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Supporting Information: Choice of Electrolyte Impacts the Selectivity of Proton-Coupled Electrochemical Reactions on Hydrogen Titanate

Evidence and attribution

Authority of statements

This corpus PDF is Supporting Information for a JPCC article (DOI in front matter). Electrochemical conclusions belong to the main article; this page documents SI ReaxFF development content excerpted in the SI text.

Summary

The SI accompanies Fortunato, Shin, Spencer, van Duin, and Augustyn’s study of proton-coupled electrochemical reactions on hydrogen titanate (H₂Ti₃O₇) in acidic media, contrasting electrolytes such as phosphoric acid versus sulfuric acid (full experimental narrative on the main article page). The SI’s “ReaxFF method” section restates the standard ReaxFF energy partitioning and notes EEM-type charge models for Coulomb interactions, emphasizing that parameters are fit to QM and/or experimental targets.

The SI’s force-field development discussion explains how the team extends a TiO/H ReaxFF suitable for Ti(IV) oxides and water chemistry on anatase, rutile, brookite, and high-pressure TiO₂ phases, and then introduces Sc–O–H parameters so that Sc(III) acts as a surrogate for Ti(III) on reduced hydrogen titanate surfaces—an expedient when direct Ti(III) training data are difficult to fold into the optimization cleanly. Training sets cited in the SI include equations of state and heats of formation for Sc₂O₃, ScO, ScO₂, hcp Sc, and molecular fragment distortion curves (Figure S1 in the SI). Additional Ti/Sc ternary oxide fits reference Ti₂ScO₅ and Ti₂Sc₂O₇ crystal phases (Figure S2). The SI also compares oxygen vacancy formation energetics on rutile with Sc substitution scenarios (Figure S3), supporting the qualitative point that vacancies near Sc analogues are stabilized differently than those near Ti(IV) sites.

Methods

Genre (supporting information, not standalone experiment)

This PDF is Supporting Information for DOI 10.1021/acs.jpcc.3c01057, containing ReaxFF development detail, QM comparison figures (S1–S3 etc.), and parameter tables—not the full electrochemical results narrative.

Force-field development content (A)

  • Baseline: Extension of a Ti/O/H ReaxFF for Ti(IV) oxides and water chemistry across TiO\(_2\) polymorphs (as cited in the SI).
  • Surrogate strategy: Sc–O–H parameters introduced so Sc(III) can mimic Ti(III) on reduced hydrogen titanate surfaces when direct Ti(III) fitting is unwieldy.
  • Training sets: Equations of state, heats of formation, and distortion curves for Sc oxides/metal and selected Ti–Sc ternary oxides (figures referenced in SI text).

Where full cells and electrochemistry live

Interfacial MD benchmarks tying anion identity to PCET selectivity, plus CV/capacity metrics, are reported in the main article—see [[2023jenelle-fortunato-j-phys-chem-choice-electrolyte]] / [[2023fortunato-x-choice-electrolyte]].

MD application (SI page — delegate to main text)

N/A — this SI file documents ReaxFF development (Ti/O/H, Sc–O–H), not a standalone MD protocol sheet for interfacial production runs. For LAMMPS+ReaxFF, PBC slab cells, NVT/NPT choices, thermostat (e.g. Nosé–Hoover in the main text), target temperature (e.g. ~300 K room-temperature MD in typical interfacial setups), timestep, ps/ns, barostat/pressure, E-field, and sampling used in the H\(_2\)SO\(_4\) vs H\(_3\)PO\(_4\) interface study, use [[2023fortunato-x-choice-electrolyte]] and the main JPCC article PDF at DOI 10.1021/acs.jpcc.3c01057.

Findings

What the SI establishes

Reproducible documentation of the ReaxFF extension and Sc-for-Ti(III) surrogate with QM comparisons for vacancy energetics and related benchmarks.

What requires the main text

H\(_3\)PO\(_4\) vs H\(_2\)SO\(_4\) selectivity, Coulombic efficiency, and capacity trends must be taken from main-text figures/discussion—not inferred from SI fragments alone. Mechanistic role of the SI: the ReaxFF extension is meant to make reduced H\(_2\)Ti\(_3\)O\(_7\) surfaces and anion-competitive adsorption comparable to QM on key training geometries—a prerequisite for interpreting interfacial MD in the main paper. Comparisons to DFT in Figure S1–S3 are internal validation of the force field, not cell polarization experiments by themselves. Sensitivity of PCET selectivity to pH/buffer/anion is an electrochemical lever reported on the VOR page, not re-derived here. Limitations & outlook: Sc as a Ti(III) surrogate can fail if electronic state space drifts outside the fitted set; however the SI still gives reproducible QM/FF gaps for main-text claims. Corpus honesty: SI-only PDF; electrochemistry “findings” live on the VOR main page.

Limitations

SI PDFs can be hard to interpret without the main article; figures are numbered separately (S1, S2, …). Surrogate elements increase risk if simulations wander outside the fitted Sc/Ti substitution regime.

Relevance to group

Stores Shin/van Duin ReaxFF development detail for titanate aqueous interfaces, relevant to energy storage and PCET mechanistic studies.

Citations and evidence anchors

  • Article DOI 10.1021/acs.jpcc.3c01057; SI PDF: papers/Fortunato_Shin_JPCC_2023_titanate_H3PO4_H2SO4_SI.pdf.
  • Excerpt alignment: normalized/extracts/2023fortunato-venue-paper_p1-2.txt.

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