Design Strategies of Transition Metal-based Electrocatalysts for Water Oxidation

Abstract

The oxygen evolution reaction occurs in the numerous aqueous-based electrochemical energy conversion and storage systems, such as hydrogen energy production, carbon dioxide reduction, and so on. However, it requires additional voltages than the theoretical values called as overpotential, which hinders the competitiveness of realistic application and commercialization. Therefore, it is necessary to develop the efficiency of this reaction by introduction of the high efficient electrocatalysts. In the case of recent research for the development of electrocatalysts, the noble metal based electrocatalyst are restricted from high cost. In this regard, the suitable and desirable strategies are needed to develop the activity of transition metal based electrocatalysts, which exhibit the relatively low electrocatalytic activity. These strategies are optimized according to the oxidative environment in water electrolysis system. In Chapter 2, the electrocatalysts based on Co/Fe PBA are synthesized and optimized for the high alkaline water electrolysis system in hydrogen energy production. In this region, the corrosive environment of long-term operation would eventually cause the oxidation of the transition metal-based catalysts, resulting in phase transformation and aggregation. To address the significant stability issues, the strategy focuses on the preservation of highly efficient electrocatalysts with the surface protective layers. For this purpose, we select the Prussian blue analogues (PBAs), a subclass of metal organic frameworks (MOFs), as an efficient template for the synthesis of the electrocatalysts. Its composition of transition metals and cyanide groups can be successfully converted into the highly conductive alloy core with nitrogen-doped graphitic layers under the optimized pyrolysis process. The derived core-shell type electrocatalysts can be a feasible strategy for the application in alkaline water oxidation. In Chapter 3, the strategy to develop the efficiency of electrocatalysts in the case of neutral media for the carbon dioxide reduction is optimized in terms of electronic structure modulation. In this region, the low concentration of the reaction intermediate inhibits the oxygen evolution reaction, resulting in the decreased efficiency. To overcome the significant activity issues, the rearrangement of the electron configuration and the introduction of cocatalyst into the electrocatalyst could be an attractive strategy. For this purpose, we select the perovskite-structured LaCoO3 and CaO as efficient precursors of the electrocatalyst and synthesize the composites via the one-pot sol-gel method. The synergistic effect between LaCoO3 (large surface area and presence of active B-site cobalt species) and CaO (alkaline property, which could help to lower the kinetic energy barriers) cause the reconstruction of the electron configuration and form the interface between the two phases for expansion of the active sites. The derived composites exhibit the improved electrochemical performance with the structural modification at electronic level. The feasible process for electrode is also introduced to boost the performance in relation to hydrophilicity with plasma treatment and high-energy ball milling. With the strategy, the composite derived the improved electrocatalytic activity. Consequently, the physicochemical and electrochemical analysis to identify the structure and activity of the aforementioned electrocatalysts confirm the improved achievement and clarify the relationship between structural characteristics and electrochemical behavior. The final aim of the thesis is to contribute to the production of next generation energy and current climate challenges through the introduction of highly efficient electrocatalysts controlled at the nanoscale electron levels.