The Fabrication of Direct Oxidation Solid Oxide Fuel Cell Anodes Using Atmospheric Plasma Spraying

Solid oxide fuel cells (SOFCs) that operate directly on hydrocarbon fuels eliminate the requirement for costly and complex external reforming systems. Atmospheric plasma spraying (APS) is an established manufacturing method that offers the potential to fabricate direct oxidation SOFC anodes in a single step, instead of the multiple steps currently required. Manufacturing by APS also allows the use of metal supports, which improve thermal shock resistance, allow rapid cell heat-up, and reduce total cost. In this study, direct oxidation SOFC anodes based on Cu and samaria-doped ceria (SDC) in combination with Co and/or Ni were investigated for their stability and performance in H2 and CH4 when plasma sprayed on ferritic stainless steel supports. Several different APS techniques were investigated. Two of these techniques were hybrid methods involving a combination of dry powder plasma spray and suspension plasma spray (SPS) processes. These techniques were proposed to help balance the degree of melting of the lower melting temperature oxides of the metals Cu, Co, and Ni with that of the higher melting temperature SDC. The use of a single suspension containing all of the anode component feedstocks was also investigated. Multi-component aqueous suspensions of CuO, Co3O4, and NiO were developed with or without the addition of carbon black and SDC. It was found that the use of a hybrid plasma spray technique can help to improve deposition efficiency (D.E.) and enhance partial melting of the low melting temperature feedstocks. However, plasma spraying all of the components in a single suspension can lead to more homogeneous mixing and greater resistance to metal coarsening at SOFC operating temperatures. In electrochemical tests of plasma-sprayed metal-supported cells containing these anodes, peak power densities as high as 0.6 W/cm2 were achieved at 750 deg C in humidified H2. In CH4, power density was limited by the activity of the anodes. Stability in CH4 was poor because of oxidation of the metal support and enhanced coking behaviour resulting from interactions between Fe in the support and Co and Ni in the anodes. When separated from the supports, the anodes demonstrated very low coking rates in thermogravimetric analysis experiments in CH4.