Thibault Broux

Oxygen diffusion mechanism in the mixed ion-electron conductor NdBaCo2O5+x

Double perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo2O5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd- and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confirms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design.

High temperature structural stability, electrical properties and chemical reactivity of NdBaCo2−xMnxO5+δ (0 ≤ x ≤ 2) for use as cathodes in solid oxide fuel cells

The effects of Mn substitution for Co on the crystal chemistry, oxygen content, thermal expansion and electrical conductivity of the NdBaCo2−xMnxO5+δ perovskites (0 ≤ x ≤ 2) have been investigated. The NdBaCo2−xMnxO5+δ samples exhibit structural changes with increasing Mn contents from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) then to cubic (1.5 ≤ x ≤ 2.0) symmetry. All the samples lose oxygen when heated in air at T > 400 °C although the degree of oxygen loss and kinetics of oxygen exchange between the gas phase and oxide decrease with increasing Mn contents. The thermal expansion coefficients evaluated from ex situ XRD and electrical resistivity decrease with increasing Mn substitution and the values for the x = 1.5 and 2.0 compositions match with those of the Ce0.8Gd0.2O1.95 (GDC) and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM) electrolytes. With electrical conductivity values of >100 S cm−1 at 800 °C and good chemical stability with GDC and LSGM, the Mn-substituted perovskites are promising cathode materials for SOFCs.

Redox Behaviour of Potential SOFC Cathode Materials La1.6Sr0.4Ni1-xCoxO4+δ (x = 0.6 and 0.8) Determined from In Situ Neutron Powder Diffraction under Flowing O2 and 5%H2

The n = 1 Ruddlesden-Popper (RP) oxides La1.6Sr0.4Ni1-xCoxO4+δ (x = 0.6 and 0.8) have been prepared by the citrate-gel method and studied using thermal analysis and in situ neutron powder diffraction (NPD) under oxygen (x = 0.6) and subsequent 5%H2 flow. On heating under O2, the x = 0.6 sample loses oxygen from the interstitial site until it is emptied at 475 °C. Subsequent heating in 5%H2 results in reduction which proceeds within two-steps; the first one occurs between ~170 and 300 °C and involves removal of the interstitial oxygen (Oint), the second step starting at ~ 400 °C and going on up to 600 °C (the highest temperature reached) involves oxygen deintercalation from the equatorial position (Oeq). After 20 min of isothermal heating at 600 °C, unless a thermodynamic equilibrium state was not reached there was no sign of impurity related to the decomposition of the sample and the composition refined to La1.6Sr0.4Ni0.4Co0.6O3.85(3). The x = 0.8 sample exhibits similar behavior under hydrogenwith slight shifts in temperature and the composition reached after the reducing cycle refined to La1.6Sr0.4Ni0.2Co0.8O3.85(1). This study demonstrates that the Co-rich La1.6Sr0.4Ni1-xCoxO4+δ oxides can withstand hydrogen reducing conditions with topotactic deintercalation of Oint and Oeq but are not stable enough at 600 °C in comparison to La1.5Sr0.5Ni0.5Co0.5O4+δ owing toincreased Sr/La ratio in the latter.

Structure and Reactivity of the La2-xSrXMnO4±δ (0.7≤ X ≤1.0) Solid Solutions in Oxidizing Condition

Single phase La2xSrxMnO4±δ (0.7 x 1.0) solid solutions were prepared via sol-gel synthesis. For all the compositions the structure turns out to be tetragonal I4/mmm. Samples were then heated for 12 h at different temperatures in O2 to investigate the structural response to oxygen uptake. On increasing the La/Sr ratio the maximum amount of oxygen inserted (δ) reaches for x = 0.7 an unprecedented value of δ ~ 0.47 in K2NiF4-structure type oxides, as evidenced from iodometric titration and thermogravimetric analysis. For the x 0.8 compositions the oxidation resulted in an intermediate orthorhombic phase.