Morphology of Nafion-Based Membranes Affects Proton Transport

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􏰁􏰂􏰃 􏰅􏰆􏰇 􏰈􏰉􏰊􏰋􏰌􏰂􏰍 polymers Article How the Morphology of Nafion-Based Membranes Affects Proton Transport † Ernestino Lufrano 1, Cataldo Simari 1 , Maria Luisa Di Vona 2, Isabella Nicotera 1,* and Riccardo Narducci 2,* Citation: Lufrano,E.;Simari,C.;Di Vona, M.L.; Nicotera, I.; Narducci, R. How the Morphology of Nafion-Based Membranes Affects Proton Transport† . Polymers 2021, 13, 359. https://doi.org/10.3390/ polym13030359 Academic Editor: Dong Jin Yoo Received: 27 December 2020 Accepted: 18 January 2021 Published: 22 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 2 * Correspondence: isabella.nicotera@unical.it (I.N.); riccardo.narducci@uniroma2.it (R.N.) † In memoriam of Prof. Giulio Alberti. Abstract: This work represents a systematic and in-depth study of how Nafion 1100 membrane preparation procedures affect both the morphology of the polymeric film and the proton transport properties of the electrolyte. The membrane preparation procedure has non-negligible consequences on the performance of the proton-exchange membrane fuel cells (PEMFC) that operate within a wide temperature range (up to 120 ◦C). A comparison between commercial membranes (Nafion 117 and Nafion 212) and Nafion membranes prepared by three different procedures, namely (a) Nafion-recast, (b) Nafion uncrystallized, and (c) Nafion 117-oriented, was conducted. Electrochemical Impedance Spectroscopy (EIS) and Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) investigations indicated that an anisotropic morphology could be achieved when a Nafion 117 membrane was forced to expand between two fixed and nondeformable surfaces. This anisotropy increased from ~20% in the commercial membrane up to 106% in the pressed membrane, where the ionic clusters were averagely oriented (Nafion 117-oriented) parallel to the surface, leading to a strong directionality in proton transport. Among the membranes obtained by solution-cast, which generally exhibited isotropic proton transport behavior, the Nafion uncrystallized membrane showed the lowest water diffusion coefficients and conductivities, highlighting the correlation between low crystallinity and a more branched and tortuous structure of hydrophilic channels. Finally, the dynamic mechanical analysis (DMA) tests demonstrated the poor elastic modulus for both uncrystallized and oriented membranes, which should be avoided in high-temperature fuel cells. Keywords: nafion; conductivity; oriented morphology; recast; uncrystallized 1. Introduction In recent years, researchers have taken an interest in the development of more sus- tainable energies, both from an economic and environmental point of view. Among the different types of fuel cells, low-medium temperature, proton-exchange membrane fuel cells (PEMFCs) are promising for the replacement of classic heat engines, especially in mo- tor vehicles [1,2]. Among the most studied and promising materials are perfluorosulfonic acid membranes (PFSA), such as long side chain (LSC) Nafion, which has, until now, been the most widely investigated ionomer, and the more recent short side chain (SSC) Aquiv- ion from Solvay [3,4]. PFSA are characterized by high proton conductivity and chemical inertness; the latter is due to the presence of fluorine. However, sometimes the mechanical and thermal stability are not enough for the present needs in automotive applications [5,6]. In particular, when relative humidity (RH)-temperature conditions overcome certain crit- ical values, (70–130 ◦C and 95–100% RH [7]), the membranes undergo some irreversible processes that induce a decrease in their through-plane proton conductivity [8]. These phenomena are due to modifications in the bulk-transport properties, and may be observed when a membrane is constrained between the electrodes and forced to swell in a plane Department of Chemistry and Chemical Technologies—CTC, University of Calabria, via Pietro Bucci, 87036 Arcavacata di Rende, Italy; ernestino.lufrano@unical.it (E.L.); cataldo.simari@unical.it (C.S.) Department of Industrial Engineering and LIME Laboratory, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy; divona@uniroma2.it Polymers 2021, 13, 359. https://doi.org/10.3390/polym13030359 https://www.mdpi.com/journal/polymers

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