1 Department of Energy Conversion and Storage, Technical University of Denmark2 Mixed Conductors, Department of Energy Conversion and Storage, Technical University of Denmark3 Imaging and Structural Analysis, Department of Energy Conversion and Storage, Technical University of Denmark4 Applied Electrochemistry, Department of Energy Conversion and Storage, Technical University of Denmark5 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
Ni/yttria stabilized zirconia (YSZ) supported solid oxide electrolysis cells (SOECs) were exposed to long-term galvanostatic electrolysis tests, under different testing conditions (temperature, gas composition, current density etc.) with an emphasis on high current density (above −1 A/cm2). Detailed post-mortem characterizations were carried out to investigate microstructural changes after long-term galvanostatic tests, focusing on the Ni/YSZ electrode. Formation of ZrO2 nano-particles on Ni surfaces was observed in cells exposed to −1 or −1.5 A/cm2 at 800 or 850°C, but not in those tested at current densities below −0.75 A/cm2. The formation of ZrO2 nano-particles deteriorates Ni percolation and presumably decreases the number of active triple phase boundaries (TPBs) and is therefore considered a degradation phenomenon. It is hypothesized that the degradation of the Ni surface is a result of Ni-YSZ interfacial reactions, taking place under the conditions prevailing under strong polarization. A mechanism for the formation of ZrO2 nano-particles on the Ni surface under the electrolysis cell testing is proposed and the possibility of Ni-YSZ interfacial reactions under such conditions (T, p(O2)) is further elucidated by thermodynamic calculations.
Journal of the Electrochemical Society, 2013, Vol 160, Issue 8