Biomedical Engineering Reference
In-Depth Information
is quite close to the result found using the supercritically drying
method [5]. It is suggested that the micropore volume has a dominant
effect on hydrogen uptake capacity when the effect of surface area
reaches saturation. Obviously the activating process will increase
the roughness of surface resulting in an increase in surface area
and pore volume. These results also suggest that a higher activating
temperature will give rise to an increase in hydrogen storage capacity,
which is attributed to an increase in microporosity.
3.4
Metal-Doped Carbon Aerogels
An area of significant interest involves the incorporation of metal
species into the CA framework with the goal of modifying its
structure and catalytic activity [51]. Metal-doped CAs have shown
an increased enthalpy of adsorption for hydrogen (see Table 3.5),
and as such have been investigated as a hydrogen storage medium
[5]. Previously, an ion exchange method has been used to uniformly
incorporate metal ions into a CA matrix.
Table 3.5
Differential enthalpy of adsorption at zero coverage and
saturation values at 77 K for activated and metal-doped CAS
(From Ref. [5])
CA ∆
H
(kJ/mol� Excess H
2
(wt.%�
Pure CA (3200 m
2
/gg) 6.2 5.3
Co-doped CA (968 m
2
/gg) 7.5 2.1
Ni-doped CA (954 m
2
/gg) 7.1 2.0
This process involves producing a potassium-doped hydrogel
via the base-catalyzed polymerization of an aqueous solution of
formaldehyde and a potassium salt. The potassium ions can then be
replaced with the desired metal ion via ion exchange and the metal-
doped hydrogels can then be pyrolyzed to generate a metal-doped
CA [51]. This metal doping method, however, lacks the crucial step
of activation that results in the high surface area and microporosity
typical of CAs. For example, it has been shown that for pure CAs,
activation leads to a 53% increase in surface area (from 1005 to
1539 m
/g) and a 31% increase in micropore volume (from 0.51
to 0.71 cm
2
3
/g) [26]. The activation process for metal doped CAs is
avoided as the high temperature that is required results in significant
