The oxygen reduction reaction (ORR), which is the cathodic reaction in a proton-exchange membrane fuel cell (PEMFC) and in several other important processes, is a widely studied reaction. From the kinetics viewpoint, Pt is the best electrocatalyst and its activity can be increased upon alloying with a 3d-transition metal (Co, Ni, Fe, Cu). Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy prove that the structural and compositional changes of Pt3Co/C nanoparticles during real-life PEMFC operation are much richer than previously thought from accelerated stress tests. Four different nanostructures are observed after 3422 h of operation in stationary mode: Pt, Pt-Co/C "hollow", Pt-Co core-shell and Pt "bulk" nanoparticles. The presence of "hollow" nanoparticles in the aged catalytic layer is accounted for by the nanoscale Kirkendall effect, a vacancy-mediated diffusion mechanism in binary alloys where one species diffuses faster than the other. The oxygen reduction reaction specific activity of the "hollow" nanostructures is 1.5-fold that of the fresh Pt3Co/C cathode catalyst and 3-fold that of Pt/C nanoparticles, thereby offering a new route to synthesize highly active and durable PEMFC electrocatalysts. (C) 2014 Elsevier B.V. All rights reserved.

Low density foams (in this work, foam density refers to apparent density) are materials of interest for fusion experiments. Low density poly(4-methyl-1-pentene)(commercial name TPX) foams have been produced for 30 years. TPX foams have been shown to have densities as low as 3 mg.cm(-3), which is very close to air density (1.2 mg.cm(-3)). Around this density foams are very light and highly fragile. Their fabrication is thus a real technological challenge.However, shrinking always appears in ranges ranking from 25% to almost 200%. As a result, the apparent density of the final foam never matches the expected value given by the precursor solution concentration. Besides, even if the mold dimensions are precisely known, shrinkage is never linear, and foams have to be machined for precise density measurement.In our work we present a fabrication process for TPX foams and discuss machining and density measuring issues.Particularly, we have found that there are volume and weight limits for a determination of density within the range of 3% uncertainty. This raises the question whether density should rather be determined directly on millimeter-sized targets or should be performed on a bigger scale sample prepared from the same batch.

Increasing lifetime and performance is critical for proton exchange membrane fuel cell (PEMFC) using stainless steel plates. A good compromise between passivity and electrical contact resistance of the plate material is required. Measuring the potential of each plate during fuel cell operation is of paramount importance to lead to relevant ex situ tests in order to investigate new materials. From a review on methods used for potential measurements, the present work focused on the realization and use of a dynamic hydrogen electrode (DHE) device as a reference electrode in a PEMFC single cell and its evaluation in terms of accuracy and drift. With classic reference electrodes introduced into the flow field, measurements were shown to be irrelevant because of the impossibility to ensure good and stable ionic conductivity between the reference electrode and the plate when operating the cell. Several examples of DHE found in the literature were reviewed and used to realize a DHE, which showed correct accuracy and stability of its potential under fully humidified conditions. The experimental device was shown to be reliable and easily adaptable for different single cells. It was used to investigate transient phenomena while cycling a cell, but needs some improvement when the cell is operated with unsaturated gases.

Bipolar plate is a key component of Polymer Electrolyte Fuel Cell (PEFC) with several essential functions, and has to resist to harsh cell operating conditions. Usually made of graphite or carbon composite, this component is studied worldwide in order to develop a commercially viable alternative: different ways have been being investigated, and to date, stainless steel (SS) appears as a good candidate material, but corrosion issues hinder its general use. This paper offers a comprehensive study of parameters impacting SS plate material corrosion through ex situ electrochemical investigations. Impact of ageing under various conditions is reported and studied in terms of film structure, semiconductivity behaviour, and cation release, on as received 316L and 904L alloys, and also surface-treated by low-cost surface modifications.

Bipolar plate represents a key component of Proton Exchange Membrane Fuel Cell (PEFC) with several essential functions, among them the electric connection of elementary cells. Usually made of graphite, this component is studied worldwide in order to develop a commercially viable alternative: different ways have been being investigated, and to date, despite corrosion issues, stainless steel (SS) appears as a good candidate material, but its Electrical Contact Resistance (ECR) can reach unacceptable values when exposed to PEFC environment. This paper offers a comprehensive study of the parameters acting on ECR when using uncoated SS in PEFC: roughness, which influences the surface contact area with carbon baking, bulk composition of the alloy, which influences only partly the nature of passive films, and the composition and structure of passive films, strongly modified by surface treatments and ageing conditions.

The parameters responsible for electrical contact resistance between a stainless steel bipolar plate and a carbon support in a PEFC has been investigated. Roughness of the bipolar plate is useful but its topography should be adapted to the type of used GDL. Bulk composition of the alloy also plays a role, because it defines the quantity of alloying elements susceptible to participate to the passive film growth, but this role appears secondary. Indeed, passivation pre-treatment can deeply modify both structure and composition of the passive layer as investigated on AISI 316L and 904L steels, until reducing the initial electrical contact resistance at a level comparable to graphite. A low-cost surface treatment could thurs represent a good compromise between using as-received stainless steel plates and deposition precious metals. Compared to bright annealed state, initial ECR is divided by a factor 20 but raised slightly after 500 hours of operation.

Increasing lifetime while maintaining performance is of major interest in the challenge for PEMFC development. One promising way when using metallic bipolar plates is to find a compromise between passivity and electrical contact resistance of the passive films formed on stainless steel. A specific test facility and testing procedure have been developed for measuring the electrical contact resistance. A parametric study has been investigated and significant differences have, for instance, been measured depending on the presence and/or the nature of the gas diffusion layer in contact with the metallic plate, on the grade and the initial surface state of the investigated 316L and 904L stainless steels. The evolution of the contact resistance is investigated after heat treatment of metallic samples to determine what in the passive film influences the contact resistance. XPS spectrometry is used to correlate electrical performance with film characteristics (thickness, composition). Ex situ ageing in a synthetic electrolyte which reproduces a cathodic PEMFC environment is being investigated.