Ab-Initio Computational Studies of the Catalysis Mechanisms on Graphene-Based Materials in Fuel Cell Applications

Abstract

Low temperature fuel cells with noble metal-based catalysts are one of the promising green technologies. However, they have obstacles to wide commercialization largely caused by catalysts with high materials costs, poor electrochemical durability, and low efficient activity. As such, it is highly demanded to develop inexpensive but efficient and stable metal-free catalysts of fuel cells. Recently, it was reported that the electrochemical activity of nitrogen-doped graphene for oxygen reduction reaction (ORR) could be comparable to Pt/C in alkaline fuel cells. In this thesis, catalytic mechanisms of graphene-based materials for Polymer Electrolyte Membrane Fuel Cells (PEMFCs) were studied using first-principles density functional theory (DFT) computations. Thesis work includes the atomic-level structural studies of graphene-based materials, charge distribution analysis by doping, interfacial water layer structures, adsorption energies of various chemical species. These quantum mechanical grade of information is used to construct thermodynamic free energy diagrams and thus, to identify the most viable ORR mechanisms. This thesis confirms that doping with heteroatom in graphene causes charge redistribution, and creates charged sites. Positively charged sites with lower electronegativity between carbon and dopants acted as O2 adsorption sites by reducing overpotential needed in the O2 adsorption. Furthermore, two-electron reaction involving hydrogen peroxide was not preferred on N, B, and P-doped graphene but on pristine graphene. Through this thesis, we found doping with heteroatoms can considerably affect the electronic structures of graphene and alter its ORR mechanisms leading to improving ORR activity. ⓒ 2013 DGIST