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...

The dilemma of employing high-capacity battery materials and maintaining the electronic and mechanical integrity of electrodes demands novel designs of binder systems. Here, we developed a binder polymer with multi-functionality to maintain high electronic conductivity, mechanical adhesion, ductility, and electrolyte uptake. These critical properties are achieved by designing polymers with proper functional groups. Through synthesis, spectroscopy and simulation, electronic conductivity is optimized by tailoring the key electronic state, which is not disturbed by further modifications of side chains. This fundamental allows separated optimization of the mechanical and swelling properties without detrimental effect on electronic property. Remaining electrically conductive, the enhanced polarity of the polymer greatly improves the adhesion, ductility, and more importantly, the electrolyte uptake to the levels of those available only in non-conductive binders before. We also demonstrate directly the performance of the developed conductive binder by achieving full-capacity cycling of silicon particles without using any conductive additive.3

In this article, a two-dimensional, multiphase, transient model is introduced and used to explore the impact of catalyst-layer thickness on performance. In particular, the tradeoffs between water production and removal through transport or evaporation are highlighted, with a focus on low-temperature performance. For the latter, a case study of an ultra-thin catalyst layer is undergone to explore how various material properties alter the steady-state and startup performance of a cell. The findings provide understanding and guidance to optimize fuel-cell performance with thin electrodes. Polymer-electrolyte fuel cells (PEFCs) have emerged as a promising zero-emission technology for energy conversion due to their thermodynamic efficiency and high energy density.1,2 However, to reduce cost, the amount of precious metal catalyst needs to be lowered. The most common strategy for such catalyst thrifting is to fabricate thinner catalyst layers. The prototypical example of this approach is the nanostructured thin-film (NSTF) electrode, which has several advantages compared to standard carbon-supported Pt electrodes.2 These stateof-the art electrodes have demonstrated improved chemical stability, durability and desired specific power and activity, at the same time having high Pt mass activities. 2,3 Common to these and other lowloaded electrodes are the issues associated with water management in thinner electrodes. Typically, thinner electrodes are susceptible to severe flooding due to their inherently low water capacity and perhaps lack of hydrophobic zones. Such phenomena are particularly pronounced when PEFCs operate at lower temperatures or during startup.Recently, several studies reported water-management mitigation strategies for thinner electrodes including modification of operating conditions and/or component morphologies to ensure successful startup and operation at low temperatures. For example, for NSTF electrodes, Steinbach et al.4,5 reported a novel water-management scheme of increasing the pressure on the cathode to drive water to the anode; coupled with a different anode design, the limiting current density at low temperatures increased by a factor of four. Kongkanand et al. 6,7 demonstrated that water removal and storage capacity of the NSTF electrode can be significantly enhanced by placing an additional Pt/C layer between the electrode and microporous layer. The various empirical findings of NSTF as well as traditional supported thin electrodes 8 can be much better understood by examining the various tradeoffs engendered and complications arising from the use of thin catalyst layers.Currently, the transport mechanisms of water removal behind possible mitigation strategies for thin electrodes are not well understood. In terms of modeling, both pore-scale and continuum models have been generated, although only the latter are germane to understanding cell water and thermal management. 9 Examples of continuum models include bilayer models, such as the one developed by Sinha et al. 10 with a membrane and wat...

Optimal water management in proton-exchange-membrane fuel cells at lower temperatures requires the efficient removal of liquid water from the cell. This pathway is intimately linked with liquid-water-droplet removal from the surface of the gas-diffusion layer (GDL) and into the flow channel. In this study, these liquid-water phenomena are investigated experimentally to improve the understanding of water transport through, and removal from, the GDL. Specifically, an experiment using a sliding-angle measurement is designed and used to quantify and measure directly the adhesion force for liquid-water droplets and to understand the droplets' growth and detachment from the GDL. The results show that unlike the static contact angle, the adhesion force, as measured by sliding angles, provides a good indicator of water-droplet removal as it is a direct measure of the dominating force that is holding a droplet on the GDL surface and preventing its detachment. It is also observed that injection through the GDL, as is representative of operating fuel cells, results in a higher adhesion force than a droplet placed on the top surface. Finally, it is shown that aged GDLs demonstrate higher adhesion forces, which dominate GDL degradation response and fuel-cell water holdup.

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Understanding dynamic liquid-water uptake and removal in gas-diffusion layers (GDLs) is essential to improve the performance of polymer-electrolyte fuel cells and related electrochemical technologies. In this work, GDL properties such as breakthrough pressure, droplet adhesion force, and detachment velocity are measured experimentally for commonly used GDLs under a host of test conditions. Specifically, the effects of GDL hydrophobic (PTFE) content, thickness, and water-injection area and rate were studied to identify trends that may be beneficial to the design of liquid-water management strategies and next-generation GDL materials. The results conclude that liquid water moving transversely through or forming at the surface of GDL may be affected by internal capillary structure. Adhesion-force measurements using a bottom-injection method were found to be sensitive to PTFE loading, GDL thickness, and injection area/rate, the latter of which is critical for defining the control-volume limits for modeling and analysis. It was observed that higher PTFE loadings, increased thickness, and smaller injection areas led to elevated breakthrough pressure; meaning there was a greater resistance to forming droplets. The data are used to predict the onset of droplet instability via a simple force-balance model with general trend agreement. Polymer-electrolyte fuel-cell (PEFC) and redox flow-battery (RFB) systems have the potential to improve energy efficiency and storage capabilities for mobile and grid-level applications in the near future. In PEFCs, the electrode structure is composed of a catalytic layer supported by porous gas-diffusion layers (GDLs) where multiphase reactant/product transport and electron conduction occur. Product liquid water can contribute to performance and degradation issues if not properly handled. Numerous studies have shown the importance of water-management strategies during start-up/shutdown and cooler operation where lower cell temperatures may lead to liquid buildup. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] In RFBs, GDLs serve a similar purpose of effective reactant distribution, especially for gaseous cells, 16,17 as well as serving as possible catalysts.18 Understanding multiphase, dynamic GDL water uptake and removal is essential to develop effective liquid-water management schemes as well as next-generation GDL materials for improved PEFC and RFB performance, stability, and component lifetimes.The influence of the porous-electrode structure on liquid/gas transport and PEFC performance has been studied by several groups focusing on the role of GDL and microporous layer (MPL) effects. [19][20][21][22] Capillary and viscous forces govern two-phase flow through GDLs; the dimensionless parameters that quantify them are the capillary number and viscosity ratio defined asandrespectively, where u is the superficial velocity of the non-wetting phase, γ is the surface tension, and μ is the wetting (wet) and nonwetting (nw) phase viscosities. 10,23 Under normal PEFC operation, capillary fo...

Interactions of the active material particles with the binder are crucial in tailoring the properties of composite electrodes used in lithium-ion batteries. The dependency of the protonation degree of the carboxyl group in the carboxymethyl cellulose (CMC) structure on the pH value of the preparation solution was investigated by Fourier transform infrared spectroscopy (FTIR). Three different distinctive chemical states of CMC binder were chosen (protonated, deprotonated, and half-half), and their interactions with different silicon single crystal facets were investigated. The different Si surface orientations display distinct differences of strength of interactions with the CMC binder. The CMC/Si adhesion forces in solution and Si wettability of the silicon are also strongly dependent on the protonation degree of the CMC. This work provides an insight into the nature of these interactions, which determine the electrochemical performance of silicon composite electrodes.

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