Edinburgh Napier University

  ABSTRACT
Solid oxide fuel cells (SOFCs) are of major interest in fuel cell development due to their high energy conversion efficiency, wide range of fuels and environmental friendliness. One important obstacle for their industrial development is their processing difficulties. These difficulties have recently been addressed by employing a novel technique namely electroless nickel - yttria-stabilised zirconia (YSZ) co-deposition which eliminates multi-layer processing and high temperature sintering.
The novel work carried out in this research programme investigates the effects of different processing parameters on the co-deposited anodes for SOFCs. In particular, YSZ particle size, electroless bath agitation method, electroless bath pH and substrate surface condition are investigated. These variables were investigated for their effect on (i) the ceramic to metal ratio – important in terms of matching the coefficient of thermal expansion of the anode and substrate, as well as providing electronic conductivity, and (ii) the porosity content in the deposited layers – required for fuel and exit gas penetration through the anode.
The experimental work was based on a full factorial Design of Experiment (DoE) approach and consisted of three phases – namely, designing, running and analysing. A 16 run 24 full factorial DoE with five replications was constructed with YSZ particle sizes of 2 and 10 µm; bath agitation of air bubbling and mechanical stirring; bath pH of 4.9 and 5.4; and substrate surface treatment of hydrofluoric acid etching and mechanical blasting. A total of 80 samples were analysed for nickel content by energy dispersive X-ray analysis and porosity content by Archimedes buoyancy measurement. The DoE was analysed by the ANOVA statistical tool in Minitab 15 software.
The co-deposition conditions that produced anodes with (i) the lowest volume percentage of nickel and (ii) the highest level of porosity were determined. Linear regression models for both nickel to YSZ content and porosity responses were built to estimate the correlation between experimental and predicted data. The coefficient of determination, R2 for nickel to YSZ content indicated a reasonable correlation between experimental and predicted values while the regression model for porosity response was less reliable.
One anode containing 50 vol.% nickel recorded an electronic conductivity at 400oC in air that is comparable to the published data. Another series of tests at higher temperatures (up to 800oC) in air and nitrogen resulted in encouraging electronic conductivities being recorded.