An open decoupled cell design achieving electricity generation and amplification through waste-to-energy conversion

Domain: neuroscience
Published: 2026-01-24
Editorial outcome: minor_revision

Abstract
Conventional closed batteries are constrained by the electrical energy efficiency of 100%, inevitably leading to the reduction of electricity storage. In contrast, open decoupled batteries offer the possibility to break this limitation, but remain unexplored. Here, we develop a highly efficient and sustainable open decoupled battery through a three-electrodynamic-potential (3E) design, simultaneously realizing waste-to-energy conversion, power generation and energy storage. For decoupled electrodes, we engineer high discharge voltage (E_D) incorporating zinc oxidation and oxygen reduction reactions, and low charge voltage (E_C) involving zinc-ion reduction and hydrazine (waste) oxidation reactions. Furthermore, we introduce reverse electrodialysis potential (E_RED) by decoupling electrolytes. Consequently, the assembled battery demonstrates stability for 1000 cycles at the fast-charging current density of 300 mA cm−2. Moreover, a scaled 20-Ah-capacity battery was performed achieving a high electrical energy efficiency of 375% at 10 mA cm−2. Techno-economic analyses reveal that storing one megawatt-hour of electricity using the open decoupled battery can reduce the cost and carbon emissions of power generation by over 80% compared to conventional batteries. This work establishes a foundation for designing electricity-amplified batteries with economic and environmental benefits.

Introduction
Energy storage systems (ESSs) are critical in developing smart-grids and the universal application of intermittent renewable energy 1–4. To mitigate energy losses (e.g., activation loss and mass transport loss) during storage, researchers have proposed various strategies, including electrodes’ material designs 5–8, electrolyte modification 9–11 and battery structure exploration 12,13, to improve ESSs’ electrical energy efficiency (η_EEE). Since conventional ESSs operate as closed systems that permit energy transfer with the external environment while preventing matter exchange, their upper limit of η_EEE is 100% (Fig. 1a) 14. By comparison, open decoupled batteries (ODBs) allow the input of free or low-cost external matter during operation, potentially realizing η_EEE greater than 100% (η_EEE > 100%) (Fig. 1b). A cell with η_EEE > 100% can output more power than its inputs, thereby enabling electricity generation and amplification, which hopefully optimizes the energy mix and accomplishes a more economically and environmentally efficient energy supply. However, designing ODBs with η_EEE > 100% has not been explored.

Results
Determination of decoupled components of ODB
The first step in developing a decoupled system is to identify whether the reactions of decoupled electrodes interfere with each other when placed in the same chamber. Therefore, a chronoamperometric curve of commercial platinum/carbon (Pt/C) catalyst was performed and exhibits the conflict between ORR and HzOR by introducing 1 mL N2H4 to 100 mL oxygen-saturated 1 mol L−1 (M) potassium hydroxide (KOH) at 300 s (Fig. 2b). Next, we evaluate the use of decoupled electrolytes to fully exploit the voltage advantage of the two electrodes. For the ORR electrode, its Pourbaix diagram suggests that a lower pH value of the electrolyte leads to a higher E_Zn-ORR (Fig. 2a). However, when pH values of electrolytes are less than 14 (corresponding to 0.01 M KOH and 0.1 M KOH), E_Zn-ORR of primary Zn-ORR batteries rapidly drops to approximately 0 V at 1 mA cm−2 (insets of Fig. 2a), probably resulting from sluggish ion transport of limited hydroxide ion (OH−). Hence, 1 M KOH is a suitable electrolyte for the ORR electrode. For the HzOR electrode using cobalt catalyst loaded on nickel foam (Co/NF, Supplementary Fig. 1), linear sweep voltammetry (LSV) curves in Fig. 2c show a higher concentration of OH− results in a lower reaction potential at the same current density, and thereby a smaller E_Zn-HzOR. In terms of Zn electrode, it is prone to passivation in 1 M KOH (Supplementary Fig. 2), resulting in a low depth of discharge (DoD), while the discharge capacity of Zn in 6 M KOH is four times that in 1 M KOH (Fig. 2d). Consequently, Zn and HzOR electrodes should be placed in one chamber, with the ORR electrode in the other. In typical alkaline Zn-based batteries (TAZBs), saturated zinc oxide is usually added to electrolytes to decrease the ionic concentration polarization, weaken the activity of water and hinder hydrogen evolution reaction (HER) that occurs during charging. Consequently, 6 M KOH, saturated zinc oxide and 0.5 M N2H4 (6 M KOH + sat. ZnO + 0.5 M N2H4) are utilized as an electrolyte both for Zn and HzOR electrodes. As shown in Fig. 2e, U_charge (equivalent to E_C as no E_RED for charging process) of Zn-HzOR are 0.33 V at 1 mA cm−2 and 0.88 V at 300 mA cm−2, far lower than that of TAZBs (~2 V) 13,20,21. Due to decoupling electrolytes, an ion exchange membrane is needed to separate them (Fig. 2f). The direction of the produced E_RED using CEM is consistent with that of E_Zn-ORR, thus boosting U_discharge; while using an anion-exchange membrane (AEM) makes the direction of E_RED opposite to that of E_Zn-ORR, cutting down U_discharge. Zn-CEM-ORR with favorable E_RED demonstrates a high voltage of 1.49 V at 1 mA cm−2 in Fig. 2g, better than that in single 1 M KOH (1.34 V) or 6 M KOH (1.44 V) in Supplementary Fig. 3. Notably, all η_EEE of ODB at different current densities ranging from 1 to 20 mA cm−2 exceed 300% and the maximum η_EEE is 445%.

Revelation of corrosion inhibition mechanism
Figure 3a and Supplementary Fig. 4 exhibit that the specific capacity and DoD of Zn-CEM-ORR with N2H4 electrolytes are both greater than those without N2H4 electrolytes, on account of suppressed Zn corrosion by N2H4 molecules. Next, corrosion potential (E_corr) and corrosion current (I_corr) were examined by Tafel plots (Fig. 3c). The E_corr of Zn electrodes in N2H4 electrolytes is −1.39 V (versus the mercury/mercury oxide electrode (Hg/HgO)), which is more positive than that of blank electrolytes (−1.41 V versus Hg/HgO). Moreover, the I_corr in N2H4 electrolytes (37.1 μA cm−2) is much lower than that in electrolytes without N2H4 (165.4 μA cm−2), demonstrating the inhibition efficiency of 77.5% and confirming the prominent corrosion resistance of Zn electrode using N2H4 electrolytes (Supplementary Fig. 5). We further investigated the inhibition mechanism by density functional theory (DFT) calculations. Corrosion processes occur at the interface between Zn electrode and electrolyte, summarized as a “trilogy”, involving water dissociation, HER and Zn dissolution (Fig. 3b). According to calculations (Supplementary Fig. 6), adsorption energies of water (−0.29 eV) and N2H4 (−0.42 eV) molecules are negative, which means that both of them spontaneously adsorb on the surface of Zn (Supplementary Data 1, 2). The differential charge density analysis shows that the adsorption of nucleophilic sites of N2H4 significantly alter the local charge distribution on Zn surface, increasing negative charge density around the adsorbed molecules (inset of Fig. 3d and Supplementary Fig. 6c). This rearranged local charge distribution resulting from N2H4 adsorption leads to three effects. First, it increases the activation energy of water dissociation (Supplementary Fig. 6d and Supplementary Data 3–5). The projected density of states (PDOS) further displays less overlap of Zn 3d and O 2p orbitals, demonstrating the diminished interaction between Zn surface and water molecule (Fig. 3d). Additionally, the calculated Milliken charges of O and Zn are both decreased, again confirming the weakened Zn-O bond to render water dissociation more challenging (Fig. 3e). Second, the increasing negative charge density strengthens the interaction between H and Zn, resulting in a higher energy barrier for HER compared to pure Zn surface (Fig. 3f and Supplementary Data 6–8). PDOS analysis also reveals that the presence of adsorbed N2H4 molecules makes Zn 3d orbital center closer to the Fermi level (Fig. 3e) and enhances the binding interaction between Zn and H*, causing H dissociation during HER process more difficult 22. Third, the Zn dissolution process involves the detachment of Zn atoms from the crystal lattice and the solvation of the detached Zn atoms 23,24. For the first process, we calculated the detachment energies of a Zn atom under the adsorption of H2O or N2H4 molecules (Fig. 3g and Supplementary Data 9, 10). For the second process, the negative solvation energies indicate that the solvation process is spontaneous (Supplementary Fig. 6e and Supplementary Data 11–13) 25,26. Hence, the whole dissolution process is determined by the first process. The stable adsorption of N2H4 induces electronic rearrangement and spatial hindrance, raising the passivation effect on the Zn surface. As a result, detachment of Zn atoms requires overcoming a higher energy barrier, thus impeding Zn dissolution.
To experimentally verify the effect of N2H4 specific adsorption on the water distribution of Zn surface, we conducted in situ Raman spectroscopy under different cathodic current densities (Fig. 3h and Supplementary Fig. 7). For 6 M KOH + sat. ZnO electrolyte, the peak of ~470 cm−1 attributed to the libration mode of H2O is evident regardless of whether a current is applied, which implies that interfacial water maintains a specially ordered structure throughout 27,28. In contrast, the peak of interfacial water is almost absent at the beginning in 6 M KOH + sat. ZnO + 0.5 M N2H4 electrolyte, which might be ascribed to the interfacial adsorption of N2H4 interfering with the formation of organized structures of water. When the current increases to 50 mA cm−2, a relatively strong peak emerges in the spectrum. We speculate that the elevated current intensifies HER, allowing more water to accumulate at the interface and thereby forming an ordered structure again. Benefiting from the suppressed corrosion, Zn electrode immersing in 6 M KOH + sat. ZnO + 0.5 M N2H4 electrolyte for 2, 5, and 10 days presents a flatter and smoother surface as shown in confocal laser scanning microscope (CLSM) images, while severe corrosion with abundant pits and protrusions is observed on Zn surface without N2H4 protection (Supplementary Fig. 8). The above evidence again identifies that N2H4 effectively mitigates Zn corrosion, improving the capacity of ODB.

ODB system performance
Unlike traditional systems where U_charge is always higher than U_discharge, the U_discharge of our system is higher than U_charge (Fig. 4a). As illustrated in Fig. 2e, U_charge of ODB is so low that fast charging is easily achieved. Specifically, ODB operates stably for 1000 cycles (1100 h) at the fast-charging current density of 100 mA cm−2 (10 mA cm−2 for discharging) with the capacity of 10 mAh cm−2 and enables recoverable η_EEE of 261% after replacing depleted electrolytes (Supplementary Fig. 9). During the cycle, η_EEE gradually decreases because of the continuous consumption of active substances in the simulated N2H4 wastewater and changes of the concentration of OH− in both electrolytes. After replacing electrolytes on both sides, the ODB nearly returns to its initial state of high U_discharge and low U_charge, indicating the long-term stability and recoverability of the system. Furthermore, even when the charging current is increased to 300 mA cm−2 (20 mA cm−2 for discharging), ODB still demonstrates durability for 1000 cycles with a high initial η_EEE of 138% (Fig. 4c). In addition, the fast charging performance of ODB with high Zn utilization of 56.7% (typically < 10%) 19 is shown in Fig. 4b, steadily working for 1000 h. Considering the operating requirements of batteries in winter, we also tested the rate performance of ODB at 0 oC. As illustrated in Fig. 4d, η_EEE > 100% is still achieved at the fast-charging current density of 100 mA cm−2 (10 mA cm−2 for discharging), demonstrating the feasibility of ODB at low temperatures. Notably, the η_EEE and lifespan performance of ODB exhibit advantages compared to previously reported flow battery systems (Fig. 4e, see Supplementary Table 1 for details) 20,21,29–35. To further validate the practical application of our system, a scaled-up ODB system is shown in Fig. 4f and Supplementary Fig. 10. Since ODB with η_EEE > 100% endows W_out greater than W_in, after satisfying charging energy requirements for treating wastes, the excess electricity (W_out − W_in) can be used for other stationary electrical equipment, such as lighting devices and charging stations. The assembled 11 × 11 cm2 ODB shows a large capacity of 20 Ah per cycle (over 18 days) (Fig. 4g).

Economic and environmental benefits of ODB
The two operational modes of the ODB are shown in Fig. 5a. On the one hand, owing to ODBs with η_EEE > 100%, all-ODB systems are self-driven, integrating power generation, energy storage and wastewater treatment. Intermittent renewable energies, such as solar energy (SE) and wind energy (WE), require supporting ESSs to ensure a continuous energy supply. Moreover, regulation equipment, such as inverters, is needed between the two, thereby increasing the complexity and cost of the entire system. For the fossil energy power systems, although they are simpler, but emit a large amount of greenhouse gases, which are harmful to the environment. In contrast, all-ODB devices not only simplify infrastructure, but also generate green electricity while treating waste, providing substantial economic and environmental benefits. On the other hand, the ODB can synergize with other power generation technologies to amplify electricity output, further reducing costs and carbon emissions. In detail, TEAs show that the required electricity generated by power generation for storing 1 MWh of electricity in ODBs with η_EEE of 300% is 70%, 73%, 75%, and 73% less than lithium-ion (Li-ion), lead-acid, nickel-iron (Ni-Fe) and all vanadium batteries 36,37, respectively, greatly cutting down the expense of power generation technologies (Fig. 5b, Supplementary Fig. 11, Supplementary Note 1 and Supplementary Tables 2–6). Furthermore, due to the significant savings in wastewater disposal, ODB can achieve considerable profits throughout its entire service life, which is totally different from the losses incurred by other ESSs, demonstrating huge potential for application. Additionally, compared to traditional methods for degrading N2H4 in wastewater through oxidants such as ozone (36.4 US$ m−3) and hydrogen peroxide (10.2 US$ m−3), the operation of all-ODB systems significantly shortens disposal expenses (5.3 US$ m−3), thus creating additional economic benefits (Fig. 5c and Supplementary Note 2). Regarding environmental impacts (Fig. 5d), the carbon emission of an ODB is 12.7 kg CO2 eq. MWh−1, better than SE (30 kg CO2 eq. MWh−1) and comparable to WE (10 kg CO2 eq. MWh−1). The combination of SE and ODB emits only 15.3% of the CO2 emissions of the combination of SE and Li-ion. It is worth mentioning that traditional natural gas energy combined with ODB reduces carbon emissions per megawatt-hour of electricity by 63.7% compared with the original all-fuel energy, which is comparable to the combination of SE and Li-ion 38–40. Based on this, we believe that scaling up ODBs can further diminish costs and emissions, enhancing their dual economic and environmental benefits.

Methods
Materials
The following chemicals and materials are all commercially available and were used as received: zinc plate (Zn, 99.9%, ZhengYinKeYan), zinc oxide (ZnO, 99%, Aladdin), potassium hydroxide (KOH, 95%, Macklin), hydrazine hydrate (N2H4·H2O, ≥80%, ShangHai LingFeng Co., Ltd), commercial platinum/carbon electrode (Pt/C, 1 mg cm−2, thickness of 0.235 mm, SEN23K, SuZhou SINERO Tech. Co., Ltd), commercial manganese dioxide electrode (MnO2, 20 mg cm−2, thickness of 1 mm, ChangSha SiPuLin), gas diffusion layer (ChangSha SiPuLin), cation-exchange membrane (CEM, Nafion 117, thickness of 0.183 mm, Dupont), anion-exchange membrane (AEM, FAA-3-PK-130, Fumasep), nickel foam (NF, thickness of 0.3 mm, ShenZhen KeJing), nickel foil (Ni foil, thickness of 0.05 mm, 99.9%, ZhengYinKeYan), acetone (AR, ShangHai LingFeng Co., Ltd), hydrochloric acid (HCl, AR, ρ=1.19 g mL−1, XiHua), ethanol (99.7%, Rhawn), deionized water (18.2 MΩ, Millipore system), cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, 99%, Macklin), ammonium fluoride (NH4F, 99.99%, Macklin), urea (CO(NH2)2, 99%, Macklin), sulfuric acid (H2SO4, AR, GuangZhou Chemical Reagent Factory).

Preparation of Co/NF electrocatalyst for HzOR
The Co precursor grown on NF as the substrate was prepared by the hydrothermal method 41. Specifically, at the beginning, the NF was thoroughly washed with acetone, 1 M HCl, ethanol and deionized water with ultrasonic treatment for 10 min to wipe off the greasy dirt and oxide layer on the surface. Subsequently, 3 mmol of Co(NO3)2·6H2O, 4 mmol of NH4F and 10 mmol of CO(NH2)2 were dissolved in 60 mL of deionized water under stirring. Then, the resulting solution and the NF (2 × 3.5 cm2) were transferred to a 100 mL Teflon-lined stainless-steel autoclave and maintained at 120 °C for 6 h. After naturally cooling to room temperature, the obtained Co precursor was washed with deionized water and ethanol three times, and dried in a vacuum. After that, the precursor was heated at 325 °C for 2 h with a heating speed of 5 °C min−1 with an Ar-5% H2 mixture atmosphere in a tube furnace. Finally, a black Co/NF was collected after cooling to room temperature. The mass loading of Co/NF was about 2 mg cm−2.

Electrochemical measurements
All electrochemical measurements were performed in a standard three-electrode system using CHI760E electrochemical workstation. A Hg/HgO electrode (1 M KOH) was used as the reference electrode and a graphite rod was used as the counter electrode, respectively. A chronoamperometry test was performed to verify whether ORR and HzOR interfere with each other at 0.74 V versus the reversible hydrogen electrode (RHE). Linear sweep voltammetry (LSV) measurements of HzOR were conducted at a scan speed of 5 mV s−1 without iR compensation. Corrosion property was tested by Tafel plot from −1.30 V to −1.50 V versus Hg/HgO with the scan speed of 5 mV s−1. The corrosion inhibition efficiency (η_CIE) is calculated as follows:
η_CIE = (I_corr-without − I_corr-with) / I_corr-without (1)
where I_corr-without and I_corr-with represent the corrosion current of Zn electrode in 6 M KOH + sat. ZnO and 6 M KOH + sat. ZnO + 0.5 M N2H4 electrolytes, respectively.

Battery performance
In a homemade ODB flow system, Zn plate (4.5 cm2, thickness of 0.3 mm) and HzOR electrode with Co/NF electrocatalyst (1 cm2) and a current collector of Ni foil were in the same chamber, using 6 M KOH + sat. ZnO + 0.5 M N2H4 (300 mL) as the HzOR electrolyte. ORR electrode including commercial Pt/C electrocatalyst (1 cm2), a gas diffusion layer and a current collector of NF was in a separate chamber, using 1 M KOH (300 mL) as the ORR electrolyte. Zn electrode was placed between HzOR electrode and ORR electrode. CEM was used to separate HzOR electrolyte and ORR electrolyte. The flow rate applied was about 52 mL min−1. Long discharge curves of primary Zn-ORR batteries were used to determine the discharge capacity of Zn in 1 M and 6 M KOH electrolytes without CEM. Galvanostatic charge-discharge measurements of ODB system were evaluated using NEWARE battery testing systems (CT-4008T-5V1A-TWS, Shenzhen, China) or LAND (CT3001A) at different current densities. The above tests were performed at 25 ± 5 oC. The rate performance of ODB at low temperature (0 oC) was measured using an electrochemical workstation (BioLogic SP-150e). The large-scale ODB device (11 × 11 cm2) was assembled using a 3 × 3 cm2 HzOR electrode, 6 × 6 cm2 Zn electrode (thickness of 1 mm) and 3 × 3 cm2 ORR electrode. The volume of the electrolyte on both sides was 300 mL and the flow rate was also set to about 52 mL min−1. In the acid-ODB flow system, except that the ORR electrolyte was changed from 1 M KOH to 0.5 M H2SO4, other conditions remained the same as in ODB system. All tests are performed in an open-air environment.
The electrical energy efficiency (η_EEE) is calculated as follows:
η_EEE = W_discharge / W_charge = (U_discharge * I * t) / (U_charge * I * t) (2)
Since the capacity (It) for charging and discharging is set to the same, the above equation is converted to:
η_EEE = U_discharge / U_charge (3)
where W, U, I, and t represent the electrical energy, voltage, current and time of the battery during charging and discharging, respectively.

Characterizations
In situ Raman spectra were conducted to characterize the interfacial water of Zn electrodes in electrolytes in order to reveal the corrosion mechanism, recorded by Raman spectrometer (HORIBA LabRAM HR800) with a 532 nm laser. The in situ three-electrode test device is shown in Supplementary Fig. 7. The Zn plate (size: 1 × 1 cm2), a Pt wire and a Hg/HgO electrode (1 M KOH) were used as the working, counter and reference electrode, respectively. The working electrode was oxidized by applying different currents (each current maintained for 6 min) through an electrochemical workstation. A confocal laser scanning microscope (CLSM, VK-X1000, KEYENCE) was used to determine the calendar corrosion of electrodes by analyzing surface roughness. The X-ray diffraction was performed on Bruker D8 Advance with Cu Kα radiation at a scan rate of 10° min−1. All tests were performed in the atmosphere at 25 ± 5 oC.

Theoretical calculations
DFT calculations were carried out using the CP2K/Quickstep software package. For electronic structure calculations, the Perdew-Burke-Ernzerhof (PBE) functional was applied, with Van der Waals forces accounted for through the Grimme D3 dispersion correction. A plane wave energy cutoff of 800 Ry was chosen to ensure precise wave function expansion, employing the Triple-Zeta Valence with Polarization (TZVP) basis sets. To achieve high accuracy in electronic calculations, the self-consistent field (SCF) convergence criterion was set at a stringent 1 × 10−6 Hartree. The Brillouin zone sampling was performed using a 3 × 3 × 1 k-point mesh. For slab relaxations, the bottom two layers were kept fixed, while the top two layers and any adsorbates were allowed to relax. Solvation effects were modeled through a hybrid computational approach combining explicit molecular representation with continuum electrostatics. The first two solvation shells were explicitly simulated using OH− and water molecules, selected for their specific coordination geometry and binding energetics with ions and critical roles in hydration thermodynamics. Surround solvent effects were incorporated via the Polarizable Continuum Model (PCM), where bulk water was represented as a uniform dielectric medium characterized by its experimentally determined static permittivity of ε = 78.5. The adsorption free energies of hydrogen and water on Zn surface in this work are calculated using the expression:
ΔG = ΔE + ΔZPE − TΔS (4)
where ΔE represents the adsorption energy of H or water. The terms ΔZPE and TΔS account for the corrections due to zero-point energy and entropy, respectively. The calculated values for and H2O, H* and H2 are consistent with previous studies 42.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript the authors have presented an ODB with >100% round-trip efficiency. The presented research is interesting, and it could have a significant impact on wastewater management and green energy. However, I have the following questions for the authors.\n1. Although the authors claim that the developed DOB can utilize wastewater containing N2H4. What if the wastewater contains other chemicals? Will other chemicals decrease cell performance?\n2. Adding ligand molecules to is an effective method to protect Zn anode (doi.org/10.1021/jacs.0c09794, 10.1002/aenm.202303998). There are two effects of the additive ligands: protecting the Zn solid surface and weakening the bonding strength between H2O and Zn2+. It seems that the authors’ theoretical modeling mainly focuses on modeling N2H4 on the Zn surface. Can the authors discuss whether adding N2H4 has any effect on Zn2+?\n3. In Figure 3e, the authors claim that the Zn-O bond is weakened. Can the authors provide the bond distance?\n4. In Figure 3g, the authors present the dissolution energy by attaching a Zn atom. Should the author consider Zn0 or Zn2+, also will Zn coordinated by OH or N2H4 be considered instead of an isolated Zn atom? The authors should provide more information about this figure.\nOverall, I’ll be happy to accept the paper if the authors properly address the comments and questions.", "reviewer_2": "Summary\nThis manuscript presents a novel and compelling open decoupled battery (ODB) design that achieves round-trip efficiencies (ηRTE) exceeding 300%, with a reported maximum of 445%. The authors implement a “three-electrodynamic-potential” (3E) design by engineering low charge voltages via hydrazine oxidation and high discharge voltages via oxygen reduction, and by introducing a reverse electrodialysis potential (ERED) through electrolyte decoupling. The system enables power generation, waste remediation, and electricity amplification.\nComprehensive experimental validation is supported by DFT analysis and techno-economic assessment (TEA), and the work culminates in a 20 Ah scaled prototype demonstrating practical potential. The manuscript is generally well written and could have the potential to be published in this journal if the following comments are addressed adequately.\nMajor Comments\n1. Hydrazine Toxicity and Practicality\nThe authors mention briefly the disposal costs of hydrazine wastewater in their TEA. However further discussion on the toxicity and practicality of using hydrazine would be nice. Hydrazine is quite a toxic and potentially carcinogenic compound. While the system degrades it, broader safety, handling, and environmental implications of deploying hydrazine in energy storage should be discussed and perhaps the authors could suggest alternatives.\n2. Electrolyte Depletion and Maintenance\nThe need to replace spent electrolytes to restore performance raises concerns about system autonomy and operational complexity. A quantitative analysis of electrolyte replacement frequency and associated costs over system lifetime should be included in the TEA.\n3. Membrane Stability and Selectivity\nThe role of the cation-exchange membrane (CEM) is central to the ERED benefit. However, the manuscript lacks data or discussion on membrane longevity, fouling, or selectivity degradation over time. Including membrane cycling tests or citing relevant lifetime studies would strengthen the system’s credibility for scale-up.\nMinor Comments\n1. Figure 2 is too dense and could benefit from being broken down into two figures.\n2. It would be helpful to include error bars or statistical variation (if any) for electrochemical performance plots to reflect reproducibility.", "reviewer_3": "The manuscript discusses the results of the development and testing of a novel battery cell with a claimed round-trip efficiency of over 300%.\nI recommend rejecting the publication of this manuscript in a highly respected journal such as Nature Communications.\nEver since the formulation of the laws of thermodynamics, it has been clear that a perpetual motion machine is not possible. The authors repeatedly claim an energy efficiency of their cell exceeding 100%, without offering any consideration of the actual energetic and material balances involved in operating such a cell. Disregarding the fundamental principles of thermodynamics is unscientific. Reports on such work have no place in any scientific publication.\nFurthermore, the manuscript is written in an overly personal tone and contains an excessive number of sub-figures within the main figures — I counted 29 sub-figures, many of which are barely mentioned in the text and so small that they are hardly legible. The cell also relies on external material flows, such as hydrazine, and apparently requires the electrolyte to be replenished multiple times during a single cycle (Fig. 4b).\nThe authors should revise their manuscript to reasonably and plausibly define the boundaries of their energetic and material assessments, as described above, in order to enable a meaningful discussion of the energy and material potentials involved."}, "author_response": "Point-by-point response to the reviewer's comments\nDear Reviewers,\nThank you very much for reviewing our manuscript. We greatly appreciate your\nprofessional comments and suggestions, which have helped us improve the scientific\nquality of this work. According to your valuable comments and suggestions, we have\ncarefully revised the manuscript and supplementary information (SI). All details of the\nrevision and our responses to the comments can be found in the following text. We\nmarked the revised parts of the manuscript and SI in red so that the corresponding\nchanges can be clearly seen.\nSincerely yours,\nGuangmin Zhou\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\nGuangmin Zhou, Ph.D, Associate Professor;\nTsinghua Shenzhen International Graduate School, Tsinghua University;\nAssociate Editor/Scientific Managing Editor, Energy Storage Materials (EnSM);\nE-mail: guangminzhou@sz.tsinghua.edu.cn\nGeneral comments: In this manuscript, the authors have presented an ODB with >100%\nround-trip efficiency. The presented research is interesting, and it could have a\nsignificant impact on wastewater management and green energy. However, I have the\nfollowing questions for the authors.\nResponse: We greatly appreciate your positive comments on this work.\nComment 1: Although the authors claim that the developed ODB can utilize\nwastewater containing N H . What if the wastewater contains other chemicals? Will\n2 4\nother chemicals decrease cell performance?\nResponse: Thanks for your thoughtful comments. First of all, we need to determine the\nmain chemicals existing in the hydrazine-containing wastewater. The industrial\nhydrazine-containing wastewater primarily originates from hydrazine production and\nits use in the chemical, medical and military industries (Industrial & Engineering\nChemistry, 1953, 45(12), 2608-2612; Biomass Conversion and Biorefinery, 2025, 1-\n22). Among them, the hydrazine production process is the dominant source of such\nwastewater, with common industrial methods including the Raschig method, urea\nmethod and peroxide-ketazine method (Chemical engineering & technology, 2010,\n33(9), 1543-1551). The chemicals in the hydrazine-containing wastewater generated\nduring the production process mainly contain hydrazine and its derivatives, salts (e.g.,\nsodium chloride (NaCl)), ammonia nitrogen (NH ·H O/NH +), and organic by-products\n3 2 4\n(e.g., urea and its derivatives, ketone compounds).\nThen, according to the reference (Patent: CN101058468A), we prepared the simulated\nwastewater-based electrolyte including 6.0 M KOH, saturated (sat.) ZnO, 0.5 M N H ,\n2 4\n2.2 M NaCl, 1.7 mM NH Cl, 17 mM acetone and 17 mM urea (Response Fig. 1a), and\n4\nperformed rate performance of ODB in this electrolyte. As shown in Response Fig. 1b,\ndischarge voltages at different current densities in the electrolyte with other waste\nchemicals are slightly higher than those in the electrolyte without other waste chemicals,\nas the addition of other waste chemicals in the electrolyte on the HzOR side leads to an\nincreased difference in solute concentration between the HzOR side and the ORR side,\nenhancing the E and thereby improving discharge voltages (Response Fig. 1d).\nRED\nCharge voltages in the electrolyte with other waste chemicals are close to those in the\nelectrolyte without other waste chemicals at current densities ranging from 1 to 100 mA\ncm−2 (Response Fig. 1c). When the current density exceeds 200 mA cm−2, charge\nvoltages in the electrolyte with other waste chemicals obviously increase, because other\nwaste chemicals significantly affect the mass transfer process at ultrahigh current\ndensity. Additionally, ODB in the electrolyte with other waste chemicals still maintains\n2\noutstanding round-trip efficiency (η ) at the commonly used current density\nRTE\n(Response Fig. 1e).\nOverall, other waste chemicals in the hydrazine-containing wastewater have little\nimpact on the ODB performance, demonstrating the huge application potential of ODB.\nResponse Fig. 1 a, Photograph of the simulated wastewater-based electrolyte including\n6.0 M KOH, sat. ZnO, 0.5 M N H , 2.2 M NaCl, 1.7 mM NH Cl, 17 mM acetone and\n2 4 4\n17 mM urea. The white solid at the bottom is NaCl and ZnO that precipitated due to\noversaturation. b, Discharge voltages of Zn-CEM-ORR at different current densities\nfrom 1 to 50 mA cm−2 in 1 M KOH electrolytes for ORR electrode and 6 M KOH+sat.\nZnO+0.5 M N H electrolytes with/without other waste chemicals for Zn electrode. c,\n2 4\nCharge voltages of Zn-HzOR at different current densities from 1 to 300 mA cm−2 in 6\nM KOH+sat. ZnO+0.5 M N H electrolytes with/without other waste chemicals. d, The\n2 4\nconfiguration schematic of the ODB designed by “3E”, including the HzOR electrode,\nZn electrode and ORR electrode. e, The calculated η of ODB in the electrolyte\nRTE\nwith/without other waste chemicals at different current densities.\nComment 2: Adding ligand molecules to is an effective method to protect Zn anode\n(doi.org/10.1021/jacs.0c09794, 10.1002/aenm.202303998). There are two effects of the\nadditive ligands: protecting the Zn solid surface and weakening the bonding strength\nbetween H O and Zn2+. It seems that the authors’ theoretical modeling mainly focuses\n2\non modeling N H on the Zn surface. Can the authors discuss whether adding N H has\n2 4 2 4\nany effect on Zn2+?\nResponse: Thanks for your valuable comments. According to your advice, we further\ndiscussed the interaction between N H and Zn2+by density functional theory (DFT)\n2 4\n3\ncalculations, as shown in Response Fig. 2. Similar to the two references you mentioned\n(doi.org/10.1021/jacs.0c09794, 10.1002/aenm.202303998), N H molecules in the\n2 4\nelectrolyte can displace water molecules from the second solvation shell of Zn2+,\nthereby modifying its solvation structure. Additionally, the previous work (Energy &\nEnvironmental Science, 2025, 18(8), 3668-3679) reported that the adsorption of similar\norganic molecules onto Zn(OH)\n4\n2− can slow down its migration in the electrolyte and\npromote uniform Zn nucleation.\nResponse Fig. 2 Free energies of different solvated structures, illustrating that four OH−\nmolecules form the first solvation shell and N H is located in the second solvation shell.\n2 4\nWe have revised our manuscript accordingly.\nRevised manuscript, Page 21, two references have been added:\nChem. Soc. 142, 21404-21409 (2020).\nReaction to Maximize Zinc Anode Stability. Adv. Energy Mater. 14, 2303998\n(2024).\nRevised SI, Page 8, the figure has been modified:\na,b, The adsorption energy of water (a) and N H (b) molecules on the surface of Zn.\n2 4\nc, The charge density difference of N H on the surface of Zn from the side view. d,\n2 4\nDFT calculations showing water dissociation barrier with and without N H adsorption.\n2 4\nIS, initial state; TS, transition state; FS, final state. e, Free energies of different solvated\nstructures, illustrating that four OH− molecules form the first solvation shell and N H\n2 4\nis located in the second solvation shell.\nComment 3: In Figure 3e, the authors claim that the Zn-O bond is weakened. Can the\nauthors provide the bond distance?\nResponse: We sincerely appreciate your advice. We have provided the Zn-O bond\ndistance as shown in Response Fig. 3. Specifically, in the absence of N H adsorption,\n2 4\nthe Zn-O bond distance is 2.46 Å. In contrast, when N H is adsorbed on the Zn surface,\n2 4\nthe Zn-O bond is weakened and its distance is extended to 2.86 Å.\nthe Zn atom in the Zn electrode surface without (a) and with (b) N H adsorption.\n2 4\nComment 4: In Figure 3g, the authors present the dissolution energy by attaching a Zn\natom. Should the author consider Zn0 or Zn2+, also will Zn coordinated by OH or N H\n2 4\nbe considered instead of an isolated Zn atom? The authors should provide more\ninformation about this figure.\nOverall, I’ll be happy to accept the paper if the authors properly address the comments\nand questions.\nResponse: We are grateful for your thoughtful comments and sorry for the confusing\npresentation of the dissolution energy calculation. The process of metal anode\ndissolution mainly involves the detachment of metal atoms from the crystal lattice and\nthe solvation of the detached atoms (Chemical Physics Letters, 2009, 469(1-3), 99-103;\nElectrochimica Acta, 2023, 437, 141443). Before analyzing the Zn dissolution process,\nit is necessary to clarify the environment on the Zn surface. In 6 M KOH+sat. ZnO or\n2 4\nsurface are OH−, H O and N H . According to the calculated adsorption energies\n2 2 4\n(Response Fig. 4a), we can conclude that N H replaces some of the H O molecules\n2 4 2\nadsorbed on the Zn surface. Hence, for the first process, we calculated the detachment\nenergies of a Zn atom under the adsorption of H\n4\nmolecules (Response Fig.\n4b). For the second process, we have added the calculations of relevant solvated\nstructure (Response Fig. 4c). Since the solvation process is spontaneous with a negative\nfree energy, the whole dissolution process is determined by the first process. Therefore,\nZn atom detachment under N H adsorption requires overcoming a higher detachment\n2 4\nenergy, thus impeding Zn dissolution.\n4\nmolecules on the\nZn surface. b, Detachment energies of Zn atom under H O or N H molecules\n2 2 4\nadsorption. c, Free energies of different solvated structures, illustrating that four OH−\nmolecules form the first solvation shell and N H is located in the second solvation shell.\n2 4\nWe have revised our manuscript accordingly.\nRevised manuscript, Page 9, several sentences have been added:\n“Third, the Zn dissolution process involves the detachment of Zn atoms from the crystal\nlattice and the solvation of the detached Zn atoms.23,24 For the first process, we\ncalculated the detachment energies of a Zn atom under the adsorption of H O or N H\n2 2 4\nmolecules (Fig. 3g). For the second process, the negative solvation energies indicate\nthat the solvation process is spontaneous (Supplementary Fig. 6e).25,26 Hence, the whole\ndissolution process is determined by the first process. The stable adsorption of N H\n2 4\ninduces electronic rearrangement and spatial hindrance, raising the passivation effect\non the Zn surface. As a result, detachment of Zn atoms requires overcoming a higher\nenergy barrier, thus impeding Zn dissolution.”\nRevised manuscript, Pages 10 and 11, the Fig. 3 has been modified:\ncapacity of Zn-CEM-ORR with 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M\nN H electrolytes at a current density of 20 mA cm−2. b, Schematic of corrosion process\n2 4\nincluding water dissociation, HER and Zn dissolution. c, Typical Tafel plots\ndemonstrating corrosion of Zn anodes in 6 M KOH+sat. ZnO and 6 M KOH+sat.\nZnO+0.5 M N H electrolytes and calculated corrosion inhibition efficiency of N H -\n2 4 2 4\ncontaining electrolyte. d, The PDOS of Zn 3d orbitals and O 2p orbitals of water with\nand without N H adsorption. Inset: the differential charge density of N H molecule on\n2 4 2 4\nZn surface. e, The calculated Milliken charges of O and Zn with and without N H\n2 4\nadsorption. f, Free energy diagram of HER process on Zn surface with and without\nN H adsorption. The insets are the corresponding optimized structural models. g,\n2 4\nDetachment energies of Zn atom with H O and N H adsorption. h, In situ Raman\n2 2 4\nspectra of interfacial water in 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M N H\n2 4\nelectrolytes measured under different current densities. The insets are the schematics of\ninterfacial molecule (H O and N H ) arrangements in different electrolytes.\n2 2 4\nRevised manuscript, Page 20, several sentences have been added:\n“while the top two layers and any adsorbates were allowed to relax. Solvation effects\nwere modeled through a hybrid computational approach combining explicit molecular\nrepresentation with continuum electrostatics. The first two solvation shells were\n8\nexplicitly simulated using OH− and water molecules, selected for their specific\ncoordination geometry and binding energetics with ions and critical roles in hydration\nthermodynamics. Surrounding solvent effects were incorporated via the Polarizable\nContinuum Model (PCM), where bulk water was represented as a uniform dielectric\nmedium characterized by its experimentally determined static permittivity of ε = 78.5.”\nRevised manuscript, Page 21, four references have been added:\ninteractions during a dissolution event as assessed from electronic structure.\nChem. Phys. Lett. 469, 99-103 (2009).\nkinetics and charge transfer. Electrochim. Acta 437, 141443 (2023).\nChem. Soc. 142, 21404-21409 (2020).\nReaction to Maximize Zinc Anode Stability. Adv. Energy Mater. 14, 2303998\n(2024).\nRevised SI, Page 8, the figure has been modified:\nSupplementary Fig. 6 | DFT calculation revealing corrosion inhibition mechanism.\na,b, The adsorption energy of water (a) and N H (b) molecules on the surface of Zn.\n2 4\nc, The charge density difference of N H on the surface of Zn from the side view. d,\n2 4\nDFT calculations showing water dissociation barrier with and without N H adsorption.\n2 4\nstructures, illustrating that four OH− molecules form the first solvation shell and N H\n2 4\nis located in the second solvation shell.\nGeneral comments: This manuscript presents a novel and compelling open decoupled\nbattery (ODB) design that achieves round-trip efficiencies (η ) exceeding 300%, with\nRTE\na reported maximum of 445%. The authors implement a “three-electrodynamic-\npotential” (3E) design by engineering low charge voltages via hydrazine oxidation and\nhigh discharge voltages via oxygen reduction, and by introducing a reverse\nelectrodialysis potential (E ) through electrolyte decoupling. The system enables\nRED\npower generation, waste remediation, and electricity amplification. Comprehensive\nexperimental validation is supported by DFT analysis and techno-economic assessment\n(TEA), and the work culminates in a 20 Ah scaled prototype demonstrating practical\npotential. The manuscript is generally well written and could have the potential to be\npublished in this journal if the following comments are addressed adequately.\nResponse: We are so grateful for your encouraging comments.\nMajor Comments\nComment 1: Hydrazine Toxicity and Practicality\nThe authors mention briefly the disposal costs of hydrazine wastewater in their TEA.\nHowever further discussion on the toxicity and practicality of using hydrazine would\nbe nice. Hydrazine is quite a toxic and potentially carcinogenic compound. While the\nsystem degrades it, broader safety, handling, and environmental implications of\ndeploying hydrazine in energy storage should be discussed and perhaps the authors\ncould suggest alternatives.\nResponse: Thanks for your valuable comments. Indeed, hydrazine is toxic and\ndangerous, and thus large volumes of hydrazine-containing wastewater generated from\nhydrazine production and usage in chemical, medical and military industries must be\ncarefully treated. In industry, there are already well-established procedures for treating\nhydrazine wastewater (China Chlor-Alkali, 2020, 9, 37-40) and corresponding safety\npolicies (e.g., China Wastewater Treatment Policy) to prevent hydrazine from harming\nhumans and the environment during wastewater treatment. Strict safety regulations are\nenforced during wastewater treatment operations (Aviat. Space Environ. Med, 1981,\n52(3), 149-53).\nThe specific processes of hydrazine-containing wastewater treatment mainly include\nphysical and chemical pretreatment, advanced oxidation process, subsequent\nprocessing and resource utilization (e.g., evaporative crystallization) (Chemosphere,\n2017, 173, 474-484; China Chlor-Alkali, 2020, 9, 37-40). Among these processes, the\nchemical oxidation is the key step for removing hydrazine, using oxidants such as\nhydrogen peroxide and ozone. The application of the proposed ODB energy storage\n11\ntechnology can significantly reduce the usage of traditional oxidants in this process\n(Response Fig. 5a). It should be noted that, in practical terms, the packaging and\nassembly details of ODB energy storage system may need to be further improved to\nensure full compliance with industrial safety regulations and process specifications for\nhydrazine treatment. Additionally, the other two electrodes (Zn electrode and air\nelectrode) in the ODB system are both environmentally friendly and intrinsically safe\nin aqueous electrolytes. Therefore, by adhering to the strict safety regulations for\nhydrazine handling in the industry, we believe that ODB integrating power generation,\nenergy storage and wastewater treatment will demonstrate significant economic and\nenvironmental benefits.\nFor alternatives: As shown in Response Fig. 5b, we listed a series of chemical\nsubstances in wastewater that need to be degraded and took the hydrazine (N H /N )\n2 4 2\nwith the most negative potential as an example to demonstrate the effectiveness and\npracticability of our design. Apart from hydrazine, we suggest that low-toxic glucose\n(C H O /C H O ) and methanol (CH OH/CO ) and might also be treated using ODB\n6 12 5 6 10 8 3 2\nconfiguration, which will be further explored in future studies.\nResponse Fig. 5 a, Disposal costs of hydrazine wastewater with different methods. b,\nDecoupling electrodes by screening appropriate redox pairs to achieve low E , high E\nC D\nand thus high η . For the charging positive electrode, the listed redox pairs all involve\nRTE\nchemicals present in the wastewater.\nComment 2: Electrolyte Depletion and Maintenance\nThe need to replace spent electrolytes to restore performance raises concerns about\nsystem autonomy and operational complexity. A quantitative analysis of electrolyte\nreplacement frequency and associated costs over system lifetime should be included in\nthe TEA.\nchemical energy in the wastewater into electrical energy by electrochemical oxidation,\nand thus simultaneously achieves additional electricity output and wastewater treatment.\nAfter the wastewater has been treated, the electrolyte will be replaced with the next\nbatch of wastewater for treatment. Electrolyte replacement frequency is related to the\nelectrolyte container used to store a batch of wastewater and the set cut-off voltage of the electro-oxidation. The larger the container and the higher the cut-off voltage, the\nlower the replacement frequency. Furthermore, electrolyte replacement is easy to\nperform, only involving pumping out the treated wastewater and then pumping in a new\nbatch of wastewater. Therefore, considering the operational situation of this work and\nother relevant references (Science, 2019, 364, eaav3506; Nature Communications,\n2021, 12(1), 4679), we have added the operation and maintenance costs containing\nelectrolyte replacement, which were taken to be 10% of the capital costs. Besides, ODB\nis capable of treating wastewater, which can significantly reduce the cost of wastewater\ndisposal using traditional oxidants. Hence, we have adjusted the cost calculation into\nthree parts: the capital cost, operation cost and saved waste disposal profit.\nThe economic analysis results have been revised. As shown in Response Fig. 6, due to\nthe significant savings in wastewater disposal, ODB can achieve considerable profits\nthroughout its entire service life, which is totally different from the losses incurred by\nother energy storage systems (ESSs), demonstrating huge potential for application.\nAdditionally, compared to traditional methods for degrading N H in wastewater\n2 4\nthrough oxidants such as ozone (36.4 US$ m−3) and hydrogen peroxide (10.2 US$ m−3),\nthe all-ODB method significantly shortens disposal expenses (5.3 US$ m−3) (Response\nFig. 7).\nResponse Fig. 6 TEAs of generating 1 MWh of electricity combining different power\ngeneration technologies with ESSs. The capital cost of ODB includes electrodes,\nelectrolytes, separators and casings.\nWe have revised our manuscript accordingly.\nRevised manuscript, Page 14, several sentences have been added:\n“On the one hand, owing to ODBs with η , all-ODB systems are self-driven,\nRTE>100%\nintegrating power generation, energy storage and wastewater treatment.”\n“Furthermore, due to the significant savings in wastewater disposal, ODB can achieve\nconsiderable profits throughout its entire service life, which is totally different from the\nlosses incurred by other ESSs, demonstrating huge potential for application.\nAdditionally, compared to traditional methods for degrading N H in wastewater\n2 4\nthrough oxidants such as ozone (36.4 US$ m−3) and hydrogen peroxide (10.2 US$ m−3),\nthe operation of all-ODB systems significantly shortens disposal expenses (5.3\nUS$ m−3), thus creating additional economic benefits (Fig. 5c and Supplementary Note\n2).”\nRevised manuscript, Page 15, the Fig. 5 has been modified:\npower generation and transportation of all-ODB system, intermittent renewable energy\nand continuous fossil energy (blue background). And schematic of electricity\namplification after combining intermittent renewable energy or continuous fossil\nenergy with ODB, and electricity loss after combining intermittent renewable energy\nand conventional ESSs (green background). b, TEAs of generating 1 MWh of\nelectricity combining different power generation technologies with ESSs.39,40 c,\nDisposal costs of hydrazine wastewater with different methods. d, CO emissions\n2\ngenerated by combining different power generation technologies and ESSs to generate\nRevised SI, Page 15, Supplementary Note 1 has been modified:\n“C is the cost per discharge energy unit (US$ MWh−1), without including the savings\n1\nfrom waste disposal costs. C is the cost per discharge energy unit (US$ MWh−1),\n2\nincluding the savings from waste disposal costs. The price of components was obtained\nfrom Alibaba.com. The detailed cost calculation of an ODB is shown in Supplementary\ncost of production in units of US$3. The total costs are broadly categorized into the\ncapital cost, operation cost and saved waste disposal profit. The capital costs are limited\nto the cost of batteries and do not include the costs of infrastructure or other equipment.\nThe operation costs were taken to be 10% of the capital costs3,4.”\nRevised SI, Page 16, Supplementary Note 2 has been modified:\n“The prices of one tonne of 30% hydrogen peroxide and ozone are 90.15 and 2273\ndollars obtained from Alibaba.com, respectively.”\nRevised SI, Page 18, Supplementary Table 2 has been modified:\nSupplementary Table 2 | Cost evaluation of the 20-Ah ODB.\nRevised SI, Page 19, Supplementary Table 3 has been modified:\nstorage systems (ESSs).\nRevised SI, Page 21, Supplementary Table 5 has been modified:\nSupplementary Table 5 | Costs and carbon emissions of various power generation\ntechnologies21,22.\nRevised SI, Page 22, Supplementary Table 6 has been modified:\nare compared with different power generation technologies as electricity input.\nRevised SI, Page 23, a reference has been added:\ncommodity chemicals and H fuel. Nat. Commun. 12, 4679 (2021).\nThe role of the cation-exchange membrane (CEM) is central to the E benefit.\nRED\nHowever, the manuscript lacks data or discussion on membrane longevity, fouling, or\nselectivity degradation over time. Including membrane cycling tests or citing relevant\nlifetime studies would strengthen the system’s credibility for scale-up.\nResponse: Thanks for your valuable suggestions. The CEM is important to design E\nRED\n.\nAt the initial stage of this work attempt, we tried the CEM (HuaMoTech 8040) based\non polyethylene material and the CEM (Nafion 117) based on perfluorosulfonic acid\n18\nmaterial, respectively. As shown in Response Fig. 8a, a higher discharge voltage is\nobtained by using CEM (Nafion 117) than CEM (HuaMoTech 8040) at 10 mA cm−2,\nillustrating that CEM (Nafion 117) provides a higher E than CEM (HuaMoTech\nRED\n8040). Therefore, we chose Nafion 117 as the CEM for subsequent ODB testing. We\nare sorry for the mistaken CEM type in Methods, and the correct specification is Nafion\n117 rather than HuaMoTech 8040.\nMany previous works reported commercial Nafion 117 membrane used as a CEM in\nalkaline systems to conduct cations such as K+, Na+ and Li+, exhibiting good\nconductivity and stability (Joule, 2018, 2(9), 1894-1906; Angewandte Chemie\nInternational Edition, 2025, 64(19), e202503138; ACS Applied Materials & Interfaces,\n2023, 15(49), 57252-57264). Additionally, we have added data about the membrane\nbefore and after 1000 cycles at current densities of 20 mA cm−2 for discharging and 300\nmA cm−2 for charging with the same capacity of 10 mAh cm−2. It can be seen that the\ncycled membrane was almost identical to the fresh membrane, except for a slight white\nfouling, indicating the membrane’s durability in this system (Response Figs. 8b,c).\nMoreover, the resistance change between the fresh and cycled membranes is very small,\nverifying the ion transport stability of the membrane (Response Fig. 8d).\nand CEM (HuaMoTech 8040) at a current density of 10 mA cm−2 in 1 M KOH\nelectrolytes for ORR electrode and 6 M KOH+sat. ZnO+0.5 M N H electrolytes for\n2 4\nZn electrode. Photographs of the CEM at the initial state (b) and after (c) 1000 cycles\nat current densities of 20 mA cm−2 for discharging and 300 mA cm−2 for charging with\nthe same capacity of 10 mAh cm−2. d, Nyquist plots of the CEM at the initial state and\nafter 1000 cycles at current densities of 20 mA cm−2 for discharging and 300 mA cm−2\nfor charging with the same capacity of 10 mAh cm−2, obtained from electrochemical\nimpedance spectroscopy with an amplitude of 5 mV at open circuit potential.\nWe have revised our manuscript accordingly.\nRevised manuscript, Page 17, a sentence has been modified:\n“cation-exchange membrane (CEM, Nafion 117, thickness of 0.183 mm, Dupont)”\nMinor Comments\nComment 4: Figure 2 is too dense and could benefit from being broken down into two\nfigures.\nResponse: Thanks for your advice. We agree that Figure 2 is too dense. However,\nFigure 2 as a whole illustrates the determination of the ODB’s components, and\nbreaking it down into two figures might reduce the presentation clarity. Therefore, we\nhave simplified Figure 2 as shown in Response Fig. 9 and hope this can satisfy your\ncomment.\nof ORR under alkaline conditions. Insets: Zn-ORR discharge voltage profiles of\ndifferent pH values (pH=12, 13 and 14) at a current density of 1 mA cm−2. b,\nChronoamperometric responses of commercial Pt/C by adding 1 mL N H (at 300 s) at\n2 4\n0.74 V (versus RHE) in 100 mL oxygen-saturated 1 M KOH. c, LSV curves of HzOR\nwith a scan rate of 5 mV s−1 in 0.5 M N H electrolytes containing different\n2 4\nconcentrations of KOH. d, Long discharge curves of Zn-ORR with 1 M and 6 M\nKOH at a current density of 20 mA cm−2. The inserted schematic illustrates that the Zn\nelectrode loses a large amount of capacity in 1M KOH electrolyte because of easy\npassivation. e, U of Zn-HzOR at different current densities from 1 to 500 mA cm−2\ncharge\nin 6 M KOH+sat. ZnO+0.5 M N H electrolytes. f, Schematics and discharge profiles\n2 4\nof decoupled Zn-ORR using CEM and AEM at a current density of 10 mA cm−2 in 1\nM KOH electrolytes for ORR electrode and 6 M KOH+sat. ZnO+0.5 M N H\n2 4\nelectrolytes for Zn electrode. g, U of Zn-CEM-ORR at different current densities\ndischarge\nfrom 1 to 50 mA cm−2 in 1 M KOH electrolytes for ORR electrode and 6 M KOH+sat.\nZnO+0.5 M N H electrolytes for Zn electrode.\n2 4\nWe have revised our manuscript accordingly.\nRevised manuscript, Pages 7 and 8, the Fig. 2 has been modified:\nunder alkaline conditions. Insets: Zn-ORR discharge voltage profiles of different pH\nvalues (pH=12, 13 and 14) at a current density of 1 mA cm−2. b, Chronoamperometric\nresponses of commercial Pt/C by adding 1 mL N H (at 300 s) at 0.74 V (versus RHE)\n2 4\nin 100 mL oxygen-saturated 1 M KOH. c, LSV curves of HzOR with a scan rate of 5\nmV s−1 in 0.5 M N H electrolytes containing different concentrations of KOH. d, Long\n2 4\ndischarge curves of Zn-ORR with 1 M and 6 M KOH at a current density of 20 mA\ncm−2. The inserted schematic illustrates that the Zn electrode loses a large amount of\ncapacity in 1M KOH electrolyte because of easy passivation. e, U of Zn-HzOR at\ncharge\ndifferent current densities from 1 to 500 mA cm−2 in 6 M KOH+sat. ZnO+0.5 M N H\n2 4\nelectrolytes. f, Schematics and discharge profiles of decoupled Zn-ORR using CEM\nand AEM at a current density of 10 mA cm−2 in 1 M KOH electrolytes for ORR\nelectrode and 6 M KOH+sat. ZnO+0.5 M N H electrolytes for Zn electrode. g, U\n2 4 discharge\nof Zn-CEM-ORR at different current densities from 1 to 50 mA cm−2 in 1 M KOH\nelectrolytes for ORR electrode and 6 M KOH+sat. ZnO+0.5 M N H electrolytes for\n2 4\nZn electrode.\nComment 5: It would be helpful to include error bars or statistical variation (if any) for\nelectrochemical performance plots to reflect reproducibility.\ndifferent current densities (Fig. 2e) and discharge voltages of Zn-ORR at different KOH\nconcentrations (Supplementary Fig. 3) were exhibited with error bars in the initial\nmanuscript and SI, as shown in Response Fig. 10. Additionally, we have added the\ndischarge voltages of ODB with error bars under different current densities, reproduced\nlong discharge curves and corresponding specific capacity with error bars, and\nreproduced Tafel plots and corresponding corrosion currents with error bars to\ndemonstrate reproducibility (Response Fig. 11).\nResponse Fig. 10 a, Charge voltages of ODB at different current densities from 1 to\n500 mA cm−2 in 6 M KOH+sat. ZnO+0.5 M N H electrolytes. Discharge voltages of\n2 4\nZn-ORR in 1 M (b) and 6 M (c) KOH electrolytes without an ion exchange membrane\nat 1 mA cm−2.\ndensities from 1 to 50 mA cm−2 in 1 M KOH electrolytes for ORR electrode and 6 M\nKOH+sat. ZnO+0.5 M N H electrolytes for Zn electrode. Reproduced long discharge\n2 4\ncurves (b) and corresponding specific capacity (c) with error bars of Zn-CEM-ORR\nwith 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M N H electrolytes at a current\n2 4\ndensity of 20 mA cm−2. Reproduced Tafel plots (d) and corresponding corrosion\ncurrents (e) with error bars of Zn electrode in 6 M KOH+sat. ZnO and 6 M KOH+sat.\nZnO+0.5 M N H electrolytes.\n2 4\nWe have revised our manuscript accordingly.\nRevised manuscript, Pages 10 and 11, the Fig. 3 has been modified:\ncapacity of Zn-CEM-ORR with 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M\nN H electrolytes at a current density of 20 mA cm−2. b, Schematic of corrosion process\n2 4\nincluding water dissociation, HER and Zn dissolution. c, Typical Tafel plots\ndemonstrating corrosion of Zn anodes in 6 M KOH+sat. ZnO and 6 M KOH+sat.\nZnO+0.5 M N H electrolytes and calculated corrosion inhibition efficiency of N H -\n2 4 2 4\ncontaining electrolyte. d, The PDOS of Zn 3d orbitals and O 2p orbitals of water with\nand without N H adsorption. Inset: the differential charge density of N H molecule on\n2 4 2 4\nZn surface. e, The calculated Milliken charges of O and Zn with and without N H\n2 4\nadsorption. f, Free energy diagram of HER process on Zn surface with and without\nN H adsorption. The insets are the corresponding optimized structural models. g,\n2 4\nDetachment energies of Zn atom with H O and N H adsorption. h, In situ Raman\n2 2 4\nspectra of interfacial water in 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M N H\n2 4\nelectrolytes measured under different current densities. The insets are the schematics of\ninterfacial molecule (H O and N H ) arrangements in different electrolytes.\n2 2 4\nRevised SI, Page 7, Supplementary Fig. 5 has been revised:\n2 4\nmethod1.\nGeneral comments: The manuscript discusses the results of the development and\ntesting of a novel battery cell with a claimed round-trip efficiency of over 300%. I\nrecommend rejecting the publication of this manuscript in a highly respected journal\nsuch as Nature Communications. Ever since the formulation of the laws of\nthermodynamics, it has been clear that a perpetual motion machine is not possible. The\nauthors repeatedly claim an energy efficiency of their cell exceeding 100%, without\noffering any consideration of the actual energetic and material balances involved in\noperating such a cell. Disregarding the fundamental principles of thermodynamics is\nunscientific. Reports on such work have no place in any scientific publication.\nResponse: We appreciate your comments, but we believe there may be a\nmisunderstanding about this work. We completely agree that a perpetual motion\nmachine that can continuously do work without consuming any energy is not possible\nbased on the laws of thermodynamics. However, the proposed open decoupled battery\n(ODB) in our work is an open system in thermodynamics (Response Fig. 12b), rather\nthan a closed system (Response Fig. 12a). An open system permits energy transfer with\nthe external environment while allowing matter exchange. For batteries, the round-trip\nefficiency is defined as the ratio of the electricity output from the storage device to the\nelectricity input to the device during one charge/discharge as follows (Nature Energy,\n2024, 9(4), 479-490; Energy & Environmental Science, 2025, 18(2), 1002-1010):\nW (UIt)\nη = discharge= discharge (1)\nRTE\nW (UIt)\ncharge charge\nSince the capacity (It) for charging and discharging is set to the same, the above\nequation is converted to:\nU\nη = discharge (2)\nRTE\nU\ncharge\nwhere W,U, I and t represent the electrical energy, voltage, current and time of the\nbattery during charging and discharging, respectively (details in Battery performance\nof Methods in the initial manuscript). Hence, the proposed ODB can convert the\nchemical energy in wastewater (hydrazine as an example) into electrical energy, thus\nachieving a round-trip efficiency (η ) of over 100% (Response Table 1).\nRTE\nAdditionally, if the chemical energy of hydrazine is completely converted into electrical\nenergy, the theoretical charging voltage according to thermodynamics should be 0.09\nV. However, the testing charging voltages are all higher than 0.3 V at different current\ndensities, indicating that there is heat energy loss and the energy conversion efficiency\nfrom chemical energy to electrical energy must be lower than 100%. Therefore, our\nwork fully complies with the laws of thermodynamics.\nRTE\nResponse Fig. 12 Design framework for ODBs. a, The exchange of matter and energy\nin conventional closed batteries with the external environment, and the corresponding\ncell reaction. b, The exchange of matter and energy in ODBs with the external\nenvironment, and the corresponding cell reactions. For designing ODBs, first of all, we\ndecouple electrodes to achieve low E , high E and thus high η . All the potentials\nC D RTE\nare relative to SHE in alkaline environments. After determining electrodes (Zn-ORR\n28\nfor the discharging reaction and Zn-HzOR for the charging reaction), we further\nimprove η by introducing E between different concentrations of electrolytes.\nRTE RED\nOverall, our “3E” design (E , E and E ) renders ODBs with outstanding η\nD C RED RTE\nexceeding 300%. W and W refer to the input electrical energy and output electrical\nin out\nenergy.\nFurthermore, the manuscript is written in an overly personal tone and contains an\nexcessive number of sub-figures within the main figures — I counted 29 sub-figures,\nmany of which are barely mentioned in the text and so small that they are hardly legible.\nResponse: We are grateful for your comments and sorry for the unclear presentation.\nWe have properly adjusted small sub-figures to make them easier to read (Response\nFigs. 13a-c), and added captions for unmarked sub-figures (Response Figs. 13b,d,e).\nResponse Fig. 13 a, The Pourbaix diagram of ORR under alkaline conditions. Insets:\nZn-ORR discharge voltage profiles of different pH values (pH=12, 13 and 14) at a\ncurrent density of 1 mA cm−2. b, Long discharge curves of Zn-ORR with 1 M and 6 M\nelectrode loses a large amount of capacity in 1M KOH electrolyte because of easy\npassivation. c, Schematics and discharge profiles of decoupled Zn-ORR using CEM\nand AEM at a current density of 10 mA cm−2 in 1 M KOH electrolytes for ORR\nelectrode and 6 M KOH+sat. ZnO+0.5 M N H electrolytes for Zn electrode. d, Free\n2 4\nenergy diagram of HER process on Zn surface with and without N H adsorption. The\n2 4\ninsets are the corresponding optimized structural models. e, In situ Raman spectra of\ninterfacial water in 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M N H\n2 4\nelectrolytes measured under different current densities. The insets are the schematics of\ninterfacial molecule (H O and N H ) arrangements in different electrolytes.\n2 2 4\nThe cell also relies on external material flows, such as hydrazine, and apparently\nrequires the electrolyte to be replenished multiple times during a single cycle (Fig. 4b).\nResponse: Thanks for your comments. Our ODB converts the chemical energy in the\nwastewater into electrical energy by electrochemical oxidation, and thus\nsimultaneously achieves additional electricity output and wastewater treatment.\nAfter the wastewater has been treated, the electrolyte will be replaced with the next\nbatch of wastewater for treatment. Electrolyte replacement frequency is related to the\nelectrolyte container used to store a batch of wastewater and the set cut-off voltage of the electro-oxidation. The larger the container and the higher the cut-off voltage, the\nlower the replacement frequency. Furthermore, electrolyte replacement is easy to\nperform, only involving pumping out the treated wastewater and then pumping in a new\nbatch of wastewater. Additionally, we have considered the operation and maintenance\ncosts containing electrolyte replacement when conducting Techno-economic analyses\n(TEAs), which were taken to be 10% of the capital costs (Science, 2019, 364, eaav3506;\nNature Communications, 2021, 12(1), 4679). As a result, due to the significant savings\nin wastewater disposal, ODB can achieve considerable profits throughout its entire\nservice life, which is totally different from the losses incurred by other energy storage\nsystems, demonstrating huge potential for application (Response Fig. 14).\nResponse Fig. 14 TEAs of generating 1 MWh of electricity combining different power\n30\ngeneration technologies with ESSs. The capital cost of ODB includes electrodes,\nelectrolytes, separators and casings.\nThe authors should revise their manuscript to reasonably and plausibly define the\nboundaries of their energetic and material assessments, as described above, in order to\nenable a meaningful discussion of the energy and material potentials involved.\nResponse: Thanks for your valuable comments. Different from conventional battery\ntechnologies that are closed systems, we clearly demonstrated that our ODB is an open\nsystem, permitting energy transfer with the external environment while allowing matter\nexchange. Therefore, the round-trip efficiency based on the electricity input/output of\nbatteries can exceed 100%, thanks to the conversion of chemical energy in hydrazine-\ncontaining wastewater into electrical energy.\nOverall, this work completely follows the laws of thermodynamics. To present our work\nmore clearly and avoid possible misunderstandings, we have revised the title as “An\nopen decoupled cell design achieving electricity generation and amplification through\nwaste-to-energy conversion”.\nWe have revised our manuscript accordingly.\nRevised manuscript and SI, Page 1, the title has been modified:\n“An open decoupled cell design achieving electricity generation and amplification\nthrough waste-to-energy conversion”\nRevised manuscript, Pages 7 and 8, the Fig. 2 has been modified:\nunder alkaline conditions. Insets: Zn-ORR discharge voltage profiles of different pH\nvalues (pH=12, 13 and 14) at a current density of 1 mA cm−2. b, Chronoamperometric\nresponses of commercial Pt/C by adding 1 mL N H (at 300 s) at 0.74 V (versus RHE)\n2 4\nin 100 mL oxygen-saturated 1 M KOH. c, LSV curves of HzOR with a scan rate of 5\nmV s−1 in 0.5 M N H electrolytes containing different concentrations of KOH. d, Long\n2 4\ndischarge curves of Zn-ORR with 1 M and 6 M KOH at a current density of 20 mA\ncm−2. The inserted schematic illustrates that the Zn electrode loses a large amount of\ncapacity in 1M KOH electrolyte because of easy passivation. e, U of Zn-HzOR at\ncharge\ndifferent current densities from 1 to 500 mA cm−2 in 6 M KOH+sat. ZnO+0.5 M N H\n2 4\nelectrolytes. f, Schematics and discharge profiles of decoupled Zn-ORR using CEM\nand AEM at a current density of 10 mA cm−2 in 1 M KOH electrolytes for ORR\nelectrode and 6 M KOH+sat. ZnO+0.5 M N H electrolytes for Zn electrode. g, U\n2 4 discharge\nof Zn-CEM-ORR at different current densities from 1 to 50 mA cm−2 in 1 M KOH\nelectrolytes for ORR electrode and 6 M KOH+sat. ZnO+0.5 M N H electrolytes for\n2 4\nZn electrode.\nRevised manuscript, Pages 10 and 11, the Fig. 3 has been modified:\ncapacity of Zn-CEM-ORR with 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M\nN H electrolytes at a current density of 20 mA cm−2. b, Schematic of corrosion process\n2 4\nincluding water dissociation, HER and Zn dissolution. c, Typical Tafel plots\ndemonstrating corrosion of Zn anodes in 6 M KOH+sat. ZnO and 6 M KOH+sat.\nZnO+0.5 M N H electrolytes and calculated corrosion inhibition efficiency of N H -\n2 4 2 4\ncontaining electrolyte. d, The PDOS of Zn 3d orbitals and O 2p orbitals of water with\nand without N H adsorption. Inset: the differential charge density of N H molecule on\n2 4 2 4\nZn surface. e, The calculated Milliken charges of O and Zn with and without N H\n2 4\nadsorption. f, Free energy diagram of HER process on Zn surface with and without\nN H adsorption. The insets are the corresponding optimized structural models. g,\n2 4\nDetachment energies of Zn atom with H O and N H adsorption. h, In situ Raman\n2 2 4\nspectra of interfacial water in 6 M KOH+sat. ZnO and 6 M KOH+sat. ZnO+0.5 M N H\n2 4\nelectrolytes measured under different current densities. The insets are the schematics of\ninterfacial molecule (H O and N H ) arrangements in different electrolytes.\n2 2 4\n33"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my questions and I'm happy to accept the paper.", "reviewer_2": "The authors have addressed all of the major concerns raised during the first round of review, and their revisions have substantially improved the manuscript. The added data on electrolyte replacement, membrane durability, and techno-economic considerations strengthen the scientific quality of the work. The manuscript is now clearly written, well-supported by experimental evidence, and presents an innovative contribution to the field.\nI am satisfied with the authors’ rebuttal and the revised version of the paper. I have no further concerns and believe the work meets the standards of Nature Communications and recommend the manuscript for publication in its current form.", "reviewer_4": "In this manuscript, the authors discuss the concept of a ZnO / Hydrazine “Open Decoupled Battery” for electricity generation and removal of hydrazine from wastewater. A prototype to demonstrate the technology was constructed and evaluated. In general, the presented work describes an interesting approach to extract electricity from wastewater processing. However, I would appreciate if the authors could address the following points, before the manuscript can be accepted:\n1. While the authors describe in sufficient detail their considerations to calculate \"round-trip efficiency\" (RTE) and also define the system boundaries, I agree with reviewer #3 that the statement of > 100% RTE is at least misleading and possibly considers the wrong system boundaries. Round-trip efficiency in battery systems is usually used to describe the performance of a closed battery system / a single reversible reaction. (Implied by the term “round-trip”.) In most systems, “efficiency” is understood as the proportion of usable (output) energy in relation to the total energy supplied (input). Now, in the proposed ODB of course part of the input energy is essentially “free”. On the other hand, in other systems with similar characteristics usually the term “efficiency” is not used. (For example, for heat pumps that can be considered forming a closed system only with the atmosphere and thus achieve “efficiencies” of > 400% only within the “open system” of the heated space, the term “coefficient of performance”, COP is usually used instead.)\nAt least the authors's usage of RTE makes a comparison of RTE with other battery systems unfair if not almost meaningless. I recommend to at least critically reflect this point in the paper and for example provide an estimation of the energy consumption of producing the hydrazine electrolyte, to be included in the overall efficiency.\nAdditionally, for example, estimates of area specific resistance (ASR) for the different reactions would help make the performance more comparable to at least other flow batteries. (Can probably be extracted from the raw data?) So far, the system performance is mostly characterized by RTE.\n2. What actually are the necessary steps to produce the electrolyte solution from hydrazine wastewater? Are there really wasterwaters with such high hydrazine concentrations? If hydrazine must be enriched first this makes my first comment even more relevant.\n3. On a related note: What is the actual abundance of hydrazine in wastewater? Since this particular ODB relies on the availability of hydrazine, can it be estimated what would be the potential of such a technology (in terms of kWh per year)? This would greatly support the usefulness / relevance of the ODB."}, "author_response": "Point-by-point response to the reviewer's comments\nDear Reviewers,\nThank you very much for reviewing our manuscript. We greatly appreciate your\nprofessional comments and suggestions, which have helped us improve the scientific\nquality of this work. According to your valuable comments and suggestions, we have\ncarefully revised the manuscript and supplementary information (SI). All details of the\nrevision and our responses to the comments can be found in the following text. We\nmarked the revised parts of the manuscript and SI in red so that the corresponding\nchanges can be clearly seen.\nSincerely yours,\nGuangmin Zhou\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\nGuangmin Zhou, Ph.D, Associate Professor;\nTsinghua Shenzhen International Graduate School, Tsinghua University;\nAssociate Editor/Scientific Managing Editor, Energy Storage Materials (EnSM);\nE-mail: guangminzhou@sz.tsinghua.edu.cn\nGeneral comments: The authors have addressed all my questions and I'm happy to\naccept the paper.\nResponse: We greatly appreciate your positive comments on our work and approval of\nthe modifications made to the manuscript.\nReviewer #2\nGeneral comments: The authors have addressed all of the major concerns raised\nduring the first round of review, and their revisions have substantially improved the\nmanuscript. The added data on electrolyte replacement, membrane durability, and\ntechno-economic considerations strengthen the scientific quality of the work. The\nmanuscript is now clearly written, well-supported by experimental evidence, and\npresents an innovative contribution to the field.\nI am satisfied with the authors’ rebuttal and the revised version of the paper. I have no\nfurther concerns and believe the work meets the standards of Nature Communications\nand recommend the manuscript for publication in its current form.\nResponse: We are sincerely grateful for your encouraging comments and professional\nsuggestions, which have helped us greatly improve the scientific quality of this work.\nAnd thank you very much for your approval of the revised manuscript.\nReviewer #4\nGeneral comments: In this manuscript, the authors discuss the concept of a ZnO /\nHydrazine “Open Decoupled Battery” for electricity generation and removal of\nhydrazine from wastewater. A prototype to demonstrate the technology was constructed\nand evaluated. In general, the presented work describes an interesting approach to\nextract electricity from wastewater processing. However, I would appreciate if the\nauthors could address the following points, before the manuscript can be accepted:\nResponse: We sincerely appreciate your positive comments on our work.\nComment 1: While the authors describe in sufficient detail their considerations to\ncalculate \"round-trip efficiency\" (RTE) and also define the system boundaries, I agree\nwith reviewer #3 that the statement of > 100% RTE is at least misleading and possibly\nconsiders the wrong system boundaries. Round-trip efficiency in battery systems is\nusually used to describe the performance of a closed battery system / a single reversible\nreaction. (Implied by the term “round-trip”.) In most systems, “efficiency” is\nunderstood as the proportion of usable (output) energy in relation to the total energy\n2\nsupplied (input). Now, in the proposed ODB of course part of the input energy is\nessentially “free”. On the other hand, in other systems with similar characteristics\nusually the term “efficiency” is not used. (For example, for heat pumps that can be\nconsidered forming a closed system only with the atmosphere and thus achieve\n“efficiencies” of > 400% only within the “open system” of the heated space, the term\n“coefficient of performance”, COP is usually used instead.)\nResponse: Thanks for your thoughtful comments. We agree that the statement of > 100%\nRTE is possibly confusing. Actually, we referred to previous works which reported\nsimilar open and hybrid systems and used the terms including “round-trip efficiency”\nand “energy efficiency” (Angewandte Chemie, 2024, 136, e202410845; Energy &\nEnvironmental Science, 2025, 18, 1002; Angewandte Chemie, 2025, e202504188).\nHowever, as these works lack an efficient design, the efficiencies of their batteries are\nall below 100%.\nBased on your suggestions, we have changed “round-trip efficiency (η )” to\nRTE\n“electrical energy efficiency (η )” in the revised manuscript and SI (marked in red).\nEEE\nIn this way, an η greater than 100% is more easily understood as an amplification of\nEEE\nelectrical energy, which originates from the conversion of other energy forms (e.g.,\nchemical energy) in the open system. We hope the modification can satisfy your\ncomment.\nAt least the authors's usage of RTE makes a comparison of RTE with other battery\nsystems unfair if not almost meaningless. I recommend to at least critically reflect this\npoint in the paper and for example provide an estimation of the energy consumption of\nproducing the hydrazine electrolyte, to be included in the overall efficiency.\nResponse: We are deeply grateful for your advice. It is meaningful to compare the RTE\nof our open battery system with those of other closed battery systems, highlighting the\nadvantage of the first electricity-amplified open batteries (Response Fig. 1). And we\nconducted techno-economic analyses (TEAs) of the electricity-amplified battery\ncompared with conventional closed batteries, demonstrating significant economic and\nenvironmental effects (Response Fig. 2).\nTo make the statement clearer, we have changed the caption of Fig. 4e from\n“Comparison of flow battery performance in this work and previous studies” to\n“Comparison of flow battery performance in our open system and typical closed\nsystems” (Revised manuscript, Page 13)\nelectricity output to the electricity input during one charge/discharge cycle (Nature\nEnergy, 2024, 9, 479; Energy & Environmental Science, 2025, 18, 1002), and it does\nnot involve the preparatory processes at the front end of battery production, such as\nelectrolyte production. When conducting TEAs, we took into account the electrolyte\ncost and considered the operation and maintenance costs including wastewater-based\nelectrolyte replacement, which were taken to be 10% of the capital costs (Science, 2019,\n364, eaav3506; Nature Communications, 2021, 12, 4679).\nThe energy consumption of producing the hydrazine electrolyte: The hydrazine-\ncontaining electrolyte in practical application can be simply prepared by adding a\ncertain amount of KOH and ZnO to the hydrazine-containing wastewater, and thereby\nthe preparation process requires little energy consumption. There are two reasons as\nfollows.\n1) The ODB system is capable of treating low-concentration hydrazine in wastewater.\nAccording to the reference (10.12345/gcjsygl.v6i11.10821), the concentration of\nhydrazine (N H ) in the industrial hydrazine-containing wastewater is about 0.16 M\n2 4\n(5000 mg L−1). As shown in Response Fig. 3, linear sweep voltammetry (LSV)\ncurves at different concentrations of N H reported in the previous work (Energy &\n2 4\nEnvironmental Science, 2022, 15, 3246) show that the potentials at 10 mA cm−2 at 0.05\nM N H and 0.4 M N H are almost identical, and the potential difference at 300 mA\n2 4 2 4\ncm−2 between 0.05 M N H and 0.4 M N H is less than 0.03 V, which can also be\n2 4 2 4\nnegligible. This indicates that the concentration of 0.05 M is sufficient for the transport\nof the N H active substance under a current density of 300 mA cm−2, and there is no\n2 4\nneed to use the concentration of 0.5 M N H as in this work (merely a prototype\n2 4\ndemonstration, the testing concentration referred to previous works (Nature\nCommunications, 2021, 12, 4182; Advanced Materials, 2024, 36, 2307979)).\nTherefore, in the practical wastewater treatment process, the hydrazine electrolyte can\nbe directly prepared using the hydrazine-containing wastewater solution, without the\nneed for hydrazine enrichment.\n2) Other chemicals in hydrazine-containing wastewater have little impact on battery\nperformance. First of all, the chemicals existing in the hydrazine-containing wastewater\nneed to be determined. The industrial hydrazine-containing wastewater primarily\noriginates from hydrazine production and its use in the chemical, medical and military\nindustries (Industrial & Engineering Chemistry, 1953, 45, 2608; Biomass Conversion\nand Biorefinery, 2025, 1). Among them, the hydrazine production process is the\ndominant source of such wastewater, with common industrial methods including the\nRaschig method, urea method and peroxide-ketazine method (Chemical Engineering &\ngenerated during the production process mainly contain hydrazine and its derivatives,\nsalts (e.g., sodium chloride (NaCl)), ammonia nitrogen (NH ·H O/NH +), and organic\n3 2 4\nby-products (e.g., urea and its derivatives, ketone compounds). Then, according to the\nreference (Patent: CN101058468A), we prepared the simulated wastewater-based\nelectrolyte including 6.0 M KOH, saturated (sat.) ZnO, 0.5 M N H , 2.2 M NaCl, 1.7\n2 4\nmM NH Cl, 17 mM acetone and 17 mM urea (Response Fig. 4a), and performed rate\n4\nperformance of ODB in this electrolyte. As shown in Response Fig. 4b, discharge\nvoltages at different current densities in the electrolyte with other waste chemicals are\nslightly higher than those in the electrolyte without other waste chemicals, as the\naddition of other waste chemicals in the electrolyte on the HzOR side leads to an\nincreased difference in solute concentration between the HzOR side and the ORR side,\nenhancing the E and thereby improving discharge voltages (Response Fig. 4d).\nRED\nCharge voltages in the electrolyte with other waste chemicals are close to those in the\nelectrolyte without other waste chemicals at current densities ranging from 1 to 100 mA\ncm−2 (Response Fig. 4c). When the current density exceeds 200 mA cm−2, charge\nvoltages in the electrolyte with other waste chemicals obviously increase, because other\nwaste chemicals significantly affect the mass transfer process at ultrahigh current\ndensity. Additionally, ODB in the electrolyte with other waste chemicals still maintains\noutstanding η at the commonly used current density (Response Fig. 4e). Overall,\nEEE\nother waste chemicals in the hydrazine-containing wastewater have almost no effect on\nthe ODB performance.\nSchematic of power generation and transportation of all-ODB system, intermittent\nrenewable energy and continuous fossil energy (blue background). And schematic of\nelectricity amplification after combining intermittent renewable energy or continuous\nfossil energy with ODB, and electricity loss after combining intermittent renewable\nenergy and conventional ESSs (green background). b, TEAs of generating 1 MWh of\nelectricity combining different power generation technologies with ESSs. c, Disposal\ncosts of hydrazine wastewater with different methods. d, CO emissions generated by\n2\ncombining different power generation technologies and ESSs to generate 1 MWh of\nelectricity.\ntypical closed systems.\nResponse Fig. 3 LSV curves in different concentrations of N H (Energy &\n2 4\nEnvironmental Science, 2022, 15, 3246).\nResponse Fig. 4 a, Photograph of the simulated wastewater-based electrolyte including\n7\n6.0 M KOH, sat. ZnO, 0.5 M N H , 2.2 M NaCl, 1.7 mM NH Cl, 17 mM acetone and\n2 4 4\n17 mM urea. The white solid at the bottom is NaCl and ZnO that precipitated due to\noversaturation. b, Discharge voltages of Zn-CEM-ORR at different current densities\nfrom 1 to 50 mA cm−2 in 1 M KOH electrolytes for ORR electrode and 6 M KOH+sat.\nZnO+0.5 M N H electrolytes with/without other waste chemicals for Zn electrode. c,\n2 4\nCharge voltages of Zn-HzOR at different current densities from 1 to 300 mA cm−2 in 6\nM KOH+sat. ZnO+0.5 M N H electrolytes with/without other waste chemicals. d, The\n2 4\nconfiguration schematic of the ODB designed by “3E”, including the HzOR electrode,\nZn electrode and ORR electrode. e, The calculated η of ODB in the electrolyte\nEEE\nwith/without other waste chemicals at different current densities.\nAdditionally, for example, estimates of area specific resistance (ASR) for the different\nreactions would help make the performance more comparable to at least other flow\nbatteries. (Can probably be extracted from the raw data?) So far, the system\nperformance is mostly characterized by RTE.\nResponse: We sincerely appreciate your suggestion. As shown in Response Fig. 2,\nit can be seen that the system performance is characterized not only by RTE, but also by\ntesting current density and cycle number. Typically, these three parameters are widely\nrecognized as the key comparison factors for battery systems. In addition to the unique\nRTE, our battery can achieve exceptional stability for 1000 cycles at the fast-charging\ncurrent density of 300 mA cm−2, demonstrating significant advantages compared to\nprevious studies.\nThe ASR is the area-normalized resistance of the cell, and is most commonly used in\nsystems where the conductivity of solids or membranes is limited, such as solid oxide\nfuel cell (Nature Communications, 2021, 12, 3979), vanadium redox flow battery\n(Advanced Energy Materials, 2025, 15, 2401375), organic redox flow battery (Nature\nEnergy, 2023, 8, 1109) and so on. However, alkaline Zn-based batteries usually do not\ncharacterize this parameter. The reasons are as follows: 1) The high-concentration\nalkaline electrolyte used is highly conductive with a low membrane impedance. The\nproportion of ohmic loss in the total polarization is small, so ASR is not regarded as a\nkey characterization parameter. 2) The Zn plating/stripping reactions easily lead to a\nchange in the true reaction area during cycling, making it difficult for ASR (normalized\nby geometric area) to calculate accurately. Typically, Zn-based flow batteries take RTE,\nrate performance (large current density) and cycle stability as their main indicators. Our\nODB uses high-concentration (6 M KOH) as the electrolyte and Zn metal as one of the\nelectrodes, thereby not suitable for using ASR as the comparison parameter. Moreover,\nmost of the references used for comparison, including Zn-Air (Nature Communications,\n8\n2024, 15, 8365), Zn-Fe (Advanced Materials, 2024, 36, e2406366), Zn/V-I (Nature\nEnergy, 2024, 9, 714; Nature Communications, 2024, 15, 3841; Nature\nCommunications, 2024, 15, 6234), Zn-Mn (National Science Review, 2024, 11,\nnwae230), and S-Fe batteries (Nature Energy, 2023, 8, 1355), did not report the ASR\nparameters. Consequently, we chose to retain the original three parameters (namely,\nRTE, operating current densities and cycle number) to compare the battery performance\nin our open system and typical closed systems. We hope that this explanation can obtain\nyour recognition.\nComment 2: What actually are the necessary steps to produce the electrolyte solution\nfrom hydrazine wastewater? Are there really wasterwaters with such high hydrazine\nconcentrations? If hydrazine must be enriched first this makes my first comment even\nmore relevant.\nResponse: Thanks for your thoughtful comments. As illustrated in Response for\nComment 1, the hydrazine-containing electrolyte in practical application can be simply\nprepared by adding a certain amount of KOH and ZnO to the hydrazine-containing\nwastewater. According to the reference (10.12345/gcjsygl.v6i11.10821), the\nconcentration of N H in the industrial hydrazine-containing wastewater is about 0.16\n2 4\nM (5000 mg L−1). As shown in Response Fig. 3, LSV curves at different concentrations\nof N H reported in the previous work (Energy & Environmental Science, 2022, 15,\n2 4\n3246) show that the potentials at 10 mA cm−2 at 0.05 M N H and 0.4 M N H are\n2 4 2 4\nalmost identical, and the potential difference at 300 mA cm−2 between 0.05 M N H\n2 4\nand 0.4 M N H is less than 0.03 V, which can also be negligible. This indicates that\n2 4\nthe concentration of 0.05 M is sufficient for the transport of the N H active substance\n2 4\nunder a current density of 300 mA cm−2, and there is no need to use the concentration\nof 0.5 M N H as in this work (merely a prototype demonstration, the testing\n2 4\nconcentration referred to previous works (Nature Communications, 2021, 12, 4182;\nAdvanced Materials, 2024, 36, 2307979)). Therefore, in the practical wastewater\ntreatment process, the hydrazine electrolyte can be directly prepared using the\nhydrazine-containing wastewater solution, without the need for hydrazine enrichment.\nComment 3: On a related note: What is the actual abundance of hydrazine in\nwastewater? Since this particular ODB relies on the availability of hydrazine, can it be\nestimated what would be the potential of such a technology (in terms of kWh per year)?\nThis would greatly support the usefulness / relevance of the ODB.\nResponse: We appreciate your comments. According to the reference\n(10.12345/gcjsygl.v6i11.10821), the concentration of hydrazine in the industrial\nhydrazine-containing wastewater is about 0.16 M (5000 mg L−1). Based on the statistics\n9\nreported on the official website (https://www.chinabgao.com/freereport/94694.html),\nthe current global production capacity of hydrazine hydrate has reached over 200,000\ntonnes per year (calculated based on a 100% hydrazine). Furthermore, according to the\nreference (Patent: CN200610043670.7), for an enterprise with an annual production\ncapacity of 12,000 tonnes of hydrazine hydrate, it will generate 300,000 steres of\nwastewater each year. Therefore, it is estimated globally that a total of 5,000,000 steres\nof wastewater will be produced annually. The annual electricity storage and generation\ncalculated is over 100 million kWh, and the saved waste disposal cost (e.g., hydrogen\nperoxide as the oxidant) is about 16.3 million dollars, demonstrating considerable\neconomic profits.\nNotably, the key innovation of this work is to propose a three-electrodynamic-potential\n(3E) design to construct ODBs that simultaneously realize waste-to-energy conversion,\npower generation and energy storage, not merely treating the hydrazine-containing\nwastewater. As shown in Response Fig. 5, we listed a series of chemical substances in\nwastewater that need to be degraded and took the hydrazine (N H /N ) with the most\n2 4 2\nnegative potential as an example to demonstrate the effectiveness and practicability of\nour design. Apart from hydrazine, many other waste substances may also be utilized\nthrough our design, demonstrating significant application potential of the ODB.\nResponse Fig. 5 Decoupling electrodes by screening appropriate redox pairs to achieve\nlow E , high E and thus high η . For the charging positive electrode, the listed redox\nC D EEE\npairs all involve chemicals present in the wastewater.\n10"}]

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