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Perspectives and Challenges
9.3 Electrode Material Challenges
9.3.1 Current Collectors
An ES electrode is traditionally composed of a conductive metallic (generally aluminum) current collector coated with an active electric double-layer
carbon or psuedocapacitive component. The primary technical challenge
in using a metallic foil current collector lies at the interface of the collector
and the active material. This interface induces a charge transfer resistance
that will inevitably increase the internal resistance of the entire system and
reduce overall ES performance.
To address this issue, increasing the active material–current collector contact area and modifying the interface are feasible methods to decrease the
charge transfer resistance. Several publications reported modifications of
aluminum current collectors by a two-step procedure [4,5]. In the first step, a
chemical or electrochemical etching procedure is carried out to induce surface roughness. This creates an increased interfacial area between the current
collector and the active material, decreasing internal resistance and allowing for higher capacitance during operation. These etching procedures have
been demonstrated to reduce the internal resistance of an ES cell from 50 to
5 Ω.cm 2 after electrochemical etching of a commercial current collector [5].
After etching, a carbonaceous coating can be applied by chemical vapor
deposition [6] or carbonaceous sol–gel deposit methods [7] deemed capable
of further decreasing the internal resistances of ES cells as low as 0.4 Ω.cm 2 .
Therefore, new and modified current collector surface treatment techniques
can promise to further decrease internal cell resistance and improve the
overall performance of modern ESs.
The development of novel current collector structures is also a feasible
method for improving ES performance. For example, utilization of a carbon
nanotube (CNT)-based current collector was reported to produce very high
capacitance values and cyclability [8]. Using highly graphitic nanostructured
materials such as CNTs can offer significant advantages including high
surface areas, electronic conductivities, electrochemical and mechanical
durability, and good compatibility with pseudocapacitor materials such as
transition metal oxides [9–11] and conductive polymers [12,13]. These standalone current collector and active material composites show great promise
and also have the potential to significantly decrease the overall weights of
electrode structures. However, further research and development efforts are
required to investigate their practical feasibility.
Electrochemical stability of the current collectors remains a significant technical challenge that must be considered throughout all aspects of
research and development efforts. For example, over prolonged periods of
operation, material corrosion may result in increased resistance, active material detachment, and inevitable significant performance loss. To address this
Perspectives and Challenges
9.3 Electrode Material Challenges
9.3.1 Current Collectors
An ES electrode is traditionally composed of a conductive metallic (generally aluminum) current collector coated with an active electric double-layer
carbon or psuedocapacitive component. The primary technical challenge
in using a metallic foil current collector lies at the interface of the collector
and the active material. This interface induces a charge transfer resistance
that will inevitably increase the internal resistance of the entire system and
reduce overall ES performance.
To address this issue, increasing the active material–current collector contact area and modifying the interface are feasible methods to decrease the
charge transfer resistance. Several publications reported modifications of
aluminum current collectors by a two-step procedure [4,5]. In the first step, a
chemical or electrochemical etching procedure is carried out to induce surface roughness. This creates an increased interfacial area between the current
collector and the active material, decreasing internal resistance and allowing for higher capacitance during operation. These etching procedures have
been demonstrated to reduce the internal resistance of an ES cell from 50 to
5 Ω.cm 2 after electrochemical etching of a commercial current collector [5].
After etching, a carbonaceous coating can be applied by chemical vapor
deposition [6] or carbonaceous sol–gel deposit methods [7] deemed capable
of further decreasing the internal resistances of ES cells as low as 0.4 Ω.cm 2 .
Therefore, new and modified current collector surface treatment techniques
can promise to further decrease internal cell resistance and improve the
overall performance of modern ESs.
The development of novel current collector structures is also a feasible
method for improving ES performance. For example, utilization of a carbon
nanotube (CNT)-based current collector was reported to produce very high
capacitance values and cyclability [8]. Using highly graphitic nanostructured
materials such as CNTs can offer significant advantages including high
surface areas, electronic conductivities, electrochemical and mechanical
durability, and good compatibility with pseudocapacitor materials such as
transition metal oxides [9–11] and conductive polymers [12,13]. These standalone current collector and active material composites show great promise
and also have the potential to significantly decrease the overall weights of
electrode structures. However, further research and development efforts are
required to investigate their practical feasibility.
Electrochemical stability of the current collectors remains a significant technical challenge that must be considered throughout all aspects of
research and development efforts. For example, over prolonged periods of
operation, material corrosion may result in increased resistance, active material detachment, and inevitable significant performance loss. To address this