Anthraquinone thin-film electrodes for reversible CO2 capture and release

Article abstract of DOI:10.1039/c8ta04817g

We report reversible electrochemical capture and release of carbon dioxide using the well-known dye precursor and industrial catalyst anthraquinone. Although quinones are well-studied for electrochemical capture and release of CO₂ in solution, we have discovered that a 100 nm film of anthraquinone can realize this in a heterogeneous fashion. In-depth spectroelectrochemical studies were performed in order to study the mechanism of this CO₂ capture and release. Anthraquinone films reached an uptake capacity of 5.9 mmolCO₂ g_AQ^-1.

Full text of DOI:10.1039/c8ta04817g

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Anthraquinone thin-film electrodes for reversible
CO2 capture and release†
Cite this: DOI: 10.1039/c8ta04817g
*
Dominik Wielend,
Dogukan Hazar Apaydin
and Niyazi Serdar Sariciftci
We report reversible electrochemical capture and release of carbon dioxide using the well-known dye
precursor and industrial catalyst anthraquinone. Although quinones are well-studied for electrochemical
capture and release of CO2 in solution, we have discovered that a 100 nm film of anthraquinone can
realize this in a heterogeneous fashion. In-depth spectroelectrochemical studies were performed in
Received 23rd May 2018
Accepted 2nd July 2018
DOI: 10.1039/c8ta04817g
order to study the mechanism of this CO2 capture and release. Anthraquinone films reached an uptake
ꢀ1
rsc.li/materials-a
capacity of 5.9 mmolCO gAQ
.
2
absorption, an energy-efficient way, for CO2 capture and
release.23
1. Introduction
Most of these systems reported in literature are either
operated in organic solvents,12–14,18,20 ionic liquids15,16,21,22 or
contain specially synthesized tailor-made compounds.19 An
industrially-scalable, cheap and abundant chemical compound
which can take up the task is still missing. In this work, we
demonstrate that the infamous, simple compound, anthraqui-
none is up to the task and capable of electrochemical capture
and release carbon dioxide efficiently.
The general strategy of Carbon Capture and Utilization (CCU)
aims to use and recycle carbon dioxide (CO2) towards useful
chemicals. Several approaches have been emerging in the
scientic community during the last decades to realize this
strategy.1–5 Before reducing CO2 one has to capture and
concentrate carbon dioxide from the atmospheric contents of
400 ppm. In addition to the use of industrial capture methods6
like amine scrubbing,7,8 organic solvents9 or physical adsorp-
tion on high surface-area materials10,11 the electrochemical
capture and release of carbon dioxide offers interesting
possibilities.
2. Experimental
A promising class of materials for electrochemical CO2
capture is the carbonyl bearing conjugated compound family.
In 1984 Harada et al. reported unsaturated a,b-ketones for
carboxylation upon electrochemical reduction.12 Some years
later quinones became well-studied in homogeneous solutions
for CO2 capture upon the formation of electrogenerated nucle-
ophiles while forming carbonate-like structures.13–16 A very
recent publication by Yin et al. describes the application of
quinones, including anthraquinone, as mediators in lithium–
CO2 batteries.17 Recent publications by our group investigated
thin lms of carbonyl pigments (quinacridone and a naphtha-
lene bisimide derivate, NBIT) for heterogeneous electro-
chemical capture and release.18,19
Electrode preparation
Glassy carbon electrodes at a size of 4 ꢁ 1 cm were initially
cleaned with 18 MU water (MQ) and acetone (VWR Chemicals,
technical grade). Aerwards each side was polished for 30 s
with a Buehler Micropolish II deagglomerated alumina in
decreasing particle sizes from 1.0 to 0.3 to 0.05 mm. Cleaning
in between was performed by sonication in iso-propanol
(VWR Chemicals, AnalaR Normapur) and MQ water. In
a last step, excess Al2O3 was removed by polishing each side
for 30 s with toothpaste followed by sonication in iso-
propanol and MQ water.
The nal step was electrochemical cleaning by sweeping the
potential between +1.5 and ꢀ1.0 V vs. Ag/AgCl/3 M KCl in 0.5 M
H2SO4 at a speed of 50 mV sꢀ1 for 30 cycles.
Glass (0.7 ꢁ 6.0 cm) and FTO/glass (0.75 ꢁ 2.5 cm)
substrates were cut into the appropriate size followed by
cleaning via sonication for 30 min each in the following media:
acetone (VWR chemicals, technical grade), 2% Hellmanex
solution (Hellma-Analytics), MQ water and iso-propanol (VWR
Chemicals, AnalaR Normapur).
Besides carbonyl compounds also electrochemically acti-
vated 4,40-bipyridinium20,21 and recently benzyldisulde22
compounds were reported to reversibly capture and release of
carbon dioxide in organic solvents and ionic liquids. Further-
more, Liu and Landskron reported supercapacitive swing
Linz Institute for Organic Solar Cells (LIOS), Institute of Physical Chemistry, Johannes
Kepler University Linz, Altenbergerstrasse 69, 4040 Linz, Austria. E-mail: dominik.
wielend@jku.at
Glass samples are further treated in an evaporation chamber
for thermal evaporation of 5 nm chromium followed by 80 nm
of gold.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta04817g
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The ATR-FTIR in situ spectroelectrochemistry studies were
performed on a Bruker IFS-66/S machine averaging over 32
scans. The ATR-FTIR spectra of pure anthracene and anthra-
quinone were recorded on a Bruker Vertex 80-ATR machine.
The mentioned in situ spectroelectrochemical investigations
were performed one time under N2 and once CO2 conditions.
Anthraquinone evaporation
1.0 g of commercially-available anthraquinone (Sigma Aldrich,
97%) were puried by sublimation at 250 ꢂC under vacuum for
20 h. This product from 2 batches was puried following this
protocol a second time before using it in an organic material
evaporator. Evaporation was performed at 57 ꢂC at ꢃ10ꢀ6 mbar
for 50 s. Under those conditions uniform AQ lms with
a thickness of 100 nm were formed. The average thickness and
evaporation speed in nm minꢀ1 respectively was determined by
a Bruker DekTak XT Prolometer.
For the CO2 capture/release quantication
a
one-
compartment cell with a total volume of 23.3 mL was used.
An AQ/Cr–Au/glass electrode acted as WE, an Ag/AgCl as RE and
a platinum wire as CE. 13.3 mL of 0.1 Na2SO4 as electrolyte
solution were used resulting in a total free headspace of 10.0
mL.
Electrochemistry
Aer purging the cell for 1 h with N2 and 1 h with CO2, two
CV cycles for capturing were performed. Aer removing the
excess CO2 by 2 h purging with N2 again two CV cycles for
release were performed. Aerwards a 2 mL aliquot of the
headspace is taken and injected into a N2 purged IR gas cell.
Measuring of the FTIR spectrum and integration of the area in
absorbance mode allows quantication of the released amount
of CO2.
A calibration curve was made by injection of dened aliquots
of a 5% CO2 in N2 mixture into the N2 ushed IR gas cell. Aer
equilibration of the conditions for 45 min in the Bruker IFS-66/S
machine to remove CO2 from the machine, IR spectra were
recorded and integrated and the calibration curve was obtained
(see ESI Fig. S1†).
For electrochemical characterization experiments a Jaissle
Potentiostat-Galvanostat 1030 PC-T was used. For simple cyclic
voltammetry (CV) experiments a two-compartment cell con-
taining a three-electrode setup was used. 20.0 mL of 0.1 M
Na2SO4 solution was used as electrolyte solution. A platinum
electrode was used as counter electrode (CE), a commercial Ag/
AgCl/3 M KCl electrode as reference electrode (RE) and a 100 nm
lm of AQ on either Cr–Au/glass or glassy carbon as working
electrode (WE).
The cell was purged with nitrogen gas (N2) for one hour prior
to electrochemical measurements. Aer CV measurement, the
cell was purged one hour with carbon dioxide (CO2). To remove
CO2 again from the electrolyte solution, another two hours
purging with N2 was applied.
In all CV measurements two cycles were recorded and due to
reason of reproducibility always the second cycles were plotted.
3. Results and discussion
The rst step to examine the application of the industrial dye
precursor anthraquinone (AQ) as potential electrochemical
carbon dioxide capture agent was performing cyclic voltam-
metry (CV) at different conditions. Thermally evaporated
lms of anthraquinone with an optimized thickness of
100 nm in aqueous 0.1 M Na2SO4 solution exhibited prom-
ising electrochemical behavior upon CV measurement as
shown in Fig. 1.
The initial nitrogen (N2) saturated conditions in Fig. 1 show
a quasi-reversible reduction feature at ꢀ0.80 V which is in
accordance to homogenous reduction of anthracene-9,10-diol
(hydroquinone) structure reported in literature.24–26 Under CO2
saturated conditions the electrochemical response of AQ nearly
vanished and only very low current was observed upon further
reduction.
UV-vis spectroelectrochemistry
For UV-vis spectroelectrochemistry a quartz cuvette lled with
2 mL of 0.1 M Na2SO4 electrolyte solution was used. An Ag/AgCl
wire was utilized as quasi reference electrode, a platinum wire
as CE and an AQ/FTO/glass as WE. A Jasco V-670 Spectropho-
tometer was used for performing the absorption experiments.
One absorption measurement took 20 s which is the reason
for setting step potentials in 0.1 V steps changing every 20 s.
Starting from 0.0 V the potential was decreased gradually to
ꢀ1.8 V (all vs. Ag/AgCl).
Under N2 conditions the nal vial setup was purged with N2
for 45 min. Under CO2 conditions aer purging 45 min with N2,
another 45 min purging with CO2 was performed.
Aer removal of unbound CO2 out of the system and satu-
ration of the solution with N2 followed by electrochemical
oxidation gave rise to a small oxidation peak at +0.27 V which
FTIR spectroscopy
For ATR-FTIR spectroelectrochemistry an AQ/Ge crystal was was assigned to the release of carbon dioxide. Aerwards,
used as WE whereas standard Ag/AgCl wire is used as RE and cycling further to negative potentials reveals once more char-
a platinum plate as CE. A constant ow of 3 mL minꢀ1 of 0.1 M acteristic quasi-reversible reductive feature of AQ. The signi-
Na2SO4 electrolyte solution was established and the same step cant decrease in current density from initial ꢀ0.68 to ꢀ0.22 mA
potential as in UV-vis spectroelectrochemistry was applied. In cmꢀ2 aer release could be explained by the dissolution of the
those experiments potentials up to ꢀ2.1 V vs. Ag/AgCl were soluble hydroquinone species from the electrode.
applied.
The vanishing of the reductive peaks behavior was already
A bottle of the electrolyte solution was purged with N2 over reported for different carbonyl bearing pigments in litera-
night. A fraction of it was taken out and in a separate bottle in ture.18,19 Like in the previous system we assigned this to the in
addition purged with CO2 again for one night.
situ formation of a carbonate-like structure illustrated in Fig. 1.
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carbon. Furthermore, the comparison of the rst and the
second cycle under CO2 is shown in the same graph.
Comparing the curves of AQ/Cr–Au in Fig. 2 to the ones of
AQ/GC in Fig. 1 revealed a nearly identical shape. The only
difference was the lack of the small oxidative release peak at
+0.27 V. To the best of our knowledge, this might be due to the
fact that glassy carbon possesses a different surface structure
than gold. We observe, that on gold a rearrangement of the AQ
was taking place which caused the lack of this peak (Fig. 3).
As discussed before, initially the AQ is reduced forming
nucleophilic hydroquinone structure which is then presumed
to bind CO2.13,14,16 This theory was supported by comparing the
rst CV cycle under CO2 with the second one in the inset of
Fig. 2. The rst cycle still showed the reductive peak of
anthraquinone, which in the second cycle disappeared due the
formation of the mentioned carbonate-like structure. Under
those CO2 conditions, this at CV behavior was observed over 50
cycles. Only aer repurging the system with N2 and releasing
CO2 by sweeping to the oxidative side, the reductive peak of
anthraquinone was recovered again.
We have also observed physical changes on the crystal
structure of AQ thin lms. Fig. 3 shows SEM image of AQ lm
directly aer evaporation (on the le) forming a quite uniform
brick-like structure. Upon performing the electrochemical
capture and release of carbon dioxide, needle-like microcrystals
(Mikado-like) were observed (on the right).
Such recrystallization of carbonyl pigments has also already
been reported.27,28 The reason for this was most probably the
partial dissolving and recrystallization of AQ molecules during
the process.
Fig. 1 CV curves of AQ on glassy carbon (GC) under different gas-
saturated conditions.
To further investigate and characterize the optical absorp-
tion changes, UV-vis spectroelectrochemistry experiments were
conducted as shown in Fig. 4.
The pictures A to D in Fig. 4 show the observed color changes
upon electrochemistry on a Cr–Au electrode. Under electro-
chemical reduction under N2 saturated conditions (Fig. 4B and
D) a color change to red-orange caused by the soluble
anthracene-9,10-diol species was observed. Besides, under CO2-
rich atmosphere (Fig. 4C) no dissolving process but a change to
a brighter yellow species was observed.
In further experiments not only glassy carbon was used as
electrode material but also thin-layers of chromium and gold
(Cr–Au) on glass substrates. The CV of AQ/Cr–Au/glass is shown
in Fig. 2 in order to conrm the similarity to the CV of AQ/glassy
These qualitative observations were justied by the UV-vis
spectroelectrochemistry. Under N2 saturated conditions three
strong absorption bands at 390, 416 and 460 nm were observed.
According to Babaei et al. and Shamsipur et al. the three bands
could be correlated to differently protonated anthracene-9,10-
diol species whereas in this aqueous surrounding the pres-
ence of a radical species was very unlikely.25,26 The absorption
maximum for AQ2ꢀ was reported at 466/470 nm and the one for
AQHꢀ at 412/414 nm. As they reported that protonation was
causing a blueshi in absorption, the detected band at 390 nm
could be assigned to the AQH2 species.25,26
In contrast, under CO2 saturated conditions the system was
forming an absorption shoulder at 440 nm which was respon-
sible for the brighter yellow color shown in Fig. 4C. To further
substantiate the proposed anthraquinone–carbonate structure
depicted in Fig. 1, in situ ATR-FTIR spectroelectrochemistry of
AQ on a germanium (Ge) reection element/electrode was
Fig. 2 Cyclic voltammetry of AQ/Cr–Au/glass in 0.1 M Na2SO4. The
insert shows the comparison of the first and the second cycle of CV
under CO2.
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Fig. 3 Scanning electron microscopy (SEM) images of the pristine anthraquinone film on Cr–Au on glass (left) and after electrochemical CO2
capture and release (right).
conducted under CO2 conditions (N2 conditions shown in the disappearing peaks could fully be correlated to anthraqui-
Fig. S2†). In Fig. 5 the characteristic region is zoomed in and the none and the characteristic C]O bands, respectively.
arising/disappearing bands were correlated with the molecular
structural changes.
The uctuations in the baseline of the measurements might
arise from the air that is trapped within the powder.
In analogy to the IR spectrum under N2 saturated conditions
The arising band at 1066 cmꢀ1 under N2 conditions was
(see ESI†) quite strong negative bands (disappearing of func- a proof for Carom–O stretching vibration in the anthracene-9,10-
tional groups) were observed upon reductive dissolving of diol structure.30 Under CO2 conditions two bands at 1051/
anthraquinone shown in Fig. 5. In order to compare the dis- 1018 cmꢀ1 indicated the formation of Carom–O single bonds.30
appearing peaks with the material, ATR-IR powder spectra of As those bands appeared at lower frequencies compared to
anthracene and anthraquinone were measured, shown in Fig. 6. nitrogen conditions, they were assumed to be caused by larger
According to the ATR-IR powder spectra of anthraquinone moieties attached to the oxygen atom. Also, a literature report
and anthracene in Fig. 6 and from the examples in literature29 from Criswell and Klanderman assigned bands in this wave-
number region from 960 to 1060 cmꢀ1 to C–O bonds in
Fig. 4 UV-vis spectra of AQ/FTO/glass electrodes under nitrogen saturated conditions (left) and under CO2 saturated conditions (right) at
various potentials vs. Ag/AgCl. On the bottom pictures of AQ/Cr–Au/glass while performing CV are shown. (A) No potential applied, (B) at ꢀ1.0 V
vs. Ag/AgCl/3 M KCl under N2, (C) at ꢀ1.0 V vs. Ag/AgCl/3 M KCl under CO2, (D) at ꢀ1.0 V vs. Ag/AgCl/3 M KCl after release under N2.
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Fig. 5 ATR-FTIR spectroelectrochemistry of AQ/Ge under CO2 saturated conditions at various potentials vs. Ag/AgCl (top). On the bottom the
proposed structure of the anthraquinone–carbonate species with assignment of functional groups to the IR spectrum.
comparable structures.31 In addition, the occurrence of the
band at 1234 cmꢀ1 was most likely caused by a C–O stretch of
a carboxylic acid. Nyquist and Potts32 reported for asymmetric
O–C–O stretches in proximity of an aromatic system values
between 1211 and 1248 cmꢀ1 which was also in accordance to
Silverstein, Bassler and Morrill.30 Furthermore, Nyquist and
Potts reported the existence of out-of-plane skeletal deforma-
tions at ꢃ800 cmꢀ1, which we correlated to the rising peak at
758 cmꢀ1
.
32
Under both, nitrogen and carbon dioxide, conditions a broad
band at 3350 cmꢀ1 from O–H bonds was appearing which did
not allow differentiation between the phenolic and carboxylic
O–H bond. Besides occurring of a C–O band from the carboxylic
acid also the appearance of a C]O band would be expected.
Due to the high degree of dissolving during the experiment we
assumed this evolving feature to be overlapping with the dis-
appearing C]O of the anthraquinone and other IR features in
this region (according to Fig. 6).
Fig.
powders.
6 ATR-IR spectra of pure anthracene and anthraquinone
Besides the different features of the evolving Carom–O under
N2 and CO2 and the rise of bands at 1234 cmꢀ1 and 758 cmꢀ1
from the carboxylic acid underlined the formation of a new
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species upon electrochemical reduction of anthraquinone and Dr Mariusz Wolff for conducting UV-vis measurements in
under CO2 saturated conditions.
their facilities is gratefully acknowledged.
In addition to investigations about the structural changes
upon the carbon dioxide capture also the quantication of the
CO2 capture process was studied. Assuming the possible uptake
of 2 moles CO2 per anthraquinone molecule resulted in a theo-
Notes and references
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retical uptake capacity of 9.6 mmolCO gAQꢀ1. Electrochemical
2
capture and release experiments in triplicates resulted in an
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ꢀ1
average experimental uptake capacity of 5.9 mmolCO gAQ
,
2
which referred to an efficiency of 61% (it was assumed that the
detected and released amount was equal to the captured
amount). As the efficiency was above 50% the assumption of
capturing two moles CO2 per molecule anthraquinone in some
parts of the electrode seemed to be plausible. This uptake
capacity value was higher than the compounds reported previ-
ously by our group18,19 for electrochemical capture and release
and comparable to the industrially-used capturing agent mon-
oethanolamine.8 Those good capture and release features could
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Authors gratefully acknowledge the funding of from FWF within
the framework of Prof. Saricici's Wittgenstein Prize (Solare 22 P. Singh, J. H. Rheinhardt, J. Z. Olson, P. Tarakeshwar,
Energie Umwandlung Z222-N19) and the FFG within the project
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