Cr-tolerance of the IT-SOFC La(Ni,Fe)O3 material

This thesis deals with a study on the Cr-tolerance of the LaNi0.6Fe0.4O3 (LNF) material. LNF is being considered for use as a current collecting layer, an interconnect protective coating and/or an electrochemically active solid oxide fuel cell (SOFC) cathode layer in an intermediate temperature IT-SOFC stack. The desired cost-effectiveness of the IT-SOFC systems can be achieved by using relatively cheap interconnect materials such as chromia forming ferritic stainless steels. However, the use of such interconnects triggers Cr-poisoning of the state-of-the-art SOFC cathodes, hence new Cr-tolerant materials are needed. The LNF material is considered as a promising candidate. Favorable properties of LNF, including high electronic conductivity, matched thermal expansion coefficient and claimed high Cr-resistance have encouraged studies described herein. The research aim is to understand the chemical stability of the LNF in the presence of Cr-species, which ultimately results in a thorough understanding of the degradation mechanisms of the LNF cathode under Cr-poisoning conditions. A final goal is to recommend feasible solutions to the Cr-poisoning issue. To meet these goals, firstly a solid-state reactivity between LNF and chromia is investigated in Chapter 2. Secondly, the influence of volatile Cr-species on the electrical properties of LNF is studied in Chapter 3. Thirdly, the impact of Cr-poisoning on the conductivity of different LNF microstructures is described in Chapter 4. Cr-poisoning of a LNF cathode under current load is discussed in Chapter 5. Finally, recommendations for a feasible application of the LNF material in IT-SOFC systems are given in Chapter 6. It can be concluded that Cr-poisoning may not be fully avoidable in the IT-SOFC systems utilizing metallic interconnects. However, a synergetic approach may be undertaken to reduce the Cr-poisoning impact: From an operational perspective - the presence of volatile Cr-species should be minimized by applying interconnect protective coatings and providing dry air at high flow velocities. From an LNF microstructural perspective - the microstructure should be adjusted according to the application: a coarse microstructure is preferred for non-electrochemical use (as a current collecting layer and/or an interconnect protective coating), and a fine microstructure is preferred for an electrochemically active cathode layer.