A unitized electrolyzer apparatus for generating hydrogen gas at high pressure. According to one embodiment, the apparatus includes a pressure-containment vessel, a water electrolyzer stack, and a water supply. The water electrolyzer stack is mounted on or in the vessel and is used to generate hydrogen gas for containment in the vessel at high pressure. The water supply is contained within the vessel, the water supply being fluidly coupled to the water electrolyzer stack to provide a cathode feeding of water to the water electrolyzer stack.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. DE-SC0001486 awarded by the Department of Energy.