183
Components and Materials for Electrochemical Supercapacitors
The disadvantages of aqueous electrolytes involve corrosion and low stability-window (ΔV) issues that affect cell performance and stability. Acidic or
basic pH conditions in a system can cause corrosion of collectors and packaging materials. Corrosive reactions detract from system performance and
reduce cycle lives. Conversely, aqueous electrolytes exhibit water decomposition, resulting in hydrogen evolution at low cell potential (around 0 V) and
oxygen evolution at high potential range (around 1.2 V) because of the poor
voltage stability of water.
Rupturing cells threaten physical safety and reduce cycle life. Precautions
must be taken with aqueous electrolyte systems to restrict the voltage window to avoid rupture. As a result, the potential window for most aqueous
systems is limited to about 1 V. The low voltage stability of aqueous electrolytes greatly restricts the energy and power density possible in an ES.
Conversely, the higher ionic conductivity and mobility of aqueous electrolytes seen in Table 4.7 translates into the best possible capacitance for an ES
and lower internal cell resistance. The low internal resistance allows quick
response time.
4.3.3 Organic Electrolytes
Organic electrolytes currently dominate the commercial ES market because
of their potential window of operation in the range of 2.2 to 2.7 V. Table 5.7
provides a list of commercialized ES devices and their performance characteristics. Each device listed uses an organic electrolyte because of the
enhanced potential window over aqueous electrolytes and moderate ion
conduction provided. Most devices utilize acetonitrile, while others employ
propylene carbonate solvents.
If organic electrolytes are used during peak operation periods, a dynamically controlled system could temporarily charge a cell to as high as 3.5 V
[2]. The higher voltage window translates to the larger energy and power
densities demanded by the consumer and industrial markets. The benefits
are compounded when larger ES modules are used. Fewer components are
needed to meet the module size requirements. Fewer cell balancing and connection components are required and less parasitic resistance arises from
interfacing individual cells.
Acetonitrile is the current solvent standard, and is used to support the
salt tetraethylammonium tetrafluoroborate (Et4NBF4, melting point >300°C)
[108]. However, its continued use brings toxicity and safety concerns. A safer
alternative is propylene carbonate, but it suffers from strong resistivity issues
compared to acetonitrile.
Table 4.7 illustrates the resistivity and potential window properties of various electrolyte solvents [8,73,109]. The resistance of organic electrolytes is
much higher than that of aqueous systems and negatively affects power and
capacitive performances. The reduction in power performance is, however,
balanced by the quadratic effect of the increased potential window [2].
Components and Materials for Electrochemical Supercapacitors
The disadvantages of aqueous electrolytes involve corrosion and low stability-window (ΔV) issues that affect cell performance and stability. Acidic or
basic pH conditions in a system can cause corrosion of collectors and packaging materials. Corrosive reactions detract from system performance and
reduce cycle lives. Conversely, aqueous electrolytes exhibit water decomposition, resulting in hydrogen evolution at low cell potential (around 0 V) and
oxygen evolution at high potential range (around 1.2 V) because of the poor
voltage stability of water.
Rupturing cells threaten physical safety and reduce cycle life. Precautions
must be taken with aqueous electrolyte systems to restrict the voltage window to avoid rupture. As a result, the potential window for most aqueous
systems is limited to about 1 V. The low voltage stability of aqueous electrolytes greatly restricts the energy and power density possible in an ES.
Conversely, the higher ionic conductivity and mobility of aqueous electrolytes seen in Table 4.7 translates into the best possible capacitance for an ES
and lower internal cell resistance. The low internal resistance allows quick
response time.
4.3.3 Organic Electrolytes
Organic electrolytes currently dominate the commercial ES market because
of their potential window of operation in the range of 2.2 to 2.7 V. Table 5.7
provides a list of commercialized ES devices and their performance characteristics. Each device listed uses an organic electrolyte because of the
enhanced potential window over aqueous electrolytes and moderate ion
conduction provided. Most devices utilize acetonitrile, while others employ
propylene carbonate solvents.
If organic electrolytes are used during peak operation periods, a dynamically controlled system could temporarily charge a cell to as high as 3.5 V
[2]. The higher voltage window translates to the larger energy and power
densities demanded by the consumer and industrial markets. The benefits
are compounded when larger ES modules are used. Fewer components are
needed to meet the module size requirements. Fewer cell balancing and connection components are required and less parasitic resistance arises from
interfacing individual cells.
Acetonitrile is the current solvent standard, and is used to support the
salt tetraethylammonium tetrafluoroborate (Et4NBF4, melting point >300°C)
[108]. However, its continued use brings toxicity and safety concerns. A safer
alternative is propylene carbonate, but it suffers from strong resistivity issues
compared to acetonitrile.
Table 4.7 illustrates the resistivity and potential window properties of various electrolyte solvents [8,73,109]. The resistance of organic electrolytes is
much higher than that of aqueous systems and negatively affects power and
capacitive performances. The reduction in power performance is, however,
balanced by the quadratic effect of the increased potential window [2].