56
Electrochemical Supercapacitors for Energy Storage and Delivery
the relative potential drop is called the relative electrode potential or electrode potential. A commonly used reference is the normal hydrogen electrode (NHE) with a potential defined as zero V that does not change with
temperature. This NHE contains a Pt or Pt black electrode immersed into a
H + solution (1.0 M) bubbled with 1.0 atm of hydrogen gas.
Another commonly used reference electrode is the saturated Calomel electrode whose electrode potential is 0.245 V versus NHE at 25°C and 1.0 atm.
The electrode reaction kinetics for these reference electrodes are fairly fast.
Even with current passing through the electrode–electrolyte interface, the
electrode potential will show an insignificant shift. This device is called a
non-polarizable electrode and will be discussed in the following section.
Any change in cell voltage will truly reflect the potential change of the targeted electrode–electrolyte interface ( Δψ M
II /S
):
d ( Δψ M
II /S ) = d V
(2.27)
Therefore, the differential capacitance of the double-layer can be rewritten
as:
dq
dq
C dl =
=
(2.28)
d Δψ
dV
II
( M /S )
Equation (2.28) suggests that the double-layer differential capacitance can be
directly measured by measuring the electrode charge change when the relative electrode potential is altered.
In supercapacitor literature, the reported electrode potentials are all relative, as expressed in Figure 2.11 in which an NHE reference electrode was
used. For the remainder of this book, all mentions of electrode potentials
refer to relative electrode potentials.
2.3.2 Double-Layer Potential Ranges or Windows
In Figure 2.1, the net charge (q) can be forced to accumulate on the electrode
to form an electrode–electrolyte double-layer using a power source such as a
battery connected in the circuit loop. Consequently, the potential drop across
the double-layer (V) forms and is expressed as Equation (2.29) if the concentration of the electrolyte is high enough to form the Helmholtz layer:
q
V =
(2.29)
C dl
Electrochemical Supercapacitors for Energy Storage and Delivery
the relative potential drop is called the relative electrode potential or electrode potential. A commonly used reference is the normal hydrogen electrode (NHE) with a potential defined as zero V that does not change with
temperature. This NHE contains a Pt or Pt black electrode immersed into a
H + solution (1.0 M) bubbled with 1.0 atm of hydrogen gas.
Another commonly used reference electrode is the saturated Calomel electrode whose electrode potential is 0.245 V versus NHE at 25°C and 1.0 atm.
The electrode reaction kinetics for these reference electrodes are fairly fast.
Even with current passing through the electrode–electrolyte interface, the
electrode potential will show an insignificant shift. This device is called a
non-polarizable electrode and will be discussed in the following section.
Any change in cell voltage will truly reflect the potential change of the targeted electrode–electrolyte interface ( Δψ M
II /S
):
d ( Δψ M
II /S ) = d V
(2.27)
Therefore, the differential capacitance of the double-layer can be rewritten
as:
dq
dq
C dl =
=
(2.28)
d Δψ
dV
II
( M /S )
Equation (2.28) suggests that the double-layer differential capacitance can be
directly measured by measuring the electrode charge change when the relative electrode potential is altered.
In supercapacitor literature, the reported electrode potentials are all relative, as expressed in Figure 2.11 in which an NHE reference electrode was
used. For the remainder of this book, all mentions of electrode potentials
refer to relative electrode potentials.
2.3.2 Double-Layer Potential Ranges or Windows
In Figure 2.1, the net charge (q) can be forced to accumulate on the electrode
to form an electrode–electrolyte double-layer using a power source such as a
battery connected in the circuit loop. Consequently, the potential drop across
the double-layer (V) forms and is expressed as Equation (2.29) if the concentration of the electrolyte is high enough to form the Helmholtz layer:
q
V =
(2.29)
C dl