Lifetime and Performance Prediction of SOFC Anodes Operated with Trace Amounts of Hydrogen Sulfide

Chemical degradation of Solid Oxide Fuel Cell (SOFC) anodes can be caused by various types of impurities present in practical fuels, as e.g. sulfur, chlorine, phosphorus and siloxane. To allow for a deeper understanding of the processes leading to sulfur poisoning, this study presents a modeling work of SOFC operating on H₂/H₂O and CH₄/H₂/H₂O gas mixtures with different hydrogen sulfide (H₂S) concentrations. In order to interpret experimental measurements, an elementary kinetic model is developed comprising a detailed multi‐step reaction mechanism of sulfur formation and oxidation at Ni/YSZ anodes coupled with channel gas-flow, porous-media transport and elementary charge-transfer chemistry.

A thermodynamic and kinetic data set of sulfur formation and oxidation is derived based upon various literature sources including a coverage-dependent description of the enthalpy of surface-adsorbed sulfur. Firstly, the developed model is validated against literature-based sulfur chemisorption isobars, and subsequently against electrochemical button-cell experiments displaying a significant influence of operation temperature and applied potential on cell performance and degradation.

It is shown that sulfur surface coverage increases with increasing current density indicating a low sulfur oxidation rate. In order to gain for an advanced fundamental understanding of sulfur poisoning, sensitivity analyses towards total anode resistance and sulfur coverage for different operating conditions will be presented.

Furthermore, the identified elementary sulfur poisoning reactions are used to extend an existing reaction mechanism for methane steam reforming which is then validated based upon a variety of electrochemical experiments. It is shown that atomically adsorbed sulfur significantly influences heterogeneous reforming chemistry, causing a substantial decrease in OCV. Under polarization, at constant current densities the cell voltage decreases in a non-linear way. After the removal of hydrogen sulfide from the feed gas the cell shows a faster recovery than in H₂/H₂O mixtures. In addition, numerical impedance simulations over a wide range of operating conditions were performed, which allows a physically-based assignment of observed gas concentration, heterogeneous chemistry and electrochemical processes.