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N. Yadav and R. Dhar
2.1.4 Potentiostatic Control
Direct measurement of impedance under potentiostatic control is a convenient technique of deducing mechanistic and kinetic parameters for a lot of electrode studies.
It is easy and simple in operation just like the bridge methods. Passive bridges
lack flexibility in terms of potential control. This direct method allows proper
control of potential. Commercially available potentiostats are calibrated for fast step
response. Although, potentiostating function introduces error for sinusoidal inputs
on increasing the frequency. Many reports on the analyses done on this type of
errors have been reported [27]. In fact, high speed potentiostats are normally not
preferred for impedance measurements at high frequencies at the interface of electrode and aqueous electrolyte. The reactive impedance decreases to that of double
layer capacitance at such kind of interfaces.
2.1.5 Oscilloscopic Techniques
A twin beam oscilloscope is used to record voltage drop across resistance in series
and the ratio of the two peak to peak voltages and observed phase angle gives the
value of impedance. A single beam oscilloscope can also be utilized to calculate
impedance by technique of Lissajous figures. Available oscilloscopes can measure
low frequencies upto 10
–2 Hz. While transmission line effects and stray capacitances
impact the high frequency measurements. Precision is the principle demerit of this
method. Linearity of the oscilloscope is mostly less than 1% and phase angles cannot
be measured with precision of more than 2º. Electrochemical instabilities can impose
limitations at low frequencies. These systems are also prone to external noise pickup.
Usage of high gains without proper filter to amplify sine wave voltage results in errors
in measurements. Electronic low pass filters can be used to reduce these errors at the
beginning of amplification.
2.1.6 Phase Sensitive Detection
Phase sensitive detector (PSD) can be used to measure the real and imaginary parts
of voltage with respect to a reference signal. Small input signals are used to measure
impedances to fulfill the need of linearity. The operation of time averaged and multiplexed circuits sequentially can help in phase sensitive detection. The multiplication
of input signal with a reference signal can be accomplished by the multiplexer. The
response of a PSD is frequency selective. It is affected by odd order harmonics of
the input signal. Commercial instruments have low frequency limit of 0.5–10 Hz.
However, this limit can be increased to below 10
–3 Hz by applying digital integration
[28]. Major drawback of these devices is that measurements have to be performed
frequency by frequency; this can be removed by using FRA.
N. Yadav and R. Dhar
2.1.4 Potentiostatic Control
Direct measurement of impedance under potentiostatic control is a convenient technique of deducing mechanistic and kinetic parameters for a lot of electrode studies.
It is easy and simple in operation just like the bridge methods. Passive bridges
lack flexibility in terms of potential control. This direct method allows proper
control of potential. Commercially available potentiostats are calibrated for fast step
response. Although, potentiostating function introduces error for sinusoidal inputs
on increasing the frequency. Many reports on the analyses done on this type of
errors have been reported [27]. In fact, high speed potentiostats are normally not
preferred for impedance measurements at high frequencies at the interface of electrode and aqueous electrolyte. The reactive impedance decreases to that of double
layer capacitance at such kind of interfaces.
2.1.5 Oscilloscopic Techniques
A twin beam oscilloscope is used to record voltage drop across resistance in series
and the ratio of the two peak to peak voltages and observed phase angle gives the
value of impedance. A single beam oscilloscope can also be utilized to calculate
impedance by technique of Lissajous figures. Available oscilloscopes can measure
low frequencies upto 10
–2 Hz. While transmission line effects and stray capacitances
impact the high frequency measurements. Precision is the principle demerit of this
method. Linearity of the oscilloscope is mostly less than 1% and phase angles cannot
be measured with precision of more than 2º. Electrochemical instabilities can impose
limitations at low frequencies. These systems are also prone to external noise pickup.
Usage of high gains without proper filter to amplify sine wave voltage results in errors
in measurements. Electronic low pass filters can be used to reduce these errors at the
beginning of amplification.
2.1.6 Phase Sensitive Detection
Phase sensitive detector (PSD) can be used to measure the real and imaginary parts
of voltage with respect to a reference signal. Small input signals are used to measure
impedances to fulfill the need of linearity. The operation of time averaged and multiplexed circuits sequentially can help in phase sensitive detection. The multiplication
of input signal with a reference signal can be accomplished by the multiplexer. The
response of a PSD is frequency selective. It is affected by odd order harmonics of
the input signal. Commercial instruments have low frequency limit of 0.5–10 Hz.
However, this limit can be increased to below 10
–3 Hz by applying digital integration
[28]. Major drawback of these devices is that measurements have to be performed
frequency by frequency; this can be removed by using FRA.