{"id":"b2952f8b-a877-417b-97ec-987b431247e6","arxiv_id":"2607.26888","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Cu-for-Cr substitution in MnFeCoNi high-entropy alloys lowers alkaline HER/OER overpotentials versus the Cr analog, with DFT linking the gain to weaker intermediate binding and post-OER Cu surface enrichment.","lead":"Replacing chromium with copper in a five-metal high-entropy alloy improves both hydrogen- and oxygen-evolution activity in alkaline water splitting. The work offers a concrete composition knob and shows copper can migrate to the surface during oxygen evolution, which matters for designing cheaper electrolyzer catalysts.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.5","headline":"DFT ranking of pristine FCC surfaces does not represent the post-OER Cu-rich shell that is the actual working OER surface.","rationale":"The reader correctly isolated the mismatch between DFT surfaces and the experimentally observed post-OER Cu shell as the weakest assumption under the strongest claim. That concern is load-bearing specifically for the OER/bifunctional half of the claim and for the design-rule language in the abstract and conclusion; the HER half is better supported by retained homogeneity and the experimental LSV/Tafel/EIS ordering. No internal contradiction falsifies the measured Cr-vs-Cu activity trend, so the verdict stays CONDITIONAL rather than REJECT—acceptance still hinges on (i) shell-aware energetics or an explicit statement that DFT explains only the pristine/HER surface, plus (ii) the missing comparative ECSA, error bars, and full OER numerics the reader already flagged. Agreement with the reader is therefore full on the identity of the soft spot; the stress test only sharpens the concrete falsification (recompute Table 1 on a Cu-shell model).","tokens_in":13854,"tokens_out":674,"duration_ms":14815,"concrete_test":"Build a Cu-terminated or Cu-rich outer-layer slab consistent with the post-OER EDS maps (Fig. 5j–k), re-adsorb O*, OH*, and OOH* with the same Monte Carlo protocol used for Fig. 8, and recompute Table 1 ΔG values (same T/pH/U). If HEA-Cu no longer shows weaker binding than HEA-Cr (or the overpotential-determining step worsens by ≳0.2 eV), the computational corroboration of the OER half of the strongest claim fails.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim is that Cr\to Cu substitution improves bifunctional activity because Cu weakens binding of H*/O*/OH*/OOH* relative to Cr (Computational Analysis; Table 1; Figs. 7–8). That electronic-structure explanation is load-bearing for OER: experiment itself shows that after OER, HEA-Cu reconstructs into a Cu-rich outer shell over a multimetallic core (Fig. 5), while HER retains homogeneous distribution (Fig. 4). The DFT models, however, use only the five most stable bulk-like FCC supercells per alloy (one structure each for OER intermediates), Monte Carlo adsorption on those terminations, and fixed corrections (T=300 K, pH=14, U=−0.80 V for HER). They never place O*/OH*/OOH* on a Cu-enriched or oxide/hydroxide shell. Table 1’s weaker |ΔG| values for HEA-Cu therefore rank idealized as-synthesized surfaces, not the operando OER surface the paper documents. If the shell’s binding energetics reverse or erase the Cu-vs-Cr ordering, the structure–property link for OER (and the bifunctional design rule) is unsupported even if the LSV ordering is real. HER is on firmer ground because elemental homogeneity is retained.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.5","summary":"The manuscript compares equiatomic CrMnFeCoNi (HEA-Cr) and MnFeCoNiCu (HEA-Cu) nanoparticles for alkaline HER and OER under matched conditions. XRD, XPS, SEM, and TEM/EDS establish single-phase FCC solid solutions with homogeneous elemental distributions as synthesized. Electrochemically, HEA-Cu outperforms HEA-Cr and pure Cu: HER onset/overpotential near 538 mV with Tafel slope 165 mV dec⁻¹ versus higher values for HEA-Cr, with analogous OER improvement; ADT, EIS, and post-test TEM show HER retains elemental homogeneity while OER drives Cu migration into a Cu-rich outer shell over a multimetallic core. DFT on selected FCC supercells with Monte Carlo adsorption sites reports weaker binding of H*, O*, OH*, and OOH* on HEA-Cu than HEA-Cr, offered as the electronic-structure rationale for the activity ranking and for single-element substitution as a bifunctional design rule.","tokens_in":14098,"tokens_out":1772,"duration_ms":42910,"significance":"If the Cr→Cu ranking and the reconstruction observation hold, the work is a useful, concrete case study of how one-element substitution in a Cantor-related MnFeCoNi framework tunes alkaline water-splitting kinetics and surface evolution. Strengths include matched synthesis/characterization of both alloys, carbon-free ink tests plus Vulcan-carbon comparisons, and especially the post-HER versus post-OER TEM/EDS contrast documenting Cu shell formation under OER. The HER performance (η ≈ 538 mV) is modest relative to state-of-the-art non-noble catalysts, so impact is primarily structure–property insight rather than a record bifunctional electrode. The DFT–experiment link is a genuine contribution only where the models represent the working surface; that link is stronger for HER than for OER given the reported reconstruction.","major_comments":[{"comment":"Computational Analysis / Table 1 / Figs. 7–8 vs Fig. 5: The load-bearing OER structure–property claim is that Cu substitution weakens binding of O*, OH*, and OOH* relative to Cr and thereby improves OER. Experiment, however, shows that after OER the working HEA-Cu surface is a Cu-rich outer shell over a multimetallic core (Fig. 5), not the homogeneous FCC termination used in DFT (one bulk-like structure per alloy; Monte Carlo sites on as-built supercells). Table 1 therefore ranks idealized as-synthesized surfaces, not the operando OER surface the paper documents. Either (i) compute intermediates on Cu-enriched / oxide–hydroxide shell models consistent with Fig. 5, or (ii) explicitly restrict the DFT OER claim to initial surfaces and separate the reconstruction discussion from the binding-energy design rule. Without one of these, the bifunctional electronic-structure explanation for OER i","section":"Computational Analysis; Table 1; Figs. 5, 7–8"},{"comment":"§2.2 / Fig. 2c–d: Numerical OER overpotentials at 10 mA cm⁻² (and, preferably, at a higher current density) for HEA-Cr, HEA-Cu, and Cu are not stated in the text with the same clarity as the HER 538 mV value. The abstract and conclusion assert bifunctional superiority and a single 538 mV figure that reads as HER-only; OER claims need explicit η and Tafel numbers in the main text for both alloys under identical loading so the Cr→Cu improvement can be evaluated quantitatively, not only from unlabeled LSV curves.","section":"§2.2 Electrochemical Characterization; Fig. 2; Abstract"},{"comment":"§2.2 / Fig. 2e–f: ECSA/C_dl is reported only for HEA-Cu (0.12 mF cm⁻², 3 cm²). Geometric j at fixed mass loading is used as the primary metric for HEA-Cr versus HEA-Cu (axiom implicit in the comparison). Without C_dl/ECSA (or another surface-normalization) for HEA-Cr under the same protocol, it is unclear whether the activity gain is intrinsic or partly area/roughness. Report ECSA for both alloys and, if feasible, ECSA-normalized LSV or specific activity at a fixed overpotential.","section":"§2.2; Fig. 2e–f"},{"comment":"Language of “high-performance” / “efficient water electrolysis” (Abstract, Conclusion) sits uneasily with HER η₁₀ ≈ 538 mV and Tafel slopes of 165–226 mV dec⁻¹, which indicate sluggish Volmer–Heyrovsky kinetics relative to competitive alkaline HER HEAs and to Pt/C in Fig. 6. The Cr→Cu ranking can stand; the performance framing should be tempered to “improved relative to HEA-Cr under these conditions,” with clearer benchmarking against recent non-noble HEA HER/OER literature beyond RuO₂/Pt/C.","section":"Abstract; Conclusion; Fig. 6"}],"minor_comments":[{"comment":"Fig. 3 caption: panels (c, d) are labeled “LSV and EIS for HER” but the text discusses OER; correct to OER.","section":"Figure 3 caption"},{"comment":"XPS narrative inconsistency: early text states Cu is predominantly Cu(I) with mixed-valence discussion later (“Fe, Co and Cu exhibit mixed-valence”); align assignments and satellite interpretation for Cu 2p / Cu LMM.","section":"§2.1; Fig. 1c"},{"comment":"Typographical/notation issues: “mVdec ¹” / missing superscripts; “cylces”; “Co–Cr–Fe–Mn–Ni” vs composition order; Ni 2p peak at 886.2 eV is unusually high and should be checked against fit/raw data.","section":"§2.1–2.2; Abstract"},{"comment":"Computational methods (main text): state functional, code, supercell size, spin treatment, how ΔG_H / ΔG_O/OH/OOH and the U = −0.80 V, pH = 14 corrections are defined (Eq. references), and why only the third-most-stable HER structures and a single OER structure per alloy are emphasized. Point readers clearly to SI methodology.","section":"§3 Computational Analysis"},{"comment":"Fig. 2a: clarify whether 0.538 V is onset or η at 10 mA cm⁻²; abstract says overpotential 538 mV while body mixes “onset potential” wording for the same numbers.","section":"Fig. 2a; Abstract; §2.2"},{"comment":"Report replicate statistics (error bars on η and Tafel) and iR-correction / RHE calibration protocol briefly in the main electrochemical section or SI cross-reference.","section":"§2.2; Supporting Information"}],"recommendation":"major_revision","confidential_remarks":"The experimental half (matched HEAs, carbon-free vs carbon inks, and especially post-OER Cu shell vs post-HER homogeneity) is the publishable core and is stronger than the DFT narrative as written. I would not reject on performance alone if the authors scope claims and fix the OER model–surface mismatch. Watch for over-claim relative to the authors’ related HEA water-splitting papers (e.g. Fe-Co-Ni-Cu-Mn/Zn); novelty is the Cr vs Cu Cantor pair plus reconstruction, which is enough if framed tightly. Fit is appropriate for a materials/electrocatalysis journal; borderline for ultra-high-impact venues unless DFT on the reconstructed shell is added."},"author_rebuttal":null,"desk_editor":{"model":"grok-4.5","letter":"The useful bit here is empirical and narrow: under matched alkaline conditions, equiatomic MnFeCoNiCu beats CrMnFeCoNi for both HER and OER, and post-OER TEM/EDS shows Cu migrating into a Cu-rich outer shell while HER keeps the alloy homogeneous. That reconstruction contrast is the clearest new observation. Single-element swaps in this family are not new (they cite Mn/Zn and related capacitance work), but a clean Cr-to-Cu electrocatalysis comparison with post-reaction microscopy was missing and is now on the table.\n\nWhat they do well is the materials side. XRD/XPS/TEM support single-phase FCC with the intended metals; LSV/Tafel/EIS/ADT tell a coherent story that HEA-Cu is better than HEA-Cr and pure Cu; carbon-free inks plus a Vulcan-carbon check is honest; and they do not hide the OER shell or the modest absolute numbers (HER η10 ≈ 538 mV). Circularity is low—the ranking is measured, not fitted. Citations sit inside an active HEA-electrocatalysis program rather than inventing a field.\n\nSoft spots, in proportion. Absolute activity is modest, so “high-performance” is relative only to HEA-Cr. Comparative ECSA, error bars, and full OER numerics for both alloys are thin, so geometric vs intrinsic activity is not fully separated. The stress-test on DFT is fair for OER: they rank H/O/OH/OOH on a few stable bulk-like FCC cells and never model the Cu-rich shell they themselves document after OER. That weakens the electronic-structure design rule for OER more than for HER, where homogeneity is retained. It does not erase the experimental Cr-vs-Cu ordering.\n\nWho it is for: people already working Cantor-derived or FeCoNi-based HEA electrodes who want a composition lever and a reconstruction caveat. Not a benchmark-displacing water-splitting paper.\n\nI would send it to referees. Ask for replicates/error bars, ECSA on both alloys, clearer OER metrics, and either shell-aware calculations or a dialed-back claim that DFT explains as-synthesized surfaces and HER more than operando OER. Worth engaging if you care about HEA composition design; skip if you only track catalysts that approach Pt/RuO2.","headline":"Solid Cr-vs-Cu head-to-head with a real OER reconstruction observation; the bifunctional ranking is credible, but the DFT story only cleanly supports HER.","tokens_in":14928,"tokens_out":591,"would_cite":false,"duration_ms":11829,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.5","headline":"Replacing chromium with copper in a MnFeCoNi high-entropy alloy improves both alkaline hydrogen and oxygen evolution.","keywords":["high-entropy alloys","alkaline water electrolysis","hydrogen evolution reaction","oxygen evolution reaction","CrMnFeCoNi","MnFeCoNiCu","single-element substitution","surface reconstruction"],"falsifier":"Measure HER and OER overpotentials and post-test elemental maps on a series of MnFeCoNi–(Cr/Cu) alloys with systematically varied Cu fraction; if activity does not track the calculated intermediate binding strengths, or if a Cu-rich shell appears without the predicted activity change, the central claim fails.","tokens_in":14633,"feed_emoji":"⚡","tokens_out":851,"duration_ms":20183,"temperature":0.7,"pith_summary":"This paper asks whether swapping one metal in a five-element high-entropy alloy is enough to change how well it splits water in base. The authors compare CrMnFeCoNi with MnFeCoNiCu under the same alkaline conditions and find that the copper version needs less extra voltage and runs with faster kinetics for both hydrogen and oxygen evolution. Calculations show copper softens how tightly the surface holds the key reaction intermediates, which matches the better experimental rates. After long oxygen-evolution testing the copper atoms migrate outward into a copper-rich shell, while hydrogen-evolution conditions keep the metals mixed. The practical point is that a single, cheap elemental swap can be used as a design knob for noble-metal-free water-splitting catalysts.","feed_headline":"Copper swap beats chromium in HEA water-splitting catalysts","feed_subtitle":"One metal change lowers overpotential and softens intermediate binding for both HER and OER","key_machinery":"Single-element Cr-to-Cu substitution inside an equiatomic FCC MnFeCoNi high-entropy alloy: copper acts as an electron acceptor that weakens intermediate binding relative to chromium-rich sites and, under OER, drives surface reconstruction into a Cu-rich outer shell.","core_discovery":"Under identical alkaline conditions, substituting Cr with Cu in the MnFeCoNi framework produces a bifunctional high-entropy alloy that outperforms the chromium analog for both HER and OER, reaching a lower HER overpotential of 538 mV and a Tafel slope of 165 mV dec⁻¹, because copper modulates the electronic structure to give more favorable binding energies for H*, O*, OH*, and OOH*.","pith_inferences":["If Cu shell formation is the main OER deactivation path, a thin protective overlayer or pulsed reverse bias might lock copper in place and extend lifetime.","The same Cr-to-Cu lever may transfer to other Cantor-derived HEAs used for CO2 reduction or nitrogen reduction where intermediate binding also limits rate.","Operando spectroscopy that tracks Cu oxidation state during the first few hundred OER cycles would test whether the calculated bulk-like sites ever exist under working conditions."],"forward_implications":["Compositional design of HEA water-splitting catalysts can start with targeted single-element swaps rather than full five-metal redesigns.","Cu-containing MnFeCoNi HEAs become candidates for low-cost bifunctional alkaline electrolyzer electrodes, with OER performance approaching RuO2 when carbon is added to the ink.","Post-OER Cu surface enrichment must be treated as part of the active catalyst state, not merely degradation.","HER stability benefits from the homogeneous multimetallic surface that persists under reducing conditions."],"fun_headline_variants":["Cu replaces Cr for superior HEA bifunctional water electrolysis","Single Cu swap beats Cr in MnFeCoNi alkaline HER and OER","HEA-Cu lowers overpotential to 538 mV versus HEA-Cr","Cu substitution tunes binding energies for better HER/OER","Cr-to-Cu switch boosts MnFeCoNi bifunctional electrolysis"],"cache_read_input_tokens":128,"weakest_assumption_plain":"The binding energies calculated on a few stable bulk-like alloy surfaces are assumed to rank the real working catalysts even though oxygen evolution drives copper to the surface and forms a shell the models do not include.","fun_headline_variants_meta":{"raw":{"variants":["Cu replaces Cr for superior HEA bifunctional water electrolysis","Single Cu swap beats Cr in MnFeCoNi alkaline HER and OER","HEA-Cu lowers overpotential to 538 mV versus HEA-Cr","Cu substitution tunes binding energies for better HER/OER","Cr-to-Cu switch boosts MnFeCoNi bifunctional electrolysis"]},"model":"grok-4.5","effort":"low","cost_usd":0.003729,"raw_usage":{"total_tokens":1258,"prompt_tokens":846,"num_sources_used":0,"completion_tokens":77,"cost_in_usd_ticks":37288000,"prompt_tokens_details":{"text_tokens":846,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":335,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":846,"tokens_out":77,"duration_ms":6338,"temperature":1.0,"reasoning_tokens":335,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-30T18:01:11.312209+00:00","model_set":{"reader":"grok-4.5"},"falsifier":"Measure HER and OER overpotentials and post-test elemental maps on a series of MnFeCoNi–(Cr/Cu) alloys with systematically varied Cu fraction; if activity does not track the calculated intermediate binding strengths, or if a Cu-rich shell appears without the predicted activity change, the central claim fails.","supporting_citations":[],"review_version":1}