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.
In situ diagnostic techniques provide a means of understanding the internal workings of fuel cells so that improved designs and operating regimes can be identified. Here, for the first time, a ...combined current density and temperature distributed measurement system is used to generate an electro-thermal performance map of an air-cooled, air-breathing polymer electrolyte fuel cell stack operating in an air/hydrogen cross-flow configuration. Analysis is performed in low- and high-current regimes and a complex relationship between localised current density, temperature and reactant supply is identified that describes the way in which the system enters limiting performance conditions. Spatiotemporal analysis was carried out to characterise transient operations in dead-ended anode/purge mode which revealed extensive current density and temperature gradients.
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•Combined current and temperature mapping as a novel performance metric.•Large current and temperature gradients form in dead-ended mode.•Localised extreme temperatures can change the current density gradients.
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The hot pressing process for fabricating polymer electrolyte fuel cells membrane electrode assemblies (MEAs) has been widely adopted, yet little is known of its effects on the ...microstructural properties of the different components of the MEA. In particular, the interaction of the electrolyte, electrode and gas diffusion layer (GDL) due to lamination is difficult to probe as conventional imaging techniques cannot access the internal structure of the MEA.
Here, a novel approach is used, which combines characterisation of hot-pressed membrane electrode assemblies using X-ray computed tomography, thermogravimetric analysis, differential scanning calorimetry and atomic force microscopy, with electrochemical performance measurements from polarisation curves and high-frequency impedance spectroscopy. Membrane electrode assemblies hot pressed at 100°C, 130°C and 170°C reveal significant differences in microstructure, which has a consequence for the performance. When hot pressed at 100°C, which is lower than the glass transition temperature of Nafion (123°C), the catalyst only partially bonds with the Nafion membrane, leading to increased Ohmic resistance. At 170°C, the Nafion membrane intrudes into the electrode, forming pinholes, degrading the catalyst layer and filling pores in the GDL. Finally, at 130°C, the interfacial contact is optimum, with similar roughness factor between the catalyst and Nafion membrane surface, indicating effective lamination of layers.
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.
Porous metal foams have been used as alternative flow-fields in proton exchange membrane fuel cells (PEMFCs), exhibiting improved performance compared to conventional ‘land and channel’ designs. In ...the current work, the mechanical behaviour of PEMFCs using metal foam flow-fields is investigated across different length scales using a combination of electrochemical testing, X-ray computed tomography (CT), compression tests, and finite element analysis (FEA) numerical modelling.
Fuel cell peak power was seen to improve by 42% when foam compression was increased from 20% to 70% due to a reduction in the interfacial contact resistance between the foam and GDL. X-ray CT scans at varying compression levels reveal high levels of interaction between the metal foam and gas diffusion layer (GDL), with foam ligaments penetrating over 50% of the GDL thickness under 25% cell compression. The interfacial contact area between the foam and GDL were seen to be 10 times higher than between the foam and a stainless-steel plate. Modelling results demonstrate highly uniform contact pressure distribution across the cell due to plastic deformation of the foam. The effect of stack over-tightening and operating conditions are investigated, demonstrating only small changes in load distribution when paired with a suitable sealing gasket material.
•X-ray computed tomography of a PEMFC with metal foam flow-fields under compression.•Intrusion of foam into GDL at high compression increases interfacial contact area.•Structural 3D model of 5 cell PEMFC stack with metal foam flow fields introduced.•Foam flow-field improves load distribution compared to land and channel design.•Energy output has been increased as the compression of metal foam increased.
The flow distribution behaviour of open-cell metallic foam fuel cell flow-fields are evaluated using ex-situ optical analysis and X-ray computed tomography (X-ray CT). Five different manifold designs ...are evaluated and flow distribution and pressure drop quantitatively evaluated with reference to applications in polymer exchange membrane fuel cells (PEMFC) and heat exchangers.
A ‘hybrid’ foam flow-field is presented consisting of flow channels pressed into the foam to promote flow distribution and reduce pressure drop. Cross- and through-channel pressure drop measurements are conducted, along with X-ray CT analysis.
Results using dyed water show that metallic foams provide excellent fluid distribution across the fuel cell flow-field, closely following the theoretical filling rate. The time for dye to cover 80% of the flow-field area was 61% faster with a foam flow-field then with no flow-field present. Pressure drop was seen to reduce with increasing foam inlet area to levels comparable to multi-serpentine flow-fields. The introduction of flow channels in the foam can further reduce pressure drop and provide more even filling of the foam, at the expense of increased residence time.
•Optical analysis of flow distribution in porous metal foam flow fields.•Findings show Increasing manifold inlet area reduces residence time and pressure drop.•New hybrid foam using pressed channels demonstrates controllable fluid distribution.•Hybrid foam performance correlated to X-ray CT scans and porosity analysis.
Polymer electrolyte membrane water electrolysers (PEMWE) are a key technology for producing clean (‘green’) hydrogen for decarbonisation of the transport sector and grid stabilisation utilising ...increasing levels of renewable energy. In this work, acoustic emission analysis is used as a non-destructive, operando diagnostic tool to provide information about the relative number and size of gas bubbles generated locally within a PEMWE, providing effective characterisation of the local flow conditions. An optically transparent single-channel PEMWE is used to investigate the relationship between the acoustic signals obtained and the two-phase flow conditions inside the cell. The number of acoustic hits, their frequency, and average peak amplitude is reported for several flow rates and current densities. Using high-speed imaging, the average bubble number and size in the flow cannels is compared to the acoustic signal. Results show good correlation between the number of acoustic ‘hits’ and the number of bubbles passing through the flow channel. The size of bubbles is also shown to affect the average frequency of the hits. Consequently, the transition between bubbly and slug flow regime can be identified by acoustic emission analysis, paving the way for a simple, low-cost, non-destructive means of mapping flow inside commercial-scale PEMWEs.
•Acoustic emission of an optically transparent PEMWE measured operando.•Size and number of gas bubbles in channels correlated with acoustic emission.•Acoustic emission can identify transition from bubbly to slug flow.
Multi‐length scale imaging of polymer electrolyte fuel cell (PEFC) membrane electrode assembly (MEA) materials is a powerful tool for studying, understanding and furthering improvements in materials ...engineering, performance and durability. A hot pressed MEA has been imaged using X‐ray micro‐ and nano‐computed tomography (CT), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and recently developed helium‐ion microscopy (HeIM). X‐ray nano‐CT captures a volume containing all of the relevant fuel cell interfaces, from the carbon fiber of the gas diffusion layer (GDL) to the Nafion membrane with a field‐of‐view of 5 µm and a pixel size of 64 nm. Features identified include linear marks on the carbon fiber surface, agglomerates of carbon nanoparticles in the microporous layer (MPL), and intrusion of the catalyst layer material into the Nafion membrane during the hot‐pressing process. HeIM has enabled imaging of a large area of MEA from tens of micrometers to sub‐nanometers pixel resolution without any sample preparation, and has captured similar features to X‐ray micro‐CT and nano‐CT. Furthermore, at its highest resolution, the platinum and carbon catalyst nanoparticles can be distinguished at the surface of the catalyst layer, overcoming the limitations of SEM and TEM.
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.
In this paper, we introduce a novel technique for pre‐filtering multi‐layer shadow maps. The occluders in the scene are stored as variable‐length lists of fragments for each texel. We show how this ...representation can be filtered by progressively merging these lists. In contrast to previous pre‐filtering techniques, our method better captures the distribution of depth values, resulting in a much higher shadow quality for overlapping occluders and occluders with different depths. The pre‐filtered maps are generated and evaluated directly on the GPU, and provide efficient queries for shadow tests with arbitrary filter sizes. Accurate soft shadows are rendered in real‐time even for complex scenes and difficult setups. Our results demonstrate that our pre‐filtered maps are general and particularly scalable.
In this paper, we introduce a novel technique for pre‐filtering multi‐layer shadow maps. The occluders in the scene are stored as variable‐length lists of fragments for each texel. We show how this representation can be filtered by progressively merging these lists. In contrast to previous pre‐filtering techniques, our method better captures the distribution of depth values, resulting in a much higher shadow quality for overlapping occluders and occluders with different depths. The pre‐filtered maps are generated and evaluated directly on the GPU, and provide efficient queries for shadow tests with arbitrary filter sizes.
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