Elementary Kinetic Numerical Simulation of Ni/YSZ SOFC Anode Performance Considering Sulfur Poisoning

Abstract

An elementary kinetic model is developed and applied to explore the influence of sulfur poisoning on the behavior of solid oxide fuel cell (SOFC) anodes. A detailed multi-step reaction mechanism of sulfur formation and oxidation at Ni/YSZ anodes together with channel gas-flow, porous-media transport and elementary charge-transfer chemistry is established for SOFCs operating on H2/H2O mixtures with trace amounts of hydrogen sulfide (H2S). A thermodynamic and kinetic data set is compiled from various literature sources. The derived chemical model, validated against sulfur chemisorption isobars taken from literature, is used to analyze performance drops of SOFCs working under typical fuel cell operating conditions. Electrochemical results show that at relatively low H2S concentrations SOFC button-cell performance can be interpreted using chemical sulfur formation. However, when the concentration is sufficiently high, the inclusion of second stage degradation and triple-phase boundary reconstruction is necessary to describe the performance decrease. Additionally, it is shown that the sulfur surface coverage increases with increasing current density. In order to shed more light on advanced fundamental understanding of cell poisoning, sensitive analyses toward total anode resistance and sulfur coverage for different operating conditions were performed.