Lung-inspired, fractal flow-fields hold great potential in improving the performance of polymer electrolyte membrane fuel cells (PEMFCs) by providing uniform gas distribution across the electrodes ...and ensuring minimum entropy production in the whole system. However, the inherent susceptibility of the fractal flow-fields to flooding renders their use inadequate at high humidity conditions. In-depth understanding of water management in lung-inspired flow-fields is indispensable for the implementation of alternative outlet channel geometries or engineered water removal strategies to alleviate flooding. Here, liquid water formation and transport across the lung-inspired and serpentine flow-field based PEMFCs are evaluated using neutron radiography. The results reveal a propensity to flooding in the interdigitated outlet channels of the fractal flow-field with N = 4 generations as a result of slow gas velocity and narrow channel dimensions, which leads to significant performance deterioration. Neutron images also elucidate the importance of ensuring a well-defined internal channel structure of the fractal flow-fields to prevent backflow of liquid water via wicking and capillary pressure build-up arising from the narrow inlet gas channels and hydrophobic gas diffusion layer.
•Neutron radiographs are presented for the lung-inspired and serpentine flow-fields.•A well-defined channel structure of the fractal flow-field is indispensable.•Water removal strategies required to alleviate flooding in the fractal flow-field.
We propose a method for improving the quantification of neutron imaging measurements with scintillator-camera based detectors by correcting for systematic biases introduced by scattered neutrons and ...other sources such as light reflections in the detector system. This method is fully experimental, using reference measurements with a grid of small black bodies (BB) to measure the bias contributions directly. Using two test samples, one made of lead alloy and having a moderate (20%) neutron transmission and one made of stainless-steel and having a very low (1%) transmission, we evaluated the improvement brought by this method in reducing both the average quantification bias and the uncertainty around this average bias after tomographic reconstruction. The results show that a reduction of the quantification bias of up to one order of magnitude can be obtained. For moderately transparent samples, little sensitivity is observed to the parameters used for the correction. For the more challenging sample with very low transmission, a correct placement of the BB grid is of utmost importance for a successful correction.
In-depth understanding of the dynamics of water formation, accumulation and removal is important for flow-field design optimization to ensure robust performance and durability of polymer electrolyte ...fuel cells (PEFCs). Here, in-operando neutron radiography is used to display and quantify liquid water distribution across the entire active area of single-, double- and quad-channel serpentine flow-fields. The results revealed that the water management and performance of PEFCs is strongly affected by the number of serpentine channels in the cathode flow-field. The single-channel serpentine-based PEFC exhibits both a better cell performance and uniformity in the local water distribution. The quad-channel based PEFC exhibits the largest voltage fluctuations caused by severe water flooding in the gas channels. However, the single-channel design leads to significantly larger pressure drop than the multiple-channel counterparts, which requires much higher parasitic power to pressurize and recirculate the reactants.
Three different regimes of operation can be defined based on the current density: gradually increasing hydration (<400 mA cm−2), flooding (400 mA cm−2 ≤ j ≤ 600 mA cm−2) and drying out (>600 mA cm−2). The reduced overall quantity of water in the channels with an increase in current density can be attributed to faster gas velocity and higher cell temperature.
•In-depth understanding of water dynamics through in-operando neutron radiography.•Single-, double- and quad-channel serpentine cathode flow-fields compared.•The single-channel exhibits better performance and water uniformity.•Three regimes identified: increasing hydration, flooding and drying out.
Proper water management plays an essential role in the performance and durability of Polymer Electrolyte Fuel Cells (PEFCs), but it is challenged by the variety of water transport phenomena that take ...place in these devices. Previous experimental work has shown the existence of fluctuations between low and high current density levels in PEFCs operated with wet hydrogen and dry air feed. The alternation between both performance states is accompanied by strong changes in the high frequency resistance, suggesting a cyclic hydration and dehydration of the membrane. This peculiar scenario is examined here considering liquid water distributions from neutron imaging and predictions from a 3D two-phase non-isothermal model. The results show that the hydration-dehydration cycles are triggered by the periodic condensation and shedding of liquid water at the anode inlet. The input of liquid water humidifies the anode channel and offsets the membrane dry-out induced by the dry air stream, thus leading to the high-performance state. When liquid water is flushed out of the anode channel, the dehydration process takes over, and the cell comes back to the low-performance state. The predicted amplitude of the current oscillations grows with decreasing hydrogen and increasing air flow rates, in agreement with previous experimental data.
•Hydration-dehydration cycles are found in PEFCs with wet anode and dry cathode feed.•The current alternates between low/high levels due to changes in anode humidification.•The cycles and performance states are analyzed by neutron imaging and modeling.•The condensation and shedding of water at the anode inlet fully humidifies the channel.•The humidification offsets the membrane dry-out induced by the cathode stream.
The water distribution across the membrane electrolyte assembly (MEA) of a working polymer electrolyte fuel cell (PEFC) was observed
in situ using neutron radiography. In order to resolve the ...distribution between the different layers of the MEA,
in plane imaging (cell membrane parallel to the beam) was used. Unprecedented spatial resolution for neutron radiography was obtained using a new detector system available at PSI combined with specific anisotropic resolution enhancement methods. A detrimental effect on performance of excessive water content in the cathode GDL was observed. Depending on the operating condition, a strong separation of the water content between ribs and channel was observed, particularly in the cathode GDL.
In this Letter, a new approach to distinguish liquid water and ice based on dual spectrum neutron radiography is presented. The distinction is based on arising differences between the cross section ...of water and ice in the cold energy range. As a significant portion of the energy spectrum of the ICON beam line at Paul Scherrer Institut is in the thermal energy range, no differences can be observed with the entire beam. Introducing a polycrystalline neutron filter (beryllium) inside the beam, neutrons above its cutoff energy are filtered out and the cold energy region is emphasized. Finally, a contrast of about 1.6% is obtained with our imaging setup between liquid water and ice. Based on this measurement concept, the temporal evolution of the aggregate state of water can be investigated without any prior knowledge of its thickness. Using this technique, we could unambiguously prove the production of supercooled water inside fuel cells with a direct measurement method.
A new experimental set-up was developed to perform simultaneous high resolution in-plane neutron imaging of six differential polymer electrolyte fuel cells (PEFCs). Beyond offering the major ...advantage of using the neutron beam time more efficiently, this set-up allows applying identical operation conditions to all cells (e.g. temperature, relative humidity, gas composition, and gas flow) and is therefore dedicated to the systematic study of different design parameters (e.g. materials, geometry, and compression).
A first set of experiments demonstrated the influence of the microporous layer (MPL) of the gas diffusion layer (GDL) on the performance and on the liquid water distribution in the porous media by means of four cells (MPL on anode, on cathode, on both sides and no MPL). The mass transport losses observed in absence of MPL on cathode side are attributed to flooding in the cathode catalyst layer (CL) and/or water film formation at the CL/GDL interface. The presence of MPL has no major influence on the saturation level of the GDLs. In particular, the insertion of a MPL on cathode side does not reduce the saturation level of the cathode GDL.
► A set-up is realized for high-resolution simultaneous neutron imaging of six cells. ► The effect of the presence of a microporous layer (MPL) is shown. ► MPLs do not change the average water content of the gas diffusion layers (GDLs). ► Absence of cathode MPL shifts the water accumulation peak towards the electrode. ► Absence of cathode MPL leads to electrode flooding and/or to water film formation.
Operando Neutron imaging is used for the investigation of a planar air-breathing array comprising multiple cells in series. The fuel cell demonstrates a stable power density level of 150 mW/cm2. ...Water distribution and quantification is carried out at different operating points. Drying at high current density is observed and correlated to self-heating and natural convection. Working in dead-end mode, water accumulation at lower current density is largely observed on the anode side. However, flooding mechanisms are found to begin with water condensation on the cathode side, leading to back-diffusion and anodic flooding. Specific in-plane and through-plane water distribution is observed and linked to the planar array design.
•An original micro fuel cell design with planar interconnections is presented.•The huge sensitivity of breathing fuel cells to hydration mechanisms is detailed.•The importance of interconnects and packaging elements is highlighted.•The water nucleation pattern is found strongly influenced by convective flows.
To overcome present limitations of the spatial resolution in neutron imaging a newly designed setup was realized at the ICON facility G. Kühne et al., ICON—the new facility for cold neutron imaging ...at the Swiss spallation neutron source SINQ, Swiss Neutron News 28 (December 2005), pp. 20–29, 〈
http://sgn.web.psi.ch/sgn/snn/snn_28.pdf〉, which is installed at the cold neutron beam line 52 at the Swiss spallation neutron source SINQ G.S. Bauer, Nucl. Instr. and Meth. A 463 (2001) 505 since 2005.
It was found by dedicated performance measurements, that the inherent spatial resolution of this locally fixed neutron imaging device is better than 50
μm, corresponding to 20 line pairs/mm in spatial frequency at 10% of the modulation transfer function (MTF). Therefore, the system has the potential to perform neutron tomography investigations with a resolution better than that previously achieved.
This article describes the design features, details of the installation and the results from first test measurements. It gives an outlook for the further potential of this technique in both radiography and tomography applications.
In-depth understanding of water management is essential for the optimization of the performance and durability of polymer electrolyte fuel cells (PEFCs). Neutron imaging of liquid water has proven to ...be a powerful diagnostic technique, but it cannot distinguish between ‘legacy’ water that has accumulated in the system over time and ‘nascent’ water recently generated by reaction. Here, a novel technique is introduced to investigate the spatially resolved water exchange characteristics inside PEFCs. Hydro-electrochemical impedance imaging (HECII) involves making a small AC-sinusoidal perturbation to a cell and measuring the consequential water generated, using neutron radiographs, associated with the stimulus frequency. Subsequently, a least-squares estimation (LSE) analysis is applied to derive the spatial amplitude ratio and phase shift. This technique provides a complementary view to conventional neutron imaging and provides information on the source and ‘history’ of water in the system. By selecting a suitable perturbation frequency, HECII can be used to achieve an alternative image ‘contrast’ and identify different features involved in the water dynamics of operational fuel cells.
•Hydro-electrochemical impedance imaging applied to water management of PEFC.•HECII distinguish between 'legacy' and 'nascent' water in the PEFC.•The perturbation frequency of HECII affects water dynamics features.
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