ES3019211T3 - Iron and cobalt molecular complexes for the selective electrochemical reduction of co2 into co, with flow cells - Google Patents

Abstract

The present invention relates to a flow cell electrolyzer (1) for electrochemically reducing reactive gas CO 2 into gaseous CO and gaseous H2, with an anodic compartment comprising an anode (2) with a current collector, and on the current collector, at least one catalyst for electrochemically oxidizing H2O to O 2 , an anodic electrolyte solution (3) at a controlled flow rate Q a , comprising: a solvent, and an anodic electrolyte, the solvent being water at neutral or basic pH; a cathode compartment comprising a cathode electrolyte solution (6), at a controlled flow rate Q c , comprising: the solvent, and a cathode electrolyte, the solvent being water at neutral or basic pH, a porous gas diffusion cathode (9) comprising, on an electrochemically inert porous gas diffusion cathode current collector, at least one molecular catalyst on a surface S for electrochemically reducing CO 2 into CO, with a by-production of H 2 , the molecular catalyst being chosen from the list with the metal chosen from: Iron, Cobalt: metallic porphyrin with one or more + N(C 1 - C 4 alkyl) 3 groups, metallic phthalocyanine, metallic phthalocyanine with one or more + N(C 1 - C 4 alkyl) 3 groups, or cobalt quaterpyridine. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Molecular complexes of iron and cobalt for the selective electrochemical reduction of CO2 to CO, with flow cells

Field of the invention

The present invention relates to molecular complexes of iron and cobalt as catalysts for the highly selective electrochemical reduction of CO2 to CO, with electrochemical cells comprising them.

State of the art

Despite the increasing use of renewable energy to generate electricity and avoid the associated production of CO2, it is reasonable to assume that CO2 emissions, particularly those resulting from energy production, will remain high in the coming decades. Therefore, it seems necessary to find ways to capture CO2 gas, whether for storage or recovery purposes.

In fact, CO2 can also be viewed not as a waste product, but rather as a source of carbon. For example, the promising production of synthetic fuels from CO2 and water has been envisioned.

However, CO2 exhibits low chemical reactivity: breaking its bonds requires an energy of 724 kJ/mol. Moreover, the electrochemical reduction of CO2 to one electron occurs at a very negative potential, requiring a high energy input and leading to the formation of a highly energetic radical anion (CO2^-). Therefore, catalysis appears mandatory to reduce CO2 and drive the process towards a multi-electron and multi-proton reduction process, in order to obtain thermodynamically stable molecules. Moreover, the direct electrochemical reduction of CO2 at inert electrodes is poorly selective, producing formic acid in water, while it produces a mixture of oxalate, formate, and carbon monoxide in low-acid solvents such as DMF. US 2018/066370 describes a flow cell electrolyzer for electrochemically reducing a gaseous reactant comprising CO2 to CO gas.

Therefore, electrochemical reduction of CO2 requires catalytic activation to reduce the energy cost of the process and increase the selectivity of the species formed in the reaction process. Several catalysts are described in US 2018/023204, Tsukasa Yoshida et al. (JEAC, (1995-01-01), pp. 209-225), Min Wang et al. (Angewandte Chemie, International Edition, Vol. 57, No. 26, (2018-05-22), pp. 7769-7773).

Molecular catalysts that combine high selectivity and high current density for the electrochemical reduction of CO2 to CO or other chemical feedstocks are in high demand. Molecular transition metal catalysts offer the distinct advantage of enabling fine-tuning of the primary and secondary coordination spheres by manipulating the chelating environment and the steric and electronic effects of the ligands. The ability to improve catalytic efficiency and product selectivity through rational optimization of the ligand structure is a feature not accessible by common solid-state catalysts in pilot-scale electrolyzer units. 1-2 There is currently a range of known molecular catalysts, including those based on noble metals (e.g., Ru, Ir, and Re) and earth-abundant ones (e.g., Co, Ni, Fe, Mn, and Cu). 1-9 These catalysts typically cause a two-electron reduction of CO2 to CO or formate with reasonable efficiencies, but in organic solvents. Experiments by Furuya et al. (JEAC, vol. 271, no. 1-2, (01-01-1989), pages 181-191) and Magdesieva et al. (JES, vol. 149, no. 6, (01-01-2002), page D89), refer to metal porphyrin and phthalocyanine catalyzed gas diffusion electrodes.

The integration of the above-described molecules by applying thin porous carbon films such as carbon powder, carbon nanotubes, or graphene to form hybrid catalytic materials has proven to be a promising strategy to selectively achieve CO production under pure aqueous conditions. Reasonable yields have been obtained under near-neutral conditions (pH 7–7.5) with good selectivity. While these yields represent advances for CO2RR (CO2 reduction reaction) catalysts, much higher current densities are required for commercial operation, as they are highly selective while operating at low overpotentials. Moreover, these current densities remain well below those obtained with state-of-the-art solid-state Ag<1213>or Au<14>nanomaterials, and have been reported to reach >150 mA/cm<2>

Summary of the invention

The invention relates to a flow cell electrolyzer for electrochemically reducing reactive gas CO2 to gaseous CO and gaseous H2, as set forth in the appended claims.

Brief description of the drawings

Other advantages and features of the devices and methods described will become apparent from reading the description, illustrated by the following Figures, where:

Figure 1. Current density and CO selectivity recorded at various potentials. Recorded in 0.5 M NaHCO<3> saturated with CO2 (pH 7.3). Each potential step was held for 20 minutes.

Figure 2. Current density and CO selectivity recorded during a 24-h electrolysis at -0.78 V versus RHE in 0.5 M NaHCO<3> saturated with CO2. The cell potential was 3.41 V.

Figure 3. The applied potential and CO selectivity recorded under chronopotentiometric conditions at 50 mAcirr<2> for 3 hours in 0.5 M NaHCO<3> saturated with CO2. The cell potential was 4.05 V.

Figure 4. The applied potential and CO selectivity recorded under chronopotentiometric conditions at 50 mAcirr<2> for 3 hours in 1.0 M NaHCO<3> saturated with CO2. The cell potential was 3.46 V.

Figure 5. (A) Current density and CO selectivity at various cell temperatures. Recorded in 0.5 M NaHCO<3> saturated with CO2. (B) Arrhenius plot constructed from the TOF values calculated for each temperature step in (A).

Figure 6. Current density and CO selectivity recorded at various potentials. Recorded in a 1.0 M KOH solution (pH 14). Each potential step was held for 20 minutes.

Figure 7. The applied potential and CO selectivity were recorded under chronopotentiometric conditions at 27 mAcirr<2> for 24 hours in 1.0 M KOH. The cell potential was 1.87 V.

Figure 8. The applied potential and CO selectivity were recorded under chronopotentiometric conditions at 50 mAcirr<2> for 3 hours in 1.0 M KOH. The cell potential was 2.36 V.

Figure 9. Step potential experiment, giving the current density recorded at various potentials. Recorded in 0.5 M NaHCO<3> saturated with CO2 (pH 7.3). Each potential step was held for 20 minutes.

Figure 10. Step potential experiment, giving the recorded current density at various potentials. Recorded in argon-saturated 0.5 M NaHCO<3> (pH 7.3) on a film formulated with the non-metallated porphyrin ligand. Each potential step was held for 20 min.

Figure 11. Current density and CO selectivity recorded during a 6-h electrolysis at -0.78 V vs. RHE in CO2-saturated 1 M NaHCO<3>.

Figure 12. Step potential experiment, giving the recorded current density at various potentials. Recorded in 1 M KOH (pH 14). Each potential step was held for 20 minutes.

Figure 13. Step potential experiment, giving the recorded current density at various potentials. Recorded in 1 M KOH (pH 7.3) on a film formulated with the non-metallated porphyrin ligand. Each potential step was held for 20 min.

Figure 14. Cross-sectional view (a) and general schematic (b) of the CO2 electrolyzer flow cell. Figure 15. a: Current density for CO production as a function of potential. b: Bulk electrolysis at fixed potential (E= -0.72 V vs. RHE) for CoPc2 on carbon black deposited on carbon paper as cathode material, in 1 M KOH. c: Co K-edge XANES profiles of CoPc2 (black dots) and CoPc2 on carbon black before and after electrolysis (E= -0.72 V vs. RHE) overlaid, in 1 M KOH solution. Figure 16. Flow cell electrolyzer setup.

Figure 17. Repetitive cyclic voltammetry for CoPc1-on-MWCNT (bottom) and CoPc2-on-MWCNT (top) films deposited on a glassy carbon electrode (d = 3 mm) in a CO2-saturated 0.5 M NaHCO<3> solution (pH 7.3); black, first scan; gray, second scan), at v = 0.1 V s<-1>.

Figure 18. SEM image of a catalytic film deposited on carbon paper.

Figure 19. Blank experiments: On the left, at E = -0.676 V vs. RHE) in the absence of catalyst in a saturated solution of 0.5 M NaHCO<3> with CO2 (pH = 7.3); on the right, at E = -0.605 V vs. RHE with CoPc2 on MWCNTs (1:15) in a saturated solution of 0.5 M NaHCO<3> with Ar (pH = 8.5). In both cases, only H2 was detected as a reaction product.

Figure 20. Variation of total current density as a function of electrolysis potential at an optimized mass ratio of CoPc to MWCNTs in a CO2 saturated solution containing 0.5 M NaHCO<3> (pH 7.3). Figure 21. XPS spectra of Co 2p (left) and N 1s (right) before and after a 7 h electrolysis (E = -0.676 V vs. RHE) with CoPc2 on MWCNTs at a ratio of 1:15 in a CO2 saturated solution with 0.5 M NaHCO<3> (pH 7.3).

Figure 22. Bulk electrolysis at E = -0.971 V vs. RHE of a saturated CO2 solution containing 0.5 M KCl (pH 4) using CoPc2 on MWCNTs as catalyst.

Figure 23. Total current density as a function of applied potential for CoPc2 on carbon black deposited on carbon paper as a cathode catalytic material, in a 1 M KOH solution (flow cell device, see main text). Each potential step was maintained for a duration of 20 min.

Figure 24. Top: K-spaced (inset) and Fourier transform EXAFS spectra of CoPc2 on carbon black before (black) and after electrolysis (E= -0.72 V vs. RHE) (gray) in 1 M KOH solution.

Bottom: Co K-edge XANES spectra of the CoPc2 electrode on carbon black after electrolysis (E = -0.72 V vs. RHE) (red) along with those of the reference compounds CoO (purple), CoOOH (orange), Co<3>O<4> (blue), and metallic Co (green).

Figure 25. Bulk electrolysis at a fixed current density (75 mA cm<-2>) with CoPc2 on carbon black deposited on carbon paper as the cathode catalytic material, in 1 M KOH solution (flow cell device, see main text). During the electrolysis, 2.5 g of KOH was added every half hour to the cathode solution to maintain pH equilibrium.

Figure 26. CO selectivity as a function of time in hours at 50 mA/cm<2> with a Co(qpy) catalyst in a gapless cell device.

Detailed description

The present invention relates to a flow cell electrolyzer 1 for electrochemically reducing a gaseous reactant comprising CO2 to gaseous CO, as specified in the appended claim 1.

The cathodic electrolyte may comprise a phosphate buffer or potassium hydroxide, and the anodic electrolyte comprises a phosphate buffer or potassium hydroxide.

The molecular catalyst is chosen from the list:

❖ metallic tetraphenylporphyrin as defined in claim 1.

❖ Metallic phthalocyanine, with the metal chosen from: iron, cobalt

❖ metallic phthalocyanine with the metal chosen from: iron, cobalt, with one or more groups from: N(C1-C<4>alkyl)<3>, F, C(CH<3>)<3>, or

❖ cobalt quaterpyridine

The flow cell electrolyzer 1 also comprises:

- an anion exchange membrane 10, impermeable to CO2, CO, H2 and O2, between the anode compartment and the cathode compartment;

- a channel 11 for flowing the reactive gas CO2, at a controlled flow rate Qg, over or through the surface S of the porous gas diffusion cathodic current collector;

- a power source that provides the energy needed to trigger the electrochemical reactions involving the reactant.

In the flow cell electrolyzer 1, all flow rates are controlled by passing the cathodic and anodic electrolytes, as well as the reactive gas, through the pumping means 12 which serve to:

° circulate by pumping the anodic electrolyte solution 3 and the cathodic electrolyte solution 6 between the inlets 4 and the outlets 5,

° pumping the reactant gas CO2 into or through the porous gas diffusion cathode 9. In one example, the reactant gas comprises at least 99% by volume CO2, or at least 99.5% by volume CO2, or at least 99.9% by volume CO2, or at least 99.99% by volume CO2.

The flow cell 1 may have a device capable of recording the current generated by the electrochemical processes.

Advantageously, but not limited to, the channel for flowing the reactive gas CO2 may comprise a flow frame that generates a turbulent flow.

Advantageously, but not limited to, the pumping means 12 recirculate the anodic electrolyte solution 3 and the cathodic electrolyte solution 6.

In a preferred embodiment, the anodic and/or cathodic electrolyte solution has a neutral or basic pH.

In yet another preferred embodiment, the cathodic electrolyte solution has a basic pH. In such an embodiment, the anodic electrolyte solution may have an acidic or neutral pH.

In particular, the anodic electrolyte solution and/or the cathodic electrolyte solution have a pH of from 9 to 14, preferably from 10 to 14, more preferably from 11.5 to 14, and most preferably from 13 to 14.

In yet another preferred embodiment, the cathodic electrolyte solution has a pH of from 9 to 14, preferably from 10 to 14, more preferably from 11.5 to 14, and most preferably from 13 to 14. In such an embodiment, the anodic electrolyte solution may have an acidic or neutral pH.

Basic, i.e. alkaline, conditions, particularly when applied to the cathode electrolyte solution, allow the flow cell electrolyzer to operate at surprisingly high current densities compared to acidic conditions.

The combination of basic conditions with the aforementioned porphyrin, phthalocyanine, or quaterpyridine molecular catalysts also results in surprisingly high selectivities for the desired conversion of CO2 to CO, with only minor or even no byproduct formation. Thus, with the high current densities and high CO selectivity, the invention enables the production of almost pure CO from CO2 at high yield rates.

Furthermore, the aforementioned molecular catalysts were found to demonstrate high long-term stability even under severe alkaline pH conditions.

The electrochemical reduction of the reagent to CO can be carried out at room temperature. The electrochemical reduction of the reagent to CO can be carried out at atmospheric pressure.

Advantageously, the cathodic current collector can be carbon paper. A porous current collector composed of multiple layers (e.g., a polytetrafluoroethylene gas distribution layer) can be filled into the carbon paper to increase the reduction rate/efficiency.

In one embodiment, the porous cathode 9 comprises an electrode film in the current collector, which at least contains polymers and the molecular catalysts. The electrode film can be filled with or grafted onto the current collector.

Tetraphenylporphyrin comprises specific groups on the phenyl moieties, with at least one or more N(C<1>-C<4>alkyl)<3> groups between the specific groups.

In a first embodiment of the flow cell electrolyzer 1, the molecular catalyst is iron porphyrin with the formula:

where:

or at least 1 and at most 8 groups between Ri and R<10> and Ri, and R<10>' which are independently a N(C1-C4 alkyl)3 group,

or the remaining groups from R<1> to R<10> and R<1> to R<10>' are independently selected from the group consisting of H, OH, F, C(CH3)3

In particular, the iron molecular catalyst may comprise:

- the other groups between R<1> and R<10> and R<1>, and R<10>' are H; or

- the other groups between R<1> and R<10> and R<1>, and R<10>' are H and F;

The iron molecular catalyst may comprise N(C1-C4 alkyl)3 groups in para or ortho position.

For example: the molecular catalyst is an iron porphyrin FeTnT of formula (II):

Or choose from these iron-containing molecules:

, preferably as its chloride salt.

With iron porphyrin as molecular catalyst, under the operating conditions: pH is between 7.3 and 14, and the potential applied to the cathode 9 is between 0 V and -1 V vs. RHE (reversible hydrogen electrode), the flow cell results can be: a current density for CO production of more than 5 and at least 150 mA.cm-2, and the selectivity of the electrochemical reaction which is between 98% and 99.9%.

In a second embodiment of the flow cell electrolyzer 1, the flow cell electrolyzer 1 comprises the molecular catalyst having the formula:

where Ri to Ri6 are independently selected from the groups consisting of H, F, C(CH3)3 or N(C1-C4 alkyl)3

In particular, Ri to Ri6 are H and the molecular catalyst is a cobalt phthalocyanine CoPc with the formula:

With phthalocyanine as a molecular catalyst, under the operating conditions: pH is between 7.3 and i4 , the potential applied to the cathode 9 is between -0.48 V and -0.98 V versus RHE, the flow cell results can be: a current density for CO production of more than i0 and at least 50 mAcm<-2>, and an electrochemical reaction selectivity between 90% and 92%.

In another variant, the cobalt molecular catalyst has at least i and at most 8 groups between Ri and Ri<6> which are independently the N(alkyl Ci -C<4>)<3> group.

In another variant, the cobalt molecular catalyst has, for one or more of the following specific groups Ri, R4, R5, R<8>, R9, Ri 2, Ri 3 and Ri<6>, a substituent N(alkyl Ci -C<4>)<3>.

For example, the molecular catalyst may be a cobalt phthalocyanine CoPc2 with the formula:

Using cobalt phthalocyanine with one or more N(C1-C4 alkyl)<3> groups as a molecular catalyst, under the operating conditions: pH between 7.3 and 14, and the potential applied to the cathode 9 between -0.3 V and -1 V versus RHE, the flow cell results can be: a current density for CO production of more than 10 and at least 150 mAcm-2, and a selectivity of the electrochemical reaction between 92% and 96%.

In a third embodiment of the flow cell electrolyzer 1, the flow cell electrolyzer 1 comprises the molecular catalyst having the formulas of cobalt quaterpyridine.

Using cobalt quaterpyridine as a molecular catalyst, under the operating conditions: pH 7.3–14, and the potential applied to cathode 9 between -0.6 V and -1 V versus RHE, the flow cell results can be: a current density for CO production between 25 and 200 mAcm-2, and an electrochemical reaction selectivity between 86.1% and 98.8%.

First realization

Preparation of hybrid materials for the gas diffusion electrode with FeTnT

7.2 mg of carbon black was dispersed in 8 ml of ethanol by sonication for 30 min. A solution of 2.08 mg of FeTn in 2 ml of ethanol was added to the carbon black suspension, followed by sonication for 30 min. 20 j l of a 5% w/w Nafion solution was added, followed by sonication for 30 min. The as-prepared ink was spotted at 65 °C on carbon paper masked with a PTFE frame to obtain a 1 x 1 cm gas diffusion current collector.

Results

The inventors included FeTNT in a flow cell configuration comprising FeTNT supported on a gas diffusion electrode as the cathode 9. Details of the setup are provided in the Supporting Information (see also Figures 14 and 16). In brief, the electrolyzer consists of a sandwich of flow frames, electrodes, gaskets, and an ion exchange membrane 10, which were assembled as schematically illustrated in Figure 14a. A CO2 gas stream is supplied from the back side of the cathode compartment and flows through the gas diffusion electrode (GDE), while the catholyte solution is circulated between the GDE and the anion exchange membrane (AEM) 10. On the other side of the AEM, the anolyte is directed between the AEM and the Pt/Ti alloy anode 2, Figure 14b. FeTNT was dispersed in a colloidal ink with carbon black and subsequently deposited on the carbon fiber paper, which composed the cathode 9.

A study of the catalytic activity as a function of applied potential was first performed in 0.5 M NaHCO<3> saturated with CO2. Figure 1 illustrates the increase in current density recorded for CO2RR as the overpotential was gradually established to more negative values in 100 mV steps, together with the selectivity for CO production (see also Figure 9). Initially, at the least negative potentials, CO was detected as the only product; whereas minor amounts of H2 arising from the HER (hydrogen evolution reaction) were observed at more negative potentials. The small amounts of the latter, even at potentials well beyond the onset of HER, reflect the ability of the catalyst to strongly suppress HER in favor of CO production. This was further verified by employing a non-catalytic film in the electrolyzer, i.e., a film formulated with the non-metallated porphyrin ligand, resulting in the exclusive production of H2 (see Figure 10). The average current density and CO selectivity at each applied potential are shown in Table 1 below.

Table 1. Current densities and CO selectivity obtained with a catalytic film with FeTNTas catalyst (pH 7.3) as a function of the potential applied at the cathode 9.

The resistance of the catalytic film was tested by performing chronoamperometric electrolysis for 24 hours. Based on the values listed in Table 1, a potential of -0.78 V versus RHE was chosen, as this potential compromises the high current density and the catalyst's ability to provide near-perfect CO production. The current density and CO selectivity obtained are shown in Figure 2, demonstrating the excellent stability of the system: a current density of 27.3 mAcirr<2> was maintained during the experiment, with an average CO selectivity of 98.2 ± 0.2%.

Under the same conditions, the cell performance was further evaluated by performing a chronopotentiometric electrolysis at a current density of 50 mAcirr<2>. As shown in Figure 3, the cell maintained this current value for 3 hours at a potential of -1.14 V vs. RHE. The product selectivity was not affected by the almost 2-fold increase in current density, as CO was produced with an average selectivity of 98.3 ± 0.3%.

To lower the cell potential, the ohmic resistance of the cell was reduced by increasing the electrolyte concentration. This was demonstrated in a 1.0 M NaHCO<3> electrolyte solution, where a gain of >8 mAcirr<2> was obtained at an applied potential of -0.78 V vs. RHE, without compromising CO selectivity (see Figure 11). In a chronopotentiometric electrolysis, see Figure 4, the applied potential at 50 mAcirr<2> was positively shifted to -0.86 V vs. RHE, and additionally, the cell potential was reduced by 590 mV. The CO product selectivity was 99.0 ± 0.2% on average.

A complementary approach to increasing cell performance was achieved by thermal activation of the catalyst. During a chronoamperometric electrolysis at -0.78 V vs. RHE, the catholyte temperature was gradually increased from 24 °C to 34 °C and finally to 40 °C. As shown in Figure 5A, an increase in current density was observed with increasing temperature, while the high CO product selectivity was maintained. Taking the TOF (in s-1) as a 1st-order apparent rate constant for CO<2> conversion (assuming steady-state conditions for the catalyst and proton source), an Arrhenius plot: ln(TOF) vs. 1/T (in K-1) was constructed (Figure 5B). From the slope, the activation energy, Ea, was calculated to be 12 kJmol-1.

Under alkaline conditions (1.0 M KOH, pH 14), cell performance was dramatically improved. Figure 6 shows the resulting current densities and CO product selectivity recorded at various potentials (see also Figure 12). A current density of 6 mAcirr2 was observed at 0.014 V vs. RHE, and 155 mAcirr2 was reached at -0.59 V vs. RHE, with excellent CO product selectivity, as summarized in Table 2. In a control experiment with the non-metallated porphyrin ligand, H<2> was the only product detected (see Figure 13).

Table 2. Current densities and CO selectivity obtained with a catalytic film with FeTNTas catalyst (pH 14) as a function of the potential applied at the cathode 9.

To investigate the catalyst's resistance under alkaline conditions, chronopotentiometric electrolysis was performed at 27 mAcirr<2> for 24 h, the same current density as that performed under neutral pH conditions. As shown in Figure 7, a potential of approximately -0.16 V was measured against RHE over the course of 24 hours, showing only a one-minute increase. The CO product selectivity remained extremely high: 99.7 ± 0.2% on average.

For direct comparison with the neutral pH 1.0 M bicarbonate system, a chronopotentiometric electrolysis was performed at 50 mAcirr<2> in 1.0 M KOH, as shown in Figure 8. Over the course of a 3 h electrolysis, the applied potential remained stable (-0.23 V vs. RHE), a very weak negative potential for such a high current density. Once again, the CO selectivity was almost perfect, with an average value of 99.8 ± 0.1%.

Second realization

The inventors included CoPc2 in the same flow cell configuration described in the first embodiment. CoPc2 was supported on a gas diffusion electrode as the cathode 9. Details of the configuration are provided in the Supporting Information (see Figure 16). CoPc2 was dispersed in a colloidal ink with carbon black and subsequently deposited onto the carbon fiber paper, which composed the cathode 9.

At -0.3 V vs. RHE and pH 14 (1 M KOH for the electrolyte), corresponding to a low overpotential of 200 mV, a high current density was achieved with j'co = 22.2 mA/cm2 (J<co is the partial current density for CO production). The current density dependence for CO production is indicated in Figure 15a (see also Figure 22). By setting the electrolysis potential to -0.72 V vs. RHE, j co was raised to 111.6 mA/cm<2> with a selectivity of 96%, while excellent stability was obtained over the course of the 3 h electrolysis (Figure 15b). The only additional by-product in the gas phase was H2 (4% selectivity), and the catholyte dissolution was carefully checked by H-NMR; Neither formate nor methanol was detected during the experiment. The j'co obtained corresponds to a turnover frequency (TOF) of 2.7 s<_1>and a turnover number (TON) of 29008. A maximum current density of 165 mA cirr<2>was obtained for CO generation at -0.92 V vs. RHE. The long-term stability of the catalytic material was assessed by applying a constant current density of 75 mA cirr<2>for 10 h, leading to a cathodic potential of -0.65 V vs. RHE (h = 540 mV) with a CO selectivity of 94% (j'<co>= 70.5 mA cm<-2>) (Figure 23). All collected data are compiled in Table 3, along with a comparison to previously reported catalysts. The cell type used (H cell vs. flow cell) is also indicated to facilitate comparison between the data. Co K-edge XANES spectra of CoPc2 on carbon black were recorded before and after electrocatalysis at E = -0.72 V vs. RHE. Figure 15c shows these spectra together with those of the starting CoPc2 complex. All these spectra exhibit the typical features expected of a cobalt(II) phthalocyanine complex, i.e., a low-intensity pre-edge peak at 7111 eV (corresponding to a 1s to 3d/4p transition) and a shoulder at 7717 eV (corresponding to a 1s to 4pz transition). Interestingly, Li et al.17 showed that the intensity of these two transitions depends on binding to a surface, i.e., the pre-edge intensity increases and the shoulder decreases upon adsorption on nanotubes, respectively. The same trend is observed in the CoPc2-on-carbon black system, with a decrease in intensities before the edge and an increase in those at the shoulder when going from the starting complex to the adsorbed species. This trend continues after catalysis, suggesting an even closer interaction with the surface while maintaining the overall structure of the molecular catalyst after the experiment. This structural conservation is further confirmed by the EXAFS spectra (Figure 24), which also display the typical features of a cobalt phthalocyanine complex.<38>Furthermore, comparison of the CoPc2-on-carbon black spectrum recorded after catalysis with that of the reference cobalt samples (Figure 24) clearly shows that the observed changes in the spectrum after catalysis are negligible with respect to the overall structure.

Surprisingly, CoPc2 remains highly selective for CO2 to CO conversion over a range of 10 pH units, from acidic (pH 4) to basic (pH 14) solutions. An average selectivity of 92% for CO2 reduction was routinely obtained with a partial current density of approximately 20 mA cm<-2> across the entire pH range, with excellent long-term stability. In near-neutral solutions (pH 7.3), CoPc2 is a significantly better catalyst than the unsubstituted phthalocyanine CoPc (see Table 3, entries 2 and 3) with approximately a 25% increase in current density at a similar overpotential, but it also outperforms the state-of-the-art tetracyano-substituted phthalocyanine (CoPc-CN, Table 3, entry 4) and the unsubstituted Co phthalocyanine polymerized around carbon nanotubes (CoPpc, Table 3, entry 5) in both current density and turnover frequency. Similar to the aforementioned cobalt phthalocyanines, CoPc2 exhibits excellent stability over time, showing no loss in performance in a 10.5 h electrolysis experiment.

Table 3. Comparison of electrolysis performances between the carbon powder CoPc2 hybrid catalyst and the previously reported state-of-the-art immobilized Co molecular catalysts and Ag nanomaterials.

A turnover frequency of up to 6.8 s<-1> was achieved, and generally, very low catalyst loading was required to obtain high jco (see, e.g., Table 3, entries 1-2). Long-term electrolysis (10 h) under basic conditions (pH 14) at -0.65 V versus RHE led to an average jco close to 70.5 mAcm<-2> and also illustrates the remarkable stability of the cobalt catalyst. The ability to implement CoPc2 under various pH conditions is also a key feature that may allow the Co catalyst to be combined with various types of anode materials to lower the overall cell potential. In particular, the excellent performance obtained at pH 14 should allow the CoPc2-loaded cathode 9 to be paired with the most efficient oxygen-evolving metal oxide anode 2 materials. At this pH, CoPc2 matches the state-of-the-art Ag-based catalyst sprayed onto a PTFE membrane, both in terms of selectivity and current density (Table 3, compare entries 10 and 12).

By introducing a positively charged trimethylammonium group onto the parent cobalt phthalocyanine, a highly efficient and versatile catalyst for the electrochemical conversion of CO2 to CO in water has been obtained. Furthermore, it operates with high selectivity (92–96%) over a wide pH range, from acidic (pH 4) to basic (pH 14) conditions. Under both acidic and neutral conditions, current densities close to 20 mA cm<-2> were routinely obtained. In a 1 M KOH electrolyte solution, the same current densities could be obtained with a very low overpotential of 200 mV, once the carbon-supported hybrid catalyst was included in a gas flow cell. At -0.92 V vs. RHE, a partial current for CO production of 165 mAcm<-2> was found with a catalytic selectivity of 94%. These results show that hybrid catalytic materials comprising only carbon and a molecular complex based on earth-abundant metals can compete with noble metal nanomaterials such as Ag and Au. This study highlights that rational tuning of the structure of simple metal complexes can enable high performance, with further improvements likely to follow. Finally, this work opens new prospects for the development of low-cost catalytic materials for inclusion in CO2 electrolyzers.

Additional information

Methods

All electrocatalytic reactions were carried out under an argon or CO2 atmosphere. Chemicals, including CoPc and supporting electrolytes, were obtained from commercial suppliers. The Supplementary Information section describes the synthesis and characterization of CoPc and FeTnT, as well as typical protocols for the electrocatalytic reduction of CO2.

Typical protocol for the electrocatalytic reduction of CO<2>. Following preparation of a catalytic ink containing FeTnT, CoPcl or CoPc2, and deposition of the material on porous carbon paper, controlled potential electrolysis was performed in a closed electrochemical cell or flow cell electrolyzer using a PARSTAT 4000 potentiostat (Princeton Applied Research). Gas chromatographic analyses of the gases evolved in the headspace during electrolysis were performed on an Agilent Technologies 7820A GC system equipped with a thermal conductivity detector. These conditions allowed simultaneous detection of H2, O2, N2, CO, and CO2. Calibration curves for H2 and CO were determined separately by injecting known amounts of pure gas.

General considerations

Chemicals and characterization methods

Chemicals and materials were purchased from Sigma-Aldrich, Fluka, TCI America, ABCR, or Alfa Aesar and used as received. All aqueous solutions were prepared with Millipore water (18.2 MQ cm). MWCNTs were purchased from Sigma-Aldrich (O.D.*L 6-9 nm*5 pm, >95%). Cobalt(II) phthalocyanine (CoPcl)(p-form, dye content 97%) was purchased from Sigma-Aldrich. Toray carbon paper (CAS number: 7782 42-5), TGP-H-60, 19 x 19 cm, was purchased from Alfa Aesar and used for the preparation of the cathodes 9 for the electrochemical cell. The 9 cathodes (gas diffusion electrodes) used in the flow cell were prepared using Freudenberg C24H5 carbon paper (21 x 29.7 cm, product code F5GDL). VULCAN® XC72R specialty carbon black was purchased from Cabot Corporation.

4-tert-Butylphthalonitrile was obtained from TCI America. 3-Nitrophthalonitrile was purchased from ABCR. All solvents were of synthetic grade. Infrared (IR) spectra were recorded on a Bio-Rad FTS 175C FTIR spectrophotometer. UV–visible absorption spectra were obtained using a Shimadzu 2001 UV spectrophotometer. High-resolution mass spectra were measured on an Agilent 6530 Accurate-Mass Q-TOF LC/MS spectrometer equipped with an electrospray ionization (ESI) source. NMR spectra were recorded in deuterated chloroform (CDCb) and THF-d8 on a Varian 500 MHz spectrometer. Melting points were recorded on a Stuart SMP apparatus.

Synthesis of FeTNT

A solution of 5,10,15,20-tetrakis(4-trimethylammoniophenyl)porphyrin tetrachloride (250 mg, 0.253 mmol, 1 eq) in water (500 ml) is prepared and placed under argon flow, Mohr's salt is added to the solution (810 mg, 3.75 mmol, 15 eq) and the solution is heated to 85 °C for 4 h. Ammonium hexafluorophosphate (2.5 g, 15 mmol, 60 eq) is added and the precipitate obtained is separated by centrifugation (10000 rpm, 30 min). The red solid obtained is rinsed once in pure water and separated again by centrifugation (10000 rpm, 30 min), then rinsed once in a 1 to 1 mixture of chloroform and acetone and separated by centrifugation (10000 rpm, 30 min). The residual solvent is evaporated under argon flow. Yield: 92% (343 mg) UV-Vis (DMF): Amax nm (log £) 407 (4), 568 (4.98).

The cobalt catalyst CoPc2 was prepared as illustrated here:

Synthesis of 3-(dimethylamino)phthalonitrile 1

3-Nitrophthalonitrile (2 g, 11.5 mmol) and dimethylamine hydrochloride (2.8 g, 34.6 mmol) were dissolved in anhydrous DMF (30 ml) under an argon atmosphere, and then finely powdered dry potassium carbonate (30 g, 217.5 mmol) was added portionwise over 15 min. The reaction mixture was stirred under argon at 65 °C for 24 h and then poured into water (250 ml). The resulting solid was collected by filtration and washed with water. After drying in vacuo, the crude product was recrystallized from ethanol. Yield: 80% (1.57 g). melting point 105 °C.<1>H-NMR (500 MHz, CdCb): 8, ppm, 3.18 (s, 6 H), 7.12 (d, 1 H), 7.15 (d, 1 H), 7.46 (t, 1 H).<13>C-NMR (125 MHz, CdCb): 8, ppm 172.4, 155.4, 133.1, 123.0, 120.5, 118.0, 116.6, 116.3, 110.0, 100.6, 42.7. FT-IR (v, cm<-1>): 2992, 2937, 2874, 2203, 1584, 1486, 1425, 1357, 1244, 1192, 1122, 1008, 792, 722.

Synthesis of phthalocyanine 3

Lithium pellets were added to anhydrous n-pentanol (10 ml). This mixture was heated to 60 °C under argon flow until complete consumption of the pellets. Then 3-(Dimethylamino)phthalonitrile1 (0.25 g, 1.46 mmol) and 4-tert-butylphthalonitrile2 (3.2 g, 17.52 mmol) were added and this reaction mixture was heated under reflux for 18 h, then cooled to room temperature and poured into an ethanol/water mixture. The resulting dark greenish-blue precipitate was filtered off, washed several times with water and dried. Phthalocyanine3 was isolated from this crude phthalocyanine mixture by chromatography on silica gel using a CH2Cl2/EtOH (100:1) mixture as eluent. Tetra-tert-butylphthalocyanine4 was eluted first and phthalocyanine3 was the second compound to elute. Yield: 15% (160 mg).<1>H-NMR (500 MHz, THF-da): 8, ppm -2.04 (s, 1H), -1.95 (s, 1H), 1.84 (m, 10H), 1.88-1.94 (m, 17H), 3.70 (s, 3H), 7.45 (d, 1H), 7.79 (m, 1H), 8.10-8.40 (m, 4H), 8.61 (d, 1H), 8.85-8.91 (m, 1H), 8.98- 9.24 (m, 4H),<13>C-NMR (125 MHz, THF-da): 8, ppm 152.97, 152.94, 152.88, 152.84, 152.61, 152.58, 150.16, 150.12, 133.67, 129.92, 127.43, 127.30, 127.22, 126.75, 125.50, 122.12, 122.07, 121.93, 121.90, 121.77, 118.69, 118.65, 118.51, 118.48, 118.41, 117.87, 117.83, 114.74, 78.54, 78.28, 78.02, 44.36, 39.98, 37.15, 37.06, 35.67, 35.60, 35.50, 31.89, 31.43, 31.32, 29.65, 29.32, 22.58, 13.43. ESI-HRMS: m/z 726.4030 [M]+ calculated for C46H47N9: 725.95. UV-Vis (DMF): Amax. nm (log e) 346 (3.90), 689 (4.87), 718 (5.38). FT-IR (v, cm<’ 1>): 3288, 2955, 2863, 2776, 1618, 1501, 1316, 1186, 1014, 828, 742.

Synthesis of phthalocyanine 5

A mixture of phthalocyanine3 (50 mg, 0.06 mmol), CoCb (18 mg, 0.12 mmol) and DBU (1 ml) in dry n-pentanol (5 ml) was heated under reflux for 18 h under argon. After cooling to room temperature, the reaction mixture was poured into an ethanol/water mixture. The resulting precipitate was filtered off and washed several times with water. Phthalocyanine5 was purified by chromatography on silica gel using a mixture of CH2Cb/EtOH (50:1) as eluent. Yield: 75% (35 mg). ESI-HRMS: m/z 782.3234 [M]+ calcd for C<46>H<45>CoNg: 782.86. UV-Vis (DMF): Amax. nm (log e) 332 (3.70), 693 (4.74); FT-IR (v, cm<-1>): 2955, 2856, 1606, 1457, 1316, 1254, 1094, 804, 737.

Synthesis of phthalocyanine CoPc2

Phthalocyanine 5 (35 mg, 0.044 mmol) was dissolved in DMF (5 mL) and methyl iodide (0.2 g, 1.5 mmol) was added. The mixture was stirred at room temperature for 16 h and then poured into diethyl ether (25 mL). The resulting blue precipitate was filtered off, washed with ether, and dried. Yield: 32 mg (88%). ESI-HRMS: m/z 797.3668 [M]+ calcd for C<47>H<48>CoNg: 797.90. Anal. calcl. for C<47>H<58>CoINgO<5>(CoPc2.5H<2>O): C, 55.28; H, 5.98; N, 11.88; Found: C, 55.62; H, 5.76; N, 12.42; UV-Vis (DMF): Amax. nm (log e) 326 (4.41), 664 (4.65). FT-IR (v, cm<-1>) 3047, 2949, 2851, 1655, 1606, 1476, 1328, 1254, 1088, 933, 829, 749.

Preparation of hybrid materials

For CV experiments and electrolysis in the closed electrolysis cell, 3 mg of MWCNTs were dispersed in 2 mL of 1:1 (v/v) ethylene glycol (EG)/ethanol (EtOH) mixture, followed by 30 min of sonication. 1 mg of cobalt (CoPcl, CoPc2) catalyst was dissolved in 1 mL of EG/EtOH mixture. Various volumes of this solution were added to the MWCNT suspension in a total volume of 3 mL, to obtain mass ratios (1:6, 1:15, and 1:30) of the catalyst. The suspension was further sonicated for 30 min. Finally, Nafion® (2.9%, 30 pl) was added, and the entire mixture was sonicated for 30 min to obtain the final catalytic ink.

For the flow cell setup, 3 mg of carbon black was dispersed in 3 mL of EtOH, followed by 30 min of sonication. 0.2 mg of CoPc2 was dissolved in 1 mL of EtOH to obtain a mass ratio (1:15) of the catalyst. The suspension was further sonicated for 30 min. Finally, Nafion® (2.9%, 30 pl) was added and the entire mixture was sonicated for 30 min to obtain the final catalytic ink. The ink was drop-cast onto carbon paper masked with a Teflon frame to obtain an electrode area of 1*1 cm<2>.

Electrochemical studies

Controlled potential electrolyses were performed using a PARSTAT 4000 potentiostat (Princeton Applied Research).

Preparative scale electrolysis

In the closed cell, experiments were carried out in a cell using a Toray carbon paper as the working electrode and an SCE reference electrode placed in close proximity to each other. The Pt grid counter electrode was separated from the cathode compartment with a glass frit. The catalytic ink was dropped onto one side of the Toray carbon paper cathode 9 (100 pl for a 0.5 cm<2> electrode) and allowed to dry under ambient conditions before use. The complete cell setup was identical to that used previously.<18>

The flow cell electrolyzer 1 (Micro Flow Cell® purchased by Electrocell) is composed of a sandwich of flow frames, electrodes, gaskets, and a membrane that, when assembled as illustrated in Figure 14, constitute a three-compartment flow cell. One compartment delivers CO2 (at 16.7 sccm) from the back and through the gas diffusion electrode (GDE, 1x1 cm<2>, fixed on a Pt frame), while another directs the catholyte solution (1 M KOH, flow rate 16 sccm) between the GDE and the anion exchange membrane 10 (AEM, Sustainion™ X37-50). On the other side of the latter, the anolyte (1 M K<o>H, flow rate of 16 sccm) is directed between the AEM and the Pt/Ti 2 alloy anode. The flow frames are made of PTFE and the gaskets are made of peroxide-cured EDPM. The catholyte and anolyte were recycled using peristaltic pumps. All tubing was made of PTFE and connected to the cell with PEEK ferrules and fittings. The complete setup is shown schematically below (Figure 16).

Gas detection

Gas chromatographic analyses of the gases sampled from the headspace during electrolysis were performed using an Agilent Technologies 7820A GC system equipped with a thermal conductivity detector. CO and H2 production were quantitatively detected using a CP-CarboPlot P7 capillary column (27.46 m length and 25 pm internal diameter). The temperature was maintained at 150 °C for the detector and 34 °C for the furnace. The carrier gas was argon flowing at 9.5 ml/min at a constant pressure of 0.4 bar. Injection was performed through a 250 pl gas-tight syringe (Hamilton) previously degassed with CO2. These conditions allowed simultaneous detection of H2, O2, N2, CO, and CO2. Calibration curves for H2 and CO were determined separately by injecting known amounts of pure gas.

XPS Analysis

A THERMO-VG ESCALAB 250 X-ray photoelectron spectrometer (K Al (1486.6 eV) RX source) was used.

XAS data collection and analysis

X-ray absorption spectra (XAS) were collected at SOLEIL's LUCIA beamline with a ring energy of 2.75 GeV and a current of 490 mA. The energy was monochromatized by a Si(111) double-crystal monochromator. Data were collected in a primary vacuum chamber as fluorescence spectra at an exit angle of 5° using a Bruker silicon offset detector. Data were normalized to the incoming incident energy intensity and processed with Athena software from the IFEFFIT package. For EXAFS analysis, an E0 value of 7722.0 eV was used for the cobalt K-edge hopping energy.

SEM analysis

Scanning electron microscopy using a field emission gun (SEM-FEG) was performed using a Zeiss Supra 40 device.

Third realization

The inventors tested cobalt quaterpyridine (Co(qpy)) in the same cell as in the first embodiment. Two types of experiments were performed with Co(qpy). The faradaic efficiency of CO was measured by chronopotentiometry at various current densities, and the catalyst stability was determined at 50 mA/cm<2>.

During the chronopotentiometry experiment, Co(qpy) showed faradaic efficiencies for CO exceeding 90% at current densities below 100 mA/cm<2>. Then, at higher current densities, the faradaic efficiency of CO decreases dramatically.

In Figure 26, Co(qpy) can maintain a faradaic efficiency above 90% for CO at 50 mAcm-2 for 12 hours. The decline is constant up to 24 hours of experiments. Then, the faradaic efficiency drops below 40% for CO.

References

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Claims (9)

CLAIMS A flow cell electrolyzer (1) for electrochemically reducing a gaseous reactant comprising CO2 to gaseous CO, with: - an anode compartment comprising: • an anode with a current collector, and on the current collector, at least one catalyst for electrochemically oxidizing H2O to O2, • an anode electrolyte solution (3), at a controlled flow rate Qa, comprising: a solvent and an anode electrolyte, the solvent being water, - an anode electrolyte solution inlet (4) and an anode electrolyte solution outlet (5) connected to the anode compartment, for circulating the anode electrolyte solution (3); - a cathode compartment comprising: • a cathode electrolyte solution (6), at a controlled flow rate Qc, comprising: a solvent and a cathode electrolyte, the solvent being water, • a porous gas diffusion cathode (9) comprising, in a porous gas diffusion cathode current collector that is electrochemically inert, at least one molecular catalyst incorporated in the porous cathode with a surface area S, for electrochemically reducing the gas comprising CO2 to gaseous CO, with a by-production of gaseous H2, the molecular catalyst being selected from the list: ❖ metallic tetraphenylporphyrin comprising at least one or more N(C<1>-C<4>alkyl)3 groups on the phenyl moieties, with the metal chosen from: iron, cobalt; wherein the iron tetraphenylporphyrin has the formula: where: °at least 1 and at most 8 groups between R<1> and R<10> and Rr and R<10>' are independently an N(C<1>-C<4>alkyl)<3> group, °the other groups R<1> to R<10> and Rr to R<10>' are independently selected from the group consisting of H, OH, F, C(CH<3)3> ❖ metallic phthalocyanine, with the metal chosen from: iron, cobalt; ❖ metallic phthalocyanine with the metal chosen from: iron, cobalt and substituted with one or more groups from: N(C1-C4)3 alkyl, F, C(CH3)3; and ❖ cobalt quaterpyridine; - an anion exchange membrane (10), impermeable to CO2, CO, H2 and O2, between the anode compartment and the cathode compartment; - a channel (11) for flowing the reactive gas CO2, at a controlled flow rate Qg, over or through the surface S of the porous gas diffusion cathode collector - a cathode electrolyte solution inlet (7) and a cathode electrolyte solution outlet (8) connected to the cathode compartment, for circulating the cathode electrolyte solution and the remaining reactive gas CO<2> and the product gas CO through the outlet (8); -pumping means (12) serving to: -circulate by pumping the anodic electrolyte solution (3) and the cathodic electrolyte solution (6) between the inlet (4, 7) and the outlet (5, 8), -pump the reactant gas CO<2>to or through the porous gas diffusion cathode (9), the pumping means being configured to control all flow rates of the cathodic and anodic electrolytes, as well as the reactant gas CO<2>passing over or through the surface S of the porous gas diffusion cathode (9); -a power source that provides the energy necessary to trigger the electrochemical reactions involving the reactant. 2. The flow cell electrolyzer (1) of claim 1, wherein the anodic and/or cathodic electrolyte solution has a neutral or basic pH. 3. The flow cell electrolyzer (1) of claim 1 or 2, wherein the anodic electrolyte solution and/or the cathodic electrolyte solution have a pH of 9 to 14. 4. The flow cell electrolyzer (1) of any one of claims 1 to 3, wherein the molecular catalyst is an iron tetraphenylporphyrin catalyst, wherein: - the other groups between R<1> to R<10> and Rr to R<10>' are H; or - the other groups between R<1> to R<10> and Rr to R<10>' are H and F. 5. The flow cell electrolyzer (1) of any one of claims 1-4, wherein the iron tetraphenylporphyrin catalyst comprises N(C<1>-C<4)3>alkyl groups in the para or ortho position. 6. The flow cell electrolyzer (1) of claims 1-3, wherein the molecular catalyst has the formulas: wherein R1 to R16 are independently selected from the groups consisting of H, F, C(CH3)3, or N(C<1>-C<4>alkyl)<3>- 7. The flow cell electrolyzer (1) of claim 6, wherein R1 to R16 are H. 8. The flow cell electrolyzer (1) of claim 6, wherein the cobalt molecular catalyst has at least 1 and at most 8 groups between R1 and R16 that are independently the N(C<1>-C<4>alkyl)<3> group. 9. The flow cell electrolyzer (1) of claim 8, wherein the cobalt molecular catalyst has one or more of the following specific groups R1, R4, R5, R<8>, R9, R12, R13, and R16, and an N(C<1>-C<4>alkyl)<3> substituent. The flow cell electrolyzer (1) of any one of claims 1-9, wherein the cathodic electrolyte comprises a phosphate buffer or potassium hydroxide, and the anodic electrolyte comprises a phosphate buffer or potassium hydroxide. The flow cell electrolyzer (1) of any one of claims 1-10, wherein the cathodic current collector is carbon paper. The flow cell electrolyzer (1) of any of claims 1-11, wherein the porous cathode (9) comprises, on the current collector, an electrode film containing polymers with the molecular catalysts and wherein the electrode film is deposited or grafted onto the current collector.