Title: Insight into the Effect of Metal Cations in the Electrolyte on Performance for Electrocatalytic CO <sub>2</sub> Reduction Reaction
Abstract: ENERGY & ENVIRONMENTAL MATERIALSEarly View HighlightFree Access Insight into the Effect of Metal Cations in the Electrolyte on Performance for Electrocatalytic CO2 Reduction Reaction Junjun Li, Junjun Li Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072 ChinaSearch for more papers by this authorZhicheng Zhang, Corresponding Author Zhicheng Zhang [email protected] orcid.org/0000-0002-2487-4250 Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072 ChinaSearch for more papers by this authorWenping Hu, Wenping Hu Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072 ChinaSearch for more papers by this author Junjun Li, Junjun Li Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072 ChinaSearch for more papers by this authorZhicheng Zhang, Corresponding Author Zhicheng Zhang [email protected] orcid.org/0000-0002-2487-4250 Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072 ChinaSearch for more papers by this authorWenping Hu, Wenping Hu Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072 ChinaSearch for more papers by this author First published: 11 October 2021 https://doi.org/10.1002/eem2.12294AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract This highlight indicates that the local electrostatic interactions between metal cations and key intermediates facilitate the electrocatalytic CO2 reduction reaction. Electrocatalytic CO2 reduction reaction (CO2RR) has been considered as a promising strategy to achieve a carbon-neutral cycle and produce valuable fuels and feedstocks.[1-7] Among various factors affecting the performance of electrocatalytic CO2RR, metal cations in the electrolyte have received increasing attention.[1, 8-10] According to previous researches, the activity trend among alkali metals follows the order of Cs+ > K+ > Na+ > Li+,[11] consistent with the order of ionic radius, while the specific molecular mechanism is still controversial. In fact, three main diverging theories that explain how cations at the electrode/electrolyte interface affect the activity and selectivity of electrocatalytic processes have been proposed. 1) Accumulation of cations near the electrode surface via non-covalent interactions contributes to the modification of the local electric field. The resultant altered potential profile could change activation energy of the overall reaction and influence the adsorption of CO2.[12, 13] 2) Larger cations would better buffer the interfacial pH, leading to lower pH changes at the interface and thus affect the CO2RR performance.[14] 3) Metal cations stabilize adsorbed CO2RR intermediates with large dipole moments through local electrostatic interactions within the electrical double layer.[15] Very recently, various advanced technologies, such as scanning electrochemical microscopy (SECM) and ab initio molecular dynamics (AIMD) simulations, have been employed to test the aforementioned models by Monteiro et al.[8] The results showed that metal cations function mainly through stabilizing CO 2 - intermediate, which matches well with the model emphasizing local electrostatic interactions between cations and key intermediates. In order to elucidate the role of metal cations in the CO2RR, cyclic voltammetry (CV) in a hanging meniscus configuration was applied. The more apparent difference between cathodic scans with or without Cs+ in a CO2 atmosphere than that in Ar implied that CO2 reduction occurs when adding Cs+ in a CO2 atmosphere. Meanwhile, anodic scans further confirmed this deduction. The observed large anodic current results from CO oxidation (COox), which was produced during the process of CO2RR. Note that COox could take place even in the absence of metal cations. Furthermore, to investigate the influence of Cs+ concentrations on CO2 reduction, Li2SO4 solutions with different Cs+ concentrations were used as electrolyte. CO production increases steeply with the addition of less than 1 mm Cs+ and gradually reaches a plateau, namely the saturation effect. In addition, the effect of different metal cations was also evaluated with 1 mM M2SO4 (M = H, Li, Na, K, or Cs) as electrolyte. It is found that the activity for CO formation is related to the M+-H2O bond length, that is, the strong correlation between activity and the solvated cation size favors the production of CO. To further verify that there is no CO produced without metal cations on Au electrode, scanning electrochemical microscopy (SECM) was employed to detect CO from CO2RR (Figure 1a). Specially, SECM was selected to detect CO owing to its higher sensitivity, compared with other analysis techniques, such as differential electrochemical mass spectrometry and gas chromatography. Besides, in such a surface-generation tip-collection mode, a platinum ultramicroelectrode (Pt-UME) served as detector for CO for extremely high sensitivity of platinum voltammetry to trace amounts of CO. As shown in Figure 1b, there is only a characteristic peak of hydrogen oxidation in Ar atmosphere when whether adding Cs+ or not. In contrast, a sharp peak of COox was observed when Cs+ were introduced in CO2 atmosphere. That is, CO is the only production on Au electrode if metal cations are added to the electrolyte, which is consistent with the conclusion deduced from CV measurements. Moreover, similar results were obtained for Ag and Cu electrodes which are not good for COox, to verify the University of the Conclusion that there is no CO produced without metal cations in electrolyte. The role of cations was further investigated via Ab initio molecular dynamics (AIMD) simulations for illustrating the formation process of Au-H2O-M+ and estimating the cation coordination with CO2. Owing to hard solvation shell attributed to small cation ionic radius, the coordination of Li+ with CO2 was limited. While for Na+, K+, and Cs+, the larger average coordination numbers than Li+ were observed, following the order of Na+ < K+ < Cs+. Subsequently, three promotional effects of partially desolvated cations enabling CO2RR were analyzed one by one. 1) From a thermodynamic point of view, with the presence of cations, the capability of a short-range M+-O(CO2) interaction to stabilize CO2 is superior to that of the solvation by water molecules. Additionally, an equivalent stabilizing effect can be achieved through a medium-range interaction between electric field induced by cations and the electric dipole of adsorbed * CO 2 - , rate-determining step for CO2RR.[16] 2) The reduction in O-C-O angle from 180° to below 140° induced by adjacent metal cations can also indicate the activation of CO2 molecule. 3) According to CO2 Bader charges close to −1.0|e−|, the electron transfer from the catalytic surface to CO2 unit was enhanced. Based on the aforementioned study, a possible reaction mechanism was proposed (Figure 1c). In brief, the metal cations can stabilize the CO2 molecules by forming a complex with CO2, which is beneficial to the formation of the CO 2 - intermediate. Substantial difference in the average coordination numbers for M+-CO2 in the 2 ps indicated that the activity of M+ for CO2 reduction is closely related to their ability of coordination with CO 2 - , as well as concentrations at the interface. Specifically, via M+-O(CO2) electrostatic interaction, compared with Li+, Na+, K+, and Cs+ are more favorable to stabilize CO2, originating from softer hydration, which accounts for higher concentration at the outer Helmholtz plane and more stable coordination with CO2. In a word, Monteiro et al. carried out CO2 reduction experiments on polycrystalline Au electrode, which yields only CO and H2 in this work, to test three main theory models: modification of the local electric field, buffering of the interfacial pH, and stabilization of reaction intermediates. Various technologies (SECM, Pt-UME, CV, etc.) were applied to reveal the important role of metal cations on the activity and selectivity of CO2RR; while the exact mechanism was studied via computational model such as AIMD. Although the other two models cannot be excluded completely, only the model focusing on stabilizing intermediates agrees well with the experimental and computational results. Notably, CO2 can only be reduced in the presence of a metal cation favoring the CO 2 - intermediate, which has important implications from both system-design and reaction modelling point of view. It is expected as a promising strategy to explore species such as large multivalent cations or surfactants, possibly possessing larger stabilizing effect on CO2 than Cs+ for promoting the development of CO2RR field. Acknowledgement This work was supported by the National Natural Science Foundation of China (22071172). Conflict of Interest The authors declare no conflict of interest. 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