Modeling Sulfur Poisoning of Ni-Based Anodes in Solid Oxide Fuel Cells

Abstract

H2S even at very low concentrations can poison Ni based SOFC anodes. This paper presents the modeling of sulfur poisoning in Ni based anodes at low H2S concentrations (1 ppm - 8 ppm). A kinetic model for predicting the fractional coverage of sulfur is developed by considering elementary step chemical reactions. The rate expression for sulfur coverage is developed by assuming the dissociation of surface adsorbed SH as the rate limiting step. It is assumed that the three phase boundary length decreases linearly with increase in fractional coverage of sulfur. The kinetic model is incorporated in to a button cell model to simulate experimentally observed impedance response. The exchange current density parameters are evaluated by calibrating the model predictions with experimentally observed dc polarization data at different temperatures. The same exchange current density parameters are used for impedance simulations as well. The impedance spectra is simulated by using a Randles circuit. The charge transfer resistances required for the Randles circuit are calculated from numerical simulation of button cell. Excellent agreement is observed between the model predictions and experimental observations.