Yttria stabilised zirconia (YSZ) membranes and their applications

1 Abstract The development of ceramic hollow-fibre membranes has gradually grown in the past decade. This specific geometry which has a high surface area per unit volume can dramatically increase the efficiency of separation processes and can be adapted to a variety of industrial applications. In addition, ceramic membranes are well known for their superior chemical and thermal stability which allows them to operate at high temperatures and/or in chemically harsh environments. Nevertheless, the main challenge for their industrial application is their insufficient mechanical strength. Yttria-stabilized zirconia (YSZ) is selected as a membrane material in this study. This is because the material has superior mechanical strength and it is relatively cheaper than other ceramic materials. The ionic conducting property of YSZ material is also a benefit when it is used in electrochemical applications. Porous and dense YSZ hollow-fibre membranes have been developed in the study using a combined phase inversion and sintering process. Different membrane morphologies, surface properties, mechanical strength and porosity could be achieved by controlling the YSZ content and sintering temperature. The developed YSZ hollow-fibre membranes with porous or dense structures show great potential for a variety of applications. Porous YSZ hollow-fibre membranes can be used as membrane contactors in aqueous media or for fluid separations in harsh environments, which most polymeric membranes cannot withstand. For the application of membrane contactors in aqueous media, the nature of the YSZ membranes must be modified from hydrophilic to hydrophobic in order to keep them non-wetted during the aqueous contacting processes. A robust and hydrophobic YSZ hollow-fibre membrane was developed by introducing a pretreatment technique, followed by a grafting procedure. The hydrophobic YSZ membrane was found to be thermally stable up to 270 °C and chemically stable in hexane for 100 h. This membrane was then applied to the absorption of carbon dioxide from a high concentration aqueous ethanolamine solution. The results demonstrated the high efficiency of the ceramic hollow-fibre membrane contactor compared to traditional devices. Dense YSZ hollow-fibre membranes with outer diameters of 1.28 mm have been used as an electrolyte support in a solid oxide fuel cell. The YSZ electrolyte-supported SOFC was prepared at relatively lower sintering temperatures and shorter sintering durations. The YSZ-based hollow fibre SOFC demonstrated its ionic stability in a redox environment and mechanical stability at temperatures up to 800 °C. The results also demonstrated its electrochemical performance at high temperature. In summary, this thesis focuses on the development of YSZ hollow-fibre membranes from the initial step of fabricating the membrane to the final step of their potential application. Different structures of YSZ hollow-fibre membranes were studied, discussed and their potential performance was compared to the achievements of others in order to gain more understanding and information on the use of the membranes for practical applications