WO2024236586A1

WO2024236586A1 - A separator electrode assembly with novel separator and a bifunctional catalyst material

Info

Publication number: WO2024236586A1
Application number: PCT/IN2023/050751
Authority: WO
Inventors: Ramaprabhu S; Anamika GHOSH; Sana Fathima T.K.
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2023-05-17
Filing date: 2023-08-04
Publication date: 2024-11-21

Abstract

The present invention provides a separator electrode assembly comprising a separator sandwiched between an anode and a cathode, wherein the separator is a cellulose based material or a synthetic polymer-based material. The separator electrode assembly electrolyzes the natural water, selected from tap water, ground water, or sea water. The assembly is a zero-gap, single flow cell membrane- less separator electrode assembly for a water electrolyzer. Further, the present invention provides a bifunctional catalyst material and method of preparing thereof. The bifunctional catalyst material is employed in Hydrogen Evolution Reaction (HER), Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR), Alkaline fuel cell, Battery, or Electrochemical sensor.

Description

A SEPARATOR ELECTRODE ASSEMBLY WITH NOVEL SEPARATOR AND A BIFUNCTIONAL CATALYST MATERIAL

FIELD OF THE INVENTION

The present invention relates to the field of electrochemistry. Particularly, the present invention relates to a separator electrode assembly (SEA) for water electrolyzer. Further, the present invention relates to a bifunctional catalyst material for cathode and anode of the separator electrode assembly and a method of preparation thereof.

BACKGROUND OF THE INVENTION

The expedited depletion of fossil fuels has urged the development of a sustainable strategy towards eco-friendly alternative- energy production. Hydrogen (H2) with its high gravimetric energy density (142 MJ/kg) and clean fuel has shown a great potential for storable and transportable fuel in future that can help combat climate change and reach zero-emission. Currently, much research is focused on the cost-effective production of H2. Water splitting driven by electricity or solar power is one of the most promising approaches for clean H2 production. Most state-of-the-art electrolyzers use expensive membranes/separators and ultrapure water for operation. There are four types of water electrolyzers: polymer electrolyte membrane water electrolyzer (PEMWE), alkaline water electrolyzer (AWE), anion exchange membrane water electrolyzer (AEMWE), and solid oxide water electrolyzer (SOWE). AWE is the most robust and commercially mature technology. It utilizes nickel-based electrodes separated by ZIRFON (Agfa, Belgium) and uses 30- 40 % KOH in deionized water (DIW) to produce hydrogen. Recently, instead of ZIRFON separator, anionic exchange membranes have also been explored. These separators/membranes are expensive and hence limit their large-scale application. On the other hand, PEMWE shows highly efficient production of high purity hydrogen (H2). However, it requires noble metal-based catalysts (such as Pt, Ru, and Ir, etc.) and costly Nafion membrane for their operation. The SOWE technology involves high temperatures for their operation and uses expensive ceramic membrane as the separator. Replacing these membranes or separators with an economical separator is the need of the hour. Additionally, water distribution issues may arise if pure or fresh water is used for large-scale electrolyzers. In all the above cases, fresh water is required to perform the electrolysis. The continuous consumption of fresh water for fuel production will cause strain on the limited water resources. Therefore, natural water resources such as seawater or groundwater based electrolyzers are rising demand.

Conventionally noble catalysts such as platinum (Pt), ruthenium oxide (Ru02), and iridium oxide (IrO2) are used in electrolysis; however, the high-cost factor and abundance still limit their application. There have been attempts in the prior art focused on designing robust and non-preci ous catalysts for water splitting.

Most of the reported catalysts used in water electrolysis are grown in situ on nickel foam commonly by the hydrothermal method. It is not feasible to design catalyst-grown metallic foam for large-scale applications. Besides, Nickel foam is also not cost-effective and suffers from chlorine corrosion.

The major challenges of sea water electrolysis observed in many prior arts arise from the impurities and multiple ions present in the water. For instance, seawater contains almost 0.5 M of sodium chloride in addition to several other ions and impurities such as bacteria and microbes. The high chlorine content leads to hypochlorite formation at the anode side and chlorine corrosion of the catalyst and metallic catalyst substrate due to continuous chlorine exposure. Several ion adsorptions on the cathode and anode surface further make both OER and HER sluggish. Such ion adsorption and blocking of active sites also occur in other natural water sources also. However, seawater is the most challenging medium and the most desirable due to its abundance in nature (96.5 % of Earth’s total water).

Therefore, to mitigate the above-stated challenges, the inventors of the present invention have developed a separator electrode assembly for electrolysis of water. Further, the inventors have also developed a novel cost-effective bifunctional catalyst material with enriched electrocatalytic active sites, which demonstrate superior OER and HER kinetics, and excellent stability over long hours.

OBJECT OF THE INVENTION An object of the present invention is to provide a separator electrode assembly for water electrolyzer comprising a separator sandwiched between an anode and a cathode, wherein the separator is a cellulose based material or a synthetic polymer-based material.

Another object of the present invention is to provide a bifunctional catalyst material.

Another object of the present invention is to provide a method of preparing the bifunctional catalyst material.

Another object of the present invention is to provide the anode and/or cathode by coating the bifunctional catalyst material on a carbon substrate.

SUMMARY OF THE INVENTION

The present invention provides a separator electrode assembly for a water electrolyzer, the assembly comprises a separator sandwiched between an anode and a cathode, wherein the separator is a cellulose based material or a synthetic polymer-based material. The separator electrode assembly electrolyzes the natural water, selected from tap water, ground water, or sea water. The assembly is a zero-gap, single flow cell membrane-less separator electrode assembly for the water electrolyzer.

Further, the present invention provides a bifunctional catalyst material and method of preparing thereof. The bifunctional catalyst material is employed in Hydrogen Evolution Reaction (HER), Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR), Alkaline fuel cell, Battery, or Electrochemical sensor.

Moreover, the present invention provides the anode and/or cathode by coating the bifunctional catalyst material on a carbon substrate.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is accompanied by following drawings, wherein:

Figure 1 represents the separator electrode assembly (SEA) for a water electrolyzer. Figure 2 represents (a) XRD and (b) SEM image of bifunctional catalyst material Ni@NFO/C.

Figure 3 represents (a) XRD and (b) SEM image of bifunctional catalyst material Ni-Co-CoO@C.

Figure 4 shows lab-scale testing of seawater electrolysis by the anode and cathode synthesized by bifunctional catalyst materials.

Figure 5 represents (a) Polarization curve, (b) stability plot of Ni@NFO/C|| Ni@NFO/C in seawater+1 M KOH.

Figure 6 represents (a) Polarization curve, (b) stability plot of Ni-Co-CoO@C ||Ni-Co-CoO@C in seawater+ 1 M KOH.

Figure 7 illustrates H2 produced over 120 days from the separator electrode assembly of active area 391 cm².

Figure 8 illustrates H2 produced over 120 days from the separator electrode assembly of active area 16 cm².

DETAILED DESCRIPTION OF THE INVENTION

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

Unless defined otherwise, technical, and scientific terms used herein have the same meaning as commonly understood by one of the ordinary skills in the art to which this invention belongs. Further, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular. As used herein, the terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises... a” does not, without more constraints, preclude the existence of other acts or additional acts.

As used herein, “Zero-gap” implies that the distance between the anode and cathode is near to zero. This minimizes the resistance of the overall cell significantly. The “Zero-gap configuration is achieved by sandwiching the cellulose based material or a synthetic polymer-based material between the bifunctional anode and cathode in a separator electrode assembly.

As used herein, “membrane-less” implies that no membrane (that allows the permeation of only OH' ions) is used. Instead, a separator is used, whose main function is to minimize or prevent the gas crossover.

As used herein “flow-cell” implies that the natural water is continuously circulated through the cathode side.

As used herein, “natural water” means tap water, ground water, or sea water.

As used herein “alkaline natural water” means natural water treated with KOH.

As used herein, the phrase “mesh” means film, sheet or layer that can be used alternatively.

As used herein, electrode means cathode and anode.

Any discussion of documents, methods, acts, materials, equipment, and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The present invention provides a simple, sustainable, and inexpensive separator electrode assembly for water electrolyzer. The separator electrode assembly is a zero-gap, single flow membrane-less separator electrode assembly. It electrolyzes natural water selected from tap water, ground water, or sea water. Oxygen production takes place at an anode and hydrogen production takes place at a cathode of the separator electrode assembly during water electrolysis.

In an embodiment, the present invention provides the use of the separator electrode assembly to prepare water electrolyzer.

In an embodiment, the present invention provides a simple, sustainable, and inexpensive alkaline natural water electrolyzer for hydrogen production which comprises the separator electrode assembly.

In an embodiment, the present invention provides the separator electrode assembly, the assembly comprises a separator sandwiched between an anode and a cathode, wherein the separator is a cellulose based material or a synthetic polymer-based material.

The cellulose-based material is coated with wax or silicone.

The cellulose-based material is butter sheet, or parchment paper.

The cellulose-based material is treated with a base, wherein the base is KOH or NaOH. The concentration of base is in range of 0.5 M to 6 M.

In an embodiment, the synthetic polymer-based material is selected from nylon cloth, polyether sulfone mesh, or polyphenylene sulphide mesh.

In a preferred embodiment, the present invention provides the separator electrode assembly, wherein the separator is a base treated butter sheet. The separator comprises one or more base treated butter sheet.

In an embodiment, the present invention provides a bifunctional catalyst material for anode and cathode.

In an embodiment, the present invention provides the anode and/or cathode, wherein the anode and/or cathode is formed by coating the bifunctional catalyst material on a carbon substrate. Any coating technique, as known to or appreciated by any person skilled in art may be used. Thus, by way of example, the coating is brush coating and spray coating. The carbon substrate is selected from carbon paper, graphite felt, carbon cloth, carbon mesh, microporous carbon coated carbon cloth or polytetrafluroethylene coated carbon cloth (gas diffusion layer).

In an embodiment, the present invention provides the use of the bifunctional catalyst material to prepare anode and/or cathode for the separator electrode assembly.

In an embodiment, the amount of the catalyst material to be applied on carbon substrate to prepare a cathode and anode is in the range of 1 to 3 mg/cm².

In an embodiment, the present invention provides the separator electrode assembly, wherein an active area of the separator electrode assembly which comprises the bifunctional catalyst material is from 1 cm² to 2000 cm².

In an embodiment, the present invention provides the bifunctional catalyst material in Hydrogen Evolution Reaction (HER), Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR), Alkaline fuel cell, Battery, or Electrochemical sensor.

In another embodiment, the present invention provides the bifunctional catalyst material which comprises a transition metal-metal oxide-carbon nanocomposite, wherein the transition metal is selected from nickel, iron, cobalt, manganese, molybdenum, copper, or combination thereof, and the transition metal oxide comprises oxide of one or more metals selected from nickel, iron, cobalt, manganese, molybdenum, or copper.

In an embodiment, the present invention provides the bifunctional catalyst material, wherein two or more phases are present in a single nanocomposite.

In an embodiment, the present invention provides the transition metal-metal oxide-carbon nanocomposite which is selected form nickel-nickel ferrite- carbon nanocomposite, cobalt-cobalt ferrite-carbon nanocomposite, manganese-manganese ferrite-carbon nanocomposite, coppercopper ferrite-carbon nanocomposite, nickel-cobalt-cobalt oxide-carbon nanocomposite, manganese-cobalt-cobalt oxide-carbon nanocomposite, copper-cobalt-cobalt oxide-carbon nanocomposite, or molybdenum-cobalt-cobalt oxide- carbon nanocomposite. In a preferred embodiment, the present invention pertains to nickel-nickel ferrite-carbon nanocomposite and nickel-cobalt-cobalt oxide-carbon nanocomposite.

In another embodiment, the present invention provides the method of preparing the bifunctional catalyst material, wherein the bifunctional catalyst material comprises a transition metal-metal oxide-carbon nanocomposite where the transition metal is selected from nickel, iron, cobalt, manganese, molybdenum, copper, or combination thereof, and the transition metal oxide comprises oxide of one or more metal selected from nickel, iron, cobalt, manganese, molybdenum, or copper.

In preferred embodiment, the present invention provides the method of preparing the bifunctional catalyst material, wherein the bifunctional catalyst material comprises a transition metal-metal oxide-carbon nanocomposite where the transition metal-metal oxide-carbon nanocomposite is selected form nickel-nickel ferrite-carbon nanocomposite, cobalt-cobalt ferrite-carbon nanocomposite, manganese-manganese ferrite-carbon nanocomposite, copper-copper ferritecarbon nanocomposite, nickel-cobalt-cobalt oxide-carbon nanocomposite, manganese-cobalt- cobalt oxide-carbon nanocomposite, copper-cobalt-cobalt oxide-carbon nanocomposite, or molybdenum-cobalt-cobalt oxide- carbon nanocomposite.

In a preferred embodiment, the present invention provides the method of preparing nickel-nickel ferrite-carbon nanocomposite and cobalt-cobalt ferrite-carbon nanocomposite.

In the present invention, the bifunctional catalyst material is prepared by two different and simple approaches: 1) one-step thermal exfoliation and 2) pyrolysis of zeolitic imidazolate framework.

In one embodiment, the bifunctional catalyst material is prepared by one-step thermal exfoliation route. The method of preparing bifunctional catalyst material comprising the steps of: a) mixing and grinding of nitrate salts of metals selected from nickel, iron, cobalt, manganese, molybdenum, cupper, or combination thereof with a carbon-based material to form a mixture, wherein the carbon-based material is selected from reduced graphene oxide, carbon nanotube, fullerene, or graphene; b) heating the mixture of step a) at a temperature range of 200°C-600 °C in inert atmosphere for a period of 1 h to 2 h to obtain the transition metal-metal oxide-carbon nanocomposite.

The weight ratio of the nitrate salts to the carbon-based material in step a) is in range of 0.7: 0.7: 1 to 2:2: 1.

In another embodiment, the bifunctional catalyst material is prepared by pyrolysis of zeolitic imidazolate framework. The method of preparing bifunctional catalyst material comprising the steps of: a) preparing an alcoholic solution of an imidazole compound having alcoholic concentration of 0.54 to 0.57 M. b) preparing an alcoholic solution of a nitrate or chloride salt of cobalt, and nitrate or chloride salt of metals selected from nickel, iron, manganese, molybdenum, copper, or combination thereof having alcoholic concentration of 27 mM to 38 mM; c) adding the alcoholic solution of step a) to the alcoholic solution of step b), followed by filtration and drying to obtain a zeolitic imidazolate framework; c) heating the zeolitic imidazolate framework at a temperature range of 400 °C to 700 °C in an inert atmosphere for a period of 0.5 h to 2 h to obtain the transition metal-metal oxidecarbon nanocomposite.

The molar ratio of the imidazole compound: nitrate or chloride salt of cobalt: nitrate or chloride salt of metals selected from nickel, iron, manganese, molybdenum, copper, or combination thereof in step b) is from 14:1: 1 to 20: 1:1.

The imidazole compound is 2-methylimidazole or 1 -methylimidazole.

In an embodiment, the nickel-nickel ferrite-carbon nanocomposite is synthesized through a one- step thermal exfoliation method and nickel-cobalt-cobalt oxide-carbon nanocomposite is synthesized by pyro lyzing nickel-exchanged zeolitic imidazolate framework (ZIF67) which is prepared by a simple coprecipitation method. Further, the anode and the cathode prepared by these bifunctional catalyst materials are utilized for seawater or ground water electrolysis. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronopotentiometry (CP) are performed to evaluate catalytic activity and stability of the separator electrode assembly.

Certain specific aspects and embodiments of the present application will be explained in greater detail with reference to the following examples, which are provided only for purposes of illustration and should not be construed as limiting the scope of the application in any manner. While particular aspects of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

EXAMPLES

The following examples are for the purpose of illustration of the invention and are not intended in any way to limit the scope of the invention.

Synthesis of bifunctional catalyst material

Example 1

Synthesis of nickel-nickel ferrite-carbon nanocomposite (Ni@NFO/C)

Nickel nitrate (Ni(NOs)3, 6H2O), Iron nitrate (Fe(NOs)3, 9H2O) and graphene oxide (GO) (1.44: 1.44: 1 weight ratio) are ground in a mortar pestle and then heated at 300 °C in a quartz boat placed inside the quartz tube for 2 h. The sample product is the nickel-nickel ferrite-carbon nanocomposite.

The nickel-nickel ferrite-carbon nanocomposite sample is characterized by using X-Ray diffraction (XRD) and scanning electron microscopy (SEM) as shown in Figure 2. The XRD confirms the Ni and NiFe2O4 phases in the nanocomposite structure. Accordingly, this bifunctional catalyst material consists of two phases - Ni and NiFe2O4 that are uniformly embedded in carbon matrix. The SEM morphology reveals a thin wrinkled sheet all over the sample.

Example 2 Synthesis of nickel-cobalt-cobalt oxide-carbon nanocomposite (Ni-Co-CoO@C)

Methanolic solution of 2-methylimidazole (0.563 M) is added to a methanolic solution of Co(NO₃)₃.6H₂O (0.036 M)and NI(NO₃)₃.6H₂O (0.036 M) to form Ni exchanged Zeolite imidazolate framework (ZIF67) which is obtained by filtration and drying of the precipitate. The molar ratios of 2-methylimidazole: cobalt nitrate: nickel nitrate in the final solution is 15.6: 1: 1. The obtained Ni exchanged Zeolite imidazolate framework (ZIF67) is then heated at 600 °C in a quartz boat placed inside a quartz tube in Argon atmosphere for 1 h. The resultant sample is nickel- cobalt-cobalt oxide-carbon nanocomposite.

The nickel-cobalt-cobalt oxide-carbon nanocomposite sample is characterized by using X-Ray diffraction (XRD) and scanning electron microscopy (SEM) as shown in Figure 3. The XRD confirms the Ni-Co, and CoO phases in the nanocomposite structure. The SEM image reveals a distorted, irregular cubic morphology. Accordingly, this bifunctional catalyst material consists of three phases - Nickel, cobalt, and cobalt oxide that are uniformly embedded in carbon matrix.

Preparation of electrode from bifunctional catalyst material

Example 3

Disperse 5 mg of the bifunctional catalyst material from Example 1 in 200pL of a suitable solvent (ethanol/isopropanol/N-methylpyrrolidone). Sonicate the dispersion for 30 minutes. Then, add 5 pL of 5 wt.% fluorinated polymer solution (Nafion™) as a binder to the dispersion. Sonicate the dispersion for 5 minutes. The prepared slurry is then coated on the carbon papers. Before coating, carbon papers are cleaned with deionized water. Dry the coated carbon paper at 60 °C for 3 h to obtain the bifunctional electrode. The loading of the catalyst on the carbon substrate is maintained at 2 mg/cm².

Example 4

The electrode is prepared with bifunctional catalyst material of Example 2 and remaining process is similar to Example 3. Preparation of Separator

Example 5

The separator is prepared by butter sheet/parchment paper/any other cellulose-based paper. This separator is soaked in 1 M KOH solution at room temperature for the period of 12 to 24 h and dried at 60 °C.

Water electrolyzer and production of hydrogen

The water electrolyzer is fabricated with the following components: a) two stainless steel (SS) endplates, b) two nickel/ copper current collectors, c) two graphite plates grooved with serpentine/ columnar flow field for water circulation and gas collection, d) separator electrode assembly 1) bifunctional catalyst material coated PTFE (10-30 wt.%) coated carbon cloth as cathode and anode, 2) KOH-treated butter sheets as separator sandwiched between anode and cathode.

A schematic illustration of the separator electrode assembly for water electrolyzer is shown in Figure 1.

Seawater is used in electrolysis to produce hydrogen. 1 M potassium hydroxide (KOH) solution of sea water is prepared in the experiment. Also termed as alkaline sea water. This 1 M KOH sea water solution is circulated through the cathode side at flow rate 0.2 mL/min to 5 mL/min. H2 and O2 are collected from the cathode side and anode side outlets grooved on the graphite plates, respectively. The following electrolytic conditions are included for electrolysis:

Current density: 10 mA/cm², Applied voltage: 2V, ambient pressure (1 atm), and temperature: 25±7°C.

Example 6

Separator electrode assembly is prepared where the two KOH treated butter sheets are sandwiched between the anode and cathode of Example 3.

Example 7 Separator electrode assembly is prepared where the two KOH treated butter sheets are sandwiched between the anode and cathode of Example 4.

Analysis

The performance of bifunctional catalyst material for cathode and anode of Examples 3 & 4 are tested. The cathode and anode are separated by a finite distance and are vertically immersed in the alkaline seawater as shown in Figure 4. This configuration is used to measure the fundamental electrochemical properties of the bifunctional catalyst material.

The polarization curves over a potential range of 0 V to 2.5 V are measured. As shown in Figure 5 (a), the bifunctional catalyst material of Example 1 shows an overall water splitting voltage of 1.73 V in IM KOH treated seawater at 26 °C with current density of 10 mA/cm² As shown in Figure 6 (a), the bifunctional catalyst material of Example 2 exhibits an overall water splitting voltage of 1.79 V in IM KOH treated seawater at 26 °C with current density of 20 mA/cm².

The stability of the bifunctional catalyst material for cathode and anode of Examples 3 & 4 are evaluated. Example 3 displays stability of 200 hrs. at current density 20 mA/cm² in IM KOH treated seawater as shown in Figure 5(b). No hypochlorite formation is observed. The durability test at 20 mA/cm² for 200 hrs. showed a negligible change in overpotential, confirming the excellent stability of the bifunctional catalyst material. Example 4 displays stability of 50 h at current density 20 mA/cm² in IM KOH treated seawater as shown in Figure 6 (b). No hypochlorite formation is observed.

The hypochlorite detection test is performed to check evaluation of chlorine in electrolysis of sea water. The hypochlorite detection test is done using a chlorine test kit (Wild). As the reagent is added to the chlorine-containing solution, the solution changes colour immediately. Depending on the colour, chlorine concentrations can be determined from the chart provided along with it. After the durability test, five drops of the reagent are added to the 5 mL electrolyte (seawater+lM KOH) solution, and colour change is monitored to check any hypochlorite formation after electrolysis. The chlorine test reveals that no hypochlorite is produced during the electrolysis of sea water, which depicts the selectivity of OER over chlorine evolution reaction (CER). The separator electrode assembly for water electrolyzer of two different dimensions are prepared a) 16 cm², and b) 391 cm² and hydrogen production rate is measured. The separator electrode assembly in Example 6 with dimension 391cm² produces hydrogen at a rate of 1 L/h at an operational voltage of 2 V at room temperature and the separator electrode assembly in Example 7 with dimension 16 cm² produces hydrogen at a rate of 200 mL/h at an operational voltage of 2 V at room temperature The fabricated separator electrode assembly in both cases show shelf-life stability of ~4 months. These results are illustrated in Figures 7 and 8, respectively. As can be seen from these figures, the hydrogen produced from the separator electrode assembly of different dimensions exhibited excellent reproducibility and stability.

The present invention provides the bifunctional catalyst materials, scalable and simple synthesis of the bifunctional catalyst materials, anode and cathode from them and the development of separator electrode assembly for the water electrolyzer for seawater, groundwater, or other natural water resources.

In the present invention, the cellulose based material with a thin coating of paraffin wax or silicone as a separator in the separator electrode assembly is cheaper than the commercially available ZIRFON (Agfa, Belgium) membrane or ion exchange membrane. Thus, the separator of the present invention drastically reduces the cost of the overall electrolyzer and can thus bring new advancements towards water electrolysis technology. Further, carbon-based support in anode and cathode helps to prevent corrosion. Thus, the present invention provides a simple, sustainable, and inexpensive alkaline natural water electrolyzer for hydrogen production which comprises cheaper separator electrode assembly of the invention.

The foregoing description of the various embodiments is provided to enable any person skilled in art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, and instead the claims should be accorded the widest scope consistent with the principles and novel features disclosed herein. While the invention has been described with reference to a preferred embodiment, it is apparent that variations and modifications will occur without departing the spirit and scope of the invention. It is therefore contemplated that the present disclosure covers any and all modifications, variations or equivalents that fall within the scope of the basic underlying principles disclosed above.

Advantages of the present invention

• Selectively produce oxygen at the anode over hypochlorite on electrolysis of sea water.

• Prevent corrosion of catalyst. • Cheaper separator for the electrode assembly

• Cost effective non-precious metal is used to prepare bifunctional catalyst material.

• Precursors are easily available and cost-effective.

• Easy and scalable synthesis, free from multiple and complex steps such as hydrothermal, solvothermal, electrodeposition etc. which are not feasible for large-scale applications.

Claims

We claim

I . A separator electrode assembly, the assembly comprises a separator sandwiched between an anode and a cathode, wherein the separator is a cellulose based material or a synthetic polymer-based material.

2. The assembly as claimed in claim 1 , wherein the cellulose-based material is coated with wax or a silicone.

3. The assembly as claimed in claim 1 to 2, wherein the cellulose-based material is butter sheet, or parchment paper.

4. The assembly as claimed in claim 1 to 3, wherein the cellulose-based material is treated with a base.

5. The assembly as claimed in claim 4, wherein the base is KOH or NaOH having a concentration in range of 0.5 M to 6 M.

6. The assembly as claimed in claim 1 , wherein the synthetic polymer-based material is selected from nylon cloth, polyether sulfone mesh, or polyphenylene sulphide mesh.

7. The assembly as claimed in claim 1 , wherein the anode and cathode are formed by coating a bifunctional catalyst material on a carbon substrate.

8. The assembly as claimed in claim 7, wherein the carbon substrate is selected from carbon paper, graphite felt, carbon cloth, carbon mesh, microporous carbon coated carbon cloth or polytetrafluroethylene coated carbon cloth (gas diffusion layer).

9. The assembly as claimed in claim 7, wherein the bifunctional catalyst material is a transition metal-metal oxide-carbon nanocomposite.

10. The assembly as claimed in claims 1 to 9, wherein the assembly electrolyzes natural water, selected from tap water, ground water, or sea water.

I I . The assembly as claimed in claims 1 to 10, wherein the assembly is a zero-gap, single flow cell membrane-less separator electrode assembly for a water electrolyzer.

12. A bifunctional catalyst material, the catalyst material comprises a transition metal-metal oxidecarbon composite, wherein the transition metal is selected from nickel, iron, cobalt, manganese, molybdenum, copper, or combination thereof, and the transition metal oxide comprises oxide of one or more metals selected from nickel, iron, cobalt, manganese, molybdenum, or copper.

13. The catalyst material as claimed in claim 12, wherein transition metal-metal oxide-carbon nanocomposite is selected from nickel-nickel ferrite-carbon nanocomposite, cobalt-cobalt ferrite-carbon nanocomposite, manganese-manganese ferrite-carbon nanocomposite, coppercopper ferrite-carbon nanocomposite, nickel-cobalt-cobalt oxide-carbon nanocomposite, manganese-cobalt-cobalt oxide-carbon nanocomposite, copper-cobalt-cobalt oxide-carbon nanocomposite, or molybdenum-cobalt-cobalt oxide- carbon nanocomposite.

14. The catalyst material as claimed in claims 12 to 13, wherein the catalyst material is employed in Hydrogen Evolution Reaction (HER), Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR), Alkaline fuel cell, Battery, or Electrochemical sensor.

15. A method of preparing bifunctional catalyst material as claimed in claim 12 comprising the steps of: a) mixing and grinding of nitrate salts of metals selected from nickel, iron, cobalt, manganese, molybdenum, cupper or combination thereof with a carbon-based material to form a mixture, wherein the carbon-based material is selected from reduced graphene oxide, carbon nanotube, fullerene, or graphene; b) heating the mixture of step a) at a temperature range of 200°C-600 °C in inert atmosphere for a period of 1 h to 2 h to obtain the transition metal-metal oxide-carbon nanocomposite.

16. The method as claimed in claim 15, wherein the weight ratio of the nitrate salts to the carbonbased material in step a) is in the range of 0.7: 0.7: 1 to 2:2: 1.

17. A method of preparing bifunctional catalyst material as claimed in claim 12 comprising the steps of: a) preparing an alcoholic solution of an imidazole compound having alcoholic concentration of 0.54 to 0.57 M. b) preparing an alcoholic solution of a nitrate or chloride salt of cobalt, and nitrate or chloride salt of metals selected from nickel, iron, manganese, molybdenum, copper, or combination thereof having alcoholic concentration of 27 mM to 38 mM; c) adding the alcoholic solution of step a) to the alcoholic solution of step b), followed by filtration and drying to obtain a zeolitic imidazolate framework; d) heating the zeolitic imidazolate framework at a temperature range of 400 °C to 700 °C in an inert atmosphere for a period of 0.5 h to 2 h to obtain the transition metal-metal oxidecarbon nanocomposite.

18. The method as claimed in claim 17, wherein the molar ratio of the imidazole compound: nitrate or chloride salt of cobalt: nitrate or chloride salt of metals selected from nickel, iron, manganese, molybdenum, copper or combination thereof in step b) is from 14: 1 : 1 to 20: 1 : 1.

19. The method as claimed in claim 17, wherein the imidazole compound is 2-methylimidazole or 1 -methylimidazole.

20. The assembly as claimed in claims 7 to 11, wherein the bifunctional catalyst material is the catalyst material as claimed in claims 12 to 13.

21. Use of the bifunctional catalyst material as claimed in claims 12 to 14 to prepare anode and/or cathode for the separator electrode assembly of claims 1 to 11.

22. Use of the separator electrode assembly as claimed in claims 1 to 11 to prepare water electrolyzer.