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

Evidence and attribution¶

Authority of statements

Prose sections below (Summary, Methods, Findings, etc.) are curated summaries of the publication identified by doi, title, and pdf_path in the front matter above. They are not new primary claims by this wiki.

For definitive numerical values, reaction schemes, and interpretations, use the peer-reviewed article (and optional records under normalized/papers/ when present)—not this page alone.

Summary¶

Experiments on five activated carbons link strong-acid group concentration to electrode “rise potential” during capacitive mixing by double-layer expansion (CDLE) in dilute electrolyte. Nitric acid oxidation shifts rise potential and boosts whole-cell voltage. Atomistic MD and metadynamics connect the trend to EDL structure: pristine-like graphene surfaces expand the diffuse layer in low-concentration solution (positive rise), whereas oxidized graphene-oxide-like surfaces compress it (negative rise), mechanistically coupling functional chemistry to salinity-gradient energy harvesting (abstract; introduction, extract pages 1–2). CDLE devices harvest energy from salinity swings by charging electrodes in concentrated brine then discharging in freshwater; rise potential is a figure of merit for how electrode chemistry shifts interfacial potential during the dilution half-cycle.

Methods¶

Experiments (macro electrochemistry)¶

Materials: Potentiometric titrations and CDLE-style rise/fall measurements on five activated carbons spanning strong-acid surface loadings from ~0.05 to ~0.36 mmol g⁻¹ (abstract); HNO₃ oxidation of YP50 provides a paired before/after functionalization case (details in the article).

ReaxFF molecular dynamics / EDL sampling (atomistic)¶

Surfaces: Pristine graphene (PG) models low strong-acid coverage, while graphene oxide (GO)-like structures (from Bagri et al., cited in the article) model high coverage (SI Figure S11 in the publication). In-plane slab footprints are ~43.35 Å × 40.04 Å with a ~16 Å solution gap as stated in the Methods narrative.
Electrolyte: KCl(aq) solutions at ~2.4 M (“HC”) and ~0.9 M (“LC”) after ion removal are used to bracket the CDLE experiment’s high- vs low-concentration half-cycles. Cross-interaction parameters follow Rahaman et al. for C/H/O (graphene–water) and Cl/O/H (chloride–water) subsets described in the paper.
Integration: NPT, Δt = 0.25 fs, Berendsen thermostat (100 fs damping) and barostat (500 fs damping); energy minimization to 0.25 kcal mol⁻¹ Å⁻¹ force norm; 50 ps equilibration at 300 K; HC EDL statistics from 50 ps production at 300 K; LC EDL from 100 ps total (50 ps equilibration + 50 ps production) per the article’s simulation schedule.
Enhanced sampling: Well-tempered metadynamics via PLUMED 1.3 coupled to LAMMPS (collective variables summarized in SI).

Coverage note¶

Full experimental titration protocols and SI tables for functional group assignments should be taken from the ES&T PDF; the checked-in text extract may truncate figure captions.

Integrated MD checklist (maps to article narrative above)¶

Engine / code: LAMMPS with PLUMED 1.3 for well-tempered metadynamics (Methods above).
System / PBC: PG vs GO-like slabs in ~43.35 Å × 40.04 Å footprints with ~16 Å solution gap (Methods).
Ensemble: NPT with Δt = 0.25 fs, Berendsen thermostat (100 fs damping) and barostat (500 fs damping) as stated.
Stages: minimization to 0.25 kcal mol⁻¹ Å⁻¹ max force; 50 ps equilibration at 300 K; HC EDL statistics from 50 ps production; LC from 100 ps total (50 ps equilibration + 50 ps production).
Temperature: 300 K for the summarized MD windows.
Pressure: controlled via NPT barostat settings above (Methods).
Electric field (applied): N/A — not part of the summarized EDL setup.
Enhanced sampling: well-tempered metadynamics (PLUMED) as stated.

Findings¶

1 — Outcomes and mechanisms¶

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 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.

2 — Comparisons¶

Experiments on five activated carbons plus HNO₃ before/after YP50; atomistic models interpret PG vs GO-like surfaces (SI Figure S11 cited in the article).

3 — Sensitivity¶

Strong-acid site loading from ~0.05 to ~0.36 mmol g⁻¹ correlates with rise potential trends (abstract).

4 — Limitations / outlook¶

Model surfaces coarse-grain carbon heterogeneity; see ## Limitations.

5 — Corpus / KB honesty¶

Full experimental protocols and metadynamics collective variables are in pdf_path and SI; extract may truncate captions.

Limitations¶

Carbon structural heterogeneity is coarse-grained into model surfaces; CDLE cycle simplifications noted in broader CDLE literature referenced in intro.

Relevance to group¶

Raju and van Duin contribute atomistic interpretation tying interfacial chemistry to electrochemical device metrics.

Retrieval note: pair this paper with other CDLE/capacitive mixing entries in the corpus when benchmarking salinity-gradient energy claims, because rise potential depends on both macroscopic flow and nanoscale EDL structure.

Citations and evidence anchors¶

Environ. Sci. Technol. 2014, 48, 14041–14048; DOI 10.1021/es5043782 (extract page 2 footer).
Abstract statistics (extract page 1).