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Characterization and Diagnosis Techniques
associated electrode layers. This is helpful for the design, development, and
screening of supercapacitor materials and components. In situ measurement
is often used for the diagnosis of a single supercapacitor cell or stack under
real operating conditions.
7.5.1 Measurement and Instrumentation
The theory and principle of EIS are beyond the scope of this chapter and
explanations can be found in the literature [8]. This section focuses on the
diagnostic applications of EIS in supercapacitors.
As a powerful diagnostic tool, one important advantage of EIS is the use of
very small AC amplitude signals to analyze electrical characteristics without
significantly disturbing the properties of the system measured. During EIS
measurement, a small AC amplitude signal is applied to a supercapacitor
cell over a frequency range from 0.001 to 3,600,000 Hz. Either voltage control
(potentiostatic) or current control (galvanostatic) mode can be used. In the
voltage control mode, an AC voltage single (usually 5 to 50mV) is applied to
disturb the supercapacitor and the current response is measured to obtain
the impedance of the system. In this mode, the frequency response analyzer
(FRA, Solartron 1260A) and potentiostat (Solartron SI 1287) are employed.
Similarly, in current control mode, an AC current single (usually 5 to 50 mA)
is applied to disturb the electrochemical system, and the voltage response is
measured to determine impedance.
Normally the EIS results measured by both control modes are consistent
and show no significant differences. The impedance responses recorded
by the EIS instrument are normally shown as Nyquist plots that illustrate
the relationship between imaginary resistance or impedance and real resistance or impedance. For example, Figure 7.9 shows the Nyquist plots (dotted points) recorded from a two-electrode symmetric supercapacitor [9]. In a
later section we will give a detailed analysis of these plots.
The three fundamental requirements of EIS measurement are: (1) the linearity between the perturbation signal and the system response, (2) the stability of the target system during measurement, (3) and the causality of the
response. In particular, the perturbation must not cause the system to shift
from its equilibrium state. To meet this requirement, it is better to measure
the EIS of a supercapacitor at its open potential voltage (OCV) rather than
under load. However, measurement at OCV does not necessarily reflect the
situation under load conditions. Due to this concern, EIS results may not be
as reliable as those obtained by CV and CDC.
7.5.2 Equivalent Circuits
Analysis of EIS data can be done by modeling or fitting of impedance spectra
with an equivalent circuit (EC). This requires the construction of a physically meaningful EC containing several elements required by the studied
Characterization and Diagnosis Techniques
associated electrode layers. This is helpful for the design, development, and
screening of supercapacitor materials and components. In situ measurement
is often used for the diagnosis of a single supercapacitor cell or stack under
real operating conditions.
7.5.1 Measurement and Instrumentation
The theory and principle of EIS are beyond the scope of this chapter and
explanations can be found in the literature [8]. This section focuses on the
diagnostic applications of EIS in supercapacitors.
As a powerful diagnostic tool, one important advantage of EIS is the use of
very small AC amplitude signals to analyze electrical characteristics without
significantly disturbing the properties of the system measured. During EIS
measurement, a small AC amplitude signal is applied to a supercapacitor
cell over a frequency range from 0.001 to 3,600,000 Hz. Either voltage control
(potentiostatic) or current control (galvanostatic) mode can be used. In the
voltage control mode, an AC voltage single (usually 5 to 50mV) is applied to
disturb the supercapacitor and the current response is measured to obtain
the impedance of the system. In this mode, the frequency response analyzer
(FRA, Solartron 1260A) and potentiostat (Solartron SI 1287) are employed.
Similarly, in current control mode, an AC current single (usually 5 to 50 mA)
is applied to disturb the electrochemical system, and the voltage response is
measured to determine impedance.
Normally the EIS results measured by both control modes are consistent
and show no significant differences. The impedance responses recorded
by the EIS instrument are normally shown as Nyquist plots that illustrate
the relationship between imaginary resistance or impedance and real resistance or impedance. For example, Figure 7.9 shows the Nyquist plots (dotted points) recorded from a two-electrode symmetric supercapacitor [9]. In a
later section we will give a detailed analysis of these plots.
The three fundamental requirements of EIS measurement are: (1) the linearity between the perturbation signal and the system response, (2) the stability of the target system during measurement, (3) and the causality of the
response. In particular, the perturbation must not cause the system to shift
from its equilibrium state. To meet this requirement, it is better to measure
the EIS of a supercapacitor at its open potential voltage (OCV) rather than
under load. However, measurement at OCV does not necessarily reflect the
situation under load conditions. Due to this concern, EIS results may not be
as reliable as those obtained by CV and CDC.
7.5.2 Equivalent Circuits
Analysis of EIS data can be done by modeling or fitting of impedance spectra
with an equivalent circuit (EC). This requires the construction of a physically meaningful EC containing several elements required by the studied