Nanoscale degradation mechanisms of solid oxide electrolysis cells with electronic- vs. mixed Ionic electronic conducting air electrodes

Abstract

Solid oxide electrolysis cells (SOECs) are key to the hydrogen economy, efficiently converting electrical energy into hydrogen. However, commercialization faces challenges, particularly air electrode delamination under high current densities, which accelerates cell degradation. While efforts have been made to develop new materials and structural enhancements, a fundamental understanding of degradation mechanisms remains limited due to the reliance on post-mortem analyses that lack insights into intermediate degradation stages. This study investigates degradation mechanisms in SOECs with electron-conducting or mixed ionic-electronic conducting (MIEC) air electrode by analyzing early-stage structural changes under varying current densities. Using advanced transmission electron microscopy (TEM), with a focus on precession electron diffraction (PED) for strain and orientation mapping, we visualize the entire delamination process, including local accumulation of oxygen ions, changes in anisotropic lattice strain, the generation of dislocations and subgrain boundaries, and nanopore alignment. The findings reveal that degradation mechanisms differ based on air electrode properties and microstructure. In SOECs with electron-conducting air electrodes, structural instability emerges at the electrode/electrolyte interface, leading to interfacial degradation. In contrast, SOECs with MIEC air electrodes experience a broader reaction zone, yet structural weakening persists due to oxygen redistribution. A critical insight is that degradation is driven by an imbalance between oxygen ion influx and consumption at triple-phase boundaries (TPBs), which disrupts interfacial stability and deteriorates electrochemical performance even before complete delamination. These results highlight the importance of microstructural factors such as porosity and electrode stability under high current densities.