Modeling Electrokinetics of Oxygen Electrodes in Solid Oxide Electrolyzer Cells

Abstract

High temperature solid oxide electrolyzers operated at >600oC are advantageous from a thermodynamic perspective since heat (TDS) can be converted into chemical energy (DH) [1]. This additional heat reduces the overall electrical power (DG) requirement, allowing an electrolysis cell to achieve ~100% efficient H2 production [1]. Since the thermal energy contribution to the electrolysis reaction can also be obtained from sensible Joule heating produced within the cell, the electrical energy demand is further reduced which decreases the H2 production price. The bulk of the heat can be provided by an external source such as nuclear power, solar thermal power, or waste heat from a chemical plant. Theoretically, there is no external heat requirement if the electrolyzers operate at the thermoneutral voltage. Due to the above advantages, the research and development on solid oxide-based water electrolyzers have received significant attention [1–6].This work focuses on physics-based modeling of high temperature solid oxide electrolysis cells (SOEC). A microscale model is presented to simulate electrode kinetics of the oxygen electrode in a solid oxide electrolyzer cell (SOEC). Two mixed ionic/electronic conducting structures are examined for the oxygen producing electrode in this work: single layer porous lanthanum strontium cobalt ferrite (LSCF), and bilayer LSCF/SCT (strontium cobalt tantalum oxide) structures. An yttrium-stabilized zirconia (YSZ) electrolyte separates the hydrogen and oxygen electrodes, as well as a gadolinium doped-ceria (GDC) buffer layer on the oxygen electrode side. Electrochemical reactions [7-8] occurring at the two-phase boundaries (2PBs) and three-phase boundaries (3PBs) of single-layer LSCF and bilayer LSCF/SCT oxygen electrodes are modeled under various SOEC voltages with lattice oxygen stoichiometry as the key output. The results reveal that there exists a competition in electrode kinetics between 2PBs and 3PBs, but 3PBs are the primary reactive sites for single-layer LSCF oxygen electrode under high voltages. These locations experience the greatest oxygen stoichiometry variations and are therefore the most likely locations for dimensional changes. By applying an active SCT layer over LSCF, the 2PBs become activated to compete with the 3PBs, thus alleviating oxygen stoichiometry variations and reducing the likelihood of dimensional change. This strategy could reduce lattice structural expansion, proving to be valuable for electrode-electrolyte delamination prevention and will be the focus of future work. Figure 1