Effect of the Side-Chain Length in Perfluorinated Sulfonic and Phosphoric Acid-Based Membranes on Nanophase Segregation and Transport: A Molecular Dynamics Simulation Approach

Abstract

The effect of side-chain length on the nanophase-segregated structure and transport in perfluorinated sulfonic acid (PFSA)-based and perfluorinated phosphoric acid (PFPA)-based membranes is investigated at 20 and 5 wt % water content conditions using a molecular dynamics simulation method. It is found using the pair correlation analysis that the longer side chain leads to more developed local water structures in the water phase at 20 wt % water content, observable in both membrane chemistries albeit more distinct in PFPA-based membranes. It is also confirmed from the structure factor analysis that large-scale nanophase segregation is enhanced with increasing side-chain length for PFPA membranes, whereas no significant change is observed at these scales for PFSA membranes. Next, it is revealed that the proton transport is increased by 0.004 S/cm in PFSA-based membranes with increasing side-chain length due to the enhanced vehicular and hopping mechanisms, whereas the proton transport in PFPA-based membranes is decreased by 0.002 S/cm despite improved nanophase segregation. As confirmed by the pair correlation function analysis, the diminished proton transport in PFPA-based membranes is attributed to the molecular association of phosphate groups with hydronium ions via hydrogen bond in the longer side-chain case, which is namely a hydronium-mediated bridge configuration. Such bridge configurations and correspondingly similar trends in proton transport are also observed at 5 wt % water content condition to a lesser extent. Our simulation study demonstrates that the proton transport is affected by the hydrogen-bonding network as well as by the nanophase segregation.

Nanostructures of Nafion Film at Platinum/Carbon Surface in Catalyst Layer of PEMFC: Molecular Dynamics Simulation Approach

“…Next, the diffusion of a proton ( D p ) was characterized by calculating the contributions from the two transport mechanisms: vehicular and hopping transport. The vehicular diffusion coefficient of the proton ( D p vehicular ) was obtained from the diffusion coefficient of hydronium ions via analyzing MSDs with eq , while the hopping diffusion coefficient of the proton ( D p hopping ) is calculated using a method developed by Deng which was successfully used to describe proton transport in the membrane systems. , The hopping diffusion coefficient ( D p hopping ) was calculated by where N is the number of protons; M is the number of water molecules; t is the hopping diffusion time; P ij is the probability with which a proton can jump from hydronium i to water j , defined as P ij = k ij /(∑ j M k ij ); and r ij is the distance between all donors and acceptors. The r ij value is obtained from ρ g H 3 O–H 2 O ( r ) data, which was calculated using the equilibrium molecular dynamics simulation trajectory.…”

Microstructures and Proton Networks of Ionomer Film on the Surface of Platinum Single Atom Catalyst in Polymer Electrolyte Membrane Fuel Cells

“…Considering the above contradictory impact of enhancing Pt single atom loading on proton transport in the Pt–ionomer interface and hydrophilic networks, in this section, we would like to dig out an ideal microstructure for proton transport in the whole ionomer film. The side chain length (defined as the distance between the carbon atoms of side chains that are attached to the backbones and the sulfur atoms of sulfonic anion groups) is an important parameter for evaluating the segregation degree of hydrophilic/hydrophobic nanophases, , which is suggested to influence the proton transport in the ionomer film. Here, the distributions of side chain lengths with the height of sulfur atoms under the loading of Pt/C 4.5 wt % are presented in Figure a (the other Pt/C loadings and Pt slab are shown in Figure S4 of Supporting Information).…”

“…Besides, the sulfonic anion groups are assumed to be fully deprotonated based on the previous experiments; , therefore, 60 hydronium ions are added to maintain electrical neutrality. Hydration level λ is also a critical factor that affects the morphology of ionomer film, ,, which represents the number of water molecules combined with hydronium ions per sulfonic anion group. However, in this study, we mainly focus on the properties of the ionomer film on the surface of Pt SACs compared to that upon Pt slab, and a hydration level of λ = 10 (540 water molecules and 60 hydronium ions) is adopt based on the range of water content that is commonly accepted in the previous research studies. , …”

“…The hydrophilic clusters contain side chains, water molecules, and hydronium ions, and it is revealed that more developed proton conduction channels will be formed with higher water content. Also, the side chain configurations are suggested to affect the hydrophilic/hydrophobic nanophase segregation and further the transport properties, , as the mobility of water molecules and hydronium ions would be restricted by the electrostatic adsorption from sulfonic anion groups and the interaction from Pt …”

Mesoscale hydrated morphology of perfluorosulfonic acid membranes

“…With the rapid development of simulation technology, researchers begin to use powerful computing function to explore proton transport in PFSA membranes, 21,22 and a multi-scale simulation method has been applied to this end. Based on atomic molecular dynamics simulation, Kuo et al 23 studied a series of hydrated PFSA membranes to evaluate the effect of water content on the morphology.…”