In the last decade X-ray tomographic microscopy (XTM) has become a powerful 3D characterization technique for polymer electrolyte fuel cells. In order to improve both spatial and temporal resolution ...for operando polymer electrolyte fuel cell (PEFC) XTM imaging, the high magnification white beam microscope capabilities at the TOMCAT beamline of the Swiss Light Source (SLS) at Paul Scherrer Institut were explored. The image quality is evaluated in terms of contrast-to-noise ratio and in-situ water detectability for three different beam configurations and two magnifications. High water detectability levels can be achieved with a voxel size of 0.4 µm at 0.25 s scan time.
On the occasion of the “Challenge Bibendum” 2004 in Shanghai, the hybrid fuel cell—supercapacitor vehicle HY-LIGHT, a joint project of Conception et Développement Michelin and the Paul Scherrer ...Institut, was presented to the public. The drive train of this vehicle comprises a 30
kW polymer electrolyte fuel cell (PEFC) and a 250
V supercapacitor (SC) module for energy recuperation and boost power during short acceleration and start-up processes. The supercapacitor module was deliberately constructed without continuous voltage balancing units. The performance of the supercapacitor module was monitored over the 2 years of operation particularly with respect to voltage balancing of the large number of SC cells connected in series. During the investigated period of 19 months and about 7000
km driving, the voltage imbalance within the supercapacitor module proved negligible. The maximum deviation between best and worst SC was always below 120
mV and the capacitor with the highest voltage never exceeded the nominal voltage by more than 40
mV.
Polymer electrolyte fuel cells (PE fuel cells) working with air at low stoichiometries (<2.0) and standard electrochemical components show a high degree of inhomogeneity in the current density ...distribution over the active area. An inhomogeneous current density distribution leads to a non-uniform utilization of the active area, which could negatively affect the time of life of the cells. Furthermore, it is also believed to lower cell performance. In this work, the homogenization of the current density, realized by means of tailored cathodes with along-the-air-channel redistributed catalyst loadings, is investigated. The air stoichiometry range for which a homogenization of the current density is achieved depends upon the gradient with which the catalyst is redistributed along the air channel. A gentle increasing catalyst loading profile homogenizes the current density at relatively higher air stoichiometries, while a steeper profile is suited better for lower air stoichiometries. The results show that a homogenization of the current density by means of redistributed catalyst loading has negative effects on cell performance. Model calculations corroborate the experimental findings on homogenization of the current density and deliver an explanation for the decrease in cell performance.
In this paper, the powertrain sizing of a fuel-cell hybrid vehicle (FCHV) is investigated. The goal is to determine the fuel-cell system (FCS) size, together with the energy storage system (ESS) ...size, which leads to the lowest hydrogen consumption. The power source (FCS + ESS) capabilities should also respect the vehicle driveability constraints. Batteries and supercapacitors are considered as ESSs. The power management strategy is a global optimization algorithm respecting charge sustaining of the ESS. The impacts of the driving cycle (urban, outer urban, and highway), ESS technology, and vehicle driveability constraints on hydrogen consumption are analyzed in detail.
Starting from subfreezing temperatures presents a challenge for PEFC since ice may form in the gas flow channel, gas diffusion layer and catalyst layer and restrict the gas transport. Under ...non-isothermal conditions, a start-up is successful if the cell temperature rises above 0°C before freezing occurs. Under isothermal conditions below 0°C cells operate for a certain time followed by a sudden drop in power. The distribution of liquid/solid water during an isothermal start at subfreezing temperatures is quantitatively mapped by means of X-ray tomographic microscopy to improve understanding the circumstances at the phase transition and its effects on gas diffusion. Evidence was found that the produced water is in supercooled state at -10 °C initially and the drop of power is associated with freezing of the water in the cell at a GDL saturation of 20 to 30%.
Measuring local currents in PE-fuel cells is an important tool for diagnostics and development. A semi-segmented cell has been developed, which can serve as a key instrument to investigate different ...phenomena in cells and stacks of technical relevance. Data with respect to water management is presented. These results show, that the local current distribution is strongly influenced by the dew point of the process air, the stoichiometry of the process air and the mode of operation.
Hydrogen fuelled vehicles with a fuel cell based powertrain are considered to contribute to sustainable mobility by reducing CO2 emissions from road transport. In such vehicles the fuel cell system ...is typically hybridised with an energy storage device such as a battery or a supercapacitor (SC) to allow for recovering braking energy and assist the fuel cell system for peak power. The direct parallel combination of a polymer electrolyte fuel cell (PEFC) and a SC without any control electronics is investigated in the present study. It is demonstrated that the combination enhances the dynamics of the PEFC significantly during load changes. However, due to the lack of a power electronic interface the SC cannot be utilised to its optimum capacity.
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