Investigation on B-Site Exsolved Titanates and its Structure-Property Relationship as Fuel Electrode in Solid Oxide Cells

Abstract

Exsolving B-site nanomaterials from A-site deficient perovskite lattice has been attracting great attention in recent years, for the development of highly efficient fuel electrode materials for solid oxide cells (SOCs). The emergent nanoparticles, such as Ni, Fe, Cu, Co or noble metals etc. are found exhibiting not only superior electrochemical catalytic activity towards fuel oxidation and reforming, but also excellent coking resistance and stability. Most recently, we discovered that electrochemical reduction can also trigger exsolution with higher efficiency and larger population than that through chemical reduction by H2. Nevertheless, the fundamental mechanism governing the exsolution with respect to the defect chemistry as well as the B-site doping level and composition is still not fully understood.In this presentation, La0.43Ca0.37Ni0.06Ti0.94O3-δ (LCNT), La0.43Ca0.37Ni0.03Fe0.03Ti0.94O3-δ (LCNFT) and La0.8Ce0.1Ni0.4Ti0.6O3 (LCeNT) are selected as model electrode materials to exsolve Ni, Ni-Fe alloy and Ni-CeO2, respectively, under reducing atmosphere. The crystallography of each material is examined by X-ray Diffractometer (XRD), followed by Rietveld refinement to reveal the detailed crystalline information. It is found that the three materials are orthorhombic structure with oxygen octahedral out-of-phase tilting observed. The doping of Fe leads to the shrinkage of LCNT unit cell, and LCeNT is found to have the largest unit cell volume, probably due to the incorporation of cerium at A-site. In addition, the conductivity test indicates that LCNFT exhibits the highest conductivity after reduction (~13 S cm-1 at 900oC), while LCeNT shows two orders of magnitude lower conductivity at the same condition (~0.1 S cm-1). In order to understand how the material behaves under different conditions, the reduction and redox stability tests are also carried out to the mentioned materials, combined with thermogravimetric analysis (TGA) to monitor real-time weight loss or gain during reduction or oxidation. X-ray photoelectron spectroscopy (XPS) is performed as well to reveal the change in element valence state and ratio from the surface and bulk of the materials. Examined by secondary electron microscopy (SEM) and transmission electron microscopy (TEM), the evolution of the morphology and composition of the exsolved nanoparticles after the redox test is anticipated to give information on the stability of the material during long-term operation.With attempt to uncover the relation between chemical and electrochemical reduction, the aim of this work is to reveal the intrinsic structure-property relationship of the titanate fuel electrode to fundamentally understand the exsolution behaviour and its impact on performance.