J.-Y. Sanchez

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.

Electrochemical study of linear and crosslinked POE-based polymer electrolytes

Abstract Lithium bis(trifluoromethane sulphone)imide, LiTFSI, complexes based on pristine POE and modified POE were prepared. Conductivity measurements were carried out by ac impedance spectroscopy. For many compositions, conductivity values are higher than 10−6 S cm−1 at 25°C in the first heating cycle. X-Ray and cyclic voltammetry studies performed on the samples prepared by the solvent casting procedure show that LiTFSI acts as a good plasticizer and also presents a wide electrochemical stability window.

Perfluorosulfonate-polyether based single ion conductors

New salts with monomer functionality and bearing a perfluorosulfonate group have been synthesized. These salts contain one or two carbon double bonds and thus can be incorporated into cross-linked solvating ionomers. Another family possesses epoxy rings and can be copolymerized with ethylene oxide. The results, in terms of ionic conductivities and thermal behavior are reported for these single-cation conductor polymer electrolytes. The conductivity values of these materials are markedly higher than those previously reported for other t + = 1 materials. The loss in conductivity, as compared to the untethered salt complexes, corresponds approximately to that attributed to the anionic conductivity. Alkali metals rank in the order σ K+ > σ Na+ > σ Li- .

Large lithium polymer battery development The immobile solvent concept

Abstract The program to develop large lithium-metal, polymer electrolyte batteries for electric traction and stand-by power is reviewed. Dry polymer electrolyte conductivity improvement through research into polymers and lithijm salts has led to a thin-film lithium polymer battery that operates in the range of 60 to 40 °C. Recent developments in large lithium polymer cell design and production are given, including preliminary results on lithium polymer cell production for the US Advanced Battery Consortium (USABC). Results of stand-by and cycle-life tests on small laboratory cells over several years are also presented confirming the electrolytes exceptional stability in the lithium rechargeable-cell environment.

Electrochemical Behavior of Lithium Electrolytes Based on New Polyether Networks

New polyethers have been prepared from polycondensation between several [alpha], [omega]-dihydroxy(oxyethylene) and 3-chloro-2-chloromethyl-1-propene (CCMP). The CCMP units decrease both the crystallinity content and the melting point of the polyether chains. The free-radical polymerization of the dangling double bonds decreases the melting point of the crystalline phase and greatly enhances the mechanical strength of the network. The conductivity as well as the thermal behavior and the electrochemical stability of lithium electrolytes based on these networks have been investigated. Conductivity values close to 10[sup [minus]5] S [center dot]cm[sup [minus]1] at 20 C were obtained for some salt concentrations without compromising the electrochemical stability. A transport number t[sub +] = 0.46 has been determined for LiTFSI/network complexes.

Membrane and Active Layer Degradation Following PEMFC Steady-State Operation II. Influence of Pt z + on Membrane Properties

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.

Comparative ion transport in several polymer electrolytes

A comparative transport number study on a wide variety of lithium salts was performed, confirming the prevalence of an anionic transport in most of the usual salts dissolved in a polyether network. A cationic transport close to 0.5 was determined for several perfluorosulfonate-based polymer electrolytes. The method used confirmed a transport number close to unity for a perfluorosulfonate-based ionomer.

Comparative ab initio calculations on several salts

Ab initio calculations performed on three anions, substituted by strong electron withdrawing groups CF 3 SO 2 , brought useful information on the anion geometries and stabilities as well as on the charge delocalization. Thus the carbanion exhibits a planar optimized geometry while, according to HOMO values, the stability vs. oxidation is best for the imide anion. Moreover the absolute hardness, calculated from the HOMO and LUMO energy levels, show that this latter is the hardest base among the three previous anions.

Conductivity measurements of LiTFSI triblock copolymers with a central POE sequence

Abstract ABA block copolymers were prepared by the addition of styrene oxide or allyl glycidyl ether (AGE) monomers to oligo poly(oxyethylene) α,ω-dialcoholates. While the PEO central block should provide good conductivity at room temperature, the lateral blocks should ensure the mechanical properties. Although an improvement in mechanical properties is obtained by the addition of poly(oxystyrene) (POS) sequences as expected, the conductivity decreases. The addition of small amounts of AGE units to the POE block, followed by free-radical curing of the allylic double bonds, results in the enhancement of the electrolyte strength. Good conductivities are observed already at room temperature.

Electrochemical and spectroscopic studies of polymethacrylonitrile based electrolytes

Abstract Polymethacrylontrile (PMAN)-based electrolytes were recently proposed as polymer matrix for ambient temperature applications. The living anionic polymerization of methacrylonitrile allowed PMAN–POE–PMAN triblock copolymers to be prepared from different starting PEGs. In order to appreciate the salt/polymer interactions, solvent-free polymer electrolytes were prepared by dissolution of lithium imide (LiTFSI) in PMAN homopolymer and copolymers. Despite the high T g of PMAN host polymer, conductivities as high as 10 −4 S cm −1 were obtained around 90°C. Infra-red spectroscopy allowed the nitrile/salt interaction to be characterized. In particular, PMAN solvation number was determined from the homopolymer complexes. The investigation on the triblock copolymer complexes allowed the solvating competition between nitrile and ether functions to be highlighted.