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This study combines local electrochemical diagnostics with ex situ analysis to investigate degradation mechanism associated to start-up/shut-down (SU/SD) of PEMFC and mitigation strategies adopted in automotive stacks. Local degradation resulting from repeated SU/SD was analyzed with and without mitigation strategies by means of a macro-segmented cell setup provided with Reference Hydrogen Electrodes (RHEs) at both anode and cathode to measure local electrodes potential and current. Accelerated Stress Test (AST) for start-up with and without mitigation strategies are proposed and validated. A ten-fold acceleration of performance loss due to un-mitigated SU/SD has been calculated with respect to AST for catalyst support. Under mitigated SU/SD, instead, a strong degradation was observed as localized at cathode inlet region (i.e. -38% ECSA loss and -22 mV voltage loss after 200 cycles) due to local potentials transient reaching up to 1.5 V vs RHE. These localized stress conditions were additionally reproduced in a zero-gradient and a new AST protocol (named start-up AST) was proposed to mimic the potential profile observed upon SU/SD cycling. Representativeness of the start-up AST for real world degradation was confirmed up to 200 cycles. Platinum dissolution and diffusion/precipitation within the polymer electrolyte was shown to be the dominant mechanism affecting performance loss. [ABSTRACT FROM AUTHOR]

The function of catalytic layers in fuel cells and electrolyzers depends on the properties of the ionically conductive phase, which are most commonly perfluorinated ionomers based on Nafion and Aquivion. An analysis by atomic force microscopy reveals that the ultrathin ionomer films around Pt/C agglomerates have a thickness distribution ranging from 3.5 nm to 20 nm. Their conductivity and gas permeation properties determine the fuel cell performance to a large extend. For electrodes in Aquivion-based membrane-electrode-assemblies operation-induced structure changes were investigated by means of material- and conductivity-sensitive atomic force microscopy, infrared spectroscopy and electron-dispersive X-ray analysis. The observed thinning of the ultrathin ionomer films was mainly caused by polymer degradation deduced from reduced swelling after long-time operation and a significant loss of ionomer with operation time detected by infrared spectroscopy. From the linear thickness increase of the ultrathin films with rising humidity, a mainly layered structure of the ionomer was deduced. An influence of thickness of such ultrathin ionomer films on fuel cell lifetime was found by analysis of differently prepared membrane-electrode-assemblies, where a linear increase of irreversible degradation rate with ionomer film thickness in the electrodes of unused membrane-electrode-assemblies was found. [ABSTRACT FROM AUTHOR]

In proton exchange membrane fuel cells (PEMFCs), platinum (Pt) from the electrodes can be deposited in the electrolyte membrane, altering its properties and causing degradation. To investigate these impacts, in some studies, membranes with artificially deposited Pt were used, however, with contradictory results. In this study, we compared membranes impregnated with 0 to 4.4 wt% Pt. Specifically, the Pt particle distribution and mass transport in the membrane as well as the membrane decomposition is investigated after performing open circuit durability tests for ~140 h. The properties of the membrane electrode assemblies (MEAs) are studied by electrochemical characterization. The changes upon degradation are investigated by thickness determination, infrared spectroscopy, fluoride emission, hydrogen permeability, and electrical conductivity. Our investigation confirmed that the particle density, not the Pt amount, is the determining factor in the extent of membrane decomposition: fluoride emission decreased exponentially with particle density. We further found a considerable difference in the particle distribution between artificial and in-situ Pt deposits. Moreover, properties of both membrane types changed contrary to each other during the durability test. In conclusion, Pt-impregnated membranes do not reflect the state of degraded membranes with in-situ formed deposits but are suitable to investigate the influence of particle distributions. [ABSTRACT FROM AUTHOR]