This invention relates to a hydrogen generator system for generating hydrogen from a water split reaction. The generator comprises a pressure container having a reactant water inlet, and a product hydrogen outlet. Pluralities of cells are vertically stacked inside the container; each cell contains a reactant compound comprising a mechanical mixture of metal and an anti-passivation material. The reactant compound produces hydrogen gas upon contact with water, and the cells are stacked such that water entering from the inlet can rise inside the container and sequentially activate each immersed cell to produce hydrogen gas.