(667d) Active and Flowable Doped-Hercynite Materials for Solarthermal Redox Processing to Split Water
Solar thermal water splitting (STWS) has the potential to efficiently produce H2 from only sunlight and water. Although an optimal redox material to drive this process has not yet been identified, substantial progress has been made in developing a doped-hercynite material (CoFe2O4:Al2O3) which can produce substantial H2 at moderate processing temperatures. Flowable particle have been synthesized by spray drying and characterized for redox water splitting using a stagnation flow reactor. These unique materials have demonstrated high H2 production capacity at near-isothermal conditions, deviating substantially from competing materials which operate using a temperature-swing scenario where the difference between thermal reduction and steam oxidation temperatures is as high as 800 C. Efforts have been made to better understand the chemical mechanism for this reaction. Our theoretical results predict that these materials operate via an O-vacancy mechanism rather than a stoichiometric mechanism. High temperature X-ray diffraction experiments during redox were carried out and show that throughout the redox cycle, aluminate phases are present and that CoFe2O4 is not present. This confirms computational predictions that the doped-hercynite cycle operates via an O-vacancy mechanism. SEM/EDS analysis of bulk material that has undergone hundreds of redox cycles indicates that no segregation of the Fe, Co, and Al cations occurs between the reduced and oxidized states, providing further evidence that the O-vacancy mechanism is active.