The rea-son is that the incorporation of Co into the fcc structure of Ptinduces contraction of the lattice. The contraction can sensitivelylead to a change in the electronic properties of platinum whichweakens the strength of the M–O bond [23]. Furthermore, the oxi-dation peak of hydrogen occurs in the region (1), and this peakarea could be used to determine the active Pt surface area by theelectrochemical method. The specific electrochemically active sur-face areas (ECSA) can be used to measure the utilized efficiency ofthe Pt nanoparticles, and this is more important to the catalyticreactions than surface area. By measuring the charges collectedin the Hupd adsorption/desorption region (1) after double-layercorrection and assuming a value of 210 C/cm2for the adsorp-tion of a hydrogen monolayer [24], the ECSA of the particleswere calculated to be 80.1 m2/g (Pt/C), 96.0 m2/g (Pt0.7Co0.3/C), and92.3 m2/g (Pt0.55Co0.45/C), respectively. The results can attributeto the better distribution and smaller particle size with addingCo content. But if the content of Co reaches a certain value(Pt0.55Co0.45), cobalt will enrich on the surface and lead to smallerECSA