Engineering Surface Dipoles on Mixed Conducting Oxides with Ultra-Thin Oxide Decoration Layers

The surfaces of mixed ionic and electronic conducting (MIEC) materials play a crucial role in a wide range of applications within energy and sensing technologies, including solid oxide cells, catalysts, and oxygen permeation membranes. Consequently, the optimization of these surfaces and interfaces has become a focal point for research efforts aimed at advancing energy materials and devices. Recent studies have unveiled the significant impact that even small amounts of surface modifications, in sub-monolayer quantities, can have on the electrochemical properties of MIEC oxide surfaces. These modifications can alter the work function and the catalytic activity, demonstrating promising potential for targeted material and device optimization [1,2,3].<br/><br/>In this contribution, we conducted a comprehensive investigation into the effects of ultra-thin oxide layers on the electronic and ionic properties of different MIEC oxide surfaces, combining experimental and computational approaches. We employed in-situ impedance spectroscopy during pulsed laser deposition (i-PLD) and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to assess the impact of surface decorations on the properties of uncontaminated, pristine surfaces of epitaxial thin films of MIEC oxides such as La_0.6Sr_0.4CoO_3-δ or Pr_0.1Ce_0.9O_2-δ. Building on these results, we employed density functional theory (DFT) to elucidate the atomic scale processes responsible for these effects.<br/><br/>Our study demonstrates that the electrochemical properties of MIEC oxide surfaces can be tailored systematically by decorating them with oxides of different ionic potential or acidity. Basic oxides with low ionic potential, such as SrO, lead to a reduction in the work function, while acidic oxides with high ionic potential, such as SnO₂, lead to an increase in the work function. As the fundamental underlying mechanism we could identify charge redistribution and dipole formation processes at the heterojunction between host material and decoration. To illustrate the potential of this surface modification method, we explored its impact on the oxygen exchange reaction kinetics on various solid oxide fuel cell (SOFC) cathode materials, highlighting the broad applicability of ultra-thin oxide layers in tuning reaction rates on MIEC surfaces.<br/><br/>[1] 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<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] Riedl, Christoph, et al. "Surface Decorations on Mixed Ionic and Electronic Conductors: Effects on Surface Potential, Defects, and the Oxygen Exchange Kinetics." ACS Applied Materials & Interfaces<i>, </i>2023