Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure-Property Relationships (original) (raw)
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ACS Applied Materials & Interfaces, 2021
Transport phenomena are key in controlling performance of electrochemical energy-conversion technologies and can be highly complex involving multiple length-scales and materials/phases. Material designs optimized for one reactant species transport however may inhibit other transport processes. We explore such trade-offs in the context of polymer-electrolyte fuel-cell (PEFC) electrodes, where ionomer thin films provide the necessary proton conductivity but retard oxygen transport to the Pt reaction site and cause interfacial resistance due to sulfonate/Pt interactions. We examine electrode overall gas-transport resistance and its components as a function of ionomer content and chemistry. Low equivalent-weight ionomers allow better dissolved-gas and proton transport due to greater water uptake and low crystallinity, but also cause significant interfacial resistance due to high density of sulfonic-acid groups. These effects of equivalent weight are also observed via in-situ ionic conductivity and CO displacement measurements. Of critical importance, the results are supported by ex-situ ellipsometry and x-ray scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Structural and resultant performance changes in the electrode are shown to occur above a threshold sulfonic-group loading highlighting the significance of ink-based interactions. Our findings and methodologies are applicable to a variety of solid-state energy-conversion devices and material designs.
Dynamic Emergence of Nanostructure and Transport Properties in Perfluorinated Sulfonic Acid Ionomers
2020
Limitations in fuel cell electrode performance have motivated the development of ion-conducting binders (ionomers) with high gas permeability. Such ionomers have been achieved by copolymerization of perfluorinated sulfonic acid (PFSA) monomers with bulky and asymmetric monomers, leading to a glassy ionomer matrix with chemical and mechanical properties that differ substantially from common PFSA ionomers (e.g., Nafion™). In this study, we use perfluorodioxolane-based ionomers to provide fundamental insights into the role of the matrix chemical structure on the dynamics of structural and transport processes in ion-conducting polymers. Through in-situ water uptake measurements, we demonstrate that ionomer water sorption kinetics depend strongly on the properties and mass fraction of the matrix. As the PFSA mass fraction was increased from 0.26 to 0.57, the Fickian swelling rate constant decreased from 0.8 s-1 to 0.2 s-1, while the relaxation rate constant increased from 3.1×10-3 s-1 to...
Langmuir, 2009
The application of sulfonic acid-functionalized multiwalled (s-MWNT) carbon nanotubes to manipulate the hydrophilic domain size of Nafion membranes is explored here as an option for tuning the proton conductivity of polymer electrolyte membranes for hydrogen-oxygen fuel cells. The electrochemical impedance experiments provide preliminary evidence of increased proton conductivity, while small-angle X-ray scattering measurements line out enhanced ionic cluster domain size in these composite membranes as the central reason for higher conductivity (70 Å for the optimum composite membrane vs 50 Å for Nafion 115) values. Scanning electrochemical microscopy indicates synergistic interaction between the sulfonic acid functional groups present in the Nafion membrane and those on the nanotube surface. More interestingly, the nanotube-tailored Nafion membranes ameliorate the performance of fuel cells as confirmed by measurements at a single-cell level, which reveal a maximum power density of 380 mW cm-2 , higher than those of Nafion 115 (250 mW cm-2) and recast Nafion (230 mW cm-2) membranes. Thus, in addition to providing an elegant means of controlling the ionic cluster size, the strategic approach of using CNT both as an anchoring backbone for-SO 3 H groups to enrich proton conductivity and as a blending agent to improve the mechanical characteristics of the Nafion phase might be helpful in alleviating many critical problems associated with the use of commercial Nafion membranes.
International Journal of Hydrogen Energy, 2014
We report on polymer electrolyte membrane fuel cells (PEMFCs) that function at high temperature and low humidity conditions based on short-side-chain perfluorosulfonic acid ionomer (SSC-PFSA). The PEMFCs fabricated with both SSC-PFSA membrane and ionomer exhibit higher performances than those with long-side-chain (LSC) PFSA at temperatures higher than 100 C. The SSC-PFSA cell delivers 2.43 times higher current density (0.524 A cm À1) at a potential of 0.6 V than LSC-PFSA cell at 140 C and 20% relative humidity (RH). Such a higher performance at the elevated temperature is confirmed from the better membrane properties that are effective for an operation of high temperature fuel cell. From the characterization technique of TGA, XRD, FT-IR, water uptake and tensile test, we found that the SSC-PFSA membrane shows thermal stability by higher crystallinity, and chemical/mechanical stability than the LSC-PFSA membrane at high temperature. These fine properties are found to be the factor for applying Aquivion™ E87-05S membrane rather than Nafion ® 212 membrane for a high temperature fuel cell.
Molecular Relaxations and Morphology of Perfluorosulfonate Ionomers for Fuel Cell Applications
2008
First, I’d like to thank Serge Pan and the Knowledge Foundation for inviting me to speak. While most of the talks so far have focused on advances in product development, I’m going to discuss some of the fundamental science regarding the membrane itself. I hope that I can clearly convey to you how the measurements we are doing will help to guide membrane technology development by understanding of structure and transport properties in these materials (Figure 1).
Acta Physica Polonica A, 2017
The proton conductivity mechanism in per-fluorinated sulfonic acid/PTFE copolymer Fumapem ® membranes for polymer electrolyte membranes has been investigated. Three samples of Fumapem ® F-950, F-1050 and F-14100 membranes with different ion exchange capacity 1.05, 0.95, and 0.71 meq/g, respectively, were used in this study after drying. The o-Ps hole volume size (VF V,Ps) was quantified using the positron annihilation lifetime technique while the proton conductivities (σ) were measured using LCR Bridge as function of temperature. It was found that as the ion exchange capacity increases, the proton conductivity increases and the free volume expands. Temperature dependences of proton conductivity and the o-Ps hole volume size (VF V,Ps) reflect the glass transition temperature of the membrane. A good linear correlation between the reciprocal of the o-Ps hole volume size (1/VF V,Ps) and log(σ) + ∆Ea/2.303kBT , (where ∆Ea is the activation energy, T is the absolute temperature and kB is the Boltzmann constant) at different temperatures indicate that the ionic motion in dry Fumapem ® is governed by the free volume. A linear relationship between the critical hole size γV * i and the ion exchange capacity was also achieved.
Perfluorinated ion exchange polymers and their use in research and industry
Macromolecular Symposia, 1994
Tetrafluoroethylene is copolymerized with a perfluorovinyl ether, containing a functional end group (-SO2F or-C02CH3) to yield a melt fabricable precursor polymer. After fabrication into the desired shape, the polymer is treated with a solution of potassium hydroxide to convert the functional group to an ion exchange site. The fundamental incompatibility of the ionic group with the perfluorinated polymer backbone results in a unique morphology, particularly in the presence of water or polar organic solvents. This has been the subject of extensive investigations by various research institutions. The most important industrial use for these polymers is in the electrolysis of sodium chloride solutions. Initially introduced in order to eliminate the environmental problems of the amalgam process, their performance has now improved to the point where they offer substantial savings, particularly in terms of energy costs, over the two older processes. exchange polymers is as an ionic conductor in fuel cells. Recent advances in this area will be discussed. An emerging, potentially very important use for perfluorinated ion
International Journal of Hydrogen Energy, 2012
We report on the preparation and characterization of perfluorosulfonic acid ionomer (PFSA) e silica composite membranes using hyperbranched polyethoxysiloxane (PEOS) as silica precursor. The membranes were formed via an in situ solegel process of PEOS in a PFSA solution and subsequent solution casting. Transmission electron microscopy and small-angle X-ray scattering data showed that at low silica content ultra-small silica particles with a size smaller than the diameter of the ionic channels were homogeneously dispersed in the PFSA matrix. Such morphology leads to enhanced proton mobility as probed by impedance spectroscopy and 1 H solid-state NMR, and eventually resulted in the improvement of fuel cell performance.
Membranes
Series of partially fluorinated sulfonated poly(arylene ether)s were synthesized through nucleophilic substitution polycondensation from three types of diols and superhydrophobic tetra-trifluoromethyl-substituted difluoro monomers with postsulfonation to obtain densely sulfonated ionomers. The membranes had similar ion exchange capacities of 2.92 ± 0.20 mmol g−1 and favorable mechanical properties (Young’s moduli of 1.60–1.83 GPa). The membranes exhibited considerable dimensional stability (43.1–122.3% change in area and 42.1–61.5% change in thickness at 80 °C) and oxidative stability (~55.5%). The proton conductivity of the membranes, higher (174.3–301.8 mS cm−1) than that of Nafion 211 (123.8 mS cm−1), was the percent conducting volume corresponding to the water uptake. The membranes were observed to comprise isolated to tailed ionic clusters of size 15–45 nm and 3–8 nm, respectively, in transmission electron microscopy images. A fuel cell containing one such material exhibited hi...