Systematic Investigation of Cobalt-Doped Barium Cerate-Zirconates for High-Purity Hydrogen Production

The prospect of using transition metals as primary dopants in ABO3-type perovskite high-temperature protonic conductors has not been extensively studied in literature so far. This work represents a pioneering effort in that direction, with a systematic investigation of homogeneous Co-doped Ba(Ce,Zr)O3 (BCZC). The goal of this dissertation was to synthesize and characterize the novel class of electro-ceramics for simultaneous electrochemical (mixed protonic-electronic conductivity) and catalytic (CH3OH partial oxidation) activity for high-purity hydrogen production, with an aim towards understanding the influence of composition on micro-structural and functional properties. ^ A material belonging to the BCZC system was initially synthesized by oxalate co-precipitation and its phase composition, elemental composition, electrochemical properties, catalytic activity and stability investigated during the first phase of this dissertation. The synthesized material, homogeneous in powder form, was observed to be catalytically active towards hydrogen generation via CH3OH partial oxidation and displayed promising electrochemical properties in the presence of wet hydrogen. Results also indicated that the material developed an intricate micro-structure upon sintering that was prone to hydrolysis and reduction under wet hydrogen at elevated temperatures.^ A series of materials belonging to the BCZC system, with varying Co doping and A:B cation mol ratios, were then synthesized by solid state reaction. Phase composition, electrochemical properties and catalytic activities of the materials were analyzed with respect to both parameters. Results indicated that sustained exposure to wet reducing atmospheres mitigated electrochemical and catalytic properties of the materials and that tailoring the composition could provide an optimum between stability and functionality.^ This dissertation was driven by the hypothesis that the use of transition metals as primary dopants for ABO3-type perovskite protonic conductors would create a unique class of mixed-conducting electro-ceramics with added catalytic activity for achieving the objective of high-purity hydrogen production. Analysis of the BCZC system confirmed the hypothesis but material stability was observed to be a significant challenge in its implementation towards the overall objective. Based upon those findings, initial synthesis and electrochemical characterization of a different material belonging to the Fe-doped BaZrO 3 (BZF) system was carried out in order to establish successful resolution of material stability and to further explore the central hypothesis of this dissertation.^ Overall, the present work introduces novel, transition metal-doped ABO 3-type perovskite protonic conductors as potential bi-functional materials for simultaneous hydrogen generation and purification. The comprehensive structural and functional investigation of Co-doped Ba(Ce,Zr)O3 also provides a basis for analyzing and optimizing other such systems in future.