3.6. Electrochemical study
Electrochemical impedance spectroscopy (EIS) is an effective method
for characterization of electrode processes. Complex plane (Nyquist)
plots of the EIS are often utilized for an easy estimate of the circuit elements.
These plots include a semicircle portion and a linear portion. The
Nyquist diameter is equal to the charge transfer resistance, Rct. Fig. 5A
and B displays the Nyquist diagram of the carbon paste electrode in
the absence and presence of SDS in solution, respectively. As we could
notice, the visible decrease in the diameter of semicircle would happen.
Rct decrease from 8.64 × 105 Ω cm2 for CPE in the absence of SDS to
1.65 × 105 Ω cm2 in the presence of SDS. The impedance data demonstrated
the facility electron transfer between BPR and electrode in the
presence of SDS. The impedance data attained, which has been analyzed
by the Randles circuit and was shown in Fig. 5 insert, offers a great fit to
the experimental data.
The influence of scan rate on the voltammetric peak was studied for
solution containing 0.08 μM of BPR in the range of 0.02 to 0.1 V s−1. It
was observed that the oxidation peak current of the BPR increased linearly
with the square root of the scan rate, which points toward a diffusion
transfer-controlled process (Fig. 6).
A Tafel plot is a useful device for estimating the kinetic parameters.
The number of electrons involved in the rate determining step can be
evaluated from the slope of the Tafel plot. The average Tafel slopes
were obtained at 0.1290 V decade−1, demonstrating a one electron process
for rate determining step. The transfer coefficient (α) is a measure
of the symmetry of the energy barrier. The value for αwas calculated to
be 0.55 using the relationship of b=2.30 RT / n(1−α)F. This value indicates
that the energy barrier is symmetrical [32].