DedicatóriaAos meus familiares, principalmente meus pais que sempre incentivaram e contribuíram muito em minha formação pessoal e acadêmica. AgradecimentosAo Prof. Dr. Edson Antonio Ticianelli por orientar o trabalho com assiduidade, dedicação e profissionalismo e principalmente a sua amizade. À minha família por tudo o que tem feito por mim. À todos os Professores que colaboraram de alguma forma na elaboração deste trabalho. Ao Instituto de Química de São Carlos-USP pelo apoio. Ao Fernando, Elki e Sydney pela paciência, pelo conhecimento transmitido e por terem contribuído para a elaboração deste trabalho. A todos os técnicos, principalmente do grupo de eletroquímica, (Valdecir, Maristela e Jonas) que colaboraram para a realização deste trabalho. Aos colegas de laboratório pelas valiosas discussões que contribuíram na minha formação científica.

In this study the activity of Pt/C, Pt 3 Co/C and PtCo/C electrocatalysts supported on high surface area carbon (Vulcan XC-72) for the oxygen reduction reaction (ORR) was correlated to their structural, morphological and compositional changes experienced after aging tests. The electrolytes were sulphuric acid at several concentrations and Nafion ® ionomer membrane. These tests are based in some different protocols that consisted in stepping the potential or keeping the electrode polarized at fixed potentials. The protocols which uses the steps consisted in stepping the potential during 1 minute successively between potentials of 0.9 and 0.1 V vs ERH, 0.9 and 0.6 V vs ERH, 1.05 and 0.10 V vs ERH or 1.05 and 0.65 V vs ERH for 15 hours and for the aging at fixed the polarizations (15 hours) the following potentials were used: 0.9, 0.6 and 0.1 V vs ERH. After the 0.9-0.1 V vs ERH aging the Pt-Co/C catalysts showed no changes in the activity, while for Pt/C an improvement was seen. However for 1.05-0.10 V vs ERH for Pt/C there was also an improvement while for the other catalysts there was a decrease of the activity. For all other protocols, a loss in activity was observed for all catalysts. A transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectroscopy (X-EDS) analyses were used to characterize the as received and aged catalysts. A particularity of this work is the use of identical location transmission electron microscopy (ILTEM) technique, with the objective of analyzing the same electrode regions or particles before and after the accelerated ageing processes, so that it was possible to follow all the morphological, structural and compositional changes caused by the catalyst aging processes. Generally it was observed that the degradation effects of the particles, correspond to the, carbon corrosion, coalescence, dissolution and re-precipitation of the catalyst particles for all aging protocols. The catalysts were compared before and after aging regarding the mean particle size, shape, particle density and composition and correlating these with the catalytic activity. The Pt/C catalyst, for example, which presented an increase of particle mean size without any negative effect of agglomeration, presented an improvement of the catalytic activity, while Pt-Co/C, in spite of the increase of the mean particle size and cobalt dissolution, presented worse or at most the same activity as that of the uncycled materials.