Two classes of hybrid/thermochemical water splitting processes for the production of hydrogen and oxygen have been proposed based on (1) metal sulfate-ammonia cycles (2) metal pyrosulfate-ammonia cycles. Methods and systems for a metal sulfate MSO4—NH3 cycle for producing H2 and O2 from a closed system including feeding an aqueous (NH3)4SO3 solution into a photoctalytic reactor to oxidize the aqueous (NH3)4SO3 into aqueous (NH3)2SO4 and reduce water to hydrogen, mixing the resulting aqueous (NH3)2SO4 with metal oxide (e.g. ZnO) to form a slurry, heating the slurry of aqueous (NH4)2SO4 and ZnO(s) in the low temperature reactor to produce a gaseous mixture of NH3 and H2O and solid ZnSO4(s), heating solid ZnSO4 at a high temperature reactor to produce a gaseous mixture of SO2 and O2 and solid product ZnO, mixing the gaseous mixture of SO2 and O2 with an NH3 and H2O stream in an absorber to form aqueous (NH4)2SO3 solution and separate O2 for aqueous solution, recycling the resultant solution back to the photoreactor and sending ZnO to mix with aqueous (NH4)2SO4 solution to close the water splitting cycle wherein gaseous H2 and O2 are the only products output from the closed ZnSO4—NH3 cycle.