Catalyst is a substance that can increase or decrease the rate of reaction without modifying the overall standard Gibbs free energy change in the reaction. Catalysis is everywhere in our daily life, and green energy related energy conversion catalytic reactions, happening in the fuel cell, have brought more and more research interests. Oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) are two critical electrochemical reactions for proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) applications, and Platinum has been found out to have the highest performance and stability among all elements to catalyze both these two reactions. The natural sacristy and high cost of Pt limit the use and spread of fuel cell applications into the public domain. At the same time, non-Pt based catalysts are under development, but it is not comparable to Pt catalyst in terms of activity. As a result, promoting the utility of Pt-based materials’ catalytic activity towards ORR and MOR has gained more and more attention and become the key challenge for catalyst design in the last decade. Currently, there are two design strategies to improve the catalytic activity of Pt-based nanomaterials, which are engineering absorption sites and modifying electronic structure. In Chapter 2 of my dissertation, through altering grain boundary density to modify the absorption site on Pt surface, we observed a positive quadratic correlation between the ORR specific activities of the Pt nanostructures and the grain boundary densities on their surfaces. Compared to commercial Pt/C, the grain-boundary-rich strain-free Pt ultrathin nanoplates demonstrated a 15.5 times higher specific activity and 13.7 times higher mass activity. Simulation studies suggested that the specific activity of ORR was proportional to the resident number and the resident time of oxygen on the catalyst surface, both of which correlate positively with grain boundary density, leading to improved ORR activities. In Chapter 3 of my dissertation, by optimizing the composition of ternary octahedral PtCuCo nanostructure, I report Pt38Cu39Co23 exhibits high stability and specific activity. The specific activity is 12.5 and 8.48-fold higher than commercial Pt/C in acidic and alkaline conditions, respectively. Specific activity retention and mass activity retention are 85.3%, and 91.3% respectively, while commercial Pt/C 10% catalyst has only a 76.9% specific activity and 58.6% mass activity retention. In Chapter 4 of my dissertation, the influence of different dispersion media, ionomer concentration, and thin film thickness on thin film rotating disk (TF-RDE) are evaluated. Firstly, I found out that pure ethanol was the best dispersion media to prepare the catalyst ink. The intrinsic property of ethanol allows a uniform ionomer coverage, which minimizes the carbon hydrophobicity and allows oxygen diffusion ability. Secondly, Ionomer concentration plays a more important role in the TF-RDE technique than the ionomer/carbon (I/C) ratio does, and an optimal 8 μL per milliliters of ethanol was successfully observed. The ORR activity would decrease with either a too low or too high ionomer concentration. Thirdly, a critical thin film thickness was observed as in between 8 to 10 μm. Due to the limited local proton and oxygen concentration, thickness above the critical thickness point would experience a decrease in activity.