Highly efficient electrochemical reduction of CO2 to CH4 in an ionic liquid using a metal–organic framework cathode

It has been discovered that Zn metal-organic framework (Zn-MOF) electrodes and ionic liquids are an excellent combination for the efficient and selective reduction of CO2 to CH4.


Introduction
Electrocatalysis combines the advantages of efficient conversion of electrical energy into chemical energy with the convenience and stability of heterogeneous catalysis, which has received much attention. 1 Transformation of CO 2 into useful chemicals and fuels is very attractive because it is a cheap and renewable carbon source, but is very difficult because CO 2 is thermodynamically stable and kinetically inert. 2 Electrochemical reduction is a promising method in CO 2 transformation. 3 It can be transformed into various products, such as CO, acids, alcohols and hydrocarbons. 4 Electrochemical reduction of CO 2 to CH 4 is an alternative route for synthesizing energy-rich and clean fuels, which however currently suffers from low activity and poor selectivity. 5 Metal-organic frameworks (MOFs) represent a class of hybrid materials comprised of ordered networks formed via combining metal ions with organic ligands. 6 MOFs are widely studied for gas storage and capture, 7 separation, 8 drug delivery 9 and catalysis. 10 In addition, MOFs have been used as efficient electrodes in fuel cell systems 11 and reduction of CO 2 in aqueous or organic electrolytes. 12 Ionic liquids (ILs) have attracted considerable attention owing to their unique properties, such as low melting point, negligible vapor pressure, high ionic conductivity, high chemical stability, and adjustable physical and chemical properties. 13 Applications of ILs in different elds have been studied extensively, 14 including those in material synthesis 15 and electrochemistry as electrolytes. 16 The electrodes and electrolytes are crucial in electrocatalysis, and different electrodes and electrolytes can induce different products. Exploring innovative combination of catalysts and electrolytes is a very interesting topic of great importance. Herein, we conducted the rst work on the electrochemical reduction of CO 2 in MOF electrode/IL electrolyte system. It was found that the combination of MOF electrodes and ILs was very effective for the electrochemical reduction of CO 2 to CH 4 , and the morphology of MOFs and the properties of the ILs affected the current density and selectivity to CH 4 signicantly.

Results and discussion
Zn-MOFs can be synthesized easily by coordination of Zn 2+ and 1,3,5-benzenetricarboxylic acid (H 3 BTC) in solution. 17 In this work, we prepared the Zn-MOFs in the mixed solvent consisting of 75 wt% 1-dodecyl-3-methylimidazolium chloride (C 12 mimCl) and 25 wt% glycerol. The mass fractions of ZnCl 2 (x) in the C 12 mimCl + glycerol + ZnCl 2 system were 0.17, 0.29, 0.38, 0.44, 0.50, respectively. The powder X-ray diffraction (XRD) patterns of the Zn-MOFs are illustrated in Fig. 1. The results indicate that the XRD patterns of the MOFs synthesized at x ¼ 0.17, 0.29, 0.38 are similar to that of the reported Zn-MOF. 18 However, at the larger x values, the Zn-MOFs showed lower crystallinity (Fig. 1).
The scanning electron microscopy (SEM) images of the Zn-MOFs are shown in Fig. S1. † The Zn-MOFs had the rod-like morphology at smaller x value, and became shorter and thicker with increasing ZnCl 2 mass fraction. When the mass fraction of ZnCl 2 reached 0.38, the Zn-MOFs had sheet-like morphology. The Zn-MOFs were spherical with further increasing the mass fraction of ZnCl 2 . It is well known that there existed ordered aggregates in ILs systems. 15 In this work, small angle X-ray scattering (SAXS) technique was used to study the microstructures of the synthetic media. 15c The results indicated that the domains in the solution varied from rod-like to sheet-like and to spherical with the increase of x values (Fig. S1 †). This demonstrates that the morphologies of the Zn-MOFs are similar to that of the domains in the solutions in which they were formed. The detailed discussion on the Zn-MOF formation and shape control are given in the ESI † in combination with SAXS study (Fig. S1-S3 † and the related discussion).
We prepared the Zn-MOF/CP cathodes by depositing Zn-MOFs on the CP using the electrophoretic deposition (EPD) method. 19 The principle of the EPD method is shown schematically in Fig. 2a and the procedures are discussed in detail in the ESI. † The Zn-MOFs synthesized can be easily deposited on CP cathode with lower voltage and short time using the electrophoretic deposition (EPD) method, the main reason is that the Zn-MOFs have partial charge on the surface due to the synthetic media. 19 From the SEM images ( Fig. 2b-d), it can be clearly seen that the Zn-MOF/CP cathode prepared using the Zn-MOF synthesized at x ¼ 0.38 had smooth surface with a thickness of about 10 mm. The Zn-MOF before and aer EPD were characterized by X-ray photoelectron spectroscopy (XPS) (Fig. S4 †). The two peaks at 1021.9 eV and 1045.1 eV, which are assigned to the 2p 3/2 and 2p 1/2 components, respectively, were not changed in the EPD process, indicating that the Zn-MOF was stable in the process. The SEM images of the surface and thickness of Zn-MOF/CP cathodes prepared using the Zn-MOFs synthesized at different x values are shown in Fig. 3. The thicknesses of the Zn-MOFs on cathodes were also about 10 mm, however the cathodes prepared using Zn-MOFs synthesized at x ¼ 0.44 and 0.5 had irregular surface. The morphologies of Zn-MOF/CP cathodes aer EPD process ( Fig. 2 and 3) were similar with the as-synthesized Zn-MOFs ( Fig. S1 †), indicating the EPD process can not destroy the morphology of Zn-MOFs. The electrochemical surface areas of the Zn-MOF/CP cathodes prepared using Zn-MOFs synthesized at x ¼ 0.17, 0.29, 0.38, 0.44 and 0.50 were examined by studying the redox reactions using cyclic voltammetry (CV) shown in Fig. S5, † which were 0.40 cm 2 , 0.47 cm 2 , 1.34 cm 2 , 1.05 cm 2 and 0.67 cm 2 , respectively. The Zn-MOF/CP cathode prepared using the Zn-MOF synthesized at x ¼ 0.38 had largest electrochemical surface area because mainly of its sheet-like structure. CO 2 reduction activities of different Zn-MOF/CP cathodes were investigated in CO 2 -saturated and N 2 -saturated IL 1-butyl-3-methylimidazolium tetrauoroborate (BmimBF 4 ), and the CV curves are shown in Fig. 4a. The reduction peak at about À2.2 V vs. Ag/Ag + can be observed for the CO 2 -saturated system, while current density of the N 2 -saturated system was negligible, indicating the reduction of CO 2 . The results in Fig. 4a also demonstrate that the morphology of the Zn-MOFs affected the current density signicantly. The sheet-like Zn-MOF synthesized at x ¼ 0.38 showed highest current density. To explore the kinetic effect of Zn-MOFs, the electrochemical impedance spectroscopy (EIS) was conducted to study the features of the Zn-MOF/CP electrodes in BmimBF 4 , and the detailed discussion are provided in the ESI (Fig. S6-S8 and Tables S1 and S2 †). The EIS result conrms that charge transfer can easily occur on the Zn-MOFs surface. The Zn-MOF/CP cathode prepared using Zn-MOF synthesized at x ¼ 0.38 had the lowest charge transfer resistance (R ct ) value due to the sheet-like structure with highest electrochemical surface area as discussed above.
In order to conrm the electrocatalytic response shown in Fig. 4a, the controlled potential electrolysis (CPE) experiments were carried out. The electrolysis device is shown schematically in Fig. S9. † The current densities using different Zn-MOF/CP cathodes are exhibited in Fig. 4b, which shows that the current  densities did not decrease with time in the electrolysis, suggesting that the Zn-MOF electrode and the IL were stable. The CV and CPE using CP as cathode were also studied ( Fig. S10 †). The CP cathode produced much lower current density than Zn-MOF/CP cathode. The Zn-MOF/CP cathodes prepared from the sheet-like Zn-MOF synthesized at x ¼ 0.38 generated highest current density due to its largest electroactive surface area. Therefore, CO 2 reduction using this cathode was further studied, and the results are discussed in the following.
Aer electrolysis of 2 h at À2.2 V vs. Ag/Ag + , the gaseous product in the headspace was collected and analyzed by gas chromatography (GC), and the liquid mixture was analyzed by 1 H-NMR to quantify liquid products (Fig. S11 †). There was no product found in the liquid phase. CH 4 was the dominate product in the gas phase with small amount of CO and H 2 . High selectivity to CH 4 is very difficult to realize using conventional electrolysis systems. We also conducted the electrolysis under different potentials, and the amount of CH 4 (A CH 4 ) is shown in Fig. 5a. It can be clearly seen that CH 4 production rate increased dramatically at the potentials less negative than À2.2 V vs. Ag/ Ag + , and rose very slowly at the potentials more negative than À2.2 V vs. Ag/Ag + . Therefore, electrolysis under À2.2 V vs. Ag/Ag + was most suitable for CH 4 production. CH 4 began to generate at À1.95 V from extrapolation method using the current densities for CH 4 under different potentials (Fig. 5b). The distinct prefeature (Fig. 4a) at the potential less negative than À1.95 V vs. Ag/Ag + was originated mainly from the generation of CO. The overpotential for CH 4 was 0.25 V for this process at À2.2 V vs. Ag/Ag + with the current density of 3.1 mA cm À2 . Moreover, the data from Tafel plot (Fig. 4c), which was obtained by electrolysis voltage, was linear in the range of h ¼ 0.19-0.37 V with the Tafel slope of 146 mV decade À1 , indicating a rate-determining initial electron transfer to CO 2 to form an adsorbed CO 2 c À intermediate. 20 XPS spectra of the Zn-MOF were given before and aer the electrolysis in Fig. S4. † The results demonstrated that the spectra did not change notably, further indicating the excellent stability of the Zn-MOF/CP cathode. In addition, the electrolyte was also stable according to the 1 H-NMR spectra (Fig. S11 †).
The property of electrolytes oen inuences an electrochemical process signicantly. It was reported that imidazolium based ILs can interact with CO 2 by physical absortption, 21 and the ILs can serve as both robust electrolytes and CO 2 activation promoters. 22 In addition, they have wide electrochemical windows and good conductivity. 23 Therefore, in this work, some  other typical imidazolium based ILs were also used as the electrolytes in the electrolysis to reduce CO 2 , including 1-butyl-3-methylimidazolium triuoromethanesulfonate (BmimOTf), 1butyl-3-methylimidazolium hexauorophosphate (BmimPF 6 ) and 1-butyl-3-methylimidazolium perchlorate (BmimClO 4 ). The CV traces and current density proles are shown in Fig. 6, and the total current densities and faradaic efficiencies for CH 4 , CO and H 2 are listed in Table 1. The ILs containing uorine such as BmimBF 4 , BmimPF 6 and BmimOTf exhibited much higher j tot than the ILs without uorine, which is partially because uorine has strong interaction with CO 2 . 24 The viscosities of the ILs also affected the j tot , as can be known from Table 1. We also conducted the electrolysis using the Zn-MOF electrode combined with other electrolytes, including DMF containing 0.01 M tetrabutylammonium tetrauoroborate (TBABF 4 ), MeCN containing 0.1 M tetrabutylammonium hexauorophosphate (TBAPF 6 ), and MeCN containing 0.1 M BmimBF 4 , and results are listed in Table S3. † The results show that the faradaic efficiency for CH 4 was very low when these electrolytes were used, indicating that imidazolium based ILs were crucial for the very high selectivity to CH 4 .
Metal electrodes are commonly used to reduce CO 2 . In this work, we carried out the electrolysis using Au, Ag, Pt, Fe, Zn, and Cu cathodes in BmimBF 4 at different voltages as well. The metal electrodes used were polished metal foils with bulk structure, therefore the effect of the surface structure can be ignored. The j tot and partial current densities of CH 4 (j CH 4 ), CO (j CO ) and H 2 (j H 2 ) over the potential range from À1.9 V to À2.5 V vs. Ag/Ag + are compared with the results obtained from the Zn-MOF cathode in Fig. 7. The CVs of different metal cathodes are shown in Fig. S12, † and the principal product, linear range in Tafel plot and Tafel slope for their main product are provided in Table  S4. † Obviously, the j tot and the selectivity to CH 4 of the Zn-MOF/ CP system were much larger than that of the metal cathode systems at the same voltage, suggesting that the Zn-MOF electrode was also very important for the high efficiency in producing CH 4 . The potential was less negative using Zn-MOF/ CP cathode than metal cathodes to reach the same current density of CH 4 , indicating that the Zn-MOF/CP cathode was more active for CH 4 generation. For comparison, Table S5 † lists the CH 4 selectivity reported in the literature for the electrochemical reduction of CO 2 . The data indicate that the selectivity to CH 4 in our work was the highest. The product contained 85% CO and 15% H 2 when CP was used as the cathode in BmimBF 4 at À2.2 V vs. Ag/Ag + , further indicating that the Zn-MOFs played a key role for producing CH 4 . These results demonstrate that the Zn-MOF/CP cathode and the IL BmimBF 4 are excellent combination for producing CH 4 from electrochemical reduction of CO 2 .
As discussed above, both Zn-MOF cathodes and imidazolium based IL electrolytes are crucial for the high yield of CH 4 . The Zn-MOF/CP cathodes and imidazolium based IL electrolytes are excellent combination for producing CH 4 . In addition, the current density reached the highest value using Zn-MOF/CP cathode prepared using Zn-MOF synthesized at x ¼ 0.38 in BmimBF 4 .
The high electrochemical activity of the Zn-MOFs in BmimBF 4 results partially from the facts that the imidazolium based ILs containing uorine can absorb and activate CO 2 , 24 and the Zn-MOFs are porous materials, which benets gas adsorption. 7 The Zn-MOFs were synthesized in imidazolium based IL mixture, which therefore has very good compatibility with ILs, which helps driving the reaction. 16 In addition, it is reported that Zn-MOFs are efficient selective adsorbent for different gases. 17 At present, there is no method to determine the gas adsorption amount on solid surface in the presence of a liquid, so we determined the adsorption amounts of the gases in the absence of IL in order to get some indirect evidence to discuss the interactions between the gases and the electrode, and the adsorption properties of the Zn-MOF for CO 2 , CO, CH 4 at 298 K (Fig. 8) were studied. The adsorption amounts of CO 2 , CO, and CH 4 on the Zn-MOF at 1 atm and 298 K are 9.7 cm 3 g À1 , 3.9 cm 3 g À1 , and 1.0 cm 3 g À1 , respectively, indicating that Zn-MOF had much stronger adsorption for CO 2 and CO than for CH 4 . Furthermore, only CO can be detected at the potential less negative than À1.95 V vs. Ag/Ag + , and CH 4 began to generate at À1.95 V and became dominate product soon (Fig. 5b), suggesting that CH 4 was derived from CO. CO 2 was absorbed on the surface of Zn-MOFs and was reduced to CO. Most of the CO molecules generated tended to be further reduced to CH 4 because the interaction between Zn-MOF and CO is stronger than that between Zn-MOF and CH 4 .
On the basis of the results of this work and the knowledge in the literature, the possible pathway for electrochemical reduction of CO 2 to CH 4 using Zn-MOF/CP cathodes in imidazolium based ILs can be discussed briey, which is shown schematically in Fig. 9. In the electrolysis, some imidazolium cations  were adsorbed on the Zn-MOF surface aer the Zn-MOF/CP cathode immersed in the IL electrolyte. Thus, CO 2 molecules were captured by the IL adsorbed on the Zn-MOF surface. One electron was transferred to a CO 2 molecule and forms CO 2 c À intermediate, and then the CO 2 c À intermediate took another electron and yields a CO molecule. In this step, the conductive Zn-MOF transferred electron to CO 2 , and imidazolium base ILs helped driving the transformation of CO 2 to CO 2 c À intermediate. CO could be desorbed from the surface of Zn-MOF or be further reduced by six electrons to generate CH 4 . 4d Due to the larger adsorption capacity of CO than CH 4 on the Zn-MOF as discussed above, the CO molecule preferred to be adsorbed on the Zn-MOF surface to be further reduced to CH 4 . More detailed mechanism is very interesting, but is challenging.
In summary, the Zn-MOF/CP cathodes and IL electrolytes have been combined for electrochemical reduction of CO 2 to CH 4 . The morphology of the Zn-MOFs has signicant effect on the electrochemical reaction. The sheet-like Zn-MOF has the highest activity in the CO 2 reduction due to its largest electroactive surface areas, and the imidazolium based ILs with uorine are more effective electrolytes because uorine has stronger interaction with CO 2 . The Zn-MOF based cathodes and the ILs are excellent combination for the efficient and selective reduction of CO 2 to CH 4 . The selectivity of CH 4 can be higher than 80% at a current density of higher than 3 mA cm À2 with an overpotential of 0.25 V. We believe that integration of MOFbased electrodes and ILs provides many opportunities for exploring efficient electrochemical reactions.

Zn-MOFs synthesis
The procedures to synthesize the Zn-MOFs were similar to that reported previously. 26 In a typical experiment, 15 g C 12 mimCl, 5 g glycerol, desired amount of ZnCl 2 and 0.8 g H 3 BTC were added into a two-necked round-bottomed ask, and the mixture was stirred vigorously at 80 C for 72 h. Aer the reaction, the obtained mixture containing the materials was mixed with 50 mL ethanol and then centrifuged with a speed of 5000 rpm. The obtained Zn-MOFs were washed with ethanol for 10 times and dried in a vacuum oven at 60 C for 24 h.

SAXS study
SAXS experiments were carried out at Beamline 1W2A at the Beijing Synchrotron Radiation Facility. The apparatus and the procedures were similar to that used in previous work. 15c The data were collected using a CCD detector (MAR) with maximum resolution of 3450 Â 3450 pixels. The wavelength of the X-ray was 1.54Å, and the distance of the sample to detector was 1.31 m. In a typical experiment, the sample was added into the sample cell, and the X-ray scattering data were recorded. The 2-D SAXS images were obtained from the detector and then transformed into the proles of intensity (I) vs. wavevector (q) by the soware FiT2D. The pair-distance distribution function p(r) was obtained from SAXS data by using Gnom application soware. 26 Materials characterization XRD analysis of the samples was performed on the X-ray diffractometer (Model D/MAX2500, Rigaka) with Cu-Ka radiation, and the scan speed was 2 min À1 . The surface morphologies of the products were characterized by a HITACHI S-4800 SEM. The surface components were characterized by XPS performed on the Thermo Scientic ESCALab 250Xi using 200 W monochromated Al Ka radiation, in which the 500 mm X-ray spot was used. The adsorption isotherms of CO 2 , CO and CH 4 of the degassed Zn-MOFs were determined at 298 K in the pressure range of 0-1 atm on a TriStar II 3020 device.

Fabrication of Zn-MOF/CP cathodes and characterization
The EPD procedure was performed on DC power supply LW6020KD (Longwei Instrument (HK) Co. Ltd). Prior to experiments, the carbon paper (CP) substrate was sonicated in acetone for 10 min, followed by washing with water and ethanol, and nally dried in N 2 atmosphere. The procedures were similar to that reported. 19 For each series of EPD experiments, 1 mg Zn-MOF powder was dispersed in 10 mL methanol/ DMF solution (the concentration of DMF was 20 wt%) prior to 10 min sonication. Then two CP substrates of the same size (0.5 cm Â 0.5 cm) were used as anode and cathode, respectively. The Zn-MOF was deposited onto anode by applying a suitable constant voltage (10-50 V) of 1 hour, and the distance between electrodes was 2 cm. The as-deposited Zn-MOF lms were washed with ethanol for several times and dried in vacuum oven at 80 C for 24 h. The morphologies of the as-synthesized Zn-MOF/CP cathodes were characterized by SEM.

Cyclic voltammetry (CV) study
An electrochemical workstation (CHI 6081E, Shanghai CH Instruments Co., China) was used for all CO 2 reduction experiments. CV measurements were carried out in a single compartment cell with three-electrode conguration, which consisted of working electrode (e.g. Zn-MOF/CP), a platinum gauze auxiliary electrode, and Ag/Ag + (0.01 M AgNO 3 in 0.1 M TBAP-MeCN) reference electrode. The reference electrode was stabilized in a glass tube with a Luggin capillary, which was lling with corresponding catholyte. The reference electrode calibration was carried out using the method reported in the literature. 16 The potential difference of the Ag/Ag + electrode in BmimBF 4 and standard hydrogen electrode (SHE) is 636 mV at 25 C. The detailed results and the discussion are given in the ESI (Fig. S13 †). Before each set of experiment, the electrolyte was bubbled with CO 2 (or N 2 ) for 30 min until CO 2 -saturated solution (or N 2 -saturated solution) was formed, which was conrmed by the fact that the CV trace was not changed with gas bubbling time. CV measurements in gas-saturated electrolyte were taken between À1.1 V and À2.5 V vs. Ag/Ag + at a sweep rate of 20 mV s À1 . For better mixing, slight magnetic stirring was applied in the process. Prior to experiments, all the metal electrodes (0.5 cm Â 0.5 cm) were polished with ne sand paper and then were sonicated in acetone for 10 min, followed by washing with water and ethanol, and nally dried in N 2 atmosphere.

Electrochemical surface area measurement
The active surface areas in the IL may be different with that determined in the aqueous solution. However, the acceptable characterization method in ILs has not been reported. Therefore, in this work, we made the characterization using the wellaccepted method as follows. 11 The electrochemical surface areas of the

EIS measurements
The measurement was performed using the Zn-MOF/CP electrodes as the reference. 28 The experimental apparatus were the same as for CV measurements. The impedance spectra was recorded in IL BmimBF 4 at an open circuit potential (OCP) with an amplitude of 5 mV of 10 À2 to 10 5 Hz. The formal potential of the system was also set at À2.0 V similar to the CO 2 reduction potential at the same experimental conditions. The data obtained from the EIS measurements were tted using the Zview soware (Version 3.1, Scribner Associates, USA).

CO 2 reduction electrolysis and product analysis
Electrolysis was performed under room temperature (25 C) in a commonly used H-type cell with an Ag/Ag + (0.01 M AgNO 3 in 0.1 M TBAP-MeCN) reference electrode, which was similar to that used by other researchers. 3, 16 The apparatus was shown schematically in Fig. S9. † The cathode and anode compartments were separated by a proton exchange membrane (Naon 117). ILs or organic solvents and H 2 SO 4 aqueous solution (0.5 M) were used as cathodic and anodic electrolytes, respectively. The proton source was from the electrolysis of water at the anode. Before electrolysis, CO 2 was bubbled through the catholyte (2 mL per min) for 30 min with stirring. Potentiostatic electrochemical reduction of CO 2 was carried out with CO 2 bubbling (2 mL min À1 ), and the gaseous product was collected in a gas bag. Aer a desired electrolysis time, the gaseous product in the gas bag was collected and analyzed by gas chromatography (GC, HP 4890D), which was equipped with TCD and FID detectors using helium as the internal standard, and the liquid mixture was analyzed by 1 H-NMR method, which recorded on a Bruker Avance III 400 HD spectrometer in DMSOd 6 with TMS as an internal standard. The amount of CH 4 and faradaic efficiency of the products were calculated on the basis of GC analysis. 3b, 29 Multiple electrolysis experiments were run at each potential and the average current density was calculated to give the data. The products of CO 2 reduction vs. hydrogen reduction were measured at each potential. The variation in partial current density vs. applied overpotential was obtained via stepped potential electrolysis. Partial current densities for CH 4 production were calculated from the GC spectra every 15 minutes and averaged over 1-2 hours. The Tafel plots were constructed from these data.