Chemistry Reference
In-Depth Information
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Figure 9.3 Measurement of electrochemical performance. (a) Charging-discharging
curves of a pyrolyzed photoresist electrode at different current densities
(0.5, 1, 5 mA cm
2
). (b) Cyclic voltammograms at different scanning rates
(0.01, 0.1, 1 V s
1
). The specific capacitance was calculated from the
corresponding values of current density based on eqn 9.3.
Reprinted from ref. 9 with permission from Elsevier.
obtained from an abrupt potential drop (Figure 9.3(a)) in the discharging
response of CDC, or from the real part of the high frequency response in
electrochemical impedance spectroscopy (EIS).
Figure 9.3 shows examples of CDC and CV curves reported by Hsia et al.
9
Their carbon-based supercapacitor electrodes were produced from a
photoresist by high-temperature pyrolysis. Indeed, the CDC curve shows an
abrupt drop at the very start of discharging, which is indicative of ESR of the
electrode. The CV responses are obviously non-ideal, and Hsia et al.
suggested that this behavior could be accounted for by redox reactions of the
aqueous electrolyte and possibly by pseudocapacitive contribution of surface
groups formed during the pyrolysis process.
.
(Pseudocapacitive charge
storage will be discussed in the next section.)
For reasonable and practical evaluation, the stored energy and the power
output are normalized with respect to extensive parameters of the super-
capacitors. Unfortunately, the evaluation standard lacks consensus. In
literature reports, researchers are still using scattered metrics, such as
gravimetric (per gram), volumetric (per cm
3
), and areal (per cm
2
) densities.
The quantity is calculated based on the single electrode or sometimes based
on the final device. For realistic performance evaluation, proper metrics
should be defined in consideration of device characteristics and perform-
ance scalability of electrode materials.
10
From eqn (9.4) and (9.5), it is obvious that specific energy and specific
power can be improved by increasing the specific capacitance and the
operation voltage window, as well as by reducing the ESR. Since the specific
capacitance is proportional to the specific surface area (SSA) (eqn (9.1)),
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