Optimization of Transports in a Proton-Exchange Membrane Fuel Cell Cathode Catalyst Layer at High Current Densities: A Coupled Modeling/Imaging Approach

International audienc

Towards the understanding of transport limitations in a proton-exchange membrane fuel cell catalyst layer: Performing agglomerate scale direct numerical simulations on electron-microscopy-based geometries

International audienc

A locally resolved investigation on direct methanol fuel cell uneven components fading: Local cathode catalyst layer tuning for homogeneous operation and reduced degradation rate

Durability issues of direct methanol fuel cell still hinder technology widespread commercialization; uneven aging of MEA components, generally harsher in air outlet region, is known to exasperate overall performance degradation. In a previous work, the authors selected a stable cathode electrode, demonstrated to fade homogenously: uneven water-related limitations, such as dehydration and flooding, were revealed to locally worsen performance at cathode inlet and outlet regions, leading to current redistribution. Aiming to reduce degradation rate, in this work homogeneous current distribution during operation is pursued by tuning MEA properties to meet local operating conditions. A properly improved 1D+1D physical model is used to support the development of a gradient MEA, featuring 1.6 mg cm−2 and 0.8 mg cm−2 of catalyst and ionomer respectively at inlet/outlet and center regions of cathode electrode. Tests based on custom macro-segmented cell demonstrated 55% more homogeneous current distribution, controllable during operation by means of cathode air stoichiometry. 500 h degradation test revealed 70% decreased degradation rate from uniform MEA (11 μV h−1) with a homogenous fading of performance. An 18% lower Pt nanoparticle growth at cathode outlet and limited ionomer degradation at cathode inlet were identified by ex-situ analyses (TEM and XPS), indicating locally mitigated fading mechanisms

Local reversed currents and degradation phenomena in long-term tested PEFC

In order to improve the long term stability of PEFC MEAs the investigation and understanding of degradation mechanisms depending on the operating conditions is of major importance. Long term tests up to 850 hours were performed in a single channel serpentine cell at constant load and in humid conditions at 80°C. The cell was equipped with a device to measure the current density distribution across the cell area with a local resolution of 0.5 cm . At regular intervals the constant operation was interrupted for a polarisation curve. The MEA was prepared at CEA using a E87-05S AQUIVION membrane with sub-gaskets, Pt/C catalyst by TKK and the GDL 25BC by SGL., In parallel flow as well as counter-flow of the gases after approximately 500 hours of operation locally reversed currents were observed at low load. The position with negative current flow was always close to the cathode inlet of the cell. At nominal load no major degradation of the current density was observed at this position in the cell. SEM, EDX, TEM, IR spectroscopy and IR microscopy measurements were performed on the MEAs after approximately 800 hours of operation investigating the GDL back side as well as cross sections and details. IR microscopy revealed very inhomogeneous PTFE distribution at the anode backing after operation. Some depositions of Si were observed that were washed out from the cell sealing but should not have major influence on the local behaviour. Different levels of Pt particle degradation were observed in the different zones of the cell. Structural damage of the membrane could be a reason of degradatio

A locally resolved investigation on direct methanol fuel cell uneven components fading: Steady state and degradation local analysis

DMFC technology widespread commercialization is still hindered by durability issues. In literature, locally resolved measurements and post-mortem analyses reveal the onset of strong heterogeneous components fading, higher at air outlet region. Local aging mechanisms could be enhanced by DMFC cathode cycling operation in highly uneven presence of water. An innovative PEM macro-Segmented Fuel Cell (mSFC) setup has been developed to investigate DMFC local performance and degradation, coupling electrochemical and ex-situ analyses as TEM and XPS. An MEA based on highly graphitized carbon supported cathode electrode, confirmed in AST to be stable to cycling operation under flooded conditions, has been implemented in DMFC operation. 500 h local degradation test, despite 32% heterogenenous current density distribution, reveals homogeneous ECSA loss, consistent with homogeneous nanoparticles growth from 3.16 to 5.4 nm, which leads to 70% lower degradation rate (31.2 μV h−1) than the reference MEA. Water-related limitations, such as dehydration and flooding, are revealed to increase local performance loss by 25% and 100% respectively at cathode inlet and outlet regions, leading to current redistribution and uneven voltage loss. Hence, local optimization of MEA properties is foreseen to permit further important durability improvements

Measurement of protonic resistance of catalyst layers as a tool for degradation monitoring

International audienceThe ionic resistance of a PEM fuel cell catalyst layer, as well as the double layer capacity and the high frequency resistance are estimated from the impedance spectra of H 2 /N 2 operated cell, thanks to a volumetric electrode model. The impedance spectra are calculated in the case of a homogeneous distribution of resistivity and capacity, and considering both trough plane and in plane heterogeneities. Then, results of the degradation of a MEA submitted to four different accelerated stress tests are presented. They consist of (i) a constant current operation, (ii) 1.2V potential hold with nitrogen and hydrogen (iii) open circuit voltage (OCV) hold with air and hydrogen, and (iv) a start-up protocol where heterogeneous degradations are observed using a linear segmented fuel cell. This technique can provide useful information about the evolution of catalyst layer microstrucure during AST