Grotthuss versus vehicular transport of hydroxide in anion-exchange membranes: insight from combined reactive and nonreactive molecular simulations
Summary¶
Joint simulations of hydrated anion-exchange membranes (AEMs) using nonreactive polarizable MD (APPLE&P) for long morphology equilibration and mapping to reactive ReaxFF runs probe whether OH⁻ transport in subnanometer water channels proceeds by vehicular diffusion or Grotthuss-like proton hopping, emphasizing bottlenecks where dehydration would penalize vehicular transport. The Letter contrasts two morphologies at matched hydration and functionalization so that pore topology—not only chemistry—controls which transport channel dominates.
Methods¶
From the J. Phys. Chem. Lett. PDF (pdf_path).
- Systems: Hydrated anion-exchange membranes based on poly(p-phenylene oxide) with quaternary ammonium headgroups; two morphologies M1 / M2 at 50% functionalization and lambda = N(H2O)/N(cation) = 10, 16 chains, n = 5 repeat units (Fig. 1).
- APPLE&P (nonreactive): Polarizable MD equilibrates membrane morphology and samples OH- transport; 7.0 ns trajectory quoted for morphology equilibration (Fig. 1 caption context in Letter). Bottlenecks where pore diameter < ~4.5 A cannot accommodate a fully solvated hydroxide.
- ReaxFF (reactive): Representative hydrated configurations mapped from APPLE&P to ReaxFF to capture Grotthuss-style Eigen-Zundel-Eigen sequences; authors report limited backbone displacement after mapping (Fig. 2 / text).
- Analysis: Bottleneck diameters are compared against solvated OH⁻ sizes to argue when vehicular paths incur desolvation penalties.
Engine, cell, and sampling: Simulations use LAMMPS (as cited in the Letter) for both APPLE&P and ReaxFF stages on hydrated anion-exchange supercells on the order of 10⁴+ atoms, with three-dimensional periodic boundary conditions. NVT trajectories near 300 K use the thermostat and timestep choices tabulated in the Letter’s Methods; morphology-focused APPLE&P segments include the ~7 ns equilibration window discussed for Fig. 1, followed by shorter ReaxFF production segments after configuration mapping. Barostat / pressure: N/A — the excerpted protocol is constant-volume NVT without NPT swelling control as the headline variable. Electric field: N/A — no applied bias field in the OH⁻ transport analysis summarized here. Replica / enhanced sampling: N/A — direct dynamics without umbrella sampling or metadynamics in the quoted workflow.
Findings¶
- Mechanism / outcomes: In nonblocky, narrow-channel AEMs, vehicular OH⁻ transport through bottlenecks requires partial desolvation, producing a large kinetic barrier; Grotthuss-like hopping lowers the effective barrier when contiguous water wires exist.
- Comparisons: M1 vs M2 morphologies at matched hydration λ compared within the Letter show that microphase layout—not only chemistry—controls dominant transport channels.
- Sensitivity: Pore diameter thresholds (~4.5 Å) relative to solvated ion size gate the vehicular penalty; functionalization 50% settings appear in Fig. 1.
- Limitations / outlook: Force-field and mapping assumptions between APPLE&P and ReaxFF remain caveats for quantitative conductivity; see Discussion in the PDF.
- Corpus honesty: Protocol fragments summarize J. Phys. Chem. Lett. 9, 825–829; confirm thermostat brands and exact timestep in the PDF before reproduction.
Limitations¶
- Force-field and mapping assumptions between APPLE&P and ReaxFF introduce uncertainty; quantitative conductivity matches to experiment are not the Letter’s sole focus.
Curation note: this Letter is the short companion to the longer Polymers multiscale membrane paper by the same collaboration; cross-link 2018dengpan-dong-in-this-stud-multiscale-modeling when retrieval questions ask for full ionomer morphology context beyond the Grotthuss bottleneck story. For MAS retrieval, keep paper_id stable and cite DOI 10.1021/acs.jpclett.8b00004 for external bibliographies. Hydration level λ and functionalization 50% settings follow the Letter’s Fig. 1 caption conventions.
Relevance to group¶
van Duin-group collaboration on membrane ion transport using ReaxFF after polarizable equilibration—useful alongside 2019fedkin-j-phys-chem-development-reaxff electrolyte work.