Metal ion cycling of Cu foil for selective C–C coupling in electrochemical CO2 reduction

© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. 1Rowland Institute, Harvard University, Cambridge, MA, USA. 2SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, CA, USA. 3Center for Nanoscale Systems, Harvard University, Cambridge, MA, USA. 4Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA. 5SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA, USA. Kun Jiang and Robert B. Sandberg contributed equally to this work. *e-mail: [email protected]; [email protected] With the fast development of advanced technologies to efficiently harvest wind or solar energies, the cost of renewable energy in the near future is expected to decrease significantly, enabling economical conversion of carbon dioxide (CO2) and water (H2O) into fuels and chemicals1–3. The electrochemical CO2 reduction reaction (CO2RR) is a promising energy conversion process due to its mild reaction conditions and high energy efficiencies4–8, but is currently challenging due to the low catalytic activity and product selectivity in aqueous solutions9–12. Catalysts with suitable electronic structures have been able to reduce CO2 in water with high Faradaic efficiencies13–24, but most of them can only catalyse two-electron reductions to carbon monoxide (CO) or formic acid (HCOOH) products, which have more facile kinetics. Further reductions to higher-value, energy-dense hydrocarbons and alcohols, and in particular C2+ products, is desirable for applications in energy storage, transportation and the chemical industry, but present significantly higher overpotentials25. This difficulty arises from the linear scaling among activation and binding energies of reaction intermediates; the catalytic surface needs to bind *CO intermediates strongly enough to build up a sufficient coverage for further reduction or C–C coupling, but the associated activation barriers also increase with stronger *CO binding26. Developing catalytic materials with appropriate electronic properties becomes critical for tuning the interplay between these two criteria for selective C–C coupling. Among all transition metals, Cu-based materials with unique electronic properties have been shown to be the most selective towards higher-value hydrocarbons beyond CO or HCOOH (refs 27,28). Cu catalysts with different morphologies or structures present varied product distributions. Polished Cu polycrystalline foils with primarily (111) facets exposed can catalyse C–C coupling for C2+ products; however, these are not the major products, with Faradaic efficiencies usually much lower than those of C1 products (HCOOH, CO, and CH4). By pre-oxidizing Cu metal to oxide under high temperature in air and reducing it in situ to Cu again under CO2 or CO reduction conditions, the C2+ selectivity was observed to be significantly improved32–35. However, the resulting Cu catalysts usually show complicated morphologies, which presents a challenge for mechanistic studies32. Electrocatalytic CO2RR on single crystal Cu facets has suggested (100) surfaces to be more selective towards C2+ products than (111)12,29,36–38, and further promoted by the introduction of steps on the (100) basal plane10,12,28,39. A deeper mechanistic understanding of C–C coupling on different Cu facets or atomic sites would provide valuable guidance for the design of catalysts for CO2 reduction to C2+ products. In this work, we first study the facet dependence of the initial C–C coupling steps on Cu in CO2 reduction using density functional theory (DFT). Several recent studies have investigated C–C coupling using an implicit description of the ion distribution in the electrolyte40–42. Here, we apply an explicit model of the electrolyte to investigate solvation and cation stabilization of initial C–C coupling intermediates. Because simulations suggest both the Cu(100) and stepped facets to be more favourable for C2+ product formation over (111), we developed a metal ion cycling method to synthesize single crystalline Cu2O nanocubes with predominantly Cu2O(100) facets. By tuning the battery cycle numbers on the Cu foil, the product distributions and reaction pathways can be effectively controlled. Under CO2RR conditions, those oxide nanocubes can be reduced to polycrystalline Cu nanocubes with preferentially exposed Cu(100) facets for C–C coupling. As a result, our 100-cycled Cu-nanocube catalyst presents a sixfold improvement in the C2+ to C1 product ratio compared with the pristine polished Cu foil, with a C2+ Faradaic efficiency of over 60% and H2 below 20%, and a corresponding C2+ partial current density of more than 40 mA cm–2.