The depletion of fossil fuels and global warming require the application of highly efficient and sustainable energy conversion devices, like fuel cells. However, the sluggish kinetics of the oxygen reduction reaction (ORR) on the cathode has significantly hindered the widespread implementation of fuel cells, which stimulates the development of highly active and durable electrocatalysts. Enormous progress has been made on both precious metal and non-precious metal catalysts, including Pt/Pd alloyed nanoparticles, transition metal oxides and nitrogen doped carbon materials. In this thesis, a bifurcated strategy involving the rational design and in situ mechanistic investigation has been proposed and conducted. Via different synthetic routes, a variety of nano-structured materials has been prepared, including Pt-based structurally ordered intermetallics, core-shell structured nanoparticles with Pt surface decoration, tri-metallic spinel oxides, bimetallic organic framework (BMOF) derived carbon materials and nitrogen-doped BMOF-derived Pt-Co nanoparticles. They are featured with distinctive properties to adapt various working conditions, for the ground transportation or stationary applications, for proton exchange membrane fuel cells (PEMFCs) or alkaline exchange membrane fuel cells (AEMFCs). For example, Pt-based catalysts exhibit excellent electrocatalytic activities with a relatively high Pt utilization efficiency, making them suitable as promising candidates in the acidic electrolyte for fuel cell vehicles. Precious-metal-free catalysts demonstrate high activity, robust durability and cost effectiveness, which can be potentially applied in alkaline media for stationary applications. The in situ mechanism analysis of these materials consists of two parts: the first reveals the structural evolution of structurally ordered bimetallic intermetallics at high temperature, from both microscopic and macroscopic levels to determine the optimized synthesis condition for the Pt3Co intermetallic nanoparticles. The second part couples the electrochemical reaction with X-ray adsorption spectroscopy to elucidate the working and degradation mechanism of trimetallic oxides, which indicates the synergistic catalysis by Co and Mn, with Fe serving as the stabilizing agent. These analyses, in return, provide valuable insights for designing and optimizing related materials in the future.