59
c
Fundamentals of Electrochemical Double-Layer Supercapacitors
TABLE 2.2
Typical Electrolyte Solvents and Their Potential Windows for Supercapacitors
Solvent
Electrolyte Salt
Temperature (°C)
Potential Range (V) c
Water
KOH, 4M
25
1
H 2 SO 4 , 2M
25
1
KCl b , 2M
25
1
Na 2 SO 4 , 1M
25
1
K 2 SO 4 , 1M
25
1
Propylene carbonate
Et 4 NBF 4, 1M
25
2.7
Acetonitrile
Et 4 NBF 4 , 1M
25
2.7
Ionic liquid a
[EtMeIm] + [BF 4 ] –
25
4
[EtMeIm] + [BF 4 ] –
100
3.25
a Ionic liquids are molten salts that at higher temperatures display stability and significantly
increase conductivity.
b Chloride ions are corrosive to metal current collectors during charging.
Potential range can be shifted due to variations in electrode material stability.
Note: See References 9 through 12.
the capacitor is used for energy storage, the capacitance is one of the most
important parameters for evaluating performance.
For example, when storing energy in a fixed size capacitor, the greater the
capacitance, the greater the performance. However, when using a smooth
electrode surface, the differential capacitance density is in the range of less
than 1.0 F.m –2 , and needs 100 m 2 of the total electrode area to obtain 100 F of
charge storage. This is not practical at all. Therefore, it is necessary to enlarge
the electrode surface without increasing the device volume when a capacitor needs to store a lot of charge. Fortunately, with the rapid development of
new materials, this surface area enlargement has become feasible for practical applications.
2.4.1 Carbon Particles and Their Associated Electrode Layers
Commercially available carbon particles such as active carbon powders are
the most common active electrode materials for double-layer supercapacitors. They can have surface areas between 1000 and 3000 m 2 .g –1 . When this
kind of carbon material is used to make the electrode layers and no electrochemical reaction is employed for charge generation, the resulting devices
are called double-layer supercapacitors.
Figure 2.12 shows an electrode layer in a double-layer supercapacitor. This
electrode layer is composed of carbon particles and a binder. In the layer,
the sources of the capacitance are the pores on the carbon particles and the
porous channels within the matrix layer. It is obvious that only locations
that are accessible by the electrolyte ions can form the electrode–electrolyte
double-layer for capacitance generation.
c
Fundamentals of Electrochemical Double-Layer Supercapacitors
TABLE 2.2
Typical Electrolyte Solvents and Their Potential Windows for Supercapacitors
Solvent
Electrolyte Salt
Temperature (°C)
Potential Range (V) c
Water
KOH, 4M
25
1
H 2 SO 4 , 2M
25
1
KCl b , 2M
25
1
Na 2 SO 4 , 1M
25
1
K 2 SO 4 , 1M
25
1
Propylene carbonate
Et 4 NBF 4, 1M
25
2.7
Acetonitrile
Et 4 NBF 4 , 1M
25
2.7
Ionic liquid a
[EtMeIm] + [BF 4 ] –
25
4
[EtMeIm] + [BF 4 ] –
100
3.25
a Ionic liquids are molten salts that at higher temperatures display stability and significantly
increase conductivity.
b Chloride ions are corrosive to metal current collectors during charging.
Potential range can be shifted due to variations in electrode material stability.
Note: See References 9 through 12.
the capacitor is used for energy storage, the capacitance is one of the most
important parameters for evaluating performance.
For example, when storing energy in a fixed size capacitor, the greater the
capacitance, the greater the performance. However, when using a smooth
electrode surface, the differential capacitance density is in the range of less
than 1.0 F.m –2 , and needs 100 m 2 of the total electrode area to obtain 100 F of
charge storage. This is not practical at all. Therefore, it is necessary to enlarge
the electrode surface without increasing the device volume when a capacitor needs to store a lot of charge. Fortunately, with the rapid development of
new materials, this surface area enlargement has become feasible for practical applications.
2.4.1 Carbon Particles and Their Associated Electrode Layers
Commercially available carbon particles such as active carbon powders are
the most common active electrode materials for double-layer supercapacitors. They can have surface areas between 1000 and 3000 m 2 .g –1 . When this
kind of carbon material is used to make the electrode layers and no electrochemical reaction is employed for charge generation, the resulting devices
are called double-layer supercapacitors.
Figure 2.12 shows an electrode layer in a double-layer supercapacitor. This
electrode layer is composed of carbon particles and a binder. In the layer,
the sources of the capacitance are the pores on the carbon particles and the
porous channels within the matrix layer. It is obvious that only locations
that are accessible by the electrolyte ions can form the electrode–electrolyte
double-layer for capacitance generation.