Effect of H2S on the Thermodynamic Stability and Electrochemical Performance of Ni Cermet-type of Anodes for Solid Oxide Fuel Cells

For SOFCs to be main means of power generation, they should be able to exploit wide variety of fuels. Among Ni-cermets, Ni-YSZ is the state-of-the-art materials for the anode in SOFC which is the fuel electrode. But sulphur impurity present in different gaseous fuels (e.g Biogas), depending on its concentration, is highly poisonous to the stability and electrochemical performance of the Ni catalyst in the cermet anodes. Thus in this study the microstructural stability of Ni-YSZ, Ni-CGO and Ni-LSGM cermets in H2S-containing hydrogen gas is studied in the intermediate temperature range of SOFC operation. Thermodynamic modelling of Ni-S-O-H quaternary system was performed for the calculation of the thermodynamic stability and sulphur-tolerance limit of Ni in the gaseous atmosphere made up of H, O and S. The effect of the presence of H2S in fuel gas, in the concentrations well below the thermodynamic tolerance limit, on the electrochemical performance of the anodes is studied by using model Ni-patterned electrodes on YSZ and LSGM. Ni-YSZ and Ni-CGO cermets were studied in H2 + H2S (1 vol%) gas in the intermediate temperature range (500-750°C). At temperatures above 901 K (calculated eutectic temperature), Ni upon its reaction with sulphur forms a eutectic melt as given on the Ni-S binary phase diagram where as at 773 K, it forms Ni3S2. YSZ in the cermets was found to be stable against H2 + H2S (1 vol%) gas in the temperature range of the study. CGO also remained stable at 500°C but was found to react with H2S to form (Ce,Gd)2O2S at 750°C. Ni-LSGM cermets were studied as a novel anode material for SOFCs. It is known from the literature that NiO and LSGM react with each other during the fabrication of the fuel cell. In this study a LaNiO3-type phase was found to form during sintering in air and in inert atmosphere. The TEM investigation of NiO-LSGM interface after sintering in air at 1350°C has revealed that the LaNiO3-type phase formed was similar to the cubic LSGM phase with a relatively high amount of Ni. During the sintering interdiffusion across the NiO-LSGM led to incorporation of Ni into LSGM. The compositional analysis across the NiO-LSGM interface revealed the interface chemistry to be Ni(Mg,Ga)O – (La,Sr)(Ga,Mg,Ni)O3-x. A further reactivity of NiO/Ni with LSGM during the reduction of the sintered NiO-LSGM samples in H2 was observed. The LaNiO3-type phase (i.e LSGM(Ni)) decomposed into La2O3 and SrLaGaO4 accompanied by diffusion of excess amount Ga into FCC-Nickel. So after