Relationship among Microstructure, Ionomer Property and Proton Transport in Pseudo Catalyst Layers

Iden, Hiroshi; Sato, Kazuyuki; Ohma, Atsushi; Shinohara, Kazuhiko

doi:10.1149/1.3598141

Cited by 69 publications

(57 citation statements)

References 25 publications

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“…As 2 and 3 layers of nodes are used in the simulations, the effective conductivity behavior quickly approaches that of the 3-D case. This is also in qualitative agreement with the existing experimental observations [76][77][78][81][82][83][84][85][86][87][88], which show conductivity approaching the bulk value for thicknesses above 100 nm or so.…”

Section: Nafion-inspired Networksupporting

confidence: 92%

“…Finally, it is of interest to examine possible proton conduction through interfacial structures or Nafion thin-films that exist within PEFC catalyst layers, where these thin films form a 3D network throughout the layer in which protons must travel [80]. These thin-films demonstrate reduced conductivity and water uptake [76][77][78][81][82][83][84][85][86][87][88] and can limit PEFC performance at low catalyst loadings as discussed recently [89], and the modeling of such transport in thin-films is of active interest [1]. It is well known that when a polymer is confined to thicknesses comparable to its characteristic domain size, its properties and morphology differ from the analogous bulk materials [90].…”

Section: Nafion-inspired Networkmentioning

confidence: 99%

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Resistor-Network Modeling of Ionic Conduction in Polymer Electrolytes

Gostick

Weber

2015

Electrochimica Acta

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A resistor- and pore-network methodology is used to examine transport of ions in various ion-conducting polymers. The model is used to examine ion conduction in random and correlated (at the mesoscale) distributions of high and low conductive domains showing the impact that defects or different conduction modes have on overall effective conductivity and percolation. The specific case of Nafion is modeled where swelling is accounted for as well as a spatially varying conductivity within the nanodomains. The model is also used to investigate conduction in thin-films, where a substantial drop in conductivity is witnessed for films less than 50 nm thick. The model shows good agreement with experimental data and provides a methodology for efficient multiscale modeling of transport in ion-conducting polymers from the nanoscale morphology through the mesoscale transport pathways to the observable macroscale properties

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Section: Nafion-inspired Networksupporting

confidence: 92%

Section: Nafion-inspired Networkmentioning

confidence: 99%

Resistor-Network Modeling of Ionic Conduction in Polymer Electrolytes

Gostick

Weber

2015

Electrochimica Acta

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“…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%

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

Weber

Borup

Darling³

et al. 2014

J. Electrochem. Soc.

498

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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“…4 Ex-situ measurements on thin-film ionomer performed by several groups have shown that the ionomer nanostructure and its properties such as water uptake, proton conduction, and O 2 permeability can vary substantially depending on its thickness, treatment history, and substrate interaction. [9][10][11][12][13][14][15][16][17] Furthermore, sulfonate groups in the ionomer can adsorb on a Pt surface and reduce the oxygen reduction reaction (ORR) activity. 18,19 Because the adsorption of the acid group immobilizes the ionomer to the Pt surface, [20][21][22] it is surmised that it will also increase O 2 transport resistance.…”

mentioning

confidence: 99%

Characterizing Electrolyte and Platinum Interface in PEM Fuel Cells Using CO Displacement

Garrick

Moylan

Yarlagadda

et al. 2016

J. Electrochem. Soc.

107

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Relatively large O 2 transport resistance at the ionomer and Pt interface has been thought to be responsible for the large performance loss at high power for a low Pt loading proton-exchange-membrane fuel cell. A facile method to characterize the interface in the fuel cell electrode is needed. In this study, the CO displacement method was explored on polycrystalline Pt and carbon-supported Pt nanoparticles. The displacement charge coverages were used to quantify the adsorption of perchlorate, sulfate, and perfluorosulfonic acid ionomer. Heavy use of platinum in the electrodes of proton-exchange membrane (PEM) fuel cells is a key challenge preventing automotive manufacturers from bringing fuel cell electric vehicles to mass market. Current state-of-the-art fuel cell vehicles use >20 g of Pt per vehicle, which is significantly higher than the internal-combustion engine (ICE) incumbent (<5 g of precious metal per vehicle).1,2 Because heavy use of Pt is needed to obtain high energy conversion efficiency in the fuel cell, improving the activity of Pt-based catalysts has continued to be a high-priority research topic for many years.On the other hand, it was found that at high power density of a low-loaded electrode (<0.10 mg Pt /cm 2 or ∼11 g Pt /vehicle), significant performance losses are observed.3-7 These large performance losses are likely due to the need to deliver more O 2 to a small area of the Pt surface. It was also found that the bulk of the observed O 2 transport resistance occurs at the interface of Pt and electrolyte, 1,4,6 which is surprising because it has generally been seen that the thickness of the ionomer coated on a Pt surface in a well optimized electrode is only a few nanometers. If one calculates the O 2 transport resistance of the thin film using known O 2 permeability of a thick ionomer membrane, 8 it would require an ionomer film with unreasonable thickness (>20 nm) in order to explain the performance loss. Ex-situ measurements on thin-film ionomer performed by several groups have shown that the ionomer nanostructure and its properties such as water uptake, proton conduction, and O 2 permeability can vary substantially depending on its thickness, treatment history, and substrate interaction.9-17 Furthermore, sulfonate groups in the ionomer can adsorb on a Pt surface and reduce the oxygen reduction reaction (ORR) activity. 18,19 Because the adsorption of the acid group immobilizes the ionomer to the Pt surface, [20][21][22] it is surmised that it will also increase O 2 transport resistance. Recent molecular dynamics and density functional theory (DFT) calculations show that ionomers fold onto the Pt surface, leading to a highly dense layer which in turn can reduce the O 2 concentration close to the Pt surface to nearly zero. 23It is also shown that the type of ionomer and operational history can affect the observed performance.1,24 Unfortunately, there is still no characterization method available that will evaluate the ionomer/Pt interface in a fuel cell electrode and in a way that can be re...

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Relationship among Microstructure, Ionomer Property and Proton Transport in Pseudo Catalyst Layers

Cited by 69 publications

References 25 publications

Resistor-Network Modeling of Ionic Conduction in Polymer Electrolytes

Resistor-Network Modeling of Ionic Conduction in Polymer Electrolytes

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

Characterizing Electrolyte and Platinum Interface in PEM Fuel Cells Using CO Displacement

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