Surface Decorations on Mixed Ionic and Electronic Conductors: Effects on Surface Potential, Defects, and the Oxygen Exchange Kinetics

Abstract

The oxygen exchange kinetics of epitaxial Pr 0.1 Ce 0.9 O 2−δ electrodes was modified by decoration with submonolayer amounts of different basic (SrO, CaO) and acidic (SnO 2 , TiO 2 ) binary oxides. The oxygen exchange reaction (OER) rate and the total conductivity were measured by in situ PLD impedance spectroscopy (i-PLD), which allows to directly track changes of electrochemical properties after each deposited pulse of surface decoration. The surface chemistry of the electrodes was investigated by near-ambient pressure XPS measurements (NAP-XPS) at elevated temperatures and by low-energy ion scattering (LEIS). While a significant alteration of the OER rate was observed after decoration with binary oxides, the pO 2 dependence of the surface exchange resistance and its activation energy were not affected, suggesting that surface decorations do not alter the fundamental OER mechanism. Furthermore, the total conductivity of the thin films does not change upon decoration, indicating that defect concentration changes are limited to the surface layer. This is confirmed by NAP-XPS measurements which find only minor changes of the Pr-oxidation state upon decoration. NAP-XPS was further employed to investigate changes of the surface potential step on decorated surfaces. From a mechanistic point of view, our results indicate a correlation between the surface potential and the altered oxygen exchange activity. Oxidic decorations induce a surface charge which depends on their acidity (acidic oxides lead to a negative surface charge), affecting surface defect concentrations, any existing surface potential step, potentially adsorption dynamics, and consequently also the OER kinetics.

Engineering surface dipoles on mixed conducting oxides with ultra-thin oxide decoration layers

“…Possibilities to alter the equilibrium reaction rate include lowering the activation barrier of the rate limiting step, changing participating defect concentrations and altering the energetics of reaction steps before the rate limiting step 23 . Considering that several previous studies have shown that surface decorations do not alter the activation energy of the oxygen exchange reaction substantially 9,14,23,[25][26][27][28] , it is unlikely that a lowered activation barrier of the rate limiting step is the primary cause for increased reaction rates. Instead, we suspect that surface decorations affects both, energetics of adsorbates on the decorated surfaces, as well as charge carrier concentrations.…”

“…While previous studies have used the Smith acidity of the decoration as a descriptor to predict these changes 9,13,14 , we introduce the ionic potential of surface cations as an alternative metric to describe surface acidity. Based on the definition by Cartledge 15 , we define the surface ionic potential φ as the ratio of ionic charge q and ionic radius r, averaged over the surface cation stoichiometry (with cation fractions x i )…”

“…The probe the applicability of the described theory, we investigated the impact of ultrathin decoration layers on the oxygen exchange kinetics of chemically and structurally different MIEC oxides. Since exchange kinetics are very prone to impurities, especially sulphur 19 , they were measured by in-situ impedance spectroscopy within the PLD chamber (i-PLD) during surface decoration, a technique whose analytic strength was proven in several previous studies 14,19,23,24 .…”

Oxygen surface exchange kinetics of La1−xSrxCoO3–δ thin-films decorated with binary oxides: links between acidity, strontium doping, and reaction kinetics

“…4–7 In two recent studies, it has been shown that surface modifications have a huge role in controlling oxygen exchange kinetics in Pr-doped ceria, and that their effect can be correlated with their acidity. 8,9…”

Internal and External Cultivation Strategy toward Efficient Electrochemical Oxygen Evolution in Cobalt Pentlandite

“…To address the above issues, there are two feasible approaches: (I) adjusting the electronic structure of cobalt pentlandite to optimize the interaction between catalytic sites and oxygen-containing intermediates; (II) modifying the surface of cobalt pentlandite catalysts to increase the number of active sites. , On the one hand, the direct use of high-valence metal cations doping such as V 3+ , W 6+ , and Mo 6+ ions have been proven to adjust the local coordination environment electronic structure, and accelerate electron transfer kinetics, leading to an optimized OER catalytic activity . Li et al designed and synthesized a novel Vanadium­(V) doped NiFe LDH, which exhibited excellent catalytic activity.…”