Page 255 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polyaniline-based nanocomposites for hydrogen storage 227

polymers before protonation at 0.12 MPa shows 0.8 wt%, but this is reduced to
0.3 wt% once it is treated with HCl. Aromatic rings are responsible for hydrogen
adsorption site in these types of materials [37–39]. It is seen that the aromatic rings
that hold electron-donating groups adsorb more hydrogen than the electron-
withdrawing group [40]. In this regard, electron-donating ability of aniline function-
ality adsorbs hydrogen more efficiently than the PANI that forms the quaternary
ammonium group upon protonation and exhibits electron-withdrawing nature. These
reveal a decreasing tendency of adsorbing hydrogen in polymers, whereas at room
temperature, there is no considerable difference in hydrogen adsorption capacity of
PANI observed [15].
The polymer that consists of stereocontorted cores with nanoporous structure is
investigated as a suitable candidate for hydrogen storage [31]. To synthesize the
cross-linked polymers, the spirobifluorene and tetraphenylmethane cores were served
as the basic units. For polymerization, trimerizations of acetylenic compounds or oxi-
dative coupling of thiophenyl compound were employed. Generally, polymers pre-
pared through this method exhibit the narrow pore size distribution with the SSA
2 1
of about 1000 m g . However, the reaction condition also influences in determining
the size of nanopores and the surface areas. Using Sievert isotherm apparatus, the
hydrogen adsorption capacities at liquid nitrogen and ambient temperatures were cal-
culated. As a result, the hydrogen adsorption capacity at liquid nitrogen temperature is
proportionate to the surface area, and ambient temperature is depending on micropo-
rosity of the material. There is no considerable variation in the adsorption of hydrogen
due to the factors like extended π-electron system and introduction of sulfur hetero-
atom. In order to considerably increase the rate of adsorptions at ambient temperature,
new functional groups with the tendency to interact strongly with hydrogen molecules
are essential. Hence, synthesized polymers with various adsorbent composition and
structural improvements explore the possibilities of obtaining efficient hydrogen
uptake in the material. Recent report by Ebrahim et al. showed the formation of
nanoporous PANI treated with 37% HCl solution at 24 h. At elevated temperature,
charging these porous nanostructured PANI at 3.0 MPa hydrogen provides 3 wt%
of hydrogen storage capacity. Also the multiple charge and discharge cycles showed
6
good retention beyond 1.8 10 s [34].

8.2.3 Activated porous PANI
Highly porous carbon materials can be synthesized through carbonization followed by
activation process of PANI structures [41–43]. These nanoporous carbon materials
derived from PANI structures exhibit larger hydrogen storage capacities due to the
increase in SSA. Hence, activated porous carbon with pore volume and SSA contrib-
utes to the hydrogen storage through physisorptive mechanism and considered as most
suitable material in hydrogen adsorbents. Chemical activation by KOH is another pro-
ductive method to obtain highly porous and large SSA carbon material [43].Asa
result of carbonization and activation process with KOH, the various rectangular
PANI tubes (RPTs) were produced using hollow carbon nanospheres (HCNSs) as