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N. Yadav and R. Dhar
also be applied to even more complex systems viz. partly conducting dielectrics
exhibiting electronic and ionic conductivity simultaneously.
The basic IS experiment consists of application of an external stimulus (a known
current or voltage) to the electrodes which are connected to the electrolytic material
under study and the resulting response is observed. Three different kinds of electrical
stimuli can be used in IS. The first approach involves an application of voltage step
function at time t = 0 to the system and measurement of the current as a function of
time. The impedance (Z) resulting from this stimulus is measured by the time varying
resistance (voltage/current). Such time dependent results are usually transformed
into the frequency domain by Laplace or Fourier-transforms to obtain impedance as
a function of frequency. This approach can be easily accomplished experimentally,
and the rate of the electrochemical reaction is governed by the independent variable
i.e. voltage. However, the requirement of Fourier transforms and changing signal
to noise ratio between different frequencies which hinders proper determination of
impedance over desired frequency are some of the demerits of this approach. Second
kind of stimulus used in IS is the application of a signal voltage comprising of random
(white) noise to the electrodes and measuring the current. Again, in this approach,
Fourier transform is used to convert the obtained results into frequency domain. The
advantage of this type of stimulus is that the data can be collected quickly since
only one signal for a short duration is applied to the electrodes. While disadvantage
is the need for a real white noise and Fourier transform to convert obtained data
into frequency domain and get impedance. White noise is usually generated by a
microcomputer which also performs further analysis of the recorded impedance data.
Better signal to noise ratio can be achieved by utilizing a sum of well-defined sine
waves as external excitation in place of white noise. The most common and standard
technique is the third one which measures impedance by applying current or voltage
having single frequency to the interface and corresponding real or imaginary parts
of current, phase shift or amplitude at that particular frequency is measured by using
either fast Fourier transform (FFT) analysis or analog circuit. Several instruments
which can automatically measure frequency dependent impedance over wide range of
frequencies (about 10 μHz to 10/40 MHz) are commercially available. In fact, broad
band impedance spectroscopy covers wide range of 10
–6 –10
12 Hz. The resulting
impedance can be interfaced effortlessly to laboratory computers for further analysis.
This approach leads to better signal to noise ratio in the desired frequency range.
Also, these three approaches can be combined to generate other variety of stimuli.
For example, the first and third approaches can be combined by ac polarography
by applying a linearly changing unipolar transient signal and a single frequency
sinusoidal signal of short duration simultaneously [2].
IS can be employed to study any intrinsic characteristic, which affects the conductivity of an electrode-materials system, or an external excitation [3–5]. IS spectra
gives several parameters which can be categorized into: (1) those important to the
material under study such as dielectric constant, conductivity, equilibrium concentration of charged particles, charge mobilities and rate of generation-recombination;
and (2) those relevant to interface of electrode and material such as capacitance of
interface region, rate constants of absorption reactions and coefficient of diffusion of
N. Yadav and R. Dhar
also be applied to even more complex systems viz. partly conducting dielectrics
exhibiting electronic and ionic conductivity simultaneously.
The basic IS experiment consists of application of an external stimulus (a known
current or voltage) to the electrodes which are connected to the electrolytic material
under study and the resulting response is observed. Three different kinds of electrical
stimuli can be used in IS. The first approach involves an application of voltage step
function at time t = 0 to the system and measurement of the current as a function of
time. The impedance (Z) resulting from this stimulus is measured by the time varying
resistance (voltage/current). Such time dependent results are usually transformed
into the frequency domain by Laplace or Fourier-transforms to obtain impedance as
a function of frequency. This approach can be easily accomplished experimentally,
and the rate of the electrochemical reaction is governed by the independent variable
i.e. voltage. However, the requirement of Fourier transforms and changing signal
to noise ratio between different frequencies which hinders proper determination of
impedance over desired frequency are some of the demerits of this approach. Second
kind of stimulus used in IS is the application of a signal voltage comprising of random
(white) noise to the electrodes and measuring the current. Again, in this approach,
Fourier transform is used to convert the obtained results into frequency domain. The
advantage of this type of stimulus is that the data can be collected quickly since
only one signal for a short duration is applied to the electrodes. While disadvantage
is the need for a real white noise and Fourier transform to convert obtained data
into frequency domain and get impedance. White noise is usually generated by a
microcomputer which also performs further analysis of the recorded impedance data.
Better signal to noise ratio can be achieved by utilizing a sum of well-defined sine
waves as external excitation in place of white noise. The most common and standard
technique is the third one which measures impedance by applying current or voltage
having single frequency to the interface and corresponding real or imaginary parts
of current, phase shift or amplitude at that particular frequency is measured by using
either fast Fourier transform (FFT) analysis or analog circuit. Several instruments
which can automatically measure frequency dependent impedance over wide range of
frequencies (about 10 μHz to 10/40 MHz) are commercially available. In fact, broad
band impedance spectroscopy covers wide range of 10
–6 –10
12 Hz. The resulting
impedance can be interfaced effortlessly to laboratory computers for further analysis.
This approach leads to better signal to noise ratio in the desired frequency range.
Also, these three approaches can be combined to generate other variety of stimuli.
For example, the first and third approaches can be combined by ac polarography
by applying a linearly changing unipolar transient signal and a single frequency
sinusoidal signal of short duration simultaneously [2].
IS can be employed to study any intrinsic characteristic, which affects the conductivity of an electrode-materials system, or an external excitation [3–5]. IS spectra
gives several parameters which can be categorized into: (1) those important to the
material under study such as dielectric constant, conductivity, equilibrium concentration of charged particles, charge mobilities and rate of generation-recombination;
and (2) those relevant to interface of electrode and material such as capacitance of
interface region, rate constants of absorption reactions and coefficient of diffusion of