The mechanism for safe and reversible storage of hydrogen in porous carbonaceous materials by electrochemical decomposition of water in alkaline electrolyte is proposed. Atomic H was found to be inserted into the microdomains of defective graphene layers. Hydrogen storage capacity increases with increasing interlayer distance between carbon sheets. Hydrogen insertion in carbonaceous materials occurs at ambient conditions. Static potential acts as an electrochemical valve which can retain the hydrogen in the carbon structure, thus preventing leakage during storage. (C) 2008 Elsevier B.V All rights reserved.