Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure-Property Relationships.

TLDR: A novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone is presented, which reveals the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder.

Abstract: Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings. In this study, we present a novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone. The novel backbone chemistry induces structural changes in the ionomer, restricting ionomer domain swelling under hydration while disrupting matrix crystallinity. These structural changes slightly reduce proton conductivity while significantly improving gas permeability. The performance implications of this trade-off are assessed, which reveal the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder. These results underscore the significance of tailoring material chemistry to specific device requirements, where ionomer chemistry should be rationally designed to match the local transport requirements of the device architecture.

Figures

Figure 2: Four reactions occur simultaneously during chain propagation in binary free-radical copolymerization (left). Each of these reactions has a characteristic rate constant, kij. The ratios of these constants, ri,j, describe the probability of each monomer incorporating into growing chains. For copolymerization of PFMMD (represented as “A”) and PFSVE (represented as “B”), rA > 1 > rB. This disparity in reactivity ratios induces compositional drift as the feedstock changes throughout the course of the reaction.

Figure 3: Numerical simulation of compositional drift in the copolymerization of PFMMD and PFSVE. (a) Polymer composition, FPFMMD, for a given feed composition, fPFMMD, changes with conversion. (b) This can be expressed as the magnitude of compositional drift, or the change in FPFMMD from its initial value. Compositional drift becomes especially severe for low fPFMMD, which introduces challenges in reaching high PFSVE fractions in the copolymer while maintaining statistical distribution of monomers. White triangles indicate the synthesis batches used to generate a range of statistical ionomer compositions. Nanostructure and Ionic Conductivity

Figure 5: (a) SAXS profiles of hydrated PFMMD-co-PFSA at PFSA mass fractions ranging from 0.26 to 0.57. A scattering peak is present at q values close to 2 nm-1, close to the ionomer peak of hydrated Nafion. As PFSA fraction increases, the intensity of this peak increases as it shifts to lower q values, indicating larger ionomer domains. (b) Ionomer domain spacing is shown for Nafion (green, from reference 5 as reproduced from reference 32) and PFMMD-coPFSA (blue) for a range of PFSA mass fractions, as calculated from SAXS scattering peaks. The domain spacings of the two ionomers exhibit a similar dependence on PFSA fraction, with PFMMD-co-PFSA displaying domains around 1 nm smaller than Nafion. (c) The WAXS profile of PFMMD-co-PFSA (0.37 PFSA mass fraction) shows similar matrix structure to the poly(PFMMD) homopolymer. Notably, the PTFE crystalline peak (denoted by dashed line) is absent. Nafion WAXS profile reproduced from reference 35, and poly(PFMMD) from reference 25. Gas-Transport Properties

Table 1: Density (g cm-3) and FFV for PFMMD-co-PFSA compositions and Nafion 1100. 120 nm films were cast on 10 MHz AT-cut Quartz Crystal Microbalance (QCM) crystals with Au electrodes and area densities were measured with dry N2 flow. Film thicknesses were measured by ellipsometry under the same conditions to calculate volumetric density. All compositions of PFMMD-co-PFSA showed lower density and higher FFV, indicating improved permeability.

Figure 7: Permeability of dehydrated PFMMD-co-PFSA was several times higher than Nafion towards a range of gases including H2, O2, Ar, and CO2. The high permeability is attributed to the increased FFV upon incorporation of PFMMD into the ionomer matrix. Measurements were performed on PTFE-supported ionomer films of 1 μm thickness in a constant-volume permeation cell.

Figure 4: (a) Proton conductivity (measured by EIS) and (b) water sorption (measured by ellipsometry) of PFMMD-co-PFSA thin-films are sensitive to the mass fraction of sulfonic-acid groups. At high PFSA fraction, the ionic conductivity of PFMMD-co-PFSA approaches that of Nafion. However, it remains lower by a factor of 2, even at higher PFSA fractions than Nafion 1100. Nafion data reproduced from reference 33. (b) Water sorption of PFMMD-co-PFSA thin films shows similar dependence on PFSA fraction as the ionic conductivity. PFMMD-co-PFSA swells less at a given PFSA fraction than Nafion.

Full text

Lawrence Berkeley National Laboratory
Recent Work
Title
Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical
Devices: Insights into Structure-Property Relationships.
Permalink
https://escholarship.org/uc/item/0rh082p6
Journal
Journal of the American Chemical Society, 142(8)
ISSN
0002-7863
Authors
Katzenberg, Adlai
Chowdhury, Anamika
Fang, Minfeng
et al.
Publication Date
2020-02-01
DOI
10.1021/jacs.9b09170
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

Highly Permeable Peruorinated Sulfonic
Acid Ionomers for Improved Electrochemical
Devices: Insights into Structure – Property
Relationships
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!!4#&7%%)3C5!
      !  <!      7
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  #!  !%
11!)   4$!   %  %%)   !
14&!#!!
1!%!%!!1<!1
=!!=1%!!)
37!5%!=!!1
767!!!!1%!%)%!!!9!
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)4&1!!7!!!1%
%!%
D-
1%!

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#!!7%%!!111!
 !%%8%!!!%!
!
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
1!!!2!1!!%%!!
!    )1      !    %  !  1  $%
!!!!%!!4#&%!2
224$!<!!!

 !2%!

 !1
%!7%!!C4)!16
!  !    !%  7  1!
1E16322.22:275F    134;5  7  
1!1C3i.e +:5
:
%!!!
1!3i.e.G.+5
 4#&  1!
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  72)  1!  )
%14;16)!
A/
 !1
  %  !      71          16  !
%1!4;2%24$%1!4;7!!
1)1!!!  1    4$
!1)1!1%%35#!!!
%)    !1  1!    12    !2!1    
%!
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?#4$!!%!'8
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 315%!!!%!4#&7
    !  1  !  %  !      7  !!      !
1%%!!!!!2!1!%!4;2%2
4$  35    )    )1    '    ()!  %1!  
1!  7  1      7    !1  %!    1)  !
1  #    %!    !  %%!%  !  !  %  
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