Biomedical Engineering Reference
In-Depth Information
2
area required for the adsorption of 1 mol of hydrogen is 85.917 m
[7]. Accordingly, based on the surface area of CAs, the storage capacity
of hydrogen at ~6 wt.% adsorbed on CAs is about 2578 m
2
/g,
which can typically be achieved at cryogenic temperatures. The
hydrogen storage capacity per unit surface area of CA is similar to
that for activated carbons, i.e., 1 wt.% for every 500 m
2
/g of surface
area, i.e., Chahine rule [5, 8]. Reversible hydrogen uptake on these
carbon materials has been consistently reported to be approximately
proportional to surface area and micropore volume [9, 10]. The
best linear correlation is usually obtained when relating hydrogen
adsorption capacity to micropore volume [11], reflecting the fact
that physisorption (and consequent hydrogen storage) is dominated
by pores having a diameter of sub-nanometer range [12]. However,
the physisorption is brought about by weak van der Waals forces. In
the absence of relatively strong polarizing centers, the interaction
between the adsorbent and nonpolar hydrogen molecules relies
on dispersion forces, which are weak; typically of the order of
4-8 kJ/mol. Hence, significant hydrogen adsorption often takes place
only at cryogenic temperatures [13]. Thermodynamic constraints
for hydrogen storage by physisorption were analyzed recently and
results indicated that the carbons are not practical for hydrogen
storage at ambient temperature [14].
3.2
Fundamentals of Adsorption
and Characterizations
3.2.1
Fundamentals of Absorption
The use of hydrogen physisorption on porous materials is one of
the main methods being considered for automobile applications.
The objective is to store large amounts of hydrogen at a near-
ambient temperature and safe pressures. Since physisorption is a
nonactivated process, fast kinetics and reversibility can be expected.
The hydrogen is physisorbed by monolayer coverage over the solid
surfaces irrespective of their pore dimensions. Therefore, solids with
high surface area and having specific affinity for hydrogen molecules
are essential for hydrogen storage. The extent of adsorption, or the
fraction of surface coverage (
θ
), as a function of pressure (
p
) can be
obtained from the Langmuir isotherm as [15]:
