A Review of the Use of Immobilized Ionic Liquids in the Electrochemical Conversion of CO 2

: This paper is a review on the application of imidazolium-based ionic liquids tethered to polymer backbones in the electrochemical conversion of CO 2 to carbon monoxide and formic acid. These tethered ionic liquids have been incorporated into novel anion ion exchange membranes for CO 2 electrolysis, as well as for ionomers that have been incorporated into the cathode catalyst layer, providing a co-catalyst for the reduction reaction. In using these tethered ionic liquids in the cathode catalyst composition, the cell operating current increased by a factor of two or more. The Faradaic e ﬃ ciencies also increased by 20–30%. This paper provides a review of the literature, in addition to providing some new experimental results from Dioxide Materials, in the electrochemical conversion of CO 2 to CO and formic acid.


Introduction
This paper is a review of the application of imidazolium-based ionic liquids tethered to polymer backbones in the electrochemical conversion of CO 2 to carbon monoxide and formic acid. The work started in 2011, when Rosen et al. [1] found that the addition of 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM + BF 4 − ) or 1-Ethyl-3-methylimidazolium chloride (EMIM + Cl − ) reduced the overpotential for CO 2 electrolysis by almost a volt. The ionic liquids also helped increase the selectivity of CO 2 to CO to above 95% [1][2][3][4][5]. Further investigation into whether the imidazolium cation was a promoter or co-catalyst in the heterogeneous/homogeneous electrocatalysis of CO 2 reduction was conducted by Zhao et al. [6]. They found that imidazolium, as well as pyrrolidium, did enhance the reduction reaction with Ag as a co-electrocatalyst, especially in the presence of water. Many investigators have been building on these findings, with the results being difficult to translate into practical devices. The presence of the ionic liquid in the solution inhibited CO 2 mass transfer into the cathode catalyst layer. Furthermore, the electrochemical cells lost most of their activity in less than about 500 h, due to the degradation of these ionic liquids. To help resolve these stability issues, researchers began investigating and synthesizing various substituted ionic liquids and determining their stability, especially in alkaline solutions. They have also been experimentally incorporating these ionic liquids, using various polymerization methods, in the search for producing alkaline stable anion exchange membranes. Some of this work is summarized below.
Hugar et al. synthesized a series of imidazolium organic cations, evaluating the impact of substitution on the imidazolium ring and its chemical stability in 1, 2, and 5 M KOH at 80 • C in 30-day tests. They found that C4-and C5-substitutions in the imidazole ring with methyl groups were C 2020, 6, 33 2 of 12 important to achieving alkaline stability. The methyl group(s) improved the ionic liquid ring stability and were better than similar substitutions using phenol groups [7].
Si et al. researched the alkaline stability of imidazolium cations having butyl groups at various substitution positions (N1-, C2-, and N3-), finding that the C2-substituted cations had a better alkaline stability in 1 M KOH in 240-h tests [9]. In addition, they also prepared cast anion exchange membranes by photo-crosslinking a mix of acrylonitrile/styrene, divinylbenzene, and photo-initiator using selected imidazolium cations. The C2-substituted [C2-BBMIM] + imidazolium cation incorporated into the anion ion exchange membranes that they prepared showed stable conductivities after 240 h in 1 M KOH at 80 • C [9].
Qiu et al. investigated the cross-linking synthesis of styrene and acrylonitrile with various alkaline imidazolium-type ionic liquids in the search for making alkaline stable anion ion exchange membranes. A variety of photo-crosslinked membranes were made, using 1-methyl-3-(4-vinylbenzyl) imidazolium chloride as the ionic liquid. The membrane conductivity stability versus time in 1 and 10 M KOH solutions were evaluated, showing stable conductivity for up to 1000 h in 1 M KOH at 60 • C [10]. The same group also did a further study, converting 1-butyl-2-methylimidazole and 1-allyl-2-methylimidazolebromide to bis-imidazoliums using bromine-based compounds and photo-crosslinking them. They produced several anion membranes with stable conductivity in 1 M KOH at 60 • C for 30 days [11].
Guo et al. prepared a novel set of anion exchange membranes based on the copolymers of 1-allyl-3-methylimidazolium chloride (AmimCl) ionic liquid with either methyl methacrylate (MMA) or butyl methacrylate (BMA), which were prepared via free radical polymerization. The anion membranes were found to be stable in 6 M NaOH for 120 h at 60 • C [12].
Marinkas et al. worked on synthesizing a series of anion conducting polymers (PPO-BIM) based on a poly-(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbone and methylated 2-mesitylbenzimidazol (BIM) functional groups. The membranes were operated in a water electrolysis cell, where the membranes were found to be stable in 0.5 M KOH at 50 • C, but not in 1 M KOH at 80 • C [13].
Tham et al. looked at the effect of substituents at the C2-and N3-positions of the imidazolium functional group of the poly(arylene ether) ketone-based anion electrolyte membranes that they prepared. The synthesized anion membranes were operated in alkaline water electrolysis zero gap cells in order to evaluate membrane and cell performance. They were able to operate the water electrolysis cell at 1.9 V at 500 mA/cm 2 . The simultaneous use of C2-methyl and N3-butyl substituents in the PAEK-APMBI membrane resulted in good chemical stability in 10 wt% NaOH at 60 • C, which retained 97% of its ion exchange capacity after 28 days [14].
Recognizing these difficulties in ionic liquid alkaline stability as well as to the need for alkaline stable anion membranes, Masel et al. [15][16][17][18][19][20] developed a new membrane system where the cation imidazoliums are tethered to a polymer backbone, as shown in Figure 1. These novel ionomers and membranes that were developed are now marketed under the Sustainion ® tradename. Other investigators, as previously discussed, have been working on different approaches in the preparation of anion membranes, using different polymer mixture components, polymer backbones, and various ionic liquids [9][10][11][12][13][14]. Unfortunately, there is no comparison data available for those and other membranes in a direct comparison with the Sustainion ® membranes in CO 2 reduction to CO.
The experimental results with the Sustainion ® -based system were a set of ionomers and membranes that were stable for at least 12,000 h at 60 • C in 1 M KOH, as shown in Figure 2. Presently, these membranes are being evaluated by multiple investigators [21][22][23][24][25][26][27][28][29]. This paper briefly reviews the results in the CO 2 conversion to CO as well as in the conversion to formic acid. Also included is a comparison set of data for the same electrochemical processes with membranes other than Sustainion ® , C 2020, 6, 33 3 of 12 with a particular focus on studies where the cells had been run for at least 100 h, as summarized by Nwabara et al. [30].
Sustainion ® , with a particular focus on studies where the cells had been run for at least 100 h, as summarized by Nwabara et al. [30]. Figure 1 shows the structure of the Sustainion ® polymers and ionomers that Dioxide Materials has made. It consists of an imidazolium cation tethered to a styrene backbone. In one of the original membrane formulations, called polystyrene vinylbenzyl methyl imidazolium chloride (PSMIM), methyl imidazolium was used as the functional group. Later, 1,2,4,5-Tetramethylimidazole was employed as the functional group. This formulation, named PSTMIM, was further improved, producing the commercial anion membrane form designated as Sustanion ® X-37. The Sustainion ® anion exchange membranes were found to be surprisingly stable under alkaline conditions. A Thermo Nicolet FTIR Spectrometer with a resolution of 4 cm −1 and scan range of 500-4000 cm −1 using an ATR mode was used to check the Sustainion ® membranes for any degradation. Figure 2 shows the FTIR spectrum of the membrane after 12180 h of operation in an alkaline exchange membrane (AEM) water electrolyzer running in 1 M KOH at 60 °C. It also shows the spectra of the two components that are used to synthesize the membrane: (i) a copolymer of styrene and vinyl benzyl chloride, and (ii) tetramethylimidazole (TMIM). The membrane structure is shown as an inset. Notice that the TMIM C-N ring modes at 1605, 1435, and 825 cm −1 and the TMIM CH3 stretches at 1211, 1165, and 975 cm −1 are still present in the membrane. The feature at 1403 cm −1 in the TMIM spectrum is missing from the spectrum of the membrane. This mode is associated with out of plane bending around the α-nitrogen in TMIM and had disappeared, since the α-nitrogen is now bound to the benzyl ligand. Other measurements showed that the membrane conductivity was unchanged during this run. Clearly, the membrane is stable under alkaline conditions.   The spectrum was taken after removing the membrane from the alkaline exchange membrane (AEM) water electrolyzer after operating at 60 °C in 1 M KOH for 12,180 h, and carefully cleaning the membrane for the FTIR tests.

Electrochemical Conversion of CO2 to CO
Sustainion ® membranes have been used by various investigators in the electrochemical conversion of CO2 to CO. Figure 3 shows a schematic of an electrolyzer for CO2 electrolysis to CO. The cell uses a titanium flow field on the anode and a 316 L stainless steel serpentine flow field on the cathode. The anode is made by depositing a mixture of IrO2 nanoparticles and a Nafion solution binder onto a Toray carbon paper, as described elsewhere [4,17,31]. The cathode was made by depositing a mixture of Ag nanoparticles and Sustainion ® XA-9 ionomer binder onto a Sigracet 39 BC GDL carbon paper, as described elsewhere, to make the gas diffusion electrode (GDE) [4,17,31]. The membrane evaluated is typically Sustainion ® X-37. The spectrum was taken after removing the membrane from the alkaline exchange membrane (AEM) water electrolyzer after operating at 60 • C in 1 M KOH for 12,180 h, and carefully cleaning the membrane for the FTIR tests. Figure 1 shows the structure of the Sustainion ® polymers and ionomers that Dioxide Materials has made. It consists of an imidazolium cation tethered to a styrene backbone. In one of the original C 2020, 6, 33 4 of 12 membrane formulations, called polystyrene vinylbenzyl methyl imidazolium chloride (PSMIM), methyl imidazolium was used as the functional group. Later, 1,2,4,5-Tetramethylimidazole was employed as the functional group. This formulation, named PSTMIM, was further improved, producing the commercial anion membrane form designated as Sustanion ® X-37.

Immobilizing Ionic Liquids to Create Functionalized Anion Conducting Membranes
The Sustainion ® anion exchange membranes were found to be surprisingly stable under alkaline conditions. A Thermo Nicolet FTIR Spectrometer with a resolution of 4 cm −1 and scan range of 500-4000 cm −1 using an ATR mode was used to check the Sustainion ® membranes for any degradation. Figure 2 shows the FTIR spectrum of the membrane after 12,180 h of operation in an alkaline exchange membrane (AEM) water electrolyzer running in 1 M KOH at 60 • C. It also shows the spectra of the two components that are used to synthesize the membrane: (i) a copolymer of styrene and vinyl benzyl chloride, and (ii) tetramethylimidazole (TMIM). The membrane structure is shown as an inset. Notice that the TMIM C-N ring modes at 1605, 1435, and 825 cm −1 and the TMIM CH 3 stretches at 1211, 1165, and 975 cm −1 are still present in the membrane. The feature at 1403 cm −1 in the TMIM spectrum is missing from the spectrum of the membrane. This mode is associated with out of plane bending around the α-nitrogen in TMIM and had disappeared, since the α-nitrogen is now bound to the benzyl ligand. Other measurements showed that the membrane conductivity was unchanged during this run. Clearly, the membrane is stable under alkaline conditions.

Electrochemical Conversion of CO 2 to CO
Sustainion ® membranes have been used by various investigators in the electrochemical conversion of CO 2 to CO. Figure 3 shows a schematic of an electrolyzer for CO 2 electrolysis to CO. The cell uses a titanium flow field on the anode and a 316 L stainless steel serpentine flow field on the cathode. The anode is made by depositing a mixture of IrO 2 nanoparticles and a Nafion solution binder onto a Toray carbon paper, as described elsewhere [4,17,31]. The cathode was made by depositing a mixture of Ag nanoparticles and Sustainion ® XA-9 ionomer binder onto a Sigracet 39 BC GDL carbon paper, as described elsewhere, to make the gas diffusion electrode (GDE) [4,17,31]. The membrane evaluated is typically Sustainion ® X-37.    Figure 4 shows some of the typical data. In this case, 1 L/min of humidified CO 2 was fed into a single cell electrolyzer with an active area of 250 cm 2 . The cell voltage was first fixed at 3.3 V, and the current reached 50 A (200 mA/cm 2 ) in less than 1 h. Then, the current was fixed at 50 A, and the cell voltage and CO selectivity monitored with time. Figure 4 shows the cell voltage and CO selectivity of the 250 cm 2 CO 2 electrolyzer operating at 200 mA/cm 2 and 35 • C. The results show that the cell voltage stabilized at 3.05 V for over 120 h, maintaining CO selectivity at 97-98%.   Table 1 summarizes the data on the electrochemical conversion of CO2 to CO and O2 using Sustainion ® membranes. Also included is a selection of examples from the literature where different membranes were used. Notice that the current is higher when the Sustainion ® anion membranes are used, only needing a lower voltage to maintain the same equivalent current density.  Table 1 summarizes the data on the electrochemical conversion of CO 2 to CO and O 2 using Sustainion ® membranes. Also included is a selection of examples from the literature where different membranes were used. Notice that the current is higher when the Sustainion ® anion membranes are used, only needing a lower voltage to maintain the same equivalent current density.

Electrochemical Conversion of CO 2 to Formic Acid
The highly conductive properties of the Sustainion ® anion ion exchange membrane allowed the development of a unique three-compartment cell, as shown in Figure 5, that can generate pure formic acid at high concentrations. The system requires only deionized water and CO 2 gas. The cell design uses a GDE cathode consisting of a nanoparticle tin oxide catalyst and carbon, using Sustainion ® XA-9 ionomer as a binder. The CO 2 is reduced to the formate ion (HCOO -) on the cathode, which then passes through the anion membrane into the center compartment [32,34,35].
The highly conductive properties of the Sustainion ® anion ion exchange membrane allowed the development of a unique three-compartment cell, as shown in Figure 5, that can generate pure formic acid at high concentrations. The system requires only deionized water and CO2 gas. The cell design uses a GDE cathode consisting of a nanoparticle tin oxide catalyst and carbon, using Sustainion ® XA-9 ionomer as a binder. The CO2 is reduced to the formate ion (HCOO -) on the cathode, which then passes through the anion membrane into the center compartment [32,[34][35].
The GDE anode consists of a nanoparticle IrO2 catalyst or an IrO2 thermal coating on sintered titanium fiber paper, in converting water into H+ ions and oxygen (O2). The H + ions pass through the adjoining cation ion exchange membrane into the center flow compartment. The H + and formate ions combine in the center compartment, directly forming a formic acid product, which is removed from the compartment using a deionized water input stream. Formic acid product concentrations of up to about 20 wt% have been produced. Figure 6 shows an example of the performance of a 5 cm 2 cell operating for 500 h, evaluating a new potential candidate cathode electrocatalyst at a current density of 200 mA/cm 2 , and producing a 10 wt% formic acid product. The operating cell voltage was constant at 3.5 V. Figure 5. The design of a three-compartment electrolyzer cell that converts CO2 and water into formic acid.  Figure 5. The design of a three-compartment electrolyzer cell that converts CO 2 and water into formic acid.
The GDE anode consists of a nanoparticle IrO 2 catalyst or an IrO 2 thermal coating on sintered titanium fiber paper, in converting water into H+ ions and oxygen (O 2 ). The H + ions pass through the adjoining cation ion exchange membrane into the center flow compartment. The H + and formate ions combine in the center compartment, directly forming a formic acid product, which is removed from the compartment using a deionized water input stream. Formic acid product concentrations of up to about 20 wt% have been produced. Figure 6 shows an example of the performance of a 5 cm 2 cell operating for 500 h, evaluating a new potential candidate cathode electrocatalyst at a current density of 200 mA/cm 2 , and producing a 10 wt% formic acid product. The operating cell voltage was constant at 3.5 V.

Summary
Laboratory test results showing that specific ionic liquids can reduce the overpotential in the electrochemical reduction of CO 2 led to the development of the Sustainion ® anion ion exchange membranes by researchers at Dioxide Materials. Specific ionic liquids, once immobilized as cations in a polymer structure, allowed for the development of anion ion exchange membranes with the properties required in developing new and efficient electrochemical processes in the conversion of CO 2 to chemicals and fuels. Two new electrochemical processes were developed using these Sustainion ® anion membranes. The first process was the electrochemical reduction of CO 2 to CO. The second process was the direct reduction of CO 2 that could directly produce a concentrated pure formic acid product. Researchers have been reporting some excellent electrolyzer performance results using these new Sustainion ® anion membranes, with some of this data tabulated in this paper.