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Summary

The authors present a graphene oxide (GrOx)-catalyzed bipolar membrane (BPM) with deliberate structural asymmetry — a thin anion-exchange layer (AEL) paired with a thicker cation-exchange layer (CEL) — engineered to sustain electrodialysis (ED) at industrially relevant current densities (1 A cm⁻²) with overpotentials below 250 mV. The central mechanistic claim is that water dissociation (WD) catalysis is localized at GrOx moieties positioned within the high-field space-charge region of the BPM junction, where second Wien effect field-enhancement of the dissociation rate constant is expected to dominate. The work integrates chronopotentiometric characterization, long-duration stability testing (>1100 h), Faradaic efficiency quantification, and a continuum (likely Nernst-Planck-Poisson or drift-diffusion) transport model to rationalize the observed kinetics and to attribute the asymmetric layer thickness to alleviating water-transport-limited operation at high j.

Major comments

1. Mechanistic attribution of the overpotential reduction (catalytic vs. field effect). The abstract frames the low overpotential as arising from "deprotonation at GrOx catalyst sites... within the high electric field... junction region," which conflates two mechanistically distinct contributions to WD kinetics in BPMs: (i) a genuine heterogeneous catalytic effect (lowering the intrinsic activation barrier for proton transfer, analogous to a Grotthuss-type or tertiary-amine/weak-acid catalytic pathway as in Neptunia-type membranes), and (ii) a field-enhancement effect on the equilibrium dissociation constant via the second Wien effect (Onsager theory), which is present in any BPM junction regardless of catalyst identity. A rigorous review should check whether the manuscript deconvolutes these two contributions — e.g., via Boltzmann/Onsager fits of ln(k_wd) vs. E² at the junction, comparison of apparent activation energies with and without GrOx, or Tafel-slope analysis distinguishing a catalytic pre-wave from the field-driven bulk WD wave. Without this deconvolution, "catalysis" risks being conflated with simple field concentration at the (thinner, more resistive) AEL/CEL interface.

2. Continuum model structure and parameter provenance. "Continuum modeling" in BPM literature typically means a 1D (occasionally 2D) Nernst-Planck-Poisson formulation with a reaction term for water dissociation/recombination (Zabolotskii-Onsager or Frumkin-Fowler-Guggenheim (FFG) kinetics) and fixed charge densities in each layer. Key items to verify in the Methods/SI:

• Whether the GrOx pKa (or the density/pKa distribution of oxygenated functional groups — carboxylic, hydroxyl, epoxide) was fit to the same chronopotentiometric dataset used to validate the model, which would be circular, or constrained independently (e.g., via titration, XPS, or FTIR quantification of surface functional group density).

• Whether the model self-consistently reproduces the space-charge layer width and the resulting field strength at 1 A cm⁻², since the second Wien effect scales with E² and is extremely sensitive to junction width assumptions.

• Whether ohmic/iR contributions from the bulk ion-exchange layers were properly subtracted before attributing residual overpotential to interfacial WD kinetics (a common source of underestimated activation overpotential in BPM papers).

3. Water transport limitation and the thin-AEL rationale. The claim that a thin AEL "overcomes water transport limitations" implies water flux to the junction (via diffusion and electro-osmotic drag) is rate-limiting at high j in symmetric BPMs. This should be substantiated quantitatively — e.g., a comparison of limiting current density (or the onset of a water-depletion plateau in the I-V curve) between the asymmetric membrane and a symmetric control, or direct water permeability/uptake measurements (gravimetric swelling, water diffusion coefficients via PFG-NMR or similar) for the two AEL thicknesses tested. Absent such a control, the water-transport argument remains inferential, reasoned from where the observed operating regime (1 A cm⁻² without a limiting-current plateau) sits relative to typical symmetric-BPM literature values rather than from a direct transport measurement on this membrane.

4. Faradaic efficiency quantification. "Near unity Faradaic efficiency" needs an explicit numerical value and associated uncertainty, along with the analytical method (acid-base titration, ion chromatography, conductivity-based back-calculation). It would also be useful to know whether FE was assessed across the full current density range (80 mA cm⁻² to 1 A cm⁻²) or only at a subset, since co-ion leakage (Donnan failure) and water dissociation side-reactions typically become more significant departures from unity FE at higher current density and should track with membrane permselectivity data if reported.

5. Interpretation of the differential stability data. The stability results (>1100 h at 80 mA cm⁻² vs. 100 h at 500 mA cm⁻²) are reported at two very different current densities and durations, which raises the question of whether the 500 mA cm⁻² test was terminated by protocol or by failure (e.g., delamination, GrOx leaching, or mechanical rupture from osmotic/electro-osmotic pressure buildup at the junction). The voltage drift rates (reported elsewhere as ~70 μV/h at 80 mA cm⁻² and ~−300 μV/h at 500 mA cm⁻², per the published abstract) show opposite sign, which is noteworthy: a positive drift (increasing overpotential) is consistent with catalyst deactivation or fouling, whereas negative drift (decreasing overpotential) at high current density may indicate junction restructuring, catalyst redistribution, or thermal effects (Joule heating altering local pKa/mobility). This sign reversal deserves explicit mechanistic comment rather than being subsumed under a single "exceptional stability" claim.

6. GrOx loading, distribution, and reproducibility. Given that catalytic performance in these systems is highly sensitive to catalyst loading and spatial distribution within the junction (as opposed to bulk incorporation), reviewers should check whether the SI reports a loading-dependence sweep (rather than a single optimized loading) and whether membrane-to-membrane reproducibility (n≥3) is reported for the key overpotential and stability metrics, given known batch variability in solution-processed GrOx.

Minor comments

• Confirm whether "freestanding architecture" is explicitly contrasted with the supported/laminated configurations more common in the BPM-ED literature, and whether any mechanical characterization (tensile strength, delamination resistance under the stack's operating pressure) is provided to substantiate stack-scale applicability.

• Check whether the continuum model results are validated against local pH or local field measurements (e.g., via a reference electrode array or fluorescent pH-sensitive probe within the stack), or whether validation is limited to global I–V and stability agreement, which is a weaker test of the proposed mechanism.

• The abstract's causal chain (thin AEL → alleviated water transport limitation → sustained 1 A cm⁻²) would benefit from an explicit statement of which limiting step (water supply, WD kinetics, or ionic ohmic resistance) is rate-determining at 1 A cm⁻² in this specific membrane, since the answer determines whether further gains should target the AEL thickness, the GrOx loading, or the CEL ionic conductivity.

Competing interests

The author declares that they have no competing interests.

Use of Artificial Intelligence (AI)

The author declares that they did not use generative AI to come up with new ideas for their review.