339
Perspectives and Challenges
Combining psuedocapacitive materials with CNTs and graphene can
help overcome the inherent challenges associated with CNTs and graphene.
The low active surfaces of CNTs available for double-layer formation can be
counteracted by the incorporation of high performance psuedocapacitive
materials, while the CNTs can maintain an electrode structure conducive
to electrolyte access. With graphene, the psuedocapacitive material can act
as a spacer to prevent agglomeration and also contribute to overall capacitance. Investigation of electric double-layer electrode materials is not limited
to CNTs and graphene. Unique template mesoporous carbons [17,18], carbon nanofibers [19] and carbon onion [20] nanostructures are other examples of carbonaceous materials that have demonstrated applicability in ES
technologies.
Further investigations are required to develop active carbon-based materials with higher capacitances and electrode arrangements favorable to mass
transport. Moreover, inexpensive, upscalable fabrication techniques for these
materials are also required to render them commercially viable.
9.3.3 Pseudocapacitor Electrode Materials
Energy storage pseudocapacitive materials that have traditionally been
investigated involve transition metal oxides and conductive polymers.
Despite promising results realized from both approaches, several technical
and cost challenges remain to be overcome.
9.3.3.1 Transition Metal Oxides
Ruthenium oxide-based materials have been extensively investigated as
psuedocapacitive materials based on their good operational stability and
extremely high theoretical capacitance values [21]. The long term applicability of these materials is limited due to the high cost and limited availability
of ruthenium. Therefore, it is of prime interest to investigate non-precious
metal oxides to capitalize on their distinct cost advantages.
Several metal oxides with varying phase structures and nanostructures
have been developed, including manganese oxide, iron oxide, molybdenum
oxide, tin oxide, and titanium oxide. The most prominent issues surrounding
transition metal oxide development efforts are (1) limited electronic conductivity and low theoretical capacitance in comparison to ruthenium oxide and
(2) poor cyclability due to their redox nature and electrochemical instabilities.
The varying electrochemically stable potential windows for some species of
metal oxides completely eliminate their potential applications in ES devices.
Several researchers investigated unique methods to overcome the poor
metal oxide electronic conductivity and energy storage limitations. Deliberate
nanostructure control is a common technique utilized to circumvent these
challenges. Particularly with manganese oxide, the specific crystal structures
have been demonstrated to exert significant impacts on specific capacitance
Perspectives and Challenges
Combining psuedocapacitive materials with CNTs and graphene can
help overcome the inherent challenges associated with CNTs and graphene.
The low active surfaces of CNTs available for double-layer formation can be
counteracted by the incorporation of high performance psuedocapacitive
materials, while the CNTs can maintain an electrode structure conducive
to electrolyte access. With graphene, the psuedocapacitive material can act
as a spacer to prevent agglomeration and also contribute to overall capacitance. Investigation of electric double-layer electrode materials is not limited
to CNTs and graphene. Unique template mesoporous carbons [17,18], carbon nanofibers [19] and carbon onion [20] nanostructures are other examples of carbonaceous materials that have demonstrated applicability in ES
technologies.
Further investigations are required to develop active carbon-based materials with higher capacitances and electrode arrangements favorable to mass
transport. Moreover, inexpensive, upscalable fabrication techniques for these
materials are also required to render them commercially viable.
9.3.3 Pseudocapacitor Electrode Materials
Energy storage pseudocapacitive materials that have traditionally been
investigated involve transition metal oxides and conductive polymers.
Despite promising results realized from both approaches, several technical
and cost challenges remain to be overcome.
9.3.3.1 Transition Metal Oxides
Ruthenium oxide-based materials have been extensively investigated as
psuedocapacitive materials based on their good operational stability and
extremely high theoretical capacitance values [21]. The long term applicability of these materials is limited due to the high cost and limited availability
of ruthenium. Therefore, it is of prime interest to investigate non-precious
metal oxides to capitalize on their distinct cost advantages.
Several metal oxides with varying phase structures and nanostructures
have been developed, including manganese oxide, iron oxide, molybdenum
oxide, tin oxide, and titanium oxide. The most prominent issues surrounding
transition metal oxide development efforts are (1) limited electronic conductivity and low theoretical capacitance in comparison to ruthenium oxide and
(2) poor cyclability due to their redox nature and electrochemical instabilities.
The varying electrochemically stable potential windows for some species of
metal oxides completely eliminate their potential applications in ES devices.
Several researchers investigated unique methods to overcome the poor
metal oxide electronic conductivity and energy storage limitations. Deliberate
nanostructure control is a common technique utilized to circumvent these
challenges. Particularly with manganese oxide, the specific crystal structures
have been demonstrated to exert significant impacts on specific capacitance