LETTER doi:10.1038/nature13249 Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper Christina W. Li1, Jim Ciston2 & Matthew W. Kanan1 The electrochemical conversion of CO and H O into liquid fuel z z { {z { 2 2 2CO 3H2O 4e ?CH3CO2 3HO is ideal for high-density renewable energy storage and could pro- ð2Þ E~0:50 V versus RHE vide an incentive for CO2 capture. However, efficient electrocata- lysts for reducing CO and its derivatives into a desirable fuel1–3 are 2 By comparing OD-Cu to electrodes comprised of commercial Cu nano- not available at present. Although many catalysts4–11 can reduce CO 2 particles, we show that CO reduction activity is not a consequence of to carbon monoxide (CO), liquid fuel synthesis requires that CO is re- 1 nanocrystallite size or morphology; instead, we propose that grain boun- duced further, using H OasaH source. Copper (Cu) is the only known 2 daries participate in the catalysis. material with an appreciable CO electroreduction activity, but in bulk OD-Cu electrodes were prepared by annealing polycrystalline Cu form its efficiency and selectivity for liquid fuel are far too low for prac- foil in air at 500 C to grow a thick Cu O layer on the surface and sub- tical use. In particular, H O reduction to H outcompetes CO reduc- u 2 2 2 sequently reducing this oxide to form Cu0 nanocrystallites. We used tion on Cu electrodes unless extreme overpotentials are applied, at which two reduction methods to vary the properties of the material. To obtain point gaseous hydrocarbons are the major CO reduction products12,13. OD-Cu 1, the Cu O precursor was reduced electrochemically in aque- Here we show that nanocrystalline Cu prepared from Cu O (‘oxide- 2 2 ous solution at ambient temperature. This procedure had previously been derived Cu’) produces multi-carbon oxygenates (ethanol, acetate and found to yield nanocrystalline Cu with maximal selectivity for CO n-propanol) with up to 57% Faraday efficiency at modest potentials 2 reduction20. To obtain OD-Cu 2, the Cu O precursor was reduced with (–0.25 volts to –0.5 volts versus the reversible hydrogen electrode) in 2 H at 130 uC. The OD-Cu electrodes were compared to commercial Cu CO-saturated alkaline H O. By comparison, when prepared by tra- 2 2 nanoparticles that had been synthesized by vapour condensation. Cu ditional vapour condensation, Cu nanoparticles with an average crys- nanoparticle electrodes were prepared by drop-casting suspensions of tallite size similar to that of oxide-derived copper produce nearly the nanoparticles and fluorinated binder onto metal substrates. exclusive H (96% Faraday efficiency) under identical conditions. 2 All three types of electrodes exhibited similar morphologies when Our results demonstrate the ability to change the intrinsic catalytic imaged by scanning electron microscopy (SEM) (Fig. 1a, d, g). Particle properties of Cu for this notoriously difficult reaction by growing sizes ranged from about 30 nm to 100 nm, and the particles formed aggre- interconnected nanocrystallites from the constrained environment gated films. Low-resolution transmission electron microscopy (TEM) of an oxide lattice. The selectivity for oxygenates, with ethanol as the confirmed the range of particle sizes observed via SEM (Fig. 1b, e, h). major product, demonstrates the feasibility of a two-step conversion High-resolution TEM of OD-Cu 1 and OD-Cu 2 showed interconnected of CO to liquid fuel that could be powered by renewable electricity. 2 nanocrystalline networks with distinct grain boundaries between nano- Cu electrodes have been thoroughly evaluated in both CO2 (refs 7, 12,13,16,17 crystallites (Fig. 1f, i). In contrast, the Cu nanoparticle electrode appeared 14, 15) and CO reduction electrolyses. In CO2-saturated aqueous as an aggregation of overlapping particles (Fig. 1c). Differential capaci- solutions, polycrystalline Cu foil produces a mixture of compounds that – tance measurements yielded electrochemical roughness factors of 135, aredominatedbyH2 at low overpotential, by CO and HCO2 at high 48 and 26 for OD-Cu 1, OD-Cu 2 and the Cu nanoparticle electrode, overpotential and by hydrocarbons and multi-carbon oxygenates at the 18,19 respectively (Extended Data Fig. 5 and Extended Data Table 2). X-ray most extreme potentials . When supplied with CO in the absence of photoelectron spectroscopy indicated that the surfaces were comprised CO2, Cu produces hydrocarbons and multi-carbon oxygenates, but very 0 of both Cu and Cu2O, the latter resulting from air exposure during the negative potentials are still required to promote CO reduction over H2 12,13 X-ray photoelectron spectroscopy sample preparation (Extended Data evolution . Large overpotentials preclude energetically efficient elec- Figs 1, 2 and 3). Grazing incidence X-ray diffraction patterns for all of trolysis and favour hydrocarbons over liquid oxygenates. We recently the electrodes exhibited peaks at the expected positions for an ideal Cu discovered that oxide-derived Cu (OD-Cu) has much higher selectivity lattice, indicating no uniform expansion or compression of the unit cell for CO2 electroreduction over H2 evolution than does polycrystalline Cu (Fig. 1j, k, l). The patterns differed substantially in their peak widths, (ref. 20). Because CO reduction activity may be obscured in the presence however. Using the Scherrer equation, both OD-Cu 2 and the Cu nano- of a large excess of CO2, we questioned whether OD-Cu would reduce CO particle electrode exhibited average crystallite sizes hiD . 100 nm, while with greater efficiency than other Cu materials when supplied with CO OD-Cu 1 had a smaller hiD of 31 nm. Additional high-resolution graz- directly. Here we show that OD-Cu reduces CO with high Faraday ing incidence X-ray diffraction patterns were collected for OD-Cu 1 efficiency at an overpotential .0.6 V lower than that of polycrystalline and OD-Cu 2 with synchrotron radiation to determine the amount of Cu foil. This improvement results both from an increase in the intrin- microstrain in each sample using Williamson–Hall analysis. This ana- sic CO reduction activity and from a decrease in intrinsic H2O reduc- lysis yielded microstrain values of 0.2% and 0.05% for OD-Cu 1 and tion activity on OD-Cu. The major CO reduction products are ethanol – – OD-Cu 2, respectively (Extended Data Fig. 4). and acetate, which correspond to 8e and 4e reductions with H2Oas 1 CO electroreduction activity was measured under steady-state con- the H source (equations (1) and (2), where Eis the equilibrium potential). ditions by performing constant-potential electrolyses for multiple hours { { in 0.1 M KOH saturated with 1 atm of CO at ambient temperature. The 2COz7H2Oz8e ?CH3CH2OHz8HO ð1Þ concentration of CO in solution is only 1 mM under these conditions. E~0:18 V versus RHE Solution-phase and gas-phase products were quantified by nuclear magnetic 1Department of Chemistry, Stanford University, Stanford 94305, California. 2National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley 94720, California. 504 | NATURE | VOL 508 | 24 APRIL 2014 ©2014 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH a b c j Cu nanoparticles 400 nm 100 nm 10 nm de f k Oxide-derived Cu 1 400 nm 100 nm 5 nm g h i l 40 60 80 100 120 140 Oxide-derived Cu 2 500 nm 200 nm 10 nm 2θ (degrees) Figure 1 | Physical characterization of Cu nanoparticle and OD-Cu b, e, h, Low-magnification TEM images. c, f, i, High-resolution TEM images. electrodes. Top row, the Cu nanoparticle electrode. Middle row, the OD-Cu 1 j, k, l, Grazing incidence X-ray diffraction patterns. electrode. Bottom row, the OD-Cu 2 electrode. a, d, g, SEM images. resonance (NMR) spectroscopy and gas chromatography, respectively. The higher Faraday efficiency for CO reduction on OD-Cu electro- Electrolyses were performed at a range of applied potentials E between des compared to a Cu nanoparticle electrode reflected higher intrinsic –0.25 V and –0.5 V versus the reversible hydrogen electrode (RHE). CO reduction activity. Figure 2b and c shows the geometric and surface- The modest reducing potential and low CO concentration provides a area-normalized current densities for CO reduction (jCOredn)versusE stringent test of a catalyst’s selectivity for CO versus H2O reduction. Under for the three electrodes. The difference in intrinsic activity between two these conditions, polycrystalline Cu foil exhibited low geometric current electrodes corresponds to the ratio of their normalized current densities densities and H2 was the only detectable product. These results are con- in the absence of mass-transport contributions to the kinetics. The rate sistent with previous results in KOH, in which much more negative of CO transport from bulk solution through the diffusion layer to the potentials (–0.7 V versus RHE) and a high flow rate of CO through the electrode surface determines the maximum attainable geometric jCOredn solution were required to attain 22% total Faraday efficiency for CO (ref. 21). The Cu nanoparticle electrode did not reach a mass-transport reduction, with only 7% Faraday efficiency for oxygenates13. A maxi- limit between –0.3 V and –0.5 V, as shown by the exponential increase 21 mum CO reduction Faraday efficiency of 65% was reported for poly- in jCOredn across this potential range. Its Tafel slope was 112 mV dec crystalline Cu foil at –0.9 V versus RHE in a mixed KCl and KHCO3 (where ‘dec’ means decade, or one order of magnitude). On account of electrolyte, although only 10% Faraday efficiency corresponded to the their higher activity, the geometric jCOredn on OD-Cu electrodes quickly formation of oxygenates12. reached a plateau value of 0.5 mA cm–2 as the overpotential was increased Larger geometric current densities were obtained with OD-Cu and (Fig.