Life Time Performance Characterization of Solid Oxide Electrolysis Cells for Hydrogen Production

Globally the amount of electricity generated from renewable energy sources such as wind or solar energy is increasing. To integrate high amount of fluctuating renewable energy into the existing energy grid, efficient and cost competitive conversion of electricity into other kinds of energy carriers is needed. Solid oxide electrolysis cells (SOECs) offer a promising technological solution for efficient energy conversion and production of hydrogen or syngas (mixture of H₂ and CO) using excess electricity from renewable energy sources. For SOECs to become commercially interesting, performance, durability, and cost are among the most critical issues. Long-term stable performance over 5-10 years of operation is generally required. The commercialization of the SOEC technology can be further promoted if SOECs can be operated at high current density, as it helps reducing electrolyser capacity cost significantly. In this work, long-term durability of Ni/yttria stabilized zirconia (YSZ) supported planar SOECs were investigated at 800 ^oC for electrolysis of steam. The cells, which represent the state-of-the-art SOEC technology at Technical University of Denmark (DTU), have a Ni/YSZ support and active fuel electrode, a YSZ electrolyte, a gadolinia doped ceria (CGO) barrier layer and a LSCF/CGO (LSCF: strontium and cobalt co-doped lanthanum ferrite) composite oxygen electrode. The cells were exposed to long-term galvanostatic electrolysis tests at different current densities from 0 (i.e. under open circuit voltage, OCV) to -1.25 A/cm². Detailed electrochemical and post-mortem characterizations were further conducted in order to clarify the cell or electrode degradation mechanisms. The cells show stable performance, with a steady-state degradation rate of up to 2 %/1000 h for electrolysis tests with current densities up to -1 A/cm². The long-term degradation is dominated by increase in serial resistance, which can be associated to a great extent with microstructure changes in the active Ni/YSZ electrode, namely Ni loss (directly reflected by an increase in the porosity) and Ni re-distribution. Operating the cells at -1.25 A/cm² causes severe and accelerated degradation, which is associated with both the Ni/YSZ fuel electrode and the LSCF/CGO oxygen electrode. Assuming an end-of-life cell voltage of 1.5 V, the cell life time is then predicted as a function of electrolysis current density. The current generation SOEC cells produced at DTU are able to be operated at current density up to ~-0.9 A/cm², in order to achieve a commercialization target of 5 years lifetime (for continuous electrolysis operation of hydrogen production). The cells can be operated at even higher current density, if the hydrogen production is intermittent. The results of the current work provide valuable technological inputs to future design of electrolysis plants for hydrogen production.