Electronic Regulation of Bismuth Oxide Via Sulfur Doping for Efficient CO2 Electroreduction to Formate

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Electrochemical CO2 reduction (ECR) to value-added fuels and chemical feedstocks driven by renewable energy sources is one promising approach to alleviating the ever-increasing atmospheric CO2 concentration and the rapid fossil fuels consumption.1,2 Among various products formed, formate is a very attractive product due to its wide applications in textile and pharmaceutical industries, and in fuel cells as the hydrogen carrier. Bi in particular has been widely investigated as advanced catalyst for formate production because of their cost-effectiveness, low toxicity, and high activity. Up to date, various strategies including size, morphology, defect, grain boundary, and heterostructure engineering have been developed to improve the catalytic performances of Bi-based catalysts. Despite some exciting achievements, the performances over Bi-based catalysts still fall behind the commercial requirements. Most existing catalysts suffer from low formate production rate (in H-type cell) because high standard of catalytic activity and product selectivity could be hardly achieved simultaneously. On the other side, desirable formate faradaic efficiency (FE) of ˃90 % can be only achieved in a narrow potential window owing to the competing hydrogen evolution reaction (HER) at high potential. Therefore, developing Bi-based catalysts for formate production via ECR with high formation rate and current density over a broad potential window is a top priority but remains as great challenges. Electronic regulation of electrocatalysts via element doping has been regarded as a powerful strategy to enhance the electrochemical activity of ECR. The introduction of heteroatoms can regulate the electron density and thus, precisely modify the electronic structures of the active sites, resulting in optimal adsorption energy of the reaction intermediates. S modification of catalysts can modulate the electronic structure of catalysts, thus boosting the electrocatalytic activity for HER, 3 oxygen evolution reaction (OER),4 and oxygen reduction reaction (ORR).5 Moreover, S doping can increase the adsorption capacity of CO2 and decrease the energy barrier for the formation of *HCOO intermediate.6 To that end, S doping is expected to be an effective strategy to improve the catalytic performance of ECR by tuning the electronic structure. However, the studies on fine-tuning the electronic structure of Bi-based catalysts for ECR by S doping have rarely been reported so far; a comprehensive understanding of the S doping effect on the ECR activity is also lacking. The study demonstrates S doped Bi2O3 nanosheets coupled with carbon nanotube (S-Bi2O3-CNT) as catalyst for ECR to formate production. The prepared S2-Bi2O3-CNT with the S doping amount of 0.7 at % achieved high FE over a wide potential range, and also achieved high partial current density (48.6 mA cm-2) and good long-term stability. Experimental results and a series of characterizations highlighted the advantages of S doping which facilitates electron transfer, increases the adsorption of CO2 and offers more undercoordinated Bi sites. Moreover, DFT results reveal that S doping induced the electronic delocalization of Bi sites, which optimized the adsorption of *CO2 and *HCOO intermediates while hindering the adsorption of *H. Therefore, S doped Bi2O3 promotes the ECR to formate by supressing the competitive HER simultaneously. This work opens up an attractive avenue for developing highly efficient electrocatalyst for ECR at atomic level.References Nielsen, D. U.; Hu, X.-M.; Daasbjerg, K.; Skrydstrup, T., Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals. Nat. Catal. 2018, 1, 244-254.Ross, M. B.; De Luna, P.; Li, Y.; Dinh, C.-T.; Kim, D.; Yang, P.; Sargent, E. H., Designing materials for electrochemical carbon dioxide recycling. Nat. Catal. 2019, 2, 648-658.Yu, J.; Guo, Y.; Miao, S.; Ni, M.; Zhou, W.; Shao, Z., Spherical ruthenium disulfide-sulfur-doped graphene composite as an efficient hydrogen evolution electrocatalyst. ACS Appl. Mater. Interfaces 2018, 10, 34098-34107. Yu, L.; Wu, L.; McElhenny, B.; Song, S.; Luo, D.; Zhang, F.; Yu, Y.; Chen, S.; Ren, Z., Ultrafast room-temperature synthesis of porous S-doped Ni/Fe (oxy) hydroxide electrodes for oxygen evolution catalysis in seawater splitting. Energy Environ. Sci. 2020, 13, 3439-3446.Wang, Y. C.; Lai, Y. J.; Song, L.; Zhou, Z. Y.; Liu, J. G.; Wang, Q.; Yang, X. D.; Chen, C.; Shi, W.; Zheng, Y. P., S‐doping of an Fe/N/C ORR catalyst for polymer electrolyte membrane fuel cells with high power density. Angew. Chem., Int. Ed. 2015, 54, 9907-9910. 6. Tian, W.; Zhang, H.; Sun, H.; Suvorova, A.; Saunders, M.; Tade, M.; Wang, S., Heteroatom (N or N‐S)‐Doping Induced Layered and Honeycomb Microstructures of Porous Carbons for CO2 Capture and Energy Applications. Adv. Funct. Mater. 2016, 26, 8651-8661.