Preliminary data on subsolidus phase equilibria in the La2O3-(Al2O3/Fe2O3)-Y2O3 and La2O3-(AI2O3/Fe2O3)-ZrO2 systems

Abstract

A fuel cell is a device for direct conversion of chemical energy into electrical energy. Oxidant is fed to the cathode and reducent (fuel) to the anode. The electrolyte, through which the ion current flows, also prevents the mixing of oxidant and fuel. The principle of fuel cell operation was reported in 1839 by Sir William Grove [1]. For a description of fuel cell development see, for example, [2]. High temperature solid oxide fuel cells (SOFC) work at temperatures of up to 1000 °C. Due to the high operating temperatures of SOFC, the choice of materials is mainly limited to ceramics. A comprehensive review of materials for SOFC was published by Minh [3]. The solid electrolyte in SOFC cells is usually yttria stabilized cubic zirconia (YSZ). The oxygen accepts electrons at the cathode and moves as an ion through the dense Z r Q ceramic. At the anode ions combine with fuel and release electrons. The fuel is either hydrogen, a H2/CO mixture, or hydrocarbons because the high temperature of operation makes possible the internal (in situ) reforming of hydrocarbons with water vapour [4]. The advantage of SOFC is their high efficiency, of 50-60%, while some estimations are even up to a yield of 70-80% [5-8]. Also, nitrous oxides are not produced and the amount of CO2 released per kilowatt hour is, due to the high yield, around 50% less than for power sources based on combustion. Annually, around 109 US$ is invested in research and development of fuel cells, and this amount is growing, mainly due to environmental concerns [9-111. Due to the high operating temperatures and oxidizing atmosphere on the cathode side, only noble metals (platinum, for example) or oxides with low resistivity can be used for electrodes. Noble metals are unsuitable, partly because of problems with long-term stability, but mostly because of their high cost. At present semiconducting oxides (perovskites) based on doped LaMnO3 or LaCoO 3 are most often used as cathode materials. The cathode must be porous to permit the diffusion of oxygen to the zirconia electrolyte, and must have high electrical conductivity. The electrode polarization losses, related to irreversibilities in the electrochemical processes, are reduced if the electrode material possesses both ionic and electronic conductivity. If the material is an electronic conductor only, the electrochemical reactions can occur solely at the three-phase boundary of the