A commercial 50 cm2 polymer electrolyte membrane (PEM) fuel cell with serpentine flow fields was operated at 2.0 bar and 60 °C with two orientations of the flow field channels with respect to ...gravity, i.e. horizontal and vertical channels. A 3 × 3 test matrix of anode and cathode reactants relative humidity was used for the performance assessment of the cell in both orientations. The cell performance and operating data, including cell voltage and resistance, were measured, and neutron radiographs were recorded during the entire operation in order to gain knowledge of the liquid water distributions within the cell for both orientations. A quantitative analysis of the results is presented in this work, comparing the cell operation for both flow field orientations. It is observed that the configuration with horizontal cathode flow field channels presents a better cell performance, and less amount of liquid water blocking the flow field channels. Thus, the results show that the selection of the cell orientation has an influence on the final performance, and it is therefore, a design parameter to be considered for a real application. The differences in the cell water content are quantitatively analyzed and discussed.
Fuel cell studies with neutrons at the PSI's neutron imaging facilities Lehmann, E.H.; Boillat, P.; Scherrer, G. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
06/2009, Letnik:
605, Številka:
1
Journal Article
Recenzirano
The aim of the current study is on water distribution inside and near the gas diffusion layers around the membrane (“in-plane” study). Because this region is normally only about 200
μm wide, a very ...high spatial resolution is required for this kind of studies.
PSI managed to build a neutron imaging setup with a stationary detection system providing best-possible conditions for high-resolution investigations
8. However, the field of view is adapted accordingly to 27
mm only. Using a special arrangement by tilting the detection plane in one direction, it ends up with nominal 2.5
μm per pixel. The real resolution was found to be about 20
μm in the relevant direction across the fuel cell plane
9.
This work will present this new setup and summarize the first promising results from fuel cell studies around the membrane region. Despite these good results with high spatial resolution, other options in fuel cell research can be used at the PSI neutron imaging facilities. Due to the large beam diameter (40
cm circular) full size cells can be studied under various conditions with moderate special and time resolution.
Advances in scintillator screen technology for neutron imaging Lehmann, E.H.; Boillat, P.
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
August 2023, 2023-08-00, Letnik:
1053
Journal Article
Recenzirano
Modern digital imaging detectors have been implemented with scintillator screens as the key component. They capture neutrons and emit light, which is registered by suitable converts into the digital ...signal, used for analyses.
The two established detector systems are either based on highly-efficient sensitive cameras or on amorphous silicon flat panels, which are in direct contact with the scintillator screen.
Because the neutron beam intensity is quite low (e.g. compared to synchrotron light), it is essential to have scintillator screens with highest light emission per neutron. This property defines the efficiency of the detection process and the needed acquisition time for a valid image.
On the other hand, highest spatial resolution is demanded in order to see smallest feature of a structure under investigation.
In order to distinguish small contrast variation in the image data, a possible highest signal-to-noise-ratio is required. These conditions cannot however be satisfied in only one unique device at the same time.
Although the principle neutron absorbers for the neutron detection and the most powerful light emitters have been known for many years, there is still potential for further improvements and optimization. Even exotic neutron absorbers and new scintillation materials are under investigation for the improvement of the scintillator screens, used in the neutron imaging detectors.
Our report will summarize the current status of the development and describe the performance of common, commercialized materials. On the other hand, we will give the outlook for further approaches and first results of running developments.
A novel method for the high accuracy measurement of water transfer in differential polymer electrolyte fuel cells (PEFCs) is proposed. This method is based on the potential difference forming in a ...H2/H2 concentration cell in the presence of humidity differences between the electrodes. In the present work, the relation between the partial pressures of hydrogen and water vapor and the potential difference over the concentration cell are verified. Furthermore, a validation experiment allowed to confirm that the targeted accuracy of 0.25% for the differential humidity can be obtained with our method, making it suitable for the measurement of water transfer in small‐scale differential cells.
► A water transfer measurement method using a H2/H2 concentration cell is proposed. ► The open cell voltage versus H2 and H2O partial pressures agrees well with theory. ► The setup is self-calibrated based on the voltage response to pressure variations. ► An accuracy of +/−0.25% on the differential humidity can be reached.
Following our two previous publications on material synthesis and on ex situ characterization, we present an experimental in situ study to evaluate the effects of using gas diffusion layers with ...patterned wettability at the cathode side of polymer electrolyte fuel cells. The operando performance was assessed using traditional electrochemical diagnostics (such as polarization curves) combined with the pulsed gas analysis (PGA) method, which allows measuring the mass transport losses. Neutron radiography was performed simultaneously in order to image the water distribution during operation. Using this methodology, the effects of changing the pattern, including a microporous layer (MPL), and varying the operation conditions (temperature and relative humidity of the cathode gas) have been systematically evaluated. It has been confirmed that water redistributes according to the engineered pattern and that the power density is significantly increased thanks to reduced mass transport losses under various conditions.
Despite being a promising technology for automotive applications, polymer electrolyte fuel cells still face challenges to reduce their complexity and cost. One challenge is to achieve good ...humidification, which is essential for a fuel cell membrane, without expensive external humidifiers. Here we present an evaporative cooling concept that manages humidification and cooling simultaneously, and does not require any additional layer to the structure of the cell. To this aim, water flows in the fuel cell itself through a small number of the flowfields' channels. Modified gas diffusion layers, with separate parallel hydrophilic regions, are capable of wicking the water from these supply channels and bring it in contact to the gas flow to evaporate, thus providing cooling and humidification. Our results show that this concept can provide the necessary cooling power and humidification for a cell with completely dry inlet gases at 80 °C, and has the potential for working at higher temperatures.
Experimental results about the impact of hydrophobic coating inside commercial gas diffusion layers (GDL) of polymer electrolyte fuel cells (PEFC) were obtained with four different combinations of ...PTFE-loadings on both anode and cathode. The measurements were performed on differential PEFCs which were characterized with high resolution neutron radiography (NR) combined with two electrochemical methods: a pulsed gas analysis (PGA) and limiting current density measurements. Using the combination of the neutron radiographs with these two electrochemical measurements, a new insight on the impact of water on mass transport losses was obtained. Under high humidification conditions, GDLs without hydrophobic coating do not accumulate more water than coated GDLs, but the water distributes differently and accumulates in the region under the channel of the flow field. Cells with high PTFE coating show higher mass transport losses despite the relatively small quantity of water, which can be explained by a different morphology of water accumulation.
We demonstrate that the use of high duty cycle time-of-flight (HDC-TOF) neutron transmission imaging allows a spatially resolved analysis of the aggregate state of water during isothermal cold starts ...of polymer electrolyte fuel cells (PEFCs). Neutron attenuation at long wavelengths depends on the mobility of protons and is therefore lower for ice compared to liquid water. With the experimental setup used in our experiment (chopper disk duty cycle of 30%) attenuation at long wavelengths normalized to attenuation at short wavelengths leads to a contrast between ice and super-cooled water higher than 6% and a measurement time of a few minutes is sufficient to detect reliably phase changes of thick water layers ( 0.5 mm), i.e. when water is present in the flow field channels. Using this method, local freezing events were identified during an isothermal cold start, which was corroborated by locally resolved measurements of the latent heat released during freezing. The impact of systematic biases such as the scattered background was evaluated, showing that a precise correction is of utmost importance if the aggregate state is to be determined in an absolute way. If such a highly precise correction is not possible, the detection of freezing events is still possible by observing relative changes during time series.
•Neutron imaging of fuel cells has interesting perspectives of further development.•Important potential of neutron imaging combination with advanced instrumentation.•Promising high resolution results ...towards the goal of catalyst layer imaging.•Use of specific neutron interactions permits to go beyond water visualization.
Neutron imaging has been used for nearly two decades to visualize the water distribution in operating polymer electrolyte fuel cells and address various issues related to water management. Its range of application ranges from small scale cells of a few cm2 to full-size technical cells, and the high penetration of neutrons through usual fuel cell structural materials make this technique particularly attractive for in situ studies. We present here an overview of the recent advances in this method and its application to fuel cell research, and discuss the prospects for the future on a selection of topics: the combination with advanced instrumentation, the imaging of catalyst layers, and the possibilities to go beyond the simple visualization of water.
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