Development of Proton Conducting Ceramic Cells in Metal Supported Architecture

Kurzfassung

Reliability, long-term stability, lower cost as well as high performance are essential for Solid Oxide Cells (SOC) in fuel-cell and electrolysis applications. Metal-Supported (MS) architecture offers various advantages over state-of-the-art ceramic supported SOCs, such as high tolerance towards thermal/redox cycling that are key features for flexible and reliable operation. Having high potential in many electrochemical applications, Proton-conducting Ceramic Cells (PCC) have been intensively studied and demonstrated promising improvements in the last decade. Different in geometric layout from SOC in terms of steam production in fuel cells and steam supply in electrolysis cells, PCC is advantageous for its low oxygen partial pressure in fuel electrodes, not merely in the electrochemical operation, also in the MS architecture. The challenge in the development of MS-PCC is to find a feasible process to fabricate gas-tight electrolyte on the porous substrate without degrading the metal support. The state-of-the-art PCC electrolyte is based on perovskite oxides that have refractory nature requesting high sintering temperature typically above 1400 °C, which makes it more challenging for MS-PCC than for MS-SOC via sintering method. Our strategy is implementing multilayers combining wet chemical processes below 1000°C for the porous electrode layers and dry Physical Vapor Deposition (PVD) techniques below 800 °C for the gas-tight electrolyte coating. The multilayer approach reducing the pore size stepwise from several tens of micrometer in the MS down to nm range in the top electrode layer, enables the deposition of 1-micrometer thick dense perovskite electrolyte by PVD. The paper presents our recent activities in the development of MS-PCC. The state-of-the-art PCC electrolyte, BaZrO3-based perovskite has been chosen and NiO cermet fuel electrodes are developed on the porous metal support. Zr4+ site of BaZrO3 is doped with 10-15 mol% Y3+ to maintain proton conductivity and is partially substituted with Ce4+ for high performance, higher thermal expansion coefficient (TEC) for better match with other components and for better sinterability of the functional layer, while higher Zr content ensures its chemical stability against CO2. Different PVD technologies, namely Pulsed Laser Deposition (PLD) and Electron-Beam Physical Vapor Deposition (EB-PVD), are applied for the electrolyte coating process. Two types of porous metal substrates are investigated: commercial porous metal substrates and metal plates with laser-drilled pores, both of which are made of inexpensive ferritic stainless steel. Pore size, pore distribution and compatibility for electrochemical applications are to be studied and adjusted/considered for improvement in terms of performance and chemical/mechanical stabilities. Protection coating on MS is necessary to prevent/ reduce Cr deposition and interdiffusion into the ceramic layers, as well as to eliminate corrosion of the metal. Obtained half cells are investigated by SEM and X-ray diffraction, some of which are tested for electrochemical performance. The results of the electrochemical tests will be presented in another paper of this conference. In this paper, the results obtained by different PVD techniques and on different types of substrates will be presented and discussed in terms of residual stress, chemical/mechanical stabilities. This work is supported by project DAICHI (EIG CONCERT-Japan), project AH2A (the Research Council of Norway, 268010/E20), project ARCADE (BMBF, 03SF0580A) and project 112CO2 (Horizon2020, 952219). The China Scholarship Council is acknowledged for the Ph.D. fellowship of Haoyu Zheng (201806160173).