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
high binder
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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].