We have carried out systematic calculations for hydrogen-adsorption and -st
orage mechanism in carbon nanotubes at zero temperature. Hydrogen atoms fir
st adsorb on the tube wall in an arch-type and zigzag-type up to a coverage
of theta = 1.0 and are stored in the capillary as a form of H-2 molecule a
t higher coverages. Hydrogen atoms can be stored dominantly through the tub
e wall by breaking the C-C midbond, while preserving the wall stability of
a nanotube after complete hydrogen insertion, rather than by the capillarit
y effect through the ends of nanotubes. In the hydrogen-extraction processe
s, H-2 molecule in the capillary of nanotubes first dissociates and adsorbs
onto the inner wall and is further extracted to the outer wall by the flip
-out mechanism. Our calculations describe suitably an electrochemical stora
ge process of hydrogen, which is applicable for the secondary hydrogen batt
ery.