Amorphous Phyllosilicate for Efficient Oxygen Evolution Reaction Catalyst

Abstract

Hydrogen is one of the most promising renewable source for next efficient energy generation because of its sustainability and higher energy density than conventional energy sources. Hydrogen can be produced from various method, however, splitting water is considered as the best solution to produce hydrogen owing to its advantages such as clean, environmentally-friendly and sustainability. In order to meet the industrial demand, integrating efficient water splitting system is crucial. Among the components of the water splitting system, the development of low-cost and efficient oxygen evolution reaction (OER) catalyst is pivotal because OER is the bottleneck of the overall water splitting reaction due to the sluggish multielectron reaction and O-O bond formation compared to hydrogen evolution reaction (HER). Many researches have been conducted to develop novel efficient OER catalysts and to understand the mechanism of OER. In the early stage of the research about OER catalysts, precious metal based electrocatalyst such as Ru, Ir and their oxide material has been widely studied owing to their superior performance. However, their scarcity has led to high prices, and this has been the biggest obstacle to commercialization. As a solution to this problem, 3d transition metal (Mn, Fe, Co and Ni) based catalysts have been proposed. Many reported transition metal-based catalysts show excellent properties and stability in neutral and alkaline electrolytes. In addition, many studies have been carried out to analyze the mechanism and the determinants of performance, but it is still controversial in many areas, and further research is needed. In this thesis, we have designed an OER catalyst with excellent performance by using the mineral which exists in nature called phyllosilicate and analyzed the effect of the elements in the crystal structure on OER. We believe that phyllosilicate-based catalysts can pave a new unexplored avenue for the design of high performance catalyst. In chapter 2, we introduced an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif as a new efficient OER catalyst. A structural investigation using X-ray absorption spectroscopy revealed that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also revealed that the local environment of the active site in the layered crystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction of η of the OER compared with that of CoOOH. Chapter 3 presented the role of iron and cobalt in the OER through amorphous cobalt-iron binary phyllosilicate (ACFP). It was confirmed that prepared ACFP had a solid solution form in which cobalt and iron were uniformly mixed. As iron was added, overpotential tended to decrease until Fe content reached 40% and the Tafel slope decreased compared to the Co phase in all regions. The calculations revealed that inactive sites for oxygen evolution in the pure cobalt phyllosilicate phase could become active by lowering its overpotential of rate determining step.