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Effect of strong acid functional groups on electrode rise potential in capacitive mixing by double layer expansion

Evidence and attribution

Authority of statements

Prose below summarizes the publication identified by doi, title, and pdf_path. The ingested PDF is a proofs file (2014-Hatzell-etal-ES&T-proofs.pdf); for pagination and publisher layout, prefer the version-of-record when available.

Summary

The work links capacitive mixing / double-layer expansion (CDLE) performance to surface chemistry of activated carbon electrodes. Experiments on five carbons show electrode rise potential in low-concentration electrolyte correlates with strong-acid surface group loading (P value stated in abstract). Nitric acid oxidation of one carbon shifts rise potential and whole-cell voltage as described in the abstract. MD and metadynamics on model carbon surfaces (pristine graphene vs oxidized / graphene-oxide-like) interpret EDL expansion vs compression in low salt, connecting functional groups to sign of rise potential.

Methods

  • Electrochemistry / materials: Potentiometric titrations and CDLE-style rise/fall tests on five activated carbons spanning strong-acid loadings from ~0.05 to ~0.36 mmol g⁻¹; HNO₃ oxidation of YP50 provides a paired before/after case (details in the article).
  • ReaxFF MD (ADF): Pristine graphene (PG) models low strong-acid coverage, while graphene oxide (GO) structures (from Bagri et al.) model high coverage (SI Figure S11). Slabs are ~43.35 Å × 40.04 Å in plane with ~16 Å solution gap; KCl solutions (~2.4 M “HC” and ~0.9 M “LC” after ion removal) bracket the CDLE experiment. Parameters follow Rahaman et al. for C/H/O graphene–water and Cl/O/H chloride–water interactions.
  • Integration: NPT, Δt = 0.25 fs, Berendsen thermostat (100 fs) and barostat (500 fs); energy minimization to 0.25 kcal Å⁻¹; 50 ps equilibration at 300 K; HC EDL from 50 ps production at 300 K; LC EDL from 100 ps total (50 ps equil + 50 ps prod). Well-tempered metadynamics uses PLUMED 1.3 via a LAMMPS fix (full CVs/collective variables in SI).

1 — MD application (atomistic dynamics). Engine / code: LAMMPS with ReaxFF and PLUMED 1.3 for well-tempered metadynamics (papers/2014-Hatzell-etal-ES&T-proofs.pdf; ingested file is proofs—prefer version-of-record for pagination). System: PG vs GO-like carbon slabs ~43.35 × 40.04 Å in-plane with ~16 Å solution region; KCl at ~2.4 M (“HC”) and ~0.9 M (“LC”) after ion removal (Methods). Boundaries: in-plane PBC with finite normal gap (slab, not bulk 3D liquid). Ensemble: NPT at 300 K for equilibration/production segments stated above. Timestep: 0.25 fs. Duration: 50 ps equil + 50 ps (HC) or 50+50 ps (LC) as quoted. Thermostat / barostat: Berendsen, damping 100 fs (T), 500 fs (P). Temperature: 300 K. Pressure: controlled via NPT barostat as above (proofs text). Electric field: N/A — not part of the summarized EDL protocol. Replica / enhanced sampling: well-tempered metadynamics (PLUMED); collective variables in SI.

2 — Force-field training: N/A — parameters follow Rahaman et al. (C/H/O graphene–water) plus Cl/O/H chloride–water interactions as cited—this work applies them to EDL structure, not a new ReaxFF fit.

Findings

  • Across carbons, electrode rise potential in LC correlates with strong-acid group concentration at P = 10⁻⁵; low-acid electrodes show positive rises (e.g., 59 ± 4 mV at ~0.05 mmol g⁻¹), whereas high-acid carbons trend negative (e.g., −31 ± 5 mV at ~0.36 mmol g⁻¹).
  • Nitric acid oxidation of YP50 shifts the rise potential from 46 ± 2 mV to −6 ± 0.5 mV and yields a whole-cell mixing potential of 53 ± 1.7 mV, about 4.4× the 12 ± 1 mV obtained with symmetric electrodes in their comparison.
  • ReaxFF MD + metadynamics link PG surfaces to EDL expansion in LC (positive-rise trend) and GO surfaces to EDL compression (negative-rise trend), matching the functional-group narrative of the experiments.

Limitations

  • Model surfaces vs real activated carbons; proof PDF may differ cosmetically from final ES&T layout.

Relevance to group

Muralikrishna Raju and Adri C. T. van Duin (Penn State) coauthor; combines carbon electrochemistry with atomistic EDL interpretation for energy from salinity gradients.

Citations and evidence anchors

  • DOI: https://doi.org/10.1021/es5043782 (papers/2014-Hatzell-etal-ES&T-proofs.pdf).