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1. Scanning Ion Conductance Microscopy of Nafion-Modified Nanopores

   https://doi.org/10.1149/1945-7111/acdd29  

   Alanis, Kristen; Siwy, Zuzanna S.; Baker, Lane A. (June 2023, Journal of The Electrochemical Society)

   Single nanopores in silicon nitride membranes are asymmetrically modified with Nafion and investigated with scanning ion conductance microscopy, where Nafion alters local ion concentrations at the nanopore. Effects of applied transmembrane potentials on local ion concentrations are examined, with the Nafion film providing a reservoir of cations in close proximity to the nanopore. Fluidic diodes based on ion concentration polarization are observed in the current-voltage response of the nanopore and in approach curves of SICM nanopipette in the vicinity of the nanopore. Experimental results are supported with finite element method simulations that detail ion depletion and enrichment of the nanopore/Nafion/nanopipette environment. 

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2. Influence of vanillic acid immobilization in Nafion membranes on intramembrane diffusion and structural properties

   https://doi.org/10.1039/D2CP01125E  

   Trusty, Blake; Berens, Samuel; Yahya, Ahmad; Fang, Junchuan; Barber, Sarah; Angelopoulos, Anastasios P.; Nickels, Jonathan D.; Vasenkov, Sergey (May 2022, Physical Chemistry Chemical Physics)

   Pulsed field gradient (PFG) NMR in combination with quasielastic neutron scattering (QENS) was used to investigate self-diffusion of water and acetone in Nafion membranes with and without immobilized vanillic acid (VA). Complementary characterization of these membranes was performed by small angle X-ray scattering (SAXS) and NMR relaxometry. This study was motivated by the recent data showing that an organic acid, such as VA, in Nafion can preserve its catalytic activity in the presence of water even at high intra-polymer water concentrations corresponding up to 100% ambient relative humidity. However, there is currently no clear understanding of how immobilized organic acid molecules influence the microscopic transport properties and related structural properties of Nafion. Microscopic diffusion data measured by PFG NMR and QENS are compared for Nafion with and without VA. For displacements smaller than the micrometer-sized domains previously reported for Nafion, the VA addition was not observed to lead to any significant changes in the water and/or acetone self-diffusivity measured by each technique inside Nafion. However, the reported PFG NMR data present evidence of a different influence of acetone concentration in the membranes with and without VA on the water permeance of the interfaces between neighboring micrometer-sized domains. The reported diffusion data are correlated with the results of SAXS structural characterization and NMR relaxation data for water and acetone. 

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3. Selective Electrocatalytic Nitrate Reduction to Ammonia Using Nafion-Covered Cu Electrodeposits

   https://doi.org/10.1021/acs.jpcc.3c01242  

   Mondol, Profulla; Thind, Jashmeen K.; Barile, Christopher J. (May 2023, The Journal of Physical Chemistry C)
4. Exceptional thermal conductivity increase of Nafion by hydrogen-bonded water molecules

   https://doi.org/10.1063/5.0217244  

   Rahbar, Mahya; Alahmad, Qusai; Bai, John; Zhang, Lijun; Wang, Xinwei (June 2024, Applied Physics Letters)

   Nafion, a widely used proton exchange membrane in fuel cells, is a representative perfluorosulfonic acid membrane consisting of a hydrophobic Teflon backbone and hydrophilic sulfonic acid side chains. Its thermal conductivity (k) is critical to fuel cell's thermal management. During fuel cell operation, water molecules inevitably enter Nafion and could strongly affect its k. In this work, we measure the k of Nafion of different water content (λ). Findings reveal that k is significantly low in a vacuum environment characterized as 0.110 W m−1 K−1, but at λ ∼1, a notable increase is observed, reaching 0.162 W m−1 K−1. Moreover, k at λ ≈ 6 is 60% higher than that of λ ∼1. This exceptional k increase is far beyond the theoretical prediction by the effective medium theory that only considers simply physical mixing. Rather this k increase is attributed to the formation of water clusters and channels with increased λ, creating thermal pathways through hydrogen bonding, thereby improving chemical connections within the Nafion structure and augmenting its k. Furthermore, it is observed that Nafion's k reaches the maximum value of 0.256 W m−1 K−1 at λ ≈ 6, with no further increase up to λ ≈ 10.5. This phenomenon is explained by the coalescence of water clusters at λ ≈ 6, forming channels that optimize heat transfer pathways and connections within the Nafion structure. Moreover, the free movement of water molecules within water channels (λ > 6) shows physical alterations in Nafion structure (significant volume increase), which have a lesser impact on k. 

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5. Mapping Microscale Chemical Heterogeneity in Nafion Membranes with X-ray Photoelectron Spectroscopy

   https://doi.org/10.1149/2.0771811JES  

   Friedman, Alicia K.; Shi, Wenqing; Losovyj, Yaroslav; Siedle, Allen R.; Baker, Lane A. (January 2018, Journal of The Electrochemical Society)

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