Interdiffusion and Charge Transport Across Surface-Modified Current Collectors in Planar SOFCs
The deposition of protective layers onto the Crofer 22 APU interconnect plates was described elsewhere . The microstructural and compositional analysis of the junctions “current collector – LSM cathode” was performed using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS, Supra 50/VP instrument). The studies of element distribution profiles across the interfaces were carried out after prolonged isothermal treatments at 850°C in air, coupled with electrical measurements as a function of time. In order to detect trace separation of new phases such as Cr2O3, micro Raman scattering spectroscopy was also employed. The area-specific resistance of the “current collector – LSM cathode” junctions was studied over long periods of time under the SOFC cathode operation conditions (atmospheric oxygen pressure, 850ºC, current density of 0.5 A/ cm2).
The results showed that the electrical resistance variations of the assemblies “Crofer 22 APU | LSM” with and without surface modification of the metallic plates can be quantitatively described in framework of the Schottky barrier model [3,4] for metal– semiconductor interfaces. The microscopic mechanism governing these changes involves metal interdiffusion between the cell components leading, in particular, to the formation of essentially immobile Cr2O3grains at the boundary between Crofer 22 APU and deposited Ni-based layer. These interfacial alterations make it possible to preserve low contact resistances during, at least, 20000 h. The junction between the current collector and SOFC cathode should be considered as forward-biased. In the case of relatively thin blocking layer, the current-voltage relationship for such a junction is
where the Richardson constant A=4πemk2/h3 comprises the Planck (h) and Boltzmann (k) constants as well as electron charge (e) and mass (m); V is the voltage drop across the forward-biased junction; F = Φmetal – ΧLSMis the difference between the metal work function and electron affinity of the LSM cathodes. Time dependencies of the contact ASR between Crofer 22 APU and LSM can be adequately described by the Schottky barrier changes, originating from changing the current-collector work function due to metal interdiffusion between the Ni-based coatings and stainless steel.
This work was supported by the Ministry of Education and Science of the Russian Federation (project 14.610.21.0007)
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