Abstract
The Rolls-Royce Integrated Planar Solid Oxide Fuel Cell (IP-SOFC) features anode, electrolyte and cathode layers of 5-20 pm in thickness, connected in series by highly-conductive precious-metal based current collecting layers of ~10 mum thickness, which are screen-printed and sintered upon a porous substrate. Replacement of the palladium- based cathode current collector material is desirable for increasing the economic and environmental potential of the IP-SOFC system, due to low resource availability, high cost and environmental degradation caused by mining of platinum-group metals. The electrically conductive ABO3 perovskite-structured lanthanum transition-metal oxide ceramics were identified as potential cathode current collector materials, and lanthanum nickel ferrite materials, of compositions LaNi0.6Fe0.4O3 and LaNi0.5Fe0.5O3, were selected for investigation based on the combined favourable properties of electrical conductivity, phase stability and compatible coefficient of thermal expansion with other cell materials. Both compositions were found to be reactive towards the IP-SOFC cathode materials, lanthanum strontium manganite (LSM) and yttria-stabilised zirconia (YSZ), and the lower conductivity of LaNi0.5Fe0.5O3 compared with LaNi0.6Fe0.4O3 meant a thicker layer would be required to meet the conductivity requirements, which negates the advantage of its more suitable coefficient of thermal expansion. It was found that a LaNi0.6Fe0.4O3 layer of ~80 mum was adequate to meet the conductivity target, and could be applied by a single stencil-print and sintered at 1125°C, which is compatible with the screen-printing and firing production line, although the manufacturing method requires optimisation to eliminate layer defects. In addition it is believed that the material can offer significant economic and environmental advantages over the present palladium-based cathode current collector. However the reaction of LaNi0.6Fe0.4O3 (LNF) with LSI'I was found to critically compromise its use in conjunction with an LSM-based cathode. Efforts to incorporate an LNF-based cathode also failed due to reaction of LNF with YSZ and gadolinium-doped ceria (CGO), and it must be concluded that the reactivity of LNF with common solid oxide fuel cell materials severely limits its potential to be used as a cathode current collector layer in the IP-SOFC.