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Abstract: In previous work we reported oxygen reduction reaction (ORR) studies on porous electrodes based on capped platinum (Pt) electrocatalyst and carbon nanotubes. These structures exhibited a significant activity but a very low platinum electrochemically active surface area (Pt-EASA) due to the grafted molecules on the platinum core nanoparticle surface. The present paper reports on thermal pre-treatment of such electrodes at moderate temperature aiming at degrading the organic capping without changing the nanoparticle Pt core feature. Using X-ray diffraction, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) it is shown that a treatment at 100°C under air fit these requirements. This results in a strong increase of the Pt-EASA. However, it is evidenced by TEM and XRD that, as soon as the organic capping is modified by partial oxidation of sulfur atoms involved in initial strong Pt-S bond, the electrochemical measurement triggers dramatic changes on the Pt dispersion and Pt core feature. Much bigger size Pt regions are formed, complete oxidation of the sulfur atoms is observed and organic capping molecules are significantly eliminated in the electrolyte. Finally, although it was not possible to prove systematically that we get rid of organic contamination after the involved treatments, the dramatic changes of Pt catalyst nanoparticles compared to the initial organically capped ones are clearly established. These original results demonstrate the essential stabilizing role of the grafted thiol molecules in the initial system and allow proposing a scenario for the ageing of these capped electrocatalyst when submitted to prolonged ORR. [Copyright &y& Elsevier]

Abstract: Our recent paper has reported the determination of diffusion area, A DiffO2, related to oxygen reduction reaction (ORR) in porous electrodes structures based on Platinum Organically Grafted Electrocatalyst (Pt-OGE) nanoparticles and carbon nanotubes. It was clearly shown that diminishing the electrocatalyst coverage density at the carbon nanotube surface resulted in a strong increase of A DiffO2, though some of the trends observed as a function of platinum loading were not clearly understood. Previous work considered the ORR selectivity, i.e. n, the average electron number exchanged per O2 molecule as being 4 whatever the Pt loading and the electrode feature. The present paper addresses this point using a recently reported new method for the determination of selectivity that can be implemented directly on porous electrodes, without using rotating disc or ring disc electrodes. Then, the paper considers the determination of specific A DiffO2 (S-A DiffO2) as a function of electrocatalyst loading on both, porous electrode based on Pt-OGE''s grafted with Low Molecular Weight Organic (Pt-LMWO) and with High Molecular Weight Organics (Pt-HMWO). For five kinds of porous electrode structures combining carbon nanotubes and involving both kinds of Pt-OGE''s, the n corrected trends for S-A DiffO2 expressed in m2 gPt−1, are discussed. It is highlighted that for a given porous Pt-OGE/carbon nanotubes electrode structure two feature parameters can be found: a S-A DiffO2 value independent of Pt loading for which the value of n is found to be ≈4, and a critical electrocatalyst loading below which n <4 and S-A DiffO2 deviates from its feature value. These trends are explained in the frame of small sized electrodes arrays at which spherical diffusion regime occurs. As a conclusion, it is finally proposed that S-A DiffO2 could be considered as a feature parameter of interest to characterize the ORR electrocatalysts in porous electrode structures. [Copyright &y& Elsevier]

Abstract: In this paper we report the determination of the diffusion area for oxygen reduction in porous electrode structure having a controlled platinum loading and based on capped platinum electrocatalysts and carbon nanotubes. Such a parameter is expected to be higher than the macroscopic geometrical area of the active porous layer. The oxygen diffusion area is determined by cyclic voltammetry after impregnation of the electrode structure by the electrolyte, and using the equations available for peak potential and peak current as a function of scan speed for irreversible redox couple. First it is found first that the oxygen diffusion area is dependent on the total amount of platinum in the electrode. Second, for a given platinum loading, the diffusion area is higher when the mass ratio of platinum to carbon nanotube decreases. This point indicates that the accessibility of platinum capped electrocatalyst is better in such cases. It is thus concluded that the oxygen diffusion area determination in porous electrode structures may be used to characterize the accessibility of the capped electrocatalysts for oxygen reduction. Even if this area is different in nature from the one calculated by Hydrogen Underpotential Deposition, we believe that its determination might be of interest for the characterization of porous electrodes structures in which the electrocatalyst is combined with a finely divided carbon support. [Copyright &y& Elsevier]