338
Electrochemical Supercapacitors for Energy Storage and Delivery
challenge, investigating unique modification techniques for use in traditional current collectors and developing novel current collector materials are
two promising approaches. The inevitable goal is to develop current collector structures that decrease internal resistance and cost without sacrificing
operational durability.
9.3.2 Double-Layer Electrode Materials
Carbonaceous materials are almost exclusively utilized as active materials
of double-layer electrodes due to their high conductivity, electrochemical
stability, and porosity. Activated carbons still constitute the most practical active carbon-based electrode materials. They have high surface areas,
are inexpensive to produce, and can be fabricated using a variety of readily
available precursor materials.
It is desirable to develop active carbon materials with higher capacitances
by increasing both surface areas and porosities. The common belief was that
increasing pore volumes to sizes that are accessible to solvated ion species
would directly increase capacitance. However, it was proposed recently that
the presence of micropores (<2 nm) could increase the capacitances of these
materials [14]. This indicates that some sort of ion desolvation mechanism
occurring at the molecular scale allows ion transport and adsorption in
these micropores.
A significant dependence on ion size was also observed. This suggests
that designing active carbon materials with tunable pore sizes that could
be optimized for specific types of ion species will be a challenge. With these
methods, it will be possible to develop electrode materials with significantly
improved capacitances through trial and error investigations supplemented
by molecular scale computational methods.
Despite the widespread applicability and high capacitance values of activated carbon materials, other electrolyte and ion transport issues may arise
and limit the performances of electrodes fabricated from these materials.
Fabricating ordered electrode structures seems promising for overcoming
this limitation. For example, CNTs have been extensively investigated due to
their one-dimensional structures that result in porous electrode networks.
The primary challenge of CNT-based electrodes is due to the low overall
surface area available for ion adsorption. Compositing CNTs with graphene
is a recently reported method for overcoming this limitation by utilizing the
expanded surface areas of graphene sheets and resulting capacitance capabilities [15]. Moreover, CNTs provide rigid conductive pathways in the active
layer and prevent the aggregation of graphene sheets that causes electrode
blockages and hinders electrolyte penetration. In fact, the aggregation of
graphene sheets is an inherent issue when using pure graphene based electrodes. Features such as unique in-plane designs [16] are required to increase
the utilization of the carbon surface area. CNTs and graphene can also be
composited with psuedocapacitive materials, as discussed in Chapter 3.
Electrochemical Supercapacitors for Energy Storage and Delivery
challenge, investigating unique modification techniques for use in traditional current collectors and developing novel current collector materials are
two promising approaches. The inevitable goal is to develop current collector structures that decrease internal resistance and cost without sacrificing
operational durability.
9.3.2 Double-Layer Electrode Materials
Carbonaceous materials are almost exclusively utilized as active materials
of double-layer electrodes due to their high conductivity, electrochemical
stability, and porosity. Activated carbons still constitute the most practical active carbon-based electrode materials. They have high surface areas,
are inexpensive to produce, and can be fabricated using a variety of readily
available precursor materials.
It is desirable to develop active carbon materials with higher capacitances
by increasing both surface areas and porosities. The common belief was that
increasing pore volumes to sizes that are accessible to solvated ion species
would directly increase capacitance. However, it was proposed recently that
the presence of micropores (<2 nm) could increase the capacitances of these
materials [14]. This indicates that some sort of ion desolvation mechanism
occurring at the molecular scale allows ion transport and adsorption in
these micropores.
A significant dependence on ion size was also observed. This suggests
that designing active carbon materials with tunable pore sizes that could
be optimized for specific types of ion species will be a challenge. With these
methods, it will be possible to develop electrode materials with significantly
improved capacitances through trial and error investigations supplemented
by molecular scale computational methods.
Despite the widespread applicability and high capacitance values of activated carbon materials, other electrolyte and ion transport issues may arise
and limit the performances of electrodes fabricated from these materials.
Fabricating ordered electrode structures seems promising for overcoming
this limitation. For example, CNTs have been extensively investigated due to
their one-dimensional structures that result in porous electrode networks.
The primary challenge of CNT-based electrodes is due to the low overall
surface area available for ion adsorption. Compositing CNTs with graphene
is a recently reported method for overcoming this limitation by utilizing the
expanded surface areas of graphene sheets and resulting capacitance capabilities [15]. Moreover, CNTs provide rigid conductive pathways in the active
layer and prevent the aggregation of graphene sheets that causes electrode
blockages and hinders electrolyte penetration. In fact, the aggregation of
graphene sheets is an inherent issue when using pure graphene based electrodes. Features such as unique in-plane designs [16] are required to increase
the utilization of the carbon surface area. CNTs and graphene can also be
composited with psuedocapacitive materials, as discussed in Chapter 3.