Fundamental studies of perovskite related oxide thin films for oxygen electrocatalysis at intermediate temperatures

Abstract

Discovering highly active and stable catalysts for electrochemical energy conversion and storage is essential to envision a new generation of renewable energy applications. Mixed ionic and electronic conductors (MIECs) such as Lai.xSrxCoO₃-[delta] (LSC₁₁₃) and Lai-xSrxCo1-yFeyO3-[delta] (LSCF₁₁₃) are currently utilized for applications including oxygen permeation membranes and solid oxide fuel cells (SOFCs), but alternative materials with higher catalytic activity and stability are required for intermediate temperature (500 - 700 °C) oxide electrocatalysts. In this thesis, two promising strategies, 1) Ruddlesden-Popper (RP) oxides and 2) surface decoration on the MIEC oxides are proposed to design highly active oxide materials and improve the fundamental understanding of the oxygen electrocatalysis at intermediate temperature. The oxygen surface exchange kinetics of a-axis-oriented La2NiO4+[delta] (LNO) thin films increases with decreasing film thickness. Increasing volumetric strains in the LNO films at elevated temperatures are correlated with increasing surface exchange kinetics and decreasing film thickness. Volumetric strains may alter the formation energy of interstitial oxygen and influence on the surface oxygen exchange kinetics of the LNO films. The effect of strontium (Sr) substitution on the oxygen electrocatalysis of RP oxides is also investigated using La2-xSrxNiO4+/-[delta] (LSNO, 0.0 </=Xsr </= 1.0) thin films. A structure reorientation occurs with increasing the Sr content, which can result from different energies in each surface. The surface exchange kinetics of LSNO is strongly dependent on the Sr content. This observed surface exchange kinetics can be attributed to the different oxygen adsorption energies and crystallographic orientations. The oxygen surface exchange kinetics of LSC₁₁₃ is significantly enhanced by La0.8Sr0.2CoO3-[delta] (LSM₁₁₃) surface decoration as shown in LSC₂₁₄-decorated LSC₁₁₃. In addition, long-term stability of LSC₁₁₃ is significantly improved by LSM₁₁₃ coverage. The suppression of Sr-enriched particles and substantial changes in the surface cationic ratios are associated with LSM₁₁₃ decoration, which can contribute the enhanced surface exchange kinetics and stability of LSM₁₁₃-decorated LSC₁₁₃. In contrast to the LSC₂₁₄-decorated LSC₁₁₃, LSC₂₁₄ decoration does not lead to the enhancement of the surface exchange kinetics and the long-term stability of LSCF₁₁₃. The change in the surface electronic structure and the suppression of the formation of secondary passive phases as a result of LSC₂₁₄ decoration can be responsible for observed oxygen surface exchange kinetics.