Solid oxide electrolysis cells (SOECs) operating at ~750-850°C are high-temperature electrochemical energy-conversion devices that efficiently produce green hydrogen. The high operating temperatures of the SOECs combined with the electrode polarizations can cause physical and chemical degradation, reducing the lifetime of the cells. To increase the durability, one can lower the operating temperature to ~700°C. However, this leads to a decrease in the electrochemical performance due to higher activation energy and lower reaction kinetics, especially in the oxygen electrode. The implementation of new materials and microstructures that would improve the electrochemical performance and durability of SOEC remains a major issue. Recently, praseodymium oxide, PrO_x, has received attention as an innovative MIEC oxygen electrode for solid oxide fuel cells, known for its very high electrocatalytic activity at low temperatures. However, its relatively poor electronic conductivity does not seem to reduce its electrochemical activity when used as an active functional layer [1].

This work is proposed to study the performance and the stability of PrO_1.833 material as a promising oxygen electrode onto a standard solid oxide electrolysis half-cell. For this purpose, a nano-structured PrO_1.833 active functional layer (AFL) was coated by electrostatic spray deposition (ESD)  following our previous microstructural optimization [1,2]. A hierarchical nanostructured porous columnar-type layer was successfully obtained. The study was performed considering this AFL topped by strontium-doped lanthanum manganite (LSM) as the current collecting layer on a standard half-cell supported, composed of a typical Ni-YSZ cermet, a thin YSZ electrolyte, and a GDC (Ce0.9Gd0.1O2-d) barrier layer. The electrochemical measurements have been investigated before and after a long-term test (at 700 °C for 1000 h) under electrolysis mode (- 0.5 A cm^‑², with 90 vol.% H₂O and 10 vol.% H₂ at the hydrogen side with a total flow rate of 15.5 Nml min^-1 cm^-2, air at the oxygen side and steam conversion (SC) of 25%). Rather good initial performance and reasonable degradation rate of 5.8 % kh^-1 were measured. Impedance spectra recorded before and after the long-term test revealed an increase in the pure ohmic resistance and the contribution at high frequencies. On the other hand, the durability test did not affect the medium and low frequencies in the impedance spectra. SEM observations have shown that a Ni particle coarsening and migration away from the hydrogen electrode/electrolyte interface occurred in operation. This microstructural evolution could explain a significant portion of the evolution of the impedance spectra. Additionally, the structural and elemental evolution of the oxygen electrode through its thickness was thoroughly investigated by synchrotron µ-X-ray diffraction and fluorescence at micro-XAS beamline at Swiss Light Source (SLS), Paul Scherrer Institute (PSI), Switzerland. The degradation of the oxygen electrode was mainly attributed to the phase transitions of PrO_1.833 while no substantial elementary interdiffusion was detected between the different layers. Finally, additional structural characterizations were performed on samples annealed at 700 °C for 1000 h and at 800 °C for 700 h without applied current. The results are discussed to provide a better understanding of the stability of the praseodymium oxide [3]. In particular, the structural analyses of the oxygen electrode aged under anodic polarization revealed the presence of GDC and an electrochemically active PrO_1.714 phase in the majority along with a small amount of PrO_{1.5 ≤ x ≤ 1.7} phase at the GDC/AFL interface. To conclude this PrO_{1.5 ≤ x ≤ 1.7} phase present in the minority in the aged SOEC could be at the origin of the increase in the ohmic resistance since it is expected to be less conductive than PrO_1.833.