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ReaxFF Reactive Molecular Dynamics Simulation of Functionalized Poly(phenylene oxide) Anion Exchange Membrane

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Summary

Alkaline anion exchange membranes (AEMs) for fuel cells require both hydroxide conductivity and chemical stability under high-pH operation. This J. Phys. Chem. C study uses ReaxFF molecular dynamics to compare quaternized poly(phenylene oxide) membranes bearing trimethylammonium (TMA), dimethylbenzylammonium (DMBA), and dimethyloctylammonium (DMOA) headgroups at controlled hydration (λ = 8.3 and 20.8 water molecules per cation). Simulations resolve membrane swelling, nanophase structure, hydroxide mobility, and degradation tendencies associated with OH\(^-\) attack on cation centers versus hydrophobic shielding by long alkyl substituents.

The comparison is structured around headgroup sterics and hydrophobicity: TMA is compact, DMBA adds an aromatic substituent, and DMOA introduces a long alkyl tail intended to shield the quaternary nitrogen from nucleophilic attack while still permitting hydration channels at high λ.

Methods

All ReaxFF reactive molecular dynamics in the main text uses ADF 2012 (papers/Zhang_Anion_Exchange_JPCC_2015.pdf, Computational Details)—molecular dynamics trajectories in the authors’ MD package, not LAMMPS unless parameters are ported. Amorphous quaternized PPO membranes carry TMA, DMBA, or DMOA headgroups at λ = 8.3 and 20.8 H₂O per cation plus OH⁻ in 3D periodic supercells (full atom counts, box sizes, and equilibration in §2 / tables). Velocity Verlet, 0.25 fs timestep, Berendsen thermostat (100 fs coupling) on the whole system for NVT-style room-temperature production (exact T in the article’s table). No barostat, controlled pressure, electric field, or enhanced sampling in the cited protocol. Analysis includes RDFs, coordination, swelling, OH⁻ MSD/diffusivity, and degradation statistics (§3).

Force-field training: N/A — a published-style ReaxFF set for PPO / quaternary ammonium / hydroxide is used; full numerical tables are in [[2015zhang-venue-microsoft-word]] (SI PDF), not a new refit narrative in the article body.

Static QM / DFT: N/A — not the dominant modality relative to ReaxFF MD (any QM benchmarks only in the PDF if present).

Findings

Higher λ swells membranes, improves water connectivity, and raises OH⁻ diffusivity; TMA shows the largest OH⁻ diffusion constant of the three headgroups at high λ in the authors’ comparison (§3). DMOA’s long alkyl tails shield the quaternary N from OH⁻, lowering degradation versus more exposed headgroups—hydrophobic protection. TMA, DMBA, and DMOA are compared head-to-head at matched λ for transport and stability. λ trades conductivity (more water aids OH⁻ transport) against OH⁻ exposure. Discussion flags missing CO₂ / carbonation chemistry for air-exposed AEMs. For machine porting, take ReaxFF numbers from [[2015zhang-venue-microsoft-word]] plus JPCC Methods—ADF inputs are primary, not assumed LAMMPS decks.

Limitations

ReaxFF organic/ion chemistry may omit carbonation, CO\(_2\), and bicarbonate chemistry relevant to air-exposed AEM operation.

Relevance to group

Penn State ReaxFF application to AEM ion transport and alkaline stability.

Citations and evidence anchors

  • DOI: https://doi.org/10.1021/acs.jpcc.5b07271 (papers/Zhang_Anion_Exchange_JPCC_2015.pdf); SI: papers/Zhang_Anion_Exchange_SI_JPCC_2015.pdf.
  • SI parameters: numerical ReaxFF tables in [[2015zhang-venue-microsoft-word]].