Biomedical Engineering Reference
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at 77 K. Hydrogen storage properties were investigated at room
temperature and liquid nitrogen temperature at pressures up to
6.5 MPa.
3.1
Introduction
Owing to the dramatic environmental impact and limited supply
of fossil fuels, the search for alternative clean fuels is becoming
increasingly important. Hydrogen has been identified as a future
clean energy carrier [1]. Hydrogen-powered fuel cells are developing
rapidly because they are more efficient than internal combustion
engines and have only water as an emission. However, use of
hydrogen as an energy carrier involves solving many problems that
relate to its production, storage, transportation, and safety. Numerous
efforts are being undertaken to develop efficient hydrogen storage
media that comply with the U.S. DOE targets. These objectives fix
a target of 2 kWh/kg (6 wt.%) and $4/kWh for 2010, and 3 kWh/
kg (9 wt.%) and $2/kWh for 2015 [2]. For practical applications,
materials should demonstrate (a) high hydrogen capacity (>6 wt.%)
(b) fast and reversible hydrogen sorption at temperatures <150°C
and reasonable pressures (<2 MPa), and (c) should be light and
environmentally friendly [3, 4]. Unfortunately, hydrogen energy
densities acceptable to the automobile industry in the short term
are presently achieved reversibly only with high pressure (>35 MPa)
gas cylinders. Promising solid state materials for hydrogen storage
include metal hydrides, complex hydrides, chemical storage, and
carbon-based porous absorbents.
Porous carbon materials, e.g., carbon aerogels (CAs), cryogels,
aerogels, and xerogels, have ultrafine cell/pore sizes, continuous
porosity and high surface areas. Among porous carbon materials,
CA materials are one of the novel carbon-based porous materials
with many fascinating properties [5, 6]. They are typically prepared
from the sol-gel polymerization of resorcinol and formaldehyde and
dried through supercritical extraction of the reaction solvent [6]. As
for nanoporous absorbents, the enhancement of hydrogen storage
capacity can be achieved by increasing surface area. In addition, the
surface should be continuous and open, which not only enhances the
hydrogen storage capacity, but also increases the hydrogen sorption
kinetics. With the assumption that the structure of the adsorbed
hydrogen is close-packed face-centered cubic, the minimum surface
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