Impact of surface chemistry on oxygen exchange in perovskite electrodes

Abstract

Solid oxide cells (SOC) enable direct andhighly efficient conversion of chemical into electrical energy and vice versa. SOC have the potential to play a vital role in future energy systems based on renewable and intermittent energy sources such as wind and solar. The insufficient long-term stability hinders the full potential of SOC technology and their widespread use. A way to mitigate the problem is to reduce the operating temperature, thereby slowing down the degradation rates. This would bring an additional benefit by enabling the use of cheaper materials for auxiliary SOC components (e.g. sealings and interconnects). However, lower operating temperatures severely impede the electrode reactions, especially oxygen reduction/evolution reaction (ORR/OER) which usually has high activation energy. Therefore, the electrode materials with better catalytic activity towards ORR/OER are needed. The focus of this thesis is the improvement of SOC oxygen electrode and the understanding of the factors determining its activity and stability. The study is based mostly on perovskite (La0.6Sr0.4)0.99FeO3-δ (LSF), which is a good electronic and ionic conductor and stable in both oxidizing and reducing atmospheres, and (La0.6Sr0.4)0.99CoO3-δ (LSC), which is a highly active ORR/OER material, but less stable compared to LSF. Many of the insights from the thesis are expected to be relevant for many other electrode materials of similar composition, and in particular for (La1-xSr1-x)FeO3-δ, (La1-xSr1-x)CoO3-δ, and(La1-xSr1-x)(Fe1-yCoy)O3-δ (LSCF). All the measurements were carried out on model electrodes which are well-suited for fundamental studies of the critical phenomena on the oxygen electrode. Throughout the thesis, the emphasis was placed on the relations between the surface properties examined by X-ray photoelectron spectroscopy (XPS) and the oxygen exchange activity probed by electrical conductivity relaxation (ECR) and electrochemical impedance spectroscopy (EIS). The results are divided into five independent manuscripts, each forming a chapter in this thesis. First, LSC and LSF electrodes were modified by introducing perovskite/Ruddlesden-Popper interfaces on the surface. In this way, oxygen exchange kinetics measured at 650°C and the pO2=0.1 bar was improved up to 2-3 times in the case of LSF and 4-5 times in the case of LSC electrodes. This study has also revealed inconsistencies in the reported literature values, which can be much larger than the improvements obtained after the intentional electrode modifications. Consequently, the following three manuscripts were devoted primarily to the investigation of non-modified electrodes. The results suggested the existence of two distinct states of perovskite surfaces. The states were labelled as ‘activated’ and ‘passive’ state and found to differ in both surface chemistry and oxygen exchange kinetics (kchem values at 650°C could be up to 60 times different). Furthermore, the transition from one state to another was found to be reversible and to be present in both thin film and bulk electrodes. After reaching the state of low oxygen activity (‘passive state’), the electrodes could be reactivated by a high-temperature thermal treatment at 1000°C or by rinsing the electrodes in deionized water. In both cases, the reactivation was correlated with the disappearance ‘non-lattice’ strontium from the surface. Finally, thin film LSF electrodes were modified by drop casting the metal-nitrate solutions of the following ions: Sr2+, Fe3+, Ce4+, Gd3+, Ba2+, and Zr4+. This study revealed a beneficial effect of barium on the activity and stability of the LSF electrodes, by removing or preventing the formation of the detrimental ‘non-lattice’ strontium.