Process for producing hydrogen gas from water is disclosed. The hydrogen gas is produced by exposing a reactive metal to an aqueous solution, where the aqueous solution is under supercritical conditions or is at a temperature of at least 200° C. and at a pressure of at least the saturated vapor pressure of water at said temperature. The reactive metals include Al, B, Mg, Si, Ti, Mn, and Zn. Heat and metal oxides and/or metal hydroxides may also be recovered from the process. The process may be used to produce hydrogen on demand in applications for producing clean energy.