Carbon deposition thresholds on nickel-based solid oxide fuel cell anodes I. Fuel utilization

• A method for representing the full range of CH4 reformate gas mixtures is presented.
• CH4-rich gas mixtures caused more cell damage than equivalent CO-rich gas mixtures.
• Measured and computed fuel utilization (Uf) coking thresholds (32%) agree at 700 °C.
• Measured (36%) and computed (46%) Uf coking thresholds disagree at 600 °C.
• Cell temperatures correlate well with enthalpy of anode gas phase reaction.

In the first of a two part publication, the effect of fuel utilization (Uf) on carbon deposition rates in solid oxide fuel cell nickel-based anodes was studied. Representative 5-component CH4 reformate compositions (CH4, H2, CO, H2O, & CO2) were selected graphically by plotting the solutions to a system of mass-balance constraint equations. The centroid of the solution space was chosen to represent a typical anode gas mixture for each nominal Uf value. Selected 5-component and 3-component gas mixtures were then delivered to anode-supported cells for 10 h, followed by determination of the resulting deposited carbon mass. The empirical carbon deposition thresholds were affected by atomic carbon (C), hydrogen (H), and oxygen (O) fractions of the delivered gas mixtures and temperature. It was also found that CH4-rich gas mixtures caused irreversible damage, whereas atomically equivalent CO-rich compositions did not. The coking threshold predicted by thermodynamic equilibrium calculations employing graphite for the solid carbon phase agreed well with empirical thresholds at 700 °C (Uf ≈ 32%); however, at 600 °C, poor agreement was observed with the empirical threshold of ∼36%. Finally, cell operating temperatures correlated well with the difference in enthalpy between the supplied anode gas mixtures and their resulting thermodynamic equilibrium gas mixtures.