Acid–Base Interaction Enhancing Oxygen Tolerance in Electrocatalytic Carbon Dioxide Reduction

Article abstract of DOI:10.1002/anie.202003093

Hybrid electrodes with improved O₂ tolerance and capability of CO₂ conversion into liquid products in the presence of O₂ are presented. Aniline molecules are introduced into the pore structure of a polymer of intrinsic microporosity to expand its gas separation functionality beyond pure physical sieving. The chemical interaction between the acidic CO₂ molecule and the basic amino group of aniline renders enhanced CO₂ separation from O₂. Loaded with a cobalt phthalocyanine-based cathode catalyst, the hybrid electrode achieves a CO Faradaic efficiency of 71 percent with 10 percent O₂ in the CO₂ feed gas. The electrode can still produce CO at an O₂/CO₂ ratio as high as 9:1. Switching to a Sn-based catalyst, for the first time O₂-tolerant CO₂ electroreduction to liquid products is realized, generating formate with nearly 100 percent selectivity and a current density of 56.7 mA cm^?2 in the presence of 5 percent O₂.

Full text of DOI:10.1002/anie.202003093

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
www.angewandte.org
Accepted Article
Title: Acid-Base Interaction Enhancing Oxygen Tolerance in
Electrocatalytic Carbon Dioxide Reduction
Authors: Pengsong Li, Xu Lu, Zishan Wu, Yueshen Wu, Richard
Malpass-Evans, Neil B. McKeown, Xiaoming Sun, and
Hailiang Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202003093
Angew. Chem. 10.1002/ange.202003093
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http://dx.doi.org/10.1002/ange.202003093

Angewandte Chemie International Edition
10.1002/anie.202003093
1
Acid-Base Interaction Enhancing Oxygen Tolerance in Electrocatalytic Carbon Dioxide
Reduction
1,2,3
2,3,*
2,3
2,3
4
Pengsong Li , Xu Lu , Zishan Wu , Yueshen Wu , Richard Malpass-Evans , Neil B.
4
1,*
2,3,*
McKeown , Xiaoming Sun and Hailiang Wang
1
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, Beijing 100029, P.R. China
2
Department of Chemistry, Yale University, New Haven, Connecticut 06520, USA
3
Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, USA
4
EastChem, School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, UK
*
Corresponding authors: xu.lu@yale.edu (Xu Lu); sunxm@mail.buct.edu.cn (Xiaoming Sun);
hailiang.wang@yale.edu (Hailiang Wang).
Abstract
Catalytic electrodes with the capability to directly convert atmospheric CO into value-added
2
products are of potential environmental and economic benefit, but entail an effective CO
selection strategy to impede the competing O reduction reaction. In this work, we have
developed a new generation of hybrid electrodes with improved O tolerance and
unprecedented capability of CO conversion to liquid products in the presence of O . We
2
2
2
2
2
introduce aniline molecules into the pore structure of a polymer of intrinsic microporosity to
expand its gas separation functionality beyond pure physical sieving. The chemical interaction
between the acidic CO
separation from O . Loaded with a cobalt phthalocyanine-based cathode catalyst, our hybrid
electrode achieves a CO Faradaic efficiency of 71% with 10% O in the CO feed gas. The
electrode can still perform CO production at an O /CO ratio as high as 9:1. Switching to a Sn-
based catalyst, we for the first time realize O -tolerant CO
2
molecule and the basic amino group of aniline renders enhanced CO
2
2
2
2
2
2
2
2
electroreduction to liquid products,
2
generating formate with nearly 100% selectivity and a current density of 56.7 mA/cm in the
presence of 5% O .
2
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Angewandte Chemie International Edition
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2
Electrochemical CO
converting CO to value-added carbon-based products. If achieved on a large scale, it could
help alleviate the global warming and ocean acidification issues. For this process to be more
commercially relevant, the CO reactant should come from practical sources such as
combustion exhaust and ambient air,
However, in a realistic electrolytic cell without mass transport limitation, the CO
2
reduction driven by renewable energy sources is an attractive strategy for
1
-7
2
8
-9
2
1
0-11
both of which contain a substantial amount of O
reduction
because of the considerable
2
.
2
reactions can be completely inhibited by as little as 5% O
2
in CO
2
1
0, 12-15
difference in their standard reduction electrode potentials.
It is therefore challenging yet
potentially highly rewarding to develop a catalytic electrode that can directly valorize O
2
-
containing CO gases without requiring additional energy input for reactant purification.
2
In our prior work, we designed the first O
design was to integrate a CO reduction electrocatalyst with a polymer of intrinsic
microporosity (PIM) layer that can selectively permeate CO
electrode was able to generate CO with a Faradaic efficiency (FE) of 75.9% from CO
containing 5% O . Despite this progress, it is still necessary to further improve O tolerance of
the electrode and expand the scope of products. Considering that PIM separates CO from O
we believe there is opportunity to
enhance the separation process by introducing chemical interactions.
2
-tolerant catalytic electrode for CO reduction. Our
2
2
1
0
2
from its O mixture. This
2
2
2
2
2
2
1
6-17
via a physical process through its size-selective pores,
Here, we report a second generation of O
2
-tolerant catalytic electrodes for CO reduction, which
2
are developed from their predecessors by introducing guest aniline molecules into the PIM
structure and by changing the electrocatalyst. Benefited from the chemical interaction between
acidic CO
2
and the basic amino group of aniline, the PIM/aniline hybrid membrane
vs O selectivity compared to pure PIM. Deployed in an
demonstrates improved CO
2
2
electrolytic flow cell, our electrode comprising such a hybrid gas selection layer and a catalyst
layer of cobalt phthalocyanine (CoPc) molecules anchored on carbon nanotubes (CNTs)
achieves a FECO of 71% in the presence of 10% O
2
in CO
2
. At a high O
2
/CO ratio of 9:1, the
2
electrode can still have net CO conversion, whereas the control electrode without aniline
2
completely loses its function. Switching the catalyst to Sn particles allows us to expand our
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Angewandte Chemie International Edition
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3
products beyond CO and realize the first selective reduction of CO
of 5% O
2
to formate in the presence
2
.
We first used a gas separation experiment (see Supporting Information for details), where a
CO /O mixture gas with a fixed volume ratio of 1:39 flows through the channel on one side of
the gas diffusion electrode (GDE) and a N carrier gas flows on the other side for gas sampling
Figure 1a), to study the amounts of CO and O penetrating the GDE. The GDE was a carbon
fiber paper (CFP) or a CFP with a gas selection layer drop-casted on the side facing the CO /O
mixture gas channel. As can be seen from the gas chromatography (GC) peak areas, the CFP
alone can reject both CO and O to some extent (Figure 1b and c). CFP coated with a layer of
PIM can more effectively limit CO and O penetration and decrease the O /CO ratio compared
to the CFP-only case. When the PIM layer is infiltrated with aniline or toluene, gas (especially
) penetration is further suppressed. In the aniline case, we observed the smallest O /CO ratio
among all the electrode configurations (Figure 1d), revealing its highest CO /O selectivity in
permeability caused by small molecule incorporation in the PIM
2
2
2
(
2
2
2
2
2
2
2
2
2
2
O
2
2
2
2
2
spite of the lower CO
2
structure (Figure 1b).
Figure 1. (a) Schematic diagram of gas separation setup for measuring the selectivity of
membrane for CO separation from O . (b) CO and (c) O signals detected by GC for the
2
2
2
2
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Angewandte Chemie International Edition
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4
effluent of the N channel with different GDEs. Error bars represent standard deviations from
2
multiple GC samplings. (d) O
2
/CO
2
volume ratios in the effluent of the N channel with different
2
GDEs.
We conducted infrared (IR) spectroscopy measurements to understand the enhancing effect of
aniline on PIM for CO separation from O . The IR spectra of aniline and toluene before and
after 1 h of CO bubbling into these two liquids are plotted in Figures 2a-d. Interestingly, the
asymmetrical stretching band of CO
2
2
2
-
1
2
is clearly observed at approximately 2335 cm for the
while no such peak is found in the case of toluene. The
peak and its decrease in wavenumber relative to a free CO molecule
is likely adsorbed by aniline via the chemical interaction
1
8-19
CO
2
-treated aniline sample,
observation of the CO
2
2
-
1 20
(
2349 cm ) indicates that CO
2
2
1-22
between the acidic CO
2
and the basic amino group of aniline.
In fact, amino groups are
adsorption
vs O
often incorporated into metal-organic framework structures to enhance their CO
2
2
3-24
capacity.
Figure 2e depicts the roles played by aniline in the improved CO
2
2
separation of the PIM/aniline material: On the one hand, the aniline molecules residing in the
pore structure of PIM create a physical barrier, which enhances the rejection of the bigger O
2
2
5
(
kinetic diameter 0.35 nm) molecules to a greater extent compared to the smaller CO
2
(0.33
2
6
nm) molecules. This would result in a lower gas permeability but a higher CO
On the other hand, the amino group of aniline can selectively enhance CO transport via acid-
base interactions. In the absence of amino groups, the PIM/toluene membrane separates CO
from O solely through physical sieving. Therefore, the CO /O selectivity of PIM/toluene is
higher than pure PIM but lower than PIM/aniline.
2
/O
2
selectivity.
2
2
2
2
2
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5
Figure 2. IR spectra of (a, b) aniline and (c, d) toluene before and after interacting with CO
e) Schematic diagram of enhanced CO /O separation and O -tolerant catalytic CO reduction
on a hybrid electrode with PIM/aniline.
2
.
(
2
2
2
2
Based on the gas separation results, we anticipate the PIM/aniline-containing GDE would
improve electrocatalytic CO reduction in the presence of O , although the separation
performance may not be directly translated into electrochemical performance because of their
different conditions. To perform electrochemical CO reduction reaction studies, we used CoPc
2
2
2
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6
3
, 10, 27-28
molecules supported on CNTs (CoPc/CNT) as the catalyst,
which was coated onto the
other side of the CFP supporting PIM/aniline (see Supporting Information). A gas-diffusion
1
0, 28
electrochemical cell as reported in our previous work was used.
optimized to be 3.4 V for achieving highest CO reduction selectivity (Figure S1). Figure 3
and Figure S2 show the FECO and total current density (jtotal) for the reduction reactions of
The cell voltage was
2
CO
2
/O
2
mixtures containing different percentages of O
2
. The PIM/aniline electrode operating
2
with 10% O
2
exhibits a FECO of 71% and a jtotal of 30.6 mA/cm , outperforming the
corresponding PIM electrode which affords a FECO of 63% under the same conditions. The
improved FECO is consistent with the improved CO vs O selectivity observed in the gas
separation experiments (Figure 1d). The reasonably high current density indicates that mass
transport of CO , although suppressed by the PIM/aniline layer (Figure 1b), is not
2
2
2
compromising the reaction rate. In contrast, the PIM/toluene electrode, which lacks chemical
2
interaction between toluene and CO
2
, shows a FECO of 33% with a jtotal of 29.3 mA/cm at the
same conditions, lower in selectivity and activity for CO production than that of the PIM
electrode. The poor CO reduction efficiency can be attributed to the low gas permeability of
the PIM/toluene membrane, which hampers CO delivery to the catalytic sites, in line with the
gas separation results (Figure 1b and c). Consistently, H evolution becomes more dominant
Figure 3b). For all these three catalytic electrodes, FECO gradually decreases when the O
content of the feed gas increases, and the PIM/aniline electrode always gives the highest FECO
among the three at any fixed O concentration (Figure 3a). Further control experiments with
2
2
2
(
2
2
PIM/benzene (Figures S3 and S4) and PIM/phenol (Figures S5 and S6) electrodes give similar
results to the PIM/toluene electrode, confirming the critical role of the amino group in the
aniline molecular structure in enhancing CO selection via acid-base interaction.
2
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Angewandte Chemie International Edition
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7
Figure 3. (a) FECO and (b) FEH2 for PIM, PIM/aniline and PIM/toluene cathodes operating with
CO /O feed gas containing different O percentages. Electrolyte: 0.5 M aqueous KHCO
Cathode catalyst: CoPc/CNT; Anode catalyst: CoO /CNT; Cell voltage: 3.4 V. Error bars
2
2
2
3
;
x
represent standard deviations from multiple measurements.
Incorporation of aniline in the PIM layer extends the range of O
2
/CO
to CO. When operating in a gas mixture
is blended in because it is a major component of
air and typical flue gases ), the PIM electrode cannot effectively catalyze CO reduction, with
its measured CO production rate at the same level as the instrument noise in this case (Figure
), whereas the PIM/aniline electrode can still produce CO at a rate that is five times higher
Figure 4). When the O /CO ratio is further increased to 9:1, the PIM/aniline electrode can
reduction at a rate significantly higher than the noise level (Figure 4). This
2
feed ratio under which
the catalytic electrode can effectively convert CO
containing 40% O , 10% CO and 50% N (N
2
2
2
2
2
2
9
2
4
(
2
2
still perform CO
2
represents another small step toward the ultimate goal of direct CO valorization from the
2
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Angewandte Chemie International Edition
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8
atmosphere.
Figure 4. CO production rate vs. volume fraction of O
and O ) with PIM or PIM/aniline as the CO /O selection layer. Electrolyte: 0.5 M aqueous
KHCO ; Cathode catalyst: CoPc/CNT; Anode catalyst: CoO /CNT; Cell voltage: 3.4 V. Volume
fraction of N in the feed gas is fixed at 50%. The dash line in the graph marks the noise level.
2
in the O
2
/CO
2
part of the feed gas (CO ,
2
N
2
2
2
2
3
x
2
Error bars represent standard deviations from multiple measurements.
The PIM/aniline gas selection layer also allows us to produce useful liquid products from
electrochemical CO
2
reduction in the presence of O , which has never been realized before. To
2
3
0
generate formate, we used Sn metal particles as the cathode catalyst and performed
electrolysis in a three-electrode cell with enhanced gas diffusion (see Supporting Information).
With 5% O
2
in the feed gas, the control Sn electrode without PIM/aniline exhibits a jtotal up to
2
2
95.0 mA/cm (Figure 5a) but produces no formate (Figure 5b and 5c) at various electrode
potentials. This is because O
2
reduction completely dominates the catalyst surface, in
1
0
consistency with our previous observation. In sharp contrast, the electrode with PIM/aniline
as the gas selection layer can catalyze CO reduction to formate with a FE close to 100% and a
2
2
j
formate of 56.7 mA/cm at a cathode potential of -2.4 V vs Ag/AgCl, despite the presence of 5%
2
O
2
. As the electrode potential is further polarized to -2.8 V, jformate increases to 73.6 mA/cm .
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9
Figure 5. (a) Total current density, (b) formate FE, and (c) formate partial current density at
various cathode potentials (without iR correction). There is 5% O in the CO feed gas.
Electrolyte: 0.5 M aqueous KHCO ; Cathode catalyst: Sn; Anode catalyst: NiFe-layered double
hydroxide.
2
2
3
In summary, we have developed a CO -selective layer by introducing aniline into the pores of
2
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Angewandte Chemie International Edition
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10
PIM and revealed that the acid-base interaction between CO
separation from O . Loaded with CO reduction electrocatalysts, the PIM/aniline catalytic
electrodes show improved O tolerance. CO in a feed gas with an O /CO ratio as high as 9:1
can be effectively reduced to CO. Formate can be produced at a near-unity FE and a high current
density from electrochemical CO reduction in the presence of O . The strategy of introducing
chemical sieving to a gas separation membrane could be useful for directly mining the
atmospheric CO for fuels.
2
and aniline enhances CO
2
2
2
2
2
2
2
2
2
2
Acknowledgements
This work was supported by the National Science Foundation (Grant CHE-1651717). Pengsong
Li acknowledges support from China Scholarship Council (CSC). Xu Lu thanks the Croucher
Fellowship for Postdoctoral Research. Neil McKeown acknowledges the Engineering and
Physical Sciences Research Council (EPSRC EP/M01486X/1).
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TOC figure and text
An aniline-infiltrated polymer-of-intrinsic-microporosity (PIM) membrane is reported for
direct valorization of CO from its mixture with O . The acid-base interaction between CO and
aniline enhances CO /O separation, enabling catalytic electrodes capable of producing CO
from a feed gas with an O /CO ratio as high as 9:1 and of reducing CO selectively to formate
in the presence of O
2
2
2
2
2
2
2
2
2
.
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