The Influence of Sulfur Poisoning and YSZ Substrate Orientation on Electrochemical Properties of Nickel Pattern Anodes

In state of the art solid oxide fuel cells (SOFCs) the anode is usually a ceramic-metal composite consisting of a network of Ni and yttria-stabilized zirconia (YSZ) grains as well as pores allowing for gas transport. This approach, while yielding excellent performance, is rather impractical for kinetic studies of the electrochemical triple phase boundary kinetics of Ni/YSZ since ionic and gaseous transport may also influence the electrode polarization resistance. These interferences make extraction of electrochemical elementary steps as well as mechanistic studies of degradation phenomena (e.g. sulfur poisoning) highly challenging. To circumvent these issues sputter deposited thin film electrodes with well-defined structures can be utilized.

In this study, pattern Ni anodes were investigated in H₂/H₂O atmosphere by means of electrochemical impedance spectroscopy (EIS). A sample pretreatment method was established to obtain stable values of the polarization resistance (R_P) and interface capacitance (C) – i.e. avoiding degradation with time – to allow comparison of different microelectrodes. The effect of different YSZ substrate orientations on R_pand C is statistically evaluated and the correlation between the two properties is discussed.

Based on this knowledge the influence of H₂S on R_p and C was investigated. The poisoning influence of sulfur on R_P as well as the regeneration behavior was found to be qualitatively consistent with cermet electrodes described in the literature. Interestingly, also the electrode capacitance was affected by H₂S: it decreased under sulfur poisoning conditions and regenerated partially after removal. Combined with dependencies of R_p and C on temperature, bias voltage and geometry these findings allow valuable insights into the electrochemical properties of Ni electrodes and their degradation.