Morphological Analyses of Polymer Electrolyte Fuel Cell Electrodes with Nano‐Scale Computed Tomography Imaging

This work demonstrates an original approach to three-dimensional, high-resolution, non-destructive visualization of fuel cells during room temperature operation using a commercial micro-X-ray computed tomography (XCT) system. The novel application of 3D printing technology was used to create a customized housing fixture that allows for non-invasive imaging of an operational fuel cell. The use of 3D print materials also allows for rapid prototyping and intricate design of any experimental fixture with minimal cost. Demonstration of imaging capabilities is shown through in situ visualization of the internal MEA components and liquid water after fuel cell operation. Water imaging in fuel cells is particularly challenging using XCT due to small differences in X-ray absorption characteristics between constituent materials. Image processing operations are discussed which allows for the improved segmentation of solid and water to calculate porosity and saturation throughout the gas diffusion layer, as reported herein. Additionally, a membrane thickness increase of approximately 25% due to swelling under 100% relative humidity is shown, which has previously been difficult to determine. Overall, the results shown here illustrate the novel use of 3D printed materials and image processing steps to be further used in understanding of fuel cell devices through non-invasive imaging procedures by use in a commercial micro-XCT system. Polymer electrolyte fuel cells (PEFC) have significant potential for automotive applications as an alternate clean energy technology with zero emissions at the point of use. The many advantages include quick start-up time, low operating temperature, low weight, high efficiency and relatively simple design as compared to other clean energy solutions.1 Despite these advantages, however, a primary reason for the delay of this technology from meeting commercial automotive application standards is cost. Significant cost reductions are possible by increasing the power density, which is currently limited by liquid water related mass transport effects at high current densities.2 Higher power density cells also have an advantage for automotive applications due to a better form factor for vehicles. In other fuel cell technologies such as back-up power in extreme climates or mobile fuel cell applications, fuel cells operating at low temperature or low gas flow rates are susceptible to flooding. Flooding, or channel clogging, may lead to significantly reduced performance from inaccessibility of reactant gas flow as well as non-uniform reactant distribution in fuel cell stacks. 3,4 Proper understanding of water management in operating fuel cells is imperative to the design and implementation of higher power density fuel cells and is thus a highly researched field of PEFC performance.Current fuel cell diagnostic methods typically involve investigation by empirical testing and ex situ analysis. A common technique for fuel cell investigatory research is to use scanning electron microscopy (SEM), which provides hi...

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“…19 studied the effect of gas diffusion layer compression and teflonation on porosity while Litster et al 22 and Andisheh-Tadbir …”

Section: -23mentioning

confidence: 99%

3D Printed Flow Field and Fixture for Visualization of Water Distribution in Fuel Cells by X-ray Computed Tomography

White

Orfino

Hannach

et al. 2016

J. Electrochem. Soc.

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“…Figure 11b was formulated by application of a compressed-surface-interaction model and experimental topographical scans of a commercial MPL and catalyst surface. 226 Based on these findings, [225][226][227][228][229][230][231] the potential interfacial gap storage volume can be significant, and depends directly on the elasticity, smoothness of the mating surface (and if there is a mismatch in roughness) and compression pressure. Thus, the worst condition is under a channel with near zero overburden (compression) pressure, with a cracked, stiff MPL with much greater surface roughness than the catalyst layer.…”

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confidence: 99%

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

Weber

Borup

Darling³

et al. 2014

J. Electrochem. Soc.

509

394

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Polymer-electrolyte fuel cells are a promising energy-conversion technology. Over the last several decades significant progress has been made in increasing their performance and durability, of which continuum-level modeling of the transport processes has played an integral part. In this review, we examine the state-of-the-art modeling approaches, with a goal of elucidating the knowledge gaps and needs going forward in the field. In particular, the focus is on multiphase flow, especially in terms of understanding interactions at interfaces, and catalyst layers with a focus on the impacts of ionomer thin-films and multiscale phenomena. Overall, we highlight where there is consensus in terms of modeling approaches as well as opportunities for further improvement and clarification, including identification of several critical areas for future research. Fuel cells may become the energy-delivery devices of the 21 st century. Although there are many types of fuel cells, polymer-electrolyte fuel cells (PEFCs) are receiving the most attention for automotive and small stationary applications. In a PEFC, fuel and oxygen are combined electrochemically. If hydrogen is used as the fuel, it oxidizes at the anode releasing proton and electrons according toThe generated protons are transported across the membrane and the electrons across the external circuit. At the cathode catalyst layer, protons and electrons recombine with oxygen to generate waterAlthough the above electrode reactions are written in single step, multiple elementary reaction pathways are possible at each electrode. During the operation of a PEFC, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. * Electrochemical Society Active Member. z E-mail: azweber@lbl.govOver the last several decades significant progress has been made in increasing PEFC performance and durability. Such progress has been enabled by experiments and computation at multiple scales, with the bulk of the focus being on optimizing and discovering new materials for the membrane-electrode-assembly (MEA), composed of the proton-exchange membrane (PEM), catalyst layers, and diffusionmedia (DM) backing layers. In particular, continuum modeling has been invaluable in providing understanding and insight into processes and phenomena that cannot be resolved or uncoupled through experiments. While modeling of the transport and related phenomena has progressed greatly, there are still some critical areas that need attention. These areas include modeling the catalyst layer and multiphase phenomena in the PEFC porous media.While there have been various reviews over the years of PEFC modeling 1-7 and issues, [8][9][10][11][12][13][14] as well as numerous books and book chapters, there is a need to examine critically the field in terms of what has been done and what needs to be done. This review serves that purpose with a focus on transport modeling of PEFCs. This is not meant to be an exhaustive review...

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“…[2][3][4][5][6][7][8][9] The diamond blade used in the microtome technique has the advantage of slicing through the electrocatalyst layer without inflicting thermal damage. However, it is difficult to cut through e.g.…”

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confidence: 99%

Correlating Cathode Microstructure with PEFC Performance Using FIB-SEM and TEM

Okumura

Noda

Matsuda

et al. 2017

J. Electrochem. Soc.

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The cathode electrocatalyst layers of polymer electrolyte membrane fuel cells (PEFCs) are quantitatively investigated for different ratios of Nafion ionomer. This is achieved using focused-ion-beam coupled scanning electron microscopy (FIB-SEM) to reconstruct the three-dimensional microstructure via tomography. Parameters such as the porosity and pore size distribution were calculated from this data. The distributions of Nafion ionomer, carbon support, and platinum nanoparticles were then further clarified using transmission electron microscopy (TEM). Changes in the PEFC performance (notably the I-V characteristics, the electrochemical surface area, the activation overvoltage, and the concentration overvoltage) are thus correlated to electrode microstructure. The electrocatalyst layers of PEFCs have complicated, threedimensional (3D) porous microstructure. This microstructure plays a key role in transporting reactant gases (i.e. hydrogen and oxygen), product water vapor, charge-carrying ions, and electrons either to or from the active sites. Therefore, the overall PEFC performance is linked inextricably to the 3D microstructure of the electrocatalyst layer, and in particular to the Nafion content.1 Making quantitative correlations between the Nafion ratio, the microstructure, and the cell performance is thus essential in order to optimize the performance and efficiency of PEFCs.The microstructure of electrocatalyst layers in PEFCs is generally studied by obtaining microtome sections, or by using focused-ionbeam (FIB) techniques.2-9 The diamond blade used in the microtome technique has the advantage of slicing through the electrocatalyst layer without inflicting thermal damage. However, it is difficult to cut through e.g. individual carbon particles using this method, meaning that the resulting cross-section is typically too rough to analyze the microstructure quantitatively.2 Meanwhile, clear and well-defined cross-sections can be obtained by FIB sectioning. However, in this case there remain difficulties in quantitative observation due to thermal damage and ion damage during the sputtering process, in which the gallium ion beam generates heat locally, melting or damaging the ionomer. This can be mitigated somewhat by cooling the sample using liquid nitrogen, as described by e.g. Katayanagi et al. 2,9Electron microscopy and X-ray computed tomography (CT) are two other common methods to observe electrocatalyst layers in PEFCs. For example, Wargo et al. observed electrocatalyst layers using both nano-CT and FIB-SEM techniques, 3 whilst Litster et al. observed electrocatalyst layers using high-resolution (50 nm) nano-CT. 4 Despite CT having the distinct advantage of being a non-destructive technique, it has the disadvantage of inferior resolution compared with SEM, TEM, or scanning transmission electron microscope (STEM).Therefore, in this study we focus on sectioning samples by FIB, and performing observation using electron microscopy. FIB-SEM has already been utilized as a powerful tool to observe the microstruct...

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Morphological Analyses of Polymer Electrolyte Fuel Cell Electrodes with Nano‐Scale Computed Tomography Imaging

Cited by 110 publications

References 33 publications

3D Printed Flow Field and Fixture for Visualization of Water Distribution in Fuel Cells by X-ray Computed Tomography

3D Printed Flow Field and Fixture for Visualization of Water Distribution in Fuel Cells by X-ray Computed Tomography

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

Correlating Cathode Microstructure with PEFC Performance Using FIB-SEM and TEM

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