Studies on the low-platinum/non-noble metal electrocatalysts for oxygen reduction reaction and carbon dioxide reduction reaction

Abstract

Recently, global warming and environmental pollution caused by greenhouse gases are causing wide-ranging and continuous problems such as climate change and ecosystem disturbance. Carbon dioxide (CO2) is pointed out as one of the main causes of global warming, and our country has an industrial structure with a high carbon emission structure in power generation, industry, and transportation. The development of innovative technologies for the enhancement and treatment of emission such as CO2 is essential. Therefore, it is important to develop clean energy such as renewable energy or fuel cells as an alternative energy source. By using fuel cells to produce eco-friendly energy sources and convert existing pollution into other useful resources, it will ultimately solve energy source problems and naturally reduce greenhouse gas emissions. The studies were conducted focusing on two reactions. One of the reactions is the oxygen reduction reaction (ORR), which is the cathodic compartment of the polymer electrolyte membrane fuel cell (PEMFC) that is safe and efficient for generating electricity. Another is a reaction that can selectively produce hydrocarbon compounds by CO2 reduction reaction (CO2RR) into a stable electrochemical method. These two reactions are promising technologies as alternative energy sources that can produce environmental energy, but the main problem is that the reactions depend on the large amount of noble metal-based catalysts such as Pt, Au, and Ag. In this thesis, we will focus on the Pt/M-TiO2 nanotube catalyst in which low-amount of Pt nanoparticles (NPs) are loaded on the metal-doped TiO2 nanotube support and the M-N-C catalyst in which nitrogen-coordinating metals (M-Nx) are embedded in carbon sites. Thus, low-Pt and non-noble metal catalysts were applied to ORR and CO2RR, respectively. Firstly, various amount of Pt NPs loaded on the Nb-doped TiO2 nanotubes were synthesized, as the ORR activity and durability were proven to be better than conventional Pt/C catalyst. Here, the Nb dopants lead to improve the electrical conductivity and strong metal support interaction (SMSI), and an appropriate amount of Pt was optimized (Chapter 2). In a second part, the effect of the metal dopants on the Pt active sites was investigated. An optimal amount of Pt was fixed and TiO2 nanotube was doped with three types of metal dopants (M = V, Nb, and Cr). As a result, it was confirmed that Pt NPs were formed in various sizes and dispersions depending on the metal dopants, which was due to the lattice contraction of M-TiO2 (Chapter 3). Lastly, metal-nitrogen coordinated on the carbon (M-Nx-C) catalysts was confirmed, as the ORR activity and durability were similar to conventional Pt/C catalyst and improved conversion CO2. Single and dual M-Nx-C catalysts (single M = Fe, Co, Ni and dual M = FeCo, FeNi, CoNi) were designed by synthesizing candidates obtained by screening various metals through DFT calculations (Chapter 4 and 5). These results provide important insight into how low-Pt and non-noble catalysts impact the role of active sites for the ORR and CO2RR activity and durability. Especially, such a mechanistic understanding can guide the design of various M-Nx-C catalysts for electrochemical reactions.