High Fuel Utilization Operation of Solid Oxide Fuel Cells - a Modeling Study

High fuel utilization (U_f) is desirable in fuel cells as it has a direct impact on efficiency – the higher the U_f  the higher the fuel cell efficiency. However, the Ni phase in Ni based SOFC anodes is susceptible to oxidation to NiO when the fuel cell is operated at high fuel utilizations. This phase transition is problematic because a) NiO is inactive to the fuel oxidation reactions, and b) the larger specific volume of NiO results in damage to the anode microstructure if this Ni to NiO transition is reversed and repeated a few times.  The second problem listed above is known as Ni redox cycling. Thus, there are conflicting objectives where the designer/operator of a SOFC system desires a high fuel utilization U_f  while avoiding Ni oxidation.

Ideally, one would like to identify a safe operating zone where the fuel utilization U_f  can be maintained in a high enough range without risking Ni oxidation. In this presentation we will use simulation results from a detailed multiphysics model to outline some of the challenges of operating a SOFC at high fuel utilization and attempt to identify the above mentioned “safe operating zone”.

The multiphysics model used in this work is for a planar anode supported geometry and includes a fully coupled description of the flow and mass transfer in the air/fuel channels and the electrodes, and the current/potential distribution in the electrodes. One of the mechanisms for Ni oxidation is through reaction (1).

Ni + H₂O ↔ NiO + H₂                                                 (1)

The favourability of reaction (1) at a particular location in the anode is dictated by the thermodynamics of the reaction and the local fuel composition. As the model is able to predict the fuel side composition as a function of position, the output of the model can be used to predict whether there is a risk of Ni oxidation in the anode at any given set of operating conditions.

We will present results from a parametric study over a wide range of different temperatures, cell voltage, as well as inlet fuel flow-rates and composition. An example set of results is given in Fig 1 where the inlet flow-rate is varied for a given cell voltage and temperature. The Ni oxidation reaction is thermodynamically favourable in the latter section of the anode where the partial pressure ratio P_H2O/P_H2 exceeds the threshold value given by the thermodynamics of the reaction and shown as a dot-dash line in the bottom plot. According to Fig 1, the threshold U_f  for these conditions is ~99%. This is a much higher value than the typical 80-85% used in most SOFC designs. Another noteworthy point is that at this high fuel utlization, the last 1/3 of the cell is producing a very small fraction of the total current.