Grégory Berthomé

Membrane and Active Layer Degradation upon PEMFC Steady-State Operation I. Platinum Dissolution and Redistribution within the MEA

We studied proton exchange membrane fuel cell membrane electrode assemblies (MEAs) degradation after fuel-cell operation. Anode and cathode pronounced degradation was monitored by chemical [energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS)], physical [scanning electron microscopy (SEM), transmission electron microscopy], and electrochemical (ultramicroelectrode with cavity) techniques. Aged MEAs underwent severe redistribution of most elements (Pt, C, F), coupled to a dramatic change of Pt particles shape, mean particle size and density over the carbon substrate. Among the various scenarios for Pt redistribution, Pt dissolution into Pt z+ species and transport in the ionomer or the proton exchange membrane play important roles. The Pt z+ dissolution/transport is likely favored by activators/ligands (F- or SO x -containing species) originating from the alteration of the polymers contained in the MEA. From SEM observations, the source of Pt z+ species is the cathode, while EDS and XPS show some SO,- and F-containing species origin from the anode. Local chemical analyses (SEM-EDS and XPS) revealed the excess Pt monitored in aged MEAs is associated with F excess. For instrumental limitation concerns, we could not detect the S element, but SO x -containing species could also act as counter ions during Pt z+ transport within the MEA. Pt corrosion/ redistribution is associated with the decrease of Pt-active area as revealed by CO ad -stripping voltammograms.

Reactivity of titanium in physiological medium. I. Electrochemical characterization of the metal/protein interface

A systematic investigation of the reactivity of a mechanically polished titanium interface in contact with calf serum with and without the addition of phosphate ions is presented. Linear sweep voltammetry experiments and electrochemical impedance spectroscopy were used to obtain information on the corrosion behavior of the titanium interface when immersed in a 21% calf serum solution diluted with phosphate buffer or water. The electrochemical analysis established a modification of the cathodic reaction processes in connection with a blocking of reactional sites due to adsorption phenomena as well as the formation of a passive film protecting the metallic titanium. Surface analysis using X-ray photoelectron spectroscopy complemented the electrochemical results and was used to characterize the composition and the morphology of the interphase. While calf serum absorbs strongly on Ti in aqueous solutions, the presence of phosphate ions limits this phenomenon. A competition between the adsorptions of calf serum and those relating to phosphate ions have to be considered. The decrease of absorbed serum in the presence of phosphate ions is believed to be related to the change in wettability of the titanium surface in the presence of phosphate. An interfacial architecture model of the titanium interface in contact with calf serum and in the presence of phosphate ions was established and consisted of an assembly of four layers: metallic Ti, titanium oxide, phosphated titanium, adsorbed calf serum.

Manganese Dioxides Surface Properties Studied by XPS and Gas Adsorption

Structurally well defined γ-MnO 2 in terms of Pr and Tw were studied using X-ray photoelectron spectroscopy (XPS) to complete surface investigations performed by high-resolution gas adsorption, water adsorption, and acid-base surface titration. The O Is s and Mn 2p spectra were deconvoluted into three components Mn-OH + 2 , Mn-OH, and Mn-O - . Analysis of the O Is spectra in energy shift and relative intensity shows that 70 to 80% of the surface groups stay as neutral OH, 5% as Mn-OH 2 and 20 to 30% as Mn-OH: their relative amount varies with Pr as well as PZC, water cross-sectional area and energetic constant C. Mn 2p spectra are far less sensitive than O is to the charge variation. Nevertheless a correlation is shown between the relative amount of surface species, the binding energy of Mn = O and O = Mn species and Tw.

Undoped TiO2 and nitrogen-doped TiO2 thin films deposited by atomic layer deposition on planar and architectured surfaces for photovoltaic applications

Undoped and nitrogen doped TiO2 thin films were deposited by atomic layer deposition on planar substrates. Deposition on 3D-architecture substrates made of metallic foams was also investigated to propose architectured photovoltaic stack fabrication. All the films were deposited at 265 degrees C and nitrogen incorporation was achieved by using titanium isopropoxide, NH3 and/or N2O as precursors. The maximum nitrogen incorporation level obtained in this study was 2.9 at. %, resulting in films exhibiting a resistivity of 115 Omega cm (+/-10 Omega cm) combined with an average total transmittance of 60% in the 400-1000 nm wavelength range. Eventually, TiO2 thin films were deposited on the 3D metallic foam template.

Wettability Study of SiC in Correlation with XPS Analysis

The wettability characteristics of both C and Si faces covered with their natural oxide of differently polished 6H-SiC wafers have been systematically determined using the contact angle measurement, before and after HF treatment. XPS was used to characterise the surface chemical state. In an original way and for the first time regarding SiC wafers, we proposed a fitting of the oxygen peak which allowed to calculate the area SiOH/SiO2 ratio. A strong correlation was found between the wettability increase induced by the HF treatment and the presence of polar hydrophilic SiOH groups on the surface.