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Title: Understanding Anion, Water, and Methanol Transport in a Polyethylene- b -poly(vinylbenzyl trimethylammonium) Copolymer Anion-Exchange Membrane for Electrochemical Applications

Abstract

Herein, we report the anion and water transport properties of an anion-exchange membrane (AEM) comprising a block copolymer of polyethylene and poly- (vinylbenzyl trimethylammonium) (PE-b-PVBTMA) with an ion-exchange capacity (IEC) of 1.08 mequiv/g. The conductivity varied little among the anions CO3 2-, HCO3 -, and F-, with a value of Ea ≈ 20 kJ/mol and a maximum fluoride conductivity of 34 mS/cm at 90 °C and 95% relative humidity. The Br- conductivity showed a transition at 60 °C. Pulsed gradient stimulated spin echo nuclear magnetic resonance (PGSE NMR) experiments showed that water diffusion in this AEM is heterogeneous and is affected by the anion present, being fastest in the presence of F-. We determined the methanol self-diffusion in this membrane and observed that it is lower than that in Nafion 117, because of the lower water uptake. This article reports the first measurements of 13C-labeled bicarbonate self-diffusion in an AEM using PGSE NMR spectrometry, which was found to be significantly slower than F- self-diffusion. Back-calculation of the bicarbonate conductivity using the Nernst-Einstein equation gave a value that was significantly lower than the measured value, implying that bicarbonate transport involves OH- in the transport mechanism. Fourier transform infrared spectroscopy, PGSE NMRmore » spectrometry, and small-angle X-ray scattering (SAXS) indicated the presence of different types of waters present in the membrane at different length scales. The SAXS data indicated that there is a water-rich region within the hydrophilic domains of the polymer that has a temperature dependence in intensity at 95% relative humidity (RH).« less

Authors:
; ; ; ; ; ; ORCiD logo
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); US Army Research Office (ARO); National Science Foundation (NSF)
OSTI Identifier:
1395847
DOE Contract Number:
AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry. C; Journal Volume: 121; Journal Issue: 4
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Sarode, Himanshu N., Yang, Yuan, Motz, Andrew R., Li, Yifan, Knauss, Daniel M., Seifert, Soenke, and Herring, Andrew M. Understanding Anion, Water, and Methanol Transport in a Polyethylene- b -poly(vinylbenzyl trimethylammonium) Copolymer Anion-Exchange Membrane for Electrochemical Applications. United States: N. p., 2017. Web. doi:10.1021/acs.jpcc.6b09205.
Sarode, Himanshu N., Yang, Yuan, Motz, Andrew R., Li, Yifan, Knauss, Daniel M., Seifert, Soenke, & Herring, Andrew M. Understanding Anion, Water, and Methanol Transport in a Polyethylene- b -poly(vinylbenzyl trimethylammonium) Copolymer Anion-Exchange Membrane for Electrochemical Applications. United States. doi:10.1021/acs.jpcc.6b09205.
Sarode, Himanshu N., Yang, Yuan, Motz, Andrew R., Li, Yifan, Knauss, Daniel M., Seifert, Soenke, and Herring, Andrew M. Thu . "Understanding Anion, Water, and Methanol Transport in a Polyethylene- b -poly(vinylbenzyl trimethylammonium) Copolymer Anion-Exchange Membrane for Electrochemical Applications". United States. doi:10.1021/acs.jpcc.6b09205.
@article{osti_1395847,
title = {Understanding Anion, Water, and Methanol Transport in a Polyethylene- b -poly(vinylbenzyl trimethylammonium) Copolymer Anion-Exchange Membrane for Electrochemical Applications},
author = {Sarode, Himanshu N. and Yang, Yuan and Motz, Andrew R. and Li, Yifan and Knauss, Daniel M. and Seifert, Soenke and Herring, Andrew M.},
abstractNote = {Herein, we report the anion and water transport properties of an anion-exchange membrane (AEM) comprising a block copolymer of polyethylene and poly- (vinylbenzyl trimethylammonium) (PE-b-PVBTMA) with an ion-exchange capacity (IEC) of 1.08 mequiv/g. The conductivity varied little among the anions CO3 2-, HCO3 -, and F-, with a value of Ea ≈ 20 kJ/mol and a maximum fluoride conductivity of 34 mS/cm at 90 °C and 95% relative humidity. The Br- conductivity showed a transition at 60 °C. Pulsed gradient stimulated spin echo nuclear magnetic resonance (PGSE NMR) experiments showed that water diffusion in this AEM is heterogeneous and is affected by the anion present, being fastest in the presence of F-. We determined the methanol self-diffusion in this membrane and observed that it is lower than that in Nafion 117, because of the lower water uptake. This article reports the first measurements of 13C-labeled bicarbonate self-diffusion in an AEM using PGSE NMR spectrometry, which was found to be significantly slower than F- self-diffusion. Back-calculation of the bicarbonate conductivity using the Nernst-Einstein equation gave a value that was significantly lower than the measured value, implying that bicarbonate transport involves OH- in the transport mechanism. Fourier transform infrared spectroscopy, PGSE NMR spectrometry, and small-angle X-ray scattering (SAXS) indicated the presence of different types of waters present in the membrane at different length scales. The SAXS data indicated that there is a water-rich region within the hydrophilic domains of the polymer that has a temperature dependence in intensity at 95% relative humidity (RH).},
doi = {10.1021/acs.jpcc.6b09205},
journal = {Journal of Physical Chemistry. C},
number = 4,
volume = 121,
place = {United States},
year = {Thu Jan 19 00:00:00 EST 2017},
month = {Thu Jan 19 00:00:00 EST 2017}
}
  • Anion exchange m em branes (AEM )are prom ising solid polym er electrolytes utilized in alkalifuelcells and electrochem icalenergy conversion devices.AEM s m ust ef ciently conductions w hile m aintaining chem icaland m echanicalstability undera range ofoperating conditions.The ionicnature ofAEM sleads to stiffand brittle m em branesunderdry conditions w hile athigher hydrations,w ater sorption causes signi cant softening and w eakening of the m em brane.In this w ork,a new polyethylene-b-poly(vinylbenzyltrim ethylam m onium ) polym er (70 kg/m ol) w as cast into large (300 cm 2),thin (127 3 m ) m em branes.These m em branes exhibitedmore » im proved elasticity over previously tested AEM s,m inim aldim ensional sw elling,and m oderate ionic conductivity (57 2 m S/cm at 50 °C,95% RH in the brom ide form ).Extensional testing indicated a 95% reduction in Young's m odulus betw een dry and hydrated states.Furtherinvestigation ofthe com plex m odulusasa function ofhydration,by dynam ic m echanical analysis,revealed a sharp decrease in m odulusbetw een dry and hydrated states.M echanicalsoftening w as reversible,but the location ofthe transition displayed hysteresis betw een hum idi cation and dehum idi cation.Conductivity increased after m em brane softening;suggesting bulk m echanicalpropertiescan identify thehydration levelrequired forim proved ion transport.Understanding the relationship betw een ion conduction and m echanical properties w illhelp guide AEM developm ent and identify operating conditions for sustained perform ance.« less
  • Anion exchange membranes (AEM) are solid polymer electrolytes that facilitate ion transport in fuel cells. In this study, a polystyrene-b-poly(vinylbenzyl trimethylammonium) diblock copolymer was evaluated as potential AEM and compared with the equivalent homopolymer blend. The diblock had a 92% conversion of reactive sites with an IEC of 1.72 ± 0.05 mmol g -1, while the blend had a 43% conversion for an IEC of 0.80 ± 0.03 mmol g -1. At 50°C and 95% relative humidity, the chloride conductivity of the diblock was higher, 24–33 mS cm -1, compared with the blend, 1–6 mS cm -1. The diblock displayedmore » phase separation on the length scale of 100 nm, while the blend displayed microphase separation (~10 μm). Mechanical characterization of films from 40 to 90 microns thick found that elasticity and elongation decreased with the addition of cations to the films. At humidified conditions, water acted as a plasticizer to increase film elasticity and elongation. While the polystyrene-based diblock displayed sufficient ionic conductivity, the films' mechanical properties require improvement, i.e., greater elasticity and strength, before use in fuel cells. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41596.« less
  • A chemically stable copolymer [poly(2,6 dimethyl 1,4 phenylene oxide)-b-poly(vinyl benzyl trimethyl ammonium)] with two ion exchange capacities, 3.2 and 2.9 meq g-1, was prepared as anion exchange membranes (AEM-3.2 and AEM-2.9). These materials showed high OH- conductivities of 138 mS.cm-1 and 106 mS.cm-1, for AEM-3.2 and AEM-2.9 respectively, at 60°C, and 95% RH. The OH- conductivity = 45 mS.cm-1 for AEM-3.2 at 60% RH and 60°C in the absence of CO2. Amongst the ions studied, only OH- is fully dissociated at high RH. The lower Ea = 10–13 kJ.mol-1 for OH- compared to F- ~ 20 kJ.mol-1 in conductivity measurements,more » and of H2O from self-diffusion coefficients suggests the presence of a Grotthuss hopping transport mechanism in OH- transport. PGSE-NMR of H2O and F- show that the membranes have low tortuosity, 1.8 and 1.2, and high water self-diffusion coefficients, 0.66 and 0.26 × 10-5 cm2.s-1, for AEM-3.2 and AEM-2.9 respectively. SAXS and TEM show that the membrane has several different sized water environments, ca. 62 nm, 20 nm, and 3.5 nm. The low water uptake, λ = 9–12, reduced swelling, and high OH- conductivity, with no chemical degradation over two weeks, suggests that the membrane is a strong candidate for electrochemical applications.« less
  • A poly(2,6 dimethyl 1,4-phenylene oxide)-b-poly(vinyl benzyl) chloride copolymer membranes was processed by solvent casting followed by melt pressing (SCMP) to provide uniformly thin films, 25 +/- 5 mu m, with improved conductivity, mechanical strength, water uptake, dimensional swelling, and chemical stability under 1 M KOH and 80 degrees C. These properties depended strongly on the length of the melt-pressing time. The solvent cast membranes melt pressing time was optimized to provided highly conductive membranes (high OH- conductivity of 75 +/- 25 mS cm(-1) for an IEC of 1.8 mmol g(-1) at room temperature in water). Membranes that were only solventmore » cast and not melt-pressed swelled excessively and had insufficient mechanical integrity for detailed study. When the copolymer powder was melt pressed (without prior solvent casting) at 240 degrees C and ca. 30 MPa for 20 minutes, membranes with high mechanical strength (tensile stress at break of 32 +/- 6 MPa at 25% RH and 29 +/- 3 MPa when 95% RH at 60 degrees C), high conductivity (Cl conductivity of 80 mS/cm at 90 degrees C and 95% RH), and lower water uptake were formed. However, melt pressing alone did not give larger then 5 cm x 5 cm area films, homogeneously thin (< 60 mu m), or mechanical defect-free membranes. The SCMP membranes were uniformly thin, and thermally crosslinked. The mass loss via dehydrochlorination indicated by TGA and elemental analysis confirmed the crosslinking via thermal melt pressing. The SCMP membranes thickness could be reduced by more than 50% (25 +/- 5 mu m) compared to melt pressing alone, and the Cl conductivity increased by 44% at 90 degrees C and 95% RH. The tensile stress at break of the SCMP membranes, however, was reduced by 50% at 25% RH.« less