Water Transport and Sorption in Nafion Membrane

Borup

Darling³

et al. 2014

J. Electrochem. Soc.

Self Cite

491

394

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...

“…[61] where [62] and η PtO and U PtO are the Pt-oxide overpotential and equilibrium potential, respectively. Another approach is to estimate it with experimental correlations.…”

Section: Basic Governing Equationsmentioning

confidence: 99%

Section: Intersection With Experimentsmentioning

confidence: 99%

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

Borup

Darling³

et al. 2014

J. Electrochem. Soc.

Self Cite

491

394

“…Membrane water uptake as a function of relative humidity (RH) was measured with a dynamic vapor-sorption (DVS) analyzer (Surface Measurement Systems, UK) at 25°C and 70°C [39,40]. First, the samples were dried in the DVS at 0% RH and 25°C for 2 hours to set a standard dry state with initial sample weight, M 0 .…”

Section: Water Uptake and Swellingmentioning

confidence: 99%

Structure/property relationship of Nafion XL composite membranes

Shi

Journal of Membrane Science

Kusoglu

2016

Self Cite

135

138

Perfluorosulfonic-acid (PFSA) ionomer membranes (most commonly Nafion ® ) are currently the prototypical proton-exchange-membrane in polymer-electrolyte fuel cells (PEFCs), for which durability still represents a technical barrier to their commercialization. In an effort to address the durability demands, PFSA membranes with reinforcement and/or stabilizers have become of great interest as they have demonstrated superior durability in PEFCs compared to their unreinforced analogues. One such particular membrane that is tailored for enhanced durability and commonly employed in PEFCs is Nafion XL, a Nafion-based ionomer membrane with mechanical reinforcement and chemical stabilizers. Despite an increasing number of recent studies demonstrating its improved lifetime in accelerated stress testing (AST), its structure and transport properties have not been investigated in a systematic fashion. In this paper, we report water uptake, dimensional change, conductivity, and mechanical properties of Nafion XL membrane, as well as its strong anisotropy, in comparison to (unreinforced) Nafion 212 membrane.Moreover, water-domain spacing and crystallinity of Nafion XL membrane, determined from small-and wide-angle X-ray scattering (SAXS/WAXS) experiments, are correlated with the measured properties to establish a structure/property relationship, and discussed within the context of composite materials. It is also found that (pre)conditioning of the membrane by heating in water at different temperatures could have significant impacts on its structure/property relationship, in particular, the mechanical stability and conductivity, which were related to morphological changes observed from microscopy 2 studies. The findings reported here not only provide a new dataset that can be used for PEFC performance and durability modeling but also benefit the efforts on developing composite ionconductive membranes. 1.Highlights  The composite Nafion XL membrane's structure/property relationship is investigated  Impact of pretreatment on membrane's transport and mechanical properties are shown  Membrane's composite morphology are revealed through TEM, SEM and SAXS/WAXS

“…The size of the pore and throats in Nafion have been intensively studied in recent years [13,14,22,55,56,[61][62][63][64][65] including some recent direct visualization of the hydrated structure [66]. The general picture is that pore bodies of mean size around 5 to 10 nm that are spaced between 12 and 16 nm apart in dry versus fully humidified membranes, respectively.…”

Section: Pore and Throat Sizesmentioning

confidence: 99%

“…The water uptake is characterized by a primary hydration of around l = 6 (i.e., 6 water molecules per sulfonic acid site), followed by a secondary swelling of the polymer matrix to a l = 14 (saturated vapor) and l = 22 (liquid water) [14,15]. Water uptake has been modeled by various means, mainly using an energy-balance approach wherein the chemical energies related to dissolution are countered by the mechanical energies related to swelling and backbone deformation [16][17][18][19][20][21][22][23]. While these models have shown significant insight into the quasi-equilibrium water content, their use as models of transport have been limited since they inherently neglect the connectivity between the water domains.…”

Section: Introductionmentioning

confidence: 99%

Resistor-Network Modeling of Ionic Conduction in Polymer Electrolytes

Gostick

2015

Electrochimica Acta

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