151
Components and Materials for Electrochemical Supercapacitors
4.2.8 EDLC Electrode Materials
4.2.8.1 Activated Carbons
Carbons act as excellent conductors, are chemically stable, and have high
surface areas, making them the preferred materials for double-layer electrodes in modern ESs. Carbon, however, comes in many varieties and not all
are applicable to electrode materials. The industrial standard and most basic
high surface area carbon material is activated carbon (AC). It is widely used
because of moderate cost and easy preparation.
AC materials can be generated from a number of precursor materials
through carbonization and high temperature annealing in an inert atmosphere. Materials are pitches or resins derived from coal and petroleum that
exhibit liquid phase shifts, allowing alignment during graphitization, creating heavily microporous areas that require activation for successful use in
ESs.
In general, high surface area alone is not easily correlated to capacitance;
pore structure seems to be critical. Mesopores are important to allow sufficient ion diffusion kinetics to support high power and area (20 to 50% suggested) [26]. Surface level micropores allow large area for storage but limit
ion mobility to loss of their solvation shells. Macropores are important to
provide high throughput ion channels.
Alternative carbon precursors are derived from more structured compounds such as wood, polymers, and hard shells that do not exhibit liquid
phases during carbonization and thus maintain alignment and rigidity.
Carbons derived from natural biomass exhibit large voids inherent to the
natural structure of the material, reducing the high volumetric area needed
for ESs. Table 4.2 lists available activated carbon powders along with their
activation pathways and properties [27]. Capacitance testing of carbon materials on the list showed that KOH activation resulted in the highest capacitance, 154 F.g -–1 in organic electrolyte.
Activated carbons can often achieve capacitances as high as 100 to 200 F.g –1
in aqueous electrolyte systems and 50 to 150 F.g –1 in organic media [28]. An
AC material prepared by Gryglewicz et al. [26] showed 160 F.g –1 in aqueous
electrolyte for a material made from coal and activated by steam (SA = 1270
m 2 .g –1 ). Wu et al. [29] prepared AC’s from firewood using steam activation
resulting in a maximum capacitance of 120 F.g -1 in acidic electrolyte.
KOH activation was used on petroleum coke by Wang et al. [30] to produce
material with 1180 m 2 .g –1 and maximum capacitance of 160 F.g –1 in aqueous
electrolyte. Kierzek et al. [31] illustrated that high surface area carbon (3000
m 2 .g –1 ) produced with KOH activation showed improved capacitance of 300
F.g –1 in aqueous electrolyte. Table 4.2 shows performances in organic electrolytes and illustrates that (1) high surface area AC does not directly correlate
to higher performance and (2) precursor materials and activation mechanisms are important to electrochemical performance.
Components and Materials for Electrochemical Supercapacitors
4.2.8 EDLC Electrode Materials
4.2.8.1 Activated Carbons
Carbons act as excellent conductors, are chemically stable, and have high
surface areas, making them the preferred materials for double-layer electrodes in modern ESs. Carbon, however, comes in many varieties and not all
are applicable to electrode materials. The industrial standard and most basic
high surface area carbon material is activated carbon (AC). It is widely used
because of moderate cost and easy preparation.
AC materials can be generated from a number of precursor materials
through carbonization and high temperature annealing in an inert atmosphere. Materials are pitches or resins derived from coal and petroleum that
exhibit liquid phase shifts, allowing alignment during graphitization, creating heavily microporous areas that require activation for successful use in
ESs.
In general, high surface area alone is not easily correlated to capacitance;
pore structure seems to be critical. Mesopores are important to allow sufficient ion diffusion kinetics to support high power and area (20 to 50% suggested) [26]. Surface level micropores allow large area for storage but limit
ion mobility to loss of their solvation shells. Macropores are important to
provide high throughput ion channels.
Alternative carbon precursors are derived from more structured compounds such as wood, polymers, and hard shells that do not exhibit liquid
phases during carbonization and thus maintain alignment and rigidity.
Carbons derived from natural biomass exhibit large voids inherent to the
natural structure of the material, reducing the high volumetric area needed
for ESs. Table 4.2 lists available activated carbon powders along with their
activation pathways and properties [27]. Capacitance testing of carbon materials on the list showed that KOH activation resulted in the highest capacitance, 154 F.g -–1 in organic electrolyte.
Activated carbons can often achieve capacitances as high as 100 to 200 F.g –1
in aqueous electrolyte systems and 50 to 150 F.g –1 in organic media [28]. An
AC material prepared by Gryglewicz et al. [26] showed 160 F.g –1 in aqueous
electrolyte for a material made from coal and activated by steam (SA = 1270
m 2 .g –1 ). Wu et al. [29] prepared AC’s from firewood using steam activation
resulting in a maximum capacitance of 120 F.g -1 in acidic electrolyte.
KOH activation was used on petroleum coke by Wang et al. [30] to produce
material with 1180 m 2 .g –1 and maximum capacitance of 160 F.g –1 in aqueous
electrolyte. Kierzek et al. [31] illustrated that high surface area carbon (3000
m 2 .g –1 ) produced with KOH activation showed improved capacitance of 300
F.g –1 in aqueous electrolyte. Table 4.2 shows performances in organic electrolytes and illustrates that (1) high surface area AC does not directly correlate
to higher performance and (2) precursor materials and activation mechanisms are important to electrochemical performance.