Direct Carbon Fuel Cells – Wetting behavior of graphitic carbon in molten carbonate

Abstract

The wetting behavior of graphitic carbon in a eutectic melt of LiK molten carbonate was investigated at two different temperatures (650 and 750 °C) under two different CO2 partial pressures. The development of the meniscus and the formation of a super-meniscus film were followed optically over a period of 24 h after partial immersion of the graphite rods. Post-test EDX analyses of the graphite surfaces exposed to the melt, above and below the meniscus, were compared to those of surfaces not exposed to the melt. The most important feature found in the EDX analyses is the non-uniform atomic distribution of oxygen both along the length of the immersed surface and in the radial cross-section of the rods exposed to the melt. In addition, the effect of pre-test exposure to gas atmospheres at various temperatures and CO2 partial pressures was investigated. The wetting behavior is found to be influenced strongly by reactions occurring at the carbon surface before and after the start of wetting. In particular, CO produced by the reverse Boudouard reaction, which appears to be physisorbed, and chemically adsorbed oxide ion, O2−, are inferred to play a dominant role in the development of the meniscus leading to a super-meniscus film on the graphitic surface. Several reaction mechanisms are examined to explain the results observed in this work, which agree with results of earlier work from this laboratory, in terms of CO and oxide ion formation.