Universal complexing agent enabling advanced iron-cerium redox flow batteries

Domain: physics
Published: 2025-12-31
Editorial outcome: minor_revision

Abstract
The development of a safe and affordable redox flow battery technology is important for storing intermittent renewable energy. Here, we design a stable aqueous organic iron-cerium redox flow battery based on the inexpensive metal iron and the abundant rare earth metal cerium, enabled by the universal complexing agent diethylenetriaminepentaacetic acid. Molecular dynamics simulations are employed to screen for carboxyl-containing ligands with different electron donating capacities, revealing that diethylenetriaminepentaacetic acid is an effective candidate to chelate iron and cerium in the negolyte and posolyte, respectively, as verified by experimental characterization. The complexing agent enhances the redox characteristics of iron and cerium and reduces osmotic water migration between the negative and positive chambers by allowing the same ligand in both electrolytes. Our iron-cerium redox flow battery achieves an energy efficiency of 87.7% at 40 mA cm−2 and 80.6% at 100 mA cm−2, while retaining 95.3% of its initial capacity and maintaining around 86.3% energy efficiency after 500 cycles under neutral environments (100% of state-of-charge). The capacity is still preserved after 1779 cycles even when cycled at high-rates (80 mA·cm−2, 70% of state-of-charge).

Introduction
The intermittency of renewable energy sources, such as solar and wind, poses significant challenges to their integration into power grids1–4. The development of large-scale energy storage systems that are safe, affordable, and efficient will play an essential part in resolving this intermittency problem5–7. Among the various technologies available, redox flow batteries (RFBs) stand out due to their inherent safety, economic viability, and the ability to independently scale power and energy capacity8–10. Currently, the vanadium redox flow battery (VRFB) is the most mature technology pathway. However, it has not been able to fulfill the accelerating demand for long-term energy storage due to the scarcity of vanadium ore, for which it also competes with the steel industry11–14. Other reported RFB chemistries include iron-chromium, zinc-bromine, zinc-iron, and aqueous organic systems. However, each of these is constrained by intrinsic limitations such as chromium deactivation, bromine’s strong corrosiveness, zinc dendrite penetration of membranes, and the high sensitivity of organic active materials to air, respectively15–22. Due to the limitations imposed by their self-discharge, RFBs are typically employed for short-term energy balancing, reducing peak demand and filling low-demand periods on the grid. Now, with the increasingly large-scale integration of renewable energy into the grid, frequency modulation demands will rise, highlighting the need for new RFB technologies that address both peak regulation and frequency modulation23,24.

Iron has been utilized in RFB technology since the 1980s, attracting attention as a resource-rich and low-cost active material25. More recently, researchers have altered the solvation structure of iron by coordinating with organic ligands to construct alkaline iron-based RFBs. Such an RFB, composed of triethanolamine (TEA) and ferrocyanide, was found to achieve a voltage of 1.3 V26. Subsequently, a 3-[bis(2-hydroxyethyl)amino]−2-hydroxypropanesulfonic acid (DIPSO) ligand that was altered by sulfonating TEA achieved even greater Fe solubility27,28. In 2024, Li’s group successfully built an iron-based flow battery in a neutral environment by using a nitrilotri-(methylphosphonic acid) (NTMPA) ligand29. A serious issue that arises with this approach is that small organic ligands such as TEA can easily traverse the ion exchange membrane between the negative and positive chambers, causing side interactions with the active substances in the other chamber, along with the formation of hydroxide precipitate, shortening the lifespan of the RFB and jeopardizing the overall safety of the battery system30. Increasing the ligand’s steric hindrance effectively mitigates this shuttle problem31, but complete prevention of shuttling is impossible when electrolyte compositions differ between the negative and positive chambers over extended periods. Therefore, employing a universal complexing agent that functions effectively in both the posolyte and the negolyte presents a compelling strategy for homogenizing the two chambers. Nevertheless, this necessitates that the two active substances that constitute an RFB have a significant potential difference. Cerium has inherent benefits as the most abundant rare earth element and an eco-friendly material, with a standard redox potential as high as 1.65 V versus (vs.) SHE, and it can combine with other redox compounds that have low potential to build a high voltage RFB32,33. In typical cerium-based flow batteries, the posolyte is often a cerous mesylate system, but its sluggish kinetics result in a low current density for the battery34,35. Furthermore, the strongly acidic environment causes low coulombic efficiency (CE), accelerated battery deterioration, and harmful hydrogen evolution side reactions. To enhance its performance and create a stable flow battery, cerium’s solvation structure in water can be altered by introducing suitable electron-donating ligands to chelate with cerium.

We have developed an aqueous organic redox flow battery which incorporates iron and cerium as its active ingredients, and uses 5-carboxyl-containing diethylenetriaminepentaacetic acid (DTPA) as a universal complexing agent ligand. After identifying the optimal ligand candidate through molecular dynamics (MD) simulations, the coordination chemistry was manipulated to alter the redox characteristics and electrochemical stability of iron and cerium. This design allows a theoretical cell voltage of 0.92 V, with solubilities reaching 0.858 M and 0.927 M in the neutral flow battery system for iron and cerium, respectively. We used this universal complexing agent in both the negolyte and posolyte, eliminating the impact of ligand transmembrane transport and minimizing water migration caused by the variations in osmotic pressure and viscosity between the negolyte and posolyte. The Fe-Ce DTPA RFB consistently maintains a CE close to 100%, with an energy efficiency (EE) that reached up to 86.3%, and a discharge capacity after cycling of 95.3% of the original capacity (100% state of charge, SOC). Our universal complexing agent RFB demonstrates excellent performance in 500-cycle testing in a neutral environment. Even at a high cycling rate of 80 mA cm−2 (70% SOC), long-term cycling stability and capacity were maintained without attenuation for 1779 cycles. Our universal complexing agent strategy, unified across the negolyte and posolyte, offers an important approach for the advancement of RFBs.

Results
Theoretical calculation and structural characterization
In aqueous solution, metal ions form a stable coordination structure by interactions between the oxygen atom’s lone pair electrons in its 2p orbital and the metal atom’s empty orbital. The carboxylic acid group (−COOH) is a common oxygen-containing electron donor. Several common ligands have −COOH in varying amounts: iminodiacetic acid (IDA, two −COOH groups), nitrilotriacetic acid (NTA, three −COOH groups), ethylene diamine tetraacetic acid (EDTA, four −COOH groups), 1,3-propylenediaminetertaacetic acid (PDTA, four −COOH groups), diethylenetriaminepentaacetic acid (DTPA, five −COOH groups), among others (Fig. 1a). The number and position of −COOH, as well as the length of the ligand backbone, influence the coordination structure by modulating the electron donation intensity and steric hindrance. These factors offer effective approaches for designing RFB electrolytes.

To identify appropriate electron donor ligands, MD simulations and density functional theory (DFT) calculations were employed to diagnose the evolution of Fe and Ce solvation structures at the atomic scale in the presence of carboxylic acid-containing ligands. The results of MD simulations, shown in Fig. 1b–e, Supplementary Figs. 1, 2, and Supplementary Data 1 reveal that the addition of ligands leads to coordination interactions between the O atom of ligands near the Fe or Ce ion in each electrolyte, reducing the coordination number (CN) of H2O. This suggests that the oxygen-containing ligands greatly reshape the Fe and Ce solvating structure. For example, the CN of Fe−H2O(O) in Fe-DTPA drops from 6 to 3.7, as illustrated in Fig. 1b, c, yet the CN of Fe−DTPA(O) rises to 0.46. Fe−DTPA(O) has a shorter radial distribution function (≈1.94 Å) than Fe−H2O(O) (≈2.11 Å), suggesting that Fe and DTPA(O) have a stronger binding strength than Fe−H2O(O). Similarly, the Ce coordination structure in Fig. 1d, e is Ce(H2O)4.55(DTPA)0.48, and the radial distribution functions of Ce−DTPA(O) and Ce−H2O(O) are slightly bigger than Fe-DTPA (2.16 Å and 2.32 Å, respectively), which is due to the increased radius of the Ce atom itself.

MD simulations of Fe and Ce coordinated with ligands IDA, NTA, EDTA, and PDTA are presented in Supplementary Figs. 1, 2, with Supplementary Table 1 summarizing the details. Although DTPA has a greater molecular size and steric hindrance than NTA, EDTA, and PDTA, which may make coordination with metal atoms more difficult, its five −COOH groups provide a stronger electron-donating capability. This means that Fe−DTPA(O) has a higher CN (0.46) than Fe−NTA(O) (0.45), Fe−PDTA(O) (0.41), and Fe−EDTA(O) (0.34). Similarly, the CN of Ce−DTPA(O) (0.48) is higher than that of Ce−NTA(O) (0.46), Ce−PDTA(O) (0.47), and Ce−EDTA(O) (0.43). Mean squared displacement (MSD) plots of K+ ions and ligand molecular sizes are shown in Supplementary Figs. 3, 4. DFT calculation furthers support these findings, showing that the binding energies of Fe-DTPA (−178 kcal mol−1) and Ce-DTPA (−1427 kcal mol−1) are lower than those of Fe or Ce with other ligands (Fig. 1f, Supplementary Data 1). Compared to ligands with fewer −COOH groups, that is IDA, NTA, EDTA, and PDTA, DTPA with its five −COOH groups has a higher binding capacity to Fe or Ce ions and generates more stable Fe−O or Ce−O interactions. Consequently, we deduce that DTPA ought to be a viable option for both the negolyte and posolyte of the flow battery.

Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) characterizations were employed to experimentally verify the coordination structures in Fe-DTPA and Ce-DTPA electrolytes. The formation of the Ce−DTPA(O) bond was confirmed by the visible peak at 450 cm−1 in the Raman spectrum (Fig. 1g), which is absent in the reference spectrum for CeCl3. Similarly, the emergence of a new Fe-DTPA peak at 450−500 cm−1 confirms the formation of the Fe−DTPA(O) bond, and the disappearance of the Fe(H2O)6 peak (≈300 cm−1) in Fe-DTPA also confirms the change in solvation structure compared with a reference FeCl3 aqueous solution. Comparison with the Raman spectrum of free DTPA confirms that the new peaks observed in Fe-DTPA and Ce-DTPA arise from Fe–DTPA(O) and Ce–DTPA(O) bonds, rather than from the uncoordinated ligand. In the FTIR spectra, both Fe-DTPA and Ce-DTPA exhibit a new peak at ~935 cm−1, attributed to the stretching vibrations of the Ce−DTPA(O) and Fe−DTPA(O) bonds (Fig. 1h). The outcomes of the Raman and FTIR measurements match with the MD simulations. Synchrotron X-ray absorption spectroscopy measurements were performed to further clarify the local environment of the metal atoms. Fourier-transformed (FT) extended X-ray absorption fine structure (EXAFS) fitting curves are presented in Fig. 1k. The prominent peak of Fe-DTPA, corresponding to the Fe−DTPA(O), has a lower radical distance than the Fe−H2O(O) peak of the FeCl3 reference. This result indicates that a strong electron-donating ability exists in the first solvation shell of the Fe ions in Fe-DTPA, compared to the Fe(H2O)6 solvation shell of the FeCl3 reference. The bond length and chemical environment of iron complexes can be obtained through least-squares EXAFS fitting, with detailed information displayed in Supplementary Table 2. The fitting reveals two Fe−O bond lengths of 1.98 Å and 2.17 Å in Fe-DTPA, which closely match the results calculated by MD (1.94 Å and 2.11 Å).

To assess each electrolyte’s facility for electron transfer, quantum chemical calculations were performed. A narrower band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicates a stronger ability to transfer electrons, as it facilitates faster oxidation and reduction processes through stronger electronic coupling and lower activation barriers, respectively. Figure 1i, j, and Supplementary Data 1 show the frontier molecular orbital diagrams of the iron and cerium electrolytes, as well as their respective LUMO and HOMO energy levels. The calculated results show that the E value (the gap between the HOMO-LUMO) of Fe-DTPA and Ce-DTPA are 4.82 eV and 0.42 eV, respectively, which are the lowest values in their respective control groups, indicating their strong electron transfer properties.

Electrostatic potential (ESP) diagrams of Fe-DTPA and Ce-DTPA, shown in Fig. 1l and Supplementary Data 1, highlight five electron-rich regions in red, mainly concentrated in the oxygen atoms of the −COOH part. These regions are strong electron-donating sites for coordination with Fe or Ce. ESP comparisons with other ligand structures (Supplementary Fig. 5) show that the electron-donating capacity of ligands is mainly positively correlated with the number of −COOH, and DTPA, with its five −COOH groups having the strongest electron-donating capacity. Taken together, these results indicate that Fe-DTPA and Ce-DTPA formed by Fe, Ce, and DTPA are the most stable coordination structures.

Optical and physicochemical property investigation
The solubility of the electrolytes is a crucial factor for determining the energy density of the battery. To evaluate this, we investigated the maximum solubility of Fe-DTPA and Ce-DTPA. The ultraviolet-visible (UV-vis) absorption spectra of Fe-DTPA at various concentrations (0.1 M to 0.8 M solution diluted 1000-fold) are shown in Fig. 2a, b. A calibration curve was established between absorbance and concentration, revealing a maximum solubility for Fe-DTPA of 0.858 M. Figure 2c displays snapshots of the solutions at various concentrations of Fe-DTPA, showing how the reddish-brown color deepens with increasing solution concentrations. Similarly, we determined that Ce-DTPA has a maximum solubility of 0.927 M according to the absorption peak at 297 nm, which is marginally greater than Fe-DTPA’s maximum solubility (Fig. 2d–f). The fitting-curve at 270 nm showed a consistent trend with that of 297 nm, and the results are similar (Supplementary Fig. 6). To further characterize the properties of the electrolytes, the viscosity and conductivity of Fe-DTPA and Ce-DTPA were investigated (Fig. 2g). Both Fe-DTPA and Ce-DTPA’s conductivity and viscosity displayed a similar trend with a change in concentration. Conductivity increased with concentration and remained high at concentrations more than 0.6 M, while the viscosity for both Fe-DTPA and Ce-DTPA remained nearly identical, making them suitable for use as the negolyte and posolyte, respectively. UV-vis spectra acquired after 3, 5, and 7 days of the diffusion experiment show negligible crossover of active species through the membrane (Fig. 2h, Supplementary Fig. 7), ensuring a stable CE for the battery. Membrane penetration is greatly reduced because of the chelation of DTPA, which introduces a significant steric hindrance effect for both Fe-DTPA and Ce-DTPA compared to just Fe3+ and Ce3+. Alongside this, the matched viscosity of negolyte and posolyte and ligands markedly lessens the diffusion driving force brought on from using different electrolyte components36.

Electrochemical property study
The electrochemical characteristics of Fe-DTPA and Ce-DTPA electrolytes were evaluated using cyclic voltammetry (CV). Fe3+ and OH− are known to combine easily to form hydroxide, which is detrimental to flow battery operation as they increase pipeline pressure, cause leaks, and reduce battery capacity. We evaluated the CV curves of Fe-DTPA at different pH levels and found that the change in current density was within an acceptable range at pH = 4 to 9 (Supplementary Fig. 8a–d). 500-cycle tests of the solution component demonstrate that the stability is still good even when the pH is raised to 10 or 12; however, the current density is much decreased, and a precipitate was observed in the solution (Supplementary Fig. 8i–l). For Ce-DTPA, Supplementary Fig. 9 shows that there is no substantial difference between the potential and current density of the CV curves at pH = 4 and pH = 7, yet there is an irreversible reduction peak at pH = 9, and the cyclic stability deteriorates. This behavior is likely due to the permanent loss of active chemicals caused by partial hydroxides and oxides of Ce that arise following a decrease in H+ concentration. In conclusion, Fe-DTPA and Ce-DTPA operate optimally within the pH range of 4 to 7.

Based on these findings, subsequent investigations were performed under neutral pH conditions. CV measurements of Fe-DTPA and Ce-DTPA electrolytes were tested at a scan rate of 40 mV s−1, revealing that Fe-DTPA and Ce-DTPA exhibit a clear reversible redox pair with formal potentials of −0.28 V and 0.64 V (vs. saturated calomel electrode (SCE)), respectively (Fig. 3a). Therefore, 0.2 M Fe-DTPA and Ce-DTPA in the presence of 1.0 M NaCl supporting electrolyte can theoretically deliver a cell voltage of about 0.92 V. CV curves for other Fe/Ce-based electrolytes with 1.0 M NaCl are shown in Supplementary Fig. 10a; their theoretical voltages (IDA: 0.43 V, NTA: 0.69 V, EDTA: 0.87 V, and PDTA: 0.31 V) are all lower than that of DTPA (0.92 V). In addition, both Fe-DTPA and Ce-DTPA exhibit the highest current densities, confirming superior electrochemical activity. The electrochemical operating window of the DTPA solution (Fig. 3b) shows no significant redox activity, or any hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) within the electrochemical window (between −1.31 V and 1.19 V vs. SCE) at pH = 7. CV tests were performed at different scan rates to further elucidate the electrolyte dynamics (Fig. 3c, f). These reveal a strong linear relationship between the scan rate and the square root of the current density (Fig. 3d, g). It is notable that both slopes of the fitted Log(I) vs. Log(v) lines are close to 0.5 (Supplementary Fig. 11), indicating a diffusion-controlled redox process. Furthermore, throughout the entire 500-cycle tests performed on Fe-DTPA and Ce-DTPA electrolytes, minimal variation can be seen between the representative 1st to 500th cycles shown in Fig. 3e, h, indicating long-term electrochemical stability (full cycle data are shown in Supplementary Figs. 12, 13).

Redox flow battery evaluation
The Fe-Ce DTPA RFB was assembled and the charge and discharge curves at various current densities (40 mA cm−2, 60 mA cm−2, 80 mA cm−2, 100 mA cm−2, and 120 mA cm−2) were measured, as shown in Fig. 4a. As the current density increases, the battery’s effective capacity decreases due to the decreased electrolyte usage caused by a higher ohmic polarization, which is a common trend in flow batteries. The open-circuit voltage (OCV) of the full cell at 50% SOC was measured to be 0.73 V (Supplementary Fig. 14a), which is lower than the output voltage obtained by CV in a single electrolytic cell (Fig. 3a). This discrepancy is likely due to the differences in electrolyte concentration, thus altering the activity ratio of oxidized and reduced species ([cOx γOx]/ [cRed γRed]) according to the Nernst equation. To confirm this, further CV measurements were performed in full cells with 0.5 M Fe-DTPA and 0.5 M Ce-DTPA without supporting electrolyte, matching the electrolytes used in full battery tests (Supplementary Fig. 14b). The modified device included a saturated calomel reference electrode, enabling us to perform CV measurements under conditions more comparable to those of the full-cell tests (i.e., with a N212 membrane and 9 cm2 graphite felt electrodes). The obtained CV curves (Supplementary Fig. 14b–f) show that the voltage derived is about 0.74 V, close to the OCV at 50% SOC (Supplementary Fig. 14a).

The as-assembled Fe-Ce RFB exhibited an EE of 87.7% and 80.6% at 40 mA cm−2 and 100 mA cm−2, respectively. The corresponding efficiency shown in Fig. 4b illustrates that the CE remained consistently at or close to 100%, indicating the significantly improved performance from the ligand chelation approach. Discharge voltage and power density plots at different SOCs demonstrate that 245 mA cm−2 provides a maximum power density of 108 mW cm−2 (Fig. 4c). The nearly linear discharge curve suggests that ohmic polarization is the primary factor governing battery discharge. Long-term battery cycling tests were used to assess the stability and longevity, as well as its capacity and EE (Fig. 4d, e). FTIR and Raman analyses after cycling confirm that the electrolyte structure remained largely unchanged, highlighting its stability (Supplementary Fig. 15). Representative cycles from a 500-cycle test (Fig. 4d) indicate minimal voltage variation throughout the course of the cycle. The discharge capacity after the cycling test retained 95.3% of the original capacity, with the EE peaking at around 86.3%, accompanied by a CE consistently close to 100% when cycled under neutral conditions (100% SOC). These performance metrics are superior to those measured for Fe-Ce DTPA RFBs operated in acidic or alkaline conditions (details are shown in Supplementary Figs. 16–18). However, testing with the thicker N117 membrane resulted in a decrease in battery efficiency (Supplementary Table 3, Supplementary Fig. 19). Comparative tests using other reference ligands cycled under identical conditions revealed varying degrees of instability (Supplementary Fig. 20).

After establishing the optimal operating parameters for a Fe-Ce DTPA RFB, we carried out a long-term cycling test at 80 mA cm−2 and 70% SOC (Fig. 4f, Supplementary Fig. 21). The results show that the capacity is almost unchanged over the 1779 cycles of operation. The 0.5 M Fe-Ce DTPA RFB also exhibited CE consistently close to 100% (Supplementary Fig. 22). Compared to other reported Ce-based RFBs, batteries featuring Fe-Ce DTPA exhibit a combination of high EE and long cycle life (Supplementary Fig. 23)37–47. In situ Raman tests provide additional evidence for the stability of the Fe-DTPA and Ce-DTPA coordination structures under different SOCs (Fig. 4g, h). Additionally, we discovered that the Ce-DTPA exhibits a noticeable color variation under various SOCs, allowing us to approximate the electrolyte’s SOC based on color, although no equivalent color change was observed for the Fe-DTPA. Self-discharge tests were performed (Supplementary Fig. 24), where the cell is fully charged at 40 mA cm−2 and then allowed to undergo static self-discharge, while the pump continues to operate until the battery voltage is lowered to below 0.3 V. The results demonstrate that the charge lasted about 0.5 h, while the self-discharge time lasted more than 351 h. We attribute this low level of self-discharge to our universal complexing agent approach. Furthermore, after fully charging the Fe-Ce DTPA RFB and resting for 3 days under open-circuit conditions, the battery retained a CE of 97.9%, further confirming its resistance to self-discharge (Supplementary Fig. 24b).

We assembled an RFB stack with five single cells and an active area of 14.5 × 7.5 cm2 to evaluate the viability of a Fe-Ce DTPA RFB (Supplementary Fig. 25). Representative cycles (Fig. 4i) show the cycling stability and utility. An EE of 80.8% was achieved at a current density of 80 mA cm−2, as illustrated in Fig. 4j. Additionally, a decoupling method with K4[Fe(CN)6]/K3[Fe(CN)6] (positioned between the potentials of Fe-DTPA and Ce-DTPA, Fig. 4k), which showed almost no change in capacity and efficiency over 2000 cycles in asymmetric battery test (Fig. 4l), was utilized to demonstrate that Fe-Ce DTPA RFB’s total EE may be further improved by enhancing both the voltage efficiency (VE) of the Ce-DTPA side and the CE of the Fe-DTPA side. Detailed discussion is provided in the Supplementary Information and Supplementary Figs. 26–29.

Methods
Chemicals
Diethylenetriamine pentaacetic acid (DTPA, AR), iminodiacetic acid (IDA, AR), nitrilotriacetic acid (NTA, AR), ethylenediaminetetraacetic acid (EDTA, AR), 1,3-propylenediaminetertaacetic acid (PDTA, AR), anhydrous potassium carbonate (K2CO3, 99%), cerium chloride heptahydrate (CeCl3‧7H2O, 99%), ferric chloride hexahydrate (FeCl3‧6H2O, 99%), potassium chloride (KCl, 99%), potassium ferrocyanide trihydrate (K4[Fe(CN)6]‧3H2O, AR), and potassium ferricyanide (K3[Fe(CN)6], AR) were purchased from Shanghai Macklin Biochemical Technology Co., LTD. Sodium hydroxide (NaOH, 99%) was bought from Shanghai Aladdin Biochemical Technology Co., LTD. All chemicals were stored in an inert gas glove box to prevent water exposure and used without any treatment. The deionized water (DIW, 18.2 MΩ cm) was boiled under reduced pressure to remove the dissolved oxygen and carbon dioxide, and stored in reagent bottles filled with nitrogen.

Commercial Nafion membranes were purchased from Chemours. For acidic environment studies, membranes were soaked at 80 °C in 5% H2O2 for 1 h, in 0.5 M H2SO4 for 1 h, and in deionized water for 0.5 h before use. For alkaline environment studies, membranes were immersed in 1 M NaOH and 1.0 M KCl solution for 2 h. Membranes were then stored in deionized water, ready for use. The membranes need to be first soaked in electrolytes before electrochemical tests.

Graphite felts were bought from Liaoning Jin Gu Carbon Material Co., Ltd. and cleaned with acetone, ethanol, and deionized water (DIW). After impurity removal, graphite felts were annealed in the air at 500 °C for 5 h. The graphite felts should be removed from the floating fiber before use and fully soaked in the electrolytes.

Flow battery (LSB-1, YTH-1, 3 cm × 3 cm) subassemblies were provided by Wuhan Zhi Sheng New Energy Co., LTD. All parts of the devices in contact with the electrolytes are made of anti-corrosion materials. Some components in our battery devices, such as gaskets and electrode frames, were purpose-built for our application. The customized gaskets are made of polytetrafluoroethylene or ethylene propylene diene monomer. The central area of customized gaskets matches the active area of the battery, and the thickness can be 0.1, 0.2, or 0.5 mm, depending on the specific requirements. The peristaltic pumps were purchased from Baoding River Fluid Technology Co., LTD. All battery tests were performed with the CT3002K battery testing system purchased from Wuhan Land Electronics Co., Ltd. The battery stack with five single cells (FSS-SK, testing bench WHZS-STS-QP) and an active area of 14.5 × 7.5 cm2 were custom-made by Wuhan Zhi Sheng New Energy Co., LTD.

Preparation of 0.5 M Fe-based electrolytes
33.79 g of FeCl3‧6H2O and 49.17 g of DTPA were added into 200 mL of deoxidized DIW and fully stirred for 10 min at 20 °C. Then 51.83 g of K2CO3 was gradually added to the solution and stirred for 24 h until a uniform solution formed. K2CO3 was used to deprotonate the DTPA ligand and enable stable metal–ligand complexation, while avoiding rapid pH rise and metal hydroxide precipitation. Under the monitoring of a pH meter, we adjusted the pH to 4, 7, or 9 with HCl or KOH solution. Finally, the volume of the solution after ultrasonic defoaming was adjusted to 250 mL with deoxidized DIW. All preparation steps were conducted under a nitrogen atmosphere to avoid oxidation and absorption of carbon dioxide. The configuration of other Fe-based electrolytes is similar to the above process, simply replacing DTPA with IDA, NTA, EDTA, PDTA and adjusting the pH to a neutral level.

Preparation of 0.5 M Ce-based electrolytes
Similar to the preparation of solution Fe-DTPA, 46.57 g of CeCl3‧7H2O and 49.17 g of DTPA were added into 200 mL of deoxidized DIW and fully stirred for 10 min at 20 °C. Then 51.83 g of K2CO3 was gradually added to the solution and stirred for 24 h until a uniform solution formed. Under the monitoring of a pH meter, we adjusted the pH to 4, 7, or 9 with HCl or KOH solution. Finally, the volume of the solution after ultrasonic defoaming was adjusted to 250 mL with deoxidized DIW. The entire configuration process was prepared in a nitrogen environment to avoid oxidation and absorption of carbon dioxide. Other Ce-based electrolytes are similar to the above process, simply replacing DTPA with IDA, NTA, EDTA, or PDTA, and adjusting the pH to neutral.

Preparation of K4Fe(CN)6 (K3Fe(CN)6) electrolytes
52.8 g of K4Fe(CN)6 (41.16 g of K3Fe(CN)6) and 9.32 g of KCl were dissolved into 200 mL deoxidized DIW and stirred for 30 min. Under the monitoring of a pH meter, the pH was adjusted to 7. The solution was adjusted to 250 mL with deoxidized DIW at normal temperature.

Electrochemical measurements
Cyclic voltammetry (CV) measurements were performed at 25 ± 1 °C at different sweep rates from 10 mV s−1 to 50 mV s−1 using a standard three-electrode system consisting of a graphite working electrode (6 mm in diameter), a saturated calomel reference electrode with KCl salt bridge, and a platinum network (1 cm2) counter electrode on a CHI660 electrochemical workstation (Shanghai Chen Hua Instrument Co., LTD). 15 mL of different Fe-based negolyte and Ce-based posolyte in the presence of 1.0 M NaCl supporting electrolyte were used for CV tests. The electrochemical cell was purged with nitrogen to remove air before the CV tests. As for evaluating the electrochemical window of DTPA electrolyte by CV, the electrolyte was prepared by dissolving 49.17 g of DTPA in 200 mL of deoxygenated deionized water under stirring for 10 min at 20 °C. Subsequently, 51.83 g of K2CO3 was gradually added to the solution, followed by continuous stirring for 24 h until a homogeneous solution was obtained. The pH of the final electrolytes was adjusted to 7 by adding HCl or KOH solution.

CV measurements were also performed in a full cell for 0.5 M Fe-DTPA and 0.5 M Ce-DTPA with a saturated calomel reference electrode under similar battery operating conditions.

Battery tests
The flow battery system was assembled by connecting a battery device to the negolyte and posolyte tanks through two peristaltic pumps and an anti-corrosion silicone tube. A battery structure with two prepared graphite felt electrodes with an effective area of 9 cm2 was used on each side of a membrane as positive and negative electrodes, and handmade Teflon gaskets were employed to control the electrode compression ratio. The entire battery system was cleaned with nitrogen before the test began to ensure the absence of oxygen and carbon dioxide. The peristaltic pump pushed the electrolyte from the tank into the battery stack at a flow rate of 100 mL min−1. In the rate performance tests, five charging and discharging cycles were evaluated at each current density, batteries were charged to 1.0 V and discharged to 0.3 V at a current density of 40 mA cm−2 to 120 mA cm−2. For the long cycle tests (under 100% SOC unless stated otherwise) at a current density of 40 mA cm−2 with 15 mL electrolytes, batteries were charged to 1.0 V and discharged to 0.3 V (N212 membrane). The longer cycle tests with 15 mL electrolytes were charged at 80 mA cm−2 to 70% SOC, or cutoff at 1 V (N211 membrane), avoiding the high-voltage regime where polarization and parasitic reactions dominate. Fe-DTPA / K4[Fe(CN)6] RFB and K3[Fe(CN)6] / Ce-DTPA RFB assembled by N212 membrane.

Computational details
To analyze the coordination structure of Fe-based or Ce-based electrolytes, molecular dynamics (MD) simulations and density functional theory (DFT) were performed using Gaussian 09 and the GROMACS package, respectively. All the geometric optimizations of each individual ion and each individual molecular compound were performed at the B3LYP/6-31+G(d,p) theoretical level, and the solution is water. The force field parameters were obtained from General Amber Force Field (GAFF) and assigned with RESP charges. The size of the box is 10 × 10 × 10 nm3, and periodic boundary conditions were set in all three directions. FeCl3 / CeCl3, IDA/NTA/EDTA/PDTA/DTPA, K+, Cl−, and H2O were randomly filled into simulated chambers with different molecular ratios. The Packmol software was used to build all the initial structures of the cell. The electrostatic interaction was calculated by the PME method. A cutoff length of 1 nm was used for the calculation of electrostatic and non-electrostatic interactions in real space. In order to obtain a reasonable initial configuration, the fastest descent integration protocol was used to minimize the energy. The system then anneals from 0 to 298/263 K in a time of 0.5 ns, and then runs another 1 ns to reach equilibrium. Finally, the Parrinello-Rahman method of 10 ns NPT was performed once, and the stable product was obtained. The final 5 ns was sampled for radial distribution function (RDF) and coordination structure counting analyses.

Physical characterization
The conductivity was measured using a DDSJ-308F conductivity meter equipped with a DJS-1VTC electrode. Prior to the measurements, the instrument was calibrated at 25 °C using standard solutions. After calibration, the electrode was thoroughly rinsed and immersed in the electrolyte samples. The measurements were carried out at a constant temperature of 25 °C, and the final conductivity value was automatically recorded once the instrument reading stabilized. To confirm the coordination structure and component of the samples, Fourier transform infrared (FTIR) spectra were measured on an FTIR spectrometer (Thermo Fisher, Nicolet 6700, USA). Raman and in situ Raman spectroscopy (inVia Reflex) were conducted to analyze the solvation structural evolution (514 nm laser wavelength). During the test, a small amount of electrolyte is exported synchronously through the branch to observe the color and electrolyte state. Then, a pipette was used to immediately take an appropriate amount of liquid and store it in a sample bottle filled with nitrogen. The X-ray diffraction (XRD) patterns of samples were collected on an X-ray diffractometer (XRD-6000) with Cu Kα radiation ranging from 10 to 90° at a scan rate of 5°/min. After the battery tests, the battery device was removed from the testing station, and the inlet and outlet ports were sealed with rubber stoppers. The battery device was then transferred into the glove box, where the electrodes were disassembled and sealed for subsequent measurements. The Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra were measured at BL01C at the Taiwan Light Source, NSRRC. Fe-DTPA solutions were directly used at room temperature without further treatment. Before the analysis at the beamline, samples were placed into aluminum sample holders and sealed using Kapton tape film. The energy resolution (ΔE/E) for incident X-ray photons was found using a Si(111) double crystal monochromator. EXAFS spectra were recorded in transmission mode and obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, the χ(k) data were Fourier transformed to real (R) space using a Hanning window (dk=1 Å−1) to separate the EXAFS contributions from different coordination shells.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Redox flow batteries (RFBs) offer advantages like flexibility and decoupled power and energy design, yet vanadium-based systems face resource limitations. While iron/cerium alternatives avoid scarcity issues, they typically suffer from capacity fade due to electrolyte crossover. This study presents an iron/cerium-based aqueous organic RFB employing 5-carboxyl-containing diethylenetriaminepentaacetic acid (DTPA) as a universal complexing agent of iron and cerium. By using DTPA ligands in both catholyte and anolyte, this work effectively eliminates the transmembrane of ligands and mitigates water migration caused by osmotic/viscosity differences. Crucially, this unified ligand strategy significantly reduces electrolyte crossover. Considering the novelty of the proposed strategy I recommend publishing this work after addressing the following minor comments.\n1.Coordination Number Discrepancy: Figure 1b shows a coordination number (CN) of 0.46 for Fe-DTPA(O), whereas the main text states 0.45. This inconsistency requires clarification.\n2.Absence of Ce-H₂O(O) Bond: The CeCl₃ electrolyte in Figure 1g does not exhibit the Ce-H₂O(O) bond. Could the authors explain why this bond is missing?\n3.Rapid Capacity Decay: The stack capacity decreases unusually fast during cycling. What factors contribute to this rapid degradation?\n4.Ambiguity in Capacity Data: Figure 4k presents efficiency and capacity for the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] electrolyte. Is this capacity specific to the Fe-DTPA/K₄[Fe(CN)₆] flow cell or the K₃[Fe(CN)₆]/Ce-DTPA system? Further clarification is needed.\n5.Conductivity Terminology and Methodology: The legend of Figure 2g refers to “electrical conductivity” of Fe-DTPA and Ce-DTPA. Should this instead be termed “ionic conductivity”? Additionally, experimental details for these measurements are not provided.\n6.Visualization and Data Mismatch: In Figure 1e, the color representation of Ce overlaps with that of C in the simulation model. Furthermore, the coordination number in the figure conflicts with the description in the main text. These issues should be addressed.", "reviewer_2": "This work by Yang et al. reports a high-performance Fe/Ce redox flow battery using DTPA as a complexing ligand for both the Fe-based anolyte and Ce-based catholyte. Compared to existing V/Ce RFBs, the proposed Fe/Ce chemistry offers advantages such as lower material costs by replacing V with Fe. Notably, the Fe/Ce RFB operates at pH 7, avoiding many technical challenges associated with the strong sulfuric acid environment of V/Ce chemistry. I support the publication of this manuscript; however, some concerns and questions regarding technical details and clarity should be addressed.\nDetailed comments:\n1. The following sentence in the introduction may need further clarification. I believe the authors are referring to TEA crossover; if so, this should be explicitly stated. Since NTMPA complexes exhibit much stronger complexation with Fe ions, crossover has not been reported as an issue in the literature.\n“However, organic ligands of small molecules traverse the ion exchange membrane between the cathode and anode chambers effortlessly, causing side interactions with the active substances in the other chamber along with the formation of hydroxide precipitate, shortening the lifespan of the RFB and jeopardizing the overall safety of the system battery”\n2. The coordination structures of Fe-DTPA and Ce-DTPA in Figures 1c and 1e are somewhat confusing. It would be helpful to present them in a larger size with correct color coding for better clarity.\n3.The coordination structures of Fe-DTPA and Ce-DTPA in Figures 1c and 1e are somewhat confusing. It would be helpful to present them in a larger size with correct color coding for better clarity.\n4.The authors calculated the band gap between the HOMO and LUMO for Fe- and Ce-based complexes with different ligands, finding the lowest values for Fe-DTPA and Ce-DTPA complexes. However, it is unclear how these results relate to the electron transfer kinetics or redox potential of the complexes.\n5. The Raman spectra of Fe-DTPA and Ce-DTPA should be compared with free DTPA ligand to see whether the complexation alters the Raman shift. The new bands reported in here may come from the free ligand.\n6. The HER and OER data presented in Figure 3b could be misleading, as the stable window for aqueous-based electrolytes is typically around 1.3 V, varying with pH. Displaying a 2.5 V stable window in Figure 3b lacks context regarding how it was measured, including details on the working electrode, pH, and salt concentration. Clarification on these parameters is needed.\n7. The self-discharge test is crucial for evaluating the stability of the Fe/Ce RFB at high SOCs. Rather than solely monitoring the battery’s OCV, a more accurate approach would be to measure the discharge capacity after holding the Fe/Ce battery at 100% SOC for a specific period, such as one week.\n8. Employing Fe-CN electrolytes to decouple Fe-DTPA and Ce-DTPA electrolytes could be approached differently, such as by conducting symmetrical flow battery tests. For example, a flow battery with symmetrical electrolytes—using either Fe-DTPA or Ce-DTPA on both the cathode and anode sides—could provide clearer insights into the performance of each electrolyte.\n9. Authors could plot Fig. 3d and g in a different way (Log(I) vs Log(scan-rate)) to see whether it is truly diffusion controlled reaction (slop should be close to 0.5).", "reviewer_3": "The design and development of a safe and low-cost redox flow battery (RFB) is garnering increasing attention to store intermittent renewable energy. Yang et al. present an interesting highly stable and affordable aqueous organic iron-cerium (Fe-Ce) RFB. A universal and unexplored complexing agent diethylenetriamine pentaacetic acid (DTPA) was demonstrated to chelate Fe and Ce in the anolyte and catholyte, respectively, as verified by a combined molecular dynamics simulations and experimental characterization. The use of DTPA was shown to boost the redox characteristics of Fe and Ce and increase the cell voltage, and also alleviate water migration resulting from osmotic pressure or viscosity differences between the anode and cathode. High energy efficiencies with impressive cycling stability exceeding 1770 cycles at a cycling rate of 80 mA·cm 2 were achieved. Overall, the results are interesting and well presented. It is therefore suitable for the broad readership of Nat. Commun. and can be considered for publication in Nat. Commun. after addressing the following comments.\n1. Did the coordination structure of the electrolytes change after extended cycles? Some spectroscopy methods such as FTIR and Raman can be used to investigate this.\n2. The colors of balls used in Fig. 1c, e, i, j, and l need to be labeled.\n3. In Figure 1, the authors have demonstrated the interaction between Fe3+, Ce3+, and DTPA, however, the relative evidence is insufficient. What are the molecular size of DTPA and the mean square displacement of K+ ions? These results may help insightfully understand the underlying mechanisms.\n4. Two UV-vis peaks were easily identified for Ce-DTPA solutions as shown in Fig. 2d and e, whereas Fig. 2e only provided the fitting results for one peak. What’s the result if the peak was used for fitting? Will the obtained result be consistent with the provided one?\n5. In Fig. 3e and h, the curves are too overlapped. It is hard to differentiate them. For clarity, the authors can provide each cycle curve in the SI.\n6. For better understanding, the authors need to provide details of specific parameters (e.g., active area, number of single cells, etc.) in Fig. 4l.\n7. Although the CV curves of IDA, NTA, EDTA, and PDTA reference species have been provided in the paper, their performance in battery tests is not shown. This needs to be added.\n8. Why did the authors use a membrane N212? Did they try thicker membranes such as N117, etc.?\n9. The authors report stable operation for 1779 cycles, but what happens after this?\n10. In Figure 1, the structures of Fe(DTPA) and Ce(DTPA) need to be further confirmed. The crystal structure of Fe(DTPA) chelate has been demonstrated in the study by Michael P. Marshak et al (ACS Energy Lett. 2020, 5, 1758−1762). The Fe(DTPA) chelate exhibits a 7-coordinate structure, binding through 3 nitrogen and 4 oxygen atoms in a pentagonal bipyramidal geometry.\n11. In Figure 2h, the authors mention that “virtually no active substances will pass through the membrane”. This conclusion requires validation through quantitative UV-Vis spectroscopy data.\n12. In Figure 4e, the discharge capacity is obviously lower than the charge capacity, yet the battery retains 95.3% of its initial capacity after 500 cycles. Was this test conducted under a capacity-imbalanced cell? The authors should provide further explanation regarding the relatively low Coulombic efficiency and outstanding capacity retention rate.\n13. In Figure 4e and Figure 4j, the authors should elucidate the underlying mechanism responsible for the observed capacity degradation."}, "author_response": "Point-by-point response to the reviewers’ comments\nManuscript ID: NCOMMS-25-10508-T\n“Unified electrolyte design for advanced redox flow batteries: a universal complexing\nagent for high performance iron-cerium redox flow batteries”\nItalic Black Text = Reviewer Comments\nBlack Text = Author Response\nBlue Text = Original Manuscript\nBlack Text Marked in Yellow = Modifications to the Manuscript\nReviewer #1 (Remarks to the Author):\nRedox flow batteries (RFBs) offer advantages like flexibility and decoupled power and\nenergy design, yet vanadium-based systems face resource limitations. While\niron/cerium alternatives avoid scarcity issues, they typically suffer from capacity fade\ndue to electrolyte crossover. This study presents an iron/cerium-based aqueous\norganic RFB employing 5-carboxyl-containing diethylenetriaminepentaacetic acid\n(DTPA) as a universal complexing agent of iron and cerium. By using DTPA ligands\nin both catholyte and anolyte, this work effectively eliminates the transmembrane of\nligands and mitigates water migration caused by osmotic/viscosity differences.\nCrucially, this unified ligand strategy significantly reduces electrolyte crossover.\nConsidering the novelty of the proposed strategy I recommend publishing this work\nafter addressing the following minor comments.\nResponse: We are very grateful to the reviewer for the positive assessment of our\nwork as well as the very helpful suggestions. The comments of the reviewer have\nhelped to enrich our paper. We respond to each comment one by one below.\nComment 1-1: Coordination Number Discrepancy: Figure 1b shows a coordination\nnumber (CN) of 0.46 for Fe-DTPA(O), whereas the main text states 0.45. This\ninconsistency requires clarification.\nResponse: Thank you very much for your detailed review. In the original manuscript,\nwe inadvertently mistakenly wrote 0.46 as 0.45, and the correct coordination number\n(CN) should be 0.46 as shown in Figure 1b. We have corrected this mistake and\namended the main text in revised manuscript.\nThe original paragraph\n“The coordination number (CN) of FeH O(O) in Fe-DTPA drops from 6 to 3.7,\n2\nas illustrated in Figure 1b and 1c, yet the CN of FeDTPA(O) rises to 0.45”\nhas been changed to\n“The coordination number (CN) of FeH O(O) in Fe-DTPA drops from 6 to 3.7,\n2\nas illustrated in Figure 1b and 1c, yet the CN of FeDTPA(O) rises to 0.46”.\nComment 1-2: Absence of Ce-H₂O(O) Bond: The CeCl₃ electrolyte in Figure 1g does\nnot exhibit the Ce-H₂O(O) bond. Could the authors explain why this bond is missing?\nResponse: Thank you for your valuable comment. The CeH O(O) bond-related\n3\nnot clearly visible in the original spectrum. In order to better visualize this feature, we\nhave performed local amplification in the corresponding region of the Raman\nspectrum. According to the study (Journal of Molecular Liquids, 2007, 136, 323330,\nDOI: 10.1016/j.molliq.2007.08.006), a weak polarized Raman band appears near\n330 cm1 in concentrated CeCl aqueous solutions. This band has been assigned to the\n3\nbreathing vibrational mode of the cerium hydrate, i.e., the CeO vibration between\nCe3+ and the water molecules in its hydration shell. The intensity of this band decreases\nwith decreasing CeCl concentration, indicating its dependence on the hydration\n3\nstructure and coordination environment of the cerium ions.\nIn our study, the weak CeH O(O) Raman feature near 330 cm1 is consistent with\n2\nthis literature. We have clarified and enhanced this region in the revised Figure 1g for\nbetter visibility. We hope this explanation addresses your concern and helps clarify the\nCeH O(O) bond.\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nComment 1-3: Rapid Capacity Decay: The stack capacity decreases unusually fast\nduring cycling. What factors contribute to this rapid degradation?\nResponse: Thank you very much for your insightful comments and suggestions. The\nrapid capacity decay observed in the stack system is primarily attributed to several\nengineering challenges encountered during the scale-up process from single cell to\nstack configuration. These include factors such as suboptimal flow field design,\nwelding quality of stack components, and increased active area, all of which can\nsignificantly affect the electrochemical performance despite using the same electrolyte.\nIn particular, the main issue identified in our stack system was the lack of an effective\ntemperature control system. During cycling, heat generated by the magnetic drive\npump and the stack’s own electrochemical reactions accumulated within the\nelectrolyte and the stack, resulting in a gradual rise in system temperature. Elevated\ntemperatures are known to accelerate side reactions, which in turn lead to partial\nelectrolyte degradation and capacity loss. In contrast, the single-cell system—due to\nits smaller volume and the use of a peristaltic pump—did not experience such\nsignificant heat buildup, and thus maintained better stability.\nTo address this issue, we have upgraded our stack testing platform by introducing a\nsemiconductor-based cooling system (Figure R1), along with a temperature controller\nto maintain the stack temperature consistently at 30 ± 1 ℃. With this improvement,\nwe re-evaluated the performance of the same electrolyte. The updated results,\npresented in the revised Figure 4j, demonstrate a significantly improved cycling\nstability and a peak EE of 80.8%, confirming the effectiveness of the thermal\nmanagement upgrade.\nFigure R1. The photograph of the semiconductor-based cooling system employed in\nthis work.\nFigure 4. (a) Voltage-capacity diagram of Fe-Ce DTPA RFB at different currents and (b)\ncorresponding efficiency and currents. (c) Discharge voltage and power densit1y plots vs.\ncurrent density. Power densities are shown for the 60%, 70%, 80%, 90%, and 100% SOC,\nrespectively. (d) Partial characteristic charge and discharge curve in full battery test. (e)\nEfficiency and capacity data of the full battery test at pH = 7. (f) Cycle test of 0.5 M Fe-Ce\nDTPA RFB and (g) the performance comparison of Fe-Ce DTPA RFB with other Ce-based\nRFBs. In situ Raman spectra on (h) Fe-DTPA and (i) Ce-DTPA obtained from 0 to 100%\nSOC, with the real-time status of the corresponding electrolyte is recorded. (j) The\nperformance of the Fe-Ce DTPA RFB stack. (k) Efficiency and capacity of\nK [Fe(CN) ]/K [Fe(CN) ] electrolyte in a 1031 h cycling test. The illustration shows the\n3 6 4 6\nredox potential, and the charge-discharge curves at the beginning and end of the test. (l)\nDigital photo of Fe-Ce DTPA RFB stack system. The battery stack was constructed with 5\nsingle cells in series with an active area of 14.5 × 7.5 cm2.\nThe sentence on page 9 in the main text “An EE of 80.8% was achieved at a current\ndensity of 80 mA·cm2, as illustrated in Figure 4j” has been updated.\nWe hope this clarification and the additional data can address your concern.\nComment 1-4: Ambiguity in Capacity Data: Figure 4k presents efficiency and\ncapacity for the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] electrolyte. Is this capacity specific to the\nFe-DTPA/K₄[Fe(CN)₆] flow cell or the K₃[Fe(CN)₆]/Ce-DTPA system? Further\nclarification is needed.\nResponse: Thank you very much for carefully reviewing our manuscript and this\nimportant question. The battery capacity shown in Figure 4k corresponds to the\nsymmetrical battery using K [Fe(CN) ]/K [Fe(CN) ] electrolytes. The capacity shown\n3 6 4 6\nis independent of the Fe-DTPA/K [Fe(CN) ] and K [Fe(CN) ]/Ce-DTPA batteries.\n4 6 3 6\nThe capacities of the decoupled Fe-DTPA/K [Fe(CN) ] and K [Fe(CN) ]/Ce-DTPA\n4 6 3 6\nbatteries are shown in Figure S23a and Figure S23b, respectively. To further clarify\nthis, the original paragraph on page 9 in main text\n“Finally, a decoupling method with K [Fe(CN) ]/K [Fe(CN) ] (Figure 4k) is\n3 6 4 6\nutilized to demonstrate that Fe-Ce DTPA RFB's total EE may be further improved\nby enhancing both the VE of Ce-DTPA side and the CE of Fe-DTPA side, which\nis what we are focusing on next (details are in Figures S12 to S14 in SI).”\nhas been changed to\n“Finally, a decoupling method with K [Fe(CN) ]/K [Fe(CN) ], which showed\n3 6 4 6\nalmost no change in capacity and efficiency over 2000 cycles of the symmetric battery\ntest (Figure 4k), is utilized to demonstrate that Fe-Ce DTPA RFB's total EE may\nbe further improved by enhancing both the VE of Ce-DTPA side and the CE of\nFe-DTPA side. Detailed discussion in this regard can be found in the SI and\nFigures S23 to S26.”\nRelated discussion in the SI can be found as follows:\nFigure S23. (a) Cycle test and (b) capacity response versus current density for decoupled Fe-\nDTPA/K [Fe(CN) ] RFB at pH = 7. (c) Cycle test and (d) capacity response versus current density\n4 6\nfor decoupled K [Fe(CN) ]/Ce-DTPA RFB at pH = 7.\n3 6\nTo better understand the battery's characteristics and further improve the\nperformance of Fe-Ce DTPA RFB in the future, a method of decoupling anolytes and\ncatholytes was adopted here. The redox potential of K [Fe(CN) ]/K [Fe(CN) ]\n3 6 4 6\n(denoted as Fe-CN) (0.27 V vs. SCE) falls between Fe-DTPA (0.28 V vs. SCE) and\nCe-DTPA (0.64 V vs. SCE) (inset of Figure 4k). In this method, Fe-Ce DTPA RFB can\nbe divided into two distinct batteries: Fe-DTPA/K [Fe(CN) ] RFB and\n4 6\nK [Fe(CN) ]/Ce-DTPA RFB, which are beneficial for studying the attenuation and\n3 6\nefficiency of the anolyte and catholyte, respectively. The decoupling method needs to\nfollow the requirement that the chosen intermediary is a highly stable active material\nthat will not breach the membrane and minimally influence the other chamber. With\nits superior stability, i.e., almost no change in capacity or efficiency over 2000 cycles\n(exceeding 1031 h) of the symmetric battery test, and no difference in voltage between\nthe first 10 and the last 10 cycles of the charge-discharge cycle test, Fe-CN is\nunquestionably a reliable candidate, as illustrated in Figure 4k. The only capacity\nvariation results from solvent volatilization during the long test period, and a small\nfraction of the electrolyte crystallizes on the wall of the electrolyte storage tank which\nthen drops and redissolves in solution. The CE of Fe-DTPA/K [Fe(CN) ] RFB\n4 6\n(~98.5%) is lower than that of K [Fe(CN) ]/CE-DTPA RFB (99.7%) when the solution\n3 6\npH is 7, as illustrated in Figure S23a and S23c, suggesting that the HER side reaction\nis more likely to occur on the Fe-DTPA side. The loss in capacity is also caused by\nsome active chemicals' failure to restore their valence states. However, the VE of\nK [Fe(CN) ]/Ce-DTPA RFB oscillates downward and fluctuates, indicating that the\n3 6\nCe-DTPA side is more prone to the side reaction that changes resistance in the battery,\nwhich is most likely caused by the generation of hydroxide/oxide at the\nelectrode/membrane interface. Figures S24 and S25 show the internal resistance of\nFe-DTPA/K [Fe(CN) ] RFB and K [Fe(CN) ]/Ce-DTPA RFB before and after charge\n4 6 3 6\nand discharge tests. The charge and discharge resistivities of Fe-DTPA/K [Fe(CN) ]\n4 6\nRFB are 0.765 and 0.783 ·cm2, respectively, prior to the cyclic test; these values rise\nto 0.864 and 0.846 ·cm2, respectively, following the test. The values of enhanced\nresistivity are 0.099 and 0.063 ·cm2, respectively. In contrast, the rise values of\ncharge and discharge resistivity of K [Fe(CN) ]/Ce-DTPA RFB before and after test\n3 6\nare 0.225 (i.e., 1.179  0.954) and 0.126 ·cm2 (i.e., 1.125  0.999), respectively,\nwhich are both significantly greater than the internal resistance change of Fe-DTPA\nside. According to the rate performances (Figure S23b and S23d), Ce-DTPA is more\nsensitive to current intensity than Fe-DTPA, hence polarization has a stronger effect\non Ce-DTPA. As a whole, the results are consistent with the full battery test results\nand exhibit a \"Buckets effect\" under neutral conditions. The Fe-Ce DTPA RFB's total\nEE may be further improved by enhancing both the VE of Ce-DTPA side and the CE\nof Fe-DTPA side.\nFigure S24. Fe-DTPA/K [Fe(CN) ] RFB internal resistance of the battery before (a and b) and\n4 6\nafter (c and d) charge and discharge tests.\nFigure S25. K [Fe(CN) ]/Ce-DTPA RFB internal resistance of the battery before (a and b) and\n3 6\nafter (c and d) charge and discharge tests.\nTo further isolate and evaluate the intrinsic electrochemical performance of Fe-\nDTPA and Ce-DTPA electrolytes, we conducted symmetrical flow battery tests. Fe3+-\nDTPA and Ce3+-DTPA were first employed as anolyte and catholyte, respectively, to\nassemble a Fe3+-Ce3+ DTPA RFB and complete the initial charging process. Following\nfull charge, Fe3+-DTPA was reduced to Fe2+-DTPA, enabling the assembly of a Fe3+/2+-\nDTPA symmetrical RFB with fresh Fe3+-DTPA as the anolyte. Similarly, Ce3+-DTPA\nwas oxidized to Ce4+-DTPA and paired with unreacted Ce3+-DTPA to form a Ce3+/4+-\nDTPA symmetrical RFB. As shown in Figure S26, both systems exhibited stable\ncycling performance. The Ce3+/4+-DTPA symmetrical RFB demonstrated a higher CE\nand superior capacity retention compared to the Fe3+/2+-DTPA system. These results\nare consistent with those obtained from the decoupled method, further validating that\nthe overall CE of the Fe-Ce DTPA system can be effectively improved through\noptimization of the Fe-DTPA side.\nFigure S26. The CE and capacity of cycle tests for Ce3+/4+-DTPA and Fe3+/2+-DTPA symmetrical\nRFB.\nComment 1-5: Conductivity Terminology and Methodology: The legend of Figure 2g\nrefers to “electrical conductivity” of Fe-DTPA and Ce-DTPA. Should this instead be\ntermed “ionic conductivity”? Additionally, experimental details for these\nmeasurements are not provided.\nResponse: We thank the reviewer for these very helpful comments and suggestions.\nAccording to his/her suggestions, we have changed the terminology “electrical\nconductivity” in the legend of Figure 2g to “ionic conductivity”.\nIn addition, we have added the experimental details of conductivity tests on page 11\nin the revised manuscript as follows:\n“The conductivity was measured using a DDSJ-308F conductivity meter equipped\nwith a DJS-1VTC electrode. Prior to the measurements, the instrument was calibrated\nat 25 ℃ using standard solutions. After calibration, the electrode was thoroughly\nrinsed and immersed in the electrolyte samples. The measurements were carried out\nat a constant temperature of 25 ℃, and the final conductivity value was automatically\nrecorded once the instrument reading stabilized.”\nComment 1-6: Visualization and Data Mismatch: In Figure 1e, the color\nrepresentation of Ce overlaps with that of C in the simulation model. Furthermore,\nthe coordination number in the figure conflicts with the description in the main text.\nThese issues should be addressed.\nResponse: We thank the reviewer very much for pointing out these important issues\nand the valuable suggestions. In response, we have modified the color of the Ce\nelement to avoid overlap with that of C and ensure clear visual distinction.\nAdditionally, we have slightly increased the model sizes of Fe and Ce atoms to\nimprove the overall clarity of the simulation figure. To further aid interpretation, we\nhave also included a color legend in the figure (Figure 1c and e) to explicitly indicate\nthe corresponding element for each sphere color used.\nThe original Figure 1\nFigure 1. (a) Molecular formulas for IDA, NTA, EDTA, PDTA, and DTPA. (b) Radial\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nhas been modified to\nFigure 1. (a) Molecular formulas for IDA, NTA, EDTA, PDTA, and DTPA. (b) Radial\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nSimilarly, the original Figures S1 and S2\nFigure S1. Radial distribution function and corresponding coordinated number of (a) Fe-IDA, (b)\nFe-NTA (c) Fe-EDTA and (d) Fe-PDTA. 3D snapshot and coordination structure of (e) Fe-IDA, (f)\nFe-NTA (g) Fe-EDTA and (h) Fe-PDTA obtained from MD simulations.\nFigure S2. Radial distribution function and corresponding coordinated number of (a) Ce-IDA, (b)\nCe-NTA (c) Ce-EDTA and (d) Ce-PDTA. 3D snapshot and coordination structure of (e) Ce-IDA,\n(f) Ce-NTA (g) Ce-EDTA and (h) Ce-PDTA obtained from MD simulations.\nhave also been modified to\nFigure S1. Radial distribution function and corresponding coordinated number of (a) Fe-IDA, (b)\nFe-NTA, (c) Fe-EDTA, and (d) Fe-PDTA. 3D snapshot and coordination structure of (e) Fe-IDA,\n(f) Fe-NTA, (g) Fe-EDTA, and (h) Fe-PDTA obtained from MD simulations.\nFigure S2. Radial distribution function and corresponding coordinated number of (a) Ce-IDA, (b)\nCe-NTA, (c) Ce-EDTA, and (d) Ce-PDTA. 3D snapshot and coordination structure of (e) Ce-IDA,\n(f) Ce-NTA, (g) Ce-EDTA, and (h) Ce-PDTA obtained from MD simulations.\nMoreover, we double checked the molecular dynamics simulation results and\nconfirmed the coordination number of 0.46. We have corrected this mistake in the\nrevised manuscript.\nOn page 3, the original text\n“The coordination number (CN) of FeH O(O) in Fe-DTPA drops from 6 to 3.7,\n2\nas illustrated in Figure 1b and 1c, yet the CN of FeDTPA(O) rises to 0.45”\nhas been changed to\n“The coordination number (CN) of FeH O(O) in Fe-DTPA drops from 6 to 3.7,\n2\nas illustrated in Figure 1b and 1c, yet the CN of FeDTPA(O) rises to 0.46”.\nWe appreciate the reviewer’s valuable comments, which help us improve the quality\nand clarity of our descriptions.\nReviewer #2 (Remarks to the Author):\nThis work by Yang et al. reports a high-performance Fe/Ce redox flow battery using\nDTPA as a complexing ligand for both the Fe-based anolyte and Ce-based catholyte.\nCompared to existing V/Ce RFBs, the proposed Fe/Ce chemistry offers advantages\nsuch as lower material costs by replacing V with Fe. Notably, the Fe/Ce RFB operates\nat pH 7, avoiding many technical challenges associated with the strong sulfuric acid\nenvironment of V/Ce chemistry. I support the publication of this manuscript; however,\nsome concerns and questions regarding technical details and clarity should be\naddressed.\nResponse: Thank you very much for reviewing our manuscript and providing valuable\ncomments and suggestions. We have revised and enriched the content of the paper\naccording to your suggestions one by one as listed below.\nComment 2-1: The following sentence in the introduction may need further\nclarification. I believe the authors are referring to TEA crossover; if so, this should\nbe explicitly stated. Since NTMPA complexes exhibit much stronger complexation with\nFe ions, crossover has not been reported as an issue in the literature.\n“However, organic ligands of small molecules traverse the ion exchange membrane\nbetween the cathode and anode chambers effortlessly, causing side interactions with\nthe active substances in the other chamber along with the formation of hydroxide\nprecipitate, shortening the lifespan of the RFB and jeopardizing the overall safety of\nthe system battery”\nResponse: Thank you very much for reviewing our manuscript and raising this\nunclear description. As the reviewer noted, we intended to express the problem of TEA\ncrossover in this sentence. To further clarify our meaning, we have revised the above\nstatement to explicitly convey the crossover of TEA molecules. The sentence in the\noriginal text on page 2\n“However, organic ligands of small molecules traverse the ion exchange membrane\nbetween the cathode and anode chambers effortlessly, causing side interactions with\nthe active substances in the other chamber along with the formation of hydroxide\nprecipitate, shortening the lifespan of the RFB and jeopardizing the overall safety of\nthe system battery”\nhas been change to\n“However, organic ligands of small molecules such as TEA traverse the ion exchange\nmembrane between the cathode and anode chambers effortlessly, causing side\ninteractions with the active substances in the other chamber along with the formation\nof hydroxide precipitate, shortening the lifespan of the RFB and jeopardizing the\noverall safety of the system battery”.\nComment 2-2: The coordination structures of Fe-DTPA and Ce-DTPA in Figures 1c\nand 1e are somewhat confusing. It would be helpful to present them in a larger size\nwith correct color coding for better clarity.\nResponse: We thank the reviewer for this good suggestion. Based on the reviewer’s\nsuggestion, we have modified the color representation of the Ce element to improve\nvisual distinction and clarity. Additionally, the sizes of the Fe and Ce atom models\nhave been appropriately enlarged to enhance readability. We have also added a legend\nin the figure to indicate the correspondence between the colors of the spheres and the\nrespective elements. These changes have been implemented in the revised Figure 1c\nand 1e.\nThe original Figure 1 on page 3\nFigure 1. (a) Molecular formulas for IDA, NTA, EDTA, PDTA, and DTPA. (b) Radial\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nhas been modified to\nFigure 1. (a) Molecular formulas for IDA, NTA, EDTA, PDTA, and DTPA. (b) Radial\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nSimilarly, the original Figures S1 and S2 in the SI\nFigure S1. Radial distribution function and corresponding coordinated number of (a) Fe-IDA, (b)\nFe-NTA (c) Fe-EDTA and (d) Fe-PDTA. 3D snapshot and coordination structure of (e) Fe-IDA, (f)\nFe-NTA (g) Fe-EDTA and (h) Fe-PDTA obtained from MD simulations.\nFigure S2. Radial distribution function and corresponding coordinated number of (a) Ce-IDA, (b)\nCe-NTA (c) Ce-EDTA and (d) Ce-PDTA. 3D snapshot and coordination structure of (e) Ce-IDA,\n(f) Ce-NTA (g) Ce-EDTA and (h) Ce-PDTA obtained from MD simulations.\nhave also been modified to\nFigure S1. Radial distribution function and corresponding coordinated number of (a) Fe-IDA, (b)\nFe-NTA, (c) Fe-EDTA, and (d) Fe-PDTA. 3D snapshot and coordination structure of (e) Fe-IDA,\n(f) Fe-NTA, (g) Fe-EDTA, and (h) Fe-PDTA obtained from MD simulations.\nFigure S2. Radial distribution function and corresponding coordinated number of (a) Ce-IDA, (b)\nCe-NTA, (c) Ce-EDTA, and (d) Ce-PDTA. 3D snapshot and coordination structure of (e) Ce-IDA,\n(f) Ce-NTA, (g) Ce-EDTA, and (h) Ce-PDTA obtained from MD simulations.\nComment 2-3: The coordination structures of Fe-DTPA and Ce-DTPA in Figures 1c\nand 1e are somewhat confusing. It would be helpful to present them in a larger size\nwith correct color coding for better clarity.\nResponse: We thank the reviewer for this comment and good suggestion. Upon\ncareful review, we noticed that Comment 2-3 overlaps with the content of Comment\n2-2. Therefore, to avoid redundancy, we kindly refer the reviewer to our detailed\nresponse provided under Comment 2-2.\nComment 2-4: The authors calculated the band gap between the HOMO and LUMO\nfor Fe- and Ce-based complexes with different ligands, finding the lowest values for\nFe-DTPA and Ce-DTPA complexes. However, it is unclear how these results relate to\nthe electron transfer kinetics or redox potential of the complexes.\nResponse: We sincerely thank the reviewer for raising this important point. In\nresponse, we have made efforts to further clarify the relationship between the\ncalculated HOMO-LUMO energy gaps and the electron transfer kinetics of the Fe-\nbased and Ce-based complexes.\nThe HOMO-LUMO energy gap plays a crucial role in determining the ease of electron\ntransfer in metal complexes. Specifically, a smaller HOMO-LUMO energy gap is\ngenerally associated with enhanced electron transfer kinetics, making oxidation and\nreduction processes more accessible. In other words, the electron transfer becomes\nmore facile when the HOMO energy level is higher (facilitating its oxidation) and the\nLUMO energy level is lower (facilitating its reduction). Transition metal complexes\nwith narrower frontier orbital gaps exhibit lower electronic excitation barriers and\nstronger orbital coupling between donor and acceptor sites, which facilitates faster\nelectron transfer (see ref. Inorg. Chem. 2021, 60, 2362−2371,\nhttps://dx.doi.org/10.1021/acs.inorgchem.0c03245). Furthermore, this observation\naligns with the Marcus theory of electron transfer, which suggests that increased\nelectronic coupling — often correlated with reduced HOMO-LUMO gaps — leads to\nlower activation barriers for charge transfer (see ref. Rev. Mod. Phys. 1993, 65, 599–\n610, DOI: https://doi.org/10.1103/RevModPhys.65.599).\nRegarding the relationship between the redox potentials and the HOMO-LUMO gap,\nwe believe it is not straightforward. The redox potential of a complex primarily\ndepends on the nature of the active metal center and the stability constants between\nthe metal in different oxidation states and the ligands, rather than directly on the\nHOMO-LUMO gap. For example, in the case of iron, its Nernst equation can be\nrewritten as follows:\nEquations illustrate that the activity coefficients (γ) and stability constants (β) of\nFe3+(L) and Fe2+(L) are dominant parameters for determining the redox potential of\nthe Fe3+(L)/Fe2+(L) couple (see ref. Energy Storage Mater. 2025, 75, 104004,\nhttps://doi.org/10.1016/j.ensm.2025.104004).\nIn conclusion, we think that while a smaller HOMO-LUMO gap is generally\nassociated with enhanced electron transfer kinetics, the redox potential of the complex\nis mainly determined by the intrinsic properties of the active metal and the stability\nconstants between the metal and its ligands. We hope that this further explanation\naddresses the reviewer's concerns and provides additional clarity regarding the\nrelationship between the HOMO-LUMO gap and the electron transfer behavior of the\ncomplexes. Related modifications have been added on page 4 in the revised\nmanuscript as follows:\n“A narrower band gap between the highest occupied molecular orbital (HOMO)\nand the lowest unoccupied molecular orbital (LUMO) indicates a stronger ability\nto transfer electrons, as it facilitates faster oxidation and reduction processes through\nstronger electronic coupling and lower activation barriers, respectively.”\nComment 2-5: The Raman spectra of Fe-DTPA and Ce-DTPA should be compared\nwith free DTPA ligand to see whether the complexation alters the Raman shift. The\nnew bands reported in here may come from the free ligand.\nResponse: We appreciate the reviewer’s careful and detailed evaluation of our\nmanuscript and the valuable suggestions provided. According to the reviewer’s advice,\nwe have conducted additional Raman spectroscopy measurements for free DTPA. The\npreparation of free DTPA samples followed the same procedure as that for the Fe-\nDTPA and Ce-DTPA electrolytes, except that no active substances (FeCl or CeCl )\n3 3\nwere added, and the pH was adjusted to 7.\nThe Raman spectra of free DTPA show two new peaks at around 350 cm1 and 420\ncm1 compared with pure DTPA. However, these peaks do not overlap with the Fe–\nDTPA(O) and Ce–DTPA(O) characteristic peaks observed in the Fe-DTPA and Ce-\nDTPA samples. This confirms that the new bands reported in our study originate from\nthe formation of new Fe–O and Ce–O bonds, rather than from the free ligand. We have\nadded the Raman spectrum of free DTPA in Figure 1g for better comparison and clarity.\nRelevant discussion has also been updated accordingly on page 4 in the revised\nmanuscript as follows:\n“Raman spectrum comparison with free DTPA confirms that the new peaks observed\nin Fe-DTPA and Ce-DTPA arise from Fe–DTPA(O) and Ce–DTPA(O) bonds, rather\nthan from the free ligand.”\nFigure 1. (a) Molecular formulas for IDA, NTA, EDTA, PDTA, and DTPA. (b) Radial\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nComment 2-6: The HER and OER data presented in Figure 3b could be misleading,\nas the stable window for aqueous-based electrolytes is typically around 1.3 V, varying\nwith pH. Displaying a 2.5 V stable window in Figure 3b lacks context regarding how\nit was measured, including details on the working electrode, pH, and salt\nconcentration. Clarification on these parameters is needed.\nResponse: Thank the reviewer for this insightful comment. We agree that the\nelectrochemical window in aqueous systems is typically limited (~ 1.23 V vs. SHE,\nvarying with pH). The electrochemical window presented in Figure 3b was determined\nusing a neutral DTPA-based electrolyte without any active material. The experimental\ndetails are as follows:\nElectrolyte preparation: 49.17 g of DTPA was dissolved in 200 mL of deoxygenated\ndeionized water and stirred for 10 minutes at 20.0 ℃. Subsequently, 51.83 g of K CO\n2 3\nwas gradually added under continuous stirring. The solution was stirred for 24 hours\nto ensure uniformity, and the pH was adjusted to 7 using a pH meter.\nElectrochemical measurements: CV was conducted at room temperature using a\nstandard three-electrode system: a graphite working electrode (diameter: 6.0 mm), a\nsaturated calomel electrode (SCE) with a KCl salt bridge as the reference electrode,\nand a platinum mesh (1.0 cm2) as the counter electrode. Approximately 15.0 mL of\nelectrolyte was used per test, and the electrochemical cell was purged with nitrogen\nbefore measurements to minimize oxygen interference.\nTo ensure the context is clearly conveyed, the above experimental details have been\nadded on page 10 in revised manuscript as follows:\n“As for evaluating the electrochemical window, the electrolyte was prepared by\ndissolving 49.17 g of DTPA in 200 mL of deoxygenated deionized water under stirring\nfor 10 min at 20.0 °C. Subsequently, 51.83 g of K₂CO₃ was gradually added to the\nsolution, followed by continuous stirring for 24 h until a homogeneous solution was\nobtained. The pH of the final electrolyte was adjusted to 7. Electrochemical\nmeasurements were carried out in a standard three-electrode system using a graphite\nworking electrode. Prior to each measurement, the electrochemical cell was purged\nwith nitrogen to remove dissolved oxygen.”\nComment 2-7: The self-discharge test is crucial for evaluating the stability of the\nFe/Ce RFB at high SOCs. Rather than solely monitoring the battery’s OCV, a more\naccurate approach would be to measure the discharge capacity after holding the\nFe/Ce battery at 100% SOC for a specific period, such as one week.\nResponse: We sincerely appreciate the reviewer’s insightful suggestion regarding the\nevaluation of the self-discharge behavior. In response, we specifically designed an\nexperiment where the Fe-Ce DTPA RFB was fully charged and then left under open-\ncircuit conditions for 3 days without any electrochemical cycling. Subsequently, we\ndischarged the battery to 0.3 V to assess the retained capacity. It is noted that the\nsystem maintained a high Coulombic efficiency of 97.9%, demonstrating minimal\nself-discharge and excellent electrolyte stability even at full charge. We chose a 3-day\nresting period as a practical balance to capture early self-discharge effects while\navoiding potential complications such as long-term side reactions or electrolyte\nevaporation that may arise during excessively prolonged tests. These findings provide\nstrong evidences that the Fe-Ce DTPA RFB system possesses robust stability against\nself-discharge at high SOC, addressing the reviewer's concern directly. The results\nhave been incorporated in Figure S22b in revised Supporting Information.\nCorresponding descriptions have been added on pages 8 and 9 in the revised\nmanuscript as follows:\n“Furthermore, after fully charging the Fe-Ce DTPA RFB and resting for 3 days under\nopen-circuit conditions, the battery retained a high CE of 97.9%, further confirming\nits excellent resistance to self-discharge (Figure S22b).”\nFigure S22. (a) Self-discharge curve and (b) long standby test profile of Fe-Ce DTPA RFB.\nComment 2-8: Employing Fe-CN electrolytes to decouple Fe-DTPA and Ce-DTPA\nelectrolytes could be approached differently, such as by conducting symmetrical flow\nbattery tests. For example, a flow battery with symmetrical electrolytes—using either\nFe-DTPA or Ce-DTPA on both the cathode and anode sides—could provide clearer\ninsights into the performance of each electrolyte.\nResponse: We appreciate the reviewer’s valuable suggestion, which indeed helps us\nto better understand the performance of each electrolyte. Following the reviewer's\nrecommendation, we have conducted additional symmetrical flow battery tests to\nfurther investigate the performances of Fe-DTPA and Ce-DTPA electrolytes\nindependently. Specifically, Fe3+-DTPA and Ce3+-DTPA electrolytes were initially\nused as anolyte and catholyte, respectively, to assemble a Fe3+-Ce3+ DTPA RFB and\ncomplete the initial charging process. After full charging, the Fe3+-DTPA electrolyte was\nconverted to Fe2+-DTPA, which was then used as the catholyte in another symmetrical\ncell, while fresh Fe3+-DTPA served as the anolyte. This setup enabled cycling tests for\nthe Fe3+/2+-DTPA symmetrical RFB. Similarly, after charging, Ce3+-DTPA was\noxidized to Ce4+-DTPA, which was then employed as the anolyte and paired with fresh\nCe3+-DTPA catholyte to form the Ce3+/4+-DTPA symmetrical RFB for subsequent\ntesting.\nThe results showed that both the Fe3+/2+-DTPA and Ce3+/4+-DTPA symmetrical RFBs\nexhibited good cycling stability. The Ce3+/4+-DTPA RFB displayed a slightly higher\nCE than the Fe3+/2+-DTPA RFB, which also led to a better capacity retention. These\nfindings are consistent with the conclusions drawn from the decoupling method,\nfurther confirming that the overall CE of the Fe-Ce DTPA system can be effectively\nimproved through optimization of the Fe-DTPA side. These new experimental results\nand related discussions have been added on page 19 in the revised Supporting\nInformation as follows:\n“To further isolate and evaluate the intrinsic electrochemical performance of Fe-DTPA\nand Ce-DTPA electrolytes, we conducted symmetrical flow battery tests. Fe3+-DTPA\nand Ce3+-DTPA were first employed as anolyte and catholyte, respectively, to\nassemble a Fe3+-Ce3+ DTPA RFB and complete the initial charging process. Following\nfull charge, Fe3+-DTPA was reduced to Fe2+-DTPA, enabling the assembly of a Fe3+/2+-\nDTPA symmetrical RFB with fresh Fe3+-DTPA as the anolyte. Similarly, Ce3+-DTPA\nwas oxidized to Ce4+-DTPA and paired with unreacted Ce3+-DTPA to form a Ce3+/4+-\nDTPA symmetrical RFB. As shown in Figure S26, both systems exhibited stable\ncycling performance. The Ce3+/4+-DTPA symmetrical RFB demonstrated a higher CE\nand superior capacity retention compared to the Fe3+/2+-DTPA system. These results\nare consistent with those obtained from the decoupled method, further validating that\nthe overall CE of the Fe-Ce DTPA system can be effectively improved through\noptimization of the Fe-DTPA side.”\nFigure S26. The CE and capacity of cycle tests for Ce3+/4+-DTPA and Fe3+/2+-DTPA symmetrical\nRFB.\nComment 2-9: Authors could plot Fig. 3d and g in a different way (Log(I) vs Log(scan-\nrate)) to see whether it is truly diffusion controlled reaction (slop should be close to\n0.5).\nResponse: Thank the reviewer for this insightful suggestion. As advised, we replotted\nFigure 3d and 3g using a log-log scale (Log(I) versus (vs.) Log(scan rate)) to further\nevaluate whether it is truly diffusion-controlled reaction. The slopes of the fitted lines\nfor Fe-DTPA oxidation and reduction are calculated to be 0.496 and 0.483,\nrespectively. The slopes of the fitted lines for Ce-DTPA oxidation and reduction are\n0.458 and 0.573, respectively. These values are both close to 0.5, suggesting that the\nelectrochemical process is predominantly diffusion-controlled. We have included the\ncorresponding new figures in Figure S11 (below). Relevant discussion has been added\non page 8 marked in yellow in the revised manuscript as follows:\n“There is a strong linear relationship between the scan rates and the square root\nof the current density (Figure 3d and 3g). It is noted that both slopes of the fitted\nLog(I) vs. Log(v) lines are close to 0.5 (Figure S11), indicating the redox intensity\nof Fe-DTPA and CE-DTPA is affected by diffusion control.”\nFigure S11. Plots of Log(I) versus Log(scan rate) for (a) oxidation and (b) reduction of Fe-DTPA,\nand (c) oxidation and (d) reduction of Ce-DTPA.\nReviewer #3 (Remarks to the Author):\nThe design and development of a safe and low-cost redox flow battery (RFB) is\ngarnering increasing attention to store intermittent renewable energy. Yang et al.\npresent an interesting highly stable and affordable aqueous organic iron-cerium (Fe-\nCe) RFB. A universal and unexplored complexing agent diethylenetriamine\npentaacetic acid (DTPA) was demonstrated to chelate Fe and Ce in the anolyte and\ncatholyte, respectively, as verified by a combined molecular dynamics simulations and\nexperimental characterization. The use of DTPA was shown to boost the redox\ncharacteristics of Fe and Ce and increase the cell voltage, and also alleviate water\nmigration resulting from osmotic pressure or viscosity differences between the anode\nand cathode. High energy efficiencies with impressive cycling stability exceeding 1770\ncycles at a cycling rate of 80 mA·cm2 were achieved. Overall, the results are\ninteresting and well presented. It is therefore suitable for the broad readership of Nat.\nCommun. and can be considered for publication in Nat. Commun. after addressing\nthe following comments.\nResponse: Thank the reviewer for carefully reviewing our manuscript and for the\npositive and encouraging comments. These have helped us to further improve the\nquality of our work. We have carefully addressed all the specific comments provided\nand revised the manuscript accordingly. We hope the revised version meets the\nreviewer’s expectations and is now suitable for publication.\nComment 3-1: Did the coordination structure of the electrolytes change after\nextended cycles? Some spectroscopy methods such as FTIR and Raman can be used\nto investigate this.\nResponse: Thank the reviewer for this insightful question and the very helpful\nsuggestion. We fully agree that understanding the stability of the electrolyte\ncoordination structure after long-term cycling is critical to evaluating the reliability of\nthe system. We adopted the reviewer’s suggestion and performed FTIR spectroscopy\nand Raman spectroscopy measurements for the electrolytes before and after 500\ncharge-discharge cycles. The results indicate that the characteristic peaks associated\nwith the coordination structures of both Fe-DTPA and Ce-DTPA remained unchanged\nafter cycling, compared with the pristine samples before tests. This suggests that the\ncoordination environment around the metal centers was well maintained during\nextended electrochemical operation. Thus, no significant degradation or\nrearrangement of the coordination structures was observed, confirming the excellent\nchemical stability of the electrolyte system.\nThe obtained Raman and FTIR results have been added in Figure S14 (listed as below)\nin the revised Supporting Information.\nFigure S14. (a) Raman and (b) FTIR spectra of electrolytes before and after cycling tests.\nRelated discussion has been added on page 8 marked in yellow in the revised\nmanuscript as follows:\n“FTIR and Raman analyses after cycling show that the electrolyte structure remained\nlargely unchanged, highlighting its outstanding stability (Figure S14).”\nComment 3-2: The colors of balls used in Fig. 1c, e, i, j, and l need to be labeled.\nResponse: Thank the reviewer for this helpful comment. In response, we have labeled\nthe colors of the balls used in Figures 1c, 1e, 1i, 1j, and 1l, and redrawn the figures to\nenhance clarity. For Figures 1i, 1j, and 1l, which share the same set of color\nrepresentations, we have included a common legend in Figure 1j to avoid redundancy.\nWe hope these revisions adequately address the reviewer’s concern.\nThe original Figure 1\nFigure 1. (a) Molecular formulas for IDA, NTA, EDTA, PDTA, and DTPA. (b) Radial\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nhas been modified to\nFigure 1. (a) Molecular formulas for IDA, NTA, EDTA, PDTA, and DTPA. (b) Radial\ndistribution function and corresponding coordinated number of Fe-DTPA. (c) 3D snapshot\nand coordination structure of Fe-DTPA obtained from MD simulations. Ce-DTPA’s (d)\nradial distribution function and corresponding coordinated number and (e) 3D snapshot and\ncoordination structure. (f) Binding energies of Fe based electrolytes (left) and Ce based\nelectrolytes (right). (g) Raman and (h) FTIR spectra of electrolytes. Calculated HOMO and\nLUMO energy levels of (i) Fe based electrolytes and (j) Ce based electrolytes. (k) Fourier-\ntransformed EXAFS fitting curves of Fe-DTPA, FeCl ·6H O, and Fe O . (l) The ESP maps\n3 2 2 3\nof Fe-DTPA and Ce-DTPA.\nComment 3-3: In Figure 1, the authors have demonstrated the interaction between\nFe3+, Ce3+, and DTPA, however, the relative evidence is insufficient. What are the\nmolecular size of DTPA and the mean square displacement of K+ ions? These results\nmay help insightfully understand the underlying mechanisms.\nResponse: Thank the reviewer for this valuable comment. In response, we have added\nthe mean square displacement (MSD) plots of K+ ions for each electrolyte in Figure\nS3 (listed as below) in the revised Supporting Information. The diffusion constants (D)\nof K+ ions were obtained by fitting the MSD curves over the time interval from 1000\nps to 9000 ps, and all MSD data were gathered over 10000 ps with 1001 restarts to\nensure statistical reliability. These results quantitatively reflect the ionic mobility and\nfurther support our proposed mechanism regarding the interaction between Fe3+, Ce3+,\nand DTPA.\nFigure S3. The mean square displacement (MSD) of K+ ions for (a) Fe-IDA, (b) Fe-NTA, (c) Fe-\nEDTA, (d) Fe-PDTA, (e) Fe-DTPA, (f) Ce-IDA, (g) Ce-NTA, (h) Ce-EDTA, (i) Ce-PDTA, and\n(j) Ce-DTPA.\nFurthermore, the approximate molecular sizes of IDA, NTA, EDTA, PDTA, and\nDTPA have been added in Figure S4 (listed as below) in the revised Supporting\nInformation.\nFigure S4. The molecular sizes of IDA, NTA, EDTA, PDTA, and DTPA.\nRelevant discussion has been added on page 4 marked in yellow in the revised\nmanuscript as follows:\n“The mean square displacement (MSD) plots of K+ ions and molecular sizes of ligands\nare shown in Figures S3 and S4.”\nWe hope these additions address the reviewer’s concern and enhance the mechanistic\nunderstanding of the system.\nComment 3-4: Two UV-vis peaks were easily identified for Ce-DTPA solutions as\nshown in Fig. 2d and e, whereas Fig. 2e only provided the fitting results for one peak.\nWhat’s the result if the peak was used for fitting? Will the obtained result be consistent\nwith the provided one?\nResponse: We thank the reviewer for the thoughtful comment. As reviewer pointed\nout, two UV-vis absorption peaks (about 270 nm and 297 nm) are clearly observed in\nFigure 2d, whereas Figure 2e only provided the fitting results for one peak (297 nm).\nIn response to the reviewer’s suggestion, we have now performed additional fitting on\nthe second peak (270 nm). The fitting results are consistent with those of the first peak,\nconfirming the same trend and supporting our original conclusion. The updated fitting\nresults and discussion have been added in the revised manuscript and Supporting\nInformation (Figure S6) as follows:\nOn page 5 in the revised manuscript,\n“Likewise, we determined that Ce-DTPA has a maximum solubility of 0.927 M\naccording to the absorption peak at 297 nm, which is marginally greater than Fe-\nDTPA's maximum solubility (Figure 2d to 2f). The fitting curve at 270 nm has a\nconsistent trend with that of 297 nm and the results are similar (Figure S6).”\nFigure S6. Standard absorbance-concentration curves of Ce-DTPA solution with different\nconcentrations (270 nm).\nComment 3-5: In Fig. 3e and h, the curves are too overlapped. It is hard to\ndifferentiate them. For clarity, the authors can provide each cycle curve in the SI.\nResponse: Thank the reviewer for carefully reviewing our manuscript and for\nproviding the good suggestion. We adopted the reviewer’s suggestion and plotted all\ncurves separately to show each CV curve clearly, which have been included in Figures\nS12 and S13 (listed as below) in the revised Supporting Information.\nFigure S12. CV curves of Fe-DTPA obtained at the (a) 1st cycle, (b) 100th cycle, (c) 200th cycle,\n(d) 300th cycle, (e) 400th cycle, and (f) 500th cycle.\nFigure S13. CV curves of Ce-DTPA obtained at the (a) 1st cycle, (b) 100th cycle, (c) 200th cycle,\n(d) 300th cycle, (e) 400th cycle, and (f) 500th cycle.\nRelevant discussion has been added on page 8 marked in yellow in the revised\nmanuscript as follows:\n“Furthermore, throughout the entire 500-cycle tests performed on Fe-DTPA and\nCe-DTPA electrolytes, there was minimal variation in the representative 1st to\n500th cycle in Figure 3e and 3h, indicating stable long-term electrochemical\ncapabilities (each cycle curve is shown in Figures S12 and S13).”\nComment 3-6: For better understanding, the authors need to provide details of\nspecific parameters (e.g., active area, number of single cells, etc.) in Fig. 4l.\nResponse: Thank the reviewer for this useful suggestion. In response, we have added\ndetailed information regarding the specific parameters used in Figure 4l, including the\nactive area of each device (14.5 × 7.5 cm²), the number of single cells assembled (5\ncells), and the method of single cells connection (in series). These details have been\nincluded in the caption of Figure 4l marked in yellow in the revised manuscript as\nfollows:\n“(l) Digital photo of Fe-Ce DTPA RFB stack system. The battery stack was constructed with 5 single cells in series with an active area of 14.5 × 7.5 cm2.”\nComment 3-7: Although the CV curves of IDA, NTA, EDTA, and PDTA reference\nspecies have been provided in the paper, their performance in battery tests is not\nshown. This needs to be added.\nResponse: Thank the reviewer for this insightful comment and suggestion. We fully\nagree that the battery performance of the reference species (IDA, NTA, EDTA, and\nPDTA) is important, and we have now conducted additional battery tests to address\nthis point. The reference battery test results have been added in Figure S19 in the\nrevised Supporting Information. It can be found that all four reference ligands\nexhibited varying degrees of instability in battery operation. Specifically, the IDA-\nbased battery (Figure S19a) was able to operate for 200 cycles but showed relatively\nlow capacity and EE below 60%. NTA-based battery (Figure S19b) was highly\nunstable, with the capacity decaying rapidly within the first 10 cycles and a continuous\ndecline in EE. The EDTA-based battery (Figure S19c) could run for 200 cycles, but\nits CE, EE, and capacity remained at low levels. Notably, it exhibited fast initial\ncapacity fading and persistent EE decline. The PDTA-based battery showed unstable\nbehavior in the early cycles, and overall EE remained below 60%, accompanied by\nrapid capacity fading.\nThese results support our hypothesis that these reference ligands, although\nelectrochemically active (as shown in the CV curves), are not ideal for stable battery\napplications under the same conditions. Relevant data and discussion have been added\non page 19 in the revised Supporting Information as follows:\n“To further evaluate the electrochemical stability and practicality of the reference\nligands, battery tests were conducted using IDA, NTA, EDTA, and PDTA under\nidentical conditions. The results are summarized in Figure S14. Among them, the\nIDA-based battery (Figure S19a) was able to operate for 200 cycles, but exhibited\nrelatively low discharge capacity and an EE below 60%, indicating limited\nreversibility. The NTA-based system (Figure S19b) displayed pronounced instability,\nwith its capacity rapidly decaying to near zero within the first 10 cycles and a\ncontinuous drop in EE throughout cycling. The EDTA-based battery (Figure S19c)\nshowed slightly improved cycling stability compared to NTA, maintaining operation\nover 200 cycles. However, it suffered from low CE, EE, and capacity. A notable fast\nfading in the initial cycles and gradual decline in EE were also observed. The PDTA-\nbased battery exhibited unstable behavior in the initial cycles and failed to achieve EE\nvalues above 60% throughout the test, with a similarly rapid capacity fade. These\nresults suggest that although the reference ligands are redox-active—as confirmed by\ntheir CV profiles—their coordination structures and molecular stability are not\nsufficient to support long-term electrochemical cycling.”\nFigure S19. Efficiency and capacity data of the full battery test of (a) IDA-based battery, (b)\nNTA-based battery, (c) EDTA-based battery, and (d) PDTA-based battery.\nIn addition, relevant discussion has been added on page 8 marked in yellow in the\nrevised main text as follows:\n“To provide a more comprehensive comparison, batteries assembled using other\nreference ligands were tested under the same conditions, which exhibited varying\ndegrees of instability (Figure S19).”\nComment 3-8: Why did the authors use a membrane N212? Did they try thicker\nmembranes such as N117, etc.?\nResponse: Thank the reviewer for this insightful comment regarding the choice of\nNafion N212 membrane. As shown in Table S3, N212 has a thickness of 50.8 μm,\nwhile N117 is significantly thicker with a thickness of 183 μm. Thicker membranes\nsuch as N117 generally provide better ion selectivity, which may improve the CE due\nto reduced crossover of active species. However, the increased thickness also leads to\nhigher internal resistance, thereby reducing the VE. Since the overall EE of a RFB is\ndetermined by the product of CE and VE, a trade-off exists between membrane\nthickness and energy performance. In our system, the active species—Fe-DTPA and\nCe-DTPA—are relatively large chelated molecules with low crossover tendencies.\nTherefore, the use of a thicker membrane is theoretically unnecessary and could\nintroduce additional ohmic losses. Moreover, using a thinner membrane like N212\nalso contributes to reducing the system cost.\nFollowing the reviewer’s suggestion, we performed additional experiments using the\nthicker N117 membrane under the same conditions. The results showed a noticeable\ndecrease in VE and a lower EE compared to the battery with N212. These findings\nconfirm that N212 is a more suitable choice for our system.\nRelevant discussion has been added on page 8 in the revised manuscript as follows:\n“However, testing with the thicker N117 membrane resulted in a decrease in battery\nefficiency (Table S3 and Figure S18).”\nThe following table and figure have been added in the revised Supporting Information.\nTable S3. The performance parameters of N212 and N117 membranes.\nTotal acid Water Water\nThickness, Density,\nMembrane capacity, content uptake\ndry, μm g∙m−2\nmeq∙g−1 25 ºC, % 25 ºC, %\nN212 50.8 100 0.95 − 1.01 5 50\nN117 183 360 0.89 5 38\nFigure S18. Efficiency and capacity data of the N117 assembled Fe-Ce DTPA RFB.\nComment 3-9: The authors report stable operation for 1779 cycles, but what happens\nafter this?\nResponse: Thank the reviewer for this valuable comment. After 1779 cycles, the\nbattery continued to operate for a period of time. However, noticeable capacity fading\nand efficiency decline were observed. This degradation is attributed to both the testing\nprogram settings and the internal resistance changes in the battery. Specifically, to\nprolong the battery's operational life and avoid severe side reactions induced by high\npotentials, the charging program settings were set to either charge up to 70% SOC or\ncut off at 1.0 V. After prolonged cycling, the accumulation of side reactions — such\nas minor water decomposition, water evaporation increasing solution resistance, and\nthe minor degradation of Ce-DTPA — led to increased internal polarization. As a\nresult, the cutoff condition shifted from 70% SOC to the voltage limit (1.0 V), causing\na reduction in charge capacity. During further cycling, the decreased charge capacity\nindirectly mitigated the increase in internal resistance, resulting in a gradual slowdown\nin the degradation rates of capacity and efficiency, eventually leading to stabilization\nat a lower operational level. The obtained data after 1779 cycles have been added in\nthe revised Supporting Information listed as below.\nFigure S20. Cycling performance of Fe-Ce DTPA RFB after 1779 cycles at 80 mA· cm‒2.\nComment 3-10: In Figure 1, the structures of Fe(DTPA) and Ce(DTPA) need to be\nfurther confirmed. The crystal structure of Fe(DTPA) chelate has been demonstrated\nin the study by Michael P. Marshak et al (ACS Energy Lett. 2020, 5, 1758−1762). The\nFe(DTPA) chelate exhibits a 7-coordinate structure, binding through 3 nitrogen and\n4 oxygen atoms in a pentagonal bipyramidal geometry.\nResponse: Thank the reviewer for this insightful comment regarding the coordination\nstructure of Fe(DTPA) in Figure 1. We have carefully considered the study by Michael\nP. Marshak et al. (ACS Energy Lett. 2020, 5, 1758−1762), which indeed reports a 7-\ncoordinated Fe(DTPA) chelate with a pentagonal bipyramidal geometry based on X-\nray diffraction (XRD) analysis. As described in their experimental section, the\nFe(DTPA) crystals were obtained through a prolonged evaporation process at room\ntemperature, followed by removal of KCl crystals and further crystallization in a near-\nwater-free environment. This approach yields the coordination geometry in the solid-\nstate crystalline phase, which is largely decoupled from an aqueous environment.\nIn contrast, our study focuses on the coordination structure of Fe(DTPA) in an aqueous\nsolution, aiming to mimic the real electrolyte environment of flow batteries. Our\nmolecular dynamics (MD) simulations were performed with FeCl , chelating ligands\n3\n(e.g., DTPA), and water molecules fully immersed in a 10 × 10 × 10 nm3 simulation\nbox under periodic boundary conditions. Additionally, the Fe K-edge extended X-ray\nabsorption fine structure conducted directly on Fe-DTPA aqueous solutions without\nfurther treatment, independently validate the coordination environment predicted by\nour MD simulations.\nTherefore, the differences observed between our results and those of Marshak et al.\narise primarily from the distinct experimental and simulation conditions—solid-state\ncrystallography versus aqueous-phase modeling and test. We believe that both sets of\ndata are internally consistent and complementary, representing coordination behavior\nunder different environmental conditions. Our findings aim to reflect the actual\ncoordination structure of Fe-DTPA in operating flow battery electrolytes. We hope\nthis explanation satisfactorily addresses the reviewer’s concern.\nComment 3-11: In Figure 2h, the authors mention that “virtually no active substances\nwill pass through the membrane”. This conclusion requires validation through\nquantitative UV-Vis spectroscopy data.\nResponse: We thank the reviewer for pointing out the need for quantitative validation\nof our claim regarding the impermeability of active substances through the membrane\nin Figure 2h. This is an important concern, and we agree that supporting this statement\nwith spectral evidence is necessary to ensure scientific rigor. To validate our\nconclusion, we conducted UV-vis spectroscopy analysis on the electrolytes after 3, 5,\nand 7 days of the diffusion experiment.\nAs shown in Figure 2a and 2d, Ce-DTPA exhibits distinct absorption peaks at 270 and\n297 nm, while Fe-DTPA shows a characteristic peak at around 259 nm. After several\ndays of permeation test, the UV-vis spectra revealed no emerging peaks at 259 nm in\nthe Ce-DTPA, nor any discernible peaks at 270 nm or 297 nm in the Fe-DTPA. This\nstrongly suggests that there is no detectable crossover of Fe-DTPA into the Ce-DTPA\nside or vice versa. Therefore, the absence of these characteristic peaks confirms that\nthe membrane effectively prevents the migration of active substances between\ncompartments within the detection limits of our instrumentation.\nWe have clarified this analysis on page 6 highlighted in yellow in the revised\nmanuscript as follows:\n“UV-vis spectra after 3, 5, and 7 days of the diffusion experiment show that almost\nno active substances pass through the membrane (Figures 2h and S7), increasing\nthe battery's CE.”\nThe corresponding UV-vis spectra have been also included as supplementary data\n(Figure S7) in revised Supporting Information to further support our conclusion.\nFigure S7. UV-vis absorption spectra of (a) Ce-DTPA and (b) Fe-DTPA after 3, 5, and 7 days\nof the diffusion experiment.\nComment 3-12: In Figure 4e, the discharge capacity is obviously lower than the\ncharge capacity, yet the battery retains 95.3% of its initial capacity after 500 cycles.\nWas this test conducted under a capacity-imbalanced cell? The authors should\nprovide further explanation regarding the relatively low Coulombic efficiency and\noutstanding capacity retention rate.\nResponse: We appreciate the reviewer’s observation. We would like to clarify that\nalthough the CE appears relatively low in Figure 4e, this is primarily due to the plotting\nscale (0 – 105% on the y-axis) and visual misrepresentation, rather than an actual\nperformance issue. In fact, the CE during the 500-cycle test starts at approximately\n98.5% in the initial cycles and gradually increases to above 99% as cycling progresses,\nindicating a consistently high level of efficiency. This result is not low compared to\nother similar flow battery systems.\nRegarding the test conditions, the battery was not operated under a capacity-\nimbalanced configuration. Both catholyte and anolyte used in Figure 4e consisted of\n0.5 M Ce-DTPA and Fe-DTPA, respectively, with equal volumes on each side. The\ntest was conducted at a relatively low current density of 40 mA·cm2. Under such\nrelatively low current conditions, the charging time is extended, which increases the\nlikelihood of side reactions per cycle compared to tests run at higher current densities\n(e.g., 80 mA·cm2), leading to a slightly lower CE.\nWe hope this clarification addresses the reviewer’s concern and meets the\nrequirements for revision.\nComment 3-13: In Figure 4e and Figure 4j, the authors should elucidate the\nunderlying mechanism responsible for the observed capacity degradation.\nResponse: Thank the reviewer for this insightful suggestion. We have carefully\nclarified these mechanisms as follows:\nFor Figure 4e, the cycling tests were conducted at a relatively low current density of\n40 mA·cm2 using a 0.5 M Fe-DTPA and Ce-DTPA electrolyte. At such low current\ndensities, the charging process is significantly prolonged, which leads to greater\naccumulation of side reactions per cycle. This partially accounts for the slightly lower\nCE. The primary contributors to capacity fading in this case are trace water\ndecomposition and gradual degradation of the Ce-DTPA species. However, Figure 2g\nshows that the conductivity of the electrolyte remains high in the 0.5 – 0.6 M\nconcentration range, and minimal polarization changes were observed over 500 cycles,\nindicating stable ionic transport. Additionally, the water-splitting-induced charge\nimbalance was partially offset by the presence of excess active material, with an\nestimated electrolyte utilization rate of ~80%. Therefore, the overall impact on\ncapacity remained negligible during the early and mid-stages of the test, although\nslight fluctuations in capacity began to appear after 450 cycles, indicating the onset of\ndegradation processes.\nFor Figure 4j, the rapid capacity decay in the stack system was primarily a result of\nengineering challenges associated with scaling up from a single cell to a multi-cell\nstack. These include suboptimal flow field design, variability in welding quality, and\nincreased electrode surface area—all of which can negatively impact system\nuniformity and electrochemical stability despite the use of the same electrolyte. The\nmost critical factor identified was the absence of an effective thermal management\nsystem. Heat generated by both the electrochemical reactions and the magnetic drive\npump was not adequately dissipated, leading to a gradual rise in the electrolyte and\nstack temperature. Elevated temperatures are known to accelerate side reactions and\nexacerbate electrolyte degradation, ultimately leading to capacity loss.\nTo address this issue, we have upgraded our stack testing platform by introducing a\nsemiconductor-based cooling system (Figure R1), along with a temperature controller\nto maintain the stack temperature consistently at 30 ± 1 ℃. With this improvement,\nwe re-evaluated the performance of the same electrolyte. The updated results,\npresented in the revised Figure 4j, demonstrate a significantly improved cycling\nstability and a peak EE of 80.8%, confirming the effectiveness of the thermal\nmanagement upgrade.\nFigure R1. The photograph of the semiconductor-based cooling system employed in\nthis work.\nFigure 4. (a) Voltage-capacity diagram of Fe-Ce DTPA RFB at different currents and (b)\ncorresponding efficiency and currents. (c) Discharge voltage and power densit1y plots vs.\ncurrent density. Power densities are shown for the 60%, 70%, 80%, 90%, and 100% SOC,\nrespectively. (d) Partial characteristic charge and discharge curve in full battery test. (e)\nEfficiency and capacity data of the full battery test at pH = 7. (f) Cycle test of 0.5 M Fe-Ce\nDTPA RFB and (g) the performance comparison of Fe-Ce DTPA RFB with other Ce-based\nRFBs. In situ Raman spectra on (h) Fe-DTPA and (i) Ce-DTPA obtained from 0 to 100%\nSOC, with the real-time status of the corresponding electrolyte is recorded. (j) The\nperformance of the Fe-Ce DTPA RFB stack. (k) Efficiency and capacity of\nK [Fe(CN) ]/K [Fe(CN) ] electrolyte in a 1031 h cycling test. The illustration shows the\n3 6 4 6\nredox potential, and the charge-discharge curves at the beginning and end of the test. (l)\nDigital photo of Fe-Ce DTPA RFB stack system. The battery stack was constructed with 5\nsingle cells in series with an active area of 14.5 × 7.5 cm2.\nThe sentence on page 9 in main text “An EE of 80.8% was achieved at a current\ndensity of 80 mA·cm2, as illustrated in Figure 4j.” has been updated.\nWe hope this clarification and the additional data adequately address the reviewer’s\nconcern.\nResponse to the reviewers’ comments\nManuscript ID: NCOMMS-25-10508-A\n“Unified electrolyte design for advanced redox flow batteries: a universal complexing\nagent for high performance iron-cerium redox flow batteries”\nItalic Black Text = Reviewer Comments\nBlack Text = Author Response\nBlue Text = Original Manuscript\nBlack Text Marked in Yellow = Modifications to the Manuscript\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nCerium-iron coupled redox flow battery is a low-cost and abundant system, which is\npromising for large-scale deployment. The authors have meticulously revised the\nmanuscript, and significant improvements have been made following the review\nprocess. I have no further issues with this paper.\nResponse: We are very grateful to the reviewer for the positive assessment of our\nwork.\nReviewer #2 (Remarks to the Author):\nIn reviewing the manuscript, I recognize the considerable effort the authors have\ninvested in addressing the comments from the reviewers, including myself. However,\nupon examining the authors' responses, I feel that further clarification is needed\nbefore the manuscript can be published.\nResponse: We sincerely thank the reviewer for the continued consideration and\nhelpful comments on our manuscript. Based on the reviewer’s suggestions, we have\ncarefully revised the manuscript to provide more detailed explanations and made\nefforts to address the remaining concerns to the best of our ability. We have revised\nand enriched the content of the paper according to your suggestions one by one below.\nComment 2-1: The cyclic voltammograms (CVs) presented for Ce-DTPA and Fe-\nDTPA in Fig. 3a with equilibrium output voltages of 0.92 V (0.64 V for Ce-DTPA and\n-0.28 V for Fe-DTPA versus SCE). Nevertheless, the voltage profiles of the full cell\ndepicted in Fig. 4a reveal a substantially reduced equilibrium voltage (~0.6 V). In\naddition, the voltage profile during the charging process is consistently below 0.9 V,\nleading to concerns about whether the redox reactions occurring in the full cell\naccurately reflect those indicated by the CVs. The voltage profiles more closely\nresemble Fe-DTPA/Fe-CN (0.56 V), which needs clarification from the authors.\nResponse: We appreciate the reviewer’s reasonable comments to clarify the\ndiscrepancy between the theoretical cell voltage inferred from CVs and the lower\nequilibrium voltage observed in full-cell operation.\nThe observed discrepancy between the redox potentials in the CV results (Fig. 3a) and\nthe voltage profile of the full cell (Fig. 4a) can be attributed to the differences in\nexperimental conditions and practical factors that affect real battery operation as\nfollows.\nWe first measured the open-circuit voltage (OCV) of the full cell at different states of\ncharge (SOC), and found that at 50% SOC the OCV was 0.73 V, which is lower than\nthe theoretical value of 0.92 V obtained from the CV results presented in Fig. 3a. We\nattribute this discrepancy to the following factors.\n1) In the CV measurements, the electrolyte concentration is 0.2 M for both anolyte\nand catholyte with 1.0 M NaCl as supporting electrolyte. The full-cell performance\nmeasurements of Fe-Ce RFB employed 0.5 M Fe-DTPA and Ce-DTPA without\nsupporting electrolyte, starting from 0% SOC. The actual electrolyte concentrations\nused in the full cell measurements differ from those in the CV tests, which alters the\nNernstian potential, especially under charge/discharge where the [𝑐  ]/[𝑐  ]\n𝑂𝑥 𝑂𝑥 𝑅𝑒𝑑 𝑅𝑒𝑑\nratio changes dynamically.\nTo directly compare with the battery tests, we performed CV measurements for 0.5 M\nFe-DTPA or Ce-DTPA without supporting electrolyte, yielding a theoretical potential\nof 0.84 V — slightly lower than the initial CV results. The results have been added in\nFigure S14b. We thus hypothesize that the electrolyte concentration and addition of\nNaCl supporting electrolyte can affect the equilibrium output voltage, likely related to\nionic strength (Phys. Chem. Chem. Phys. 25, 7832–7846 (2023),\nhttps://doi.org/10.1039/D3CP00098B). However, even this corrected theoretical value\n(0.84 V) is still higher than the OCV measured at 50% SOC (0.73 V, Figure S14a),\nsuggesting that additional factors as below may also contribute to the observed\ndifference.\n2) Another possible reason for the difference between the equilibrium potential\nderived from CV and the full-cell charge/discharge voltage is i) the neglect of proton\nactivity and ii) the Donnan potential across the membrane, which results from proton\nconcentration gradients (Electrochem. Commun. 13, 342–345 (2011),\nhttps://doi.org/10.1016/j.elecom.2011.01.020).\n3) Ohmic resistance, interfacial polarization, and mass transfer limitations in the flow\ncell can contribute to further voltage losses.\nV = V − (IR + η + η )\ncell rev ohm act conc\nwhere:\nIR is the bulk ohmic drop (i.e., electrolyte, membrane, current collector, and circuit\nohm\nresistance).\nη encompasses activation overpotentials at both electrodes.\nact\nη arises from concentration polarization and mass‐transport limitations.\nconc\nEspecially, due to the neutral electrolyte used during the test and the introduction of\norganic ligands on both anolytes and catholytes, the R of the system has\nohm\nsignificantly increased (1.379 cm2 in charge process and 1.817 cm2 in discharge\nprocess, Figure S14c and d), compared to all-vanadium redox flow batteries (e.g.\nSmall 16, 1907333 (2020), https://doi.org/10.1002/smll.201907333).\n4) The slow kinetics and relatively large overpotential of Ce(IV)/Ce(III) couple\nsuppress the practical charge voltage.\n5) The voltage profile reflects not the theoretical redox gap but the real operating\nvoltage window under load and flow conditions. It is noted that this discrepancy\nbetween theoretical and practical voltages has been also observed in many other types\nof RFBs, such as Fe-Cr flow batteries (affording an actual equilibrium voltage of ~1.05\nV at 40 mA·cm2 in contrast to the theoretical voltage of 1.18 V as shown in Chem.\nEng. J. 421, 127855 (2021), https://doi.org/10.1016/j.cej.2020.127855), Zn-Ce flow\nbatteries (with an equilibrium voltage of ~2.3 V at 20 mA·cm2 in contrast to the\ntheoretical voltage of 2.4 V, as shown in J. Power Sources 196, 5174−5185 (2011),\nhttps://doi.org/10.1016/j.jpowsour.2011.01.095), and all-iron flow battery systems\n(with an equilibrium voltage of ~1.3 V at 40 mA·cm2 for a Fe(TEA)MM/Fe(CN)\n6\nfull cell in contrast to the theoretical voltage of 1.373 V, as shown in Angew. Chem.\nInt. Ed. 63, e202316593 (2024), https://doi.org/10.1002/anie.202316593).\nBased on the above analysis, we have made corresponding changes to clarify the\nvoltage difference between the value derived from the CV tests and the battery\nperformance measurements marked in yellow in the revised manuscript and SI as\nfollows:\nOn pages 7 and 8 in the main text,\n“Therefore, 0.2 M Fe-DTPA and Ce-DTPA in the presence of 1.0 M NaCl\nsupporting electrolyte can theoretically produce a voltage of about 0.92 V. The\nCV curves of the other Fe / Ce based electrolytes with addition of 1.0 M NaCl\nsupporting electrolyte are shown in Figure S10a.”;\n“The open-circuit voltage (OCV) of the full cell at 50% SOC was measured to be\nabout 0.73 V (Figure S14a), differing from the equilibrium potential derived from\nCV tests (Figures 3a and S14b). This difference can be attributed to several causes,\nincluding not accounting for proton activity and the Donnan potential across the\nmembrane resulting from the proton concentration gradients, ohmic resistance,\ninterfacial polarization and mass transfer limitations in the flow cell (Figure S14c\nand d), and also the slow kinetics and relatively large overpotential of\nCe(IV)/Ce(III).”\nFigure S14. (a) The plot of OCV of the full cell versus SOC. (b) CV measurements\nof 0.5 M Fe-DTPA and Ce-DTPA without supporting electrolyte. (c, d) The internal\nresistance of the 0.5 M Fe-Ce DTPA RFB at pH = 7. The R of the system has\nohm\nsignificantly increased (1.379 cm2 in charge process and 1.817 cm2 in discharge\nprocess), compared to all-vanadium redox flow batteries.\nOn page 10 in the main text,\n“Electrochemical measurements\nCyclic voltammetry (CV) measurements were performed at room temperature with\ndifferent sweep rates from 10 mV·s−1 to 50 mV·s−1 using a standard three-electrode\nsystem consisting of a graphite working electrode (6 mm in diameter), a saturated\ncalomel reference electrode with KCl salt bridge, and a platinum network (1 cm2)\ncounter electrode on a CHI660 electrochemical workstation (Shanghai Chen Hua\nInstrument Co., LTD). 15 mL of different Fe-based anolytes and Ce-based catholytes\nwere used for CV tests. The electrochemical cell was purged with nitrogen to remove\nair before the CV tests. As for evaluating the electrochemical window of the Fe-Ce\nDTPA RFB by CV, the electrolyte was prepared by dissolving 49.17 g of DTPA in\n200 mL of deoxygenated deionized water under stirring for 10 min at 20 °C.\nSubsequently, 51.83 g of K₂CO₃ was gradually added to the solution, followed by\ncontinuous stirring for 24 h until a homogeneous solution was obtained. The pH of the\nfinal electrolyte was adjusted to 7 by adding HCl or KOH solution.”\nWe sincerely appreciate the reviewer’s constructive comments, which have helped us\nclarify this important point and improve the quality of the manuscript.\nComment 2-2: I appreciate the authors for their efforts in testing symmetric cells\nusing Ce-DTPA and Fe-DTPA electrolytes respectively. However, I am surprised by\nthe rapid capacity degradation observed in both symmetric cells (Fig. S26), which\ncontradicts the stable performance reported in Fig. 4e and 4f. This discrepancy\nrequires explanation to ensure consistency in the reported data.\nResponse: We thank the reviewer for the valuable comment.\nThe observed discrepancy between the symmetric cell results (original Fig. S26) and\nthe full-cell performance in Fig. 4e and 4f is primarily due to the experimental\nlimitations during symmetric cell assembly.\nTo construct the symmetric cells, Fe3+-DTPA and Ce3+-DTPA were first employed as\nanolyte and catholyte, respectively, to assemble a Fe3+-Ce3+ DTPA RFB and complete\nthe initial charging process. Following full charge, Fe3+-DTPA was reduced to Fe2+-\nDTPA, which was then used as the catholyte in another symmetrical cell, while fresh\nFe3+-DTPA served as the anolyte. This setup enabled cycling tests for the Fe3+/2+-\nDTPA symmetrical RFB. Similarly, after charging, Ce3+-DTPA was oxidized to Ce4+-\nDTPA, which was then employed as the anolyte and paired with fresh Ce3+-DTPA\ncatholyte to form the Ce3+/4+-DTPA symmetrical RFB for subsequent testing.\nHowever, during disassembly and reassembly, it is difficult to completely avoid\nexposure to ambient air, especially for Fe2+-DTPA, which is highly sensitive to\noxidation by dissolved oxygen. This unintended exposure leads to partial oxidation of\nFe2+-DTPA prior to test, resulting in artificial capacity fading. Moreover, even trace\namounts of residual air in the symmetric cell chamber can continuously affect redox-\nactive species during cycling. Similar air-sensitivity issues have also been reported in\nother Fe-based aqueous RFB systems, e.g., Angew. Chem. Int. Ed., 63, e202316593\n(2024), https://doi.org/10.1002/anie.202316593.\nIn the initial submission, we used a simplified decoupling strategy with relatively short\ncycle numbers (200 cycles) to highlight the distinct CE and VE contributions of Fe-\nDTPA and Ce-DTPA, which are more prominent during early cycling—particularly\nfor Fe-DTPA.\nTo address the reviewer’s concern and ensure consistency with the full-cell results,\nwe have re-performed the symmetric cell experiments under improved conditions. All\nelectrolyte preparation, cell disassembly, and reassembly were carried out strictly\nunder nitrogen atmosphere using a glovebox setup to minimize oxygen interference.\nFurthermore, we extended the cycling duration to 500 cycles to better match the test\nconditions in Fig. 4e and 4f. Under these improved conditions, the capacity retention\nis significantly enhanced. The updated symmetric cell results are shown in the revised\nFig. S27 as follows.\nThe old Fig. S26\n“\nFigure S26. The CE and capacity of cycle tests for Ce3+/4+-DTPA and Fe3+/2+-DTPA\nsymmetrical RFB.\n”\nhas been changed to\n“\nFigure S27. The CE and capacity of cycle tests for Ce3+/4+-DTPA and Fe3+/2+-DTPA\nsymmetrical RFB. All electrolyte preparation, cell disassembly, and reassembly were\ncarried out strictly under nitrogen atmosphere using a glovebox setup to minimize\noxygen interference.\n”\nWe appreciate the reviewer’s comment again, which helps us refine the experimental\nprocedures and improve the clarity and reliability of our data.\nComment 2-3: It escaped my attention during the first-round review, but I now notice\nthat the authors have added 51.83 g of K CO to both Ce-DTPA and Fe-DTPA\n2 3\nelectrolytes. I am curious about the reasons behind this addition. Does the presence\nof carbonate anions play a pivotal role in facilitating the redox reactions involved?\nResponse: We sincerely thank the reviewer for this insightful question. The addition\nof K CO to both Ce-DTPA and Fe-DTPA electrolytes was intended to neutralize the\n2 3\ncarboxylic acid groups (–COOH) of the DTPA ligand, facilitating the formation of the\nmetal–ligand complexes in their deprotonated form (–COO). During this\nneutralization process, carbonate ions (CO 2) react with the liberated protons (H+) to\n3\nform CO gas, which is gradually released from the solution. Finally, hydrochloric\n2\nacid is used to adjust the pH to neutral to ensure that there is no CO 2.Therefore, after\n3\nthe complexation is complete, CO 2 ions are essentially removed from the system and\n3\ndo not participate in the redox reactions.\nFurthermore, K CO was deliberately chosen over strong bases such as KOH, as it\n2 3\nenables a more controlled pH adjustment. KOH can induce a sharp rise in pH,\npotentially leading to the premature precipitation of Fe(OH) before the coordination\n3\nof Fe3+ with DTPA occurs. The use of K CO thus ensures the stability of the Fe-\n2 3\nDTPA complex by allowing for a gradual and homogeneous pH adjustment process\n[as also shown in a prior literature: ACS Energy Lett. 5, 1758−1762 (2020),\nhttps://dx.doi.org/10.1021/acsenergylett.0c00761].\nWe provided additional clarifications in the methods section:\n“Then 51.83 g of K CO was gradually added to the solution and stirred for 24 h until\n2 3\na uniform solution formed. K CO was used to deprotonate the DTPA ligand and\n2 3\nenable stable metal–ligand complexation, while avoiding rapid pH rise and metal\nhydroxide precipitation.”\nWe hope this clarification addresses the reviewer’s concerns.\nComment 2-4: Regarding the extended cycling test, the authors have indicated a\ncharging limit of up to 70% state of charge (SOC), with a cutoff at 1 V. Is there a\nspecific rationale for capping the capacity at 70%? From the observed voltage profiles,\nit appears that the full cell has been charged to full capacity unless significant voltage\nplateaus exist above 1 V, which would justify such limitations. If so, I would encourage\nauthors to show the testing results by increasing the cut off voltage to a high voltage\nvalue, such as 1.5 V.\nResponse: We thank the reviewer for this important question and suggestion.\nOur limiting of charging to 70% SOC (cutoff at 1.0 V) is based on commonly used\noperating procedures in iron-based aqueous RFBs since full-depth cycling\n(approaching 100% SOC) often leads to excessive overpotential, enhanced side\nreactions (e.g., water splitting), and accelerated capacity fade. In practice, the usable\ncapacity of Fe–Ce DTPA systems is below the theoretical 100% SOC, a 70% SOC\nwindow captures nearly all the reversible capacity while avoiding the high-voltage\nregime where polarization and parasitic reactions dominate. As the cell approaches\nfull charge, the required charging voltage rises sharply due to increased activation and\nconcentration overpotentials. This high-voltage regime not only accelerates undesired\nreactions (notably water splitting) but also impacts the DTPA complex stability.\nLimiting SOC to ~70% maintains the cell within a relatively low overpotential\nwindow, minimizing side reactions and extending cycle life—an approach also\nadopted in other iron-based flow battery studies (e.g., Angew. Chem. Int. Ed., 63,\ne202316593 (2024), https://doi.org/10.1002/anie.202316593; ACS Energy Lett. 9,\n3859−3868 (2024), https://doi.org/10.1021/acsenergylett.4c01550).\nWe have added such information on page 11 marked in yellow in the revised\nmanuscript as follows:\n“The longer cycle tests with 15 mL electrolytes were charged at 80 mA·cm−2 to 70%\nSOC, or cutoff at 1 V (N211 membrane), avoiding the high-voltage regime where\npolarization and parasitic reactions dominate.”\nAfter extended cycling, the polarization increases further, pushing the charge voltage\nabove the water-splitting threshold. By enforcing a 1.0 V cutoff, we avoid entering the\nregion where oxygen evolution would occur, thereby protecting both electrode\nintegrity and electrolyte composition. This behavior is reflected in the gradual capacity\ndecline observed in Figure S21, where a small drop corresponds to polarization growth\ntriggering the voltage cutoff.\nWe adopted the reviewer’s suggestion and performed additional cycling with the\ncutoff voltage raised to 1.5 V. The results are shown in the following Figure R1. It can\nbe observed that although this higher cutoff does allow access to the remaining\ncapacity, both the CE and EE have decreased, suggesting that high-voltage operation\nexacerbates side reactions and will shorten cycle life. Therefore, a 1.0 V cutoff was\nused for our study.\nFigure R1. Cycle test results with 1.5 V cutoff.\nReviewer #3 (Remarks to the Author):\nThe authors have addressed the reviewer's mentioned comments, and this manuscript\ncan be published.\nResponse: We sincerely thank the reviewer for the positive evaluation and recognition\nof our work.\nResponse to the reviewers’ comments\nManuscript ID: NCOMMS-25-10508B\n“Unified electrolyte design for advanced redox flow batteries: a universal\ncomplexing agent for high performance iron-cerium redox flow batteries”\nItalic Black Text = Reviewer Comments\nBlack Text = Author Response\nBlue Text = Original Manuscript\nBlack Text Marked in Yellow = Modifications to the Manuscript\nREVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\nI appreciate the authors for their thorough efforts in addressing the comments I\nprovided during the previous round of review. The majority of their responses\nwere comprehensive and convincing. However, one key concern remains\nunresolved, which I believe is critically important and requires further\nclarification. At this stage, I recommend that the authors expand the main\nmanuscript to explicitly address the discrepancy between the theoretical cell\nvoltage derived from cyclic voltammograms (CVs) and the voltage observed\nduring the full cell test. Additionally, I encourage the authors to provide more\ndetailed explanations to elucidate this inconsistency in the main manuscript.\nResponse: We sincerely thank the reviewer for highlighting this important point.\nWe have carefully reconsidered the issue and conducted additional experiments\nto better clarify the discrepancy between the cell voltage derived from cyclic\nvoltammograms (CVs) and the voltage observed during full-cell measurements.\nSpecifically, we modified the flow battery device by incorporating a reference\nelectrode, enabling us to perform CV measurements under conditions more\ncomparable to those of the full-cell tests (i.e., with a membrane and graphite felt\nelectrodes). The corresponding results have been added and are now presented in\nthe new Figure S14b to S14f.\nFigure S14. (a) The plot of OCV of the full cell versus SOC. (b) CV curves of Fe-DTPA and Ce-\nDTPA without supporting electrolyte obtained at 40 mV s‒1 in a full cell. CV curves of (c) Fe-\nDTPA electrolyte and (e) Ce-DTPA electrolyte recorded at different scan rates in a full cell, and\n(d, f) corresponding linear fitting curves of the square root of the scan rate to the current density.\nThe electrolytes were adjusted at pH = 7 with a concentration of 0.5 M for all the tests.\nCompared with the CV results shown in Figure 3a (0.92 V, 0.2 M Fe-DTPA / Ce-\nDTPA in 1.0 M NaCl, with single electrolytic cell), the potential obtained in the\nfull cell device (0.74 V, 0.5 M Fe-DTPA / Ce-DTPA without supporting electrolyte)\nis much closer to the OCV values observed during full-cell operation. The\nstandard redox potential derived from CV shown in Figure R1 (0.84 V, 0.5 M Fe-\nDTPA / Ce-DTPA without supporting electrolyte in a single electrolytic cell)\ndiffers from the value shown in Figure 3a by ~0.08 V, mainly due to the difference\nin the actual electrolyte concentrations, thus altering the activity ratio of oxidized\nand reduced species ([𝑐  ]/[𝑐  ]) according to the Nernst equation.\n𝑂𝑥 𝑂𝑥 𝑅𝑒𝑑 𝑅𝑒𝑑\nFigure R1. CV measurements of 0.5 M Fe-DTPA and 0.5 M Ce-DTPA without supporting\nelectrolyte in a single electrolytic cell.\nA potential difference of about 0.1 V between Figure R1 and Figure S14b was\nobserved. We believe the main cause for this originates from the different type of\nworking electrode. In Figure R1, a graphite working electrode (6 mm in diameter)\nwas used, whereas in the full-cell measurements (Figure S14b), an activated\ngraphite felt electrode (5 mm in thickness with an area of 9 cm2) was employed.\nGraphite felt electrodes are expected to have more surface defects than graphite\nelectrodes, resulting in more background current and thus affecting the precise\ndetermination of Faraday current to estimate the standard redox potential.\nHowever, the influence of electrode materials on CV measurements is not the\nprimary focus of this study and will be investigated in future work.\nBased on the above analysis, we have revised the manuscript and Supporting\nInformation (SI) to include these results and discussion as follows:\nOn page 8 in the main text,\n“The open-circuit voltage (OCV) of the full cell at 50% SOC was measured to be\nabout 0.73 V (Figure S14a). This differs from the output voltage obtained by CV\nin a single electrolytic cell (Figure 3a). This discrepancy is likely due primarily\nto the differences in electrolyte concentration, thus altering the activity ratio of\noxidized and reduced species ([𝑐  ]/[𝑐  ]) according to the Nernst\n𝑂𝑥 𝑂𝑥 𝑅𝑒𝑑 𝑅𝑒𝑑\nequation. To check if this is the case, we further performed CV measurements in\na full cell with 0.5 M Fe-DTPA / 0.5 M Ce-DTPA without supporting electrolyte,\nequivalent with the electrolytes used in full battery tests (Figure 4a and S14a).\nThe obtained CV curves are shown in Figure S14b to S14f. It can be observed\nthat the voltage derived is about 0.74 V, close to the OCV at 50% SOC (Figure\nS14a).”\nRelevant experimental details have been added on page 11 in the main text as\nfollows”\n“CV measurements were also performed in a full cell for 0.5 M Fe-DTPA and 0.5\nM Ce-DTPA with a saturated calomel reference electrode under similar battery\noperating conditions.”\nIn the revised SI, the original Figure S14\n“\nFigure S14. (a) The plot of OCV of the full cell versus SOC. (b) CV measurements of 0.5\nM Fe-DTPA and Ce-DTPA without supporting electrolyte. (c, d) The internal resistance of\nthe 0.5 M Fe-Ce DTPA RFB at pH = 7. The R of the system has significantly increased\nohm\n(1.379 cm2 in charge process and 1.817 cm2 in discharge process), compared to all-\nvanadium redox flow batteries.”\nhas been change to\n“\nFigure S14. (a) The plot of OCV of the full cell versus SOC. (b) CV curves of Fe-DTPA and Ce-\nDTPA without supporting electrolyte obtained at 40 mV s‒1 in a full cell. CV curves of (c) Fe-\nDTPA electrolyte and (e) Ce-DTPA electrolyte recorded at different scan rates in a full cell, and\n(d, f) corresponding linear fitting curves of the square root of the scan rate to the current density.\nThe electrolytes were adjusted at pH = 7 with a concentration of 0.5 M for all the tests.\n”\nMoreover, we have carefully refined our manuscript and added more specific\ndetails for clarity. The changes have been highlighted in yellow in the revised\nmanuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Cerium-iron coupled redox flow battery is a low-cost and abundant system, which is promising for large-scale deployment. The authors have meticulously revised the manuscript, and significant improvements have been made following the review process. I have no further issues with this paper.", "reviewer_2": "In reviewing the manuscript, I recognize the considerable effort the authors have invested in addressing the comments from the reviewers, including myself. However, upon examining the authors' responses, I feel that further clarification is needed before the manuscript can be published.\n1. The cyclic voltammograms (CVs) presented for Ce-DTPA and Fe-DTPA in Fig. 3a with equilibrium output voltages of 0.92 V (0.64 V for Ce-DTPA and -0.28 V for Fe-DTPA versus SCE). Nevertheless, the voltage profiles of the full cell depicted in Fig. 4a reveal a substantially reduced equilibrium voltage (~0.6 V). In addition, the voltage profile during the charging process is consistently below 0.9 V, leading to concerns about whether the redox reactions occurring in the full cell accurately reflect those indicated by the CVs. The voltage profiles more closely resemble Fe-DTPA/Fe-CN (0.56 V), which needs clarification from the authors.\n2. I appreciate the authors for their efforts in testing symmetric cells using Ce-DTPA and Fe-DTPA electrolytes respectively. However, I am surprised by the rapid capacity degradation observed in both symmetric cells (Fig. S26), which contradicts the stable performance reported in Fig. 4e and 4f. This discrepancy requires explanation to ensure consistency in the reported data.\n3. It escaped my attention during the first-round review, but I now notice that the authors have added 51.83g of K2CO3 to both Ce-DTPA and Fe-DTPA electrolytes. I am curious about the reasons behind this addition. Does the presence of carbonate anions play a pivotal role in facilitating the redox reactions involved?\n4. Regarding the extended cycling test, the authors have indicated a charging limit of up to 70% state of charge (SOC), with a cutoff at 1V. Is there a specific rationale for capping the capacity at 70%? From the observed voltage profiles, it appears that the full cell has been charged to full capacity unless significant voltage plateaus exist above 1 V, which would justify such limitations. If so, I would encourage authors to show the testing results by increasing the cut off voltage to a high voltage value, such as 1.5 V.", "reviewer_3": "The authors have addressed the reviewer's mentioned comments, and this manuscript can be published."}, "author_response": "Response to the reviewers’ comments\nManuscript ID: NCOMMS-25-10508-A\n“Unified electrolyte design for advanced redox flow batteries: a universal complexing\nagent for high performance iron-cerium redox flow batteries”\nItalic Black Text = Reviewer Comments\nBlack Text = Author Response\nBlue Text = Original Manuscript\nBlack Text Marked in Yellow = Modifications to the Manuscript\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nCerium-iron coupled redox flow battery is a low-cost and abundant system, which is\npromising for large-scale deployment. The authors have meticulously revised the\nmanuscript, and significant improvements have been made following the review\nprocess. I have no further issues with this paper.\nResponse: We are very grateful to the reviewer for the positive assessment of our\nwork.\nReviewer #2 (Remarks to the Author):\nIn reviewing the manuscript, I recognize the considerable effort the authors have\ninvested in addressing the comments from the reviewers, including myself. However,\nupon examining the authors' responses, I feel that further clarification is needed\nbefore the manuscript can be published.\nResponse: We sincerely thank the reviewer for the continued consideration and\nhelpful comments on our manuscript. Based on the reviewer’s suggestions, we have\ncarefully revised the manuscript to provide more detailed explanations and made\nefforts to address the remaining concerns to the best of our ability. We have revised\nand enriched the content of the paper according to your suggestions one by one below.\nComment 2-1: The cyclic voltammograms (CVs) presented for Ce-DTPA and Fe-\nDTPA in Fig. 3a with equilibrium output voltages of 0.92 V (0.64 V for Ce-DTPA and\n-0.28 V for Fe-DTPA versus SCE). Nevertheless, the voltage profiles of the full cell\ndepicted in Fig. 4a reveal a substantially reduced equilibrium voltage (~0.6 V). In\naddition, the voltage profile during the charging process is consistently below 0.9 V,\nleading to concerns about whether the redox reactions occurring in the full cell\naccurately reflect those indicated by the CVs. The voltage profiles more closely\nresemble Fe-DTPA/Fe-CN (0.56 V), which needs clarification from the authors.\nResponse: We appreciate the reviewer’s reasonable comments to clarify the\ndiscrepancy between the theoretical cell voltage inferred from CVs and the lower\nequilibrium voltage observed in full-cell operation.\nThe observed discrepancy between the redox potentials in the CV results (Fig. 3a) and\nthe voltage profile of the full cell (Fig. 4a) can be attributed to the differences in\nexperimental conditions and practical factors that affect real battery operation as\nfollows.\nWe first measured the open-circuit voltage (OCV) of the full cell at different states of\ncharge (SOC), and found that at 50% SOC the OCV was 0.73 V, which is lower than\nthe theoretical value of 0.92 V obtained from the CV results presented in Fig. 3a. We\nattribute this discrepancy to the following factors.\n1) In the CV measurements, the electrolyte concentration is 0.2 M for both anolyte\nand catholyte with 1.0 M NaCl as supporting electrolyte. The full-cell performance\nmeasurements of Fe-Ce RFB employed 0.5 M Fe-DTPA and Ce-DTPA without\nsupporting electrolyte, starting from 0% SOC. The actual electrolyte concentrations\nused in the full cell measurements differ from those in the CV tests, which alters the\nNernstian potential, especially under charge/discharge where the [𝑐  ]/[𝑐  ]\n𝑂𝑥 𝑂𝑥 𝑅𝑒𝑑 𝑅𝑒𝑑\nratio changes dynamically.\nTo directly compare with the battery tests, we performed CV measurements for 0.5 M\nFe-DTPA or Ce-DTPA without supporting electrolyte, yielding a theoretical potential\nof 0.84 V — slightly lower than the initial CV results. The results have been added in\nFigure S14b. We thus hypothesize that the electrolyte concentration and addition of\nNaCl supporting electrolyte can affect the equilibrium output voltage, likely related to\nionic strength (Phys. Chem. Chem. Phys. 25, 7832–7846 (2023),\nhttps://doi.org/10.1039/D3CP00098B). However, even this corrected theoretical value\n(0.84 V) is still higher than the OCV measured at 50% SOC (0.73 V, Figure S14a),\nsuggesting that additional factors as below may also contribute to the observed\ndifference.\n2) Another possible reason for the difference between the equilibrium potential\nderived from CV and the full-cell charge/discharge voltage is i) the neglect of proton\nactivity and ii) the Donnan potential across the membrane, which results from proton\nconcentration gradients (Electrochem. Commun. 13, 342–345 (2011),\nhttps://doi.org/10.1016/j.elecom.2011.01.020).\n3) Ohmic resistance, interfacial polarization, and mass transfer limitations in the flow\ncell can contribute to further voltage losses.\nV = V − (IR + η + η )\ncell rev ohm act conc\nwhere:\nIR is the bulk ohmic drop (i.e., electrolyte, membrane, current collector, and circuit\nohm\nresistance).\nη encompasses activation overpotentials at both electrodes.\nact\nη arises from concentration polarization and mass‐transport limitations.\nconc\nEspecially, due to the neutral electrolyte used during the test and the introduction of\norganic ligands on both anolytes and catholytes, the R of the system has\nohm\nsignificantly increased (1.379 cm2 in charge process and 1.817 cm2 in discharge\nprocess, Figure S14c and d), compared to all-vanadium redox flow batteries (e.g.\nSmall 16, 1907333 (2020), https://doi.org/10.1002/smll.201907333).\n4) The slow kinetics and relatively large overpotential of Ce(IV)/Ce(III) couple\nsuppress the practical charge voltage.\n5) The voltage profile reflects not the theoretical redox gap but the real operating\nvoltage window under load and flow conditions. It is noted that this discrepancy\nbetween theoretical and practical voltages has been also observed in many other types\nof RFBs, such as Fe-Cr flow batteries (affording an actual equilibrium voltage of ~1.05\nV at 40 mA·cm2 in contrast to the theoretical voltage of 1.18 V as shown in Chem.\nEng. J. 421, 127855 (2021), https://doi.org/10.1016/j.cej.2020.127855), Zn-Ce flow\nbatteries (with an equilibrium voltage of ~2.3 V at 20 mA·cm2 in contrast to the\ntheoretical voltage of 2.4 V, as shown in J. Power Sources 196, 5174−5185 (2011),\nhttps://doi.org/10.1016/j.jpowsour.2011.01.095), and all-iron flow battery systems\n(with an equilibrium voltage of ~1.3 V at 40 mA·cm2 for a Fe(TEA)MM/Fe(CN)\n6\nfull cell in contrast to the theoretical voltage of 1.373 V, as shown in Angew. Chem.\nInt. Ed. 63, e202316593 (2024), https://doi.org/10.1002/anie.202316593).\nBased on the above analysis, we have made corresponding changes to clarify the\nvoltage difference between the value derived from the CV tests and the battery\nperformance measurements marked in yellow in the revised manuscript and SI as\nfollows:\nOn pages 7 and 8 in the main text,\n“Therefore, 0.2 M Fe-DTPA and Ce-DTPA in the presence of 1.0 M NaCl\nsupporting electrolyte can theoretically produce a voltage of about 0.92 V. The\nCV curves of the other Fe / Ce based electrolytes with addition of 1.0 M NaCl\nsupporting electrolyte are shown in Figure S10a.”;\n“The open-circuit voltage (OCV) of the full cell at 50% SOC was measured to be\nabout 0.73 V (Figure S14a), differing from the equilibrium potential derived from\nCV tests (Figures 3a and S14b). This difference can be attributed to several causes,\nincluding not accounting for proton activity and the Donnan potential across the\nmembrane resulting from the proton concentration gradients, ohmic resistance,\ninterfacial polarization and mass transfer limitations in the flow cell (Figure S14c\nand d), and also the slow kinetics and relatively large overpotential of\nCe(IV)/Ce(III).”\nFigure S14. (a) The plot of OCV of the full cell versus SOC. (b) CV measurements\nof 0.5 M Fe-DTPA and Ce-DTPA without supporting electrolyte. (c, d) The internal\nresistance of the 0.5 M Fe-Ce DTPA RFB at pH = 7. The R of the system has\nohm\nsignificantly increased (1.379 cm2 in charge process and 1.817 cm2 in discharge\nprocess), compared to all-vanadium redox flow batteries.\nOn page 10 in the main text,\n“Electrochemical measurements\nCyclic voltammetry (CV) measurements were performed at room temperature with\ndifferent sweep rates from 10 mV·s−1 to 50 mV·s−1 using a standard three-electrode\nsystem consisting of a graphite working electrode (6 mm in diameter), a saturated\ncalomel reference electrode with KCl salt bridge, and a platinum network (1 cm2)\ncounter electrode on a CHI660 electrochemical workstation (Shanghai Chen Hua\nInstrument Co., LTD). 15 mL of different Fe-based anolytes and Ce-based catholytes\nwere used for CV tests. The electrochemical cell was purged with nitrogen to remove\nair before the CV tests. As for evaluating the electrochemical window of the Fe-Ce\nDTPA RFB by CV, the electrolyte was prepared by dissolving 49.17 g of DTPA in\n200 mL of deoxygenated deionized water under stirring for 10 min at 20 °C.\nSubsequently, 51.83 g of K₂CO₃ was gradually added to the solution, followed by\ncontinuous stirring for 24 h until a homogeneous solution was obtained. The pH of the\nfinal electrolyte was adjusted to 7 by adding HCl or KOH solution.”\nWe sincerely appreciate the reviewer’s constructive comments, which have helped us\nclarify this important point and improve the quality of the manuscript.\nComment 2-2: I appreciate the authors for their efforts in testing symmetric cells\nusing Ce-DTPA and Fe-DTPA electrolytes respectively. However, I am surprised by\nthe rapid capacity degradation observed in both symmetric cells (Fig. S26), which\ncontradicts the stable performance reported in Fig. 4e and 4f. This discrepancy\nrequires explanation to ensure consistency in the reported data.\nResponse: We thank the reviewer for the valuable comment.\nThe observed discrepancy between the symmetric cell results (original Fig. S26) and\nthe full-cell performance in Fig. 4e and 4f is primarily due to the experimental\nlimitations during symmetric cell assembly.\nTo construct the symmetric cells, Fe3+-DTPA and Ce3+-DTPA were first employed as\nanolyte and catholyte, respectively, to assemble a Fe3+-Ce3+ DTPA RFB and complete\nthe initial charging process. Following full charge, Fe3+-DTPA was reduced to Fe2+-\nDTPA, which was then used as the catholyte in another symmetrical cell, while fresh\nFe3+-DTPA served as the anolyte. This setup enabled cycling tests for the Fe3+/2+-\nDTPA symmetrical RFB. Similarly, after charging, Ce3+-DTPA was oxidized to Ce4+-\nDTPA, which was then employed as the anolyte and paired with fresh Ce3+-DTPA\ncatholyte to form the Ce3+/4+-DTPA symmetrical RFB for subsequent testing.\nHowever, during disassembly and reassembly, it is difficult to completely avoid\nexposure to ambient air, especially for Fe2+-DTPA, which is highly sensitive to\noxidation by dissolved oxygen. This unintended exposure leads to partial oxidation of\nFe2+-DTPA prior to test, resulting in artificial capacity fading. Moreover, even trace\namounts of residual air in the symmetric cell chamber can continuously affect redox-\nactive species during cycling. Similar air-sensitivity issues have also been reported in\nother Fe-based aqueous RFB systems, e.g., Angew. Chem. Int. Ed., 63, e202316593\n(2024), https://doi.org/10.1002/anie.202316593.\nIn the initial submission, we used a simplified decoupling strategy with relatively short\ncycle numbers (200 cycles) to highlight the distinct CE and VE contributions of Fe-\nDTPA and Ce-DTPA, which are more prominent during early cycling—particularly\nfor Fe-DTPA.\nTo address the reviewer’s concern and ensure consistency with the full-cell results,\nwe have re-performed the symmetric cell experiments under improved conditions. All\nelectrolyte preparation, cell disassembly, and reassembly were carried out strictly\nunder nitrogen atmosphere using a glovebox setup to minimize oxygen interference.\nFurthermore, we extended the cycling duration to 500 cycles to better match the test\nconditions in Fig. 4e and 4f. Under these improved conditions, the capacity retention\nis significantly enhanced. The updated symmetric cell results are shown in the revised\nFig. S27 as follows.\nThe old Fig. S26\n“\nFigure S26. The CE and capacity of cycle tests for Ce3+/4+-DTPA and Fe3+/2+-DTPA\nsymmetrical RFB.\n”\nhas been changed to\n“\nFigure S27. The CE and capacity of cycle tests for Ce3+/4+-DTPA and Fe3+/2+-DTPA\nsymmetrical RFB. All electrolyte preparation, cell disassembly, and reassembly were\ncarried out strictly under nitrogen atmosphere using a glovebox setup to minimize\noxygen interference.\n”\nWe appreciate the reviewer’s comment again, which helps us refine the experimental\nprocedures and improve the clarity and reliability of our data.\nComment 2-3: It escaped my attention during the first-round review, but I now notice\nthat the authors have added 51.83 g of K CO to both Ce-DTPA and Fe-DTPA\n2 3\nelectrolytes. I am curious about the reasons behind this addition. Does the presence\nof carbonate anions play a pivotal role in facilitating the redox reactions involved?\nResponse: We sincerely thank the reviewer for this insightful question. The addition\nof K CO to both Ce-DTPA and Fe-DTPA electrolytes was intended to neutralize the\n2 3\ncarboxylic acid groups (–COOH) of the DTPA ligand, facilitating the formation of the\nmetal–ligand complexes in their deprotonated form (–COO). During this\nneutralization process, carbonate ions (CO 2) react with the liberated protons (H+) to\n3\nform CO gas, which is gradually released from the solution. Finally, hydrochloric\n2\nacid is used to adjust the pH to neutral to ensure that there is no CO 2.Therefore, after\n3\nthe complexation is complete, CO 2 ions are essentially removed from the system and\n3\ndo not participate in the redox reactions.\nFurthermore, K CO was deliberately chosen over strong bases such as KOH, as it\n2 3\nenables a more controlled pH adjustment. KOH can induce a sharp rise in pH,\npotentially leading to the premature precipitation of Fe(OH) before the coordination\n3\nof Fe3+ with DTPA occurs. The use of K CO thus ensures the stability of the Fe-\n2 3\nDTPA complex by allowing for a gradual and homogeneous pH adjustment process\n[as also shown in a prior literature: ACS Energy Lett. 5, 1758−1762 (2020),\nhttps://dx.doi.org/10.1021/acsenergylett.0c00761].\nWe provided additional clarifications in the methods section:\n“Then 51.83 g of K CO was gradually added to the solution and stirred for 24 h until\n2 3\na uniform solution formed. K CO was used to deprotonate the DTPA ligand and\n2 3\nenable stable metal–ligand complexation, while avoiding rapid pH rise and metal\nhydroxide precipitation.”\nWe hope this clarification addresses the reviewer’s concerns.\nComment 2-4: Regarding the extended cycling test, the authors have indicated a\ncharging limit of up to 70% state of charge (SOC), with a cutoff at 1 V. Is there a\nspecific rationale for capping the capacity at 70%? From the observed voltage profiles,\nit appears that the full cell has been charged to full capacity unless significant voltage\nplateaus exist above 1 V, which would justify such limitations. If so, I would encourage\nauthors to show the testing results by increasing the cut off voltage to a high voltage\nvalue, such as 1.5 V.\nResponse: We thank the reviewer for this important question and suggestion.\nOur limiting of charging to 70% SOC (cutoff at 1.0 V) is based on commonly used\noperating procedures in iron-based aqueous RFBs since full-depth cycling\n(approaching 100% SOC) often leads to excessive overpotential, enhanced side\nreactions (e.g., water splitting), and accelerated capacity fade. In practice, the usable\ncapacity of Fe–Ce DTPA systems is below the theoretical 100% SOC, a 70% SOC\nwindow captures nearly all the reversible capacity while avoiding the high-voltage\nregime where polarization and parasitic reactions dominate. As the cell approaches\nfull charge, the required charging voltage rises sharply due to increased activation and\nconcentration overpotentials. This high-voltage regime not only accelerates undesired\nreactions (notably water splitting) but also impacts the DTPA complex stability.\nLimiting SOC to ~70% maintains the cell within a relatively low overpotential\nwindow, minimizing side reactions and extending cycle life—an approach also\nadopted in other iron-based flow battery studies (e.g., Angew. Chem. Int. Ed., 63,\ne202316593 (2024), https://doi.org/10.1002/anie.202316593; ACS Energy Lett. 9,\n3859−3868 (2024), https://doi.org/10.1021/acsenergylett.4c01550).\nWe have added such information on page 11 marked in yellow in the revised\nmanuscript as follows:\n“The longer cycle tests with 15 mL electrolytes were charged at 80 mA·cm−2 to 70%\nSOC, or cutoff at 1 V (N211 membrane), avoiding the high-voltage regime where\npolarization and parasitic reactions dominate.”\nAfter extended cycling, the polarization increases further, pushing the charge voltage\nabove the water-splitting threshold. By enforcing a 1.0 V cutoff, we avoid entering the\nregion where oxygen evolution would occur, thereby protecting both electrode\nintegrity and electrolyte composition. This behavior is reflected in the gradual capacity\ndecline observed in Figure S21, where a small drop corresponds to polarization growth\ntriggering the voltage cutoff.\nWe adopted the reviewer’s suggestion and performed additional cycling with the\ncutoff voltage raised to 1.5 V. The results are shown in the following Figure R1. It can\nbe observed that although this higher cutoff does allow access to the remaining\ncapacity, both the CE and EE have decreased, suggesting that high-voltage operation\nexacerbates side reactions and will shorten cycle life. Therefore, a 1.0 V cutoff was\nused for our study.\nFigure R1. Cycle test results with 1.5 V cutoff."}]

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