First-Principles Molecular Dynamics Study of a Hydrocarbon Copolymer for Use in Polymer Electrolyte Membrane Fuel Cells

Abstract

The structural and dynamic properties of a brush-type hydrocarbon copolymer are investigated using first-principles molecular dynamics simulations. Two model compounds, one with mainly hydrophilic domains and one with mainly hydrophobic domains, were selected and used in the simulations. A series of radial distribution functions of different groups, such as water-water, sulfonic group-hydrogen, and etherhydrogen, is obtained to investigate the structure of the whole systems. The radial distribution functions of sulfonic groups, g S−S (r), and the structure of water clusters indicate the formation of a well-developed water channel in the studied copolymer.Analysis of proton dissociation reveals that the protons in both systems are not completely dissociated when the number of water molecules per sulfonic group is equal to 4. The low dissociation nature of this copolymer compared with that of Nafion is explained by its intrinsic acid strength and the presence of ineffective hydrogen bonds in the system, where ineffective hydrogen bonds indicate hydrogen bonds that do not contribute strongly to proton transport. The proton conductivity of this copolymer is comparable to that of Nafion, which is ascribed to the formation of good water channels. In addition, the calculated electrical conductivity of the two model compounds shows good agreement with the measured proton conductivity of this copolymer.

Insights from ab initio molecular dynamics simulations for a multicomponent oxide glass

“…The computational costs of Fourier transforms are considerable, therefore, ab initio calculation using finite‐element basis functions in real space can be used as an alternative approach to express localized wave functions for amorphous materials and reduce computational costs without the need for Fourier transforms. Application of this original approach has been successful for relatively large atomic systems, including oxide glass and polymers . A simple question is whether the present situation for AIMD has the ability to reproduce atomic structures consistent with the most advanced diffraction and NMR data.…”

EPR Study on the Oxidative Degradation of Phenyl Sulfonates, Constituents of Aromatic Hydrocarbon-Based Proton-Exchange Fuel Cell Membranes

“…Chain cleavage (dealkylation/dearylation) and the loss of protogenic sulfonate groups have been proposed as follow-up or parallel degradation reactions, leading to inevitable membrane failure. − Theoretical studies involving DFT calculations hypothesized that desulfonation is the energetically favorable degradation route for polyaromatic model compounds and that moieties containing heteroatoms are possible weak points susceptible to chain scission . We recently showed that an increase in electron density of the aromatic ring has a favorable impact on the stability against radical-induced degradation …”

“… 20 − 22 Theoretical studies involving DFT calculations hypothesized that desulfonation is the energetically favorable degradation route for polyaromatic model compounds and that moieties containing heteroatoms are possible weak points susceptible to chain scission. 23 We recently showed that an increase in electron density of the aromatic ring has a favorable impact on the stability against radical-induced degradation. 16 …”

Topology, Oxidation States, and Charge Transport in Ionic Conductors

“…In the literature, one can find ab initio simulations employing both the rigorous quantum mechanical definition of the electric flux of Equation ( 4), [24][25][26] as well as others where integer charges are assigned to atoms in motion. [27][28][29] Both approaches lead to satisfying results: while in the former case this is, of course, to be expected, since it makes use of the exact formulae, the latter could be thought reasonable in strongly ionic cases-where electronic polarization effects are allegedly small and effective charge tensors could possibly be approximated to integer multiples of the identity-but questions might arise in all the other cases, where covalent effects are not negligible.…”