Electrochemical Reduction of Amine-Captured Carbon Dioxide Catalyzed by Transition-Metal Substituted Polyoxometalates

Abstract

The low temperature electrocatalysis of CO₂ to CO typically requires purified CO₂, adding complexity, cost, and energy penalties due to the need for separate CO₂ capture and purification processes. Electrochemical reactive capture (e-RCC) is emerging as a simplification of the process. The method combines CO₂ capture with amines to form ammonium carbamates which are then reduced to produce CO and H₂. Heterometallic polyoxometalate catalysts were evaluated for e-RCC by cyclic voltammetry showing that [SiCu₂^IIGa^III(H₂O)₃W₉O₃₇]⁹⁻ had the lowest overpotential and highest turnover frequency. A combination of cyclic voltammetry and controlled potential electrolysis and transport measurements of carbamates through Nafion membranes revealed that commonly used small-molecule carbamates easily traversed membranes and were oxidized at the anode to CO and CO₂. To avoid anodic amine oxidation, polyammonium carbamates were prepared, and the method provides an efficient way to convert CO₂ captured into valuable carbon monoxide. Controlled potential electrolysis confirmed that [SiCu₂^IIGa^III(H₂O)₃W₉O₃₇]⁹⁻ reduced polyammonium carbamate at a negative potential of −1 V vs. Ag/AgCl, forming CO and H_2, while other polyoxometalate catalysts yield CO and H₂ at more negative potentials of −1.3 V. This research combines the use of polyoxometalates for electrocatalysts for e-RCC and provides a potential pathway to avoid detrimental amine/carbamate anodic oxidation.

1. Introduction

The rising concentration of CO₂ in Earth’s atmosphere is a critical factor in the ongoing challenges of climate change and sustainability [1]. Among the strategies for converting excess CO₂ into valuable chemicals is the low-temperature electrochemical reduction of CO₂ (e-CO₂RR) [2,3], where advances toward commercialization have been made using nanoparticle catalysts [4]. Concurrently to the use of metal nanoparticle catalysts for e-CO₂RR, homogeneous transition-metal catalysis has also been broadly investigated following earlier seminal research by the groups of Lehn, Saveant, and Kubiak among many others [5,6,7]. Molecular catalysts may have advantages in active site design and control and spectroscopic elucidation of reaction intermediate, especially toward the two-electron reduced products CO and formate [8].

Typically, e-CO₂RR requires the use of a relatively high purity CO₂ feedstock, and therefore e-CO₂RR needs to be coupled to carbon capture and utilization (CCU) technologies that still are challenging particularly in terms of high costs, energy efficiency, and the technological maturity required to scale these solutions [9]. The need for high-purity CO₂ for e-CO₂RR has spurred more recent investigations into the possibility of utilizing low-purity CO₂ for e-CO₂RR either by integrating CCU and e-CO₂RR methods yielding so-called electrochemically reactive capture (e-RCC) systems [10,11] or similar development of direct e-CO₂RR [12]. Although the study of integrated e-RCC systems has not yet matured [13,14], they have the perceived and calculated advantage of reducing the overall cost of CO₂ reduction through the elimination of costs related to the formation of high-purity CO₂ [15,16,17,18,19]. In addition, scenarios predict an advantage through an increase in cathodic CO₂ concentrations thereby potentially increasing faradaic efficiencies and product yields.

Two concepts have been investigated toward e-RCC reactions. The first relies on the formation of bicarbonate using a hydroxide base. Since no catalyst has been reported to directly electrochemically reduce bicarbonate to two-electron products such as CO or formate, the concept relies on in situ release of CO₂ by protons formed by the anodic four-electron oxidation of water [20,21,22,23]. It is reported that the pH at the cathode surface significantly influences the product selectivity, and pH control is crucial for obtaining reactivity [24,25]. The second pathway is based on the broadly used method of CO₂ capture with primary and secondary amines, notably diethanolamine (DEA), monoethanolamine (MEA), and methyldiethanolamine (MDEA), among others, to yield mostly corresponding carbamates. In typical CO₂ purification units, the carbamates are thermally decomposed back to CO₂ and amine [26,27,28]. In e-RCC reactions the carbamates formed in situ have been shown to be reduced on various cathode materials to yield mostly the two-electron reduced CO and formate, along with significant amounts of H₂ [10,11,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. Despite the advances made, product selectivity, reduced Faradaic efficiencies (FE) compared the reactions without amines, higher H₂ yields through the competing hydrogen evolution reaction (HER), and corrosion of metallic catalysts under reducing potentials all remain significant challenges toward viable e-RCC technology. Importantly, it should be noted that in all the e-RCC reactions described in the literature, the formation of CO and/or CO₂ by anodic oxidation of small molecule amines and/or the corresponding carbamates has not been considered. This is certainly the case for reactions carried out in undivided cell configurations, but also for the divided cell configuration (H-cell or electrolyzers) where the permeability of small-molecule amines and/or the corresponding carbamates through proton exchange membranes (PEMs) or anion exchange membranes (AEMs) has not been considered.

In our laboratory we have recently investigated the use of transition-metal substituted polyoxometalates for ambient temperature e-CO₂RR. For example, [SiW₉Cu₃(H₂O)₃O₃₇]^10– anions were shown to be highly effective catalysts, >95% FE, for the selective electrocatalytic reduction of CO₂ gas to CO in acetonitrile, albeit at rather negative potentials, −2.5 V vs. Fc/Fc⁺ [46]. More recently, it was shown that the introduction of a Lewis acid into the polyoxometalate framework, notably Ga(III), yielding [SiW₉Cu^IIFe^IIIGa^III(H₂O)₃O₃₇]^8–, can lead to electrocatalytic reduction of CO₂ to CO at low overpotentials of only 250–300 mV [47]. Thus, it was of interest to see if similar polyoxometalate compounds would be active catalysts toward CO formation in e-RCC reactions of carbamates.

The research described herein asks several questions. First, are transition-metal-substituted polyoxometalates, previously studied for e-CO₂RR [47], also active catalysts for e-RCC reactions and then which are the most active? Second, do small-molecule amines and their corresponding carbamates diffuse through commonly used PEMs or AEMs from the cathode to anode, and if so, what is a strategy to mitigate this diffusion? Third, are small-molecule amines and their corresponding carbamates oxidized at the anode under typical e-RCC cell potentials? As will be shown below and summarized in Figure 1, [SiCu₂^IIGa^III(H₂O)₃W₉O₃₇]⁹⁻ was found to be the most active for e-RCC reactions, but carbamates based on small molecule amines quickly traverse PEMs and AEMs and are oxidized at the anode to form CO and CO₂. To mitigate the problem of carbamate diffusion from cathode to anode cell compartments, high-molecular-weight amines were used to capture CO₂, and the resulting carbamate solution was reduced to CO with the co-formation of H₂ as a by-product.

Figure 1. Reduction of branched polyethylenimine-derived carbamates to CO catalyzed by [SiCu₂^IIGa^III(H₂O)₃W₉O₃₇]⁹⁻. W: dark grey; Cu: blue; Ga: turquoise; O: red. For clarity the branched polyethylenimine is presented as a linear polymer.

2. Results and Discussion

To evaluate potential polyoxometalate compounds as catalysts for the reduction of ammonium carbamates, CO₂ was reacted with 1:1 aqueous solutions of ethanolamine (MEA) and diethanolamine (DEA) to form the corresponding ammonium carbamates, as seen in Scheme 1.

Scheme 1. Reaction of CO₂ with MEA and DEA.

As can be seen from the ¹H and ¹³C NMR measurements, depicted in Figures S1 and S2, CO₂ was introduced into 1:1 by volume aqueous solutions containing either a primary amine, MEA, or a secondary amine, DEA, to afford in situ formation of aqueous ammonium carbamate species via the reaction of approximately one molar equivalent of CO₂ with two molar equivalents of amine. After about 20 min the formation of the MEA and DEA carbamates was completed, and further addition of CO₂ led to the decrease in pH and the formation of a small amount of bicarbonate and some dissolved CO₂. Also, one can observe that MEA is notably more efficient in forming MEA-carbamate than DEA, and thus MEA was used to evaluate the electrocatalytic activity of polyoxometalates. Initial cyclic voltametric (CV) measurements were carried out under N₂ and MEA carbamate (MEAC) for various polyoxometalates [SiCu₂^IIGa^III(H₂O)₃W₉O₃₇]⁹⁻ {SiCu₂GaW₉}, [SiCu^IIFe^IIIGa^III(H₂O)₃W₉O₃₇]⁸⁻ {SiCuFeGaW₉}, [SiCu₂^IIZn^II(H₂O)₃W₉O₃₇]¹⁰⁻ {SiCu₂ZnW₉}, and [SiCu^IIFe^IIIZn^II(H₂O)₃W₉O₃₇]⁹⁻ {SiCuFeZnW₉} chosen based on previous research that showed such polyoxometalates to be the most reactive [47], Figure 2.

Figure 2. Cyclic voltammograms of four different polyoxometalates in 1:1 aqueous MEA under N₂ and after formation of MEAC, by bubbling CO₂ for 20 min. Conditions: 2 mM polyoxometalate, 3 mL MEA, 3 mL of DDW, glassy carbon working electrode (WE), stainless-steel wire counter electrode (CE), and Ag/AgCl reference electrode (RE), at room temperature, with scan rate 100 mV/s.

No reduction peaks were observed of MEA-H₂O solutions under N₂, but after formation of MEAC, {SiCuFeGaW₉}, {SiCu₂ZnW₉}, and {SiCuFeZnW₉} and others, Figure 3 shows catalytic peaks with onset reduction potentials at −1.18 to −1.22 V. Surprisingly, {SiCu₂GaW₉} showed significantly lower reduction onset potentials of −0.68 V and significantly higher reduction currents. This contrasts with e-CO₂RR catalyzed by these polyoxometalates where the highest activity was observed at similar potentials as those obtained with {SiCuFeGaW₉} [47]. To support a postulate that the observed reactivity of {SiCu₂GaW₉} originated predominantly from the carbamate species rather than dissolved CO₂ or bicarbonate, solutions were (a) treated with N₂ following CO₂ saturation (20 min), and the CV was subsequently recorded. There is no change in the onset potential and only a very slight decrease in the reduction potential was observed and (b) CV measurements after extending the CO₂ bubbling time to up to 40 min with a concurrent reduction in pH (Figure S1) resulted in a progressive decrease in the catalytic peak current, as depicted in Figure S3. This sensitivity to the decreasing pH in solution is likely due to a decrease in the relative amount of carbamate on the electrode during the CV measurement because of competition with bicarbonate and/or lower reactivity of CO₂ formed in situ. CVs as a function of scan rate are shown in Figure S3 and show slightly decreasing current with the increase in scan rate likely because of incomplete regeneration of active species after electron transfer with increasing scan rates.

Figure 3. Catalytic Tafel plots for the cathodic MEAC reduction by various polyoxometalate electrocatalysts. Data was taken from cyclic voltammograms (Figure 2 and Figure S4–S7). Structures for the various complexes are presented in Figure S8.

One way to determine the maximum turnover frequencies, (TOF_max), and compare the electrocatalytic activity of the various polyoxometalates is by calculating TOF_max and the overpotential, η, using a catalytic Tafel plot [48,49,50], as in Equation (1) and Figure 3.

$$TOF={\displaystyle \frac{2k_{cat}}{1+\exp \left[\frac{F}{RT}\left(E_{MEAC}^0-E_{cat}^0\right)\right]\exp \left(-\frac{F}{RT}\mathrm{ɳ}\right)}}$$

(1)

The k_cat values were obtained from the CV measurements, as depicted in Figure 2 and Figures S4–S7, and E⁰_MEAC is the standard potential for the cathodic reaction, as demonstrated in Equation (2), and at pH = 10 it is calculated to be −0.91 V (see further details in the Materials and Methods Section). E⁰_cat is the standard potential of the catalyst under N₂.

MEAC + 2 H⁺ + 2 e^– ⟶ 2 MEA + CO + H₂O

(2)

The results show that {SiCu₂GaW₉} has the lowest overpotential (η = 0.54 V) at the highest log TOF_max = 5.17 while the other catalysts have overpotentials of η = 0.69 V with a range of lower TOF_max values.

Initial controlled potential electrolysis (CPE) experiments were carried out in an undivided cell to sample the products of a MEAC reduction reaction. For example, a CPE reaction of a 6 mL 1:1 solution of MEA: H₂O treated for 20 min with CO₂ containing 2 mM {SiCu₂GaW₉}, at room temperature for 15 h at −1.5 V versus Ag/AgCl using a glassy carbon (d = 3 mm) WE and a stainless-steel CE, yielded 25 µmol CO and 25 µmol H₂ as only products. Analysis was by gas chromatography with a thermal conductivity detector for the gas phase (GC-TCD) and ¹H NMR for the liquid phase. To verify that the CO formed was indeed from the in situ prepared MEAC, an identical experiment was performed where MEAC was prepared using ¹³CO₂. Analysis by GC with a mass selective detector (MSD) revealed the formation of >90% unlabeled ¹²CO₂ indicating an anodic oxidation of MEA and/or MEAC. To further understand this anodic oxidation, (1) the CV for the oxidation of MEA, DEA, diethylamine and their corresponding carbamates were tested, and (2) the transport of MEAC was conducted through a Nafion membrane often used in divided cell configurations. One can see in Figure S9 that the amines themselves are quite stable to oxidation up to 1.5 V versus Ag/AgCl; however, the corresponding carbamates are oxidized at this potential; diethylamine carbamate > DEAC > MEAC. Furthermore, the transport of MEAC through a Nafion membrane in a H-cell configuration was measured by ¹H NMR spectra as a function of time under open circuit conditions starting with MEAC in the cathode cell only, as depicted in Figure S10. As can be seen MEAC easily traverses the Nafion membrane within a short period of time. It should also be noted that upon crossing the Nafion membrane, MEAC was quantitatively hydrolyzed to MEA. Finally, reduction of MEAC was carried out in both an H-cell and a membrane electrode assembly (electrolyzer). The anode used for H₂O oxidation was IrO₂ on platinized Ti felt. The cathode was a polyoxometalate catalyst supported on a porous carbon gas diffusion electrode, and Sustainion^® X37-50 grade 60 and Pipirion^® were used as AEMs to prevent MEAC decomposition. The headspace above the cathode and anode cells was analyzed separately by GC-TCD. The results showed that CO was found almost exclusively in the anode cell, supporting the above-mentioned indications that CO can be formed from MEAC oxidation rather than MEAC reduction. H₂ was formed exclusively at the cathode. See the Supplementary Materials for more details, specifically Figure S11.

Given that the catalytic reduction of MEAC as observed by CV could not be translated to a divided cell configuration because of fast transport of such small-molecule-derived carbamates through commercial PEMs and AEMs, the solution to this problem would be to use high-molecular-weight polyamines as CO₂-capturing agents that would form carbamate units on the polymer chain and would not traverse PEMs or AEMs. Common and commercial branched polyethylenimine polymers (PEIs) that contain ~25% tertiary, ~50% secondary, and ~25% primary amines, already known as carbon-capture compounds [51,52,53,54], and available in a range of molecular weights from 800 to 1,000,000, were used for the e-RCC reaction in a membrane electrode assembly electrolyzer. The amount of CO₂ bound by treatment of 1:1 PEI:H₂O was ~80 mg CO₂/g 25 K PEI (7.8 mol%/amine unit) and ~90 mg CO₂/g 1 M PEI (8.9 mol%/amine unit), similar to previous reports [52,53].

In the first step using the polyanionic properties of the polyoxometalate compounds and the polycationic properties of typical anion exchange ionomers based on quaternary ammonium sites, polyoxometalates were heterogenized onto porous carbon supports as gas diffusion layers (GDLs). The GDL electrode was fabricated by preparing an ink containing 80 wt% polyoxometalate, 10 wt% Aemion^® ionomer, and 10 wt% carbon black, dispersed in a mixture of isopropanol and water. The suspension was homogenized by ultrasonication for 30 min and subsequently spray-coated onto a carbon paper substrate (Freudenberg H23C6 GDL with a microporous layer and hydrophobic surface treatment) using an air gun operated at 2 bar pressure. The catalyst morphology was evaluated on a high-resolution scanning electron microscope (HRSEM). The HRSEM images at 500× magnitude showed a very good dispersion of {SiCu₂GaW₉} on the GDL, as depicted in Figure 4 in comparison to the untreated carbon paper GDL in Figure S12. EDS measurements showed homogeneous coverage of {SiCu₂GaW₉} on the GDL as would be expected for a molecular compound, as depicted in Figure S13.

Figure 4. HRSEM image of {SiCu₂GaW₉} on a carbon GDL using an Aemion^® ionomer-based ink.

Initial exploratory CPE reactions were carried out in an cell using {SiCu₂GaW₉}, {SiCuFeGaW₉}, or {Ni₄(PW₉)₂} on a 1 cm² carbon paper GDL as described above and pictured in Figure 4 as WE, a IrO₂ on Pt-Ti felt separated by a glass frit as CE, and a Ag/AgCl RE. CPE was carried out in a 18 mL glass vial containing 1 mL of PEI (molecular weight 1 million), 3 mL of DDW, 2 mL of Na₇DTPMP (heptasodium diethylenetriaminepentamethylenephosphonate 25% (w/v) in H₂O), and 0.8 M Na₂SO₄·10H₂O under 1 bar CO₂ reacted at −1.5 V, −1.3 V, and −1.0 V versus Ag/AgCl for 15 h at room temperature. CO and H₂ were the only observed products. Although the separation of the in situ formed carbamate of PEI from the anode was only by a frit, the importance of this measurement lies in the observation that {SiCu₂GaW₉} was active down to −1 V, while {SiCuFeGaW₉} and {Ni₄(PW₉)₂} were active at only more negative potentials, with {Ni₄(PW₉)₂} as the poorest catalyst, as depicted in Figure S14. This supports the finding that {SiCu₂Ga W₉} is uniquely active at lower overpotentials as observed in the CV measurements. A control experiment to verify the source of CO from CO₂ using PEI-carbamate prepared with ¹³CO₂ showed formation of ¹³CO only.

To solidify the advantage of the use of PEI for e-RCC, CPE was carried out in a membrane electrode assembly electrolyzer. Controlled-voltage electrolysis (CVE) was carried out in 4 cm² electrolyzer using {SiCu₂GaW₉}, {SiCuFeGaW₉}, or {Ni₄(PW₉)₂} on a GDL cathode as described above (1 cm² area) as WE. The catholyte was prepared from 1 mL PEI and 3 mL H₂O, 2 mL Na₇DTPMP 25% (w/v) in H₂O, and 0.8 M Na₂SO₄·10H₂O. The catholyte was treated for 20 min under a flow of CO₂ gas to yield the corresponding PEI-carbamate and then purged with N₂ and kept under 2 bar N₂. The anolyte was Na₇DTPMP 25% (w/v) in H₂O and 0.8 M Na₂SO₄·10H₂O. The anolyte and catholyte solutions were separated by a Nafion 107 membrane. CVE was carried out at 2.7, 2.5, and 2.3 V for 15 h at room temperature. Analysis of the gas phase by GC-TCD showed that CO and H₂ were the only gaseous products formed while analysis by ¹H NMR of the liquid phase from reactions carried out in D₂O showed no discernible formation of any soluble products. The results are summarized in Figure 5.

Figure 5. Controlled-voltage electrolysis comparing {SiCu₂GaW₉}, {SiCuFeGaW₉}, or {Ni₄(PW₉)₂} in an electrolyzer. Cathode: 1 cm² carbon paper GDL from ink containing 10 µmol polyoxometalate, 2 mg Aemion^® ionomer, and 5 mg carbon black. Anode: IrO₂ on Pt-Ti felt as counter electrode. Catholyte: 1 mL PEI (molecular weight 1 M), 3 mL H₂O, 2 mL Na₇DTPMP 25% (w/v) in H₂O, and 0.8 M Na₂SO₄·10H₂O treated for 20 min under a flow of CO₂. Anolyte was Na₇DTPMP 25% (w/v) in H₂O and 0.8 M Na₂SO₄·10H₂O. Nafion 107 membrane, room temperature, and time of 15 h. Reactions were carried out in duplicate with minimal differences. Total faradaic efficiencies were moderate and typically less than 40%; see Table S1 for detailed data.

Post-reaction analysis by NMR showed that no polyethyleneimine or corresponding carbamate had diffused into the anolyte solution, as depicted in Figure S15. The {SiCu₂GaW₉} catalyst appeared to be stable after 15 h under CVE conditions, as depicted in Figure S16. There was no evidence of dissolution of the polyoxometalate or component elements as measured by ICP-MS. There were only statistically insignificant (±5%) amounts of Cu, Fe, Ga, or Ni above the background amounts of the elements measured in solution after the reactions. The results indeed confirm that {SiCu₂GaW₉} can yield product at the lowest overpotential; although, at more negative potentials of −2.7 V, {SiCuFeGaW₉} yielded similar results. For both polyoxometalates the CO:H₂ ratios hovered around 2:1. On the other hand, {Ni₄(PW₉)₂} was less reactive and yielded H₂ in preference to CO. A comparison of the use of different molecular weight PEI, 1,000,000 versus 25,000, showed that the former yields better results, as depicted in Figure S17. Tabulated results including the FE (%), charge density (J, mA/cm²), and total charge (Q, C) can be found in Table S1. Using the conditions described in Figure 5, control experiments (1) without catalyst (just GDL) showed no product formation; (2) PEI not treated with CO₂ also showed no product formation; (3) experiments with CO₂ dissolved in water yield only small trace amounts of CO; and (4) an experiment with ¹³CO₂ showed only the formation of ¹³CO.

3. Materials and Methods

Polyoxometalate Synthesis. The alkali metal salts of {SiCu₂ZnW₉}, {SiCu₂GaW₉}, {SiCuFeGaW₉}, {SiCuFeZnW₉}, and {SiCuNiZnW₉} were synthesized and analyzed as previously reported [47]. Similarly, the alkali metal salts of {Ni₄(PW₉)₂}, {Cu₄(PW₉)₂}, and {Fe₄(FeW₉)₂} were also prepared by procedures reported in the literature [55,56,57].

Electrochemistry. Electrochemical experiments were carried out using a Biologic, Seyssinet-Pariset, France multichannel VSP 201 potentiostat. Cyclic voltammograms of 2 mM polyoxometalate in 3 mL water and 3 mL MEA were measured at different scan rates under 1 bar N and after formation of MEAC by treatment with a flow of 1 bar CO₂ (99.99%) for 20 min. Glassy carbon (d = 3 mm) WE, a stainless-steel wire CE, and Ag/AgCl (3M KCl) RE. Similar CV measurement to evaluate the oxidation of MEA and MEAC were carried out in the same manner by exchanging the WE and CE. To extract catalytic parameters from the CV measurements, we used the equation proposed by Kubiak and coworkers for deriving the catalytic rate constant, k_cat [48], namely Equation (1), where R is the universal gas constant, T = temperature, F = Faraday constant, ʋ = scan rate, n is the number of electron-transfer processes that occur at the electrode per catalyst and equals 2; n′ is the number of catalysts equivalents required per catalysts and equals 1; i_cat is the current under CO₂; i_p is the current under N₂; and k_cat is the catalytic rate constant.

$${\displaystyle \frac{i_{cat}}{i_p}}={\displaystyle \frac{1}{0.446}}\sqrt{{\displaystyle \frac{RT}{nF\mathrm{ʋ}}}{n}^{\prime }{k}_{cat}}$$

(3)

The interdependency of turnover frequency (TOF) and overpotential (η) are based on the intrinsic properties of the catalyst and can be calculated from the difference between the applied potential and the standard potential for the reduction of CO₂ to CO, independent of contingent factors such as the cell characteristics. The relevant catalytic Tafel plots were calculated using Equation (3) [49,50]. E⁰_MEAC, the standard potential for the reduction of carbamate to CO under the specific conditions of catalysis, was calculated using literature data for ΔG_f values and is shown in Figure 6; E⁰_MEAC = −0.91 V. The pH = 10 value was the one measured before the CV measurements (Figure S1). The presence of bicarbonate or dissolved CO₂ was not observed and therefore not considered.

Figure 6. Calculation of E⁰_MEAC from literature ΔG_f values of ΔG_f for MEA, CO₂, CO, and H₂O were taken directly from the literature [58,59]. The ΔG_f for 2 H⁺ + 2 e^– was calculated from E = 0.0591 V × pH. The ΔG_f of MEAC was deduced from DFT-calculated literature values [60].

Controlled-voltage electrolysis. Reactions in 4 cm² electrolyzer (with 1 cm² active area). Cathode: a carbon-paper GDL electrode was fabricated by preparing an ink containing 80 wt% polyoxometalate cesium salts of ({SiCu₂GaW₉}, {SiCuFeGaW₉}, or {Ni₄(PW₉)₂}), 10 wt% Aemion ionomer, and 10 wt% carbon black, dispersed in a mixture of isopropanol and water. The suspension was homogenized by ultrasonication for 30 min and subsequently spray-coated onto a carbon-paper substrate (Freudenberg H23C6 GDL with a microporous layer and hydrophobic surface treatment) using an air gun operated at 2 bar pressure. Catholyte: 1 mL PEI, 3 mL H₂O, 2 mL Na₇DTPMP 25% (w/v) in H₂O, and 0.8 M Na₂SO₄·10H₂O treated for 20 min under a flow of CO₂. Anode: IrO₂/Pt-Ti felt. Anolyte: 3 mL DDW, 2 mL Na₇DTPMP (25% w/v), and 0.8 M sodium sulfate decahydrate. Reactions were carried out under 2.5 bar N₂ for 15 h at room temperature at cell voltages ranging from 2.3 to 2.7 V, respectively, with an anodic potential of ∼1.0 V versus SHE. The analysis of the products at the gas phase was conducted by using a HP 6890 GC-TCD, Santa Clara, CA, USA using a ShinCarbon ST 80/100 micropacked 2 m × 0.53 mm ID column with He as a carrier gas. Hydrogen was analyzed separately using a GOWMAC GC-TCD, Bethlehem, PA, USA, configured with two columns in series (4′ by 1/8” Hayesep T; 10′ by 1/8” Molecular sieve 5A) with Ar as a carrier gas carrier. CO and H₂ were observed as the only products. Experiments using ¹³CO₂-labeled carbamates were analyzed by GC-MS using a molecular sieve PLOT column (30 m × 0.32 mm)

¹H NMR spectra were recorded on a Bruker Avance III 300 MHz spectrometer, Ettlingen, Rheinstetten, Germany at 298 K and referenced to the solvent shift. Data manipulations were completed using MestReNova 14.3.2 and TopSpin3.7 software.

HRSEM imaging was carried out on a Zeiss MERLIN high-resolution scanning electron microscope, Oberkochen, Germany containing an Inlens detector for higher energy secondary electrons for clear topographic mapping.

4. Conclusions

Others have noted an e-RCC reaction of captured carbamate with small-molecule amines such as MEA to form MEAC as substrate for the formation of CO to indirectly reduce low atmospheric concentrations of CO₂. We have found, however, that such carbamates easily diffuse through membranes in divided cell configurations, e.g., H-cells or membrane assembly electrolyzers, and are oxidized at the anode. To circumvent this problem, we have shown that high-molecular-weight PEI can be used as a proven CO₂ capturing agent. The corresponding carbamates can yield CO along with H₂ in ~2:1 ratio in a CVE reaction using a membrane electrode assembly electrolyzer with a supported {SiCu₂GaW₉} catalyst. The experiments demonstrated that {SiCu₂GaW₉} reduces polyammonium carbamates at a significantly less negative potential (–1.0 V vs. Ag/AgCl), compared to other tested polyoxometalate catalysts, and produces CO with H₂ as a co-product. Typically, current densities of 1–2 mA/cm² were obtained with only moderate faradaic efficiencies. It is not yet possible to benchmark these results since previous e-RCC research using amine capture has not considered anodic oxidation of the small-molecule carbamates used as substrates, typically MEAC. Considering the too-low faradaic efficiencies and current densities, and limited actual turnover numbers, much more research is still required to optimize such polyamine-based e-RCC reactions.