167
Components and Materials for Electrochemical Supercapacitors
materials leads to a loss of carefully selected morphology, reducing capacitance and leading to a greatly reduced cycle life.
EDLC-based electrodes can handle over a half million cycles, while high
energy batteries may manage a few hundred to a few thousand charge
cycles. One of the design challenges for pseudocapacitive electrodes is
to avoid compromising cycle life. Electrolyte choice can also affect operating voltage range, reactivity, and power output of a total system and
further alter the cycle life of a pseudocapacitive material and its performance. Inherently the presence of redox reactions and high energy storage will prevent use over half a million cycles as seen with EDLC devices.
However, it is expected that 100,000 cycles would be sufficient for many
ES applications and that should be achievable with proper electrode
design [73].
Pseudocapacitive reactions involving underpotential deposition and partial electron transfer from anion chemisorption are present in all ESs and
account for a small percentage of charge storage [74,75]. However, in pseudocapacitors, these processes can account for a much larger percentage of
overall capacitance. More importantly, interfacial redox reactions involving transition oxide thin films store large amounts of charge through easily accessible surface oxidation states. Some of the more desirable transition
metal oxide materials exhibit multiple oxidation or adsorption states within
the range of EC operation, further boosting pseudocapacitive energy storage.
In conducting polymers, the ion uptake during doping allows ion intercalation throughout the electrode and exposes the electrolyte ions to more
than just the surface interface of the material. The result is capacitance that
can reach as high as 10 times that of carbon materials [74]. Unfortunately,
ion uptake into the electrode material during oxidation causes swelling
and strain on the material. This internal stress can result in material shifts,
cracks, reduced contact with the substrate, and dislocation of crystal structures to more energetically favorable positions.
These changes can reduce the surface area and ion accessibility for future
charge cycles. To deal with the challenges of pseudocapacitance, thin layers
of material are frequently used on carbon supports that provide both high
area for performance reasons and space for the materials to swell without
generating damage. Stable crystal morphology is also important to resisting
degradation over time.
4.2.9.2 Transition Metal Oxides
Transition metal oxides derived from ruthenium (RuO 2 ), iron (Fe 3 O 4 ), vanadium (V 2 O 5 ), tin (SnO 2 ), and manganese (MnO 2 ) are widely used in the
research and application of pseudocapacitance [76–78]. Oxide materials are
known to exhibit multiple oxidation states at specific potentials, and selection of materials with multiple stable states within an electrolyte’s potential
window allows maximum capacitance to develop.
Components and Materials for Electrochemical Supercapacitors
materials leads to a loss of carefully selected morphology, reducing capacitance and leading to a greatly reduced cycle life.
EDLC-based electrodes can handle over a half million cycles, while high
energy batteries may manage a few hundred to a few thousand charge
cycles. One of the design challenges for pseudocapacitive electrodes is
to avoid compromising cycle life. Electrolyte choice can also affect operating voltage range, reactivity, and power output of a total system and
further alter the cycle life of a pseudocapacitive material and its performance. Inherently the presence of redox reactions and high energy storage will prevent use over half a million cycles as seen with EDLC devices.
However, it is expected that 100,000 cycles would be sufficient for many
ES applications and that should be achievable with proper electrode
design [73].
Pseudocapacitive reactions involving underpotential deposition and partial electron transfer from anion chemisorption are present in all ESs and
account for a small percentage of charge storage [74,75]. However, in pseudocapacitors, these processes can account for a much larger percentage of
overall capacitance. More importantly, interfacial redox reactions involving transition oxide thin films store large amounts of charge through easily accessible surface oxidation states. Some of the more desirable transition
metal oxide materials exhibit multiple oxidation or adsorption states within
the range of EC operation, further boosting pseudocapacitive energy storage.
In conducting polymers, the ion uptake during doping allows ion intercalation throughout the electrode and exposes the electrolyte ions to more
than just the surface interface of the material. The result is capacitance that
can reach as high as 10 times that of carbon materials [74]. Unfortunately,
ion uptake into the electrode material during oxidation causes swelling
and strain on the material. This internal stress can result in material shifts,
cracks, reduced contact with the substrate, and dislocation of crystal structures to more energetically favorable positions.
These changes can reduce the surface area and ion accessibility for future
charge cycles. To deal with the challenges of pseudocapacitance, thin layers
of material are frequently used on carbon supports that provide both high
area for performance reasons and space for the materials to swell without
generating damage. Stable crystal morphology is also important to resisting
degradation over time.
4.2.9.2 Transition Metal Oxides
Transition metal oxides derived from ruthenium (RuO 2 ), iron (Fe 3 O 4 ), vanadium (V 2 O 5 ), tin (SnO 2 ), and manganese (MnO 2 ) are widely used in the
research and application of pseudocapacitance [76–78]. Oxide materials are
known to exhibit multiple oxidation states at specific potentials, and selection of materials with multiple stable states within an electrolyte’s potential
window allows maximum capacitance to develop.