Abstract
The electrochemical conversion of CO2 and H2O into liquid fuel is ideal for high-density renewable energy storage and could provide an incentive for CO2 capture. However, efficient electrocatalysts for reducing CO2 and its derivatives into a desirable fuel1,2,3 are not available at present. Although many catalysts4,5,6,7,8,9,10,11 can reduce CO2 to carbon monoxide (CO), liquid fuel synthesis requires that CO is reduced further, using H2O as a H+ source. Copper (Cu) is the only known material with an appreciable CO electroreduction activity, but in bulk form its efficiency and selectivity for liquid fuel are far too low for practical use. In particular, H2O reduction to H2 outcompetes CO reduction on Cu electrodes unless extreme overpotentials are applied, at which point gaseous hydrocarbons are the major CO reduction products12,13. Here we show that nanocrystalline Cu prepared from Cu2O (‘oxide-derived Cu’) produces multi-carbon oxygenates (ethanol, acetate and n-propanol) with up to 57% Faraday efficiency at modest potentials (–0.25 volts to –0.5 volts versus the reversible hydrogen electrode) in CO-saturated alkaline H2O. By comparison, when prepared by traditional vapour condensation, Cu nanoparticles with an average crystallite size similar to that of oxide-derived copper produce nearly exclusive H2 (96% Faraday efficiency) under identical conditions. Our results demonstrate the ability to change the intrinsic catalytic properties of Cu for this notoriously difficult reaction by growing interconnected nanocrystallites from the constrained environment of an oxide lattice. The selectivity for oxygenates, with ethanol as the major product, demonstrates the feasibility of a two-step conversion of CO2 to liquid fuel that could be powered by renewable electricity.
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Acknowledgements
We thank Stanford University and the NSF (CHE-1266401) for support of this work. C.W.L. gratefully acknowledges an NSF Predoctoral Fellowship. A portion of this work was performed at NCEM, which is supported by the Office of Science, Office of Basic Energy Sciences of the US Department of Energy under contract number DE-AC02-05CH11231. We thank M. Toney and B. Shyam for assistance with grazing incidence X-ray diffraction performed at SSRL, a national user facility operated by Stanford University on behalf of the Office of Basic Energy Sciences of the US Department of Energy.
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C.W.L. and M.W.K. designed the experiments. C.W.L. prepared and characterized all electrodes and performed all electrochemical experiments; J.C. obtained all TEM images; C.W.L. and M.W.K. wrote the manuscript. All authors contributed to the overall scientific interpretation and edited the manuscript.
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C.W.L. and M.W.K. have filed a patent application (WO 2013-US25791, US) covering oxide-derived Cu and other oxide-derived catalysts for electrochemical fuel synthesis.
Extended data figures and tables
Extended Data Figure 1 Additional physical characterization of OD-Cu 1.
a, X-ray photoelectron spectroscopy survey spectrum. b, High-resolution X-ray photoelectron spectrum of the Cu LMM region. c, High-resolution X-ray photoelectron spectrum of the Cu 2p peaks. d, Low-resolution SEM image.
Extended Data Figure 2 Additional physical characterization of OD-Cu 2.
a, X-ray photoelectron spectroscopy survey spectrum. b, High-resolution X-ray photoelectron spectrum of the Cu LMM region. c, High-resolution X-ray photoelectron spectrum of the Cu 2p peaks. d, Low-resolution SEM image.
Extended Data Figure 3 Additional physical characterization of Cu nanoparticle electrodes.
a, X-ray photoelectron spectroscopy survey spectrum. b, High-resolution X-ray photoelectron spectrum of the Cu LMM region. c, High-resolution X-ray photoelectron spectrum of the Cu 2p peaks. d, Low-resolution SEM image.
Extended Data Figure 4 Additional grazing-incidence X-ray diffraction pattern data, collected using synchrotron X-rays at 11.5 keV.
a, X-ray diffraction patterns for OD-Cu 1 and OD-Cu 2. b, c, Williamson–Hall plots for OD-Cu 1 (b) and for OD-Cu 2 (c), where B = integral breadth of the peak, and the points highlighted in red have been excluded. To calculate crystallite size and strain, the following relationships were used: B = Kλ/<D>cosθ + 4εtanθ, where <D> is the average crystallite size, λ is the wavelength, ε is the non-uniform strain (microstrain), and the Scherrer constant K ≈ 1.
Extended Data Figure 5 Electrochemical surface area measurement.
a, b, Determination of double-layer capacitance over a range of scan rates for an OD-Cu 1 electrode after 1 h bulk electrolysis. c, d, Determination of double-layer capacitance over a range of scan rates for an OD-Cu 2 electrode after 12 h bulk electrolysis at –0.3 V versus RHE in 0.1 M KOH. a, c, Cyclic voltammagrams taken over a range of scan rates. b, d, Current due to double-layer charging plotted against cyclic voltammetry scan rate.
Extended Data Figure 6 Representative bulk-electrolysis data for CO reduction on OD-Cu 1.
a, Current density over time for the reduction of Cu2O to form active OD-Cu. b, Current density over time (left) for OD-Cu 1 at –0.4 V versus RHE in 0.1 M KOH, saturated with 1 atm CO and Faraday efficiency over time (right) for H2 (green), C2H4 (red), and C2H6 (blue). Efficiencies for EtOH and AcO– were obtained at the end of the electrolysis.
Extended Data Figure 7 Representative NMR spectrum for an OD-Cu 1 bulk electrolysis at –0.5 V versus RHE in 0.1 M KOH, saturated with 2.4 atm CO.
DMSO, dimethyl sulphoxide.
Extended Data Figure 8 Additional Tafel data collected in 0.1 M KOH, saturated with 1 atm CO.
a, Geometric current density for CO reduction versus potential for OD-Cu 2 and Cu nanoparticles. b, Surface-area-normalized current density for H2 evolution versus potential for OD-Cu 1, OD-Cu 2, Cu nanoparticles and polycrystalline Cu foil.
Extended Data Figure 9 CO reduction bulk electrolysis data for OD-Cu 1 in 1 M KOH, saturated with 1 atm CO.
a, Faraday efficiency for various products versus potential. b, Total current density and partial current density for CO reduction versus potential. c, d, SEM images of OD-Cu 1 after electrolysis in 1 M KOH.
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Li, C., Ciston, J. & Kanan, M. Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper. Nature 508, 504–507 (2014). https://doi.org/10.1038/nature13249
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DOI: https://doi.org/10.1038/nature13249
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