150
Electrochemical Supercapacitors for Energy Storage and Delivery
mV/s); see Figure 4.7, bottom. The double oxidation creates a large capacitance within the peak range that averages 370 F.g –1 for 11 wt% AQ—more
than three times the 120 F.g -–1 produced by the unmodified AC carbon fabric (1500 m 2 .g –1 ) [22]. This high capacitance in the narrow voltage region
highlights AQ-modified materials for positive electrodes in asymmetric cell
designs [22].
4.2.7 Series Resistance in EDLC Design
The series resistance (ESR) affects power loss and stems from the internal resistances within capacitor materials. Traditionally, the equivalent series resistance
represents deviation from the pure ideal capacitor (90° phase, no resistance)
due to the presence of an ohmic resistance generated in the dielectric. At very
high frequencies, a phase delay occurs because the molecular relaxation of the
dielectric is kinetically limited and polarization cannot occur quickly enough
[25]. In supercapacitors, the dielectric losses within the double-layer start to
occur only when frequencies greater than hundreds of megahertz are used—
the ohmic losses do not dominate. Larger ESR resistances are generated due to
the nature of the porous structure and multi-component cell design:
• External contact resistances
• Electrolyte solution resistance
• Separator transport resistance due to scattering and insufficient
ion conduction
• Internal electrode resistance due to insufficient electron conduction
• Internal interparticle contact resistance
• Electrode–collector contact resistance
The reported effective ESR is taken from a Nyquist plot of real versus imaginary impedance. At high frequency approaching infinity, the imaginary
impedance tends to zero as the capacitor becomes an AC short circuit. The
ESR can be taken from the phase delay or intercept of the real impedance
at high frequency. Alternatively, the ESR can be determined from the voltage drop seen during constant current discharge. These techniques are discussed in more detail in Chapter 7. As previously discussed, the ESR present
in ESs is a very important factor in device performance.
Power loss due to resistance creates a significant amount of heat that can
limit operating power output and performance characteristics. As devices
age, degradation sets in, causing an increase in series resistance. Continued
device operation at extreme temperatures or high power, lack of proper cooling, or contamination through gas leakage or permeation will increase degradation rates and the ESR will increase, leading to reduced performance
and shorter cycle life.
Electrochemical Supercapacitors for Energy Storage and Delivery
mV/s); see Figure 4.7, bottom. The double oxidation creates a large capacitance within the peak range that averages 370 F.g –1 for 11 wt% AQ—more
than three times the 120 F.g -–1 produced by the unmodified AC carbon fabric (1500 m 2 .g –1 ) [22]. This high capacitance in the narrow voltage region
highlights AQ-modified materials for positive electrodes in asymmetric cell
designs [22].
4.2.7 Series Resistance in EDLC Design
The series resistance (ESR) affects power loss and stems from the internal resistances within capacitor materials. Traditionally, the equivalent series resistance
represents deviation from the pure ideal capacitor (90° phase, no resistance)
due to the presence of an ohmic resistance generated in the dielectric. At very
high frequencies, a phase delay occurs because the molecular relaxation of the
dielectric is kinetically limited and polarization cannot occur quickly enough
[25]. In supercapacitors, the dielectric losses within the double-layer start to
occur only when frequencies greater than hundreds of megahertz are used—
the ohmic losses do not dominate. Larger ESR resistances are generated due to
the nature of the porous structure and multi-component cell design:
• External contact resistances
• Electrolyte solution resistance
• Separator transport resistance due to scattering and insufficient
ion conduction
• Internal electrode resistance due to insufficient electron conduction
• Internal interparticle contact resistance
• Electrode–collector contact resistance
The reported effective ESR is taken from a Nyquist plot of real versus imaginary impedance. At high frequency approaching infinity, the imaginary
impedance tends to zero as the capacitor becomes an AC short circuit. The
ESR can be taken from the phase delay or intercept of the real impedance
at high frequency. Alternatively, the ESR can be determined from the voltage drop seen during constant current discharge. These techniques are discussed in more detail in Chapter 7. As previously discussed, the ESR present
in ESs is a very important factor in device performance.
Power loss due to resistance creates a significant amount of heat that can
limit operating power output and performance characteristics. As devices
age, degradation sets in, causing an increase in series resistance. Continued
device operation at extreme temperatures or high power, lack of proper cooling, or contamination through gas leakage or permeation will increase degradation rates and the ESR will increase, leading to reduced performance
and shorter cycle life.