WO2026009233A1

WO2026009233A1 - Low-cost zinc-polyiodide redox flow battery

Info

Publication number: WO2026009233A1
Application number: PCT/IN2025/050873
Authority: WO
Inventors: Kothandaraman Ramanujam; Rubi .; Harun Khan
Original assignee: Indian Institute of Technology Madras
Current assignee: Indian Institute of Technology Madras
Priority date: 2024-07-05
Filing date: 2025-06-11
Publication date: 2026-01-08
Anticipated expiration: 2027-01-05

Abstract

The present invention relates to redox flow battery system. In particular, the present invention relates to low-cost Zinc-polyiodide redox flow battery (ZIFBs) with a low-cost electrolyte additive and a composite membrane comprising a microporous membrane and a cation conducting membrane.

Description

LOW-COST ZINC-POLYIODIDE REDOX FLOW BATTERY

FIELD OF THE INVENTION

[001] The present disclosure relates to redox flow battery system. In particular, the present disclosure relates to low-cost zinc-polyiodide redox flow battery (ZIFBs) with a low-cost electrolyte additive and a composite membrane comprising a microporous membrane and a cation conducting membrane.

BACKGROUND OF THE INVENTION

[002] ZIFBs have gained considerable attention because of their exceptional volumetric energy density (~ 250 Wh L'¹ of iodide electrolyte) attributed to the high solubility of active species (potassium iodide, up to 7.5 M) and favorable electrochemical kinetics of I", b" redox species. The electrolyte contains redox active species dissolved in water, which plays a crucial role in deciding the electrochemical performance of the battery.

[003] ZIFBs face challenges like zinc dendrite formation, low ionic conductivity, poor cyclability, electrolyte imbalances, precipitation of insoluble iodine, and the issue of crossover, i.e. electrolyte shifting from catholyte to anolyte side.

[004] Separators used in flow battery generally include an ion-exchange membrane, conducting charge-carrier ions preventing crossover of active species. However, active species in anolyte/ catholyte may still slowly diffuse through the separators to the opposite side, causing self-discharge and reducing the coulombic efficiency of the battery.

[005] Significant advancement has been made in developing separators which are more sophisticated such as composite and hybrid membranes. Porous polyolefin membranes have been explored to partly replace cation conducting membranes such as Nafion series proton exchange membranes to reduce the cost of the battery. This prevents zinc dendrites because the pores of the membrane are filled with triiodide ions reacting with zinc dendrites reaching membrane, forming soluble Znh, thereby physically limiting the dendrite growth reaching cathode. Although they have achieved a high energy density, the columbic efficiency and operating discharge voltage are low due to the self-discharge. [006] The mechanism of capacity fade during extended cycling of ZIFBs with a porous polyethylene separator and an ammonium-based, decoupled Zn²⁺ and I" electrolyte has also been investigated. The findings demonstrate that fluctuations in the viscosity and density of the anolyte and catholyte disturb the pressure balance, leading to heightened electrolyte transport and significant (poly)iodide crossover. Consequently, this phenomenon contributes to capacity fade.

[007] Researchers have also fabricated ZIFBs using Znh electrolyte and achieved high energy density due to the high solubility of redox-active species. However, as one-third of I" combines with h to form soluble b" ion, its molar capacity is limited. Further, at high SOC, I2 solid precipitates are formed resulting in their accumulation and subsequently clogging the pores of the electrode, leading to the decay of battery performance. To address this issue, researchers have explored the incorporation of Br" and Cl" ions which can form complexes with I2 to liberate I" ions during the charging process, thereby enhancing the electrolyte's utilization and molar capacity in ZIFBs. However, such addition of ions may lead to side reactions leading to electrolyte instability. Researchers have reported ZIFBs by replacing Znh with KI + ZnBn, the advantage of adding KI is that it can avoid the formation of zinc oxide and zinc hydroxide precipitates during cycling and can increase the cycling stability, however, such combination may result in crossover issues of active species.

[008] Solutions such as incorporating Br" and Cl" ions, employing electrolyte compositions of KI, ZnBn, and KC1, in the presence of a porous or composite and polyolefin membranes have been explored in the past. Many researchers also investigated solving the crossover issue by improving designs of the separators, including functionalization of membrane surfaces, pore-size control of nanoporous membranes, and coating on membrane surfaces. However, solving crossover issue with high selectivity separator may lead to high polarization of flow battery, which reduces energy efficiency. Moreover, high cost of high-performance ionexchange membranes may work against the economic feasibility of the flow batteries.

[009] Nafion ionomer coating on the porous membrane in the past has been carried out by a dip coating method. Due to the difference in the swelling extent of the porous membranes compared to Nafion ionomer with cycling, debonding/delamination can happen between Nafion ionomer coating and porous membrane. The porous membranes employed in the past had a thickness of the order of about 900 micrometers. The higher thickness could lead to more IR drop across the membrane, resulting in decreased voltage efficiency and energy efficiency.

[0010] Despite all the research, issues like low coulombic efficiency (C.E) and cycle life persist. In the ZIFBs available, the electrolyte is not circulated in the iodide side (cathode), and hence, the amount of electrolyte absorbed by the electrode material is limited. Consequently, the capacity and energy density of zinc-polyiodide redox flow batteries (ZIFBs) is constrained. Further, although KC1 has been employed to increase the conductivity of electrolytes, its impact on shifting of electrolytes from the positive to the negative side of the cell or the morphology of deposited zinc has not been studied.

[0011] Widespread implementation of ZIFB technology is therefore hindered to some extent by electrolyte imbalances, low conductivity, and crossover issues occurring in the anolyte and catholyte solutions throughout charge/discharge cycles. These obstacles must be addressed to ensure the progression of ZIFBs. Electrolyte shifting and control on the morphology of deposited zinc is critical for improving the performance of ZIFBs or any other zinc-based electrochemical energy storage systems needed which affects the performance of the cell. Therefore, further research in this direction is necessary to improve the long-term stability and scalability of Zn-based energy storage technologies.

SUMMARY OF THE INVENTION

[0012] This summary is intended to introduce, in simplified form, a selection of concepts that are further described in the detailed description. This summary is merely presented as a brief overview of the subject matter described and claimed herein and does not aid in determining the scope of the claimed subject matter.

[0013] The present disclosure provides zinc-polyiodide redox flow batteries (ZIFBs) with enhanced cell voltage, coulombic efficiency, and energy efficiency resulting in enhanced cycle life that is achieved through a low-cost electrolyte additive, and a composite membrane comprising a microporous membrane and a cation conducting membrane.

[0014] In one aspect, the present disclosure provides ZIFBs comprising a low-cost electrolyte additive selected from potassium chloride (KC1) at a concentration of IM to 3M. Additionally, the invention employs a composite membrane comprising a combination of a microporous membrane and a cation conducting membrane. The present disclosure aims to address the issue of electrolyte shifting, low ionic conductivity and to reduce the cost of ZIFBs.

[0015] ZIFBs with enhanced cell voltage, coulombic and energy efficiency and cycle life of the present disclosure comprise:

• carbon based positive and negative electrodes;

• a membrane sandwiched between the positive and negative electrodes;

• electrolyte; and

• optionally a peristaltic pump to circulate the electrolytes, wherein, the electrolyte composition includes a zinc salt, a precursor for iodide, and KC1 as additive at a concentration of IM to 3M concentration; and wherein the membrane is a composite membrane composed of a microporous membrane and a cation conducting membrane.

[0016] In another aspect, the present disclosure relates to a method for preparation of low-cost zinc-polyiodide redox flow batteries (ZIFB).

[0017] The method for preparing the low-cost ZIFB comprises:

• preparing an electrolyte solution by adding precursor for iodide, zinc salt, and potassium chloride (KC1) at a concentration of IM to 3M into a solvent;

• providing a composite membrane comprising a microporous membrane and a cation conducting membrane, with the microporous membrane placed towards the anode side and the cation conducting membrane facing the cathode;

• providing carbon-based electrodes as the positive and negative electrodes of the battery, with the composite membrane sandwiched between the positive and negative electrodes; and

• assembling the electrolyte, composite membrane, and positive and negative electrodes into the zinc-polyiodide flow battery.

[0018] In yet another aspect, the present disclosure pertains to application of the ZIFBs for large-scale energy storage applications, storing energy harvesting from solar, wind, and tidal energies. [0019] The present disclosure aims to address the issue of electrolyte shifting and to reduce the cost of ZIFBs. It involves introducing KC1 as an additive and composite membrane comprising a microporous membrane and a cation conducting membrane. The disclosure of the present application holds promise in improving cell voltage, coulombic and energy efficiency and reducing the overall cost of ZIFBs.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0020] In order to facilitate a comprehensive understanding and practical implementation of the disclosure, reference will now be made to exemplary embodiments illustrated in the accompanying figures. The figures together with detailed description below, are incorporated into and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where:

Fig. 1 (a) illustrates FT-IR spectra of various membranes dipped in different electrolytes, (b) illustrates magnified spectra of symmetric stretching of SCh-, (c) illustrates magnified spectra of bending of OH and (d) illustrates magnified spectra of stretching vibration of OH.

Fig- 2 illustrates (a) voltage vs time, (b) voltage vs capacity, (c) discharge capacity vs cycle number and (d) efficiency vs cycle number plot of ZIFBs at current density of 50 mA cm'² with ZIK2 electrolyte assembled with porous + thinner (20 pm) Nafion® combo membrane.

Fig- 3 illustrates (a) GCD at different SOC at 20 mA cm'² current density, (b) discharge capacity vs cycle number, (c) areal capacity of deposited zinc vs cycle number and (d) efficiency vs cycle number ZIFBs with 4 M KI+ 2 M ZnBn+l M KC1 electrolyte.

Fig. 4 illustrates SEM images of electrodeposited zinc with electrolyte containing (a) with KC1 and (b) without KC1. Optical image of anode side electrode of ZIFBs after 10 GCD cycles (c) with ZI electrolyte and (d) with ZIK2 electrolyte.

Fig- 5 illustrates cycling performance of ZIFBs with and without KC1 at current density of 20 mA cm'² (a) couloumbic efficiency vs. cycle number and (b) polarization curves and calculated power density curves of ZIFBs with different electrolytes assembled with 2 pieces of N-212 as membrane. Fig. 6 illustrates (a) GCD at current density of 20 mA cm "² and (b) efficiency plot with ZIK2 electrolyte assembled with Daramic membrane.

Fig- 7 illustrates (a) GCD at the current density of 20 mA cm "² and (b) efficiency plot with ZIK2 electrolyte assembled with combo membrane comprising of Daramic membrane + thinner Nafion membrane (20 pm).

DESCRIPTION OF THE INVENTION

[0021] The objective of the present disclosure is to address the cross-over issue in ZIFBs, leading to improved coulombic and energy efficiency, higher discharge voltage, and extended cycle life for ZIFBs. Additionally, it promises to alleviate capacity decay, enhance areal capacity, and reduce the overall cost of ZIFBs.

[0022] The present disclosure can be understood more readily by reference to the following description, taken in conjunction with the accompanying Figures and Examples, all of which form a part of this disclosure.

[0023] At the very outset of the detailed description, it may be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment, and without intending to imply any limitation on the scope of this invention. Accordingly, the description is to be understood as an exemplary embodiment and teaching of invention and not intended to be taken restrictively.

[0024] Before the present disclosure or methods of the present disclosure are described in greater detail, it is to be understood that the specific products, methods, processes, conditions or parameters, are not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

[0025] Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the methods. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the methods. Certain ranges are presented herein with numerical values being preceded by the term “about.” The term “about” is used herein to provide literal support for the exact number that it precedes, as well as a number that is near to or approximately the number that the term precedes. For example, "about" can mean within one or more standard deviations, or within ± 30%, 25%, 20%, 15%, 10% or 5% of the stated value.

[0026] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. For the purposes of the present invention, the following terms are defined below.

[0027] It is appreciated that certain features of the methods, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the methods, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. All combinations of the embodiments are specifically embraced by the present invention and are disclosed herein just as if each and every combination was individually and explicitly disclosed, to the extent that such combinations embrace operable processes and/or composites/scaffolds.

[0028] The articles "a" and "an" are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

[0029] As used herein, the term "comprises", "comprising", or “comprising of’ is generally used in the sense of include, that is to say permitting the presence of one or more features or components. The term "comprises", "comprising", or “comprising of’ when placed before the recitation of steps in a process or method means that the process or method encompasses one or more steps that are additional to those expressly recited, and that the additional one or more steps may be performed before, between, and/or after the recited steps. [0030] Reference throughout this specification to “certain embodiments”, “further embodiments”, “specific embodiments”, “further specific embodiment”, “one embodiment”, “a non-limiting embodiment”, “an exemplary embodiment”, “some instances”, or “further instances”, means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure.

[0031] As used herein, the terms ‘include’, ‘have’, ‘comprise’, ‘contain’ etc. or any form of said terms such as ‘having’, ‘including’, ‘containing’, ‘comprising’ or ‘comprises’ are inclusive and will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0032] The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illustrate the disclosure and does not pose a limitation on the scope of the disclosure otherwise claimed.

[0033] As used herein, the term “invention”, “present invention”, “disclosure” or “present disclosure” as used herein is a non-limiting term and is not intended to refer to any single embodiment of the particular invention but encompasses all possible embodiments as described in the specification.

[0034] The terms “process(es)” and “method(s)” are considered interchangeable within this disclosure.

[0035] In one aspect, the present disclosure provides ZIFBs comprising a low-cost electrolyte additive selected from potassium chloride (KC1) at a concentration of IM to 3M. Additionally, the invention employs a composite membrane comprising combination of a microporous membrane and a cation conducting membrane. The present disclosure aims to address the issue of electrolyte shifting and to reduce the cost of ZIFBs.

[0036] The zinc-polyiodide redox flow batteries of the present invention provide a method to elevate the cell voltage of ZIFBs and to address electrolyte transfer issues by the addition of IM to 3M concentrations of potassium chloride (KC1). This strategy facilitates the formation of heterogeneous polyhalides, such as BQ", BBr", etc. The ensuing redox reactions are outlined as follows:

At anode: Zn Zn²⁺ + 2e~

At cathode: where X- = I~, Br~, Cl~

[0037] KC1 is employed as an electrolyte additive in an amount of about IM to 3M (molarity), preferably 2M.

[0038] This innovative strategy of electrolyte optimization can effectively unlock additional battery capacity, improve cell voltage, and prolong cell life, contributing to the advancement of ZIFBs technology.

[0039] Although the addition of KC1 offers benefits in terms of increased ionic conductivity, electrolyte stabilization, and improved zinc deposition, to effectively use these benefits in improving the battery performance, it has to be combined with additional features of efficient ion transport and electrolyte distribution. Such additional features can reduce the formation of dendrites and enhance the cycle life of the battery. These additional benefits can be achieved using an appropriate membrane to separate the electrodes. The membrane while allowing selective ion transport between the anode and cathode compartments should also improve electrolyte flow and ion distribution across the electrodes.

[0040] The structure and the composition of the membrane is tailored to balance ionic conductivity with the need for electrolyte separation. This ensures that the ionic transport resistance is minimized, while simultaneously preventing electrolyte mixing that could otherwise interfere with the reactions occurring at the electrodes.

[0041] The present invention employs a composite membrane comprising a combination of a microporous membrane and a cation conducting membrane.

[0042] The microporous membrane can be selected from but are not limited to hydrocarbon based microporous membranes such as polypropylene, polyethylene, polystyrene, polyvinylidene fluoride, and the like; ceramic membranes such as zirconia, alumina, silica and the like. [0043] The cation conductive membranes may be selected from, but are not limited to perfluorosulfonic acid membrane (Nafion), polymer electrolyte membranes (PEMs), polyethylene oxide (PEO), sulfonated polymers such as sulfonated poly ether ether ketone (SPEEK), sulfonated polyvinylidene fluori de-hexafluor opropylene and the like.

[0044] In an embodiment, the composite membrane has a bilayered structure wherein the microporous membrane is placed towards the anode, and the cation conducting membrane is placed towards the cathode. The thickness of the microporous membrane ranges from about 10 pm to about 500 pm or from about 10 pm to about 300 pm and thickness of the cation conducting membrane ranges from about 20 pm to about 500 pm, or from about 20 pm to about 300 pm. The porosity of membrane ranges from about 20 to 100%, or from about 10 to 80 %, or from about 30 to 60 %.

[0045] In a specific embodiment, the composite membrane is combination of a hydrocarbon based microporous membrane having a thickness of about 10 pm to about 500 pm or from about 10 pm to about 300 pm and a perfluorosulfonic acid membrane having a thickness of about 20 pm to about 500 pm, or from about 20 pm to about 300 pm. The invention of the present disclosure aims to address the issue of electrolyte shifting and to reduce the cost of ZIFBs.

[0046] In a further specific embodiment, the composite membrane is combination of a polyethylene membrane (Daramic) having a thickness of about 10 pm to about 500 pm or from about 10 pm to about 300 pm and a Nafion membrane having a thickness of about about 20 pm to about 500 pm, or from about 20 pm to about 300 pm. The invention of the present disclosure aims to address the issue of electrolyte shifting and to reduce the cost of ZIFBs.

[0047] In preferred embodiment, the composite membrane is combination of a polyethylene membrane (Daramic) having a thickness of about 10 pm to about 500 pm or from about 10 pm to about 300 pm and a Nafion membrane having a thickness of about about 20 pm to about 500 pm, or from about 20 pm to about 300 pm; wherein the composite membrane has a bilayered structure and wherein the microporous membrane is placed towards the anode, and the cation conducting membrane is placed towards the cathode. [0048] In a specific embodiment, ZIFBs of the present application with enhanced cell voltage and cycle life of the present disclosure comprise:

• carbon based positive and negative electrodes;

• a composite membrane sandwiched between the positive and negative electrodes;

• electrolyte; and

• optionally a peristaltic pump to circulate the electrolytes, wherein, the electrolyte composition includes a zinc salt, a precursor for iodide ions, and KC1 as additive at a concentration of IM to 3M concentration; and wherein the membrane is a composite membrane composed of a microporous membrane and a cation conducting membrane.

[0049] The carbon-based electrode may be selected from graphite felt, carbon cloth, activated carbon, carbon paper etc. In a specific embodiment, the carbon-based electrode is graphite felt.

[0050] The electrolyte comprises a zinc salt such as zinc iodide, zinc sulfate or zinc bromide which participates in the electrochemical reaction at the anode. The electrolyte further comprises a precursor of iodide ions (T) to facilitate the redox reactions at the cathode. The zinc salts and iodide precursors are dissolved in an appropriate solvent such as water, or organic solvents such as acetonitrile, or propylene carbonate. The solvent must be capable of dissolving the zinc salts and iodide species, while also facilitating ionic conductivity. Additives such as KC1 (potassium chloride) or other salts are introduced into the electrolyte to improve ionic conductivity, electrolyte stability, or to control zinc deposition on the anode, preventing dendrite formation. The additive is added in an amount of IM to 3M (molarity).

[0051] The peristaltic pump helps circulate the electrolytes between the anode and cathode, ensuring that the electrolyte remains evenly distributed and that the electrochemical reactions can occur effectively.

[0052] A combination of a low-cost electrolyte additive and a composite membrane comprising a microporous membrane and a cation conducting membrane results in reduction in the cost associated with ZIFBs and allows flow of the electrolyte on both sides of the cell. Therefore, energy storage is not limited by the volume of the porous electrode. [0053] In a specific embodiment, ZIFBs of the present application with enhanced cell voltage and cycle life of the present disclosure comprise:

• carbon based positive and negative electrodes;

• electrolyte; and

• optionally a peristaltic pump to circulate the electrolytes, wherein, the electrolyte composition includes zinc bromide, KI, and KC1 as additive at a concentration of IM to 3M concentration; and wherein the membrane is a composite membrane composed of polyethylene membrane (Daramic) having a thickness of about 10 pm to about 500 pm or from about 10 pm to about 300 pm.

[0054] In another aspect, the present disclosure provides a method for preparing a ZIFB comprising:

• assembling the electrolyte, membrane, and positive and negative electrodes into the zinc-polyiodide flow battery.

[0055] In certain embodiments, the reagents and reaction parameters employed in the above method are the same as defined in the preceding embodiment s).

[0056] The composite membrane may be prepared using solution casting method, hot pressing methods or any other methods known in the art. When the membrane is a bilayer membrane, the membranes may be place one over the other. [0057] In a specific embodiment, the present disclosure provides a method for preparing a ZIFB comprising:

• preparing an electrolyte solution by adding potassium iodide (KI), zinc bromide (ZnBr), and potassium chloride (KC1) at a concentration of IM to 3M into a solvent;

• providing a composite membrane comprising a polyethylene membrane (DARAMIC) and a perfluorosulfonic acid membrane (NATION), with the microporous membrane placed towards the anode side and the cation conducting membrane facing the cathode;

• providing carbon-based electrodes as the positive and negative electrodes of the battery, with the membrane sandwiched between the positive and negative electrodes; and

[0058] The ZIFBs exhibit a coulombic efficiency (CE) nearing about 90% to 99%, a voltage efficiency (VE) of about about 90% to 99%, and an energy efficiency (EE) of about about 90% to 99%, at a current density of about 20 to 50 mA cm ² with the ZIK2 electrolyte and a composite membrane comprising hydrocarbon based microporous membrane at a thickness of 10 to 500 gm and thinner Nafion-XL having a thickness in the range of 20 to 500 gm.

[0059] The present invention focuses on enhancing cell voltage and cycle life in energy-dense zinc-polyiodide redox flow batteries through a low-cost electrolyte additive. The introduction of IM to 3M KC1 as an additive and the substitution of the thicker Nafion membrane with a combination of a hydrocarbon-based microporous membrane (DARAMIC) having a thickness of about 10 pm to about 500 pm and a Nafion membrane having a thickness of about 20 to 500 pm serves this purpose.

[0060] Nafion exhibits varying behaviour with different alkali metal ions. Considering the interaction of the Nafion membrane with different cations, the novel approach involves adding potassium chloride (KC1) to improve cell voltage and address electrolyte transfer issues, unlocking additional capacity, preventing water crossover, and extending the cell life. This invention in the present disclosure aims to optimise and incorporate KC1 as an additive to mitigate the electrolyte imbalance. Besides, a combination of the hydrocarbon based microporous membrane such as DARAMIC and thinner Nafion membranes are used to reduce the membrane cost associated with ZIFBs. [0061] In a specific embodiment, to reduce the cost associated with ZIFBs, the cell is assembled using a bilayer membrane comprising a low-cost porous DARAMIC membrane of the order of about 10 to about 500 pm and a thinner Nafion membrane of the order of about 20 pm to about 500 pm, or from about 20 pm to about 300 pm. The Daramic membrane is placed towards the zinc side and Nafion facing cathode to present any percolation of iodine related species. The incorporation of the Nafion membrane prevents self-discharge due to the crossover of iodide species.

[0062] The present disclosure is employed for large-scale energy storage applications, storing energy harvesting from solar, wind, and tidal energies.

[0063] The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other redox flow batteries (RFBs) such as Vanadium RFBs, Zinc-Bromine RFBs, iron chromium RFBs, Aluminum-vanadium, and other organic RFBs. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein.

[0064] The present disclosure is further described with reference to the following examples, which are only illustrative in nature and should not be construed to limit the scope of the present disclosure in any manner.

EXAMPLES

[0065] The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention in any way.

Comparative Example 1: Importance of KC1, and cell performance of with Nafion membrane.

Electrolyte preparation [0066] Electrolyte solutions were prepared by adding calculated amounts of potassium iodide (KI), zinc bromide (ZnBn), and different concentrations of potassium chloride KC1 (1 M, 2 M, and 3 M), as well as sodium chloride (NaCl), into deionized water. The same electrolyte solution was consistently applied to both the catholyte and anolyte sides of the cell. The various compositions of prepared electrolytes are shown in Table.1

Table.l Compositions of various electrolytes.

Membrane activation

[0067] The Nafion membrane is pretreated, following a well-known procedure in the literature, to increase the water uptake capacity of the membrane or to increase the number of water molecules per sulphonic group of the membrane. The membrane is first heated with 3 w/v % of EECh for 30 minutes at 80° C to eliminate the organic impurities. Then it was heated and treated with 3M H2SO4 acid for 30 minutes at 80° C. Then it was washed several times with water and dipped in the electrolytes (each one of those in Table 1) for 24 h to replace protons with K⁺, Na⁺ and Zn²⁺ ions.

Energy Dispersive X-ray Spectroscopy (EDS) analysis of membrane in different electrolytes

[0068] To evaluate the K⁺/Zn²⁺/Na⁺ ratio in membranes equilibrated with different ions, EDS is conducted, and the results are shown in Table 2. The data indicates a higher K⁺ content compared to Zn²⁺, attributed to Nafion's increased selectivity for K⁺ ions.

Table 2 EDS of various membranes. Atomic Weight (%)

ZIN 65.48 9.99 24.52

[0069] With an increase in KC1 concentration in the electrolyte, there is a corresponding increase in K⁺ content within the membrane. In the ZIK3 electrolyte scenario, nearly all H⁺ ions are substituted by K⁺ ions, emphasizing the membrane's strong affinity for potassium ions over other ions.

[0070] In electrolytes comprising K⁺, Na⁺, and Zn²⁺, the observed ratio stands as K⁺ > Na⁺ > Zn²⁺, underscoring the membrane's inclination and selectivity in favour of potassium, followed by sodium and zinc ions. It is well known that hydration shell of Zn²⁺ ions are bigger than K⁺ and Na⁺ ions therefore Zn²⁺ ion can carry large amount of water during transportation through the membrane. Since K⁺ ions are present in the electrolyte, preferentially K⁺ will move through the membrane and water transport can be minimised.

[0071] The interaction of different ions (Na+, K+, Zn2+) in electrolytes with the Nafion membrane follows the observed ratio as K+ > Na+ > Zn2+, underscoring the membrane's inclination and selectivity in favor of potassium, followed by sodium and zinc ions. Good discharge voltage and maximum power density confirm that IM to 3 M KC1, and preferably 2M is the optimized concentration for running the cell. Moreover, the cell achieves a substantial areal capacity of 100 mAh cm'² with ZIK2 electrolyte (2 M KI, 1 M ZnB , 2 M KC1).

FTIR analysis

[0072] To see the interaction of the Nafion membrane with different types of cations present in the electrolyte, FT-IR spectra of various membranes equilibrated with different ions (K⁺, Na⁺, Zn²⁺) were recorded over the range 500-4000 cm'¹. The spectra are shown in Fig.1(a). The strong absorption band around 1100-1200 cm'¹ is attributed to the overlapping of CF2 and CF3 stretching bands. The absorption band, around 1050-1065 cm'¹, originates from the symmetric stretching of SOs'. A more pronounced cation-sulfonate interaction induces polarization of the S-0 dipole, causing a positive shift in the vibration of SOs' with higher concentration of KC1. The increase in KC1 concentration is correlated with an increase in K⁺ content in the membrane, as confirmed by EDS, increasing the polarization of the S-0 dipole. Consequently, this leads to a higher vibration frequency of the SOs' group, as illustrated in Fig.1(b). Due to the lower surface charge density of K⁺ ions compared to Na⁺ ions, the polarization of the S-0 dipole is less evident in the case of K⁺ ions. Consequently, in the presence of ZIN electrolyte, the symmetric stretching of SOs' is identified at a higher frequency. Fig. 1 shows FT-IR spectra of various membranes dipped in different electrolytes, (b) Magnified spectra of symmetric stretching of SON, (c) Magnified spectra of bending of OH and (d) Magnified spectra of stretching vibration of OH.

[0073] Two distinct absorption peaks for the OH group are identified — one at a lower frequency (-1620 cm'¹) attributed to the bending of the OH group and the other at a higher frequency (3600-3300 cm'¹) to the stretching of the OH' group. The amplified spectra of both peaks are depicted in Fig. 1(c) and Fig. 1(d). The increase of KC1 concentration, which results in an increase in K⁺ content in the membrane, leads to a reduction in hydrogen bonding of the -OH group, consequently causing a rise in the stretching and bending frequency of the OH group. Additionally, due to the higher hydration energy of Na⁺ions compared to K⁺ ions, there is a greater possibility of hydrogen bonding in the ZIN electrolyte, leading to a lower stretching and bending frequency of the OH group compared to other electrolytes containing K⁺.

Conductivity of membrane

[0074] To measure the ionic conductivity of membranes, the membranes equilibrated with different ions was sandwiched between two graphite blocks of dimensions 7.5 x 7.5 cm and Electrochemical impedance spectroscopy (EIS) was recorded in the frequency range of 200 kHz to 1 Hz by applying an AC perturbation of 10 mV. The conductivity of membranes soaked in different electrolyte solutions is represented in Table 3. As K⁺ is preferred by the Nafion membrane, an elevation in the KC1 concentration in the electrolyte leads to an increase in K⁺ content within the membrane. Simultaneously, the water uptake of the membrane decreases in comparison to the neat hydrated Nafion membrane, resulting in a reduction in the membrane’s ionic conductivity.

[0075] In contrast to membranes exposed to KC1, those exposed to NaCl exhibit higher conductivity. This contrast arises from the higher hydration energy associated with Na⁺ ions compared to K⁺ ions.

Table. 3 Measured ionic conductivity of various membranes dipped in various electrolytes.

Electrolyte Membrane conductivity (mS cm'¹)

Cell performance

[0076] Galvanostatic charge-discharge (GCD) tests were carried out with a 25 cm² single ZIFBs flow cell. 2 pieces of Nafion® 212 membranes together were sandwiched between two graphite felt electrodes. Either 23 mL, 80 or 130 mL of electrolyte was circulated on either side of the cell by a peristaltic pump at 20 mL min'¹ flow rate. Charge discharge tests were carried out at 20, 40 and 50 mA cm'² current density upto varying charging capacity of 500, 1500, and 2500 mAh. When normalized with the cell electrode geometrical area of 25 cm², they correspond to 20, 60 and 100 mAh/cm².

[0077] Fig. 2(a) illustrates the voltage vs. time profile of ZIFBs utilizing ZIK2 electrolyte under a current density of 50 mA cm'², with a charging duration of 2 h. The cell achieves a substantial areal capacity of 100 mAh cm'², as depicted in Fig. 2(b). The discharge capacity vs. cycle number and efficiency plots are presented in Fig. 2(c) and 2(d) respectively. Notably, the cell exhibits a CE nearing 98%, a VE of 81%, and an EE of 80%. This membrane combination is highly effective for the cell's operation, with no observable capacity decay or electrolyte transfer.

[0078] To increase the volumetric capacity and to check the effectiveness of KC1 at higher concentrations of electroactive species, the ZIFBs is assembled with 4 M KI+ 2 M ZnBn+l M KC1 electrolyte. At high electrolyte concentration, ZIFBs exhibit the volumetric capacity of 52 Ah L'¹ with 73 % electrolyte utilisation at the current density of 20 mA cm'² as shown in Fig. 3 (a). Fig. 3 (b) and (c) represent discharge capacity vs cycle number and areal capacity of electrodeposited zinc vs cycle number. There is no capacity decay observed over 100 GCD cycles. ZIFBs with high electrolyte concentration demonstrate CE of 99.3 %, VE of 84% and EE of 84% as shown in Fig. 3(d).

[0079] To see the effect of KC1 on the morphology of zinc deposition, zinc was electrodeposited on graphite felt in cell mode during the first charging at the current density of 20 mA cm'² for 1 h from ZI and ZIK1 electrolytes. The areal capacity of deposited zinc was 20 mAh cm'². The SEM images of zinc deposited from ZI and ZIK1 electrolytes are shown in Fig. 4(a) and (b). In the ZIK1 electrolyte, the deposited zinc exhibits a compact and dense deposition, whereas, in the ZI electrolyte, an undesired fluffy morphology consisting of nanoflakes is evident which further confirms the importance of KC1 in smooth deposition of zinc.

[0080] Fig. 4(c) and (d) represent the optical image of anode side graphite felt after 10 GCD cycles (after discharge). The white precipitates formed due to side reaction with zinc. However, the extent of these side reactions appears to be notably reduced in the case of the ZIK2 electrolyte. This finding serves as further evidence supporting the efficacy of KC1 in promoting reversible plating and stripping of zinc, thereby minimizing undesired side reactions.

[0081] These studies reveal the importance of KC1 in addressing the cross-over issue. KC1 is not only able to stop electrolyte transfer completely but also helps to improve power density, discharge voltage and cycle life of ZIFBs. The strategy to utilize porous and thinner Nafion membrane definitely a good choice for sustainable and economically viable ZIFBs. Comparative Example 2: Cell performance of Daramic membrane and a combo membrane of Daramic + thinner Nafion membrane:

[0082] Galvanostatic charge-discharge (GCD) of ZIFBs was performed by using a 25 cm² cell. Daramic membrane and a combo membrane of Daramic + thinner Nafion membrane (20 m) were sandwiched between two graphite felts. On each side, 23 mL of ZIK2 electrolyte was circulated by a peristaltic pump at a 20 mL min'¹ flow rate. The cell was charged at 20 mA cm' ² current density for 1 h corresponding to the total capacity of 500 mAh.

[0083] The GCD curve of ZIFBs using a Daramic membrane is presented in Fig. 6(a). Due to its porous nature, the Daramic membrane permits the passage of all ions. During charging the pores of the membrane are filled by h' ions which can react with deposited zinc, leading to selfdischarge. As a result, the cell exhibits lower coulombic efficiency (CE) along with reduced voltage (VE) and energy efficiency (EE), as illustrated in Fig. 6(b).

[0084] To reduce self-discharge and enhance the CE of the cell, the Daramic membrane was replaced with a combo membrane comprising Daramic and thinner Nafion (20 pm). The GCD profile of ZIFBs with this combo membrane is shown in Fig. 7(a). This modification improves the electrochemical performance of ZIFBs, achieving CE, VE, and EE 99, 91 and 90 % respectively as illustrated in Fig. 7(b).

[0085] Moreover, the electrochemical performance of ZIFBs with the composite membrane is comparable to that of ZIFBs cell employing 2 pieces of Nafion-212 membranes as shown in Fig. 5(b). However, replacing the thicker Nafion membrane with a thinner alternative significantly lowers the overall cost of the ZIFB system while maintaining equivalent performance.

Claims

We Claim:

1. A zinc-polyiodide redox flow battery comprising potassium chloride (KC1) at a concentration of IM to 3M as an electrolyte additive, and a composite membrane comprising a combination of a microporous membrane and a cation conducting membrane.

2. The zinc-polyiodide redox flow battery as claimed in claim 1, wherein the hydrocarbon based microporous membrane has a thickness of about 10 pm to about 500 pm or from about 10 pm to about 300 pm and the perfluorosulfonic acid membrane has a thickness of about 20 pm to about 500 pm, or from about 20 pm to about 300 pm.

3. The zinc-polyiodide redox flow battery as claimed in claim 1, wherein the microporous membrane is a hydrocarbon based microporous membranes selected from polypropylene, polyethylene, polystyrene or ceramic membranes selected from zirconia, alumina, silica, and the cation conductive membrane is selected from perfluorosulfonic acid membrane (NATION), polymer electrolyte membranes (PEMs), polyethylene oxide (PEO), and sulfonated polymers such as sulfonated poly ether ether ketone (SPEEK).

4. The zinc-polyiodide redox flow battery as claimed in claim 1, wherein the composite membrane is a bilayer membrane with the microporous membrane placed towards the anode side and the cation conducting membrane facing the cathode.

5. The zinc-polyiodide redox flow battery as claimed in claim 1, wherein the microporous membrane is a polypropylene membrane (DARAMIC), and the cation conductive membrane is perfluorosulfonic acid membrane (NATION).

6. The zinc-polyiodide redox flow battery as claimed in claim 1 comprising:

• positive and negative electrodes;

• electrolyte; and

7. The zinc-polyiodide redox flow battery as claimed in claim 6, wherein the zinc salt is zinc bromide, the precursor of iodide is potassium iodide (KI), the microporous membrane is a polypropylene membrane (DARAMIC), and the cation conductive membrane is perfluorosulfonic acid membrane (NATION).

8. The zinc-polyiodide redox flow battery as claimed in claim 7, wherein the polypropylene membrane (DARAMIC) has a thickness of about 10 pm to about 500 pm or from about 10 pm to about 300 pm and the perfluorosulfonic acid membrane (NATION) has a thickness of about 20 pm to about 500 pm, or from about 20 pm to about 300 pm.

9. A method for preparing a ZITB comprising:

• preparing an electrolyte solution by adding precursor for iodide ions, zinc salt, and potassium chloride (KC1) at a concentration of IM to 3M into a solvent;

10. The method of claim 9, wherein the zinc salt is zinc bromide, the precursor of iodide is potassium iodide (KI), the microporous membrane is a polypropylene membrane (DARAMIC), and the cation conductive membrane is perfluorosulfonic acid membrane (NATION).