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Title: Molecular Structure and Transport Dynamics in Perfluoro Sulfonyl Imide Membranes

Abstract

We report a detailed and comprehensive analysis of the nanostructure, transport dynamics of water and hydronium and water percolation in hydrated perfluoro sulfonyl imides (PFSI), a polymer considered for proton transport in PEM fuel cells, using classical molecular dynamics simulations. The dynamical changes are related to the changes in the membrane nanostructure. Water network percolation threshold, the level at which a consistent spanning water network starts to develop in the membrane, lies between hydration level (λ) 6 and 7. The higher acidity of the sulfonyl imide acid group of PFSI compared to Nafion reported in our earlier ab initio study, translates into more free hydronium ions at low hydration levels. Nevertheless, the calculated diffusion coefficients of the H3O+ ions and H2O molecules as a function the hydration level were observed to be almost the same as that of Nafion, indicating similar conductivity and consistent with the experimental observations. This research was performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a U.S. Department of Energy (DOE) national scientific user facility located at the Pacific Northwest National Laboratory (PNNL). This work was supported by the US Department of Energy Basic Energy Sciences'more » Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less

Authors:
; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
1015894
Report Number(s):
PNNL-SA-73759
40083; KC0302020; TRN: US201112%%37
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physics. Condensed Matter, 23(23):Article No. 234106; Journal Volume: 23; Journal Issue: 23
Country of Publication:
United States
Language:
English
Subject:
30 DIRECT ENERGY CONVERSION; DIFFUSION; FUEL CELLS; HYDRATION; IMIDES; MEMBRANES; MOLECULAR STRUCTURE; OXONIUM IONS; PH VALUE; POLYMERS; PROTON TRANSPORT; TRANSPORT; WATER; proton transport; polymer electrolyte membranes; molecular dynamics; Environmental Molecular Sciences Laboratory

Citation Formats

Idupulapati, Nagesh B., Devanathan, Ramaswami, and Dupuis, Michel. Molecular Structure and Transport Dynamics in Perfluoro Sulfonyl Imide Membranes. United States: N. p., 2011. Web. doi:10.1088/0953-8984/23/23/234106.
Idupulapati, Nagesh B., Devanathan, Ramaswami, & Dupuis, Michel. Molecular Structure and Transport Dynamics in Perfluoro Sulfonyl Imide Membranes. United States. doi:10.1088/0953-8984/23/23/234106.
Idupulapati, Nagesh B., Devanathan, Ramaswami, and Dupuis, Michel. 2011. "Molecular Structure and Transport Dynamics in Perfluoro Sulfonyl Imide Membranes". United States. doi:10.1088/0953-8984/23/23/234106.
@article{osti_1015894,
title = {Molecular Structure and Transport Dynamics in Perfluoro Sulfonyl Imide Membranes},
author = {Idupulapati, Nagesh B. and Devanathan, Ramaswami and Dupuis, Michel},
abstractNote = {We report a detailed and comprehensive analysis of the nanostructure, transport dynamics of water and hydronium and water percolation in hydrated perfluoro sulfonyl imides (PFSI), a polymer considered for proton transport in PEM fuel cells, using classical molecular dynamics simulations. The dynamical changes are related to the changes in the membrane nanostructure. Water network percolation threshold, the level at which a consistent spanning water network starts to develop in the membrane, lies between hydration level (λ) 6 and 7. The higher acidity of the sulfonyl imide acid group of PFSI compared to Nafion reported in our earlier ab initio study, translates into more free hydronium ions at low hydration levels. Nevertheless, the calculated diffusion coefficients of the H3O+ ions and H2O molecules as a function the hydration level were observed to be almost the same as that of Nafion, indicating similar conductivity and consistent with the experimental observations. This research was performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a U.S. Department of Energy (DOE) national scientific user facility located at the Pacific Northwest National Laboratory (PNNL). This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.},
doi = {10.1088/0953-8984/23/23/234106},
journal = {Journal of Physics. Condensed Matter, 23(23):Article No. 234106},
number = 23,
volume = 23,
place = {United States},
year = 2011,
month = 5
}
  • A study of proton conductivity in a commercial sample of Nafion{reg_sign} 117 and a structurally similar bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane under variable temperature and humidity conditions is reported. The sulfonyl imide ionomer was synthesized using a novel redox-initiated emulsion copolymerization method, and conductivities were measured using a galvanostatic four-point-probe electrochemical impedance spectroscopy technique. Both materials exhibited a strong dependence of conductivity on temperature and humidity, with conductivity in both cases being strongly diminished with decreasing humidity (at constant temperature) and increasing temperature (at constant water partial pressure). The observed behavior is consistent with a liquid-like mechanism of proton conductivity whereby protonsmore » are transported as hydrated hydronium ions through water-filled pores and channels in the ionomer.« less
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  • Molecular dynamics simulations of n-hexane adsorbed onto the interface of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([bmim][Tf{sub 2}N]) are performed at three n-hexane surface densities, ranged from 0.7 to 2.3 {mu}mol/m{sup 2} at 300 K. For [bmim][Tf{sub 2}N] room-temperature ionic liquid, we use a non-polarizable all-atom force field with the partial atomic charges based on ab initio calculations for the isolated ion pair. The net charges of the ions are {+-}0.89e, which mimics the anion to cation charge transfer and polarization effects. The OPLS-AA force field is employed for modeling of n-hexane. The surface tension is computed using the mechanical route and itsmore » value decreases with increase of the n-hexane surface density. The [bmim][Tf{sub 2}N]/n-hexane interface is analyzed using the intrinsic method, and the structural and dynamic properties of the interfacial, sub-interfacial, and central layers are computed. We determine the surface roughness, global and intrinsic density profiles, and orientation ordering of the molecules to describe the structure of the interface. We further compute the survival probability, normal and lateral self-diffusion coefficients, and re-orientation correlation functions to elucidate the effects of n-hexane on dynamics of the cations and anions in the layers.« less