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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Selective Formation of C2 Products from Electrochemical CO2 Reduction over Cu1.8Se Nanowires Yuying Mi, Xianyun Peng, Xijun Liu,* and Jun Luo* Center for Electron Microscopy and Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
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S Supporting Information *
ABSTRACT: Electrochemical reduction of CO2 into higher-value C2 products (e.g., C2H4 and EtOH) is desirable for applications in chemical and fuel industries. Herein, we report that Cu1.8Se nanowires supported on Cu foam can function as an efficient electrocatalyst for selective reduction of CO2 to C2 products, with the Faradaic efficiencies for C2H4 and EtOH production reaching 55% and 24% at −1.1 V, respectively. Additionally, these nanowires also demonstrate a good longterm stability. We believe that the high C2/C1 product selectivity exhibited by the resulting catalyst is more likely due to a high CO2 pressure and the presence of Se species. KEYWORDS: Cu1.8Se nanowires, electrocatalysis, selectivity, CO2 reduction, C2 products, high CO2 pressure
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nanoparticles catalyst showed an FE of 44% C2H4, which was up to 4 times higher than that at 1 atm (10.81%).13 This is owing to an oversupply of CO2 pressure giving rise to a high CO2 concentration around the catalysts and increasing CO surface coverage, promoting CO dimerization and facilitating the formation of the C2 products.14,15 Inspired by these studies, we first designed and investigated CO2 electrochemical reduction on Cu1.8Se NWs grown on 3D Cu foam. It is expected that Se species on the Cu1.8Se NWs under a high CO2 pressure are beneficial to C2 product selectivity. As expected, the catalyst exhibited much better activity and selectivity for C2H4 and EtOH with higher FEs of 55% and 24% at −1.1 V, respectively. Robust stability is also demonstrated by 25 h of stable operation with C2 products’ current density of −14.6 mA cm−2 at −1.1 V. The simplified synthesis route of Cu1.8Se NWs is illustrated in Scheme 1. Typically, the Cu1.8Se NWs were first synthesized through simple chemical oxidization and selenization processes (see details in the Supporting Information). Successful realization of this strategy was clearly perceivable by the color evolution of the samples from reddish brown to light blue after oxidation for the formation of Cu(OH)2 NWs and then to dark brown after selenization treatment, as shown in the digital photograph (Supporting Information Figure S1). As shown in scanning electron microscopy (SEM) images in Figure 1a,b, the entire surface of the Cu foam was covered uniformly by densely packed Cu(OH)2 NWs (Figure 1a). A
lectrochemical reduction of CO2 (ERC) to value-added carbon-based products, which can be used directly as fuel and chemical feedstock, provides a “clean” and efficient way to mitigate energy shortage and to lower the global carbon footprint.1 In a typical ERC, H2O acts as a proton source and CO2 is reduced to various products based on a multielectron transfer mechanism.2,3 The main target products are often classified into C1 product (e.g., CO, CH4, CH3OH, and HCOOH, etc.) and C2 product (e.g., C2H4, C2H5OH, and CH3COOH, etc.). C2 products are more desirable for applications because of their higher value and energy density compared to the C1 products.4 Therefore, the development of catalysts with appropriate electronic properties becomes critical and more industrially desirable for tuning the selectivity for C2 products. Cu-based materials exhibit high activity and selectivity toward valuable multicarbon products (for example, C2H4 and EtOH) due to their unique electron properties.5 Recently, chalcogen species have been proposed to facilitate the C2 selectivity of Cu-based catalysts,6−11 which was mainly attributed to the adsorption of chalcogen species decreasing the surface concentration of CO and suppressing the formation of C1 products. For example, oxygen species on a Cu nanocube catalyst played the even more important role than Cu(100) facets for the formation of C2 products.6 The presence of oxygen species on the catalyst made the Faraday efficiency (FE) up to ∼45% for C2H4 and ∼22% for EtOH. Similarly, the introduction of sulfide species on a Cu electrode also enhanced C2 product selectivity with an FE of 30%, whereas that for methane formation remarkably decreased.11 Moreover, the selectivity of C2 products can be also tuned as a function of CO2 pressure.12−15 For example, at 9 atm, a Cu © XXXX American Chemical Society
Received: May 10, 2018 Accepted: October 1, 2018 Published: October 1, 2018 A
DOI: 10.1021/acsaem.8b00744 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 2a. As shown in Figure 2a, the three strong diffraction peaks labeled as # come from the Cu foam (JCPDS No. 85-
Scheme 1. Schematic Illustration of the Synthetic Process of Cu1.8Se NWs
close view (Figure 1b) revealed such NWs extend vertically to the Cu foam with diameters of about 140.6 nm and lengths of up to several micrometers (Figure S2). After selenization, the appearance of the Cu1.8Se NWs (Figure 1c,d) are rough and much more protuberances can be observed. Compared with the smooth Cu(OH)2 NWs, plentiful particles are formed on the surface of the NWs due to the deformation caused by stretching in the selenization process. These features can be further evidenced by transmission electron microscopy (TEM; Figure 1e). Corresponding high-resolution TEM analysis (Figure 1f) at the middle section of an individual NW exhibits clear lattice fringes, and the lattice spacing is measured to be about 0.33 nm, which can be assigned to the interplanar separation of the (111) plane of the Cu1.8Se phase. The presence of an element on the NWs is confirmed using EDS elemental mapping (Figure 1f), which shows Cu and Se elements are uniformly distributed along the whole NW. These results imply that the presynthesized Cu(OH)2 are completely converted into Cu1.8Se phase. X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the detailed phase and composition of the asprepared samples. Typical XRD patterns of the pure Cu foam, Cu(OH)2, and Cu1.8Se samples are displayed in Figure S3 and
Figure 2. Structural characterization of Cu1.8Se NWs: (a) XRD pattern; (b) Raman spectra; (c, d) XPS spectra of (c) Cu 2p and (d) Se 3d.
1326) (Figure S3). All other diffraction peaks can be indexed to orthorhombic Cu(OH)2 (JCPDS No. 35-0505).16 After selenization, the sharp and strong diffraction peaks marked as ″$″ in Figure 2a can be assigned to Cu1.8Se phase (JCPDS No. 71-0044), indicating its high crystallinity. The Raman spectrum of the Cu1.8Se NWs (Figure 2b) shows a distinct peak located at 258.2 cm−1, which could be assigned to the lattice vibrations of Cu1.8Se.17 In addition, no Cu−O vibrations
Figure 1. Morphological characterizations: (a, b) SEM images of Cu(OH)2 NWs; (c, d) SEM images of Cu1.8Se NWs; (e) TEM image of the Cu1.8Se NWs; (f) high-resolution TEM image and element mapping of Cu1.8Se NWs. B
DOI: 10.1021/acsaem.8b00744 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 3. ERC electrochemical characterization: (a) LSV curves measured in Ar or CO2-saturated solutions at a scan rate of 10 mV s−1 under 10 atm Ar or CO2 atmosphere pressure; (b) FE of C1 and C2 products at various applied potential; (c) FE of formate, CO, C2H4, and EtOH products at various potentials for Cu1.8Se catalyst; (d) relative ratios of the FE; (e) chronoamperometry results at −1.1 V and FE of C2 product; and (f) SEM image of Cu1.8Se NWs after 25 h electrolysis.
7.4 times higher than that of Cu(OH)2 foam (5.2 mF cm−2). The high ECSA Cu1.8Se NWs are attributed to the formation of nanoparticles on the Cu1.8Se NWs in the selenization process, which can significantly increase catalytically active sites. The possible products of ERC were analyzed using an online gas chromatograph and a nuclear magnetic resonance (NMR) spectrometer. As displayed in Figure S6, the main ERC product of the pure Cu foam is hydrogen (H2), which from HER competes with the ERC in the aqueous electrolyte.21 The carbonaceous product on the Cu foam includes a small quantity of C1 and C2 products with the FEs of 12% and 4% at −1.1 V, respectively. However, a dramatically improved C−C coupling selectivity for C2 products is observed on Cu1.8Se catalyst (Figures 3b and S7). As shown, the main products are still an abundance of H2 (56%) (Figure S7) and 42% C1 + C2 (Figure 3b) at the potential of −0.7 V. Beyond this potential, a substantial rise in C2 FE (79%) is observed, whereas the FE of C1 products is only 5%. When the applied potential continues up to −1.3 V, the C1 formation is almost completely suppressed (
