Porous electrodes based on platinum capped electrocatalyst: Combining thermal treatment XPS analysis and electrochemistry give evidence for the stabilizing role of the thiol capping agent on the Pt dispersion and core feature
Résumé
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.