Modeling Electrokinetics of Oxygen Electrodes in Solid Oxide Electrolyzer Cells

Abstract

A microscale model is presented in this study 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. A 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 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.

High Temperature Solid Oxide Electrolysis – Technology and Modeling

“…This raises the complexity of the interplay between oxygen exchange kinetics, microstructure and diffusion. Therefore, kinetics and 3D microstructural models are directly coupled to enable distinction between microstructural and material influences on the electrode behavior [64,76,[99][100][101][102][103][104][105][106]. An overview on oxygen electrode modeling is given in a recent review by [107].…”

Crack Growth Rate at Oxygen Electrode/Electrolyte Interface in Solid Oxide Electrolysis Cells Predicted by Experiment Coupled Multiphysics Modeling

“…3,11,16 In the present work, it is considered that lattice oxygen stoichiometry variation at the interface of the OE/ EL causes lattice structural contraction and eventually leads to delamination. 17 Mixed electronic and ionic conductors (MIECs) such as La 1-x Sr x Co 1-y Fe y O 3-δ (LSCF) are common OE of choice for SOECs. One of the advantages of the MIEC is that it extends the electrochemically active sites to the entire electrode instead of limiting them to triple phase boundary (TPB -i.e., the boundary where electrode, electrolyte, and gas phase meet).…”

“…18,19 However, such cells still suffer from short lifetimes with current densities over 1 A cm −2 . In our early work, 17 we showed through parametric studies that LSCF skeleton coated with SCT (SrCo 0.9 Ta 0.1 O 3-δ ) layer (denoted as bilayer design) exhibits a significantly higher oxygen evolution rate than LSCF only (termed as single layer baseline design). The bilayer OE design inherently alleviates delamination owning to their fast oxygen evolution reaction (OER) rate.…”

“…A detailed explanation about these reactions is provided in our previous work. 17 To make this article self-explanatory, the governing equations with boundary conditions are listed in Table I. These governing equations specify transport of charged species, such as oxygen vacancies (V o ⋅⋅ ), electron (e') and hole (h ⋅ ) expressed using Kröger-Vink notations, 24 within the electrolysis cell.…”

A kinetic study on oxygen redox reaction of a double-perovskite reversible oxygen electrode— Part II: Modelling analysis

“…In the case of composite and infiltrated electrodes, the model formulation has been improved by expressing the kinetic parameters as a function of the microstructure [12][13][14]. However, an effective ORR-OER kinetic formulation for MIEC materials has to consider two possible reaction paths, identified as the surface path (TPB) and the bulk path (DPB), which can occur in parallel or prevail depending on the specific working point [15][16][17]. The TPB path assumes the development of electrochemical reactions at the electron conductor (electrode)-ionic conductor (electrolyte) interface correlated with oxygen adsorption/desorption.…”