Prakt. Met. Sonderband 46 (2014) 163 
nad to De ENHANCED OXYGEN SURFACE EXCHANGE KINETICS OF 
ol. CERAMICS BY DEPOSITION OF A NANO-LAYER OF SILVER 
rates. As a , 
of 15-20%. A. Egger, W. Sitte 
ge is rather . vn i | ; 
jith 50 vol% Montanuniversität Leoben, Chair of Physical Chemistry 
about 13%, Franz-Josef-Straße 18, 8700 Leoben, Austria 
layers. The 
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ABSTRACT 
The oxygen surface exchange kinetics of the mixed ionic-electronic conducting ceramic 
LazNiO4+5 has been enhanced by coating the material with a 200 nm thin Ag-layer. Due to 
the catalytic activity of silver the surface exchange coefficient (kchem) was strongly increased 
coordinated up to one order of magnitude. This effect has been exploited to reduce the limiting role of 
thank Prof. the surface redox kinetics in the overall oxygen exchange process, thus permitting the 
_eganes, for experimental determination of chemical diffusion coefficients of oxygen (Dchem) by means of 
the conductivity relaxation technique. Measurements were performed between 600 and 
850°C and at oxygen partial pressures of 0.1 and 0.01 bar. In the course of a complete 
temperature cycle the silver layer was removed at high temperatures as evidenced by an 
irreversible change in kchem between the heating and cooling run. Depth profiles of the 
surface recorded after testing by X-ray photoelectron spectroscopy showed no evidence of 
silver within the topmost 500 nm, indicating that silver was completely removed via gas 
phase transport in the high-temperature segment of the measurements and did not diffuse 
ringer, New into the material. The morphology of deposited silver films was studied by scanning electron 
microscopy after short-term annealing between 500 and 800°C. 
lls based on 
, Goteborg, 
sel Cells 11, 1. INTRODUCTION 
LazNiO4+5 is a mixed ionic-electronic conducting ceramics that has been considered for 
ells (SOFC) applications such as electrodes for solid oxide fuel cells (SOFCs) and solid oxide 
pan, JPMA, electrolyser cells (SOECs) as well as for oxygen-permeable membranes. In such 
applications, fast ionic transport of oxygen within the bulk is a key property for the 
. | corrosion functionality of the ceramics. However, in addition to large oxygen diffusivities, high oxygen 
ort”, (2014), exchange rates with the surrounding atmosphere are required. For example, in oxygen 
conducting membranes molecular oxygen has to be incorporated into the material at the 
sel by image high-pressure side and — after ambipolar diffusion as oxide ions and electrons through the 
membrane — is released as O2 on the low pressure side. Even though the surface exchange 
of La2NiOu+5 is rather fast, it dominates the overall oxygen transport from the gas phase 
through the bulk at temperatures below 700°C [1]. This places a limit to the oxygen flux 
through a SOFC electrode or ceramic membrane, which cannot be alleviated by a decrease 
in particle size or membrane thickness. In this work the oxygen surface exchange kinetics 
of LazNiOa+s has been improved by covering the surface with a 200 nm layer of silver. Using 
this approach, reliable values for chemical diffusion coefficients of oxygen were obtained for 
LazNiO4+5 in the IT-regime between 600 and 850°C. Furthermore, chemical surface 
exchange coefficients of oxygen were determined after removal of the silver layer [2].