Letter pubs.acs.org/acscatalysis
Electrochemical Reduction of CO2 at Copper Nanofoams Sujat Sen,† Dan Liu,† and G. Tayhas R. Palmore*,†,‡ †
Department of Chemistry and ‡School of Engineering, Brown University, Providence, Rhode Island 02912, United States
ACS Catal. 2014.4:3091-3095. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/23/19. For personal use only.
S Supporting Information *
ABSTRACT: We report the electrochemical reduction of CO2 at copper foams with hierarchical porosity. We show that both the distribution of products formed from this reaction and their faradaic efficiencies differ significantly from those obtained at smooth electropolished copper electrodes. We attribute these differences to be due to high surface roughness, hierarchical porosity, and confinement of reactive species. We provide preliminary evidence in support of these claims. KEYWORDS: CO2 reduction, copper foam, formic acid, nanoporous, confinement effects
E
significantly, whereas C2 and C3 products such as ethane and propylene are produced in small but detectable quantities. Although ethane has been observed at very low yields,18 propylene has not been observed previously at copper electrodes. The presence of ethane and propylene suggests that copper foams provide both the nanostructured surfaces and cavities that facilitate the reaction between adsorbed CO2 and hydrogen species to generate higher-order hydrocarbons during the electrochemical reduction of CO2. Three-dimensional foams of copper were electrodeposited onto mechanically polished copper substrates using a reported procedure.25 This procedure is simple to perform and results in an electrode with hierarchical porosity. Evolution of hydrogen gas at an electrode surface is significant during the electrodeposition of copper when a high current density is maintained (typically >0.5 A/cm2). The evolution of hydrogen gas impedes electrodeposition of copper directly onto the cathode by temporarily preventing contact between the copper cathode and the electrolyte that contains copper sulfate. Eventually, a thin film of electrolyte surrounding a H2 bubble comes into contact with the cathode, which completes the electrochemical circuit and allows for the electrodeposition of copper. The resulting foam is a connected network of copper pores templated by H2 bubbles. Shown in Figure 1 are typical SEM images of copper foams electrodeposited for different amounts of time. Copper foams appear reddish when freshly electrodeposited (inset) but gradually dull with exposure to air as the copper is oxidized. Nanoscale dendritic structures protrude from the walls of the pores (Figure 1f). The pore diameter (20−50 μm) can be controlled by electrodeposition parameters such as concentration of copper salts, pH, and deposition time.25 Shown in Figure 2a is the X-ray diffractogram (XRD) of electrodeposited copper foam on an aluminum substrate, which reveals that copper foams have face-centered cubic structure
lectrochemical reduction of CO2 has been investigated at a variety of metallic electrodes, and a number of reports and reviews have been published on this subject.1−9 Among the metals studied, copper generates significant quantities of hydrocarbons such as methane and ethylene in aqueous media.7 Hori et al.3,10−14 conducted extensive studies on the electrochemical reduction of CO2 and CO at copper electrodes and concluded that the product distribution reflected a sensitivity of adsorbed hydrogen species to the underlying structure of the copper electrode and that “surface roughening likely introduced surface defects such as steps and vacancies that are favorable for reaction of adsorbed hydrogen atoms”.12,13 Several other groups have reported on the electrochemical reduction of CO2 at copper electrodes in aqueous and nonaqueous media with various supporting electrolytes.7,15−17 Nanoparticulate and nanoporous electrode surfaces of copper and other metals have been used to study effects of particle size and porosity.18−20 Norskov et al.18 studied copper electrodes with three different morphologies (electropolished, sputter coated, and nanoparticle coated) for their selectivity toward CO2 reduction. They found that the latter two morphologies were more selective toward hydrocarbon generation and attributed this effect to the greater abundance of uncoordinated sites.18 DFT calculations further suggested that these sites are the most likely sites involved in CO2 activation and reduction. Other computational studies have been performed to explain the catalytic behavior and selectivity of copper toward CO2 reduction.21−24 Recent work23 that describes a novel approach to the fabrication of metal foams with hierarchical porosity provides an excellent opportunity for testing the effect of threedimensional nanostructured metal surfaces and their corresponding cavities on the products produced during the electrochemical reduction of CO2. In this paper, we show that the electrochemical reduction of CO2 at copper foams yields formic acid at a lower onset potential with faradaic efficiencies that are 10−20% higher than other reported values. In comparison to smooth copper electrodes, the faradaic efficiencies of CO, methane, and ethylene are reduced © 2014 American Chemical Society
Received: April 19, 2014 Revised: June 12, 2014 Published: August 8, 2014 3091
dx.doi.org/10.1021/cs500522g | ACS Catal. 2014, 4, 3091−3095
ACS Catalysis
Letter
Shown in Figure 3 are the faradaic efficiencies of the various products obtained from the electro-reduction of CO2 at a
Figure 1. SEM images of electrodeposited copper foams on a copper substrate for (a) 5s; (b) 10s; (c) 15s; (d) 30s; and (e) 60s; (f) nanostructure of the electrodeposited foams. Inset of (a) is a photo of a copper electrode immediately after electrodeposition of the copper foam. Figure 3. Product distribution as a function of applied potential during the electrochemical reduction of CO2. The working electrode was a copper nanofoam electrodeposited for 15 s. Data for the electrochemical reduction of CO2 to formate at a smooth copper electrode (both from our laboratory and from the literature) are included for comparison.
copper nanofoam (15 s electrodeposit) plotted as a function of applied voltage. The sum of the faradaic yield for all products approached 100% across the entire potential range. Major products were HCOOH, H2, and CO, whereas minor products (
