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Highly Dense Cu Nanowires for Low-overpotential CO2 Reduction David Raciti, Kenneth J. Livi, and Chao Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b03298 • Publication Date (Web): 09 Sep 2015 Downloaded from http://pubs.acs.org on September 13, 2015
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Highly Dense Cu Nanowires for Low-overpotential CO2 Reduction David Raciti†, Ken Livi‡, Chao Wang†,* †
Department of Chemical and Biomolecular Engineering and ‡High Resolution Analytical Electron Mi‐
crobeam Facility, Johns Hopkins University, 3400 N Charles Street, Baltimore, MD 21218.
ABSTRACT: Electrochemical reduction of CO2, an artificial way of carbon recycling, represents one promising solution for energy and environmental sustainability. It is, however, challenged by the lack of active and selective catalysts. Here we report a two-step synthesis towards highly dense Cu nanowires as advanced electrocatalysts for CO2 reduction. CuO nanowires were first grown by oxidation of Cu mesh in air and then reduced by either annealing in the presence of hydrogen or applying a cathodic electrochemical potential to produce Cu nanowires. The two reduction methods generated Cu nanowires with similar dimensions but distinct surface structures, which have provided an ideal platform for comparative studies of the effect of surface structure on the electrocatalytic properties. In particular, the Cu nanowires generated by electrochemical reduction were highly active and selective for CO2 reduction, requiring an overpotential of only 0.3 V to reach 1 mA/cm2 electrode current density and achieving Faradaic efficiency toward CO as high as ~60%. Our work has advanced the understanding of the structure-property relationship of Cu-based nanocatalysts, which could be valuable for the further development of advanced electrocatalytic materials for CO2 reduction.
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Keywords: carbon dioxide, CO2 reduction, Cu nanowires, copper nanocatalysts, electrocatalysis, pH effect
Introduction Approximately 80% of the world’s primary energy consumption comes from fossil fuels today. As the demand for energy continues growing, affordable and feasible fossil fuel sources are increasingly depleted.1 The burning of fossil fuels also leads to anthropologic accumulation of CO2 in the atmosphere, causing various climate and environmental consequences.2 Electrochemical reduction of CO2, an artificial way of carbon recycling, represents one promising solution for energy and environmental sustainability.3 By using electricity generated from solar cells and wind turbines, electrochemical CO2 reduction can store these renewable but intermittent energies in chemical forms. With water as the reducing agent, this process can generate a wide range of reduced carbon compounds, ranging from carbon monoxide (CO) and formic acid to methane, methanol ethanol, etc.4 These molecules can either be directly used as fuels, such as ethanol being used to partially substitute gasoline for internal combustion engines,5 or be converted into liquid fuels and other valuable chemicals through further processing, e.g., Fischer-Tropsch process for CO.6 Despite the great potential, electrochemical reduction of CO2 is however challenged by the absence of efficient catalysts. Copper (Cu) is one of the most studied materials for catalyzing CO2 reduction, in particular for the production of hydrocarbons, but it still requires large overpotentials; e.g., to reach a total current density of 1 mA/cm2 on polycrystalline Cu electrode, it typically requires an overpotential of >0.5 V for producing CO and HCOOH (two-electron processes) and >0.8 V for further reduced products such as CH4 and C2H4.7-10 Moreover, hydrogen evolution
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competes with CO2 reduction and reduces the Faradaic efficiencies (FEs) towards carbon-containing products, e.g., 60% FE) for Cu nanoparticles in the size range of 2-15 nm.11 These reports indicate that, although generally more active than their bulk counterparts owing to the high surface areas, Cu nanocatalysts could exhibit distinct selectivities depending on the nanostructures. Systematic study of the structure-property relationship of Cu nanocatalysts, especially the impact of surface structure on the catalytic selectivity, is thereby important for further development of CO2 reduction electrocatalysis. Here we report the synthesis of highly-dense Cu nanowires as advanced electrocatalysts for CO2 reduction. CuO nanowires were first grown by annealing Cu mesh in air and then reduced by either annealing in the presence of hydrogen or applying a cathodic electrochemical potential to produce Cu nanowires (Fig. 1, also see the experimental methods in the Supporting Information). The obtained Cu nanowires were then applied as electrocatalysts for CO2 reduction, targeting high selectivities in the low-overpotential region, i.e., E ≥ −0.5 V (corresponding to overpotentials no more than 0.39 V, considering E0(CO2/CO) = −0.109 V). Electrocatalytic studies were performed in 0.1
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M KHCO3 by using an electrolysis cell with the cathode and anode compartments separated by an anion exchange membrane (Selemion®). The gas and liquid-phase products were analysed by a gas chromatography (GC) equipped with a mass spectrometer (MS) detector and nuclear magnetic resonance (NMR) spectroscopy, respectively. CuO nanowires were grown by annealing Cu mesh in air at 600 oC, following the method reported by Xia et al.23 (Figs. 1 a-d). The annealed mesh was found to be entirely covered by dense nanowires. Typical diameters of the obtained nanowires were in the range of 50-100 nm, while the length varied from 10-50 μm. X-ray diffraction (XRD) analysis showed that the product was dominated by CuO and Cu2O phases, albeit a small amount of metallic Cu was still present after the annealing (Fig. S2). According to the previous report, these CuO nanowires were grown via a solid-vapor mechanism and presumably possessed a bicrystal structure divided by a (111) twin plane parallel to the longitudinal axis.23 The presence of Cu2O in the product could be understood by considering the subsequent oxidation of Cu to Cu2O and then CuO during the nanowire growth; the surface of the Cu mesh was likely covered by a layer of Cu2O film after the growth. 23 The as-grown CuO nanowires were reduced to Cu either by further annealing at 300 oC in a forming gas (N2 + 5% H2) or by applying a negative electrochemical potential at −0.4 V in 0.1 M KHCO3. The forming gas and electrochemically reduced nanowires are denoted as “FGR” and “ECR” nanowires in the following discussion, respectively. The electrochemical reduction was monitored by recording the cathodic current in time, where the current reaching a near-zero steady state was used as the sign for achieving complete reduction (Fig. S1). It was found that both the FGR and ECR nanowires mostly preserved the dimensions of the CuO nanowires, despite some deformation of the head of the wires (Figs. 1 e-h). XRD patterns collected after the reduction treatments showed only peaks of Cu, except that the ECR nanowires exhibited a weak Cu2O (hkl)
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peak at 36-37o. This small amount of Cu2O is believed to be due to re-oxidation of small Cu crystalline domains on the surface when the ECR nanowires were exposed to air, which was further supported by the transmission electron microscopy (TEM) characterization to be discussed in the following. While the two types of nanowires had similar dimensions, they were found to possess substantially different crystalline structures at the nanoscale. This was depicted by the selected area electron diffraction (SAED) patterns and dark-field TEM images collected for the two types of nanowires (Fig. 2). Besides bright spots corresponding to the crystal planes of face-centered cubic (fcc) Cu, the SAED pattern of the ECR nanowires also showed rings at 4.0 and 6.5 nm-1, which can be assigned to the reflections of the (111) and (220) planes of Cu2O (Figs. 2 a and c). On the other side, SAED pattern of the FGR nanowires only showed the spots corresponding to metallic Cu (Figs. 2 b and d). This observation, in combination with the XRD results presented above, suggests that a small amount of Cu2O exists in the ECR nanowires. For a closer look at the crystalline structures inside the ECR nanowires, separate dark-field TEM imaging was performed by using the electron beams associated with the Cu and Cu2O diffraction spots.24 It was found that the bulk of the ECR nanowires is mainly polycrystalline metallic Cu with the domain size ranging from 20-50 nm (Fig. 2e). The surface is covered by a thin layer of Cu2O nanocrystals that are smaller than 10 nm (Fig. 2g). Here the oxide is believed to be residues from the electrochemical reduction (carried out completely as indicated by Fig. S1), as otherwise the oxide would be CuO instead of Cu2O and present inside the nanowires rather than on the surface. In contrast, the FGR nanowires were found to be composed of large Cu crystal segments of >100 nm in size and without Cu2O present on the surface (Figs. 2 f and h). The dissimilar crystalline nanostructures are likely a result of the different reducing conditions.
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Compared to the annealing in forming gas at 300 oC, the electrochemical reduction was carried out at room temperature and the Cu atoms may not have possessed sufficient mobility to reach the equilibrium state, resulting in much smaller crystal domains and rougher surfaces. It is believed that small Cu crystal domains, also in the size range of −0.5 V. At potentials more negative than −0.5 V, HCOOH was produced with ~20% FE on both the pristine Cu gauze and the FGR nanowires. Small amounts of ethylene and ethane were detected below −0.6 V on the FGR nanowires, but the total FEs of C2 species were generally below 10%. No significant signal was observed for C2 species on the pristine Cu gauze. Compared to the previously reported results for extended-surface polycrystalline Cu electrodes, the ECR Cu nanowires reported here are much more active and selective at low overpotentials. CO2 reduction on polycrystalline Cu was negligible at E > −0.5 V, and it reached the maximum FE of 20% for CO at −0.8 V (vs. 61.8% at −0.4 V for the ECR nanowires).8, 26 The difference in
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selectivity is better visualized by plotting the partial current densities associated with CO production (jCO) and total current densities for CO2 reduction (jCO2) of different Cu electrodes together. Fig. 4 summarizes the comparison of the catalytic performance among the Cu nanowires and polycrystalline Cu,10 as well as the oxide derived Cu (OD-Cu) reported by Li et al.12 CO2 reduction to CO had an onset potential of about −0.3 V (say, e.g., to reach 0.1 mA/cm2) by the ECR Cu nanowires and the OD-Cu, as compared to −0.6 V for the FGR nanowires and −0.7 V for the polycrystalline Cu (Fig. 4a). At −0.5 V, jCO of the ECR nanowires reached ~1.2 mA/cm2, corresponding to an improvement factor of ~2 versus the OD-Cu (~0.59 mA/cm2) and ~18 versus the FGR nanowires (0.067 mA/cm2). Even considering the difference in roughness factor (see the Supporting Information, section S5), the ECR nanowires still shows an improvement factor of almost 2 compared to the FGR nanowires. The ECR nanowires are also much more selective for producing CO in the low-overpotential region, which had an FE towards CO of ~60% at −0.3 ~ −0.5 V in comparison to ~40% for the OD-Cu and 10 mA/cm2),12 as otherwise one would not expect to see the decrease of jCO (Fig. 4a). The drop of jCO observed here was likely because CO was further reduced and C−C bond formed, as evidenced by the production of C2 species below −0.5 V. From Figs. 4 a and b, it can also be seen that the pristine Cu gauze was much less active than polycrystalline Cu, indicating that the contribution of the gauze itself was negligible in the catalysts with Cu nanowire. The high activity and selectivity of the ECR Cu nanowires for CO2 reduction are intriguing, especially when compared to the FGR nanowires. Considering that these two types of nanowires
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had similar dimensions, it is believed that the high selectivity of the ECR nanowires originated from their unique surface structures. As discussed above for Fig. 2, the near-surface region of the ECR nanowires were composed of small (5 mA/cm2) (Fig. 3a). The observation of predominantly C2 products (ethane, ethylene and ethanol)
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and negligible amount of methane at the high overpotentials on the ECR Cu nanowires is also consistent with the results reported by Hori et al..10 However, the local pH effect may not be significant at the low-overpotential region, where the electrode current density is rather small (
