Cooperative CO2-to-ethanol conversion via enriched intermediates

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Articles Cooperative CO2-to-ethanol conversion via enriched intermediates at molecule–metal catalyst interfaces Fengwang Li 1,3, Yuguang C. Li 1,3, Ziyun Wang 1,3, Jun Li 1,2,3, Dae-Hyun Nam 1, Yanwei Lum1, Mingchuan Luo1, Xue Wang 1, Adnan Ozden2, Sung-Fu Hung1, Bin Chen1, Yuhang Wang 1, Joshua Wicks1, Yi Xu 2, Yilin Li1, Christine M. Gabardo 2, Cao-Thang Dinh 1, Ying Wang 1, Tao-Tao Zhuang1, David Sinton 2 and Edward H. Sargent 1* Electrochemical conversion of CO2 into liquid fuels, powered by renewable electricity, offers one means to address the need for the storage of intermittent renewable energy. Here we present a cooperative catalyst design of molecule–metal catalyst inter- faces with the goal of producing a reaction-intermediate-rich local environment, which improves the electrosynthesis of ethanol from CO2 and H2O. We implement the strategy by functionalizing the copper surface with a family of porphyrin-based metallic complexes that catalyse CO2 to CO. Using density functional theory calculations, and in situ Raman and operando X-ray absorp- tion spectroscopies, we find that the high concentration of local CO facilitates carbon–carbon coupling and steers the reaction pathway towards ethanol. We report a CO2-to-ethanol Faradaic efficiency of 41% and a partial current density of 124 mA cm−2 at −0.82 V versus the reversible hydrogen electrode. We integrate the catalyst into a membrane electrode assembly-based system and achieve an overall energy efficiency of 13%. The renewable-electricity-powered CO2 reduction reaction (CO2RR) offers a means to store intermittent electricity as 1,2 dispatchable fuels and valuable chemical feedstocks . Among the various products (CO, formic acid, methane, ethylene, ethanol and 1-proponal) formed from CO2RR, ethanol (a liquid fuel or fuel additive) is desired because it is energy dense by volume and because it levers extensive existing infrastructure for the storage and distribution of carbon-based fuels3,4. However, electrochemical CO2-to-ethanol conversion relies on multiple proton and electron transfers that involve multiple intermediates5–7, making the devel- opment of more efficient electrocatalysts an important but also challenging problem. Cu-based heterogeneous materials are distinctive among metal catalysts in their ability to reduce CO2 to products with two or more carbon atoms (C2+). Ethylene has, in past reports, been favoured two to fourfold over ethanol8–11. To steer the selectivity towards ethanol, the focus has been on the tuning of binding strength of reaction intermediates on Cu via doping using elements such as silver12,13, and through the creation of grain boundaries14 and vacancies15. These advances have increased the Faradaic efficiency (FE) towards ethanol to an impressive 30%. Technoeconomic assessment4,16 sug- gests that further progress on FE, as well as on lowering the overpo- tential (today above 1 V at current densities above 10 mA cm–2), are urgently needed; indeed, it is desired to increase half-cell cathodic energy efficiencies (CEEs) from today’s level of ≤15% (refs. 11–13,15,17– 20) to >20% and beyond. Scaling relations account for the fact that binding strengths of different intermediates on the same site (that is Cu) are correlated, which means that optimizing for one adsorbed species will typically take the other steps away from their optima21,22. We sought therefore to add a further degree of freedom in catalyst design that would influence predominantly one step. Specifically, we strove to increase the reaction rate of one step without strongly modulating the oth- ers, by judiciously increasing the coverage of one key intermedi- ate, yet not interfering with the electronic structure (hence binding strength) of Cu, with the goal of circumventing the scaling relations. Cu-based bimetallic catalysts have been reported to generate, on the doping metal sites, high-concentration CO, a key intermediate along C2+ pathways23–25, to spill over to the Cu sites for improved carbon–carbon (C–C) coupling and further reduction to C2+ prod- ucts20,26,27. However, the ratio of the heteroatom (Au, Ag, Zn) to Cu on the catalyst surface is low (for example, <2% in the case of Au and Cu (ref. 27)), resulting in a limited proportion of interfaces that offer local enrichment of intermediates. Further increasing this ratio modulates the electronic structure of Cu and then reduces selectivity towards C2+ products27–29. Here we present instead a molecule–metal composite material in which the molecular adsorbate generates a high concentration of the key early intermediate, CO, yet does not modulate the metal- lic active sites germane to the crucial C–C coupling step. Density functional theory (DFT) calculations indicate that the increased coverage of CO on Cu, achieved by increasing its concentration near the Cu surface, not only decreases the reaction energy for the C–C coupling step, but also steers the selectivity from ethylene to etha- nol. We implement this concept experimentally by functionalizing a family of porphyrin-based metallic complexes capable of catalys- ing CO2RR to CO on the Cu surface. The near-unity coverage of the complexes maximizes the local concentration of CO for the 1Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada. 2Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada. 3These authors contributed equally: Fengwang Li, Yuguang C. Li, Ziyun Wang. *e-mail: NATurE CATALYSiS | VOL 3 | JanUarY 2020 | 75–82 | 75

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