Quantitative Electro-Reduction of CO2 to Liquid Fuel over Electro-Synthesized Metal–Organic Frameworks

Efficient electro-reduction of CO2 over metal–organic framework (MOF) materials is hindered by the poor contact between thermally synthesized MOF particles and the electrode surface, which leads to low Faradaic efficiency for a given product and poor electrochemical stability of the catalyst. We report a MOF-based electrode prepared via electro-synthesis of MFM-300(In) on an indium foil, and its activity for the electrochemical reduction of CO2 is assessed. The resultant MFM-300(In)-e/In electrode shows a 1 order of magnitude improvement in conductivity compared with that for MFM-300(In)/carbon-paper electrodes. MFM-300(In)-e/In exhibits a current density of 46.1 mA cm–2 at an applied potential of −2.15 V vs Ag/Ag+ for the electro-reduction of CO2 in organic electrolyte, achieving an exceptional Faradaic efficiency of 99.1% for the formation of formic acid. The facile preparation of the MFM-300(In)-e/In electrode, coupled with its excellent electrochemical stability, provides a new pathway to develop efficient electro-catalysts for CO2 reduction.


■ INTRODUCTION
Efficient conversion of CO 2 into chemical feedstocks and fuels is a highly desirable but extremely challenging target. 1−3 Reduction of CO 2 into useful chemicals via thermo-catalysis and photocatalysis has been studied very widely. 4−7 However, the former often requires both high temperature and pressure to activate CO 2 , while the latter relies heavily upon the use of sacrificial agents, typically organic amines, thus limiting longterm applications. Electro-catalysis enables the storage of intermittent renewable energy into chemical energy, 8−10 and there are powerful drivers for the development of active, selective and stable electro-catalysts for the efficient reduction of anthropogenic CO 2 emission via conversion to valuable chemicals. 11−14 Electrocatalytic reduction of CO 2 normally relies upon the use of active transition metals (e.g., Cu, Co, and Pd) to reduce the high overpotentials required to activate CO 2 to the CO 2 •− radical anion or other intermediates that can be converted further. 15−17 Metal−organic frameworks (MOFs) are crystalline hybrid materials constructed from metal ions or clusters bridged by polydentate organic ligands. 18−20 Compared with conventional electro-catalysts, MOFs have unique features for the electrochemical reduction of CO 2 . 21,22 Metal sites (e.g., Co, Cu, Fe, and Ni) that show activity for CO 2 electro-reduction 23−26 can be readily incorporated into MOF structures via single-site dispersion. Additionally, the intrinsic and tunable porosity of MOFs can lead to high capacity adsorption and selectivity to CO 2 , 27,28 thus promoting activation and further conversion. Gaseous products obtained from electro-reduction of CO 2 (e.g., CO and CH 4 ) often show lower adsorption in MOFs than CO 2 and can thus be readily recovered and the MOF electrode regenerated. 21 MOFs have inherent design flexibility via choice of metal ions and organic ligands, and therefore, their function and activity for electro-reduction of CO 2 can be optimized. 29 To date, universal approaches to fabricate efficient MOFbased electrodes showing high charge-transfer capacity have been rarely reported. 30−33 Normal methods are based upon the doping of MOF materials onto an electrode substrate. 34 Although various electrodes loaded with MOFs have been studied for the electro-reduction of CO 2 , the poor contact between the catalyst and substrate surface limits their performance. 21 Indium-based materials have demonstrated excellent performance for the electro-reduction of CO 2 . 35,36 Although electrodeposition of MOFs, particularly Cu-and Znbased systems, has been described, 37−40 the electro-synthesis of In-based MOFs has not been reported previously. We therefore sought to develop an In electrode decorated with a MOF film that might incorporate active defect sites and show high charge transfer capacity.
Here, we report a facile route to the fabrication of such decorated electrodes via electro-synthesis of MOFs onto a metal foil substrate. The MFM-300(In)-e/In electrode ("e" is short for electro-synthesis) prepared in this way shows a 1 order of magnitude enhancement in electrical conductivity compared with electrodes prepared using MOFs prepared by thermo-chemical methods. More importantly, exceptional activities for the electro-reduction of CO 2 are observed with a current density of 46.1 mA cm −2 under an applied potential of −2.15 V vs Ag/Ag + . A Faradaic efficiency of 99.1% for formic acid after 2 h of electrolysis is observed. We also found that the electro-synthesized MOF incorporates structural defects in the form of additional framework In 3+ sites, which, coupled with improved charge transfer capacity, greatly promote the activation of CO 2 to the radicals, consistent with the observed excellent electrocatalytic activity and stability of the catalyst. Density functional theory (DFT) calculations are applied to reveal the mechanism of catalysis.
■ RESULTS AND DISCUSSION Materials Preparation and Characterizations. Samples of MFM-300(In)-t ("t" is short for thermo-synthesis) were obtained by solvothermal reaction of In(NO 3 ) 3 ·5H 2 O and biphenyl-3,3′,5,5′-tetracarboxylic acid (H 4 L; Figure S1) in a mixture of DMF and MeCN at 85°C for 3 days. 41 The crystal structure of MFM-300(In) is shown in Figure S2. The electrosynthesis of MFM-300(In)-e was achieved using indium foil (In-foil) as both the cathode and anode in a solvent mixture of DMF/dioxane/water containing H 4 L with 1-ethyl-3-methylimidazolium acetate (EmimOAc, structure shown in Figure  S3) as the supporting electrolyte ( Figure 1a). The formation of MOF occurred rapidly at an applied potential of 10 V and 60°C , and the In-foil anode was covered by particles of MFM-300(In)-e within 200 s (Figure 1b). The electro-synthesis was then terminated as the surface of In-foil became fully covered with MOF particles thus limiting the supply of further In 3+ ions from the anode for MOF synthesis. No MOF particle was detected in the electrolyte, indicating that particles of MFM-300(In)-e have a strong interaction with the In-foil substrate. The MFM-300(In)-e/In electrode was then immersed in acetone for 2 h and dried before being used as the MFM-300(In)-e/In electrode. Scanning electron microscopy confirms that the In-foil metal surface is coated uniformly with MFM-300(In)-e, which has an octahedral morphology with a narrow particle size distribution centered at ∼500 nm (Figures 1c−e). By comparison, MFM-300(In)-t shows cube-shaped crystals with an average particle size of ∼2.5 μm ( Figure S4). These differences in morphology result only in small differences in the relative intensity of powder X-ray diffraction (PXRD) peaks for MFM-300(In)-t and MFM-300(In)-e ( Figure 2a).
Powder crystalline samples of MFM-300(In)-e were carefully scraped off the In-foil for characterisations. The crystal structure and phase purity of MFM-300(In)-e were confirmed by PXRD ( Figure 2a); the increased peak width of MFM-300(In)-e compared with MFM-300(In)-t is consistent with their particle size distributions. Full chemical analysis suggests that the indium contents are 35.3 and 32.5 wt % in MFM-300(In)-e and MFM-300(In)-t, respectively. Negligible N content was detected in MFM-300(In)-e, indicating that the ionic liquid cations were removed completely during the solvent exchange process. The higher content of In 3+ in MFM-300(In)-e suggests that acetate anions OAc − introduced in the synthesis with EmimOAc are bound to the framework to balance the charge. The slightly higher residual solid observed in the thermogravimetric analysis (TGA) is consistent with the presence of additional In 3+ in the framework of MFM-300(In)e ( Figure S5), while the slightly lower stability of MFM-300(In)-e compared with MFM-300(In)-t originates most likely from defects in the framework structure. The band observed near 1550 cm −1 by Fourier-transform infrared (FTIR) spectroscopy is assigned to the vibration of the carboxylate group, but has an apparent red shift (20 cm Figure 2d) confirm a micropore profile centered at 6.5 Å. In addition, there are two small peaks centered at 9.5 and 11.5 Å in MFM-300(In)-e owing to the presence of defects in the framework. During the electro-synthesis, we propose that the In-foil is oxidized to In 3+ which binds OAc − anions from the EmimOAc electrolyte at the surface of the electrode thus preventing full dissolution of In 3+ . Binding of the tetracarboxylate ligand to In 3+ occurs at the surface either directly or via replacement of OAc − to assemble the framework structure uniformly across the In-foil surface. 42 Since there are abundant In 3+ ions at and around the anode, the as-synthesized MFM-300(In)-e contains excess In 3+ sites with some associated OAc − . Both MFM-300(In)-t and MFM-300(In)-e show typical features for In 3+ at the surface as characterized by X-ray photoelectron spectroscopy (XPS) ( Figure S6 and Table  S1). Electrochemical Reduction of CO 2 . Carbon paper (CP) has a very coarse surface that can support materials to fabricate electrodes. Samples of MFM-300(In)-t and MFM-300(In)-e were loaded onto CP substrates using Nafion D-521 as a binder to f abricate MFM-300(In)-t /CP a nd MFM-300(In)-e/CP electrodes, respectively. Indium foil has a very smooth surface, and we were thus unable to fabricate MFM-300(In)-t/In since loading thermally prepared MFM-300(In)-t onto this metal surface was unsuccessful. The surface structures of MFM-300(In)-e/In, MFM-300(In)t/CP and MFM-300(In)-e/CP were characterized by XPS ( Figure S7 and Table S1). These spectra show typical features of In 3+ cations, and the shifts in binding energy [+0.5 and −0.5 eV for MFM-300(In)-t and MFM-300(In)-e, respectively] compared to powder samples originate from the different conductivities of the surface substrates.
All three electrodes were investigated for electrochemical reduction of CO 2 in an H-type cell ( Figure S8) with 0.5 M EmimBF 4 (1-ethyl-3-methylimidazolium tetrafluoroborate) in MeCN and 0.5 M H 2 SO 4 as catholyte and anolyte, respectively. Cyclic voltammetry (CV) of MFM-300(In)-e/In was studied as a function of gas loading (Figure 3a). Large current densities were generated in the CO 2 -saturated electrolyte, while negligible densities were observed for the electrolyte saturated with N 2 . Controlled potential electrolysis at −2.0∼−2.3 V vs Ag/Ag + was conducted at room temperature under a flow of CO 2 into the electrolyte. All liquid and gas-phase products were quantified by 1 H NMR spectroscopy and gas chromatography (GC), respectively. Interestingly, only H 2 was observed in the gas phase and formic acid was identified as the sole product in the liquid phase. The electrolysis was performed for 2 h and results are summarized in Figure 3b, 3c. The current density increases with increasing applied potential in all cases and the Faradaic efficiency for formic acid (FE HCOOH ) reaches 99.1% with a current density of 46.1 mA cm −2 at −2.15 V vs Ag/Ag + for MFM-300(In)-e/In. Significantly, MFM-300(In)-e/In shows a higher current density compared with MFM-300(In)-e/CP and MFM-300(In)-t/CP at −2.15 V vs Ag/Ag + ; the latter gives the lowest FE HCOOH at all potentials. The Faradaic efficiency for hydrogen (FE H2 ) has an inverse trend over these electrodes ( Figure S9) with MFM-300(In)-t/CP higher than MFM-300(In)-e/CP, which is higher than MFM-300(In)-e/In. Infoil, as a smooth metallic electrode (Figure S10), exhibits a current density of 19.7 mA cm −2 and FE HCOOH of 50.7% in 0.5 M EmimBF 4 /MeCN at −2.15 V vs Ag/Ag + , lower than that  Table 1). The dependence of current density and FE HCOOH on time was studied further (Figure 3d). N 2 was first charged into the electrolyte, and an imperceptible current density was observed for all electrodes investigated. Significant increases in current density were observed over all electrodes on charging CO 2 into the electrolyte, with MFM-300(In)-e/In showing the strongest response to CO 2 and reaching a maximum current density most rapidly. No obvious inflection point was observed for the Journal of the American Chemical Society pubs.acs.org/JACS Article MFM-300(In)-t/CP electrode, indicating that H 2 evolution occurred initially, accompanied by CO 2 reduction throughout. The electro-reduction of CO 2 was conducted for 6 h at −2.15 V vs Ag/Ag + to assess the long-term electrochemical stability of the electrodes (Figure 3e, 3f). After 6 h of electrolysis, the current density of MFM-300(In)-e/In had increased slowly to 57.6 mA cm −2 with a slightly decrease in FE HCCOH to 91.2%. The average rate of production of formic acid is estimated to be 46 mg cm −2 h −1 for MFM-300(In)-e/In under these conditions ( Figure S11). In contrast, the current density increased rapidly for MFM-300(In)-t/CP from 24.1 mA cm −2 (1 h) to 52.3 mA cm −2 (6 h) and for MFM-300(In)-e/CP from 24.1 mA cm −2 (1 h) to 49.0 mA cm −2 (6 h) but with notable decreases in FE HCCOH to 52.9% and 66.8%, respectively. No carbon-containing product was detected in the gas phase product. The increased current density is assigned to H 2 evolution over the extended period of electrolysis, but MFM-300(In)-e/In retains a high selectivity toward the activation of CO 2 vs H 2 evolution ( Figure S12). A comparison of the electrolytic performance for the formation of formic acid for MFM-300(In)-based electrodes and other state-of-the-art electrodes in organic electrolytes is given in Table 1, with MFM-300(In)-e/In showing the best performance. 43−47 Reusability of Electrodes. All three electrodes were reused over five cycles for electro-reduction of CO 2 ( Figure  3g,h). Both the current density and Faradaic efficiency for formation of formic acid show excellent stability for all three electrodes. During the electrolysis over MFM-300(In)-e/In, the surface of the electrode is rendered flat as confirmed by SEM (Figures 1d and S13). PXRD studies confirm retention of the structure of MFM-300(In)-e in cycled samples after electrolysis ( Figure S14). XPS was used to study the surface properties of used MFM-300(In)-t/CP and MFM-300(In)-e/In electrodes ( Figure S15). During the electrolysis, both MFM-300(In)-t/CP and MFM-300(In)-e/In electrodes evolved and the change of surface structure was accompanied by shifts of the In 3d peak (−0.5 eV and +0.5 eV, respectively), moving to the energy of the as-prepared MOF sample (Table  S1). Before the electrolysis, MOF particles are relatively isolated (Figure 1d), and intercrystallite charge-transfer is thus restricted, resulting in the difference observed in XPS spectra. As the MOF surface evolves during the electrolysis, the surface becomes flatter and increasingly uniform ( Figure S13), and the energy difference is therefore minimized. The absence of In 0 at the electrode surface suggests that In 3+ sites in MFM-300(In) are not reduced during electrolysis, 48 consistent with the excellent electrochemical stability of MFM-300(In)-e/In.
Mechanistic Studies. Density functional theory (DFT) calculations afford the Gibbs free energy for the electroreduction of CO 2 to formic acid over the pristine and defective MFM-300(In), representing MFM-300(In)-t and MFM-300(In)-e, respectively (Figure 4a). The formation of *COOH over pristine MFM-300(In) (* indicates an adsorption site on the MOF) involves a high energy barrier, indicating that this process is the rate-determining step. In contrast, the formation of *COOH is spontaneous over defective MFM-300(In)-e. The In 3+ defects promote the adsorption of intermediate species, and therefore, higher Gibbs free energies are required for the desorption of *COOH and *HCOOH. The calculated reaction pathway of the electroreduction of CO 2 over defective MFM-300(In) is shown in Figure S16. Furthermore, the Gibbs free energy for the formation of *COOH over In 2 O 3 , a benchmark indium-based catalyst, is very high (Figure S17), indicating that defective MFM-300(In) is a better candidate for the electro-reduction of CO 2 . The adsorbed *COOH intermediate could be further reduced to *HCOOH or *CO. 10 The calculation of Gibbs free energy of electro-reduction of CO 2 to CO over defective MFM-300(In) has also been conducted, and we find that the *HCOOH intermediate is more readily formed than *CO due to the lower energy barrier for the former. This rationalizes the formation of formic acid as the main product over MFM-300(In)-based electrodes.
We have used 5,5-dimethyl-1-pyrroline N-oxide (DMPO) 49 as a spin trap to identify any radical species involved in the catalytic reaction. Figure 4b shows the EPR spectra of the reaction solution measured after electrolysis at −2.15 V vs Ag/Ag + using the different decorated electrodes. The EPR spectrum after electrolysis using MFM-300(In)-e/In shows three sets of radicals ( Figure S19): two weak signals are assigned to oxidized DMPO radical ·DMPO-OX, a N = 1.5 mT, and to ·DMPO−OH radicals, a N = 1.43 mT, a H = 1.33 mT. 50−52 Based upon established pathways for CO 2 reduction, ·DMPO−COOH radicals are the intermediate products ( Figure S16). 53,54 In the above electrolysis we observe the formation of ·COOH with a H = 2.21 mT. 52 The EPR spectra for the electrolysis reactions using MFM-300(In)t/CP and MFM-300(In)-e/CP electrodes and their simulations are shown in Figures S20 and S21. The concentration   Journal of the American Chemical Society pubs.acs.org/JACS Article indicating that the initial electron-transfer to generate surfaceadsorbed CO 2 •− species is the rate-determining step. 12 Electrolysis using the MFM-300(In)-e/In electrode gives the lowest slope for the Tafel plot, indicating a higher absolute current density for a given overpotential compared with the other two electrodes. The MFM-300(In)-e/In electrode also shows more rapid kinetics for generation of radicals, consistent with the higher current density and higher FE HCOOH for the electrolysis.
Electrochemical impedance spectroscopy (EIS) was conducted to investigate the properties of the electrode/electrolyte interface (Figures 4e, 4f). 43 Values for the interfacial chargetransfer resistance (R ct ) were obtained by fitting the experimental impedance data using Randles' equivalent circuit ( Figure S22). The interface between MOF particles and the substrate is a significant barrier to charge transfer. The electrosynthesized MOF on In-foil gives a better interfacial contact, resulting in a lower R ct for MFM-300(In)-e/In (9.5 Ω cm 2 ) compared with MFM-300(In)-e/CP (74.1 Ω cm 2 ). The lower R ct for MFM-300(In)-e/CP (74.1 Ω cm 2 ) compared to MFM-300(In)-t/CP (178.2 Ω cm 2 ) is consistent with the presence of In 3+ defect sites, which enhances the charge transfer of the former electrode. The higher double-layer capacitance value of MFM-300(In)-e/In (1.06 mF cm −2 ) compared with MFM-300(In)-t/CP (0.31 mF cm −2 ) and MFM-300(In)-e/CP (0.46 mF cm −2 ), as measured by CV curves at −0.6 to −0.65 V vs Ag/Ag + at different scan rate ( Figure S23), indicates that MFM-300(In)-e/In has higher electrochemical surface area. 19 CO 2 adsorption isotherms of desolvated MFM-300(In)-t and MFM-300(In)-e show uptakes of 87.6 and 78.3 cm 3 g −1 , respectively, at 1 bar and 298 K (Figure 4g). The isosteric heat (Q st ) and entropies (ΔS) of adsorption ( Figure S24) were determined by fitting of the Van't Hoff isochore from the adsorption isotherms of CO 2 at different temperatures ( Figure  S25). The value of Q st of MFM-300(In)-e is higher than that of MFM-300(In)-t suggesting a stronger interaction of CO 2 with MFM-300(In)-e.
Synchrotron FTIR microspectroscopy of MFM-300(In)-e was recorded as a function of CO 2 loading at 298 K ( Figure  4h). The change in the stretching mode ν(μ 2 −OH) at 3600 cm −1 in the presence of CO 2 can provide direct insights into the strength of the host−guest binding. MFM-300(In)-t exhibits a red-shift of 5 cm −1 of the ν(μ 2 −OH) stretching band on binding of CO 2 . 55 In contrast, a red-shift of 25 cm −1 is observed for this stretching vibration on CO 2 -loading into MFM-300(In)-e under the same conditions, suggesting that MFM-300(In)-e interacts more strongly with CO 2 . This is consistent with the DFT calculation and Q st analysis.

■ CONCLUSIONS
We have described a simple and effective strategy to prepare MOF-based electrodes via rapid electro-synthesis template by an ionic liquid. The as-prepared MFM-300(In)-e/In electrode incorporates active defect In 3+ sites and shows high capacity for charge transfer and high electrochemical stability. This decorated electrode catalyzes the electro-reduction of CO 2 to formic acid with a current density of 46.1 mA cm −2 and a Faradaic efficiency, FE HCOOH of 99.1% in organic electrolyte. To the best of our knowledge, this performance exceeds all other MOF systems. Overall, the excess In 3+ sites in MFM-300(In)-e combined with a strong contact between the MOF and the indium foil leads to reduction in interfacial resistance resulting in exceptional activity, selectivity and stability for the electro-reduction of CO 2 .