Protonation and structural/chemical stability of Ln2NiO4+δ ceramics vs. H2O/CO2: High temperature/water pressure ageing tests

Abstract

Mixed ionic-electronic conductors (MIEC) such as rare-earth nickelates with a general formula Ln2NiO4+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, La2NiO4+δ (LNO), Pr2NiO4+δ (PNO) and Nd2NiO4+δ (NNO) have been exposed during 5days to high water vapor pressure (40bar) 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 (<0.5wt% 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.