(168e) Aerosol Spray Pyrolysis Synthesis of Synthesis of Water-Splitting Ferrites for Solar Hydrogen Production

Aerosol Spray Pyrolysis (ASP) was employed for the synthesis of mixed ferrite systems to be used as redox materials for the production of Hydrogen via a two-step solar thermochemical water-splitting cycle. In the first step (regeneration) the oxide is reduced by delivering some of its lattice oxygen; in the second step (water splitting) it is re-oxidized by taking oxygen from water and producing hydrogen. ASP is an attractive method for the synthesis of such systems since it combines the potential for nanoparticle synthesis, ease for precisely controlling doping and concentration of oxygen vacancies through the use of reducing reagents and/or atmosphere and very short synthesis times. The ASP experiments took place in an aerosol reactor consisting of a stainless steel tube, heated between 350-1070 oC. The residence time varied between 1 and 0.7 sec. A filter at the end of the reactor was used to collect the synthesized particles. Metal nitrates of Fe, Zn, Mn and Ni were used as precursor materials to produce ferrite powders, employing three kinds of aqueous solutions: ?plain?, glycine-additized and citric acid-additized, all with total metal ions concentration of 1M, atomized with the aid of an in-house air-assisted atomizer. Organic additions in the precursor solutions (in particular citric acid), by consuming some of the oxygen during synthesis, favor the formation of oxygen vacancies in the products, a fact rendering them more active water splitters. For such citric-acid-synthesized powders, the effect of metal dopant and stoichiometry on water conversion at 800 oC, was studied. Results show that a high percentage of Zn dopant favors water conversion; the latter having reached 81 % for the most active system tested so far, a value much higher than that of systems of the same composition synthesized through solid-state routes.