Non-Precious Metal Catalysts Based on Carbon Nanomaterials for Oxygen and Hydrogen Electrocatalysis

The environmental issues caused by the hydrocarbon energy sources have emerged as one of the most urgent challenges in 21st century. The development of clean and renewable energy technologies is critical to meet both the environment regulations and to circumvent dependence upon the fossil fuels. This situation has brought a new idea about the future society solely driven by hydrogen-based energy infrastructures, so-called hydrogen economy. The production and utilization of hydrogen via water electrolysis and fuel cells, respectively, are key ingredients to realize the hydrogen economy. However, the high cost of those devices hinders their wide adoption, which can be attributed primarily to the use of precious metal electrocatalysts such as Pt and Ir that are required for efficient operation. In this context, the development of active non-precious metal catalysts (NPMCs) is of great significance. \nIn this dissertation, new NPMCs based on carbon nanotube (CNT) have been designed and prepared for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), where the ORR is an important half-reaction that critically affects the fuel cell performance, while the OER & HER are involved in water electrolysis. CNT was selected as the carbon support owing to its high conductivity, chemical stability, and surface tunability, advantageous for electrocatalysis. \nIn Chapter 2, we developed a facile and scalable synthetic method for carbon nanostructures comprising active heteroatom-doped carbon (HDC) layers coated on CNT (CNT/HDC), which was exploited as a metal-free ORR electrocatalyst. The preparation involves the adsorption of heteroatom-containing ionic liquid (IL) on the CNT walls via van der Waals and cationic-?? interactions and subsequent carbonization, yielding CNT/HDC core???sheath nanostructures. The design enables both the efficient utilization of surface active sites of HDC layers and high electric conductivity of the CNT core. The CNT/HDC catalyst exhibited high ORR activity and reaction kinetics comparable to a commercial Pt/C catalyst in alkaline media, and an excellent anion exchange membrane fuel cell (AEMFC) performance. The IL-derived CNT/HDC catalysts could be prepared using various types of IL precursors. \nIron and nitrogen codoped carbon (Fe???N/C) catalysts have emerged as the most promising electrocatalysts for the ORR among various classes of NPMCs. A growing body of literature suggests that Fe???Nx species are major active sites in a Fe???N/C catalyst. Chapter 3 presents a general ???silica-protective-layer-assisted??? approach that can preferentially generate the catalytically active Fe???Nx sites in Fe???N/C catalysts while suppressing the formation of less-active large Fe-based particles. The catalyst preparation consisted of the adsorption of iron porphyrin precursor on CNT, silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layers of porphyrinic carbon (CNT/PC) catalysts. We found that the silica-coating step plays a decisive role in preferentially generating catalytically active Fe???Nx coordination sites, as revealed by temperature-controlled in situ X-ray absorption spectroscopy (XAS). The CNT/PC catalyst contained higher concentration of active Fe???Nx sites compared to the CNT/PC prepared without silica coating. The CNT/PC showed very high ORR activity and excellent stability in alkaline media. Importantly, an alkaline AEMFC with a CNT/PC-based cathode exhibited the highest current and power densities among NPMC-based AEMFCs. In addition, a CNT/PC-based cathode exhibited a high volumetric current density of 320 A cm???3 in acidic proton exchange membrane fuel cell. We also demonstrated the general applicability of this synthetic strategy to other carbon supports. \nChapter 4 describes the investigation of active site structures of bifunctional oxygen electrode catalysts based on cobalt oxide (CoOx) under reaction conditions. Size-controlled (3???10 nm) cobalt oxide nanoparticles (CoOx NPs) supported on CNT were prepared, and served as model catalysts. Electrochemical in situ XAS suggested that the initial Co3O4 or CoO phase was transformed to Co3O4???CoOOH core???shell structures under the ORR and OER conditions regardless of particle sizes. Combined with the in situ XAS, cyclic voltammetry study revealed that Co2+/Co3+ and Co3+/Co4+ redox transitions are involved in the ORR and OER, respectively. We further examined the size-dependent electrocatalytic activities. The OER activity increased with decreasing NP size, which correlated to the larger amount of Co(III) species and larger surface area in smaller NPs. For the ORR, no particle size dependence was found