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Effects of interlayer confinement and hydration on capacitive charge storage in birnessite

Summary

The paper resolves how layered birnessite stores charge in neutral aqueous electrolytes by combining ex situ XRD, EQCM, in situ Raman, operando AFM dilatometry, DFT, and ReaxFF grand canonical Monte Carlo and molecular dynamics to connect interlayer expansion, mass transport, and atomistic intercalation pathways involving confined water and cations. The central narrative is that apparently capacitive voltammetry can coexist with massive ion and water reorganization in confined galleries—blurring the line between EDLC-like and intercalation-like storage in layered oxides.

Methods

Experiments (electrochemical, diffraction, gravimetry, imaging)

  • Electrodes and electrolyte: Electrodeposited birnessite on conductive substrates, cycled in aqueous electrolytes (e.g. 0.5 M K₂SO₄ at near-neutral pH as in the main text) with cyclic voltammetry (CV) over controlled sweep rates at 298 K (room-temperature class; see pdf_path for exact thermal protocol) (see pdf_path for exact T and pH); a non-aqueous electrolyte with bulky tetrabutylammonium is used as a control to disentangle intercalation-like mass changes from EDL-like response.
  • XRD / Raman / AFM: Ex situ XRD tracks the (001) interlayer d-spacing vs potential; Raman follows Mn–O phonon bands; operando AFM dilatometry measures out-of-plane height changes in electrolyte.
  • EQCM: Gravimetric electrochemistry maps mass flux to charge; DFT-informed interpretation in the SI (per the Nature Materials article) supports assigning H₂O and K⁺ stoichiometry in the same intercalation picture the authors promote.

MD application (ReaxFF + grand canonical sampling)

  • Engine / code: ReaxFF-based molecular dynamics and GCMC-style intercalation sampling (as described in the main text and supplementary materials/videos) for K⁺ and H₂O in the MnOₓ interlayer. N/A—whether the production code is LAMMPS-only vs a linked PuReMD/ReaxFF path: follow pdf_path and the supplementary protocol for exact executables and input decks.
  • System size & composition: Birnessite-like interlayer models; K⁺ and H₂O insertion from dehydrated and partly hydrated starting interlayers. Atom counts, NₓNᵧ in-plane repeats, and H₂O:K ratios N/A—copy from pdf_path/SI when reproducing trajectories.
  • Boundaries / periodicity: 2D PBC in the a–b plane of the layered model with vacuum or explicit electrolyte treatment as given in the SI (N/A—cell vectors not duplicated here).
  • Ensemble / control: The MD segments follow NVT/NVT+MC stages reported in the article (the exact NVT vs NVE for short relaxations N/A—in SI). There is no macroscopic continuum NPT barostat on the electrochemical cell: E-field/ionic double layer in experiment is not represented as a classical E-field in every listed MD segment (N/A—treat the MD as a chemical potential/grand-canonical sampling complement to the XRD data).
  • Timestep / duration / thermostat: pdf_path + SI give femtosecond timestep, equilibration and production-stage MD lengths, and NVT-compatible thermostat use where applicable. N/A—numeric values not duplicated in this wiki (copy from the version-of-record and supplementary videos). N/A for replica exchange and metadynamics-style enhanced sampling in the primary workflow.

Static DFT in this work

  • DFT (when split out from ReaxFF): K⁺/H₂O-dependent interlayer relaxations/energetics in condensed models as referenced (functional/basis: N/A—pulled in full in pdf_path/SI, not in this one-line note).

Findings

Pseudocapacitive CVs and anomalously high gravimetric capacitance relative to EDL benchmarks: K⁺-based XRD (001) peaks shift reversibly with potential, and EQCM shows H₂O/cation co-transport (not a single K⁺-only Faradaic line). The combined XRD/Raman/AFM/EQCM package supports co-intercalation in confined H₂O-rich interlayers that still appear like EDLC-like kinetics. ReaxFF+GCMC/MD movies in the supplementary materials illustrate how H₂O+K⁺ can enter the interlayer together rather than a pure K⁺-for-H exchange, tying ion+water reorganization to the XRD (001) sweep and the AFM dilation signal. For citation-ready concentration and H₂O/cation number assignments, use the pdf_path and SI tables/figures; this wiki is a retrieval summary, not a substitute for the full data product.

Limitations

Thin-film morphology (island structures) and electrolyte scope (focus on neutral pH sulfate) limit direct transfer to all device configurations; force-field models simplify electronic transitions of Mn oxidation states. Operando AFM and EQCM coupling can also introduce instrument-specific artifacts that must be separated from intrinsic intercalation mass changes when interpreting gravimetric capacitance. ReaxFF GCMC/MD provides atomistic hypotheses for ion and water uptake that should be cross-checked against crystallography and spectroscopy constraints from the same electrode films. Nature Materials supplementary videos illustrate intercalation pathways that are easier to qualify than to reduce to a single order parameter.

Relevance to group

Adri van Duin is a co-author; ReaxFF GCMC/MD supports intercalation mechanisms for birnessite supercapacitors.

Citations and evidence anchors