EP3770302A1

EP3770302A1 - Iron and cobalt molecular complexes for the selective electrochemical reduction of co2 into co, with flow cells

Info

Publication number: EP3770302A1
Application number: EP19305971.4A
Authority: EP
Inventors: Waldemar Braun; Dorian JOULIE; Benjamin BOUDY; Kristian TORBENSEN; Marc Robert
Original assignee: Centre National de la Recherche Scientifique CNRS; Air Liquide SA; Universite Paris Diderot Paris 7; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Centre National de la Recherche Scientifique CNRS; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Universite Paris Cite
Priority date: 2019-07-22
Filing date: 2019-07-22
Publication date: 2021-01-27
Also published as: US20220251715A1; WO2021013530A1

Abstract

The present invention concerns a flow cell electrolyzer (1) to electrochemically reduce reagent gas CO2 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 a catalyst to electrochemically oxidize H2O to O2, an anodic electrolyte solution (3) at a controlled flow rate Qa, comprising: a solvent, and an anodic electrolyte, the solvent being water at neutral or basic pH ; a cathodic compartment comprising a cathodic electrolyte solution (6), at a controlled flow rate Qc, comprising: the solvent, and a cathodic electrolyte, the solvent being water at neutral or basic pH, a gas diffusion porous cathode (9) which comprises, on a gas diffusion porous current cathode collector which is electrochemically inert, at least a molecular catalyst on a surface S to electrochemically reduce CO2 into CO, with a by-production of H2, the molecular catalyst being chosen between the list with the metal chosen among: Iron, Cobalt: metal porphyrin with one or several +N(C1-C4 alkyl)3 groups, metal phthalocyanine, metal phthalocyanine with one or several +N(C1-C4 alkyl)3 groups, or cobalt quater pyridine.

Description

FIELD OF THE INVENTION

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

STATE OF ART

Despite the increasingly frequent use of renewable energies to produce electricity and avoid concomitant production of CO₂, it is reasonable to consider that CO₂ emissions, in particular resulting from energy production, will remain high in the next decades. Thus, it appears necessary to find ways to capture CO₂ gas, either for storing or valorization purposes.
Indeed, CO₂ can also be seen, not as a waste, but on the contrary as a source of carbon. For example the promising production of synthetic fuels from CO₂ and water has been envisaged.
However, CO₂ exhibits low chemical reactivity: breaking its bonds requires an energy of 724 kJ/mol. Moreover, CO₂ electrochemical reduction to one electron occurs at a very negative potential, thus necessitating a high energy input, and leads to the formation of a highly energetic radical anion (CO₂ ^.-). Catalysis thus appears mandatory in order to reduce CO₂ and drive the process to multi-electronic and multi-proton reduction process, in order to obtain thermodynamically stable molecules. In addition, direct electrochemical reduction of CO₂ at inert electrodes is poorly selective, yielding formic acid in water, while it yields a mixture of oxalate, formate and carbon monoxide in low-acidity solvents such as DMF.
CO₂ electrochemical reduction thus requires catalytic activation in order to reduce the energy cost of processing, and increase the selectivity of the species formed in the reaction process.
Molecular catalysts that combine high selectivity and high current density for CO₂ electrochemical reduction to CO or other chemical feedstocks, are in high demand. Molecular transition metal catalysts offer the distinct advantage of allowing for the 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 the rational optimization of the ligand structure is a feature not accessible to the solid-state catalysts common to pilot-scale electrolyser units.^1-2 There now exists a range of known molecular catalysts, including those based on noble (e.g. Ru, Ir and Re) and earth-abundant metals (e.g. Co, Ni, Fe, Mn and Cu). ^1-9 These catalysts typically trigger a two electron reduction of CO₂ to either CO or formate with reasonable efficiencies, but in organic solvents.
The integration of 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 in pure aqueous conditions. Reasonable performances have been obtained in nearly neutral conditions (pH 7-7.5) with good selectivity.^10,11 While these performances represent advances for CO2RR (CO₂ reduction reaction) catalysts, much higher current densities are required for commercial operation, being highly selective and operating at low overpotentials at the same time. Moreover, these current densities remain far below those obtained with state-of-the-art solid-state Ag^12,13 or Au¹⁴ nanomaterials have been reported to reach >150 mA/cm².

SUMMARY OF THE INVENTION

Here, the invention presents a flow cell electrolyzer to electrochemically reduce reagent gas CO₂ into gaseous CO and gaseous H₂, with:

an anodic compartment comprising:
- an anode with a current collector, and on the current collector, at least a catalyst to electrochemically oxidize H₂O to O₂,
- an anodic electrolyte solution, at a controlled flow rate Q_a, comprising: a solvent, and an anodic electrolyte, the solvent being water,
the anodic compartment being connected to an anodic electrolyte solution inlet and an anodic electrolyte solution outlet;
a cathodic compartment comprising:
- a cathodic electrolyte solution, at a controlled flow rate Q_c, comprising: a solvent, and a cathodic electrolyte, the solvent being water, the cathodic compartment being connected to a cathodic electrolyte solution inlet and a cathodic electrolyte solution outlet,
- a gas diffusion porous cathode which comprises, on a gas diffusion porous current cathode collector which is electrochemically inert, at least a molecular catalyst incorporated in the porous cathode with a surface S, to electrochemically reduce CO₂ into CO, with a by-production of H₂,
the molecular catalyst being chosen between the list:
- ❖ metal porphyrin with one or several +N(C₁-C₄ alkyl)₃ groups, with the metal chosen among: Iron, Cobalt;
- ❖ metal phthalocyanine, with the metal chosen among: Iron, Cobalt;
- ❖ metal phthalocyanine with the metal chosen among: Iron, Cobalt, with one or several groups among: +N(C₁-C₄ alkyl)₃, F, C(CH₃), or
- ❖ cobalt quater pyridine
an anion exchange membrane, impermeable to CO₂, CO, H₂ and O₂, between the anodic compartment and the cathodic compartment;
a channel for flowing the reagent gas CO₂, at a controlled flow rate Q_g, onto or through the surface S of the gas diffusion porous current cathode collector;
all flow rates are controlled by passing the cathodic and anodic electrolytes as well as the reagent gas through pump means serving to:
- ∘ circulate by pumping the anodic electrolyte solution and the cathodic electrolyte solution between the inlet and the outlet,
- ∘ flow by pumping the reagent gas CO₂ onto or through the gas diffusion porous cathode;
a power supply providing the energy necessary to trigger the electrochemical reactions involving the reagent.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the disclosed devices and methods 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 CO₂ saturated 0.5 M NaHCO₃ (pH 7.3). Each potential step was maintained for 20 min.
Figure 2. Current density and CO selectivity recorded during a 24 hour electrolysis at -0.78 V vs RHE in CO₂ saturated 0.5 M NaHCO₃. The cell potential was 3.41 V.
Figure 3. Applied potential and CO selectivity recorded under chronopotentiometric conditions at 50 mAcm^-2 for 3 hours in CO₂ saturated 0.5 M NaHCO₃. The cell potential was 4.05 V.
Figure 4. Applied potential and CO selectivity recorded under chronopotentiometric conditions at 50 mAcm^-2 for 3 hours in CO₂ saturated 1.0 M NaHCO₃. The cell potential was 3.46 V.
Figure 5. (A) Current density and CO selectivity at various cell temperatures. Recorded in CO₂ saturated 0.5 M NaHCO₃. (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 1.0 M KOH solution (pH 14). Each potential step was maintained for 20 min.
Figure 7. Applied potential and CO selectivity recorded under chronopotentiometric conditions at 27 mAcm^-2 for 24 hours in 1.0 M KOH. The cell potential was 1.87 V.
Figure 8. Applied potential and CO selectivity recorded under chronopotentiometric conditions at 50 mAcm^-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 CO₂ saturated 0.5 M NaHCO₃ (pH 7.3). Each potential step was maintained for 20 min.
Figure 10. Step potential experiment, giving the current density recorded at various potentials. Recorded in argon saturated 0.5 M NaHCO₃ (pH 7.3) at a film formulated with the non-metalated porphyrin ligand. Each potential step was maintained for 20 min.
Figure 11. Current density and CO selectivity recorded during a 6 hour electrolysis at -0.78 V vs RHE in CO₂ saturated 1 M NaHCO₃.
Figure 12. Step potential experiment, giving the current density recorded at various potentials. Recorded in 1 M KOH (pH 14). Each potential step was maintained for 20 min.
Figure 13. Step potential experiment, giving the current density recorded at various potentials. Recorded in 1 M KOH (pH 7.3) at a film formulated with the non-metalated porphyrin ligand. Each potential step was maintained for 20 min.
Figure 14. Cross-sectional view (a) and general scheme (b) of the CO₂ electrolyzer flow cell.
Figure 15. a: Current density for CO production as a function of the potential. b: Bulk electrolysis at fixed potential (E = - 0.72 V vs. RHE) for CoPc2@carbon black deposited onto a carbon paper as cathodic material, in 1 M KOH. c: Co K-edge XANES profiles of CoPc2 (black dots), and CoPc2@carbon black before and after electrolysis (E = - 0.72 V vs. RHE) which are superposed, in 1 M KOH solution.
Figure 16. Flow cell electrolyzer set-up.
Figure 17. Repetitive cyclic voltammetry for CoPc1@MWCNTs (bottom) and CoPc2@MWCNTs (top) films deposited onto a glassy carbon electrode (d = 3 mm) in 0.5 M NaHCO₃ solution saturated with CO₂ (pH 7.3; black, 1^st scan; grey, 2^nd scan), at v = 0.1 V s^-1.
Figure 18. SEM image of a catalytic film deposited onto carbon paper.
Figure 19. Blank experiments: Left, at E = -0.676 V vs. RHE) in the absence of catalyst in CO₂ saturated 0.5 M NaHCO₃ solution (pH =7.3); right, at E = -0.605 V vs. RHE with CoPc2@MWCNTs (1:15) in Ar saturated 0.5 M NaHCO₃ solution (pH =8.5). In both cases, only H₂ was detected as reaction product.
Figure 20. Variation of the total current density as a function of the electrolysis potential at optimized mass ratio for CoPc1@MWCNTs in a CO₂ saturated solution containing 0.5 M NaHCO₃ (pH 7.3).
Figure 21. Co 2p (left) and N 1s (right) XPS spectra before and after a 7h electrolysis (E = -0.676 V vs. RHE) with CoPc2@MWCNTs with a 1:15 ratio in a CO₂ saturated solution with 0.5 M NaHCO₃ (pH 7.3).
Figure 22. Bulk electrolysis at E= -0.971 V vs. RHE of a CO₂ saturated solution containing 0.5 M KCI (pH 4) using CoPc2@MWCNTs as catalyst.
Figure 23. Total current density as a function of the applied potential for CoPc2@carbon black deposited onto a carbon paper as cathodic catalytic material, in 1 M KOH solution (flow cell device, see main text). Each potential step was held for a duration of 20 min.
Figure 24. Top: K-space (insert) and Fourier transform EXAFS spectra of CoPc2@carbon black before (black) and after electrolysis (E= -0.72 V vs RHE) (grey) in 1 M KOH solution.
Bottom: Co K-edge XANES spectra of the CoPc2@carbon black electrode after electrolysis (E = -0.72 V vs RHE) (red) together with those of reference compounds CoO (purple), CoOOH (orange), Co₃O₄ (blue) and metallic Co (green).
Figure 25. Bulk electrolysis at fixed current density (75 mA cm^-2) with CoPc2@carbon black deposited onto a carbon paper as cathodic catalytic material, in 1 M KOH solution (flow cell device, see main text). During the electrolysis, 2.5 g KOH were added every half hour into the cathode solution to maintain the pH balance.
Figure 26. CO Selectivity as a function of time in hours at 50 mA/cm² with Co(qpy) catalyst in a zero gap cell device.

DETAILED DESCRIPTION

The present invention concerns: a flow cell electrolyzer 1 to electrochemically reduce a gas reactant comprising CO₂, 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 a catalyst to electrochemically oxidize H₂O to O₂,
- an anodic electrolyte solution 3, at a controlled flow rate Q_a, comprising: a solvent, and an anodic electrolyte, the solvent being water, the anodic compartment being connected to an anodic electrolyte solution inlet 4 and an anodic electrolyte solution 3 outlet;
a cathodic compartment comprising:
- a cathodic electrolyte solution 6, at a controlled flow rate Q_c, comprising: a solvent, and a cathodic electrolyte, the solvent being water, the cathodic compartment being connected to a cathodic electrolyte solution inlet 7 and a cathodic electrolyte solution outlet 8,
- a gas diffusion porous cathode 9 which comprises, on a gas diffusion porous current cathode collector which is electrochemically inert, at least a molecular catalyst incorporated in the porous cathode with a surface S, to electrochemically reduce CO₂ into CO, with a by-production of H₂.

The cathodic electrolyte can comprise a phosphate buffer or potassium hydroxide, and the anodic electrolyte comprises a phosphate buffer or potassium hydroxide.
The molecular catalyst is chosen between the list:

❖ metal porphyrin with one or several +N(C₁-C₄ alkyl)₃ groups, with the metal chosen among: Iron, Cobalt
❖ metal phthalocyanine, with the metal chosen among: Iron, Cobalt
❖ metal phthalocyanine with the metal chosen among: Iron, Cobalt, with one or several groups among : +N(C₁-C₄ alkyl)₃, F, C(CH₃)₃, or
❖ cobalt quater pyridine

The flow cell electrolyzer 1 comprises also:

an anion exchange membrane 10, impermeable to CO₂, CO, H₂ and O₂, between the anodic compartment and the cathodic compartment;
a channel 11 for flowing the reagent gas CO₂, at a controlled flow rate Q_g, onto or through the surface S of the gas diffusion porous current cathode collector;
a power supply providing the energy necessary to trigger the electrochemical reactions involving the reagent.

In the flow cell electrolyzer 1, all flow rates are controlled by passing the cathodic and anodic electrolytes as well as the reagent gas through pump means 12 serving to:

∘ circulate by pumping the anodic electrolyte solution 3 and the cathodic electrolyte solution 6 between the inlets 4, and the outlets, 5,
∘ flow by pumping the reagent gas CO₂ onto or through the gas diffusion porous cathode 9;

In an example, the gas reactant comprises at least 99 % per volume CO₂, or at least 99,5 % per volume CO₂, or at least 99,9 % per volume CO₂, or at least 99,99 % per volume CO₂.
The flow cell 1 can have a device capable of recording the current generated by the electrochemical processes.
Advantageously, but in a non-limiting way, the channel for flowing the reagent gas CO₂ can comprise a flow frame generating a turbulent flow.
Advantageously, but in a non-limiting way, pumping means 12 to 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 has a pH from 9 to 14, preferably from 10 to 14, more preferably from 11.5 to 14, and more preferably from 13 to 14.
In yet another preferred embodiment, the cathodic electrolyte solution has a pH from 9 to 14, preferably from 10 to 14, more preferably from 11.5 to 14, and more 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, in particular when applied to the cathodic electrolyte solution, allow operating the flow cell electrolyzer at surprisingly high current densities in comparison to acidic conditions.
The combination of basic conditions with the aforementioned molecular porphyrin, phthalocyanine or quater pyridine catalysts further result in surprisingly high selectivities for the desired conversion of CO₂ to CO, with only minor or even no side product formation. Thus, with the high current densities and high CO selectivity, the invention allows production of close-to-pure CO from CO₂ at high throughput rates.
Furthermore, it was found that the aforementioned molecular catalysts demonstrate high long-term stability even at harsh alkaline pH conditions.
The electrochemical reduction of the reagent into CO and H₂ can be carried out at ambient temperature.
The electrochemical reduction of the reagent into CO and H₂ can be carried out at atmospheric pressure.
Advantageously, the cathodic current collector can be carbon paper. On the carbon paper, a composite multilayered porous current collector (for instance a polytetrafluoroethylene gas distribution layer) can be filed to increase the reduction rate/efficiency.
In a realization, the porous cathode 9 comprises on the current collector an electrode film which, at least, contains polymers and the molecular catalysts. The electrode film can be filed in or grafted on the current collector.
Advantageously, when the molecular catalyst is a metal porphyrin, the metal porphyrin is a tetraphenylporphyrin which comprises specific groups on the phenyl moieties, with at least one or several +N(C₁-C₄ alkyl)₃ groups among specific groups.
In a first embodiment of the flow cell electrolyzer 1, the molecular catalyst is the iron porphyrin with the formula:
Wherein:

∘ at least 1 and at most 8 groups among R₁ to R₁₀ and R_1' to R_10' being independently +N(C₁-C₄ alkyl)₃ group,
∘ the remaining groups R₁ to R₁₀ 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 can comprise:

the other groups among R₁ to R₁₀ and R_1' to R_10' are H; or
the other groups among R₁ to R₁₀ and R_1' to R_10' are H and F;

₁

₄

₃

For instance: the molecular catalyst is an iron porphyrin FeTNT of formula (II):
Or chosen among these iron containing molecules:

,preferably as its chloride salt.
With the iron porphyrin as molecular catalyst, at the operational conditions: the pH is between 7.3 and 14, and the potential applied to the cathode 9 is between 0 V and -1 V versus RHE (Reversible Hydrogen Electrode), the results of the flow cell 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 which presents the formula:
wherein R₁ to R₁₆ are independently selected from the groups consisting of H, F, C(CH₃)₃ or +N(C₁-C₄ alkyl)₃
In particular, R₁ to R₁₆ are H, and the molecular catalyst is a cobalt phthalocyanine CoPc of formula:
With the phthalocyanine as molecular catalyst, at the operational conditions: the pH is between 7.3 and 14, the potential applied to the cathode 9 is between -0.48 V and -0.98 V versus RHE, the results of the flow cell can be: a current density for CO production of more than 10 and at least 50 mAcm^-2, and a selectivity of the electrochemical reaction between 90% and 92%.
In another variant, the cobalt molecular catalyst presents at least 1 and at most 8 groups among R₁ to R₁₆ being independently +N(C₁-C₄ alkyl)₃ group.
In another variant, the cobalt molecular catalyst presents for one or several of the specific following R₁, R₄, R₅, R₈, R₉, R₁₂, R₁₃ and R₁₆ groups, a +N(C₁-C₄ alkyl)₃substituent.
For instance the molecular catalyst can be a cobalt phthalocyanine CoPc2 of formula:
or the molecular catalyst is a cobalt phthalocyanine CoPc3 of formula:
With the cobalt phthalocyanine with one or several +N(C₁-C₄ alkyl)₃ groups, as molecular catalyst, at the operational conditions: the pH is between 7.3 and 14, and the potential applied to the cathode 9 is between -0.3 V and -1 V versus RHE, the results of the flow cell 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 is between 92 % and 96 %.
In a third embodiment of the flow cell electrolyzer 1, the flow cell electrolyzer 1 comprises the molecular catalyst which presents the formulae of the cobalt quater pyridine
With the cobalt quater pyridine as molecular catalyst, at the operational conditions: the pH is between 7.3 and 14, and the potential applied to the cathode 9 is between -0.6 V and -1 V versus RHE, the results of the flow cell can be: a current density for CO production is between 25 and 200 mAcm^-2, and the selectivity of the electrochemical reaction is between 86.1 % and 98.8 %.

FIRST EMBODIMENT

Preparation of the hybrid materials for the gas diffusion electrode with FeTNT

7.2 mg carbon black was dispersed in 8 mL ethanol by sonication for 30 min. A solution of 2.08 mg FeTNT in 2 mL ethanol was added to the carbon black suspension followed by sonication for 30 min. 20 µL of a 5% w/w Nafion solution was added followed by sonication for 30 min. The as prepared ink was disposed at 65°C on carbon paper masked with a PTFE frame to obtain a 1x1 cm² gas diffusion current collector.

Results

The inventors included FeTNT into a flow cell setup 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). Briefly, 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 gas flow of CO₂ is delivered from the back side of the cathodic compartment and flows through the gas diffusion electrode (GDE), while the catholyte solution is circulated in between the GDE and the anion exchange membrane 10 (AEM). 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, composing the cathode 9.
A survey of the catalytic activity as a function of applied potential was first performed in CO₂ saturated 0.5 M NaHCO₃. Figure 1 illustrates the increase in current density recorded for the CO₂RR as the overpotential was gradually set to more negative values by steps of 100 mV, along with the selectivity for CO production (see also Figure 9). Initially, at the less negative potentials, CO was detected as the only product; whereas minor amounts of H₂ stemming from the HER (Hydrogen Evolution Reaction) was observed at more negative potentials. The small amounts of the latter, even at potentials well beyond the HER onset, reflects the ability of the catalyst to strongly suppress the HER in favour of CO production. This was further verified by employing a non-catalytic film in the electrolyzer, i.e., a film formulated with the non-metalated porphyrin ligand, resulting in H₂ production exclusively (see Figure 10). The average current density and the CO selectivity at each applied potential is collected in Table 1 hereafter. 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.

E/V vs RHE j_av/mAcm^-2 % CO

-0.38 7.2 99.9

-0.48 9.8 99.9

-0.58 14.8 99.8

-0.68 20.7 99.5

-0.78 27.2 99.3

-0.88 34.9 98.9

-0.98 42.1 98.6
The endurance of the catalytic film was tested by performing a chronoamperometric electrolysis for 24 hours. Based on the values listed in Table 1, a potential of -0.78 V vs RHE was chosen as this potential compromises between high current density and the ability of the catalyst to provide a nearly perfect CO production. The current density and the CO selectivity that have been obtained are shown in Figure 2, demonstrating the excellent stability of the system: a current density of 27.3 mAcm^-2 was maintained through the experiment with an average selectivity for CO 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 mAcm^-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 current density increase, as CO was produced with a 98.3 ± 0.3% average selectivity.
In order to diminish the cell potential, the ohmic resistance of the cell was reduced by increasing the electrolyte concentration. This was demonstrated in a 1.0 M NaHCO₃ electrolyte solution, where a gain of >8 mAcm^-2 was obtained at an applied potential of -0.78 V vs RHE, without compromising the CO selectivity (see Figure 11). In a chronopotentiometric electrolysis, see Figure 4, the applied potential at 50 mAcm^-2 shifted positively to -0.86 V vs RHE, and in addition the cell potential was reduced by 590 mV. The CO product selectivity was 99.0 ± 0.2% on average.
A complementary approach to boost the cell performance was achieved by thermally activating the catalyst. During a chronoamperometric electrolysis at -0.78 V vs RHE, the catholyte temperature was incrementally increased from 24 °C to 34 °C and finally to 40 ºC. As shown in Figure 5A, an increase in current density was observed upon increasing the temperature, while maintaining the high CO product selectivity. Taking the TOF (in s^-1) as an apparent 1^st order rate constant for the CO₂ conversion (assuming steady state conditions for the catalyst and proton source), an Arrhenius plot: In(TOF) vs 1/T (in K^-1) was constructed (Figure 5B). From the slope, the activation energy, Ea, was calculated to 12 kJmol^-1.
In alkaline conditions (1.0 M KOH, pH 14) the cell performance was drastically 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 mAcm^-2 was observed at 0.014 V vs RHE, and 155 mAcm^-2 was reached at -0.59 V vs RHE, with an excellent CO product selectivity, as summarized in Table 2. In a control experiment with the non-metalated porphyrin ligand, H₂ was the only detected product (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.

E/V vs RHE j_av/mAcm^-2 % CO

0.014 6.0 99.9

-0.086 16.5 99.9

-0.19 37.6 99.9

-0.29 62.9 99.8

-0.39 89.7 99.5

-0.49 122.4 99.2

-0.59 155.2 98.1
To investigate the endurance of the catalyst in alkaline conditions, a chronopotentiometric electrolysis was performed at 27 mAcm^-2 for 24h, the same current density value as performed in pH-neutral conditions. As shown in Figure 7, a potential of ca. -0.16 V vs RHE was measured over the 24 hour course, showing only a minute increase. The CO product selectivity remained extremely high: 99.7 ± 0.2% on average.
For a direct comparison with the pH-neutral 1.0 M bicarbonate system, a chronopotentiometric electrolysis was performed at 50 mAcm^-2 in 1.0 M KOH, as shown in Figure 8. Over the course a 3 hours electrolysis, the applied potential remained stable (-0.23 V vs RHE), a very weakly negative potential for such high current density. Again, the CO selectivity was nearly perfect, with an average value of 99.8 ± 0.1%.

SECOND EMBODIMENT

The inventors included CoPc2 into the same flow cell setup described in the first embodiment. CoPc2 was supported on a gas diffusion electrode as the cathode 9. Details of the setup are provided in the Supporting Information (see Figure 16). CoPc2 was dispersed in a colloidal ink with carbon black and subsequently deposited on the carbon fiber paper, composing the cathode 9.
At -0.3 V vs. RHE and pH 14 (1 M KOH for the electrolyte), which corresponds to a low 200 mV overpotential, a high current density with j _CO = 22.2 mA/cm² was achieved (j _CO is the partial current density for CO production). The dependence of the current density for CO production is reported in Figure 15a (see also Figure 22). Upon setting the electrolysis potential at -0.72 V vs. RHE, j _CO raised to 111.6 mA/cm² with 96% selectivity, while excellent stability over the course of the 3h electrolysis was obtained (Figure 15b). The only additional gas phase by-product was H₂ (4% selectivity) and the catholyte solution was carefully checked by ¹H NMR; no formate nor methanol was detected during the experiment. The obtained j _CO 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 cm^-2 for CO generation was obtained at -0.92 V vs. RHE. Long term stability of the catalytic material was assessed upon applying a constant current density of 75 mA cm^-2 for 10h, which led to a cathodic potential of -0.65 V vs. RHE (h = 540 mV) with 94% selectivity of CO (j _CO = 70.5 mA cm^-2) (Figure 23). All the collected data are compiled in Table 3, along with a comparison of previously reported catalysts. The type of cell used (H cell vs. flow cell) is also indicated to ease comparison between data. The Co K-edge XANES spectra of CoPc2@carbon black were recorded before and after electrocatalysis at E= -0.72 V vs. RHE. Figure 15c shows these spectra together with that of the starting CoPc2 complex. All these spectra present the typical features expected for 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 4p_z transition). Interestingly, the intensity of these two transitions were shown by Li et al ¹⁷ to depend on the attachment to a surface, i.e. the pre-edge intensity increases and the shoulder decreases upon adsorption onto nanotubes, respectively. The same trend is observed in the CoPc2@carbon black system, with decreased pre-edge and increased shoulder intensities on 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 present the typical features of a cobalt phthalocyanine complex.³⁸ In addition, comparison of the spectrum of CoPc2@carbon black recorded after catalysis with those of reference cobalt samples (Figure 24) clearly shows that the changes observed on the spectrum after catalysis are insignificant as far as the overall structure is concerned.

Remarkably, CoPc2 remains highly selective for the CO₂-to-CO conversion across an interval of 10 units of pH, extending from acidic (pH 4) to basic solutions (pH 14). An averaged 92% selectivity for CO₂ reduction with partial current density of ca. 20 mA cm^-2 were routinely obtained in the whole domain of pH values with excellent stability over time. In close to neutral solutions (pH 7.3), CoPc2 is a significantly better catalyst than the non-substituted phthalocyanine CoPc (see Table 3, entries 2 and 3) with a ca. 25% increase in current density at similar overpotential, but it also surpasses state-of-the art tetra-cyano substituted phthalocyanine (CoPc-CN, Table 3, entry 4) and unsubstituted Co phthalocyanine polymerized around carbon nanotubes (CoPpc, Table 3, entry 5), both in terms of current density and turnover frequency. Similarly to the previously reported cobalt phthalocyanines mentioned above, CoPc2 exhibits excellent stability over time, showing a 10.5 h electrolysis experiment, with no loss of performance.

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

Entry	Catalyst	E (V vs. RHE) [overpotential (mV)]	Electrolyte	jCO (mA cm^-2)	TOF (s^-1)	CO sel. (%)	Cell type	Ref.
1	CoPc2	-0.97 [836]^a	0.5 M KCI	16.3	6.1	92	H-cell	this work
2	CoPc2	-0.67₅ [539]^a	0.5 M NaHCO₃	18.1	6.8	93	H-cell	this work
3	CoPc	-0.67₅ [546]^a	0.5 M NaHCO₃	13.1	4.1	92	H-cell	this work
4	CoPc-CN	-0.63 [520]	0.1 M KHCO₃	14.7	4.1	98	H-cell	15
5	CoPpc	-0.61 [500]	0.5 M NaHCO₃	18	1.4	ca. 90	H-cell	16
6	Coqpy	-0.55 [440]	0.5 M NaHCO₃	19.9	12	99	H-cell	10
7	CoPc2	-0.31 [200]^b	1 M KOH	22.2	0.54	93	Flow-cell	this work
8	CoPc2	-0.65 [540]^b	1 M KOH	70.5	1.67	94	Flow-cell	this work
9	CoPc2	-0.72 [610]^b	1 M KOH	111.6	2.7	96	Flow-cell	this work
10	CoPc2	-0.92 [810]^b	1 M KOH	165	3.9	94	Flow-cell	this work
11	CoPc-CN	-0.66 [550]	1 M KOH	31	/	94	Flow-cell	11
12	Ag^c	-0.81 [700]	1 M KOH	156.5	/	92	Flow-cell	13

Entry 1: ┌ = 14.4 nmol cm^-2 (pH 4, 2h electrolysis), entry 2: ┌ = 14.4 nmol cm^-2 (pH 7.3, 1h electrolysis), entry 3: ┌ = 23.3 nmol cm^-2 (pH 7.3, 1h electrolysis), entries 7-10: ┌ = 0.216 µmol cm^-2 (pH 14, for 0.5, 10, 3 and 0.3 h electrolysis respectively). Typical uncertainty on j _CO and TOF values is ±5%.
^acorrected from ohmic drop (uncompensated solution resistance of ca. 3 W, electrode surface 0.5 cm²)
^buncorrected from ohmic drop
^ccarbonate-derived Ag nano-catalyst (500 nm thickness), see reference 13 for details.

A turnover frequency up to 6.8 s^-1 was reached and, generally, a very small loading of the catalysts was necessary to obtain high j _CO (see for example Table 3, entries 1-2). Long term electrolysis (10h) in basic conditions (pH 14) at -0.65 V vs. RHE led to an average j _CO close 70.5 mAcm^-2 and also illustrates the remarkable stability of the cobalt catalyst. The ability to implement CoPc2 in various pH conditions is also a key feature that may allow for combining the Co catalyst to various types of anodic materials in order to decrease the overall cell potential. In particular, the excellent performance obtained at pH 14 should permit to pair the CoPc2 loaded cathode 9 with the most efficient oxygen evolving metal oxide anode 2 materials. At this pH, CoPc2 matches the state-of-the art Ag based catalyst sputtered onto a PTFE membrane, both in terms of selectivity and current density (Table 3, compare entries 10 and 12).
Upon introducing a positively charged trimethylammonium group on the parent cobalt phthalocyanine, a highly efficient and versatile catalyst for the CO₂-to-CO electrochemical conversion in water has been obtained. Furthermore, it operates with high selectivity (92 to 96%) in a broad range of pHs, extending from acidic (pH 4) to basic conditions (pH 14). In acid and neutral conditions, current densities close to 20 mA cm^-2 were routinely obtained. In a 1 M KOH electrolyte solution, same current densities could be obtained at a very low overpotential of 200 mV, once the hybrid catalyst mixed with carbon support 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 94% catalytic selectivity. These performances show that hybrid catalytic materials including only carbon and an earth abundant metal based molecular complex can rival noble metal nanomaterials such as Ag and Au. This study highlights that rational tuning of the structure of simple metal complexes may allow for high performance, and it is likely that further improvement is yet to come. Finally, this work opens new perspectives for the development of low-cost catalytic materials to be included in CO₂ electrolysers.

Supplementary Information

Methods

All electrocatalytic reactions were carried out under an atmosphere of argon or CO₂. Chemicals, including CoPc1 and supporting electrolytes were obtained from commercial suppliers. Supplementary Information section describes the synthesis and characterization of CoPc2 and FeTNT, as well as typical protocols for electrocatalytic CO₂ reduction.
Typical protocol for electrocatalytic CO₂ reduction. After preparation of a catalytic ink containing either FeTNT, CoPc1 or CoPc2, and deposition of the material onto porous carbon paper, controlled potential electrolysis were performed either in a closed electrochemical cell or in a flow cell electrolyser using a PARSTAT 4000 potentiostat (Princeton Applied Research). Gas chromatography analyses of gas evolved in the headspace during the electrolysis were performed with an Agilent Technologies 7820A GC system equipped with a thermal conductivity detector. Conditions allowed detection of both H₂, O₂, N₂, CO, and CO₂. Calibration curves for H₂ and CO were determined separately by injecting known quantities 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 MΩ cm). The MWCNTs were purchased from Sigma-Aldrich (O.D. × L 6-9 nm × 5 µm, > 95%). The cobalt (II) phthalocyanine (CoPc1) (β-form, dye content 97%) was purchased from Sigma-Aldrich. Toray Carbon Paper (CAS Number: 7782-42-5), TGP-H-60, 19x19cm was purchased from Alfa Aesar and used for preparation of the cathode 9s for the electrochemical cell. The cathode 9s (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 Speciality 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 spectra (IR) 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 electrospray ionization (ESI) source. NMR spectra were recorded in deuterated chloroform (CDCl₃) 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 put under argon flux, 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 obtained precipitate is separated by centrifugation (10000 rpm, 30 min). The red solid obtained is rinsed one time in pure water and separated again by centrifugation (10000 rpm 30 min), then rinsed one time in 1 to 1 mixture of chloroform and acetone and separated by centrifugation (10 000 rpm 30 min). Residual solvent is evaporated under Argon flux. Yield: 92 % (343 mg)
UV-Vis (DMF): λ_max nm (log ε) 407 (4), 568 (4.98).
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 argon atmosphere then finely powdered dry potassium carbonate (30 g, 217.5 mmol) was added portion-wise over 15 min. The reaction mixture was stirred under argon at 65 °C for 24 h then poured into water (250 mL). The resulting solid was collected by filtration and washed with water. After drying in vacuum, the crude product was recrystallized from ethanol. Yield: 80 % (1.57 g). m.p. 105 °C. ¹H NMR (500 MHz, CDCl₃): δ, ppm 3.18 (s, 6H), 7.12 (d, 1H), 7.15 (d, 1H), 7.46 (t, 1H). ¹³C NMR (125 MHz, CDCl₃): δ, 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

Granules of lithium were added to anhydrous n-pentanol (10 mL). This mixture was heated to 60 °C under argon flux until total consumption of the granules. 3-(dimethylamino)phthalonitrile 1 (0.25 g, 1.46 mmol) and 4-tert-butylphthalonitrile 2 (3.2 g, 17.52 mmol) were then added and this reaction mixture was refluxed for 18 h, then cooled to room temperature and poured to an ethanol/water mixture. The resulting dark blue-green precipitate was filtered off, washed several times with water and dried. Phthalocyanine 3 was isolated from this crude mixture of phthalocyanines by chromatography on silica gel using a mixture of CH₂Cl₂/EtOH (100:1) as the eluent. The tetra-tert-butylphthalocyanine 4 was first eluted and the phthalocyanine 3 was the second eluted compound. Yield: 15 % (160 mg). ¹H NMR (500 MHz, THF-d₈): δ, 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), ¹³C NMR (125 MHz, THF-ds): δ, 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): λ_max nm (log ε) 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 phthalocyanine 3 (50 mg, 0.06 mmol), CoCl₂ (18 mg, 0.12 mmol), and DBU (1 mL) in dried n-pentanol (5 mL) was heated to 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. Phthalocyanine 5 was purified by chromatography on silica gel using a mixture of CH₂Cl₂/EtOH (50:1) as the eluent. Yield: 75 % (35 mg). ESI-HRMS: m/z 782.3234 [M]⁺ calculated for C₄₆H₄₅CoN₉: 782.86. UV-vis (DMF): λ_max nm (log ε) 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 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]⁺ calculated for C₄₇H₄₈CoN₉: 797.90. Anal. calcd for C₄₇H₅₈CoIN₉O₅ (CoPc2.5H₂O): C, 55.28; H, 5.98; N, 11.88. Found: C, 55.62; H, 5.76; N, 12.42. UV-vis (DMF): λ_max nm (log ε) 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 the hybrid materials

For CV experiments and electrolysis in the closed electrolysis cell, 3 mg of MWCNTs were dispersed in 2 mL ethylene glycol (EG)/ethanol (EtOH) 1:1(v/v) mixture followed by 30 min of sonication. 1 mg of the cobalt catalyst (CoPc1, CoPc2) was dissolved in 1mL EG/EtOH mixture. Various volumes of this solution were added to the MWCNTs suspension in a total volume of 3 mL, so as to get mass ratio (1:6, 1:15 and 1:30) of the catalyst. The suspension was further sonicated for 30 min. Finally, Nafion® was added (2.9 %, 30 µL) and the complete mixture was sonicated for 30 min to obtain the final catalytic ink.
For the flow cell set-up, 3 mg of carbon black were dispersed in 3 mL EtOH followed by 30 min of sonication. 0.2 mg of CoPc2 was dissolved in 1mL EtOH so as to get a mass ratio (1:15) of the catalyst. The suspension was further sonicated for 30 min. Finally, Nafion® was added (2.9 %, 30 µL) and the complete mixture was sonicated for 30 min to obtain the final catalytic ink. The ink was drop casted on carbon paper masked with a Teflon frame to obtain an electrode area of 1x1 cm².

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 working electrode, and a SCE reference electrode closely positioned one from the other. The Pt grid counter electrode was separated from the cathodic compartment with a glass frit. The catalytic ink was dropped on one face of the Toray carbon paper cathode 9 (100 µL for a 0.5 cm² electrode), and allowed to dry under ambient conditions prior to use. The full cell setup was identical to the one used previously.¹⁸
The flow cell electrolyzer 1 (Micro Flow Cell® purchased by Electrocell) is composed by a sandwich of flow frames, electrodes, gaskets and a membrane, which, when assembled as illustrated in Figure 14, constitute a three-compartment flow cell. One compartment delivers the CO₂ (at 16.7 sccm) from the back side and through the gas diffusion electrode (GDE, 1x1 cm², fixated in a Pt frame), while another directs the catholyte solution (1 M KOH, flow rate of 16 sccm) in between the GDE and the anion exchange membrane 10 (AEM, Sustainion™ X37-50). On the other side of the latter, the anolyte (1 M KOH, flow rate of 16 sccm) is directed between the AEM and the Pt/Ti alloy anode 2. The flow frames are made of PTFE, and the gaskets of peroxide cured EDPM. Catholyte and anolyte were recycled using peristaltic pumps. All tubings were made of PTFE and connected to the cell with PEEK ferrules and fittings. The whole setup is schematically shown below (Figure 16).

Gas Detection

Gas chromatography analyses of gas sampled from the headspace during the electrolysis were performed with an Agilent Technologies 7820A GC system equipped with a thermal conductivity detector. CO and H₂ production was quantitatively detected using a CP-CarboPlot P7 capillary column (27.46 m in length and 25 µm internal diameter). Temperature was held at 150 °C for the detector and 34 °C for the oven. The carrier gas was argon flowing at 9.5 mL/min at constant pressure of 0.4 bars. Injection was performed via a 250-µL gas-tight (Hamilton) syringe previously degassed with CO₂. Conditions allowed detection of both H₂, O₂, N₂, CO, and CO₂. Calibration curves for H₂ and CO were determined separately by injecting known quantities of pure gas.

XPS analysis

An X-Ray Photoelectron Spectrometer THERMO-VG ESCALAB 250 (RX source K Al (1486.6 eV)) was used.

XAS data collection and analysis

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

SEM analysis

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

THIRD EMBODIMENT

The inventors tested the cobalt quater pyridine (Co(qpy)) in the same cell as the first embodiment. Two types of experiments have been performed on the Co(qpy). The faradaic efficiency for CO has been measured chronopotentiometry at several current densities and the stability of the catalyst has been determined at 50 mA/cm².
During chronopotentiometry experiment the Co(qpy) showed faradaic efficiencies for CO higher than 90 % at current densities under 100 mA/cm². Then, at higher current densities, the faradaic efficiency for CO decreases drastically.

Cell potential (V) j/mA.cm^-2 % CO

1.71 25 96.6

1.80 50 98.7

1.88 75 98.8

2.24 100 91.6

2.60 125 86.1

2.71 150 64.6

2.73 175 34.7

2.74 200 16.6
In figure 26, Co(qpy) can maintain at 50 mAcm^-2 a faradaic efficiency higher than 90 % for CO for 12 hours. The decrease is steady until 24 hours of experiments. Then, the faradaic efficiency drops under 40 % for CO.

References

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Claims

A flow cell electrolyzer (1) to electrochemically reduce a gas reactant comprising CO₂, 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 a catalyst to electrochemically oxidize H₂O to O₂,

• an anodic electrolyte solution (3), at a controlled flow rate Q_a, comprising: a solvent, and an anodic electrolyte, the solvent being water,

- An anodic electrolyte solution inlet (4) and an anodic electrolyte solution outlet (5) connected to the anodic compartment, to circulate the anodic electrolyte solution (3);

- a cathodic compartment comprising:
• a cathodic electrolyte solution (6), at a controlled flow rate Q_c, comprising: a solvent, and a cathodic electrolyte, the solvent being water,

• a gas diffusion porous cathode (9) which comprises, on a gas diffusion porous current cathode collector which is electrochemically inert, at least a molecular catalyst incorporated in the porous cathode with a surface S, to electrochemically reduce the gas comprising CO₂ into gaseous CO, with a by-production of gaseous H₂,
the molecular catalyst being chosen between the list:
❖ metal porphyrin with one or several +N(C₁-C₄ alkyl)₃ groups, with the metal chosen among: Iron, Cobalt;

❖ metal phthalocyanine, with the metal chosen among: Iron, Cobalt;

❖ metal phthalocyanine with the metal chosen among: Iron, Cobalt, with one or several groups among: +N(C₁-C₄ alkyl)₃, F, C(CH₃)₃, or

❖ cobalt quater pyridine

- a channel (11) for flowing the reagent gas CO₂, at a controlled flow rate Q_g, onto or through the surface S of the gas diffusion porous current cathode collector;

- a cathodic electrolyte solution inlet (7) and a cathodic electrolyte solution outlet (8) connected to the cathodic compartment, to circulate the cathodic electrolyte solution, and the remaining reagent gas CO₂ and the product gas CO by the outlet (8);

- an anion exchange membrane (10), impermeable to CO₂, CO, H₂ and O₂, between the anodic compartment and the cathodic compartment;

- 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),

- flow by pumping the comprising gas CO₂ passing through the gas diffusion porous cathode (9);
the pumping means being configured to control all flow rates of the cathodic and anodic electrolytes as well as the reagent gas CO₂ passing through the surface S of the gas diffusion porous cathode (9);
- a power supply providing the energy necessary to trigger the electrochemical reactions involving the reagent.
The flow cell electrolyzer (1) of claim 1, wherein the anodic and/or cathodic electrolyte solution has a neutral or basic pH.
The flow cell electrolyzer (1) of claim 1 or 2, wherein the anodic electrolyte solution and/or the cathodic electrolyte solution has a pH from 9 to 14, preferably from 10 to 14, more preferably from 11.5 to 14, and more preferably from 13 to 14.
The flow cell electrolyzer (1) of claims 1-3, wherein the molecular catalyst is the iron porphyrin with the formulae:

Wherein:
∘ at least 1 and at most 8 groups among R₁ to R₁₀ and R_1' to R_10' being independently +N(C₁-C₄ alkyl)₃ group,

∘ the other groups R₁ to R₁₀ and R_1' to R_10' are independently selected from the group consisting of H, OH, F, C(CH₃)₃
The flow cell electrolyzer (1) of claim 4, wherein the iron molecular catalyst comprises:
- the other groups among R₁ to R₁₀ and R_1' to R_10' are H; or

- the other groups among R₁ to R₁₀ and R_1' to R_10' are H and F;
The flow cell electrolyzer (1) of claims 4-5, wherein the iron molecular catalyst comprises the +N(C₁-C₄ alkyl)₃ groups in the para or ortho position.
The flow cell electrolyzer (1) of claims 1-3, wherein the molecular catalyst presents the formulae:
wherein R₁ to R₁₆ are independently selected from the groups consisting of H, F C(CH₃)₃ , or +N(C₁-C₄ alkyl)₃
The flow cell electrolyzer (1) of claim 7, wherein R₁ to R₁₆ are H.
The flow cell electrolyzer (1) of claim 7, wherein the cobalt molecular catalyst presents at least 1 and at most 8 groups among R₁ to R₁₆ being independently +N(C₁-C₄ alkyl)₃ group.
The flow cell electrolyzer (1) of claim 9, wherein the cobalt molecular catalyst presents for one or several of the specific following R₁, R₄, R₅, R₈, R₉, R₁₂, R₁₃ and R₁₆ groups, a +N(C₁-C₄ alkyl)₃ substituent.
The flow cell electrolyzer (1) of any of claims 1-10, 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 of claims 1-6, with the iron porphyrin as molecular catalyst, wherein:
- the pH is between 7.3 and 14

- the potential applied to the cathode 9 is between 0 V and -1 V versus RHE, and

- the current density for CO production is at least 150 mAcm^-2, and

- the selectivity of the electrochemical reaction is between 98% and 99.9%
The flow cell electrolyzer (1) of any of claims 7-8, with the phthalocyanine, wherein:
- the pH is between 7.3 and 14,

- the potential applied to the cathode 9 is between -0.48 V and -0.98 V versus RHE,

- the current density for CO production is at least 50 mAcm^-2,

- the selectivity of the electrochemical reaction is between 90% and 92%.
The flow cell electrolyzer (1) of any of claims 7, 9, with the cobalt phthalocyanine with one or several +N(C₁-C₄ alkyl)₃ groups, wherein:
- the pH is between 7.3 and 14,

- the potential applied to the cathode 9 is between -0.3 V and -1 V versus RHE,

- the current density for CO production is at least 150 mAcm^-2,

- the selectivity of the electrochemical reaction is between 92 % and 96 %.
The flow cell electrolyzer (1) of claims 1-3, with the cobalt quater pyridine, wherein:
- the pH is between 7.3 and 14,

- the potential applied to the cathode 9 is between -0.6 V and -1 V versus RHE,

- the current density for CO production is at least 150 mAcm^-2,

- the selectivity of the electrochemical reaction is between 86.1 % and 98.8 %.
The flow cell electrolyzer (1) of any of claims 1-15, wherein the cathodic current collector is carbon paper.
The flow cell electrolyzer (1) of any of claims 1-16, wherein the porous cathode (9) comprises on the current collector, an electrode film which contains polymers with the molecular catalysts and wherein the electrode film is deposited or grafted on the current collector.
The flow cell electrolyzer (1) of claims 1-17, wherein pumping means (12) recirculate the anodic electrolyte solution (3) and the cathodic electrolyte solution (6).