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Title:
MEMBRANE-FREE ZN/MNO2 FLOW BATTERY FOR LARGE-SCALE ENERGY STORAGE
Document Type and Number:
WIPO Patent Application WO/2020/214604
Kind Code:
A1
Abstract:
This disclosure provides novel batteries, e.g., redox flow batteries, and methods of making and operating batteries. Certain embodiments include redox flow battery comprising: a housing; a first electrode disposed in the housing; a second electrode disposed in the housing and facing the first electrode, the first electrode is spaced from the second electrode by a gap without an intervening ion selective membrane; and a fluid conveyance mechanism connected to the housing and configured to convey an electrolyte across the gap between the first electrode and the second electrode.

Inventors:
LI GUODONG (US)
CHEN WEI (US)
CUI YI (US)
Application Number:
PCT/US2020/028141
Publication Date:
October 22, 2020
Filing Date:
April 14, 2020
Export Citation:
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Assignee:
UNIV LELAND STANFORD JUNIOR (US)
International Classes:
H01M8/18; H01M4/38; H01M4/86; H01M4/96; H01M8/023; H01M8/04186; H01M8/04276; H01M12/08
Foreign References:
US20120135282A12012-05-31
US20120045680A12012-02-23
US20050221169A12005-10-06
US8951673B22015-02-10
US20190006701A12019-01-03
Attorney, Agent or Firm:
SHELTON, Daniel R. et al. (US)
Download PDF:
Claims:
What is claimed is:

1. A redox flow battery comprising:

a housing;

a first electrode disposed in the housing;

a second electrode disposed in the housing and facing the first electrode, the first electrode is spaced from the second electrode by a gap without an intervening ion selective membrane; and

a fluid conveyance mechanism connected to the housing and configured to convey an electrolyte across the gap between the first electrode and the second electrode.

2. The redox flow battery of claim 1, wherein the first electrode is a cathode, and the second electrode is an anode.

3. The redox flow battery of any of claims 1-2, wherein the first electrode includes a porous, conductive support.

4. The redox flow battery of claim 3, wherein the porous, conductive support is a carbonaceous fibrous support.

5. The redox flow battery of any of claims 1-4, wherein the second electrode includes zinc.

6. The redox flow battery of claim 5, wherein the second electrode includes a zinc foil.

7. The redox flow battery of any of claims 1-6, wherein the fluid conveyance mechanism includes a container configured to store the electrolyte, and a pump connected between the container and the housing and configured to convey the electrolyte from the container into the housing.

8. A method of operating a redox flow battery, comprising:

providing a first electrode;

providing a second electrode facing the first electrode, the first electrode is spaced from the second electrode by a gap; and conveying an aqueous electrolyte across the gap between the first electrode and the second electrode, the aqueous electrolyte includes manganese ions and zinc ions.

9. The method of claim 8, wherein the first electrode is spaced from the second electrode by the gap without an intervening ion selective membrane.

10. The method of any of claims 8-9, wherein the first electrode is a cathode, and the second electrode is an anode.

11. The method of any of claims 8-10, wherein the first electrode includes a porous, conductive support.

12. The method of claim 11, wherein the porous, conductive support is a carbonaceous fibrous support.

13. The method of any of claims 8-12, wherein the second electrode includes zinc.

14. The method of claim 13, wherein the second electrode includes a zinc foil.

15. The method of any of claims 8-14, wherein the electrolyte is configured to support reversible precipitation and dissolution of manganese at the first electrode and reversible precipitation and dissolution of zinc at the second electrode.

16. The method of any of claims 8-15, wherein a concentration of the manganese ions is in a range of about 0.1 M to about 5 M, about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M.

17. The method of any of claims 8-16, wherein a concentration of the zinc ions is in a range of about 0.1 M to about 5 M, about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M.

18. The method of any of claims 8-17, wherein the aqueous electrolyte has a pH of about 7 or below, about 6.5 or below, about 6 or below, about 5.5 or below, about 5 or below, about 4.5 or below, or about 4 or below.

Description:
MEMBRANE-FREE ZN/MNO 2 FLOW BATTERY FOR LARGE-SCALE ENERGY

STORAGE

Cross-reference to related applications

[0001] This application claims the benefit of U.S. Provisional Patent Application No.

62/835,954, filed April 18, 2019, which is incorporated by reference herein in its entirety.

Statement Regarding Federally Sponsored Research or Development

[0002] This invention was made with Government support under contract DE-AC02-76- SFO0515 awarded by the Department of Energy. The Government has certain rights in the invention.

Background

[0003] Grid-scale energy storage has attracted great attention due to the expansion of intermittent renewable energy sources, and requests for services of power quality and energy management. It urges the development of energy storage systems with low cost, good safety, high energy density and scalability. Among various energy storage systems, lithium ion batteries are attractive due to high energy densities, but the availability of some element resources and the use of flammable, toxic and expensive organic electrolytes remain as concerns. In contrast, aqueous rechargeable batteries have favorable characteristic of low cost, high ionic conductivity, high safety and environmental friendliness. The available aqueous batteries include Zn/LiM O^ Zn/Mn0 2 , Zn/NiOOH, lead acid (Pb acid), metal hydride (Ni-MH), nickel- iron (Ni-Fe), and nickel-cadmium (Ni-Cd); however, these batteries display issues of inadequate cycling stability and constrained energy density (thus affecting cost per stored energy over the lifetime of batteries), seriously restricting their large-scale applications.

[0004] Different from solid-state active materials as secondary battery electrodes, redox flow battery has a prominent ability to tailor the energy capacity independently from the power output. Generally, liquid electrolyte and electroactive materials are stored externally, and the role of an electrode is to provide the electrochemically active surface for redox reaction to take place, such as vanadium redox battery, zinc/bromine battery, lead acid, and alkaline quinone flow battery. These characteristics make them promising for large scale energy storage. However, there are still some issues in these systems such as low active material concentration, low energy density, high environmental toxicity, high cost of ion selective membranes and high cost of battery system. Therefore, it is highly desirable to develop an improved flow battery with low cost, good safety and scalability.

[0005] Zn/MnC aqueous battery has attracted great interest due to its low cost, high safety, high output voltage and environmental friendliness. However, primary Zn/MnCk batteries are still dominant in market rather than the rechargeable ones, mainly owing to the poor reversibility of cathode reaction during cycling, thus leading to serious capacity decay.

[0006] It is against this background that a need arose to develop the embodiments described in this disclosure.

Summary

[0007] This disclosure provides novel batteries, e.g., redox flow batteries, and methods of making and operating batteries.

[0008] Certain embodiments include redox flow battery comprising: a housing; a first electrode disposed in the housing; a second electrode disposed in the housing and facing the first electrode, the first electrode is spaced from the second electrode by a gap without an intervening ion selective membrane; and a fluid conveyance mechanism connected to the housing and configured to convey an electrolyte across the gap between the first electrode and the second electrode. In some embodiments, the first electrode is a cathode, and the second electrode is an anode. In some embodiments, the first electrode includes a porous, conductive support. In some embodiments, the porous, conductive support is a carbonaceous fibrous support. In some embodiments, the second electrode includes zinc. In some embodiments, the second electrode includes a zinc foil. In some embodiments, the fluid conveyance mechanism includes a container configured to store the electrolyte, and a pump connected between the container and the housing and configured to convey the electrolyte from the container into the housing.

[0009] Other embodiments include a method of operating a redox flow battery, comprising: providing a first electrode; providing a second electrode facing the first electrode, the first electrode is spaced from the second electrode by a gap; and conveying an aqueous electrolyte across the gap between the first electrode and the second electrode, the aqueous electrolyte includes manganese ions and zinc ions. In some embodiments, the first electrode is spaced from the second electrode by the gap without an intervening ion selective membrane. In some embodiments, the first electrode is a cathode, and the second electrode is an anode. In some embodiments, the first electrode includes a porous, conductive support. In some embodiments, the porous, conductive support is a carbonaceous fibrous support. In some embodiments, the second electrode includes zinc. In some embodiments, the second electrode includes a zinc foil. In some embodiments, the electrolyte is configured to support reversible precipitation and dissolution of manganese at the first electrode and reversible precipitation and dissolution of zinc at the second electrode. In some embodiments, a concentration of the manganese ions is in a range of about 0.1 M to about 5 M, about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M. In some embodiments, a concentration of the zinc ions is in a range of about 0.1 M to about 5 M, about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M. In some embodiments, the aqueous electrolyte has a pH of about 7 or below, about 6.5 or below, about 6 or below, about 5.5 or below, about 5 or below, about 4.5 or below, or about 4 or below.

[0010] Other aspects and embodiments of this disclosure are also contemplated. The foregoing summary and the following detailed description are not meant to restrict this disclosure to any particular embodiment but are merely meant to describe some embodiments of this disclosure.

Brief Description of Drawings

[0011] Figure 1 shows an embodiment of a redox Zn/MnC flow battery. Figure la shows configuration of a membrane-free redox flow aqueous battery. Figure lb shows charge and discharge curves obtained at a constant charge current of about 2 mA and discharge current of about 2 mA (It is equal to about 4C; a rate of nC corresponds to a full discharge in 1/n h). Figure lc shows charge and discharge curves obtained at a constant charge voltage of about 2.0 V vs. Zn 2+ /Zn and a discharge rate of about 4C, and Figure Id shows corresponding capacity retention over 1000 cycles (data shown with an interval of 25 cycles).

[0012] Figure 2 shows an embodiment of characterization of samples obtained from a redox flow Zn/MnC battery embodiments. Figure 2a shows a SEM image of carbon felt after first charge. SEM images of carbon felt obtained at first discharge voltages of Figure 2b, about 1.6 V, Figure 2c, about 1.3 V, Figure 2d, about 1.0 V vs. Zn 2+ /Zn in sequence (see Figure lc). Figure 2e shows XRD pattern and Figure 2f, XPS spectra of cathode materials deposited on the carbon felt surface after first charge in Figure 2a.

[0013] Figure 3 shows charge and discharge curves of redox flow in an Zn/MnC battery embodiment at various discharge rates and corresponding XPS characterizations of different samples. Figure 3a shows a first charge and discharge curves at various discharge rates when the charge voltage is about 2.0 V vs. Zn 2+ /Zn. Figure 3b shows a tenth charge and discharge curves at different discharge rates when the charge voltage is about 2.0 V vs. Zn 2+ /Zn. Figure 3c and Figure 3d show XPS spectra of samples obtained at first different discharge end voltages of about 1.4 V, about 1.3 V and about 1.0 V vs. Zn 2+ /Zn (marked in Figure 3a) when the discharge rate is about 0.5C. Figure 3c, Mn 2p and Figure 3d Zn 2p. Numbers adjacent to dashed lines represent the peaks’ binding energy values.

[0014] Figure 4 shows influences of Mn 2+ ion concentration and pH value on the cell performance of an embodiment. Charge and discharge curves when electrolyte contain Figure 4a, about 0.5 M Mn 2+ and about 1 M Zn 2+ ion with a pH value of about 4.1, and Figure 4b, about 3 M Mn 2+ and about 1 M Zn 2+ ion with a pH value of about 3.0. Figure 4c, Figure 4d, Charge and discharge curves when adding concentrated sulfuric acid (H2SO4) to adjust the pH value of electrolyte to about 2.2 and about 1.8.

[0015] Figure 5 shows scale up of redox flow of an Zn/MnCk battery embodiment. Figure 5a shows charge and discharge curves at a specific capacity of about 1 mAh/cm 2 , and Figure 5b shows corresponding capacity retention over 500 cycles. Figure 5c shows charge and discharge curves at a capacity of about 10 mAh (carbon felt area: about 10 cm 2 ), and Figure 5d shows corresponding capacity retention over 500 cycles (data shown in Figure 5b, Figure 5d with an interval of 25 cycles).

[0016] Figure 6 shows construction of an embodiment of a bench-scale cell of about 1.2 Ah as well as its electrochemical performance. Figure 6a show the cell is composed of 6 Zn foils as anodes, 10 carbon felts as cathodes, 5 carbon coated Ti mesh as cathode current collectors, and 10 PMMA water-diversion channels. Figure 6b shows the digital picture and Figure 6c shows a cross-section schematic of the cell, where the electrolyte flow propelled by the impeller is indicated by green arrow. Figure 6d shows charge the cell at about 2.0 V to about 1.2 Ah and then discharge at about 500 mA to about 1.0 V. Figure 6e shows capacity retention over 500 cycles when charging the cell at about 2.0 V to about 1.2 Ah and then discharge at about 1000 mA to about 1.0 V.

[0017] Figure 7a shows the morphology of an embodiments of pure carbon felt in an SEM image with a scale bar of 50 pm. Figure 7b shows the morphology of an embodiments of pure carbon felt in an SEM image with a scale bar of 5 pm. [0018] Figure 8 shows a cyclic voltammogram (CV) of zinc anode and cathode at about 2 mV/s in aqueous electrolyte containing about 1 M MnSCL and about 1 M ZnSCL (pH of about 3.8). The tests were carried out in a two electrode setup by taking glassy carbon as cathode and Zn foil as anode electrode.

[0019] Figure 9 shows the operation of a light-emitting diode (LED) by using an embodiment of a Zn/MnCL flow battery.

[0020] Figure 10 shows TEM characterization of deposited MnCL of an embodiments of carbon felt surface after a first charge. Figure 10a shows HR-TEM image of the sample. Figure 10b shows the corresponding energy dispersive X-ray spectroscopy (EDX) measurement.

[0021] Figure 11 shows XPS spectra of the deposited samples from figure 10 after first charge. It can be seen that there is no Zn element in the deposited sample after first charge

[0022] Figure 12 shows cyclic voltammograms (CV) of zinc anode and cathode via three electrode test at about 2 mV/s in aqueous electrolyte containing about 1 M MnSCL and about 1 M ZnSCL (pH of about 3.8). The tests were carried out in a three electrode setup by taking glassy carbon as working electrode, Pt foil as counter electrode and saturated calomel reference (SCE) as reference electrode. The SCE reference electrode was calibrated with respect to reversible hydrogen electrode (RHE) in ¾ saturated about 1 M KOH electrolyte, yielding a relation of E(RHE) = E(SCE) + 1.06 V.

[0023] Figure 13a shows the morphology of a carbon felt after 1000 cycles. Figure 13b shows an enlarged image of the morphology of a carbon felt after 1000 cycles.

[0024] Figure 14 shows characterization of an embodiment of a Zn anode electrode. Figure 14a shows fresh Zn foil. Figure 14b shows Zn after 1000 cycles. Figure 14c is an enlarged image of the Zn after 1000 cycles. Figure 14d is the corresponding XRD pattern of the Zn in Figures 14b and 14c.

[0025] Figure 15 shows charge and discharge curves of Zn/MnCL battery at different discharge rates. The discharge rate of Figure 15a is about 0.5C, the discharge rate of Figure 15b is about 1C, the discharge rate of Figure 15c is about 2C, the discharge rate of Figure 15d is about 3C, the discharge rate of Figure 15e is about 4C, the discharge rate of Figure 15f is about 6C, the discharge rate of Figure 15g is about 8C and the discharge rate of Figure 15h is about IOC. [0026] Figure 16 shows morphology of carbon felt of an embodiment obtained at first discharge process when the discharge rate is about 0.5C. Figure 16a shows discharge end voltage of about 1.4 V vs. Zn 2+ /Zn. Figure 16b shows discharge end voltage of about 1.3 V vs. Zn 2+ /Zn. Figure 16c shows discharge end voltage of about 1 V vs. Zn 2+ /Zn. Scale bar stands for 2 pm.

[0027] Figure 17 shows TEM Characterization of sample on the carbon felt of an

embodiment surface at the first discharge end voltage of about 1.3 V vs. Zn 2+ /Zn when the discharge rate is about 0.5C. Figure 17a shows HR-TEM image of the sample and figure 17b shows a corresponding EDX spectrum.

[0028] Figure 18 shows XPS spectra of the sample charged at the second cycle when the discharge rate is about 0.5C. It can be seen that when the cell is recharged, the formed Zh c Mh2- c q4 can release Zn element and generate MnC again. Figure 18a shows Mn and figure 18b shows Zn.

[0029] Figure 19 shows an embodiment of influence of charge voltage on a cell performance. Charge voltage of figure 19a is about 1.9 V, figure 19b is about 1.95 V, figure 19c is about 2.0 V and figure 19d is about 2.1 V vs. Zn 2+ /Zn under the conditions of discharge rate of about 4C and end discharge voltage of about 1.0 V vs. Zn 2+ /Zn.

[0030] Figure 20 shows morphology of carbon felt of an embodiment after first charge at different voltage. Figure 20a and figure 20b show SEM images for charge at about 1.9 V vs. Zn 2+ /Zn. Figure 20c and figure 20d show SEM images for charge at about 1.95 V vs.

Zn 2+ /Zn. Figure 20e and figure 20f show SEM images for charge at about 2.0 V vs. Zn 2+ /Zn. Figure 20g and figure 20h show SEM images for charge at about 2.1 V vs. Zn 2+ /Zn.

[0031] Figure 21 shows charge and discharge curves obtained at a constant charge voltage of about 2 V vs. Zn 2+ /Zn and a discharge rate of about 4C for a particular embodiment. Figure 21a shows discharge end voltage of about 1.5 V vs. Zn 2+ /Zn, figure 21b shows discharge end voltage of about 1.4 V vs. Zn 2+ /Zn, and figure 21c shows discharge end voltage of about 1.3 V vs. Zn 2+ /Zn.

[0032] Figure 22 shows influence of Mn 2+ ion concentration on the performance of Zn/MnC battery of an embodiment. Figure 22a shows about 0.5 M Mn 2+ and about 1 M Zn 2+ ion, pH value of electrolyte: about 4.1. Figure 22b shows about 1 M Mn 2+ and about 1 M Zn 2+ ion, pH value of electrolyte: about 3.8. Figure 22c shows about 3 M Mn 2+ and about 1 M Zn 2+ ion, pH value of electrolyte: about 3.0. [0033] Figure 23 shows influence of Zn 2+ ion concentration on the performance of Zn/MnC battery of an embodiment. Figure 23a shows about 0.5 M Mn 2+ and about 0.5 M Zn 2+ ion, pH value of electrolyte: about 4.3. Figure 23b shows about 1 M Mn 2+ and about 0.5 M Zn 2+ ion, pH value of electrolyte: about 4.2. Figure 23c shows about 1 M Mn 2+ and about 1 M Zn 2+ ion, pH value of electrolyte: about 3.8. Figure 23d shows about 1 M Mn 2+ and about 3 M Zn 2+ ion, pH value of electrolyte: about 3.2.

[0034] Figure 24 shows morphology of a carbon felt of an embodiment. Figure 24a shows Charge the cell to about 1 mAh/cm 2 at first cycle and Figure 24b shows after first discharge process.

[0035] Figure 25 shows scale up Zn/MnC flow battery of an embodiment. Figure 25a shows charge and discharge curves at a specific capacity of about 2 mAh/cm 2 , and Figure 25b shows corresponding capacity retention over 500 cycles (data shown with an interval of 25 cycles).

[0036] Figure 26 shows morphology of the carbon felt of an embodiment. Figure 26a shows Charge the cell to about 2 mAh/cm 2 at first cycle and figure 26b shows after first discharge process.

[0037] Figure 27 shows scale up Zn/MnC flow battery of an embodiment. Figure 27a shows Charge and discharge curves at a capacity of about 5 mAh (carbon felt area: about 10 cm 2 ), and figure 27b shows corresponding capacity retention over 500 cycles (data shown with an interval of 25 cycles).

[0038] Figure 28 shows a demonstration of a rotational cathode Zn/MnC flow cell. Figure 28a shows a schematic and Figure 28b shows a picture of the cell. Figure 28c shows charge- discharge curves when charging at 2 V for 1 mAh per carbon felt cathode and discharging at 1 mA to 1 V. Inset shows the cycle performance of the cell. Figure 28d shows charge- discharge curves when charging at 2 V for 2 mAh per carbon felt cathode and discharging at 1 mA to 1 V.

Detailed Description

[0039] Disclosed herein are novel batteries, e.g., redox flow batteries, and methods of making and operating batteries.

Redox flow battery

[0040] The present disclosure includes embodiments directed to a redox flow battery including: (1) a housing; (2) a first electrode disposed in the housing; (3) a second electrode disposed in the housing and facing the first electrode, the first electrode is spaced from the second electrode by a gap without an intervening ion selective membrane; and (4) a fluid conveyance mechanism connected to the housing and configured to convey an electrolyte across the gap between the first electrode and the second electrode.

[0041] In some embodiments of the redox flow battery, the first electrode is a cathode, and the second electrode is an anode. In some embodiments, the porous, conductive support included in the first electrode includes a carbonaceous fibrous support, such as carbon cloth, carbon paper, or carbon felt, although other carbonaceous or non-carbon-based fibrous supports can be used. In some embodiments, the cathode is constituted to allow manganese- based chemistry with the reversible Mn 2+ /Mn0 2 deposition/stripping reaction. In some embodiments, the cathode can be carried out by reversible transformation between soluble ion and solid via a two-electron transfer reaction.

In some embodiments of the redox flow battery, the second electrode includes zinc, or another metal or a combination of metals. In some embodiments, the second electrode includes a metallic foil or sheet, such as a zinc foil or sheet. In some embodiments, the anode is constituted to allow zinc-based chemistry with the reversible Zn 2+ /Zn deposition/stripping reaction.

[0042] In some embodiments, the fluid conveyance mechanism includes one or more of a container configured to store the electrolyte, and a pump connected between the container and the housing and configured to convey the electrolyte from the container into the housing.

[0043] In some embodiments of the redox flow battery, the electrolyte is configured to support reversible precipitation and dissolution of manganese at the first electrode and/or reversible precipitation and dissolution of zinc at the second electrode.

[0044] In some embodiments of the redox flow battery, the electrolyte is an aqueous electrolyte. In some embodiments, the aqueous electrolyte includes manganese ions and zinc ions. In some embodiments, the manganese ions include Mn 2+ , although manganese ions having other oxidation states can be included. In some embodiments, a concentration of the manganese ions is in a range of about 0.1 molar (M) to about 5 M, such as about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M. In some embodiments, the zinc ions include Zn 2+ . In some embodiments, a concentration of the zinc ions is in a range of about 0.1 M to about 5 M, such as about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M. In some embodiments, the aqueous electrolyte has a pH of about 7 or below, such as about 6.5 or below, about 6 or below, about 5.5 or below, about 5 or below, about 4.5 or below, or about 4 or below, and down to about 3 or below, down to about 2.5 or below, or down to about 2 or below.

[0045] Some embodiments of the redox flow battery include batteries, can demonstrate a high discharge voltage, e.g., of about 1.78 V, excellent cycling stability (e.g., 1000 cycles without noticeable decay) and/or good rate capability, e.g., up to about IOC

[0046] Some embodiments of the redox flow battery include a membrane-free aqueous flow Zn/Mn0 2 battery, where the anode has the zinc-based chemistry with the reversible Zn 2+ /Zn deposition/stripping reaction, and the cathode is based on the dissolution-precipitation reaction (Mn 2+ /Mn0 2 ). Both the anode and the cathode can be based on low-cost materials. The cell is composed of MnSCC solution as the catholyte and ZnSCC solution as the anolyte, but they are mixed together without using any membrane between the two electrodes (carbon felt as the cathode collector, Zn metal foil as the current collector) (Figure la). The working principle of the Zn/Mn0 2 battery is described in the following reactions.

Cathode: Mn 2+ + 2H 2 0 - 2e < Mn0 2 + 4H + , 1.23 V versus SHE (1)

Anode: Zn 2+ + 2e < Zn, -0.76V versus SHE (2)

Overall: Mn 2+ + Zn 2+ + 2H 2 0 < Mn0 2 + 4H + + Zn, 1.99 V (3)

[0047] At charge, highly soluble Mn 2+ ions can diffuse and electrochemically deposit on the cathode in the form of solid Mn0 2 (equation 1, 1.23 V versus standard hydrogen electrode (SHE), theoretical capacity of 616 mAh/g Mn o 2 based on the two-electron transfer reaction), while Zn 2+ ions are reduced to Zn on the anode (equation 2, -0.76 V versus SHE, 820 mAh/gz n ). Therefore, the full cell voltage is 1.99 V, at the charge state, and both the anode and the cathode are in solid state, which are not in physical contact with each other and can allow omission of an ion selective membrane. This is a difference from comparative flow batteries where all redox molecules are in liquid phase. At discharge, the formed Mn0 2 reversibly dissolve into soluble Mn 2+ ions and revert back into the electrolyte, and the deposited Zn dissolves into Zn 2+ ions. Impressively, the cathode can be carried out by reversible transformation between soluble Mn 2+ ion and solid Mn0 2 via a two-electron transfer reaction, which is different from and superior to a comparative cathode that is cycled between Mn0 2 and MnOOH via a one-electron transfer reaction. It is significant that the theoretical capacity of the cathode reaction (616 mAh/g Mn o 2 ) is twice that of the comparative Zh/Mhq2 cell (308 mAh/g Mn c > 2). It is noted that the long cycle life (about 10,000 cycles) of Mn 2+ /Mn0 2 dissolution/precipitation chemistry is possible.

[0048] Herein it is demonstrated that embodied batteries, e.g., Zn/MnC batteries, can demonstrate a high discharge voltage, e.g., of about 1.78 V, excellent cycling stability (e.g., 1000 cycles without noticeable decay) and/or good rate capability, e.g., up to about IOC. Moreover, the theoretical energy density for an embodied battery is calculated at the substantially equal concentration of Mn 2+ and Zn 2+ ions in the electrolyte. The theoretical volumetric energy density could be modulated from about 46.4 Wh L 1 for about 0.5 M electrolyte to about 370.9 Wh L 1 for about 4 M saturated electrolyte (Table 1).

Methods of operating a redox flow battery

[0049] Additional embodiments are directed to a method of operating a redox flow battery including: (1) providing a first electrode; (2) providing a second electrode facing the first electrode, the first electrode is spaced from the second electrode by a gap; and (3) conveying an aqueous electrolyte across the gap between the first electrode and the second electrode, the aqueous electrolyte includes manganese ions and zinc ions.

[0050] In some embodiments of the method, the first electrode is spaced from the second electrode by the gap without an intervening ion selective membrane.

[0051] In some embodiments of the method, the first electrode is a cathode, and the second electrode is an anode.

[0052] In some embodiments of the method, the first electrode includes a porous, conductive support. In some embodiments, the porous, conductive support included in the first electrode is a carbonaceous fibrous support, such as carbon cloth, carbon paper, or carbon felt, although other carbonaceous or non-carbon-based fibrous supports can be used.

[0053] In some embodiments of the method, the second electrode includes zinc, or another metal or a combination of metals. In some embodiments, the second electrode includes a metallic foil or sheet, such as a zinc foil or sheet.

[0054] In some embodiments of the method, the electrolyte is configured to support reversible precipitation and dissolution of manganese at the first electrode and reversible precipitation and dissolution of zinc at the second electrode.

[0055] In some embodiments of the method, the manganese ions include Mn 2+ , although manganese ions having other oxidation states can be included. In some embodiments, a concentration of the manganese ions is in a range of about 0.1 molar (M) to about 5 M, such as about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M. In some embodiments, the zinc ions include Zn 2+ . In some embodiments, a concentration of the zinc ions is in a range of about 0.1 M to about 5 M, such as about 0.5 M to about 4 M, about 0.5 M to about 3 M, about 0.5 M to about 2 M, or about 0.5 M to about 1 M. In some embodiments, the aqueous electrolyte has a pH of about 7 or below, such as about 6.5 or below, about 6 or below, about 5.5 or below, about 5 or below, about 4.5 or below, or about 4 or below, and down to about 3 or below, down to about 2.5 or below, or down to about 2 or below.

[0056] Other aspects and embodiments of this disclosure are also contemplated. The foregoing description and the following examples are not meant to restrict this disclosure to any particular embodiment but are merely meant to describe some embodiments of this disclosure.

Examples

Configuration of aqueous rechargeable Zn/MnO? flow battery

[0057] In some embodiments, a redox flow Zn/MnC battery is constructed by an aqueous electrolyte containing about 1 M MnSCC and about 1 M ZnSQr (pH value of about 3.8), a blank carbon felt (about 6.35 mm in thickness, Figure 7) as a current collector at cathode side, a Zn foil anode (about 0.1 mm in thickness). A notable feature here is that no ion selective membrane is included between the two electrodes (Figure la). The electrodes are both rectangles (about 1 x about 2 cm 2 ) and half-immersed into the electrolyte in a Pyrex glass vial, and an inter-electrode gap is about 4 mm. The conductive electrode area in contact with electrolyte is about 1 cm 2 and the upper half of electrodes are connected to titanic wires.

[0058] To realize the possible electrochemical behaviors, cyclic voltammogram (CV) test is carried out in the two-electrode full cell. Redox peaks are observed, indicating the occurrence of electrochemical deposition and dissolution of MnC (Figure 8). Moreover, the

electrochemical deposition of MnC occurs at about 1.9 V vs. Zn 2+ /Zn and the deposition peak is at about 2.4 V vs. Zn 2+ /Zn, while the reduction peak for dissolution of MnCT is at about 1.78 V vs. Zn 2+ /Zn. Next, two different charge modes including constant current and contact voltage methods are applied for this cell. Figure lb shows the charge and discharge curves when constant current of about 2.0 mA is used for charge and discharge. The charge voltage is up to about 2.0 V at a specific capacity of about 0.5 mAh/cm 2 , and the corresponding average discharge plateau is about 1.78 V. Furthermore, the maximum

Coulombic efficiency is up to about 92% during the first ten cycles (Coulombic efficiency (%) = discharge capacity/charge capacity x 100%, which is equal to capacity retention due to the same charge capacity for every cycle). Figure lc shows the charge and discharge curves when constant voltage of about 2.0 V vs. Zn 2+ /Zn is used for charge and constant current of about 2 mA for discharge (corresponding to a discharge rate of about 4C; a rate of nC corresponds to a full discharge in 1/n h). This cell also exhibits a well-defined discharge potential plateau of about 1.78 V vs. Zn 2+ /Zn (Figure 9). The initial Coulombic efficiency is about 90%, while the subsequent values can reach up to about 100%, which is slightly higher than that obtained via constant current method. It is also noticed that the average discharge plateau is higher than the output voltage of about 1.5 V vs. Zn 2+ /Zn in the comparative primary Zn/MnCk cell, due to a higher reduction potential for cathode reaction (about 1.23 vs. about 0.95 V (SHE)).

[0059] The electrochemical energy storage mechanism of this system is further investigated when constant voltage of about 2.0 V vs. Zn 2+ /Zn is used for charge. After first charge, there is a uniform thin layer of MnCk coated on carbon felt surface (Figure 2a). Powder X-ray diffraction (XRD) pattern demonstrates that the deposited samples are gamma-phase MnCk (JCPDS No. 14-0644) (Figure 2e), and they exhibit high crystallinity with interplanar spacing of about 0.26 nm, representing the (031) plane of

http://en.wikipedia.org/wiki/Periodic_table_%28crystal_st ructure%29 - Face_centered_cubic_.28cubic_close_packed.29y-Mn02 (Figure 10).

[0060] X-ray photoelectron spectroscopy (XPS) measurements further manifest that tetra- valent Mn is solely present in the sample and no noticeable Zn element is found (Figure 2f and Figure 11). At discharge, the deposited MnCk can dissolve gradually (Figure 2b and c). When the discharge voltage reaches about 1 V vs. Zn 2+ /Zn, most of the deposit dissolves into soluble Mn 2+ ions, but very little solid remains in patches on carbon felt surface (Figure 2d), thus leading to the low initial Coulombic efficiency (about 90%, Figure Id).

[0061] It is further noted that the actual amount of deposited MnCkon the carbon felt surface is about 0.79 mg at a capacity of about 0.5 mAh at first charge (corresponding to about 630 mAh/g), very close to the theoretical value (about 0.81 mg) according to Faraday’s laws of electrolysis. This result also indicates that although standard reduction potentials of MnCk and O2 versus SHE are similar (about 1.23V), no noticeable O2 evolution occurs during this process, possibly owing to the high overpotential for oxygen evolution reaction. This hypothesis is also confirmed by three-electrode cyclic voltammetric experiments and corresponding working potential window is about 2.35 V for redox reactions of Mn 2+ and Zn 2+ ions during charge and discharge processes (Figures 8 and 12).

[0062] More importantly, this aqueous Zn/MnCk flow battery exhibits excellent cycling stability with a high-capacity retention of about 100% over 1000 cycles at a discharge rate of about 4C (Figure Id). Furthermore, no noticeable change is observed for the carbon felt after long-term test and just a small amount of deposition is left on its surface (Figure 13). As for Zn foil anode, its original flat surface is dramatically changed and some nanoflakes are formed rather than Zn dendrites (Figure 14). Moreover, just Zn is present and no side products are observed, indicating that the reversible conversion between Zn and Zn 2+ ion occurs during cycling. Altogether, this redox flow battery can exhibit excellent stability of the cathode in respect with a comparative primary battery and also have great potential to achieve large-scale energy storage.

Rate capability of redox flow Zn/MnO battery

[0063] Figure 3a and b show the rate capability of the redox flow aqueous Zn/MnCk battery when constant voltage of about 2.0 V vs. Zn 2+ /Zn is used for charge (Figure 15). At the discharge rates of about 0.5C, about 1C and about 2C, all the curves exhibit similar characteristics and two continuous discharge plateaus are observed. One plateau is with the average discharge voltage of about 1.78 V vs. Zn 2+ /Zn, and the other one is dependent on the discharge rate, such as about 1.3 V at about 0.5C, about 1.25 V at about 1C and about 1.2 V at about 2C. The initial Coulombic efficiencies are about 60%, about 80% and about 85%, respectively, and corresponding tenth values are about 68%, about 85% and about 91%. With increasing the discharge rate to about 3C and about 4C, the discharge plateau at higher voltage becomes longer and the other one at lower voltage becomes shorter. Furthermore, when the discharge rates are about 6C, about 8C and about IOC, just the discharge plateau at higher voltage is observed, indicating that just dissolution of MnCk into soluble Mn 2+ ion occurs at discharge (equation 1).

[0064] To further understand the electrochemical behavior at low discharge rates, the cathode materials at first discharge end voltages of about 1.4, about 1.3 and about 1.0 V vs. Zn 2+ /Zn are obtained with a discharge rate of about 0.5C (Figure 3a and Figure 16), and then subjected to XPS characterizations. The results indicate that the mixture of Mn 4+ and Mn 3+ ions together with Zn 2+ ion are present in all the samples (Figure 3c and d). Furthermore, with decreasing the discharge end voltage, peak intensities of Mn 3+ and Zn 2+ ions become stronger and stronger, indicating that more Zn 2+ ions can intercalate into MnCF crystal structure. High- resolution transmission electron microscopy (HR-TEM) image and Energy-dispersive X-ray spectroscopy (EDX) measurements show that the obtained sample has high crystallinity with interplanar spacings of about 0.49, about 0.31, about 0.27 and about 0.25 nm, representing the (112), (103), (211) and (321) planes of

http://en.wikipedia.org/wiki/Periodic_table_%28crystal_st ructure%29 - Face_centered_cubic_.28cubic_close_packed.29ZnMn204 (JCPDS No. 24-1133) (Figure 17). This phenomenon is also consistent with the comparative cells that use Zn 2+ ion intercalation into cathode MnCE and allow the cells to work when the aqueous electrolyte contains zinc salts. It is noted that the intercalated Zn 2+ ion can also be released again at charge to form MnCE (Figures 8 and 18). It is noted that at discharge, the proton H + and Zn 2+ ion

competitively react with MnCF to form Mn 2+ ion and ZnM CF. Furthermore, the former reaction rate is higher than the latter because just dissolution of MnCF into Mn 2+ ion is observed at higher discharge rates of about 6C, about 8C and about IOC (Figure 3a and b). Therefore, it is of importance to reasonably control the discharge rate in order to achieve high Coulombic efficiency for the cell.

[0065] It is also noted that at discharge rates of about 3C, about 4C and about 6C, the initial Coulombic efficiencies are about 89%, about 90% and about 94%, respectively, and the tenth values can reach up to about 100%. With further increasing the discharge rate to about 8C and about IOC, the initial Coulombic efficiencies are about 90% and about 88%, respectively, and corresponding tenth values are about 98% and about 94%. Altogether, this redox flow battery can operate very well within a wide range of discharge rate and exhibit excellent discharge behaviors at fast discharge rates.

Effect of charge and discharge voltage on cell performance

[0066] The influence of charge voltage is also investigated on the cell performance (Figure 19). The average overpotentials are about 125 mV at about 1.9 V, about 170 mV at about 1.95 V, about 220 mV at about 2.0 V and about 320 mV at about 2.1 V vs. Zn 2+ /Zn, indicating that charge voltage has almost no noticeable effect on the discharge plateaus.

However, they have some influences on the Coulombic efficiency. The initial Coulombic efficiencies are about 80% at about 1.9 V, about 88% at about 1.95 V, about 90% at about 2.0 V and about 90% at about 2.1 V, respectively, and corresponding tenth values can reach up to about 90%, about 94%, about 100% and about 100%. The reason for the low Coulombic efficiencies at about 1.9 V and about 1.95 V vs. Zn 2+ /Zn might be that carbon felt has a relatively smooth surface (Figure 7), and a higher current density is involved for effective deposition of MnC on its surface (Figure 20). Altogether, the charge voltage of about 2.0 V vs. Zn 2+ /Zn is more suitable for this aqueous flow battery.

[0067] In addition, the influence of discharge end voltage on the cell performance is also investigated (Figure 21). When the discharge end voltages are about 1.5, about 1.4 and about 1.3 V vs. Zn 2+ /Zn, just one discharge plateau is found and no noticeable side reaction is observed at a discharge rate of about 4C; however, the Coulombic efficiency becomes lower and corresponding initial values are about 78%, about 78% and about 87%. This is mainly caused by incomplete dissolution of the formed MnC with higher discharge end voltages. It is also noticed that with increasing cycle number, the Coulombic efficiencies can reach up to about 100%, since the more MnC left on the carbon felt surface, the more protons are produced in the aqueous solution, which can promote the dissolution of MnC into soluble Mn 2+ ion according to the chemical equilibrium principle (equation 1).

Effect of active material concentration on cell performance

[0068] Figure 4a and b show the influence of Mn 2+ ion concentration on the cell performance (Figure 22). When Mn 2+ ion concentrations are about 0.5 M (pH of about 4.1) and about 1 M (pH of about 3.8), the charge and discharge behaviors are similar, and the initial Coulombic efficiency is about 90% and subsequent values can be increased to about 100%. However, with further increasing its concentration to about 3 M (pH of about 3.0), the average discharge plateaus of about 1.78 V vs. Zn 2+ /Zn becomes shorter with increasing cycle number and the other discharge plateau at low voltage disappears. The similar phenomenon is also observed when fixing MnSQr concentration at about 1 M and changing ZnSQr concentration from about 0.5 M (pH of about 4.3) to about 3.0 M (pH of about 3.2) (Figure 23). These may be relevant with various pH values of different aqueous solutions, because at discharge, protons react with MnC to produce Mn 2+ ion (equation 1). To confirm this, the influence of pH value on the cell performance is investigated via adding concentrated sulfuric acid (H2SO4) into the electrolyte with pH value of about 3.8. When the pH value is adjusted to about 2.2 and about 1.8, the discharge plateau at higher voltage becomes shorter and the other one at lower voltage disappears, both of which further confirm the presence of competitive reaction of proton and Zn 2+ ion with MnC at discharge. Furthermore, the average

overpotential becomes smaller with decreasing pH value, and they are about 170 mV at the pH of about 2.2 and about 140 mV at the pH of about 1.8, indicating that more protons can promote the dissolution of MnCk and reduce the overpotential of this cell (equation 1).

Therefore, it is of importance to control the pH value of electrolyte in a reasonable range.

Scale up of Zn/MnO flow battery

[0069] To satisfy high demand for energy storage, it is desired to enlarge the specific capacity of per unit (cm 2 ). The specific capacity of this cell is increased from about 0.5 to about 1.0 and about 2.0 mAh/cm 2 . At a specific capacity of about 1.0 mAh/cm 2 , the initial Coulombic efficiency is about 87% (Figure 5a). With increasing cycle number, the Coulombic efficiency is gradually enhanced and finally is kept at about 97% over 500 cycles without any noticeable capacity decay (Figure 5b, and Figure 24). Similarly, at a specific capacity of about 2 mAh/cm 2 , the initial Coulombic efficiency is about 78% and then it can be increased to about 95%. Furthermore, it can be kept at this value over 500 cycles without any noticeable capacity decay (Figures 25 and 26). To further increase absolute energy output, the immersed area of current collector is increased from about 1.0 to about 10.0 cm 2 . When the total capacity is set as about 5 and about 10 mAh, respectively, and corresponding initial

Coulombic efficiencies are about 91.6% and about 89.3% (Figure 27a and Figure 5c). With increasing cycle number, the Coulombic efficiencies can be increased and kept at about 95% for about 5.0 mAh over 500 cycles (Figure 27b) and about 93% for about 10.0 mAh over 500 cycles without any noticeable capacity decay (Figure 5d).

[0070] In addition, another scale up strategy that transports active material via rotation of carbon felt electrode is applied for this novel redox flow battery, in which a hexagonal substrate for the decoupling of energy and power demonstration is used (Figure 28a). The cathode of the rotating cell was built by affixing six 1 cm 2 carbon cathodes (6.35 mm in thickness, the same as the one used in the other cells) onto six sides of a stainless steel hexagon rotator, with 2 cm in length and 1.5 cm in width for each side. The carbon felts were affixed onto hexagon substrates by graphite emulsion and heating of the substrate at 150°C for 4 hours to cure the binder. The hexagon substrate was partially immersed into the electrolyte to keep only one of the six electrodes soaked. After each charging/discharging, the substrate was clockwisely rotated 60 degree to get the charged/discharged electrode out and rolling another electrode into the electrolyte (Figure 28b). When charged at constant voltage of 2.0 V and discharged at constant current of 1 mA for each electrode, the whole set of the six electrodes was charged for 6 mAh and it achieved discharge capacity of 5.91 mAh with a total Coulombic efficiency of 98.5% (Figure 28c). Furthermore, after successive 20 cycles, this rotation cell exhibited excellent stability with Coulombic efficiency of 98.5 %. Further increasing the capacity up to 2 mAh for each cathode collector, the whole cell was charged to a capacity of 12 mAh and it delivered discharge capacity of 11.9 mAh, with a total

Coulombic efficiency of 99.1% (Figure 28d). The above rotational cell demonstrated the decoupling of energy and power of the rechargeable Zn/MnCk flow system and the scaling of the capacity by a simple rotation cathode.

[0071] Based on above, to further demonstrate the benefits and potential of this redox flow battery for grid scale energy storage, fabrication of a bench-scale cell of about 1.2 Ah is performed, which was constructed by 6 Zn foils (about 7.1 x about 7.1 cm 2 ) as anodes, 5 carbon coated Ti mesh as cathode current collectors, 10 carbon felts (about 7.1 x about 7.1 cm 2 ) as cathodes, 10 poly(methyl methacrylate) (PMMA) water-diversion channels, and a PMMA box of about 1.02 L (Figure 6a and Table 2). The electrode plates stand upright and are substantially parallel with each other. The cathode and anode plates are separated by about 2.0 mm gap of upward-flowing electrolyte created with the about 2.0 mm thick PMMA dowel rods (Figure 6b). The electrode plates were sealed inside the PMMA box filled with electrolyte, which was pumped into the bottom of this box by a motor drive impeller, causing the electrolyte to flow upward between the electrode plates and then circulated through the pump again (green arrows in Figure 6c). At the top of these channels, the flow was restricted by a narrow gap, which homogenized the flow of electrolyte with the same velocity in each channel. The pumping of the electrolyte ensures the flow rate of about 0.4 cm s 1 on the electrode surface, which is comparable to the value in other reported flow cells. The two terminal ports on the top lid were collected directly from Zn foil and carbon coated Ti mesh for energy storage tests. The result demonstrates that when the cell is charged to about 1.2 Ah and then discharged at the current rate of about 500 mA, the discharge capacity is about 1.104 Ah and corresponding Coulombic efficiency is about 92.0%. Moreover, cycling test shows that when the bench scale cell is charged to about 1.2 Ah and then discharged at about 1000 mA, the Coulombic efficiency is about 89.7% after 500 cycles. This indicates the longevity of this approach. After cycling test, it is found neither apparent passivation of the electrodes (no huge voltage drops during discharge) nor zinc particulates that obstructed the gap between the electrodes, ensuring the long cycle life.

[0072] Altogether, the energy output of the redox flow Zn/MnCk battery can be readily scaled up by increasing either the specific capacity or the working area of electrodes, both of which exhibit excellent cycling stability. Further, it is desirable to optimize potential current collectors with high specific surface area, good conductivity and excellent hydrophilicity for large-scale energy storage.

[0073] In summary, fabrication is performed of a membrane-free aqueous Zn/MnC flow battery by using MnSC solution as a catholyte and metallic Zn foil as an anode. At the cathode side, Mn 2+ ions are transformed into g-Mhq2 at charge, and reversibly dissolve into Mn 2+ ions at discharge. At the anode side, reversible transformation between Zn and Zn 2+ ion occurs on Zn foil surface. Furthermore, Zn 2+ ions against protons competitively react with MnC at low discharge rate. Impressively, this aqueous flow battery exhibits a high discharge voltage of about 1.78 V, good rate capability (from about 0.5C to about IOC) and excellent capacity retention of about 100% over 1000 cycles for about 0.5 mAh/cm 2 , and about 95% over 500 cycles for about 2 mAh/cm 2 . The scale up on absolute energy output is also evidenced by a bench scale cell of about 1.2 Ah with good capacity retention over 500 cycles. This approach provides a foundation for developing the next-generation low cost and safe energy storage system for grid-scale application.

Methods

[0074] Chemicals. MnSCC-FbO (> about 99%) and ZnSCC-VFbO (> about 99%) were purchased from Sigma- Aldrich. Carbon felt (about 6.35 mm in thickness with the purity of about 95%) and Zn foil (about 0.1 mm in thickness with the purity of about 99.98%) were purchased from Alfa Aesar. All chemicals were used directly without further purification. Ultrapure water (about 18 MW) used in the experiments was supplied by a Millipore System (Millipore Q).

[0075] Characterization. X-ray diffraction was conducted by PANalytical X’Pert

diffractometer using copper K-edge X-rays. X-ray photoelectron spectroscopy (XPS) was performed on SSI S Probe XPS spectrometer with A1 Ka source. SEM (FEI XL30 Sirion) and TEM (FEI Tecnai G2 F20 X-TWIN) were used to characterize the sample morphology and micro structure.

[0076] Electrochemical measurements. Galvanostatic experiments were performed using multi-channel potentiostat, VMP3 (Bio-Logic). The rechargeable Zn/Mn0 2 battery is constructed by an aqueous electrolyte containing about 1 M MnS0 4 and about 1 M ZnS0 4 (pH value of about 3.8), a blank carbon felt (about 6.35 mm in thickness) as a current collector at cathode side, a Zn foil anode (about 100 pm in thickness), and with no separator between the two electrodes. The electrodes are both rectangles (about 1 x about 2 cm 2 ) and half- immersed into electrolyte in a round, Pyrex glass vial, and an inter-electrode gap is about 4 mm. The conductive electrode area in contact with the electrolyte is about 1 cm 2 and the upper halves of the electrodes not in contact with the electrolyte are connected to titanic wire current collectors. The carbon felt is subjected to wash with methanol and ultrapure water before experiments. The influences of charge voltage and discharge voltage, discharge rate capability, Mn 2+ and Zn 2+ ion concentration, pH value on the cell performances were also investigated. All electrochemical measurements were carried out in the Pyrex glass vial.

Additional Information

Table 1. Theoretical volumetric energy densities of aqueous rechargeable Zn/MnO flow batteries.

[0077] The energy density of a redox flow battery is determined by the number of transferred electrons, the concentration of active species in the electrolyte, and the cell voltage, as be described by eq 1.

where n c and n a are the number of electrons involved in the redox reactions on the cathode and anode, respectively, C c and C a are maximum concentrations of the less soluble of charged and discharged active redox species in catholyte and anolyte, respectively, F is the Faraday constant (26.8 Ah/mol), and V is the voltage of the cell. For the membrane-free Zn/MnC flow battery, where catholyte and anolyte concentrations equals with each other, and the volumetric energy density of the cell is determined by the lesser one between the two ions (eq 2). C is the concentration of the lesser one and n is 2. E = nCFV eq 2

[0078] The calculation of the theoretical volumetric energy densities of Zn/MnC flow batteries as a function of its concentration is listed in Table 1. The calculation is based on the solution of MnSCC and ZnSCC in water at room temperature (about 25°C) and a discharge potential of about 1.73 V at about 4C rate (Fig. lc).

Table 2 | Bench scale cell design.

[0079] Cyclic voltammogram (CV) obtained via two electrode test shows that redox peaks are observed, indicating the occurrence of electrochemical deposition and dissolution of MnC . Moreover, the electrochemical deposition of MnC may occur at about 1.9 V vs. Zn 2+ /Zn (Figure 8). The deposition peak is about 2.4 V Zn 2+ /Zn, and no noticeable O2 evolution is found. The reduction peak at about 1.78 V vs. Zn 2+ /Zn should be ascribed to dissolution of MnC into Mn 2+ ion, while at about 1.2 V vs. Zn 2+ /Zn, the intercalation of Zn ion into the undissolved MnC occurs. It is also noticed that another oxidation peak at about 1.64 V vs. Zn 2+ /Zn should be ascribed to the deintercalation of Zn ion from the formed Zh c Mh2- c q4 to generate MnC again.

[0080] HR-TEM image exhibits high crystallinity with interplanar spacing of about 0.258 nm, representing the (031) planes of

http://en.wikipedia.org/wiki/Periodic_table_%28crystal_st ructure%29 - Face_centered_cubic_.28cubic_close_packed.29y-Mn02. EDX spectrum further confirms that Mn and O elements are solely present in the sample after first charge and without any noticeable Zn element. [0081] The cyclic voltammetric experiments via three electrodes indicate that the working potential window is about 2.35 V for redox reactions of Mn 2+ and Zn 2+ ions during the charge and discharge processes and no noticeable ¾ and O2 evolution is observed (Figure 12).

[0082] After 1000 cycles, most of the deposit has dissolved into soluble Mn 2+ ions, but some poorly conducting solid clearly remains in patches on the carbon felt surface.

[0083] As for Zn foil anode, its original flat surface is dramatically changed and some nanoflakes are formed with no noticeable Zn dendrites (Figure 14). However, Zn is solely present and no side products are observed, indicating that the transformation between Zn and Zn 2+ ion occurs during the cycling.

[0084] At the discharge rates of about 0.5C, about 1C, and about 2C, all the curves exhibit similar characteristics and two continuous discharge voltage plateaus are observed. One plateau is with the average discharge voltage of about 1.78 V vs. Zn 2+ /Zn, and the average potential for the other plateau is dependent on the discharge rate, such as about 1.3 V at about 0.5C, about 1.25 V at about 1C and about 1.2 V at about 2C. The initial Coulombic efficiencies are about 60%, about 80% and about 85%, respectively, and the tenth values are about 68%, about 85% and about 91%. With increasing the discharge rate to about 3C and about 4C, the first discharge plateau becomes longer and the second discharge plateau becomes shorter. Furthermore, when the discharge rates are about 6C, about 8C and about IOC, just one plateau at high discharge voltage is observed, indicating that dissolution of MnCk into soluble Mn 2+ ion occurs at discharge.

[0085] It can be seen from Figure 16 that when the discharge end voltage is about 1.4 V vs. Zn 2+ /Zn, the formed MnCk is dissolved gradually and some blank carbon felt is observed. With decreasing the discharge end voltage, more blank carbon felt is observed, and at the voltage of about 1 V vs. Zn 2+ /Zn, small amount of samples is left on the surface, which is more than that obtained at a discharge rate of about 4C (Figure 2d).

[0086] HR-TEM image exhibits high crystallinity with interplanar spacings of about 0.49, about 0.313, about 0.27 and about 0.256 nm, representing the (112), (103), (211) and (321) planes of http://en.wikipedia.org/wiki/Periodic_table_%28crystal_struc ture%29 - Face_centered_cubic_.28cubic_close_packed.29ZnMn204 (JCPDS No. 24-1133). EDX spectrum further confirms that Zn, Mn and O elements are present in the sample.

[0087] The average overpotentials are about 125 mV at about 1.9 V, about 170 mV at about 1.95 V, about 220 mV at about 2.0 V and about 320 mV at about 2.1 V vs. Zn 2+ /Zn, indicating that charge voltage has almost no effect on the discharge plateaus. However, they have some influences on the Coulombic efficiency. The initial Coulombic efficiencies are about 80% at about 1.9 V, about 88% at about 1.95 V, about 90% at about 2 V and about 90% at about 2.1 V, respectively, and corresponding tenth values can reach up to about 90%, about 94%, about 100% and about 100%. Therefore, under these conditions the charge voltage of about 2 V vs. Zn 2+ /Zn is more suitable for this aqueous Zn/MnCk battery. The reason for the lower Coulombic efficiency at about 1.9 V and about 1.95 V vs. Zn 2+ /Zn might be that carbon felt has a very smooth surface (Figure 7), and a higher current density is involved for effective deposition of MnCk on its surface (Figure 20).

[0088] When the discharge end voltages are about 1.5 V, about 1.4 V and about 1.3 V vs. Zn 2+ /Zn (Figure 21), no noticeable side reaction is found at a discharge rate of about 4C; however, the Coulombic efficiency becomes lower and corresponding initial values are about 78%, about 78% and about 87%. This is mainly caused by incomplete dissolution of the formed MnCk with higher discharge end voltage.

[0089] When Mn 2+ ion concentrations are about 0.5 M and about 1 M, the charge and discharge behaviors are similar, and the initial Coulombic efficiency is about 90% and subsequent values can be increased to about 100%. However, with further increasing its concentration to about 3 M, the average discharge plateaus of about 1.78 V vs. Zn 2+ /Zn becomes shorter with increasing cycle number and the other discharge plateau at low voltage disappears.

[0090] When Zn 2+ ion concentrations are about 0.5M and about 1 M, the charge and discharge behaviors are the similar, and the initial Columbic efficiency is about 90% and subsequent values can be increased to about 100% (Figures 23a, 23b and 23c). However, with further increasing its concentration to about 3 M, the average discharge plateaus of about 1.78 V vs. Zn 2+ /Zn become short gradually and the second discharge plateau at low voltage disappears (Figure 23d).

[0091] After discharge, most of the deposit has dissolved into soluble Mn 2+ ions, but some poorly conducting solid clearly remains in patches on the carbon felt surface.

[0092] At a specific capacity of about 2 mAh/cm 2 , the initial Coulombic efficiency is about 78% and then it can be increased to about 95%. Furthermore, it can be kept at this value over 500 cycles without any noticeable capacity decay (Figures 25 and 26). [0093] After discharge, most of the deposit has dissolved into soluble Mn 2+ ions, but some poorly conducting solid clearly remains in patches on the carbon felt surface.

[0094] When the total capacity is set as about 5 mAh, the initial Coulombic efficiencies are about 91.6%, (Figure 27a). With increasing cycle number, the Coulombic efficiencies can be increased and kept at about 95% for about 5 mAh over 500 cycles (Figure 27b).

[0095] As used herein, the singular terms“a,”“an,” and“the” may include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object may include multiple objects unless the context clearly dictates otherwise.

[0096] As used herein, the terms“substantially,”“substantial,” and“about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

[0097] As used herein, the term“size” refers to a characteristic dimension of an object. Thus, for example, a size of an object that is circular can refer to a diameter of the object. In the case of an object that is non-circular, a size of the non-circular object can refer to a diameter of a corresponding circular object, where the corresponding circular object exhibits or has a particular set of derivable or measurable characteristics that are substantially the same as those of the non-circular object. When referring to a set of objects as having a particular size, it is contemplated that the objects can have a distribution of sizes around the particular size. Thus, as used herein, a size of a set of objects can refer to a typical size of a distribution of sizes, such as an average size, a median size, or a peak size.

[0098] Additionally, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

[0099] While the disclosure has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the disclosure as defined by the appended claim(s). In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, operation or operations, to the objective, spirit and scope of the disclosure. All such modifications are intended to be within the scope of the claim(s) appended hereto. In particular, while certain methods may have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the disclosure. Accordingly, unless specifically indicated herein, the order and grouping of the operations is not a limitation of the disclosure.