Synchrotron Radiography Research Articles

Investigating the Effect of Channel Width and Surface Wettability on Liquid Water Content in PEMFCs Using Synchrotron Radiography

Polymer electrolyte membrane (PEM) fuel cells show promise as electrochemical energy conversion devices for transport, stationary, and portable applications. Despite their potential for high efficiencies, the widespread adoption of PEM fuel cells has been hindered by their current durability and cost. These obstacles are related to a lack of fundamental understanding of the water transport mechanisms in the gas diffusion layers (GDLs) [1]. Another aspect in the improvement of fuel cell performance requires an understanding of the effects of flow channel geometry and wettability on water management in the PEM fuel cells. For instance, liquid water transferred from the GDL to the flow channel may be constrained by water slugs in the channel, which may result in increased saturation inside the GDL [2,3]. Moreover, liquid water flux from the GDL can be influenced by the rib/channel geometry due to reaction distribution, heat transport, and water transport [2,3].In this study, a PEM fuel cell, with an active area of 0.48 cm2, was designed for synchrotron imaging to acquire through-plane radiographs of the cell. Using Toray GDLs coated with an MPL (TGP-H-60), each fuel cell experiment was performed at specified operating conditions controlled with a Scribner 850e test station. Two different channel geometries were studied: one with a channel width of 0.6 mm; the other, 0.2 mm. In addition, the effect of channel wettability was also investigated. For each channel geometry, two sets of bipolar plates were prepared; they were either coated with a hydrophilic or hydrophobic coating.Visualizations of the dynamic liquid water content in operating fuel cells were performed at the Biomedical Imaging and Therapy – Bending Magnet (05B1-1) beamline at the Canadian Light Source Inc. (Saskatoon, Canada) using conventional absorption imaging. A monochromatic X-ray beam set to an energy level of 18 keV was used to acquire two- dimensional radiographs. By applying the principles of the Beer-Lambert law, the raw radiographs were processed to isolate the water content, in the form of a water thickness distribution.The results of the current study aim to improve the understanding of how the channel surface wettability and channel dimensions influence the water management of PEM fuel cells. This is important as the channel water morphology can have an effect on the GDL water distribution, which is linked to the overall fuel cell performance. As the interface between the liquid water in the GDL and the liquid water in the channel is not fully understood, this study will shed light on interface effect by taking advantage of the fine spatial and time resolutions of the synchrotron X-ray radiography.

Impact of MPL Thickness on Water Management of PEMFC By Synchrotron X-ray Radiography

The polymer electrolyte membrane fuel cell (PEMFC) is an efficient and clean energy conversion device. The performance of PEMFCs at high power operation is governed by product water management. The gas diffusion layer (GDL), a porous carbon fiber sheet, is sandwiched between a catalyst layer and a flow field, providing diffusive pathways for reactant gases and product water. A transient microporous layer (MPL) characterized by sub-micron size pores and hydrophobic contents applied on the GDL or catalyst layer is known to extend limiting current density. Past research [1] has shown that there exists an optimal thickness of MPL. However, the effect of varying MPL thickness has not been studied by visual inspection.Our research group has successfully investigated liquid water dynamic and distribution in an operating PEMFC using synchrotron radiography [2-3]. For this study, a miniature fuel cell with an active area of 0.48cm2and 0.2mm channel and rib width was designed for improved image quality. MPLs of varying thickness (no MPL, 50μm, 100μm, and 150μm) were coated on TGP-H-60 (Toray Industries Inc.). Each fuel cell was operated according to a pre-determined testing scheme on a Scribner 850e fuel cell testing station (Scribner Inc.). Visualizations were performed at the Biomedical Imaging and Therapy Bending Magnet (05B1-1) beamline at Canadian Light Source Inc. (Saskatoon, Canada). Spatial and temporal resolutions were approximately 10μm and 0.3fps, respectively. Image processing was carried out according to the Beer-Lambert law. The processed image provides information on liquid water profile across components of the fuel cell (Figures 1 and 2). For each types of GDL, liquid water content at the interfaces and within microstructures will be analyzed to isolate effect of MPL thickness on water distribution and performance of the fuel cell.

In situ study of granular micromechanics in semi-solid carbon steels

The granular micromechanics of semi-solid steel at ∼80% solid are studied by synchrotron radiography. A particulate soil mechanics approach to image analysis shows that deformation occurs by the translation and rotation of quasi-rigid grains under the action of contact forces, and that the changes in directional fabric and grain–grain contacts occur by mechanisms similar to those of highly compacted soils including “locked sands”. Grain-scale phenomena are then linked to the macroscopic displacement and strain fields and it is shown that shear-induced dilation is a fundamental response at both the grain and macro scales. Based on this, recommendations are made on future rheology experiments.

Influence of cracks in the microporous layer on the water distribution in a PEM fuel cell investigated by synchrotron radiography

Water evolution in the gas diffusion layer of a polymer electrolyte membrane fuel cell was visualized in situ by means of synchrotron X-ray radiography. Cracks in the microporous layer were identified as start points of efficient liquid water transfer paths through the gas diffusion layer. Quantitative analysis of the water flow rate through those arbitrarily distributed cracks into the gas channel revealed that they have a strong influence on the overall liquid water transport. This could find entry into future material design and simulation.

Investigation of Fuel Cell Materials and Liquid Water Transport by Means of Synchrotron Imaging

Synchrotron imaging allows addressing various important issues in fuel cell research, for example water distribution and transport. The water distribution in polymer electrolyte membrane fuel cells (PEMFCs) was observed quasi in-situ directly after operation by means of synchrotron tomography. The 3D data set was compared with the tomogram of a dry cell in order to separate the water distribution from cell materials. Engineered transport pathways realized by perforating holes through the gas diffusion layer (GDL) are a recent approach to optimize water transport and cell performance. For some parameter sets a cell performance increase and an improvement of stabilization have already been proven. We present high resolution investigations of the water distribution in perforated GDLs of operating PEMFCs by means of in-situ synchrotron radiography. The surrounding areas of the holes exhibited a distinct hydrophilic character.

Synchrotron radiography and tomography of water transport in perforated gas diffusion media

Water transport in gas diffusion media (GDM) is investigated by synchrotron radiography and tomography. It is demonstrated that micro porous layer (MPL) cracks improve the water management in polymer electrolyte membrane (PEM) fuel cells. A further treatment by means of laser perforation is expected to enhance this effect. The radiography analysis reveals that water transport is practically not influenced by perforations applied only to the MPL. In contrast, perforations through the whole GDM (including the MPL) have a strong influence on the overall water transport behavior and are therefore considered for a deeper analysis. Performance measurements show a correlation between the perforation size and the fuel cell power density. An optimum is found for a perforation diameter of 60 μm. Synchrotron tomography analysis reveals that this optimum is due to an improved draining effect on the area around the perforation. Moreover, SEM and EDX analysis show a loss of PTFE on the GDM surface in the vicinity of the perforation due to the laser processing. The tomography images reveal water accumulations in this area that can be explained by the hydrophilic surface.

The influence of porous transport layer modifications on the water management in polymer electrolyte membrane fuel cells

In this study, the influence of modifications of the porous transport layer (PTL) on the water management in polymer electrolyte membrane fuel cells is investigated. Laser perforation and milling are used to locally remove either the whole PTL or only the micro porous layer in the PTL. The changed liquid water distribution is visualized using synchrotron radiography and ESEM liquid water imaging. The observed effects are correlated with in-situ performance characteristics, pre-assembly and post-mortem analysis to examine on the beneficial and obstructive effects of the modifications.The analysis reveals that laser-perforation results in PTFE loss and hydrophilic regions which results in a good performance at dry conditions but serious flooding at high humidification conditions.Machined perforations show beneficial effects in the in-situ performance characteristics at high humidification conditions. A draining of the vicinity of the perforations is observed which results in an improved oxygen diffusivity. Water balancing and synchrotron visualization indicate that under the analyzed operating conditions the MPL increases the humidification of the ionomer on the one hand but creates an additional diffusion barrier by liquid water in the interface between the cathode catalyst layer and the MPL on the other hand.

Initiation of detonation of a porous high explosive by a high-enthalpy gas flow

Loose packed PETN initiation by a hot gas flow generated by an explosion of an active charge separated by an air gap is studied. Experimental data obtained by means of synchrotron radiography are compared with simulations employing a two-phase two-velocity two-temperature model. Reasonable agreement is reached by taking into account two processes accelerating the reaction: particle fragmentation during powder compaction and combustion intensification due to instability of the evaporating surface layer excited by a high-velocity gas flow around the particles.

<i>In Situ</i> Study of the Altering Globule Packing-Density during Semisolid Alloy Deformation

Synchrotron radiography experiments are overviewed that directly image semi-solid deformation at the globule-scale. Globular Al-15Cu at 50-60% solid was deformed in direct-shear at 10-2 s-1. Deformation is shown to occur by globule rearrangement without discernible deformation of the individual globules. Globules were found to translate and rotate as quasi-discrete bodies in response to forces acting at globule-globule contacts, similar to liquid-saturated granular materials such as water-saturated sand. Rearrangement caused the globule packing-density (the solid fraction) to adjust by local compaction and local dilation of the globule assembly, and deformation is highly inhomogeneous. During shear, there was a net dilation and strain began to localize into a shear band of decreased solid fraction by the end of the experiments.

Coalescence analysis for evolving foamsviaoptical flow computation on projection image sequences

A novel image-processing procedure is proposed for the analysis of sequences of two-dimensional projection images. Sudden events like the merging of bubbles in an evolving foam can be detected and spatio-temporally located in a given projection image sequence. The procedure is based on optical flow computations extended by a forward-backward check for each time step. Compared with prior methods, efficient suppression of noise or false events is achieved owing to uniform foam motion, and the reliability of detection is thus increased. The applicability of the proposed procedure in combination with synchrotron radiography is illustrated by a series of characteristic studies of foams of different kind. First, the detection of single-bubble collapses in aqueous foams is considered. Second, a spatial distribution of coalescence events in metals foamed in casting molds is estimated. Finally, the structural stability of polymer foams containing admixed solid nanoparticles is examined.

Synchrotron radiography of direct-shear in semi-solid alloys

Understanding phenomena occurring at the scale of the crystals during the deformation of semi-solid alloys is important for the development of physically-based rheological models. A range of deformation mechanisms have been proposed including agglomeration and disagglomeration, viscoplastic deformation of the solid skeleton, and granular phenomena such as jamming and dilatancy. This paper overviews in-situ experiments that directly image crystal-scale deformation mechanisms in equiaxed Al alloys at solid fractions shortly after the crystals have impinged to form a loose crystal network. Direct evidence is presented for granular deformation mechanisms including shear-induced dilation in both equiaxed-dendritic and globular microstructures. Modelling approaches suitable for capturing this behaviour are then discussed.

A generalised version of an Ivantsov-based dendrite growth model incorporating a facility for solute measurement ahead of the tip

Extensions to the Ivantsov dendrite growth model are presented, which allow for increased solute levels in the field ahead of the dendrite tip. This model is intended for use with video-microscopy experimental results taken from synchrotron radiography sources. Ideally, the composition and composition gradient at the solid–liquid interface would be measured. However, due to the spatial resolution limitations, it is difficult to accurately measure the composition at the interface. This paper sets out an extended description of the theory where measurements ahead of the tip determine the operating conditions at the tip. This extended model of dendritic growth may be incorporated into solidification process models, thus improving predictions where the solute field ahead of the tip may change due to solutal build up from neighbouring dendrites. It is shown that growth arrest is predicted in cases where local solutal enrichment reaches a critical level. A distinction between global and local tip undercooling is made. The usefulness of this model is demonstrated with an application to experimental results for a binary (Al–Ge) alloy.

Diagnostics of coated fuel particles by neutron and synchrotron radiography

The nondestructive monitoring of coated fuel particles has been performed using contact neutron radiography and refraction radiography based on synchrotron radiation. It is shown that these methods supplement each other and have a high potential for determining the sizes, densities, and isotopic composition of the particle components.

Pitting corrosion of stainless steel: measuring and modelling pit propagation in support of damage prediction for radioactive waste containers

In situ synchrotron radiography has been used to observe the evolution of two-dimensional pits growing in stainless steel foils under electrochemical control in chloride solutions. A method for extracting the key kinetic parameters from radiographs is under development to provide data for validating and calibrating a two-dimensional finite element model previously developed by Laycock and White. The local current density along the boundary of a pit is directly measured from the radiographs. Then, the local metal ion concentration and potential drop inside the pit cavity are backcalculated using transport equations and the requirement to maintain charge neutrality, giving the relationship between current density, solution composition and interfacial potential. Preliminary comparisons show qualitative correlation between the model and extracted data; quantitative comparison is under way.

Investigation of Water Evolution and Transport in a PEM Fuel Cell with Synchrotron Radiography

not Available.

Investigation of PEM Fuel Cell Transport Material Flooding and Microstructure with Synchrotron Radiography

not Available.

In situ viscosity measurements of liquid Fe–S alloys at high pressures

The viscosities of liquid Fe73S27 and Fe90S10 alloys were measured by combining a falling-sphere viscometer based on Stokes’ law with the technique of synchrotron radiography to up to 9.4 GPa and 2023 K. A high-speed CCD camera (125 frames/s) enabled the precise terminal velocity of the falling sphere to be determined over pressure and temperature ranges higher than those of previous studies. The effect of pressure on the viscosities was quite small, whereas temperature had a large effect. The apparent activation energies of Fe73S27 and Fe90S10 were quite similar, which indicates that the effect of sulfur content on liquid Fe alloys was negligible. The small effects associated with pressure and sulfur content on the viscosities of liquid Fe alloys show that iron is the dominant component of the viscous flow unit under high pressures. The addition of sulfur may not have a serious effect on the viscous flow of the Earth's outer core.

Liquid decomposition, droplet coagulation and droplet-interface interactions in hypermonotectic Al-Bi alloys

Liquid decomposition in slightly hypermonotectic Al-Bi alloys has been investigated by synchrotron radiography during directional solidification. The L→L 1+L 2 reaction was found to be almost completely suppressed, appearing in close proximity to or simultaneous with the monotectic reaction, corresponding to a L(Bi)-supersaturation of 0.3. The liquid supersaturation also undercooled the monotectic reaction, where gravity, thermo-solutal Marangoni and Stokes drag set up a force-balance that control a strongly size dependent L 2 droplet motion. The size dependant droplet motion gives rise to complex meso- and micro scale hydrodynamics that together with short-range non-hydrodynamic and hydrodynamic L 2-L 2 interactions have been identified as the main mechanisms behind droplet coagulation. The relatively substantial coagulation dynamics result in a bimodal droplet size distribution at the advancing solidification front. By varying the experimental parameters it was possible to study the droplet-α-Al-liquid interface interactions with both planar and curved interface morphologies. These interactions will be decisive for the final Bi particle distribution in the α-Al matrix. In contrast to what is most commonly assumed for pushing/engulfment of solid particles it was found that the larger droplets tended to be pushed by the front while smaller droplets are more frequently engulfed or entrapped. This result could be ascribed to effects on the droplets caused by the presence of solutal gradients ahead of the solidification front as well as dropletdroplet interactions.

The kinetics of Fe-rich intermetallic formation in aluminium alloys: In situ observation

In situ synchrotron radiography of Fe-rich intermetallic formation was performed during the solidification of an Al–7.5Si–3.5Cu–0.8Fe (wt.%) alloy. Growth kinetics was quantified by segmenting the Fe-rich intermetallic phases and nucleation temperatures were determined by extrapolating to zero size. Fe-rich β-intermetallics nucleated between 550 and 570 °C, growing initially at an instantaneous tip velocity of 100 μm s−1, and slowing to 10 μm s−1 towards the end of growth. Final plate size was controlled by local Fe concentration and α-Al impingement.

In Situ Observation of Liquid Water Evolution and Transport in PEM Fuel Cells by Synchrotron Radiography

not Available.