One still unsolved problem for the use of hydrogen as dean fuel is the safe and efficient storage of hydrogen. Currently cryo tanks, gas cylinders or metal hydrides are used. Important parameters for a hydrogen storage system ate weight and volume density, cost and safety. Recent publications [1-3] claimed that large amounts of hydrogen can be stored reversibly in carbon nanotubes from the gas phase. Similar to a gas phase experiment where the storage material absorbs hydrogen as a function of pressure the hydrogen absorption in a electrochemical system is controlled by the potential. We report results of experiments with carbon nanotubes charged reversibly with hydrogen. The density of hydrogen per weight exceeds the storage density of metal hydrides. Samples with different degrees of purity are compared.