Water Uptake of Fuel-Cell Catalyst Layers

Kusoglu, Ahmet; Kwong, Anthony; Clark, Kyle T.; Gunterman, Haluna P.; Weber, Adam Z.

doi:10.1149/2.031209jes

Cited by 109 publications

(119 citation statements)

References 40 publications

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“…However, recent studies on the underlying knowledge of the structure/function relationships of ionomer thin-films in the catalyst layers are beginning to show that such an assumption is poor as discussed in more detail below. 157,[298][299][300][301][302][303][304][305] The kinetic expressions, equations 58 and 60 for the HOR and ORR, respectively, result in a transfer current between the electronicconducting (1) and ionic-conducting (2) phases (see equation 10), which is related to the current density in the two phases through equation 23 (neglecting double-layer charging)…”

Section: Catalyst-layer Modelingmentioning

confidence: 99%

“…Water-uptake isotherms for Nafion in various PEFC catalyst layers as a function of relative humidity compared with that of bulk Nafion membrane. 157 where φ is the dimensionless Thiele modulus…”

Section: Catalyst-layer Modelingmentioning

confidence: 99%

“…As shown in Figure 20, water-uptake isotherms for Nafion in the catalyst layers show much lower water contents compared to bulk Nafion membrane. 157 Also, water uptake in catalyst layers has been shown to increase with increasing loading of platinum, which improves wetting properties and hydrophilicity, 157,326 and with increasing ionomer content, 326,327 although in general the catalyst layer's ionomer water uptake has been consistently shown to be depressed. 157,327 With the lower water contents (and perhaps the existence of different morphology and confinement driving the lower water uptake), it is not surprising that the ion conductivity is likewise depressed, [327][328][329] and is a function of ionomer content.…”

Section: Catalyst-layer Modelingmentioning

confidence: 99%

“…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%

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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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Section: Catalyst-layer Modelingmentioning

confidence: 99%

Section: Catalyst-layer Modelingmentioning

confidence: 99%

Section: Catalyst-layer Modelingmentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

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

Weber

Borup

Darling³

et al. 2014

J. Electrochem. Soc.

Self Cite

509

394

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show abstract

“…The catalyst layer effective conductivity exhibits stronger water activity dependence than the membrane conductivity, which is consistent with literature reports on conductivity, 36 but at first glance appears contradictory with literature reports finding lower ionomer hydration in catalyst layers. 38 However, even with reduced hydration levels, the catalyst layer ionomer will swell with increasing relative humidity. The resulting increase in volume fraction and decrease in tortuosity appears to dominate any difference in bulk vs. thin-film ionomer conductivity.…”

Section: Resultsmentioning

confidence: 99%

A Physics-Based Impedance Model of Proton Exchange Membrane Fuel Cells Exhibiting Low-Frequency Inductive Loops

Setzler

Fuller

2015

J. Electrochem. Soc.

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A physics-based impedance model of a proton exchange membrane fuel cell is developed, incorporating a coupled oxide growthoxygen reduction reaction kinetic model. The oxide layer is shown to produce a low frequency inductive loop that agrees quantitatively with the experimental inductive loop at current densities as high as 800 mA/cm 2 , even when kinetic and mass-transfer parameters are fit from polarization curves and cyclic voltammetry instead of electrochemical impedance spectroscopy. The importance of the inductive loop in explaining both AC and DC results is discussed. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0361506jes] All rights reserved.Manuscript submitted October 31, 2014; revised manuscript received February 16, 2015. Published March 3, 2015 Electrochemical impedance spectroscopy (EIS) is a widely used technique in proton exchange membrane fuel cells (PEMFCs) that separates differential contributions to cell overpotential by characteristic time constants. Analysis of EIS is difficult because there may be dozens of processes involved. It is standard to analyze EIS using equivalent electrical circuit models of the PEMFC, in which several key processes are represented by circuit elements such as the resistor, capacitor, Warburg element, and constant phase element. Fitting the experimental EIS data with an equivalent circuit model and obtaining circuit parameters is straightforward. Often, a close fit can be achieved with relatively few components, especially when constant phase elements are used. However, the challenge becomes determining which processes to include and converting the electrical circuit parameters back to meaningful cell parameters.Although the basic building blocks of equivalent circuits, for example the Randles circuit, have an exact physical interpretation, these elements are often applied to more complex processes without accounting for the differences. The circuits may fit the data perfectly, but the parameters have lost their original meaning. Often, some transport processes are faster than double layer charging, and as a result, are inseparable from charge-transfer resistance. Additionally, slow transport processes often contribute impedance that scales with chargetransfer resistance, rather than being independent. Unfortunately, it is common in the literature to see circuit parameters reported as results without a translation to meaningful physical parameters.An alternative approach is to model the physical processes occurring in an electrochemical cell during EIS experiments. This approach was pioneered by De Levie, 1 who calculated an analytical solution for the impedance of a porous electrode with a potential gradient, assuming linear kinetics and no concentration gradient. Keddam et al. 2 cons...

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Impact of Substrate and Processing on Confinement of Nafion Thin Films

Kusoglu

Kushner

Paul

et al. 2014

Adv Funct Materials

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Thin films of ion‐conducting polymers are an important area of study due to their function in many electrochemical devices and as analogues for interfacial phenomena that occur in bulk films. In this paper, the properties of Nafion, a prototypical ionomer, are investigated as thin films (4 to 300 nm) on carbon, gold, and platinum substrates that are fabricated using different casting methods and thermal histories. Specifically, water uptake, swelling, and morphology are investigated by quartz‐crystal microbalance, ellipsometry, and grazing‐incidence X‐ray scattering to develop structure/property/processing relationships. For all substrates, as the films' thickness decreased, there is an initial decrease in swelling followed by a subsequent increase for film thicknesses below ≈20 nm due to a disordering of the film hydrophilic/hydrophobic structure. Decreased swelling and less structural order is observed on gold for spin‐cast films compared to self‐assembled films; the opposite effect is observed for films on carbon. The presented systematic data set and analyses represent a thorough study of the behavior of Nafion thin films on model substrates of interest in metal catalyst/carbon electrodes, and these insights help to elucidate the underlying polymer physics and confinement effects in these and related systems.

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Water Uptake of Fuel-Cell Catalyst Layers

Cited by 109 publications

References 40 publications

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

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

A Physics-Based Impedance Model of Proton Exchange Membrane Fuel Cells Exhibiting Low-Frequency Inductive Loops

Impact of Substrate and Processing on Confinement of Nafion Thin Films

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