FIELD OF THE INVENTION
[0001] The present invention relates to iron and cobalt molecular complexes as catalysts
for the highly selective electrochemical reduction of CO
2 into CO, with electrochemical cells comprising them.
STATE OF ART
[0002] Despite the increasingly frequent use of renewable energies to produce electricity
and avoid concomitant production of CO
2, it is reasonable to consider that CO
2 emissions, in particular resulting from energy production, will remain high in the
next decades. Thus, it appears necessary to find ways to capture CO
2 gas, either for storing or valorization purposes.
[0003] Indeed, CO
2 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 CO
2 and water has been envisaged.
[0004] However, CO
2 exhibits low chemical reactivity: breaking its bonds requires an energy of 724 kJ/mol.
Moreover, CO
2 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 (CO
2.-). Catalysis thus appears mandatory in order to reduce CO
2 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 CO
2 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.
[0005] CO
2 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.
[0006] Molecular catalysts that combine high selectivity and high current density for CO
2 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 CO
2 to either CO or formate with reasonable efficiencies, but in organic solvents.
[0007] 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 (CO
2 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 Au
14 nanomaterials have been reported to reach >150 mA/cm
2.
SUMMARY OF THE INVENTION
[0008] Here, the invention presents a flow cell electrolyzer to electrochemically reduce
reagent gas CO
2 into gaseous CO and gaseous H
2, 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 Qa, 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 Qc, 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 Qg, 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
[0009] 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, 1st scan; grey, 2nd 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
[0010] The present invention concerns: a flow cell electrolyzer 1 to electrochemically reduce
a gas reactant comprising CO
2, into gaseous CO and gaseous H
2, 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, 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 Qc, 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.
[0011] The cathodic electrolyte can comprise a phosphate buffer or potassium hydroxide,
and the anodic electrolyte comprises a phosphate buffer or potassium hydroxide.
[0012] 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
[0013] 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 Qg, 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.
[0014] 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;
[0015] In an example, the gas reactant comprises at least 99 % per volume CO
2, or at least 99,5 % per volume CO
2, or at least 99,9 % per volume CO
2, or at least 99,99 % per volume CO
2.
[0016] The flow cell 1 can have a device capable of recording the current generated by the
electrochemical processes.
[0017] Advantageously, but in a non-limiting way, the channel for flowing the reagent gas
CO
2 can comprise a flow frame generating a turbulent flow.
[0018] Advantageously, but in a non-limiting way, pumping means 12 to recirculate the anodic
electrolyte solution 3 and the cathodic electrolyte solution 6.
[0019] In a preferred embodiment, the anodic and/or cathodic electrolyte solution has a
neutral or basic pH.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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 CO
2 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 CO
2 at high throughput rates.
[0025] Furthermore, it was found that the aforementioned molecular catalysts demonstrate
high long-term stability even at harsh alkaline pH conditions.
[0026] The electrochemical reduction of the reagent into CO and H
2 can be carried out at ambient temperature.
[0027] The electrochemical reduction of the reagent into CO and H
2 can be carried out at atmospheric pressure.
[0028] 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.
[0029] 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.
[0030] 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(C
1-C
4 alkyl)
3 groups among specific groups.
[0031] In a first embodiment of the flow cell electrolyzer 1, the molecular catalyst is
the iron porphyrin with the formula:

[0032] Wherein:
∘ at least 1 and at most 8 groups among R1 to R10 and R1' to R10' being independently +N(C1-C4 alkyl)3 group,
∘ the remaining groups R1 to R10 and R1' to R10' are independently selected from the group consisting of H, OH, F, C(CH3)3
[0033] In particular, the iron molecular catalyst can comprise:
- the other groups among R1 to R10 and R1' to R10' are H; or
- the other groups among R1 to R10 and R1' to R10' are H and F;
The iron molecular catalyst can comprise the +N(C
1-C
4 alkyl)
3 groups in para or ortho position.
[0035] 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%.
[0036] In a second embodiment of the flow cell electrolyzer 1, the flow cell electrolyzer
1 comprises the molecular catalyst which presents the formula:

wherein R
1 to R
16 are independently selected from the groups consisting of H, F, C(CH
3)
3 or +N(C
1-C
4 alkyl)
3
[0037] In particular, R
1 to R
16 are H, and the molecular catalyst is a cobalt phthalocyanine CoPc of formula:

[0038] 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%.
[0039] In another variant, the cobalt molecular catalyst presents at least 1 and at most
8 groups among R
1 to R
16 being independently +N(C
1-C
4 alkyl)
3 group.
[0040] In another variant, the cobalt molecular catalyst presents for one or several of
the specific following R
1, R
4, R
5, R
8, R
9, R
12, R
13 and R
16 groups, a +N(C
1-C
4 alkyl)
3substituent.
[0041] 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(C
1-C
4 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 %.
[0042] 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
[0043] 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 cm
2 gas diffusion current collector.
Results
[0044] 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 CO
2 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.
[0045] A survey of the catalytic activity as a function of applied potential was first performed
in CO
2 saturated 0.5 M NaHCO
3. Figure 1 illustrates the increase in current density recorded for the CO
2RR 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 H
2 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 H
2 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 |
jav/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 |
[0046] 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%.
[0047] 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.
[0048] 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 NaHCO
3 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.
[0049] 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 CO
2 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.
[0050] 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, H
2 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 |
jav/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 |
[0051] 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.
[0052] 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
[0053] 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.
[0054] 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
jCO = 22.2 mA/cm
2 was achieved (
jCO 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,
jCO raised to 111.6 mA/cm
2 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 H
2 (4% selectivity) and the catholyte solution was carefully checked by
1H NMR; no formate nor methanol was detected during the experiment. The obtained
jCO 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 (
jCO = 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.
[0055] Remarkably,
CoPc2 remains highly selective for the CO
2-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 CO
2 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 |
Agc |
-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 jCO 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. |
[0056] 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
jCO (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
jCO 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).
[0057] Upon introducing a positively charged trimethylammonium group on the parent cobalt
phthalocyanine, a highly efficient and versatile catalyst for the CO
2-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 CO
2 electrolysers.
Supplementary Information
Methods
[0058] All electrocatalytic reactions were carried out under an atmosphere of argon or CO
2. 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 CO
2 reduction.
[0059] 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 H
2, O
2, N
2, CO, and CO
2. Calibration curves for H
2 and CO were determined separately by injecting known quantities of pure gas
General Considerations
Chemicals and characterization methods
[0060] 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.
[0061] 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 (CDCl
3) and THF-d
8 on a Varian 500 MHz spectrometer. Melting points were recorded on a Stuart SMP apparatus.

Synthesis of FeTNT
[0062] 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).
[0063] Cobalt catalyst
CoPc2 was prepared as illustrated here:

Synthesis of 3-(dimethylamino)phthalonitrile 1
[0064] 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, CDCl
3): δ, ppm 3.18 (s, 6H), 7.12 (d, 1H), 7.15 (d, 1H), 7.46 (t, 1H).
13C NMR (125 MHz, CDCl
3): 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
[0065] 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 CH
2Cl
2/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-d
8): 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 C
46H
47N
9: 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
[0066] A mixture of phthalocyanine
3 (50 mg, 0.06 mmol), CoCl
2 (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 CH
2Cl
2/EtOH (50:1) as the eluent. Yield: 75 % (35 mg). ESI-HRMS: m/z 782.3234 [M]
+ calculated for C
46H
45CoN
9: 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
[0067] 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 C
47H
48CoN
9: 797.90. Anal. calcd for C
47H
58CoIN
9O
5 (CoPc2.5H
2O): 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
[0068] 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.
[0069] 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 cm
2.
Electrochemical studies
[0070] Controlled potential electrolyses were performed using a PARSTAT 4000 potentiostat
(Princeton Applied Research).
Preparative Scale Electrolysis
[0071] 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 cm
2 electrode), and allowed to dry under ambient conditions prior to use. The full cell
setup was identical to the one used previously.
18
[0072] 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 CO
2 (at 16.7 sccm) from the back side and through the gas diffusion electrode (GDE, 1x1
cm
2, 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
[0073] 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 H
2 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 CO
2. Conditions allowed detection of both H
2, O
2, N
2, CO, and CO
2. Calibration curves for H
2 and CO were determined separately by injecting known quantities of pure gas.
XPS analysis
[0074] An X-Ray Photoelectron Spectrometer THERMO-VG ESCALAB 250 (RX source K Al (1486.6
eV)) was used.
XAS data collection and analysis
[0075] 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 E
0 value of 7722.0 eV was used for the cobalt K-edge jump energy.
SEM analysis
[0076] Scanning electron microscopy using a field emission gun (SEM-FEG) was performed using
a Zeiss Supra 40.
THIRD EMBODIMENT
[0077] 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/cm
2.
[0078] During chronopotentiometry experiment the Co(qpy) showed faradaic efficiencies for
CO higher than 90 % at current densities under 100 mA/cm
2. 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 |
[0079] 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.
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1. A flow cell electrolyzer (1) to electrochemically reduce a gas reactant comprising
CO
2, into gaseous CO and gaseous H
2, 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,
- 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 Qc, 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 Qg, 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 CO
2 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.
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 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.
4. 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 R1' to R10' being independently +N(C1-C4 alkyl)3 group,
∘ the other groups R1 to R10 and R1' to R10' are independently selected from the group consisting of H, OH, F, C(CH3)3
5. The flow cell electrolyzer (1) of claim 4, wherein the iron molecular catalyst comprises:
- the other groups among R1 to R10 and R1' to R10' are H; or
- the other groups among R1 to R10 and R1' to R10' are H and F;
6. 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.
7. The flow cell electrolyzer (1) of claims 1-3, wherein the molecular catalyst presents
the formulae:

wherein R
1 to R
16 are independently selected from the groups consisting of H, F C(CH
3)
3 , or +N(C
1-C
4 alkyl)
3
8. The flow cell electrolyzer (1) of claim 7, wherein R1 to R16 are H.
9. 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.
10. 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.
11. 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.
12. 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%
13. 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%.
14. The flow cell electrolyzer (1) of any of claims 7, 9, with the cobalt phthalocyanine
with one or several +N(C
1-C
4 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 %.
15. 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 %.
16. The flow cell electrolyzer (1) of any of claims 1-15, wherein the cathodic current
collector is carbon paper.
17. 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.
18. 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).