Biomedical Engineering Reference
In-Depth Information
where
the gas constant
(8.314 J/mol/K). Typical adsorption enthalpies range from 4 kJ/mol to
8 kJ/mol, which demonstrates a very weak interaction between
a poorly polarizable hydrogen molecule and the pore surface [7].
Further, thermodynamic constraints for hydrogen storage by
physisorption were analyzed recently by Bhatia and Myers with a
focus on porous carbon [14]. They showed that the optimum temp-
erature for hydrogen adsorption on carbon was 114.4 K [14, 17, 18].
T
is the temperature in Kelvin and
R
3.2.1.2
Isosteric Enthalpy of Adsorption
The heats of adsorption were determined from the temperature
dependence of the isotherms via the Clausius-Clapeyron equation
[19, 20]
R
ln(
P
/
P
)
2
1
Q
(3.3)
st
(1
T
1
T
)
1
2
where
are hydrogen pressures
obtained from the hydrogen isotherms at temperatures
Q
is the heat of adsorption,
P
and
P
st
1
2
T
and
T
,
1
2
respectively.
If
isotherms
at
several
temperatures
are
available,
the
appropriate form is
Q
(3.4)
st
ln
P
const.
RT
where
Q
represents an isosteric heat of adsorption. The relation
st
between
for a given amount adsorbed can be plotted from
the isotherms for a series of temperatures. The heat of adsorption is
then determined from the slope of the isosteric plot.
P
and
T
3.2.2 Characterizations Techniques
To characterize the properties of porous hydrogen storage
materials, parameters such as (i) surface area, (ii) pore volume, (iii)
adsorption/desorption cycle life, and (iv) enthalpy of adsorption
were focused upon. Our aim was to develop a simple, safe, and cheap
method for the preparation of CAs with high surface areas, large
pore volume, and a pore size distribution dominated by micropores,
to facilitate thermodynamic changes due to the size of encapsulated
nano particles. The method is also expected to incorporate
