112
Electrochemical Supercapacitors for Energy Storage and Delivery
When θ Pt–H = 0.5, the maximum pseudocapacitance will be induced; the
result is the same as that when E E
o
+
= H Pt− predicted by Equation (3.15).
/
H
Note that the treatment above is based on the totally reversible electrochemical adsorption and desorption of H + (or an ideal Langmuir isotherm model
with a monolayer adsorption) on a metal surface.
As coverage increases, repulsions occur between the orbitals of the adsorbate and metal. Surface electron redistribution will also occur locally along
the surface. Depending on these interactions, the capacitance will decrease
and spread over a larger potential window as shown in Figure 3.1. High levels of surface interaction during charging can also reduce the reversibility of
the deposition.
The cyclic voltammograms shown in Figure 3.3 illustrate that, even for
monocrystalline structures, multiple states are represented by many peaks
in the curve. This means that there are multiple states below a monolayer
with quasi-stable surface organization or by partial charge transfer from the
anion [2,22].
As noted previously, the high maximum capacitance induced by these
types of redox processes is not maintained over the available voltage range,
which is normally limited to 0.6 V for most materials. Further the twodimensional surface limitation, high costs of noble and rare metals, and low
available surface area reduce usefulness for energy storage compared to
other pseudocapacitive materials.
3.2.3 Pseudocapacitance Induced by Lithium Intercalation
Cathode materials that exhibit lithium ion intercalation represent an interesting transition technology between standard battery and ES behavior. In
a battery, the lithium is driven thermodynamically into a graphitic anode
material and upon discharge the lithium reacts and inserts itself into the
intercalation host cathode where it is more thermodynamically stable. The
host cathode is a crystalline transitional metal oxide or sulfide (e.g., MoS 2 ,
TiS 2 , and V 6 O 13 ). The cathode intercalation mechanism requires a faradic
charge or driving force for the lithium cation to absorb and deposit into the
cathode material lattice structure. Therefore, electrode potential increases
linearly with the occupancy fraction (state of charge) of the three-dimensional lattice sites. An example of this behavior (Figure 3.4) illustrates a near
pseudocapacitive nature of the sorption isotherm.
The pseudocapacitance of the intercalation cathode can be utilized in ES
through combination with another lithium intercalation electrode, a lithium source electrode, or a capacitive double-layer electrode for the anode.
Capacitance for a TiS 2 electrode can reach as high as 500 F.g –1 over a large
potential range, providing high energy density [24]. Intercalation exhibits a
clear state separation during absorption seen in Figure 3.5 for a MoO 2 host
material [2].
Electrochemical Supercapacitors for Energy Storage and Delivery
When θ Pt–H = 0.5, the maximum pseudocapacitance will be induced; the
result is the same as that when E E
o
+
= H Pt− predicted by Equation (3.15).
/
H
Note that the treatment above is based on the totally reversible electrochemical adsorption and desorption of H + (or an ideal Langmuir isotherm model
with a monolayer adsorption) on a metal surface.
As coverage increases, repulsions occur between the orbitals of the adsorbate and metal. Surface electron redistribution will also occur locally along
the surface. Depending on these interactions, the capacitance will decrease
and spread over a larger potential window as shown in Figure 3.1. High levels of surface interaction during charging can also reduce the reversibility of
the deposition.
The cyclic voltammograms shown in Figure 3.3 illustrate that, even for
monocrystalline structures, multiple states are represented by many peaks
in the curve. This means that there are multiple states below a monolayer
with quasi-stable surface organization or by partial charge transfer from the
anion [2,22].
As noted previously, the high maximum capacitance induced by these
types of redox processes is not maintained over the available voltage range,
which is normally limited to 0.6 V for most materials. Further the twodimensional surface limitation, high costs of noble and rare metals, and low
available surface area reduce usefulness for energy storage compared to
other pseudocapacitive materials.
3.2.3 Pseudocapacitance Induced by Lithium Intercalation
Cathode materials that exhibit lithium ion intercalation represent an interesting transition technology between standard battery and ES behavior. In
a battery, the lithium is driven thermodynamically into a graphitic anode
material and upon discharge the lithium reacts and inserts itself into the
intercalation host cathode where it is more thermodynamically stable. The
host cathode is a crystalline transitional metal oxide or sulfide (e.g., MoS 2 ,
TiS 2 , and V 6 O 13 ). The cathode intercalation mechanism requires a faradic
charge or driving force for the lithium cation to absorb and deposit into the
cathode material lattice structure. Therefore, electrode potential increases
linearly with the occupancy fraction (state of charge) of the three-dimensional lattice sites. An example of this behavior (Figure 3.4) illustrates a near
pseudocapacitive nature of the sorption isotherm.
The pseudocapacitance of the intercalation cathode can be utilized in ES
through combination with another lithium intercalation electrode, a lithium source electrode, or a capacitive double-layer electrode for the anode.
Capacitance for a TiS 2 electrode can reach as high as 500 F.g –1 over a large
potential range, providing high energy density [24]. Intercalation exhibits a
clear state separation during absorption seen in Figure 3.5 for a MoO 2 host
material [2].