In situ formation and hydrolysis of Zn nanoparticles for H2H2 production by the 2-step ZnO/Zn water-splitting thermochemical cycle

The production of solar hydrogen via the Zn/ZnO water-splitting thermochemical cycle consists of a 1st-step solar endothermic dissociation of ZnO and a 2nd-step non-solar exothermic hydrolysis of Zn. We report on a novel combined process for the efficient execution of the second step that encompasses the formation of Zn nanoparticles followed by their in situ hydrolysis for H2H2 generation. The advantages of using Zn-nanoparticles are three-fold: (1) their inherent high specific surface area augments the reaction kinetics, heat transfer, and mass transfer; (2) their large surface to volume ratio favors complete or nearly complete oxidation; and (3) their entrainment in a gas flow allows for simple, continuous, and controllable feeding of reactants and removal of products. This combined process is experimentally demonstrated using a tubular aerosol flow reactor featuring Zn-evaporation, steam-quenching, and Zn/H2OZn/H2O-reaction zones. When the Zn-evaporation zone was operated at 1023 K and the Zn/H2OZn/H2O reaction zone was operated continuously just below the Zn(g) saturation temperature, Zn particles of 69 nm average crystallite size were formed and in situ hydrolyzed by up to 83% degree of chemical conversion, while the H2H2 yield reached up to 70% after a single pass of H2OH2O of 0.85 s residence time.