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Abstract

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.

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