Environmental Engineering Reference
In-Depth Information
Nafion composite membrane, fabricated by a simplistic solution casting method. This novel nanocomposite material demon-
strates a significant proton conductivity improvement (four times) over the unmodified one and can be considered as a potential
proton exchange membrane replacement for applications having low humidity of 30% and high temperature of 120°C. The
nanocomposite prepared by the covalent functionalization of multiwalled CNTs by grafting sulfanilic acid and their dispersion
into sulfonated poly(ether ether ketone) was investigated by Tripathi et al. [78] as a novel membrane material for applications
in alcohol or hydrogen fuel cells.
Some recent investigations show advantages of track-etched membranes in fuel cell applications such as no need to wet the
membrane, larger proton conductivity, resistance to extreme conditions, etc. [11]. The ion-track technology is based on the irra-
diation of thin films of various materials with accelerated heavy ions [79]. heavy ions produce so-called latent tracks with high
density of matter radiation damage along their path in the film with a diameter between 1 and 10 nm depending on the ion type
and its kinetic energy. After the etching procedure, the latent tracks may be used for the production of track-etched membranes
[80]. The grafting functionalization of the ion tracks in polymer thin films leads to the formation of highly proton-conductive
electrolyte membranes for fuel cell applications [81]. Such ion track-grafted membranes have lower crossover of methanol,
same proton conductibility, lower ion-exchange capacity, and superior mechanical properties compared to Nafion membranes,
which make them promising materials for widespread application in direct methanol fuel cells [82]. Track-etched membranes
may be also used as a host matrix for the preparation of polymer-polymer nanocomposite membranes with polyelectrolyte
nanodomains oriented normal to the plane of the membrane [12]. The membranes synthesized in this way demonstrate the
ability to enhance transport in the desired direction.
28.6
NaNomaterials For hydrogeN storage
Although hydrogen is widely recognized as a promising energy carrier for the transportation sector, widespread adoption of
hydrogen and fuel cell technologies depends critically on the ability to store hydrogen at adequate densities. At present, no
known material or storage means exists that satisfies all requirements to enable high-volume automotive application [83].
hydrogen can be stored as pressurized gas, cryogenic liquid, or as a suitable solid-state material [84]. high-pressure gaseous
hydrogen storage offers the simplest solution in terms of infrastructure requirement and has become the most popular and highly
developed method [85]. recent progress in cryogenic storage of liquid hydrogen has been reviewed by ho and rahman [86].
Solid-state storage is potentially the most convenient and safest method from a technological point of view, because this storage
technology supposes the presence of near-ambient temperatures and pressures [87]. moreover, storage of hydrogen in liquid or
gaseous form requires a large container and poses important safety problems for onboard transport applications. An attempt to
receive some insight into almost all classes of known hydrogen storage materials was made by Pukazhselvan et al. [88].
The hydrogen storage materials in solid state can be divided into several categories in terms of the strength of hydrogen
bonding: chemisorption, physisorption, and quasi-molecular bonding, which is the intermediate one between chemisorption
and physisorption [14]. in the case of chemisorption, the h
2
molecule dissociates into individual atoms, migrates into the
material, and binds chemically with a binding energy lying in the 2-4 eV range. The bonding is strong, and desorption takes
place at high temperatures. This category of hydrogen storage materials consists of simple metal and intermetallic hydrides
with maximum emphasis on magnesium-based hydrides, which can be treated as promising candidates for competitive
hydrogen storage with reversible hydrogen capacity up to 7.6 wt% for automotive applications. A recent review [89] reports
current developments of metal hydrides in properties including hydrogen storage capacity, kinetics, cyclic behavior, toxicity,
pressure, and thermal response.
A group of mg-based nanostructured hydrides is under intensive investigation at present. The hydrogen sorption property of
magnesium mgh
2
in the form of sandwiched Pd/mg/Pd films of nanoparticles with sizes on the order of 50 nm was studied by
Barcelo et al. [90]. A nanostructured mgh
2
/Tih
2
composite was synthesized [91] directly from mg and Ti metals by ball milling
under an initial hydrogen pressure of 30 mPa. The desorption temperature of this composite is more than 100°C lower compared
to commercial mgh
2
. The improved properties are due to the catalyst and nanostructure introduced during high-pressure ball
milling. The synthesis, hydrolysis, structure, dynamics, intermediate compound, and improvement of de/rehydrogenation prop-
erties of liBh
4
as hydrogen storage material with high gravimetric hydrogen capacity of 18.4 wt% were investigated by li et al.
[92]. The hydrogen desorption properties of the nanostructured mgh
2
-Ti alloy composite for hydrogen storage produced via
combined vacuum arc remelting and mechanical alloying were studied and compared with pure magnesium hydride [93].
The physisorption of hydrogen in porous materials is a viable mechanism for hydrogen storage in automotive applications.
This storage mechanism has the advantage of possessing fast kinetics, low heat of adsorption, and being completely reversible.
The investigation of hydrogen adsorption capability of different nanostructured carbon materials is one of the ongoing strategic
research areas in science and technology [94]. The challenges, distinguishing traits, and apparent contradictions of carbon-based
