Interdiffusion and Charge Transport Across Surface-Modified Current Collectors in Planar SOFCs

Ferritic stainless steel with high chromium content (>16 wt%) are regarded as promising interconnects materials for solid oxide fuel cell applications. However, particularly in oxidizing atmospheres, the formation of chromia scales leads to high contact resistances harmful to SOFC performance. At the same time, up to now one of main challenges in SOFC technology developments is to suppress the degradation processes, often associated with the stainless-steel interconnect materials, and to provide low contact resistivity in oxidizing atmospheres. In previous works [1,2] we proposed a new approach for forming barrier layers impeding Cr diffusion to the metallic current-collector surface. The deposition of Ni-based layers with subsequent thermal treatments was shown to result in unique interfacial microstructures, where the formation of an internal oxidized Cr layer prevents the segregation of insulating Cr₂O₃ phase on the stainless steel surface and reduces electrical resistivity [2]. The present work, focused on the analysis of near-surface interdiffusion phenomena in Crofer 22 APU ferritic steel interconnects with Ni-based protective layers, summarizes our developments in this field. Particular emphasis is centered on the studies of time dependencies of the area-specific resistance (ASR) between the current collector and standard La_0.8Sr_0.2MnO₃(LSM) cathodes, depending on the protective inter-layer composition. A theoretical model describing the contact resistivity behavior was proposed and validated.

The deposition of protective layers onto the Crofer 22 APU interconnect plates was described elsewhere [2]. 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

J=AT²(Ve/kT)exp( -F/kT),

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 = Φ_metal – Χ_LSMis 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)

References

[1]. N. Ledukhovskaya, E. Frolova, G. Strukov, D. Matveev, S. Bredikhin “New Type of Current Collectors with Modified Near-Surface Layer” ECS Transactions, 25 (2) 1523-1528 (2009).

[2]. N. Demeneva, S.Bredikhin “Improvement of Oxidation Resistance of Crofer 22 APU with Modified Surface for Solid Oxide Fuel Cell Interconnects” ECS Transactions, 57 (1) 2195-2201 (2013)

[3]. S. Bredikhin, T. Hattori, M. Ishigame “Schottky barriers and their properties in superionic crystals” Rhys. Rev. B, 50, p. 2444 – 2449 (1994)

[4]. S.I. Bredikhin, V.N. Bondarev, A.V. Boris, P.V. Pikhisa, W. Weppner “Electronic conductivity and current instability in superionic crystals” Solid State Ionics 81. 19 -28 (1995).