Redox dynamics of sulphur with Ni/GDC anode during SOFC operation at mid- and low-range temperatures: An operando S K-edge XANES study

Abstract

Sulphur poisoning of nickel–based solid oxide fuel cell (SOFC) anode catalysts is a well-documented shortcoming, but not yet fully understood. Here, a novel experiment is demonstrated to obtain spectroscopic information at operando conditions, in particular the molecular structure of sulphur species in the sulphur K-shell X-ray absorption near edge structure (XANES) region for a SOFC anode under realistic operando conditions, thus, with the flux of O2− from cathode to anode. Cooling from T = 550 °C stepwise down to 250 °C, 5 ppm H2S/H2 reacting with Ni-gadolinium doped ceria (GDC) anode resulted in several sulphur species in different oxidation states (6+, 4+, 0, −2) and in amounts being at a minimum at high temperature. According to sulphur speciation analysis, the species could either relate to –SO42− or SO3 (g), –SO32− or SO2 (g), S2 (g) or surface-adsorbed S atoms, and, Ni or Ce sulphides, respectively. The coexistence of different sulphur oxidation states as a function of temperature was analysed in the context of thermodynamic equilibrium calculations. Deviations between experimental results and calculations are most likely due to limitations in the speed of some intermediate oxidation steps as well as due to differences between stoichiometric CeO2 used in calculations and partially reduced Ce0.9Gd0.1O2−δ.