CO ) or formate (FE formate ) depending on the alkali metal cations. For example, the product could be switched from CO (FE CO =97.3 %) to formate (FE formate =93.5 %) by changing the electrolyte from 0.1 M KBr‐0.5 M 1‐octyl‐3‐methylimidazolium bromide (OmimBr) to 0.1 M CsBr‐0.5 M OmimBr aqueous solutions over pristine Cu foil electrode. In situ spectroscopy and theoretical calculations reveal that the ordered structure generated by the assembly of Omim + under an applied negative potential alters the hydrogen bonding structure of the interfacial water, thereby inhibiting the HER. The difference in selectivity in the presence of different cations is attributed to the hydrogen bonding effect caused by Omim + , which alters the solvated structure of the alkali metal cations and thus affects the stabilization of intermediates of different pathways.
Open AccessCCS ChemistryCOMMUNICATIONS1 Mar 2024Tuning the Surface Field by Embedding Cations into Metals to Direct the Reaction Pathway of CO2 Electroreduction Pei Zhang, Meng Zhou, Shiqiang Liu, Xueqing Xing, Jiahao Yang, Peng Chen, Yaoyu Yin, Yingying Cheng, Xing Tong, Jun Ma, Qinggong Zhu, Xiaofu Sun, ZhongJun Chen, Xinchen Kang and Buxing Han Pei Zhang Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Meng Zhou Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Shiqiang Liu Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Xueqing Xing Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 , Jiahao Yang Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Peng Chen Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemistry, University of Chinese Academy of Sciences, Beijing 100049 , Yaoyu Yin Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemistry, University of Chinese Academy of Sciences, Beijing 100049 , Yingying Cheng Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Xing Tong Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemistry, University of Chinese Academy of Sciences, Beijing 100049 , Jun Ma Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Qinggong Zhu Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemistry, University of Chinese Academy of Sciences, Beijing 100049 , Xiaofu Sun Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemistry, University of Chinese Academy of Sciences, Beijing 100049 , ZhongJun Chen Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 , Xinchen Kang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemistry, University of Chinese Academy of Sciences, Beijing 100049 and Buxing Han *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Center for Carbon Neutral Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 School of Chemistry, University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.023.202303394 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The creation of universal strategies to affect the reaction route of the electroreduction of CO2 is critical. Here, we report the first work to introduce cations into diverse metals such as Cu, Bi, In, and Sn via the electroreduction of related metallic oxides in quaternary ammonium surfactant solutions. Compared to their physical adsorption, cations embedded into the electrodes have a more pronounced impact on the electrical field, which effectively influences the adsorption state of intermediates. With the increase of surface field, the hydrogen evolution reaction and *COOH route are significantly reduced, favouring the *OCHO pathway instead. As a result, hydrogen, CO, and C2+ products almost completely vanish at −0.5 V versus RHE in 0.1 M Na2SO4 in an H-type cell after enough cations are embedded into the Cu electrode, and the faradaic efficiency of formate rises from 18.0% to 99.5% simultaneously. Download figure Download PowerPoint Introduction The electrocatalytic CO2 reduction reaction (CO2RR) in aqueous electrolyte provides an environmentally benign route for converting CO2 into renewable fuels and feedstocks.1–3 By far, catalysts have shown excellent performance for CO2RR in aqueous electrolytes.4–7 Although complicated design and modification of electrode materials help to generate exclusive carbonous products and suppress the hydrogen evolution reaction (HER),8,9 the catalytic efficiency and selectivity of this method are still far from satisfactory. Furthermore, the lack of universality of most reported methods makes it difficult to provide explicit guidance for materials design in CO2RR. Therefore, proposing universal methodologies for materials design to further direct the reaction pathway of CO2RR is of great significance. The formation of *COOH (the CO path) and *OCHO (the formate path) represents the first electron and proton transfer step of CO2RR, and it is generally recognised that the binding of intermediates on catalysts determines the reaction pathway.10,11 Apart from CO and formate, the CO intermediate undergoes further reduction to form products which require more than two electrons to transfer.12 Most metal electrodes are capable of generating formate and CO simultaneously, along with the intense competition of HER in aqueous electrolyte,13–16 rendering a low faradaic efficiency (FE) for a given product. Thus, proposing a universal methodology is an efficient way to steer the selectivity of carbonous products and suppress HER.17–19 Cations in the electrolyte play significant roles during CO2RR by altering the electrical field, local pH, interfacial water, and so on.20,21 Among these, the electrical field generated by interfacial cations has shown a particularly significant impact on the reaction route of CO2RR.22–24 Drawing inspiration from the cation effect, we are aiming to realise direct introduction of cations into electrodes since cations in electrodes generate stronger electrostatic interactions than those in the electrolyte due to the shorter distance. Herein, we designed a strategy to prepare cation-embedded metal electrodes via electroreduction of related metal oxides in quaternary ammonium surfactant solutions. Compared with pristine electrodes, cation-embedded electrodes demonstrate an improved ability to simultaneously produce formate and suppress HER. The current density and FEformate are 42.9 mA cm−2 and ∼100%, respectively, over a cation-embedded Cu electrode at −0.5 V versus reversible hydrogen electrode (RHE) in 0.1 M Na2SO4 in an H-type cell, which is much higher than most reported results in the literature under the same conditions. In contrast, dominant hydrogen and various carbonous products are generated over a Cu electrode without decoration of cations. We clearly observed that the cationic charges on the electrode direct the reaction route. A related mechanism was confirmed by control experiments in combination with zeta-potential tests, in situ small angle X-ray scattering (SAXS)-X-ray absorption spectroscopy (XAS), theoretical simulation, and so on. Results and Discussion Preparation and characterisations of Cu electrodes Cu-foam with porous and bulky structure was firstly calcinated in air at 800 °C for 24 h to obtain CuO-foam ( Supporting Information Figure S1).25 Compared with Cu-foam, the resulting CuO-foam maintains its original porous structure but has a rough surface composed of particles ranging from several micrometres to nanometres in size, as shown by scanning electron microscopy (SEM) images ( Supporting Information Figures S2 and S3). Subsequently, the calcinated CuO-foam was electroreduced in various aqueous electrolytes, including 100 mM Na2SO4, 1 mM cetyltrimethylammonium bromide (CTAB), 10 mM CTAB, 50 mM CTAB, and 100 mM CTAB at −1.9 V versus RHE, respectively (Figure 1a). The corresponding electrodes (Cu-X) obtained are denoted as Cu-Na2SO4, Cu-CTAB-1, Cu-CTAB-10, Cu-CTAB-50, and Cu-CTAB-100, respectively. All crystal structures of as-prepared Cu-X electrodes are identical to that of pristine Cu-foam as confirmed by X-ray diffraction (XRD) patterns. All Cu-X materials exhibit the cubic structure, and Cu(111) is the main crystal plane (Figure 1b). Figure 1 | Preparation and structural characterisations of Cu-X electrodes. (a) Schematic diagram for the preparation. (b) Powder XRD patterns. (c,d) HAADF-STEM images of Cu-Na2SO4 (c) and Cu-CTAB-100 (d). (e, f) SAED images of Cu-Na2SO4 (e) and Cu-CTAB-100 (f). The scale bars in c, d, e, f are 20 nm, 20 nm, 5 nm−1, and 5 nm−1, respectively. Download figure Download PowerPoint Further investigations were conducted to examine the fine structures of Cu-Na2SO4 and Cu-CTAB-100. Cu-X electrodes exhibit a porous structure with microstructure evolving on the walls ( Supporting Information Figures S4 and S5). Furthermore, the lack of interface in Cu-X electrodes can be observed through the side-view SEM images ( Supporting Information Figure S6). The high-angle annular dark-field (HAADF) images obtained from scanning transmission electron microscopy (STEM) indicate that the fine surface morphologies of Cu-Na2SO4 and Cu-CTAB-100 are similar, as shown in Figure 1c,d. The selected area electron diffraction (SAED) illustrates the same polycrystalline structure of Cu-foam-Na2SO4 and Cu-foam-CTAB (Figure 1e,f). Therefore, it can be concluded that the structure and morphology are hardly affected by electrolytes during the reduction of CuO-foam. Electrochemical reduction of CO2 CO2RR was carried out over various Cu electrodes in 0.1 M Na2SO4 aqueous solution. All electrolysis was conducted in an H-type cell, which is more suitable for mechanism study. Electrolysis over different electrodes at potentials ranging from −0.1 to −0.7 V versus RHE were conducted for 5 h. All liquid and gaseous products were quantitatively analysed by 1H NMR spectroscopy and gas chromatography, respectively. Cu-X electrodes show much higher current density compared with original Cu-foam (Figure 2a). Although similar current density over Cu-X electrodes is obtained, the selectivity of products is totally different (Figure 2b). The Cu-Na2SO4 electrode achieves a current density of 39.6 mA cm−2 at −0.5 V versus RHE with various products, including hydrogen, formate, CO, ethanol, acetic acid, and propanol. The FEformate and the FEH2 are 18.0% and 75.3%, respectively. Cu-CTAB-1, prepared in low-concentration CTAB solution, exhibits a product distribution similar to Cu-Na2SO4. The production of formate is gradually boosted while HER, and the production of other carbonous products is obviously suppressed on going from Cu-CTAB-1 to Cu-CTAB-100 (Figure 2c). Particularly, Cu-CTAB-100 obtains a FEformate of 99.5% with a current density of 42.9 mA cm−2 at −0.5 V versus RHE. Control experiments in N2-saturated electrolyte show undetectable carbonous products, validating the fact that these products are specifically derived from CO2. Besides, Cu-Na2SO4 and Cu-CTAB-100 were used for CO2RR in different electrolytes ( Supporting Information Table S1), which shows that the electrolyte used does not influence the product distribution. Figure 2 | Comparisons of electrochemical properties of different electrodes. (a) Plots of current density versus potential of CO2 electrolysis. (b) Plots of FEformate versus potential of CO2 electrolysis. (c) FE of different products at −0.5 V versus RHE. (d) Plots of current density (curve) and FEformate (dots) versus time at −0.5 V versus RHE. (e) Nyquist plots. (f) Tafel plots. The lines and dots in d correspond to the current density and FEformate over different electrodes, respectively. Download figure Download PowerPoint Figure 2d shows that the current density and FEformate over all electrodes exhibit indistinct change after 48 h of electrolysis, suggesting excellent stability. As revealed from the Nyquist plots at an open circuit potential in Figure 2e, Cu-foam, Cu-Na2SO4, and Cu-CTAB-100 electrodes exhibit charge transfer resistances (Rct) of approximately 68.1, 3.7, and 2.7 Ω cm2, respectively. The Tafel slopes over Cu-foam, Cu-Na2SO4, and Cu-CTAB-100 electrodes are 143, 139, and 86 mV dec−1, respectively (Figure 2f). The lower Rct value and Tafel slope indicate that Cu-CTAB-100 is endowed with excellent capabilities for CO2 activation and charge transfer. Charge characterisation of different Cu electrodes Under normal conditions, as is typical, the amphipathic CTAB can adsorb on a metal surface through electrostatic adsorption to form a bilayer structure.26,27 In this way, cetyltrimethylammonium cation (CTA+) cations are first electrostatically adsorbed on the metal surface covered with primarily adsorbed bromide ions. Differently, under negative potential, CTA+ cations can feasibly assemble onto the cathode surface directly, while Br− ions depart from the cathode during the electroreduction in CTAB solution, as illustrated in Supporting Information Figure S7. The zeta-potential test shows a significant increase in positive charge on Cu-CTAB (36.1 mV) compared with Cu-Na2SO4 (5.9 mV). In addition, Fourier transform infrared spectroscopy (FTIR) confirms the existence of CTA+ in Cu-CTAB-100 (Figure 3a), consistent with the occurrence of the peak from the N 1s X-ray photoelectron spectroscopy (XPS) spectra ( Supporting Information Figure S8) and the even distribution of C and N from elemental distribution mappings ( Supporting Information Figure S9). As expected, Br− anion is not detected in either XPS or elemental distribution mappings. Interestingly, Cu-Na2SO4 and Cu-CTAB-100 exhibit indistinctive discrepancy in the surface wettability as illustrated by contact angle measurements ( Supporting Information Figure S10). The similar wettability indicates that the concentration of embedded CTA+ is too low to produce the hydrophobic effect, which is very different from the conventional adsorption of CTAB on a surface.17 Moreover, the value of the zeta potential becomes increasingly positive on going from Cu-CTAB-1 to CTAB-100 (Figure 3b). Thereby, the embedding of a small amount of CTA+ cations in this manner significantly enhances the electropositivity of the electrode surface. However, Cu-CTAB-100 shows a slightly higher zeta potential than Cu-CTAB-50, indicating that zeta potential does not increase evidently when the CTAB concentration increases to a certain extent. After Cu-CTAB-100 was sonicated in water for 30 min, no CTAB was detected in solution by FTIR or 1H NMR, indicating that CTAB is embedded into Cu-CTAB-100 rather than being physically adsorbed on it. Figure 3 | Characterisations of Cu-Na2SO4 and Cu-CTAB-100 and their formation process. (a) FTIR spectra of Cu-Na2SO4 and Cu-CTAB-100. (b) Zeta potentials of Cu-X. (c, d) In situ SAXS curves during the electroreduction of CuO-foam in 100 mM Na2SO4 (c) and 100 mM CTAB (d) at −1.9 V versus RHE. (e, f) In situ EXAFS spectra in R-space during the electroreduction of CuO-foam in 100 mM Na2SO4 (e) and 100 mM CTAB (f) at −1.9 V versus RHE. Download figure Download PowerPoint In situ SAXS-XAS In situ SAXS-XAS experiments were carried out to understand the mechanism of charge formation over Cu-X electrodes during electroreduction of CuO-foam in 100 mM Na2SO4 and 100 mM CTAB.28 Change of SAXS curves generated in Na2SO4 is negligible but very evident in CTAB (Figure 3c,d). The ever-increasing scattering intensity of SAXS curves during the electroreduction in CTAB implies that CTA+ cations are continuously adsorbed on the surface of CuO-foam (Figure 3d). The Fourier transform curves at the Cu K-edge of extended X-ray adsorption fine structure (EXAFS) spectra were further fitted to investigate the environment of Cu–O during reduction of CuO. Peaks at ∼1.5 and ∼2.2 Å (Figure 3e,f) in R-space from k3-weighted Fourier transform of EXAFS spectra refer to Cu–Cu and Cu–O bonds, respectively. The gradually increased intensity of Cu–Cu bonds and decreased intensity of Cu–O bonds suggest the reduction of CuO to Cu. The length of the Cu–O bond is steadily prolonged during the reduction in the CTAB solution but remains unchanged in Na2SO4 solution, confirming that the CTA+ cations have very strong interaction with the oxygen of the Cu–O bond at negative potential. The interaction between CTA+ and oxygen decreases the rate for the fracture of the Cu–O bond and reduction of Cu2+ ( Supporting Information Figure S11). Normally, it is feasible for long-chain surfactant to be adsorbed on the surface of materials to assemble into a bilayer, including CTA+ cations and Br− anions, which can also feasibly disassemble in aqueous solution.29,30 However, under the electrical field, CTA+ cations are easily adsorbed onto the CuO surface and participate in the transformation of Cu–O to Cu–Cu. Due to the strong interaction between CTA+ and oxygen, it is feasible for CTA+ cations to be entrained in the gap of Cu grains upon completion of the reconstruction, which form a stable embedding of CTA+ in Cu nanocrystals ( Supporting Information Figure S7). Thus, Cu-CTAB-100 with obviously increased zeta potential is obtained after washing it thoroughly, as illustrated in Figure 3b. The composition, morphology, crystal structure, and facets of electrodes are all impact factors for determining the selectivity of CO2 electroreduction.31,32 Cu-Na2SO4 and Cu-CTAB-100 electrodes exhibit similar current density but very different selectivity towards products (Figure 2a–c). Considering that the structure and morphology of Cu-Na2SO4 and Cu-CTAB-100 are similar, the surface charge of electrodes is the only visible difference between Cu-Na2SO4 and Cu-CTAB-100. Therefore, it can be speculated that the charge characteristic on the surface plays a significant role in determining the reaction pathway. Stability of the surface charge of electrodes Maintaining the high efficiency of electrodes for CO2RR depends on the stability of the surface charge. The zeta potential of Cu-CTAB-100 changes imperceptibly after being used five times ( Supporting Information Figure S12), indicating that the high stability of CTA+ on the surface of Cu-CTAB-100. CTA+ cations are entrained into the gap of Cu nanoparticles instead of being physically adsorbed on their surface during the reconstruction of CuO-foam into Cu in CTAB solution, which results in the stable embedding. Correspondingly, current density is nearly unchanged, and FEformate remains ∼100% over Cu-CTAB-100 after five cycles of electrolysis, displaying excellent stability ( Supporting Information Figure S13). Furthermore, for comparison, Cu-Na2SO4-CTAB is prepared by immersing Cu-Na2SO4 into 100 mM CTAB to verify the effect of physically adsorbed CTAB ( Supporting Information Figure S14). A zeta-potential value of 35 mV, close to that of Cu-CTAB-100, is obtained, signifying the formation of an adsorbed CTAB layer on the surface of the Cu electrode ( Supporting Information Figure S15a). The surface amount of CTAB on Cu-Na2SO4-CTAB is much higher than that on Cu-CTAB-100, as illustrated by the FTIR spectra ( Supporting Information Figure S15b). FEformate over Cu-Na2SO4-CTAB is 84.5% at −0.5 V versus RHE for the first cycle, much higher than that over Cu-Na2SO4 ( Supporting Information Figure S16). However, FEformate over Cu-Na2SO4-CTAB decreases notably after further cycles, and the zeta potential of the electrode follows a similar trend ( Supporting Information Figures S16 and S17), illustrating the desorption of CTAB from Cu-Na2SO4-CTAB during CO2RR. As CTAB is physically adsorbed on the surface of the Cu-Na2SO4-CTAB electrode, the interaction between CTAB and the electrode surface is weak. The control experiment confirms the significant impact of surface charge on the reaction pathway. Embedding of different cationic cations into Cu electrode More Cu-X electrodes were prepared by electroreduction of CuO-foam in various 100 mM alkyltrimethylammonium bromide (ATAB) solutions, including tetramethylammonium bromide (TMAB), butyltrimethylammonium bromide (BTAB), octyltrimethylammonium bromide (OTAB), and CTAB. This shows that the alkyl chain length of ATAB obviously affects the amount of positive charge on the surface of Cu-X electrodes. We observed that ATAB with long alkyl chains can be effectively embedded into the surface of Cu-X electrodes, whereas ATAB with a short alkyl chain is not conducive to the cationic decoration ( Supporting Information Figure S18).33 Consequently, Cu-X electrodes with small zeta-potential values were obtained when CuO-foam was electroreduced in ATAB solutions with short alkyl chains (such as TMAB, BTAB, and OTAB). As a result, FEformate is lower, and HER is more serious over Cu-X electrodes with smaller zeta-potential value ( Supporting Information Figure S19). However, total current densities over these electrodes exhibit negligible difference ( Supporting Information Figure S20). These results further verify the crucial role of surface charge on promoting formate selectivity and suppressing HER. Universality of charge control strategy To further verify the influence of surface charge on the reaction route of CO2RR, other metals such as In, Bi, and Sn, which mainly produce formate with competing HER, were chosen.34–36 In2O3, Bi2O3, and SnO were loaded onto carbon paper (CP) to prepare metal (M) oxide/CP electrodes, which were then electroreduced in 100 mM Na2SO4 and 100 mM CTAB to form M-Na2SO4/CP and M-CTAB-100/CP, respectively. XRD patterns and SEM images confirm that M-Na2SO4/CP and M-CTAB-100/CP exhibit identical structure and similar morphology ( Supporting Information Figures S21–S32). As expected, M-CTAB-100/CP shows significantly higher zeta-potential value than M-Na2SO4/CP ( Supporting Information Figure S33). This proves that positive charge can be effectively embedded into electrodes using this strategy. Subsequently, M-Na2SO4/CP and M-CTAB-100/CP electrodes were applied in CO2RR. Although current densities generated by M-Na2SO4/CP and M-CTAB-100/CP are similar, FEformate over all M-CTAB-100/CP electrodes is evidently larger than that over M-Na2SO4/CP, indicating that positively charged electrodes are indeed powerful in generating formate and suppressing HER ( Supporting Information Table S2). As an example, FEformate over Bi-Na2SO4/CP and Bi-CTAB-100/CP are 38.7% and 98.1%, respectively. Despite the improvement of FEformate, the overpotentials over these as-prepared CP-supported electrodes are much larger than that of Cu-CTAB-100 in the case of improving FEformate, which also elucidates that the substrate-free Cu-CTAB-100 electrode is obviously effective for CO2RR. Comparing with Cu electrodes investigated in this study and those reported in the literature, the overall catalytic performance of Cu-CTAB-100 in generating formate in aqueous solution is among the best results to date ( Supporting Information Table S3).37–40 Charge effect on CO2RR The adsorption of *COOH (the CO path), *OCHO (the formate path), and *H (the HER path) steers the product selectivity. As oxygen is the negative charge centre, it can be easily adsorbed by the positively charged electrode surface. Therefore, high FEformate is feasible to realise over a positively charged electrode. To confirm this speculation, density functional theory (DFT) simulations were further conducted. The catalyst models of Cu(111) without or with cation modification were constructed. Pristine Cu(111) and cation-decorated Cu(111) represent Cu-Na2SO4 and Cu-CTAB-100, respectively. A tetramethyl ammonium cation model was employed to represent CTA+ for simulation. Cations around an electrode produce a strong electrical field, and the intensity is influenced by the position of cations. The electrical field intensity increases as the distance between the cation and the Cu surface decreases (Figure 4a). Since cations are embedded into the electrode rather than physically adsorbed on the surface of the electrode, the electrical field over decorated Cu(111) is very strong. In order to study the surface field effect on the selectivity of CO2RR, the relation between the binding energy of different intermediates and the field was interrogated by field-corrected DFT calculations on Cu(111) (Figure 4b). Au and Sn were used as descriptors for product preference as reported since they are on the peak positions of CO and formate volcanos, respectively.41 At the low surface field, Cu(111) shows adsorption energies of *COOH and *OCHO that are far from the optimal Au and Sn references. Meanwhile, the ΔG*H is very close to 0 eV that benefits HER.42,43 Thus, HER is dominant, and various carbonous products are also generated over the Cu(111) electrode of the low surface field. Figure 4 | Mechanism for the electroreduction of CO2 over different electrodes. (a) Electrical field intensity as a function of distance between Cu(111) and cations. (b) The adsorption energy of intermediates at different electrical fields on Cu(111). (c, d) DFT calculations of Gibbs free energy of electroreduction of CO2 over pristine Cu(111) and decorated Cu(111). (e, f) Schematic diagrams for electroreduction of CO2 over pristine electrode (e) and cation-doped electrode (f). Download figure Download PowerPoint The surface field increases as the cations gradually approach the electrode surface (Figure 4a). With the surface field increasing, the adsorption of *OCHO on Cu(111) gradually shifts towards the optimal strength of the Sn reference, while the adsorption of *COOH deviates from the optimal value of Au reference. Accordingly, the continuously increased field produced by the embedding of cations leads to the increased selectivity of formate against CO on Cu(111). At the high surface field, ΔG*H is far from 0 eV that results in suppressed HER. In addition, the embedding of cations enhances the electron density on Cu(111) that favours CO2RR ( Supporting Information Figure S34). Further, Gibbs free energy with respect to potential reaction steps was modelled. The plausible reaction pathways via *COOH and *OCHO were both considered over these pristine and decorated Cu(111) electrodes. The formation of *OCHO requires lower Gibbs free energy than that of *COOH at pristine Cu(111). However, the further reduction of *OCHO to *HCOOH is more difficult than that of *COOH to *CO (Figure 4c). *CO is the