Solid Oxide Fuel Cells


A solid oxide fuel cell (SOFC) is constructed as a multilayered ceramic material, comprising a ceramic cathode and anode with a solid oxide electrolyte used to conduct negative oxygen ions between the two.

The electrolyte material has a significant influence on the overall performance of the SOFC; it is constructed of a dense layer of ceramic that conducts oxygen ions but maintains a low electronic conductivity to prevent losses from leakage currents. High operating temperatures are required to enable the kinetics of oxygen ion transport to be sufficiently high for good cell performance. As with other SOFC materials, the electrolyte must be chemically, thermally and structurally stable across a wide temperature range, hence yttria stabilised zirconia has emerged as one of the most suitable materials for this application.

Yttria loadings of 8-10 mol% provide the dual benefit of stabilising zirconia into a cubic phase at high temperatures and providing the necessary oxygen vacancies for ionic conductivity, at a rate of one vacancy per mole of dopant. The high density of cubic yttria zirconia is also important to physically separate the gaseous fuel from oxygen; without this separation, the electrochemical system would produce no electrical power. The disadvantage of yttria zirconia systems is their ionic conductivity decreases significantly over time when working at high operating temperatures. Nanostructured thin-film electrolytes in anode-supported SOFC cells reportedly show significantly lower ohmic losses, facilitating lower operating temperatures and, therefore, longer term fuel cell stability.

One area of current research into maintaining operating performance at lower temperatures (ca. 500ºC) is in thin film deposition, which has been shown to provide the necessary control of the nano-structure of the fine electrolyte grains to achieve a ‘fine-tuning’ of electrical properties and grain structures that are less resistive to ionic transport. Ultra-fine grained ceramics from consolidated nano-structured powders has generated considerable interest due to the impact of the increased number of grain boundaries on the total conductivity of the 8YSZ. Nano-crystalline yttria zirconia exhibits better mechanical, electrical and thermal properties with enhanced ionic conduction attributed to high defect densities and increased oxygen vacancy density at those grain boundaries. The specific grain boundary ionic conductivity of nano-crystalline yttria stabilised zirconia has been reported as 2 orders of magnitude higher than that comprising micrograin yttria stabilised zirconia.

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