Bifunctional electrocatalyst for metal-air electrochemical device

Abstract

The present invention generally relates to electrochemical energy storage and battery technology. Specifically, the present invention relates to bifunctional electrocatalyst for metal-air electrochemical device. More particularly, the present invention relates to Nickel-Iron metal¬ organic framework (NF-MOF) as bifunctional electrocatalyst for zinc-air battery application. Further, the present invention relates to synthesis of Nickel-Iron metal-organic framework (NF- MOF) by refluxing method.

Description

[0001] BIFUNCTIONAL ELECTROCATALYST FOR METAL-AIR ELECTROCHEMICAL DEVICE

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention generally relates to electrochemical energy storage and battery technology. Specifically, the present invention relates to bifunctional electrocatalyst for metal-air electrochemical device. More particularly, the present invention relates to Nickel-Iron metalorganic framework (NF-MOF) as bifunctional electrocatalyst for metal-air battery application. Further, the present invention relates to synthesis of Nickel-Iron metal-organic framework (NF- MOF) by refluxing method.

[0004] BACKGROUND OF THE INVENTION

[0005] Energy storage devices play a crucial role in the modern world due to extensive use of electrically powered devices like mobile phones, laptops, electric vehicles, and various portable devices. Additionally, energy storage devices provide stability to electrical grids by balancing supply and demand of electricity. The need for energy storage devices is becoming increasingly evident as available lithium-ion technology has its own limitations, such as limited reservoirs and the complex process of lithium extraction. This has led to a growing interest in alternative energy storage technologies within the research community.

[0006] Amidst the exploration of alternative energy storage technologies, sodium has emerged as a promising candidate due to its abundance in nature and chemical similarities to lithium. In this context, primary zinc-air (ZAB) batteries have been commercially available for some time. However, there is a strong interest among researchers in developing secondary zinc-air batteries due to their distinct advantages over other metal-based batteries. Zinc-air battery technology offers benefits such as high energy density, environmental friendliness, and cost-effectiveness. Notably, zinc is the only metal that can be safely handled in open air and effectively operate batteries with aqueous electrolytes, making it a compelling choice for energy storage applications.

[0007] The secondary zinc-air batteries typically consist of zinc metal as the anode, an aqueous electrolyte, and air as the cathode. During a discharging process, zinc metal undergoes a reaction with an alkaline electrolyte to produce zinc oxide as the discharge product, while the air cathode reduces oxygen from air, combining it with water to form hydroxide ions. In a reverse process during charging, zinc oxide is converted back into metallic zinc, and hydroxide ions are oxidized to evolve oxygen. The efficiency and performance of the battery rely significantly on the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) that occur at the air electrode during discharging and charging processes.

[0008] However, zinc-air batteries face several challenges, including electrolyte leakage, limited cycle life, degradation of the air electrode, and overpotential, which can hinder their overall efficiency and performance. Researchers have been actively exploring various strategies to address these issues, such as air electrode engineering, electrolyte modifications, and the development of porous zinc anodes. Efforts to overcome these challenges are crucial for advancing the practicality and effectiveness of secondary zinc-air batteries in energy storage applications.

[0009] Precious noble metal-based electrocatalyst, Transition metals, Transition metal oxides, and their hydroxides have been widely used as air electrodes. Zhang, Yidan, et al., Energy & Fuels 34.8, 2020, 10170-10177, reported cerium-doped lanthanum manganese oxide as an ORR electrocatalyst with an onset potential of 0.92 V and fabricated ZAB with such an electrocatalyst was compared with ZAB fabricated with platinized carbon. In another work, Guo, Beibei, et al., Nano-Micro Letters 12, 2020, 1-13, synthesized nitrogen-doped carbon which shows a half-wave potential of 0.82 V. However, these literature lack bifunctionality of the electrocatalyst and synthesis methods provided are complex and time consuming.

[0010] Therefore, there is a need in the art to provide a novel electrocatalyst for bifunctional or dual activity with easy synthesis for overall ORR / OER activity, which is essential for metal-air battery.

[0011] OBJECTS OF THE INVENTION

[0012] An object of the present invention is to develop an electrocatalyst that exhibits bifunctional electrocatalytic activity for both ORR and OER, thereby improving the overall efficiency of zinc- air batteries.

[0013] Another object of the present invention is to synthesize a nickel-iron metal-organic framework (MOF), which can be easily and cost-effectively produced for commercial applications.

[0014] Another object of the present invention is to enhance stability and performance of zinc-air batteries by utilizing the synthesized electrocatalyst. Another object of the present invention is to provide a method for synthesis of the electrocatalyst that is efficient, scalable, and suitable for large-scale production in the industry.

[0015] SUMMARY OF THE INVENTION

[0016] The present invention generally relates to bifunctional electrocatalyst for metal-air electrochemical device. More particularly, the present invention relates to nickel-iron metal-organic framework (NF-MOF) as bifunctional electrocatalyst for zinc-air battery application. The present invention also relates to synthesis of Nickel-Iron metal-organic framework (NF-MOF) by refluxing method. The invention involves synthesis of a nickel-iron metal-organic framework grown in the pores of nitrogen doped carbon, which exhibits bifunctional electrocatalytic activity for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalyst aims to improve the efficiency and cost-effectiveness of zinc-air batteries by reducing overpotential and enhancing overall performance.

[0017] In an aspect, the present invention provides a bifunctional electrocatalyst for metal air battery, comprising: i. metal M 1 ; ii. metal M2; iii. metal-organic framework (MOF); and iv. nitrogen doped carbon, wherein the metal Ml and M2 are bonded to oxygen forming a metal organic framework.

[0018] In an embodiment, the nitrogen doped carbon of the bifunctional electrocatalyst is mesoporous.

[0019] In an embodiment, the bifunctional electrocatalyst comprises morphology of rods.

[0020] In an embodiment, the M 1 , M2 and metal-organic framework are grown inside or encapsulated in pores of the nitrogen doped carbon.

[0021] In an embodiment, the Ml metal is selected from nickel, cobalt, manganese, zirconium, zinc, and molybdenum.

[0022] In an embodiment, the M2 metal is selected from iron, cobalt, manganese, zirconium, zinc, and molybdenum.

[0023] In an embodiment, the bifunctional electrocatalyst comprises oxidation state(s) of the Ml and M2 metals in the range of +1 to +3. In an embodiment, the metal-organic framework comprises branches attached to the metals Ml and / or M2.

[0024] In another aspect, the present invention provides a process for synthesis of the bifunctional electrocatalyst, comprising: i. preparing nitrogen doped porous carbon (NPC) solution by mixing NPC in an organic solvent; ii. reacting Ml metal precursor and M2 metal precursor in presence of a linker and an organic solvent under stirring for a duration of 30 to 60 minutes to obtain a solution A; iii. mixing the NPC solution of step i) with solution A of step ii) to obtain a solution B; iv. adding a pH regulator to the solution B of step iii) to maintain pH of the solution in a range between 4 to 6, followed by stirring the mixture for time period in the range of 15-30 minutes to obtain a solution C; and v. heating and stirring the solution C of step iv) at temperature in the range of 150 to 200 °C under reflux for a duration of 45 to 90 minutes followed by cooling the mixture to a temperature of 35 °C, washing and drying to obtain the bifunctional electrocatalyst.

[0025] In an embodiment, the Ml metal precursor and M2 metal precursor are in a weight ratio in the range of 2:0.5 to 4: 1.5.

[0026] In various embodiments, Ml metal precursor and M2 metal precursor are in a ratio of 3: 1.

[0027] In an embodiment, the Ml metal precursor is selected from chloride, nitrate and acetate of metal selected from nickel, cobalt, manganese, zirconium, zinc, and molybdenum.

[0028] In an embodiment, the M2 metal precursor is selected from chloride, nitrate and acetate of metal selected from iron, cobalt, manganese, zirconium, zinc, and molybdenum.

[0029] In various embodiments, Ml metal precursor is nickel nitrate (hexahydrate) and M2 metal precursor is iron chloride (hexahydrate).

[0030] In certain embodiments, Ml metal precursor is selected from a group consisting of Nickel chloride, nickel nitrate, and nickel acetate etc.

[0031] In certain embodiments, M2 metal precursor is selected from a group consisting of Iron chloride, iron nitrate, and iron acetate etc.

[0032] In an embodiment, the linker is selected from a group consisting of 2-amino-terephthalic acid, 2- nitro-terephthalic acid, hydroxy-terephthalic acid and terephthalic acid. In an embodiment, the organic solvent used in step i) and ii) is selected from N, N- dimethylformamide .

[0033] In an embodiment, the pH regulator is tetrabutylammonium hydroxide (TBAOH), tetraethylammonium hydroxide (TEAOH), and tetramethylammonium hydroxide (TMAOH).

[0034] In another aspect, the present invention provides a metal air battery, comprising: a) the bifunctional electrocatalyst 1 coated onto a cathode; b) an anode; c) a current collector; d) an electrolyte; and e) a separator.

[0035] In an embodiment, the metal-air battery is selected from zinc-air battery, aluminum-air battery, iron-air battery, and magnesium-air battery.

[0036] In specific aspect, the present invention provides a bifunctional electrocatalyst including Ml -M2 metal-organic framework (M1-M2-M0F) grown or deposited or encapsulated in the pores of nitrogen doped carbon, wherein Ml is nickel and M2 is iron. The nickel-iron metal-organic framework grown in the pores of nitrogen doped carbon is termed as NF-MOF.

[0037] In various embodiments, carbon utilized for growing the bifunctional electrocatalyst is mesoporous.

[0038] In various embodiments, Ml and M2 are selected from a group consisting of cobalt, manganese, zirconium, zinc, and molybdenum etc.

[0039] In certain embodiments, the nickel-iron metal-organic framework grown in the pores of nitrogen doped carbon enhances the electron transfer hence the ORR and OER activity in zinc-air batteries. In various embodiments, the bifunctional electrocatalyst is synthesized by refluxing method with reduced reaction time.

[0040] In certain embodiments, the bifunctional electrocatalyst of the present invention has onset potential for OER of 1.57 V at 10 mAcm'2.

[0041] In various embodiments, the bifunctional electrocatalyst exhibit bifunctional catalytic activity with onset potential for OER of 1.63 V at 10 mAcm-2and half wave potential of 0.76 V for ORR.

[0042] In yet another specific aspect, the present invention provides a metal air battery comprising said bifunctional electrocatalyst coated onto the cathode, zinc foil as an anode, a current collector, KOH as electrolyte, and a separator. The metal air battery also includes positive case and negative case along with spacer and spring. The metal air battery is zinc-air battery.

[0043] In certain embodiments, the zinc-air battery fabricated using bifunctional electrocatalyst of the present invention shows stable performance up to 500 cycles.

[0044] In various embodiments, the said bifunctional electrocatalyst is also used for aluminum-air, iron- air, and magnesium-air batteries etc.

[0045] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

[0046] BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 illustrates schematic of process for synthesis of bifunctional electrocatalyst by refluxing method.

[0048] FIG. 2 illustrates a) FESEM of NF31, b) XPS of Ni2p in NF31, c) XPS of Fe2p in NF31, d) FESEM of NF31 @NPC, and e) Elemental Mapping of NF31 @NPC.

[0049] FIG. 3 illustrates a) Comparison of ORR activity of MOFs with Pt / C, b) Comparison of OER activity of MOFs with Ru02, and c) Tafel Plot.

[0050] FIG. 4 illustrates a) air battery setup made up of Teflon and silicon gaskets, b) fabricated zinc-air battery (side view), c) fabricated zinc-air battery (bottom view), and d) charge discharge curves at 10 mAcm'2.

[0051] FIG. 5 shows line drawing of zinc air battery of the present invention.

[0052] FIG. 6 shows NF31 rods embedded into porous nitrogen doped carbon structure.

[0053] FIG. 7 shows XRD pattern for NF-MOF and NF-MOF@NPC.

[0054] FIG. 8 shows FTIR spectra for NF-MOF and NF-MOF@NPC.

[0055] FIG. 9 shows raman spectra for NF-MOF and NF-MOF @NPC

[0056] DETAILED DESCRIPTION OF THE INVENTION

[0057] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims. Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”

[0058] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and / or” unless the content clearly dictates otherwise.

[0059] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be constructed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.

[0060] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it is individually recited herein.

[0061] All processes described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 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 illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any nonclaimed element essential to the practice of the invention.

[0062] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0063] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0064] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0065] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description that follows, and the embodiments described herein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.

[0066] It should also be appreciated that the present invention can be implemented in numerous ways, including as a system, a method or a device. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes. In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.

[0067] The terms “bifunctional electrocatalyst” or “NF-MOF” or “NF31-M0F” or “NF31 @NPC” are used herein interchangeably with same meaning throughout the specification.

[0068] The present invention generally relates bifunctional electrocatalyst for metal-air electrochemical device. More particularly, the present invention relates to nickel-iron metal-organic framework (NF-MOF) as bifunctional electrocatalyst for zinc-air battery application. The present invention also relates to synthesis of Nickel-Iron metal-organic framework (NF-MOF) by refluxing method. The invention involves synthesis of a nickel-iron metal-organic framework grown in the pores of nitrogen doped carbon, which exhibits bifunctional electrocatalytic activity for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalyst aims to improve the efficiency and cost-effectiveness of zinc-air batteries by reducing overpotential and enhancing overall performance.

[0069] In an aspect, the present invention provides a bifunctional electrocatalyst including Ml -M2 metalorganic framework (M1-M2-M0F) grown or deposited or encapsulated in the pores of nitrogen doped carbon, wherein Ml is nickel and M2 is iron. The nickel-iron metal-organic framework grown in the pores of nitrogen doped carbon is termed as NF-MOF.

[0070] The Ml and M2 metals are bonded to the oxygen of linker molecule forming a metal organic framework which provides additional stability to system in highly alkaline solution.

[0071] The MOF is grown inside the porous carbon which increases the accessible active sites. These features prevent the passivation of active sites and enhances the catalyst durability hence increases the performance of metal-air battery (e.g. zinc air battery).

[0072] In various embodiments, carbon utilized for growing the bifunctional electrocatalyst is mesoporous.

[0073] In various embodiments, Ml and M2 are selected from a group consisting of cobalt, manganese, zirconium, zinc, and molybdenum etc.

[0074] In certain embodiments, the nickel-iron metal-organic framework grown in the pores of nitrogen doped carbon enhances the electron transfer hence the ORR and OER activity in zinc-air batteries. In another aspect, the present invention relates to a process for synthesis of said bifunctional electrocatalyst, including the steps of: a) reacting M 1 metal precursor and M2 metal precursor in organic solvent under stirring for a duration of 30 to 60 minutes followed by addition of linker to obtain a solution; b) adding nitrogen doped porous carbon (NPC) to the solution of step a) to start nucleation inside the pores followed by adding precursors of NF31 and carrying out the reaction; c) adding a pH regulator to the solution of step b) to maintain pH of the reaction followed by stirring into oil bath at 170°C for a duration of 1 hour followed by naturally cooling; d) washing the solution of step c) with DI water and ethanol followed by drying at 80°C to obtain the bifunctional electrocatalyst (NF31-MOF).

[0075] In various embodiments, Ml metal precursor and M2 metal precursor are in a ratio of 3: 1.

[0076] In various embodiments, Ml metal precursor is nickel nitrate (hexahydrate) and M2 metal precursor is iron chloride (hexahydrate). In certain embodiments, Ml metal precursor is selected from a group consisting of Nickel chloride, nickel nitrate, and nickel acetate etc.

[0077] In certain embodiments, M2 metal precursor is selected from a group consisting of Iron chloride, iron nitrate, and iron acetate etc.

[0078] In various embodiments, the linker is selected from a group consisting of 2-amino-terephthalic acid, 2-nitro-terephthalic acid, hydroxy-terephthalic acid and terephthalic acid.

[0079] In an embodiment, the solvent is selected from N, N-Dimethylformamide.In certain embodiments, the pH regulator is tetrabutylammonium hydroxide (TBAOH).

[0080] In various embodiments, the bifunctional electrocatalyst is synthesized by refluxing method with reduced reaction time.

[0081] In certain embodiments, the bifunctional electrocatalyst of the present invention has onset potential for OER of 1.57 V at 10 mAcm'2.

[0082] In various embodiments, the bifunctional electrocatalyst exhibit bifunctional catalytic activity with onset potential for OER of 1.63 V at 10 mAcm-2and half wave potential of 0.76 V for ORR.

[0083] In yet another aspect, the present invention provides a metal air battery comprising said bifunctional electro catalysts coated onto the cathode, zinc foil as an anode, a current collector, KOH as electrolyte, and a separator. The metal air battery also includes positive case and negative case along with spacer and spring. The metal air battery is zinc-air battery.

[0084] In certain embodiments, the zinc-air battery fabricated using bifunctional electrocatalyst of the present invention shows stable performance up to 500 cycles.

[0085] In various embodiments, the said bifunctional electrocatalyst is also used for aluminum-air, iron- air, and magnesium-air batteries etc.

[0086] EXAMPLES

[0087] The present invention is further explained in the form of the following examples. However, it is to be understood that the following examples are merely illustrative and are not to be taken as limitations upon the scope of the invention.

[0088] Example 1: Material Synthesis

[0089] To synthesize nickel-iron metal-organic framework, nickel nitrate (hexahydrate) and iron chloride (hexahydrate) are taken as sources for metal ions, 2 amino terephthalic acid as a linker, and dissolved in DMF in the ratio of (Nickel: Iron-3: 1, 0.75 mmol: 0.25 mmol) and stirred for 30 min. TBAOH was added to maintain the pH of the reaction and again stirred for a few min. The obtained solution was transferred to a round bottom flask and kept in an oil bath at 170° C for 1 hour. After 1 hour mixture was allowed to cool naturally and washed with DI water and ethanol and dried at 80° C. The sample was named NF31.

[0090] To synthesise the bifunctional electrocatalyst, nitrogen doped porous carbon (NPC) was added before addition of NF31 precursors, so that to start nucleation inside the pores. After dispersing the NPC in DMF, the precursors of NF31 were added and procedure mentioned above was followed. Here N,N-Dimethylformamide (DMF) is used as solvent. The prepared material was named as NF31 @NPCand characterised further.

[0091] Example 2: Morphological Studies

[0092] Synthesized NF31 MOF shows the rod like morphology in the FESEM image shown in FIG. 2(a). Also, XPS analysis shows the presence of Ni2+& Fe2+ions which can be attributed to the bonding between metal and organic linker shown in FIG. 2(b) and FIG. 2(c). The presence of Fe3+suggests the creation of branches in a metal-organic framework. In table 1 , elemental composition of NF31 is shown and contains 2.8 % Nickel and 3.58 % iron which will create active sites for OER in the material.

[0093] Table 1

[0094] In case of NF31 @NPC, MOF rods can be seen to be grown in the porous carbon uniformly shown in FIG. 2(d) and FIG. 2(e) shows FESEM images of elemental mapping. Also, in table 2, the elemental composition for the NF31 @NPCshows a less decreased amount of nickel 0.35 % and iron 2.85 % due to the addition of NPC which enhances the percentage of carbon to 69.69 %.

[0095] Table 2

[0096] Additionally, The HRTEM image in FIG. 6 shows that the NF31 rods formed are embedded into porous nitrogen doped carbon structure.

[0097] Also, the comparative characteristic analytical data are provided in FIGs. 7 to 8, which confirms the formation of said electrocatalyst containing NF31 , NPC and MOF, wherein the FIG.7 shows XRD pattern for NF-MOF and NF-MOF@NPC; FIG. 8 shows FTIR spectra for NF-MOF and NF- MOF@NPC; and FIG. 9 shows raman spectra for NF-MOF and NF-MOF@NPC, which are self- explanatory and evident as shown in the FIGs.

[0098] Example 3: Electrochemical characterizations

[0099] All the electrochemical tests were done in 1 M KOH with three electrode system using rotating disk electrode (RDE) technique. Slurry was prepared using 5mg active material in 400 uL IPA and lOOuL of nafion (1%) solution was added as binder. LSV graphs were recorded for all samples to study ORR and OER activity shown in FIG. 3(a) and FIG. 3(b) respectively. FIG. 3(c) shows the Tafel plot which is plotted between over potentials with respect to the log of current density. Tafel slope is considered a measure of OER activity. The Tafel slope is a key parameter in electrochemistry, representing the slope of a Tafel plot. It's the change in overpotential (voltage) required to increase the current density by a factor of 10, typically expressed in mV / decade. A low Tafel slope suggests a more active catalyst or a faster reaction rate, as a smaller overpotential is needed to achieve a higher current. In present case, the tafel slope is lesser in case of the claimed electrocatalyst (126 mV / decade) than the standard Ru02 (189 mV / decade), refer, FIG. 3c.

[0100] The zinc-air battery was fabricated using the Teflon setup for ZAB shown in FIG. 4(a), 4(b) and 4(c) with zinc foil as an anode and KOH as electrolyte. In FIG. 4(d) ZAB performance of NF31 @NPChas been compared with ZAB performance of Pt / C+RuO2 and data for 500 cycles is provided where present study shows stable performance for more than 170 hours. Overall, NF31 shows good OER activity with onset potential of 1.57 V at 10 mAcm’2. NF31 @NPCshows bifunctional catalytic activity with 0.76 V half wave potential for ORR and 1.63 V onset potential for OER. NF31 @NPCshows stable ZAB performance up to 170 hrs.

[0101] ADVANTAGES OF THE INVENTION:

[0102] 1. The nickel-iron metal-organic framework (MOF) grown in the pores of carbon exhibits low overpotential for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), leading to improved efficiency and performance of zinc-air batteries.

[0103] 2. The electrocatalyst demonstrates bifunctional electrocatalytic activity, making it suitable for use in both the cathode and anode of zinc-air batteries, thereby simplifying the battery design and improving overall efficiency.

[0104] 3. The synthesis method for the electrocatalyst is described as easy and quick, utilizing relatively inexpensive materials such as nickel nitrate, iron chloride, and 2-amino terephthalic acid, leading to a cost-effective production process, making the technology more accessible for commercialization.

[0105] 4. The electrocatalyst shows stable performance in zinc-air batteries, indicating its potential for long-term use and durability in energy storage applications.

[0106] 5. The synthesis process provided by present invention is suitable for large-scale production, making it feasible for commercial applications and contributing to the scalability of zinc-air battery technology.

[0107] 6. The zinc air battery fabricated using said bifunctional electrocatalyst shows the superior stability of up to 170 hours.

Claims

We Claim:

1. A bifunctional electrocatalyst for metal air battery, comprising: i. metal M 1 ; ii. metal M2; iii. metal-organic framework MOF; and iv. nitrogen doped carbon, wherein the metal Ml and M2 are bonded to oxygen forming a metal organic framework.

2. The bifunctional electrocatalyst as claimed in claim 1 , wherein the nitrogen doped carbon of the bifunctional electrocatalyst is mesoporous; the bifunctional electrocatalyst comprises morphology of rods; and the Ml, M2 and metal-organic framework are grown inside or encapsulated in pores of the nitrogen doped carbon.

3. The bifunctional electrocatalyst as claimed in claim 1, wherein the Ml metal is selected from nickel, cobalt, manganese, zirconium, zinc, and molybdenum; and the M2 metal is selected from iron, cobalt, manganese, zirconium, zinc, and molybdenum.

4. The bifunctional electrocatalyst as claimed in claim 1, wherein the oxidation state of the Ml and M2 metals in the range of +1 to +3.

5. A process for synthesis of the bifunctional electrocatalyst as claimed in claim 1, comprising the steps of: i. preparing nitrogen doped porous carbon (NPC) solution by mixing NPC in an organic solvent; ii. reacting Ml metal precursor and M2 metal precursor in presence of a linker and an organic solvent under stirring for a duration of 30 to 60 minutes to obtain a solution A; iii. mixing the NPC solution of step i) with solution A of step ii) to obtain a solution B;iv. adding a pH regulator to the solution B of step iii) to maintain pH of the solution in a range between 4 to 6, followed by stirring the mixture for time period in the range of 15-30 minutes to obtain a solution C; and v. heating and stirring the solution C of step iv) at temperature in the range of 150 to 200 °C under reflux for a duration of 45 to 90 minutes followed by cooling the mixture to a temperature of 35 °C, washing and drying to obtain the bifunctional electrocatalyst.

6. The process as claimed in claim 5, wherein the Ml metal precursor and M2 metal precursor are in a weight ratio in the range of 2:0.5 to 4: 1.5.

7. The process as claimed in claim 5, wherein the Ml metal precursor is selected from chloride, nitrate and acetate of metal selected from nickel, cobalt, manganese, zirconium, zinc, and molybdenum; and the M2 metal precursor is selected from chloride, nitrate and acetate of metal selected from iron, cobalt, manganese, zirconium, zinc, and molybdenum.

8. The process as claimed in claim 5, wherein the linker is selected from a group consisting of 2-amino-terephthalic acid, 2-nitro- terephthalic acid, hydroxy-terephthalic acid and terephthalic acid; the organic solvent used in step i) and ii) is N, N-Dimethylformamide; and the pH regulator is tetrabutylammonium hydroxide (TBAOH), tetraethylammonium hydroxide (TEAOH), and tetramethylammonium hydroxide (TMAOH).

9. A metal air battery, comprising: a) the bifunctional electrocatalyst as claimed in claim 1 coated onto a cathode, b) an anode, c) a current collector, d) an electrolyte, and e) a separator.5 10. The metal air battery as claimed in claim 9, wherein the metal-air battery is selected from zinc- air battery, aluminum-air battery, iron-air battery, and magnesium-air battery.

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