Polymeric ionic liquid and carbon black composite as a reusable supporting electrolyte: Modification of the electrode surface

Article abstract of DOI:10.1002/anie.201410207

One of the major impediments to using electroorganic synthesis is the need for large amounts of a supporting electrolyte to ensure the passage of charge. Frequently this causes separation and waste problems. To address these issues, a polymeric ionic liqu

Full text of DOI:10.1002/anie.201410207

Angewandte
Chemie
DOI: 10.1002/anie.201410207
Ionic Liquids
Polymeric Ionic Liquid and Carbon Black Composite as a Reusable
Supporting Electrolyte: Modification of the Electrode Surface**
Seung Joon Yoo, Long-Ji Li, Cheng-Chu Zeng, and R. Daniel Little*
Abstract: One of the major impediments to using electro-
organic synthesis is the need for large amounts of a supporting
electrolyte to ensure the passage of charge. Frequently this
causes separation and waste problems. To address these issues,
a polymeric ionic liquid–Super P carbon black composite has
been formulated. The system enables electrolyses to be
performed without adding an additional supporting electrolyte,
and its efficient recovery and reuse. In addition, the ability of
the composite to modify the electrode surface in situ leads to
improved kinetics. A practical consequence is that one can
decrease catalyst loading without sacrificing efficiency.
polymers, we turned our attention to the development of
[
5]
a polymeric ionic liquid (PIL) and carbon nanoparticle
composite featuring conductive Super P carbon black (CB).
We report that the composite can be 1) used as a substitute for
traditional supporting electrolytes, 2) recovered and reused,
thereby simplifying product isolation and minimizing waste,
and, with the formation of a composite dispersion, 3) used to
avoid the high viscosity and low conductivity that is fre-
quently encountered with ionic liquids. We also describe the
dramatic change in current intensity and curve shape that is
observed when voltammograms are recorded in the compo-
site dispersion.
E
lectroorganic chemistry provides a benign and practical
We selected the polymeric form of diallyldimethylammo-
nium (PDDA) chloride, because the pyrrolidinium cation
offers access to a wide electrochemical potential window.
Formation of the composite dispersion begins with an anion
exchange to convert PDDA chloride to the easily handled
[1]
approach to synthesis. Unfortunately some applications
necessitate the use of large amounts of supporting electrolyte
to ensure ionic conductivity and the efficient movement of
charge. As a result one can be faced with a tedious workup. In
an effort to overcome this problem and the waste issues that
accompany the use of supporting electrolytes, researchers
have devised many solutions, including the use of ionic
[6]
solid, PDDA(Tf N), that is subsequently dissolved in aceto-
2
[
7,8]
nitrile (Scheme 1).
After the addition of CB to the
[
2]
liquids. While attractive, their high viscosity and low
conductivity diminish the mass transport rate of the electro-
active species, leading to low current flow and detracting from
[3]
their utility. Since the electroactive species must reach or be
nearby the surface of an electrode for a heterogeneous
electron transfer to occur, the overall efficiency of an
electrolysis depends on the rate at which the species reach
the surface.
Scheme 1. Formation of composite dispersion.
Inspired by the discovery of an enhancement in the self-
diffusion coefficient and conductivity of an ionic liquid
containing nanoparticles, and the intrinsic advantages of
solution, the resulting mixture is transformed to a dispersion
[9]
by sonication, and is stable for more than a year (see the
Supporting Information (SI) for details). The composite
system is electrochemically inert over a broad range of
[
4]
potentials (ca. + 2.8 V to À2.7 V vs. Ag/AgNO ), and there-
3
[
*] S. J. Yoo, Prof. Dr. R. D. Little
fore may constitute a reaction medium for a variety of
applications (Figure S2).
Department of Chemistry and Biochemistry
University of California Santa Barbara
Santa Barbara, CA 93106-9510 (USA)
E-mail: little@chem.ucsb.edu
To study the influence of the composite dispersion upon
voltammetry and to explore the possibility that the carbon
nanoparticles might electrochemically deposit onto the elec-
L.-J. Li, Prof. Dr. C.-C. Zeng
[
10]
trode surface, we chose the redox mediator tris(p-bromo-
College of Life Science and Bioengineering
Beijing University of Technology
Beijing, 100124 (China)
[11]
phenyl)amine (TBPA) for our investigation.
Voltammograms of TBPA were obtained beginning with
À1
[
**] We thank the US National Science Foundation PIRE-ECCI Program
a scan rate of 500 mVs (Figure 1; curve a) and decreasing to
À1
(
Partnership for International Research and Education—Electron
5
0 mVs . Once completed, 300 additional cycles were
Chemistry and Catalysis at Interfaces) for a fellowship to S.J.Y. and
for their support of our research. C.C.Z. thanks the National Science
Foundation of China (No. 21272021, 21472011) for support. S.J.Y.
thanks Dr. Young-Si Jun of the Stucky group at UCSB for his gift of
Super P carbon black, and Brian Ie for assistance with the drawing
of Figure 5.
[12]
recorded to allow time for the electrophoretic deposition
of nanoparticles to occur. Thereafter, the scan at 500 mVs
À1
was repeated (Figure 1; curve b) and compared with curve a.
The contrast was dramatic, showing a significant increase in
current intensity and change of curve shape.
We suggest that the changes are due to the formation of
a nanoparticle assembly that creates a new conducting surface
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410207.
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a manifestation of an increase in the double-layer charging
[14]
(non-Faradaic current),
two chronoamperometry experi-
ments were performed in the composite dispersion, one in the
presence of TBPA, and the other in its absence. We
capitalized upon the fact that the non-Faradaic current
decays much more rapidly (1/exp(t); t = time) than the
1
/2 [14]
Faradaic current (1/(t) ). In each experiment, the potential
was stepped from 0.5 V to 1.1 V, a potential in excess of that
required to oxidize TBPA, and the current decay was
monitored for 5 seconds. Next, repetitive CV scans were
performed using the sequence described previously, to allow
time for surface modification to occur. Figure 3 clearly shows
Figure 1. CV of TBPA in the composite dispersion. Pt working elec-
trode, Pt wire counter electrode, and Ag/AgNO reference electrode.
3
on the electrode, accompanied by an increase in the surface
area, through in situ electrophoretic deposition of CB. The
modification leads to a change in the flux of TBPA toward the
surface through a combination of linear and radial diffu-
[13]
sion, and to an amplified current and change in the shape of
the voltammogram. In contrast to the behavior just described,
when the cyclic voltammetry (CV) of TBPA was examined in
the PIL dissolved in acetonitrile, there was no change in the
current intensity or the shape of the voltammogram (Fig-
ure S5). We conclude that in the absence of carbon nano-
particles the surface of the electrode remains unchanged.
To visually support our observations and rationale, we
used an optical microscope to observe the surface changes on
a carbon-coated aluminum foil electrode. Figure 2 shows the
changes on the surface of the electrode after the application
of a potential of 1.0 V (vs. Ag/AgNO ) in the composite
3
dispersion for 10 min. In contrast, no surface changes were
observed when an electrolysis was conducted in a solution of
the PIL dissolved in acetonitrile containing CB (no compo-
site). The electrophoretic deposition of CB presumably
results from attractive interactions between the anode surface
and ions of PIL, as well as a cation–p interaction between the
pyrrolidinium cation and p-electron-rich surface of CB in the
composite dispersion.
To determine whether the change in current intensity was
mainly due to an increased Faradaic current of TBPA or was
Figure 3. Chronoamperograms of the composite dispersion in the
presence (top) and absence (bottom) of TBPA. Pt working electrode,
[
9]
Pt wire counter electrode, and Ag/AgNO reference electrode.
3
that the current decays at a rate that is consistent with
a Faradaic decay when TBPA is present and that the Faradaic
current intensity is substantially higher with the surface-
modified electrode (Figure 3; top). Meanwhile the use of
a surface-modified electrode, but in the absence of TBPA,
shows no difference in the current intensity over time,
indicating that the current decays at a rate that is consistent
with a non-Faradaic process (Figure 3; bottom). Together,
these experiments illustrate that the increase is due to an
increase of Faradaic current owing to an enhanced mass
transport rate of TBPA to the surface of the in situ-modified
electrode.
To explore the applicability of the composite dispersion in
synthesis, the voltammetric behavior of TBPA, and TBPA in
the presence of 4-methoxybenzyl alcohol and 2,6-lutidine
Figure 2. Optical microscope images of the electrode surface before
(
left) and after (right) application of a potential in the composite
dispersion. Scale bar on each image indicates 500 mm.
2
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Figure 4. Comparison of catalytic effects; grey line: 2 mm TBPA; black
solid line: 2 mm TBPA+50 mm 4-methoxybenzyl alcohol+100 mm
2
2
,6-lutidine in the PDDA(Tf N) dissolved in acetonitrile; dashed line:
mm TBPA+50 mm 4-methoxybenzyl alcohol+100 mm 2,6-lutidine in
2
the composite dispersion with surface modified electrode; scan rate,
À1
n=100 mVs ; all currents are normalized to the oxidation peak
current Ip,ox (28.1 mA) of the mediator (dotted line).
Figure 5. The workup procedure.
were examined in a) PDDA(Tf N) dissolved in acetonitrile,
2
and b) in the composite dispersion in which the surface of the
working electrode was modified in situ. Figure 4 portrays
examples clearly show that the composite can serve in place
of a traditional supporting electrolyte and be reused with little
sacrifice in efficiency.
[15]
a dramatic increase in catalytic current encountered when
the CV of TBPA in the presence of substrate and base was
measured in the composite dispersion with the surface-
modified electrode (compare the black dashed and solid
curves). The evidence suggests that the use of composite
dispersion and in situ-modified electrode provides improved
catalytic activity, and is kinetically more effective in the
catalytic process.
To expand the scope of the composite system, we
conducted oxidations of several dissimilar substrates
(Table 1, see SI for details). Thus, the electrochemically
mediated oxidation of 4-chlorobenzyl alcohol and an unsym-
metrically substituted ether (entry 1 and 3), the arylation of
[
17]
an epoxy chalcone (entry 2), and an oxidative amination of
[
18]
benzoxazole (entry 4), each proceeded efficiently in con-
trolled potential and constant current electrolyses. The
oxidation of 4-chlorobenzyl alcohol is of special interest,
Encouraged by these results, we envisioned that the
composite could be used in place of a traditional supporting
electrolyte in preparative scale electrolyses. We performed an
anodic oxidation of 4-methoxybenzyl alcohol in the compo-
site dispersion to isolate 4-methoxybenzaldehyde in
because we operated at 0.9 V (vs. Ag/AgNO ) and reduced
3
the mediator loading to 5 mol%. Gratifyingly, the substrate
[16]
7
6% yield. Importantly, by following the simple
Table 1: Expanding the scope in synthesis.
workup procedure described below, a 95% recovery
of the composite was achieved (Figure 5).
Entry Substrate
Ep,ox vs.
Product
Yield[%]
Ag/
The workup illustrated in Figure 5 exploits that
AgNO [V]
3
PDDA(Tf N) is insoluble in dichloromethane
2
(
DCM), but is soluble in acetonitrile. Thus, evapo-
[
a]
ration of the solvent is followed by suspension of the
1
2
3
1.94
1.03
74
[
[
b]
b]
62
4
(
5
1.44
1.02
composite by adding DCM and filtration. The crude
product(s) remains dissolved in DCM and can be
isolated using standard workup procedures. Mean-
while, the recovered composite is simply redispersed
and is ready for its next use. To explore its reusability,
three successive oxidations were performed. Each
[c]
74)
[d]
[a]
4
60
1
[a] Yields are of isolated products. [b] The yield was determined by H NMR
spectroscopy. [c] The yield is based on the recovered starting material. [d] Constant
led to product yields in the range of 65 to 76%, and current electrolysis was conducted using nBu NI as a mediator; otherwise, controlled
4
to ca. 95% recovery of the composite. These potential electrolysis using TBPA as a mediator was performed.
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was converted to 4-chlorobenzaldehyde in 74% yield reveal-
ing that the enhanced kinetics enabled a ca. 1.0 V thermody-
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1
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In conclusion, a composite electrolyte has been developed
that combines the features of a PIL with the properties of
conductive CB. The result is a stable electrolyte that can be
efficiently recovered and reused, and also improves the mass
transport rate through modification of the electrode surface.
Our approach promises to provide a general solution to
problems associated with the use of conventional supporting
electrolytes and ionic liquids. The system can be applied to
a variety of synthetic transformations and will undoubtedly
find use in a host of other applications that necessitate the use
of an electrolyte.
[
[8] To demonstrate that the ion mobility of the PIL is competitive
with that of a traditional supporting electrolyte, a CV of TBPA
was measured in PDDA(Tf N) and in LiClO , each dissolved in
2
4
acetonitrile. The results demonstrate little sacrifice in efficiency
using PDDA(Tf N) as an electrolyte (see SI for details).
2
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Received: October 17, 2014
Revised: December 4, 2014
Published online: && &&, &&&&
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[
12] This term refers to the migration and deposition of colloidal CB
onto the electrode.
Keywords: electrochemistry · electrolysis · ionic liquids ·
nanoparticles · waste prevention
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[19] No conversion reported previously, because the oxidation
potential difference between TBPA and substrate was “too
[
16]
large”.
4
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Communications
Ionic Liquids
S. J. Yoo, L.-J. Li, C.-C. Zeng,
R. D. Little*
&&& — &&&
Polymeric Ionic Liquid and Carbon Black
Composite as a Reusable Supporting
Electrolyte: Modification of the Electrode
Surface
A composite electrolyte that combines
the features of a polymeric ionic liquid
with the properties of carbon nanoparti-
cles enables electrochemical oxidations
without an additional supporting electro-
lyte. The composite can be reused in
subsequent electrolyses. In addition, the
composite electrolyte can improve the
kinetics through modification of the
electrode surface.
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