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Chapter 3-Section 3.3
3.3 Electrochemical
Response of Conducting Polymers
One unique feature of many
conjugated organic polymers like polypyrrole and polythiophene is their
ability to reversibly change their electrical properties. In their
reduced form, they are insulators. Upon oxidation, they are transformed
into an electrically conducting material. As seen in Figure 3.2, poly(3-hexyl
thiophene), a soluble conducting polymer which was spray coated on
a Au electrode can be electrochemically doped. Spray coating is used
for deposition because polymerization of the monomer species requires
potentials too high to be used with high-temperature superconductor
electrodes. A pre-formed polymer such as poly(3-hexyl thiophene),
can be chemically synthesized and subsequently can be spray coated
onto the electrode. Unlike the electropolymerization of polypyrrole,
the spray-coating technique does not place requirements on the electrical
properties of the substrate material for polymer deposition.
Figure 3.2:The electrochemical
response of a thick film (~1-10mm) of poly(3-hexyl thiophene) in a 0.1
M Bu4NBF4 / CH3CN solution on a
Au coated glass electrode at a scan rate of 50 mV/sec.
Figure 3.3: Cyclic
voltammetry shows the oxidation and reduction of a polypyrrole coated
YBa2Cu3O7-x thin film electrode placed
in a 0.1 M Et4NBF4 / CH3CN solution
at a scan rate of 5 mV/sec.
Figure 3.4:
Chronoamperometry showing the doping and undoping of polypyrrole on
a YBa2Cu3O7-x electrode in a 0.1 M
Et4NBF4 / CH3CN solution.
The availability
of this solution processing method expands the diversity in the type
of polymer films placed on superconductors.
When the polymer is oxidized, it becomes
positively charged and the conjugated backbone is delocalized. To compensate for the charge, a
tetrafluoroborate anion is inserted into the polymer matrix. The cyclic
voltammetry shows a clear oxidation and reduction of the poly(3-hexyl
thiophene) coated gold film in an electrolytic acetonitrile solution
of tetraethylammonium tetrafluoroborate. This behavior demonstrates
the reversibility and the redox process. It also shows that there is
clear electron transfer between the poly(3-hexyl thiophene) and the underlying
gold. If the surface of the gold had been coated with an insulating
layer, this electron transfer would not have taken place. Similarly,
if there had been an insulating barrier coating the surface of YBa2Cu3O7-x in
a polypyrrole coated YBa2Cu3O7-x electrode,
electron transfer at the interface would be hindered. As seen in Figure
3.3, when proper conditions are used, a reversible electrochemical response
from a polypyrrole coated YBa2Cu3O7-x thin
film electrode can be obtained. Chronoamperometry can also be used to
oxidize and reduce the conducting polymer as seen in Figure 3.4. The
data here provides additional evidence for the reversible charge flow
between the polymer and the cuprate conductor. While there were reports
of conducting polymers deposited on bulk high-temperature superconductors,
we were the first to report the electrochemical response of a conducting
polymer deposited onto a thin film as well as to explore the polymer’s
charging characteristics on the underlying superconductor’s properties
(vide infra).49
