Implications of surface defects on oxygen electrocatalysis in perovskite-type oxides

Abstract

Formation of crystal defects is an indispensable process for thermodynamic stability, as crystalline materials go through high temperature heat treatment. As a result, various defects are distributed not only inside the crystal lattice but also on the surface, and since these defects have a structure different from that of the bulk, they are factors to be controlled according to the purpose of utilization. In terms of electrochemical reactions, previous studies have been reported that defects can act as active sites due to high free energy. In this regard, it has been reported that defects such as oxygen vacancies or strain influence the improvement of oxygen catalytic activities. However, many studies have analyzed and approached in entire bulk-scale, even though catalytic activity is a surface reaction, which has limitations in identifying the exact mechanism. Therefore, we observed changes in the physical and chemical properties of perovskite oxide used as oxygen electrocatalyst in atomic-scale, and systemically control the defects to identify the exact cause for the electrochemical properties of the defects. In the first part, changes in electrochemical properties related to oxygen electrocatalytic activities and their causes at the surface grain boundaries of perovskite oxides were analyzed in terms of the structural aspect in atomic-scale. By using sintered LaCoO3 and LaMnO3 polycrystalline bulk with grains of various sizes, it was confirmed that the grain boundaries could act as an active site of the oxygen electrocatalysis. In addition, by using a bicrystal LaCoO3 epitaxial thin film, it was verified that there was a significant improvement in catalytic activity even at a single grain boundary. And also, we demonstrated that the increases in oxygen catalytic activity at grain boundaries were due to the specific structure of grain boundaries, through a combination of atomic scale STEM imaging and density functional theory (DFT) calculations. The displacements of B-site metal and oxygen ions occurred near the grain boundaries from observation of atomic observations, resulted in significant separation of degenerate B-site metal 3d orbitals, which caused a significant improvement in charge transfer. Further, based on the DFT calculations, it was verified that a noticeable improvement in oxygen electrocatalysis could be seen in perovskite oxide-based catalysts under symmetrical shifts occurrence in octahedra rather than all the displacements. As an extension of the conclusion that the grain boundary can act as an active site of the oxygen electrocatalysis, an attempt was made to manipulate the structure of grain boundary in atomic-scale to maximize the efficiency. When aliovalent ions are doped into crystalline oxides and annealed at high temperature, segregation which is enrichment of cation is occurred on the surface and interface, and it has different composition with the bulk. In the second part, divalent alkali-earth metal ions (Ca, Sr, Ba) were substituted for the A-site of LaCoO3 known as a high-efficiency perovskite oxide catalyst for formation of chemical composition changes. In addition, the characteristics of the material appearing in this structure were analyzed. After heat treatment at high temperature, the degree of segregation is controlled by a strain recognized as driving force, which is listed in sequence from large to small ionic radius. As a result, it was confirmed that the activity of the grain boundaries was significantly improved in the Ba doped case, which formed more segregation at the grain boundaries. According to the XPS results, the chemical shifts of Co were not occurred regardless of the dopant, so the valence state is maintained. Therefore, the distribution of solute ions in certain defects also includes the possibility that they can play a critical role in electrocatalysis. Additionally, physical properties change considerably as oxygen vacancies are created, and electrical conductivity is one of them. The oxygen vacancy effect on OER treated in previous studies did not deal with the intrinsic conductivity of the material itself in depth. In this study, it was found that the electrical conductivity changed according to the content of the substituted acceptor (alkali-earth metal cation) influences the tendency of the intrinsic activity of the catalyst materials. Furthermore, by utilizing a thin film-type catalyst electrode with a conductive layer, it was shown that over a certain degree of electrical conductivity must be maintained in order to exhibit the oxygen vacancy effect on OER.