In situ high temperature neutron powder diffraction study of La 2Ni 0.6Cu 0.4O 4+ δ in air: Correlation with the electrical behaviour

Abstract

The knowledge of the thermal evolution of the crystal structure of a cathode material across the usual working conditions in solid oxide fuel cells is essential to understand not only its transport properties but also its chemical and mechanical stability in the working environment. In this regard, high-resolution neutron powder diffraction (NPD) measurements have been performed in air from 25 to 900 °C on O 2-treated (350 °C/200 bar) La 2Ni 0.6Cu 0.4O 4+ δ . The crystal structure was Rietveld-refined in the tetragonal F4/ mmm space group along all the temperature range. The structural data have been correlated with the transport properties of this layered perovskite. The electrical conductivity of O 2-treated La 2Ni 0.6Cu 0.4O 4+ δ exhibits a metal (high T)-to-semiconductor (low T) transition as a function of temperature, displaying a maximum value of 110 S cm −1 at around 450 °C. The largest conductivity corresponds, microscopically, to the shortest axial Ni–O2 distance (2.29(1) Å), revealing a major anisotropic component for the electronic transport. We have also performed a durability test at 750 °C for 560 h obtaining a very stable value for the electrical conductivity of 87 S cm −1. The thermal expansion coefficient was 12.8 × 10 −6 K −1 very close to that of the usual SOFC electrolytes. These results exhibit La 2Ni 0.6Cu 0.4O 4+ δ as a possible alternative cathode for IT-SOFC.