Investigation of Ca-doped Yttrium Iron Garnet for Solid Oxide Fuel Cells

Abstract

Solid oxide fuel cells (SOFCs) are among the next-generation electrochemical energy conversion devices with higher energy efficiency. A major drawback hindering the mass commercialization of SOFCs is the slow oxygen reduction reaction kinetics at the cathode when the operating temperature is lowered from a high temperature (~1000 °C) to an intermediate temperature (500–750 °C). While many perovskite materials have been considered for use in novel SOFC cathodes, there remains a number of issues, such as large thermal expansion coefficient of Ba0.5Sr0.5Co0.8Fe0.2O3-δ due to the presence of Co and chemical instability under humidity and CO2- containing atmosphere of various perovskites. Therefore, the search, characterization and optimization of new cathode materials is of vital importance to the development of SOFCs. In this work, garnet-type Y3-xCaxFe5O12-δ was investigated as a promising candidate for a novel SOFC cathode. Structural variations from Ca-substitution in the parent phase Y3Fe5O12 was studied using powder X-ray diffraction. Different charge compensation mechanisms because of aliovalent doping of Ca were studied and validated through iodometric titration and X-ray absorption spectroscopy. It was suggested that formation of oxide ion vacancy and electron-hole co-exist in Ca-doped garnet samples. The maximum electrical conductivity was found in the x = 0.1 garnet composition, whose ionic transport number was revealed to be about 1.82×10-5 at 750 °C. Symmetrical cells with garnet and La0.8Sr0.2Ga0.8Mg0.2O3-δ composite electrode were fabricated and evaluated for their electrochemical properties at 600–900 °C in air and at 750 °C in various oxygen partial pressures. At 750 °C, the lowest area-specific resistance in air was obtained by x = 0.3 garnet composition, with dissociation and partial reduction of adsorbed oxygen identified as rate-limiting steps.