Environmental Engineering Reference
In-Depth Information
Energy storage components
Battery
Capacitor
Primary
Secondary
(rechargeable)
Electrostatic
Electrolytic
Electrochemical
Lead-
acid
NiCd
NMH
Li ion
Symmetric
Asymmetric
Aqueous
electrolyte
Organic
electrolyte
Organic
electrolyte
Aqueous
electrolyte
Most popular today
Potential for bulk storage
Active research
Figure 10.20 Family tree of electrochemical energy storage technologies
To understand the EDLC effect in more detail, consider the graphics in
Figure 10.21 in which an electron distribution is assumed in the activated carbon (AC)
and cations are present in the electrolyte, TEATFB/AN (reads tetraethylammonium-
tetrafluoroborate in acetonitrile as the salt in a solvent). We are interested in under-
standing in more depth the reason for the compact layer (i.e. the Helmholtz layer) and
for this a digression into electrochemistry is in order, specifically the governing rela-
tionship for the electron-ion charge separation distance. This distance is now more
appropriately referred to as the Debye length,
d
c
[14].
The electric field strength shown in Figure 10.21 across this boundary is high
as depicted in the right hand vector plot for charges in non-conducting medium.
The electrostatic forces are very high for nanoscale charge separation distances, but
are balanced by van der Waals nuclear forces as the spacing becomes very small
(nanometres). For this situation the Debye length is defined as
s
e
r
e
0
RT
2
F
2
C
0
¼
6
:
6
7
10
9
d
c
¼
ð
10
:
24
Þ
10
12
,
R
where
e
r
¼
37.5,
e
0
¼
8.854
¼
8.314 J/K-mol,
T
¼
300 K,
F
¼
96,474,
C
0
¼
1 mol/L.
From (10.24) it is clear that a charge separation distance of 6.67 nm can be
expected in an electrolyte concentration of monovalent salts when the salt
concentration is 1 molar at room temperature. An example will help illustrate
the applicability of Debye length in the calculation of electrode capacitance of a
carbon-carbon symmetric ultra-capacitor.
