Biomedical Engineering Reference
In-Depth Information
than 0.5 wt.% (for pressures below 6 MPa), although the activated
samples do show a reasonable room temperature hydrogen uptake
due to their high surface area, as shown in Fig. 3.4c.
Figure 3.4
Hydrogen sorption curves of (a) CA900-A and (b) CA900
measured at 77 K, (c) CA900-A measured at room temperature
[26].
The hydrogen sorption at room temperature is much lower than
at 77 K because physisorption is a function of van der Waals forces
which are not as dominant at higher temperatures. The hydrogen
adsorption isotherm of the CAs at room temperature shows a linear
Henry-type behavior. Furthermore, the hydrogen storage capacity
and the shape of the isotherm are independent of the number of
adsorption cycles which clearly indicates that hydrogen is stored
reversibly in CAs. The hydrogen sorption results at 77 K for the
activated and nonactivated CA samples are presented in curves (a)
and (b) in Fig. 3.4. Both samples display a reasonably high uptake at
77 K with a maximum hydrogen sorption of 3.6 wt.% at 2.5 MPa for
the activated sample which has a larger uptake due to its increased
surface area. The measured excess wt.% of hydrogen is lower than
that of activated CA with a surface area of 3200 m
2
/g [5] and higher
than that of activated carbons with surface areas of 49 - 3000 m
2
/g
(ranging from 0.5 to 2.5 wt.% at
= 0.1 MPa) [9, 33]. The rule of
thumb relationship between the hydrogen storage capacity (wt.%)
and the surface area of porous materials is 1 wt.% of hydrogen per
500 m
P
2
/g of surface area, i.e., “Chahine rule” [5, 8]. The ratio of
the hydrogen storage capacity to surface area of the activated CA
presented here is similar to the above rule of thumb.
