Three-Dimensional Hierarchical Copper-Based Nanostructures as

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Three-Dimensional Hierarchical Copper-Based Nanostructures as Advanced Electrocatalysts for CO2 Reduction David Raciti, Yuxuan Wang, Jun Ha Park, and Chao Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00356 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Energy Materials

Three-Dimensional Hierarchical Copper-Based Nanostructures as Advanced Electrocatalysts for CO2 Reduction David Raciti,1 Yuxuan Wang,1, 2 Jun Ha Park,1 Chao Wang1,* 1

Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore,

Maryland 21218 2 College of Chemistry, Nankai University, Tianjin, 300071, China. *Email: [email protected]

1 ACS Paragon Plus Environment

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Abstract Cu-based nanomaterials have received increasing interests for electrocatalytic applications in the CO2 reduction reaction. It is however challenging to design nanostructured Cu electrodes to improve both the chemical kinetics and molecular transport under the reaction conditions. Here we report on a new type of three-dimensional Cu-based nanostructures as advanced electrocatalysts for CO2 reduction. Driven by thermal oxidation, CuO nanowires and/or porous structures are grown on commercial Cu foams with three-dimensional (3D) frameworks. An electrochemical method is used to reduce CuO to Cu with the structural features largely preserved. The derived Cu-based hierarchical nanostructures are demonstrated to be highly active and selective for CO2 reduction, achieving >80% Faradaic efficiency and ~3 times of enhancement in terms of CO2 conversion rate as compared to the Cu nanowires grown on planar electrodes. Our work highlights the great potential of 3D Cu nanostructures for improving the energy efficiency and power performance of CO2 electrolysis.

Keywords: CO2 reduction, copper nanostructures, Cu nanowires, porous structures, electrocatalysis


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Electrochemical reduction of CO2 represents a promising solution for mitigation of the greenhouse gas emission and solar-fuel conversion.1–3 With water as the proton source, this process can generate a wide range of valuable products, such as carbon monoxide (CO) and ethylene (C2H4) that can be further processed to make chemical commodities and ethanol that can be directly used as fuels.4 The foreground of CO2 electroreduction is, however, built upon efficient catalysts capable of selective reducing CO2 into the desired products. Copper (Cu) has been one of the most extensively studied materials for CO2 reduction. Early studies on extended surfaces show that it requires high overpotentials for CO2 reduction, ca. 0.5 V (versus reversible hydrogen electrode, RHE; the same in the following discussion) to produce CO and formate (COOH) and more negative potentials than 0.8 V for hydrocarbons such as methane (CH4) and C2H4.4,5 Later work on high-surface-area Cu electrodes achieved lowoverpotential reduction of CO2, i.e., with onset potentials as low as 0.2 V for production of CO and 0.6 V for C2 species.6–9 Inspired by the potential for energy-efficient conversion of CO2, significant efforts have been dedicated to the development of Cu-based nanomaterials for electrocatalytic applications, with the catalytic activity and selectivity of CO2 reduction found to be dependent on the particle size10, shape11,12 and composition13,14. Besides surface structure, transport is also known to play a vital role in the electroreduction of CO2, where the limited supply of reactant to the electrode surface places a hurdle on the current density and efficiency of CO2 reduction.15,16 Moreover, the CO2 reduction reaction, as well as the undesired side reaction – hydrogen evolution, produces hydroxide anions (OH–) and raises the local pH near the electrode surface, further impeding the mass transfer owing to the shifted equilibrium of CO2 hydrolysis toward (bi)carbonates.15,17 It thus becomes important for the design of CO2 reduction



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electrocatalysts to improve both the chemical kinetics and molecular transport under the reaction conditions. Here we report on a new type of Cu-based hierarchical nanostructures (HNs) as advanced electrocatalysts for CO2 reduction. Driven by thermal oxidation, CuO nanowires and/or porous structures are grown on commercial Cu foams with three-dimensional (3D) frameworks. Growth conditions such as temperature, reaction time and atmosphere are systematically studied to understand the growth mechanism of these CuO-HNs. The obtained CuO-HNs are further converted into metallic Cu-HNs by using electrochemical reduction, which are then applied as electrocatalysts for the CO2 reduction reaction. The structure-performance relationship is evaluated to demonstrate the advantages of the hierarchical architecture for electrocatalytic applications. Commercial Cu foams are used as a precursor for the synthesis of CuO-HNs. These metallic foams have a thickness of ~1.6 mm, ≥95 porosity and a surface density of 350 g/cm2. Scanning electron microscopy (SEM) images show that the Cu foam possesses ribbons of ~50 μm in diameter and inter-ribbon spacing on the order of 100 μm (Figure 1a). The CuO-HNs were grown by annealing Cu foam in air. The the oxidation of copper to cuprous (Cu2O) or cupric (CuO) oxide resulted in the formation of nanowires and/or porous structures on the ribbons. Figures 1b-g show the SEM images of the CuO-HNs obtained after annealing at 500 – 700 o

C for 8 hours (the products are denoted as, e.g., CuO-HNs-500C for the material obtained at 500

o

C). Highly dense nanowires about ca. 10 μm long and ~150 nm in diameter were found on CuO-

HNs-500C (Figure 1e). In contrast, almost no nanowires were found on CuO-HNs-700C, and instead, pores were observed on the ribbons which were not present on the pristine Cu foams (Figure 1g). For the intermediate temperature, 600 oC, both nanowires and pores were grown on 4 ACS Paragon Plus Environment

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the ribbons of the foams, albeit that the nanowires are a little shorter and less dense than the case for CuO-HNs-500C (Figure 1f). A closer look at the two porous nanostructures reveal that the pores have an average diameter of 400 – 500 nm (Figures 2 a-e). Cross-sectional imaging shows that the pores are not throughout the ribbons but have a penetration depth of a few micrometers (Figure 2f). The pores were found to exhibit a dumbbell-like shape below the surface of the ribbons, with the diameter reaching ~1,000 nm at the bottom. X-ray diffraction (XRD) patterns show that CuO is the dominant phase in CuO-HNs-600C and CuO-HNs-700C, while CuO and Cu2O co-exist in CuO-HNs-500C, suggesting the order of Cu → Cu2O → CuO for the oxidation reactions as the annealing temperature increases (Figure 1h). To better understand the evolution of nanostructures, we have carried out time-dependent studies of the CuO-HNs growth. Our focus was placed on whether the nanowires or porous structure was developed first during the growth of CuO-HNs-600C. The Cu ribbons in the pristine Cu foam had a granular but yet smooth surface, typical of polycrystalline materials (Figure 3a). After 1 h of annealing in air at 600 oC, the surface became substantially roughened and abundant with sub-micrometer pores, and yet only a few short nanowires were observed at this stage (Figure 3b). XRD analysis shows that metallic Cu is still the dominant phase and Cu2O is more than CuO at this stage (Figure 3e). As the growth continued, more and longer nanowires appeared, while the porous structure of the ribbon persisted (e.g., Figure 3c at 4 h, Figure 1f at 8 h, and Figure 3d at 15 h). After 4 h of annealing, no more metallic Cu was left and CuO became the dominant oxide form (Figure 3e). Only CuO was found after 8 h or longer annealing (Figures 1h and 3e). The growth of CuO nanowires during thermal oxidation of Cu has been extensively studied on various types of substrates (Cu mesh,7,18 foil,19–21 thin films,22,23 wires,24 etc.). It was believed in early studies that a vapor-solid (VS) phase transition accounts for the growth of CuO 5 ACS Paragon Plus Environment


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nanowires,18 but later studies are more inclined to a stress-driven grain-boundary diffusion mechanism20,25–27. In the latter mechanism, a CuO-Cu2O-Cu three-layer structural model has been proposed to explain the accumulation of compressive stress during the oxidation process and the fast diffusion of Cu atoms through grain boundaries to form bi-crystal nanowires on the surface.28,29 The growth of nanowires in our synthesis of CuO-HNs is likely also via the stressdriven grain-boundary diffusion mechanism. As shown in Figure 4, we have conducted elemental analysis to map the spatial distribution of the two oxide phases in the CuO-HNs. CuO and Cu2O are distinguished by the Cu/O atomic ratios as determined from the energy-disperse X-ray (EDX) spectra, namely a ratio of ~1 being indicative of CuO and ~2 for Cu2O.30 In the case of CuO-HNs500C (after 8 h of annealing in air), a Cu2O/CuO core/shell-like structure was observed for the ribbons of the foam (Figures 4a, b), whereas only CuO was found in the CuO-HNs derived at higher temperatures (Figures 4c, d). These observations are in line with the crystal phase information derived from the XRD patterns (Figure 1h) and indicate that the growth of CuO nanowires could be associated with the formation of a CuO/Cu2O interface. This correlation is further evidenced by the control experiments conducted in different atmospheres. For Cu foam annealed in Ar at 600 oC (for 8 h), no nanostructures were observed (Figure 4e), whereas only porous structures were obtained from the treatment in O2 (Figure 4f). The latter finding is in sharp contrast to the observation of both nanowires and porous structures after the treatment in air (Figure 1f). The suppression of CuO nanowire growth in this case could be attributed to the high chemical potential of oxygen, which could have either directly (or very fast, without allowing for the relaxation of stress through diffusion) converted Cu to CuO or made the more homogeneous lattice diffusion (versus preferential diffusion through the grain boundaries) dominant during the oxidation process25,27,28,31.


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Besides driving the growth of nanowires, the fast diffusion of Cu cations through the grain boundaries could have also left a large number of vacancies in the copper (or copper oxide) lattice, leading to the formation of porous structures on the ribbons. This mechanism is supported by the observation of pores open to the surface and the dumbbell-like shape of the pores (Figure 2). Since the diffusion coefficient of oxygen anions inward is slower than that for the outward diffusion of Cu cations in the CuO and Cu2O lattices,32 Kirkendall effect is also likely to play a role during the oxidation process, causing the formation of voids buried in the CuO-HNs (see the yellow arrows labeled in Figures 4a and c). Unlike the growth of CuO nanowires, the formation of pores do not necessarily go through the Cu2O phase and thus still takes place at high temperatures (≥700 oC) or in pure oxygen. For electrocatalytic applications, the CuO-HNs were reduced to Cu-HNs by applying a cathodic potential (−0.4 V) in 0.1 M KHCO3. The XRD patterns collected after the electrochemical treatment confirmed the reduction of CuO to metallic Cu (Figure S1) The presence of a weak Cu2O (111) peak at 36-37° in the XRD patterns can be ascribed to re-oxidation of the highly roughened surface when the Cu-HNs were exposed to air.7 The electrochemical reduction (ECR) method has previously been used to reduce CuO nanowires grown on Cu mesh to produce Cu nanowires, where it was shown to be able to preserve the dimensions of the nanostructures and expose open facets that are active for CO2 reduction.7,33 Electrochemical reduction also preserved the morphologies of the CuO-HNs to a large extent, with highly dense Cu nanowires found on Cu-HNs-500C, porous structures on Cu-HNs-700C with the pore diameter ranging from a few hundred nanometers to ~1 μm (Figures 5a, c). While the porous structure is less discernible than the nanowires on Cu-HNs600C (Figure 5b), the presence of both features in this case is supported by the larger electrochemically active surface area (ECSA, estimated from the non-Faradaic electrochemical



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capacitance measured using cyclic voltammetry, see Figure S2) of Cu-HNs-600C than the other two types of Cu-HNs. For the three types of nanostructures, Cu-HNs-600C has the largest surface roughness factor of 637 (in reference to the extended surface of polycrystalline Cu electrode), which is ~18 times rougher than the pristine Cu foam. Cu-HNs-500C and -700C have lower surface roughness factors than Cu-HNs-600C, estimated to be 358 and 328, respectively (Figure 5d). The electroreduction of CO2 was studied in a customized electrolysis cell, with 0.1 M KHCO3 solution being used as the electrolyte (Figure S3). Gas- and liquid-phase products were analysed using gas chromatograph-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy, respectively. The total electrode current density per geometric area (Jtot, Figure 6a) exhibits nearly an exponential increase as the potential becomes more negative. The Tafel slopes (in terms of partial current density toward CO production at low overpotentials) are determined to be 146, 180 and 133 for the Cu-HNs-500C, -600C and -700C, respectively (Figure S4). Throughout the potential regime (–0.2 ~ –0.7 V) studied here, Jtot of the Cu-HNs are typically 10 – 20 times larger than the pristine Cu foam, indicating the enhanced catalytic activities of the hierarchical nanostructures compared to the macroporous foams. At −0.5 V, the most active CuHNs-600C reaches ~9.77 mA/cm2geo, as compared to 4.97 and 4.15 mA/cm2geo for Cu-HNs-500C and Cu-HNs-700C nanowires, respectively. The trend in Jtot, Cu-HNs-600C > Cu-HNs-500C ≈ Cu-HNs-700C, is well in line with that for surface roughness (Table S1), suggesting that the different types of Cu-HNs may have rather similar specific activities, namely current density per electrochemically active surface area (ECSA) of Cu (jtot). This deduction was confirmed by the calculated specific activities, with the three types of electrocatalysts all having similar values of jCO2, e.g., ~13-14 μA/cm2Cu at −0.5 V (Figure 6b).


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The Cu-HNs are much more selective than the pristine Cu foam for CO2 reduction. On the Cu foam, hydrogen evolution is dominant and formate is the only CO2 reduction product detected at significant amounts, the Faradaic efficiency (FE) of which is less than 20% throughout the studied potential regime (Figure 6c). The three Cu-HNs exhibit similar potential-dependent behaviours (Figures 6d-f): at low overpotentials (more positive than −0.5 V), CO is the dominant product (with FEs as high as ~60%); at potentials more negative than −0.5 V, formate selectivity increases to 30-40%. Multi-carbon products (denoted as C2+, including ethane, ethene, ethanol, acetic acid and 1-propanol) start to appear at −0.4 V ~ −0.5 V and increase as the potential becomes more negative. Out of the three electrocatalysts, Cu-HNs-700C possesses the highest peak selectivity of CO2 reduction, with the total FE (FECO2) reaching 82.5% at −0.55 V (Figure 6f), as compared to 74.9% (at −0.55 V) for Cu-HNs-500C (Figure 6d) and 77.7% (at −0.5 V) for CuHNs-600C (Figure 6e). Noticeably, both Cu-HNs-500C and Cu-HNs-700C show drops of FECO2 as the potential goes more negative than −0.55 V, which is much less significant in the case of CuHNs-600C. The electroreduction of CO2 at large current densities is known to be subjected to transport limitations, especially for nanostructured electrodes.34 To visualize the mass transfer effects in the 3D hierarchical nanostructures, we have plotted the total CO2 reduction current densities (JCO2, per geometric area) of the Cu-HNs and calculated the total CO2 conversion rate per electrode area (see the details of calculation in the Supporting Information). We then compared the Cu-HNs to the ECR-Cu-Mesh from our previous reports,7,33,35 which possesses Cu nanowires grown on planar, two-dimensional (2D) Cu mesh and has surface roughness factor of ~356 (~0.11 mm in thickness, vs. 1.6 mm for the Cu foam). It can be seen that Cu-HNs-500C and Cu-HNs-700C give rather similar behaviours of JCO2 as compared to ECR-Cu-Mesh (albeit with slightly higher values), but



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Cu-HNs-600C outperforms in the series (Figure 7a). Cu-HNs-600C achieves JCO2 of 7.57 mA/cm2geo at −0.5 V and 20.32 mA/cm2geo at −0.7 V, versus 1.81 and 9.5 mA/cm2geo at these potentials by the ECR-Cu-Mesh, representing improvement factors of ~4 and ~2, respectively. The CO2 conversion rate by Cu-HNs-600C does not show further increase as the potential becomes more negative than −0.65 V, which is a signature of diffusion limitation (Figure 7b). In contrast, Cu-HNs-500C and Cu-HNs-700C do not show diffusion-limitation behavior throughout the potential region. The onset potential for diffusion limitation was previously reported to be at ca. −0.8 V for the ECR Cu nanowires.34 It is also worth mentioning that the maximum CO2 conversion rate reached by Cu-HNs-600C, ~101 nmol s-1 cm-2geo, is about three times of that by the ECR-CuMesh (~35 nmol s-1 cm-2geo). At −0.75 V, Cu-HNs-500C and Cu-HNs-700C reach their maximum CO2 conversion rates of ~82 and ~73 nmol s-1 cm-2geo, respectively. By correlating the catalytic activities (JCO2) and CO2 conversion rates to the nanostructures, it can be seen that the Cu-HNs have more efficient mass transfer of CO2 than the ECR-Cu-Mesh, which can be ascribed to the high porosity of the 3D frameworks inherited from the Cu foams. Furthermore, the superior performance of Cu-HNs-600C is likely a result of the integration of both nanowires and porous ribbons, which has given rise to higher ECSAs as compared to the other two types of Cu-HNs. In summary, CuO nanowires and/or porous structures were grown on commercial Cu foams as driven by thermal oxidation. The growth of CuO nanowires was attributed to a stressdriven grain-boundary diffusion mechanism, while the formation of porous structure on the ribbons can be a result of both grain-boundary diffusion and the Kirkendall effect. An electrochemical method was used to reduce CuO to Cu with the structural features largely preserved. The derived hierarchical Cu-based nanostructures were demonstrated to be highly active and selective for CO2 reduction, achieving >80% Faradaic efficiency and ~3 times of


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enhancement in terms of CO2 conversion rate as compared to the Cu nanowires grown on planar electrodes. Our work highlights the great potential of 3D Cu nanostructures for improving the energy efficiency and power performance of CO2 electrolysis. ASSOCIATED CONTENT Supporting Information Available: **Synthesis, Characterization, Estimation of Surface Roughness, Electrochemical Methods, Supplemental Electrocatalytic Results, and Calculation of CO2 Conversion Rates** ACKNOWLEDGEMENTS This work was supported by the National Science Foundation (CHE-1437396) and the Discovery Award of Johns Hopkins University. This study made use of the Johns Hopkins University Department of Chemistry Core Facilities and the authors would like to acknowledge the NMR manager Dr. Joel A. Tang for his assistance.



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Figure 1. SEM images of (a, b) the pristine, untreated Cu foam and (c, f) CuO-HNs-500C, (d, g) CuOHNs-600C and (e, h) CuO-HNs-700C. (i) XRD patterns of the CuO-HNs.


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Figure 2. SEM images for analyzing the porous structures on (a-c) CuO-HNs-600C and (d-f) CuO-HNs700C.



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Figure 3. Time-dependent study of the CuO-HNs growth at 600 oC: (a) 0 h (

Three-Dimensional Hierarchical Copper-Based Nanostructures as

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