Interrelationship between Extended Transport Pathways of Mixed Conducting Electrode and Delamination in Solid Oxide Electrolyzer Cells
Authors: Jin Xinfang , Cook Korey , Huang Kevin 
Published: July 2021

Generating electricity using renewable energy such as solar and wind rather than fossil fuels like coal and natural gas reduces CO2 emissions and addresses climate change. However, due to the intermittency nature of renewables, highly efficient and massive scale energy storage method is the major enabler for its integration into the grid. Solid oxide electrolyzer cell (SOECs) (1), which is an electrochemical device to convert electricity into chemicals (such as hydrogen, syngas, ammonia or even methane, ethane, and ethylene), have received widespread attention. There have been tremendous improvements of the technology during the past 10 to 15 years. On the other hand, to drive the technology for commercialization, durability for long-term operation is still facing significant challenges (2). One degradation mechanism that has been widely reported is the delamination of the oxygen electrode/electrolyte interface. Pure ionic conductors such as 8 mol% Y2O3 doped ZrO2 (YSZ) and Gadolinium-doped ceria (GDC) are usually used as the dense electrolyte; mixed electronic and ionic conductors (MIEC) such as La1-xSrxMnO3-d(LSM), La1-xSrxCo1-yFeyO3-d (LSCF), Sm0.5Sr0.5CoO3 (SSC) are widely used as the oxygen electrode (3).

The most popular delamination mechanism (2, 4) is attributed to the buildup of oxygen partial pressure at the LSM/YSZ interface and growth of microcracks caused by exceeding the fracture strength of the electrolyte. Another interesting explanation (5) from atomic level is that under high temperature the inter-diffusion of different cations across the LSM/YSZ interface significantly affect oxygen transport which develops pressure buildup and delamination. A mechanism (6) from a different angle has been proposed by analyzing SEM images of the LSM/YSZ interface at different stages of polarization. It is shown that the formation of nanoparticles caused by the migration or incorporation of oxygen ions from the electrolyte into the electrode, leading to the shrinkage of LSM lattice.

Meanwhile, there have been modeling and experimental investigations of extended charge transport pathways from triple phase boundary (3PB) to two phase boundary (2TP) (3, 7) in MIEC electrodes, such as LSM and LSCF, as well as the multi-step charge transfer reaction (8-10) at gas/MIEC interface. It has been demonstrated that there is a transition voltage between two different transport pathways, and it can cause significant oxygen vacancy concentration variation at the electrode/electrolyte interface. In this work, we explore the correlation between the transition of transport pathways and the delamination phenomenon. We assume that large oxygen vacancy concentration variation will cause chemical-expansion-mismatch at the electrode/electrolyte interface, which leads to delamination of oxygen electrode from the electrolyte.

Figure 1 shows a schematic of model domain constructed with details at the interface. A bilayer oxygen electrode structure consists of a commercial LSCF-GDC porous backbone coated by a thin SrCo0.9Ta0.1O3-
d (SCT10) film. The new oxygen electrode has an inherently fast oxygen evolution reaction (OER) electrokinetics (or high oxygen evolution rate) and is capable of mitigating the delamination problem by minimizing the accumulation of oxygen in OER electrode lattice and thus chemical stresses. To understand the fundamental Electro-Chemical-Mechanical coupling mechanism and its contribution to SOEC performance degradation, we develop a rigorous Multiphysics model to explore the interplays between reaction rates and transport for two different pathways in delamination probability of the pristine and bilayer OER electrodes. The work will lead to a better understanding of the delamination mechanisms and effective mitigation plan and provide guidelines for the experimental optimization of bilayer OER electrode processing with improved electrochemical and mechanical performance.

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Figure 1