A tunable metal-polyaniline interface for efficient carbon dioxide electro-reduction to formic acid and methanol in aqueous solution

Article abstract of DOI:10.1039/C6CC07212G

It is reported that metals on polyaniline (PANI) prepared by a simple method can exhibit excellent activity in the electro-reduction of CO₂ to HCOOH or CH₃OH due to tunable properties: N atoms on PANI capture CO₂ through a strong Lewis acid-base interaction while Pd atoms, amongst Pd, Pt, and Cu studied, facilitate the fastest proton and electron transfers along PANI to the CO₂ trapped sites to give rise to the best HCOOH yield in a highly cooperative manner.

Full text of DOI:10.1039/C6CC07212G

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Tunable Metal-Polyaniline Interface for Efficient Carbon Dioxide
Electro-reduction to Formic acid and Methanol in Aqueous Solution
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
c
c
a
Weiran Zheng, Simantini Nayak, Weizi Yuan, Zhiyan Zeng, Xinlin Hong, Kylie A. Vincent, and
a
Shik Chi Edman Tsang *
DOI: 10.1039/x0xx00000x
www.rsc.org/
It is reported that metal on polyaniline (PANI) prepared by a water. Further reduction of the formic acid to hydrocarbons
3
simple method can give excellent electro-reduction activity of CO
to HCOOH or CH OH due to tunable properties: N atoms on PANI
capture CO through a strong Lewis acid-base interaction while Pd
atoms, amongst Pd, Pt, Cu, studied, facilitate the fastest proton
and electron transfers along PANI to the CO trapped sites to form
the good HCOOH yield in highly cooperative manner.
2
and higher alcohols over metals was also demonstrated.
However, the use of metal electrodes suffered from the use of
high over-potential (almost 1 V) due to weak adsorption of
3
2
CO
2
, and poor product selectivity (a range of C1 to C3 products
2
which included methanol, formic acid, alcohol and propanol
3
b
together with a large quantity of H2). While the choice of
metal catalyst can undoubtedly affect efficiency and selectivity
of the products, organic molecules incorporated into the
electrode, such as pyridine and N-containing polymers, can
Catalytic conversion of carbon dioxide (CO ) is considered to
2
be a realistic solution in the reduction of its greenhouse effect.
The formation of valuable chemicals or fuels from this route
can offset some of the costs for its fixation. Although various
conversion methods including thermal catalytic processes are
widely explored, the concerns of secondary CO2 emission
prevail, because many of the suggested processes unavoidably
involve the input of intense energy in the form of high
4
play additional role(s) for the electro-reduction. Nitrogen
atoms are known to lower activation energy due to efficient
5
at the electrode surface. On the
trapping of gaseous CO
2
other hand, the use of organic layer(s) as electrodes showed
improved current efficiency and product selectivity, but lower
temperature and pressure, etc. On the other hand, the intrinsic reaction rates were commonly achieved, presumably
electrochemical reduction of CO driven by renewable energy due to their poor electronic and H conductivity. Thus, it
2
sources (wind or solar) appears to be promising. Particularly, appears that a composite electrode with metal and N-
the recent dwindling cost of grids from nuclear, hydropower containing conjugated polymer at the interface could be a
and biomass makes the process even more attractive.
1
useful combination for CO reduction. In addition, size- and
2
However, problems of high over-potential, low current shape- controlled metal nanoparticles (NPs) may be
efficiency and poor product selectivity over reported synthesized by the use of polymer as stabilizer. However, there
electrocatalysts remain to be solved. The key issue lies in the have been few studies in using metal/conjugated polymer
6
poor understanding of fundamental chemical and composite for electrochemical CO reduction in the literature.
2
electrochemical steps in the reduction of CO
2
over a selected For example, a preliminary study of polyaniline(PANI)/Cu O
2
catalyst on an electrode. If the underlying factors for the composite was reported, which indicated the formation of
6
a
multifunctional properties (i.e. CO
2
uptake, H and electron HCOOH and CH COOH in methanol, but no work studying the
3
transfers and surface rearrangement) were understood, which interactions of the catalyst components has yet been initiated.
could lead a rational design of structure and layout of a
Here, we report a systematic synthesis of a series of PANI
loaded with metal NPs (Pt, Pd, Cu) to establish the composites
2
2
composite catalyst for more efficient CO electro-reduction.
Early studies showed that Pb, Zn, Pd, and Cu metal electrodes with extensive metal-PANI interface, which give exceptional
can produce formic acid from the electro-reduction of CO in activity in the electro-reduction of CO to HCOOH and CH OH.
2
2
3
7
PANI was synthesized using a rapidly mixed reaction in HCl ,
which was obtained in proton-doped form. The as-synthesized
-
PANI was washed with NH ·H O to remove Cl (the de-doping
3
2
process). Notice that PANI contains conjugated benzenoid and
quinoid units (Scheme S1) at different ratios to give molecular
structures dependent on oxidation states. This ranges from a
fully protonated form, the leucoemeraldine (LEB), partially
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oxidized emeraldine base (EB) to fully oxidized pernigraniline V. This is presumably due to trace CO produced during the
8
10
(
PB), as shown in Fig. S1. For metal/PANI synthesis, PANI (EB reduction of CO
form) was dispersed in DMF with metal precursor (metal/N = current efficiency for HCOOH is over 22%, over Pd/PANI at -1.1
:5). NaBH was added to reduce the metal ions to form metal V, which shows Pd is the best amongst all the three metals
NPs on PANI at room temperature. studied. Thus, Pd somehow dramatically promotes the
First, our XPS suggested that Cl was totally removed during formation of HCOOH rather than the H formation (hydrogen
the de-doping process (Fig. S1). The SEM image (Fig. S2a) evolution reaction) which is typical of metal catalysts without
2
that poisons the Pt surface. The highest
1
4
-
2
3
shows that the nanofiber-like morphology with a diameter of the polymer support (also see ESI) The total CO
2
conversion
150 nm to 300 nm, indicative of an ordered structure. After in rate reaches 94.6 mmol/ g·h with 94% selectivity towards
situ synthesis of Pd NPs, the metal-PANI composite maintains HCOOH (Tables S1, S2). As far as we are aware, such current
its shape (Fig. S2b) without changing the PANI structure, as efficiency and particularly the ultra-high selectivity towards
illustrated in Fig. S2c. Similar images were also observed for HCOOH represent one of the best reported electro-catalytic
Cu/PANI and Pt/PANI. The TEM images show that the metal performancesfor the CO fixation in this area. Interestingly, as
2
particles are in the nanoscale range with an average diameter stated, CH OH is only formed over the metal (Cu, Pd) /PANI
3
ranging from 15.4 nm for Cu NPs (Figure S2d), 2.44 nm for Pd but not at the unmodified PANI (Fig. 1b). Notice that Pd and
NPs (Figure S2e) to 1.11 nm for Pt NPs (Figure S2f). Their especially Cu offer active and selective surfaces in thermal
1
narrow size distributions are given in Fig. S3. Here, the size of catalytic conversion of CO /H to CH OH. This suggests the
1
2
2
3
metal NPs is much smaller than that of the previous reports metal NPs surface in the electrocatalyst is also critical for the
6
a,9
due to careful in situ synthesis. It is likely that PANI acts as a formation of methanol. With potential driving force is
stabilizer during the formation of metal NPs. TEM also shows increased from -0.5 V to -1.1 V, Cu/PANI shows increasing
that the metal NPs adopt regular shapes: polyhedral Cu NPs, efficiency. Pd/PANI reaches its highest efficiency at -0.9 V, in
wormlike Pd NPs and spherical Pt NPs.
contrast to Pt/PANI. It is exciting to note that the highest
current efficiency for CH OH is observed at -0.9 V when the
3
mixture of Pd/PANI and Cu/PANI (molar ratio is 1:1 but keep
the same total amount of catalyst) is used. It is apparent from
the above study that there are synergetic effects between
metal NPs and PANI in the reduction of CO to products, giving
2
methanol productivity of 8.62 mmol/ g·h at 10% current
efficiency (Tables S1, S2). We did not get any direct evidence
on the formation of alloy (from XRD) during testing the
mixture. On the other hand, we cannot exclude the possibility
of such formation in small quantity at the materials’ interface.
Nevertheless, it is envisaged that Pd surface produces [H] at a
close proximity to Cu surface which is selective for ‘methoxy’
Fig. 1. Current efficiency of (a) HCOOH and (b) CH
amount of PANI was used. RDE experiments were run in 0.5 M H
kept at the target potential for 120 min; the rotating speed was set at 800 rpm.
3
OH over PANI and metal/PANI. Same
2
SO and electrode
4
The CO
of the samples was evaluated on a rotating disk electrode
RDE) in order to minimise mass transfer limitations. The cyclic
voltammetry (CV) plot of CO reduction is shown in Fig. S4.
2
electrochemical reduction activity in acidic solution
11,12
adsorption
for the final methanol desorption. Work is in
(
progress to shed more light on this mixture. It would be
important to elucidate such interactions, which may lead to
2
Typically, each sample was tested for 2 hours (Pd/PANI as
shown in Fig. S5) and then collected for characterization: it
rational design of new catalysts for efficient CO fixation. Fig.
2
2
a shows the XRD patterns of PANI and metal/PANI. It is well
showed no structural destruction (Fig. S6). Apart from H
CO in gas phase, only two main liquid products, namely
HCOOH and CH OH were identified (HCOOH was exclusively
2
and
accepted that PANI possesses regular chemical backbones
1
2
2
(Scheme S1) hence can be crystallized via H-bonding linkage.
3
1
The broad peak located at 20.5° (d = 4.6 Å) is a characteristic
peak of PANI, which is comparable to the literature. Apart
from additional diffraction peaks of metals (for Pt/PANI, no
metal peak is observed due to the severe peak broadening due
to the small Pt NPs size (see Fig. S2f)), it is interesting to note
that the peak of PANI in these composites clearly shifts to a
larger 2θ value. In the case of Pt/PANI, the peak shifts from
formed over the un-promoted PANI) by GC-MS and H-NMR.
The current efficiencies of the two products at different
potentials, are shown in Fig. 1. According to the figure, the
PANI and metal-PANI composites clearly display the ability to
produce HCOOH from CO
2
at potentials more negative than
-0.5 V vs SHE (Standard hydrogen electrode). Two unique
features deserve attention: this system gives significantly
lower over-potential and higher selectivity than many reported
6
catalysts indicative of the impressive nature of the PANI
2
0.5° to 22.0° and in Pd/PANI, further shifts to 22.6°. This
implies the highest degree of intermolecular H-bonding
packings of PANI units (increasing benzenoid/quinoid ratios)
results during the reduction of Pd NPs from the precursor. It is
2
based catalysts for CO reduction (Tables S1, S2). It is apparent
that the composites containing metal NPs (in particular Pd) can
greatly promote the HCOOH yield at -0.7 V compared to
unmodified PANI (Fig. 1a). By applying more negative
potentials, a higher yield is obtained for most of the materials,
although Pt/PANI shows a decreasing yield from -0.7 V to -1.1
-1
known that a vibrational peak around at 1497 cm assigned to
-1
C=C (benzenoid) and a peak at 1588 cm assigned to C=N
quinoid) can be used to assess the redox state of the PANI
(
1
3
according to their area ratio (Scheme S1). FTIR and Raman
spectra suggest that the polymer chain remains intact during
2
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1
6
+
metal NPs formation (Figs. S7, S8), showing the same band and ionic conductivity dependent on oxidation state and pH.
positions of benzenoid and quinoid structures. After the Pt and This observation is believed to relate to the facilitated H
Pd metal NPs incorporation, a significant increase in their peak formation from H
2
on Pd surface due to readily electronic flow
area ratios is evident (Table S3). This once again suggests the through conjugation together with high proton flux to PANI via
extensive reduction of the polymer in the presence of metal metal-polymer interface during the facile oxidation. Similarly,
NPs. In order to find out how the metal NPs interact with PANI, Pd/PANI and Pt/PANI display higher over-potential for HER
2
the XPS region of N1s was analyzed. There are three nitrogen- than that of Pd/C in N (Fig. S11) due to resilience of proton
containing groups within the polymer chain (Fig. 2b): imine (- reduction with reduced PANI of lower electronic conductivity.
N= at 398.4 eV), amine (-NH- at 399.6 eV) and small amount of
+
14
positive charged nitrogen (N at 400.4 eV). With the
introduction of Cu NPs, the binding energy (B.E.) of -N=
and -NH- remain almost unchanged, while for Pd NPs, the B.E.
of -N= clearly shifts to 398.1 eV and -NH- to 399.4 eV (electron
promotion from metal to nitrogen). Similarly, there is a small
degree of peak shift towards lower B.E. in the Pt NPs indicative
of the electron promotion to nitrogen but the easy fouling of
Pt surface by contaminants in laboratory (O and CO) despite
2
the careful handling of the samples in inert gas is noted.
Nevertheless, the pronounced peak shift in Pd suggests that
there is a significant electronic interaction between Pd NPs
with the surface nitrogen of PANI nanofibers. But it should be
noted that XPS may not be sensitive to reflect the electronic
interaction since the majority of nitrogen atoms are not in
direct contact with metal NPs (Figs. S2d, S2e and S2f).
Characterization of Pd/PANI compared to unmodified PANI by
1
Fig. 2. (a) XRD of PANI, Pt/PANI, Pd/PANI and Cu/PANI; (b) XPS of N1s of PANI, Pt/PANI,
H-NMR was also conducted (Fig. S9). The broader shoulder
Pd/PANI and Cu/PANI; (c) FTIR of PANI and Pd/PANI before and after treatment with
th
peaks of hydrogen in -NH- group (7.0–7.2 ppm) suggests a
wider and increasing population of H atoms in this catalyst
during the formation of Pd NPs. Fig. 2c shows the FTIR and CV
of PANI and Pd/PANI before and after the treatment with H2.
There is no change of PANI before or after treatment with H2.
Interestingly, for Pd/PANI after H2 treatment, there is a
distinctive IR peak shift (C=C stretching vibrations of benzenoid
H
2
; (d) CV plots of PANI and Pd/PANI in N
2
and H
2
saturated 0.5 M H
2
SO
4
(10 scan
shown), scan rate=10 mV/s; rotating speed is 500 rpm.
-
1
-1
structure) from 1497 to 1509 cm . The peak of 1591 cm
assigned to C=N stretching vibration (quinoid structure) also
completely disappears after it is in contact with H . It is well
2
accepted that Pd NPs can dissociate H gas and form surface
2
1
atomic H even at sub-ambient temperature. But, the
5
2
Fig. 3. In situ electrochemical ATR-IR of (a) Pd/PANI and (b) PANI under H .
complete reduction of PANI by H to form leucoemeraldine
2
Fig. 3a shows the in situ ATR-IR of Pd/PANI and Fig. 3b for
(LEB) is still incredible since Pd NPs could not be in touch with
PANI at different potentials from -0.3 to 0.5 V at a scan rate of
all the quinoid units. There implies a long-range efficient H
transfer from the metal must occur to lead to extensive
reduction of the polymer in remote locations. Fig. 2d shows
the CV plots of PANI and Pd/PANI in acidic solution saturated
1
2 2
0 mV/s under H (spectra under N are shown in Fig. S12). All
the spectra are processed against a reference recorded at the
initial potential, -0.3 V. The peak intensity of C=N peak (PANI:
-1
558 cm , Pd/PANI: 1565 cm ) increases at the expense of the
-1
1
with N
2
and H
2
. Clearly, the protonic benzenoid -NH groups of
-1
-1
C=C peak (PANI: 1505 cm , Pd/PANI: 1501 cm ) during
oxidation (from -0.2 V to 0.5 V), indicative of increasing
quinoid units at the consumption of benzenoid units, and vice
versa during the reduction. The peak intensity of the N-H
PANI can be completely oxidized to form quinoid with two
oxidation steps between -0.4 V to 0.5 V through its conjugative
system: 1. oxidation from the fully reduced form (LEB) to the
half-oxidized form (emeraldine salt, ES) at 0 V; 2. oxidation
-1
-1
deformation (Pd/PANI: 1601 cm , PANI: 1595 cm ) also
decreases during the oxidation. The new peak (Pd/PANI: 1611
-1
cm , PANI: 1614 cm ) is considered to be a characteristic
from ES form to fully oxidized form (pernigraniline, PB) at 0.5
8
V. Under N
2
, both PANI and Pd/PANI show no hydrogen
-1
evolution or oxidation peak in the given range of potentials.
However, under H , Pd/PANI shows a sharp peak at -0.04 V,
vibrational band of PB and can be assigned to a –C=C-C=N-
17
asymmetrical vibrational mode.
2
Comparing the peak
close to the SHE potential. This is a much lower potential for
the oxidation of pre-adsorbed H atoms on Pd NPs to protons
compared to that of Pd/C of about -0.35 to -0.4 V (Fig. S10).
PANI exhibits interesting intra- and intermolecular electronic
intensity at 0.5 V, PANI is more oxidized than Pd/PANI,
suggesting Pd can help to transfer H to PANI for its extensive
reduction via high electronic and proton conductivity at the
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+
interface (taking up H
2
and forming more N-H structure). effects to conduct H and electron from water decomposition
formation via
to 0.5 V to -0.2 V), but it is reversible in the case of Pd/PANI, surface H recombination on the metal surface. The fast but
showing the improved stability with Pd. Thus, Pd metal NPs long-range reduction of the adsorbed CO of the trapped sites
Moreover, the PANI suffers change after one cycle (from -0.2 V with PANI to diminish the more favorable H
2
2
can exert strong electronic effects with PANI, facilitating of the PANI polymer will thus give the observed high selectivity
+
electron and H migration at long range during the redox cycle. towards HCOOH. The slightly higher activation (lower potential
of -0.7 V) compared with thermodynamic value of -0.61 V, high
selectivity towards HCOOH due to selective trapping and high
+
activity for the fast electron and H migration in the presence
of Pd metal make this system unique. Although Pt shows
similar properties as Pd, it is easily fouled during CO
HCOOH. Thermodynamically, CO should be reduced more
easily to CH OH than HCOOH, as shown below (pH = 7 in
2
to
2
3
1
9
aqueous solution versus SHE, 25 °C) :
+
-
o
CO
CO
2
+ 2H + 2e => HCOOH
+ 6H + 6e => CH
E = - 0.61 V
O E = - 0.38 V
Fig. 4. (a) CO
metal/PANI before (solid line) and after CO
2
TPD plots of PANI and metal/PANI; (b) FTIR spectra of PANI and
adsorption (dotted line).
+
-
o
OH + H
2
2
2
3
However, ‘formate’ would need to reorganize to ‘methoxy’
over the metal surface under reduction conditions before
methanol can be produced and Cu is well-known to provide
The next key questions are: where are the sites for CO
capture and why the composites are excellent for CO
2
2
reduction? Thus, CO
2
adsorptions on PANI and metal/PANI
-temperature programed
1
2
such selective surface. As a result, our Pd/PANI provides the
best activity for the first stage CO reduction, followed by
Cu/PANI for the further reduction of HCOOH to CH OH.
were characterized using CO
2
2
desorption (TPD) and FTIR (Fig. 4). From the thermogravimetric
analysis (TGA), both PANI and metal/PANI are stable until 270
3
In summary, metal/PANI gives high proton and electron
migrations between metal NPs and PANI during CO electro-
reduction in water with high over-potential for hydrogen
evolution. Thus, it can facilitate high activity and selectivity
°
C (Fig. S14). TPD of PANI depicts two main desorption peaks
2
of 96.6 °C and 167.4 °C, respectively. Pd/PANI also gives the
two peaks but at slightly higher temperatures. In cases of
Pt/PANI and Cu/PANI, we can only see one dominating signal,
3 2
towards HCOOH or CH OH without much production of H .
1
95.0 °C and 125.7 °C, respectively. Thus, PANI can interact
st
2
with CO mainly in two ways: weak physisorption peak (1
nd
peak) and stronger chemisorption peak (2 peak). The size of
the physisorption peak appears to be related to the surface
Notes and references
1
E. E. Benson, C. P. Kubiak, A. J. Sathrum, J. M. Smieja, Chem. Soc.
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2
3
J. Qiao, Y. Liu, F. Hong, J. Zhang, Chem. Soc. Rev. 2014, 43, 631.
a)M. Azuma, K. Hashimoto, M. Hiramoto, J. Electrochem. Soc.
2
area of PANI. The strength of CO chemisorption could be
reflected by peak temperature (higher electronic density of N
atoms promoted by metal gives higher temperature). Clearly,
the introduction of metal alters both the adsorption modes.
The in situ doping on PANI significantly lowers its surface area
1
990, 137, 1772. b)K. Kuhl, et al., JACS. 2014, 136, 14107.
4
5
H. Yang, S. Qin, H. Wang, J. X. Lu, Green Chem. 2015, 17, 5144.
a)L. M. Aeshala, R. Uppaluri, A. Verma, Phys. Chem. Chem. Phys.
2014, 16, 17588. b)A. K. Mishra, S. Ramaprabhu, J. Mater. Chem.
2
a)A. N. Grace, S. Y. Choi, M. Vinoba, M. Bhagiyalakshmi, D. H.
Chu, Y. Yoon, S. C. Nam, S. K. Jeong, Appl. Energy 2014, 120, 85.
b)C. Zhao, Z. Yin, J. Wang, ChemElectroChem 2015, 2, 1974. c)K.
012, 22, 3708.
(
aggregation as evidenced by TEM), which decreases the
st
6
relative physisorption peak size (1 peak, Fig. S15). As to
chemisorption of CO , Pd and Pt NPs shift the peak to higher
temperatures. Significant chemisorption of CO by the metal
surface on its own is ruled out since without extensive H,
2
2
Ogura, N. Endo, M. Nakayama, J. Electrochem. Soc. 1998, 145
3801. d)R. Aydin, F. Koleli, J. Electroanal. Chem. 2002, 535, 107.
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A. G. Macdiarmid et al., Mol. Cryst. Liq. Cryst. 1985, 121, 173.
,
1
8
7
8
9
adsorption of CO
required for electro-chemical reduction of CO
electrodes is partly due to its poor adsorption. On the other
hand, a significant and comparable quantity of CO
taken up by PANI and Pd-PANI (Fig. S15). The chemisorbed CO
2
is not facile. The high over-potential
2
over metal
B. J. Gallon, R. W. Kojima, R. B. Kaner, P. L. Diaconescu, Angew.
Chem., Int. Ed. 2007, 46, 7251.
2
can be 10 S. Akhade, W. Luo, X. Nie, N. J. Bernstein, A. Asthagiri, M. J.
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1 L. C. Grabow, M. Mavrikakis, ACS Catal. 2011, , 365.
2
nd
1
1
1
peak (2 peak size) shifts to higher temperature over Pd/PANI
and Pt/PANI as compared to PANI and Cu/PANI. FTIR spectra in
2 J. P. Pouget, M. E. Jozefowicz, A. J. Epstein, X. Tang, A. G.
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3 E. T. Kang, K. G. Neoh, K. L. Tan, Prog. Polym. Sci. 1998, 23, 277.
-1
Fig. 4b show the appearance of the peak at 1656 cm after
CO
structure. This confirms that CO
1
2
adsorption, which indicates the formation of stable O-C-O 14 S. golczak, A. Kanciurzewska, M. Fahlman, K. Langer, J. Langer,
2
chemisorption takes place at
Solid State Ionics 2008, 179, 2234.
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1
PANI, Pd/PANI and Cu/PANI (Pt/PANI, similar to Pd/PANI, not
shown) through the Lewis acid-base interaction with the
5
66, 703.
1
1
6 W. W. Focke, G. E. Wnek, Y. Wei, J. Phys. Chem. 1987, 91, 5813.
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polymer N atoms. As a result, the strong CO
2
adsorption by the
Lewis basic sites in combination with rapid proton and
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1
will assist the fast CO
2
hydrogenation to HCOOH. Pd
2
apparently is the best candidate to exert the strong synergetic
4
| J. Name., 2012, 00, 1-3
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