Rapid and Scalable Synthesis of Cuprous Halide-Derived Copper

Subscriber access provided by ALBRIGHT COLLEGE

Communication

Rapid and Scalable Synthesis of Cuprous Halide-Derived Copper NanoArchitectures for Selective Electrochemical Reduction of Carbon Dioxide Huan Wang, Edward Matios, Chuanlong Wang, Jianmin Luo, Xuan Lu, Xiaofei Hu, and Weiyang Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01197 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Rapid and Scalable Synthesis of Cuprous Halide-Derived Copper NanoArchitectures for Selective Electrochemical Reduction of Carbon Dioxide Huan Wang,† Edward Matios,† Chuanlong Wang,† Jianmin Luo,† Xuan Lu,† Xiaofei Hu,† Weiyang Li†,* †Thayer

School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, New

Hampshire 03755, USA *To

whom correspondence may be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Electrochemical reduction of carbon dioxide (CO2) into value-added chemicals and fuels provides a promising pathway for environmental and energy sustainability. Copper (Cu) demonstrates a unique ability to catalyze the electrochemical conversion of CO2 into valuable multicarbon products. However, developing a rapid, scalable and cost-effective method to synthesize efficient and stable Cu catalysts with high selectivity towards multicarbon products at a low overpotential is still hard to achieve and highly desirable. In this work, we present a facile wet chemistry approach to yield well-defined cuprous halide (CuX, X=Cl, Br or I) microcrystals with different degrees of truncations at edges/vertices, which can be ascribed to the oxidative etching mechanism of halide ions. More importantly, the as-obtained cuprous halides can be electrochemically transformed into varied Cu nano-architectures, thus exhibiting distinct CO2 reduction behaviors. The CuI-derived Cu nanofibers composed of self-assembled nanoparticles are reported for the first time, which favor the formation of C2+3 products at a low overpotential with a particular selectivity towards ethane. In comparison, the Cu nanocubes evolved from CuCl are highly selective towards C1 products. For CuBr-derived Cu nanodendrites, C1 products are subject to form at a low overpotential, while C2+3 products gradually become dominant with a favorable formation of ethylene when the potential turns more negative. This work explicitly reveals the critical morphology effect of halide-derived Cu nanostructures on the CO2 product selectivity, and also provides an ideal platform to investigate the structure-property relationship for CO2 electroreduction. KEYWORDS: electrochemical reduction of carbon dioxide, rapid and scalable synthesis, cuprous halide, copper nano-achitectures, morphology effect

1


Page 2 of 23

Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

TOC

2


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Modern energy and chemical industries highly depend on fossil resources, resulting in massive emission of carbon dioxide (CO2), which is known as a significant contributor to greenhouse effect.1 Mitigating greenhouse emission by converting CO2 into value-added products is critical for environmental protection and energy sustainability. The CO2 electrochemical reduction reaction (CO2RR) is one of the most attractive approaches as it facilitates a sustainable redox cycle using only water and electricity as inputs at room temperature, ambient pressure and in neutral pH media.2,3 Linear CO2 molecule is extremely stable, hence highly efficient and stable catalysts are required for the conversion process to ensure the efficiency and selectivity of the electrochemical transformations.4,5 Studies in recent years have advanced the development of various electrocatalysts, such as metals,6-11 metal oxides and chalcogenides,12-14 metal complexes,15,16 and carbon-based materials.17,18 Among these electrocatalysts, metallic copper (Cu) stands out due to its unique capability of converting CO2 into high-value multicarbon products.19 Nevertheless, bulk Cu is neither efficient nor selective for CO2RR due to its weakly coordinated surface sites.20 Thus, a variety of Cu micro- and nano-structures have been developed through different processes, including reduction of Cu oxides,21-23 wet chemistry syntheses,24-26 and electrodeposition,27-29 which showed improved activity and selectivity towards the production of hydrocarbons or oxygenates. Specifically, direct utilization of Cu foil through high-temperature annealing or oxygen plasma treatment to first obtain oxidized Cu followed by the reduction back to Cu (the so-called oxide-derived Cu) has been considered as a practical and scalable approach to prepare efficient Cu catalysts for CO2RR.21-23 Using oxide-derived Cu foil can avoid the employment of extra substrates or binders and skip the slurry preparation procedures of loading the solution-synthesized Cu micro- or nano-structures onto the electrode. However, the pre-oxidization methods always require a high-temperature or high-energy treatment, and the resulting structures generally possess a poorly defined morphology,21-22 making it rather difficult to study the structure-property relationship for CO2RR. Compared to oxygen, halogens (right next to the oxygen family in the periodic table), particularly chlorine, bromine and iodine, can also easily combine with Cu element, forming cuprous halides, which are promising candidates as precursors to synthesize Cu electrocatalysts. So far, very few methods have been reported on synthesizing uniform and well-defined cuprous halide micro- or nano-structures, not to mention the studies through direct use of cuprous halides

3


Page 4 of 23

Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

as starting materials to synthesize Cu nanostructures for CO2RR. For halide-ion-involved CO2RR, several prior studies showed that plasma-activated or anodized Cu in the presence of potassium chloride (KCl) could improve the product selectivity towards multicarbon products.24,30-32 However, the influence of Cu morphology on the resulting products still remains elusive. This is because the initial state of Cu was either under plasma treatment or electric field, which cannot rule out the effect of Cu2O on Cu surface. Recent work reported selective C2H4 production on bromide (Br)-promoted Cu dendrites by adding 0.1 M potassium bromide (KBr) into the electrolyte with a maximum Faradaic efficiency of 57%, but with fast structural degradation and performance decay possibly due to the use of acidic electrolyte.33 Meanwhile, the high concentration of Br- or Cl- anions added into the electrolyte makes it hard to determine which factor (such as, structural morphology, electrolyte pH/composition, or Cu oxidation state) plays the dominant role in influencing the product selectivity. A systematic study of the morphology/shape dependence of Cu catalysts on the CO2RR behaviors is still lacking. To clearly reveal the morphology effect, the CO2RR needs to be conducted in a neutral electrolyte and be separated from the synthesis process of the starting material, which can exclude the contributions from the high-concentration halide anions adsorption or the oxidation state of Cu. In addition, a rapid, scalable and cost-effective synthesis of highly stable Cu-based catalysts with high CO2RR selectivity and efficiency towards multicarbon products is still highly desirable. Herein, we employed a facile wet chemistry method to rapidly synthesize three types of well-controlled cuprous halide (CuX, X=Cl, Br or I) microcrystals. Interestingly, compared to the tetrahedral shapes of CuCl and CuBr microcrystals with respective severe and slight truncations at edges/vertices, CuI microcrystals exhibit tetrahedral morphology with sharp edges/vertices. The different degrees of truncations of the as-obtained cuprous halide microcrystals can be ascribed to the differences in the oxidative etching ability of halide ions in the presence of oxygen (O2). More importantly, we find that different cuprous halides result in varied Cu nano-architectures after electroreduction, exhibiting distinguished CO2RR behaviors. The Cu nanocubes derived from CuCl favor the formation of C1 products with two-electron transfer over the measured voltage range from -0.95 V to -0.5 V (vs. reversible hydrogen electrode, RHE), while the Cu nanofibers derived from CuI show high selectivity toward C2+3 products at the same voltage range with the highest Faradaic efficiency of 57.2% at -0.735 V vs. RHE. As to Cu nanodendrites derived from CuBr, C1 products are predominant at a low

4


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

overpotential while C2+3 products gradually dominate as the voltage turns more negative, where the selectivity of C2+3 products finally reaches its highest Faradaic efficiency of 53.5% at -0.85 V vs. RHE. Moreover, regarding the components of C2+3 products, Cu nanofibers exhibit a distinct capability in producing ethane (C2H6) while Cu nanodendrites are inclined to promote C2H4 formation. We also reveal that the morphology effect of Cu nanostructures evolved from different cuprous halides dominates the CO2RR product selectivity over the role of halide ions. Our study provides a promising alternative to develop efficient and cost-effective catalysts with high selectivity towards multicarbon products, and also offers an ideal platform to evaluate the critical role of catalyst morphology on the CO2RR performance. Figure 1a schematically illustrates the rapid and scalable aqueous-solution-based synthesis of cuprous halides as well as the conversion to different nanostructured Cu as catalysts for CO2RR with distinct product selectivity. Typically, one piece of electropolished Cu foil was immersed, for around 5 minutes, in the aqueous solution consisting of copper sulfate (CuSO4), sulfuric acid (H2SO4) and corresponding halide salts (NaCl, KBr or KI), leading to the formation of CuX (X=Cl, Br or I). The reaction can be expressed by the following equations: Cu+CuSO4+H2SO4+4X- → 2[CuX2]- + 2SO42- +2H+

(1)

[CuX2]- → CuX↓+X- (X=Cl, Br or I)

(2)

Through the above simple chemical reactions in the solution at room temperature, microcrystals of cuprous halides with well-controlled tetrahedral shapes but different degrees of truncations at edges/vertices are achieved (grown onto the Cu foil). Specifically, the CuCl microcrystals exhibit high degree of truncation at both edges and vertices, while the CuBr ones present slightly truncated edges/vertices. In contrast, the CuI tetrahedrons show relatively sharp edges and vertices. After electrochemical reduction, three distinct Cu nanostructures including Cu nanocubes, Cu nanodendrites and Cu nanofibers are formed from the as-prepared CuCl, CuBr and CuI microcrystals. These three different Cu nanostructures exhibit distinctive selectivity in terms of carbon monoxide (CO), C2H4 and C2H6, respectively, as well as towards C1 and C2+3 products. Figure 1b displays the typical photographs of a piece of large-sized Cu foil (1.5 inches  1.5 inches) before and after the formation of CuI tetrahedrons and Cu nanofibers, showing the highly scalable synthesis of Cu-based nanocatalysts. It can be clearly observed that the surface of the electropolished Cu foil before treatment exhibits shiny gold color, and then becomes brickcolor without metallic luster when CuI tetrahedrons are formed on the surface. The color finally

5


Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

turns black once CuI is electrochemically reduced to Cu nanofibers. Note that the as-formed surface shows a uniform color, indicating the homogenous formation and distribution of CuI tetrahedrons and Cu nanofibers over a large area. Scanning electron microscopy (SEM) characterization was carried out to reveal the morphology of the as-obtained samples before and after electrochemical reduction. As shown in Figure 2a-c, the three types of cuprous halide microcrystals exhibit a similar profile of a tetrahedron-dominated morphology as illustrated in Figure 1. The degree of truncation increases in the order of CuII-,35 which is also consistent with the different degrees of truncation of the as-obtained microcrystals. The energy dispersive spectroscopy (EDS) mappings of element Cl, Br, I for CuCl, CuBr, CuI samples are shown in Figure S1, revealing the composition profiles of respective microcrystals. Insets in Figure 2a-c are the high-resolution transmission electron microscope (TEM) images of CuCl, CuBr, and CuI samples, which present interlayer spacing of 0.310 nm, 0.320 nm and 0.345 nm, corresponding to the (111) planes of CuCl, CuBr and CuI, respectively. This indicates that these cuprous halide tetrahedrons or truncated tetrahedrons are mainly enclosed by {111} facets. Meanwhile, the tetrahedral shape of cuprous halides cannot be obtained if there is no CuSO4 in the solution (Figure S2), which verifies that the reaction follows the above-mentioned chemical equations. To achieve active catalysts, a constant potential was applied on the samples in the presence of CO2-saturated potassium bicarbonate (KHCO3) electrolyte to reduce the cuprous halides. Interestingly, the tetrahedral microcrystals are electrochemically transformed into three types of Cu nanostructures with completely different morphologies, respectively (Figure 2d-f). For CuCl, uniform cubes with a size of around 800 nm are formed after electroreduction. With respect to CuBr, dendrite-like nanostructures are obtained. In terms of CuI, bundles of nanofibers can be clearly observed in a relatively lowmagnification SEM image (Figure 2f). These nanofibers are composed of nanoparticles with an average size of around 50 nm, which could be identified in the corresponding high-resolution

6


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SEM and TEM images (Figure S3 and the inset of Figure 2f). The difference in the final morphology of Cu nanostructures electrochemically reduced from cuprous halide microcrystals may originate from their respective stability in CO2-saturated electrolyte as well as the induced effect by the corresponding halide anions, which will be discussed in detail below. To evaluate the stability of cuprous halides before electrochemical reduction, SEM characterization was performed on the as-received samples that were immersed in the CO2saturated KHCO3 electrolyte for 15 mins. As shown in Figure S4, the morphologies of CuBr and CuI microcrystals could be well maintained, indicating that the resulting nanodendrites and nanofibers should evolve from CuBr and CuI, respectively. While the CuCl microcrystals change into cubic structures, which is consistent with the previous report.24 These results suggest that the as-formed CuBr and CuI are stable in the above aqueous solution, while CuCl is unstable before transformation into Cu nanocubes. To further determine the crystallinity and compositions of cuprous halides at different evolution stages, X-ray diffraction (XRD) analysis was conducted on the samples before and after electroreduction, as well as those immersed in the CO2-saturated KHCO3 electrolyte for 15 mins. Before immersing into the electrolyte, the major characteristic peaks of CuX (X=Cl, Br or I) can be easily detected in the XRD patterns (Figure 2g-i), along with strong diffraction peaks from the Cu substrate (marked with green square). After immersion in the electrolyte but before electroreduction, all the characteristic peaks of CuBr and CuI could still be detected, therefore confirming that both compositions and structures of CuBr and CuI remain unchanged. In comparison, all the characteristic peaks associated with CuCl have disappeared after immersion, and two new peaks appear, corresponding to the (021) plane of Cu(OH)2 (PDF no. 3-307) and the (111) plane of Cu2O (PDF no. 2-1067), respectively. This indicates that CuCl is unstable in the CO2-saturated KHCO3 electrolyte and can easily transform into other phases prior to the reduction. After a constant potential was applied to complete the electrochemical reduction process, Cu peaks become dominant. Since Cu nanofibers composed of self-assembled nanoparticles are reported for the first time in this work, we further investigated the morphology evolution of tetrahedral CuI microcrystals over the electrochemical reduction process to understand the formation process of the as-received Cu nanofibers (Figure S5). After electrochemical reduction for 30 s, the tetrahedral shape can still be maintained. While after 60 s and 120 s of reduction, nanoparticles form on the surface and gradually assemble into fibers. After 300 s, the nanofibers fully cover

7


Page 8 of 23

Page 9 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

the surface. Extensive high-resolution TEM images (Figure 3a-b and Figure S6) all show an interplanar spacing of 0.203-0.206 nm measured by integrating the pixel intensities over ten atomic layers. This is in agreement with the theoretical lattice parameter of metallic Cu (111), which indicates that a significant portion of Cu (111) facets exposed on the Cu nanoparticles that constitute the Cu nanofibers derived from CuI. Cu Auger LMM XPS spectrum was further carried out to identify the valence state of Cu in the sample of nanofibers after 30 minutes of electrochemical reduction. No peaks at ~916.9 eV corresponding to Cu+ could be observed (Figure 3c), which suggests the complete transformation from CuI tetrahedrons to metallic Cu nanofibers via electrochemical reduction in 30 minutes. Previous work reports that the presence of I- anions can promote the anisotropic growth of gold nanoparticles.36 In our case, we hypothesize that the minute amount of residual I- anions derived from CuI on the Cu surface could direct the self-assembly of nanoparticles and promote the growth of anisotropic structure into nanofibers via cathodic reduction with a negative potential. Even with the absence of CO2 in the solution, the CuI tetrahedrons can remain stable (Figure S7) and the formation of Cu nanofibers derived from CuI microcrystals undergoes a similar evolution process (Figure S8), indicating that the morphological evolution is not related to CO2 reduction. In this case, wellaligned Cu nanofiber arrays are more likely to be obtained possibly due to the absence of CO2 bubbles perturbation. Moreover, extensive SEM images were taken on the inch-sized foil in Figure 1b before and after reduction (Figures S9 and S10), indicating that the Cu foil is uniformly covered by the highly dense CuI microcrystals and Cu nanofibers, respectively. The representative cross-section SEM images of Cu nanofibers (Figure 3d and Figure S11) cut by focused ion beam (FIB) confirm the dense distribution of nanostructures with a thickness of around 4 m. X-ray photoelectron spectroscopy (XPS) was performed before and after the reduction process to examine the halide anions distribution. Obvious peaks associated with Cl, Br, I in CuCl, CuBr, and CuI, respectively, are clearly observed (Figure 3e and Figure S12). The signals of both Cl and Br become extremely weak after 30 minutes of electrochemical reduction (Figure S12), indicating a trace amount of Cl- or Br- anions residue on the surface of the Cu nanocubes and nanodendrites. But peaks corresponding to I- can still be detected at different time points during reduction and the intensities become around 10%, 7%, 5% and 2% after 0.5 hours, 2 hours, 5 hours and 8 hours, respectively, compared to the peak intensities before reduction

8


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(Figure 3e-f), while the valence states of Cu remain unchanged (Figure S13). This indicates that longer reduction time was necessary for the removal of I- anions absorbed on the Cu nanoparticles due to a stronger binding energy of I- anions towards the surface.37 Assuming that CuI microcrystals can be completely reduced to metallic Cu nanofibers and all the I- anions in CuI can diffuse into the electrolyte, the I- concentration is calculated to be around 0.6 mM. This is around 500 times lower than that of the previous work on studying the halide effect on CO2RR performance by adding very high concentration of halide ions (0.3 M).30 For simplicity, the Cu nanocubes derived from CuCl, the Cu nanodendrites evolved from CuBr and the Cu nanofibers derived from CuI, are denoted as Cu NCs (Cl), Cu NDs (Br), Cu NFs (I), respectively, in the following content. These three types of Cu nanostructures provide ideal platforms to explore the structureproperty relationships and to reveal the correlation between product selectivity and Cu morphology. The CO2RR catalytic activity and selectivity based on electropolished Cu foil, Cu NCs (Cl), Cu NDs (Br), Cu NFs (I) were evaluated under identical condition of CO2-saturated 0.1 M KHCO3 aqueous electrolyte at a potential range between -0.5 V to -0.95 V (vs. RHE). The gas and liquid products were quantified by gas chromatography (GC) and nuclear magnetic resonance (NMR) spectroscopy, respectively. Figure 4 compares the CO2RR catalytic activity and selectivity using the above-mentioned catalysts as well as Cu foil as a control. The potentialdependent current densities of the Cu NCs (Cl), Cu NDs (Br), Cu NFs (I) and Cu foil, were recorded in order to evaluate the respective electrocatalytic activity. In comparison with Cu foil, Cu NCs (Cl), Cu NDs (Br), Cu NFs (I) all show much higher current densities, which can be partially attributed to the significantly increased surface roughness for increased catalytic activities (Figure S14). The surface roughness factor can be estimated by comparing the measured double-layer capacitance derived from cyclic voltammetry (CV) curves with the value for the electropolished Cu electrode (Figure 4a and Figure S14). Among the three kinds of catalysts, Cu NFs (I) catalyst shows the highest geometric current density over the entire voltage region due to the highest roughness, which results in the largest amounts of electrochemically active sites. The Faradaic efficiencies of CO2RR products at various potentials are presented in Figure 4c-f. Hydrogen (H2) gas was also detected from the competing H2 evolution reaction (Figure 4b). No methane (CH4) was detected in the systems using the four types of catalysts over the applied

9


Page 10 of 23

Page 11 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

potential region. For Cu foil, the Faradaic efficiency of H2 is the highest over the measured potential range, where formate is the major CO2RR product (42.0% of Faradaic efficiency at 0.82 V vs. RHE). We find that the Cu nanostructures derived from cuprous halides can not only improve the CO2RR activity of Cu, but also have a remarkable effect on the product selectivity. The onset potentials for all the CO2RR products shift to lower overpotentials compared to Cu foil. For Cu NCs (Cl), the selectivity towards formate and CO can reach a maximum Faradaic efficiency of 37.4% and 17.2%, respectively, at -0.6 V and -0.72 V vs. RHE. The Faradaic efficiency of CO from Cu NCs (Cl) is around 2-3 times higher than the one from Cu NDs (Br) and the one from Cu NFs (I). As to Cu NDs (Br), we observe a high selectivity towards C2H4, reaching 38.5% at -0.85 V vs. RHE. An increasing trend of C2H4 selectivity in the Cu NCs (Cl) sample is observed at -0.8 V vs. RHE and more negative potential, which is in line with the reported results about Cu nanocubes for CO2RR.24,38 Interestingly, Cu NFs (I) exhibits a unique selectivity towards C2H6 (30% of Faradaic efficiency at -0.735 V vs. RHE). In contrast, C2H6 is observed as a minor product for Cu NCs (Cl) or Cu NDs (Br) over the entire potential region. Additionally, it was reported that no C2H6 is detected in the presence of I- as an electrolyte additive using plasma-activated Cu catalyst.30 Therefore, we believe the unique selectivity towards C2H6 in the sample of Cu NFs (I) is attributed to its distinct morphology of nanofibers derived from its starting material of CuI microcrystals. C2H6 is not a common product of CO2RR, which can only be observed in the samples with very rough surface. However, in most cases, C2H6 is mainly detected as a minor CO2RR product, which exhibits a relatively low Faradaic efficiency. The proposed formation mechanism of C2H6 on Cu (111) facets involves the following two possible pathways: i) the dimerization of adsorbed -CH3,39,40 and ii) the hydrogenation of C-C coupling (Figure S15).41-45 As -CH3 is proposed as the only intermediate for CH4 formation,41,46 the absence of CH4 product in the sample of Cu NFs (I) indicates that the C2H6 formation is unlikely to follow the first pathway. For the second pathway, the final hydrogenation to C2H6 is generally hard to take place on Cu (111) facets at a neutral pH, but could be activated by specifically designed Cu nanostructures.45 In our case, we deduce that the distinct morphology of the Cu nanofibers (composed of self-assembled Cu nanoparticles), including anisotropic property and high surface roughness, could suppress the fast β-elimination process and thus enable the C2H6 production. This also agrees with the explanation reported by Broekmann et al.41 Meanwhile, compared to

10


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C2H4, C2H6 formation is more favorable at the potential above -0.735 V vs. RHE, and then gradually reverses at more negative potential, which suggests that the C2H6 is formed at the expense of C2H4. Hence, the selective formation of C2H6 in the sample of Cu NFs (I) can be attributed to the hydrogenation of C2H4. We hypothesize that the rough surfaces of nanofibers improve the trapping efficiency of adsorbed H2, which is a very important intermediate for C2H6 formation. The obvious drop in H2 formation at around -0.735 V vs. RHE, where the C2H6 selectivity reaches the highest also supports our hypothesis. To further reveal that the specific morphology of Cu nanofibers is the key factor for producing C2H6, the as-prepared Cu nanofibers after 2 hours of electrochemical reduction were sonicated in CO2-saturated KHCO3 aqueous electrolyte for around 12 hours to obtain relatively homogenous Cu nanoparticles. After lyophilization, no obvious structures of Cu nanofibers can be observed in the SEM images, leaving Cu nanoparticles dispersed in a homogenous manner with a tiny amount (~2%) of I- anions residue (Figure S16 and S17). Then the catalyst ink was prepared via mixing the as-obtained Cu nanoparticles, Nafion binder in ethanol/water solution, and then drop-casted onto the Cu substrate followed by drying. As a result, the morphology becomes much smoother with a much lower surface roughness (Figure S18 and S19), exhibiting a high selectivity towards C2H4 with C2H6 as a very minor product (Figure S20). This is very similar to the CO2RR behavior for the sample of Cu nanocubes derived from CuCl, which is possibly due to the similar surface roughness. This confirms that the decisive factor for the production of C2H6 is mainly ascribed to the specific morphology of Cu nanofibers, rather than the I- adsorbates. To verify that the halide anions make an insignificant contribution in CO2RR performance, we changed the electrolyte after CuI was electrochemically reduced for around 2 hours in CO2saturated KCHO3 electrolyte and used newly prepared CO2-saturated KCHO3 electrolyte for subsequent CO2RR. The Cu nanofibers still exhibit a unique selectivity towards C2H6 (Figure S21), which is very similar to the results without changing the electrolyte. This demonstrates that the halide ions in our system make no difference in the CO2RR behaviors. For the formation of ethanol (C2H5OH) and n-propanol (n-C3H7OH) (Figure 4g and 4h), the halide-derived Cu nanostructures all display enhanced Faradaic efficiencies compared to Cu foil. Notably, the overall current densities normalized to electrochemically active surface areas (ECSAs) for the above three kinds of Cu nano-architectures are similar (Figure S22), and the

11


Page 12 of 23

Page 13 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

actual increased ratios of current density are smaller than those of the respective ECSAs. This suggests that the improvement in CO2RR performance of three distinct Cu nanostructures is mostly ascribed to the enhanced inherent activity. Figure 5a and 5b compare the total Faradaic efficiencies of C2+3 and C1 products, respectively. Compared with Cu foil and Cu NCs (Cl), a drastic increase in C2+3 production is observed in the samples of Cu NDs (Br) and Cu NFs (I). For halide-derived Cu nanostructures, the onset potentials of C2+3 product formation all shift to positive direction (Figure 5a). For the sample of Cu NFs (I), the Faradaic efficiency of C2+3 products can reach 11.3% at only -0.58 V vs. RHE, indicating that onset potential is significantly reduced. Beyond this potential, a substantial rise in C2+3 Faradaic efficiency is observed with the highest selectivity (57.2% of Faradaic efficiency) achieved at -0.735 V vs. RHE. This is approximately five times higher than that of C1 products (12.4% of Faradaic efficiency) with two-electron transfer, including CO and formate (Figure 5b). Compaed to the previously reported catalysts in neutral pH aqueous media (Figure 5c),22,25-26,28,30-32,39-41,47-50 the highest selectivity towards C2+3 products in this study is competitive at a much lower overpotential. The sample of Cu NDs (Br) also exhibits its highest selectivity towards C1 products (52.0% of Faradaic efficiency) at -0.56 V vs. RHE, which gradually decreases as the potential turns more negative with the enhanced C2+3 production. At 0.85 V vs. RHE, the selectivity towards C2+3 products reaches its highest Faradaic efficiency of 53.5%, where the C1 selectivity decreases to 18.1%. In contrast, the sample of Cu NCs (Cl) exhibits a high selectivity toward C1 products at potential range from -0.5 V to -0.95 V (vs. RHE), and the selectivity of which attains its maximum value of 49.3% at -0.67 V. An increase in C2+3 selectivity can be observed as potentials negatively shift, indicating the Cu nanocubes favor the formation of C1 products at a smaller overpotential. In addition to Faradaic efficiency, long-term stability of the catalysts is another crucial factor to evaluate the CO2RR performance. As the C2H4 and C2H6 are the major and distinct products for the sample of Cu NDs (Br) and Cu NFs (I), we monitored the respective product evolution for 500 minutes of continuous catalysis. As shown in Figure 5d, the current density of Cu NDs (Br) is relatively stable. The higher current density at initial stages is due to the reduction of CuBr to Cu nanodendrites. Moreover, the C2H4 selectivity consistently maintains at around 38±2%. The product selectivity in the sample of Cu NDs (Br) is in accordance with the results on Br-promoted Cu dendrites by adding 0.1 M potassium bromide (KBr) into the

12


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrolyte. But in our case, the Cu NDs (Br) exhibit much better electrochemical stability, which is possibly due to the use of neutral electrolyte without the addition of high concentration of Branions.33 This also verifies our points that it is essential to separate the CO2RR from the synthesis process of the starting material. A similar phenomenon is also observed for the case of Cu NFs (I), indicating the good stability of halide-derived Cu nanostructured catalysts. Notably, even the concentration of I- anions dramatically decreases as the reduction time proceeds (Figure 3e-f), the CO2RR catalytic performance of Cu NFs (I) towards C2H6 can remain stable for over 8 hours (Figure 5e), further confirming the negligible effect of I- anions ions in promoting the C2H6 formation. Hence, in our case, it is reasonable to exclude the dominant role of halide ions on the CO2RR behavior. In summary, we report a scalable and rapid route for the synthesis of three kinds of cuprous halide microcrystals including CuCl, CuBr and CuI on Cu foil through a facile wet chemistry reaction. The as-obtained cuprous halide microcrystals exhibit a tetrahedron-dominated morphology but with different degrees of truncation at edges/vertices. This could be explained by the differences in the etching ability of halide ions in the presence of O2. Furthermore, three kinds of distinguished Cu nano-architectures including nanocubes, nanodendrites and nanofibers are acquired by electrochemically reducing the as-prepared CuCl, CuBr, CuI microcrystals, respectively, revealing entirely different CO2RR behaviors. CuI-derived Cu nanofibers, as the first reported Cu nanostructures via the self-assembly of nanoparticles, exhibit high selectivity towards C2+3 products at a relatively low overpotential with C2H6 as the major product. In contrast, Cu nanocubes derived from CuCl favor the formation of C1 products. For CuBr-derived Cu nanodendrites, C1 products are predominant at a low overpotential, while C2+3 products gradually dominate as the potential turns more negative with a unique selectivity towards C2H4. Our findings reveal that the morphology effect of halide-derived Cu nanostructures plays a dominant role in influencing the CO2RR product selectivity over the role of halide ions. This work provides an ideal platform to relate the critical dependence of CO2RR selectivity on the catalyst morphology. ASSOCIATED CONTENT

13


Page 14 of 23

Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Supporting Information. Details about material synthesis, characterization and electrocatalytic performance of CO2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors greatly acknowledge the support from the start-up funds at Thayer School of Engineering, Dartmouth College. The authors thank Prof. Jifeng Liu and Mr. Sidan Fu for their help on the operation of focused ion beam (FIB). The authors also thank Dr. Min Li from Yale University on the assistance of XPS characterization.

REFERENCE (1) Pacala, S.; Socolow, R. Science 2004, 305, 968-972. (2) Montoya, J. H.; Seitz, L. C.; Chakthranont, P.; Vojvodic, A.; Jaramillo, T. F.; Norskov, J. K. Nat. Mater. 2017, 16, 70-81. (3) Liu, C.; Colon, B. C.; Ziesack, M.; Silver, P. A.; Nocera, D. G. Science 2016, 352, 1210-1213. (4) Zhu, D. D.; Liu, J. L.; Qiao, S. Z. Adv. Mater. 2016, 28, 3423-3452. (5) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.; Peden, C. H. F.; Portis, A. R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek, J. N. H.; Seefeldt, L. C.; Thauer, R. K.; Waldrop, G. L. Chem. Rev. 2013, 113, 6621-6658. (6) Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 14, 1695-1698. (7) Medina-Ramos, J.; Pupillo, R. C.; Keane, T. P.; DiMeglio, J. L.; Rosenthal, J. J. Am. Chem. Soc. 2015, 137, 5021-5027. 14


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8) Gao, D. F.; Zhou, H.; Wang, J.; Miao, S.; Yang, F.; Wang, G. X.; Wang, J. G.; Bao, X. H. J. Am. Chem. Soc. 2015, 137, 4288-4291. (9) Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G. G.; Jiao, F. Nat. Commun. 2014, 5, 3242. (10) Jiang, K.; Siahrostami, S.; Zheng, T.; Hu, Y.; Hwang, S.; Stavitski, E.; Peng, Y.; Dynes, J.; Gangisetty, M.; Su, D.; Attenkofer, K.; Wang, H. Energy Environ. Sci., 2018, 11, 893-903. (11) Zheng, T.; Jiang, K.; Ta, N.; Hu, Y.; Zeng, J.; Liu, J.; Wang, H. Joule, 2019, 3, 265-278. (12) Gao, S.; Lin, Y.; Jiao, X. C.; Sun, Y. F.; Luo, Q. Q.; Zhang, W. H.; Li, D. Q.; Yang, J. L.; Xie, Y. Nature 2016, 529, 68-71. (13) Asadi, M.; Kim, K.; Liu, C.; Addepalli, A. V.; Abbasi, P.; Yasaei, P.; Phillips, P.; Behranginia, A.; Cerrato, J. M.; Haasch, R.; Zapol, P.; Kumar, B.; Klie, R. F.; Abiade, J.; Curtiss, L. A.; Salehi-Khojin, A. Science 2016, 353, 467-470. (14) Xu, J. Q.; Li, X. D.; Liu, W.; Sun, Y. F.; Ju, Z. Y.; Yao, T.; Wang, C. M.; Ju, H. X.; Zhu, J. F.; Wei, S. Q.; Xie, Y. Angew. Chem. Int. Ed. 2017, 56, 9121-9125. (15) Angamuthu, R.; Byers, P.; Lutz, M.; Spek, A. L.; Bouwman, E. Science 2010, 327, 313-315. (16) Weng, Z.; Wu, Y. S.; Wang, M. Y.; Jiang, J. B.; Yang, K.; Huo, S. J.; Wang, X. F.; Ma, Q.; Brudvig, G. W.; Batista, V. S.; Liang, Y. Y.; Feng, Z. X.; Wang, H. L. Nat. Commun. 2018, 9, 415. (17) Varela, A. S.; Sahraie, N. R.; Steinberg, J.; Ju, W.; Oh, H. S.; Strasser, P. Angew. Chem. Int. Ed. 2015, 54, 10758-10762. (18) Wu, J. J.; Ma, S. C.; Sun, J.; Gold, J. I.; Tiwary, C.; Kim, B.; Zhu, L. Y.; Chopra, N.; Odeh, I. N.; Vajtai, R.; Yu, A. Z.; Luo, R.; Lou, J.; Ding, G. Q.; Kenis, P. J. A.; Ajayan, P. M. Nat. Commun. 2016, 7, 13869. (19) Kuhl, K. P.; Cave, E. P.; Abram, D. N.; Jaramillo, T. F. Energy Environ. Sci. 2012, 5, 70507059. (20) Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Norskov, J. K. Energy Environ. Sci. 2010, 3, 1311-1315. (21) Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 7231-7234. (22) Mistry, H.; Varela, A. S.; Bonifacio, C. S.; Zegkinoglou, I.; Sinev, I.; Choi, Y. W.; Kisslinger, K.; Stach, E. A.; Yang, J. C.; Strasser, P.; Cuenya, B. R. Nat. Commun. 2016, 7, 12123.

15


Page 16 of 23

Page 17 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(23) Raciti, D.; Livi, K. J.; Wang, C. Nano Lett. 2015, 15, 6829-6835. (24) Roberts, F. S.; Kuhl, K. P.; Nilsson, A. Angew. Chem. Int. Ed. 2015, 54, 5179-5182. (25) Kim, D.; Kley, C. S.; Li, Y. F.; Yang, P. D. Proc. Natl. Acad.Sci. 2017, 114, 10560-10565. (26) De Luna, P.; Quintero-Bermudez, R.; Dinh, C. T.; Ross, M. B.; Bushuyev, O. S.; Todorovic, P.; Regier, T.; Kelley, S. O.; Yang, P. D.; Sargent, E. H. Nat. Catal. 2018, 1, 103-110. (27) Zhao, Z.; Peng, X.; Liu, X.; Sun, X.; Shi, J.; Han, L.; Li, G.; Luo, J. J. Mater. Chem. A. 2017, 5, 20239. (28) Jiang, K.; Sandberg, R. B.; Akey, A. J.; Liu, X. Y.; Bell, D. C.; Norskov, J. K.; Chan, K. R.; Wang, H. Nat. Catal. 2018, 1, 111-119. (29) Yang, K. D.; Ko, W. R.; Lee, J. H.; Kim, S. J.; Lee, H.; Lee, M. H.; Nam, K. T. Angew. Chem. Int. Ed. 2017, 56, 796-800. (30) Gao, D. F.; Sohoken, F.; Cuenya, B. R. ACS Catal. 2017, 7, 5112-5120. (31) Gao, D.; Zegkinoglou, I.; Divins, N. J.; Scholten, F.; Sinev, I.; Grosse, P.; Cuenya, B. R. ACS Nano 2017, 11, 4825−4831. (32) Lee, S.; Kim, D.; Lee, J. Angew. Chem. Int. Ed. 2015, 54, 14701-14705. (33) Reller, C.; Krause, R.; Volkova, E.; Schmid, B.; Neubauer, S.; Rucki, A.; Schuster, M.; Schmid, G. Adv. Energy Mater. 2017, 7, 1602114. (34) Wiley, B.; Herricks, T.; Sun, Y. G.; Xia, Y. N. Nano Lett. 2004, 4, 2057-2057. (35) Pahlavan, S.; Moazen, S.; Taji, I.; Saffar, K.; Hamrah, M.; Moayed, M. H.; Beidokhti, S. M. Corros. Sci. 2016, 112, 233-240. (36) Millstone, J. E.; Wei, W.; Jones, M. R.; Yoo, H.; Mirkin, C. A.; Nano Lett., 2008, 8, 25262529. (37) Peng, H-C.; Li, Z.; Aldahondo, G.; Huang, H.; Xia, Y.; J. Phys. Chem. C 2016, 120, 1175411761. (38) Grosse, P.; Gao, D.; Scholten, F.; Sinev, I.; Mistry, H.; Cuenya, B. R.; Angew. Chem. Int. Ed. 2018, 57, 6192 –6197. (39) Handoko, A. D.; Chan, K. W.; Yeo, B. S. ACS Energy Lett. 2017, 2, 2103-2109. (40) Ma, M.; Djanashvili, K.; Smith, W. A. Angew. Chem. Int. Ed. 2016, 55, 6680-6684. (41) Dutta, A.; Rahaman, M.; Luedi, N. C.; Broekmann, P. ACS Catal. 2016, 6, 3804-3814. (42) Goncalves, M. R.; Gomes, A.; Condeco, J.; Fernandes, T. R. C.; Pardal, T.; Sequeira, C. A. C.; Branco, J. B. Electrochim. Acta. 2013, 102, 388-392.

16


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(43) Kas, R.; Kortlever, R.; Milbrat, A.; Koper, M. T. M.; Mul, G.; Baltrusaitis, J. Phys. Chem. Chem. Phys, 2014, 16, 12194-12201. (44) Chen, C. S.; Wan, J. H.; Yeo, B. S. J. Phys. Chem. C. 2015, 119, 26875-26882. (45) Xiao, H.; Cheng, T.; Goddard, III, W. A.; J. Am. Chem. Soc. 2017, 139, 130-136. (46) Schouten, K. J. P.; Kwon, Y.; van der Ham, C. J. M.; Qin, Z.; Koper, M. T. M.; Chem. Sci., 2011, 2, 1902-1909. (47) Varela, A. S.; Ju, W.; Reier, T.; Strasser, P. ACS Catal. 2016, 6, 2136-2144. (48) Jeon, H. S.; Kunze, S.; Scholten, F.; Cuenya, B. R. ACS Catal. 2018, 8, 531-535. (49) Ren, D.; Ang, B. S. H.; Yeo, B. S. ACS Catal. 2016, 6, 8239-8247. (50) Ren, D.; Deng, Y. L.; Handoko, A. D.; Chen, C. S.; Malkhandi, S.; Yeo, B. S. ACS Catal. 2015, 5, 2814-2821.

17


Page 18 of 23

Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1. (a) Schematic illustration showing the rapid and scalable synthesis of CuCl, CuBr, CuI microcrystals and the corresponding Cu nanostructures after electrochemical reduction, as well as the product differences of CO2 electrochemical reduction due to the distinct Cu morphologies. (b) Photographs of large-area Cu foil (1.5 inches  1.5 inches) before and after the formation of CuI microcrystals and Cu nanofibers.

18


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a-c) SEM images of CuCl, CuBr and CuI microcrystals. Scale bars: 2 µm. Insets: corresponding high-resolution TEM images. Scale bars: 2 nm. (d-f) SEM images of Cu nanocubes, nanodendrites and nanofibers derived from (a-c), respectively. Scale bars: 2 µm. Inset in (f) is the corresponding TEM image. Scale bar: 50 nm. (g-i) XRD profiles of CuCl, CuBr and CuI microcrystals before and after electrochemical reduction as well as those after immersion in CO2-saturated KHCO3 electrolyte for 15 mins.

19


Page 20 of 23

Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3. (a) High-resolution TEM image of an individual Cu nanoparticle that constitutes the nanofiber. Scale bar: 2 nm. (b) The lattice spacing measured by integrating ten atomic layers indicated in (a). (c) Cu Auger LMM XPS spectrum of the Cu nanofibers. (d) Cross-section SEM image of Cu nanofibers cut by FIB. (e) XPS signals of I 3d for CuI samples before and after electrochemical reduction at different time points. (f) The relative intensity of I with increasing reduction time (the intensity of I is normalized to that of I in CuI before reduction).

20


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Electrocatalytic performance of CO2 on Cu foil, Cu NCs (Cl), Cu NDs (Br), Cu NFs (I). (a) Current densities versus applied potentials between -0.5 V to -0.95 V (vs. RHE). Faradaic efficiencies versus applied potentials for (b) H2, (c) CO, (d) formate, (e) C2H4, (f) C2H6, (g) C2H5OH and (h) n-C3H7OH.

21


Page 22 of 23

Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5. Total Faradaic efficiency of (a) C2+3 products and (b) C1 products as a function of applied potential. (c) Comparison of the total Faradaic efficiency of C2+3 products obtained by Cu NDs (Br) and Cu NFs (I) with those reported Cu-based catalysts produced by different methods in neutral pH aqueous media, stars of 1-14 indicate the values from the references of [22, 25-26, 28, 30-32, 39-41, 47-50], respectively (see Table S1 for details). (d) Long-term electrocatalysis at -0.85 V vs. RHE with C2H4 product measured every 30 minutes for the sample of Cu NDs (Br). (e) Long-term electrocatalysis at -0.73 V vs. RHE with C2H6 product measured every 30 minutes for the sample of Cu NFs (I).

22


Rapid and Scalable Synthesis of Cuprous Halide-Derived Copper

Recommend Documents