Chromium Poisoning : The Needle in the SOFC Stack

This thesis focuses on Cr-poisoning in solid oxide fuel cells (SOFC), which currently presents a key challenge for the development of this technology. By the implementation of dedicated experimental tools, this work offers a new access to, and comprehension of, electrochemical performance degradation caused by Cr contamination accumulation. An experimental setup for the in situ assessment of Cr vapor concentrations within the hot air flux of an SOFC system inlet gives direct proof, and a measure of severity, of the Cr contamination issue. An energy-dispersive X-ray spectroscopy (EDX) based Cr quantification methodology leads this work to objective and therefore comparable data from post-test observations performed by scanning electron microscopy (SEM).Moreover, the developed methods are time-efficient. To understand Cr-poisoning, in particular the deposition mechanisms of Cr gVI to CrsIII , electronic conducting cathode materials such as (La,Sr)MnO3 as well as mixed ionic electronic conductors (MIEC) such as (La,Sr)CoO3, (La,Sr)(Co,Fe)O3, (La,Sr)MnO3-(Y,Zr)O2 and Nd1.95NiO4+δ are investigated during and after medium- to long-term electrochemical testing involving deliberate exposure to Cr contamination in button cell and stack test arrangements. The deposition rate of both chemically-driven and electrochemically-driven cathode mechanisms related to Cr-poisoning depends on the material, its operating conditions as well as on superimposed degradation phenomena, such as sulfur-poisoning. Investigation and subsequent results at the SOFC stack level combine the different aspects of Cr contamination encountered within this work. The severity of Cr-poisoning of a cathode, depending on the electrode overpotential, guides the development of less-sensitive materials towards high performing cathodes, in particular MIEC electrode materials at lower temperature. The crucial role of electrode proximal layers within the cathodic half-cell, such as current collectors and protective coatings of metallic interconnects (MIC) is found to be adequately dispatched by present SOFC technology. Their role is to lower the concentration of Cr vapor species reaching electrochemically active cathode regions both by diffusion resistance and reactive trapping. In contrary, sealing materials do not achieve satisfactory tightness to hydrogen diffusion into the cathode compartment, causing aggravated Cr-poisoning by local steam generation and hence increased Cr-evaporation. As the protection against Cr evaporation of the entire balance-of-plant (BoP) of a real SOFC system, or its construction with non-emitting components regarding Cr evaporation, is not practicable, a Cr-getter-based air filter, developed within this work and validated in situ, offers a suitable solution for BoP-caused Cr pollution.