Abstract
Hydrogen adsorption at near-ambient temperatures on ultramicroporous carbon (UMC),
derived through secondary chemical activation from a wood-based activated carbon was
studied using volumetric and gravimetric methods. The results showed that physisorption
is accompanied by a process of different nature that causes slow uptake at high pressures
and hysteresis on desorption. In combination, this results in unusually high levels of
hydrogen uptake at near-ambient temperatures and pressures (e.g. up to 0.8 wt % at 25 oC
and 2 MPa). The heat of adsorption corresponding to the slow process leading to high
uptake (17 – 20 kJ/mol) is higher than usually reported for carbon materials, but the
adsorption kinetics is slow, and the isotherms exhibit pronounced hysteresis. These
unusual properties were attributed to contributions from polarization-enhanced
physisorption caused by traces of alkali metals residual from chemical activation. The
results support the hypothesis that polarization-induced physisorption in high surface area
carbons modified with traces of alkali metal ions is an alternate route for increasing the
hydrogen storage capacity of carbon adsorbents.
