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the capacitor, the positive electrode voltage increases and concurrently the negative
electrode voltage decreases, both at approximately the same rate. Provided the rest
potential (zero charge) of the electrode material in the electrolyte is midway between its
stability limits, both reach their potential limits at the same state of charge. This allows
the full operating voltage window to be realized. In practical implementations, the type I
capacitors usually do not have a rest potential in the middle of the voltage window and do
not have exactly the same capacitance in each electrodes. This reduces the maximum
operating voltage in some cases from 1.2 V to below 1 V.
For type II capacitors, with organic electrolytes, voltages are higher and the window is
increased. Instead of 0.8V operation as shown in the figure, this voltage could be perhaps
2.3 to 2.7 V. Also, in practical products the non-aqueous symmetric designs make up for
Symmetric
Aqueous
C + C -
C + = C - = C o
C total
= 1 / 2 C o
+ -
operates at ~ 0.8 V = V o
E = 1 / 2
( 1 / 2
C o
)V
2
o
E = 1 / 4
C o
V
2
o
up to 16 times more
Upper Limit
+ -
Asymmetric
Aqueous
C +
C - =2 C o
C + >> C -
E = 4C o
V
2
o V +
C total = C - = 2C o
operates at ~ 1.6 V = 2V o
E = 1 / 2
(2C o
)(2V o
) 2
V + - Lower Limit
-
Q
Q
Upper Limit
Lower Limit
some of the difference in capacitance by choice of electrode material.
Figure 33 Comparison of type I and type III electrochemical capacitor energy calculation 4
Type IV electrochemical capacitors operate in the same fashion as type III devices except
the operating voltage can be higher due to the use of an organic electrolyte. Some type
IV capacitors have been reported to operate at 4.0 V or higher in contrast to the 2.7 V
value for the highest type II products. Consequently, the V 2 dependence of energy on the
operating voltage represents at least a two-fold increase in energy density. The net
4 The figure is intended to illustrate a fundamental difference between the symmetric and asymmetric
capacitor designs. Specific energy in practical products also depends on specifics of the electrode and
electrolyte materials, operating voltage per cell, device design life, and type of packaging.
Electrochemical Capacitors 62