New Type of Current Collectors with Modified Near-surface Layer

Abstract

One challenge for advancing solid oxide fuel cell (SOFC) technology is to develop interconnect materials with adequate conductivity and stability for efficient long-term use. Proposed here is a technology for effectively modifying the surface of SOFC current collectors and significantly reducing the oxidation rate of ferritic stainless steel at elevated temperatures and high current densities. The formation of "Kirkendall voids" at the Crofer 22 APU ferritic stainless steel-intermetallic compound interface was observed and investigated. The Kirkendall voids play the role of a barrier layer blocking Cr diffusion to the surface and suppressing the formation of the resistive Cr2O3 surface layer. Long-term test measurements showed that the ASR value of the current collector-LSM junction decreases two times during the first 3000 hours and subsequently remains constant.

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

“…During the initial interval (marked as I), the resistivity decreases. The duration of this period of time was shown to increase with increasing thickness of the protective coatings (7,8). One should also mention that the ASR values observed at the end of the first period of time were always similar to one another, 5-7 mOhm×cm 2 , irrespective of the starting ASR and Ni layer thickness.…”

“…The discs were dipped into a saturated NiCl 2 solution in aqueous HCl, heated up to 70-80°C for surface oxide film removal and adhesion improvement. Then Ni layers (thickness of 6-16 μm) were applied as described elsewhere (7,8). The Ni-coated Crofer 22APU samples were annealed in a vacuum furnace at 1050°C.…”

“…This problem may only be solved employing protective coatings (5). As the interconnect oxidation at the cathode side is responsible for over 30% SOFC stack degradation, the protective coatings minimizing the ASR should provide lifetimes of, at least, 30000 hours (6)(7)(8)(9)(10).…”

“…In previous works (7,8), a new approach for forming barrier layers impeding Cr diffusion to the metallic current collector surface was proposed and tested. The resultant Ni coatings were found to provide low ASR of the «current collector -LSM cathode» junctions, varying in the range 5 -10 mOhm•cm 2 during prolonged tests (up to 30,000 hours) at 850°C in air under a load current of 0.5 A/cm 2 .…”

Improvement of Oxidation Resistance of Crofer 22APU With Modified Surface for Solid Oxide Fuel Cell Interconnects

“…(-) {Current collector}/ {La 0.8 Sr 0.2 MnO 3 (LSM)} [1] (+) {La 0.8 Sr 0.2 MnO 3 (LSM)} / { Current collector} [2] The electrical polarity of first junction [1] corresponds to the working polarity of the junction under the SOFC operation. During the measurements experimental cell was placed in a quartz reactor, which was inserted to the furnace, heated to 850°C.…”