Modulating coordinate site occupancy in high-entropy spinel electrocatalysts

Domain: biology
Published: 2026-03-20
Editorial outcome: major_revision

Abstract
High entropy spinel oxides provide a versatile platform for electrocatalysis because multiple metal cations can be incorporated into a single crystalline lattice, enabling tunable electronic structures. However, controlling how these cations distribute between tetrahedral and octahedral coordination sites remains a major challenge, limiting rational catalyst design. Here, we modulate cation coordination site occupancy between tetrahedral and octahedral sites in a Co–Fe–Cr–Mn–Ni framework by introducing a sixth cation (Zn, Ga, Mg, or Al) with distinct site preference energies. Using density functional theory, synchrotron X-ray absorption spectroscopy, and magnetic circular dichroism, we demonstrate that Zn preferentially occupies tetrahedral sites, driving increased octahedral occupancy of cobalt. This redistribution increases the population of octahedrally coordinated cobalt in mixed oxidation states, enhances electrical conductivity, and improves oxygen evolution reaction activity. Our findings establish coordination site occupancy as a critical design parameter, providing a strategic pathway for tailoring multicomponent spinel electrocatalysts with optimized performance.

Introduction
High-entropy materials (HEMs), distinguished by entropy-driven phase stabilization and compositional complexity, have rapidly emerged as versatile functional materials across applications in energy storage and conversion.1–5 A common assumption is that equimolar mixing of metal precursors yields a single-phase solid solution with uniformly distributed cations. This assumption is often supported by bulk-level characterizations such as X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) mapping in transmission electron microscopy (TEM).6 However, recent studies employing advanced techniques have challenged this view. For example, high-entropy alloys (HEAs) have been shown to exhibit nanoscale chemical heterogeneity and element al clustering, indicating that equimolar input does not guarantee atomic-scale homogeneity.7,8 In parallel, site engineering and vacancy control have been actively explored in various HEMs to tailor functionalities including mechanical strength, electrical properties, and catalytic activity.9–14 Notable efforts include short-range ordering in HEAs,15,16 oxygen vacancy tuning in rock salt-type high-entropy oxides (HEOs),10,17,18 and B-site modulation in perovskite-type HEOs.19,20 These studies have expanded our understanding of how compositional complexity can be harnessed to control materials’ behavior. However, much less is understood about site-specific occupation in more complex structures, where the interplay between multiple crystallographic sites adds further degrees of freedom, and complexity.

Spinel-type HEOs are a particularly intriguing structure, featuring two distinct cation sites—tetrahedral (T ) and octahedral (O ) in a 1:2 ratio—whose occupancy is governed by oxidation states, ionic radii, and electronic configurations.21–23 This dual-site nature enables additional avenues for property tuning, but also complicates the control of cation distribution.24 Most conventional techniques lack the resolution to distinguish between T and O site occupancies at the bulk level, and few studies have demonstrated the importance of site-specific cation engineering in high-entropy spinel oxides (HESOs).25,26 Adjusting the occupancy of Co between O and T sites was shown to tune Co-O bond polarity and hence enhance redox kinetics in lithium-sulfur batteries.27,28 Separately, compositional tuning in HESOs with selective redistribution of transition metals across crystallographic sites has enabled control over ferrimagnetic ordering.29 Despite growing interest in HESOs for different applications,21,22,24,26–39 there remains a lack of direct correlation between synthetic composition, cation site distribution, and catalytic properties. Establishing such correlations would provide substantial strategies for catalyst design through targeted site engineering.

In this study, we address the gap in understanding and controlling coordination site distribution in HESOs, arising from the structural complexity of dual T and O sites, limited site-specific characterization techniques, and the lack of established correlations between composition, site occupancy, and catalytic performance. We introduce a strategy based on a targeted addition of site-selective cations (Zn, Ga, Al, or Mg) into a Co–Fe–Cr–Mn–Ni HESO framework to modulate the occupancy of key active elements (e.g., Co and Fe) between T and O sites. The high configuration entropy stabilized HESO lattice allows dopant introduction to drive significant Co migration without compromising structural stability due to multiple redox-active and redox-stable transition metals that collectively buffer the charge redistribution. We use synchrotron-based spectroscopy and density functional theory (DFT), showing that compositional design governs site-specific cation occupancy that influences electronic structure and electrocatalytic properties. Particularly, Zn addition promotes the redistribution of active cations into octahedral sites with more favorable adsorption energetics and higher electrical conductivity, which correlates with the improved oxygen evolution reaction (OER) activity. This establishes coordination site occupancy as a key design parameter and offers a broadly applicable strategy for tuning functionality in multicomponent electrocatalysts.

Results
Design and Structural Characterization of High-Entropy Spinel Oxides
Fig. 1 illustrates cation engineering in HESOs with detailed structural characterization. All HESOs were synthesized using the previously reported solution-flame synthesis method with fast heating/cooling rates and high temperature, facilitating rapid single-phase formation.40 Our reference HESO comprises equimolar Co–Fe–Cr–Mn–Ni. Typically, these metal cations are assumed to be evenly distributed in both O and T sites (Fig. 1a),29,40 but detailed material characterizations, discussed later, reveal that the total cations are equimolar but they have uneven distributions in O and T sites.29,41 This uneven distribution is caused by site preference energies for various cations in spinel structures calculated from the crystal field theory.42 Cations with more positive energy (e.g., Zn2+) prefer the T site, and cations with more negative energy (e.g., Cr3+) prefer the O site (Fig. 1b).41 Thus, HESO does not have equimolar cations on O and T sites, presenting an opportunity to control configurational entropy by achieving a more uniform cation distribution on both T and O sites, leading to more stable structure. Our strategy is to intentionally introduce additional cations with distinct site preference energies to force the original cations into other sites. In our Co–Fe–Cr–Mn–Ni HESO, Ni and Cr exhibit strong O site preference, while Co, Fe, and Mn have more flexible coordination. As we incorporate a sixth element into the HESO: Zn (favoring T ), Al (favoring O ), and Ga/Mg (flexible occupancy),41 the distribution of Co–Fe–Cr–Mn–Ni in O and T sites is finely tuned.

For Zn-HESO, we expect that Zn prefers T sites, and more Co will be distributed to O sites (Fig. 1c). The as-synthesized Zn-HESO maintains a spinel structure with clear identification of O and T sites, as shown by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging along [110] zone axis (Fig. 1d, Supplementary Fig. 1). The cations in Zn-HESO exhibit uniform distribution as shown by HAADF-STEM-EDS (energy dispersive X-ray spectroscopy) mapping (Fig. 1e, Supplementary Fig. 2).

Zn-HESO in high-resolution TEM images in Fig. 1f shows d-spacings of 0.246 nm for the (311) and 0.292 nm for the (220) planes, compared with those of reference spinel NiFe O (0.251 nm for (311) and 0.295 nm for (220)) and pristine HESO (0.250 nm for (311) and 0.294 nm for (220)). This corresponds to an overall lattice contraction of approximately 1–2%. Such contraction is consistent with the incorporation of Zn ions, which preferentially occupy T sites in spinel lattices, replacing or redistributing larger divalent cations (e.g., Mn2+) within the T sublattice, thereby reducing the average metal–oxygen bond length. Similar variations in d-spacing are observed locally, as highlighted in the inset of Fig. 1f, where the measured d-spacings span 0.23–0.27 nm for the (311) and 0.27–0.32 nm for the (220) planes, reflecting compositional and local structural heterogeneity intrinsic to high-entropy spinels. In contrast, Ga-, Mg-, and Al-substituted HESOs exhibit slightly different average d-spacings, with values of 0.254–0.263 nm for the (311) and 0.291–0.297 nm for the (220) planes (Supplementary Figs. 3–5). These differences reflect variations in the average cation distribution and site-specific ionic radii within the spinel lattice. For Ga3+ and Mg2+, which possess relatively larger ionic radii compared to some of the host cations in O or T coordination, subtle lattice expansion or reduced contraction may occur depending on their preferred coordination environment. Notably, Al-HESO shows a distinct behavior compared to Ga- and Mg-containing samples. Al3+ ions have a relatively small ionic radius in O coordination (~0.535 Å) and form strong Al–O bonds, which can locally stabilize the spinel framework without necessarily inducing uniform lattice contraction. As a result, the observed d-spacing in Al-HESO likely reflects a balance between local lattice distortion, cation redistribution, and strong metal–oxygen bonding, rather than a simple ionic-size-driven expansion or contraction.

X-ray diffraction (XRD) (Fig. 1g, Supplementary Fig. 6) also shows that both pristine and Zn-HESO maintain pure spinel structures. Zn-HESO shows a slight shift in (311) due to the lattice contraction from a smaller ionic radius of Zn. The fast Fourier transform (FFT) in Fig. 1f confirms that the diffraction pattern matches the expected spinel structure and is consistent with XRD results, and X-ray photoelectron spectroscopy (XPS) (Supplementary Figs. 7–9) shows near-equimolar ratios of Co–Fe–Cr–Mn–Ni with additional substituent (Zn, Ga, Mg, or Al), similar to the precursor ratios, at the surface.

Characterization of Electronic Structures for M-HESOs
To assess oxidation states and local coordination of M-HESOs, X-ray absorption spectroscopy (XAS) K-edge measurements, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were performed at 3d transition metals K-edges (Supplementary Figs. 10–24). Detailed discussion of K-edge analysis is included in Supplementary Note 1, which shows that the sixth cation addition (Zn, Ga, Mg, and Al) induces edge shifts for Co and Fe, indicating oxidation state changes and T /O coordination. To further investigate the effect of sixth cation addition on cation redistribution, synchrotron XAS and X-ray magnetic circular dichroism (XMCD) were employed to examine electronic states and coordination (Supplementary Figs. 25–31). Figs. 2a, 2b show Co L-edge XAS and XMCD spectra of HESOs, together with the reference spectra of Co3+ O , Co2+ O and Co2+ T . All HESOs exhibit Co2+ and Co3+ features, but with different fine peak features. For example, Zn-HESO and Ga-HESO show distinct XAS and XMCD signatures. The XAS spectra show peak shift and broadening compared to the other HESOs, and XMCD provides element-specific magnetic contrast and differentiates oxidation states and site occupancies. Notably, the XMCD spectra for Zn- and Ga-HESO show clear signal combinations due to the co-existence of different valence occupancies. Spectral quantification using XAS spectra (Fig. 2c, Supplementary Fig. 26) shows that Al-HESO and Mg-HESO have more Co2+ in T sites, but Ga-HESO and Zn-HESO have more Co2+ and Co3+ in O sites. This is consistent with the cation preference (Fig. 1b) as Zn2+ and Ga3+ have a stronger T preference than Co ions. Figs. 2d-f show Fe L-edge XAS and XMCD with the reference spectra of Fe2+ O , Fe3+ O , and Fe3+ T , and fitted results for HESO and M-HESOs. HESO has the most Fe3+ in T sites and Fe2+ in O sites, while M-HESOs show an increased amount of Fe3+ in O (Fig. 2f). Similarly, XAS and XMCD analysis of Mn, Ni, Cr, and O (Supplementary Note 2) reveal that Mn, Cr, and Ni maintain their oxidation states and site occupancies across different HESOs. The modified cation distribution leads to a change in configurational entropy (ΔS ) (Fig. 2g-i). The pristine HESO has ΔS of 1.07R and 1.47R for T and O sites, while those for Zn-HESO are increased to 1.37R and 1.59R due to a more balanced cation distribution. The increased ΔS values for Zn-HESO are expected to increase phase stability (Supplementary Fig. 32).

Theoretical Investigation of the Effect of Zn Addition on Co Cation Distribution in HESO
To verify the experimental observation that Zn addition displaces Co from T to O sites (Fig. 2h), DFT calculations were performed. We employed the simple active site local impurity model from our previous work on HEOs.40 This model is based on a (100) facet surface slab of bulk Co O spinel, with a Co O active site fixed on the surface whose neighboring metals are interchanged amongst five elements (Fig. 3a). The local impurity model maintains the Co active site in Zn-HESO but substitutes Zn for one neighboring atom, either in O (Fig. 3b) or T (Fig. 3c). For the 120 pristine HESO Co-active-site permutations, Zn can either occupy O or T sites for each permutation, leading to 240 local impurity Zn-HESO configurations. Fig. 3d (Supplementary Table 1) shows total-energy histograms for Zn replacing Co, Ni, Mn, Fe, or Cr in T (top row) and O (bottom row) sites. Energies were corrected by adding 1/3 the energy of the pure M O spinel for each metal atom M replaced by Zn to allow comparison across different stoichiometries. Across all five substitutions, the mean energy of Zn in T sites is ~1.155-1.360 eV lower than in the corresponding Zn in O cases. Additionally, the structures where Zn replaces T Co sites show the lowest mean energy, 0.14 eV lower than the second lowest energy structures (Mn T sites). This indicates that Zn likely preferentially occupies T over O sites, consistent with site preference energy from crystal field theory (Fig. 1b),42 and that Zn may specifically displace Co from T , consistent with XAS/XMCD (Fig. 2h).

While the local impurity Zn-HESO model enables direct comparison with our previous HESO study, it simplifies the real structure by using Co O (100) as the support and assuming a normal spinel. To validate this model, we removed these simplifications (Supplementary Note 5) and developed a full high-entropy bulk Zn-HESO model. Fig. 3e shows a representative six-metal rhombohedral primitive cell with Zn and Fe in T and Co, Mn, Cr, and Ni in O . The energy histograms (Fig. 3f) show Zn T has a lower average energy than Zn O , in agreement with the simple-model result (Fig. 4d). We evaluated converged bulk configurations spanning normal, mixed normal/inverse, and inverse spinels with both CFT and DFT, and they show similar trends. The five most stable structures in descending order are (Co2Zn2) (Cr3Mn4Fe3Ni2) , (Fe3Zn2) (Cr3Mn3Co3Ni2) , (Fe3Zn2) (Cr3Mn4Co2Ni2) , (Ni2Zn2) (Cr3Mn4Fe3Co3) , and (Fe2Zn2) (Cr3Mn4Co3Ni2) (Supplementary Table 4). In this notation, superscripts represent oxidation state; the first two elements in T positions and the last four in O positions. In all five structures, Zn occupies the T site and Co2+ prefers the O site, matching XAS and XMCD element distributions (Fig. 2c). Mn4+ prefers the O site, consistent with XAS analysis (Supplementary Fig. 27). Notably, fully high-entropy bulk structures have similar bond lengths/angles as pure spinels (Co O , Fe O , Ni O , Mn O , Cr O ), indicating changes in OER behavior of Zn-HESO compared to pure spinels arise from electronic effects.

Theoretical DFT Investigation of the OER Activity for Zn-HESO
Since Co3+ is an established active site for OER,43 we further study how Zn-HESO-induced modulation in Co distribution affects its OER activity through DFT calculations and experiments. For DFT calculations, we used a fully high-entropy bulk model of the most stable Zn-HESOs (Fig. 3e) to form an O -terminated (100) surface (Fig. 4a), analogous to the simplified local impurity Zn-HESO model (Fig. 3b-c). We calculated O*, OH*, and selected OOH* adsorption energies on the most stable surfaces of both fully high-entropy Zn-HESO (Supplementary Table 4) and local impurity Zn-HESO model (Supplementary Table 1). Calculations follow the single-site OER pathway (Fig. 4b): a clean active site sequentially binds OH*, O*, and OOH*, then releases O to regenerate the site, recovering a clean catalyst surface. Although a direct O-O coupling route can be favored over the single-site mechanism, the overpotential calculated serves as a good lower bound on activity.

The volcano plot in Fig. 4c summarizes OER overpotentials for Co and non-Co active sites in HESO and Zn-HESO. It includes 24 local impurity Zn-HESO cases with Zn substituting T Co (Fig. 3c), paired with 24 pristine HESO surfaces from previous work40 to see the Zn substitution effect. Fig. 4c also includes 12 stable fully high-entropy Zn-HESO surfaces with Co and other cations as active sites, and the use of a fully high-entropy model to check the accuracy of the simpler impurity model. The calculated OER activity from the Co site local impurity model shows that Zn-HESOs (pink squares) are comparable to pristine HESOs (blue circles) (Supplementary Fig. 33-34), indicating that Zn does not have a large impact on the activity of individual Co sites. The scaling relationship between O*/OH*/OOH* (Supplementary Fig. 35) remains largely unchanged with a few Co outliers. We used the fully high-entropy models to evaluate Zn-HESOs with Co at O . The calculated overpotentials of Co3+ sites range from 350-540 mV, compared to 410-940 mV for Co sites in the local impurity model, indicating a smaller lower bound than the fully high-entropy model. The Co3+ O in the complex Zn-HESO model has an average overpotential of 453 mV, about 788 mV lower than that for Co2+. We also considered the possibility of other cations as potential active sites, with the most stable Zn-HESO with Co T , where it is not available as an active site. Among the calculated activities of four O sites (Ni, Fe, Mn, Cr), Cr has the lowest overpotential of 670 mV, but it is still larger than the best Co3+. The overpotentials of Fe, Mn, and Ni sites are 760 mV, 940 mV, and 1120 mV, respectively. Altogether, these results indicate that Co3+ O is the most active site for OER in Zn-HESO. Since Zn-HESOs have a slightly higher amount of Co3+ O than HESOs (Fig. 2c), we expect that Zn-HESO will be more active for OER (Figs. 5a-5c). While more Co3+ O sites will increase the activity, aligning with our expectations of improvement of Zn-HESO compared to pristine HESO, it is also important to note that the likely optimal Co occupation is less than 100%, as having the high entropy structure does lead to lower overpotentials than a pure Co spinel.

As electrical conductivity also impacts electrocatalytic performance, we examined the projected density of states (PDOS) of the five most stable bulk Zn-HESOs (Supplementary Fig. 36) to evaluate the impact of Zn substitution on conductivity. For four structures with O Co ((Fe3Zn2) (Cr3Mn3Co3Ni2) , (Fe3Zn2) (Cr3Mn4Co2Ni2) , (Ni2Zn2) (Cr3Mn4Fe3Co3) , and (Fe2Zn2) (Cr3Mn4Co3Ni2) ), the presence of Co states near the Fermi energy indicates that Co has available electrons for OER active site and is likely an active site. Additionally, PDOS plots for Co2+ O in (Fe3Zn2) (Cr3Mn4Co2Ni2) , have a much smaller band gap than those with Co3+ O sites in (Fe3Zn2) (Cr3Mn3Co3Ni2) , (Ni2Zn2) (Cr3Mn4Fe3Co3) , and (Fe2Zn2) (Cr3Mn4Co3Ni2) . This suggests that HESOs with more Co2+ O rather than Co 3+ are more conductive, because Co2+ provides an extra electron in comparison to Co3+ in n-type semiconductors like Zn-HESOs. As our experiments have shown an increase in Co2+ O from 20% in pristine HESOs to 60% in Zn-HESOs (Fig. 2c), we expect that Zn-HESO will show higher electrical conductivity than HESO (Fig. 5d).

Electrochemical performance of high-entropy spinel oxides for OER
To validate the DFT predictions that Zn-HESOs show more active and conductive, we experimentally measured OER properties of pristine HESO and M-HESOs (M = Zn, Ga, Mg, Al) in 1 M KOH electrolyte (Fig. 5). Fig. 5a plots iR-corrected polarization curves for pristine HESO, M-HESOs, and two references (CoFe O and Co O ). Among the substituted catalysts, Ga-HESO shows slightly lower onset overpotential at low current densities, consistent with its relatively strong tetrahedral site preference energy, which promotes effective redistribution of Co and Fe into octahedral coordination. However, this advantage is primarily observed in the low-current region. In contrast, Zn-HESO exhibits the lowest average overpotentials at both 10 mA cm-2 (η , 295 mV) and 100 mA cm-2 (η , 314 mV) (Supplementary Fig. 37), indicating great performance across a broader operational current range. The kinetic advantage of Zn-HESO is further evidenced by its lowest Tafel slope (25 mV dec-1) compared to Ga-HESO (37 mV dec-1), pristine HESO (33 mV dec-1), Mg-HESO (40 mV dec-1), and Al-HESO (45 mV dec-1) (Fig. 5b). This trend is directly reflected in the systematic increase in Tafel slopes from Zn- to Al-substituted systems, indicating progressively slower OER kinetics. In parallel, electrochemical impedance spectroscopy (Fig. 5c) shows that Zn-HESO possesses the smallest charge transfer resistance (R ) from the Nyquist semicircle in electrochemical impedance spectroscopy (EIS, Fig. 5c), and the double-layer capacitance (C ) of 1.8 mF cm-2 (Supplementary Figs. 38-39), indicating a larger electrochemical surface area (ECSA) of ~6 cm2. Importantly, after normalization by ECSA, Zn-HESO still exhibits a comparable or higher current density in the OER-relevant potential region. This observation suggests that factors beyond the electrochemically accessible surface area may influence the measured current density, such as differences in intrinsic catalytic reactivity, charge-transfer kinetics, surface electronic structure, or local reaction environments under OER conditions, as well as variations in ion accessibility, and the effective utilization of the catalyst surface. Benchmarking η , η , and Tafel slopes against literature-reported high-entropy OER catalysts ( Supplementary Fig. 40) positions Zn-HESO with one of the lowest Tafel slopes among state-of-the-art materials in both spinel and perovskite HEO classes.

The OER-related properties rank in the order of Zn-HESO > Ga-HESO > HESO > Mg-HESO > Al-HESO, the same order as the T preference energy of those added cations (Fig. 1b). This trend makes sense as DFT suggests that Co3+ O sites are the primary active sites for OER; the ranking reflects the ability of those added cations in pushing Co into O sites. The difference is more obvious at higher current density, which comes from the conductivity difference. The bulk electrical conductivity of Zn-HESO measured using the two-terminal method (Fig. 5d) is 0.17 mS cm-1, higher than pristine HESO (0.12 mS cm-1). This result is consistent with DFT DOS calculation (Supplementary Fig. 36) as structures with higher amounts of Co2+ O had a higher conductivity than structures with less Co2+ O . As the DFT calculation suggests that Co3+ O (Fig. 4c) is the dominant active site, both η and η of pristine and M-HESOs indeed show clear correlations with Co3+ and Co2+ O occupancy (Fig. 5e and Supplementary Fig. 41), but no clear correlation with Fe2+ and Fe3+ in either O or T (Supplementary Fig. 42). These results demonstrate that targeted enrichment of catalytically active Co in O correlates with enhanced OER performance. Finally, long-term durability was assessed via chronopotentiometry dynamically at 10 and 100 mA cm-2 (Fig. 5f). Zn-HESO maintains a stable overpotential over 80 hours of continuous operation (Fig. 5g). A slight and abrupt potential change observed at approximately 70 h arises from a routine maintenance step during the measurement, during which accumulated oxygen bubbles were removed from the electrode surface and the electrolyte was refreshed to ensure stable mass transport, rather than from catalyst degradation. Extended measurements further confirm similar stability up to ~120 hours at 10 mA cm-2 (Supplementary Fig. 43). In contrast, pristine HESO exhibits gradual performance degradation, particularly at high current density. The enhanced OER stability in Zn-HESO may be associated with a more homogeneous cation distribution across the spinel lattice (Fig. 2h), which could mitigate local compositional fluctuations and thereby improve structural robustness under OER conditions.

Operando XAS for Redox Dynamics of Co and Ni in Zn-HESO under OER Conditions
To probe redox dynamics of active sites for Zn-HESO during OER, we performed operando XAS in total fluorescence yield under electrochemical bias. Fig. 6a shows a custom-built three-electrode flow cell (Fig. 6a), where Zn-HESO-coated Ti/Pt/Si N electrodes were irradiated with X-rays, and Co and Ni L -edge signals were collected with circulating electrolyte. CV of Zn-HESO in 0.05 M KOH (Fig. 6b) revealed characteristic redox peaks between 0.8–1.2 V , indicating electrochemical activation of the catalyst surface, with OER onset beyond 1.45 V . Operando Co L -edge spectra (Fig. 6c) were recorded during a potential sweep starting from open circuit voltage (OCV, ~0.8 V ), increasing anodically to 1.45 V , and reversing back to reduction potentials. At an initial 0.8 V , Co L -edge peaks at ~779.2 eV, corresponding to mixed Co2+/Co3+ species. As potential increases, Co peak shifts progressively to higher photon energies, reaching a maximum at 1.45 V , indicating oxidation of Co2+ to Co3+ and formation of CoOOH-like species under OER-relevant conditions.44,45 During the cathodic sweep, Co L -edge shifts back toward lower energies, reflecting partial reduction to Co2+. Notably, Co spectra at 0.8 V after the cathodic sweep did not fully recover to the initial state, suggesting partial irreversibility or restructuring of the Co environment. This partial irreversibility indicates that while Co undergoes dynamic redox transitions, some oxidized Co species remain stabilized after high-potential cycling, potentially contributing to catalyst activation. Similarly, Ni L -edge spectra (Fig. 6d) exhibit oxidation from Ni2+ to Ni3+ under anodic polarization, with the most significant shifts near 1.2–1.45 V , and partial reduction during the cathodic sweep with some hysteresis.45,46 Such observed redox hysteresis likely reflects structural or electronic reconfigurations associated with transition metal (oxy)hydroxide formation under OER. While Co has been widely identified as the primary redox-active species, our operando XAS measurements also reveal a subtle shift in Ni edge position, indicating its partial involvement in electronic reorganization. This suggests that Ni may play a secondary or synergistic role during catalysis. The complex redox behavior in high-entropy systems reveals limitations of attributing catalytic activity to a single element, emphasizing the need for comprehensive, site-resolved, multi-element analysis.

Consistent with these operando L-edge observations, previously reported operando K-edge XANES studies on closely related high-entropy spinel oxides show that the Co K-edge shifts toward higher photon energies under anodic polarization, while the Ni K-edge exhibits a weaker but discernible shift toward higher oxidation states. These K-edge results further support the interpretation that Co undergoes pronounced redox evolution under OER conditions, with Ni contributing subtly to the overall electronic reorganization.47

Integrating experiments and theory, we showed that coordinate-site engineering enables controlled tuning of cation distributions in HESOs. Zn, with the strongest T preference, is associated with increased Co occupancy in O . Quantitative site occupancies aligned well with DFT and suggest Co3+ at O as the dominant active site for OER. The high-current performance of Zn-HESO is consistent with higher electronic conductivity, which correlates with a greater fraction of electron-donating Co2+ at O . Fully high-entropy models indicate mixed normal/inverse spinel configurations whose site populations are adjustable via Zn, offering a potential explanation for experimentally observed site distributions. Lastly, operando spectroscopy reveals partially reversible Co/Ni oxidation under OER, supporting sustained activity. Our investigations challenge assumptions of sublattice homogeneity and establish targeted elements as an effective lever for site and valence distributions and enhance catalytic performance.

Methods
Synthesis of spinel oxides and HEOs.
All chemicals were used as purchased without further purification. The metal precursors—chromium (III) nitrate nonahydrate, manganese (II) nitrate tetrahydrate, iron (II) nitrate hexahydrate, nickel (II) nitrate hexahydrate, and cobalt (II) nitrate (all from Sigma-Aldrich)—were used to prepare the spinel oxides. Absolute ethanol (99.5+%, Acros Organics) was used as the solvent. Potassium hydroxide (reagent grade, 90%, Sigma-Aldrich) was used as the electrolyte for electrochemical measurements. All spinel oxides studied in this work were synthesized using a previously reported, rapid sol–gel flame method. Equimolar concentrations (0.1 M) of each metal precursor were dissolved in ethanol and sonicated for 30 minutes to ensure complete mixing. The precursor solution was then spin-coated onto clean fluorine-doped tin oxide (FTO) glass substrates (7–8 Ω/sq, MSE Supplies) at 3000 rpm for 30 seconds, followed by annealing on a hot plate at 120 °C. This spin-coating and annealing process was repeated twice to achieve uniform film coverage. The precursor-coated FTO/glass substrates were annealed in the post-flame region of a co-flow premixed flat-flame burner (McKenna Burner). The burner operated using premixed methane (CH ) as fuel and air as oxidizer, with a fuel-to-oxygen equivalence ratio of 1.0. The substrates were positioned 3 cm above the burner surface and exposed to the flame for 45 seconds, during which the local gas temperature reached approximately 1000 °C, as measured by a K-type thermocouple (1/16 in. bead size, Omega Engineering, Inc.). After flame annealing, the samples were removed from the flame and allowed to cool naturally to room temperature (25 °C). This rapid high-temperature heating and cooling cycle facilitates homogeneous multi-elemental mixing and stabilizes the high-entropy spinel phases.

Materials characterizations.
The crystal structure of the synthesized samples was characterized using X-ray diffraction (XRD, PANalytical Empyrean) equipped with a Cu Kα radiation source (λ = 1.54 Å). Surface chemical composition and elemental quantification were analyzed by X-ray photoelectron spectroscopy (XPS, PHI VersaProbe 3) using a monochromated Al Kα source (1486 eV). High-resolution imaging, dark-field imaging, and elemental mapping via energy-dispersive X-ray spectroscopy (EDS) were performed using a scanning transmission electron microscope (STEM, Thermo Fisher Spectra).

Electronic structure analysis through synchrotron X-ray absorption spectroscopy at the K-edge
To investigate the bulk valence states of Fe and Co, X- ray absorption spectroscopy (XAS) measurements were performed in transmission mode at beamline 4-1 of the Stanford Synchrotron Radiation Lightsource (SSRL). Samples were sealed under argon in pouch cells to prevent air exposure. A Si (220) monochromator (ϕ = 90°) was used and detuned to 50–60% of the peak intensity to suppress higher-order harmonics. Photon energy calibration was carried out using Fe reference foils by setting the zero-crossing point of the second derivative of the absorption spectrum to 7112 eV. Three ion chambers arranged in series were used to record the incident (I ), transmitted (I ), and reference (I ) intensities. Spectrum alignment and normalization were performed using the Athena software package, while XAS data fitting and simulations were carried out using Artemis.

Electronic structure analysis through synchrotron X-ray absorption spectroscopy and X-ray magnetic circular dichroism at the L-edge
To obtain element-specific magnetic information, we performed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements at the L -edges of all transition metals using the total electron yield (TEY) detection mode. All experiments were carried out at beamline 13-3 of the Stanford Synchrotron Radiation Lightsource (SSRL). Measurements were conducted in normal incidence geometry with an incident angle set at 90°. The samples were cooled in a closed-cycle helium liquefied system and maintained at a base temperature of 13.5 K,48 which is well below Curie temperature (T ). We obtained the absorption spectra by reversing the X-ray polarity (right and left circular – RCP and LCP) of the incident photon beam, while a constant external magnetic field of approximately 0.4 T along the surface normal direction was applied using a neodymium permanent magnet. The XMCD signal was obtained by taking the difference between RCP and LCP spectra. To quantitatively determine the valence- and site-specific distributions of Co and Fe in the spinel lattice, Co and Fe L-edge XAS spectra were analyzed using a linear-combination fitting approach based on ligand-field multiplet–simulated reference spectra. Although experimentally measured Co and Fe L-edge spectra of representative spinel oxides were acquired and are shown for qualitative comparison (Supplementary Fig. 25, 30, 31), such reference materials inherently contain mixed tetrahedral and octahedral site occupancies as well as mixed oxidation states and therefore cannot serve as isolated standards for site-resolved quantification. Accordingly, site- and valence-specific reference spectra corresponding to Co3+(O ), Co2+(O ), Co2+(T ), Fe2+(O ), Fe3+(O ), or Fe3+(T ) were taken from well-established ligand-field multiplet–simulated datasets reported in the literature.49–51 For the quantification, all experimental L-edge spectra were energy-aligned, background-subtracted, and normalized to the edge jump prior to analysis. The experimental spectrum for each sample, W(E), was then modeled as a linear combination of the normalized reference spectra according to
𝑊(𝐸) = ∑ 𝑎 𝑋 (𝐸) (1)
, where X(E) denotes the reference spectra and a are non-negative fitting coefficients constrained to sum to unity. The fitting was performed by least-squares minimization using the Trust-Region Reflective algorithm implemented in MATLAB. The uncertainties in the extracted site fractions were determined as the 95% confidence intervals derived from the regression covariance matrix. XMCD spectra were additionally used as a complementary constraint to distinguish tetrahedral and octahedral contributions, owing to their distinct magnetic signatures.

Operando XAS on Co and Ni Redox Dynamics in Zn-HESO
Operando X-ray absorption spectroscopy (XAS) experiments were carried out at beamline 8.0.1 of the Advanced Light Source (ALS) located at Lawrence Berkeley National Laboratory (LBNL). During measurements, the storage ring operated at an energy of 1.9 GeV with a beam current close to 500 mA. Spectra were collected in total fluorescence yield (TFY) mode. The energy windows used for data acquisition were 774–790 eV for the Co L₃-edge and 845–860 eV for the Ni L -edge. To ensure accurate normalization, the incident X-ray intensity (I ) was continuously monitored using the drain current of an upstream gold-coated mesh while TFY signals were recorded simultaneously. A custom electrochemical flow cell was employed for operando measurements. The cell incorporated a 100 nm-thick silicon nitride membrane that maintained separation between the liquid electrolyte and the high-vacuum chamber. To prepare the working electrode, a 5 nm titanium adhesion layer was first deposited onto the membrane, followed by a 10 nm platinum conductive layer. The active material, Zn-HESO, was subsequently loaded onto the platinum surface through a drop-casting process. For ink preparation, Zn-HESO powder was dispersed in ethanol (3 mg mL-1) and ultrasonicated until a uniform suspension was obtained. A 10 μL portion of the suspension was then deposited onto the electrode surface and dried at 60 °C. This coating procedure was repeated 25 times to achieve sufficient loading, after which the electrode underwent thermal treatment at 150 °C for 1 hour.

Electrochemical testing was performed using a three-electrode configuration consisting of an Ag/AgCl reference electrode and a platinum counter electrode. All measurements were conducted in 0.05 M KOH electrolyte with a BioLogic SP-200 potentiostat. Images of the operando XAS electrochemical flow-cell assembly are presented in Supplementary Fig. 46.

Electrochemical measurement
All electrochemical measurements—including linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS)—were performed using a potentiostat (Interface 1010, Gamry Instruments) in a standard three-electrode setup. The electrolyte used was 30 mL of 1 M KOH (pH ~14), and its pH was measured using a calibrated benchtop pH meter (Mettler Toledo F20-Kit). Calibration was conducted prior to measurements using standard buffer solutions of pH 4, 7, and 10. The 1 M KOH electrolyte was prepared by dissolving potassium hydroxide (reagent grade, 90%, Sigma-Aldrich) in deionized water. The solution was freshly prepared prior to each experiment and stored in a tightly sealed polypropylene bottle to minimize CO absorption. The synthesized spinel oxides served as working electrodes with a geometric area of 0.196 cm-2. A platinum wire and an Hg/HgO electrode were used as counter and reference electrodes, respectively. All measured potentials versus Hg/HgO were converted to the reversible hydrogen electrode (RHE) scale using the equation:
E = E + 0.059 × pH + 0.098 (2)
LSV curves were recorded at a scan rate of 10 mV s-1 and corrected for 85% of the solution resistance (iR compensation). Overpotentials (η) were calculated as:
η=E −1.23 V (3)
Tafel plots were constructed by plotting overpotential (η) versus the logarithm of the current density (log j). Electrochemical impedance spectroscopy (EIS) was carried out over a frequency range of 20 kHz to 0.05 kHz at 1.54 V . The EIS measurements consist of solution resistance (R ), charge-transfer resistance (R ), and double-layer capacitance (C ). The HESO and M-HESO electrodes exhibit R values in the range of 15–25 Ω, which were used for the iR compensation.

Long-term electrochemical stability was evaluated using chronopotentiometry at initial current densities of ~10 and 100 mA cm-2. The electrochemical surface area (ECSA) was estimated via CV measurements in non-Faradaic regions at varying scan rates and calculated using the equation; ECSA=C /Cs, where C is the specific capacitance of a smooth surface in alkaline media, taken as 60 μF cm-2.

Electrical conductivity measurements
The electrical conductivity of pressed powder pellets was measured using a two-electrode configuration on a potentiostat (Interface 1010, Gamry Instruments). Powder samples were first pressed into cylindrical pellets using a uniaxial press at a load corresponding to 10 MPa. Before each measurement, the baseline resistance of the measurement system was determined by assembling the fixture without a sample. The fixture consisted of samples embedded in a Teflon die. The thickness between the metal plates was measured with calibrated digital calipers (repeated 5 times to average), ensuring that the calipers were zeroed before each measurement.

For conductivity measurements, the pellet was placed between the metal electrode and the stopper in the Teflon holder, then compressed to 10 MPa. The load was maintained for approximately 1 min to allow stabilization, with additional pressure applied if the value decreased. The sample thickness was measured at the same points as in the baseline measurement, and the baseline thickness was subtracted to obtain the net sample thickness.

Linear sweep voltammetry (LSV) was performed to determine the total system resistance, with sweep rate parameters adjusted according to the expected sample conductivity (e.g., 1000 mV s-1 for low-conductivity samples, 10 mV s-1 for high-conductivity samples).

The slope of the current–voltage curve was determined for both the baseline and the sample measurement. The sample contribution was calculated according to:
𝑠𝑎𝑚𝑝𝑙𝑒 𝑠𝑙𝑜𝑝𝑒 = (1/𝑠𝑎𝑚𝑝𝑙𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 𝑠𝑙𝑜𝑝𝑒 − 1/𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 𝑠𝑙𝑜𝑝𝑒) (4)
The electrical conductivity, 𝜎 (S cm⁻¹), was then obtained using:
σ = (𝑠𝑙𝑜𝑝𝑒 [A/V] × 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 [𝑐𝑚]) / 𝑎𝑟𝑒𝑎 [𝑐𝑚]2 (5)

Computational Details. We follow a similar DFT calculation scheme and parameters as in our previous HEO spinel work, which we briefly summarize here.40 We performed spin-polarized density functional theory calculations with a plane-wave basis set using the Vienna ab initio simulation package (VASP)52,53 and project augmented wave (PAW) pseudopotential,54 and we conducted and analyzed all calculations using the atomic simulation environment (ASE).55 We described electronic exchange and correlation using the Perdew-Burke-Ernzerhof functional.56 We applied the DFT+U method and set the energy cutoff to 600 eV for all calculations. The complex six-element Zn-HESO bulk structures were optimized using 5x5x5 k-point mesh sampling, and considered their bulk structures optimal when the energies and forces dropped below 10-5 eV and 0.02 eV/Å, respectively. Fully high-entropy Zn-HESO OER calculations were performed on (100) surfaces with three frozen atomic layers and two free layers. Initial magnetic moments were applied to achieve targeted spin states on each element by setting the occupation matrix of 3d-electron localization, followed by optimizing the geometry.57

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a thorough study on tuning cation site occupancy in high-entropy spinel oxides via incorporation of a sixth element (Zn, Ga, Al, Mg). Using synchrotron XAS/XMCD and DFT calculations, the authors demonstrated Zn's preference for tetrahedral sites, which redistributed Co to octahedral sites, enhancing the electronic conductivity and OER performance. This work is well-supported by extensive characterization data and addresses an important topic in catalyst design.\nHowever, the electrochemical analysis lacks rigor, with serious flaws in iR compensation, ECSA determination, and stability testing undermining the reliability of the performance claims. The justification for using high-entropy systems over simpler analogs also requires strengthening. While the core findings are valuable, these issues must be addressed to validate the conclusions.\n1. The manuscript should provide a stronger justification for employing HESO in this coordination site study, explicitly addressing why this complex platform offers advantages over simpler, well-defined binary or ternary spinel systems. Additionally, the potential influence of the sixth cation on other key OER-active elements (Ni, Fe) should be thoroughly addressed.\n2. The LSV curves in Figure 5a appear to be overcompensated, resulting in distorted data. The resistance value used for iR-correction must be determined by electrochemical testing, and the specific value should be clearly annotated in the figure. The inset in Figure 5g also shows clear signs of overcompensation.\n3. The Tafel slopes in Figure 5b appear to be derived from an inappropriate fitting range, potentially including data from both the low-current-density and high-current-density non-linear regions. This practice can distort the slope value and misrepresent the catalytic kinetics.\n4. The data presented do not sufficiently support the conclusion that Zn-HESO is the optimal catalyst. As shown in Figure 5a, Ga-HESO clearly demonstrates superior activity at lower current densities, an observation further compounded by the apparently overcompensated LSV curve of Zn-HESO. This performance trend is corroborated by the Tafel analysis in Figure 5b. The authors should either re-perform these electrochemical measurements to ensure data integrity or provide additional analyses to substantiate the claimed performance hierarchy.\n5. For the operando sXAS analysis in Figure 6, a photograph of the actual electrochemical flow cell setup used for the measurements should be provided in the supplementary information.\n6. The selection of scan rates (20, 50, 100, 200, and 300 mV/s) in Supplementary Fig. 29 appears to be nonstandard for capacitance determination, and the varying voltage windows across different materials undermine meaningful ECSA comparisons. The absence of 20 mV/s data for HESO requires explanation. These issues should be addressed to validate the ECSA analysis.\n7. The abrupt change in the chronopotentiometry data at approximately 70 hours in Figure 5g requires explanation.", "reviewer_2": "This manuscript presents an extensive study on coordination-site engineering in high-entropy spinel oxides of Co-Fe-Cr-Mn-Ni by introducing an element of Zn, Ga, Mg, or Al. The authors carried out XAS/XMCD, DFT calculations, and electrochemical measurements to show that Zn incorporation redistributes Co from tetrahedral to octahedral sites enhancing OER activity. The concept of entropy-driven site modulation is very interesting and innovative. I recommend that this manuscript can be considered for publication in Nature Communications after appropriate revisions. Several key points need to be reconsidered or revised, as detailed below.\n1) While the authors mention that Co L3-edge and Fe L3-edge XAS spectra were obtained for reference samples of Co3+ (Oh), Co2+(Oh), Co2+(Td), Fe2+(Oh), Fe3+(Oh), and Fe3+(Td)” in Fig. 2, the manuscript does not specify which compounds were used to obtain these reference spectra. The corresponding reference materials should be clearly described in the main manuscript.\n2) All relevant spinel-type reference compounds should be measured by both hard XAS and soft XAS for all elements. For example, in the case of Fe K-edge and Fe L-edge XAS, MnFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4 would be useful to discuss Fe valence and tetrahedral/octahedral coordination. Similarly, for Ni K-edge and L-edge XAS, NiFe2O4 is also informative. I recommend that the authors discuss the valence states and Td/Oh coordination environments in more detail using such references.\n3) In the Co K-edge and Fe K-edge XAFS spectra in Figures S9 and S10, the differences among Zn-, Ga-, Mg-, Al-substituted and pristine HESO samples are not clearly visible.\nIt is difficult to identify the valence shift of Co and Fe induced by the sixth-element doping.\n4) In the EXAFS spectra in Figures S9-S13, spectra of appropriate reference compounds should be included for comparison of local coordination structures.\n5) Did the authors simulate the second-nearest coordination shell around 3A in Figure S14? This region appears to change depending on the type of sixth-element doping in Figures S9-S13.\n6) The authors should perform detailed curve-fitting simulations for all samples (Zn-, Ga-, Mg-, Al-doped and pristine HESO) to quantitatively evaluate the structural parameters.\n7) In this study, structural information of Zn and Ga is crucial. Please measure the Zn and Ga K-edge EXAFS spectra to confirm the local coordination of each element. The central hypothesis relies on Zn occupying Td sites, but the manuscript currently does not provide direct experimental verification.\n8) While the authors claim a clear redistribution of Co and Fe between tetrahedral and octahedral sites upon Zn substitution, the quantitative reliability of the XAS/XMCD fitting is not fully demonstrated. The deconvolution and fitting procedures should be described in sufficient detail. This is quite important.\n9) In Figure 6, the operando XAS spectra are not very clear due to the broad peaks. In the Co L3-edge spectra, a new feature appears near 782 eV at 0.8 V in the final measurement.\n10) The authors should compare the operando Co K-edge and Ni K-edge XAFS spectra with the corresponding Co L-edge and Ni L-edge spectra in terms of valence shifts and local structural changes. Reference spectra of Co(OH)2, CoOOH, Ni(OH)2, and NiOOH would be useful for this discussion.\n11) The authors should clarify how the L-edge XAS spectra allow distinguishing between octahedral and tetrahedral sites, since these spectral features are often similar and difficult to separate.", "reviewer_3": "In this study, Baet et al. investigate the effect of introducing a sixth cation into a Co-Fe-Cr-Mn-Ni HESO to modulate cation site occupancy. Among five candidate cations examined, Zn is identified as the most effective in altering the distribution of Co2+ and Co3+ ions between Td and Oh sites. The authors employ a combination of advanced characterization techniques and theoretical calculations to build a fundamental understanding of coordination environment control in HEOs. While the study is intriguing and the narrative is generally well-constructed, the OER performance of the optimized Zn-HESO catalyst falls short of the benchmark expected for publication in this journal. Therefore, despite the scientific merit of the work, I cannot recommend its publication in its current form. My major concerns are detailed below.\nMajor Comments:\n1. The authors should provide details regarding the identification of Td and Oh sites in these HEOs via STEM.\n2. TEM images for Al-HESO are missing. Based on HR-TEM, the authors identified that the interplanar spacing at the (311) plane for Mg-HESO and Ga-HESO is larger than that of HESO. However, the XRD patterns show that the (311) diffraction peak for both Mg-HESO and Ga-HESO shifts to higher angles, indicating a decrease in their interplanar spacing. Please clarify this apparent discrepancy.\n3.In this work, the authors synthesized four distinct cation-engineered materials: Mg-HESO, Ga-HESO, Al-HESO, and Zn-HESO, each with different site occupancies. However, the discussion section primarily focuses on comparisons between HESO and Zn-HESO, lacking a detailed comparative discussion highlighting the differences among these high-entropy oxides.\n4.The iR-corrected OER polarization curves presented in this manuscript show that Zn-HESO requires an overpotential of 295 mV to achieve 10 mA cm⁻2. Furthermore, the demonstrated stability duration is relatively short. Compared to similar OER studies published in Nature Communications, these performance metrics do not appear competitive.\n5.The combined experimental and theoretical results indicate that Co3+ at the Oh site in Zn-HESO is the primary active center. Given this, is there an optimal proportion for the Co3+ Oh content in Zn-HESO?\nMinor Comments:\n1. The abbreviation ‘HEOs’ is used without being defined upon its first appearance in the introduction part.\n2.The elemental percentages/compositions for Mg-HESO, Ga-HESO, and Al-HESO should also be provided, as demonstrated in supplementary Fig. 8."}, "author_response": "Point-by-point response letter for the reviewers’ comments\nWe sincerely appreciate all reviewers providing valuable comments. We have conducted\nadditional experimental characterizations to respond to reviewers’ comments. We believe the\nreviewers’ comments improved our manuscript significantly. Our response to reviewer\ncomments will be in blue color and revised parts in red color, which are highlighted in yellow\ncolor in the revised manuscript.\nReviewer #1 (Remarks to the Author):\nThis manuscript presents a thorough study on tuning cation site occupancy in high-entropy\nspinel oxides via the incorporation of a sixth element (Zn, Ga, Al, Mg). Using synchrotron\nXAS/XMCD and DFT calculations, the authors demonstrated Zn's preference for tetrahedral\nsites, which redistributed Co to octahedral sites, enhancing the electronic conductivity and\nOER performance. This work is well-supported by extensive characterization data and\naddresses an important topic in catalyst design. However, the electrochemical analysis lacks\nrigor, with serious flaws in iR compensation, ECSA determination, and stability testing\nundermining the reliability of the performance claims. The justification for using high-entropy\nsystems over simpler analogs also requires strengthening. While the core findings are valuable,\nthese issues must be addressed to validate the conclusions.\nResponse: We sincerely appreciate the reviewer for the careful and constructive evaluation of\nour manuscript. We thank the recognition of the value of our coordination-site tuning concept\nand the strength of the spectroscopic and computational analysis. At the same time, we fully\nacknowledge the reviewer’s concerns regarding the rigor of electrochemical measurements and\nthe need for a clearer justification for employing a high-entropy spinel platform. In the revised\nmanuscript, we have thoroughly addressed these points by (i) re-visiting the uncompensated\nresistance and all iR-corrected LSV curves, (ii) standardizing the ECSA protocol and repeating\nthe capacitance measurements, (iii) describing the stability data with clarified procedures, and\n(iv) expanding the rationale for using HESO by providing mechanistic evidence showing\nentropy-enabled cation redistribution not accessible in simpler spinel oxides. These revisions\ngreatly strengthen the reliability and interpretability of electrochemical analysis and the overall\nconclusions. We are grateful for the reviewer’s feedback, which has helped us to significantly\nimprove the manuscript.\n1\n1. The manuscript should provide a stronger justification for employing HESO in this\ncoordination site study, explicitly addressing why this complex platform offers advantages over\nsimpler, well-defined binary or ternary spinel systems. Additionally, the potential influence of\nthe sixth cation on other key OER-active elements (Ni, Fe) should be thoroughly addressed.\nResponse: We appreciate the reviewer for this important comment. We have significantly\nexpanded the justification for employing a high-entropy spinel oxide (HESO) platform for\ncoordination-site engineering.\n- Why HESO is essential for this coordination-site engineering study:\nAlthough four core effects and consequent structural stability are a well-established advantage\nof high-entropy oxides, our main motivation extends beyond structural stability. A HESO\nlattice provides a unique chemical environment that enables us to probe coordination-site\nmodulation in ways that binary or ternary spinels cannot. We explicitly clarify the following\npoints in the revised manuscript.\nWe believe high configurational entropy enables a flexible cation rearrangement. Our XAS\n(XANES, EXAFS) and XMCD analyses show a substantial Co redistribution from T to O\nd h\nsites upon Zn substitution (as well as Ga due to their higher tetrahedral preference energy than\nCo), as shown in Fig. 2a-c, and Supplementary Figs. 17, 18, 26. Importantly, while Zn\nintroduction in simple spinel oxides (e.g., CoCr O , CoMn O , CoFe O ) may also drive Co\n2 4 2 4 2 4\nmigration from T to O sites, such redistribution often forces large and localized changes in\nd h\nvalence states that destabilize the lattice and accelerate cation dissolution during OER. In\ncontrast, the high-entropy spinel lattice possesses multiple redox-active and redox-stable\ntransition elements that collectively buffer the charge redistribution. The other elements\ncompensate for valence changes, while Ni and Cr remain electronically invariant anchor\ncations. This multi-element charge-compensation capability allows the HESO system to\naccommodate substantial Co site redistribution without compromising structural stability.\nFurthermore, the HESO system is advantageous over a simple spinel Co system because the\nactivity of the OER sites has a wider distribution in HESOs, allowing the activity of some of\nthe Co active sites to be higher than in Co O , which has one fixed theoretical overpotential\n3 4\nvalue. Thus, HESO enables simultaneous enhancement of activity (through Co (O ) enrichment)\nh\nand preservation of lattice robustness, which is an emergent benefit not accessible in simpler\nspinel oxides.\n- Influence of the sixth cation on the other elements, especially Fe and Ni:\nthe other elements. First, Fe XAS analysis (Fig. 2d-f and Supplementary Figs. 19, 20, 25,\nand 26) shows that Fe exhibits partial redistribution toward octahedral sites upon Zn\nsubstitution, whereas Ni and Cr remain electronically stable, consistent with its strong Oh-site\npreference. We expect that Mn can be the valence buffer for this redistribution due to its various\nvalence states. These findings support the interpretation that Co and Fe are responsive to\nentropy-driven perturbations.\nRegarding the reviewer’s request to clarify how the sixth cation influences other OER-relevant\nelements (Ni and Fe), we have expanded our discussion accordingly. For Fe, we note that the\nmanuscript already includes a quantitative correlation between Fe valence/site redistribution\nand OER performance (Supplementary Fig. 41). This analysis shows that Zn substitution\ninduces a modest but measurable shift of Fe toward the octahedral environment, but we could\nnot conclude its correlation with the OER activity due to its vague tendency. For Ni, a\ncorrelation plot is not presented because our analysis found that Ni2+ consistently occupies\noctahedral sites in spinel oxides and does not undergo site redistribution upon doping. This\nbehavior is confirmed by XAS (Supplementary Fig. 28), where Ni exhibits invariant spectral\nfeatures across all M-HESOs. Ni therefore acts as a structurally stable anchor cation whose\ncoordination environment remains unchanged even under entropy-driven perturbations.\nWe emphasize that we do not claim that Co (O ) is the only active site in the system. Rather,\nh\namong the multiple cations present in the high-entropy lattice, Co in octahedral coordination\nemerges as the statistically dominant active center, as supported by the strongest correlation\nwith activity descriptors and the most pronounced spectral evolution under Zn substitution.\nComputationally, we also note that Zn inclusion does not strongly influence the Ni occupation,\nas Ni strongly prefers octahedral sites even without Zn. We see a split between Fe occupation\nin octahedral and tetrahedral sites, both with and without Zn. We also calculate the OER activity\nof a Ni and Fe active site for the fully high entropy model case of (Co2Zn2) (Cr3Mn4Fe3Ni2) ,\nTd Oh\nshown in Fig. 4. They both show high overpotentials compared to Co active sites, with the Fe3+\nsite having an overpotential of 760 mV and the Ni2+ site having an overpotential of 1120 mV\n(compared to a range of 350-540 mV for Co3+ sites for the fully high entropy model).\nThis\nindicates that Fe and Ni are not the primary OER active sites in the Zn-HESO; as such, they\nare not explored further for overpotential changes caused by the Zn doping in Zn-HESO. We\nhave added more details to the manuscript accordingly.\nintroduction to drive significant Co migration without compromising structural stability due to\nmultiple redox-active and redox-stable transition metals that collectively buffer the charge\nredistribution.”\n[Page 8] “We used the fully high-entropy models to evaluate Zn-HESOs with Co at O . The\nh\ncalculated overpotentials of Co3+ sites range from 350-540 mV, compared to 410-940 mV for\nCo sites in the local impurity model, indicating a smaller lower bound than the fully high-\nentropy model. The Co3+ O in the complex Zn-HESO model has an average overpotential of\nh\n453 mV, about 788 mV lower than that for Co2+. We also considered the possibility of other\ncations as potential active sites, with the most stable Zn-HESO with Co T , where it is not\nd\navailable as an active site. Among the calculated activities of four O sites (Ni, Fe, Mn, Cr), Cr\nh\nhas the lowest overpotential of 670 mV, but it is still larger than the best Co3+. The\noverpotentials of Fe, Mn, and Ni sites are 760 mV, 940 mV, and 1120 mV, respectively.\nAltogether, these results indicate that Co3+ O is the most active site for OER in Zn-HESO.\nh\nSince Zn-HESOs have a slightly higher amount of Co3+ O than HESOs (Fig. 2c), we expect\nh\nthat Zn-HESO will be more active for OER (Figs. 5a-5c). While more Co3+ O sites will\nh\nincrease the activity, aligning with our expectations of improvement of Zn-HESO compared to\npristine HESO, it is also important to note that the likely optimal Co occupation is less than\n100%, as having the high entropy structure does lead to lower overpotentials than a pure Co\nspinel.”\n2. The LSV curves in Figure 5a appear to be overcompensated, resulting in distorted data. The\nresistance value used for iR-correction must be determined by electrochemical testing, and the\nspecific value should be clearly annotated in the figure. The inset in Figure 5g also shows clear\nsigns of overcompensation.\nResponse: We thank the reviewer for raising this important point regarding iR compensation.\nWe agree that inappropriate correction can lead to distorted polarization curves, and therefore\nwe carefully revisited our iR-correction protocol.\nIn our analysis, the uncompensated resistance (R ) for each electrode was experimentally\nu\ndetermined by electrochemical impedance spectroscopy, following the standard procedure\ndescribed in the Gamry technical note on iR compensation. Specifically, R was extracted from\nu\n4\nthe high-frequency intercept of the Nyquist plot, where capacitive and faradaic contributions\nare negligible. These values were then used for post-measurement of iR correction.\nWe applied a partial iR correction of 85%, rather than full compensation, to account for ohmic\nlosses while avoiding potential amplification of noise and instability that can arise from full\ncorrection under dynamic OER conditions. This iR correction lies within the commonly\naccepted range (≈85–95%) used in the different OER literature. Importantly, the same\ncorrection protocol was applied consistently to all samples, ensuring that the relative activity\ntrends are not affected by the choice of correction percentage.\nIndeed, partial or full iR compensation is commonly applied in recent OER studies when the\nuncompensated resistance is well defined (Fig. R1). For example, Wu et al. (Nat. Mater. 22,\n100–108, 2023) applied 100% iR compensation for all measured potentials in rotating disk\nelectrode tests (Fig. R1a). Similarly, Zhu et al. (Nat. Commun. 14, 5365, 2023) reported 95%\niR-compensated linear sweep voltammetry curves (Fig. R1b). We still agree that, in some\nreports, aggressive correction schemes are adopted to emphasize intrinsic catalytic kinetics,\nwhich can lead to visually steeper polarization curves.\nIn our study, we now explicitly annotate the experimentally determined Ru values and the\napplied iR-correction percentage in the captions and Methods section of the revised manuscript.\nRegarding the inset in Fig. 5g, we note that its appearance is sensitive to axis scaling; however,\nthe same iR-correction protocol based on EIS-derived Ru was applied consistently. We believe\nthat these clarifications adequately address the reviewer’s concern and strengthen the rigor of\nelectrochemical analysis.\nRevision: [Method section, Page 16] “LSV curves were recorded at a scan rate of 10 mV s⁻¹\nand corrected for 85% of the solution resistance (iR compensation). Overpotentials (η) were\ncalculated as:\nη=E −1.23 V\nRHE\nTafel plots were constructed by plotting overpotential (η) versus the logarithm of the current\ndensity (log j). Electrochemical impedance spectroscopy (EIS) was carried out over a\nfrequency range of 20 kHz to 0.05 kHz at 1.54 V . The EIS measurements consist of solution\nRHE\nresistance (R ), charge-transfer resistance (R ), and double-layer capacitance (C ). The HESO\ns ct dl\nand M-HESO electrodes exhibit R values in the range of 15–25 Ω, which were used for the iR\ns\ncompensation.”\n(85% iR-corrected) showing OER performance of various substituted-HESO samples\ncompared with reference catalysts CoFe O and Co O in alkaline media. b Corresponding\n2 4 3 4\nTafel plots derived from polarization curves, indicating the reaction kinetics of OER for each\ncatalyst; Tafel slopes (mV dec-1) obtained from linear fitting are included. c Nyquist plots from\nEIS measurements, with an equivalent circuit inset showing electrolyte resistance (R ), charge-\ns\ntransfer resistance (R ), and double-layer capacitance (C ). The HESO and M-HESOs anodes\nct dl\nshow the Rs ranges of 15-25 Ω. d Conductivity measurements (σ), comparing Zn-HESO and\nHESO. e Comparison of overpotential at 10 mA cm-2 (η ), at 100 mA cm-2 (η ), and Tafel\n10 100\nslopes for various catalysts from this work against previously reported state-of-the-art high-\nentropy catalysts. f Correlation plots showing relationships between Co3+ or Co2+ in octahedral\nsite occupancy and catalytic overpotential at 100 mA cm-2 showing structure-performance\ntrends. g Long-term stability tests performed by chronopotentiometry at constant current\n6\ndensities (10 mA cm-2 and 100 mA cm-2), showing superior stability of Zn-HESO compared to\npristine HESO; inset shows the LSV curves before and after the stability test.\n[FIGURE REDACATED]\nFigure R1. Representative examples of iR-corrected OER polarization curves reported in\nrecent literature.\n3. The Tafel slopes in Figure 5b appear to be derived from an inappropriate fitting range,\npotentially including data from both the low-current-density and high-current-density non-\nlinear regions. This practice can distort the slope value and misrepresent the catalytic kinetics.\nResponse: We thank the reviewer for this insightful comment. We agree that the original Tafel\nanalysis may have included portions of the LSV curves outside the linear kinetic region. To\naddress this, we re-processed all Tafel plots using a more rigorous fitting protocol. Specifically,\nwe identified the true kinetic region by excluding (i) the low-current region where the log(j)–\nη relation is non-linear and (ii) the high-current region where mass transport and bubble\naccumulation can cause deviations. The revised Tafel slopes were determined exclusively from\nthe linear middle-current region (typically 10 mA cm-2), in accordance with standard\nelectrochemical practice. Fig. 5b has been revised accordingly.\nRevision: [Page 9] “The kinetic advantage of Zn-HESO is further evidenced by its lowest Tafel\nslope (25 mV dec-1) compared to Ga-HESO (37 mV dec-1), pristine HESO (33 mV dec-1), Mg-\nHESO (40 mV dec-1), and Al-HESO (45 mV dec-1) (Fig. 5b). This trend is directly reflected in\n7\nthe systematic increase in Tafel slopes from Zn- to Al-substituted systems, indicating\nprogressively slower OER kinetics.”\n4. The data presented do not sufficiently support the conclusion that Zn-HESO is the optimal\ncatalyst. As shown in Figure 5a, Ga-HESO clearly demonstrates superior activity at lower\ncurrent densities, an observation further compounded by the apparently overcompensated LSV\ncurve of Zn-HESO. This performance trend is corroborated by the Tafel analysis in Figure 5b.\nThe authors should either re-perform these electrochemical measurements to ensure data\nintegrity or provide additional analyses to substantiate the claimed performance hierarchy.\nResponse: We appreciate the reviewer for pointing out the better performance of Ga-HESO at\nlow current densities. We agree that Ga is another good example of a sixth-element candidate\nand that its slightly lower onset activity originates from its relatively strong tetrahedral site\npreference energy, which drives pronounced redistribution of Co and Fe toward octahedral\ncoordination. This effect is consistent with our XAS/XMCD analysis and aligns well with the\nreviewer’s observation.\nHowever, while Ga-HESO exhibits favorable activity at low current densities, the overall\ncatalytic performance must also be evaluated in terms of reaction kinetics and charge-transport\nproperties. As shown in Fig. 5b, Zn-HESO exhibits a substantially lower Tafel slope (25.2 mV\ndec-1) compared to Ga-HESO (36.7 mV dec-1), indicating more favorable OER kinetics. In\naddition, Zn-HESO displays higher bulk conductivity and lower charge-transfer resistance (Fig.\n5c,d), which becomes increasingly important as the current density increases.\nAs a result, although Ga-HESO shows slightly better onset behavior, Zn-HESO consistently\noutperforms Ga-HESO in the moderate-to-high current density regime (~100 mA cm-2), which\nis more relevant to practical OER operation. These advantages arise from the combined effects\nof efficient Co (O ) enrichment, improved electronic transport, and cooperative multi-cation\nh\nredistribution enabled by Zn incorporation.\nAccordingly, we emphasize that Ga-HESO is a promising formulation, but when considering\nboth intrinsic kinetics and high-current performance, the collective electrochemical and\nstructural evidence supports Zn-HESO as the optimal catalyst within the investigated\ncomposition space. We have clarified this comparison and explicitly discussed the role of Tafel\nslopes in the revised manuscript.\noverpotential at low current densities, consistent with its relatively strong tetrahedral site\npreference energy, which promotes effective redistribution of Co and Fe into octahedral\ncoordination. However, this advantage is primarily observed in the low-current region. In\ncontrast, Zn-HESO exhibits the lowest average overpotentials at both 10 mA cm-2 (η , 295\n10\nmV) and 100 mA cm-2 (η , 314 mV) (Supplementary Fig. 37), indicating superior\n100\nperformance across a broader operational current range. The kinetic advantage of Zn-HESO is\nfurther evidenced by its lowest Tafel slope (25 mV dec-1) compared to Ga-HESO (37 mV dec-\n1), pristine HESO (33 mV dec-1), Mg-HESO (40 mV dec-1), and Al-HESO (45 mV dec-1) (Fig.\n5b). This trend is directly reflected in the systematic increase in Tafel slopes from Zn- to Al-\nsubstituted systems, indicating progressively slower OER kinetics.\nIn parallel, electrochemical impedance spectroscopy (Fig. 5c) shows that Zn-HESO possesses\nthe smallest charge transfer resistance (R ) from the Nyquist semicircle in electrochemical\nct\nimpedance spectroscopy (EIS, Fig. 5c), and the highest double-layer capacitance (C ) of 1.8\ndl\nmF cm-2 (Supplementary Figs. 38-39), indicating larger electrochemical surface area (ECSA)\nof ~6 cm2.”\n5. For the operando sXAS analysis in Figure 6, a photograph of the actual electrochemical flow\ncell setup used for the measurements should be provided in the supplementary information.\nResponse: We thank the reviewer for this helpful suggestion. A photograph of the operando\nXAS electrochemical flow-cell setup used at the beamline has now been included in the\nSupplementary Information (Supplementary Fig. 45). We believe this addition provides a\nclearer context for the measurement configuration.\nRevision: [Method section, Page 15] “All operando X-ray absorption spectroscopy (XAS)\nmeasurements were performed on the beamline 8.0.1 of the Advanced Light Source (ALS),\nLawrence Berkeley National Laboratory (LBNL). The storage ring was operated at 1.9 GeV\nwith a beam current of approximately 500 mA. XAS spectra were recorded in the total\nfluorescence yield (TFY). Co L -edge and Ni L -edge spectra were acquired over the\n3 3\ncorreponding energy ranges of 774-790 eV and 845-860 eV, respectively. The incident beam\nintensity (I ) was measured simultaneously with TFY signals via the drain current from an\n0\nupstream Au-coated mesh and used for signal intensity normalization. The electrochemical\nflow cell features a 100 nm-thick Si N membrane window, which separates the liquid sample\n3 4\n9\nenvironment from the high-vacuum condition of the experimental chamber. A 5 nm Ti adhesive\nlayer was first deposited on the Si N window, followed by a 10 nm Pt conductive layer\n3 4\ndeposition to serve as the working electrode. Zn-HESO was deposited onto the Pt layer via the\ndrop-casting method. Specifically, Zn-HESO was dispersed in ethanol at a concentration of 3\nmg/mL, sonicated to form uniform ink, and 10 μL of the ink was drop-casted onto the electrode\nand then dried on a 60 °C hotplate. This process was repeated for 25 times, and the electrode\nwas then annealed at 150 °C for 1 hour. The electrochemical cell included an Ag/AgCl\nreference electrode and a Pt counter electrode. All electrochemical measurements were carried\nout in 0.05 M KOH using a BioLogic SP-200 potentiostat. The photographs of the XAS\nelectrochemical flow-cell setup are shown in Supplementary Fig. 45.”\nSupplementary Fig. 45 Photograph of the operando XAS electrochemical flow-cell setup. a\ncell sealing parts with 1*1cm wafer in the center as working electrode, b Inner side of the flow\ncell with Pt wire counter electrode, Ag/AgCl reference electrode, and a metal ring as contact\nfor working electrode, c Outside view of the flow-cell, d sample holder view after assembly of\nparts a and b, with 1*1mm Si N window in the center for X-ray transmission.\n3 4\n10\n6. The selection of scan rates (20, 50, 100, 200, and 300 mV/s) in Supplementary Fig. 29\nappears to be nonstandard for capacitance determination, and the varying voltage windows\nacross different materials undermine meaningful ECSA comparisons. The absence of 20 mV/s\ndata for HESO requires explanation. These issues should be addressed to validate the ECSA\nanalysis.\nResponse: We thank the reviewer for pointing out the nonstandard scan-rate selection and the\ninconsistent potential windows in our original capacitive-current measurements, which could\ncompromise a meaningful ECSA comparison. In response, we repeated the entire C and ECSA\ndl\nmeasurement using a standardized protocol across all catalysts.\nSpecifically, we (i) selected non-Faradaic potential windows that are free of discernible redox\nfeatures for all samples, and (ii) recorded CVs at a consistent set of scan rates (10, 20, 50, and\n100 mV/s), which are commonly used for double-layer capacitance extraction and minimize\nnon-capacitive contributions (e.g., iR drop and pseudocapacitance) that can become\npronounced at higher scan rates. Importantly, the previously missing 20 mV/s CV for HESO\nhas now been newly acquired under the same conditions, and all datasets were processed using\nan identical fitting procedure. For the updated analysis, the capacitive current was extracted at\nthe midpoint potential within the selected non-Faradaic region and plotted as Δj = (j −\nanodic\nj )/2 versus scan rate; C was obtained from the slope of the linear fit. The revised CV\ncathodic dl\ncurves and corresponding linear fittings are provided in Supplementary Figs. 38–39, where\nall samples are now directly comparable because they were measured within the non-Faradaic\npotential window, at the same scan rates, and analyzed with the same data-processing workflow.\nIn addition, the detailed discussion has been added to the main manuscript to reflect the updated\nECSA-normalized analysis.\nRevision: [Page 9] “the double-layer capacitance (C ) of 1.8 mF cm-2 (Supplementary Figs.\ndl\n38-39), indicating a larger electrochemical surface area (ECSA) of ~6 cm2. Importantly, after\nnormalization by ECSA, Zn-HESO still exhibits a comparable or higher current density in the\nOER-relevant potential region. This observation suggests that factors beyond the\nelectrochemically accessible surface area may influence the measured current density, such as\ndifferences in intrinsic catalytic reactivity, charge-transfer kinetics, surface electronic structure,\nor local reaction environments under OER conditions, as well as variations in ion accessibility,\nand the effective utilization of the catalyst surface.”\nGa-HESO, and e Zn-HESO at scan rates of 10, 20, 50, and 100 mV/s\nSupplementary Fig. 39 a Double-layer capacitance calculated extracted from scan-rate-\ndependent CV measurements, b ECSA, and c normalized current densities by ECSA for\ndifferent spinel oxides, including pristine and M-HESOs.\n7. The abrupt change in the chronopotentiometry data at approximately 70 hours in Figure 5g\nrequires explanation.\nResponse: We thank the reviewer for raising this point. The apparent potential change observed\nat around 70 h corresponds to a routine maintenance step during the long-term\nchronopotentiometry measurement, during which accumulated oxygen bubbles were removed\n12\nfrom the electrode surface and the electrolyte was refreshed. This procedure was performed to\nensure stable mass transport over prolonged operation and does not indicate catalyst\ndegradation. We have clarified this point and added a more detailed description of the stability\ntest protocol in the revised manuscript.\nRevision: [Page 10] “A slight and abrupt potential change observed at approximately 70 h\narises from a routine maintenance step during the measurement, during which accumulated\noxygen bubbles were removed from the electrode surface and the electrolyte was refreshed to\nensure stable mass transport, rather than from catalyst degradation. Extended measurements\nfurther confirm similar stability up to ~120 hours at 10 mA cm-2 (Supplementary Fig. 42).”\nThis manuscript presents an extensive study on coordination-site engineering in high-entropy\nspinel oxides of Co-Fe-Cr-Mn-Ni by introducing an element of Zn, Ga, Mg, or Al. The authors\ncarried out XAS/XMCD, DFT calculations, and electrochemical measurements to show that\nZn incorporation redistributes Co from tetrahedral to octahedral sites, enhancing OER activity.\nThe concept of entropy-driven site modulation is very interesting and innovative. I recommend\nthat this manuscript be considered for publication in Nature Communications after appropriate\nrevisions. Several key points need to be reconsidered or revised, as detailed below.\nResponse: We sincerely appreciate the reviewer for the positive evaluation of our work and for\nrecognizing the novelty of entropy-driven coordination-site modulation in high-entropy spinel\noxides. We appreciate the constructive suggestions provided. In the revised manuscript, we\nhave addressed all points by clarifying the reference compounds, expanding the XAS/EXAFS\nanalysis, including Zn and Ga K-edges, and strengthening the discussion of Td/Oh site\nassignment. These revisions improve the rigor and completeness of our structural interpretation.\nWe are grateful for the reviewer’s supportive recommendation.\n1) While the authors mention that Co L3-edge and Fe L3-edge XAS spectra were obtained for\nreference samples of Co3+(Oh), Co2+(Oh), Co2+(Td), Fe2+(Oh), Fe3+(Oh), and Fe3+(Td)”\nin Fig. 2, the manuscript does not specify which compounds were used to obtain these reference\nspectra. The corresponding reference materials should be clearly described in the main\nmanuscript.\nResponse: We thank the reviewer for requesting clarification on the reference spectra and the\ndeconvolution procedure used for the Co and Fe L -edge analysis. We have acquired Co and\n25, 30, and 31 for qualitative comparison. However, as widely recognized in ferrite and spinel\nliterature, such experimental reference materials intrinsically contain mixed tetrahedral and\noctahedral site occupancies as well as mixed oxidation states and therefore cannot serve as\nisolated standards for site- and valence-resolved analysis.\nTo enable quantitative deconvolution, we followed established methodologies reported for\nspinel ferrites and high-entropy spinel oxides and employed ligand-field multiplet–simulated\nL-edge reference spectra corresponding to Co3+(O ), Co2+(O ), Co2+(T ), Fe2+(O ), Fe3+(O ),\nh h d h h\nor Fe3+(T ). These reference spectra were generated using ligand-field multiplet calculations\nd\n14\nthat explicitly include crystal-field symmetry (O vs T ), spin–orbit coupling, and metal–ligand\nh d\ncharge-transfer effects, as described in prior studies (Ref. 49-51: J. Am. Chem. Soc. 2022, 144,\n20590–20600; Acta Mater. 226 (2022) 117581; J. Magn. Magn. Mater. 460 (2018) 243–252).\nAll experimental L-edge spectra were energy-aligned and normalized to the L -edge area.\nexperimental spectrum was modeled as a weighted sum of the simulated reference spectra. The\nfitting coefficients were constrained to be positive and to sum to unity. XMCD spectra were\nused as an additional constraint, as tetrahedral and octahedral sites exhibit distinct XMCD\nsignatures and opposite magnetic contributions. The uncertainties in the extracted site fractions\nwere estimated from the covariance matrix of the fits and from reproducibility across repeated\nmeasurements.\nWe have now explicitly described this procedure, the origin of the reference spectra, and the\nrelevant literature sources in the main manuscript and Methods section. We believe that this\nclarification addresses the reviewer’s concern and demonstrates the quantitative reliability of\nthe L-edge deconvolution.\nRevision: [Page 14] “To quantitatively determine the valence- and site-specific distributions\nof Co and Fe in the spinel lattice, Co and Fe L-edge XAS spectra were analyzed using a linear-\ncombination fitting approach based on ligand-field multiplet–simulated reference spectra.\nAlthough experimentally measured Co and Fe L-edge spectra of representative spinel oxides\nwere acquired and are shown for qualitative comparison (Supplementary Fig. 25, 30, 31),\nsuch reference materials inherently contain mixed tetrahedral and octahedral site occupancies\nas well as mixed oxidation states, and therefore cannot serve as isolated standards for site-\nresolved quantification. Accordingly, site- and valence-specific reference spectra\ncorresponding to Co3+(O ), Co2+(O ), Co2+(T ), Fe2+(O ), Fe3+(O ), or Fe3+(T ) were taken from\nh h d h h d\nwell-established ligand-field multiplet–simulated datasets reported in the literature.49–51”\n2) All relevant spinel-type reference compounds should be measured by both hard XAS and\nsoft XAS for all elements. For example, in the case of Fe K-edge and Fe L-edge XAS,\nMnFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4 would be useful to discuss Fe valence and\ntetrahedral/octahedral coordination. Similarly, for Ni K-edge and L-edge XAS, NiFe2O4 is\nalso informative. I recommend that the authors discuss the valence states and Td/Oh\ncoordination environments in more detail using such references.\nspinel-type reference compounds in both hard and soft X-ray absorption spectroscopy (XAS)\nto more rigorously discuss cation valence states and tetrahedral/octahedral (Td/Oh)\ncoordination environments.\nFollowing this recommendation, we clarify that an extensive set of reference-based analyses\nhas now been incorporated and expanded throughout the revised Supplementary Information.\nFirst, hard XAS analyses for Fe, Co, Mn, Cr, and Ni are already presented in Supplementary\nFigs. 10–14, where K-edge XANES and EXAFS trends are compared across all substituted\nHESO samples. To further strengthen the reference-based interpretation, we newly added\nexpanded XANES half-step edge analyses for Fe and Ni (Supplementary Fig. 15) and\nadditional EXAFS fitting results using spinel-type references (Supplementary Fig. 16).\nFor soft XAS, we have incorporated several spinel-type reference compounds, as suggested by\nthe reviewer, to enable direct comparison of L-edge spectral features. In particular, NiCo O ,\n2 4\nFeCr O , and ZnFe O are included as experimental benchmarks representing Co3+ in\n2 4 2 4\ntetrahedral coordination, Fe2+ in a normal-spinel tetrahedral environment, and Fe3+\npredominantly in octahedral coordination, respectively (Supplementary Fig. 25). These\nreferences allow clearer interpretation of both valence states and site occupancies based on L-\nedge peak positions and line shapes.\nUsing these references, we explicitly compare the HESO spectra with established spinel\nstandards. The Co L-edge spectra indicate that Co in pristine HESO predominantly occupies\ntetrahedral sites, while Zn substitution induces a redistribution of Co toward octahedral\ncoordination. A similar evolution is observed for Fe: upon Zn substitution, Fe L-edge features\nshift away from Fe2+-like tetrahedral signatures and toward Fe3+-like octahedral characteristics,\nindicating enhanced octahedral occupancy.\nTo quantitatively assess oxidation-state evolution, we expanded the hard XAS K-edge half-step\nedge analysis for Fe and Ni (Supplementary Fig. 15). While the overall changes in oxidation\nstate are modest, Fe in all M-HESOs exhibits a slightly higher average oxidation state than in\npristine HESO, with Zn-HESO showing the largest shift. The Fe valence in Zn-HESO lies\nbetween those of Fe O and Fe O . In contrast, Ni shows only minimal edge shifts upon\n3 4 2 3\nsubstitution, and all M-HESOs maintain an average Ni oxidation state close to +2, indicating\nthat Ni valence is largely insensitive to sixth-cation addition.\nNiFe O was selected as a reference compound for both Ni and Fe due to the close similarity\n2 4\nof its R-space EXAFS features to those of the M-HESOs (Supplementary Fig. 16). EXAFS\nfitting reveals that Fe exhibits pronounced sensitivity to compositional tuning: pristine HESO\ncontains Fe distributed between both Td and Oh sites, Zn substitution drives Fe toward\nincreased octahedral coordination, while Al-HESO shows a relative increase in tetrahedral Fe.\nGa- and Mg-HESOs exhibit Fe coordination environments comparable to that of pristine HESO\n(Supplementary Figs. 19–20). In contrast, Ni remains largely unaffected by substitution and\nconsistently prefers octahedral coordination in pristine HESO and all M-HESOs\n(Supplementary Figs. 21–22). By comparison, Mn shows a systematic reduction in\ncoordination number upon sixth-cation substitution, as revealed by Mn K-edge EXAFS\nanalysis (Supplementary Figs. 23–24). Quantitative EXAFS fitting indicates that Mn in\npristine HESO exhibits an average Mn–O coordination number of ~6, consistent with\npredominantly octahedral coordination. In contrast, all substituted M-HESOs (Al-, Ga-, Mg-,\nand Zn-HESOs) show reduced Mn–O coordination numbers in the range of ~4.5–5.3,\nindicating a deviation from ideal octahedral coordination. In addition, we have measured the\nK-edge (and when available, L-edge) spectra of the substituent cations themselves (Zn, Ga,\nMg, Al), which are now presented in Supplementary Fig. 29. Now, we have added all details\nto the Supplementary Notes.\ncorresponding fits for reference spinel oxides, HESO, and Zn-HESO.\ncorresponding fits for reference spinel oxides, HESO, and Zn-HESO.\n3) In the Co K-edge and Fe K-edge XAFS spectra in Figures S9 and S10, the differences among\nZn-, Ga-, Mg-, Al-substituted and pristine HESO samples are not clearly visible. It is difficult\nto identify the valence shift of Co and Fe induced by the sixth-element doping.\nResponse: We thank the reviewer for this helpful comment. To resolve the subtle but systematic\ndifferences in the Co and Fe K-edge XANES among the substituted HESO samples, we\nreplotted the spectra with expanded edge-onset regions and quantified the absorption-edge\npositions using half-step height analysis (Supplementary Fig. 15).\nrelative to pristine HESO, indicating a slight increase in the average Co oxidation state upon\nsixth-cation substitution. Among the doped samples, Zn-HESO shows the largest Co edge shift,\nfollowed by Ga-, Mg-, and Al-HESOs, while pristine HESO displays the lowest edge energy.\nThe Co K-edge positions for all samples remain close to that of CoFe O , confirming that Co\n2 4\nremains predominantly in a Co2+-like state with a modest increase in Co3+ fraction. A similar\nbut more pronounced trend is observed for Fe K-edge XANES. All substituted M-HESOs show\na positive Fe edge shift compared to pristine HESO, indicating an increase in average Fe\noxidation state. The Fe K-edge positions systematically progress from pristine HESO toward\nhigher energies in the order Al-/Mg-/Ga-HESO < Zn-HESO, with Zn-HESO again exhibiting\nthe highest Fe edge energy. Importantly, the Fe edge positions for all M-HESOs fall between\nthose of Fe O and Fe O , indicating mixed-valence Fe states rather than full oxidation to Fe3+.\n3 4 2 3\nIn contrast, the Ni K-edge XANES shows negligible edge shifts across all compositions. The\nNi edge positions of pristine HESO and all M-HESOs overlap closely with those of NiFe O ,\n2 4\nindicating that the average Ni oxidation state remains near Ni2+ and is largely insensitive to\nsixth-cation substitution. Likewise, Cr K-edge XANES exhibits minimal variation among\npristine HESO and the M-HESOs, with all edge positions closely matching that of NiCr O ,\n2 4\nconsistent with a stable Cr3+ oxidation state. Mn K-edge XANES shows only minor edge shifts\nupon substitution, with all samples remaining within the energy window defined by Mn O and\n3 4\nMnO references, indicating that Mn oxidation states are only weakly perturbed by sixth-\n2\nelement incorporation.\nOverall, the half-step height analysis in Supplementary Fig. 15 demonstrates that sixth-cation\nsubstitution primarily affects the redox-flexible Co and Fe centers, leading to systematic but\nmodest oxidation-state increases, while Ni, Cr, and Mn remain comparatively insensitive.\nAmong all dopants, Zn induces the largest oxidation-state shifts for both Co and Fe, showing\nits strongest perturbative effect on the electronic structure of the HESO lattice.\nRevision: [Supplementary Note 1] “Upon elemental substitution, all five constituent cations\nexhibit a discernible positive shift in their X-ray absorption near-edge structure (XANES) edge\nenergies, reflecting an overall increase in their effective oxidation states. This observation is\nconsistent with charge compensation mechanisms in HEOs, where the introduction of\naliovalent species, such as Zn2+, necessitates oxidation of neighboring transition metals to\nmaintain electroneutrality. Notably, Zn-HESO demonstrates the most significant edge shifts for\nboth cobalt and iron, indicating that these two elements are particularly sensitive to the\n20\nchemical environment perturbations induced by Zn incorporation. This suggests a preferential\noxidation of Co and Fe cations, potentially due to their higher redox flexibility compared to\nother constituent metals. In parallel, the pre-edge peak intensity, associated with 1s → 3d\nelectronic transitions, exhibits a marked decrease following Zn substitution. Since pre-edge\nfeatures are sensitive to both oxidation state and local coordination symmetry, the observed\nsuppression points toward a transition from lower symmetry environments to higher symmetry\nconfigurations, most plausibly reflecting the stabilization of Co3+ and Fe3+ in centrosymmetric\nO sites. This assignment is supported by ligand field considerations, where octahedral\nh\ncoordination reduces p-d orbital mixing, thereby diminishing the pre-edge intensity.\nIn Supplementary Fig. 15, upon sixth-cation substitution, Co, Fe, and Ni exhibit systematic\npositive shifts in XANES edge energies, whereas Cr and Mn show little or no measurable\nchange in their effective oxidation states. This element-specific response reflects differences in\nredox flexibility and site preference within the high-entropy spinel lattice. Co and Fe possess\naccessible mixed-valence manifolds (Co2+/Co3+ and Fe2+/Fe3+) and occupy both tetrahedral and\noctahedral sites in pristine HESO; as a result, they readily accommodate charge compensation\ninduced by compositional perturbations through partial oxidation (Co2+ → Co3+ and Fe2+ →\nFe3+), particularly when dopants such as Zn2+ promote redistribution toward octahedral\ncoordination. Consequently, Co and Fe act as the primary redox-active species accommodating\ndopant-induced charge compensation, and Cr and Ni remain effectively unchanged in average\noxidation state.”\nmetal edges in doped HESO.\n4) In the EXAFS spectra in Figures S9-S13, spectra of appropriate reference compounds should\nbe included for comparison of local coordination structures.\nResponse: We thank the reviewers for raising this point. To address it, we have added reference\ncompounds to the EXAFS spectra for all elements to enable direct comparison of local\ncoordination environments (Supplementary Fig. 16). Comparison with these references\nshows that the Co K-edge EXAFS of HESO closely resembles that of CoFe O , the Fe K-edge\n2 4\nis similar to NiFe O , the Ni K-edge also resembles NiFe O , and the Mn K-edge is comparable\n2 4 2 4\nto Mn O . Based on these comparisons, we selected reference compounds with similar first-\n3 4\nshell R-space features to perform quantitative EXAFS fitting of coordination numbers for\nHESO and M-HESOs (Supplementary Figs. 17-24). The relevant explanations were handled\nin comments #2, 5, 6, and Supplementary Notes.\n5) Did the authors simulate the second-nearest coordination shell around 3A in Figure S14?\nThis region appears to change depending on the type of sixth-element doping in Figures S9-\nS13.\nResponse: We appreciate the reviewer for raising this point. In high-entropy spinel oxides,\nquantitative fitting of the second-shell EXAFS is intrinsically unreliable and therefore not\npursued. Unlike the first coordination shell, which corresponds to well-defined metal-oxygen\nbonds, the second cation shell represents a chemically and structurally disordered ensemble of\nmultiple metal-metal pairs with similar scattering properties but distributed interatomic\ndistances. The random occupation of tetrahedral and octahedral sites by multiple transition\nmetals introduces substantial static disorder, leading to large Debye-Waller factors that strongly\ndamp higher-R EXAFS oscillations. In addition, second-shell single-scattering contributions\nsignificantly overlap with multiple-scattering paths inherent to the spinel framework, further\nobscuring path-specific signatures in R space. Combined with the limited backscattering\ncontrast among neighboring 3d transition metals and strong parameter correlations in multi-\npath fitting, these factors render the extracted coordination numbers and distances non-unique\nand highly model-dependent. As a result, we think the second-shell EXAFS in this system is\nnot appropriate for quantitative fitting.\n23\n6) The authors should perform detailed curve-fitting simulations for all samples (Zn-, Ga-, Mg-,\nAl-doped and pristine HESO) to quantitatively evaluate the structural parameters.\nResponse: We appreciate the reviewer for pointing out the missing EXAFS analysis for the\nother M-HESOs. To systematically quantify coordination numbers, as previously reported for\nselected compositions (Supplementary Figs. 17 and 19), we first identified appropriate\nreference compounds with similar first-shell R-space features in the EXAFS spectra\n(Supplementary Fig. 16). Using these references, we expanded the EXAFS fitting to include\nall M-HESOs at the Co, Fe, Ni, and Mn K-edges, enabling a consistent comparison of local\ncoordination environments across the entire compositional series (Supplementary Figs. 17-\n24).\nThe expanded EXAFS analysis reveals that site redistribution in doped high-entropy spinel\noxides is highly element-specific and governed by an interplay of crystal-field stabilization,\nredox flexibility, and dopant site preference. Co is the most sensitive to sixth-cation substitution:\nin pristine HESO, Co predominantly occupies tetrahedral (Td) sites, while Zn and Ga\nsubstitution progressively drive Co toward octahedral (Oh) coordination, with Zn-HESO\nexhibiting the most pronounced shift due to the strong tetrahedral site preference of Zn2+.\nIn contrast, Al- and Mg-HESOs show a slight redistribution of Co back toward Td sites, reflecting\nthe weaker ability of these dopants to displace Co from tetrahedral coordination. Fe exhibits a\nsimilarly strong but dopant-dependent response. Zn substitution promotes Fe redistribution\ntoward octahedral sites, consistent with oxidation from Fe2+ toward Fe3+ and stabilization in\nhigher-symmetry environments, whereas Al substitution favors increased tetrahedral Fe\noccupancy. Ga- and Mg-substituted HESOs induce minimal changes in Fe coordination,\nyielding environments comparable to pristine HESO.\nIn comparison, Ni remains largely insensitive to compositional tuning. Across pristine HESO\nand all M-HESOs, the fitted Ni-O coordination number remains close to six, indicating a\npersistent preference for octahedral coordination. This invariance reflects the strong octahedral\ncrystal-field stabilization and limited redox flexibility of Ni2+, which renders coordination-site\nredistribution energetically unfavorable.\nMn displays the opposite behavior, acting as a structurally flexible, charge-buffering species.\nIn pristine HESO, Mn predominantly occupies octahedral sites, stabilized through cooperative\ninteractions within the equimolar multi-cation framework. Upon sixth-cation substitution (Zn,\nimmobility of Cr2+ and Ni2+, creates energetic congestion in the octahedral sublattice. Mn has\na weak intrinsic site preference and multiple accessible oxidation states, so it accommodates\nthis perturbation by shifting toward increased tetrahedral occupancy, with Al-HESO exhibiting\nthe highest Td/Oh ratio among the substituted compositions. This redistribution is reflected by\nreduced Mn-O coordination numbers in EXAFS, while Mn L-edge sXAS remains largely\ninvariant due to the mixed-valent nature of Mn and the limited site sensitivity of the technique.\nWe have added all the details to the Supplementary Note.\nRevision: [Supplementary Note 1] “For the EXAFS analysis, to quantitatively elucidate\nchanges in the local coordination environment, extended X-ray absorption fine structure\n(EXAFS) analysis was performed. Reference compounds—CoCr O and CoFe O for Co,\n2 4 2 4\nNiFe O for Fe, NiFe O for Ni, and Mn O for Mn—were employed to calibrate the amplitude\n2 4 2 4 3 4\nreduction factor (S 2), ensuring accurate fitting of experimental data. These S 2 values, once\no o\ndetermined, were held constant across fitting procedures to reliably extract first-shell\ncoordination numbers. (Supplementary Figs. 17–24) The Fourier-transformed EXAFS\nspectra reveal that the first major peak corresponds to metal–oxygen (M–O) scattering paths,\nreflecting the immediate oxygen coordination shell around metal cations. Given the inherent\nmixed occupancy of tetrahedral (CN = 4) and octahedral (CN = 6) sites in the spinel lattice, the\naverage metal–oxygen coordination number is expected to fall between these two canonical\nvalues.\nPost Zn-substitution, the Co–O coordination number increases from 4.1 to 4.8, while Fe–O\nincreases from 5.5 to 5.8. These subtle yet significant increments suggest a redistribution of Co\nand Fe species from tetrahedral to octahedral sites, aligning with the pre-edge analysis\nindicating enhanced local symmetry. Collectively, the XANES edge shifts, pre-edge\nsuppression, and EXAFS coordination analysis converge to a consistent interpretation: Zn\nsubstitution in HESO preferentially oxidizes Co and Fe cations while simultaneously\npromoting their stabilization in octahedral coordination environments.\nIn addition, EXAFS analysis reveals distinct and complementary roles of Ni and Mn in\naccommodating dopant-induced perturbations in HESOs. Ni remains predominantly\noctahedrally coordinated across pristine HESO and all doped M-HESOs, with a fitted Ni-O\ncoordination number close to six. This invariance reflects the strong octahedral site preference\nof Ni2+, arising from its large crystal-field stabilization energy in octahedral coordination,\nwhich renders site exchange energetically unfavorable. In contrast to Co and Fe, Ni exhibits\n25\nlimited redox flexibility and does not gain energetic stabilization by redistributing between\ntetrahedral and octahedral sites during charge compensation. Consequently, Ni acts as an\noctahedral structural anchor within the high-entropy lattice, accommodating dopant-induced\nperturbations primarily through minor changes in effective oxidation state rather than\ncoordination-site redistribution, consistent with the nearly invariant Ni L-edge sXAS line\nshapes and minimal shifts observed in Ni K-edge XANES.\nIn contrast, Mn displays pronounced site flexibility. In pristine HESO, Mn predominantly\noccupies octahedral sites, stabilized through cooperative interactions within the equimolar\nmulti-cation framework. Upon introduction of a sixth cation (Zn, Mg, Ga, or Al), site-\npreference competition and charge-compensation requirements drive redistribution of Co and\nFe, while Cr and Ni remain locked in octahedral coordination. Mn, having a weaker intrinsic\nsite preference and multiple accessible oxidation states, accommodates the resulting\nperturbation by shifting toward increased tetrahedral coordination, thereby relieving octahedral\nsite congestion and stabilizing the overall lattice. This redistribution is manifested as a reduced\nMn-O coordination number in EXAFS, whereas Mn L-edge sXAS remains largely invariant\ndue to the mixed-valent nature of Mn and the limited site sensitivity of the technique.”\nSupplementary Fig. 18 Extended X-ray absorption fine structure (EXAFS) Co spectra fitting\nfor Al-HESO, Ga-HESO and Mg-HESO.\nfor Al-HESO, Ga-HESO and Mg-HESO.\nSupplementary Fig. 22 Extended X-ray absorption fine structure (EXAFS) Ni spectra fitting\nfor Al-HESO, Ga-HESO and Mg-HESO.\nfor Al-HESO, Ga-HESO and Mg-HESO.\n7) In this study, structural information of Zn and Ga is crucial. Please measure the Zn and Ga\nK-edge EXAFS spectra to confirm the local coordination of each element. The central\nhypothesis relies on Zn occupying Td sites, but the manuscript currently does not provide direct\nexperimental verification.\nResponse: We appreciate the reviewer’s suggestion. While we agree that element-specific local\nstructural information is in principle valuable, quantitative Zn and Ga K-edge EXAFS analysis\nis intrinsically limited in the present system due to the low concentrations of these elements in\nthe high-entropy spinel oxide. EXAFS probes only the absorbing atoms, and at dilute\nconcentrations, the absorption edge jump becomes very small, leading to a poor signal-to-noise\nratio. As a result, high-k oscillations are strongly suppressed, the effective k-range is\nsignificantly reduced, and the number of independent fitting parameters becomes insufficient\nfor reliable coordination analysis. In addition, the large static disorder inherent to high-entropy\noxides further damps EXAFS oscillations, exacerbating the signal degradation for dilute\nelements. Consequently, any extracted coordination numbers or site occupancies for Zn and Ga\nwould be highly model-dependent and not quantitatively reliable. For this reason, we do not\nattempt Zn- or Ga-edge EXAFS fitting in this work and instead rely on complementary\nevidence (DFT calculation) to support Zn occupancy of tetrahedral sites. Our DFT calculations\nshow strong tetrahedral site preference for Zn atoms; as such, our experimental measurements\n28\nof the change in other elements’ site occupations when Zn is doped into the HESO is our best\nverification of the placement of Zn.\n8) While the authors claim a clear redistribution of Co and Fe between tetrahedral and\noctahedral sites upon Zn substitution, the quantitative reliability of the XAS/XMCD fitting is\nnot fully demonstrated. The deconvolution and fitting procedures should be described in\nsufficient detail. This is quite important.\nResponse: We thank the reviewer for emphasizing this important point. In the revised\nmanuscript, we have substantially expanded the description of our XAS deconvolution and\nfitting procedures. In detail, all Co and Fe L-edge spectra were energy-aligned, background-\nsubtracted, and normalized to the edge jump before analysis. The experimental spectra were\nthen fitted by a linear combination of normalized reference spectra corresponding to Co3+(Oh),\nCo2+(Oh), Co2+(Td), Fe2+(Oh), Fe3+(Oh), or Fe3+(Td), which were taken from well-established\nsimulated datasets. (Ref 48. J. Am. Chem. Soc. 2022, 144, 45, 20590–20600 / Ref 49. Acta\nMaterialia 226 (2022) 117581 / Ref 50. J. Magn. Magn. Mater. 460 (2018) 243–252) The\ndetails on each L-edge spectrum were mentioned in Comment #1.\nFor each sample, the measured spectrum W(E) was modeled as\n𝑊(𝐸) = ∑𝑎 𝑋 (𝐸)\ni 𝑖\ni\nwhere X(E) are the reference spectra and are non-negative fitting coefficients constrained to\ni\nsum to unity. The fits were obtained by least-squares minimization in MATLAB (Trust Region\nReflective algorithm), and uncertainty was determined as the 95% parameter confidence\ninterval using the regression covariance matrix. We note that the shift in distribution of Co\natoms from 70% in tetrahedral sites (30% in octahedral sites) in the pristine HESO to 20% in\ntetrahedral sites (80% in octahedral sites) in the Zn-HESO is much more significant than the\nerror bar margin, visualized in Fig. 2c. This suggests that data fitting is sufficiently reliable for\nour conclusion of a significant systematic shift of Co atoms from tetrahedral to octahedral sites\nin Zn-HESO. A more detailed description of the fitting workflow and constraints has also been\nadded to the Methods section.\nRevision: [Page 14] “For the quantification, all experimental L-edge spectra were energy-\naligned, background-subtracted, and normalized to the edge jump prior to analysis. The\n29\nexperimental spectrum for each sample, W(E), was then modeled as a linear combination of\nthe normalized reference spectrum according to\n𝑊(𝐸) = ∑𝑎 𝑋 (𝐸)\ni 𝑖\ni\n, where X(E) denotes the reference spectra and a are non-negative fitting coefficients\ni i\nconstrained to sum to unity. The fitting was performed by least-squares minimization using the\nTrust-Region Reflective algorithm implemented in MATLAB. The uncertainties in the\nextracted site fractions were determined as the 95% confidence intervals derived from the\nregression covariance matrix. XMCD spectra were additionally used as a complementary\nconstraint to distinguish tetrahedral and octahedral contributions, owing to their distinct\nmagnetic signatures.”\n9) In Figure 6, the operando XAS spectra are not very clear due to the broad peaks. In the Co\nL3-edge spectra, a new feature appears near 782 eV at 0.8 V in the final measurement.\nResponse: We appreciate the reviewer’s careful observation. The broad spectral features\nobserved are characteristic of soft X-ray absorption at liquid-solid interfaces, as distinct from\nthe sharp peaks seen in crystalline solid phase measurements. This is consistent with\nestablished literature using this specific flow-cell setup (e.g., Velasco-Velez et al., Science\n2014). To improve the clarity of the operando Co L-edge spectra in Fig. 6, we have revised the\nfigure by adding explicit peak assignments for the main electronic features, namely the Co3+-\nrelated multiplet peaks, the lower-energy Co2+ contribution, and the high-valence Co3+/Co4+-\nlike shoulder that emerges near 782 eV under anodic potentials. These labels make the spectral\nevolution more readily interpretable despite the inherent broadness of the operando features.\nThe shoulder at ~782 eV observed at 1.45 V is now highlighted in the revised figure and text.\nWe attribute this feature to the growth of high-valence Co species and increased Co–O\ncovalency under oxidative conditions, which is consistent with prior reports on operando Co\nK-edge behavior. (Ref. 47: https://doi.org/10.26434/chemrxiv-2025-396l8)\n10) The authors should compare the operando Co K-edge and Ni K-edge XAFS spectra with\nthe corresponding Co L-edge and Ni L-edge spectra in terms of valence shifts and local\nstructural changes. Reference spectra of Co(OH)2, CoOOH, Ni(OH)2, and NiOOH would be\nuseful for this discussion.\nbetween operando Co and Ni K-edge XAFS and the corresponding L-edge spectra using\nreference compounds such as Co(OH) /CoOOH and Ni(OH) /NiOOH. We agree that such a\n2 2\ncombined hard–soft XAS analysis would provide valuable insight into valence evolution and\nlocal structural changes under operando conditions. However, operando Co and Ni K-edge\nXAFS measurements could not be conducted at this stage due to experimental and beamtime\nlimitations.\nNevertheless, to place our results in context, we refer to a recently reported operando K-edge\nXAFS study on high-entropy spinel oxides composed of Mn, Fe, Co, Ni, and Zn, which closely\nresemble the compositional and structural motifs of our HESO system, shown in Fig. R2. (Ref.\n47: https://doi.org/10.26434/chemrxiv-2025-396l8) In this work, operando Co and Ni K-edge\nXANES spectra were collected under anodic polarization, allowing direct tracking of\noxidation-state evolution as a function of applied potential. Notably, the reported data show\nthat the Co K-edge exhibits a clear positive edge shift with increasing anodic potential,\nindicating progressive oxidation of Co during electrochemical operation, and the Ni K-edge\ndisplays a smaller, still positive edge shift over the same potential range. This behavior closely\nmirrors the trend observed in our L-edge analyses, where Co shows redox sensitivity across\nHESO compositions.\nThe operando K-edge XANES evolution of HESO can be interpreted by direct comparison\nwith hydroxide and oxyhydroxide reference compounds (Fig. R3 and R4). Specifically, the\nCo K-edge shift observed for HESO under anodic polarization follows the same direction as\nthe edge shift from Co(OH) to CoOOH, indicating progressive oxidation of Co toward higher\n2\noxidation states (Energy Environ. Sci., 2019, 12, 739-746). Likewise, although less pronounced,\nthe Ni K-edge also shifts toward higher energies in the same direction as the Ni(OH) to\n2018, 3, 6, 1373-1380). We have already added references used for oxidation-state comparison;\nin addition, we will include the appropriate references and explanation for the Co and Ni K-\nedge shifts.\nRevision: [Page 11] “Consistent with these operando L-edge observations, previously reported\noperando K-edge XANES studies on closely related high-entropy spinel oxides show that the\nCo K-edge shifts toward higher photon energies under anodic polarization, while the Ni K-\nedge exhibits a weaker but discernible shift toward higher oxidation states. These K-edge\n31\nresults further support the interpretation that Co undergoes pronounced redox evolution under\nOER conditions, with Ni contributing subtly to the overall electronic reorganization.47”\n[FIGURE REDACATED]\nFigure R2. In-situ Co and Ni K-edge XANES spectra of the HESO collected under the rest\npotential and potential applied conditions from 1.2 V to 1.53 V vs RHE. (Ref. 47)\n[FIGURE REDACATED]\nFigure R3. In-situ Co K-edge XANES spectra of the CoOOH collected under the rest potential\nand potential applied conditions from 1.3 V to 1.53 V vs RHE. (Energy Environ. Sci., 2019, 12,\n739-746)\n[FIGURE REDACATED]\nFigure R4. Ni K-edge XANES spectra of the Ni(OH) . (ACS Energy Lett. 2018, 3, 6, 1373-\n2\n1380)\n32\n11) The authors should clarify how the L-edge XAS spectra allow distinguishing between\noctahedral and tetrahedral sites, since these spectral features are often similar and difficult to\nseparate.\nResponse: We agree with the reviewer that L-edge XAS alone often does not provide a fully\ndefinite distinction between tetrahedral (T ) and octahedral (O ) sites, because the spectral\nd h\nenvelopes of spinel oxides can overlap significantly. In our analysis, the site assignment was\ntherefore not based solely on raw L-edge line shapes. Instead, we combined two\ncomplementary approaches that are widely used for spinel systems:\nFirst, multiplet simulations of reference T /O environments. Using established simulated\nd h\nreference spectra for Co2+(T ), Co2+(O ), Co3+(O ), Fe3+(T ), Fe2+(O ), and Fe3+(O ), we\nd h h d h h\nperformed linear combination fitting of the experimental L edge. The characteristic differences,\n3\nsuch as the relative intensities of the t –e multiplet peaks and the branching ratio, allowed the\n2g g\nTd and Oh components to be deconvoluted.\nSecond, XMCD polarity and magnitude as a site-selective probe. In spinels, the moments on\nthe Td and Oh sublattices are antiparallel. As reported in earlier studies of high-entropy spinel\noxides (e.g., Acta Mater. 2022, Sarkar et al.), the sign of the XMCD peak provides a direct\nindicator of whether a cation predominantly occupies the Td or Oh site. In our case, Co and Fe\nshowed XMCD sign reversal consistent with the expected antiparallel coupling of Td and Oh\ncations, enabling a robust assignment of the site fractions. We have included the Methods and\nDiscussion sections to clarify this procedure.\nIn this study, Baek et al. investigate the effect of introducing a sixth cation into a Co-Fe-Cr-\nMn-Ni HESO to modulate cation site occupancy. Among five candidate cations examined, Zn\nis identified as the most effective in altering the distribution of Co2+ and Co3+ ions between\nTd and Oh sites. The authors employ a combination of advanced characterization techniques\nand theoretical calculations to build a fundamental understanding of coordination environment\ncontrol in HEOs. While the study is intriguing and the narrative is generally well-constructed,\nthe OER performance of the optimized Zn-HESO catalyst falls short of the benchmark\nexpected for publication in this journal. Therefore, despite the scientific merit of the work, I\ncannot recommend its publication in its current form. My major concerns are detailed below.\nResponse: We truly appreciate the reviewer for the thoughtful evaluation of our work and for\nrecognizing the fundamental interest of coordination-site modulation in high-entropy oxides.\nWe appreciate the reviewer’s concerns regarding the OER performance of Zn-HESO and the\nneed for stronger evidence to support our conclusions. We would like to clarify that the primary\naim of this study is to uncover the fundamental thermodynamic and electronic principles\ngoverning cation redistribution in high-entropy spinel oxides. Our focus is on understanding\nhow entropy-driven coordination-site modulation occurs and how these structural and\nelectronic changes correlate with catalytic behavior. While we agree that further optimization\nof morphology, surface engineering, and nanoscale engineering could significantly enhance the\nabsolute OER activity, such catalyst-design efforts lie beyond the scope of the present\nmechanistic study. Instead, our goal here is to provide a foundational and theory-based\nunderstanding of site occupancy, electronic structure evolution, and the resulting activity trends\nwithin a chemically complex multi-cation lattice. We hope this clarification helps contextualize\nthe performance levels observed and the scientific intent of the manuscript.\nMajor Comments:\n1. The authors should provide details regarding the identification of Td and Oh sites in these\nHEOs via STEM.\nResponse: We thank the reviewer for pointing this out. The Td and Oh labels in Fig. 1d are not\nderived from atomic-resolution chemical identification, but from the projected geometry of the\nspinel lattice along the [110] zone axis, which is well established in the literature for spinel\noxides. In a normal or inverse spinel viewed along [110], the oxygen sublattice forms a\n34\ncharacteristic zig-zag arrangement, and the tetrahedral and octahedral cation sites project into\ndistinct crystallographic positions in the image. Accordingly, the Td and Oh sites marked in\nFig. 1d correspond to theoretical lattice positions expected for the spinel structure when viewed\nalong [110], as confirmed by: (1) the crystallographic model of the spinel unit cell, (2) slice\nimage simulations of spinel viewed along [110] in Fig. 1c, and (3) prior reports demonstrating\nthe recognizable Td/Oh projected positions in spinels (e.g., Nanoscale Res. Lett. 2013, 8, 374).\nImportantly, we clarify that these labels are only structural guides, indicating the expected\ncrystallographic positions of T and O sites. Quantitative determination of site populations\nd h\nwas instead obtained from L-edge multiplet fitting, XMCD, and K-edge XANES/EXAFS,\nas detailed in the main text.\n2. TEM images for Al-HESO are missing. Based on HR-TEM, the authors identified that the\ninterplanar spacing at the (311) plane for Mg-HESO and Ga-HESO is larger than that of HESO.\nHowever, the XRD patterns show that the (311) diffraction peak for both Mg-HESO and Ga-\nHESO shifts to higher angles, indicating a decrease in their interplanar spacing. Please clarify\nthis apparent discrepancy.\nResponse: We thank the reviewer for this important comment. First, we have remeasured the\nXRD patterns for all samples to clarify this point. The apparent inconsistency arises because\nXRD provides ensemble-averaged lattice parameters over the entire sample, and the\nremeasured XRD (311) peak shifts are actually very small (i.e., subtle changes in the bulk-\naveraged lattice parameter), whereas HR-TEM lattice-fringe measurements reflect local d-\nspacings from selected nanoscale regions that can be affected by local strain, compositional\nfluctuation, and measurement uncertainty. According to Bragg’s law, a shift of the (311) peak\ntoward a higher 2θ indeed corresponds to a decrease in the average d(311). In our updated\nXRD patterns, the (311) reflections of M-HESOs show slight shifts to higher angles compared\nto pristine HESO, indicating a minor bulk-averaged contraction of the spinel lattice, while the\noverall phase and peak set remain unchanged, confirming that the spinel framework is\npreserved.\nIn HR-TEM, we measured local (311) lattice fringes and obtained d(311) = 0.254 nm (Ga-\nHESO), 0.257 nm (Mg-HESO), 0.263 nm (Al-HESO), and 0.246 nm (Zn-HESO), all of which\nfall within the characteristic range of the spinel (311) plane. Importantly, these values represent\nlocal spacings in specific crystallites/regions rather than bulk averages. In compositionally\ncomplex spinels, nanoscale strain accommodation and local lattice relaxation can produce\nlocally expanded fringes even when the bulk-averaged lattice parameter from XRD shows a\nslight contraction. In addition, the small XRD peak shifts correspond to sub-percent changes\nin d(311), which can be comparable to the uncertainty in extracting lattice fringes from HR-\nTEM (e.g., local thickness/tilt/FFT selection), further amplifying the apparent difference when\ncomparing a single local value to an ensemble-averaged XRD position.\nTherefore, the HR-TEM and XRD results are not contradictory but complementary: XRD\ncaptures the ensemble-averaged crystallographic response, whereas HR-TEM highlights local\nstructural heterogeneity. We have revised the manuscript to explicitly clarify that HR-TEM-\nderived d-spacings should not be interpreted as bulk lattice parameters. In addition, we now\ninclude the missing HR-TEM images for Al-HESO, and the extracted d(311) is reported\nalongside the other compositions.\nRevision: [Page 4] “Zn-HESO in high-resolution TEM images in Fig. 1f shows d-spacings of\n0.246 nm for the (311) and 0.292 nm for the (220) planes, compared with those of reference\nspinel NiFe O (0.251 nm for (311) and 0.295 nm for (220)) and pristine HESO (0.250 nm for\n2 4\n(311) and 0.294 nm for (220)). This corresponds to an overall lattice contraction of\napproximately 1–2%. Such contraction is consistent with the incorporation of Zn ions, which\npreferentially occupy T sites in spinel lattices, replacing or redistributing larger divalent\nd\ncations (e.g., Mn2+) within the T sublattice, thereby reducing the average metal–oxygen bond\nd\nlength. Similar variations in d-spacing are observed locally, as highlighted in the inset of Fig.\n1f, where the measured d-spacings span 0.23–0.27 nm for the (311) and 0.27–0.32 nm for the\n(220) planes, reflecting compositional and local structural heterogeneity intrinsic to high-\nentropy spinels. In contrast, Ga-, Mg-, and Al-substituted HESOs exhibit slightly different\naverage d-spacings, with values of 0.254–0.263 nm for the (311) and 0.291–0.297 nm for the\n(220) planes (Supplementary Figs. 3–5). These differences reflect variations in the average\ncation distribution and site-specific ionic radii within the spinel lattice. For Ga3+ and Mg2+,\nwhich possess relatively larger ionic radii compared to some of the host cations in O or T\nh d\ncoordination, subtle lattice expansion or reduced contraction may occur depending on their\npreferred coordination environment. Notably, Al-HESO shows a distinct behavior compared to\nGa- and Mg-containing samples. Al3+ ions have a relatively small ionic radius in O\nh\ncoordination (~0.535 Å) and form strong Al–O bonds, which can locally stabilize the spinel\nframework without necessarily inducing uniform lattice contraction. As a result, the observed\nd-spacing in Al-HESO likely reflects a balance between local lattice distortion, cation\n36\nredistribution, and strong metal–oxygen bonding, rather than a simple ionic-size-driven\nexpansion or contraction.\nX-ray diffraction (XRD) (Fig. 1g, Supplementary Fig. 6) also shows that both pristine and\nZn-HESO maintain pure spinel structures. Zn-HESO shows a slight shift in (311) due to the\nlattice contraction from a smaller ionic radius of Zn. The fast Fourier transform (FFT) in Fig.\n1f confirms that the diffraction pattern matches the expected spinel structure and is consistent\nwith XRD results.”\nSupplementary Fig. 6 X-ray diffraction of different HESOs and enlarged view of the (311)\ndiffraction peaks\n3. In this work, the authors synthesized four distinct cation-engineered materials: Mg-HESO,\nGa-HESO, Al-HESO, and Zn-HESO, each with different site occupancies. However, the\ndiscussion section primarily focuses on comparisons between HESO and Zn-HESO, lacking a\ndetailed comparative discussion highlighting the differences among these high-entropy oxides.\nprovide a complete comparison among all substituted HESOs; however, because the\nmanuscript was already extensive and lengthy, we prioritized presenting Zn-HESO, which is\nthe composition that exhibited the most pronounced redistribution effect, as the representative\ncase for detailed mechanistic discussion. We agree that this focus may have underrepresented\nthe important differences among Mg-, Ga-, Al-, and Zn-HESO. Following the reviewer’s\nsuggestion, we have expanded the revised Discussion to include a clear comparative analysis\nacross all four substituted HESOs in the Supplementary Note, highlighting:\n- how each sixth cation perturbs Co/Fe site occupancy to different extents,\n- how their coordinate-site-preference energies correlate with redistribution behavior,\n- how the resulting electronic structure differences manifest in XAS trends, and\n- how these structural/electronic differences relate to the observed OER activities.\nThese additions will provide a more comprehensive interpretation of the entire material library.\nWe appreciate the reviewer for pointing out this improvement.\nRevision: [Supplementary Note 6] “To provide a comprehensive understanding of how\ndifferent sixth cations influence coordination-site redistribution and OER performance in high-\nentropy spinel oxides, we present here a detailed comparative analysis of Mg-, Ga-, and Al-\nsubstituted HESOs, based on the combined XAS/XMCD, electrochemical, and DFT results.\nAmong the investigated compositions, Al-HESO exhibits the weakest coordination-site\nperturbation. Al3+ has a strong intrinsic preference for octahedral coordination in spinel lattices,\nwhich limits its ability to displace other cations from tetrahedral sites. As a result, XAS and\nXMCD analyses show minimal redistribution of Co and Fe toward octahedral coordination in\nAl-HESO (Figs. 2c, f). The fraction of Co3+(O ), identified as the dominant OER-active site in\nh\nthis study, remains low, while a significant portion of Co persists in tetrahedral coordination.\nThis limited redistribution is reflected in electrochemical behavior. Al-HESO shows the highest\nTafel slope (45 mV dec-1) and the largest charge-transfer resistance among the substituted\nHESOs (Fig. 5b, c), indicating sluggish OER kinetics. Additionally, the absence of significant\nCo2+(O ) enrichment leads to relatively poor electronic conductivity, further limiting\nh\nperformance at elevated current densities. These results indicate that Al substitution in HESO\nprimarily stabilizes the lattice without inducing substantial electronic or catalytic enhancement.\nMg-HESO exhibits intermediate behavior between Al- and Ga-substituted systems.\nMg2+ possesses greater coordination flexibility than Al3+, and XAS/XMCD results indicate a\n39\nmodest redistribution of Co and Fe toward octahedral sites compared to pristine HESO.\nHowever, the extent of this redistribution remains limited, reflecting the relatively weak site-\npreference energy of Mg in the spinel lattice. Electrochemically, Mg-HESO shows moderate\nOER activity and kinetics, with Tafel slopes and overpotentials lower than Al-HESO but clearly\ninferior to Ga- and Zn-substituted systems (Fig. 5a, b). The partial increase in Co3+(O )\nh\ncontributes to some enhancement in activity, but the lack of significant Co2+(O ) enrichment\nh\nrestricts improvements in bulk conductivity and charge transport. Consequently, Mg-HESO\ndoes not sustain strong performance gains at higher current densities.\nGa-HESO represents a distinct regime in which substantial coordination-site\nredistribution is achieved. Ga3+ exhibits a strong tetrahedral site preference relative to Co and\nFe, effectively driving Co and Fe toward octahedral coordination. L-edge XAS deconvolution\nreveals a pronounced increase in the Co3+(O ) fraction in Ga-HESO (Figs. 2c, f), which\nh\nexplains its relatively low onset overpotential and favorable activity at low current densities\n(Fig. 5a). However, despite effective generation of catalytically active Co3+(O ) sites, Ga-\nh\nHESO displays a higher Tafel slope value (36.7 mV dec-1) and larger charge-transfer resistance\nthan Zn-HESO (Fig. 5b, c). This suggests that while Ga substitution efficiently promotes\nactive-site formation, it does not comparably enhance electronic conductivity or charge\ntransport. Taken together, Mg-, Ga-, and Al-HESOs demonstrate that the effectiveness of\ncoordination-site engineering in high-entropy spinel oxides depends not only on the ability of\nthe substituent cation to promote active site formation, but also on how the resulting\nredistribution influences electronic transport.”\n4. The iR-corrected OER polarization curves presented in this manuscript show that Zn-HESO\nrequires an overpotential of 295 mV to achieve 10 mA cm⁻2. Furthermore, the demonstrated\nstability duration is relatively short. Compared to similar OER studies published in Nature\nCommunications, these performance metrics do not appear competitive.\nResponse: We appreciate the reviewer’s comment regarding the OER benchmarking. While\nthe overpotential of ~295 mV at 10 mA cm-2 is not among the very best values reported in\nNature Communications, the primary goal of this study is to establish a clear structure–property\ncorrelation between entropy-driven cation redistribution and OER behavior in high-entropy\nspinel oxide structures. Electrochemical performance is therefore used as a diagnostic probe to\nvalidate the redistribution mechanism. We emphasize that the present materials were evaluated\n40\nwithout any morphology engineering, surface-area enhancement, or nano-structuring. These\nwell-known strategies typically yield substantial improvements in OER performance, and we\nexpect that applying them to Zn-HESO and Ga-HESO would further reduce overpotential.\nHowever, such optimization falls outside the mechanistic scope of the current work. Also,\nanother long-term stability data (Supplementary Fig. 42) shows continuous operation for\n~120 hours at 10 mA cm-2 for both pristine HESO and Zn-HESO, with no observable\ndegradation trend.\n5. The combined experimental and theoretical results indicate that Co3+ at the Oh site in Zn-HESO is the primary active center. Given this, is there an optimal proportion for the Co3+ Oh content in Zn-HESO?\nResponse: We thank the reviewer for this thoughtful question. We agree that understanding\nwhether an “optimal” Co (Oh) proportion exists is an important consideration. Although the\nhigh-entropy lattice involves multi-cation interactions that prevent Co (Oh) from serving as a\nsole activity descriptor, we performed an additional control experiment to examine the trend in\nsimpler systems.\nSpecifically, we synthesized four distinct cobalt spinel oxides with well-established Co site\ndistributions, Co O , CoFe O (already measured in Fig. 5a), ZnCo O , or NiCo O , and\n3 4 2 4 2 4 2 4\nmeasured their OER polarization curves under identical conditions. These materials span a\nbroad range of Co occupancy in Oh sites and thus provide a useful reference set for comparison.\nThe LSV results show that the trend is not strictly monotonic, illustrating additional factors,\nsuch as conductivity and metal–oxygen covalency, also play substantial roles.\nThis observation is consistent with the behavior of our HESO system, where Co (Oh)\nenrichment contributes positively to activity, but the optimal configuration arises from the\ncombined electronic structure of all constituent cations, rather than from Co site occupancy\nalone. This is also reflected in the DFT calculations of Co sites within the local high entropy\nmodel, which show a wide distribution of activities compared to the single Co site theoretical\nactivity in the pure Co spinel, with some of the overpotentials for HESO Co sites being lower\nthan the pure Co spinel. At the same time, Co sites showed higher activity than any other\npossible active site in our HESOs, indicating that more Co sites would increase the performance.\nTaken together, these two ideas indicate that there is some optimal Co concentration less than\n100% occupancy that allows for improvement from the pure Co spinel overpotential and still\n41\nmaximizes the number of available Co sites. We have added a brief clarification of this\ncomparison in the revised manuscript.\nRevision: [Page 8] “The volcano plot in Fig. 4c summarizes OER overpotentials for Co and\nnon-Co active sites in HESO and Zn-HESO. It includes 24 local impurity Zn-HESO cases with\nZn substituting T Co (Fig. 3c), paired with 24 pristine HESO surfaces from previous work40\nd\nto see the Zn substitution effect. Fig. 4c also includes 12 stable fully high-entropy Zn-HESO\nsurfaces with Co and other cations as active sites, and the use of a fully high-entropy model\nchecks the accuracy of the simpler impurity model. The calculated OER activity from the Co\nsite local impurity model shows that Zn-HESOs (pink squares) are comparable to pristine\nHESOs (blue circles) (Supplementary Fig. 34), indicating that Zn does not have a large impact\non the activity of individual Co sites. The scaling relationship between O*/OH*/OOH*\n(Supplementary Fig. 35) remains largely unchanged with a few Co outliers. We used the fully\nhigh-entropy models to evaluate Zn-HESOs with Co at O . The calculated overpotentials of\nh\nCo3+ sites range from 350-540 mV, compared to 410-940 mV for Co sites in the local impurity\nmodel, indicating a smaller lower bound than the fully high-entropy model. The Co3+ O in the\nh\ncomplex Zn-HESO model has an average overpotential of 453 mV, about 788 mV lower than\nthat for Co2+. We also considered the possibility of other cations as potential active sites, with\nthe most stable Zn-HESO with Co T , where it is not available as an active site. Among the\nd\ncalculated activities of four O sites (Ni, Fe, Mn, Cr), Cr has the lowest overpotential of 670\nh\nmV, but it is still larger than the best Co3+. The overpotentials of Fe, Mn, and Ni sites are 760\nmV, 940 mV, and 1120 mV, respectively. Altogether, these results indicate that Co3+ O is the\nh\nmost active site for OER in Zn-HESO. Since Zn-HESOs have a slightly higher amount of Co3+\nO than HESOs (Fig. 2c), we expect that Zn-HESO will be more active for OER (Figs. 5a-5c).\nh\nWhile more Co3+ O sites will increase the activity, aligning with our expectations of\nh\nimprovement of Zn-HESO compared to pristine HESO, it is also important to note that the\nlikely optimal Co occupation is less than 100%, as having the high entropy structure does lead\nto lower overpotentials than a pure Co spinel.”\nMinor Comments:\n1. The abbreviation ‘HEOs’ is used without being defined upon its first appearance in the\nintroduction part.\n“HEOs” (high-entropy oxides) at its first appearance in the Introduction.\n2.The elemental percentages/compositions for Mg-HESO, Ga-HESO, and Al-HESO should\nalso be provided, as demonstrated in supplementary Fig. 8.\nResponse: We appreciate the reviewer’s comment. We have now added the elemental\ncompositions of Mg-HESO, Ga-HESO, and Al-HESO, measured by XPS (Supplementary Fig.\n9).\nRevision: [Page 5] “X-ray photoelectron spectroscopy (XPS) (Supplementary Figs. 7–9)\nshows near-equimolar ratios of Co–Fe–Cr–Mn–Ni with additional substituent (Zn, Ga, Mg, or\nAl), similar to the precursor ratios, at the surface.”\nSupplementary Fig. 9 Quantification of atomic percentage through X-ray photoelectron\nspectroscopy high-resolution elemental spectra for pristine HESO and M-HESOs.\n43"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed the reviewers' comments and clarified the issues raised. I have no further suggestions at this stage.", "reviewer_2": "The authors have provided a thoughtful response to the reviewer’s comments. In the revised manuscript, they clarified the origin of the reference spectra used in the L-edge analysis and provided a more rigorous description of the fitting procedure. In addition, they incorporated additional hard and soft X-ray absorption analyses with relevant spinel-type reference compounds to better support the assignments of cation valence states and site occupancies.\nThese revisions significantly improve the clarity and reliability of the structural interpretation. In my opinion, the authors have satisfactorily addressed the concerns raised in the initial review. The manuscript is now suitable for publication. I therefore recommend acceptance of the manuscript in its present form.", "reviewer_3": "The authors have addressed most of my concerns, I accept the revision for this version."}, "author_response": ""}]

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