294
Electrochemical Supercapacitors for Energy Storage and Delivery
For pseudosupercapacitors, charging–discharging curves can also be used
to measure capacitance, maximum energy and power densities, and equivalent series resistance by simulating experimental charging and discharging
curves. For more information, see Chapter 3.
Similar to the situation with CV, using different charging and discharging rates (current densities) can provide some information about the mass
transfer kinetics of an electrode layer. Tsay et al. [1] demonstrated that the
specific capacitance would decrease with increasing charging rate; similar
to the result shown in Figure 7.7b. The decrease in specific capacitance with
increasing charging rate is due to the limited transfer of ions to the carbon
particle surface or pores, leading to inaccessible pore portions of the electrode layer at high charging rates.
7.4.2 Cycle Life Measurement Using Charging–Discharging Curves
The most reliable way to investigate the degradation of a supercapacitor is
to charge and discharge it over many cycles (one cycle equals one charge
plus one discharge) to observe the changes in both specific capacitance and
equivalent series resistance. Normally, supercapacitors have much longer
cycle lives than batteries. For example, a commercially available lithium ion
battery has a normal life of 400 to 1200 cycles. A supercapacitor can have a
life as long as 100,000 cycles. Fortunately, the fast charging and discharging
rates of supercapacitors can reduce testing times dramatically. For example,
if one charging–discharging cycle takes 10 seconds, 100,000 cycles will take
only ~11.6 days.
Normally, with prolonged charging–discharging cycling, the capacitance
of a supercapacitor will gradually reduce while the equivalent series resistance will increase, leading to decreases in both the energy and power densities of the device. If the degradation rate is defined as the energy or power
density loss per cycle, the degradation rate can be calculated according to the
number of cycles and the measured difference between the energy or power
density before the cycle life test and after the test.
7.5 Electrochemical Impedance Spectroscopy (EIS)
EIS, also known as AC impedance spectroscopy, is a powerful technique for
characterizing the properties of electrode–electrolyte interfaces related to
metal corrosion and electrodeposition for batteries, fuel cells, and supercapacitors [5]. In characterizing supercapacitors, both capacitance and equivalent series resistance can be obtained from EIS [8].
EIS includes both ex situ and in situ measurements. The ex situ measurement is mainly used to characterize single-electrode materials and their
Electrochemical Supercapacitors for Energy Storage and Delivery
For pseudosupercapacitors, charging–discharging curves can also be used
to measure capacitance, maximum energy and power densities, and equivalent series resistance by simulating experimental charging and discharging
curves. For more information, see Chapter 3.
Similar to the situation with CV, using different charging and discharging rates (current densities) can provide some information about the mass
transfer kinetics of an electrode layer. Tsay et al. [1] demonstrated that the
specific capacitance would decrease with increasing charging rate; similar
to the result shown in Figure 7.7b. The decrease in specific capacitance with
increasing charging rate is due to the limited transfer of ions to the carbon
particle surface or pores, leading to inaccessible pore portions of the electrode layer at high charging rates.
7.4.2 Cycle Life Measurement Using Charging–Discharging Curves
The most reliable way to investigate the degradation of a supercapacitor is
to charge and discharge it over many cycles (one cycle equals one charge
plus one discharge) to observe the changes in both specific capacitance and
equivalent series resistance. Normally, supercapacitors have much longer
cycle lives than batteries. For example, a commercially available lithium ion
battery has a normal life of 400 to 1200 cycles. A supercapacitor can have a
life as long as 100,000 cycles. Fortunately, the fast charging and discharging
rates of supercapacitors can reduce testing times dramatically. For example,
if one charging–discharging cycle takes 10 seconds, 100,000 cycles will take
only ~11.6 days.
Normally, with prolonged charging–discharging cycling, the capacitance
of a supercapacitor will gradually reduce while the equivalent series resistance will increase, leading to decreases in both the energy and power densities of the device. If the degradation rate is defined as the energy or power
density loss per cycle, the degradation rate can be calculated according to the
number of cycles and the measured difference between the energy or power
density before the cycle life test and after the test.
7.5 Electrochemical Impedance Spectroscopy (EIS)
EIS, also known as AC impedance spectroscopy, is a powerful technique for
characterizing the properties of electrode–electrolyte interfaces related to
metal corrosion and electrodeposition for batteries, fuel cells, and supercapacitors [5]. In characterizing supercapacitors, both capacitance and equivalent series resistance can be obtained from EIS [8].
EIS includes both ex situ and in situ measurements. The ex situ measurement is mainly used to characterize single-electrode materials and their