Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell

Article abstract of DOI:10.1002/chem.202000160

Molecular catalysts have been shown to have high selectivity for CO₂ electrochemical reduction to CO, but with current densities significantly below those obtained with solid-state materials. By depositing a simple Fe porphyrin mixed with carbo

Full text of DOI:10.1002/chem.202000160

A Journal of
Accepted Article
Title: Iron Porphyrin Does Fast and Selective Electrocatalytic
Conversion of CO2 to CO in a Flow Cell
Authors: Kristian Torbensen, Cheng Han, Benjamin Boudy, Niklas von
Wolff, Caroline Bertail, Waldemar Braun, and Marc Robert
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To be cited as: Chem. Eur. J. 10.1002/chem.202000160
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Iron Porphyrin Does Fast and Selective Electrocatalytic
Conversion of CO2 to CO in a Flow Cell
Kristian Torbensen,[a] Cheng Han,[a],[b] Benjamin Boudy,[a] Niklas von Wolff,[a] Caroline Bertail,[c]
Waldemar Braun[d] and Marc Robert*[a]
Abstract: Molecular catalysts have been shown to have high
selectivity for CO2 electrochemical reduction to CO, but with current
densities significantly below those obtained with solid-state materials.
By depositing a simple Fe porphyrin mixed with carbon black onto a
carbon paper support, we have obtained a catalytic material that could
be used in a flow cell for fast and selective conversion of CO2 to CO.
At neutral pH (7.3) a current density as high as 83.7 mA cm-2 was
obtained with a CO selectivity close to 98%. In basic solution (pH 14),
a current density of 27 mA cm-2 was maintained for 24 h with 99.7%
selectivity for CO at only 50 mV overpotential, leading to a record
energy efficiency of 71%. In addition, a current density for CO
production as high as 152 mA cm-2 (˃98% selectivity) was obtained at
a low overpotential of 470 mV, outperforming state-of-the-art noble
metal based catalysts.
dogma can be overcome by applying the complex into a flow cell,
generating CO with 94% selectivity at 165 mA/cm2 current density
at basic pH (14).[21-22] Such performances approach those
obtained with Ag and Au based catalytic materials.[23-25] However,
the Co complex, like the above mentioned solid nano-catalysts,
operates at quite large overpotential, in the range of 800 mV at
comparably current densities. The selectivity could also be still
improved, while the exclusive use of earth abundant elements
instead of expensive metals is mandatory. In this context, Fe is
an ideal candidate being the most abundant transition metal. As
previously shown, tetraphenyl Fe porphyrin substituted with
trimethyl ammonium groups at the para position of each of the
four phenyl groups (FeP, Scheme 1 (left),[26-27] is able to produce
CO with a selectivity of 90% at neutral pH and at an overpotential
of 450 mV. However, maximum current density obtained in an H-
cell device was in the range of 1 mA cm-2.27 Using FeP/graphene
framework28 or FeP/graphene hydrogel29 assemblies also led to
high selectivity for CO (96-98.7%) and a maximum current density
of about 1.7 mA cm-2 at ca. 430 mV overpotential was shown.
CO2 may be used as renewable feedstock for producing fuels
or commodity chemicals with renewable electricity as energy
source.[1-2] However, selective, robust and fast electrochemically
driven conversion of CO2 remains a great challenge in the
scientific field and as well as in the industrial environment.
Therefore, new approaches are needed to develop electrolyzers
operating at industrial relevant scales while fulfilling customer
needs and requirements. Regarding the catalysts employed,
molecular complexes have been widely investigated for the
carbon dioxide reduction reaction (CO2RR) due to the possibility
of fine tuning the ligand structure (steric and electronic effects, as
well as second coordination sphere effects).[3-6] Earth abundant
metal-based catalysts have been shown to efficiently catalyze the
production of CO and formate in organic solvents and in water,[7-
11] even if examples in pure aqueous solutions are less numerous.
In the latter case, the catalysts are typically dispersed into thin
conductive films usually made of carbon nanomaterials, such as
carbon black or carbon nanotubes.[12-20] Until very recently, these
molecular catalysts were deemed to be incapable to operate in
flow cells at both high product selectivity and conversion rate.
Using Co phthalocyanine as catalysts, we have shown that this
Supramolecular
porous
structures
made
of
Fe
tetraphenylporphyrin units have also been used as catalysts.[30-31]
After deposition at the surface of a carbon electrode, it led to good
selectivity for CO production (ca. 90%), with a current density of
1.2 mA cm-2 at ⁓ 500 mV overpotential. With these encouraging
preliminary results at hand, and keeping in mind the idea of a
simple, cheap and easy-to-prepare material, we manufactured
gas diffusion electrodes by depositing a thin nanoscaled film
containing FeP and carbon particles onto a carbon paper support.
When implemented in an electrolyzer, this catalytic material
proved to be exceptionally active for the CO2RR, even surpassing
the performance of noble metal based nanomaterials.
[a]
Dr., K., Torbensen, Dr., C., Han, Mr. B., Boudy, Dr., N., von Wolff,
Prof., Dr. M., Robert
Université de Paris, Laboratoire d’Electrochimie Moléculaire, CNRS,
F-75013 Paris, France
E-mail: robert@univ-paris-diderot.fr
Dr., C., Han
College of Aerospace Science and Engineering, National University
of Defense Technology, 109 Deya Road, Changsha, Hunan 410073,
P. R. China
Scheme 1. Structure of the catalyst FeP (left, counterions omitted for clarity)
and schematic view of the flow cell operating in pH neutral conditions (right).
[b]
[c]
[d]
Dr. C., Bertail,
The catalytic ink was prepared from a suspension of carbon
black and FeP. It allows to ensure good electronic conduction
between the molecular catalyst and the support. The ink was
deposited onto carbon paper by drop casting, leading to a 1 cm2
gas diffusion electrode (GDE), with a FeP loading of 2.08 mg cm-
2. SEM images of the film are shown in Figure S1. Qualitative EDX
elemental mapping analysis was performed and a homogeneous
Air Liquide Research&Development Paris Innovation Campus, F-
78354 Jouy en Josas, France
Dr. W., Braun
Air Liquide Forschung und Entwicklung GmbH, Gwinnerstraße 27-
33, 60388 Frankfurt, Germany
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.202000160
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distribution of the catalyst at the GDE (Figure S2) was found. We
then verified that immobilization of the catalyst did not alter its
molecular integrity. Fe K-edge X-Ray absorption near-edge
structure spectra recorded at a FeP loaded GDE and from a FeP
pellet did not show any significant difference (Figure S3A), while
UV-vis spectra of FeP was similar to the spectra obtained after
extracting the complex from a GDE using DMF as solvent (Figure
S3B). Cyclic voltammetry was also performed at a GDE to verify
that the FeP was electrochemically active. However, due to the
hydrophobic nature of the film, only a small fraction of the total
amount of catalyst could be addressed when interrogated from
the front side of the GDE (see Figure S4 for further explanation).
Upon introducing CO2 to the electrochemical cell, a clear catalytic
response was observed (Figure S5). The catalyst coated GDE
was then inserted into a flow cell setup, Scheme 1 (right). The
electrolyzer consists of a sandwich of mounted flow frames,
electrodes, gaskets and an ion exchange membrane, which were
assembled as schematically illustrated in Figure S6. CO2 is
allowed to flow through the gas diffusion electrode, while the
catholyte solution is circulated in between the GDE and the anion
exchange membrane (AEM). Within the GDE, CO2 undergoes a
2-electron-2-proton reduction forming CO and OH-. In the anodic
compartment, water is oxidized to dioxygen and H+ at a Pt/Ti alloy
electrode. The AEM allows for diffusion of OH-, thereby
maintaining overall electroneutrality upon reacting with protons. In
the following electrolysis experiments, no liquid CO2 reduction
products could be detected by NMR or ion chromatography.
verified by inserting in the electrolyzer a carbon film with non-
metalated porphyrin, which resulted in exclusive H2 production
(Figure S7).
The endurance of the catalytic film was tested by performing
a 24 h chronoamperometric electrolysis at -0.78 V vs RHE, so as
to compromise between high rate and selectivity. As shown in
Figure 2, excellent stability was obtained during the whole course
of the electrolysis, with an average current density of 27.3 mA cm-
2
and 98.2 ± 0.2% selectivity for CO. It corresponds to a
production of 0.5 mmol h-1 CO, which, based on the total catalyst
loading (1.37 µmol cm-2), gives a lower limit for the TON of 8765
and TOF of 365 h-1. The TON and TOF values reported herein are
thus significantly underestimated. The cell potential (Ecell) was 3.4
V and an energy efficiency (EE) of 39% was calculated from EE
= FE% x E0/Ecell, where E0 is the apparent standard potential
difference between the anodic and cathodic reactions (1.34 V).
EDX elemental mapping analysis was performed at the FeP
loaded GDE after the 24 h electrolysis, showing almost no change
in elemental composition and no aggregation of iron, which
remains homogeneously dispersed in the film (Figure S8).
XANES of the GDE and UV-vis of catalyst extracted from the
GDE after electrolysis further proved the molecular integrity of
FeP intact (Figures S9A and S9B).
0
-5
100.0
99.5
99.0
98.5
98.0
97.5
97.0
96.5
96.0
-10
-15
-20
-25
-30
-35
-40
Catalytic activity was first investigated as a function of the
applied potential in a CO2-saturated 0.5 M NaHCO3 solution.
Figure 1 illustrates the current density increase as a function of
the potential, along with the selectivity for CO production (see also
Table S1). At a low overpotential () of 270 mV (no IR drop
correction was applied throughout the paper), CO was detected
as the only product (99.9% selectivity) with a total current density
of 7.2 mA cm-2. Selectivity for carbon monoxide remained above
or equal to 99% up to 700 mV overpotential, and H2 was the only
minor by-product that could be detected. At a potential of -0.98 V
vs. RHE, the current density reached 42.1 mA cm-2 along with
98.6% CO selectivity. The catalyst ability to suppress the H2
evolution reaction (HER) in favor of the CO2RR was further
0
5
10
Time/h
15
20
25
Figure 2. Current density (left axis) and CO selectivity (right axis) during a 24 h
electrolysis at E = -0.78 V vs RHE in 0.5 M NaHCO3 electrolyte solution.
Encouraged by these results, a high current density of 50 mA
cm-2 could be sustained for about 3 hours at a potential of -1.14 V
vs RHE (η = 1.03 V), with 98.3 ± 0.3% average selectivity and a
production of 0.9 mmol h-1 CO (Figure S10). This was obtained at
the expense of an increase of the cell potential (4.05 V), lowering
the EE to 33%. The Ohmic resistance of the electrolyzer cell could
be reduced upon increasing the electrolyte concentration. Using
a 1.0 M NaHCO3 electrolyte solution, a current density of 50 mA
cm-2 was sustained for 3 h at only -0.86 V vs RHE (η = 0.75 V),
reducing the cell potential by 590 mV and increasing the energy
efficiency up to 38% (Figure S11). The CO product selectivity was
maintained at a high value of 99.0 ± 0.2% on average. Increasing
the temperature from 24 °C up to 40 °C led to larger current
densities, as shown in Figure 3, while maintaining a high CO
selectivity. At E = -0.78 V vs RHE, the current density raised from
28 to 39 mA cm-2 (in 0.5 M NaHCO3). It was verified that the pH
of the electrolyte solution was about constant within this
temperature range. Regarding thermodynamics, increasing the
temperature amounts to decreasing the overpotential since the
20
10
101.0
100.5
100.0
99.5
99.0
98.5
98.0
97.5
97.0
96.5
96.0
0
-10
-20
-30
-40
-50
-60
-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3
E/V vs RHE
Figure 1. Current density (left axis) at the gas diffusion electrode and CO
selectivity (right axis) as a function of the applied potential in 0.5 M NaHCO3
electrolyte solution.
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COMMUNICATION
as summarized in Table 1. As recently suggested by Chan et al.,34
cations having the smallest hydration radius, which is inversely
proportional to the ionic radius, may lead to the strongest
interfacial electric field, favoring CO2 adsorption at catalytic sites.
Alkaline conditions allowed to boost the performance a step
further. Using 1 M KOH as electrolyte, a current density of 37.6
mA cm-2 was obtained at 70 mV overpotential with perfect
selectivity for CO (99.9%). A current density of 155 m Acm-2 was
reached at  = 470 mV with a selectivity for CO of 98.1%,
corresponding to a partial current density for CO (jCO) of 152 mA
cm-2 (see Figure 4 and Table S3). Such current density for CO
production is significantly higher than that obtained with a state-
of-the art noble metal based material (500 nm thick Ag film mixed
with carbon particles and deposited at a PTFE membrane) which
gave a jCO of ca. 90 mA cm-2 at   480 mV.24 Figure 4 display the
current density increase as a function of the potential, along with
the selectivity for CO production (see also Table S3). A control
experiment with the non-metalated porphyrin ligand only afforded
H2 as reduction product (Figure S17).
-15
-20
100.0
99.5
99.0
98.5
98.0
97.5
97.0
96.5
96.0
-25
24oC
24oC
-30
-35
34oC
40oC
-40
-45
0
50
100
Time/min
150
200
Figure 3. Current density (left axis) and CO selectivity (right axis) obtained at
various temperatures as indicated. Electrolysis performed at E = -0.78 V vs RHE
in 0.5 M NaHCO3 electrolyte solution.
apparent standard potential for converting CO2 into CO is E0 –
(RT ln10/2F) x pH. Thus, the observed rate increase could be
attributed to enhanced kinetics.
As recently demonstrated,32 the GDE likely counter-balances
CO2 solubility decrease when T is raised, preventing catalysis
limitation by mass transport and ensuring a constant (steady
state) CO2 concentration at the catalytic sites. TOF values are
hence equivalent to pseudo first order rate constant for the CO
production. An Eyring plot provided an activation enthalpy of 9.5
kJ mol-1 and an activation entropy of -231 Jmol-1 K-1, suggesting
an associative rate determining step, possibly CO2 binding to the
reduced FeP, as already proposed from electrochemical studies
(Figure S12). A prolonged electrolysis was performed for 3 h at a
constant current density of 50 mA cm-2 (in 0.5 M NaHCO3) at 40°
C (Figure S13), leading to 98.6 ± 0.1% selectivity for CO at an
average overpotential of just 730 mV (compared to 1.03 V at 24°
C), illustrating the positive impact of such small temperature
increase.
50
0
101.0
100.5
100.0
99.5
99.0
98.5
98.0
97.5
97.0
96.5
96.0
-50
-100
-150
-200
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
E/V vs RHE
Figure 4. Current density (left axis) at the gas diffusion electrode and CO
selectivity (right axis) as a function of the applied potential in 1 M KOH
electrolyte solution.
Besides increasing the electrolyte salt concentration and
temperature, it was recently shown that the cation size of the
electrolyte may also impact the catalytic performance.[33-34] Using
a 0.5 M CsHCO3 as electrolyte solution (see Figure S14 and Table
S2), a significant performance increase was obtained with a
current density of 9 mA cm-2 at 270 mV overpotential (99.9% CO
selectivity) and a current density (at  = 870 ) of 83.7 mA cm-2
(97.6% selectivity), a value two times larger than the one obtained
using a 0.5 M NaHCO3 solution. Comparative electrolysis at 50
mA cm-2 (Figure S15) led to a 390 mV overpotential decrease
along with a slight CO selectivity increase and an EE gain of 8%,
To compare the endurance of FeP in similar conditions with
the pervious results gathered at pH close to neutral, a
chronopotentiometric electrolysis was performed at 27 mA cm-2
for 24 h. As shown in Figure 5, an average potential of -0.16 V vs
RHE (η = 0.05 V) was measured, showing only a minute increase
over time. The CO product selectivity was 99.7 ± 0.2% on average
with a total CO production of 0.5 mmol h-1, a TON of 8719 and a
1.0
0.8
101.0
100.5
100.0
99.5
Table 1. Catalytic performances for 3h electrolysis at 50 mA cm-2 in a 0.5 M
electrolyte solution (NaHCO3, KHCO3 or CsHCO3).
0.6
0.4
Cation
E/V
η/V
CO
EE/%
vs RHE
Selectivity/%
0.2
0.0
Na+
K+⁕
-1.14
-0.95
-0.75
1.03
0.84
0.64
98.3
98.6
98.9
33
36
41
99.0
-0.2
-0.4
-0.6
98.5
Cs+
98.0
0
5
10
Time/h
15
20
25
⁕Electrolysis in 0.5 M KHCO3 electrolyte (Figure S16) was performed so as to
confirm the trend described in the text.
Figure 5. Applied potential (left axis) and CO selectivity (right axis) for a 24 h
electrolysis performed at 27 mA cm-2 in 1 M KOH electrolyte solution.
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10.1002/chem.202000160
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COMMUNICATION
TOF of 363 h-1. The cell potential was only 1.87 V, corresponding
to an EE of 71%. EDX elemental mapping of the GDE after the 24
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electrolysis showed that FeP was still present and
homogeneously dispersed in the film (Figure S18). XANES of the
GDE and UV-vis of catalyst extracted from the GDE after
electrolysis further proved the molecular integrity of FeP intact
(Figures S19A and S19B). Finally, at 50 mA cm-2 current density,
a remarkably low overpotential of 120 mV was maintained during
a 3 h electrolysis, producing 0.9 mmol h-1 CO with 99.8 ± 0.2%
selectivity, a TON of 2023 and a TOF of 674 h-1, along with an EE
of 57% (Figure S20).
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In conclusion, FeP appears as an exceptionally efficient
supported homogeneous catalyst for the conversion of CO2 to CO
in water once inserted in a flow cell. From neutral pH to alkaline
conditions, selectivities larger than 98% were systematically
obtained, thanks to the high reactivity of the catalyst with CO2 and
the low overpotential values that makes the HER pathway
unfavorable. The catalytic material proved to be stable under
operating conditions over prolonged electrolysis time. Slight
increase in temperature (from 25 to 40 °C) as well as increasing
the electrolyte concentration and the cation size were beneficial
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conversion and the energy efficiency. Remarkably, a very high
current density for CO could be obtained at a low overpotential
(jCO > 150 mA cm-2 at η = 470 mV), outperforming state of the art
silver based nanomaterials. These results illustrate that molecular
catalysts can be used in flow cell devices and are candidates for
being included into large scale CO2 electrolyzers.
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The work described in this project was financed in part by Air
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Text for Table of Contents
Kristian Torbensen, Cheng Han,
Benjamin Boudy, Niklas von Wolff,
Caroline Bertail, Waldemar Braun and
Marc Robert*
Page No. – Page No.
Iron Porphyrin Does Fast and
Selective Electrocatalytic Conversion
of CO2 to CO in a Flow Cell
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