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Fundamentals of Electrochemical Pseudocapacitors
redox mechanism. Depending on the material it is possible that charge is
stored by electron transfer to the polymer with ion pairing to stabilize the
charge. Alternatively, the nature of the polymer system may allow resonance
throughout the ring structure along the polymer backbone. This creates a
delocalization of charge allowing n or p doping within the polymer. The
doped polymer can act as a double-layer system.
In both cases, the process must involve ion intercalation into three-dimensional chain entanglements of the polymers [2]. In general, polymers offer
competitive capacitance values, advantages in cost, and a variety of deposition targets and techniques. Their potential window is normally around 0.7
to 0.8 V, putting them on par with most metal oxides. However, chemical
degradation of the polymer and swelling from ion intercalation may reduce
their cycle lives compared to that of reversible pseudocapacitance created by
redox on metal oxides.
In addition, overcharging can create polymer degradation, and in the
undoped state, polymer conductivity is much lower [32]. This low conductivity can create a charge isolation state that at practical charge rates can lead to
irreversible degradation to the polymer structure. To reduce the effect of the
isolation state and reduce damage from swelling, a polymer can be coupled
with carbon supports in thin films and high-area nanostructures similar to
high-capacitance metal oxides.
3.2.6 Coupling of Differential Double-Layer and Pseudocapacitance
In Section 3.2.1, the ideal coupling of double-layer and pseudocapacitance
was discussed briefly. In practice, one important factor to consider when
looking to optimize capacitance is the interaction between faradic charge
transfer and double-layer charge storage. In general, it has been shown theoretically that coupling between the two capacitive types is possible [1,33,34].
In composite carbon or oxide electrodes, it can be difficult to quantify coupling effects because of overlapping potential regions. In addition to coupling
between charge types, it is also important to recognize that some improvement can be attributed to increasing the reversibility of the pseudocapacitive
component. High conductivity carbon support also contributes to boosting
the fraction of active pseudocapacitive material. An example that allows us
to observe this relationship can be seen in Figure 3.10 [35]. The pure PANI
electrode could not effectively charge at the 10 mV/s scan rate applied. After
introduction of the conductive CNT–graphene, the PANI is able to charge
effectively but quantifying the contribution of the double-layer charge to the
system remains difficult.
Coupling effects are much more pronounced for carbon electrodes that
contain small amounts of surface functionality. These functionalities can
induce a clear pseudocapacitive component that significantly boosts capacitance even for carbons with lower surface areas. Heteroatom functional
groups induced by specific carbonization procedures or doping reactions
Fundamentals of Electrochemical Pseudocapacitors
redox mechanism. Depending on the material it is possible that charge is
stored by electron transfer to the polymer with ion pairing to stabilize the
charge. Alternatively, the nature of the polymer system may allow resonance
throughout the ring structure along the polymer backbone. This creates a
delocalization of charge allowing n or p doping within the polymer. The
doped polymer can act as a double-layer system.
In both cases, the process must involve ion intercalation into three-dimensional chain entanglements of the polymers [2]. In general, polymers offer
competitive capacitance values, advantages in cost, and a variety of deposition targets and techniques. Their potential window is normally around 0.7
to 0.8 V, putting them on par with most metal oxides. However, chemical
degradation of the polymer and swelling from ion intercalation may reduce
their cycle lives compared to that of reversible pseudocapacitance created by
redox on metal oxides.
In addition, overcharging can create polymer degradation, and in the
undoped state, polymer conductivity is much lower [32]. This low conductivity can create a charge isolation state that at practical charge rates can lead to
irreversible degradation to the polymer structure. To reduce the effect of the
isolation state and reduce damage from swelling, a polymer can be coupled
with carbon supports in thin films and high-area nanostructures similar to
high-capacitance metal oxides.
3.2.6 Coupling of Differential Double-Layer and Pseudocapacitance
In Section 3.2.1, the ideal coupling of double-layer and pseudocapacitance
was discussed briefly. In practice, one important factor to consider when
looking to optimize capacitance is the interaction between faradic charge
transfer and double-layer charge storage. In general, it has been shown theoretically that coupling between the two capacitive types is possible [1,33,34].
In composite carbon or oxide electrodes, it can be difficult to quantify coupling effects because of overlapping potential regions. In addition to coupling
between charge types, it is also important to recognize that some improvement can be attributed to increasing the reversibility of the pseudocapacitive
component. High conductivity carbon support also contributes to boosting
the fraction of active pseudocapacitive material. An example that allows us
to observe this relationship can be seen in Figure 3.10 [35]. The pure PANI
electrode could not effectively charge at the 10 mV/s scan rate applied. After
introduction of the conductive CNT–graphene, the PANI is able to charge
effectively but quantifying the contribution of the double-layer charge to the
system remains difficult.
Coupling effects are much more pronounced for carbon electrodes that
contain small amounts of surface functionality. These functionalities can
induce a clear pseudocapacitive component that significantly boosts capacitance even for carbons with lower surface areas. Heteroatom functional
groups induced by specific carbonization procedures or doping reactions