Page 259 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polyaniline-based nanocomposites for hydrogen storage 231

subsequently high-hydrogen storage capacity at normal circumstances. Further, stor-
ing hydrogen at ambient conditions can be improved through hydrogen spillover
mechanism via dispersing different types of carbon-based materials with transition
metal (e.g., palladium and platinum) nanoparticles. Hydrogen spillover mechanism
is a chemisorption process that encompasses the scission of chemically absorbed
hydrogen molecules over the metal catalysts supported on a carbon substrate as atomic
hydrogen, followed by the migration of atomic hydrogen onto the surface of the car-
bon support and finally reversible storing of hydrogen [46]. For instance, at 298 K, the
hydrogen storage capacity at 1.9 MPa of a microporous organic polymer material was
improved to 0.21 wt% from 0.12 wt% by the incorporation of Pt (2 wt%). This
improvement in the weight percentage at room temperature was mainly ascribed to
the spillover mechanism of hydrogen on the platinum nanoparticles in the doped
porous polymer matrix [47]. Sequences of works were conducted for dispersing PANI
with Pd nanoparticles (Pd NP) for various applications [48,49]. Recently, at 298 K,
hydrogen storage at 8 MPa was investigated for Pd NP that uniformly decorated
PANI-NF (PANI-NF-Pd NP) with a mean particle size of 4–5nm [30]. Results
suggested that the hydrogen storage capacity of PANI-NF-Pd NP is observed to be
0.12 wt%; unexpectedly, it is lesser than that of undecorated PANI-NF (0.46 wt%).
Furthermore, the hydrogen uptake of the composite PANI-NF-Pd NP with PPY coated
was improved to 0.47 wt%, which suggests that active sites were generated after the
polymerization of the PPY layer over PANI-NF-Pd NP. The influence of PPY layer is
more effective than the PANI-NF-Pd NP composite in terms of hydrogen uptake.
Actually, the storing capacity of the composite PANI-NF-Pd NP after coating the thin
PPY layer was enhanced from 0.12 to 0.47 wt%, which is nearly fourfold increment
[30]. Hydrogen storage property of the PANI is further improved by either catalytic
metal doping or inclusion of nanovariants in PANI matrix. Nanocomposite consisting
of a PANI matrix and catalytic metal doping is widely studied as promising hydrogen
storage materials. Skowronski and Urbaniak synthesized metal and PANI composite
for hydrogen storage application. In this composite, they sandwiched carbon with Ni
and Pd (i.e., Ni/C/Pd) and studied their hydrogen storage mechanism through electro-
sorption method [50].

8.3.2 PANI-carbon nanocomposites
Hydrogen storage capacity was investigated in different types of carbon-based mate-
rials like activated carbon, carbon nanotubes, and carbon. However, at normal circum-
stances, not only the storage capacities of carbon materials are <1 wt%, but also it
varies with the temperature. At 77 K, microporous activated carbons with
high-porosity show better hydrogen storage at low cost. At moderate temperature,
physisorption mechanism is responsible for storing the hydrogen in carbon-based
materials due to its weak van der Waals interactions between adsorbate and adsorbent
[51]. Fig. 8.11A and B shows the relation between hydrogen storage capacity and their
SSA and with their pore volume of the different carbon samples at 77 K. Here, it can
be concluded that H 2 uptake capacity is proportionate to the pore volume and surface
areas of the analyzed materials [52]. Fig. 8.12 shows the hydrogen sorption isotherms