Investigating Point Defect Concentrations and Their Impact on Surface Exchange Reaction Rates of Mixed Ionic and Electronic Conductors

Abstract

The oxygen exchange reaction (OER) is an important reaction for various applications in energy- and environment-related technologies. For example, during operation of low and intermediate temperature solid oxide fuel cells (SOFCs), the kinetics of this fundamental reaction is often regarded as the bottleneck for the overall performance [1]. To tackle this issue, research and development of new SOFC cathode materials are focused on high intrinsic catalytic activity and high degradation stability. For promising cathode materials, this degradation is commonly attributed to poisoning effects due to different elements in the environment like S, Si or Cr, as well as to cation segregation, for example Sr segregation for perovskites like La0.6Sr0.4CoO3-δ (LSC) and SrTi0.3Fe0.7O3-δ (STF). In this contribution, we report the results of experiments on truly pristine surfaces of different SOFC cathode materials, performed with a novel experimental technique called “in-situ impedance spectroscopy during pulsed laser deposition” (i-PLD) [2]. These measurements revealed an astonishing catalytic performance of all materials, with polarization resistances being up to 2 orders of magnitude lower than previously observed in conventional ex-situ setups. The reason behind this performance difference was found in Sulphur adsorbates which accumulate very quickly on cathode surfaces due to trace amounts of Sulphur compounds in nominally pure measurement gases (~0.5 ppmv). Impedance spectroscopy during ambient pressure XPS measurements could unambiguously correlate increasing Sulphur adsorbate coverage with an increase of the polarization resistance. Moreover, long-time degradation experiments revealed that the presence of these Sulphur impurities leads to a further strong performance decrease and even to SrSO4 particle formation. As this discovery puts previously reported effects of Sr segregation in question, further experiments were performed to assess the effect of surface Sr on the oxygen exchange kinetics of SOFC cathode materials. Indeed, in-situ surface decoration experiments with SrO even revealed a performance increase of different cathode materials. Such effects on cathode performance by surface modifications could thereupon be correlated with a model based on the acidity/basicity of the investigated surface [3], which can either be advantageous or detrimental for the kinetics of the oxygen exchange reaction.

[1] Adler S.B. "Factors governing oxygen reduction in solid oxide fuel cell cathodes." Chemical reviews 104.10 (2004): 4791-4844. [2] Siebenhofer M. et al. "Oxygen exchange kinetics and nonstoichiometry of pristine La0.6Sr0.4CoO3−δ thin films unaltered by degradation." Journal of Materials Chemistry A 8.16 (2020): 7968-7979. [3] Nicollet C. et al. "Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides." Nature Catalysis 3.11 (2020): 913-920.