English

• A series of problems such as the continuous overconsumption of fossil fuels, global warming, ocean acidification, and glacier melting have emphasized the importance and urgency of carbon neutralization by artificially converting carbon dioxide (CO₂) into chemical feedstock. Electrocatalytic CO₂ reduction (ECO₂R) is the most attractive carbon reduction method due to its advantages such as mild reaction conditions, controllable reaction rate and low cost [1-3]. Copper (Cu) has a wide distribution of reduction products and poor product selectivity due to its moderate binding strength to *CO reaction intermediates [4,5]. Hence, researchers have focused on preparing Cu-based catalysts with high product selectivity and stability in recent years [6-10]. In addition, since liquid products such as formic acid, methanol and ethanol dissolve in the cathode electrolyte during the generation process, they need to go through complicated separation and purification steps in practical applications, which will undoubtedly increase the cost. Hence, gas products will be more popular and economically valuable compared to liquid products [11].

  The ECO₂R reaction involves two key steps, the solvation transport of CO₂ and the reduction reaction. Consequently, the cathodic electrolyte, as the key site where the reaction occurs, will directly affect the selectivity and reactivity of products. Unfortunately, most current studies only focus on the development of advanced Cu-based catalysts, and there is no uniform standard for the selection of electrolyte types. More studies refer to the results reported by previous authors, and few studies focus on the effect of the electrolyte itself on the reduction reaction. Currently, KHCO₃ [12,13] and NaHCO₃ [14,15] are the two most commonly used buffered electrolytes for ECO₂R. While NaCl [16,17] and KCl [18,19] are the other two common unbuffered electrolytes. Even so, there is no uniform basis for the selection of electrolyte concentration. Hence, the study of electrolyte dependence is important to gain insight into the reduction behavior of CO₂ over Cu-based catalysts and to improve the selectivity of the reduction products.

  In this study, a systematic study of the electrolyte dependence of ECO₂R, including different cations, anions, electrolyte concentrations and potential dependence, was carried out using unmodified Cu foam (CF) as the working electrode. First, the effects of different cations (K⁺, Na⁺) and anions (Cl⁻, HCO₃⁻) on the selectivity of ECO₂R gas products were investigated. Then, the changes in actual pH and solution resistance of buffered (KHCO₃, NaHCO₃) and unbuffered solutions (KCl, NaCl) during prolonged ECO₂R were compared. Finally, the effects of different concentrations of electrolytes (0.1, 0.5 and 1.0 mol/L) on the ECO₂R reaction were explored. The results are expected to provide guidance for the design of ECO₂R electrolyte systems in the future.

  Fig. 1 shows the gas product selectivity and partial current densities of CF in four electrolytes (KCl and NaCl; KHCO₃ and NaHCO₃) at 0.5 mol/L. As shown in Fig. 1a, at lower potentials, the selectivity of CF for CO in KCl solution was much higher than that of NaCl. At −1.5 V vs. Ag/AgCl (Subsequent potentials are vs. Ag/AgCl unless otherwise noted), the CO FE (FE_CO) of CF was 18.5% in KCl solution and only 1.6% in NaCl solution. The FE_CO in KCl solution decreased rapidly with more negative potentials. As shown in Fig. 1b, at lower potentials, the CH₄ FE (FE_CH4) of CF in KCl solution is higher than that in NaCl. When the potential was negative than −1.8 V, FE_CH4 in NaCl solution increased rapidly and reached a maximum value of 26.1% at −2 V, corresponding to the CH₄ partial current density of −17.54 mA/cm². Generation of the gaseous C₂ product C₂H₄ involves a critical C-C coupling step. As shown in Fig. 1c, the C₂H₄ FE (FE_C2H4) of CF and the partial current density of this process in KCl solution were significantly higher than in NaCl at full potentials of −1.5 V ~ −2.1 V. FE_C2H4 was highest at 25.8% (−1.8 V) in KCl solution. While it was highest at 12.2% in NaCl solution and the reduction potential was more negative (−2 V). Since C-C coupling requires the consumption of a large amount of the key reaction intermediate *CO, the rapid decrease of FE_CO with increasing potential in KCl solution is observed in Fig. 1a. Hydrogen evolution reaction (HER) is a competitive side effect of ECO₂R, so it is desirable that HER be as weak as possible. As shown in Fig. 1d, significantly lower HER was observed in KCl solution, especially at lower reaction potentials (−1.5 V ~ −1.9 V). At a potential of −1.6 V, the H₂ FE (FE_H2) of CF in NaCl solution was 84.1%, which was 2.8 times higher than that in KCl (30.1%).

  Figure 1

  

  Figure 1.  Gas product selectivity and partial current density changes (effect of cations) of CF in 0.5 mol/L electrolyte at different potentials. KCl/NaCl solution: (a) FE_CO and j_CO, (b) FE_CH4 and j_CH4, (c) FE_C2H4 and j_C2H4, (d) FE_H2 and j_H2. KHCO₃/NaHCO₃ solution: (e) FE_CO and j_CO, (f) FE_CH4 and j_CH4, (g) FE_C2H4 and j_C2H4, (h) FE_H2 and j_H2.

  DownLoad: Full-Size Img PowerPoint

  It was observed in KCl and NaCl solutions that K⁺ promoted ECO₂R and especially contributed to the enhancement of FE_C2H4. It is due to the fact that potassium has less hydration energy than sodium, less hydronium ions are available for reduction with respect to sodium ion [20]. In addition, during ECO₂R, as the size of the alkali metal cation increases, the reaction overpotential of the CO₂ reduction product decreases and the partial current density at the same voltage is elevated, while the HER is suppressed [21]. Since the ionic radius of K⁺ is larger than that of Na⁺, the inhibition of HER in KCl solution is observed. Meanwhile, CO₂ is less soluble in NaCl solution than in KCl [22]. Consequently, NaCl solution promotes HER compared to KCl (Fig. 1d), which leads to a much lower efficiency of ECO₂R in NaCl solution compared to KCl. However, when the reaction systems were buffered KHCO₃ and NaHCO₃ (Figs. 1e–h), while Na⁺ was similarly observed to promote CH₄ generation at high reduction potentials, as shown in Fig. 1f, the FE_CH4 and partial current densities in the NaHCO₃ solution increased continuously. However, the difference in the selectivity of CO and C₂H₄ between the two buffer solutions was not significant. In addition, NaHCO₃, in contrast to KHCO₃, inhibited HER, contrary to what was observed in KCl and NaCl solutions. This may be related to the strong dipolar interaction between Na⁺ and adsorbed substances on the surface of the double-layer [23]. All the above reaction trends were also observed in 0.1 mol/L and 1.0 mol/L solutions (Figs. S1-S4 in Supporting information), but the electrolyte concentrations affected ECO₂R performance, this phenomenon will be analyzed in detail later. To sum up, when the anion of electrolytes is Cl⁻, K⁺ is more able to promote ECO₂R, especially to improve the selectivity to C₂H₄, and significantly inhibit HER compared to Na⁺. In contrast, when the anion of electrolytes is HCO₃⁻, Na⁺ is able to significantly improve the selectivity to CH₄ and inhibit HER compared to K⁺. Hence, metal cations cannot be singularly considered to promote or inhibit ECO₂R, requiring a combination of the buffering properties of the electrolyte and anionic effects. We next tested the effect of anions on the gas product selectivity of the electrolyte and the pH change of the solution during prolonged ECO₂R.

  Fig. 2 depicts the effect of anions (Cl⁻ and HCO₃⁻) in 0.5 mol/L electrolyte on the gas product selectivity and partial current density of ECO₂R at different reaction potentials. As shown in Figs. 2a–d, CF in KCl solution had significantly enhanced ECO₂R activity compared to KHCO₃, and its FE_CO, FE_CH4, FE_C2H4 and partial current densities were much higher than those in KHCO₃ solution in the range of −1.5 V ~ −2.1 V. FE_C2H4 in KCl solution reached >13% in all cases, while FE_C2H4 in KHCO₃ solution was almost zero. In addition, KCl solution also showed inhibitory effect on HER. The highest value of FE_H2 in KCl solution was 56.7% (at −2 V) in the range of −1.5 V ~ −2.1 V, which was much smaller than the lowest value of 73.9% (at −1.5 V) in KHCO₃ solution. Perfectly consistent results were observed in 0.1 mol/L and 1.0 mol/L electrolytes (Figs. S5 and S6 in Supporting information). The gas product selectivity and partial current density of ECO₂R in 0.5 mol/L NaCl and NaHCO₃ solutions were compared in Figs. 2e-h. The FE_CO of CF in NaCl solution showed a tendency to increase and then decrease, while it remained relatively stable in NaHCO₃ solution. When the gas products were CH₄ and C₂H₄, NaCl solution showed higher selectivity for both. When the reduction potential was −2 V, the FE_CH4 and partial current densities of CF in NaCl solution were 26.1% and −17.54 mA/cm², respectively, while in NaHCO₃ solution they were only 13.49% and −8.38 mA/cm². For HER, CF at low potentials (−1.5 V ~ −1.8 V) showed similar H₂ selectivity in NaCl and NaHCO₃ solutions. When the reaction potential was further increased, the HER in NaCl solution was inhibited to a certain extent. Comparing the reduction of CO₂ in 0.1 mol/L NaCl and NaHCO₃ solutions (Fig. S7 in Supporting information), it was found that the decrease in the electrolyte concentration affects the selectivity trend of the gas products, which will be focused later. Whereas, when the solution concentration was increased to 1.0 mol/L (Fig. S8 in Supporting information), the overall trend of the distribution of gas reduction products was similar to that in 0.5 mol/L electrolyte.

  Figure 2

  

  Figure 2.  Gas product selectivity and partial current density changes (effect of anions) of CF in 0.5 mol/L electrolyte at different potentials. KCl/KHCO₃ solution: (a) FE_CO and j_CO, (b) FE_CH4 and j_CH4, (c) FE_C2H4 and j_C2H4, (d) FE_H2 and j_H2. NaCl /NaHCO₃ solution: (e) FE_CO and j_CO, (f) FE_CH4 and j_CH4, (g) FE_C2H4 and j_C2H4, (h) FE_H2 and j_H2.

  DownLoad: Full-Size Img PowerPoint

  Combining Fig. 1 with Figs. S1–S8, the four electrolytes were more favorable for CH₄ generation when the cation was Na⁺ compared to K⁺, while the KCl solution had higher selectivity for C₂H₄. In addition, Cl⁻ in the electrolytes was able to increase the ECO₂R activity of CF and inhibit HER compared to HCO₃⁻. This is due to the fact that KHCO₃ and NaHCO₃ are buffer solutions, and the locally generated OH⁻ during ECO₂R can be neutralized by HCO₃⁻, resulting in a localized pH lower than that of the unbuffered KCl and NaCl solutions. The lower local pH of the HCO₃⁻ solution leads to a drastic decrease in C₂H₄ production and facilitates the HER to occur, since this reaction requires a large amount of H⁺ [24]. The presence of Cl⁻ inhibits HER even at high overpotentials, which is thought to be the result of Cl⁻ specific adsorption effect. Strongly adsorbed Cl⁻ promotes the transfer of electrons from the electrode to CO₂ and inhibits proton adsorption, leading to higher HER overpotentials [25,26]. In addition, it has been shown in various studies that halogen atoms can act as stabilizers for key reaction intermediates, thus indirectly increasing the catalytic activity of the electrode [27-29].

  The importance of pH for ECO₂R is mentioned above. We expect the overall pH of the electrolyte during ECO₂R to be in a relatively steady state, which can ensure the accuracy of the experiment and the stability of the overall device. Due to the buffering properties of HCO₃⁻, most ECO₂R studies are currently conducted in such electrolytes to ensure stable operation of the system for a long time. However, the ECO₂R process involves the generation and consumption of H⁺ and OH⁻, and the actual change of solution pH is more complicated. As shown in Fig. 3, continuous 2 h reduction at higher potentials (−2 V) was carried out in four solutions at 0.5 mol/L and the pH changes of the solutions during the process were monitored. The pH of the KHCO₃ and NaHCO₃ solutions in Figs. 3a and b remained relatively stable throughout. As shown in Table S1 (Supporting information), the pH changes of 0.5 mol/L KHCO₃ and NaHCO₃ solutions before and after the reaction were only 0.17 and 0.14, respectively, and both remained overall at the neutral condition of 7.5. In contrast, the pH of the unbuffered solutions KCl and NaCl increased rapidly at the beginning of the reaction (the first 30 min) (Figs. 3c and d). pH of the KCl solution increased from 3.79 to 6.23, and that of the NaCl solution from 4.95 to 6.38. Surprisingly, the pH of both electrolyte solutions remained relatively stable at around 6.7 in subsequent tests, which belonged to a neutral solution environment. This is consistent with the findings of Liu et al. [30]. The change in pH from weakly acidic to neutral is caused by a decrease in the concentration of H⁺. For the two buffered solutions, the reaction process can counteract and mitigate the effect of H⁺ or OH⁻ on the acidity or alkalinity of the solution, thus keeping the pH of the solution relatively stable. For the two non-buffered solutions, due to the large amount of CO₂ passed into the solution at the beginning of the reaction, a part of the CO₂ reacted with water to form H₂CO₃, which made the solution weakly acidic. Subsequently, as the ECO₂R reaction proceeded, the electron gaining reactions shown in Eqs. 1-3 may had been carried out in the solution to generate products such as CO, CH₄, and C₂H₄. Hence, the non-buffered solution reaction process is accompanied by a continuous consumption of H⁺, which leads to a rapid increase in the pH of the solution. In addition, the electron and proton transfer processes involved in ECO₂R generated OH⁻ in the KCl solution, and a small amount of CO₂ reacted with it to be further converted to HCO₃⁻, making the solution buffered. Hence, the solution is ultimately stabilized at near-neutral conditions.

  $ \mathrm{CO}_2+2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightleftharpoons \mathrm{CO}+\mathrm{H}_2 \mathrm{O} $

  (1)

  $ \mathrm{CO}_2+8 \mathrm{H}^{+}+8 \mathrm{e}^{-} \rightleftharpoons \mathrm{CH}_4+2 \mathrm{H}_2 \mathrm{O} $

  (2)

  $ \mathrm{CO}_2+12 \mathrm{H}^{+}+12 \mathrm{e}^{-} \rightleftharpoons \mathrm{C}_2 \mathrm{H}_4+4 \mathrm{H}_2 \mathrm{O} $

  (3)

  Figure 3

  

  Figure 3.  Changes of pH and current density in 0.5 mol/L electrolyte during ECO₂R. Electrolyte: (a) KHCO₃, (b) NaHCO₃, (c) KCl, and (d) NaCl.

  DownLoad: Full-Size Img PowerPoint

  KCl and NaCl solutions exhibited more stable I-t curves than KHCO₃ and NaHCO₃ solutions at large reduction potentials. The above results indicate that stable ECO₂R can be achieved in KCl and NaCl cathode electrolytes. It should be noted that large amounts of H₂ are generated from KHCO₃ and NaHCO₃ solutions due to buffering effects (Eqs. 4-9). In addition, when a large amount of CO₂ is passed into the system, the solution pH remains close to neutral and the HCO₃⁻ concentration is high, which also favors HER (Eqs. 4-6) [31]. This is consistent with the experimental results of this study.

  $ \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}^{+}+\mathrm{HCO}_3^{-} $

  (4)

  $ \mathrm{H}^{+}+\mathrm{HCO}_3^{-} \rightleftharpoons 2 \mathrm{H}^{+}+\mathrm{CO}_3^{2-} $

  (5)

  $ 2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_2 $

  (6)

  $ \mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} \rightleftharpoons{ }^* \mathrm{H}+\mathrm{OH}^{-} $

  (7)

  $ \mathrm{HCO}_3^{-}+\mathrm{e}^{-} \rightleftharpoons{ }^* \mathrm{H}+\mathrm{CO}_3^{2-} $

  (8)

  $ 2^{~ *} \mathrm{H} \rightarrow \mathrm{H}_2 $

  (9)

  The linear sweep voltammetry (LSV) curves of the four electrolytes under CO₂-saturated were further tested. As shown in Fig. S9 (Supporting information), when the concentrations were different, the activities of the four solutions were different, which was manifested by the different magnitudes of current densities of the electrolytes with different concentrations at the same potential. When the concentration was 0.1 mol/L, the activity of CF in the four solutions did not differ much. When the concentration was 0.5 mol/L, the trend of CF current density growth in KCl solution was the largest as the potential increased. In conjunction with the above, this reflected the superior ECO₂R performance of the electrode at this time. While when the concentration was increased to 1.0 mol/L, the current densities in KHCO₃ and NaHCO₃ solutions were the largest, combining with their actual ECO₂R performances, suggesting that the HER dominated at this time. In addition, the current densities at the same potential all increased significantly with increasing cathodic electrolyte concentration, which might be related to the different solution resistances of the electrolytes in H-cell. Hence, the variation of solution resistance during the reaction was further monitored (Fig. S10 in Supporting information). As shown in Fig. S10a and Table S2 (Supporting information), the initial resistances of the solutions were 17 Ω (KCl), 21 Ω (KHCO₃), 20.2 Ω (NaCl), and 22.5 Ω (NaHCO₃), respectively, when the concentration of the electrolyte was 0.1 mol/L. As the reaction proceeded, the solution resistance decreased rapidly and dropped by 33.5% (KCl), 42.9% (KHCO₃), 33.2% (NaCl) and 38.7% (NaHCO₃) after 2 h, respectively. Since a typical H-cell reactor was used in this work, the working electrode chamber and the counter electrode chamber were separated by a Nafion 117 proton exchange membrane to prevent the oxidation of the reduction products at the counter electrode. Hence, during the continuous catalytic process, K⁺ or Na⁺ in the anodic chamber will continuously diffuse across the membrane to the cathodic electrolyte [32], resulting in an increase in the solution conductivity and thus a continuous decrease in the solution resistance. However, the decrease in solution resistance was considerably limited when the concentration of the electrolyte was increased to 0.5 mol/L and 1.0 mol/L. The solution resistance remained essentially stable after a period of reaction. This is attributed to the fact that when the concentration of the electrolyte increases, its own conductivity increases dramatically, causing the initial solution resistance to decrease (Figs. S10b and c). In addition, since the concentration of conducting ions in the cathode electrolyte is much larger compared to the 0.1 mol/L electrolyte, the effect of subsequent K⁺ diffusion across the membrane on the solution conductivity will be much smaller. Since IR compensation (where R is the solution resistance of the cathode electrolyte and I is the working current) needs to be considered in ECO₂R experiments, this requires that the change of solution resistance must be monitored in real time for a certain period of time and the solution resistance value must be updated according to the results, otherwise, it is easy to cause potential overcompensation when the resistance drops. Considering that long-time catalytic tests (e.g., stability tests) often need to be kept continuous, the solution resistance should be kept as stable as possible. Hence, the electrolyte with higher concentration should be selected for ECO₂R experiments as much as possible to ensure the stability of the solution resistance during the test. However, it is worth noting that when the concentration of the electrolyte is too high, the attachment of salt on catalyst can also seriously affect its performance and stability. Consequently, the above factors should be considered for the selection of cathode electrolyte.

  From the above, it can be seen that different electrolyte concentrations not only change the FE of each gas product of ECO₂R, but also change the overall distribution trend of the gas products. Therefore, this study further compared the gas product selectivity of CF in electrolytes of different concentrations. Due to the gap in ionic conductivity of electrolytes with different concentrations, it leads to a huge difference in current density at same potential. Hence, for comparison, we compared the ECO₂R performance over the same current density range. As shown in Fig. 4, CF was more favorable for CO and CH₄ in 0.1 mol/L KCl solution, but its selectivity for C₂H₄ was much lower than that of high-concentration KCl solution. In addition, HER was more favorable to proceed in 0.1 mol/L KCl solution, which was detrimental to ECO₂R. CF exhibited relatively high C₂H₄ selectivity in both 0.5 mol/L and 1.0 mol/L KCl. However, compared to the lower FE_C2H4 in 1.0 mol/L KCl at low current densities, FE_C2H4 in 0.5 mol/L KCl was more stable over a wide current density interval and its HER was weaker. The gas product distribution of CF in the other three cathode electrolytes was further analyzed. As shown in Fig. S11 (Supporting information), CF exhibited superior gas product selectivity and weaker HER in 0.1 mol/L and 1.0 mol/L NaCl solutions when the current density was in the range of 0 ~ −45 mA/cm². However, when the current density was increased above −45 mA/cm², ECO₂R performance of CF in 0.5 mol/L NaCl solution was rapidly enhanced, mainly in terms of the high selectivity for CH₄ and C₂H₄. Its FE_CH4 exceeded up to 25% and FE_C2H4 could reach >10%. In Fig. S12 (Supporting information), 0.1 mol/L KHCO₃ showed higher selectivity for both CO, CH₄ and C₂H₄, and relatively better inhibition of HER, and the above results were especially shown at lower current densities. The difference in performance between 0.5 mol/L and 1.0 mol/L KHCO₃ was not significant, with HER dominating. Similar results were also reflected in the NaHCO₃ solution. As shown in Fig. S13 (Supporting information), over a wide range of current densities, CF in 0.1 mol/L NaHCO₃ solution had much higher concentrations of FE_CO, FE_CH4 and FE_C2H4 than the other two concentrations, as well as lower FE_H2. CF in 0.1 mol/L NaHCO₃ solution was even able to reach >25% of FE_CH4.

  Figure 4

  

  Figure 4.  Variation of gas product selectivity of CF with current density in different concentrations of KCl solution. (a) FE_CO, (b) FE_CH4, (c) FE_C2H4, (d) FE_H2.

  DownLoad: Full-Size Img PowerPoint

  Considering the effects of the aforementioned changes in solution resistance on the experiments, as well as the actual catalytic effect of CF at different electrolyte concentrations, we believe that it is more favorable to achieve high ECO₂R activity of the electrodes in KCl and NaCl solutions when the electrolyte concentration is 0.5 mol/L. In particular, it improves the selectivity to CH₄ or C₂H₄ and inhibits HER. In addition, for both buffer solutions, the test results indicate that the electrolyte concentration of 0.1 mol/L is more favorable for ECO₂R, while 0.5 mol/L and 1.0 mol/L are more favorable for HER. This may be due to the fact that the high concentration of HCO₃⁻ leads to an increase in the pH of solutions, resulting in the evolution of carbonate and bicarbonate crystals on the surface of the electrodes, which can seriously affect the ECO₂R performance of the catalysts and improve the HER.

  In this work, the effects of common electrolyte types (KCl, NaCl, KHCO₃ and NaHCO₃) and different concentrations (0.1, 0.5, and 1.0 mol/L) of ECO₂R on the CF surface were systematically investigated. The results showed that KCl and NaCl solutions were able to inhibit HER significantly compared to KHCO₃ and NaHCO₃ solutions. the KCl solution favored the generation of C₂H₄, while the NaCl and NaHCO₃ solutions containing Na⁺ were highly selective for CH₄. In addition, the selectivity of KCl and NaCl solutions for C₂H₄ and CH₄ was optimized at 25.8% (−1.8 V) and 26.1% (−2.0 V) when the electrolyte concentration was 0.5 mol/L, respectively. In contrast, CF had ECO₂R activity only in low concentrations of KHCO₃ and NaHCO₃ solutions (0.1 mol/L). When concentrations were increased, the HER dominated. Meanwhile, we found that CF can also maintain a relatively stable neutral pH after reacting in KCl and NaCl solutions for a period of time as in buffer solutions, resulting in a stable ECO₂R. In addition, considering the necessity of keeping the solution resistance stable for long-time catalytic tests, the electrolyte with higher concentration should be selected as much as possible in ECO₂R experiments. This work is expected to provide some reference basis for the selection of electrolyte system for subsequent ECO₂R. However, it should be emphasized that all the results of this study are based on unmodified CF as the working electrode. Considering the numerous types of Cu-based catalysts, including alloyed, single-atom, and porous structures, etc., the reduction properties of different catalysts in the above solutions may differ from the results of this study.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Zekun Zhang: Writing – review & editing, Methodology, Conceptualization. Shiji Li: Software, Methodology, Investigation, Conceptualization. Qian Zhang: Writing – review & editing, Methodology, Conceptualization. Shanshan Li: Writing – review & editing, Supervision, Formal analysis. Liu Yang: Resources, Methodology, Investigation. Wei Yan: Writing – review & editing, Supervision, Conceptualization. Hao Xu: Writing – review & editing, Validation, Supervision, Conceptualization.

  Acknowledgments

  The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (No. 52270078) and the Fundamental Research Funds for the Central Universities (No. xzy022023039), as well as the instrumental support from Instrumental Analysis Center of Xi'an Jiaotong University.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110742.

  
  
• 1. [1]

     Z.K. Zhang, X.S. Jing, H. Xu, Z.C. Li, W. Yan, Chin. J. Anal. Chem. 51 (2023) 316–330.

  2. [2]

     B.Q. Miao, W.S. Fang, B. Sun, et al., Chin. J. Struct. Chem. 42 (2023) 100095.

  3. [3]

     Z.K. Zhang, S.J. Li, Q. Zhang, et al., Small 21 (2025) 2409001. doi: 10.1002/smll.202409001

  4. [4]

     A.A. Peterson, J.K. Nørskov, J. Phys. Chem. Lett. 3 (2012) 251–258. doi: 10.1021/jz201461p

  5. [5]

     M.H. Jing, H.Z. Wang, M.F. Zhu, et al., Chem. Soc. Rev. 53 (2024) 5149–5189. doi: 10.1039/D3CS00857F

  6. [6]

     Z.K. Zhang, S.J. Li, Y.F. Rao, et al., Chem. Eng. J. 479 (2024) 147376. doi: 10.1016/j.cej.2023.147376

  7. [7]

     N. Sakamoto, K. Sekizawa, S. Shirai, et al., Nat. Catal. 7 (2024) 574–584. doi: 10.1038/s41929-024-01147-y

  8. [8]

     C.J. Chen, L.S. Huang, Y.L. Jiang, Y. Zheng, S.Z. Qiao, Nano Energy 126 (2024) 109656. doi: 10.1016/j.nanoen.2024.109656

  9. [9]

     Z.Z. Liu, L. Song, X.M. Lv, et al., J. Am. Chem. Soc. 146 (2024) 14260–14266. doi: 10.1021/jacs.4c03830

  10. [10]

      J.Q. Feng, L.M. Wu, X.N. Song, et al., Nat. Commun. 15 (2024) 4821. doi: 10.1038/s41467-024-49308-8

  11. [11]

      A.Q. Chen, B.L. Lin, Joule 2 (2018) 594–606. doi: 10.1016/j.joule.2018.02.003

  12. [12]

      Y. Lu, H.J. Wang, P.F. Yu, et al., Nano Energy 77 (2020) 105158. doi: 10.1016/j.nanoen.2020.105158

  13. [13]

      L. Jia, M.Z. Sun, J. Xu, et al., Angew. Chem. Int. Ed. 60 (2021) 21741–21745. doi: 10.1002/anie.202109288

  14. [14]

      R. Hegner, L.F.M. Rosa, F. Harnisch, Appl. Catal. B Environ. 238 (2018) 546–556. doi: 10.1016/j.apcatb.2018.07.030

  15. [15]

      B.B. Pan, X.R. Zhu, Y.L. Wu, et al., Adv. Sci. 7 (2020) 2001002. doi: 10.1002/advs.202001002

  16. [16]

      X.Y. Tang, C. Yu, X.D. Song, et al., Adv. Energy Mater. 11 (2021) 2100075. doi: 10.1002/aenm.202100075

  17. [17]

      F.J. Quan, D. Zhong, H.C. Song, F.L. Jia, L.Z. Zhang, J. Mater. Chem. A 3 (2015) 16409–16413. doi: 10.1039/C5TA04102C

  18. [18]

      X. Zhang, J.C. Li, Y.Y. Li, et al., J. Am. Chem. Soc. 143 (2021) 3245–3255. doi: 10.1021/jacs.0c13427

  19. [19]

      C. Zhu, G.F. Wu, A.H. Chen, et al., Energy Environ. Sci. 17 (2024) 510–517. doi: 10.1039/d3ee02867d

  20. [20]

      S. Ringe, E.L. Clark, J. Resasco, et al., Energy Environ. Sci. 12 (2019) 3609–3610. doi: 10.1039/c9ee90056j

  21. [21]

      B.B. Pan, Y.H. Wang, Y.G. Li, Chem Catal. 2 (2022) 1267–1276.

  22. [22]

      Y.H. Liu, M.Q. Hou, G.Y. Yang, B.X. Han, J. Supercrit. Fluid. 56 (2011) 125–129. doi: 10.1002/9781118095249.ch11

  23. [23]

      M.R. Thorson, K.I. Siil, P.J.A. Kenis, J. Electrochem. Soc. 160 (2013) F69. doi: 10.1149/2.052301jes

  24. [24]

      Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc. Faraday Trans. 1 85 (1989) 2309–2326. doi: 10.1039/f19898502309

  25. [25]

      K. Ogura, J.R. Ferrell, A.V. Cugini, E.S. Smotkin, M.D. Salazar-Villalpando, Electrochim. Acta 56 (2010) 381–386. doi: 10.1016/j.electacta.2010.08.065

  26. [26]

      Y.C. Hsieh, S.D. Senanayake, Y. Zhang, W. Xu, D.E. Polyansky, ACS Catal. 5 (2015) 5349–5356. doi: 10.1021/acscatal.5b01235

  27. [27]

      L. Hao, L. Kang, H.W. Huang, et al., Adv. Mater. 31 (2019) 1900546. doi: 10.1002/adma.201900546

  28. [28]

      T. Wang, J.D. Chen, X.Y. Ren, et al., Angew. Chem. Int. Ed. 62 (2023) e202211174. doi: 10.1002/anie.202211174

  29. [29]

      Q.J. Wu, D.H. Si, Q. Wu, et al., Angew. Chem. Int. Ed. 62 (2023) e202215687. doi: 10.1002/anie.202215687

  30. [30]

      W. Liu, P.B. Zhai, A.W. Li, et al., Nat. Commun. 13 (2022) 1877. doi: 10.1038/s41467-022-29428-9

  31. [31]

      P.K. Jiwanti, Y. Einaga, Chem.: Asian J. 15 (2020) 910–914. doi: 10.1002/asia.201901669

  32. [32]

      A.R. Heenan, J. Hamonnet, A.T. Marshall, ACS Energy Lett. 7 (2022) 2357–2361. doi: 10.1021/acsenergylett.2c00800

• 

  Figure 1  Gas product selectivity and partial current density changes (effect of cations) of CF in 0.5 mol/L electrolyte at different potentials. KCl/NaCl solution: (a) FE_CO and j_CO, (b) FE_CH4 and j_CH4, (c) FE_C2H4 and j_C2H4, (d) FE_H2 and j_H2. KHCO₃/NaHCO₃ solution: (e) FE_CO and j_CO, (f) FE_CH4 and j_CH4, (g) FE_C2H4 and j_C2H4, (h) FE_H2 and j_H2.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Gas product selectivity and partial current density changes (effect of anions) of CF in 0.5 mol/L electrolyte at different potentials. KCl/KHCO₃ solution: (a) FE_CO and j_CO, (b) FE_CH4 and j_CH4, (c) FE_C2H4 and j_C2H4, (d) FE_H2 and j_H2. NaCl /NaHCO₃ solution: (e) FE_CO and j_CO, (f) FE_CH4 and j_CH4, (g) FE_C2H4 and j_C2H4, (h) FE_H2 and j_H2.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Changes of pH and current density in 0.5 mol/L electrolyte during ECO₂R. Electrolyte: (a) KHCO₃, (b) NaHCO₃, (c) KCl, and (d) NaCl.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Variation of gas product selectivity of CF with current density in different concentrations of KCl solution. (a) FE_CO, (b) FE_CH4, (c) FE_C2H4, (d) FE_H2.

  下载: 全尺寸图片 幻灯片
•