Correlation between Structural and Transport Properties of Ca-Doped La Nickelates and Their Electrochemical Performance

Abstract

This work presents the results from a study of the structure and transport properties of Ca-doped La2NiO4+δ. La2−xCaxNiO4+δ (x = 0–0.4) materials that were synthesized via combustion of organic-nitrate precursors and characterized by X-ray diffraction (XRD), in situ XRD using synchrotron radiation, thermogravimetric analysis (TGA) and isotope exchange of oxygen with C18O2. The structure was defined as orthorhombic (Fmmm) for x = 0 and tetragonal (I4/mmm) for x = 0.1–0.4. Changes that occurred in the unit cell parameters and volume as the temperature changed during heating were shown to be caused by the excess oxygen loss. Typical for Ruddlesden–Popper phases, oxygen mobility and surface reactivity decreased as the Ca content was increased due to a reduction in the over-stoichiometric oxygen content with the exception of x = 0.1. This composition demonstrated its superior oxygen transport properties compared to La2NiO4+δ due to the enhanced oxygen mobility caused by structural features. Electrochemical data obtained showed relatively low polarization resistance for the electrodes with a low Ca content, which correlates well with oxygen transport properties.

Unusual Lattice Parameters Behavior for La1.9Ca0.1NiO4+δ at the Temperatures below Oxygen Loss

“…In our previous work [36], an unusual behavior of the La 1.9 Ca 0.1 NiO 4.11 sample was observed. The sample exhibited a structure unexpectedly more stable than it was assumed to be from the calculations of the dependence of δ on Ca content.…”

“…The LCNO_01 material for the study was prepared using a nitrate combustion method from the starting materials of Ni(NO 3 ) 2 •4H 2 O (>99% of purity), Ca(NO 3 ) 2 •4H 2 O (>98.5%) and La(NO 3 ) 3 •6H 2 O (99.99%) according to the procedure described in detail elsewhere [36]. As a fuel, a mixture of glycine and glycerol in a ratio of 1.5:0.5 to nitrate groups was taken.…”

“…However, from an energetic point of view, Ca-doping is less destructive to oxygen transport compared to doping with Ba or Sr. Moreover, Ca-doping in a low amount (5 mol%) was shown to facilitate the oxygen-transport properties compared to undoped La 2 NiO 4+δ [36].…”

“…The concentration of electron holes mainly determines the superior electronic conductivity of the considered materials [31]. Nevertheless, oxygen-transport properties and ionic conductivity deteriorate with doping as demonstrated in a number of studies [32][33][34][35][36]. It was shown in [37] that in the La 2−x M x NiO 4+δ (M = Ca, Sr, Ba) series, doping with an alkaline earth metal (12.5 mol%) increases migration energy barriers and also alters the migration energy ratio between different interstitial oxygen-diffusion paths.…”

“…Furthermore, La 2−x Ca x NiO 4+δ complex oxides, exhibiting a tetragonal structure in the entire range of the dopant concentrations up to the solubility limit (~25 mol%), were previously studied in [28,36,[38][39][40]. The tolerance factors for La 1.9 Ca 0.1 NiO 4+δ calculated for stoichiometric composition (δ = 0) and for composition with δ = 0.11 [36] are equal to 0.889 and 0.897, respectively, and illustrate the increasing stability of the oxide structure in the case of both excess interstitial oxygen accommodation and substitution of lanthanum by calcium.…”