The effect of specific adsorption of halide ions on electrochemical CO 2 reduction† Tenghui Yuan, abc Tuo Wang, abc Gong Zhang, abc Wanyu Deng, abc Dongfang Cheng, abc Hui Gao, abc Jing Zhao, abc Jia Yu, abc Peng Zhang abc and Jinlong Gong * abcd In the electrochemical CO 2 reduction reaction (CO 2 RR), halide ions could impose a significant effect on multi-carbon (C 2+ ) product production for Cu-based catalysts by a combined contribution from various mechanisms. However, the nature of specific adsorption of halide ions remains elusive due to the difficulty in decoupling different effects. This paper describes a facile method to actively immobilize the morphology of Cu-based catalysts during the CO 2 RR, which makes it possible to reveal the fundamental mechanism of specific adsorption of halide ions. A stable morphology is obtained by pre-reduction in aqueous KX (X ¼ Cl, Br, I) electrolytes followed by conducting the CO 2 RR using non-buffered and non- specifically adsorbed K 2 SO 4 as the supporting electrolyte, by which the change of local pH and cation concentration is also maintained during the CO 2 RR. In situ spectroscopy revealed that the specific adsorption of halide ions enhances the adsorption of *CO intermediates, which enables a high selectivity of 84.5% for C 2+ products in 1.0 M KI. Introduction The electrochemical CO 2 reduction reaction (CO 2 RR) driven by renewable electricity, such as solar and wind power, holds great potential to close the carbon cycle. 1–3 Up to now, Cu-based materials have attracted extensive attention since they are the only transition metal-based catalysts known to catalyze the high- rate electroreduction of CO 2 to multi-carbon (C 2+ ) products (e.g., C 2 H 4 and C 2 H 5 OH). 4,5 The composition of the aqueous electrolyte has been widely recognized as a critical factor affecting the catalytic activity and selectivity of copper. 6–13 For cations, Hori et al. and Bell et al. ascribed the promoter effect of alkali metal cations to the change of outer Helmholtz plane potential 14 and the interfacial electric eld, 8,9 respectively. Recently, Xu et al. revealed that the increase in cation concentration promotes the formation of C 2+ . 15 For anions, 16,17 Hori et al. reported that non- buffered anions (Cl  , ClO 4  , and SO 4 2 ) enhance the CO 2 RR selectivity towards C 2+ while the buffered anions (HCO 3  and HPO 4 2 ) promote the formation of H 2 and CH 4 , and their production rates increase with the increasing concentration of buffered anions, which can be ascribed to the local pH during the CO 2 RR, because higher local pH facilitates the production of C 2+ . 18–20 Thus, the anions and cations in aqueous electrolytes may affect the CO 2 RR through various pathways. Among different ions in aqueous electrolytes, halide ions have attracted broad interest due to their specic adsorption on catalysts. It has been reported that the reconstruction of the catalyst surface and charge transfer induced by specic adsorp- tion of halide ions could enhance the selectivity/activity towards C 2+ and/or other CO 2 RR products, making it a promising approach to tune the product distribution of the CO 2 RR by optimizing the type and concentration of halide ions. 17,21–26 It has been widely accepted that halide ions could easily induce the reconstruction of Cu, 13,21–24 leading to the changes in the strain effect, exposed active sites, surface roughness, etc. Moreover, the reconstructed morphologies vary with the types of halide ions, resulting in different activity/selectivity towards the CO 2 RR, making it difficult to compare the CO 2 RR activity and selectivity in different aqueous halide containing electrolytes. 13,23,25 In addition to inducing morphological changes, the specic adsorption of halide ions on Cu is also reported to interact with reaction intermediate species directly and affect the product distribution. A lot of insightful understandings of this halide- intermediate interaction have been elegantly reported, but more studies are still needed to reach a denite conclusion. Strasser et al. proposed that the interaction between Cu and I  favors the a School of Chemical Engineering and Technology, Key Laboratory for Green Chemical Technology of Ministry of Education, Tianjin University, Tianjin 300072, China. E-mail: jlgong@tju.edu.cn b Collaborative Innovation Center for Chemical Science & Engineering, Tianjin 300072, China c Haihe Laboratory of Sustainable Chemical Transformations, Tianjin 300192, China d Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207, China † Electronic supplementary information (ESI) available: Catalyst morphology evolution in different electrolytes, the inuence of cation concentration and pre-treatment conditions, the detailed activity and ATR-SEIRAS spectra in different electrolytes. See https://doi.org/10.1039/d2sc02689a Cite this: Chem. Sci. , 2022, 13, 8117 All publication charges for this article have been paid for by the Royal Society of Chemistry Received 14th May 2022 Accepted 27th May 2022 DOI: 10.1039/d2sc02689a rsc.li/chemical-science © 2022 The Author(s). Published by the Royal Society of Chemistry Chem. Sci. , 2022, 13, 8117–8123 | 8117 Chemical Science EDGE ARTICLE Open Access Article. Published on 28 June 2022. Downloaded on 7/17/2022 11:30:36 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue