Elisabeth Rossinot

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.

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.

In Situ Quantification of Electronic Short Circuits in PEM Fuel Cell Stacks

There is a need for increasing the durability of proton exchange membrane fuel cell systems. Membrane failure is usually generated by a chemical attack led by peroxide radical species. The chemical attack promotes pinhole formation that ultimately induces the shutdown of the fuel cell. An electronic short circuit between the anode and the cathode through the membrane has also been identified as a failure mode. However, the current resulting from H2 crossover is estimated to be an order of magnitude larger than the electronic current. As a consequence, the electronic short circuit is often disregarded. In the present work, we revealed numerous local hotspots in pristine membrane electrode assemblies (MEAs) that are sensitive to short circuits. Catalyst layer heterogeneities were found responsible for these hotspots. In use, these flaws do not directly impact the global performance but may induce premature degradation. With the use of an electrical passive technique, one can identify the electronic short-circuit resistance of membranes simply by charging and discharging the double-layer capacitor of the MEA. The integration of this technique into fuel cell systems was possible, and measurements were performed at different ageing times. They revealed a gradual increase in the number of cells with short circuit annunciating the failure of the entire stack.

Durability of Pt3Co/C Cathodes in a 16 Cells PEMFC Stack: Degradation Mechanisms and Modification of the ORR Electrocatalytic Activity

In this work, we operated a 16-cells stack based on Pt/C anode and Pt3Co/C cathode under H2/air at constant current (j = 0.6 A cm-2) for 1124 h. The MEA were characterized with special emphasis on the cathode electrocatalyst, using physical, chemical and electrochemical techniques after different life stages: as received (fresh: 0 h), conditioned (17 h) and aged for 504 or 1124 h. The fresh Pt3Co/C cathode catalyst is not stable during PEMFC operation and suffers compositional changes. In particular, the dissolution of Co atoms at the surface yields to the formation of a Pt-shell/Pt-Co alloy core structure. The high electrochemical potential of the cathode induces formation of surface oxides and drives continuous Co surface segregation. The dissolution of Co atoms at the surface increases the affinity of the surface to oxygenated and hydrogenated species and renders it less active towards the ORR.

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.