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Tin dioxide is a promising catalyst support to improve the stability of proton-exchange membrane fuel cells (PEMFC) cathodes at high voltages. However, optimizing the catalytic activity for the oxygen reduction reaction (ORR) of tin dioxide based electrocatalyst still remains challenging. In this study, an antimony doped tin dioxide (ATO) aerogel featuring suitable physico-chemical properties for application at a PEMFC cathode was successfully synthetized. Two platinum nanoparticles deposition methods were tested and compared. One is a chemical reduction route (using ethylene glycol, EG), the other uses ultraviolet (UV) irradiation followed by different thermal post-treatments. Electrocatalysts structure and morphology were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments. The highest ORR mass activity measured in liquid electrolyte at 25°C was obtained for Pt/ATO from EG method (32 A gPt -1 versus 27 A gPt -1 for a reference Pt/HSAC 40 wt%). Pt/ATO turned out to be more stable than Pt/HSAC during an accelerated stress test composed of 5,000 potential cycles between 1.0 and 1.5 V vs. RHE at 80°C. [ABSTRACT FROM AUTHOR]

In this work, we operated a 16-cell proton-exchange membrane fuel cell (PEMFC) stack based on a Pt/C anode and a Pt3Co/C cathode under H2/air at constant current (j=0.6 A cm-2), temperature (T=70°C), relative humidity (RH) (anode: 0% RH, purge mode; cathode: 65% RH at the inlet), and pressure (1.3/1.1 bars absolute at the anode/cathode, respectively) for 1124 h. The membrane electrode assemblies were characterized with physical [field-emission gun-scanning electron microscopy (FEG-SEM), high resolution transmission electron microscopy, X-ray diffraction], electrochemical (cyclic voltammetry), and chemical (inductively coupled plasma-atomic emission spectrometry and X-ray energy-dispersive spectroscopy) techniques after different life stages, with special emphasis on the cathode electrocatalyst: as received (0 h), conditioned (17 h), and after 504 or 1124 h of aging. At the micrometer scale, FEG-SEM images reveal a very subtle degradation of the catalytic layers (thickness and density of defects). At the nanometer scale, the cyclic voltammograms of the cathode electrocatalyst point toward fast leaching of Co and formation of a "Pt skeleton" structure at the topmost surface layer within the first hours of operation. When the proton-exchange membrane fuel cell is operated for longer durations, two phenomena increase the "Pt character" of the electrocatalyst surface: Co surface segregation/leaching and Ostwald ripening. Consequently, the Pt-skeleton particles slowly evolve toward "Pt-shell/Pt-Co alloy core" structures with depleted Co content and a Pt-enriched shell (in the order of 2 atomic monolayers after 1124 h of operation). [ABSTRACT FROM AUTHOR]

This paper (Part II) aims to provide an understanding of the degradations originating in the membrane or the binder and which dramatically affect the membrane electrode assembly performance and therefore the lifespan of protein exchange memory fuel cells. In Part I, evidence was provided of the corrosion of the platinum catalysts and the diffusion/migration through the membrane of platinum cations. In this paper we establish that these platinum cations induce physical cross-links that modify both the thermomechanical properties and the conductivities of the membrane. Since physical cross-links are reversible, ex Situ experiments were performed to recover the performance of pristine membranes by acidic treatment. Membrane performance was found to be much more affected in the case of membranes aged in the hot zone rather than in the cold zone. [ABSTRACT FROM AUTHOR]

The physical and chemical state of proton exchange membrane fuel cell (PEMFC) electrocatalysts (Pt/C) were investigated after 529 h of operation under fuel cell relevant conditions (333 K, 0.12 W cm-2) and 123 h of rest time under inert atmosphere (N2). Upon aging, pronounced corrosion of the cathode electrocatalyst (carbon-supported platinum nanoparticles: Pt/C) was evidenced by field-emission gun scanning electron microscopy (FEG-SEM), high-resolution transmission electron microscopy (HRTEM), and electrochemical techniques. Carbon corrosion was witnessed by the decrease of the cathode thickness (-60%) and by the presence of nonsupported Pt particles inside the electrode. At the cathode, the corrosion of Pt nanoparticles produces Pt(*)z+ ions which diffuse in the ionomer phase or in solution [(*) stands for ionic species present in the ionomer phase or in solution]. These ions are highly mobile inside the membrane electrode assembly (MEA) and may cross over from the cathode to the anode through the PEM. The driving force for that is the electro-osmotic drag (if Pt(*)z+ ions combine with anions and carry a negative charge) and the chemical diffusion (concentration gradient of oxidized platinum species). Consequently, Pt(*)z+ ions were detected by ultraviolet (UV) spectroscopy in the PEM. Due to the high mobility of Pt(*)z+ ions, FEG-SEM and HRTEM analysis of the cross sections of MEAs revealed a pronounced change of Pt distribution after operation. Size distributions of both anode and cathode electrocatalysts evidenced an increase of the mean particle size, tailing toward large particle sizes and particle agglomeration. Nonspherical Pt nanoparticles were detected inside the PEM, the size and shape distribution of which strongly depend on the distance from the cathode. We believe PtPt(*)z+ ions are reduced chemically inside the membrane by H2 crossing over the PEM and both chemically and electrochemically at the anode/cathode. [ABSTRACT FROM AUTHOR]