In Situ Optical Studies of Carbon Accumulation with Different Molecular Weight Alkanes on Solid Oxide Fuel Cell Ni Anodes

The addition of steam to carbon containing fuels is a common method for minimizing carbon accumulation on anodes in solid oxide fuel cells (SOFCs).  The required steam to fuel ratio depends on fuel type where higher molecular weight hydrocarbons require higher steam concentrations.  The mechanism(s) responsible for carbon suppression with humidified fuels remain speculative, however, and motivate the need for direct observation of the surface chemistry on SOFC anodes during operation. Experiments presented in this work utilize in situ Raman spectroscopy coupled with voltammetry measurements to examine the extent of carbon accumulation on Ni-based anodes of electrolyte supported SOFCs exposed to dry and humidified fuels (methane, propane and n-hexane) at 700ᵒC.  Vibrational Raman spectra show that highly ordered graphite forms on the anodes exposed to methane and propane while disordered graphite forms during exposure to hexane.  The carbon structure was unaffected when 3% steam was added to the fuel and the amount of carbon accumulation was reduced for humidified methane (figure 1).  Electrochemical data showed rapidly diminishing performance and continued cell degradation for anodes exposed to propane and hexane compared to experiments with methane.  From these data, we quantify carbon induced degradation of SOFC anodes exposed to the various fuels.

Figure 1. Representative Raman spectra collected on SOFC anodes exposed to (a) dry and (b) humidified methane (red), propane (green) and n-hexane (blue) at 700ᵒC.  The G and 2D peaks at 1561 cm^-1 and 2695 cm^-1, respectively, are associated with the vibrational modes of highly ordered graphite.  The D peak at 1346 cm^-1 is associated with the vibrational mode of disordered graphite.  The Raman feature at 2915 cm^-1 corresponds to the C-H vibrational mode and is present for spectra recorded during fuel exposure with methane.  The spectrum recorded on a pristine anode that has been fully reduced with hydrogen (black) is shown as a reference.   Spectra have been off set on the y-axis for clarity.