The sluggish kinetics of oxygen reduction reaction (ORR) contributes significantly to the loss of cathode overpotential in fuel cells, thus requiring high loadings of platinum (Pt), which is an expensive metal with limited supply. However, Pt and Pt-based alloys are still the best available electrocatalysts for ORR thus far. The research presented in this dissertation focuses on the study of the enhanced electrocatalytic activity of Pt-based cathode catalysts by alloying Pt with another transition metal, Co, toward the ORR and the improved performance and stability of this alloy by the addition of Mo, another transition metal. In the first study, the X-ray photoelectron spectroscopy (XPS) and electrochemical techniques, such as cyclic voltammetry (CV with a rotating disk electrode (RDE), have been used to study the surface structure, stability, and electrocatalytic activity of Pt[subscript 3]Co alloy nanoparticles toward ORR (compared to that of Pt black) in both alkaline and acidic media. According to the XPS data, two species of cobalt, namely metallic cobalt and cobalt oxides, coexist in the as-received Pt[subscript 3]Co alloy nanoparticle sample. The XPS and electrochemical data also revealed that the cobalt species in Pt[subscript 3]Co are stable in alkaline media, whereas there is a dissolution of cobalt oxides upon exposure in acidic media. As the cobalt oxides dissolve from the catalyst surface, a metallic Co core protected by a pure Pt shell remains. This is the so-called "Pt skin" in which metallic Co exerts an electronic effect on Pt and causes an increase in the catalyst's ORR activity over pure Pt. The electronic effect was observed in the Pt 4f binding energy upshift of about 0.2 eV vs. that of pure Pt, which is consistent with previous reports. The binding energy upshift of Pt 4f is correlated to the downshift of Pt d-band center. In acidic media, Pt[subscript 3]Co showed a slightly higher activity than Pt black. However, in alkaline media, the adsorption of OH species on Co shielded the Pt active sites, causing Pt[subscript 3]Co to be less active than Pt black. Notably, following an electrochemical pretreatment in acidic media to remove cobalt oxides and create the "Pt skin," the catalytic activity of the Pt[subscript 3]Co alloy sample can be restored equivalent to that of Pt black in alkaline media. These results, along with the d-band theory, present a new understanding in terms of fine tuning and designing a suitable electrocatalyst with less content of Pt while maintaining the its activity for fuel cells. In addition to overcoming the slow kinetics of the ORR, another critical issue to low temperature fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst under the harsh operating conditions in fuel cells. In the second part of my study, the performance, stability, and durability of four different Pt-based cathode catalysts were investigated in a microfluidic hydrogen-oxygen (H[subscript 2]/O[subscript 2]) fuel cell platform with a flowing acidic electrolyte. The studied catalysts include Pt black, as-received unsupported commercial Pt[subscript 3]Co alloy nanoparticles, acid-treated Pt[subscript 3]Co (Pt[subscript 3]Co-at), and Mo-modified Pt[subscript 3]Co nanoparticles (Pt[subscript 3]Co/Mo). The addition of Mo to the Pt 3Co nanoparticles was confirmed by XPS and electrochemical data. Adding Mo to Pt[subscript 3]Co nanoparticles does not alter the electronic effect of Co exerted on Pt, but significantly improved the performance and stability of the electrocatalyst. The binding energy of Pt 4f also demonstrated an upshift of 0.2 eV vs. that of pure Pt, similar to that of the nanoparticles without Mo. Here, a novel application of the rotating disk electrode was used to study the electrocatalytic activity of relevant electrocatalysts directly on gas diffusion electrodes, indicating an oxygen reduction activity of Pt3 Co/Mo which is slightly better than that of Pt[subscript 3]Co and Pt black. In terms of fuel cell performance, all the cathode catalysts showed good short-term stability and electroactivity. However, in accelerated aging studies,Pt[subscript 3]Co/Mo showed a superior improved long-term stability over 10,000 potential cycles in acidic solution over the other catalysts studied. This enhancement of Pt[subscript 3]Co/Mo was attributed as both enhanced catalytic stability and electrode durability via an electrochemical impedance spectroscopy study. Therefore, building on these promising results, further development of these catalysts may lead to significant performance enhancements. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]