WO2025196817A1 - Enhanced oxygen electrochemistry in flexible rechargeable zinc-air batteries with f-doped catalyst and quasi-solid state membrane - Google Patents

Abstract

The present invention generally relates to a field of advanced energy storage systems. Specifically, the present invention relates to a development of a synergistic F-doped catalyst and quasi-solid state electrolyte membrane for flexible and rechargeable zinc-air batteries. More particularly, the present invention relates to a bifunctional electrocatalyst including f- doped cubic nickel cobalt oxide (NiCoO₂)-heteroatom doped mesoporous carbon composite and anion conducting polymer electrolyte membrane for zinc air battery application. Further, the resultant bifunctional electrocatalyst and quasi-solid state electrolyte membrane exhibit an enhanced bifunctional activity and significantly enhance the oxygen evolution reaction and/or hydrogen evolution reaction. In addition, considering the increasing interest in flexible electronics developing reliable anion exchange membranes for ZABs with ionic conductivity matching liquid electrolytes is also essential.

Description

ENHANCED OXYGEN ELECTROCHEMISTRY IN FLEXIBLE RECHARGEABLE ZINC-AIR BATTERIES WITH F-DOPED CATALYST AND QUASI-SOLID STATE

MEMBRANE

FIELD OF THE INVENTION

The present invention generally relates to a field of advanced energy storage systems. Specifically, the present invention relates to the development of a synergistic F-doped catalyst and quasi-solid state electrolyte membrane for flexible and rechargeable zinc-air batteries. More particularly, the present invention relates to a bifunctional electrocatalyst including f- doped cubic nickel cobalt oxide (NiCoO2)-heteroatom doped mesoporous carbon composite and anion conducting polymer electrolyte membrane for zinc-air battery application. Further, the resultant bifunctional electrocatalyst and quasi-solid state electrolyte membrane exhibit an enhanced bifunctional activity and significantly enhance the oxygen evolution reaction and/or hydrogen evolution reaction. In addition, considering the increasing interest in flexible electronics developing reliable anion exchange membranes for ZABs with ionic conductivity matching liquid electrolytes is also essential.

BACKGROUND OF THE INVENTION

Metal-air batteries, particularly zinc-air batteries (ZABs), have gained significant research interest due to the high theoretical energy density offered by zinc metal. The growing demand for flexible electronics has led to a focus on developing flexible devices, and ZABs have emerged as a suitable candidate for such applications. However, there are challenges in developing flexible and rechargeable ZABs (RZABs), specifically in terms of finding an electrocatalyst that exhibits improved activity for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Additionally, an appropriate anion-conducting electrolyte with good conductivity, low cost, and long life is needed.

Noble metals like platinum (Pt), ruthenium (Ru), and iridium (Ir) are often used as catalysts for oxygen reactions. However, these metals are expensive, non-durable in harsh conditions, and lack bifunctionality. To address these issues, metal-free heteroatom-doped carbon-based systems have been explored as ORR catalysts. The incorporation of transition metal oxides, hydroxides, chalcogenides, and phosphides (such as nickel, cobalt, iron, and manganese) into heteroatom-doped carbon can introduce multifunctional activity. Encasing these active catalysts in heteroatom-doped carbon enhances stability against active site corrosion and allows for performance modulation through synergistic effects.

Anion engineering is often used to tune the electronic structure and improve the activity of transition metal composites by increasing active sites and improving electrical, optical, and structural properties. Fluorine doping (F-doping) has been found to be advantageous in enhancing oxygen catalytic activity due to the unique properties incorporated into the system, such as high electronegativity, low polarizability, and the ability to replace oxygen with fluorine in the crystal lattice.

In addition to developing a bifunctional catalyst, designing an efficient anion-conducting electrolyte is crucial for flexible and rechargeable device fabrication. While synthetic polymer- based electrolytes have been extensively investigated, concerns about their environmental impact and non-biodegradability have led to an increased interest in natural polymer-based systems. Chitosan, a renewable polysaccharide, is a promising biomaterial due to its cationic nature, film-forming property, alkaline resistance, and functionality. Chitosan can be grafted with various monomers to form quatemised chitosan membranes, which can serve as well- structured anion exchange membranes capable of transporting hydroxyl anions.

Xu, Hui-Min, et al., Science China Materials 66.8 (2023), 2953-3003, “Noble metal-free N- doped carbon-based electrocatalysts for air electrode of rechargeable zinc-air battery” disclosed synthesis and characterization of N-doped carbon-based electrocatalyst for the air electrode of ZABs. The catalyst was prepared by a one-step pyrolysis of a metal-organic framework (MOF) precursor. The resulting catalyst exhibited a high surface area and a high nitrogen content, and it showed excellent electrocatalytic activity for the ORR in both acidic and alkaline media. However, they do not discuss about bifunctionality of the electrocatalyst that exhibits improved activity for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Wei, Yanan, et al., ACS applied materials & interfaces 10.35 (2018), 29593-29598, “Alkaline exchange polymer membrane electrolyte for high performance of allsolid-state electrochemical devices” disclosed composite membrane, where quaternary ammonium containing allyl monomer is not grafted but crosslinked directly using glutaraldehyde. However, they do not disclose grafting experiment but mixing, the interaction between chitosan and quaternary ammonium monomer is through glutaraldehyde. This can be called as interpenetrating networks, making process complicated. Therefore, there is a need to provide synergistic F-doped catalyst and quasi-solid state electrolyte membrane for flexible and rechargeable zinc-air batteries, thereby improving their performance and stability.

OBJECTIVES OF THE INVENTION

An objective of the present invention is to develop a bifunctional electrocatalyst that exhibits improved activity for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER).

Another objective of the present invention is to develop quaternised chitosan membranes as well-structured anion exchange membranes for improved anion conduction in the zinc-air battery.

Another objective of the present invention is to demonstrate the feasibility of combining F- doped metal oxides, heteroatom-doped carbon-based catalysts, and chitosan-based electrolytes to create a flexible and rechargeable zinc-air battery (RZAB) with enhanced performance, stability, and cost-effectiveness.

Yet another objective of the present invention is to explore fluorine doping (F-doping) as a means to enhance the oxygen catalytic activity of the electrocatalyst.

SUMMARY OF THE INVENTION

The present invention generally relates to a development of a synergistic F-doped catalyst and quasi-solid-state electrolyte membrane for flexible and rechargeable zinc-air batteries. More particularly, the present invention relates to a bifunctional electrocatalyst and anion conducting polymer electrolyte membrane for specifically zinc air battery application. The bifunctional electrocatalyst and quasi-solid state electrolyte membrane exhibit an enhanced bifunctional activity and significantly enhance the oxygen evolution reaction and/or hydrogen evolution reaction.

In an aspect, the present invention relates to a bifunctional electrocatalyst for zinc-air batteries, comprising: a) Ml-M2-bimetals in alloy and/or cubic oxide form; b) halogen; and c) heteroatom doped mesoporous carbon as support; wherein the Ml metal is nickel, and the M2 metal is cobalt, the halogen is doped into said Ml -M2- bimetals oxide, and the halogen doped Ml -M2- bimetals oxide is supported onto said support.

In an embodiment, the halogen is selected from but not limited to fluorine, chlorine and iodine.

In an embodiment, the bifunctional electrocatalyst is a composite material comprising multiple components either supported or doped onto or into one another.

In an embodiment, the heteroatom in said heteroatom doped mesoporous carbon is selected from nitrogen, phosphorous and sulphur.

In an embodiment, the bifunctional electrocatalyst comprises particles having granular morphology, with globule size ranging between 80 to 120 nm.

In an embodiment, the halogen doped Ml -M2- bimetals oxide is uniformly distributed over said support.

In an embodiment, the halogen doped Ml -M2- bimetals oxide is having particles with average size in the range of 2.2-2.7 nm.

In an embodiment, the bifunctional catalyst comprises NiCo metals in alloy form, and NiCoCh in cubic phase.

In an embodiment, the bifunctional catalyst comprises uniform distribution of the Ni, Co, and F, specifically over the particles of halogen doped Ml -M2- cubic bimetals oxide, along with the C, N, O, P, and S, which are distributed uniformly throughout the material.

In another aspect, the present invention relates to a process for preparation of the bifunctional electrocatalyst, including a step of: a) preparing a solution comprising Ml -metal precursor and M2-metal precursor in water, and NaBF4 as F-doping agent and a reducing agent; b) adding oxalic acid and a base to the solution of step a) followed by sonicating for 3-10 minutes to obtain a solution A; c) mixing isopropyl alcohol and pyrrole, followed by adding phytic acid (PA), and cooling to temperature in the range of 0-5 °C to obtain a solution B; d) mixing the solution A and the solution B to obtain a solution C, followed by adding ammonium persulfate (APS) solution into the solution C; e) allowing to complete the polymerization in the solution C obtained in step d) at temperature in the range of 25-40 °C for time period in the range of 2-4 hours to obtain a hydrogel; f) transforming the hydrogel of step e) into an aerogel by ice templating followed by freeze- drying or oven drying to obtain dry aerogel or xerogel without the need of removal of excess phytic acid and/or without the need of purification; g) mixing the dry aerogel or xerogel with N-Methyl pyrrolidone (NMP) followed by heating in a microwave for time period in the range of 4-10 minutes at power of 200-400 W to obtain a mixture; h) repeatedly rinsing the mixture with ethanol and water mixture followed by drying at temperature in the range of 60-80 °C for time period in the range of 16-20 h to obtain crude bifunctional electrocatalyst; and i) annealing the crude bifunctional electrocatalyst obtained in step h) at temperature in the range of 500-700 °C for time period in the range of 1-3 h at a ramping rate of 5 °C per minute to obtain the bifunctional electrocatalyst.

In an embodiment, the Ml -precursor is selected from but not limited to chloride of Ml metal, sulfate of Ml metal, acetate of Ml -metal and so on.

In an embodiment, the Ml metal is selected from nickel.

In an embodiment, the M2-precursor is selected from but not limited to chloride of M2 metal, sulfate of M2 metal, acetate of M2-metal and so on.

In an embodiment, the M2 metal is selected from cobalt.

In an embodiment, the ice templating is done by cooling the hydrogel in liquid nitrogen for time period in the range of 5-10 min. In an embodiment, the freeze-drying is done in a lyophilizer at pressure in the range of 70-90 bar for time period in the range of 8-10 h.

In an embodiment, the oven drying is done at temperature in the range of 75-80 °C for time period in the range of 16-20 hours.

In an embodiment, the reducing agent as well as the halogen doping agent is selected from but not limited to sodium tetrafluoroborate (NaBF4), Lithium tetrafluorob orate (LiBF4), and so on.

In an embodiment, the base is selected from but not limited to sodium hydroxide, potassium hydroxide, potassium carbonate and so on.

In various embodiments, the freeze drying done in step f) provides the aerogel or aerogel- derived carbon (ADC), which finally provides the bifunctional catalyst in step i) (termed as NCOF-ADC-600).

In various embodiments, the oven drying done in step f) provides the xerogel or xerogel- derived carbon (XDC), which finally provides the bifunctional catalyst in step i) (termed as NCOF-XDC-600).

In another aspect, the present invention provides a zinc air battery, comprising: a) the bifunctional electrocatalyst; and b) an anion exchange polymer electrolyte membrane comprising quaternary ammonium group onto the biodegradable chitosan membrane.

In an embodiment, the zinc air battery is selected from but not limited to rechargeable zinc-air battery, quasi-solid-state rechargeable zinc-air battery, flexible and rechargeable zinc-air battery, and a three-electrode half-cell setup.

In an embodiment, the rechargeable zinc-air battery comprises air cathode comprising the bifunctional electrocatalyst as a slurry onto a gas diffusion layer and zinc as anode.

In an embodiment, the air cathode is prepared by coating the slurry of the bifunctional electrocatalyst onto the gas diffusion layer followed by drying at temperature in the range of 50-70 °C in vacuum oven, wherein the slurry comprises dispersing the bifunctional catalyst in an isopropyl alcohol-water mixture with Fumion solution as a binder. In an embodiment, the quasi-solid-state rechargeable zinc-air battery comprises air cathode comprising the bifunctional electrocatalyst as a slurry onto a gas diffusion layer and zinc as anode.

In an embodiment, the flexible and rechargeable zinc-air battery comprises air cathode comprising the bifunctional catalyst-coated onto carbon cloth, and zinc as anode.

In an embodiment, the three-electrode half-cell setup for ORR and OER activity, comprising: a) electrolyte, b) reference electrode (RE), and c) the bifunctional catalyst-coated glassy carbon electrode as the working electrode (WE) and graphite rod as counter electrode (CE).

In an embodiment, the electrolyte is selected from potassium hydroxide, sodium hydroxide and potassium carbonate.

In an embodiment, the concentration of said electrolyte is in the range of 0.1-1 M.

In an embodiment, the reference electrode is selected from Hg/HgO electrode, Ag/Ag2O electrode, and so on.

In an embodiment, the anion exchange polymer electrolyte membrane exhibits superior ionic conductivity, mechanical stability, and electrolyte retention as a quasi-solid electrolyte.

In an embodiment, a process of preparation of said anion exchange polymer electrolyte membrane, comprises steps of: a) mixing chitosan and aqueous acetic acid solution followed by stirring at temperature in the range of 25-40 °C for time period in the range of 8-12 h to obtain a first solution; b) purging the first solution with inert gas for a time period in the range of 1-3 hours at temperature in the range of 25-35 °C to remove dissolved oxygen followed by adding potassium persulfate to the first solution at temperature in the range of 50-60 °C for time period in the range of 4-6 minutes to obtain a mixture; c) dropwise adding a 3-acrylamidopropyltrimethylammonium chloride (APTMAC) monomer in the mixture of step b) followed by continuing the reaction for a duration of 5-8 hours with stirring and at temperature in the range of 25-40 °C to obtain a crude graft polymer; d) recovering a pure graft polymer by precipitating the crude graft polymer in a solvent followed by filtering, and drying the precipitate in a vacuum oven at temperature in the range of 35-50 °C for time period in the range of 16-20 h; e) mixing the pure graft polymer in a water to obtain a homogeneous solution followed by casting the homogeneous solution onto a petri dish and allowed to dry to obtain a 1^st film; f) mixing glutaraldehyde solution into the homogeneous solution of the step e) under stirring at temperature in the range of 25-40 °C for time period in the range of 1.5-3 minutes followed by casting the mixture of glutaraldehyde solution into the homogeneous solution onto a petri dish and allowed to dry inside a fume hood to obtain a 2^nd film; and g) soaking the 1^st film and/or the 2^nd film in a solution comprising potassium hydroxide and electrolyte to obtain the anion exchange polymer electrolyte membrane.

In an embodiment, the 1^st film is designated as Chi-A-l:X film, wherein X corresponds to the amount of APTMAC grafted onto chitosan.

In an embodiment, the 2^nd film is designated as Chi-A-l:X-CL film.

In an embodiment, the electrolyte additive used in step g) is selected from but not limited to zinc acetate, zinc oxide (ZnO), and so on.

In an embodiment, the inert gas is selected from but not limited to nitrogen gas, helium gas and argon gas.

In an embodiment, the solvent is selected from but not limited to acetone, isopropyl alcohol, and so on.

In an embodiment, the electrolyte additive is selected from but not limited to zinc acetate, zinc oxide (ZnO), and so on.

In various embodiments, the delayed addition of APTMAC at step c) (rather than in step a) is intentionally followed to avoid possible homo polymerization of APTMAC.

In an embodiment, the APTMAC is grafted into the chitosan in said anion exchange polymer electrolyte membrane. In an embodiment, the anion exchange polymer electrolyte membrane comprises crosslinked network formation of APTMAC with chitosan.

In an embodiment, the anion exchange polymer electrolyte membrane is hydrophilic in nature than chitosan due to increased grafting density of APTMAC. crosslinked network formation. Chi-A-1:6-CL has the highest graft density of APTMAC; hence, it

In another aspect, the present invention relates to a battery or cell comprising the F-doped cubic nickel cobalt oxide (NiCoO2)-heteroatom doped mesoporous carbon composite as a bifunctional electrocatalyst and the anion exchange polymer electrolyte membrane, wherein the battery or cell exhibits an enhanced bifunctional activity and significantly enhances the oxygen evolution reaction or hydrogen evolution reaction.

In various embodiments, the fluorine-atom is doped into the metal oxide lattice of cubic nickel cobalt oxide (NiCoCh) and further composited with hetero atom doped carbon to obtain the desired catalyst processing bifunctional activity and hence a suitable candidate or rechargeable zinc-air battery applications.

In certain embodiments, the anion exchange polymer electrolyte membrane is foldable/flexible in nature.

In certain embodiments, the bifunctional electrocatalyst and the anion exchange polymer electrolyte membrane used together in the zinc-air battery provide an improved and superior cell/battery that has higher stability for up to 8.5 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a schematic representation of stages and processes involved in the synthesis of the NCOF-ADC-600 electrocatalyst, APTMAC -grafted chitosan membrane, along with the fabrication and demonstration of the final device (battery).

Figure 2 illustrates (a) FESEM images (b) TEM images, (c) histogram showing the particle size distribution of NCOF over ADC, (d) the HRTEM image recorded at the 5 nm scale depicting the lattice fringe pattern and d-spacing, (e) table showing atomic % of different elements in the material from EDAX and (f) the STEM-EDS elemental mapping images corresponding to the Co, Ni, F, C, N, O, P and S for NCOF- ADC-600 samples.

Figure 3 illustrates (a) XRD profiles recorded for NCOF-ADC-600, NCO-ADC-600, NCOF- 600, ADC-600, and the corresponding JCPDS files for the cubic NiCoCh and metallic Co and Ni, (b) the Raman spectra recorded for NCOF-ADC-600 and NCO-ADC-600, and the high- resolution deconvoluted XPS spectra of (c) C is, (d) O ls, and (e) F Is corresponding to NCOF- ADC-600.

Figure 4 illustrates (a) The comparative LSV profiles of NCOF-ADC-600 and the control samples recorded with respect to Pt/C at 1600 rpm of the working electrode, (b) the RRDE data of NCOF-ADC-600 showing the electron transfer number (n-value) and the percentage of the H2O2 generated during the ORR process, (c) the ADT data of NCOF-ADC-600 for ORR taken after the 5000 CV cycles, (d) The LSV profiles of NCOF-ADC-600, RuCh, and the control samples recorded in 1.0 M KOH solution at a scan rate of 10.0 mV s^-1, (e) the ADT data of NCOF-ADC-600 for OER (f) Comparative bifunctional LSV plots for Pt/C-RuO2, NCOF- ADC-600, NCO-ADC-600, XDC-600 and NCOF-600.

Figure 5 illustrates RZAB performance analysis of NCOF-ADC-600 in comparison with the state-of-the-art Pt/C-RuO2 as the cathode catalyst a) the current (I)-voltage (V) polarization plots recorded for the RZABs, b) the Nyquist plots obtained from the EIS analysis of the NCOF-ADC-1000 and Pt/C-RuCL cells at the OCV condition with the corresponding equivalent circuit model fit, c) the galvanostatic discharge capacities of the fabricated RZABs after normalizing with the consumed amount of zinc during the discharge process at 5 mA cm' ², d) the comparative galvanostatic charge-discharge plots recorded at 5 mA cm^-2 displaying voltage efficiency for NCOF-ADC-600 and Pt/C-RuCL systems.

Figure 6 illustrates (a) swelling studies performed by measuring the % of swelling as a function of the time when the membranes were soaked in 6 M KOH + 0.2 M Zn(OAc)2 electrolyte, (b) contact angle measurement on the various membranes, and (c) the FTIR data for the crosslinked and the various APTMAC grafted crosslinked chitosan membranes along with the pristine chitosan membrane.

Figure 7 illustrates (a) The plots representing the Zeta potential values measured on the various membranes, (b) the data showing viscosity as a function of the membrane composition, (c) the tensile strength measured for the various membranes, (d) the stress vs strain plots for the prepared membranes, (e) the XRD profiles of the membranes, and (f) the Arrhenius plots.

Figure 8 illustrates the QS-RZAB performance analysis of NCOF- DC-600 in comparison with the state-of-the-art Pt/C-RuCh as the cathode catalyst a) the current (I)-voltage (V) polarization plots recorded for the RZABs, b) the galvanostatic discharge plot at 5 mA cm'², c) the corresponding galvanostatic discharge capacities of the fabricated QS-RZABs after normalizing with the consumed amount of zinc during the discharge process at 5 mA cm'², d) the comparative galvanostatic charge-discharge plots recorded at 5 mA cm^-2 displaying voltage efficiency for NCOF- ADC-600 and Pt/C-RuO2 systems.

Figure 9 illustrates a performance analysis of the flexible, rechargeable ZAB (f-RZAB) with (a) the current (I) voltage-(V) polarisation plot recorded for f-RZAB, (b) the comparative galvanostatic charge-discharge plots recorded at 2 mA cm'², (c) the rate capability studies for f-RZAB recorded at the current densities of 1, 2, 3, 5 and 10 mA cm'², and (d) the photographic image showing the LED illuminated by two as fabricated f-RZAB devices connected in series, e) the galvanostatic charge-discharge curve recorded at 2 mAcm'², at various bending angles portraying the flexibility of the device.

DETAILED DESCRIPTION OF THE INVENTION

The terms “bifunctional electrocatalyst” or “electrocatalyst” or “NCOF-ADC-600” are used herein interchangeably with same meaning throughout the specification.

The terms “quasi-solid-state electrolyte membrane” or “anion exchange electrolyte membrane” or “electrolyte membrane” or “electrolyte” or “membrane” are used herein interchangeably with same meaning throughout the specification.

The term “aerogel” in the context is used to mention that the material obtained after slow drying is the hydrogel by freeze drying approach.

The term “xerogel” denotes the solid mass prepared by rapid oven drying approach.

The present invention generally relates to the development of a synergistic F-doped catalyst and quasi-solid-state electrolyte membrane for flexible and rechargeable zinc-air batteries. More particularly, the present invention relates to a bifunctional electrocatalyst and anion conducting polymer electrolyte membrane for specifically zinc air battery application. The bifunctional electrocatalyst exhibit an enhanced bifunctional activity and significantly enhance the oxygen reduction reaction and/or oxygen evolution reaction and the quasi-solid state electrolyte membrane exhibited superior ionic conductivity, mechanical stability and electrolyte uptake.

In an aspect, the present invention relates to a bifunctional electrocatalyst for rechargeable Quasi-solid state zinc-air batteries, wherein the bifunctional electrocatalyst includes f-doped cubic nickel cobalt oxide (NiCoChkheteroatom doped mesoporous carbon composite.

In another aspect, the present invention relates to a process for synthesis of the bifunctional electrocatalyst (NCOF-ADC-600) (figure 1), including a step of: a) preparing a solution by adding NiCh and C0CI2 in DI water and NaBF4 (1 M); b) adding oxalic acid and NaOH to the solution of step a) followed by sonicating for 5 minutes to obtain a solution A; c) mixing isopropyl alcohol and pyrrole, followed by adding phytic acid (PA), and cooling to 0-5 °C in an ice bath to obtain a solution B; d) mixing the solution A and the solution B to obtain a solution C, followed by adding ammonium persulfate (APS) in DI water to the solution C; e) allowing to complete the polymerization of pyrrole in the solution obtained at step d) for 3 hours to obtain a hydrogel; f) transforming the hydrogel into an aerogel (NCOF-PPy) by ice templating followed by freeze-drying (lyophilization) without removing the excess PA or employing any purification step; g) mixing the dry aerogel with N-Methyl pyrrolidone (NMP) followed by heating in a microwave for 5 minutes at 300 W to obtain a mixture; h) repeatedly rinsing the mixture with ethanol and water followed by drying at 70 °C in an oven; and i) annealing the sample obtained at step h) at 600 °C in an argon atmosphere for 2 h at a ramping rate of 5 °C per minute to obtain the NCOF-ADC-600. In various embodiment, the NCOF-XDC-600 sample is prepared using the same process as synthesizing the NCOF-ADC-600 and using regular oven drying instead of freeze-drying. The carbon that is directly acquired via freeze-drying and then annealing at 600 °C is termed aerogel-derived carbon (ADC-600), whereas the carbon that is obtained through conventional oven drying and further annealing at 600 °C is referred to as xerogel-derived carbon (XDC- 600).

In another aspect, the present invention relates to an anion exchange polymer electrolyte membrane grafted using the quaternary ammonium group onto the biodegradable chitosan membrane as quasi- solid- state electrolyte membrane, wherein the anion exchange polymer electrolyte membrane exhibit superior ionic conductivity, mechanical stability, and electrolyte retention as a quasi-solid electrolyte.

In yet another aspect, the present invention relates to a process for preparation of the anion exchange polymer electrolyte membrane (figure 1), including a step of: a) mixing chitosan and 1% acetic acid aqueous solution followed by stirring overnight for complete dissolution to obtain a first solution; b) purging the first solution with nitrogen for a duration of 1 hour at 30 °C to remove dissolved oxygen followed by adding K2S2O8 to the first solution at increased temperature of 56 °C; c) adding a predefined amount of APTMAC monomer after 5 minutes using a drop-wise method followed by continuing the reaction for a duration of 6 hours with stirring and nitrogen purging to obtain a graft polymer; d) recovering the graft polymer by precipitation in acetone followed by drying in a vacuum oven at 40 °C; e) mixing the recovered graft polymer and DI water to obtain a homogeneous solution followed by casting the homogeneous solution onto a teflon petri dish and allowed to dry inside a fume hood to obtain films, wherein the films are designated as Chi-A-l:X films, Where X corresponds to the amount of APTMAC grafted onto chitosan; f) mixing glutaraldehyde solution into the homogeneous solution of the step e) while stirring for 2 minutes followed by casting the mixture of glutaraldehyde solution into the homogeneous solution onto a teflon petri dish and allowed to dry inside a fume hood to obtain films, wherein the films are designated as Chi-A-l:X-CL films; and g) soaking the Chi-A-1 :X films and the Chi-A-1 :X-CL films in 6 M KOH + 0.2 M Zn(OAc)2 electrolyte to obtain anion conducting Chi-A-1 :X-CL membranes.

In various embodiments, the delayed addition of APTMAC at step c) is intentionally followed to avoid possible homo polymerization of APTMAC.

In various embodiments, the prepared grafted chitosan membranes are further soaked in 6 M KOH solution to make it OH- conductive membranes, hence making it useful as an anion- conducting polymer electrolyte membrane for zinc-air battery application.

In various embodiments, the grafting of 3-acrylamidopropyltrimethylammonium chloride (APTMAC) onto the chitosan chain is to create a quaternary ammonium group in the chitosan chain. In the subsequent step, the prepared dry membranes are soaked in 6 M KOH electrolyte, where the quarternary ammonium groups act as positively charged sites attracting the OH- thus invoking anion conductivity to the prepared membranes for application as a quasi- solid- state electrolyte for zinc-air battery application.

In another aspect, the present invention relates to a battery or cell (figure 1) comprising the F- doped cubic nickel cobalt oxide (NiCoO2)-heteroatom doped mesoporous carbon composite as bifunctional electrocatalyst and the anion exchange polymer electrolyte membrane, wherein the battery or cell exhibit an enhanced bifunctional activity and significantly enhances the oxygen evolution reaction or hydrogen evolution reaction.

In various embodiments, the fluorine-atom is doped into the metal oxide lattice of cubic nickel cobalt oxide (NiCoCh) and further composited with hetero atom doped carbon to obtain the desired catalyst processing bifunctional activity and hence a suitable candidate or rechargeable zinc-air battery applications.

In certain embodiments, the anion exchange polymer electrolyte membrane is foldable/flexible in nature.

In certain embodiments, the bifunctional electrocatalyst and the anion exchange polymer electrolyte membrane used together in the zinc-air battery provide an improved and superior cell/battery that has higher stability for up to 8.5 hours. EXAMPLES

C0CI2, NiCh, NaBF4, NaOH, and oxalic acid were purchased from Sigma Chemicals and N- Methyl pyrrolidone (NMP) from Thomas Baker. All the chemicals are used without further purification. Chitosan, 3-acrylamidopropyltrimethylammonium chloride (APTMAC), potassium persulphate, and glutaraldehyde were purchased from Sigma Aldrich and used as such without any purification. The commercial 40% Pt/C catalyst was purchased from Alfa- Aesar, and RuO2 was procured from Sigma Chemicals.

Example 1: Synthesis of NCOF-ADC-600:

NCOF-ADC-600 was prepared by taking 0.1 mmol NiCh and 0.1 mmol C0CI2 in 0.6 mL DI water and 0.4 mL NaBF4 (1 M). To this solution, 0.3 mmol of oxalic acid and 0.1 mL of NaOH were added and sonicated for 5 min. The above solution is labelled as Soln. A. Further, to about 0.125 mL isopropyl alcohol, 0.042 mL of pyrrole is added, followed by 0.092 mL of phytic acid (PA), and is cooled to 0-5 °C in an ice bath and labelled as Soln. B. Subsequently, the cooled solutions of A and B were mixed to obtain Soln. C. To this solution, a cooled solution of 0.135 g of ammonium persulfate (APS) in 0.250 mL DI water was added and kept for 3 h to complete the polymerization of pyrrole. The resulting hydrogel was then transformed into an aerogel (NCOF-PPy) by ice templating followed by freeze-drying (lyophilization) without removing the excess PA or employing any purification step. The dry aerogel was mixed with NMP and microwaved for five minutes at 300 W. The mixture was repeatedly rinsed with ethanol and water before being dried at 70 °C in an oven. Finally, the obtained sample was annealed at 600 °C in an argon atmosphere for 2 h at a ramping rate of 5 °C per min. to obtain NCOF-ADC-600. Similarly, the NCOF-XDC-600 sample was prepared using same procedure using regular oven drying instead of freeze-drying. The carbon that is directly acquired via freeze-drying and then annealing at 600 °C is termed aerogel-derived carbon (ADC-600), whereas the carbon is obtained through conventional oven drying and further annealing at 600 °C is referred to as xerogel-derived carbon (XDC-600).

Morphological analysis of the materials prepared with and without F-doping

The morphological analysis of the materials prepared with and without F-doping was performed by field emission scanning electron microscopy (FESEM). The FESEM image of NCOF-ADC-600 in Figure 2a clearly shows a granular morphology, with globule sizes ranging from 80 to 120 nm. The TEM image presented in Figure 2b validates the uniform distribution of NCOF over the carbon support. The HRTEM images indicate that the NCOF particles have an average size of around 2.5 nm, which is supported by the particle size histogram presented in Figure 2c. As shown in Figure 2d, the lattice fringe pattern from the HRTEM images with the d spacing values of 0.213 nm and 0.246 nm match with that of the (200) and (111) planes of the cubic NiCoCh and 0.206 nm corresponding to the (111) planes of the NiCo alloy, indicating that both NiCo and cubic NiCoCh coexist in the catalyst. A slight enlargement in the d-spacing has been observed in the lattice fringes in the case of NCOF compared to that of NiCoO2, implying the F-doping into the lattice. It is evident from the EDAX (Figure 2e) that nearly 0.46 % of the F is present in the NCOF-ADC-600 catalyst, which is attributed to its doping in the NCO lattice. The elemental mapping further confirms the uniform distribution of the Ni, Co, and F, specifically over the particles of NCOF, along with the C, N, O, P, and S, which are distributed uniformly throughout the material (Figure 2f).

XRD profiles, Raman spectra, and XPS profiles

Figure 3a depicts the XRD profiles recorded for NCOF-ADC-600, NCOF-600, ADC-600, NCOF, ADC-600, and the corresponding JCPDS files for the cubic NiCoO2 and metallic Co and Ni. The specific 29 values of NCOF-600 corroborate the formation of cubic NiCoO2, wherein the peaks at 36.6°, 42.7°, 61.8°, 74°, and 78.2° correspond to the (111), (220), (322), (431), and (522) planes of NiCoO2 (JCPDS No. 10-0188), respectively, and indicate that the F-doping does not affect the phase structure of NiCoO2. In addition to this, the three diffraction peaks at the 29 values of 44.3°, 51.7°, and 76.2° correspond to the (111), (200), and (220) planes of the fee (Face centered cubic crystal structure) NiCo alloy, which can be well indexed to the diffractions corresponding to the metallic Ni [JCPDS No. 04-0850] and Co [JCPDS No. 04-0806]. Thus, the XRD analysis of NCOF-600 confirms the coexistence of the NiCo alloy phase along with the NiCoCh cubic structure in the system, thereby substantiating the results obtained from the HRTEM analysis. The XRD spectra of ADC-600, on the other hand, show a broad peak at the 29 value of 20-30°, implying that the graphitic (002) plane of the carbon support shifted slightly to the left from the anticipated angle of 26°. This shift indicates a substantial amount of disordered carbon in the amorphous face. At the same time, the NCOF- ADC-600 spectra show all the peaks similar to that of NCOF-ADC-600, excluding the graphitic carbon peak corresponding to ADC-600, thus indicating that the composite is formed without causing any particular changes to the distinctive phases in their dispersed states. Finally, The XRD spectrum was obtained for NCO-ADC-600, and the corresponding XRD pattern is well in agreement with that of NCOF-ADC-600, mirroring all the peaks. This confirms that the doping of F in its substantially low amounts is not enough to provide a separate phase or distortion in the metal oxide lattice to make any evident characteristic in the XRD spectrum. The Raman spectra of NCOF-ADC-600 and NCO-ADC-600 shown in Figure 3b reveal the presence of the well-defined peaks at 1355 and 1575 cm ¹, corresponding to the disordered (D band) and graphitized (G band) regions in the carbon sample. The intensity ratios of the D and G bands (ID/IG ratio) for NCOF-ADC-600 and NCO-ADC-600 are 1.13 and 1.09, respectively. It is observed that during the F-doping onto the NiCoO2 lattice, a small amount of F-doping also happens onto the carbon support, which results in a higher ID/IG ratio in NCOF- ADC-1000 compared to NCO-ADC-600. This heteroatom doping onto the carbon enables the enhancement of the defect in the carbon, resulting in abundant active sites and hence enhanced oxygen catalytic activity. To further elucidate the chemical composition of the samples and to explore the change in the valance state with the F-doping, the X-ray photoelectron spectroscopic (XPS) analysis of the samples was performed. The deconvoluted C Is, O Is, and F Is spectra of NCOF-ADC-600 have been used to confirm the presence of F-doping. The deconvoluted Cis spectra of NCOF-ADC-600 (Figure 3c) show the peaks corresponding to the C-C/C=C bond (284.0 eV), C-O/C=O bond (285.0 eV) the COO bond (287.0 eV), 290.0 and 292.0 eV corresponding to the C-F and C-F2 bonds, thus confirming the doping of the F in the material. Furthermore, the deconvoluted O ls XPS spectra reveal that the metal-oxygen bond (NiCo-O), which is typically detected at 529.5 eV, is instead observed at 530.9 eV for NCOF-ADC-600 (Figure 3d). The presence of F-doping in NCOF-ADC-600 is further supported by the corresponding shift in the M-0 bond to the higher binding energy which can be attributed to the higher electronegativity of the F than O. Additionally, the two deconvoluted peaks appear in the O Is spectra of the materials corresponding to the chemisorbed and physisorbed OH. Further, the deconvoluted F Is spectra of NCOF-ADC-600 in Figure 3e show peaks corresponding to the semi-ionic C-F bond (688.5 eV) and ionic C-F bond (682.5 eV), confirming the successful incorporation of the F in the lattice.

Example 2: Synthesis of APTMAC-grafted Chitosan as solid electrolyte:

The graft copolymer was synthesized by grafting APTMAC onto the chitosan using potassium persulfate (K2S2O8) as a thermal initiator. An accurately weighed 2 g of chitosan was transferred to a double-jacketed reactor, placed on a magnetic stirrer, and the temperature was controlled with a circulating water bath. 1% acetic acid aqueous solution was added to this solution and stirred with a magnetic bar overnight for complete dissolution. After purging the solution with nitrogen for 1 h at 30 °C to remove dissolved oxygen, the temperature of the bath was increased to 56 °C, and 120 mg of K2S2O8 was added. After 5 min., an appropriate amount of APTMAC monomer was added drop-wise. This delayed addition was intentionally followed to avoid possible homo polymerization of APTMAC. The reaction was continued for 6 h with stirring and nitrogen purging, and the obtained graft copolymer was recovered by precipitation in acetone, followed by drying in a vacuum oven at 40 °C. Three graft polymers with different feed weight % of chitosan and APTMAC (Chitosan: APTMAC 1:2, 1:4 and 1:6 designated as Chi-A 1:2, Chi-A 1:4 and Chi-A 1:6) were synthesized, and the details are listed in Table 1.

Table 1: Volume of APTMAC for different feed weight ratios of chitosan

After precisely weighing 500 mg of the synthesized graft copolymer in a vial, 50 ml of DI water was added, and the solution was stirred till it yielded a homogeneous solution. The prepared solution was cast onto a Teflon Petri dish with a diameter of 8.7 cm and allowed to dty inside a fume hood. These films were carefully removed from the Petri dish and designated Chi-A- 1 :X films, Where X corresponds to the amount of APTMAC grafted onto chitosan. After precisely weighing 500 mg of the synthesized graft copolymer in a vial, 50 ml of DI water was added, and the solution was stirred till it yielded a homogeneous solution. After adding 50 pl of glutaraldehyde solution to the chitosan solution and stirring it for 2 min., the mixture was transferred to a Teflon Petri dish of 8.7 cm diameter and was allowed to dry inside a fume hood. These films were carefully removed from the Petri dish and designated Chi-A- 1 :X-CL films (CL stands for crosslinked). Further, the obtained films were soaked in 6 M KOH + 0.2 M Zn(OAc)2 electrolyte to obtain anion conducting Chi-A- LX-CL membranes.

Swelling studies, contact angle analysis and IR spectra of prepared polymer films:

The swelling studies of the prepared polymer films were conducted in 6 M KOH + 0.2 M Zn(OAc)2 electrolyte (Figure 6a). The swelling data profile in Figure 4a shows that all the films reach equilibrium swelling in 24 h. The Chi-1:2-CL film swells maximum, and Chi-A- 1:6-CL depicts minimum swelling. Generally, for any polyelectrolytes, it is observed that with the increase in the charge density, the hydrophilicity also increases, resulting in enhanced swelling. Also, the swelling is generally inversely proportional to the degree of crosslinking. During glutaraldehyde crosslinking in grafted chitosan membrane, in addition to the -NH2 group of chitosan, the amide group in the APTMAC moiety can also undergo a crosslinking reaction. Thus, the higher graft density of APTMAC may result in increased crosslinked network formation. Chi-A-1:6-CL has the highest graft density of APTMAC; hence, it attains relatively higher crosslinking density than the other graft copolymers in the study, resulting in a lower swelling profile, thus contrasting the general trend in the swelling profile. The contact angles measured for the different films are presented in Figure 6b, depicting 52.4°, 50.4°, 34.2°, and 25.1°, respectively, for the chitosan, Chi 1:2 CL, Chi 1:4 CL, and Chi 1:6 CL membranes. The study infers that the contact angle values of the modified chitosan films decrease with increased grafting density, corresponding to the increased hydrophilicity with increasing the charge density as expected in polyelectrolytes. The characteristic vibration of the quaternary ammonium group was observed at 1483 cm ¹ in FTIR spectra (Figure 6b). The FT-IR data points out that the band corresponding to the quaternary ammonium group is absent in the chitosan spectrum, whereas the intensity of this vibration increases from Chi 1:2 CL to Chi 1:6 CL. This confirms the grafting of APTMAC onto chitosan.

Zeta potential, viscosity, mechanical stability and XRD measurements:

To confirm the extent of APTMAC grafting onto the chitosan, the zeta potential measurements of the chitosan and the chitosan grafted copolymers films were conducted (Figure 7a). The obtained data clearly defined as grafting feed weight ratio increases from Chi- A- 1:2 to Chi-A- 1:6 the zeta potential values increase thus confirming that more APTMAC gets incorporated onto the chitosan which increases its cationic charge. The viscosity of chitosan and chitosan grafted with APTMAC were measured at 26 °C. The results in Figure 7b clearly show decreased viscosity with increasing grafting. This grafting might alter the mobility of the polymer and reduce the viscosity. Tensile properties of pristine, crosslinked chitosan and chitosan grafted copolymers were determined by using Universal Testing Machine (UTM) (Figure 7c and d). From the mechanical studies, it is clear that chitosan has a higher tensile strength in the case of both pristine and crosslinked films. With the increase in grafting, tensile strength decreased. However, with crosslinking, the tensile strength of the films has further increased. X-ray diffraction analysis of pristine chitosan and grafted chitosan copolymers (Chi, Chi-A-1:2, Chi-A-1:4, and Chi-A-1:6) were obtained from 5° to 40° (Figure7e). Reflections in the 5° to 15° range indicate crystalline peaks of pristine chitosan. With the increase in the grafting density of APTMAC, crystalline domains in chitosan have been reduced gradually. This is due to the increased repulsive electrostatic interaction between APTMAC units. The ionic conductivity of the prepared membranes is evaluated through-plane conductivity studies using electrochemical impedance spectroscopy (EIS). For this, the membranes, after soaking in 6 M KOH+ 0.2 M Zn(OAc)2, are placed between two stainless steel, and the Nyquist plots are obtained at different temperatures. The Chi-A-1:2-CL system has a higher conductivity of 4.71 x 10’² S cm ¹ owing to the enhanced swelling shown by the system. The ionic conductivity of the polymer electrolytes based on the Chi-A-1:2-CL membrane is superior to that of the other two at all temperatures, suggesting a faster ion transport for the Chi-A-1:2-CL membranes. Thus, considering the superior properties and performance of the Chi-A-1:2-CL membrane, it was considered for device-level studies.

Example 3: Bifunctional activity of the prepared catalyst (half-cell/three electrodes setup data):

The bifunctional activity of the prepared catalyst was analyzed electrochemically by evaluating its performance towards both ORR and OER. The ORR performance was evaluated using a standard three-electrode setup by employing 0.1 M KOH as the electrolyte for ORR and 1 M KOH as electrolyte for OER, Hg/HgO as the reference electrode (RE), and the catalyst-coated glassy carbon electrode as the working electrode (WE). At 1600 RPM of the WE in an oxygen atmosphere, the linear sweep voltammograms (LSVs) corresponding to ORR were acquired at a voltage scan rate of 10 mVs ¹. The NCOF-ADC-600 exhibits an onset potential of 0.96 V (40 mV lower than Pt/C catalyst) with the E1/2 and limiting current values of 0.83 V and 5 mA cm' ², respectively (Figure 4a). The heteroatom doped carbon control sample, ADC-600, which was made via the freeze-drying technique, exhibits comparatively good ORR activity, with an onset potential of 0.93 V, an E1/2 value of 0.78 V, and a limiting current of 4.5 mA cm'² However, in terms of the key performance metrics, NCOF-600, the F-doped metal oxide catalyst without carbon support, shows substantially lower ORR activity. Moreover, the control sample prepared without the F-doping, NCO-ADC-600, which displayed an onset potential of 0.84 V along with the E1/2 and the limiting current values of 0.68 V and 4.3 mA cm'², respectively. The amount of H2O2 generated and the number of electrons engaged (n) in the process were estimated by RRDE analysis (Figure 4b). With an estimated n value of 3.8, NCOF-ADC-600 is found to be following the ORR process dominated by the 4-electron reduction process usually displayed by the Pt/C catalyst. This has been further supported by the significantly low H2O2 yield of less than 5% estimated from the ring current for NCOF- ADC-600, excluding the possibility of any significant contribution from the parasitic 2-electron reduction pathway, which usually yields H2O2 as the major product. Further, the ORR stability profiling of NCOF-ADC-600 is done by subjecting the catalyst for 5000 CV cycles in the kinetic region at a scan rate of 100 mV s ¹ under oxygen-enriched conditions. The ADT is found to be reducing the E1/2 only by 26 mV in the case of NCOF-ADC-600 (Figures 4c).

Subsequent to the ORR performance mapping, OER experiments were conducted on NCOF- ADC-600 to elucidate its bifunctional activity and determine its suitability as an air electrode for RZABs. The CV and LSV plots were recorded in 1 M KOH solution using a 3-electrode setup consisting of Hg/HgO as the RE, a graphite rod as the CE, and the catalyst-coated glassy carbon as the WE. The LSV profiles presented for NCOF-ADC-600, along with the standard RUO₂ catalyst and the control samples presented in Figure 4d, reveal that NCOF-ADC-600 has the highest activity, exhibiting an overpotential of 340 mV at 20 mA cm^-2, which is comparable 337 mV displayed by RuO2 measured at the same current density. It should also be noted that NCO-ADC-600 (the catalyst without the F-doping) exhibited inferior performance with the overpotential values of 369 mV, at 20 mA cm'² The OER durability of the prepared catalyst was analyzed through ADT analysis, where the catalytic activity retention was monitored by cycling the catalyst in a corrosion-triggered condition at the kinetic region (Figure 4e). The NCOF-ADC-600 catalyst displayed only a difference of 20 mV shift in potential at 20 mA cm'² after ADT, thus revealing the better structural endurance and activity retention of the catalyst. Further, the bifunctional activity exhibited by the catalysts was evaluated from the difference in E1/2 and the potential at 20 mA cm'² (AE) for ORR and OER, respectively (Figure 4f). The calculations confirm that the NCOF-ADC-600 shows a AE value of 0.76 V, comparable to the slale-of-lhe-Av Pt/C-RuO2 catalyst pair (0.74 mV) and much lower than the other control samples.

Example 4: The rechargeable zinc air battery (RZAB) performance analysis:

Finally, a liquid-state RZAB battery was fabricated by using zinc foil (0.5 mm) as an anode, NCOF-ADC-600 coated over GDL or Pt/C-RuO2 coated over GDL as the cathode, and 6 M KOH + 0.2 M Zn (OAc)2 as the electrolyte. The catalyst slurry was prepared dispersing 5 mg of catalyst in an isopropyl alcohol- water mixture with 10% Fumion solution as a binder. The fabricated RZABs depicted an open circuit voltage (OCV) of 1.40 and 1.38 V and a peak power density of 105 and 96 mW cm'² for Pt/C-RuO2 and NCOF-ADC-600, respectively (Figure 5a). The fitted Nyquist plots (Figure 5b) display a slightly lower equivalent series resistance (Ro) and a comparable charge transfer resistance (RCT) for NCOF-ADC-600 with the Pt/C-RuCh, pointing out to the better interface formation and ion transport attributed through the F-doping in the metal oxide system. Upon discharging at a current density of 5 mA cm^-2 (Figure 5c), the RZABs displayed sharp discharge plateaus at 1.25 (Pt/C-RuCh) and 1.18 V (NCOF-ADC-600). The corresponding specific capacities are 690 and 741 mAh g'¹, respectively, for NCOF-ADC- 600 and Pt/C-RuCh. Finally, long-term charge-discharge cycling of the Pt/C-RuCh RZAB and NCOF-ADC-600 RZAB performed at a current density of 5 mA cm^-2 with 5 min charge and discharge clearly shows the improved cycling performance of the former for 90 h with a constant voltage window, while the latter cycled for about 70 h with a gradually increasing voltage gap (Figure 5d).

Example 7: Performance analysis of the quasi-solid-state rechargeable ZAB:

To demonstrate the grafted chitosan membrane’s capability as an anion exchange polymer electrolyte membrane, a quasi-solid-state rechargeable zinc-air battery (QS-RZAB) was fabricated with Chi-A-1 :2-CL membrane as a solid-state electrolyte, zinc powder as anode and GDL coated with Pt/C-RuCh and the presently prepared NCOF-ADC-600 as cathodes viz; QS- RZAB -Pt/C-RuCh (this battery is provided for comparison) and QS-RZAB-NCOF-ADC-600 (battery of the present invention). The polarization plot reveals that the QS-RZAB-NCOF- ADC-600 exhibited a comparable peak power density of 192 mW cm'² with that of QS-RZAB- Pt/C-RuCh of 207 mW cm'² (Figure 8a). The QS-RZABs were discharged at a current density of 5 mA cm'² (Figure 8b), and the RZABs displayed sharp discharge plateaus at 1.30 V (Pt/C- RuCh) and 1.23 V (NCOF-ADC-600) with a discharge time of 26 h and 25 h respectively. The corresponding specific capacities (Figure 8c) and energy densities calculated for QS-RZAB- Pt/C-RuCh were 710 mAh g'¹ and 923 Wh kg'^1, respectively, and for QS-RZAB-NCOF-ADC- 600 were 680 mAh g'¹ and 836.4 Wh kg'¹, respectively. A comparison of the fitted impedance spectra obtained at OCV condition for the QS-RZABs reveals a lower Ro as well as RCT values for QS-RZAB-NCOF-ADC-600, implying better interface formation incurred from the porous nature of the catalyst as well as lower charge transfer resistance due to the enhanced electronic conductivity attributed by F-doping (Figure 8d). The rate capability profile displayed in Figure 8e shows an excellent rate performance with a small voltage drop with increasing the current density from 5.0 to 30.0 mA cm'² with similar voltage retention when cycled back to 5.0 mA cm'². The long-time charge-discharge cycling of the above-fabricated QS-RZABs was conducted at 5 mA cm'² current density for 5 minutes of charge and discharge cycles. The results revealed that the QS-RZAB-NCOF-ADC-600 displayed constant cycling for about 19.4 h with initial and final discharge potential of 1.23 and 0.86 V, while QS-RZAB-Pt/C-RuCh displayed lower cycling time of 13.5 with initial and final discharge potential of 1.3 and 0.97

V (Figure 8f). The better cycling stability displayed by QS-RZAB-NCOF-ADC-600 over QS- RZAB-Pt/C-RuCh is due to the better stability of NCOF- ADC-600 for ORR as well as OER as evident from the ADT analysis. Further, the voltage efficiency was calculated for the QS- RZAB-NCOF -ADC-600 depicting an initial voltage efficiency of 70%, which was reduced to 42 % after cycling for 19.4 h with a slightly rising voltage gap. QS-RZAB-Pt/C-RuCF depicted a voltage efficiency of 73%, which is reduced to 60% after 10 h, with continuously drooping charge-discharge profiles.

Example 8: Performance analysis of the flexible and rechargeable ZAB:

Finally, a flexible and rechargeable ZAB (f-RZAB) of 2 x 4 cm^-2 area was demonstrated by combining the NCOF-ADC-600 and Chi-A-1:2-CL. Here, zinc foil (100 pm) is used as anode and the NCOF-ADC-600 coated over carbon cloth as cathode, and Chi-A-1:2-CL soaked in 6 M KOH + 0.2 M Zn(OAc)2 as quasi-solid state electrolyte. The fabricated flexible device displayed an OCV of 1.36 V. The obtained polarisation plot shows a peak power density of 46 mW cm^-2 (Figure 9a). The galv anostatic charge-discharge cycling profile at 5 mA cm^-2 showed stable cycling for 8.5 h, maintaining a constant voltage gap (Figure 9b). The rate capability analysis displayed an excellent rate performance with a low voltage drop with increasing current density from 1.0 to 10.0 mA cm^-2 with similar voltage retention when cycled back to 2.0 mA cm^-2 (Figure 9c). Finally, two fabricated f-RZAB devices were connected in series, and a realistic application-based demonstration was performed by illuminating an LED of 1.4

V (Figure 9d). Furthermore, the f-RZAB was cycled at a bending angle of 0°, 60°, 90° and 180° (Figure 9e). The device charge-discharge profiles showed an almost stable voltage gap value of 0.38 V (0°), 0.36 V (60°), 0.35 V (90°), and 0.32 V (180°).

The present invention involves the novel strategy for developing a fluorine-doped cubic nickel cobalt oxide (NiCoCE) composite with heteroatom-doped metal-free carbon (NCOF-ADC- 600) in combination with a quatemised chitosan membrane (Chi-A-1:2-CL) as the quasi-solid- state electrolyte for designing flexible and rechargeable zinc-air battery (f-RZAB). F-doping onto the metal oxide lattice of cubic nickel cobalt oxide enhances the electronic conductivity, thereby boosting oxygen electrocatalytic performance. The NCOF-ADC-600 catalyst thus exhibited impressive ORR as well as OER activity in the half-cell analysis, showing a marginal shift of 50 mV in the onset potential compared to the state-of-the-art- Pt/C catalyst in ORR and an overpotential of 340 mV in OER at 20 mA cm'². Besides, NCOF-ADC-600 also showed better durability in retaining the electrochemical activity under corrosion-triggered conditions than the state-of-the-art electrocatalysts. Additionally, quaternary ammonium-grafted chitosan membranes soaked in alkaline electrolytes exhibited good ionic conductivity and mechanical stability when used as quasi-solid-state membranes. Finally, the flexible device (f-RZAB) exhibited a superior peak power density of 46 mW cm'² and good cycling stability for 8.5 h. with superior flexibility and excellent rate capability. These results provide a new strategy for rationally designing an anion exchange membrane and electrocatalyst to develop high- performance, flexible, and rechargeable batteries.

ADVANTAGES OF THE INVENTION:

1. The electrocatalyst’s oxygen electrocatalytic activity is significantly enhanced by doping the metal oxide lattice with fluorine, offering a more cost-effective alternative to noble metal catalysts. The resulting electrocatalyst exhibits improved performance, leading to higher power output and longer battery life.

2. The development of a quasi-solid state electrolyte membrane addresses the issues associated with liquid electrolytes.

3. The grafting of the quaternary ammonium group onto the biodegradable chitosan membrane provides a safer and more stable electrolyte option. This quasi-solid-state membrane also simplifies battery design and operation, making it more practical for portable and wearable electronics.

4. By improving the electrocatalyst’ s performance and developing a more stable and safer electrolyte, the invention seeks to enhance the efficiency, reliability, and costeffectiveness of zinc-air batteries, making them a more viable power source for portable and wearable electronics.

5. The invention provides the bifunctional electrocatalyst that exhibits improved activity for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), enhancing battery performance, efficiency, and overall energy storage capabilities.

Claims

WE CLAIM:

1. A bifunctional electrocatalyst for zinc-air battery, comprising: a) Ml-M2-bimetals in alloy and/or cubic oxide form; b) halogen; and c) heteroatom doped mesoporous carbon as support; wherein the Ml metal is nickel, and the M2 metal is cobalt, the halogen is doped into said Ml-M2-bimetals oxide, and the halogen doped Ml-M2-bimetals oxide is supported onto said support.

2. The bifunctional electrocatalyst as claimed in claim 1, wherein the halogen is selected from fluorine, chlorine and iodine; the heteroatom in said heteroatom doped mesoporous carbon is selected from nitrogen, phosphorous and sulphur; and the bifunctional electrocatalyst comprises particles having granular morphology, with globule size ranging between 80 to 120 nm.

3. The bifunctional electrocatalyst as claimed in claim 1, wherein the halogen doped Ml- M2- bimetals oxide is uniformly distributed over said support, and is having particles with average size in the range of 2.2-2.7 nm.

4. A process for preparation of the bifunctional electrocatalyst as claimed in claim 1, comprising: a) preparing a solution comprising Ml -metal precursor and M2-metal precursor in water, and a reducing agent; b) adding oxalic acid and a base to the solution of step a) followed by sonicating for 3-10 minutes to obtain a solution A; c) mixing isopropyl alcohol and pyrrole, followed by adding phytic acid (PA), and cooling to temperature in the range of 0-5 °C to obtain a solution B; d) mixing the solution A and the solution B to obtain a solution C, followed by adding ammonium persulfate (APS) solution into the solution C; e) allowing to complete the polymerization in the solution C obtained in step d) at temperature in the range of 25-40 °C for time period in the range of 2-4 hours to obtain a hydrogel; f) transforming the hydrogel of step e) into an aerogel by ice templating followed by freeze-drying or oven drying to obtain dry aerogel or xerogel without the need of removal of excess phytic acid and/or without the need of purification; g) mixing the dry aerogel or xerogel with N-Methyl pyrrolidone (NMP) followed by heating in a microwave for time period in the range of 4-10 minutes at power of 200-400 W to obtain a mixture; h) repeatedly rinsing the mixture with ethanol and water mixture followed by drying at temperature in the range of 60-80 °C for time period in the range of 16-20 h to obtain crude bifunctional electrocatalyst; and i) annealing the crude bifunctional electrocatalyst obtained in step h) at temperature in the range of 500-700 °C for time period in the range of 1-3 h at a ramping rate of 5 °C per minute to obtain the bifunctional electrocatalyst.

5. The process as claimed in claim 4, wherein the Ml -precursor is selected from chloride of Ml metal, sulfate of Ml metal, and acetate of Ml -metal; the M2-precursor is selected from chloride of M2 metal, sulfate of M2 metal, and acetate of M2 -metal; the reducing agent is selected from sodium tetrafluoroborate (NaBF4), and lithium tetrafluoroborate (LiBF4); and the base is selected from sodium hydroxide, potassium hydroxide, and potassium carbonate.

6. The process as claimed in claim 4, wherein the ice templating is done by cooling the hydrogel in liquid nitrogen for time period in the range of 5-10 min; the freeze-drying is done in a lyophilizer at pressure in the range of 70-90 bar for time period in the range of 8-10 h; and the oven drying is done at temperature in the range of 75-80 °C for time period in the range of 16-20 hours.

7. A zinc air battery, comprising: a) the bifunctional electrocatalyst as claimed in claim 1 ; and b) an anion exchange polymer electrolyte membrane comprising quaternary ammonium group onto the biodegradable chitosan membrane.

8. The zinc air battery as claimed in claim 7, wherein the zinc air battery is selected from rechargeable zinc-air battery, quasi- solid- state rechargeable zinc-air battery, flexible and rechargeable zinc-air battery, and a three-electrode half-cell setup.

9. The zinc air battery as claimed in claim 7, wherein a process of preparation of said anion exchange polymer electrolyte membrane, comprises steps of: a) mixing chitosan and aqueous acetic acid solution followed by stirring at temperature in the range of 25-40 °C for time period in the range of 8-12 h to obtain a first solution; b) purging the first solution with inert gas for a time period in the range of 1-3 hours at temperature in the range of 25-35 °C to remove dissolved oxygen followed by adding potassium persulfate to the first solution at temperature in the range of 50- 60 °C for time period in the range of 4-6 minutes to obtain a mixture; c) dropwise adding a 3-acrylamidopropyltrimethylammonium chloride (APTMAC) monomer in the mixture of step b) followed by continuing the reaction for a duration of 5-8 hours with stirring and at temperature in the range of 25-40 °C to obtain a crude graft polymer; d) recovering a pure graft polymer by precipitating the crude graft polymer in a solvent followed by filtering, and drying the precipitate in a vacuum oven at temperature in the range of 35-50 °C for time period in the range of 16-20 h; e) mixing the pure graft polymer in a water to obtain a homogeneous solution followed by casting the homogeneous solution onto a petri dish and allowed to dry to obtain a 1^st film; f) mixing glutaraldehyde solution into the homogeneous solution of the step e) under stirring at temperature in the range of 25-40 °C for time period in the range of 1.5- 3 minutes followed by casting the mixture of glutaraldehyde solution into the homogeneous solution onto a petri dish and allowed to dry inside a fume hood to obtain a 2^nd film; and g) soaking the 1 ^st film and/or the 2^nd film in a solution comprising potassium hydroxide and electrolyte to obtain the anion exchange polymer electrolyte membrane.

10. The zinc air battery as claimed in claim 9, the electrolyte additive used in step g) is selected from zinc acetate, and zinc oxide; the inert gas is selected from nitrogen gas, helium gas and argon gas; and the solvent is selected from acetone, and iso propyl alcohol.