Steering CO2 electroreduction toward ethanol production

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Steering CO2 electroreduction toward ethanol production by a surface-bound Ru polypyridyl carbene catalyst on N-doped porous carbon Yanming Liua,b, Xinfei Fanc, Animesh Nayakb, Ying Wangb, Bing Shanb, Xie Quana, and Thomas J. Meyerb,1 aKey Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China; bDepartment of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; and cCollege of Environmental Science and Engineering, Dalian Maritime University, Dalian 116024, China Contributed by Thomas J. Meyer, November 10, 2019 (sent for review May 6, 2019; reviewed by Andrew B. Bocarsly and Clifford P. Kubiak) Electrochemical reduction of CO2 to multicarbon products is a sig- nificant challenge, especially for molecular complexes. We report here CO2 reduction to multicarbon products based on a Ru(II) poly- pyridyl carbene complex that is immobilized on an N-doped porous carbon (RuPC/NPC) electrode. The catalyst utilizes the synergistic effects of the Ru(II) polypyridyl carbene complex and the NPC interface to steer CO2 reduction toward C2 production at low overpotentials. In 0.5 M KHCO3/CO2 aqueous solutions, Faradaic efficiencies of 31.0 to 38.4% have been obtained for C2 production at −0.87 to −1.07 V (vs. normal hydrogen electrode) with 21.0 to 27.5% for ethanol and 7.1 to 12.5% for acetate. Syngas is also pro- duced with adjustable H2/CO mole ratios of 2.0 to 2.9. The RuPC/ NPC electrocatalyst maintains its activity during 3-h CO2-reduction periods. CO2 reduction | electrocatalysis | Ru(II) polypyridyl complex | porous carbon Electrocatalytic reduction of CO2 to useful fuels and chemical feedstocks is a promising strategy for carbon utilization and greenhouse gas mitigation. Among the CO2-reduction products including CO, formate, methanol, methane, acetate, ethanol, etc., liquid multicarbon products such as ethanol and acetate are desirable because of their high energy densities and economic values (1, 2). A variety of electrocatalysts have been explored for CO2 reduction, including metals (3, 4), metal oxides (5), heteroatom-doped carbon nanomaterials (6), molecular com- plexes (7–9), immobilized molecular complexes (10), and hybrid catalysts (11). Manipulation of morphology (12, 13), oxidation state (14), and introduction of dopants (15), alloys (16), and single-metal atoms (17, 18) have been employed to control overpotential, steer product distributions, enhance activity, and control selectivity toward specific products. Significant progress has been made, but, to date, the most common products for CO2 electroreduction are CO and formate. Immobilized molecular complexes such as porphyrins (19, 20), phthalocyanines (21), polypyridyl carbenes (22), and their hybrid catalysts (23, 24) have been investigated for electrochemical reduction of CO2. They offer the merits of tailorable catalytic sites and molecular structures for enabling electrocatalytic per- formance optimization. By immobilizing molecular complexes on the electrode surface, the catalysts are easy to reuse and show improved CO2-reduction performance (25, 26). In previous works, we have demonstrated that the Ru(II) polypyridyl carbene complex [RuII(tpy)(Mebim-py)(H2O)]2+ (tpy, 2,2′:6′,2′′-terpyridine; Mebim-py, 3-methyl-1-pyridyl-benzimidazol-2-ylidene) is an effective catalyst for electrochemical reduction of CO2 to CO with high selectivity in solution and on surfaces (22, 27, 28). CO2 reduction occurs by proton-coupled electron transfer through a carbene complex sta- bilized intermediate [RuII(tpy)(Mebim-py)(CO)]+ to give CO as the product, which could reduce overpotential for CO2 reduction. A significant challenge that remains is development of electro- catalysts that steer CO2 reduction toward multicarbon products with high selectivity at low overpotentials. Formation of C–C bonds necessitates coupling reactions between a CO intermediate and/or intermediates from CO protonation (29–31). Assembling the Ru(II) polypyridyl carbene on an electrode surface that is capable of C–C dimerization offers an attractive strategy for reducing CO2 to multicarbon products. N-doped carbon nanomaterials have been widely used for electroreduction due to their electrocatalytic activity and low cost. N-doped carbon electrodes have been shown to be capable of C–C dimerization (6, 32). Combining the Ru(II) polypyridyl carbene catalyst with an N-doped porous carbon electrode (RuPC/NPC) provides an appealing approach to facilitate CO2 electroreduction toward multicarbon products. The immobilized Ru(II) polypyridyl carbene can provide atomically distributed active sites for electro- catalysis. The large surface area and porous structure of NPC favors Ru(II) polypyridyl carbene attachment at catalytic inter- faces and exposes reactive sites. In the experiments described here, the pyrene-derivatized Ru(II) polypyridyl carbene complex was attached to NPC by π–π stacking on the surface. As noted below, the catalytic results were notable in identifying a greatly enhanced reactivity toward the formation of ethanol as a major product at relatively low overpotentials. Results and Discussion Synthesis and Characterization of the RuPC/NPC Electrode. The RuPC/ NPC hybrid catalyst was prepared by attaching the pyrene-derivatized Ru(II) polypyridyl carbene (22, 27, 28, 33) on NPC bonded by π–π Significance Electrochemical reduction of CO2 can convert CO2 emission back to value-added fuels and chemicals and store renewable elec- tricity. Reducing CO2 to multicarbon products has attracted great interest because of their higher energy densities and associated economic values. We report here a promising strategy for steering CO2 electroreduction toward ethanol production by exploiting a surface-bound Ru polypyridyl carbene catalyst on an N-doped porous carbon electrode. We show the synergistic ef- fects of Ru polypyridyl carbene for CO intermediate production with a porous carbon for C–C coupling that could boost ethanol production at relatively low overpotentials. The strategy pro- vides insights on how to improve selectivity and efficiency for CO2 reduction toward multicarbon products. www.pnas.org/cgi/doi/10.1073/pnas.1907740116 PNAS Latest Articles | 1 of 6 Author contributions: Y.L. and T.J.M. designed research; Y.L., X.F., A.N., and Y.W. per- formed research; Y.L., X.F., A.N., Y.W., B.S., X.Q., and T.J.M. analyzed data; and Y.L., X.F., A.N., Y.W., B.S., X.Q., and T.J.M. wrote the paper. Reviewers: A.B.B., Princeton University; and C.P.K., University of California San Diego. The authors declare no competing interest. Published under the PNAS license. 1To whom correspondence may be addressed. Email: tjmeyer@unc.edu. This article contains supporting information online at https://www.pnas.org/lookup/suppl/ doi:10.1073/pnas.1907740116/-/DCSupplemental. Downloaded by guest on May 6, 2021 CHEMISTRY

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