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Abstract
In the present work, thermochemical water splitting with siliconized silicon carbide (SiSiC) honeycombs coated with a zinc ferrite redox material was investigated. The small scale coated monoliths were tested in a laboratory test-rig and characterized by X-ray diffractometry (XRD) and Scanning Electron Microscopy (SEM) with corresponding micro analysis after testing in order to characterize the changes in morphology and composition. Comparison of several treated monoliths revealed the formation of various reaction products such as SiO2, zircon (ZrSiO4), iron silicide (FeSi) and hercynite (FeAl2O4) indicating the occurrence of various side reactions between the different phases of the coating as well as between the coating and the SiSiC substrate. The investigations showed that the ferrite is mainly reduced through reaction with silicon (Si), which is present in the SiSiC matrix, and silicon carbide (SiC). These results led to the formulation of a new redox mechanism for this system in which Zn-ferrite is reduced through Si forming silicon dioxide (SiO2) and through SiC forming SiO2 and carbon monoxide. A decline of hydrogen production within the first 20 cycles is suggested to be due to the growth of a silicon dioxide and zircon layer which acts as a diffusion barrier for the reacting specie.
Highlights
The production of hydrogen without deploying fossil resources is one of the main challenges that have to be overcome for building a future hydrogen economy
Monolith 3 and 4 parametric studies concerning the effect of water splitting temperature on the hydrogen production rate were performed
The fact that no oxygen was detected in the off-gas of the reactor during the regeneration step must be due to the fact that the ferrite is reduced through reaction with Si and silicon carbide (SiC), forming SiO2
Summary
The production of hydrogen without deploying fossil resources is one of the main challenges that have to be overcome for building a future hydrogen economy. Thermochemical cycles split water in several steps and enable hydrogen generation at moderate temperatures, which are manageable by today’s technical equipment Another major advantage is that hydrogen and oxygen are produced in separate steps, i.e., no separation of hydrogen and oxygen is needed. Comparative studies that have been published showed that thermochemical cycles have the potential of a higher efficiency than electrolysis and the potential to reduce the production costs of hydrogen from water [4,5,6]. Concentrated solar irradiation and on the other hand provide the necessary surface area for the chemical reaction The advantages of such a concept are its simplicity, as it has no moving parts, and its scalability. First studies with coated SiSiC monoliths were presented in a previous work showing the influence of water splitting temperature and water concentration on the hydrogen production rate [34]. In the present work, coated SiSiC honeycomb structures with a zinc ferrite redox material were analyzed with different material characterization techniques after they were cycled in a laboratory test set-up in order to resolve any interactions between the substrate and the coating
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