La2−Pr NiO4+ as suitable cathodes for metal supported SOFCs

Abstract

Aiming at a tradeoff between the chemical stability of La 2 NiO 4+ δ (LNO) and the high electrochemical performances of Pr 2 NiO 4+ δ (PNO), La 2 − x Pr x NiO 4+ δ mixed nickelates, further referred as LPNO, were studied as possible oxygen electrodes for solid oxide fuel cells (SOFCs). LPNO phases were synthesized using the modified citrate-nitrate route followed by a heat treatment at 1200°C for 12 h under air. Structural characterizations of those K 2 NiF 4 -type compounds show the existence of two solid solutions with orthorhombic structure, namely a La-rich one from x = 0 to 0.5 with Fmmm space group, and a Pr-rich one from x = 1.0 to 2.0 with Bmab space group. The mixed ionic and electronic conducting (MIEC) properties of LPNO phases were investigated through the evolution of the oxygen over-stoichiometry, δ, measured as a function of temperature and pO 2 , the electrical conductivity, the diffusion and surface exchange coefficients versus x, showing that all compositions exhibit suitable characteristics as cathode materials for SOFCs. In particular, the electrochemical performances measured in symmetrical cells using LPNO materials sintered under low pO 2 , as requested in metal supported cell, (MSC-conditions) confirmed a decrease in polarization resistance values, R p , from 0.93 Ω cm 2 (LNO) down to 0.15 Ω cm 2 (PNO) at 600°C with increasing x.

Structural transformations of the La2−xPrxNiO4+δ system probed by high-resolution synchrotron and neutron powder diffraction

“…Additional synthesis details were described previously. 17 Highly hygroscopic Pr6O11 and La2O3 were first pre-fired at 900 °C overnight to remove residual water before weighing targeted precursor mixtures. Final annealing was performed at 1200 °C for 12 h in air, leading to well crystallized phases.…”

“…Details of pellet sintering and physical property measurements, including electrical conductivity values determined under air using the four-probe technique, and polarization resistance values obtained by fitting of Nyquist impedance diagrams, have been reported previously. 17…”

“…[10][11][12][13][14] For this reason, there has been recent interest in mixed PNO-LNO compositions which could offer a compromise of chemical stability and electrochemical performance. [15][16][17][18][19] However, despite investigations spanning three decades, the phase diagram of the La2-xPrxNiO4+δ system; as a function of composition, oxygen stoichiometry, and temperature, is still unclear. In the current work, a fundamental crystallographic study of the La2-xPrxNiO4+δ series has been performed using high resolution synchrotron and neutron powder diffraction data.…”

In-situ study of cationic oxidation states in Pr2NiO4+δ using X-ray absorption near-edge spectroscopy

“…Among them, La 2 NiO 4+ is the most investigated material. However the best electrochemical results have been obtained when using Pr 2 NiO 4+ [4] For instance, among the different nickelates, the lowest polarization resistance values are measured in the intermediate temperature range (T ≤ 800°C) under zero dc conditions, in air when porous Pr 2 NiO 4+ is screen-printed on a thin (2 -4 μm) and almost dense doped (yttrium or gadolinium) ceria layer [4,5], while dense 8 mol% yttria-doped zirconia (8YSZ) is the main electrolyte. Even, better results are obtained when Pr 2 NiO 4+ is infiltrated in a porous Gd-doped ceria backbone [6], then leading to a composite oxygen electrode.…”

Recent developments in metal‐supported solid oxide fuel cells

“…Mixing the two materials as La 2−x Pr x NiO 4+δ , mixed nickelates were synthesized (LPNO), and studied as possible oxygen electrodes for SOFCs. Electrochemical characterization of half‐cells confirmed the following: The ratio of La/Pr, as determined by x , strongly determines electrochemical performance, e.g., La rich phases ( x < 0.5) may provide higher stability while 0.5 < x < 1 generates better performance (lower ASR). All the lanthanum–praseodymium nickelates are overstoichiometric in oxygen in the whole temperature range (up to 1000°C, from TGA data) in air and in Argon. Other properties such as—electrical conductivity, CTE's, oxygen diffusion and surface exchange coefficients appear to be adequate for the application of LPNO for MS‐SOFCs. …”