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Components and Materials for Electrochemical Supercapacitors
4.2.8.6 Carbon Nanofibers
Carbon nanofibers (CNFs) are long hollow fibers composed of ordered
arrangements of graphene sheets. CNFs are of two types: highly graphitic
and lowly graphitic. The highly ordered graphitic CNFs are produced by
catalytic CVD using a metal catalyst such as nickel, iron, or cobalt particles
in solution to promote CNF growth [69]. Alternatively, fixed catalysts on a
support can be used to create a highly graphitic CNF array. Highly graphitic
CNFs exhibit strong conductivity of 1000 mS.cm –1 , but suffer from stacked
graphite planes in the walls of the tubules.
The stacking leads to low surface area of only 10 to 50 m 2 .g –1 and restricts
capacitance to only 1 to 10 F.g –1 [69]. Further, the tight stacking and stability
of the graphitic walls prevent efficient activation of the CNF and reduce the
usefulness of highly graphitic production methods in ES applications. Lowly
graphitic CNFs are mostly amorphous and are created by the carbonization
of polymer precursors. One production method is to use polymer blends and
remove one polymer during carbonization to produce a CNF web template
with 100 to 500 m 2 .g –1 . However, the reduction in graphitic character restricts
conductivity and capacitance to 100 mS.cm –1 and below 100 F.g –1 in aqueous
electrolyte. An effective alternative is to use electrospinning. Subsequent
carbonization of the polymer nanofiber webs could be created [69].
Electrospun polymers exhibit low levels of molecular defects, thus optimizing strength and creating order that leads to higher conductivity of 700
to 900 mS.cm –1 after carbonization at temperatures between 700 and 800°C
[69]. The amorphous character of the CNFs allows effective functionalization
and activation of the lowly graphitic form. Lowly graphitic PAN-based CNFs
synthesized by Kim et al. with steam activation (1100 m 2 .g –1 ) at 750°C showed
120 F.g –1 at 1 A.g –1 in KOH electrolyte [70]. They also showed that polyamic
acid (PAA) fibers steam activated at 750°C (1400 m 2 .g –1 ), produced 160 F.g –1 at
1 A.g –1 in KOH electrolyte [71].
Barranco et al. [69] electrospun CNF fibers by using a polymer blend containing a phenolic resin and a high density polyethylene (PE). The blend was
carbonized at 800°C producing CNFs of 450 m 2 .g –1 (700 mS.cm –1 ) and then
KOH activated at 750°C to increase surface area to 1500 m 2 .g –1 . The KOH activation reduced conductivity to 400 mS.cm –1 but boosted capacitance to 180
F.g –1 (from 50 F.g –1 not activated) at 1 A.g –1 (20 mA.cm –2 ) in KOH electrolyte [69].
Yan et al. [72] utilized the accessible webs created by the long fibers to
derive a CNF composite with PANI (Figure 4.19). The base capacitance of the
CNFs inactivated managed 310 F.g –1 in H 2 SO 4 at 2 A.g –1 and electrical conductivity was 950 mS.cm –1 . The improved performance is likely due to improved
layer-by-layer methodology for collection of the nanofibers and illustrates
the macroscopic mechanical control over optimizing performance of CNFs
produced by electrospinning. The CNF paper (10 to 15 μm) produced by
layer-by-layer electrospinning required no conductive carbon black additives and was very flexible. After a simple rapid mixture polymerization,
Components and Materials for Electrochemical Supercapacitors
4.2.8.6 Carbon Nanofibers
Carbon nanofibers (CNFs) are long hollow fibers composed of ordered
arrangements of graphene sheets. CNFs are of two types: highly graphitic
and lowly graphitic. The highly ordered graphitic CNFs are produced by
catalytic CVD using a metal catalyst such as nickel, iron, or cobalt particles
in solution to promote CNF growth [69]. Alternatively, fixed catalysts on a
support can be used to create a highly graphitic CNF array. Highly graphitic
CNFs exhibit strong conductivity of 1000 mS.cm –1 , but suffer from stacked
graphite planes in the walls of the tubules.
The stacking leads to low surface area of only 10 to 50 m 2 .g –1 and restricts
capacitance to only 1 to 10 F.g –1 [69]. Further, the tight stacking and stability
of the graphitic walls prevent efficient activation of the CNF and reduce the
usefulness of highly graphitic production methods in ES applications. Lowly
graphitic CNFs are mostly amorphous and are created by the carbonization
of polymer precursors. One production method is to use polymer blends and
remove one polymer during carbonization to produce a CNF web template
with 100 to 500 m 2 .g –1 . However, the reduction in graphitic character restricts
conductivity and capacitance to 100 mS.cm –1 and below 100 F.g –1 in aqueous
electrolyte. An effective alternative is to use electrospinning. Subsequent
carbonization of the polymer nanofiber webs could be created [69].
Electrospun polymers exhibit low levels of molecular defects, thus optimizing strength and creating order that leads to higher conductivity of 700
to 900 mS.cm –1 after carbonization at temperatures between 700 and 800°C
[69]. The amorphous character of the CNFs allows effective functionalization
and activation of the lowly graphitic form. Lowly graphitic PAN-based CNFs
synthesized by Kim et al. with steam activation (1100 m 2 .g –1 ) at 750°C showed
120 F.g –1 at 1 A.g –1 in KOH electrolyte [70]. They also showed that polyamic
acid (PAA) fibers steam activated at 750°C (1400 m 2 .g –1 ), produced 160 F.g –1 at
1 A.g –1 in KOH electrolyte [71].
Barranco et al. [69] electrospun CNF fibers by using a polymer blend containing a phenolic resin and a high density polyethylene (PE). The blend was
carbonized at 800°C producing CNFs of 450 m 2 .g –1 (700 mS.cm –1 ) and then
KOH activated at 750°C to increase surface area to 1500 m 2 .g –1 . The KOH activation reduced conductivity to 400 mS.cm –1 but boosted capacitance to 180
F.g –1 (from 50 F.g –1 not activated) at 1 A.g –1 (20 mA.cm –2 ) in KOH electrolyte [69].
Yan et al. [72] utilized the accessible webs created by the long fibers to
derive a CNF composite with PANI (Figure 4.19). The base capacitance of the
CNFs inactivated managed 310 F.g –1 in H 2 SO 4 at 2 A.g –1 and electrical conductivity was 950 mS.cm –1 . The improved performance is likely due to improved
layer-by-layer methodology for collection of the nanofibers and illustrates
the macroscopic mechanical control over optimizing performance of CNFs
produced by electrospinning. The CNF paper (10 to 15 μm) produced by
layer-by-layer electrospinning required no conductive carbon black additives and was very flexible. After a simple rapid mixture polymerization,