Description:FIELD OF THE INVENTION
The present invention pertains to an electrocatalyst. More specifically, the present invention pertains to an electrocatalyst comprising a molybdenum-based alluaudite framework material and a process for preparation of the molybdenum-based alluaudite framework material. The electrocatalyst comprising a molybdenum-based alluaudite framework material is used for oxygen evolution reaction and oxygen reduction reaction and further in zinc-air batteries.

BACKGROUND OF THE INVENTION
Photocatalytic water splitting to hydrogen and oxygen forms a key concept in renewable energy sector, which can be exploited for practical applications. However, their large-scale commercialization is hindered by sluggish kinetics of oxygen evolution reaction (OER). OER is a multi-step four-electron transfer process in an alkaline medium given by,

?4OH?^-=2H_2 O+4e^-+O_2 ; E=1.23 V vs R.H.E

Although precious metal oxides like RuO2 and IrO2 show excellent OER activity in both acidic and alkaline mediums, the limited abundance and high cost restrict their practical usage [Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J.; Chen, H. M. Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chem. Soc. Rev. 2017, 46 (2), 337]. It is ideal to develop efficient, durable, and economic alternatives. In this spirit, several materials based on 3d transition metals (Mn, Fe, Co, Ni) have been explored involving different oxides, hydroxides, phosphates, pyrophosphates, fluorophosphates, perovskites, chalcogenides and so on.

In parallel, oxygen reduction reaction (ORR) is pivotal for fuel cell devices. In ORR, the molecular oxygen is electrochemically reduced by four protons and electrons to form water accompanied by the generation of an electrical potential.18 At the anode of the fuel cell, hydrogen is oxidized to produce electrons and protons that are transferred to the cathode. The equation can be given by (in acidic medium):

H_2?2H^++2e^-

At the cathode, oxygen is reduced by a reaction with protons and electrons to produce water:

1/2 O_2+2H^++ 2e^-? H_2 O

Both the anode and the cathode consist of highly dispersed Pt-based nanoparticles on carbon-black to promote the hydrogen oxidation reaction (HOR) as well as the oxygen reduction reaction. While the kinetics of HOR is extremely fast at the anode side, the reaction rate of ORR at the cathode side is the extremely slow which requires a higher Pt-loading to achieve a desirable cell performance [Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent advances in electrocatalysts for oxygen reduction reaction. Chem. Rev. 2016, 116 (6), 3594]. But Pt is rare metal and expensive, which needs low cost and efficient alternatives. Like the oxygen evolution reaction (OER), 3d transition metal-based compounds have been widely explored for oxygen reduction reaction. Some such compounds can act as bifunctional electrocatalysts showing both ORR and OER activity [Zhan, Y.; Lu, M.; Yang, S.; Xu, C.; Liu, Z.; Lee, J. Y. Activity of Transition-Metal (Manganese, Iron, Cobalt, and Nickel) Phosphates for Oxygen Electrocatalysis in Alkaline Solution. ChemCatChem 2016, 8 (2), 372]. However, the overpotential related to the benchmark electrocatalysts like RuO2 and Pt/C is very high. Also, they are based on precious metals having high cost. Therefore, there is a constant search of new electrocatalysts which can be perform closely to the benchmarks catalysts but are also economic.

Alluaudites or alluaudite structured materials, named after its discoverer François Alluaud II, are group of alkali metal phosphates based naturally occurring minerals having open frameworks. [Fisher, D. J. Alluaudite. Am. Min. 1955, 40 (11-12), 1100]. The general formula can be written as A(1)A(2)M(1)M(2)2(XO4)3, where A denotes the alkali ions, M denotes the transition metal ions, and X could be S, P, As, V, Mo, or W [Dwibedi, D.; Barpanda, P.; Yamada, A. Alluaudite Battery Cathodes. Small Methods 2020, 4 (7), 2000051]. Generally, the A and M(1) sites are occupied by monovalent (Na+, Li+) or divalent cations (Ca2+, Mg2+) while the M(2) site can be filled with divalent (Mn2+, Fe2+) or trivalent cations (Mn3+, Fe3+, In3+). The MO6 octahedra and XO4 tetrahedra are connected to form two tunnels along the c direction within the structure. The alluaudites are mainly known for their excellent performance in battery application due to the presence of alkali ions at the A sites which is present in the tunnels making easier for (de)intercalation without changing the structure.

Further, transition-metal oxide and phosphate materials, commonly used for lithium battery devices, are active as oxygen evolution reaction (OER) catalysts under alkaline and neutral solution conditions. [Lee, S. W.; Carlton, C.; Risch, M.; Surendranath, Y.; Chen, S.; Furutsuki, S.; Yamada, A.; Nocera, D. G.; Shao-Horn, Y. The nature of lithium battery materials under oxygen evolution reaction conditions. J. Am. Chem. Soc. 2012, 134 (41), 16959]. Though molybdenum (Mo)-based alluaudite framework material has been explored as sodium battery insertion material [Barman, P.; Jha, P. K.; Chaupatnaik, A.; Jayanthi, K.; Rao, R. P.; Sai Gautam, G.; Franger, S.; Navrotsky, A.; Barpanda, P. A new high voltage alluaudite sodium battery insertion material. Mater. Today Chem. 2023, 27, 101316], the electrocatalytic activity of the molybdenum-based alluaudite framework materials has not been disclosed.
OBJECTIVES OF THE INVENTION
The main objective of the present invention is to provide an electrocatalyst comprising a molybdenum-based alluaudite framework material.

Another objective of the present invention is to provide a process for the preparation of the molybdenum-based alluaudite framework material.

Another objective of the present invention is to utilize the electrocatalyst for oxygen evolution reaction and oxygen reduction reaction.

Another objective of the present invention is to utilize the electrocatalyst comprising the molybdenum-based alluaudite framework material in zinc-air batteries.

SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended to determine the scope of the invention.

The present invention provides an electrocatalyst comprising a molybdenum-based alluaudite framework material having general formula NaxMy(MoO4)z wherein M is a transition metal; x has a value selected from 2.4, and 3.36; and y has a value selected from 0.8, and 1.32; z is selected from 2, and 3.

The present invention provides a process for the preparation of an electrocatalyst ink comprising a molybdenum-based alluaudite framework material, the process comprises:
adding the molybdenum-based alluaudite framework material and carbon super P in a solvent;
adding a binder to form a slurry; and
sonicating the slurry to obtain the electrocatalyst ink.

The present invention also provides a zinc-air battery characterize in comprising an electrocatalyst cathode comprising a molybdenum-based alluaudite framework material having general formula NaxMy(MoO4)z wherein M is a transition metal; x has a value selected from 2.4, and 3.36; and y has a value selected from 0.8, and 1.32; z is selected from 2, and 3.

BRIEF DESCRIPTION OF THE DRAWINGS:
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 depicts a) Rietveld refinement of powder X-ray diffraction (PXRD) pattern of alluaudite-type Na2.4Ni0.8(MoO4)2 collected at room temperature (?=1.5405 Å) assuming a monoclinic (space group: C2/c) crystal structure where the red dots denote the observed XRD pattern, black line denotes the calculated pattern, the purple bars depict the Bragg positions for the reference pattern and finally the green line denotes the difference between observed and the calculated data. b-d) Structural illustration of the compound showing a 3D tunnel type structure.
Figure 2 depicts a) SEM image shows the micrometric alluaudite particles. b) SAED pattern. c) HRTEM image shows the lattice fringes. d-h) HAADF-EDS images confirm the uniform distribution of the elements.
Figure 3 depicts a) XPS survey spectrum of Na2.4Ni0.8(MoO4)2 which confirms Ni2+ ¬and Mo6+ oxidation state. b) FT-IR spectrum shows the (a)symmetric stretching modes of the target alluaudite. c) Raman spectra shows the stretching and bending modes.
Figure 4 depicts a) Linear sweep voltammetry (LSV) curves at different rotation rate of RRDE for ORR activity of the target alluaudite. b) LSV curve of the same after SPEX milling the sample for 5 minutes.
Figure 5 depicts a) LSV curve of NNMo and benchmark RuO2 catalyst at 1600 rpm in 0.1 M KOH showing an onset potential of 1.52 V and 1.42 V, respectively. The corresponding CV is given in the inset showing the redox from Ni2+ to Ni3+. b) The Tafel slope calculated for RuO2 and NNMo. c) LSV curves after 5, 100, and 500 cycles of CV. (inset) Chronoamperometric curve showing a stability up to 12 h. d) comparison of onset potential values and the potential at 10 mA/cm2 for RuO2 and NNMo.
Figure 6 depicts Impedance spectra of the NNMo taken at its OCV. (inset) corresponding circuit diagram.
Figure 7 depicts a) SAED pattern taken after performing 100 cycles of CV in OER which attests the intact crystallinity of the sample. b) HRTEM image of the sample after 100 cycles of CV in OER.
Figure 8 depicts a) XPS spectrum of Ni 2p showing a slight shift towards higher binding energy side after 100 cycles of OER. b) XPS spectrum of Mo 3d showing no change even after 100 cycles attesting redox inactivity of Mo.
Figure 9 depicts a) Operando Raman spectroscopy study of NNMo compound with increasing the potential from OCV to 1.79 V (vs RHE). b) Stability of the ?-NiOOH phase with time after turning off the applied potential.
Figure 10 depicts a) Rietveld refined XRD pattern of alluaudite Na3.36Co1.32(MoO4)3 assuming a monoclinic (space group: C2/c) crystal structure. b) The alluaudite structure illustrated along c direction.
Figure 11 depicts a) Linear sweep voltammetry curves at different rotation rates of NCMo alluaudite for its ORR activity. (inset) corresponding CV with and without O2-saturation. b) LSV curve of NCM showing the OER activity. (inset) corresponding CV. c) Galvanostatic charge-discharge profile of the metal-air battery. d) The stripping and plating performance of the Zn metal-air battery at 1mA/ cm2 current density.

DETAILED DESCRIPTION OF THE INVENTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments in the specific language to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated process, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The composition, methods, and examples provided herein are illustrative only and not intended to be limiting.

The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The term “some” as used herein is defined as “none, or one, or more than one, or all”. Accordingly, the terms “none”, “one”, “more than one”, “more than one, but not all” or “all” would all fall under the definition of “some”. The term “some embodiments” may refer to no embodiments or to one embodiment or to several embodiments or to all embodiments. Accordingly, the term “some embodiments” is defined as meaning “no embodiment, or one embodiment, or more than one embodiment, or all embodiments”.

More specifically, any terms used herein such as but not limited to “includes”, “comprises”, “has”, “consists” and grammatical variants thereof is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. The specification will be understood to also include embodiments which have the transitional phrase “consisting of” or “consisting essentially of” in place of the transitional phrase “comprising”. The transitional phrase “consisting of” excludes any element, step, or ingredient not specified in the claim, except for impurities associated therewith. The transitional phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed invention.

Whether or not a certain feature or element was limited to being used only once, either way it may still be referred to as “one or more features” or “one or more elements” or “at least one feature” or “at least one element”. Furthermore, the use of the terms “one or more” or “at least one” feature or element do NOT preclude there being none of that feature or element, unless otherwise specified by limiting language such as “there NEEDS to be one or more” or “one or more element is REQUIRED”.

Use of the phrases and/or terms such as but not limited to “a first embodiment”, “a further embodiment”, “an alternate embodiment”, “one embodiment”, “an embodiment”, “multiple embodiments”, “some embodiments”, “other embodiments”, “further embodiment”, “furthermore embodiment”, “additional embodiment” or variants thereof do NOT necessarily refer to the same embodiments. Unless otherwise specified, one or more particular features and/or elements described in connection with one or more embodiments may be found in one embodiment, or may be found in more than one embodiment, or may be found in all embodiments, or may be found in no embodiments. Although one or more features and/or elements may be described herein in the context of only a single embodiment, or alternatively in the context of more than one embodiment, or further alternatively in the context of all embodiments, the features and/or elements may instead be provided separately or in any appropriate combination or not at all. Conversely, any features and/or elements described in the context of separate embodiments may alternatively be realized as existing together in the context of a single embodiment.

The terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and does not limit, restrict, or reduce the spirit and scope of the invention.

The present invention provides an electrocatalyst comprising a molybdenum-based alluaudite framework material having general formula NaxMy(MoO4)z wherein M is a transition metal; x has a value selected from 2.4, and 3.36; and y has a value selected from 0.8, and 1.32; z is selected from 2, and 3.

In an embodiment of the present invention, the transition metal is selected from nickel (Ni), and cobalt (Co).

In an embodiment of the present invention, the molybdenum-based alluaudite framework material is selected from Na2.4Ni0.8(MoO4)2 and Na3.36Co1.32(MoO4)3.

The molybdenum-based alluaudite framework material Na2.4Ni0.8(MoO4)2 has a particle size in a range of 1 to 10 µm; and Na3.36Co1.32(MoO4)3 has the particle size in a range of 10 to 20 µm.

In an embodiment of the present invention, the electrocatalyst is used in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), wherein the electrocatalytic (oxygen evolution and reduction) activity of Na2.4Ni0.8(MoO4)2 and Na3.36Co1.32(MoO4)3 alluaudite framework material lies with Ni and Co redox centre.

The electrocatalyst Na2.4Ni0.8(MoO4)2 has a current density of 16.5 mA/cm2 at an onset potential of 1.52 V with respect to reversible hydrogen electrode in 0.1 M KOH.

The electrocatalyst Na3.36Co1.32(MoO4)3 has a current density of about 3.5 mA/cm2 at the onset potential of ~0.85 V with respect to reversible hydrogen electrode in 0.1 M KOH.

The present invention provides a process for the preparation of an electrocatalyst ink comprising the molybdenum-based alluaudite framework material, the process comprises:
adding the molybdenum-based alluaudite framework material and carbon super P in a solvent;
adding a binder to form a slurry; and
sonicating the slurry to obtain the electrocatalyst ink.

The molybdenum-based alluaudite framework material to carbon super P are in a ratio of 3:1; wherein the solvent is a mixture of water and isopropyl alcohol having a ratio of 3:1; wherein the binder is NAFION.

In an embodiment of the present invention, the molybdenum-based alluaudite framework material is prepared by a process comprises:
combining a molybdate precursor, a precursor of a transition metal, and sodium nitrate with a fuel to form a mixture;
combusting the mixture to obtain a combustion product; and
annealing the combustion product to obtain the molybdenum-based alluaudite framework material.

In an embodiment of the present invention the molybdate precursor, the precursor of the transition metal, and sodium nitrate to the fuel has a weight ratio of 1:1. The fuel is selected from glycine and citric acid.

In an embodiment of the present invention, the molybdate precursor, the precursor of the transition metal, and sodium nitrate are added in stoichiometric ratio according to the chemical formula of molybdenum-based alluaudite framework material.

In an embodiment of the present invention the molybdate precursor is ammonium molybdate tetrahydrate ((NH4)6Mo7O24.4H2O) and MoO3.

The transition metal is selected from a group comprises Ni, and Co. The precursor of the transition metal nickel (Ni) is selected from a group comprises nickel nitrate tetrahydrate (Ni(NO3)2.4H2O), nickel oxides, nickel carbonates, nickel acetates, and nickel sulfates. The precursor of the transition metal cobalt (Co) is selected from a group comprises cobalt nitrate hexahydrate (Co(NO3)2.6H2O), cobalt oxides, cobalt carbonates, cobalt acetates, and cobalt sulfates.

In an embodiment of the present invention, the mixture is prepared by dissolving the molybdate precursor, the precursor of the transition metal, and sodium nitrate in water and adding the fuel to form a solution; followed by heating the solution at a temperature in a range of 100 to 200 ? for 1 hour.

In another embodiment of the present invention, the mixture is prepared by grinding the molybdate precursor, the precursor of the transition metal, and sodium nitrate with the fuel. The grinding is carried out in mortar pestle.

In an embodiment of the present invention, the mixture is grounded and shaped into a pellet for combustion.

The mixture is combusted at a temperature in a range of 200 to 250 ? for 1 to 2 hours to obtain the combusted product.

The annealing of the combusted product is carried out at a temperature in a range of 600 to 700 ? for 1 min to 50 hours.

The present invention provides a zinc-air battery characterize in comprising an electrocatalyst cathode comprising a molybdenum-based alluaudite framework material having general formula NaxMy(MoO4)z wherein M is a transition metal; x has a value selected from 2.4, and 3.36; and y has a value selected from 0.8, and 1.32; z is selected from 2, and 3.

In an embodiment of the present invention the zinc-air battery comprises:
a cathode, wherein the cathode is an electrocatalyst comprising a molybdenum-based alluaudite framework material having general formula NaxMy(MoO4)z wherein M is a transition metal; x has a value selected from 2.4, and 3.36; and y has a value selected from 0.8, and 1.32; z is selected from 2, and 3;
an anode; and
an electrolyte.

In an embodiment of the present invention, the molybdenum-based alluaudite framework material of the electrocatalyst cathode is Na3.36Co1.32(MoO4)3.

In an embodiment of the present invention, the anode is Zn foil, and the electrolyte is selected from a potassium hydroxide (KOH) solution and a zinc acetate dihydrate [Zn(CH3COO)2·2H2O] solution.

The zinc-air battery has an open circuit voltage (OCV) of 1.42 V.

The zinc-air battery has high galvanostatic charge-discharge cyclic stability over 50 cycles and demonstrates high reversibility.

EXAMPLES:
The present disclosure with reference to the accompanying examples describes the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. It is understood that the examples are provided for the purpose of illustrating the invention only and are not intended to limit the scope of the invention in any way.

The electrocatalysis properties as well as the mechanism were tested for two compounds: Na2.4Ni0.8(MoO4)2 and Na3.36Co1.32(MoO4)3. Some salient features are highlighted below.

Example 1: Na2.4Ni0.8(MoO4)2 (NNMo) alluaudite material as OER electrocatalyst
Synthesis and structure: The target compound was prepared by solution combustion synthesis. Sodium nitrate, nickel nitrate, and ammonium-hepta-molybdate were taken in a stoichiometric ratio according to the formula as the oxidizers and citric acid was used as the fuel in this synthesis by maintaining the oxidizer to fuel ratio of 1:1. The annealing was conducted at 700 °C for 10-12 h. The chemical equation can be expressed as:

NaNO_3+?Ni(NO_3 )?_2.4H_2 O+?(NH_4)?_6 ?Mo?_7 O_24.4H_2 O??Na?_2.4 ?Ni?_0.8 ?(MoO_4)?_2

Structural analysis was performed using powder X-ray diffraction (at 25 °C) with a Panalytical X-ray diffractometer with Cu Ka source. Rietveld refinement (Fig. 1a) confirms the target phase crystallizes in a monoclinic crystal system (Space Group: C2/c) with a classical alluaudite type structure. It is a 3D tunnel type of structure. Two NiO6 octahedra form Ni2O10 dimers which are forming a chain along c direction; the chains are connected to each other by MoO4 tetrahedra to form a 2D sheet along bc plane. Finally, those sheets are connected to each other by MoO4 tetrahedra to form 3D tunnel type structure. There are two tunnels present along c direction within the structure (Fig. 1b-d).

Physical characterization: NNMo sample has micrometric (1~10 µm) particles with irregular shaped morphology (Fig. 2a). The selected area diffraction pattern (SAED) taken along [010] zone axis (Fig. 2b) confirms the monoclinic crystal structure. High resolution transmission electron microscopy (HRTEM) image (Fig. 2c) shows clear lattice fringes. An overall Na-rich and Ni-deficient composition, similar to synthetic alluaudites, with uniform distribution of all constituent elements (Na, Ni, Mo) was observed by energy dispersive (EDS) elemental analysis (Fig. 2d-h).

The chemical state of the constituent transition elements was detected in XPS spectrum that consists signals from Na 1s, Ni 2s, Ni 2p, O 1s, Mo 3s, Mo 3p, Mo 3d and C 1s located at 1073 eV, 1010 eV, 857 eV, 532 eV, 496 eV, 398 eV, 232 eV and 285 eV respectively (Fig. 3a) confirming the presence of Ni2+ and Mo6+ in the compound. FTIR spectra (Fig. 3b) was examined after keeping it in air for 2-3 days. The absence of a broad peak at around 3500 cm-1 (characteristic peak of O-H bond) proved the absence of any structural or adsorbed water in the compound. Two distinct kinds of vibrational modes of MoO4 tetrahedra were captured: ?1~ 929 cm-1 = symmetric stretching mode and ?2 ~ 842 cm-1, 788 cm-1 and 731 cm-1 = triply degenerate asymmetric stretching mode. The bending mode of the MoO42- couldn’t be captured in FTIR technique but the same has been found out in the Raman spectra having three different peaks arising from the vibrations of MoO42- tetrahedra (Fig. 3c). The bands above 750 cm-1 stem from the stretching modes, while the bands below 400 cm-1 correspond to the bending mode of MoO4 tetrahedra in accordance with the FT-IR spectra.

Electrocatalytic Measurement: The electrocatalytic activity of Na2.4Ni0.8(MoO4)2 was tested in 0.1 M KOH solution with a rotating ring disc electrode (RRDE) with a CHI instrument. The electrocatalyst ink was prepared by taking the active material and carbon super P in a 3:1 ratio in a small vial followed by dispersing them in a mixture of DI water and IPA (3:1 ratio). Finally, 10 µL of NAFION binder was added in the slurry and it was sonicated for an hour. While the material was found to be ORR (oxygen reduction reaction) inactive (Fig. 4), it has shown exciting performance as an OER (oxygen evolution reaction) active electrocatalyst.

LSV (linear sweep voltammetry) of NNMo showed a current density of 16.5 mA/cm2 with an onset potential of 1.52 V with respect to reversible hydrogen electrode which is also compared with the activity of benchmark RuO2 electrocatalyst having an onset potential of 1.42 V with a current density reaching 17 mA/cm2 (Fig. 5a). The above-mentioned current density was obtained directly from the slurry prepared by pristine sample and carbon without any particle size optimization. The Tafel slope of the target alluaudite material (136 mV/decade) was found to have a slightly lower value than that of RuO2 (Fig. 5b). The stability of the material was also measured during the experiment. The current density dropped 15% after 100 cycles and 34% after 500 cycles of CV (Fig. 5c). Electrocatalytic stability was validated by chronoamperometric technique (i-t curve) at 1.45 V over 10 h targeting a descent current density of 2 mA/cm2 in 0.1 M of KOH electrolyte solution with continuous stirring at 1600 rpm (Fig. 5c inset) without any current loss. At the very beginning, there is a steep current loss which is attributed to the huge O2 bubble formation as soon as the test started which inhibited the surface activity and after that there is a gradual increase in the current density which can be possible since the catalyst is getting activated with time. The alluaudite molybdate showed an overpotential value of 390 mV as compared to that of RuO2 with an overpotential of 350 mV (Fig. 5d) at a saturation current of 10 mA/cm2.

The high efficiency of this catalyst can be due to the very low charge-transfer resistance (RCT ~ 95 O) associated with the material at OCV (Fig. 6).

Post-Mortem Analysis: The changes in the crystallinity of the catalyst before and after the cycling was probed by using electron microscopic tools (SEM and TEM). The crystallinity of the material was studied using TEM after performing 100 cycles of cyclic voltammetry in OER. The local crystallinity stays intact which is evident from SAED pattern showing clear diffraction spots (Fig. 7a), that can be indexed to the planes of pristine Na2.4Ni0.8(MoO4)2. The clear lattice fringes observed in HRTEM image (Fig. 7b) corresponding to the (020) planes of alluaudite molybdate complements the same. The amorphous region in the HRTEM image is coming from the carbon (Super-P) which was introduced while preparing the catalyst ink in accordance with the bright field image (Fig. 7a inset), the low contrasted periphery region.

Alluaudite Na2.4Ni0.8(MoO4)2 is made up of two redox centers; Ni and Mo. XPS was carried out to the pristine sample as well as after performing 100 cycles of cyclic voltammetry of OER to investigate the surface composition and to check the change in the valence state of the redox element. The Ni 2p XPS spectrum shows two main peaks at ~856.6 eV and ~874.29 eV which correspond to the two peaks of Ni 2p3/2 and Ni 2p1/2 respectively confirming the presence of Ni2+ (Fig. 8a) in the pristine material. The spin-energy separation of around 17.7 eV is a significant characteristic of Ni2+ present in the pristine sample. It was found out that after 100 cycles of CV there is a slight shift of Ni 2p3/2 peak at ~856.6 eV towards higher B.E side at ~857 eV indicating the oxidation of Ni2+ to Ni3+. Basically, the Ni 2p3/2 and Ni 2p1/2 peaks remained almost unchanged. There might be a possibility of gradual formation of Ni-OOH species at the surface of the catalyst with gradual cycling. The Mo XPS spectrum shows two distinguished peaks at 232.9 eV and 236.3 eV corresponding to Mo 3d3/2 and Mo 3d1/2 peaks respectively which is in well accordance with the presence of Mo6+ in the pristine sample (Fig. 8b). There is no change in the XPS peak of Mo after 100 cycles of OER which proves that Mo is not involved in any redox process.

Operando Raman Spectroscopy: Further to probe the actual active catalyst structure in action and underlying reaction mechanism, operando Raman spectroscopy was carried out during OER operation. The electrode bias was progressively altered from potential regions where the catalyst is inactive to a potential region where OER occurs. The evolution of this active material was also studied when the bias was turned off and open circuit was created. From Fig. 9a, at OCP, there is one broad peak at around 460 cm-1 which corresponds to the characteristic Ni-O(H) stretching mode of Ni(OH)2. (Ni2+ containing species). With progressive increase of bias, two new peaks appeared in the range of 450-600 cm-1 at +1.74 V. These correspond to the Ni-O stretch of ?-NiOOH phase which contains Ni3+ species. This operando data directly confirms the formation of Ni3+ species on the OER active catalyst surface. Once the applied potential is turned off, this active ?-NiOOH species was found to be metastable and could be observed upto 30 minutes on the electrode. Beyond this time, the peaks at 480 cm-1 and 565 cm-1 vanished and the peak at 450 cm-1 reappeared conforming the formation of Ni2+ containing Ni(OH)2 species. Operando spectra indicates that the overall material (NNMo) has a potential dependent identity and under conditions of OER it functions as ?-NiOOH. The overall excellent OER activity can be assigned to in situ developed ?-NiOOH within a NNMo matrix.

Example 2: Na3.36Co1.32(MoO4)3 as a bifunctional electrocatalyst
Synthesis and structure: The target NCMo compound was prepared by solution-combustion as well as solid-state synthesis. The precursors were NaNO3, Co(NO3)2 and (NH4)6Mo7O24 as the oxidizers and glycine as the fuel.

Solution-combustion process: NaNO3, Co(NO3)2.6H2O and (NH4)6Mo7O24.4H2O were taken in a stoichiometric ratio according to the formula and dissolved in distilled water to form a solution. Glycine was added as a fuel in the solution. The solution was evaporated at 100 ? and then was heated at 200 ? for an hour. After that, the intermediate powder was ground, pelletized, and annealed at 600 degree for 1 minute to get the pure phase of the product. The material was directly quenched from 600 ? to room temperature.

Solid-state route: NaNO3, Co(NO3)2.6H2O, and (NH4)6Mo7O24.4H2O were taken in a stoichiometric ratio according to the formula and ground together in mortar pestle with fuel and then pelletized and annealed at 600 ? for 50 hours and quenched to room temperature. It was ground again and pelletized and annealed again at 600 ? for 50 hours and quenched.

Rietveld refinement confirmed the monoclinic alluaudite framework as shown in Fig. 10. The chemical equation can be written as follows:

NaNO_3+?Co(NO_3 )?_2.6H_2 O+?(NH_4)?_6 ?Mo?_7 O_24.4H_2 O??Na?_3.36 ?Co?_1.32 ?(MoO_4)?_3

Electrocatalytic Measurement: The electrocatalytic measurements were studied with a RRDE connected to the CH potentiostat instrument (660C) in 0.1 M KOH solution. The presence of Co centre can impart efficient bifunctional electrocatalytic activity. The ORR activity was checked in between 0.25 V to 0.95 V against standard hydrogen electrode. The NCMo showed a current density around 3.5 mA/cm2 at 1600 rpm which is also close to the commercial state-of-the-art 20% Pt/C with 4.5 mA/cm2 current density (Fig. 11a). The onset potential was found to be ~0.85 V (vs RHE) in O2-saturated 0.1 M KOH, close to the onset of standard Pt/C (ca. 0.95 V). Excellent stability was observed for the Co-analogue for up to 20 h in the chronoamperometric experiment. Further, the OER activity was checked in between 0.9 V to 1.7 V (vs RHE). It showed a moderate activity reaching a current density up to 12 mA/cm2 with an onset potential of 1.57 V (Fig. 11b). It showed an overpotential of 0.42 V, which is slightly higher than that of benchmark RuO2 catalyst.

Performance of Metal-Air Battery: A zinc-air battery was constructed for metal-air battery applications utilizing NCMo as a cathode and zinc (Zn) metal as anode. A 6 M potassium hydroxide (KOH) and 0.1 M Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] solution was used as the electrolyte. The cathode carried out the oxygen evolution reaction (OER) during the charging process, while the oxygen reduction reaction (ORR) took place during discharge. The charge-discharge polarization was conducted at a current density of 1 mA/cm2, and the corresponding results are depicted in Fig. 11c. The open circuit voltage was determined to be 1.42 V, which is marginally lower than the theoretical open circuit potential (OCV) of a zinc-air battery, which is typically 1.65 V. This discrepancy is attributed to the overpotential associated with the electrode. Additionally, the galvanostatic charge-discharge cyclic stability of the battery was evaluated over 50 cycles (Fig. 11d), demonstrating exceptional stability with high reversibility. Even after 50 cycles, no significant changes were observed. A stable cycling for 60 h was noticed without any change in the current. , Claims:1. An electrocatalyst comprising a molybdenum-based alluaudite framework material having general formula NaxMy(MoO4)z wherein M is a transition metal; x has a value selected from 2.4, and 3.36; and y has a value selected from 0.8, and 1.32; z is selected from 2, and 3.

2. The electrocatalyst as claimed in claim 1, wherein the transition metal is selected from nickel (Ni), and cobalt (Co).

3. The electrocatalyst as claimed in claims 1 and 2, wherein the molybdenum-based alluaudite framework material is selected from Na2.4Ni0.8(MoO4)2 and Na3.36Co1.32(MoO4)3.

4. The electrocatalyst as claimed in claims 1 and 2, wherein the molybdenum-based alluaudite framework material Na2.4Ni0.8(MoO4)2 has a particle size in a range of 1 to 10 µm; and Na3.36Co1.32(MoO4)3 has a particle size in a range of 10 to 20 µm.

5. The electrocatalyst as claimed in claims 1- 4, wherein the Na2.4Ni0.8(MoO4)2 has a current density of 16.5 mA/cm2 at an onset potential of 1.52 V with respect to reversible hydrogen electrode in 0.1 M KOH; and Na3.36Co1.32(MoO4)3 has a current density about 3.5 mA/cm2 at the onset potential of ~0.85 V with respect to reversible hydrogen electrode in 0.1 M KOH.

6. A process for the preparation of an electrocatalyst ink comprising a molybdenum-based alluaudite framework material, the process comprises:
i. adding the molybdenum-based alluaudite framework material and carbon super P in a solvent;
ii. adding a binder to form a slurry; and
iii. sonicating the slurry to obtain the electrocatalyst ink.

7. The process as claimed in claim 6, wherein the molybdenum-based alluaudite framework material to carbon super P are in a ratio of 3:1; wherein the solvent is a mixture of water and isopropyl alcohol having a ratio of 3:1; wherein the binder is NAFION.

8. The process as claimed in claim 6, wherein the molybdenum-based alluaudite framework material is prepared by a process comprises:
i. combining a molybdate precursor, a precursor of a transition metal, and sodium nitrate with a fuel to form a mixture;
ii. combusting the mixture to obtain a combustion product; and
iii. annealing the combustion product to obtain the molybdenum-based alluaudite framework material.

9. The process as claimed in claim 8, wherein the molybdate precursor, the precursor of the transition metal, and sodium nitrate to the fuel has a weight ratio of 1:1; wherein the fuel is selected from glycine and citric acid.

10. The process as claimed in claim 8, wherein the molybdate precursor is ammonium molybdate tetrahydrate ((NH4)6Mo7O24.4H2O) and MoO3.

11. The process as claimed in claim 8, wherein the transition metal is selected from a group comprises nickel (Ni), and cobalt (Co).

12. The process as claimed in claim 8 and 11, wherein the precursor of the transition metal nickel (Ni) is selected from a group comprising nickel nitrate tetrahydrate (Ni(NO3)2.4H2O), nickel oxides, nickel carbonates, nickel acetates, and nickel sulfates; and the precursor of the transition metal cobalt (Co) is selected from a group comprising cobalt nitrate hexahydrate (Co(NO3)2.6H2O), cobalt oxides, cobalt carbonates, cobalt acetates, and cobalt sulfates.

13. The process as claimed in claim 8, wherein the mixture is prepared by dissolving the molybdate precursor, the precursor of the transition metal, and sodium nitrate in water and adding the fuel to form a solution; followed by heating the solution at a temperature in a range of 100 to 200 ? for 1 hour.

14. The process as claimed in claim 8, wherein the mixture is prepared by grinding the molybdate precursor, the precursor of the transition metal, and sodium nitrate with the fuel.

15. The process as claimed in claim 8, wherein the mixture is combusted at a temperature in a range of 200 to 250 ? for 1 to 2 hours.

16. The process as claimed in claim 8, wherein the annealing is carried out at a temperature in a range of 600 to 700 ? for 1 min to 50 hours.

17. A zinc-air battery characterize in comprising an electrocatalyst cathode comprising a molybdenum-based alluaudite framework material having general formula NaxMy(MoO4)z wherein M is a transition metal; x has a value selected from 2.4, and 3.36; and y has a value selected from 0.8, and 1.32; z is selected from 2, and 3.

18. The zinc-air battery as claimed in claim 17, wherein the molybdenum-based alluaudite framework material is Na3.36Co1.32(MoO4)3.

19. The zinc-air battery as claimed in claim 17, wherein the zinc-air battery has an open circuit voltage (OCV) of 1.42 V.