Electronic and Ionic Effects of Acidic Adsorbates on Solid Oxide Fuel Cell Cathode Surfaces

The oxygen exchange reaction 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. 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, such as S, Si or Cr, as well as to cation segregation, for example Sr segregation for perovskites like La_0.6Sr_0.4CoO_3-δ (LSC) and SrTi_0.3Fe_0.7O_3-δ (STF). In particular, it was recently shown that acidic adsorbates, such as SO₄^2-, originating from trace impurities, which are omnipresent in nominally pure measurement atmospheres, readily adsorb on the surfaces of different SOFC cathode materials and cause a sudden and strong degradation [1]. However, the underlying mechanism of these effects has so far been unclear.<br/><br/>In this contribution, the effects of adsorbed SO₂ and other typical solid oxide fuel cell (SOFC) poisons, such as CO₂ and CrO₃, on the electronic and ionic properties of an SrO-terminated (La,Sr)CoO_3-_δ (LSC) surface and on its oxygen exchange kinetics have been investigated experimentally with near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), low energy ion scattering (LEIS) and impedance spectroscopy, as well as computationally with density functional theory (DFT) [2]. The investigations revealed that acidic adsorbates induce the formation of a surface dipole and extract charge from the LSC surface. As a result, they strongly increase the surface work function and alter the energetics of molecular oxygen adsorbates and oxygen vacancies in LSC.<br/><br/>Interestingly, the total work function changes as well as the redistributed charge for different acidic adsorbates scale quantitatively with the Smith acidity of the adsorbed oxide, which has recently been identified as a descriptor for the effect of binary oxide infiltrations on the oxygen exchange kinetics of Pr_0.1Ce_0.9O_2-_δ [3]. We suspect that the here presented concept of charge redistribution and its impact on the oxygen exchange reaction is highly relevant for multiple phenomena on mixed conducting surfaces and that it is a further step on the way to a detailed understanding of the oxygen exchange reaction on these surfaces.<br/><br/>[1] Riedl, Christoph, et al. "In situ techniques reveal the true capabilities of SOFC cathode materials and their sudden degradation due to omnipresent sulfur trace impurities." Journal of Materials Chemistry A, 2022<br/><br/>[2] Siebenhofer, Matthäus, et al. "Electronic and ionic effects of sulphur and other acidic adsorbates on the surface of an SOFC cathode material." Journal of Materials Chemistry A, 2023<br/><br/>[3] Nicollet, Clement, et al. "Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides." Nature Catalysis, 2020