Perovskite microtubular membranes for pure hydrogen production from water splitting

Abstract

The purpose of this thesis is to investigate the feasibility of producing hydrogen with microtubular membranes made of mixed ionic electronic conducting perovskite by membrane-based steam reforming. This process involves water splitting in one side of the membrane followed by oxygen ions transport across the membrane to react with methane in the membrane reaction side. The overall process produces two separate streams of pure hydrogen and syngas. Initial experiments were performed using temperature programmed redox (water oxidation and methane reduction) cycles to investigate the feasibility of three perovskites (Ba0.5Sr0.5Co0.8Fe0.2O3-δ, La0.6Sr0.4Co0.2Fe0.8O3-δ and La0.7Sr0.3FeO3-δ) to produce hydrogen from water splitting. Membrane fragments and powder materials were used during these tests, resulting in La0.6Sr0.4Co0.2Fe0.8O3-δ and La0.7Sr0.3FeO3-δ powder materials showing better activity for hydrogen production than Ba0.5Sr0.5Co0.8Fe0.2O3-δ. However La0.6Sr0.4Co0.2Fe0.8O3-δ presented better performance among all membrane fragments tested under the experiments conditions. Preliminary oxygen permeation and hydrogen production experiments using membranes systems were also carried out with all perovskites; Ba0.5Sr0.5Co0.8Fe0.2O3-δ microtubes presented high oxygen permeation, however low activity for hydrogen production from water splitting. La0.7Sr0.3FeO3-δ microtubes presented low oxygen permeation rates and no activity for hydrogen production, post-operation analysis showed the presence of a strontium/sulfur layer on the microtubes surfaces which may have affected permeation. La0.6Sr0.4Co0.2Fe0.8O3-δ microtubes presented better potential for oxygen permeation and hydrogen production among the other membranes; hence these microtubes were selected for further long term experiments. La0.6Sr0.4Co0.2Fe0.8O3-δ microtubular membrane reactors were tested for long term oxygen permeation followed by membrane-based steam reforming. The membranes were subjected to two known axial temperature profiles in the temperature of 900°C and 960°C. The microtubes showed good stability under reaction conditions, operating over a total operation period of ca 400 hours of oxygen permeation followed by ca 400 hours of steam reforming. The outlet gas composition from both sides (methane and water side) of the membranes were analysed which allowed material a balance. This indicated that the hydrogen production occurred due to oxygen flux across the membrane and not ii just surface reaction. Post-operation analysis of the microtubes revealed the presence of a strontium-enriched dense layer on the water-exposed membrane surface and of crystallites enriched with cobalt and sulfur on the methane feed side surface.