PVD Gd-doped Ceria Sr-Diffusion Barrier Layers for Electrolyte Supported SOCs: Stack Test

Abstract

High performance and reliable long-term stability of Solid Oxide Cell (SOC) stacks under industrial operating conditions are crucial factors for the widespread commercialization of the technology. Stacks with electrolyte supported cells, which consist of a Ni-GDC fuel electrode, a 3YSZ electrolyte, a Gd-doped Ceria (GDC) Sr-diffusion barrier layer and and a LSCF-GDC air electrode, belong to the most established and robust SOC architectures for electrochemical applications, such as combined heat power supply or high temperature electrolysis. Aiming to increase the overall performance and lower the degradation of the SOC stacks, two modifications on state-of-the-art technologies were brought into a rainbow stack with 30 repeat unites (RUs): (1) replacing the conventional screen-printed and sintered GDC Sr-diffusion barrier layers by thin-film GDC of 0.5 µm in thickness, and (2) adding MgAl2O4 protective layers with a thickness of 2 µm on metallic interconnects. In the tested rainbow stack, the GDC Sr-diffusion barrier layers were either fabricated by screen-printing method for state-of-the-art reference RUs or electron beam physical vapor deposition (EB-PVD) for the modified RUs. With a fuel gas composition of 40 % H2 + 60 % N2 and air as oxidant gas, the stack was operated in SOFC mode under a current density of 227 mA cm−2 and 75% fuel utilization for over 5000 h. In this paper, the investigation mainly focuses on the electrochemical characteristics of RUs containing the thin-film GDC layers during the stack testing. The performance and degradation behavior of the stack and representative RUs were investigated by means of polarization curves and electrochemical impedance spectroscopy (EIS). The initial performance, voltage and resistance degradation of the stack and the RUs were determined and discussed to evaluate the effectiveness of the modifications.

The Federal Ministry for Economic Affairs and Energy (BMWi) is acknowledged for the financial support of this work within the KOSOS Project under grant number 03ETB005C.