Task-specific functionalization of graphene for use as a cathode catalyst support for carbon dioxide conversion

Article abstract of DOI:10.1039/c4ta05087h

This study describes the potential advantages of task-specific functionalization of graphene for carbon dioxide capture and conversion. An ionic liquid was employed as a functional moiety on a graphene catalyst support since it shows reversible interactions with CO₂ molecules. In this study, graphene was synthesized by a hydrogen-assisted low-temperature-exfoliation method and functionalized using a polymerized ionic liquid (PIL). Adsorption analysis shows that PIL functionalization improves the CO₂ adsorption capacity of graphene significantly. Catalytic nanoparticles were dispersed over the catalyst supports by a microwave assisted polyol reduction method and the catalysts were employed as the cathode catalyst in a polymer electrolyte membrane (PEM) CO₂ conversion cell. The cell hydrogenates CO₂ into formic acid at the cathode, which was quantified by spectrophotometry. The PIL functionalized support shows a higher formic acid formation rate compared to a pure support under similar experimental conditions since PIL functionalization improves the interactions between the catalyst support and the CO₂ molecules. The cells were tested with discontinuous and continuous CO₂ supplies. This journal is

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Page 1 of 27
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Task-specific functionalization of graphene for cathode catalyst
support in carbon dioxide conversion
P. Tamilarasan and S. Ramaprabhu
Alternative Energy and Nanotechnology Laboratory (AENL),
Nano Functional Materials Technology Centre (NFMTC),
5
Department of Physics, Indian Institute of Technology Madras, Chennai – 600036, India
*
Email:ꢀ ramp@iitm.ac.in
ABSTRACT
This study describes the potential advantages of taskꢀspecific functionalization of graphene in
carbon dioxide capture and conversion. Ionic liquid was employed as functional moiety on
1
1
2
0
5
0
graphene catalyst support, since it shows reversible interaction with CO molecules. In this
2
study, graphene was by hydrogenꢀassisted lowꢀtemperatureꢀexfoliation method and
functionalized using polymerized ionic liquid (PIL). Adsorption analysis shows that PIL
functionalization improves the CO adsorption capacity of graphene, significantly. Catalytic
2
nanoparticles were dispersed over the catalyst supports by microwave assisted polyol reduction
method and employed as cathode catalyst in polymer electrolyte membrane (PEM) CO2
conversion cell. The cell hydrogenates CO into formic acid at cathode, which was quantified by
2
spectrophotometry. PIL functionalized support shows higher formic acid formation rate than
pure support at similar experimental conditions, since PIL functionalization improves the
interaction between catalyst support and CO2 molecules. The cells were tested with
discontinuous and continuous CO supply.
2
Keywords: Carbon dioxide; conversion; graphene; ionic liquid; functionalization
1

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1
. INTRODUCTION
Carbon dioxide valorization as useful chemical/chemical – feed – stokes is an economically
1
2
5
beneficial way to manage greenhouse gases. Protonation of CO to formic acid is recognized as
2
an efficient way to store hydrogen for certain applications, since formic acid has a volumetric
2
ꢀ3
hydrogen density of 43.5 g of H per kilogram. In addition, formic acid is a lowꢀtoxic liquid
2
ꢀ1
with 1700 Wh kg energy density, which is recognized as one of the efficient fuels in carbon
4
ꢀ7
energy cycle (carbon + energy → Fuel → carbon + energy).
3
0
Polymer electrolyte membrane (PEM) cells are widely used for hydrogen production by
electrolysis. It is an advanced technology to use the protons before recombination for CO2
protonation. In this regards, CO conversion using PEM cells at room temperature and ambient
2
8
ꢀ9
pressure has been extensively studied. Various configurations of PEM CO conversion cells
2
1
0
are constructed and evaluated by Charles Delacourt and coꢀworkers. Here, the cell keeps the
products of conversion away and prevents the reoxidation at cathode, which is a main
3
5
9
advantage.
The CO conversion product and its rate are sensitive to local pH of the medium and CO
2
2
1
1
concentration. Generally, sodium carbonate or bicarbonate solutions are used as electrolytes,
which have very low available free CO , viz. 0.15 and 0.041 mM, respectively, which offers very
2
1
2
4
0
small amount of CO available for the reaction. But, CO solubility in pure water is 33 mM and
2 2
the pH is also in acidic range (pH= 5 – 6), which facilitate high conversion efficiency.
In PEM CO conversion cells, the choice of electrolyte and cathode catalysts is unique, which
2
1
3
decides the final product and its efficiency. Narayanan et al., have reported the production of
formic acid with proton exchange membrane based electrochemical cell, where indium was used
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5
as cathode catalyst. The gas phase conversion of CO using a carbon fiber/Ni gas diffusion
2
1
4
electrode (GDE) produces synꢀgas as reaction product. Similar experiment carried out by
Kobayashi et al., shows higher methanol production efficiency on Cu/Zn/Al alloy electrode, even
1
5
at relatively lower current densities. AlvarezꢀGuerra et al., have studied the continuous
electrochemical conversion of CO into formate using PEM cell, where lead or tin plates were
2
1
6ꢀ17
5
0
used as cathode, individually.
Improved catalytic activity with supported nanoparticles has been well realized in fuel cells,
since it offers large number of surface active sites. In the choice of catalyst support, major
importance has been given to the factors, such as surface area, anchoring sites and electrical
conductivity of the catalyst support. Carbon nanomaterials, particularly graphene, offer an
1
8
5
5
improved nanoscale 3D architecture, which has proved their promising role as catalyst support.
The porous architecture allows a better threeꢀphase contact and improves the performance.
Moreover, it allows functionalising (covalent or nonꢀcovalent) its surface with wide variety of
1
9
moieties.
The recombination of protons with CO is the rateꢀdetermining step in electrochemical
2
2
0
6
0
conversion, which competes with hydrogen generation. Here, the local concentration of
reactants around the active catalyst site is an important factor to be considered. Xu et al., have
employed CaO as a highly affine catalyst support, which accumulates CO neighbourhood to the
2
21
catalyst and improves carbon nanotubes production. This suggests that taskꢀspecific surface
modification of catalyst support can increase the local reactant concentration and thus the
2
2
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5
reaction rate.
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3ꢀ25
Carbon dioxide has good, reversible solubility in ionic liquids (ILs).
We have realized the
potential advantages of surface functionalization of graphene with ILs in high pressure CO2
2
6
adsorption applications in our previous study. The good affinity of IL moieties accumulates
CO on the cathode catalyst support. In addition, ILs are well known for their thermal and
2
2
7
7
7
8
0
5
0
electrochemical stability. Zhang et al., have reported ionic liquid assisted CO protonation to
2
2
8ꢀ29
formic acid.
Several model cathode electrocatalysts, such as In, Pb, Pt, Pd, Cu, Fe, Zn etc., were used in
3
0
electrochemical CO conversion cells. Especially, indium gives better formic acid yield than Pt
2
3
1
or PtRu cathode catalyst. Typically, noble metal catalysts (Pt, Ru, Rh and their alloys) have
high H dissociation ability, and hence they have been used as efficient catalysts for CO2
2
protonation. Nevertheless, we chose Pt as model catalyst in order to project the advantage of
32
ionic liquid functionalization.
In the present study, graphene surface is functionalized by polymerized ionic liquids (PIL) in
order to make it more affine towards CO molecules, while platinum was taken as a model
2
catalyst. The novelty of the present study is the use of PIL for CO adsorption, which might be
2
useful to accumulate CO and enhance the conversion efficiency due to enhanced mass transfer
2
3
3
at the reactive site. To the best of our knowledge, this is the first report on PIL functionalized
catalyst support for CO conversion applications.
2
2
. EXPERIMENTAL SECTION
8
5
2.1 Graphene synthesis
Graphene has been synthesized by a two step process. In the first step, graphite was oxidized by
Hummers’ method. Briefly, pure graphite was mixed with waterꢀfree mixture of concentrated
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4
sulphuric acid, hydrogen peroxide, sodium nitrate and potassium permanganate in an ice bath.
Graphite oxide (GO) was obtained as a brown powder. In the second step, GO was heated at 200
ºC under hydrogen atmosphere in a tubular furnace, which results in exfoliation of few layered
9
0
3
5
reduced graphene. The presence of hydrogen rapidly removes oxygen containing functional
groups from graphene lattice at high temperature and results in separation of layers. The
graphene synthesized by hydrogen exfoliation technique (denoted as HEG) has wrinkled
3
5
structure due to the rapid removal of oxygen containing functional groups. The asꢀexfoliated
2
ꢀ1
3
ꢀ1
9
5
HEG has the surface area as high as 443 m g and electrical conductivity of 1.64 x 10 S m ,
3
6
reported, elsewhere.
2.2 Graphene functionalization
The surface of HEG was functionalized using polymerized ionic liquid by free radical
3
7
polymerization, reported elsewhere. HEG (100.0 mg) was dispersed in 25.0 ml of
1
00
dimethylformamide (DMF) by ultrasonic irradiation, followed by the addition of [VMIM][BF4]
(200 mg) and 2,2’ꢀazobisisobutyronitrile (7 mg) under stirring. The mixture was loaded into a
roundꢀbottom flask (50 ml) equipped with a reflux condenser and refluxed for 16 h at 80 ⁰C
under vigorous stirring in N atmosphere. Then the sample was cooled down to room
2
temperature and poured into 100 ml methanol, which precipitates the PIL functionalized HEG
along with certain amount of free polymer chains. The precipitate was washed with deionized
water and methanol several times in order to remove free polymer chains and unreacted
monomer. Finally, the product (HEGꢀPIL) was filtered through a nylon 66 membrane and dried
over night at 60 ⁰C under reduces pressure.
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10
2.3 Platinum nanoparticle decoration
Platinum nanoparticles were decorated over the surface of catalyst support (HEG and HEGꢀPIL)
3
7
by microwave assisted polyol reduction method (Pt/HEG or Pt/HEGꢀPIL). Briefly, 100 mg of
catalyst support was dispersed in 100 ml ethylene glycol. Aqueous standard (3.6 mg Pt in 1 ml
solution) solution of H PtCl .(H O) (5.54 ml) was slowly added into the dispersion under
2
6
2
6
1
15
string. The thoroughly mixed solution was irradiated to microwave radiation (800 W) for 2 min,
which reduces, nucleates and grows Pt ions as nanocrystals on the surface of catalyst support.
Since microwave reduces the ions rapidly, we maintained 10 s pulse rate of irradiation, i.e., 5 s
“ON” and 5 s “OFF”, in order to reduce the particle size. The completion of reduction was
confirmed by color change from yellow to colorless. The solution was cooled down to room
temperature and aged for 6 h at room temperature. A black precipitate was obtained, which was
further diluted with deionized water, washed, filtered through a Nylon 66 membrane and dried at
1
20
60 ⁰C and reduced pressure.
2.4 Experimental techniques
Molecular vibrational analysis of catalyst support was carried out by PerkinꢀElmer Fourier
transform infrared (FTIR) spectrometer. The structural analysis of synthesized materials was
carried out by X’ Pert Pro PANalytical powder Xꢀray diffractometer and the data are analyzed by
X’Pert HighScore Plus package. The surface morphology of nanomaterials were carried out
1
25
using a Technai Gꢀ20 transmission electron microscope (TEM). Low pressure CO adsorption
2
isotherms (Fig. 4) were determined by a surface area analyzer (Micromeritics ASAP 2020) at
298 K sample temperature for various pressures up to 100 kPa. Carbon dioxide conversion
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analysis has been carried out by PEM cell, which mimics a conventional fuel cell. Concentration
of formic acid was determined by PerkinꢀElmer spectrophotometer.
2
.5 PEM CO conversion cell fabrication and analysis method
2
1
1
1
1
35
40
45
50
The supported electrocatalysts are used without any further modification to construct the PEM
cell. The physical makeup of a PEM CO conversion cell would be much similar to PEM fuel
2
cell (PEMFC) and it functions similar to PEM water electrolysis cells used for hydrogen
ꢀ2
ꢀ2
production. The Pt loading was maintained to be 0.5 mg cm at anode and 1 mg cm at cathode
for all the experiments. We chose deionised water as catholyte base, because the pH of CO2
saturated water is 5ꢀ6, at 1 bar partial pressure. At higher pH values (pH>6), interaction of CO2
3
8
and its intermediates with Pt surface is poor. Carbon dioxide saturated DI water contains 33
mM dissolved CO (i.e., 1.45 g/L) at 298 K temperature in equilibrium at ambient pressure.
2
Catholyte was continuously circulated using a peristaltic pump, which removes the products and
feeds fresh reactants to the electrode. Dissolved CO has advantage over gas phase, as the liquid
2
can closely reach the catalyst by wetting the gas diffusion layer. In this regards, high purity CO2
(grade 4.5) was dissolved in DI water by bubbling for an hour. In a typical experiment, the
cathode reservoir was filled with 40 ml of CO saturated DI water and the anode reservoir with
2
pure DI water. Since the CO flow was stopped after saturation, this mode is denoted as
2
Discontinuous Flow Mode (DF mode). In another mode, CO gas flow was continues throughout
2
the experiment, which was denoted as Continuous Flow Mode (CF mode). These cells were
powered with an external source. We chose potentiostatic method of operation to limit the
product distribution. Brief description on cell fabrication and analysis method are given in ESI
1†.
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3
. RESULTS AND DISCUSSIONS
1
55
3.1 Molecular vibration spectrum analysis
Primarily, molecular vibrational characteristics of catalyst support were determined to confirm
the presence of PIL moieties on the surface of HEG (Fig. 1). The stretching vibrations of
hydroxyl group due to the presence of moisture and surface hydroxyl functionalities produces
ꢀ1
signal at ~3400 cm in both materials. The antiꢀsymmetric and symmetric =CH vibrations at
2
ꢀ
1
1
1
1
60
65
70
2923 and 2854 cm of HEGꢀPIL is quite stronger than that of HEG, which can be imputed to the
ꢀ1
presence of PIL moieties. The peaks at the wavenumber region 500 – 2000 cm can be assigned
to various modes of stretching and bending vibrations of surface functional groups (=CH , ꢀOH, ꢀ
2
COOH, >C=O) and vibrations emerged from PIL structure. Furthermore, inꢀplane bending
ꢀ1
vibrations of aromatic rings (1050 cm ) occur in both samples, from the hexagonal honeycomb
ꢀ1
lattice, which is merged with another peak at ~ 1250 cm . The later one can be assigned to the
CꢀN stretching vibrations emerge from imidazolium ring. The FTIR spectrum of HEG shows
ꢀ1
peaks at 1580 and 1760 cm corresponding to aromatic ring stretching from benzene (hexagonal
lattice) and C=O stretching vibrations, respectively. These peaks are merged in the spectrum of
ꢀ1
HEGꢀPIL due to the raise of new peak at ~1650cm , corresponding to –C=NꢀC from
2
6, 39
imidazolium ring of ionic liquid.
Since ammonium ions produce multiple broad peaks
−
1
ꢀ1
between 2400–3200 cm , the broad signal found between 2000ꢀ2300 cm in HEGꢀPIL can be
4
0ꢀ41
assigned to the cationic nitrogen in imidazolium backbone.
confirms the presence of PIL moieties on HEG surfaces.
Conclusively, FTIR spectrum
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HEG
HEG-PIL
1
80
3
000
2000
1000
-1
Wavenumber (cm )
Fig. 1. FTIR analysis of HEG and HEGꢀPIL
3.2 Structural analysis
1
85
Xꢀray diffraction patterns of HEG and Pt NPs decorated HEG and HEGꢀPIL are presented in Fig.
. The XRD pattern of HEG shows a broad (002) diffraction peak, cantered at 24⁰,
2
corresponding to ~0.37 nm interꢀlayer distance. The relatively higher interꢀplanar distance in
comparison with graphite (0.34 nm) can be imputed to the large disorder, wrinkles and residual
functional groups in the structure. The broadening and low intensity of (002) peak suggests the
highly disordered graphitic structure. The XRD pattern of HEGꢀPIL (ESI 2†) shows no
difference from that of HEG, since PIL moieties are in amorphous state. The characteristic peaks
of Pt are located at 2θ values of around 40°, 46°, 68°, 81° and 86° are found with both HEG and
HEGꢀPIL supports along with (002) peak of exfoliated graphene. The peak broadening in
Pt(111) peak is almost same with both HEG and HEGꢀPIL support material, which reveals the
particle sizes of 2.8 and 2.4 nm, respectively. Since HEG itself has large amount of residual
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90
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functional groups, the Pt nanoparticles are anchored/nucleated in a well distributed manner.
Thus, PIL functionalization did not show much significant change in particle size. However, this
small change in particle size also can be attributed to the presence of PIL as suggested by Wu
4
2
and coꢀworkers.
2
00
(a)
(b)
(c)
2
2
05
10
3
0
60
90
2θ (degree)
Fig. 2. Xꢀray diffraction pattern of (a) HEG, (b) Pt/HEG and (c) Pt/HEGꢀPIL.
.3 Morphological analysis
3
Figure 3 reveals the morphological change in HEG and HEGꢀPIL upon Pt nanoparticles
decoration. TEM (Fig. 3a) image of HEG confirms the disorder induced in graphite structure
resulting in the form of sheets by exfoliation. The transparency of the layers suggests that the
graphene is fewꢀlayered and highly wrinkled. The rapid removal of intercalated functional
2
15
3
5
groups during exfoliation results in wrinkled structure to graphene sheets. Platinum
nanoparticles are uniformly dispersed on both substrates. Although the surface is not
functionalized, HEG also gives better dispersion due to the presence of uniformly distributed
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anchoring sites (Fig. 3b). The surface defects (wrinkles), residual functional groups and PIL
4
3
2
20
moieties present on the surface lead to a better dispersion of metal nanoparticles. In addition
the particle distribution is not much different upon PIL functionalization (Fig. 3c). This is in
good agreement with the XRD data.
2
25
Fig. 3. TEM images of (a) HEG, (b) Pt/HEG and (b) Pt/HEGꢀPIL.
3.4 Carbon dioxide adsorption analysis
Since the main claim in this work is based on the affinity of catalyst towards CO , it is important
2
2
30
to determine the CO adsorption behavious of support materials. We have reported improvement
2
in CO adsorption capacity of graphene by ionic liquid functionalization, in our previous study.
2
The isosteric heat of adsorption of IL functionalized graphene suggested the physical interaction
26
of CO with sorbent, i.e., no chemical interaction between CO and ionic liquid moieties. This
2
2
motivated functionalizing catalyst support with PIL in order to enrich CO concentration at local
2
2
35
conversion sites.
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6
5
4
3
2
1
00
00
00
00
00
00
00
0
HEG
HEG-PIL
2
40
0
20
40
60
80
100
Pressure (kPa)
2
45
Fig. 4. CO adsorption properties of HEG and HEGꢀPIL at 298 K.
2
In the present study, we have determined the CO adsorption capacity of HEG and HEGꢀPIL at
2
low pressures (up to 100 kPa), since 100 kPa is the maximum CO equilibrium pressure in
2
conversion studies. HEGꢀPIL clearly shows 28 % higher CO capturing capacity than that of
2
HEG due to the better interaction between CO molecules and PIL moieties. Here, PIL moieties
2
2
50
accumulate CO on the surface of HEG, which improves formic acid formation. HEGꢀPIL shows
2
large hysteresis (adsorbate retention) upon desorption under reduced pressure, since the binding
energy of CO with PIL moieties is higher than that with pure HEG surface.
2
3
.5 CO conversion – Half cell measurement
2
The electrochemical behaviour of prepared materials was carried out by three electrode system,
where modified glassy carbon electrode was used as working electrode along with Pt wire
counter electrode and Ag/AgCl reference electrode. A typical working electrode was prepared by
drop casting catalyst on glassy carbon electrode. Briefly, 1 mg of catalyst was dispersed in 0.25
2
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ml of 0.5 wt. % Nafion in isopropanol + DI water solution by ultrasonic irradiation. The slurry
ꢀ2
2
was drop casted (~52 µg cm Pt loading) on glassy carbon electrode (0.076 cm ). The analysis
ꢀ1
2
60
was carried out with CO saturated 0.5 M KHCO electrolyte at 50 mV s scan rate. Cyclic
2 3
voltammograms of Pt/HEG and Pt/HEGꢀPIL (Fig. 5) show two reduction peaks at around ꢀ0.6
3
2
and ꢀ0.48 V vs NHE. These peaks can be assigned to the formation of HCOOH and HCHO,
respectively. The peak current is significantly improved upon PIL functionalization. This may be
attributed to the higher concentration of reactant (CO ) at the conversion site.
2
2
65
Multiple peaks occur between ꢀ0.4 and ꢀ0.2 V in the forward scan may be assigned to the
4
4
electrosorption of reduced CO (*CO radical) on catalyst. Pt/HEGꢀPIL shows high reduction
2
2
peak current than that of pure Pt/HEG, at their position. The double layer contribution in
graphene based electrode is not much improved upon PIL functionalization. As HEG itself has
higher surface area, PIL moieties may not further improve the accessible surface area,
considerably, and thus the double layer contribution. It has to be pointed out here that the
electrochemical behaviour of material has been obtained by three electrode system in KHCO3
solution, where the mass transfer behaviour will be different in comparison with PEM CO2
conversion cell (full cell measurement). Hence, the electroreduction conditions cannot be
2
70
directly obtained to PEM CO conversion cell from half cell measurement.
2
2
75
It is reported that the reduction of CO on platinum surface occurs at more positive potentials in
2
45
ionic liquids than in aprotic solvents. We observed a slight positive shift in peak position upon
PIL functionalization. This can be attributsed to the stabilization of charged intermediates by
4
6
ionic liquid, which shifts the potential of rateꢀlimiting step to more positive potentials.
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2
80
20
10
0
-
-
-
-
-
10
20
30
40
50
Pt/HEG
Pt/HEG-PIL
2
85
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential (V)
Fig. 5. Cyclic voltammogram of CO reduction on Pt/HEG and Pt/HEGꢀPIL electrode in CO2
2
ꢀ1
saturated 0.5 M KHCO at 50 mV s scan rate.
3
3
.6 CO conversion analysis
2
2
90
A positive potential was applied to anode with respect to cathode, which produces protons,
electrons and gaseous oxygen at anode by water electrolysis as given in eq. 1.
+
−
2
H O → 4H + 4e + O
(1)
2
2
Protons pass through electrolyte, while the electrons move in the external circuit under applied
potential. At cathode, protons and electrons recombine with CO molecules to produce
2
2
95
hydrocarbon. Further, the colourless cathode reservoir solution was analysed by
spectrophotometer, which shows a strong peak at 230ꢀ240 nm and matches well with commercial
formic acid (see Fig. S6†). This is in good agreement with literature, where Narayanan et al.,
9
have obtained sodium formate with sodium ion conducting membrane.
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+
−
H + e → H
ads
CO + H → HCOOads
2
ads
(2)
HCOO + H → HCOOH
ads
ads
Hads + Hads → H2
3
00
The formation of formic acid is a three step reaction as given in eq. 2. Briefly, a proton and an
electron combine on catalyst site and form a hydrogen atom, which is adsorbed on the catalyst
surface (H ). Typically, electroreduction of CO on Pt surface proceeds by the interaction of
ads
2
CO molecules with hydrogen (H ) at the three phase boundary and forms an adsorbed formate
2
ads
3
8
radical (HCOO ). Further addition of H to adsorbed formate radical produces formic acid
ads
ads
3
05
molecules. Thus, 2 protons and 2 electrons (or one water molecule) converts a waste CO2
molecule to useful fuel. The other possible competing reaction is the recombination of two Hads
to produce one H molecule. This can be viewed as the reverse of hydrogen spill over on catalyst
2
surfaces.
The cathode reservoir solution was further analyzed by FTIR spectroscopy (Fig. 6), in order to
confirm the conversion of CO into hydrocarbon molecule. Deionized water and CO saturated
3
10
2
2
deionized water are also analyzed by the same method, which shows the traditional stretching
ꢀ1
ꢀ1
vibrations of –OH group at around 3300 cm and the bending vibrations at ~ 1650 cm . CO
2
ꢀ1
saturated deionized water shows at strong signal at 2345 cm , corresponding to the asymmetric
stretching vibrations of CO molecule. Moreover, there are two weak signals at 2926 and 2853
2
ꢀ1
3
15
cm corresponding to symmetric and asymmetric stretching vibrations of CꢀH group. This may
obviously arise from the trace amount of carbonic acid present in carbonated water. Upon
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protonation of CO into hydrocarbon, these peaks are strengthened. Furthermore, the FTIR
2
spectrum of cathode reservoir solution after 30 min of reaction shows relatively weak peak at
ꢀ1
47
~
2330 cm , which is an evidence for the conversion of CO into formic acid. Interestingly, the
2
3
20
asymmetric stretching signal is split into two signals, which can be attributed into the charged
38
intermediates (*CO ) in the solution.
2
3
25
DI Water
CO Saturated DI Water
2
Cathode reservoir solution
1000 1500 2000 2500 3000 3500
-1
Wavenumber (cm )
Fig. 6. FTIR spectrum of CO saturated deionized water (dotted line) and cathode reservoir
2
3
30
solution (solid line).
Hence, the spectrophotometer was calibrated to formic acid by serial dilution method and used to
determine the formic acid concentration in cathode reservoir solution at various point of reaction
48
time, where CO saturated deionised water was taken as reference (ESI 3†). Here, the optical
2
spectra of standard solutions were fitted to Gaussian function and the peak height was taken for
calibration (Fig. S7†). The calibration curve is given in Fig. S8†.
3
35
1
6

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The cell potential is one of the deciding factors of formic acid formation rate (FFR). From Fig. 5,
hydrogenation of CO to formic acid occurs at ꢀ0.6 V vs NHE, i.e., vs hydrogen adsorption
2
potential on Pt surface. Hence, hydrogenation potential of CO to formic acid is ꢀ0.6 V with
2
respect to cathode catalyst (Pt) surface. Similarly, the theoretical oxidation potential of water is
+1.23 V w.r.t NHE. But, it is extensively reported that practical open cell potential of PEMFCs
are less than 1.23 V, due to the overpotentials. In the present case, the device mimics a
conventional PEMFC and it works in the reverse principle of PEMFCs. Hence, the water
electrolysis potential in the present case is expected to be higher than the theoretical value. In
order to find the optimum potential for better FFR, the cells were operates at various cell
potentials and found the potential at which high FFR is achieved, while all other parameters are
unchanged.
3
40
3
45
The conversion efficiency at various cell potential in discontinuous flow mode is presented in
Fig. 7, which shows that the FFR reaches its maximum at 1.5 V vs NHE. Hence, 1.5 V positive
potential was applied potentiostatically to anode with respect to cathode catalyst in all
experiments, i.e., totally the anode is +2.1 V positive with respect to cathode. The efficiency was
significantly less at lower potentials, may be due to the insufficiency in applied potential to
overcome the resistive loses and overpotential. No such change in efficiency at higher potentials
is observed. A similar nature has been observed for Pt/HEGꢀPIL.
3
50
3
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20
16
12
8
Pt/HEG
Pt/HEG-PIL
3
60
1.9
2.0
2.1
2.2
Potential (V)
Fig. 7. Determination of cell potential of PEM CO conversion cell by DF mode with 150 min
2
3
65
conversion time per each data point.
3
.7 CO conversion – Full cell measurement
2
The cell was activated by stepwise (0.1 V) increasing the cell potential from 1.9 to 2.2 V with 15
min duration stepꢀwidth, where both reservoirs were filled with DI water. This step is analogous
to the activation step in fuel cell. The cathode reservoir solution was sampled at certain time
intervals up to 150 min and analysed by calibrated spectrophotometer (see ESI 4†). Fig. S9 &
3
70
S10† are the typical set of absorption spectra of Pt/HEG and Pt/HEGꢀPIL based CO conversion
2
cell in DF mode. A gradual increase in intensity was observed with time. All these spectra were
fitted to Gaussian function after baseline correction. The peak height of the fitted peak is taken as
peak intensity and correlated with the calibration curve (Fig. S8†) to calculate concentration of
HCOOH in cathode reservoir solution. The given values are averaged from three to six
measurements.
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75
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8

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20
15
10
5
0
DF Mode
3
80
Pt/HEG
Pt/HEG-PIL
0
30
60
90
120
150
Time (min)
3
85
Fig. 8. Electrochemical formic acid (FA) formation from CO on Pt/HEG and Pt/HEGꢀPIL
2
cathode PEM cell in discontinuous flow (DF) mode.
Fig. 8 shows the change of formic acid concentration in cathode reservoir solution as a function
of reduction time in DF mode. The cell with Pt/HEGꢀPIL cathode produces 19 ± 3 mM solution
of HCOOH by 150 min reaction in DF mode PEM cell, while that of Pt/HEG is 9 ± 1 mM.
Clearly, PIL functionalized HEG support shows double the FFR of pure HEG support. Since the
size of Pt nanoparticles on both supports were almost same, the improvement has to be imputed
to the PIL functionalization only. Briefly, PIL moieties on the surface of catalyst support
3
90
accumulates CO molecules around them due to the better affinity as discussed in CO2
2
adsorption analysis, i.e., the reactant concentration is improved near the active conversion sites
(Pt sites). According to the rate law, the rate of reaction is proportional to the concentration of
reactant. Thus the FFR of HEGꢀPIL was improved.
3
95
Moreover, it is important to notice that Pt/HEGꢀPIL produces HCOOH equivalent to more than
5
0 % of supplied CO and the curve did not tend to saturate. This indicates that HCOOH is the
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9

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major product with appreciably narrow product distribution. Here, it is believed that the presence
3
3
4
00
of PIL moiety assists the product selectivity.
In DF mode, CO concentration was gradually reduced as it is converted. In the second part of
2
evaluation, the cells were tested with constant reactant concentration by continuously supplying
CO gas, i.e., in CF mode. The same operating potential (2.1 V positive to anode) is applied in
2
CF mode also. Similar data acquisition and processing method has been followed as of DF mode.
As observed in DF mode, the intensity of optical absorption peak is gradually increased with
time in CF mode as well (see Fig. S11 & S12†). But, the intensities are higher than those
obtained in DF mode at all points of time.
4
05
CF Mode
40
4
10
30
2
0
0
0
1
Pt/HEG
Pt/HEG-PIL
0
30
60
90
120
150
Time (min)
4
15
Fig. 9. Electrochemical formic acid (FA) formation from CO on Pt/HEG and Pt/HEGꢀPIL
2
cathode PEM cell in continuous flow (CF) mode.
Once again the potential of PIL functionalization in CO conversion has been recognized. The
2
concentration of formic acid in the catholyte has been presented as a function of reaction time
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(Fig. 9). In CF mode, Pt/HEG electrode material gives 25 ± 3 mM, while it is 38 ± 5 mM for
4
20
Pt/HEGꢀPIL based cell after 150 min reaction. This is almost 2.8 and 2 times that obtained with
DF mode, respectively. This improvement can be attributed to the continuous CO supply
2
throughout the experiment.
3.8 Mechanism
4
25
In a recent study, Aeshala et al., have experimentally observed 50 % less H generation along
2
with improved hydrogenation of CO , when anchoring functional groups are introduced in the
2
4
9
solid polymer electrolyte (i.e., at three phase boundary). Based on this observation, we propose
a correlation between the improvement in conversion rate and the presence of PIL moieties.
+
Briefly, PIL moieties on the catalyst support accumulate H (by reduced H generation) and CO
2
2
4
30
molecules in the three phase boundary, which increase the localized concentration of reactants
H and CO ) around metallic catalyst (Fig. 10). This significantly increases the FFR (i.e., better
(
2
2
proton utilization), since reaction rate is proportional to the concentration of reactants, according
to rate law of chemical reactions.
4
35
Fig. 10. Schematic representation of the mechanism
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Since PIL shows good affinity towards CO molecules and is integrated in the cathode, HEGꢀPIL
2
will be a promising catalyst support in devices, that can simultaneously capture CO from
2
atmosphere at ambient conditions and convert into fuels by (photo)electrochemical process. Such
device will be the next generation technology for CO management in an economically
2
4
40
beneficial way.
4
. CONCLUSION
The improved interaction between catalyst support and CO molecules upon polymerized ionic
2
liquid functionalization has been demonstrated. PIL functionalized graphene has been employed
as cathode catalyst support for PEM CO conversion cell, where Pt nanoparticles were used as
2
4
45
model catalyst. The formic acid formation rate of the cells with HEG and HEGꢀPIL supported
cathode catalysts was tested in DF and CF modes, where formic acid formation rate is higher in
CF mode than DF mode. In addition, formic acid formation rate of PIL functionalized graphene
is much higher than the pure graphene support. For example, the formic acid formation rate of
HEG based cell has been improved threeꢀfold in DF mode and twoꢀfold in CF mode upon
functionalizing HEG with PIL moieties. The improvement has been attributed to the
4
50
accumulation of CO molecules on PIL sites, where the active protonation sites (Pt) are present.
2
5
. ACKNOWLEDGEMENT
Authors acknowledge SAIFꢀIITM for FTIR analysis. One of the authors (Tamilarasan)
acknowledges Indian Institute of Technology Madras (IITM) for the financial supports (senior
research fellowship).
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