Laetitia Dubau

Identical-Location Transmission Electron Microscopy Study of Pt/C and Pt-Co/C Nanostructured Electrocatalyst Aging: Effects of Morphological and Compositional Changes on the Oxygen Reduction Reaction Activity

In this study, the activity for the oxygen reduction reaction (ORR) and the structural, morphological, and compositional changes of Pt, Pt3Co, and PtCo nanoparticles deposited on high surface area carbon (Vulcan XC72) are investigated after they were submitted to accelerated electrochemical aging tests. These tests consisted of stepping the potential in 1xa0min successively between potentials of 0.9 and 0.1xa0V vs. reversible hydrogen electrode (RHE) or between 0.9 and 0.6xa0V vs. RHE for 15xa0h in 1.0xa0M H2SO4 at 60xa0°C. Transmission electron microscopy, identical-location transmission electron microscopy, X-ray energy-dispersive spectroscopy analyses, and in situ X-ray absorption spectroscopy are used to characterize the changes in the morphological, compositional, and in the Pt 5d electronic states before and after the electrochemical aging tests. The experimental results show that the Pt/C and Pt–Co/C electrocatalysts are modified upon aging, following changes in particle size, geometry, and composition. The 0.9- to 0.6-V protocol is more aggressive in reducing the ORR activity, and this seems to be strongly related to the carbon corrosion and not to changes in the metallic particles. After the 0.9- to 0.1-V aging procedure, the ORR activity of the Pt/C particles is improved, while that of Pt3Co/C particles only slightly changes. In the case of Pt/C, these effects are related to the increase of the particle sizes, surface reconstruction, and, therefore, smaller oxide formation, which potentially induces an increase of the ORR activity. On the contrary, on Pt3Co/C, these positive effects are counterbalanced by a detrimental (and large) effect of Co dissolution.

Accelerated Stress Tests of Pt/HSAC Electrocatalysts: an Identical-Location Transmission Electron Microscopy Study on the Influence of Intermediate Characterizations

The influence of intermediate characterizations used in long-term accelerated stress tests (ASTs) to monitor the changes of the electrochemically active surface area of carbon-supported Pt nanoparticles (Pt/HSAC) was investigated. Our results indicate that, in the studied experimental protocol (potentiostatic polarization at Eu2009=u20091.0xa0V vs. RHE during 96xa0h), the loss of the electrochemically active surface area is greatly exacerbated by intermediate characterizations such as cyclic voltammetry or stripping of a saturated COad surface layer. These results can be understood in view of the breakdown of the passivation layer formed on the Pt/HSAC electrocatalyst during the polarization at Eu2009=u20091.0xa0V vs. RHE. By using identical location transmission electron microscopy, the structural modifications of the Pt/HSAC nanoparticles could be monitored. The migration/agglomeration of Pt nanocrystallites, the growth of Pt nanocrystallites by electrochemical Ostwald ripening, and the corrosion of the high surface area carbon support are more pronounced when cyclic or COad stripping voltammograms are implemented in the AST. A detailed analysis of the identical-location transmission electron microscopy images also indicates that adsorbed CO molecules minor the dissolution of Ptz+ ions into the electrolyte.

First Insight into Fluorinated Pt/Carbon Aerogels as More Corrosion-Resistant Electrocatalysts for Proton Exchange Membrane Fuel Cell Cathodes

This study evaluates the fluorination of a carbon aerogel and gives first insights into its durability when used as platinum electrocatalyst substrate for proton exchange membrane fuel cell (PEMFC) cathodes. Fluorine has been introduced before or after platinum deposition. The different electrocatalysts are physico-chemically and electrochemically characterized, and the results discussed by comparison with commercial Pt/XC72 from E-Tek. The results demonstrate that the level of fluorination of the carbon aerogel can be controlled. The fluorination modifies the texture of the carbons by increasing the pore size and decreasing the specific surface area, but the textures remain appropriate for PEMFC applications. Two fluorination sites are observed, leading to both high covalent C-F bonds and weakened ones, the quantity of which depends on whether the treatment is done before or after platinum deposition. The order of the different treatments is very important. Indeed, the presence of platinum contributes to the fluorination mechanism, but leads to amorphous platinum, which is demonstrated rather inactive towards the oxygen reduction reaction. On the contrary, a better durability was demonstrated for the fluorinated and then platinized catalyst compared both to the same but not fluorinated catalyst and to the reference commercial material (based on the loss of the electrochemical real surface area after accelerated stress tests).

Influence of PEMFC Operating Conditions on the Durability of Pt3Co/C Electrocatalysts

This work is dedicated to the characterization of structure and composition change of the membrane electrode assembly (MEA) upon on-site proton-exchange membrane fuel-cell (PEMFC) operation (constant current load and start/stop conditions), with special emphasis on the Pt3Co/C cathode electrocatalyst. A fast degradation of the cathode catalytic layer was monitored. It yielded to the redistribution of both Pt and Co elements within the whole MEA, decrease of the MEA thickness and a continuous leaching of the surface/bulk Co atoms. The cell performance decay is discussed in the light of these changes.

Tools and Electrochemical In Situ and On-Line Characterization Techniques for Nanomaterials

In the last century, progress in electrochemistry and electrocatalysis was very spectacular due to the remarkable evolution in surface science, chemistry, and physics. Electrochemical study of perfect smooth or bulk materials was the usual way to understand the interaction between the surfaces of such materials with their close environment. Therefore, any modification of the surface structure or composition provides change in the material behavior and the nature of the adsorbed species or near. Usually, the modification of smooth surface consists in the increase of its roughness factor through the deposition of sublayer or layer of metal particles. The deposition can be done on a well-defined surface (model electrode with a known crystallographic structure) [1]. Then, surface modification becomes a way of creating new active sites to enhance the reactivity of molecules. The development of nanoscale materials has changed the approach of studying and identifying active sites in heterogeneous catalysis, and particularly in electrocatalysis. Indeed, electrocatalysis uses the surface of a material, which is submitted to an electrode potential, as the reaction site. Therefore, the material structure, morphology and its composition are the key parameters to control the electrochemical process [2]. The nature of the active site depends on these parameters. Furthermore, the assessment of the nature of the active site before, during, and after the electrocatalytic reaction becomes a huge challenge. Thereby, electrochemical tools like cyclic voltammetry, underpotential deposition of a monolayer of a species [3–5], the specific adsorption of species or molecule, and CO stripping [6] were the first approaches. It is the basic measurement of the electrons flow through the surface per unit of time during the reaction at the surface. Therefore, the electric current per area unit represents the charge transfer reaction that occurs at a metal-solution interface. Since the middle of the last century, an increase in the development of several in situ spectroscopic techniques was observed due to the need of understanding the structure of the interface between electrodes and solutions. Indeed, coupling the electrochemistry measurements to other techniques such as Fourier Transform Infrared Spectroscopy (FTIRS), X-Ray Diffraction (XRD) [7, 8], Transmission Electron Microscopy (TEM) [9], Scanning Tunneling Microscopy (STM) [10], Surface-Enhanced Raman Scattering (SERS) [11] becomes a suitable approach to assess in real time at the electrified interface electrode-solution; some relevant data on electrocatalysts structure, morphology, composition, and stability of materials; and on the reaction intermediates and products. The identification of the surface state in addition to that of adsorbed species, intermediates, and products of the reaction process have permitted to determine a mechanistic pathway which is essential for enhancing the material performance and selectivity. It appears obvious that the identification of surface state of a nanomaterial under realistic electrochemical reaction conditions represents a noble scientific breakthrough. In the present chapter, for the first time some techniques coupled with electrochemistry able to characterize nanomaterials as electrodes will be extensively addressed. This chapter will also show the progress in in situ electrochemical approaches. One motivated approach is to be able to characterize electrochemically and experimentally the surface of the nanoparticle. Therefore, in the first part of the chapter, an example of a pure electrochemical tool, which permits to probe the nanoelectrocatalyst surface, is discussed. Although the progress in nanotechnology increases rapidly, various tools have been developed in electrochemistry for understanding the reaction pathway, intermediates, and products formation.