Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
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Selective Electrochemical Reduction of Carbon Dioxide Using Cu Based Metal Organic Framework for CO2 Capture Yan-Ling Qiu,† He-Xiang Zhong,†,§ Tao-Tao Zhang,†,‡ Wen-Bin Xu,† Pan-Pan Su,† Xian-Feng Li,*,†,§ and Hua-Min Zhang*,†,§ †
Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Collaborative innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, China S Supporting Information *
ABSTRACT: The conversion efficiency and product selectivity of the electroreduction of carbon dioxide have been largely limited by the low CO2 solubility in aqueous solution. To relieve this problem, Cu3(BTC)2 (Cu-MOF) as CO2 capture agent was introduced into a carbon paper based gas diffusion electrode (GDE) in this study. The faradaic efficiencies (FEs) of CH4 on GDE with Cu-MOF weight ratio in the range of 7.5−10% are 2−3-fold higher than that of GDE without CuMOF addition under negative potentials (−2.3 to −2.5 V vs SCE), and the FE of the competitive hydrogen evolution reaction (HER) is reduced to 30%. This work paves the way to develop GDE with high catalytic activity for ERC. KEYWORDS: carbon dioxide electroreduction, CO2 solubility, CO2 capture, Faradaic efficiency, gas diffusion electrode 100 mA cm−2.10 In addition, good stability could also be achieved when metal particles supported on GDE were utilized.13 Methane and ethylene were produced as the main products of ERC over GDE prepared using a blend of nanometer Cu powder and carbon powder of XC-72R, while multicomponents (CH4, C2H4, C2H5OH, CO, n-C3H7OH, and formate) were obtained on GDE fabricated using a mixture of Cu powder and CuO/ZnO powder.14 Therefore, the structure of active components and the GDE has great influence on ERC production selectivity and the stability of electrode. Although GDE can improve the ERC reaction rate owing to its effectively extended TPB, this advantage cannot be fully embodied due to the low solubility of CO2 in water (0.759 mL−1 (H2O)) at 25 °C under 1 atm.15 The CO2 reduction reaction on Cu was found to be mass-transfer controlled by CO2 present in the electrolyte or adjacent to Cu surface.5,16 Mass transfer polarization will become serious when CO2 is rapidly consumed under high overpotentials or high reaction rates, which results in the poor ERC reaction efficiency and favors the hydrogen evolution reaction (HER). In order to increase CO2 concentration in aqueous solution and improve the reaction rate of ERC, it seems beneficial to carry out the ERC reaction under high CO2 pressure. The ERC reaction rate increases with increasing CO2 pressure, and the
1. INTRODUCTION The electrochemical reduction of carbon dioxide (ERC) on various metal electrodes in aqueous electrolytes has been extensively studied. It is generally known that hydrocarbons can only be effectively produced on copper electrodes.1 However, one of the essential problems producing hydrocarbons in ERC technology is the low reaction rate, which is correlated with the low reaction area of the bulk Cu electrodes, and the total current density of ERC in aqueous solution at 1 atm is usually lower than 100 mA cm−2 under moderate potentials. To solve this problem, gas diffusion electrodes (GDE) and modified Cu electrodes (cuprous oxide or copper oxide-derived nanostructured morphology) have been introduced into the ERC research,2−9 and while most modified Cu electrodes catalyzed CO2 into carbon monoxide and formic acid, only a small amount of hydrocarbons were obtained. GDE is a kind of electrically conductive composite constructed on porous materials. GDE has the advantage of establishing stable and extended three-phase boundaries (TPB) of gas−liquid−solid interface, shortening the gas diffusion path and improving the electrode reaction rate, and thus has been extensively applied in fuel cells and energy storage batteries. Researchers have also introduced GDE into the electrochemical reduction of carbon dioxide (ERC) to improve its reaction efficiency and modulate the product selectivity.10,11 With GDE as cathode, the current density of ERC could be increased by 1 to 2 orders of magnitude,12 current efficiency close to 100% could be obtained under reaction current density more than © 2017 American Chemical Society
Received: October 8, 2017 Accepted: December 21, 2017 Published: December 21, 2017 2480
DOI: 10.1021/acsami.7b15255 ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
Research Article
ACS Applied Materials & Interfaces
Figure 1. Structural schematic diagram of GDE with Cu3(BTC)2 as CO2 capture agent.
exposed unsaturated Cu sites, thus appropriately increasing CO2 concentration adjacent the catalyst (40 nm Cu particle) in 0.5 M NaHCO3 aqueous solution, and the main ERC product is hydrocarbons. The CO2 capture capacity of the prepared CuMOF reaches up to 1.8 mmol g−1 under 1.0 bar at room temperature. The faradaic efficiencies (FEs) of CH4 on GDE with Cu-MOF weight ratio in the range of 7.5−10% are 2−3fold more than GDE without Cu-MOF addition under relatively negative potentials (as low as −2.3 to −2.5 V vs SCE), and the FE of the competitive HER is decreased to 30%. This implies that the CO2 concentration on the interface between GDE and the electrolyte can be increased via the capturing properties of Cu-MOF, and the ERC performance can be modulated according to CO2 concentration. This work paves the way developing GDE with high catalytic activities for ERC.
partial current density for the main products exceeded several hundred milliampere per square centimeter under pressurized CO2,14,17,18 which is at least one order higher than that under ambient CO2 pressure. Furthermore, it was reported that high CO2 pressure will suppress H2 evolution.19 These results suggest that the concentration of CO2 in electrolyte is an important factor determining ERC reaction rate and product selectivity. Another effective method of increasing CO2 concentration is to introduce a CO2 capturer into the GDE structure. Studies on metal−organic frameworks (MOFs) demonstrated that the topological structure of MOFs makes these kinds of materials very suitable for gas storage and separation. CO2 can be effectively trapped and separated when its molecular structure coordinates with the unsaturated metal sites in the MOF structure owing to the very small kinetic diameter of CO2 (0.33 nm).20−25 The CO2 trapping capacity of Mg-MOF-74 was reported up to 8.9 wt %,21 and 87% trapped CO2 could be released at room temperature, indicating the selective capture capacity of this kind of MOF material. In addition, MOF-74 is reported to have CO2 uptake more than 7 mmol g−1,26,27 implying the extraordinary CO2 adsorption capacity of this kind of materials. Cu3(BTC)2, as one kind of typical MOFs, has been reported to possess CO2 uptake of more than 5 mmol g−1,28,29 and has demonstrated selectivity to oxalic acid30 and alcohols31 when it was used as the active component to catalyze ERC in CO2 saturated solution. Extremely high catalytic selectivity (FE = 88.3 ± 3.8%) toward CH4 for ERC on Zn3(BTC)2 (Zn-MOFs) has been obtained in ionic liquid,32 indicating these kinds of metal−organic frameworks having potential application in increasing CO2 concentration and catalyzing ERC. Inspired by these results, if MOF with appropriate structure and stable performance is introduced into GDE as CO2 capture agent, CO2 concentration adjacent to the GDE surface would be increased as a result, which would be beneficial for relieving the mass transfer limitation due to the low CO2 solubility, and enhancing the ERC reaction rate. Product selectivity as well as the inhibition of HER would also be changed accordingly. In this work, Cu3(BTC)2 (Cu-MOF) was synthesized and introduced into carbon paper based GDE for CO2 capture, and Cu nanoparticles act as the active component (Figure 1). The main differences between our work from earlier research reports about Cu3(BTC)2 in ERC lie in the role of the Cu3(BTC)2 and the ERC product. In the earlier reports, Cu3(BTC)2 was used as the active material to catalyze CO2 reduction, and the main ERC product are alcohols or oxalic acid in aqueous solution. In this work, the role of the Cu3(BTC)2 is to capture CO2 by gas coordination with the
2. RESULTS AND DISCUSSION 2.1. Cu-MOF Structure and Adsorption Ability for CO2. The synthesized Cu-MOF exhibits vivid blue color with regular octahedral morphology and smooth surfaces (Figure S1), which is agree with previous report observations.23,33 The average size of the Cu-MOF crystals is ca. 20 μm. The XRD pattern (Figure 2) shows that all diffraction peaks of the synthesized Cu3(BTC)2 are consistent with ref 23, suggesting the successful synthesis of Cu3(BTC)2.
Figure 2. XRD pattern of the synthesized Cu3(BTC)2. 2481
DOI: 10.1021/acsami.7b15255 ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
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Figure 3. (a) CO2 adsorption isotherms at 25 °C and 1.0 bar and (b) N2 adsorption−desorption isotherm at 77 K of the prepared Cu3(BTC)2 particles.
Figure 4. Activity measurements of GDE-Cu-MOF in CO2 saturated 0.5 M NaHCO3 solution: (a) LSV curves, (b) current variation versus time at the potential of −2.0 V (vs SCE).
2.2. GDE Catalytic Performance for ERC. 2.2.1. Electrocatalytic Activity of GDE with CO2 Capture Agent. The catalytic activity of GDE-CuMOF-10 was evaluated in CO2 saturated 0.5 M NaHCO3 solution by linear sweep voltammetry (LSV) technique (Figure 4a), which represent the overall ERC reaction rates occurring on the GDE surface under dynamic potential conditions. For comparison, the voltammogram for GDE without Cu-MOF is also measured. The reduction current associated with two parallel competing processes: hydrogen evolution reaction (HER) and ERC. As shown in Figure 4a, the presence of Cu-MOF improves the onset potential (Eonset) and the catalytic activity of GDE in a specific potential range. At low overpotentials, promotion of the reduction process resulting in the positive shift of the onset potential was observed on GDE-CuMOF-10, corresponding to −0.91 V (vs SCE), which is about 230 mV (from −1.35 V to −0.91 V) more positive than that on GDE-Blank. Furthermore, GDE-CuMOF-10 exhibits higher reduction current than that of the GDE-Blank when the applied potential is more positive than −2.19 V, indicating the faster reaction rate on GDE with the Cu-MOF addition. However, a distinct decrease of the reduction current on GDE-CuMOF-10 was observed compared with the GDE-Blank when the applied potential is lower than −2.2 V (vs SCE), which means that some of the reduction processes occurring at negative potential, such as HER, are suppressed by the introduction of Cu-MOF into GDE. In other
The CO2 adsorption isotherm of the Cu3(BTC)2 in Figure 3a and Figure S2 shows adsorption quantity of 1.8 mmol g−1 at 298 K under 1.0 bar, about 70% CO2 adsorption capacity of the N-doped hollow carbon sphere,34 and lower than the reported CO2 uptake under the same conditions.28,29 This indicates that the synthesized Cu3(BTC)2 has moderate CO2 capture capability, which can be attributed to the relatively lower surface area (728 m2 g−1, Figure 3b). However, high content of micropores (micropore area: 619 m2 g−1, about 85% of the total surface, Figure S3) can still provide CO2 adsorption capacity which will meet the provision of the ERC requirement. In addition, saturation cannot be observed up to a pressure of 100 kPa, implying that higher CO2 adsorption capacity can be achieved at a higher pressure environment (i.e., GDE). The calculated average Qst for CO2 adsorption on Cu-MOF is ca. −28 kJ mol−1 (Figure S4), which is in the typical range for physisorption and close to the previous report.29 The moderate Qst value implies the relatively weaker adsorption strength between Cu-MOF and CO2. This is beneficial to our ERC system which requires the captured CO2 desorption from the Cu-MOF in time to meet the provision for the ERC reaction, especially at higher current densities. It is assumed that CO2 adsorption is the prerequisite to participate in the subsequent reactions, and hence the largely increased CO2 adsorption amount could continually provide reactant for subsequent ERC reaction. 2482
DOI: 10.1021/acsami.7b15255 ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
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Figure 5. Effect of content of Cu-MOF on the faradaic efficiency of the ERC main products after 15 min electrolysis at constant potentials in CO2 saturated 0.5 M NaHCO3: (a) CH4, (b) C2H4, (c) H2.
words, sharp increases of the current on GDE-Blank would be mainly attributed to HER because ERC was inhibited due to the mass transport limitation of CO2. As for the GDE-CuMOF10, the effective CO2 capturing of Cu-MOF can dramatically inhibit HER, which is beneficial for CO 2 reduction. Furthermore, the catalytic active in N2 saturated 0.5 M NaHCO3 is higher than that in CO2 saturated 0.5 M NaHCO3 under potential more negative than −1.88 V, implying the selective CO2 capture property of Cu-MOF rather than capturing N2, and HER prevails under more negative potentials. To evaluate the stationary catalytic performance of GDE with different contents of Cu-MOF, chronoamperometric electrolysis was performed continually for 16 min and the results were shown in Figure 4b. Stable currents are obtained for GDE with different Cu-MOF contents, indicating the relatively robust structure of Cu-MOF under ERC reaction conditions. The structure variation of the GDE-CuMOF-10 after different ERC electrolysis time further verified this statement (Figure S5). In addition, no characteristic peaks of Cu(OH)2 are detected on all of the GDE-CuMOF-10 samples (Figure S5a and b), which implies that the Cu-MOF structure is relatively stable in the ERC reaction environment,31 and the CO2 capture capacity is still maintained during the ERC electrolysis. From Figure 4b, we can predict that the presence of Cu-MOF can also affect the overall catalytic activity of GDE in stationary reduction condition. During ERC reaction, two effects induced by the introduction of Cu-MOF to GDE coexist. One is the insulating effect to block the electron transfer to the Cu active sites, and
the reaction rate will be decreased. Another is the improvement of CO2 concentration at the interface of the electrode and the electrolyte, which can facilitate the ERC reaction rate and inhibit the HER. The compromise of the two effects will finally determine the reaction rate and the product selectivity as well as the product yields. In Figure 4b, the reaction currents on GDEs with various Cu-MOF contents are lower than the GDEBlank, indicating the lower activity of ERC or HER on these electrodes. Especially, the reduction current on GDE with 10 wt % Cu-MOF is 26.5% lower than that on the GDE-Blank, implying the effect of Cu-MOF on the reduced amount of Cu active sites and/or the dramatic inhibition of HER induced by the capturing effect of Cu-MOF in GDE. In conjunction with the LSV results, it can be inferred that the reduced current is attributed to the prominent inhibition of the competitive HER. As the Cu-MOF content further increases, the reaction current tends to increase, just as shown in the GDEs with 15 and 20 wt % Cu-MOF contents in Figure 4b. In particular, GDE-CuMOF20 presents 24% higher reduction current than GDE-CuMOF10, yet about 9% lower than GDE-Blank. Whether the increased current coming from the improved HER or ERC cannot be determined from the active results in Figure 4b; however, the product selectivity of ERC could provide efficient information about this current increase. 2.2.2. Cu-MOF Effect on the Selectivity of Cu-GDE. The data presented in Figure 4 evidenced that Cu-MOF has an obvious effect on the catalytic activity of Cu-GDE for the ERC. In particular, Cu-GDEs with 10 and 20 wt % Cu-MOF addition appeared to have the most prominent beneficial or harmful 2483
DOI: 10.1021/acsami.7b15255 ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
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distinct pathway from that of CH4. In addition, the increase of CO2 concentration induced by the capturing effect of Cu-MOF does not obviously favor the key reaction process to produce C2H4. This result further implies that the main intermediate adsorption properties to C2H4 formation on the GDE might not be influenced distinctly, especially under the medium and higher overpotentials. The optimum Cu-MOF content based on the GDE for the FE of C2H4 is 10 wt % in Figure 5b. Harmful effects of Cu-MOF on C2H4 selectivity appear when Cu-MOF in GDE exceeds this threshold, indicating that the amount of exposed active sites of GDE have critical influence on C2H4 production, and the CO2 concentration is not very pivotal. Therefore, research on improving C2H4 selectivity in the ERC reaction should focus on the preparation method to increase the active area as much as possible, which has been obtained through controlling the Cu morphology, such as nanocube,36−38 nanowire,9 etc. The competitive HER is distinctly suppressed by introducing an adequate amount of Cu-MOF into GDE catalytic layers in Figure 5c. The FE for H2 demonstrates a declining tendency under the three potentials, and the most distinct inhibition of HER occurred on GDE-CuMOF-10. Compared with that of GDE-Blank, the FEs of H2 on GDE-CuMOF-10 are decreased by 35.4%, 26.7%, and 25.5% under −1.8, −2.0, and −2.5 V, respectively. This result verifies our design concept, i.e., the competitive HER can be effectively inhibited via increasing CO2 concentration (or partial pressure) in electrolyte which is realized by introducing Cu-MOF into GDE. According to ERC reaction conditions, the following factors are closely related with the reaction rate of the competitive HER: The first one is the electrolysis potential. H+ arising from water electrolysis in anode migrates to the cathode, where part of H+ will be consumed by CO2 electroreduction on the cathode surface, and the left H+ will be reduced to hydrogen atom and escaped as H2 under appropriate potentials. In general, HER is inevitable in aqueous solution due to the sluggish ERC reduction (i.e., H+ consumption rate is low for CO2 reduction) and the rapid mitigation of proton mobility (3.625 × 10−3 cm2·V−1·s−1).39 Therefore, hydrogen evolution will be more serious as the electrolysis potential become more negative. This factor can explain the most serious HER occurring at −2.5 V and the weakest HER at −1.8 V in Figure 5c on GDEs with or without Cu-MOF addition. Another factor influencing the HER is the impurities in the electrocatalyst or in electrolyte.12,40,41 Among several kinds of impurities, carbon is the most common one, and almost has no catalytic effect for ERC. However, previous studies reported that carbon can provide abundant active sites for HER under much more negative potentials,42−44 which is unfavorable for ERC. The Cu-MOF includes a large amount of ligands (such as benzoic acid), where abundant active sites for HER might be provided by the carbon atoms.45 Under negative potentials, the HER rate is the competitive result of the inhibition effect by the increased CO2 concentration and the promotion effect by the carbon sources. When Cu-MOF content is lower than the threshold (10 wt %), the inhibition effect occupy the domination for the number of the carbon sites is small, and the FE for HER tends to decline with Cu-MOF content increase. On the contrary, the promotion effect for HER gradually becomes predominant when Cu-MOF content exceeds 10 wt %, and serious HER will occur as a result. This speculation has been verified in Figure 5c). It is noteworthy that the total faradaic efficiency of gas products (Figure 5) is far from 100%, suggesting that some
effects on the processes occurring at high overpotential, respectively, such as improving CO2 concentration of the interface of the GDE and electrolyte and affecting the subsequent product formation, inhibiting the competitive HER. To clarify these points, further chronoamperometry electrolysis at different constant potentials were carried out and the faradaic efficiencies (FE) of the main products were calculated (Figure 5). Combining this testing, the Cu-MOF effect on the product selectivity of ERC and inhibition of HER as well as the potential window of Cu-MOF in ERC could be revealed. The results in Figure 5 confirm that the Cu-MOF addition into GDE not only affects the reaction activity, but also modulates the product selectivity of ERC reaction. According to Figure 5a, dramatic improvement of CH4 selectivity can be observed with the introduction of Cu-MOF into GDE. For example, the FEs for CH4 on GDEs with Cu-MOF content in the range of 7.5−15 wt % are 2−3-fold higher than that on GDE-Blank under the medium and higher overpotentials. However, a different tendency is observed under the lower overpotential, i.e., −1.8 V (vs SCE), in which the optimum CuMOF content is 5 wt %, and the lowest effect is shown on GDE-CuMOF-10. The different tendency for CH4 selectivity under various overpotentials might be the integrated results of the following factors: one is the driving force applied by the electrolysis potential, the second is the concentration of CO2 adjacent to the interface of GDE and electrolyte induced by the strong CO2 capture ability of Cu-MOF. The third factor is the amount of effective Cu active sites which is related to the blocking effect of the Cu-MOF.35 Under the lower overpotentials (i.e., −1.8 V), the electrochemical driving force is relatively small, the reaction rate is mainly determined by the total amount of active sites and CO2 concentration. When CuMOF content in GDE is lower than 5 wt %, the Cu-MOF blocking effect is relatively weak, and CO2 provision is abundant for the lower ERC reaction rate. As Cu-MOF content increased from 5 wt % to 10 wt %, more CO2 can be provided to the interface of GDE and electrolyte as the result of CO2 capturing effect of the Cu-MOF. However, the harmful blocking effect of the Cu-MOF might overwhelm, in which case the effective Cu active sites are prevented from participating in the ERC reaction. As the content of Cu-MOF in GDE further increases to 15 wt % and even to 20 wt %, the beneficial effect of CO2 capture by Cu-MOF will dominate the ERC process over the blocking effect of Cu-MOF, thus ERC product selectivity is increased. When the overpotential increases continually, the driving force of ERC increases accordingly, and the CO2 concentration required for maintaining the ERC reaction process should also be increased. The competing results of the beneficial effect of the CO2 concentration improvement and the blocking effect on the Cu active sites induced by the Cu-MOF would mitigate the ERC selectivity to the GDE with more Cu-MOF content. The optimum CH4 selectivity is obtained on GDE with Cu-MOF content more than 10 wt %. In Figure 5b, FE for C2H4 demonstrates a different trend compared with FE for CH4 under the studied potentials, i.e., GDE-CuMOF-5 presents the lowest selectivity for C2H4, even lower than GDE-Blank. Another distinct phenomenon is the higher FE for C2H4 under −1.8 V than those under −2 V and −2.5 V, suggesting that higher overpotential would be against C2H4 formation. This rule is quite different from the FE for CH4 in Figure 5a, implying that C2H4 formation follows a 2484
DOI: 10.1021/acsami.7b15255 ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
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ACS Applied Materials & Interfaces
Figure 6. EIS of the GDE with and without Cu-MOF under different electrolysis potentials in CO2 saturated 0.5 M NaHCO3 solution: (a) −1.8 V, (b) −2.2 V.
results are shown in Figure 7. The Tafel slope is 113 mV decade−1 for GDE-CuMOF-10, about 17% lower than 134 mV
liquid products such as C1 and C2 species were produced during ERC processes.30,31 2.2.3. Cu-MOF Effect on ERC Resistance. To further characterize the beneficial effect of Cu-MOF on the catalytic activity and product selectivity of ERC, the EIS spectra of GDE-CuMOF-10 and GDE-Blank were tested in 0.5 M NaHCO3 solution under different potentials (−1.8 V and −2.2 V), and the result is shown in Figure 6. Under −1.8 V in Figure 6a, the ohmic resistance (RΩ) of the reaction system with GDE-CuMOF-10 increases about 10% compared with that of GDE-Blank, and a significant increase of the charge transfer resistance (Rct) also appeared. These results should be ascribed to the low electric conductivity property of Cu-MOF. At −1.8 V, the electrochemical driving force is small due to the lower overpotential of ERC reaction; the product yield is very rare under such a low reaction rate, and CO2 dissolved in the electrolyte can basically meet the requirements of the ERC. However, the decrease of the electrode conductivity and the number of exposed active sites by CuMOF introduction might be obvious, which is reflected in the EIS spectrum. As the electrolysis potential is controlled at −2.2 V (Figure 6b), an adverse phenomenon is observed for both RΩ and Rct on the two GDEs, i.e., RΩ of GDE-CuMOF-10 is about 5% lower than that of GDE-Blank, and Rct also tends to be reduced compared with GDE-Blank. These results suggest that the increased CO2 captured by Cu-MOF can effectively reduce the resistance of the ERC reaction system and promote the reactive charge transfer through suppression of the competitive HER. Based on the results in Figure 5c, we can infer that the addition of Cu-MOF can effectively inhibit HER, and more serious HER will occur on the surface of GDE-Blank, which will lead to the increase of charge transfer resistance. The EIS results further evidence the beneficial effect of Cu-MOF as CO2 capture agent to ERC reaction, especially more prominently under higher overpotential. It should be pointed out that a stable EIS spectrum cannot be obtained under electrolysis potential more negative than −2.2 V, which may be due to the disturbance of the large amount of bubbles escaping from the electrode surface. 2.2.4. ERC Mechanism on GDE-CuMOF. In order to study the Cu-MOF effect on the ERC mechanism, especially on the formation pathway of hydrocarbon, the Tafel slopes for the partial current density of CH4 (jCH4) on GDE-CuMOF-10 and GDE-Blank are analyzed under the low overpotentials and the
Figure 7. Tafel plot with linear fit at low current densities for GDE with and without Cu-MOF.
decade−1 for GDE-Blank, indicating a little faster kinetic reaction of ERC on the GDE-CuMOF ascribed to the increased CO2 concentration. In addition, this result suggests that the active sites in GDE-CuMOF-10 are not obviously blocked by Cu-MOF, and also implies the almost unaffected catalytic properties of Cu-MOF on the formation pathway of CH4. It is generally thought that the rate-limiting step for CH4 generation on copper foils or Cu particles larger than 35 nm involves a single electron transfer to CO2 which is characterized with 120 mV decade−1 as the Tafel slope.46 The fact that the Tafel slopes on both GDE-blank and GDE-Cu-MOF are close to 120 mV decade−1 suggests the sluggish reaction kinetics of ERC on relatively large Cu nanoparticles (40 nm). Therefore, future studies should focus on small Cu nanoparticles and the introduction method of CO2 capture agent into GDE. 2.2.5. Stability of GDE-CuMOF. The rapid deactivation of the Cu-based electrodes (typically shorter than 2 h) is a major obstacle impeding their application in the CO2 electroreduction reaction.12,39 Therefore, it is important to evaluate the stability of the Cu-based GDE with Cu-MOF over a prolonged CO2 electroreduction process. The electrocatalytic durability of the prepared GDE electrode was evaluated at −2.5 V (vs SCE) for 2485
DOI: 10.1021/acsami.7b15255 ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
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ACS Applied Materials & Interfaces
Figure 8. Stability of GDE with Cu-MOF as CO2 capture agent in CO2 saturated 0.5 M NaHCO3 solution at ambient pressure and room temperature. Electrolysis potential is −2.5 V (vs SCE): (a) current, (b) variation of FE for CH4 versus ERC time.
obvious fluctuations, which first decrease with small amplitude (ca. 20%), then significantly increase (ca. 50%), and reach a peak value at 2.3 h. After this turning point, the FECH4 decreased slowly, and is close to its initial value (about 17%) at 6.3 h. It is inferred that the addition of Cu-MOF would change the number of active sites on the GDE surface or the adsorption strength of the key intermediates, such as CO* and CHO* on the Cu nanoparticles, and the latter case might be the main factor to influence the catalytic activity of GDE4.1
ca. 6 h in CO2 saturated NaHCO3 solution. The cathodic current and FE for CH4 as a function of time are shown in Figure 8. Compared with GDE-blank, the GDE-CuMOF-10 displayed more superior and stable catalytic activity for ERC demonstrated by the stable cathodic current (Figure 8a) and an overall stable FE for CH4 (Figure 8b). After 6 h ERC electrolysis, the rise of catalytic current on GDE-CuMOF-10 is 6.5%, about 50% lower than that on GDE-Blank. However, the Cu-MOF structure degraded gradually as the electrolysis time was prolonged, and the crystallinity reduction of the Cu-MOF is observed. In order to clearly and accurately reflect the structure evolution of Cu-MOF during ERC reaction process, the infrared spectra of GDE-CuMOF-10 after different ERC reaction times were measured (Figure S6). The functional groups of the GDE-CuMOF-10-as prepared exhibit relatively weak vibration signals in the FTIR spectra for the low Cu-MOF content in the GDE, as shown in Figure S6a). The vibration signal gradually weakened as the ERC reaction proceeded (Figure S6b), implying a slow degradation of Cu-MOF occurred during the electrolysis. Especially, the main signal attributed to the Cu-MOF in the FTIR spectroscopy diminishes after 24 h electrolysis in 0.5 M NaHCO3 aqueous solution. The structure evolution in the FTIR spectra is attributed to the limited stabilities in water, in which Cu-MOF undergoes hydrolysis or phase transformations. However, the relatively stable electrocatalytic activities compared with GDEBlank in Figure 8a suggests that the preservation structure of Cu-MOF can meet the requirement of CO2 capture to meet ERC reaction, which can maintain the ERC efficiency. On GDE-Blank, the cathodic current first shows a declining tendency, then a distinct upward trend. This phenomenon may be due to the insufficient provision of CO2 on the interface of the GDE and the electrolyte, which will lead to the carbon deposition and poison the electrode.4 Correspondingly, the obvious current increase mainly comes from the competitive HER on the poisoned GDE surfaces. During the stability test, the CH4 selectivity on the two GDEs exhibits a different tendency. The FE for CH4 on GDEBlank tends to become stable after 2 h ERC reaction, and remains almost constant close to its initial value (about 3%) during the subsequent 4 h. This result reveals that the number of active sites to catalyze CO2 to CH4 is almost unchanged. However, the FE for CH4 on GDE-CuMOF-10 exhibits
3. CONCLUSIONS In this study, we demonstrated that the introduction of CuMOF into Cu nanoparticles based GDE could efficiently improve the selectivity of CH4 for the electrochemical reduction of CO2, besides more positive onset potential, and excellent stability. The synthesized Cu-MOF exhibited much high performance for CO2 capture which could continually provide CO2 for subsequent ERC reaction and favor the ERC product formation. The onset potential for GDE with 10 wt % Cu-MOF was 230 mV more positive than that for the GDEBlank. Addition of an adequate amount of Cu-MOF into GDE can improve the FE for CH4 by 2−3-fold compared to GDEBlank under the medium and higher overpotentials. In addition, the competitive HER can be suppressed remarkably, and FE for HER can be reduced up to 30% at −1.8 V (vs SCE). Furthermore, GDE-CuMOF exhibits relatively stable catalytic activity of ERC and selectivity for CH4 during 6 h continuous stability test. The improved ERC performance of GDE with Cu-MOF is a result of the enhanced CO2 concentration on the interface of GDE and the electrolyte favored by the CO2 capture properties of Cu-MOF. However, more than 10 wt % of Cu-MOF addition can lead to the decrease of GDE active sites and provide more carbon sources for HER. This study paves a new way of improving the ERC performance through introducing CO2 capture agent to increase CO2 concentration adjacent the the interface of the electrode and electrolyte, including electrode modification and introducing some kinds of additive into the electrolyte. 4. EXPERIMENTAL METHODS 4.1. Cu-MOF Synthesis. Cu3(BTC)2 was synthesized based on the proposed constant pressure method by Yaghi’s group.47 In brief, 1,3,5benzene tricarboxylic (BTC, 2.38 mmol) and Cu(NO3)2 (3.89 mmol) were dissolved separately in the mixed solvent of DMF/EtOH/H2O 2486
DOI: 10.1021/acsami.7b15255 ACS Appl. Mater. Interfaces 2018, 10, 2480−2489
Research Article
ACS Applied Materials & Interfaces (12 mL with volume ratio of 1:1:1) under stirring. Then, the Cu(NO3)2 solution was dropped into the BTC solution with the speed of 2 mL min−1, and blue liquid was obtained after 30 min. The mixture was crystallized for 24 h in static conditions under circulating condensation at 85 °C. The blue crystals produced were cleaned with DMF and centrifuged at 8000 rpm. Third, the blue crystals were soaked in acetone to replace molecular DMF. The replacement process was repeated 9 times with 3 times per day. The blue crystals were finally dried and activated at 170 °C for 24 h in a vacuum oven with −0.1 MPa. The synthesized Cu3(BTC)2 samples were denoted as Cu-MOF and stored in a desiccator to avoid moisture adsorption. 4.2. Characterization of Cu-MOF. To characterize the crystal structure of the synthesized sample, the powder X-ray diffraction (XRD) patterns of the Cu-MOF were carried out using a X-ray diffractometer (X-Pert Pro) equipped with a Cu Kα radiation (λ = 0.1542 nm). The surface area and pore distribution of the Cu-MOF were investigated by N2 adsorption−desorption isotherms (ASAP 2020 Surface Area and Porosity Analyzer, Micromeritics Corp.), and the CO2 capture capability was evaluated by CO2 sorption isotherm at 273 and 298 K and up to 1.0 bar (ASAP 2020 Surface Area and Porosity Analyzer, Micromeritics Corp.). The morphology of the CuMOF was studied by the back scattered-electron imaging (JEOL JCM6000, JEOL Ltd.). 4.3. Preparation of Gas Diffusion Electrodes and Pretreatment. A gas diffusion electrode with Cu-MOF as CO2 capture agent consisted of porous substrate and catalyst layer. TGP-H-030 (Toray Co., Japan) was used as the porous substrate. In the catalyst layer, Cu nanoparticles (NPs) with diameter of 40 nm were used as active component. Nafion (Dupont corp., US) ionomer was used as the binder. The mass percent of Nafion was 20% in all of the GDEs. The mass ratio of Cu-MOF was designed as 0%, 5%, 7.5%, 10%, 15%, and 20%, which was named as GDE-Blank, GDE-CuMOF-5, GDECuMOF-7.5, GDE-CuMOF-10, GDE-CuMOF-15, and GDECuMOF-20, respectively. During the GDE preparation, Cu NPs and Cu-MOF were first mixed with isopropanol alcohol, and well dispersed in ultrasonic bath. Second, Nafion solution (DupontIncor., DE521) was added to the aforementioned mixture to obtain a homogeneous slurry. Third, the slurry was coated on the surface of TGP-H-030, and dried under 60 °C to obtain GDE with different contents of Cu-MOF. Before electrochemical measurement, the GDE samples were immersed into concentrated hydrochloric acid to remove the oxide layer covering the Cu nanoparticles. 4.4. FTIR Spectra Measurement. The IR spectra were recorded with a Fourier Transform Infra spectrometer (Nicolet iS50, Thermo Fisher, USA) in the 2000−400 cm−1 wavenumber region with transmittance technique. For the GDE-CuMOF-10-as prepared and those undergone different ERC electrolysis time (−2.5 V vs SCE), the particles were first scrapped from the GDE surface, then mixed with KBr, and compressed to the testing sample under pressure. 4.5. ERC Onset Potential Testing of GDE. The electrocatalytic activity of GDEs toward ERC was studied by linear sweep voltammetry (LSV) in CO2 (99.999%) saturated 0.5 M NaHCO3 solution (pH 7.2). LSV curves were measured in the potential range of −0.6 to −2.65 V (vs SCE) at the scan rate of 10 mV s−1. The onset potential was determined from the current density of 0.1 mA cm−2. 4.6. Chronoamperametry Electrolysis. The electrochemical reduction of CO2 was carried out through chronomamperametry technique by a Potentiostat/Galvanostat (EG&G 2273, Princeton Applied Research) in a homemade H-type cell with two compartments separated by Nafion115 membrane. The supporting electrolyte was 0.5 M NaHCO3 (180 mL). GDE sample with 3 cm2 geometric surface area as the working electrode and SCE reference electrode were both placed in the cathodic compartment, and platinum foil (99.99%, Tianjin Aida Corp.) as counter electrode was positioned in the anode compartment. Before each measurement, CO2 was continuously bubbled into the cathode solution at least 40 min with flow rate of 60 mL min−1. ERC was conducted in potential range between −1.7 V and −2.8 V (vs SCE) under room temperature and absolute pressure of 1 atm. The electrolysis time for each applied potential was 16 min.
When ERC reaction proceeded to 15 min, the outlet gas of the cathodic compartment was online introduced into gas chromatography (GC-2014, Shimadzu). Gas products, including CH4, C2H4, C2H6, and H2 were analyzed. The volume of the sample loop is 1.0 mL.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15255. Morphology of the synthesized Cu-MOF; CO2 adsorption isotherms; micropore and mesopore distribution of prepared particles; isosteric heat for Cu-MOF; structure variation; FTIR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yan-Ling Qiu: 0000-0003-4492-0274 Xian-Feng Li: 0000-0002-8541-5779 Author Contributions
The manuscript was written through contributions of all authors. Yan-Ling Qiu designed the experiments and wrote the paper. Tao-Tao Zhang and Wen-Bin Xu performed part of the experiments. Yan-Ling Qiu carried out the XRD and SEM imaging analysis. He-Xiang Zhong and Pan-Pan Su discussed the results and contributed to the modification of the manuscript. Xian-Feng Li and Hua-Min Zhang guided the work. All authors discussed the results and have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the financial grants from the National Natural Science Foundation of China (No. 21577141 and No. 21576255).
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REFERENCES
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