Protonation and structural/chemical stability of Ln₂NiO_4+δ ceramics vs. H₂O/CO₂: High temperature/water pressure ageing tests

Highlights: • High temperature/water pressure autoclave is used to study the reaction/corrosion at SOFC/HTSE electrode. • High stability of Pr₂NiO_4+δ (PNO) and Nd₂NiO_4+δ (NNO) dense ceramics vs. water pressure is demonstrated. • Protonated rare-earth nickelates retain the perovskite-type structure and their H-content is determined. • Very low laser illumination power is required to avoid RE nickelate phase transition. • Nickelates show increasing stability from La to Pr/Nd vs. CO₂-rich high temperature water vapor. - Abstract: Mixed ionic-electronic conductors (MIEC) such as rare-earth nickelates with a general formula Ln₂NiO_4+δ (Ln = La, Pr, Nd) appear as potential for energy production and storage systems: fuel cells, electrolysers and CO₂ converters. Since a good electrode material should exhibit important stability in operating conditions, the structural and chemical stability of different nickelate-based, well-densified ceramics have been studied using various techniques: TGA, dilatometry, XRD, Raman scattering and IR spectroscopy. Consequently, La₂NiO_4+δ (LNO), Pr₂NiO_4+δ (PNO) and Nd₂NiO_4+δ (NNO) have been exposed during 5 days to high water vapor pressure (40 bar) at intermediate temperature (550 °C) in an autoclave device, the used water being almost free or saturated with CO₂. Such protonation process offers an accelerating stability test and allows the choice of the most pertinent composition for industrial applications requiring a selected material with important life-time. In order to understand any eventual change of crystal structure, the ceramics were investigated in as-prepared, pristine state as well as after protonation and deprotonation (due to thermal treatment till 1000 °C under dry atmosphere). The results show the presence of traces or second phases originating from undesirable hydroxylation and carbonation, detected in the near-surface layers. The proton/water insertion modifies the structure symmetry and the unit-cell volume whatever the low amount (<0.5 wt% equivalent H₂O). This result is consistent with long range interaction and in contradiction with the formation of hydroxyl species hypothesis. The reaction mechanisms evidenced after autoclave treatment may be useful to understand the reaction occurring at the electrode surface in SOFC/HTSE systems