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Components and Materials for Electrochemical Supercapacitors
4.2.3 Electrode Conductivity
Good electrical conductivity is important in enabling ES devices to operate
at high power. During discharge, charges stored within the material must
effectively maneuver through the thick electrode layers and out to the circuit. Electron transport is dependent upon the quality of conductive pathways within the material and the conductivity of the material used in the
electrode. If the conduction through the electrode is not strong enough or the
path is too tortuous, performance will suffer.
High power will not be achievable, as charges will not be able to respond
quickly enough to match the rated load of the circuit. This is because the high
capacitance provided by a poorly conductive material at very low power will
be much lower at a desirable power output. Charges cannot organize efficiently
to match the load and some charges will dissipate as heat to meet the current
demanded by the circuit load. Eventually the device will no longer be able
to meet the power demands and the ES will no longer store any charge. For
poorly conducting electrodes, this will happen at much lower current density.
Metals used in traditional capacitors utilize highly conductive metals as
electrode materials that can achieve very high power; however, ESs commonly use carbons as the active electrode components. Not all carbon is sufficiently conductive to support high power operation. Graphitic planes are
highly conductive but temperatures used in the activation processes are limited to prevent complete restructuring into nonporous graphite.
A balance is achieved between conductivity of the active carbon and porosity, which in effect translates to better capacitive performance. The result is that
many active carbon materials lack sufficient conductivity to support long range
or short range conduction of current within the electrode layer. To restrict the
range over which carbon conduction must occur, metal collectors are used as
supports for the carbon layer. Further, when the active carbon is unable to effectively conduct over short distances, specifically designed small carbon additives
(e.g., carbon black, Super P) are used to increase the conduction of the electrode.
4.2.4 Surface Area for EDLC Design
Increased electrode surface area plays an important role in performance.
Enhanced area allows more electrolyte ions to organize at the electrode surface. Larger pores and channels in the electrode layer increase the accessibility
and speed at which ions can organize onto the electrode pores from bulk
electrolyte. Area plays a significant role for deciding on the materials for the
electrode layers and the electrolyte. The pore structures of materials vary
(macroporous >50 nm, mesoporous <50 nm, microporous <2 nm) and the
ability to control the type of porous area available will lead to increased optimization between accessible power and maximum charge storage.
Maximizing the amount of surface area provides the greatest number of
active sites in the material and improves the performance of the ES. However,
Components and Materials for Electrochemical Supercapacitors
4.2.3 Electrode Conductivity
Good electrical conductivity is important in enabling ES devices to operate
at high power. During discharge, charges stored within the material must
effectively maneuver through the thick electrode layers and out to the circuit. Electron transport is dependent upon the quality of conductive pathways within the material and the conductivity of the material used in the
electrode. If the conduction through the electrode is not strong enough or the
path is too tortuous, performance will suffer.
High power will not be achievable, as charges will not be able to respond
quickly enough to match the rated load of the circuit. This is because the high
capacitance provided by a poorly conductive material at very low power will
be much lower at a desirable power output. Charges cannot organize efficiently
to match the load and some charges will dissipate as heat to meet the current
demanded by the circuit load. Eventually the device will no longer be able
to meet the power demands and the ES will no longer store any charge. For
poorly conducting electrodes, this will happen at much lower current density.
Metals used in traditional capacitors utilize highly conductive metals as
electrode materials that can achieve very high power; however, ESs commonly use carbons as the active electrode components. Not all carbon is sufficiently conductive to support high power operation. Graphitic planes are
highly conductive but temperatures used in the activation processes are limited to prevent complete restructuring into nonporous graphite.
A balance is achieved between conductivity of the active carbon and porosity, which in effect translates to better capacitive performance. The result is that
many active carbon materials lack sufficient conductivity to support long range
or short range conduction of current within the electrode layer. To restrict the
range over which carbon conduction must occur, metal collectors are used as
supports for the carbon layer. Further, when the active carbon is unable to effectively conduct over short distances, specifically designed small carbon additives
(e.g., carbon black, Super P) are used to increase the conduction of the electrode.
4.2.4 Surface Area for EDLC Design
Increased electrode surface area plays an important role in performance.
Enhanced area allows more electrolyte ions to organize at the electrode surface. Larger pores and channels in the electrode layer increase the accessibility
and speed at which ions can organize onto the electrode pores from bulk
electrolyte. Area plays a significant role for deciding on the materials for the
electrode layers and the electrolyte. The pore structures of materials vary
(macroporous >50 nm, mesoporous <50 nm, microporous <2 nm) and the
ability to control the type of porous area available will lead to increased optimization between accessible power and maximum charge storage.
Maximizing the amount of surface area provides the greatest number of
active sites in the material and improves the performance of the ES. However,