Carbon Hybrids Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

Abstract

The oxygen reduction reaction (ORR) is a key reaction in fuel cells and metal-air batteries. This process involves multi-electron transfer and is kinetically sluggish due to the high-energy barriers required to break the O-O bond. Current commercial Pt-C electrocalysts for the ORR suffer from high-cost and easy degradation; therefore, the development and rational design of alternative electrocatalysts for the ORR are important. Numerous efforts have been devoted to the development of environmentally benign electrocatalysts of high-performance and low cost. Among those, carbon-based materials have been considered promising alternatives for Pt-free ORR due to their distinct advantages such as high electrical conductivity, low cost, stable physical, and chemistry properties. This thesis includes the synthesis of hydrothermal carbon-graphene hybrid materials as electrocatalysts for ORR, and the engineering on the interface of carbon electrocatalysts at the triple point for enhanced ORR performance. The fundamental knowledge and the research background are introduced and summarized in Chapter 1 and Chapter 2, Chapter 3, respectively. The innovative research work is presented in the subsequent chapters: In Chapter 5, N-doped nanocarbon/graphene composites were carefully designed as electrocatalysts in ORR, in order to decouple the influence of active sites and electric conductivity, and investigate the underlying relationships between them. Results show that a low conductivity limits the exertion of active sites and results in a conductivity-dependent ORR activity. However, when the conductivity reaches critical value, the active sites can be fully utilized and contribute to a positively correlated ORR activity In Chapter 6, an effective strategy was proposed to enhance the oxygen reduction reaction (ORR) performance of MWCNTs in both acid and alkaline electrolytes by coating them with a layer of biomass derivative N-doped hydrothermal carbons. The N-doped amorphous carbon (NC) coating plays triple roles: it (i) promotes the assembly of MWCNTs into a 3D network therefore improving the mass transfer, thus increasing the catalytic activity; (ii) protects the surface present Fe-containing active sites on the MWCNTs from H2O2 poisoning; (iii) creates nitrogenated active sites and hence further enhances ORR activity and robustness. In Chapter 7, a novel and general concept was reported to improve the performance of Pt-free electrocatalysts in oxygen reduction reaction. This concept is based on the addition of oxygenophilic and hydrophobic ionic liquids (ILs) into Pt-free carbon catalyst to form a thin passivating layer at the triple point between the electrocatalyst-electrolyte-gas interface. The IL layer at the catalyst’s surface provides a water-equilibrated secondary medium with a higher O2 solubility, while its hydrophobic nature prevents water from building-up locally. It was also confirmed that the use of a protic ILs favours the ORR in both acid and alkaline media. This concept not only shows significant improvements in ORR in both alkaline and acid electrolyte, but also represents a promising method to be implemented in other renewable energy technologies (i.e. metal-air batteries, supercapacitors) where nanocarbons with improved surface properties are required.