Aqueous electrocatalytic CO2 reduction using metal complexes dispersed in polymer ion gels

We use fac-[Re(bpy)(CO)3Cl] ([Re-Cl]) dispersed in polymer ion gel (PIG) ([Re]-PIG) to carry out electrocatalytic CO2 reduction in water. Electrolysis at -0.68 V vs. RHE in a CO2-saturated KOH and K2CO3 solution produces CO with over 90% Faradaic efficiency. The PIG electrode is readily combined with water oxidation catalysts to generate a full electrochemical cell. Additionally, we provide evidence that the PIG electrode can be implemented with other molecular catalysts.

Development of photoelectrochemical catalysts for reduction of CO 2 is an increasingly important area of research due to the shortage of fossil fuels and global warming. Developing a photosynthetic system for solar fuel generation which produces organic products from CO 2 , H 2 O, and sunlight is one of the most promising technologies to help overcome these issues.
Polymer ion gels (PIG) are composed of an ionic liquid and a polymer and have unique properties compared to hydrogels. Previous studies have used PIGs as CO 2 gas separation films, 23,24 as actuators, 25 as electrolytes for lithium-ion batteries, 26 and as high mechanical strength gels. 27 Recently, PIGs were embedded with [Re-Cl] and used for electrocatalytic CO 2 reduction under CO 2 atmosphere in the absence of additional solvent. 28 However, due to the lack of a proton source, the current density of the PIG/[Re-Cl] composite was smaller than that observed in organic solvent. CO 2 reduction activity is drastically affected by both the solvent and the supporting electrolyte. 4,29 Although [Re-Cl] can act as a photocatalyst for CO 2 reduction in DMF/TEOA solution, hydrogen is the main product from [Re-Cl] in a THF/TEOA solution. 4 Recently, it was reported that electrocatalytic CO 2 reduction activity can be improved by combining molecular catalysts with carbon-based supports. [30][31][32] For instance, [Re{4,4'-di(1H-pyrrolyl-3-propyl carbonate)-2,2'bipyridine}(CO) 3 (Cl)] cannot act as a CO 2 reduction photoor electrocatalyst in organic solvent, because the reduction potential of the catalyst is too small. However, this Re complex can act as an electrocatalyst for CO 2 reduction in water in the presence of a carbon support. 32 Previous reports have also shown how ionic liquids can improve electrocatalytic CO 2 reduction activity of metal complexes, making PIGs good candidates as solid supports for CO 2 reduction. 13,14,33 This paper describes the electrocatalytic CO 2 reduction of [Re-Cl] using a PIG support in aqueous solution. The PIG is composed of acrylamide polymer with silicone rubber and ionic liquid, imparting stability to the gel in aqueous solution. Herein, we demonstrate that [Re-Cl] embedded in a PIG electrode ([Re]-PIG) selectively catalyzes CO 2 reduction at the same applied potential as that observed in non-aqueous systems. Thus, stable CO 2 reduction activity in aqueous solution can be improved by PIG supports, opening the way for use of other molecular catalysts in aqueous solution and in tandem CO 2 reduction and water oxidation systems.   Figure 1b shows the electrocatalytic formation of CO on the [Re]-PIG electrode at −0.68 V vs. RHE using an electrochemical flow cell over one hour. The main product detected by GC-FID-TCD was CO, with a Faradaic efficiency (FE) of 90 ± 5%. H 2 was not detected during electrolysis. To assess the durability of the [Re]-PIG electrode, electrolysis was performed for 24 hours ( Figure S1). Although FE for CO production was over 90% in the first six hours, CO efficiencies decreased to about 50% after 24 hours. [Re-Cl] may have decomposed during electrolysis, as evidenced by a color change from yellow to colorless. We also identified by ATR-IR that the CO peaks of [Re-Cl] decreased in intensity after electrolysis, further supporting catalyst decomposition ( Figure  S2). However, the TON for CO was 76 after 24 hours in aqueous solution, which is higher than any TON observed for [Re-Cl] in non-aqueous solvent (TON = 15). [1][2][3][4] The results for electrochemical CO 2 reduction conducted at various overpotentials for [Re]-PIG, [Re]/CC, PIG/CC and CC are shown in Table 1. The [Re]-PIG electrode under N 2 atmosphere and the PIG/CC and CC electrodes under CO 2 atmosphere did not produce any quantifiable CO 2 reduction products. Only H 2 production was observed for these electrodes. Although CO production was observed using the [Re]/CC electrode, H 2 was the primary product, and the current density was lower than [Re-Cl]/PIG at the same overpotential.
Carbon-based supports such as carbon cloth are known to improve CO 2 reduction activity for molecular catalysts. [30][31][32] It is well known that the CO 2 reduction activity of [Re-Cl] is greatly decreased in aqueous environments. In contrast, the [Re]-PIG electrode had a higher selectivity and current density than other electrodes. The [Re]-PIG electrode can act as an electrocatalyst for CO 2 reduction at potentials cathodic of -0.68 V vs. RHE, which is near the reduction potential of [Re-Cl]. 34 On the other hand, it shows poor CO 2 reduction activity at -0.58 V vs, RHE because the potential is smaller than the reduction potential of [Re-Cl]. A current density of 1.19 mA cm −2 and an FE value of 90.8 % for CO were achieved at −0.68 V vs. RHE over [Re]-PIG electrode, accompanied by the generation of 20.1 μmol of CO after electrolysis for one hr. At −0.78 V vs. RHE, the current density was 2.54 mA cm −2 , which generated 46.6 μmol of CO with a FE of 98.7 % after electrolysis for one hour. Although there is one report for electrocatalytic CO 2 reduction in water using Re(bpy)(CO) 3 Br ([Re-Cl] and [Re(bpy)(CO) 3 Br] exhibit similar behavior for CO 2 reduction) with Nafion, H 2 was the primary product (FE for CO production was less than 30%). 35 These results suggest that PIG drastically improves the CO 2 reduction activity of [Re-Cl] in aqueous solution.
The [Re]-PIG electrode can act as an electrocatalyst for CO 2 reduction at nearly the same reduction potential as [Re-Cl]. It has been reported that ionic liquids can lower the overpotential of CO 2 reduction. 13,14 The suppression of H 2 production can also be explained by the presence of ionic liquid in the PIG support. 13,14,33 Two mechanisms for electrocatalytic CO 2 reduction by [Re-Cl] exist in the literature. 22 The first is the one electron reduction pathway, which is two molecular reactions of Re complexes. The other pathway is the two electron reduction pathway. CO 2 reduction in [Re]-PIG electrode can operate by the one electron reduction pathway, because CO 2 reduction is able to proceed at the first reduction potential of [Re-Cl]. Assuming the reaction proceeds by the one electron reduction pathway, electrocatalytic activity should depend on the concentration of catalyst because this mechanism requires two Re complexes. Table S1 shows the results of CO 2 reduction using several concentrations of [Re-Cl] in the PIG electrode. A low concentration of [Re]-PIG electrode resulted in poor CO 2 reduction activity. Due to the low concentration of Re complex and low diffusion coefficients of PIG, 28 the Re carbonate dimer cannot be produced efficiently due to the low concentration of Re complexes. Based on these results, we propose that the reaction mechanism of [Re]-PIG electrode proceeds by the one electron reduction pathway with some involvement of the ionic liquid in facilitating CO 2 reduction. The PIG support is applicable to CO 2 reduction with other molecular catalysts because it imparts favorable conditions for CO 2 reduction in aqueous solution. To demonstrate the applicability of the PIG support on molecular catalysts, we also tested an Ir catalyst, [Ir(tpy)(ppy)Cl] ([Ir], tpy: terpyridine, ppy: 2-phenylpyridine). Similar to [Re-Cl], [Ir] acts as a photocatalyst for CO 2 reduction in non-aqueous solvents. 36 [Ir] with PIG ([Ir]-PIG, method of preparation given in ESI) exhibited electrocatalytic CO 2 reduction activity in aqueous solution ( Figure S3). The PIG support can also be implemented with cobalt tetraphenylporphyrin (Co(TPP)), another CO 2 reduction catalyst ( Figure S4). Thus, the PIG support can also be implemented with other catalysts for aqueous CO 2 reduction.
One of the goals of CO 2 reduction technology is to develop a full electrochemical cell with water oxidation at the anode to create a biomimetic system. To this end, we attempted to combine the [Re]-PIG electrode and a water oxidation catalyst. Numerous water oxidation catalysts have been reported in the literature. [37][38][39] For example, Ni foam electrodes exhibit good electrocatalytic activity for water oxidation in basic solution. 38 However, the water oxidation activity of Ni foam in neutral pH has been shown to be drastically attenuated. [40][41][42][43] Thus, we selected CoO(OH), which has been shown to catalyze water oxidation under neutral pH conditions. 41 Figure 2a shows the chronoamperomogram of CoO(OH) on carbon paper (CoO(OH)/CP; detail of preparation in ESI) electrode in 0.1 M KOH and 0.1 M K 2 CO 3 aqueous solution under CO 2 atmosphere at +1.82 V vs. RHE. Stable current was observed under CO 2 atmosphere and oxygen production was confirmed after electrolysis for one hour (Figure 2b). Combined with a [Re]-PIG cathode, a cell potential over 2.3 V can be obtained ( Figure  S5).
Tandem electrocatalytic CO 2 reduction and water oxidation was carried out using a full cell system (schematic illustration of full cell system in Figure 3a). A current density of −2.3 to −1.7 mA cm −2 was obtained at a cell voltage of 2.5 V for one hour ( Figure S6). CO and O 2 were produced during electrolysis, generating 30.45 μmol of CO and 15.97 μmol of O 2 (Figure 3b). The FEs for CO and O 2 production were 90.4 % and 94.6 %, respectively. Generally, CO 2 reduction is disturbed by the reduction of O 2 as a competitive reaction. However, we found that the amount of O 2 was approximately stoichiometric with the amount of CO produced. Therefore, the PIG can act as a catalyst support and prevent parasitic O 2 reduction at the cathode, similar to molecular catalyst-carbon materials. 30,43 This behaviour is also consistent with the utility of PIG films as CO 2 gas separation films. 23,24 Therefore, we confirmed that the electrocatalytic CO 2 reduction using [Re]-PIG electrode proceeds even in the presence of oxygen ( Figure S7). These results suggest that the PIG support can provide a platform for integrating selective CO 2 reduction and water oxidation in a two-electrode configuration. Figure 3. (a) Schematic illustration of the full-cell configuration for CO 2 reduction using water as the electron donor. (b) Time courses of CO (black), H 2 (blue) and O 2 (red) production during electrolysis at 2.5 V for one hour under CO 2 atmosphere in 0.1 M KOH + 0.1 M K 2 CO 3 mixed solution.
To verify the carbon source for the generation of CO, an isotope tracer experiment with 13 CO 2 was performed using [Re]-PIG electrode ( Figure S8). CO (m/z = 29) was the main product, confirming that the CO detected in these