189
Components and Materials for Electrochemical Supercapacitors
Lian et al. [121–123] investigated a composite solid state polymer that utilized PVA for its good film formability. The solid PVA–PWA and PVA–SiWA
electrolytes have good film making properties and exhibit strong stability
at high relative humidity. Nafion ® is another proton conducting polymer
that has good film forming properties and also exhibits high conductivity at
room temperature, but its conductivity decreases significantly with decreasing humidity and cell moisture [122]. Stability means that HPA materials
can be processed in ambient environments; they simplify packaging procedures and create leak-proof, corrosion-free cell designs. The PVA–SiWA solid
state electrolyte exhibited 11 mS.cm –1 and provided capacitance (50 mF.cm –2 )
comparable to an aqueous H 2 SO 4 electrolyte (70 mF.cm –2 ) when tested with
a symmetrical ruthenium dioxide cell (60 μm thick electrodes) [121]. Further
optimization showed that an even mix of PWA and SiWA created a synergetic effect when combined with PVA and increased conductivity to 13
mS.cm –1 [122].
4.4 Separator Structures
Separators play a role in preventing contact and electron transfer between
anode and cathode. A separator must be mechanically strong to provide
device durability and prevent migration of high carbon particles over time.
It is important that the material possesses strong ion conductance and electronic insulating capability. High ionic conductivity is promoted by high
porosity and low tortuosity [2]. Resistances to ion flow and interfacial contact
resistance with an electrode can also be improved when separators exhibit
sufficient wettability [114]. A separator film should be thin, while maintaining mechanical stability. Separators must be chemically resistant to corrosion from electrolytes and by-products of electrode degradation. It is also
very important that separators prevent migration of active materials in order
to eliminate short circuiting.
Batteries and ESs that operate near ambient temperatures often use materials such as cellulose paper, polymer, and glass wool. However, commercial
separators vary based on electrolyte choice and temperature of operation. ESs
represent a developing market that utilizes many common electrolytes used
in battery systems. For this reason, separator choices closely mimic choices for
batteries. Organics utilize microporous polymers and cellulose paper separators, whereas aqueous devices traditionally utilize glass, mica, and ceramic
separators [124]. However, paper-based separators suffer from poor mechanical strength and durability in high temperature operation environments.
Polyolefin-based microporous separators (Figure 4.32) continue to displace
natural materials such as glass and cellulose fibers because of high porosity,
low cost, flexibility, corrosion resistance, and improved mechanical strength
Components and Materials for Electrochemical Supercapacitors
Lian et al. [121–123] investigated a composite solid state polymer that utilized PVA for its good film formability. The solid PVA–PWA and PVA–SiWA
electrolytes have good film making properties and exhibit strong stability
at high relative humidity. Nafion ® is another proton conducting polymer
that has good film forming properties and also exhibits high conductivity at
room temperature, but its conductivity decreases significantly with decreasing humidity and cell moisture [122]. Stability means that HPA materials
can be processed in ambient environments; they simplify packaging procedures and create leak-proof, corrosion-free cell designs. The PVA–SiWA solid
state electrolyte exhibited 11 mS.cm –1 and provided capacitance (50 mF.cm –2 )
comparable to an aqueous H 2 SO 4 electrolyte (70 mF.cm –2 ) when tested with
a symmetrical ruthenium dioxide cell (60 μm thick electrodes) [121]. Further
optimization showed that an even mix of PWA and SiWA created a synergetic effect when combined with PVA and increased conductivity to 13
mS.cm –1 [122].
4.4 Separator Structures
Separators play a role in preventing contact and electron transfer between
anode and cathode. A separator must be mechanically strong to provide
device durability and prevent migration of high carbon particles over time.
It is important that the material possesses strong ion conductance and electronic insulating capability. High ionic conductivity is promoted by high
porosity and low tortuosity [2]. Resistances to ion flow and interfacial contact
resistance with an electrode can also be improved when separators exhibit
sufficient wettability [114]. A separator film should be thin, while maintaining mechanical stability. Separators must be chemically resistant to corrosion from electrolytes and by-products of electrode degradation. It is also
very important that separators prevent migration of active materials in order
to eliminate short circuiting.
Batteries and ESs that operate near ambient temperatures often use materials such as cellulose paper, polymer, and glass wool. However, commercial
separators vary based on electrolyte choice and temperature of operation. ESs
represent a developing market that utilizes many common electrolytes used
in battery systems. For this reason, separator choices closely mimic choices for
batteries. Organics utilize microporous polymers and cellulose paper separators, whereas aqueous devices traditionally utilize glass, mica, and ceramic
separators [124]. However, paper-based separators suffer from poor mechanical strength and durability in high temperature operation environments.
Polyolefin-based microporous separators (Figure 4.32) continue to displace
natural materials such as glass and cellulose fibers because of high porosity,
low cost, flexibility, corrosion resistance, and improved mechanical strength