Page 285 - Polymer-based Nanocomposites for Energy and Environmental Applications

256 Polymer-based Nanocomposites for Energy and Environmental Applications

As mentioned earlier, Si-based materials show the highest possible theoretical spe-
1
cific capacity of 4200 mAh g . But they suffer from mechanical and electric degra-
dation due to the stress generation as a result of large volume expansion (>300%)
during intercalation/deintercalation process, which leads to poor cyclic stability
[171]. Polymer nanocomposite of the Si nanoparticles with a less active and flexible
material that can accommodate volume changes is a viable option. Kummer et al. syn-
thesized silicon nanoparticles using the gas-phase synthesis, followed by the prepara-
tion of PANI/Si nanocomposite via in situ polymerization of the monomer. One
percent boron was added to the Si nanoparticles to improve its electric conductivity.
It resulted into a polymer nanocomposite with 50% polyaniline and 50% Si of 40 nm
size. A composite electrode with 20% of the n-Si/PANI showed an initial capacity of
561 mA g 1 at 0.1 C. After 300 cycles, the electrode showed high capacity of
390 mAh g 1 at 0.5 C discharge rate [172]. The improved performance was ascribed
to the presence of PANI that increased the electric conductivity and adds flexibility in
the structure to buffer stress due to volume changes (Table 9.2).

9.3.2 PNCs based materials for supercapacitors
Supercapacitors or ultracapacitors are another popular electric energy storage device.
They offer high power output ( 10 Kw/kg) at the cost of low energy density
1
( 5–10 Wh kg ) [174,175]. Therefore, they are mostly found in applications where
“power bursts” is required; that is, it is required to deliver large amounts of electric
energy in a very short period of time, such as in military application, satellite launch,
powering remote areas, and hybrid electric vehicles, but large energy density is of sec-
ondary importance [176]. Supercapacitors provide a practical compromise between
conventional dielectric capacitors (high power output and low energy storage) and
batteries (high storage capacity and low power output) [177]. Apart from this, super-
capacitors show very short discharging and charging time because of the physical
nature of the storage mechanism and absence of slow electrochemical reactions. In
comparison with the batteries that can at best deliver power for 1000 cycles, super-
capacitors can maintain performance for millions of charging/discharging cycles
because of complete absence of any irreversible chemical reaction (faradaic process)
taking place at the electrodes in an electric double-layer capacitor [177–179].
Table 9.3 highlights the comparison between the battery and supercapacitors.
A supercapacitor consists of two electrodes, which are separated by a dielectric
electrolyte [176,179]. Depending on the way charge is stored, supercapacitors can
be classified as (1) electrochemical double-layer capacitors (EDLCs) and (2) pseudo-
capacitors. The working principle of both the types of supercapacitors is shown in
Fig. 9.4 [180]. Electrochemical double-layer capacitors store charge electrostatically
through a reversible physical adsorption/desorption of ions from the electrolyte on the
electrode/electrolyte interface. This is essentially a nonfaradic process. Working prin-
ciple of EDLCs is shown in Fig. 9.4A. Since the charge is stored momentarily at the
interface of the active material (electrode) and the electrolyte, the electrode should
have high surface area per unit mass and high electric conductivity to facilitate the
formation of double layer [181]. Moreover, the capacitance in EDLCs depends on