Influence of the substitution with rare earth elements on the properties of layered lanthanum nickelate - Part 1: Structure, oxygen transport and electrochemistry evaluation

This study is aimed at studying the properties of La2NiO4+delta (LNO) substituted with rare earth elements to increase its oxygen over-stoichiometry and enhance oxygen transport thus improving electrochemical properties of the related electrodes. Materials of the La(1.6)Ln(0.4)NiO(4+delta) series (where Ln = Pr (LPNO), Nd (LNNO), Sm (LSNO), Eu (LENO), Gd (LGNO)) are synthesized by a nitrate combustion technique and their structural features, oxygen over-stoichiometry, and oxygen transport properties are investigated by different methods such as X-ray diffraction, TGA, temperature programmed isotopic oxygen exchange with (CO2)-O-18 (TPIE) in a flow reactor. The unit cell volume in the La(1.6)Ln(0.4)NiO(4+delta) series decreases from Pr (V = 188.3 angstrom(3)) to Gd (V = 186.7 angstrom(3)). According to the TGA data, the LNO value of oxygen over-stoichiometry, delta, is equal to 0.15, while all substituted materials, except LGNO, show higher values: 0.16 for LENO, 0.17 for LSNO and LNNO, and the highest value equal to 0.18 has been revealed for LPNO. Oxygen mobility and surface reactivity are studied by the temperature-programmed isotope exchange of oxygen with (CO2)-O-18 in a flow reactor. All samples can be characterized by a fast oxygen diffusivity provided by cooperative mechanism of oxygen migration involving both regular and highly mobile interstitial oxygen. Variation of oxygen tracer diffusion coefficient with dopant cation nature is probably associated with interstitial oxygen content. The best characteristics have been acquired for Ln = Eu (D* = 2.4.10(-9) cm(2)/s at 700 degrees C). Electrochemical properties of the related La(1.6)Ln(0.4)NiO(4+delta) electrodes are investigated in a contact with a Ce0.8Sm0.2O1.9 solid-state electrolyte. The influence of the temperature of the electrode formation and the effect of the collector layer on the electrode electrochemical activity are considered. The LENO, LPNO and LSNO-based electrodes demonstrate low polarization resistance related to the enhanced oxygen diffusion properties. Substitution with Gd, contrarily, significantly deteriorates electrochemical properties of LNO.

Automated summary

This study investigates the properties of La2NiO4+delta (LNO) substituted with rare earth elements and their effects on oxygen transport and electrochemical properties. The results show that substitution with rare earth elements can increase the oxygen over-stoichiometry and improve the oxygen transport properties of LNO, thus enhancing the electrochemical performance of the related electrodes.