The complex dynamic response of a 1‐dimensional polymer electrolyte fuel cell to load changes is numerically and experimentally investigated, in order to understand the transient transport processes ...in the different domains of the cell through the membrane‐electrode assembly. The analysis focuses on the comparison of cell response to step load changes, at different levels of reactants humidification. To render the model capable of capturing the cell behaviour at different operating conditions, a multiparameter optimisation procedure is performed to determine the most sensitive transport parameters. The optimisation procedure converges towards a well‐defined solution and the resulting parameters are compared to those available in the literature. Model results indicate that the time required to reach the new steady‐state after a load increase is controlled by the water transport in the membrane. The influence of the water diffusion coefficient at the different operating conditions is further investigated numerically.
Synchrotron radiation X-ray tomographic microscopy (SRXTM) allows the simultaneous in-situ visualization of the water and carbonaceous structures in the gas diffusion layer (GDL) of polymer ...electrolyte fuel cells (PEFC) on the pore scale. Recently, the TOMCAT beamline of the Swiss Light Source (SLS) was upgraded with a new CMOS camera, enabling ultra fast in-situ XTM investigations of PEFC with temporal resolution of about 8 s. This acquisition speed limits irradiation of the cell, preventing radiation damage and subsequent measurement bias. In this way, PEFC can be imaged several times before deterioration is arising. The stability of the 3D liquid water distribution in porous GDL structures was studied with a temporal resolution of 10 minutes and liquid water fraction data is presented.
In order to better understand the two phase properties of the porous GDL, the liquid water pattern and local saturation in the Toray 060 gas diffusion layer (GDL) is visualized in-situ in cells close ...to the optimal performance point at 80 {degree sign}C by X-ray tomographic microscopy. A recording time of about 10 s is achieved. At the optimal performance point (dew point of reactant gases 74 {degree sign}C) the water volume fraction observed depends on the rib width. A variation of {plus minus} 8 {degree sign}C of the feed gas dew point leads to significant saturation and performance changes. For different current densities the water volume fraction and permeability of the liquid phase of the cathode are determined and analyzed.
In X-ray tomography experiments, a significant decay of the performance of operating PEFC was observed when the entire active area is irradiated. Therefore, the dose dependent influence of ...synchrotron radiation at 13.5 keV on the mechanical and chemical properties of membranes, gas diffusion layers, electrodes and PTFE is investigated. The PEFC performance decays when exposed for more than about 100 s. On the same timescale, the strength of break of PTFE and membranes and the equivalent weight of the membrane decreases. IR-spectra of irradiated membranes show new features, increasing with exposure time and the wetting properties are influenced. The integrity of the catalyst layer on the electrodes, as well as on the CCM suffers to a large extent.
Membrane defects are a frequent failure mode of polymer electrolyte fuel cells. To understand the degradation mechanisms at defects, small pinholes are implemented artificially in membranes and their ...evolution is monitored during accelerated stress tests by online gas permeation analysis, ex-situ X-ray tomographic microscopy and local temperature analysis. Membrane degradation at defects is controlled primarily by the synergetic effect of chemical and mechanical degradation processes. The elimination of the synergism mitigates degradation significantly. Further, local temperatures of up to 140 degree celsius induce catalyst agglomeration and carbon oxidation of the MPL in the vicinity of the membrane defects.
In-situ synchrotron-based tomographic microscopy (SRXTM) with a spatial resolution in the order of 1μm and sensitivity for carbon and liquid water, has the potential to provide fundamental ...information for the understanding of the wetting properties of gas diffusion layer (GDL) materials on the pore level. This is important for the understanding of the solid-water interactions in the porous structures since water transport in GDLs is considered a key transport mechanism polymer electrolyte fuel cells (PEFC). However SRXTM of PEFC is a major experimental challenge. To obtain quantitative results, a complete cell needs to be operated under realistic conditions in the constrained space of the small field of view on the beamline sample stage without disturbing the sample rotation.
The resistance of the Nafion® 117 membrane in
H
2
O
2
and
H
2
air
polymer electrolyte fuel cells (PEFCs) has been measured in situ using fast current pulses. The dependence of the membrane resistance ...on current density, temperature, pressure and flow-field design was investigated. It was found that, independent of other variations, the resistance increases with increasing current density. When the current density in the cell is increased from 0.2 to 0.7 A cm
−2, the membrane resistance increases by up to 22%. Even on open circuit the resistance at 60°C is 15%–35% higher than that measured ex situ, indicating that the membrane is not fully hydrated under the fuel cell operating conditions. The resistance on open circuit also depends on the design of the flow field. In a design with forced gas convection the resistance at 60°C is substantially higher (210 mΩ cm
2) than in a design without forced convection (186 mΩ cm
2).
Transient Bi-Domain 1D PEFC Model Zaglio, M.; Roth, J.; Wokaun, A. ...
ECS electrochemistry letters,
01/2012, Letnik:
1, Številka:
1
Journal Article
Recenzirano
A bi-domain transient 1D PEFC model is developed which, while retaining the low computational cost of 1D models, takes into account the different pathways for multiphase transport under the rib and ...channel domains. This concept allows setting different boundary conditions for liquid transport at the channel-GDL and the rib-GDL interfaces. The model is thus suitable for studying the temporal cell behavior more accurately than 1D models, but still with comparable computation times.
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