| 초록 |
In recent years, many attempts for clean and sustainable energy system have been progressed to cope with rapid climate changes and global warming. Among them, electrocatalysis, which is key factor of renewable energy systems related to water splitting, metal-air batteries, and fuel cells, has been suggested as promising research field. Particularly, oxygen evolution reaction (OER) which is anodic half-reaction of water splitting can strongly affect the performances of devices. However, OER typically needs to enhance the efficiency of water splitting due to the slow kinetics of OER. As a result, efficient electrocatalysts lowering the overpotential for OER are practically used in industry. Iridium (Ir) is considered as one of most efficient electrocatalysts for OER due to its superior activity. However, it is one of the rarest elements in earth and therefore numerous studies using Ir composites with other transition metals or metal oxides have been reported to reduce the amount of Ir. Especially, cobalt oxide (Co3O4) is one of non-precious metal oxide that shows relatively good and stable OER activity. In this work, by introducing metal-organic frameworks (MOFs) which have ultrahigh porosity and high surface area, we synthesized the ultrasmall iridium oxide nanoparticles encapsulated Co3O4 nanosheets (ICNs) via solvothermal methods and heat treatment. Using ZIF-67, which is one of MOFs, composed of cobalt ions with organic ligand, we can uniformly encapsulate iridium nanoparticles (Ir NPs) into the cavity of ZIF-67 by NaBH4 reduction, and subsequent solvothermal reaction makes Ir NPs encapsulated 2D nanosheets providing exposed Ir atom sites. Since the limited size of cavity, encapsulated Ir NPs are less than 2 nm. Followed by heat treatment, this composite material turns into oxide and forms numerous mesopores by decomposition of organic components. As a result, it shows more exposed active sites and makes rapid interfacial charge transfer by providing intimate contact with electrolyte. Even with a lower amount of Ir content, ICNs exhibited higher OER activity than that of 20 wt% Ir/C, in terms of overpotential, Tafel slope, and stability. These results demonstrated the effective way to reduce the amount of precious electrocatalysts with enhanced OER activity by using MOF template. |
