Improved modeling and understanding of diffusion-media wettability on polymer-electrolyte-fuel-cell performance

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...

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“…[19][20][21][22] Numerous aspects of the model have Figure 1. Model geometry where different water-transport pathways are shown.…”

Section: Model Descriptionmentioning

confidence: 99%

Understanding Impacts of Catalyst-Layer Thickness on Fuel-Cell Performance via Mathematical Modeling

Zenyuk

Das

Weber

2016

J. Electrochem. Soc.

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147

136

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“…14,15 Weber et al, 11,16 Mateo et al 13 and Mulone et al 12 have treated hydrophilic (HI) and hydrophobic (HO) pore-networks separately either by introducing a continuous wettability distribution, i.e. Ref.…”

Section: -7mentioning

confidence: 99%

A Mixed Wettability Pore Size Distribution Based Mathematical Model for Analyzing Two-Phase Flow in Porous Electrodes

Zhou

Pütz

Secanell

2017

J. Electrochem. Soc.

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A multi-dimensional, non-isothermal, two-phase membrane electrode assembly (MEA) numerical model is developed where the micro-structure of the porous layers is characterized by a mixed wettability pore size distribution (PSD). The PSD model is used to predict local water saturation based on gas and liquid pressures, and can be used to study the effect of varying pore size and wettability. The MEA model accounts for gas transport via molecular and Knudsen diffusion, liquid water transport, sorbed water transport by back-diffusion, electro-and thermo-osmosis, and heat generation and transport. Multi-step kinetic models are used to predict anode and cathode electrochemical reactions. Local transport losses are accounted for using a local transport resistance. The PSD model is used to predict capillary pressure vs. saturation and saturation vs. relative liquid permeability curves based on PSDs from several GDLs obtained using mercury intrusion porosimetry. The PSD-based MEA model electrochemical performance predictions are also compared to experimental data from the literature. Results show that the numerical model is able to capture the performance changes associated with varying temperate and the introduction of a micro-porous layer. Improving the performance of polymer electrolyte fuel cells (PEFCs) at high current density is critical for reducing stack size, cost and weight in automobile applications. Water accumulation in the electrode however, limits fuel cell performance at high current densities. [1][2][3] In order to mitigate the excessive water buildup in the cell, which hinders mass transport, gas diffusion layer (GDL) and catalyst layer (CL) microstructures and wettabilities must be advanced to achieve sufficient water retention in the electrolyte, reject excess water in vapor form and alleviate complete flooding of the electrode. For example, by modifying the weight percentage (wt%) of the hydrophobic content in the GDL, Wang et al. 4 reported a 10%-20% increase in performance. To understand fuel cell behavior in the two-phase region and optimize the design of GDL and CL, a two-phase model which includes microstructural information of the porous layers is needed.Two approaches have commonly been used to study liquid water transport in fuel cells. The first approach involves the solution of a saturation equation obtained by replacing liquid pressure by saturation in Darcy's law. [5][6][7] In this approach the Levertt J-function is commonly used to relate saturation with capillary pressure. The Leverett J-function is however, an empirically measured curve for a given material and it is obtained from porous media structure analysis. Thus, it is difficult to optimize the structure of the porous layers using Leverett J-functions. The second approach, known as the multi-phase approach, involves solving mass and momentum equations for gas and liquid phases independently and then relating the capillary pressure to saturation using a closure equation based on micro-structural information such as a pore size dist...

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“…The functional form of the saturation dependence on capillary pressure can be measured [154][155][156][157][158][159] or derived using various simplistic models 141 or more complicated pore-network and other models as discussed in the next section. Figure 4 displays exemplary capillarypressure -saturation relationships.…”

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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.

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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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Improved modeling and understanding of diffusion-media wettability on polymer-electrolyte-fuel-cell performance

Cited by 78 publications

References 91 publications

Understanding Impacts of Catalyst-Layer Thickness on Fuel-Cell Performance via Mathematical Modeling

Understanding Impacts of Catalyst-Layer Thickness on Fuel-Cell Performance via Mathematical Modeling

A Mixed Wettability Pore Size Distribution Based Mathematical Model for Analyzing Two-Phase Flow in Porous Electrodes

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

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