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Penetration of Hydrogen Into Polymer Electrolyte Membrane for Fuel Cells by Quantum and Molecular Dynamics Simulations polymers Article Penetration of Hydrogen into Polymer Electrolyte Membrane for Fuel Cells by Quantum and Molecular Dynamics Simulations JinHyeok Cha * , Wooju Lee and Jihye Baek Institute of Fundamental and Advanced Technology, Hyundai Motor Company, 37 Cheoldobangmulgwan-ro, Uiwang-si 16082, Gyeonggi-do, Korea; [email protected] (W.L.); [email protected] (J.B.) * Correspondence: [email protected] Abstract: The advent of the Hydrogen Society created great interest around hydrogen-based energy a decade ago, with several types of vehicles based on hydrogen fuel cells already being produced in the automotive sector. For highly efficient fuel cell systems, the control of hydrogen inside a polymer-based electrolyte membrane is crucial. In this study, we investigated the molecular behavior of hydrogen inside a polymer-based proton-exchange membrane, using quantum and molecular dynamics simulations. In particular, this study focused on the structural difference of the pendent- like side chain polymer, resulting in the penetration ratio of hydrogen into the membrane deriving from the penetration depth of the membrane’s thickness while keeping the simulation time constant. The results reveal that the penetration ratio of the polymer with a shorter side chain was higher than that with the longer side chain. This was justified via two perspectives; electrostatic and van Citation: Cha, J.; Lee, W.; Baek, J. der Waals molecular interactions, and the structural difference of the polymers resulting in the free Penetration of Hydrogen into Polymer Electrolyte Membrane for volume and different behavior of the side chain. In conclusion, we found that a longer side chain Fuel Cells by Quantum and is more trembling and acts as an obstruction, dominating the penetration of hydrogen inside the Molecular Dynamics Simulations. polymer membrane. Polymers 2021, 13, 947. https:// doi.org/10.3390/polym13060947 Keywords: polymer electrolyte membrane for fuel cell; molecular dynamics simulations; side chain; penetration Academic Editors: Célio Bruno Pinto Fernandes, Salah Aldin Faroughi, Luís L. Ferrás and Alexandre M. Afonso 1. Introduction Hydrogen, as an energy resource, can make life significantly more eco-friendly. Al- Received: 24 February 2021 though the hydrogen fuel cell was introduced by William Robert Grove 180 years ago [1], Accepted: 17 March 2021 numerous potential applications have recently raised extreme engagement from many Published: 19 March 2021 researchers, in various industrial sectors [2]. Automotive systems based on hydrogen fuel cells can already be found on the road, with examples such as “NEXO” produced by Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Hyundai Motor Company or “MIRAI” by Toyota representing the upcoming hydrogen- published maps and institutional affil- based automotive future. iations. A hydrogen fuel cell works based on electrochemistry, by passing hydrogen through the anode and oxygen through the cathode of the cell. More specifically, electrical power is generated via three simple steps: (i) the dissociation of hydrogen to the proton and electron; (ii) the conduction of electrons through electronic channels and protons through a proton-exchange membrane; and (iii) the synthesis of water by a proton, electron, and Copyright: © 2021 by the authors. oxygen. A polymer electrolyte membrane for fuel cell (PEMFC) is commonly used in Licensee MDPI, Basel, Switzerland. This article is an open access article vehicles due to better operation at relatively low temperature, while other types of fuel distributed under the terms and cells (such as alkaline cell, phosphoric acid fuel cell, molten carbonate cell, direct methanol conditions of the Creative Commons fuel cell and solid oxide cell) require a higher operation temperature [3,4]. Attribution (CC BY) license (https:// A stack, one of the main components of PEMFCs, comprises of serially connected unit creativecommons.org/licenses/by/ cells generating electricity. Each of them features a bipolar gas diffusion layer (GDL) and a 4.0/). membrane electrode assembly (MEA), which are directly responsible for the electrochemical Polymers 2021, 13, 947. https://doi.org/10.3390/polym13060947 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 947 2 of 12 Polymers 2021, 13, 947 2 of 11 and a membrane electrode assembly (MEA), which are directly responsible for the elec- reaction.trochemical Figure reaction.1a shows Figure a schematic 1a shows ofa schema a protontic exchangeof a proton membrane exchange membrane in the fuel in cell the system.fuel cell The system. proton The exchange proton membraneexchange membrane is a key component is a key component of PEMFCs, of acting PEMFCs, excellently acting towardsexcellently gas towards blocking gas and blocking proton conduction. and proton Inconduction. particular, In gas particular, diffusion gas behavior diffusion inside be- MEAhavior has inside a direct MEA impact has a on direct the performanceimpact on the of performance the fuel cell, of as the the fuel diffusion cell, as of the hydrogen diffusion andof hydrogen oxygen at and both oxygen the anode at both and cathodethe anode determines and cathode the efficiencydetermines of the the efficiency fuel cell, hence of the shortenedfuel cell, hence diffusion shortened time induces diffusion faster time proton induces transport faster proton and water transport conversion and water [5,6]. con- In addition,version [5,6]. gas crossover In addition, through gas crossover the PEMFC through during the long-term PEMFC membraneduring long-term operation membrane causes theoperation degradation causes of the the degradation MEA, due toof the MEA, combustion due to reactionthe combustion between reaction H2 and between O2 gases H2 andand the O2 formationgases and ofthe reactive formation oxygen of reactive radicals oxygen (HO• radicalsand HOO (HO•• radicals) and HOO• which radicals) attack membranewhich attack linkages membrane [7–11 ].linkages [7–11]. FigureFigure 1. 1.(a ()a Schematic) Schematic of of the the operation operation of aof fuel a fuel cell; cell; (b) snapshot(b) snapshot of the of unitthe unit cell for cell the for molecular the molecular dynamic dynamic (MD) simulation(MD) simu- + oflation the morphological of the morphological effect of effect the side of the chain side on chain gas penetration, on gas penetration, containing containing H3O and H H3O2+O and with H2 theO with hydration the hydration number num- of 3; (cber) index of 3;x (c(backbone)) index x (backbone) and y (side and chain) y (side of thechain) chemical of the chemical structure structure of perfluorosulfonic of perfluorosulfonic acid determine acid determine the length the of length side chain;of side and chain; (d) schematic and (d) schematic of the MD of simulationthe MD simulation for hydrogen for hydrogen molecules’ molecules’ penetration penetration into the ionomer into the region.ionomer region. GasGas is is transported transported through through carriers carriers of of polymeric polymeric surrounding, surrounding, suchsuch asas aa polymer- polymer- basedbased binder binder supporting supporting catalysts catalysts at bothat both electrodes, electrodes, and aand polymeric a polymeric electrolyte electrolyte membrane. mem- Therefore,brane. Therefore, the interaction the interaction between between the gas the molecules gas molecules and polymer and polymer needs needs to be clarified,to be clar- inified, order in toorder control to control the gas the flow gas in flow and in out. and The out. transport The transport of gas moleculesof gas molecules in the PEMin the dependsPEM depends on the on geometric the geometric properties properties of polymeric of polymeric media media as well as aswell atomic as atomic interactions interac- betweentions between gas and gas polymer and polymer molecules. molecules. The geometrical The geometrical effects effects of polymeric of polymeric media media can becan analyzedbe analyzed by using by using a theoretical a theoretical model model of diffusion of diffusion in porous in porous media. media. Several Several studies studies have beenhave reported been reported on the on diffusion the diffusion phenomena phenomen in thea in randomly the randomly distributed distributed microstructure microstruc- byture employing by employing fractal fractal geometry geometry theory theory [12,13 [1],2,13], and those and those studies studies show show a good a agreementgood agree- withment the with available the available experimental experimental data and data existing and existing models models reported repo inrted theliterature. in the literature. Even theEven theoretical the theoretical model ofmodel diffusion of diffusion in the porous in the media porous well media predicts well the predicts effective the transport effective propertiestransport ofproperties gas molecules, of gas themolecules, effect of the molecular effect of interactions molecular shouldinteractions be fully should understood be fully to design polymeric materials in molecular-level (e.g., chain length, electrostatic interaction, understood to design polymeric materials in molecular-level (e.g., chain length, electro- radius of gyration of polymer formed by different chain structures). static interaction, radius of gyration of polymer formed by different chain structures). To explore the molecular behavior of gas (e.g., hydrogen at the anode and oxygen at the cathode) [14], numerous studies have employed molecular dynamic (MD) simulations Polymers 2021, 13, 947
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