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High-activity Cu nanowires electrocatalysts for CO2 reduction
Journal of CO₂ Utilization 20 (2017) 27–33
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
High-activity Cu nanowires electrocatalysts for CO2 reduction Peng Huang
a,b
a,⁎
b,c
b,c
a
, Suqin Ci , Genxiang Wang , Jingchun Jia , Jiangwei Xu , Zhenhai Wen
b,c,⁎
MARK
a
Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, PR China Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China c Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: CO2 reduction CuO nanowires Electrocatalysis Energy conversion
The daunting challenge in electrochemical conversion of CO2 into carbon resource is to explore high-activity and high-selectivity electrocatalysts for CO2 reduction reaction (CO2RR). Here, we report a water-bath route for tunable synthesis of a set of CuO nanostructures including nanowires (NWs), microsphere and nanoflake. The CuO NWs show a unique one-dimensional (1D) structure constructed by nanoparticles building block and manifest a significantly enhanced electrochemical surface area compared with the counterparts and the bulk CuO. Systematic electrochemical studies demonstrate the CuO NWs, as electrocatalyst of CO2RR, manifests a highly electrocatalytic activity, as demonstrated by reaching a current dentistry of 2 mA cm−2 at a relative low onset potential and delivering a catalytic current density of up to 100 mA cm−2 at −1.6 V. The nanoparticles assembled 1D structure provides a large amount of edge active sites and thus contributes to such a high electrocatalytic activity. Besides a small quantity of HCOOH, the major products of CO2RR are H2, CO, with tunable ratio of H2/CO in the range of 1.75:1 to 2.75:1 depending on the applied potential; such H2/CO mixture product are potentially used for the syngas that are key feedstocks to produce a variety of liquid fuels in industry.
1. Introduction Carbon dioxide (CO2) is the final product of the burning of fossil fuels process, the increasing accumulation of CO2 in the atmosphere has been cited as a major contributor to the greenhouse effect [1]. To reduce CO2 emissions and change the structure of the energy consumption, great efforts have been thrown into this field in recent years. One promising strategy to alleviate this concern is to convert CO2 into useful carbon-based products by electrochemical method. The particularly appealing aspects of this technology can be attributed to its intrinsic advantages [2]: (1) the electrochemical reduction reaction occurs under mild pressure and temperature; (2) a high energy utilization efficiency can be achieved in electrochemical system; (3) the reaction rate and product selectivity can be easily regulated by adjusting the applied potential. Given the fact that CO2 is extremely stable, the ideal electrocatalysts should be able to promote the kinetic of such sluggish reduction process. Researchers have devoted many efforts to develop highperformance electrocatalysts for CO2RR. A variety of electrocatalysts, including transition metals and their oxides, and engineering carbon nanostructures, have been extensively studied for catalyzing CO2RR
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Corresponding authors. E-mail addresses: [email protected] (S. Ci), [email protected] (Z. Wen).
http://dx.doi.org/10.1016/j.jcou.2017.05.002 Received 7 February 2017; Accepted 3 May 2017 Available online 15 May 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
[3–8]. In particular, copper or copper compounds, have recieved tremendous attentions since copper based materials showed high activity for yielding carbon-based compounds products and high selectivity due to high overpotential toward hydrogen evolution reaction (HER) [9]. For instance, Raciti et al. [10] found the copper nanowires showed high activity for CO2RR, which requiring an overpotential of only −0.3 V (vs. RHE) to reach a current density of 1.0 mA cm−2. Roberts et al. [11] reported that the copper nanocube manifested high selectivity for the formation of ethylene. Copper based materials have achieved great progress for CO2RR, yet, as far as we know, they are highly susceptible to poisoning and deactivation within a short time from the start of CO2RR [12]. As a result, low catalytic activity was obtained for most of copper based materials. Besides, the fundamental mechanism upon electrocatalytic process at Cu based electrocatalysts remain to be further clarified [13–16]. Here, we developed a simple water-bath method to prepare copper oxides nanostructures with tunable morphology, including microsphere (three-dimensional 3D), nanoflake (2D) and nanowires (1D). The electrocatalytic properties of the as-prepared CuO samples toward CO2RR were systematically studied, and we found that the CuO NWs presented an outstanding electrocatalytic activity and good products
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Fig. 1. SEM images of (a) CuO DBNWs, (b) CuO STNWs, (c) CuO microsphere, and (d) CuO nanoflake.
tained using an ESCALAB 250Xi. To characterize the CuO NWs after CO2RR, the catalyst on working electrode was peeled off and sonicated in absolute ethyl alcohol solution for a few minutes. Subsequently, the solution contains catalyst was collected, then placed a few drops of it on a silicon substrate for XPS and SEM characterization.
proportion in CO2RR. The possible pathway is proposed to help understand the reaction mechanism in this work simultaneously. 2. Experimental section 2.1. Material and nanowires synthesis
2.3. Electrochemical testing All chemicals were purchased from Shanghai Chemical Reagent Co. Ltd. Doubly distilled deionized water (DW, 18.2 MΩ) was used for all synthesis. Simply, a mixture of CuCl2·2H2O (0.43 g) and polyvinyl pyrrolidone (PVP, 0.15 g) were dissolved in 50 mL of DW to form a homogeneous solution through vigorous magnetic stirring at room temperature. Subsequently, the solution was sonicated for 10 min after adding 1.0 mL of aqueous ammonia (25%–28%). 15 mL NaOH (1.0 M) was added into the mixed solution slowly in a water-bath until the blue precursor precipitated, then the colloid solution maintained at the temperature of 45 °C for 45 min. Next the products were filtered, alternately washed with DW and absolute ethanol for several times before drying at 60 °C in vacuum. Finally, the dried products were transformed into CuO samples upon thermal annealing for 2 h at 300 °C in air. The synthetic procedures of different morphologies of CuO were similar by adjusting the ratio of the synthetic raw materials and changing the reaction conditions (Please see detail in Table S1).
Glassy carbon electrode (GCE) (rGCE = 3 mm) was polished by 0.3 and 0.05-μm alumina slurry successively followed by rinsing thoroughly with DW. 5 mg sample, 50 μL nafion (5% w/w%), 50 μL ethanol and 400 μL DW were mixed together to prepare the catalyst slurry. The mixture was sonicated for 20 min until it formed a uniform suspension. 6 μL of the suspension was dropped on GCE and dried under ambient temperature. A standard three-electrode airtight electrochemical cell configuration with two compartments (H-type cell) was used for all tests. The cathode and anode compartments were separated with a cation exchange membrane (nafion 117) to prevent oxidation of the reduced CO2 products by the oxygen from anode. Ag/AgCl electrode and platinum filament electrode were used as reference electrode and counter electrode, respectively. Before the measurements, Ar or CO2 was bubbled into the 0.5 M KHCO3 for at least 30 min under magnetically stirring. All of the electrochemical sweep tests were performed at potential ranging from −0.6 V to −1.6 V (vs. Ag/AgCl). All electrochemical experiments were performed through a computer-controlled CHI 660E potentiostation. For the whole reduction reaction, gaseous and liquid products were detected respectively by using gas chromatography (GC) and nuclear magnetic resonance (NMR).
2.2. Characterization Crystal structure of CuO was determined by using a Bruker D8 Advance X-ray diffractometer (XRD) equipped with Cu-Kα (λ = 0.154 nm) radiation. The morphology of CuO catalysts were analyzed by scanning electron microscopy (SEM, SU-8010). The particle size and dispersion on the support were explored by using transmission electron microscopy (TEM, Tecnai F20) operated at 200 kV. X-ray photoelectron spectra (XPS) measurements were ob-
3. Results and discussion The morphology and structure of CuO products can be facilely tuned by adjusting the synthetic parameter, such as reaction time, concentra28
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Fig. 2. TEM images of (a) CuO DB NWs and (c) CuO ST NWs. HRTEM images of (b) CuO DB NWs and (d) CuO ST NWs. The crystal face of CuO NWs are mainly (−111) and (111).
growth until the CuO DBNWs architectures were finally formed. For CuO STNWs, with extension of reaction time and decreasing temperature, the CuO DBNWs can resolve into scattered nanowires. The increased concentration of ammonia can frizzle the nanowires into microspheres, while enhancing reaction time can help the fusion of nanowires to form nanoflakes structure. Fig. 3 shows the powder X-ray diffraction patterns (XRD) for all of the CuO samples. All diffraction peaks can be well indexed to the monoclinic-phase CuO (PDF#44-0706). No impurity peaks were observed, confirming the high purity of the four CuO products. The diffraction peaks of CuO DBNWs are sharper than the CuO STNWs, indicating that the CuO STNWs has smaller particle diameter than CuO DBNWs [18]. The particle size may cause an essential effect on catalytic activity for CO2RR [19]. The catalytic activity of CuO samples for CO2RR were evaluated by cyclic voltammetry (CV) technique, which was performed in 0.5 M KHCO3 solution saturated with CO2 at the scan rate of 5 mV/s. Fig. 4a depicts the CV curves for CuO bulk and the four CuO nanostructures. Obviously, the CuO STNWs possess the highest activity among them, as manifest a lower onset potential and a higher catalytic current density. The CuO STNWs delivered a current density of about −100 mA cm−2 at −1.6 V. By contrast, the CuO DBNWs and the CuO bulk attained
tion and temperature. The CuO NWs (1D) bundles with a dumbbell-like morphology (CuO DBNWs) can be obtained with adding NaOH (1.0 M) to the copper ammonia solution at 60 °C (Fig. 1a). When the reaction time increased to 45 min with decreasing reaction temperature to 45 °C, the CuO DBNWs should be disaggregated into many scattered nanowires (CuO STNWs), as shown in Fig. 1b. In addition, two different nanostructures of CuO, microsphere (Fig. 1c) and nanoflake (Fig. 1d), were synthesized by changing the addition of ammonia and reaction time. In this way we can study the favorable nanostructure (or dimensionality) of the CuO nanostructures upon catalyzing CO2RR. It’s found that the nanowires are actually composed of CuO nanoparticles like string of pearls, and it was further supported by the TEM characterization (Fig. 2a and c). In Fig. 2b and d, obvious lattice fringes of CuO are observed and the average lattice spacing are ∼0.26 nm and ∼0.23 nm, respectively, which can be assigned to the reflections of the (-111) and (111) planes of CuO. A possible growth mechanism was proposed to explain the formation of CuO DBNWs [17]. CuO nanoparticles were formed in consequence the dehydration of Cu(OH)2 after adding NaOH at the early stage. Then the nanoparticles self-assemble along the identical direction as the building blocks. The subsequent lateral attachments of nanoparticles occurred along the side defects accompanying with a parallel 29
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hydrogen evolution reaction (HER). The CuO STNWs show a significantly higher current density for both CO2RR and HER than the other four catalysts. Notably, all the CuO nanostructures show a distinct positively shift in onset potential for CO2RR compared with HER except the bulk CuO (Table S3). The onset potential of CO2RR for CuO STNWs is −0.97 V, this value is more positive than that of CuO DBNWs (−1.12 V), CuO microsphere (−1.15 V) and CuO nanoflake (−1.16 V). Moreover, the CuO STNWs and the CuO DBNWs show 140 mV (Fig. 4c) and 70 mV (Fig. 4d) lower on the onset potential in CO2-saturated electrolyte as compared with those in Ar-saturated electrolyte, suggesting that the two CuO nanostructures has a favorable catalytic activity and smaller overpotential for CO2RR than HER. This is probably caused by the lesser barrier of the initial electron transfer to form a CO2* (* represents the adsorbed species) intermediate on CuO STNWs [22]. To achieve a better understanding of what lead to the enhanced activity for CuO STNWs, electrochemical active surface area (ECSA) [23] was determined by measuring the double layer capacitance (Cdl) and surface roughness factor (Rf). Fig. 5a shows the slope from the linear relationship of the current density against the scan rate (Fig. S2). The CuO DBNWs and CuO STNWs show capacitance of ∼9.54 mF/cm2 and ∼13.99 mF/cm2, respectively, indicating that CuO STNWs have a relative larger ECSA than CuO DBNWs. N2 adsorption-desorption test was further performed to study the specific surface area and pore size distribution. The CuO STNWs reveal a type III curve with an H3 type hysteresis loop (Fig. 5b). According to the testing report, the CuO STNWs show a specific surface area of 46.6 m2 g−1, higher than that of the CuO DBNWs (38.7 m2 g−1, Fig. S3). Electrochemical impedance spectroscopy (EIS, Fig. 5c) were also performed to measure the charge transfer resistance (Rct) [24]. Remarkably, the Rct of CuO STNWs is lower than that of the CuO DBNWs. The particle size has an essential effect on catalytic activity for CO2RR in previous report [19]. It presented a dramatic increase in overall catalytic activity due to the
Fig. 3. XRD patterns of CuO DBNWs, CuO STNWs, CuO nanoflake and CuO microsphere.
current density of about −30.5 mA cm−2 and −8.0 mA cm−2, respectively. Furthermore, a total current density of 3.62 mA cm−2, 1.05 mA cm−2, 0.35 mA cm−2 were obtained at the potential of −1.0 V for the CuO STNWs, the CuO DBNWs and the bulk CuO, respectively. It should be pointed out that the Cu electrode normally require a potential of > 1.1 V to reach a current density of 1 mA cm−2 [20,21]. According to the CV curves, both of CuO STNWs and CuO DBNWs present higher current density than the other three catalysts. Compared with the previously reported Cu based materials, the CuO STNWs (1D) show the highest activity in CO2RR (Table S2). Fig. S1a and S1b record the linear sweep voltammetic (LSV) curves of the CuO samples in CO2-saturated and Ar-saturated electrolyte, respectively. Fig. S1b basically reflects the catalytic activity toward
Fig. 4. (a) CV of five CuO in 0.5 M KHCO3 saturated with CO2 at the scan rate of 50 mV/s. (b) LSV of CuO DBNWs and CuO STNWs in 0.5 M KHCO3 saturated with CO2 at 5 mV/s scan rate. (c) LSV of CuO STNWs in 0.5 M KHCO3 saturated with CO2 or Ar at 5 mV/s scan rate. (d) LSV of CuO DBNWs in 0.5 M KHCO3 saturated with CO2 or Ar at 5 mV/s scan rate.
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Fig. 5. (a) CV of CuO STNWs measured at different scan rate from 2 to 10 mV/s. Inset of (a): plot of the current density at 0.95 V vs the scan rate. (b) specific surface area of CuO STNWs. (c) EIS of CuO DBNWs and CuO STNWs recorded at −1.2 V with the frequency range from 0.1 Hz to 1.0 MHz. (d) CO2RR current density as a function of time at different voltages for CuO STNWs.
finally disappeared, which can be attributed to the reduction reaction of Cu(II) to Cu(I) or Cu(0). Depending on the situation, we conducted amperometric test at an applied potential of −1.0 V in 0.5 M KHCO3 solution saturated with CO2. Fig. 6 shows the high resolution XPS spectrum of Cu element for the CuO STNWs samples before and after the test. Both samples show a major peak of Cu (2p3/2) at around 932.9 eV that is ascribed to copper (0). Furthermore, another peak for Cu (2p1/2) corresponding to Cu (0) was observed at around 952.5 eV. Peaks associated with Cu (II) in the spectra prior to the experiment were rarely observed. Previous study indicated that Cu (I) and Cu (0) would be overlaped for Cu (2p3/2) peaks [26]. Consequently, it is difficult to rule out the presence of Cu (I). Energy Dispersive Spectrometer (EDS) results (Fig. S6) show that there was a trace amount of oxygen left in the catalyst, which is likely to be a result of the inevasible exposure of the samples to air and the necessity of being in a dried form for the XPS test. Notably, the color of the catalyst changed from black to golden brown during the test, demonstrating the reduction conversion of CuO into Cu (0). All these results prove that Cu (0) plays a leading role in CO2RR [25], being consistent with the previous studies reported that Cu catalyst reduced from itself oxide has a better catalytic activity [10]. Gas chromatography (GC) and nuclear magnetic resonance (NMR) were used to detect the gaseous products and liquid products of CO2RR. The main products include H2, CO, and HCOOH for CuO STNWs (Fig. 7a). The Faradic efficiency (FE) of CO almost keeps unchanged at around 20% in potential range, and there seems to be a complementary FE effect between HCOOH and H2. By tuning the potential from −1.0 V to −1.6 V, the H2/CO ratio can be adjusted in a minor range between 1.75:1 and 2.75:1. Especially at −1.4 V, the ratio is close to 2 (Fig. 7b), this can be applied in fischer-tropsch synthesis
particle size effect, which means higher activity arising from smaller nanoparticles in the size range. In this view, the CuO STNWs may have a higher catalytic activity than the CuO DBNWs. The above all analysis indicate that the CuO STNWs have a unique activity catalytic than CuO DBNWs for CO2RR. The catalytic stability was also studied through potentiostatic technique. Fig. 5d shows the I-t curves of CuO STNWs at a different applied potential for CO2RR. The CuO STNWs behaves feeble activity at the potential of −0.8 V and −1.0 V, which reaching a steady current density of −0.17 mA cm−2 and −0.65 mA cm−2, respectively. When the potential increased to −1.2 V, the CuO STNWs exhibits significantly higher reduction current density during the first about 20 s before stabilizing to about −6 mA cm−2. The steady current density is not consistent with the current density of CV at −0.8 V, −1.0 V and −1.2 V. This is caused by the lots of tiny bubbles (gaseous products) quickly generated as a result of high activity of the CuO STNWs at the negative potential. The tiny bubbles accumulated together to form large bubbles, whereafter, they blocked the part of working electrode surface, leading to dramatic decline in activity. We also researched the morphology of the CuO nanowires after amperometric test (Fig. S4). With only a slight change in morphology before and after the test for CuO STNWs (Fig. S4c and Fig. S4d), while the CuO DBNWs were embedded and blended together to form larger group after operation for catalyzing CO2RR (Fig. S4a and Fig. S4b). These results show that the CuO STNWs possess a good stability in CO2RR. It should be pointed out that the oxides were in-situ reduced to Cu (I) or Cu (0) during CO2RR [25]. As show in Fig. S5, a reduction peak occurred at the potential of −0.8 V to −1.0 V during CO2RR. With the continuous scanning, the reduction peak gradually decreased and 31
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Fig. 6. High-resolution of XPS spectrums of Cu element for CuO DBNWs before (a) and after (b) i–t test, and CuO ST NWs before (c) and after (d) i–t test.
or HOCOH*. The HOCOH* is chemically labile and dehydrated to form CO in the end. For HER, two proton-electron pair turn into adsorbed hydrogen species on the working electrode, and then they bind each other to produce H2 with CO2 does not participate in the reaction.
according to the reaction process: nCO + 2n H2 → nCH3OH [27]. Interestingly, a trace amount of ethanol and acetate was detected unexpectedly in the liquid products only at −1.6 V (Fig. S7), being basically consistent with the previous reports [28,29]. However, the reaction mechanism of CO2RR remains mysterious for a certain product at present [30]. Herein, the possible pathway is proposed to help understand the reaction mechanism in this work. Oxygen formed at the anode in the process of CO2RR [31], which could react according to: H2 O − 2e− → 2H+ + 1/2O2 [32]. H+ can be transferred from anode compartment to cathode compartment through Nafion 117 to take part in CO2RR or HER [33]. Thus, we can assume that every step of CO2RR involves the transfer of one proton-electron pair (H+ + e−). Under this assumption, the CO2 and a proton-electron pair are first adsorb as a carboxyl species, then the addition of a second proton-electron pair to this adsorbate lead to the production of HCOOH
4. Conclusions In summary, we report a set of CuO nanostructures, including nanowires, microsphere and nanoflake. The present work demonstrates the morphology and structure of CuO have a significant impact on the activity and illustrates the nanowires have an advantage for CO2RR. The CuO STNWs showed the best activity with excellent stability among these CuO catalysts. The improved catalytic activity can be attributed to the increased active sites, 1D structure, high surface area. In addition, the gas products of CO2RR at the CuO STNWs electrocatalyst are H2 and
Fig. 7. (a) Faradaic efficiencies of products at each given potential and (b) The ratio of the Faraday efficiency of H2 and CO for CuO STNWs.
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CO with the liquid product of HCOOH. The ratio of the produced H2 and CO is close to 2 at −1.4 V, which can be potentially applied in fischer-tropsch synthesis in industry.
[15]
[16]
Acknowledgements
[17]
This work was supported by the National Natural Science foundation of China (21566025) and the Natural Science Foundation of Jiangxi Province (20152ACB21019), the Graduate Student Innovation Foundation of Nanchang Hangkong University (YC2015017).
[18]
[19]
Appendix A. Supplementary data
[20]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcou.2017.05.002.
[21]
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
High-activity Cu nanowires electrocatalysts for CO2 reduction Peng Huang
a,b
a,⁎
b,c
b,c
a
, Suqin Ci , Genxiang Wang , Jingchun Jia , Jiangwei Xu , Zhenhai Wen
b,c,⁎
MARK
a
Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, PR China Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China c Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: CO2 reduction CuO nanowires Electrocatalysis Energy conversion
The daunting challenge in electrochemical conversion of CO2 into carbon resource is to explore high-activity and high-selectivity electrocatalysts for CO2 reduction reaction (CO2RR). Here, we report a water-bath route for tunable synthesis of a set of CuO nanostructures including nanowires (NWs), microsphere and nanoflake. The CuO NWs show a unique one-dimensional (1D) structure constructed by nanoparticles building block and manifest a significantly enhanced electrochemical surface area compared with the counterparts and the bulk CuO. Systematic electrochemical studies demonstrate the CuO NWs, as electrocatalyst of CO2RR, manifests a highly electrocatalytic activity, as demonstrated by reaching a current dentistry of 2 mA cm−2 at a relative low onset potential and delivering a catalytic current density of up to 100 mA cm−2 at −1.6 V. The nanoparticles assembled 1D structure provides a large amount of edge active sites and thus contributes to such a high electrocatalytic activity. Besides a small quantity of HCOOH, the major products of CO2RR are H2, CO, with tunable ratio of H2/CO in the range of 1.75:1 to 2.75:1 depending on the applied potential; such H2/CO mixture product are potentially used for the syngas that are key feedstocks to produce a variety of liquid fuels in industry.
1. Introduction Carbon dioxide (CO2) is the final product of the burning of fossil fuels process, the increasing accumulation of CO2 in the atmosphere has been cited as a major contributor to the greenhouse effect [1]. To reduce CO2 emissions and change the structure of the energy consumption, great efforts have been thrown into this field in recent years. One promising strategy to alleviate this concern is to convert CO2 into useful carbon-based products by electrochemical method. The particularly appealing aspects of this technology can be attributed to its intrinsic advantages [2]: (1) the electrochemical reduction reaction occurs under mild pressure and temperature; (2) a high energy utilization efficiency can be achieved in electrochemical system; (3) the reaction rate and product selectivity can be easily regulated by adjusting the applied potential. Given the fact that CO2 is extremely stable, the ideal electrocatalysts should be able to promote the kinetic of such sluggish reduction process. Researchers have devoted many efforts to develop highperformance electrocatalysts for CO2RR. A variety of electrocatalysts, including transition metals and their oxides, and engineering carbon nanostructures, have been extensively studied for catalyzing CO2RR
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Corresponding authors. E-mail addresses: [email protected] (S. Ci), [email protected] (Z. Wen).
http://dx.doi.org/10.1016/j.jcou.2017.05.002 Received 7 February 2017; Accepted 3 May 2017 Available online 15 May 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
[3–8]. In particular, copper or copper compounds, have recieved tremendous attentions since copper based materials showed high activity for yielding carbon-based compounds products and high selectivity due to high overpotential toward hydrogen evolution reaction (HER) [9]. For instance, Raciti et al. [10] found the copper nanowires showed high activity for CO2RR, which requiring an overpotential of only −0.3 V (vs. RHE) to reach a current density of 1.0 mA cm−2. Roberts et al. [11] reported that the copper nanocube manifested high selectivity for the formation of ethylene. Copper based materials have achieved great progress for CO2RR, yet, as far as we know, they are highly susceptible to poisoning and deactivation within a short time from the start of CO2RR [12]. As a result, low catalytic activity was obtained for most of copper based materials. Besides, the fundamental mechanism upon electrocatalytic process at Cu based electrocatalysts remain to be further clarified [13–16]. Here, we developed a simple water-bath method to prepare copper oxides nanostructures with tunable morphology, including microsphere (three-dimensional 3D), nanoflake (2D) and nanowires (1D). The electrocatalytic properties of the as-prepared CuO samples toward CO2RR were systematically studied, and we found that the CuO NWs presented an outstanding electrocatalytic activity and good products
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Fig. 1. SEM images of (a) CuO DBNWs, (b) CuO STNWs, (c) CuO microsphere, and (d) CuO nanoflake.
tained using an ESCALAB 250Xi. To characterize the CuO NWs after CO2RR, the catalyst on working electrode was peeled off and sonicated in absolute ethyl alcohol solution for a few minutes. Subsequently, the solution contains catalyst was collected, then placed a few drops of it on a silicon substrate for XPS and SEM characterization.
proportion in CO2RR. The possible pathway is proposed to help understand the reaction mechanism in this work simultaneously. 2. Experimental section 2.1. Material and nanowires synthesis
2.3. Electrochemical testing All chemicals were purchased from Shanghai Chemical Reagent Co. Ltd. Doubly distilled deionized water (DW, 18.2 MΩ) was used for all synthesis. Simply, a mixture of CuCl2·2H2O (0.43 g) and polyvinyl pyrrolidone (PVP, 0.15 g) were dissolved in 50 mL of DW to form a homogeneous solution through vigorous magnetic stirring at room temperature. Subsequently, the solution was sonicated for 10 min after adding 1.0 mL of aqueous ammonia (25%–28%). 15 mL NaOH (1.0 M) was added into the mixed solution slowly in a water-bath until the blue precursor precipitated, then the colloid solution maintained at the temperature of 45 °C for 45 min. Next the products were filtered, alternately washed with DW and absolute ethanol for several times before drying at 60 °C in vacuum. Finally, the dried products were transformed into CuO samples upon thermal annealing for 2 h at 300 °C in air. The synthetic procedures of different morphologies of CuO were similar by adjusting the ratio of the synthetic raw materials and changing the reaction conditions (Please see detail in Table S1).
Glassy carbon electrode (GCE) (rGCE = 3 mm) was polished by 0.3 and 0.05-μm alumina slurry successively followed by rinsing thoroughly with DW. 5 mg sample, 50 μL nafion (5% w/w%), 50 μL ethanol and 400 μL DW were mixed together to prepare the catalyst slurry. The mixture was sonicated for 20 min until it formed a uniform suspension. 6 μL of the suspension was dropped on GCE and dried under ambient temperature. A standard three-electrode airtight electrochemical cell configuration with two compartments (H-type cell) was used for all tests. The cathode and anode compartments were separated with a cation exchange membrane (nafion 117) to prevent oxidation of the reduced CO2 products by the oxygen from anode. Ag/AgCl electrode and platinum filament electrode were used as reference electrode and counter electrode, respectively. Before the measurements, Ar or CO2 was bubbled into the 0.5 M KHCO3 for at least 30 min under magnetically stirring. All of the electrochemical sweep tests were performed at potential ranging from −0.6 V to −1.6 V (vs. Ag/AgCl). All electrochemical experiments were performed through a computer-controlled CHI 660E potentiostation. For the whole reduction reaction, gaseous and liquid products were detected respectively by using gas chromatography (GC) and nuclear magnetic resonance (NMR).
2.2. Characterization Crystal structure of CuO was determined by using a Bruker D8 Advance X-ray diffractometer (XRD) equipped with Cu-Kα (λ = 0.154 nm) radiation. The morphology of CuO catalysts were analyzed by scanning electron microscopy (SEM, SU-8010). The particle size and dispersion on the support were explored by using transmission electron microscopy (TEM, Tecnai F20) operated at 200 kV. X-ray photoelectron spectra (XPS) measurements were ob-
3. Results and discussion The morphology and structure of CuO products can be facilely tuned by adjusting the synthetic parameter, such as reaction time, concentra28
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Fig. 2. TEM images of (a) CuO DB NWs and (c) CuO ST NWs. HRTEM images of (b) CuO DB NWs and (d) CuO ST NWs. The crystal face of CuO NWs are mainly (−111) and (111).
growth until the CuO DBNWs architectures were finally formed. For CuO STNWs, with extension of reaction time and decreasing temperature, the CuO DBNWs can resolve into scattered nanowires. The increased concentration of ammonia can frizzle the nanowires into microspheres, while enhancing reaction time can help the fusion of nanowires to form nanoflakes structure. Fig. 3 shows the powder X-ray diffraction patterns (XRD) for all of the CuO samples. All diffraction peaks can be well indexed to the monoclinic-phase CuO (PDF#44-0706). No impurity peaks were observed, confirming the high purity of the four CuO products. The diffraction peaks of CuO DBNWs are sharper than the CuO STNWs, indicating that the CuO STNWs has smaller particle diameter than CuO DBNWs [18]. The particle size may cause an essential effect on catalytic activity for CO2RR [19]. The catalytic activity of CuO samples for CO2RR were evaluated by cyclic voltammetry (CV) technique, which was performed in 0.5 M KHCO3 solution saturated with CO2 at the scan rate of 5 mV/s. Fig. 4a depicts the CV curves for CuO bulk and the four CuO nanostructures. Obviously, the CuO STNWs possess the highest activity among them, as manifest a lower onset potential and a higher catalytic current density. The CuO STNWs delivered a current density of about −100 mA cm−2 at −1.6 V. By contrast, the CuO DBNWs and the CuO bulk attained
tion and temperature. The CuO NWs (1D) bundles with a dumbbell-like morphology (CuO DBNWs) can be obtained with adding NaOH (1.0 M) to the copper ammonia solution at 60 °C (Fig. 1a). When the reaction time increased to 45 min with decreasing reaction temperature to 45 °C, the CuO DBNWs should be disaggregated into many scattered nanowires (CuO STNWs), as shown in Fig. 1b. In addition, two different nanostructures of CuO, microsphere (Fig. 1c) and nanoflake (Fig. 1d), were synthesized by changing the addition of ammonia and reaction time. In this way we can study the favorable nanostructure (or dimensionality) of the CuO nanostructures upon catalyzing CO2RR. It’s found that the nanowires are actually composed of CuO nanoparticles like string of pearls, and it was further supported by the TEM characterization (Fig. 2a and c). In Fig. 2b and d, obvious lattice fringes of CuO are observed and the average lattice spacing are ∼0.26 nm and ∼0.23 nm, respectively, which can be assigned to the reflections of the (-111) and (111) planes of CuO. A possible growth mechanism was proposed to explain the formation of CuO DBNWs [17]. CuO nanoparticles were formed in consequence the dehydration of Cu(OH)2 after adding NaOH at the early stage. Then the nanoparticles self-assemble along the identical direction as the building blocks. The subsequent lateral attachments of nanoparticles occurred along the side defects accompanying with a parallel 29
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hydrogen evolution reaction (HER). The CuO STNWs show a significantly higher current density for both CO2RR and HER than the other four catalysts. Notably, all the CuO nanostructures show a distinct positively shift in onset potential for CO2RR compared with HER except the bulk CuO (Table S3). The onset potential of CO2RR for CuO STNWs is −0.97 V, this value is more positive than that of CuO DBNWs (−1.12 V), CuO microsphere (−1.15 V) and CuO nanoflake (−1.16 V). Moreover, the CuO STNWs and the CuO DBNWs show 140 mV (Fig. 4c) and 70 mV (Fig. 4d) lower on the onset potential in CO2-saturated electrolyte as compared with those in Ar-saturated electrolyte, suggesting that the two CuO nanostructures has a favorable catalytic activity and smaller overpotential for CO2RR than HER. This is probably caused by the lesser barrier of the initial electron transfer to form a CO2* (* represents the adsorbed species) intermediate on CuO STNWs [22]. To achieve a better understanding of what lead to the enhanced activity for CuO STNWs, electrochemical active surface area (ECSA) [23] was determined by measuring the double layer capacitance (Cdl) and surface roughness factor (Rf). Fig. 5a shows the slope from the linear relationship of the current density against the scan rate (Fig. S2). The CuO DBNWs and CuO STNWs show capacitance of ∼9.54 mF/cm2 and ∼13.99 mF/cm2, respectively, indicating that CuO STNWs have a relative larger ECSA than CuO DBNWs. N2 adsorption-desorption test was further performed to study the specific surface area and pore size distribution. The CuO STNWs reveal a type III curve with an H3 type hysteresis loop (Fig. 5b). According to the testing report, the CuO STNWs show a specific surface area of 46.6 m2 g−1, higher than that of the CuO DBNWs (38.7 m2 g−1, Fig. S3). Electrochemical impedance spectroscopy (EIS, Fig. 5c) were also performed to measure the charge transfer resistance (Rct) [24]. Remarkably, the Rct of CuO STNWs is lower than that of the CuO DBNWs. The particle size has an essential effect on catalytic activity for CO2RR in previous report [19]. It presented a dramatic increase in overall catalytic activity due to the
Fig. 3. XRD patterns of CuO DBNWs, CuO STNWs, CuO nanoflake and CuO microsphere.
current density of about −30.5 mA cm−2 and −8.0 mA cm−2, respectively. Furthermore, a total current density of 3.62 mA cm−2, 1.05 mA cm−2, 0.35 mA cm−2 were obtained at the potential of −1.0 V for the CuO STNWs, the CuO DBNWs and the bulk CuO, respectively. It should be pointed out that the Cu electrode normally require a potential of > 1.1 V to reach a current density of 1 mA cm−2 [20,21]. According to the CV curves, both of CuO STNWs and CuO DBNWs present higher current density than the other three catalysts. Compared with the previously reported Cu based materials, the CuO STNWs (1D) show the highest activity in CO2RR (Table S2). Fig. S1a and S1b record the linear sweep voltammetic (LSV) curves of the CuO samples in CO2-saturated and Ar-saturated electrolyte, respectively. Fig. S1b basically reflects the catalytic activity toward
Fig. 4. (a) CV of five CuO in 0.5 M KHCO3 saturated with CO2 at the scan rate of 50 mV/s. (b) LSV of CuO DBNWs and CuO STNWs in 0.5 M KHCO3 saturated with CO2 at 5 mV/s scan rate. (c) LSV of CuO STNWs in 0.5 M KHCO3 saturated with CO2 or Ar at 5 mV/s scan rate. (d) LSV of CuO DBNWs in 0.5 M KHCO3 saturated with CO2 or Ar at 5 mV/s scan rate.
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Fig. 5. (a) CV of CuO STNWs measured at different scan rate from 2 to 10 mV/s. Inset of (a): plot of the current density at 0.95 V vs the scan rate. (b) specific surface area of CuO STNWs. (c) EIS of CuO DBNWs and CuO STNWs recorded at −1.2 V with the frequency range from 0.1 Hz to 1.0 MHz. (d) CO2RR current density as a function of time at different voltages for CuO STNWs.
finally disappeared, which can be attributed to the reduction reaction of Cu(II) to Cu(I) or Cu(0). Depending on the situation, we conducted amperometric test at an applied potential of −1.0 V in 0.5 M KHCO3 solution saturated with CO2. Fig. 6 shows the high resolution XPS spectrum of Cu element for the CuO STNWs samples before and after the test. Both samples show a major peak of Cu (2p3/2) at around 932.9 eV that is ascribed to copper (0). Furthermore, another peak for Cu (2p1/2) corresponding to Cu (0) was observed at around 952.5 eV. Peaks associated with Cu (II) in the spectra prior to the experiment were rarely observed. Previous study indicated that Cu (I) and Cu (0) would be overlaped for Cu (2p3/2) peaks [26]. Consequently, it is difficult to rule out the presence of Cu (I). Energy Dispersive Spectrometer (EDS) results (Fig. S6) show that there was a trace amount of oxygen left in the catalyst, which is likely to be a result of the inevasible exposure of the samples to air and the necessity of being in a dried form for the XPS test. Notably, the color of the catalyst changed from black to golden brown during the test, demonstrating the reduction conversion of CuO into Cu (0). All these results prove that Cu (0) plays a leading role in CO2RR [25], being consistent with the previous studies reported that Cu catalyst reduced from itself oxide has a better catalytic activity [10]. Gas chromatography (GC) and nuclear magnetic resonance (NMR) were used to detect the gaseous products and liquid products of CO2RR. The main products include H2, CO, and HCOOH for CuO STNWs (Fig. 7a). The Faradic efficiency (FE) of CO almost keeps unchanged at around 20% in potential range, and there seems to be a complementary FE effect between HCOOH and H2. By tuning the potential from −1.0 V to −1.6 V, the H2/CO ratio can be adjusted in a minor range between 1.75:1 and 2.75:1. Especially at −1.4 V, the ratio is close to 2 (Fig. 7b), this can be applied in fischer-tropsch synthesis
particle size effect, which means higher activity arising from smaller nanoparticles in the size range. In this view, the CuO STNWs may have a higher catalytic activity than the CuO DBNWs. The above all analysis indicate that the CuO STNWs have a unique activity catalytic than CuO DBNWs for CO2RR. The catalytic stability was also studied through potentiostatic technique. Fig. 5d shows the I-t curves of CuO STNWs at a different applied potential for CO2RR. The CuO STNWs behaves feeble activity at the potential of −0.8 V and −1.0 V, which reaching a steady current density of −0.17 mA cm−2 and −0.65 mA cm−2, respectively. When the potential increased to −1.2 V, the CuO STNWs exhibits significantly higher reduction current density during the first about 20 s before stabilizing to about −6 mA cm−2. The steady current density is not consistent with the current density of CV at −0.8 V, −1.0 V and −1.2 V. This is caused by the lots of tiny bubbles (gaseous products) quickly generated as a result of high activity of the CuO STNWs at the negative potential. The tiny bubbles accumulated together to form large bubbles, whereafter, they blocked the part of working electrode surface, leading to dramatic decline in activity. We also researched the morphology of the CuO nanowires after amperometric test (Fig. S4). With only a slight change in morphology before and after the test for CuO STNWs (Fig. S4c and Fig. S4d), while the CuO DBNWs were embedded and blended together to form larger group after operation for catalyzing CO2RR (Fig. S4a and Fig. S4b). These results show that the CuO STNWs possess a good stability in CO2RR. It should be pointed out that the oxides were in-situ reduced to Cu (I) or Cu (0) during CO2RR [25]. As show in Fig. S5, a reduction peak occurred at the potential of −0.8 V to −1.0 V during CO2RR. With the continuous scanning, the reduction peak gradually decreased and 31
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Fig. 6. High-resolution of XPS spectrums of Cu element for CuO DBNWs before (a) and after (b) i–t test, and CuO ST NWs before (c) and after (d) i–t test.
or HOCOH*. The HOCOH* is chemically labile and dehydrated to form CO in the end. For HER, two proton-electron pair turn into adsorbed hydrogen species on the working electrode, and then they bind each other to produce H2 with CO2 does not participate in the reaction.
according to the reaction process: nCO + 2n H2 → nCH3OH [27]. Interestingly, a trace amount of ethanol and acetate was detected unexpectedly in the liquid products only at −1.6 V (Fig. S7), being basically consistent with the previous reports [28,29]. However, the reaction mechanism of CO2RR remains mysterious for a certain product at present [30]. Herein, the possible pathway is proposed to help understand the reaction mechanism in this work. Oxygen formed at the anode in the process of CO2RR [31], which could react according to: H2 O − 2e− → 2H+ + 1/2O2 [32]. H+ can be transferred from anode compartment to cathode compartment through Nafion 117 to take part in CO2RR or HER [33]. Thus, we can assume that every step of CO2RR involves the transfer of one proton-electron pair (H+ + e−). Under this assumption, the CO2 and a proton-electron pair are first adsorb as a carboxyl species, then the addition of a second proton-electron pair to this adsorbate lead to the production of HCOOH
4. Conclusions In summary, we report a set of CuO nanostructures, including nanowires, microsphere and nanoflake. The present work demonstrates the morphology and structure of CuO have a significant impact on the activity and illustrates the nanowires have an advantage for CO2RR. The CuO STNWs showed the best activity with excellent stability among these CuO catalysts. The improved catalytic activity can be attributed to the increased active sites, 1D structure, high surface area. In addition, the gas products of CO2RR at the CuO STNWs electrocatalyst are H2 and
Fig. 7. (a) Faradaic efficiencies of products at each given potential and (b) The ratio of the Faraday efficiency of H2 and CO for CuO STNWs.
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CO with the liquid product of HCOOH. The ratio of the produced H2 and CO is close to 2 at −1.4 V, which can be potentially applied in fischer-tropsch synthesis in industry.
[15]
[16]
Acknowledgements
[17]
This work was supported by the National Natural Science foundation of China (21566025) and the Natural Science Foundation of Jiangxi Province (20152ACB21019), the Graduate Student Innovation Foundation of Nanchang Hangkong University (YC2015017).
[18]
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcou.2017.05.002.
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