It is hoped that a hydrogen-on-demand generator may one day start with just the turn of an ignition key, if the reaction kinetics is accelerated for the production of hydrogen gas from water. Our quantum molecular dynamics simulations have revealed the atomistic mechanism of rapid hydrogen production from water using aluminum nanoclusters, Al-n (n = 16, 17, and 18). We have found a low activation-barrier mechanism of hydrogen production, in which a pair of Lewis add and base sites on the nanocluster surface plays a crucial role. Hydrogen production is assisted by rapid proton transport in water via a chain of hydrogen-bond switching events similar to the Grotthuss mechanism. The solvation shell has been shown to greatly reduce the energy barrier. We have also found that the reaction rate does not depend strongly on the cluster size n, in contrast to the existence of magic numbers in gas-phase reaction. This work paves a way for a rational design of hydrogen-on-demand technologies. (C) 2013 Elsevier B.V. All rights reserved.