Defect Engineering And Surface Functionalization Of Carbon Materials As Electrocatalyst For Oxygen Reduction Reaction

The cathodic oxygen reduction reaction (ORR) is a key process in the fuel cells. However, the poor reaction kinetics of ORR has significantly limited the commercialization of fuel cell. With different electrocatalysts, the ORR process could proceed either through an efficient one step four-electron pathway or a less efficient two-step, two-electron pathway. A direct, four-electron pathway of ORR would lead to high performance of fuel cells. Commercially, the most commonly used electrocatalysts for ORR are the state-of-the-art precious Pt-based nanomaterials. However, such electrocatalysts derived from the limited precious metal resources with extremely high cost, susceptibility to fuel crossover and poor stability, which have tremendously limited the development and application of fuel cells. Alternatively, numerous studies have been performed to design and prepare precious metal-free or even metal-free electrocatalysts with comparable electrocatalytic activity towards ORR. Carbon materials doped with heteroatoms have been extensively investigated as efficient, stable, low-cost and green metal-free electrocatalysts for ORR. For heteroatom doping to improve the ORR activity is attributed to changes in the surface electronic structure of carbon, regulating carbon material with negative surface also could improve the O2 adsorption to further enhance the ORR catalysis. Although the heteroatom-doped carbon have shown reasonable electrochemical activity towards ORR, the preparation of doped carbon usually requires high temperature, or complicated equipment associated with safety concerns. Therefore, there is still much research space when it can actually be applied to the fuel cell for the carbon material as a catalyst for ORR. Meanwhile, doping is not the only way to surface-modified carbon material, regulatory function of the carbon material with small molecule and creating more the active site at the edge of carbon materials are both alternative strategies as more modest ways to enhance the ORR performance of carbon materials.Firstly, in chapter I of this paper, we introduced the mechanism of fuel cell and summarized the development of cathodic ORR electrocatalysts in the recently years. In Chapter II, a micro apparatus for the electrochemical studies on the ORR with highly oriented pyrolytic graphite (HOPG) as the working electrode and electrocatalysts is presented. An air-saturated droplet of around 15 um in diameter was deposited using a microinjection system onto the specified location of the surface of HOPG. The ORR characterizations were performed with the HOPG as the working electrode, the air-saturated droplet as the electrolyte, the Pt wire and Ag/AgCl wire in a capillary tube connected with the droplet as the counter and reference electrodes, respectively. With this technique, we could deposit the air-saturated droplet onto any specified locations of the HOPG surface to investigate the ORR activity at different locations. Particularly, we compared the ORR activities of the edge and the basal plane of HOPG and found that the edge is much more active than the basal plane. The conclusion that the edge is much more active than the basal plane of graphite leads us to design bulk graphite powder with enriched edges as efficient metal-free electrocatalysts for ORR. The as-prepared graphite powder by ball-milling indeed showed significantly enhanced catalytic activity for ORR. The DFT calculation was also performed to investigate the effect of the edge carbon, confirming that the higher charge densities on the edge carbon contributed to the improved catalytic activity for ORR. This work shows a clear image on where the active sites are on graphite for ORR.In Chapter III, our strategy to design metal-free electrocatalyst is to use small molecules (tetracyanoethylene, TCNE) with strong electron-accepting ability to non-covalently functionalize graphene. This process is very simple by just dispersing graphene in proper solvent in the presence of the small molecules. Since graphene is an electron-rich material and the chosen small molecules have strong electron-accepting ability, the charge transfer process would occur from graphene to small molecules in the functionalized graphene materials. In addition, the well-defined structure of TCNE makes the theoretical calculation easier to fundamentally understand the charge transfer behaviour. The functionalized graphene was used as metal-free electrocatalyst for ORR, showing enhanced electrocatalytic activity relative to the unfunctionalized graphene. Our theoretical calculation confirms that the charge transfer process could alter the electron density of carbon atoms in the functionalized graphene, which would promote its electrocatalytic activity towards ORR. Furthermore, we provide a new design principle to the efficient carbon-based metal-free electrocatalysts for ORR and will significantly contribute to this research area.