TERM reaction mechanism, surface morphology, kinetics of film



This paper reviews
detailed work of two research groups hereby mentioned as Pletcher et. al. and
Marino et. al. on the specific topic of electrodeposition of Pyrrole from
aqueous solutions. Both the groups analyze the oxidation potentials for the
oxidative polymerization of Pyrrole, the microstructure, effect of pH, Pyrrole
conc., and the supporting electrolyte conc. on the film formation. While
Pletcher et. al. layout the fundamentals of oxidative polymerization of pyrrole
by establishing the relationship between oxidative potential and the relative
film formation, they fail to put forward details accounting to features on the potentiostatic
transients and not enough analysis the role of solution on the final properties
of the film. Marino et. al use this fundamental knowledge to further analyze
the current transients and establish the reaction mechanism, surface
morphology, kinetics of film growth and effect of varying the solution
parameters on the electrodeposition of the film.

Significance and Interests:

Polypyrrole (Ppy.)
was the first of the conducting polymers that shows relative high conductivity
in the range of 1 – 100 Scm-1 and it attains this conductivity due
to interchain hopping of electrons and hence is an inherently conducting
polymer. It has drawn attention because of its simple preparation, stability
and good mechanical properties. Electrodeposited free-standing films could be
used in several applications such as energy storage devices, organic
electronics, capacitive touch panels and in Bio-medical applications or as modified
electrodes for enzymosensors(1) .
Electrochemical approach for making electroactive/conductive films is very
versatile and provides a facile way to vary film properties by introducing
either a different dopant anion (NO3-, SO4-2
etc.), system into the electrolytic solution or a different polymer system
within the same to electrodeposit a composite solution for tailoring the final
product properties according to your requirements, eg: – Polypyrrole – graphene
oxide nanocomposite films.


                                                    Figure 1:
PolyPyrrole repeating unit.(2) 


Both Pletcher et.
Al and Marino. Et. Al used similar experimental setup and techniques. Pletcher
et. Al used a three electrode, three compartment cells with working Pt. gauze
secondary electrodes, SCE reference electrode which was connected to the
anolyte compartment by a Luggin Capillary. The working electrodes were mostly
Pt disc (area 0.2 cm2). The anolyte compartment used Dimethyl
Formamide (DMF) as an internal standard. Fresh electrodes were used for each
experimental run to keep reproducibility of the results and the polishing
procedure was repeated in between each experiment. Triply distilled water and
Analar grade electrolytes were used to prepare the solution, the pyrrole
monomer was distilled twice in vacuum and then stored under nitrogen. KNO3
was used as a supporting electrolyte. The group uses Cyclic voltammetry,
Coulometry and Gas Liquid Chromatography for analysis.

Marino et. Al used
an all glass, air tight three-compartment cell. The working electrodes employed
were Vitreous Carbon or gold which were sealed into glass with the exposed are
of 7.07 X 10-2 cm2 and 7.85 X 10-3 cm2
respectively. The electrodes were treated for residue removal before each
experiment reported in the literature to ensure absence of Polypyrrole. The
counter electrode was Platinum wire sealed in glass in a separate compartment,
a SCE electrode was used for reference which was sealed in a different
compartment and was connected to the working electrode with the help of a
Luggin Capillary. Solutions were prepared from triply distilled water and
analar grade electrolytes, Pyrrole was distilled twice under Nitrogen and
immediately used to prepare solutions which contained 1 M KNO3 which
acts as a supporting electrolyte.

Result, analysis,
similarities and contradictions:

Pletcher et. al.
reported that at electrode potential higher than 0.6 V vs SCE, a Polypyrrole
film could be observed on the electrode surface. The physical observations such
as color depended on the thickness of the film in general thick films are black
and thin are yellow. Other factors that could be attributed to various other
factors which include time of deposition and time of oxidation.  

Cyclic Voltammetry
studies as show in Figure 2 (i) illustrates the effect of potential scan rate
and the difference between 1st and 2nd cycles on Pt.
electrodes. It is seen that the oxidation of pyrrole gives a completely
irreversible peak, Ep = 0.90 V vs. SCE at 100 mVs-1 which
leads to Polypyrrole film formation on Pt. surface. In the figure below, it is
seen that the direction of the scan was reversed (a) at a potential before the
peak (b) potential just beyond the peak (c) reversed after the complete run. It
is in both (a) and (b) that the current on the reverse sweep is higher than on
the forward sweep which is a common observation for a phase growth by
nucleation, also led to the analysis that Pyrrole oxidizes readily on the
Polypyrrole film surface than on Pt. confirming the previous observation about
nucleation phase growth. In part (c) on extending the positive limit, it is
seen that the anodic peak is very broad correlating to the several overlapping
oxidation peaks after which the current drops to very low values depicting the
loss of activity leading to the conclusion that taking the potential higher
than 1.0 V does irreversible damage.



Figure 2 (i):
The effect of the positive
potential limit at potential scan rate of 100 mVs-1 on CV of pyrrole
at 53 mmol dm-3 in aqueous KNO3 (1 mol dm-3),
Pt electrode (       ) 1st
cycle, (—) 2nd cycle. (ii) (-
– -) pH 1.7, (       ) pH 13.2, (…..) pH 6.2. (2)

Pletcher et. al.
studied the effect of pH by using KNO3 acidified by Nitric acid to a
pH of 1.7 and by adding potassium Hydroxide, pH of 13.2 at 53 mmol dm-3 at
100 mVs-1 as shown in Figure
2(ii). It is observed that I-E curves for both acidic and neutral solutions
show similar behavior in all behavior which is associated to the release of
protons during the oxidation of Pyrrole causing the layer of solution near the
electrode surface to become acidic even when a neutral electrolyte is used. The
group does not provide reason for the nature of the CV curve in a basic
solution. Polypyrrole deposition was further investigated by several series of
potential step techniques. It was observed as seen in Figure. 3 that the timescale of transients decreases, and the
magnitude of current increases with increase in Pyrrole concentration, deposition
potential is made more positive and the decrease in the pH value of the
electrolyte. The trends shown in Figure 3(b) are in line with the those
expected for a nucleation and growth of a new conducting phase on an inert
electrode surface. The shapes of the transients were analyzed by finding the
best linear fits of the plot I vs t2 which shows that such a
relationship is possible when the growth of the new phase is determined by the
rate of electron transfer at the surface of the expanding phase relating
instantaneous nucleation with a three-dimensional growth.



(a)                                                                                 (b)

Figure 3. (a) I-t transients in response to potential
steps from 0.0 – 0.7 V vs SCE for on Pt. electrode in 1 mmol dm-3
KNO3 solution containing 23 mmol dm-3 pyrrole. (      ) pH
13.2, (     ) pH 6.2, ( …..)  pH 1.7. (b) I-t transients in response to
potential steps to a series of potentials for Carbon electrode.(2)

Marino et. al.
delver deeper onto the points left out by Pletcher. Et.al in explaining the I-t
transients and what the curve depicts in terms of Ppy film formation at the
liquid solution and electrode interface. Miller et. al. does experimental
analysis in similar fashion as done by Pletcher. Et.al. using similar cell
construction and electrolyte solution with KNO3 acting as a
supporting electrolyte. The quantitative reproducibility of the current
transients from the Pletcher et.al. was questioned as it did not follow the
similar power law i.e. i-tx during every experiment(3) . Marino’s
group further studies the effect of stirred solution on the Polypyrrole film
deposition on the electrode surface by studying the current-potential traces in
presence of NO3- ions at low scan rates for stagnant
solution and by incorporating 0.1 M HNO3 in 1M KNO3 soln.
with varying pyrrole conc. They deduced that the rate of oxidation of Pyrrole
is not significantly dependent on the pH of the soln. Marino et. al reference
Miller et. al. as to why film formation does not take place on the electrode
surface from stirred solutions even at high current densities though polymerization
mechanism proceeds in the solution which was attributed to the formation of
Pyrrole oligomers and coloring of the electrolytic soln.

Marino et. al.
state that at sufficient Pyrrole conc., the rise of the current transients
relates to increasing area available for electrochemical reaction as pyrrole is
deposited on the surface of the electrode. However, in very dilute solutions
that are below 1 X 10-3 M, the rising transients were not observed
even after longer durations as seen in Fig 4 (a).  Similar current transient curves can be seen
in between Marino et. al. and Pletcher. Et. al however, Marino et. al. ascribes
the reason for decaying currents at short times in 0-5 seconds as seen in
Figure 4(b) is due to the charging of the double layer which could also be
accompanied by adsorption of Pyrrole at the interface or to its oxidation to
Oligomeric forms, soluble species followed by increase in the current
transients which relate to the growth of Ppy. centers on the electrode surface,
followed by decaying current at higher times or at higher potential voltages as
discussed earlier due to processes that limit the rate of growth of films. They
also state that the nature of the substrate only influences the birth of the
film after which it becomes less important on longer time scale. While Pletcher
et. al. speculates about this decline in the current-transient curve and relate
it to the nucleophilic properties of the anions at potentials higher than 1.2
V, for time scale longer than 10s as seen in Figure 4(b), Marino et. al.
identifies the relations between i & t given below which is contrary to the
equation used by Pletcher et.al.


The decaying
portions at the long times of current transients for Ppy film growth were
fitted to the equation above with least-squares procedure the results of which
are represented by the dots in Figure 4(b). this eventually lead them to deduce
the reason for decline was decline in conductivity of growing Ppy films is
lower and hence RDS for rate of growth is limited by film resistance.


Figure 4(a)
effect of Pyrrole conc. on oxidation of pyrrole in 1M KNO3 at 740
mV(SCE) on Vitreous C electrode. (b)
current transients for Ppy film formation on vitreous carbon from 28.9 X 10-3
M soln. of pyrrole in aq. 1M KNO3 at different step potentials
indicated in the figure in mV i.e 20 mV potential steps.(4)

Marino et. al and Pletcher et. al confirm the fact that growth of Ppy. film om
electrode surface is not diffusion controlled which refutes the claim by certain
groups. However Marino et. al. provide logical reasoning and analysis to prove
otherwise. While pletcher et. al are unable to identify the  complete reaction mechanism for oxidation of
Pyrrole to Ppy, effect of pH, presence of oligomers, soluble species, solution
stirring in the film deposition, Marino et. al. clearly outline the reaction
order, reaction procedure and the effect of pH, However both the groups do not
analyze with a varying pH range within the acidic range.

Conclusion and
Future work:

The papers
satisfactorily analyze the PolyPyrrole film formation on the electrode surface
by oxidative polymerization of Pyrrole monomer. However, experiments such as
weight gain analysis of the film on the electrode surface can be carried out by
using multiple working electrodes of the same material, with similar surface
area, by keeping the time of voltage and current constant while varying the time
and interchanging the variables. This might also help explain the conversion
rate of Pyrrole to Ppy. Furthermore SEM, XPS can be used to analyze
microstructure at different growth intervals and finally FTIR for surface group




1. Ravindra P. Singh. Prospects of
Organic Conducting Polymer Modified Electrodes: Enzymosensors. International
Journal of Electrochemistry. 2012;2012:1-14.
2. Asavapiriyanont S, Chandler GK,
Gunawardena GA, Pletcher D. The electrodeposition of Polypyrrole films from
aqueous solutions. Journal of Electroanalytical Chemistry. 1984;177(1):229-44.
3. Miller LL, Zinger B, Zhou QX.
Electrically controlled release of hexacyanoferrate(4-) from Polypyrrole.
Journal of the American Chemical Society. 1987 Apr;109(8):2267-72.
4. Scharifker BR, García-Pastoriza E,
Marino W. The growth of Polypyrrole films on electrodes. Journal of
Electroanalytical Chemistry. 1991;300(1):85-98.



Declaration of Plagirism:

This paper was
written has been written on basis of my personal understanding and citing the
two main research papers and a little supplementary research papers to help
strengthen the arguments/observations and have been cited. I, hereby declare
that no previously published text/materials have been copied to a level of
infringement except for some technical terminologies, figures and procedures
which could not be paraphrased so far.