FIELD OF THE INVENTION

[0001] The present invention relates to a bifunctional electrocatalyst for all-solid- state rechargeable zinc-air battery. In particular, the present invention relates to an electrocatalyst for bifunctional oxygen reaction at the air cathode interface comprising manganese-cobalt-based bimetallic spinel oxide deposited on N-doped 3D porous entangled graphene (NEGF). The invention further provides fabricated all- solid- state rechargeable zinc-air batteries (ZABs) comprising said electrocatalyst coated air cathode that delivers a higher power density with stable cyclic stability. The invention finds immense application in the field of energy storage, particularly mobile as well as stationery (also renewable) applications. The invention shall help attain the 7 ^th sustainable development goal of affordable and clean energy.

BACKGROUND AND PRIOR ART OF THE INVENTION

[0002] The all-solid-state rechargeable zinc-air batteries (ZABs) have gained appreciable interest for large-scale energy storage applications in portable electronic devices in order to address future energy and environmental challenges. The Li-air batteries are known but have safety considerations and Li abundance is low which escalates the cost of Li-air batteries. The abundance and availability of zinc is one reason for the popularity of zinc -air batteries. Moreover, the all- solid- state rechargeable ZABs have several advantages over existing metal-air batteries such as high theoretical energy density and use of safe aqueous electrolyte. The practical applications of ZABs are however impoverished by their low power density, deficient charge-discharge voltage, and overall lower output energy efficiency. These limitations are mainly attributed to the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the air cathode. [0003] Conventionally, the spherical-shaped platinum nanoparticles supported by carbon (Pt/C) and RuCh are mostly used as electrocatalysts for ORR and OER processes in ZABs but they are costly and not durable. The major obstacle with the air cathode in the ZABs is the restricted mass transport of reactant/products gas molecules and electrolytes due to the comparatively lower access of active sites and imbalanced hydrophilicity/hydrophobicity of the electrocatalyst-coated gas diffusion layer (GDL) interface.

[0004] In order to develop high-performance air electrode catalysts, a series of nonnoble metal-based catalytic materials with excellent intrinsic activities are reported (e.g., transition metal oxides/hydroxides/chalcogenides/heteroatom doped carbonbased materials and hybrids of these materials) in the art. Among them, the transition metal oxides (Fe, Co, Mn, Ni) have received enormous attention as ORR/OER electrocatalysts due to their ease of synthesis, stability, and structural flexibility.

[0005] Spinel oxides (A ^II B ^II B ^III O4) are mostly being explored as an electrocatalyst, in which mixed-valence metal ions are distributed in octahedral and tetrahedral sites, respectively. The mixed valency metal ions in a spinel oxide crystal structure provide a preferable electron transport channel improving the electrochemical activity [Zang, M.; Xu, N. et.al', ACS Catal. 2018, 8, 5062-5069]. Recently, the present inventors have reported the nano -rod- shaped spinel cobalt oxides with promising ORR performance [Manna, et.al in Zinc- Air Batteries Catalyzed Using CO3O4 Nanorod-Supported N-Doped Entangled Graphene for Oxygen Reduction Reaction. ACS Appl. Energy Mater. 2021, 4, 5, 4570-4580]. To overcome the electronic conductivity issue of the metal oxide catalysts, carbon support incorporation as active sites have been adopted, this simultaneously prevents the aggregation of nanoparticles. Most of the conducting carbon support used for the spinel oxides support are ID and 2D materials with poorly established triple phase boundaries (TPB) at the electrochemical interface.

[0006] Regardless of the importance of TPB in the electrocatalyst, the interfacial engineering in rechargeable ZAB’s air cathodes has received diminutive attention. Although significant research has been done on conventional air cathode fabrication by metal-oxide carbon composite -based bifunctional catalyst layer on the surface of a hydrophobic gas diffusion electrode (GDL), this air cathode structure provides an almost 2D multiphase interface that is confined to the limited space between the porous GDL and electrocatalyst layer. In this configuration, most of the electrolytes and gaseous reactants cannot reach out to the catalytic sites. Thus, the traditional air cathode structure in ZABs inevitably gives rise to sluggish reaction kinetics for ORR and OER, which significantly reduces the ZAB performance. Thus aircathode interface engineering with good balance between hydrophobicity and hydrophilicity are vital for better mass transport.

[0007] Accordingly, keeping in view the drawbacks of the hitherto reported prior art, there exists a dire need to improve intrinsic bifunctional activity of spinel oxides, which can be done by way of providing an electrocatalyst for bifunctional oxygen reaction at the air cathode interface comprising manganese-cobalt-based bimetallic spinel oxide, wherein the intrinsic bi-functional activity of spinel oxides can be improved by morphology and compositional tuning.

OBJECTIVES OF THE INVENTION

[0008] The main objective of the present invention is therefore to provide an electrocatalyst for bifunctional oxygen reaction at the air cathode interface comprising manganese-cobalt-based bimetallic spinel oxide deposited on N-doped 3D porous graphene (NEGF) and solvothermal preparation process thereof.

[0009] Another objective of the present invention is to provide all- solid- state rechargeable zinc-air batteries (ZABs) comprising said electrocatalyst coated air cathode that delivers a higher power density with stable cyclic stability.

SUMMARY OF THE INVENTION

[0010] In an aspect, the present invention provides an electrocatalyst for bifunctional oxygen reaction at the air cathode interface comprising manganese- cobalt-based bimetallic spinel oxide deposited on N-doped 3D porous entangled graphene (NEGF).

[0011] Preferably, the present invention provides MnCo2O4/3D NEGF electrocatalyst which represents the self-assembly structure of nitrogen-doped three-dimensionally oriented graphene. The MnCo2O4is uniformly distributed over the N-doped 3D graphene.

[0012] In an aspect, the MnCo2O4 is spherical in shape with the size ranging between 30-60 nm.

[0013] In another aspect, the pore size of MnCo2O4/NEGF catalytic material ranges between 2-16nm; and has a BET surface area in the range of 300 - 320 m ² g ^-1 .

[0014] In another aspect, the present invention provides solvothermal process for synthesis of said electrocatalysts (MnCo2O4/3D NEGF) as coated material on air cathode, process comprising; i. dispersing the graphene oxide (GO) synthesized via improved Hummer’s method in water and ammonia solution (30% v/v) to obtain viscous graphene oxide solution; ii. adding Co ²⁺ and Mn ²⁺ metal salts to the viscous graphene oxide solution of step (i) in 2:1 ratio at constant stirring followed by probe sonication; iii. transferring the solution of step (ii) to a Teflon-lined autoclave and heating followed by cooling and washing to remove excess ammonia; and iv. freeze-drying the mixture of step (iii) under high vacuum pressure to obtain the electrocatalyst.

[0015] In another preferred aspect, the present invention provides an all- solid- state rechargeable zinc-air battery (ZAB) comprising: a. MnCo2O4/NEGF electrocatalyst coated on gas diffusion layer (GDL) in an air cathode; b. an anode; and c. an electrolyte placed between the air cathode and anode; wherein, the NEGF and GDL interact at the reactive interface of the air cathode delivering a higher power density with stable cyclic stability of ZAB .

[0016] The anode material is zinc material, which is more abundant, cheap, and non-toxic.

[0017] The electrolyte is selected from polyvinyl alcohol (PVA), potassium hydroxide (KOH), and the combination of PVA-KOH gel, where said materials are cheap and easily polymerized to fabricate in the ZAB device. [0018] The electrocatalyst slurry was brush-coated over a gas diffusion layer (GDL) and was dried at 60 °C for 12 h to achieve a catalyst loading of 1.0 mg cm' ² (electrode area = 1.0 cm ² ). A VMP-3 model Bio-Logic Potentiostat/Galvanostat was used to evaluate the ZAB set-up at room temperature.

[0019] These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0020] The following drawings form a part of the present specification and are included to further illustrate aspects of the present disclosure. The disclosure may be better understood by reference to the drawings in combination with the detailed description of the specific embodiments presented herein.

[0021] Fig 1 : shows (a) the field-emission scanning electron microscopy (FESEM) images of MnCo2O4/NEGF, displaying the porous architecture of the entangled 3D graphene sheets; (b) magnified FESEM image of MnCo2O4/NEGF; (c) 3D micro- CT images of MnCo2O4/NEGF, showing the porous structure of 2D sheets are connected; (d) TEM image of MnCo2O4/NEGF, shows the uniform distribution of MnCo2O4 over the N-doped of 3D graphene; (e) High-resolution transmission electron microscopic (HRTEM) image of MnCo2O4/NEGF, clearly shows the d- spacing for MnCo2O4, Inset showing the crystal nature of MnCo2O4; and (f-k) elemental mapping for Co, Mn, N, O and C respectively, in accordance with an embodiment of the present disclosure.

[0022] Fig 2: shows (a) the comparative pore size distribution of NEGF, MnCo2O4, and MnCo2O4/NEGF materials; (b) comparative BET adsorption and desorption isotherm of NEGF, and MnCo2O4/NEGF, showing type-IV isotherm, in accordance with an embodiment of the present disclosure. [0023] Fig 3 : shows the XPS analysis of MnCo2O4/NEGF; (a) comparative survey scan spectra of NEGF, CO3O4/NEGF, and MnCo2O4/NEGF showing the presence of C, N, O, Co, and Mn in the respective catalysts; (b) deconvoluted spectra of Co2p showing presence of two spin-spin splitting peaks reveals the +2 and +3 oxidation state of Co in MnCo2O4/NEGF; (c) deconvolutes Mn spectra, shows two peaks corresponding two oxidation state of Mn, +2 and +3; (d) deconvoluted Nls spectra, confirm the presence of four types of nitrogen, in accordance with an embodiment of the present disclosure.

[0024] Fig 4: depicts the electrocatalytic rotating disc electrode (RDE) performance analysis of NEGF, CO3O4/NEGF, Mn3O4/NEGF, and MnCo2O4/NEGF towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in comparison to the state-of-the-art (Pt/C) and RuO2 catalyst respectively; (a) comparable linear sweep voltammetry (LSV) profiles for NEGF, CO3O4/NEGF, MnCo ₂ O4/NEGF, and Pt/C in O ₂ sat 0.1 M KOH recorded at an RPM of 1600 of the WE displaying the onset potentials at 1.0, 0.94, 0.80 and 0.65 mV, respectively, with respect to RHE; (b) comparable LSV profiles for NEGF, CO3O4/NEGF, MnCo ₂ O4/NEGF, and Pt/C in O ₂ sat 0.1 M KOH recorded at an RPM of 1600 of the WE displaying the onset potentials at 1.0, 0.94, 0.80 and 0.65 mV, respectively, with respect to RHE; (c) comparable bifunctional activity LSV profiles of NEGF, CO3O4/NEGF, Mn ₃ O ₄ /NEGF, and MnCo ₂ O ₄ /NEGF displaying the onset potentials at 1.0, 0.94, 0.80 and 0.65 mV, respectively, with respect to RHE; (d) Comparative onset, half-wave potential and bifunctional activity for NEGF, CO3O4/NEGF, Mn ₃ O ₄ /NEGF, and MnCo ₂ O ₄ /NEGF; (e) Tafel plot analysis for NEGF, CO3O4/NEGF, Mn ₃ O ₄ /NEGF, and MnCo ₂ O ₄ /NEGF for ORR activity; (f) Tafel plot analysis for NEGF, CO3O4/NEGF, Mn3O4/NEGF, and MnCo2O4/NEGF for OER activity, in accordance with an embodiment of the present disclosure.

[0025] Fig 5. shows (a) the cross-sectional FESEM image of the bare GDL with the same in inset image; (b) the cross-sectional FESEM image of the GDL coated with MnCo2O4/NEGF, inset image gives better clarity of the surface containing the 3D self-assembled structure of the coated layer of MnCo2O4/NEGF; (c) and (d) the 3D tomogram cross-section images of the bare GDL and the MnCo2O4/NEGF- coated GDL, respectively; (e) contact angle (CA) data corresponding to the bare GDL; and (f) the contact angle (CA) data corresponding to MnCo2O4/NEGF-coated surface of the GDL shows a water contact angle of 109.2°, in accordance with an embodiment of the present disclosure.

[0026] Fig 6: shows all-solid-state rechargeable zinc-air battery (ZAB) performance evaluation for MnCo2O4/NEGF and Pt/C+RuO2 as the air electrodes: (a) polarization plots recorded on the ZABs fabricated by employing MnCo2O4/NEGF and Pt/C+RuO2 as the air electrodes; (b) comparative impedance plot recorded for ZAB set-up constructed with MnCo2O4/NEGF and Pt/C+RuO2 as the air electrodes; (c) galvanostatic charge-discharge plot for MnCo2O4/NEGF and Pt/C +RUO ₂ , shows the higher potential window for Pt/C+RuO2 compared to MnCo2O4/NEGF; (d) the galvanostatic charge-discharge cycling curves at 10 mA cm ^-2 , shows in case of Pt/C+ Ru02, asymmetric charge-discharge plateau; and (e) galvanostatic discharge capacity of the battery at the various current density of 5, 10,20, 30 mA cm ^-2 , in accordance with an embodiment of the present disclosure.

[0027] Fig 7 provides schematic illustration of the stages involved in the stepwise synthesis of MnCo2O4/NEGF as an ORR/OER bifunctional electrocatalyst, and demonstration of its application as the air-electrode for the Solid-State Rechargeable Zn-Air Battery, in accordance with an embodiment of the present disclosure.

[0028] Fig 8 shows an illustration of all solid-state rechargeable Zn-air battery set up, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated. Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such steps, features, compositions, and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such steps or features.

Definitions

[0030] For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are delineated here. These definitions should be read in light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.

[0031] 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.

[0032] The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”.

[0033] The term "at least one" is used to mean one or more and thus includes individual components as well as mixtures/combinations.

[0034] Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

[0035] The term “including” is used to mean “including but not limited to”, “including” and “including but not limited to” are used interchangeably.

[0036] The term “electrocatalyst” refers to a catalyst which takes part in the oxidation and/or reduction processes in an electrochemical system enhancing the rate of electrochemical reactions occurring on the electrode surface. In the present disclosure, the terms “catalyst” and “electrocatalyst” are used alternatively. In an aspect of the present disclosure, the electrocatalyst is a bifunctional electrocatalyst comprising bimetallic spinel oxide for bifunctional oxygen reaction at the air cathode interface.

[0037] In an embodiment, the present invention discloses a bifunctional electrocatalyst for bifunctional oxygen reaction at air cathode interface comprising manganese-cobalt-based bimetallic spinel oxide deposited on N-doped 3D porous graphene (NEGF).

[0038] In a preferred embodiment, the present invention relates to MnCo2O4/3D NGr (NEGF) electrocatalyst which represents the self-assembly structure of nitrogen-doped three-dimensionally oriented graphene. The MnCo2O4is uniformly distributed over the N-doped 3D graphene.

[0039] In another embodiment, the electrocatalysts (MnCo2O4/3D NEGF) as air cathode material is prepared by solvothermal process comprising; i. dispersing the graphene oxide (GO) synthesized via improved Hummer’s method in water and ammonia solution (30% v/v) to obtain viscous graphene oxide solution; ii. adding Co ²⁺ and Mn ²⁺ metal salts to the viscous graphene oxide solution of step (i) in 2:1 ratio at constant stirring followed by probe sonication; iii. transferring the solution of step (ii) to a Teflon-lined autoclave and heating followed by cooling and washing to remove excess ammonia; and iv. freeze-drying the mixture of step (iii) under high vacuum pressure to obtain the electrocatalyst.

[0040] The freeze-drying of hydrothermally treated catalytic material is a crucial step that induces the homogeneous porosity to the N-doped reduced graphene oxide which is clearly evidenced in the FESEM (field emission scanning electron microscopy) images (Figs la and lb).

[0041] In still another embodiment, the pore size of MnCo2O4/NEGF catalytic material ranges between 2-16 nm; and has a BET surface area in the range of 300 - 320 m ² g ^-1 .

[0042] The XRD pattern (Fig 2b) of MnCo2O4/NEGF discloses a series of peaks at 20 = 18.3, 30.2, 35.6, 37.0, 43.2, 53.8, 57.2, 62.7 and 74.0°, which are ascribed to (I l l), (220), (311), (400), (422), (511), (440) and (533) diffraction peaks corresponding to the spinel structure MnCo2O4. After incorporating spherical shaped MnCo2O4 over NEGF, a graphitic (002) plane shift towards a lower diffraction angle compared to the NEGF is observed is ascribed to the increasing the d-spacing of the nitrogen-doped graphene sheets.

[0043] The extent of the defects to the graphitic nature of the employed conducting support is measured by calculating the ID/IG ratio using Raman spectroscopy analysis. In the Raman spectra, the D-band expresses the defects in the graphene lattice structure, and the G band represents the orderliness in the graphene. The D- band peak that appeared at 1350 cm ¹ corresponds to the graphitic lattice vibration mode with the Ai _g symmetry, while the G-band peak appeared at 1590 cm ¹ corresponds to the E2 _g symmetry graphitic lattice vibration mode. Raman spectra of NEGF, and MnCo2O4/NEGF catalyst with the ID/IG values of 1.25, and 1.31, respectively. The increased ID/IG value from GO (~1.0) to NEGF catalyst clearly indicated the creation of new defect sites with the introduction of doped nitrogen into the graphitic lattice structure through solvothermal treatments at 180°C. The introduced defective sites in the N-doped graphene sheets were helpful for metal oxides nucleation. The defective sites were higher in MnCo2O4/NEGF than its counterpart NEGF support which must have been introduced during the in-situ growth of metal oxides. The higher defective sites observed in the case of metal oxides supported NEGF stood out to assist the system towards catalytic activity enhancement. The total loading of the spinel oxide active site, which suppressed the BET surface area in MnCo2O4/NEGF, was determined by the thermogravimetric analysis (TGA). TGA was done under an oxygen atmosphere in the temperature range of 25 to 900 °C at a scan rate of 10 °C per minute. TGA weight loss profile for MnCo2O4/NEGF, indicating the MnCo2O4 loading of ~45 wt.% over the nitrogen-doped carbon. The observed higher loading of MnCo2O4 nanoparticles suppressed the overall surface area of the prepared MnCo2O4/NEGF electrocatalyst to 300 m ² g ¹ . The achieved higher loading of MnCo2O4 (45%) over conducting support maintained the overall conductivity and active sites density of the catalyst required for better electrochemical activity. [0044] In a further embodiment, the electrochemical ORR and OER performance was measured using an aqueous solution of 0.1 M KOH and 1 M KOH respectively. The intrinsic ORR activity of the catalysts was measured by linear sweep voltammetry (LSV) analysis in 0.1 M KOH at the scan rate of 10 mV sec ¹ under O2 atmosphere to maintain the working electrode rotation at 1600 RPM. The comparative LSV profile (Fig 4a) evidences the superior ORR performance achieved by MnCo2O4/NEGF compared to the control samples, i.e., NEGF (0.86 V), CO3O4/NEGF (0.89 V), and Mn ₃ O ₄ /NEGF (0.85 V). In addition, the ORR performance of MnCo2O4/NEGF (0.93 V) was observed close to state-of-the-art catalysts (Pt/C), showing its higher catalytic potential (0.99 V). Similarly, the OER activity was for NEGF, CO3O4/NEGF, Mn3O4/NEGF, MnCo2O4/NEGF, and RuO2 in 1 M KOH at a scan rate of 10 mV sec ¹ under N2 atmosphere. The LS Vs recorded for MnCo2O4/NEGF (Fig 4b) showed better electrochemical OER activity compared to NEGF, CO3O4/NEGF, and Mn3O4/NEGF. The superior performance of MnCo2O4/NEGF catalyst towards both oxygen reactions (ORR/OER) was observed in LSV analysis. The overall bifunctional activity (ORR-OER) of MnCo2O4/NEGF was found to be 0.82 V which was comparable or better than previously reported various bifunctional electrocatalysts (Table 1). The observed higher bifunctional oxygen reaction activity of the prepared catalyst was attributed to the bimetallic composite of Mn and Co spinel oxides and nitrogen doped 3D carbon support providing effective TPB formation for better mass transport properties.

Table 1 -Comparison of the bifunctional oxygen activity of the non-noble metalbased electrocatalysts and electrocatalyst of present invention.

[0045] From table 1, it is evident that lower E1/2 value means better activity; higher Ej higher means the improved limiting current; and lower AE value means the better bifunctional activity for catalysts of present application.

[0046] In another embodiment, the MnCo2O4/NEGF catalyst of the present invention was stable up to 5000 cycles evidenced by the cyclic durability study (Fig 5a-c).

[0047] In another preferred embodiment, the present invention relates to all- solid state rechargeable zinc air battery (ZAB) comprising; a. MnCo2O4/NEGF electrocatalyst coated on gas diffusion layer (GDL) in an air-cathode; b. an anode; and c. an electrolyte placed between the air cathode and anode; wherein, the NEGF and GDL interact at the reactive interface of the air cathode delivering a higher power density with stable cyclic stability of ZAB.

[0048] In still another embodiment, the anode material is zinc material, which is more abundant, cheap, and non-toxic.

[0049] In yet another embodiment, the electrolyte is selected from polyvinyl alcohol (PVA), potassium hydroxide (KOH), and the combination of PVA-KOH gel, where said materials are cheap and easily polymerized to fabricate in the ZAB device.

[0050] In still another embodiment, the gel electrolyte for all- solid- state rechargeable zinc-air battery (ZAB) is prepared by the process comprising: a. dissolving PVA powder in ultrapure water and agitating vigorously at a temperature ranging between 80-100°C until a translucent gel solution is formed; and b. adding a base to the above solution drop wise at the same temperature and storing in the refrigerator to obtain the desired product.

[0051] In yet another embodiment, the catalyst slurry of MnCo2O4/NEGF for coating on to the GDL electrode is prepared by the process comprising: i. adding MnCo2O4/NEGF to the mixture of IPA and water (1:4) and sonicating; ii. adding 10 wt% Fumion solution to the dispersion of step (i) and sonicating until complete dispersion is obtained; and iii. coating the catalyst slurry over the gas diffusion layer (GDL) and drying to achieve a catalyst loading of 1.0 mg cm' ² .

[0052] Figure 7 depicts a simplified illustration of the stages involved in the stepwise synthesis of MnCo2O4/NEGF as an ORR/OER bifunctional electrocatalyst and demonstration of its application as the air-electrode material for the rechargeable ZAB. In brevity, the aqueous solution of the graphene oxide (GO) synthesized via the improved Hummer’s method was mixed well with Co ²⁺ and Mn ²⁺ metal precursors (2:1) at constant stirring for 6 h. Ammonium hydroxide (~30% v/v) was added to the metal ion-anchored GO solution with continuous stirring for 6 h, followed by probe sonication for 10 min. Depending on the nature of the functional groups present in the GO and the binding strength of carbon-carbon bonds, the doped nitrogen exists in various forms such as pyrrolic, pyridine, graphitic, and quaternary states. This creates asymmetric carbon centers with some differences in the electronegativity in the system. At high temperatures and pressure of the solvothermal treatment, the metal hydroxides gradually decompose and nucleate at the asymmetric carbon centers, resulting in the formation of the spherically shaped spinel oxide (MnCo2O4) nanoparticles anchored over the N-doped reduced graphene oxide’s surface. The solvothermal reaction is followed by the freeze-drying process, which plays an important aspect in establishing the 3D geometrical orientation and restructuring of the graphene sheets bearing the bimetallic spinel oxide nanoparticles. This electrocatalyst consisting of the entangled graphene framework with homogeneously dispersed Co-Mn spinel oxide nanoparticles (MnCo2O4/NEGF) possesses a high surface area and catalytic site-accessible porous architecture. The resulting catalyst was coated over a porous carbon gas diffusion layer (GDL) in combination with PVA-KOH gel electrolyte, and a solid-state rechargeable ZAB device was fabricated and demonstrated.

[0053] In another embodiment, the performance of all-solid-state rechargeable ZAB is shown in Fig 6(a-e), with the open-circuit voltage (OCV) values of 1.31 and 1.20 V, respectively, for MnCo2O4/NEGF and Pt/C+RuO2 coated electrodes. The comparative steady-state cell polarization leads to the maximum power density (Pmax) of 110 and 200 mW cm ^-2 for the ZABs based on Pt/C+RuO2 and MnCo2O4/NEGF, respectively. The cathode catalysts show superior performance for the prepared electrocatalyst (MnCo2O4/NEGF) compared to Pt/C+RuO2, which is ascribed to be better interface formation in the former catalyst. Galvanostatic charge/discharge curve measured at 10 mA cm ^-2 is shown in Fig 6c. The observed difference between the charging and discharging voltages of ZAB on MnCo2O4/NEGF during the initial process was 0.84 V which was lower than 0.91 V of the Pt/C + RUO ₂ . After 50 h of continuous charge-discharge cycles, a nominal voltage difference increased by 0.10 V on ZAB consisting of MnCo2O4/NEGF compared to 1.1 V after 15 h cycle operation on Pt/C+ RuO2 ZAB. Moreover, the magnified image showed (Fig 6d) that in case of MnCo2O4/NEGF, chargedischarge voltage plateau are more symmetric but in the case of Pt/C+RuO2 deficient asymmetric charge-discharge curve is observed. This feature revealed the better bifunctional activity at ZAB air cathode interface in case of MnCo2O4/NEGF compared to Pt/C+RuO2.

[0054] The application of MnCo2O4/NEGF as an air electrode to function in the discharging (ORR) and charging (OER) modes for a solid-state ZAB was demonstrated by employing the catalyst-coated gas diffusion electrode (GDE) as the cathode. Prior to the fabrication of the cell and its testing, the catalyst-coated GDL surface was characterized by using FESEM and X-ray CT mapping to check the 3D micro structure of the resulting electrodes (Fig la-d). Fig 5a and the inset image show the cross-sectional FESEM image of the bare GDL, revealing the mostly flat structure of the surface. However, in the case of the GDL coated with M11CO2O4/NEGF, a thick layer with 3D structure (indicated by the dotted lines) is observed (Fig 5b). The inset of Fig 5b gives better clarity of the surface of the GDL containing the 3D self-assembled structure of the coated layer of MnCo2O4/NEGF. This 3D micro structured catalyst layer over the GDL has a significant advantage for achieving improved TPB with better active interface and mass transfer characteristics. The 3D CT tomography imaging of the commercial bare GDL consists of two parts (indicated by the dotted lines in Fig 5c and 5d, z.e., the oxygen catalytic face (OCF) and the gas diffusion face (GDF) towards the inner and outer side of the air-electrode, respectively. At OCF, the carbon fibers are coated with the hydrophobic PTFE, which prevents the flooding of the microporous surface of the GDL. Fig 5c and 5d showed the 3D tomogram cross-section images of the bare GDL and the MnCo2O4/NEGF-coated GDL, respectively. The tomography image in Fig 5c showed the two distinct phases of OCF and GDF (marked with the dotted lines) of the GDL as already indicated in the FESEM image of the corresponding sample presented in Fig 5a. On the other hand, in the case of the 3D CT image of the catalyst-coated GDL (Fig 5d), the 3D micro structure formation of the layer of MnCo2O4/NEGF is clearly evident and is demarcated with the dotted line.

[0055] The 3D porous morphology of the MnCo2O4/NEGF layer in the electrode is beneficial for improving the electrode-electrolyte interface formation. However, to realize this advantage significantly, the porous layer also should retain the optimum intrinsic wettability of the electrocatalyst even after it was subjected to the coating protocol during the electrode fabrication process. Surprisingly, the MnCo2O4/NEGF-coated surface of the GDL shows a water contact angle of 109.2° (Fig 5f). CA data corresponding to bare GDL is presented in Fig 5e. From these results, it is readily inferred that while aqueous electrolyte hardly wet bare GDL, GDL based on MnCo2O4/NEGF coating possesses balanced hydrophilic/hydrophobic characteristic, which is expected to result in optimum wettability at interface.

[0056] The ORR process is more sensitive to the TPB (triple phase boundary) interface during the discharge process than the OER reaction. The discharge curve at various current densities 5, 10, 20, and 30 mA cm ^-2 were recorded for M11CO2O4/NEGF and Pt/C + RUO ₂ catalysts for 1 h. When the current density was increased from 5.0 to 10.0 mA cm' ² , the charge voltage with the MnCo ₂ O4/NEGF cathode decreased from 1.25 to 1.24 V. However, it decreased significantly from 1.1 to 0.2 V with the Pt/C+RuO ₂ . Even at 30.0 mA cm ² , the former has a charge voltage of 1.10 V, which is about 210 mV higher than the Pt/C + RuO ₂ . The ZAB based on a 3D nitrogen-doped containing catalyst has a relatively small voltage gap of 0.11, 0.12, 0.13, and 0.15 V at 5.0, 10.0, 20.0, and 30.0 mA cm' ² . However, the ZAB with Pt/C + RUO ₂ catalyst are 1.05, 0.14, 0.15, and 0.60 V, respectively. Even in the case of Pt/C+RuO ₂ at a higher current density of 10 mA cm' ² sudden drop of potential is observed. The catalyst (MnCo ₂ O4/NEGF) coated air cathode benefits more from its higher ORR kinetics at the ZAB interfaces showing the vast advantage of the 3D porous architecture in terms of better oxygen gas transport and kinetics. Furthermore, the galvanostatic discharge curve recorded for MnCo ₂ O4/NEGF and Pt/C + RuO ₂ at 10 mA cm' ² catalyst has a discharge time of about 48 h and 40 h, respectively. Hence, in the longer run, the MnCo ₂ O4/NEGF catalyst is observed to outperform the Pt/C + RuO ₂ system both in terms of performance and long-term durability under a realistic ZAB system.

[0057] In a nutshell, the present invention provides electrode material consisting of manganese-cobalt-based bimetallic spinel oxide (MnCo ₂ O4)- supported nitrogen- doped entangled graphene (MnCo ₂ O4/NEGF) with multiple active sites responsible for facilitating both OER and ORR has been prepared. The porous 3D graphitic support significantly affects the bifunctional oxygen reaction kinetics and helps the system display a remarkable catalytic performance. The air electrode consisting of the MnCo ₂ O4/NEGF catalyst coated over the gas diffusion layer (GDL) ensures the effective TPB, and this feature works in favor of the rechargeable ZAB system under the charging and discharging modes. As an important structural and functional attribute of the electrocatalyst, the porosity and nitrogen doping in the 3D conducting support play a decisive aspect in controlling the surface wettability (hydrophilicity/hydrophobicity) of the air electrode. The fabricated solid-state rechargeable ZAB device with developed electrode displayed a maximum peak power density of 202mW cm ^-2 , which is significantly improved as compared to one based on Pt/C + RuCh standard catalyst pair(124 mWcm ^-2 ). Solid-state device displaying an initial charge-discharge voltage gap of only 0.7 V at 10 mA cm ^-2 showed only small increment of 86 mV after 50 h.

[0058] Although the present disclosure has been described in considerable detail with reference to certain embodiments and implementations thereof, other embodiments are possible to cover the modifications and variations of the present disclosure.

EXAMPLES

[0059] The following examples are given by way of illustration only and therefore should not be construed to limit the scope of the present invention in any manner. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods, devices, and materials are described herein. It is to be understood that this disclosure is not limited to particular methods, and experimental conditions described, as such methods and conditions may apply.

Materials:

[0060] For the purposes of the present disclosure, the materials and chemicals such as graphite, potassium permanganate (KMnCU), manganese acetate tetrahydrate [Mn(OAC)2.4H2O], cobalt acetate tetrahydrate [Co(OAc)2-4H2O], ammonium hydroxide (NH4OH), zinc acetate and potassium hydroxides were purchased from Sigma- Aldrich. Sulphuric acid (H2SO4) and phosphoric acid (H3PO4) were acquired from Thomas Baker. All the chemicals were used as such without any further purification.

Example 1:

(a) Synthesis of Graphene Oxide (GO): [0061] An improved Hummer’s method was employed to synthesize graphene oxide (GO). Firstly, (1:6) graphite powder and KMnO4 were well mixed using a mortar and pestle. The resulting solid mixture was slowly added to the round bottom flask containing a mixture of H3PO4:H2SO4 (1:9) solution kept in the ice bath. After complete transfer of solid mixture, the reaction solution was kept on stirring for 12 h at a constant temperature of 60°C. After the reaction was completed, the mixture was allowed to cool to room temperature. The resultant product was slowly poured into ice-cold water containing 3% H2O2 resulting in a yellowish solution. The resulting solution was then rinsed several times with a copious amount of distilled water followed by centrifugation at 10000 rpm. The collected residue solid was washed with 30 percent HC1 to remove any metal impurities, then washed with plenty of water to neutralize the acidic pH and wash away the impurities. Finally, the dark chocolate-colored, highly viscous solution was collected and cleaned with ethanol and diethyl ether before drying at 40°C to produce GO powder.

(b) Synthesis of MnCo2O4 Supported N-doped entangled 3D Graphene (M11CO2O4/NEGF):

[0062] The as-prepared GO (example la) was dispersed in water (3 mg/ml) via overnight stirring and water-bath sonication. After the complete dispersion of GO in water, ammonia solution (30 % v/v) was added and kept for constant stirring. After the formation of highly viscous graphene oxides solution, Mn(OAc)2.4H2O and CO(OAC)2.4H2O was added to the solution with a 1:2 ratio, and kept stirring for another 6h followed by sonication by using probe sonication. After the metal ions had been thoroughly mixed, the reaction mixture was transferred to a Teflon-lined autoclave and heated at 180°C for 12 hours. After that, the autoclave was allowed to cool and the sample was washed with water 5-6 times to remove the excess ammonia. The resulting reaction mixture was then freeze-dried for lOh at -52°C under high vacuum pressure. The sample was taken after the freeze-drying procedure was completed, and it had a black color flaky structure. The obtained sample was named as MnCo2O4/NEGF. For comparison, the controlled samples such as N-doped entangled graphene (NEGF), MnsCU supported N-doped entangled 3D graphene (Mn3O4/NEGF), and CO3O4 supported N-doped entangled 3D graphene (CO3O4/NEGF) was also synthesized. The NEGF, MnsO NEGF, CO3O4/NEGF was prepared by using the same methods without adding any metal precursor and graphene oxide, with the addition Co(OAc)2.4H2O, Mn(OAc)2.4H2O respectively, keeping all the other parameters as such.

[0063] Fig 7 provides schematic illustration of the stages involved in the stepwise synthesis of MnCo2O4/NEGF as an ORR/OER bifunctional electrocatalyst, and demonstration of its application as the air-electrode for the Solid-State Rechargeable Zn-Air Battery.

(c) Preparation of physically mixed composite of M11CO2O4 and N-doped Entangled 3D Graphene (MnCo2O4@NEGF):

[0064] To prepare the physically mixed composite of MnCo2O4 and NEGF, 100 mg of the as-prepared NEGF and 50 mg of MnCo2O4 were mixed with the help of a mortar and pestle.

Example 2: Physical Characterization a) Field emission scanning electron microscopy (FESEM) analysis:

[0065] Fig 1(a) showed the FESEM image of the MnCo2O4/NEGF, which represented the self-assembly structure of nitrogen-doped three-dimensionally oriented graphene and displaying the porous architecture of the entangled 3D graphene sheets. The magnified image of MnCo2O4/3D NEGF shown in Fig 1(b) indicated the interconnected two-dimensional nitrogen-doped graphene. Fig 1 (c) depicted 3D micro-CT images of MnCo2O4/NEGF, showing the porous structure of 2D sheets were connected. b) Transmission electron microscopy (TEM) imaging:

[0066] Transmission electron microscopy (TEM) imaging was performed to visualize the distribution of MnCo2O4 nanoparticles over 3D NEGF support (Fig Id). The TEM analysis showed that the spheric al- shaped MnCo2O4 nanocrystals were uniformly distributed over individual sheets of N-doped graphene. The controlled distribution of the metal oxide nanoparticles was credited to the doped- N in the graphene sheets, which generated asymmetric carbon centers helping in the creation of homogeneous nucleation sites for growth of metal oxide nanoparticles. A fraction of metal oxide nanoparticles were distributed at the inner surface of 3D graphene, which were protected by the thin layer of graphene sheets providing better stability and preventing the chances of self-agglomeration of nanoparticles. The size of the spherical nanoparticles was distributed mostly in the range of 30-60 nm. Fig 1(e) showed the high-resolution transmission electron microscopy (HRTEM), elucidating that the metal oxides were crystalline in nature. The metal oxide nanoparticles were having lattice fringe widths of d-spacing 0.25 and 0.21 nm, which was ascribed to the (311) and (211) facets suggesting the formation of cubic MnCo2O4 spinel phase. The selected area electron diffraction (SAED) pattern shown in Fig l(f-k), was the elemental mapping of the MnCo2O4/3D NEGF catalyst. Elemental mapping exhibited presence and distribution of Co, Mn, O, C, and N, which was in line with the chosen composition of the catalyst. The presence of elemental cobalt and manganese in same positions with almost double intensity of cobalt clearly supports bimetallic structured Co and Mn formation. c) Pore size:

[0067] Fig 2(a) showed the comparative pore size distribution of NEGF, MnCo2O4, and MnCo2O4/NEGF materials where the pores were distributed in the region of 2- 20 nm for NEGF and 2-16 nm for MnCo2O4/NEGF. However, MnCo2O4 showed a significantly lower pore size distribution. The significantly suppressed pore size in the case of CoMn2O4/NEGF catalyst was in the range of 16-20 nm was mostly due to the agglomerated nonporous structure of spinel oxides (MnCo2O4). The Type-IV isotherms were seen in both NEGF and MnCo2O4/NEGF, Fig 2(b). Moreover, the higher BET surface area of NEGF (450 m ² g ¹ ) confirmed the highly porous nature of NEGF as observed in the FESEM and CT-tomography image analysis. A reduction in BET surface area of MnCo2O4/NEGF to 300 m ² g ^-1 showed that some of the metal oxide species were lying in the microspores obscuring porous surface. The large specific surface area of catalyst would be beneficial towards establishment of effective TPB in catalysis process suitable for fabrication of air electrodes of rechargeable ZAB . d) X-ray diffraction (XRD) analysis: [0068] X-ray diffraction (XRD) analysis of NEGF displayed the broad diffraction peaks at 29 values of 26° and 43° corresponding to the (002) and (100) graphitic diffraction planes, respectively. The absence of any metallic peaks in the spectra suggested the higher purity level of the prepared nitrogen-doped 3D graphene. The XRD pattern of CO3O4/NEGF showed a comparatively intense peak at 29 values of 35° corresponds to (311) plan for CO3O4. However, after the incorporation of Mn into the spinel structure of CO3O4, the resulting MnCo2O4/NEGF showed almost similar peaks intensity with a small shift in the peak position. The XRD pattern of MnCo2O4/NEGF confirmed a series of peaks at 29 = 18.3, 39.2, 35.6, 37.9, 43.2, 53.8, 57.2, 62.7 and 74.0°, which was ascribed to (111), (220), (311), (400), (422), (511), (440) and (533) diffraction peaks corresponding to spinel structure MnCo2O4(JCPDS No.23-1237). After incorporating spherical shaped MnCo2O4 over NEGF, a graphitic (002) plane shift towards lower diffraction angle compared to NEGF was observed. This was ascribed due to incorporation of spherical MnCo2O4 nanoparticles between graphene layers which increased d-spacing of nitrogen-doped graphene sheets. e) X-ray photoelectron spectroscopy (XPS):

[0069] X-ray photoelectron spectroscopy (XPS) measurements had been employed in Fig 3 (a-d). Fig 3 : shows the XPS analysis of MnCo2O4/NEGF; (a) comparative survey scan spectra of NEGF, CO3O4/NEGF, and MnCo2O4/NEGF showed the presence of C, N, O, Co, and Mn in the respective catalysts; (b) deconvoluted spectra of Co2p showed presence of two spin- spin splitting peaks revealed the +2 and +3 oxidation state of Co in MnCo2O4/NEGF; (c) deconvoluted Mn spectra, showed two peaks corresponding two oxidation state of Mn, +2 and +3; (d) deconvoluted Nls spectra, confirmed the presence of four types of nitrogen. The survey scan spectra of NEGF, CO3O4/NEGF, and MnCo2O4/NEGF shown in the Fig 3 confirmed the presence of Mn, Co, O, N, and C in the respective materials. The characteristic Co 2p XPS peaks corresponding to CO3O4/NEGF and MnCo2O4/NEGF appeared at the binding energy (B.E.) value of 784.2 eV and 795.5 eV and 783.5eV and 796.5eV, respectively. The characteristic peak separation (-15.84 eV) between two peaks remained the same for spinel oxides. However, the shift in the binding energy after incorporating Mn into the spinel oxides CO3O4 evidenced the formation of bimetallic (MnCo2O4) spinel oxides. The observed negative shift in the binding energy of MnCo2O4 compared to CO3O4 might be due to the charge transfer from Co to Mn. Furthermore, deconvoluted XPS spectra of Co 2p in MnCo2O4/NEGF showed two doublet peaks at the B.E. values of 783.1 and 798.8eV with a band separation of ~15.7eV pointing towards the existence of the +2 and +3 oxidation states of Co. In addition, the deconvoluted Mn spectra showed the two spin-spin coupling peaks at the B.E. values of 783.1 and 798.8eV corresponding to the Mn 2p3/2 and Mn 2pi/2 states of Mn also confirmed the existence of the +2 and +3 oxidation states. Moreover, the deconvoluted N Is spectra of the MnCo2O4/NEGF displayed the peaks at pyridinic-N at 398.6 eV and the pyrrolic-N at 399.7eV as the major moieties along with smaller proportions from the graphitic-N at 400.5eV and NFU ⁺ at 405.5eV. The presence of nitrogen doping in the conducting support was mostly responsible for improving surface wettability of electrocatalysts, thereby enhancing electrocatalytic activity. f) Wettability of the Electrocatalyst:

[0070] Hydrophilicity and Hydrophobicity property was found to be an important aspect to maintain the effective electrochemical triple phase boundary (TPB) during electrochemical process. The contact angle (CA) measurement was performed in Fig. 5e to check the surface wettability of MnCo2O4/EGF and MnCo2O4/NEGF catalysts. Fig. 5(f) the contact angle (CA) data corresponding to MnCo2O4/NEGF- coated surface of the GDL shows a water contact angle of 109.2°. The lower contact angle value of 24° for MnCo2O4/NEGF confirmed the higher hydrophilicity of the catalyst, which could easily wet the catalyst surface resulting in water flooding, thereby hindering the mass transfer due to excessive wettability of the surface. After N doping into the 3D structure of graphene, the contact angle value for MnCo2O4/NEGF reached the value of 42°. Optimum contact angle value implied that appropriate hydrophilicity/hydrophobicity of catalytic material was more conducive to form the gas-liquid-solid TPBs during the electrochemical reaction.

Example 3: Electrochemical Half-cell Studies

(a) Rotating Disk Electrode Study: [0071] The electrochemical analysis was done by a couple of electrochemical techniques such as voltammetry. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and impedance techniques were adopted. A rotating disc electrode (RDE) set-up (Pine Instrument) was employed for the LSV measurements. The electrochemical cell was made of a set-up of a three-electrode used with an SP-300 model BioLogic potentiostat. A glassy carbon electrode was used as the working electrode, whereas a graphite rod (Alfa Aesar, 99.99%) and Hg/HgO were employed as the counter and reference electrodes, respectively. For the comparison of the ORR and OER performance of the prepared electrocatalyst in half-cell studies, the ORR activity of 20% Pt/C and the OER activity of RuO2 were included. The electrocatalyst slurry was prepared by mixing the catalyst (5 mg) in 1 mL isopropyl alcohol-water (3:2) solution and 40 pL of Nafion solution (5 wt%, Sigma- Aldrich) using approximately 1 h water-bath sonication. After that, 10 pL of the catalyst slurry was drop-coated on the surface of the working electrode, which was polished with 0.3 pm alumina slurry in DI water followed by cleaning with DI water and acetone. The electrode was then dried under an IR-lamp for 1 h. The experiment was carried out in an aqueous solution of 0.1 M KOH for ORR and 1 M KOH for OER performance measurements.

(b) Solid-state ZAB demonstration:

(b)-l: Preparation of the Gel Electrolytes:

[0072] 2 g PVA powder (MW205000, Sigma-Aldrich) was typically dissolved in 16 mL ultrapure water at 90 °C with vigorous agitation. When a translucent gel solution was formed, 4 mL of 9 M KOH solution was added dropwise and the mixture was stirred for 20 min. at 90 °C. The gel solution was put into a petri dish (2 cm in diameter), and then stored in the refrigerator at -20°C for 1 h and then at 0°C for another 1 hour. After that, a thin sheet structure of the gel electrolyte was formed, which was used as the electrolyte for the fabrication of the solid-state ZAB device.

(b)-2: Assembly and test of solid-state ZAB device:

[0073] The solid-state rechargeable ZAB was assembled by utilizing Zn powder as the anode, MnCo2O4/NEGF-coated GDL as the air-cathode, and PVA/KOH gel as the electrolyte in an electrochemical ZAB device set up (MTI Corporation). For the preparation of the catalyst slurry, MnCo2O4/NEGF was added to the 1: 4 ratio mixture of isopropyl alcohol and water followed by keeping for sonication for 1 h. To the resulting dispersion, 10 wt% Fumion solution was added, and the mixture was sonicated for an additional 1 h. After the complete dispersion, the catalyst slurry was brush-coated over a gas diffusion layer (GDL) and was dried at 60 °C for 12 h to achieve a catalyst loading of 1.0 mg cm ^-2 (electrode area = 1.0 cm ² ). A multichannel VMP-3 model Bio-Logic Potentiostat/Galvanostat was used to evaluate the ZAB set-up at room temperature. The ZAB was analyzed by steadystate polarization at a scan rate of 5 mV/s. The polarization analysis and EIS studies were performed at a constant voltage of 1.0 V with an amplitude of 20 mV; the galvanostatic discharge and discharge-charge cycling (5 min discharge followed by 5 min charge) tests were carried out by a Bio-Logic potentiostat.

(c) Electrocatalytic rotating disc electrode (RDE) performance analysis:

[0074] The intrinsic ORR activity of the catalysts was measured by linear sweep voltammetry (LSV) analysis in 0.1 M KOH at the scan rate of 10 mV sec ¹ under O2 atmosphere to maintain the working electrode rotation at 1600 RPM. The comparative LSV profile (Fig 4a) evidences the superior ORR performance achieved by MnCo2O4/NEGF compared to the control samples, i.e., NEGF (0.86 V), CO3O4/NEGF (0.89 V), and Mn ₃ O ₄ /NEGF (0.85 V). In addition, the ORR performance of MnCo2O4/NEGF (0.93 V) was found to be close to that of the conventional catalysts (Pt/C), showing its higher catalytic potential (0.99 V). Similarly, the OER activity was also measured for NEGF, CO3O4/NEGF, Mn ₃ O4/NEGF, MnCo2O4/NEGF, and RuO2 in 1 M KOH at a scan rate of 10 mV sec ^-1 under N2 atmosphere (Fig 4b). LSVs recorded for MnCo2O4/NEGF showed better electrochemical OER activity compared to NEGF, CO3O4/NEGF, and Mn3O4/NEGF. The superior performance of MnCo2O4/NEGF catalyst towards both oxygen reactions (ORR/OER) was observed in LSV analysis. Moreover, differences in OER potential (Ej@ 10 mA cm ^-2 ) and ORR half-wave potential (E1/2) were generally used to evaluate the performance of bifunctional catalyst. Overall bifunctional activity (ORR-OER) of MnCo2O4/NEGF was found to be 0.82 V (Fig 4c), which was comparable or better than previously reported various bifunctional electrocatalyst. Observed higher bifunctional oxygen reaction activity of prepared catalyst was attributed to bimetallic composite of Mn and Co spinel oxides and nitrogen doped 3D carbon support providing effective triple phase boundary (TPB) formation for better mass transport properties.

Example 4: Fabrication of All-solid-state ZAB Device a) Preparation of Gel Electrolytes:

[0075] 2 g PVA powder (MW205000, Sigma-Aldrich) was dissolved in 16 mL ultrapure water at 90 °C with vigorous agitation. When translucent gel solution was formed, 4 mL of 9 M KOH solution was added dropwise and stirred for 20 minutes at 90°C. The gel solution was put into a petri dish (2 cm in diameter), and stored in refrigerator at -20°C for 1 hour and then at 0°C for 1 hour. After that, thin sheet structure of gel electrolyte was formed and used as electrolyte for all-solid-state ZAB device fabrication. b) All-solid-state ZAB Device:

[0076] All- solid- state rechargeable ZAB was assembled by utilizing Zn powder as the anode, MnCo2O4/NEGF coated GDL as the air cathode electrodes, and PVA/KOH gel as an electrolyte, respectively, in an electrochemical ZAB device set up (MTI corporation) depicted in Fig 8. For the preparation of the electrocatalyst slurry, MnCo2O4/NEGF was added to the mixture of (1: 4) ratio isopropyl alcohol and water and kept for 1 hr sonication. To the resulting dispersion, 10 wt% Fumion solution was added, and the mixture was sonicated for an additional 1 hour. After the complete dispersion, the catalyst slurry was brush coated over the gas diffusion layer (GDL) and dried at 60 °C for 12 hours to achieve a catalyst loading of 1.0 mg cm-2 (electrode area = 1.0 cm ² ). The ZAB was analyzed by steady-state polarization at a scan rate of 5 mV/s. The air electrode for all- solid- state ZAB contained a porous catalyst layer coated onto diffusion layer (GDL) with hydrophobic PTFE pointed on the air-facing side. A solution consisting of 6 M KOH and 0.1 M Zn(Ac)2 was added during the fabrication of PVA as gel electrolyte. The polarization analysis and EIS studies were performed at a constant voltage of 1.0 V with an amplitude of 20 mV; the galvanostatic discharge and discharge- charge cycling (5 min discharge followed by 5 min charge) tests were carried out by Bio-Logic potentiostat. c) Characterization of ZAB Assembly:

[0077] The application of MnCo2O4/NEGF as an air electrode to function in the discharging (ORR) and charging (OER) modes for a solid-state ZAB was demonstrated by employing the catalyst-coated gas diffusion electrode (GDE) as the cathode. Prior to the fabrication of the cell and its testing, the catalyst-coated GDL surface was characterized by using FESEM and X-ray CT mapping to check the 3D micro structure of the resulting electrodes (Fig 1). Fig 5a and the inset image showed the cross-sectional FESEM image of the bare GDL, revealing the mostly flat structure of the surface. However, in the case of the GDL coated with MnCo2O4/NEGF, a thick layer with 3D structure (indicated by the dotted lines) was observed (Fig 5b). The inset of Fig 5b gave better clarity of the surface containing the 3D self-assembled structure of the coated layer of MnCo2O4/NEGF. Compared to the highly porous nature of the MnCo2O4/NEGF layer on the GDL, the catalyst layer of Pt/C+RuO2 was found to be significantly less porous. Fig 5c and 5d showed the 3D tomogram cross-section images of the bare GDL and the MnCo2O4/NEGF- coated GDL, respectively. d) Electrochemical performance of all-solid-state rechargeable ZAB device:

[0078] Fig 6(a-e) showed the performance of all-solid-state rechargeable ZAB with the open-circuit voltage (OCV) values of 1.31 and 1.20 V, respectively, for MnCo2O4/NEGF and Pt/C+RuO2 coated electrodes. The comparative steady- state cell polarization led to the maximum power density (P _m ax) of 110 and 200 mW cm' ² for the ZABs based on Pt/C+RuO2 and MnCo2O4/NEGF, respectively. The cathode catalysts showed superior performance for the prepared catalyst (MnCo2O4/NEGF) compared to Pt/C+RuO2, which was ascribed to be better interface formation in the former catalyst. The performance of the fabricated all- solid-state ZAB was comparable and even superior to some of the reported all-solid- state ZABs in the literature (Table 2). The impedance spectra for all- solid- state ZABs were significantly different for MnCo2O4/NEGF and Pt/C+RuO2, which might be due to better interface formation between the 3D porous structure of the electrocatalyst and GDL surface compared to the two-dimensional architecture of Pt/C+RuCh catalyst. Furthermore, the galvanostatic charge/discharge curve measured at 10 mA cm ^-2 was shown in Fig 6c.

Table 2 - Comparison of the performance of the solid-state ZAB systems based on the non-precious metal-based electrocatalysts.

[0079] The observed difference between the charging and discharging voltages of ZAB on MnCo2O4/NEGF during the initial process was 0.84 V which was lower than 0.91 V of the Pt/C + RuO2. After 50 h of continuous charge-discharge cycles, a nominal voltage difference increased by 0.10 V on ZAB consisting of MnCo2O4/NEGF compared to 1.1 V after 15 h cycle operation on Pt/C+ RuO2 ZAB . The higher stability of MnCo2O4/NEGF based air cathode might be due to better air cathode interface formation via the porous and stable 3D structure of nitrogen- doped carbon. Moreover, the magnified image showed (Fig 6d) NEGF-based that in the case of MnCo2O4/NEGF, the charge-discharge voltage plateau were more symmetric but in the case of Pt/C+RuO2 deficient asymmetric charge-discharge curve. This feature reveals the better bifunctional activity at the ZAB air cathode interface in the case of MnCo2O4/NEGF compared to Pt/C+RuCh.

[0080] The ORR process was found to be more sensitive to the TPB interface during the discharge process than the OER reaction. Discharge curves for the ZABs at various current densities were collected to analyze the influence of the 3D micro structure on ORR kinetics at ZAB air cathode (Fig 6e). So, the discharge curve at various current densities 5, 10, 20, and 30 mA cm ^-2 were recorded for MnCo2O4/NEGF and Pt/C + RuO2 catalysts for 1 h. When the current density was increased from 5.0 to 10.0 mA cm' ² , the charge voltage with the MnCo2O4/NEGF cathode decreased from 1.25 to 1.24 V. However; it decreased significantly from 1.1 to 0.2 V with the Pt/C+RuO2. Even at 30.0 mA cm ² , the former had a charge voltage of 1.10 V, which was about 210 mV higher than the Pt/C + RuO2. The ZAB based on a 3D nitrogen-doped containing catalyst had a relatively small voltage gap of 0.11, 0.12, 0.13, and 0.15 V at 5.0, 10.0, 20.0, and 30.0 mA cm' ² . However, the ZAB with Pt/C + RUO ₂ catalyst were 1.05, 0.14, 0.15, and 0.60 V, respectively. Even in the case of Pt/C+RuO2 at a higher current density of 10 mA cm' ² sudden drop of potential was observed. The catalyst (MnCo2O4/NEGF) coated air cathode benefited more from its higher ORR kinetics at the ZAB interfaces which showed the vast advantage of the 3D porous architecture in terms of better oxygen gas transport and kinetics. This demonstrated that these kinds of air cathode had outstanding application potential under high current density. Furthermore, the galvanostatic discharge curve recorded for MnCo2O4/NEGF and Pt/C + RuO2 at 10 mA cm' ² catalyst had a discharge time of about 48h and 40h, respectively. Hence, in the longer run, the MnCo2O4/NEGF catalyst was expected to outperform the Pt/C + RUO ₂ system both in terms of performance and long-term durability under a realistic ZAB system. The remarkable high-performance of rechargeable all-solid- state ZAB was attributed to the suitable air cathode interface design, sufficient active sites, and efficient mass transfer properties of MnCo2O4/NEGF.

ADVANTAGES OF THE INVENTION [0081] 3D porous architecture of N-doped graphene supported MnCo2O4 nanosphere viz. MnCo2O4/NEGF (N-doped 3D porous entangled graphene) as an air cathode in all-solid-state Zinc-air batteries (ZABs).

[0082] Features like porous 3D architecture of the catalyst, balanced hydrophilic/hydrophobic characteristics, and optimal oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER) activity are found to be favorably helping the system as an air-electrode for the rechargeable ZAB application.

[0083] The 3D structure of the catalyst greatly helps the system in mass transfer and active site accessibility in the electrode. At the same time, the optimal hydrophilicity, originating from the functional attributes of the support surface, is found to play a significant role in constructing an effective interface for the catalyst and the electrolyte.

[0084] In terms of activity of MnCo2O4/NEGF toward said reactions, overpotential values are found closely comparable to respective state of art systems(Pt/C for ORR & RUO ₂ for OER).

[0085] The demonstration of a solid-state rechargeable ZAB device with MnCo2O4/NEGF as the air electrode delivered a maximum peak power density of 200 mWcm" ² , with good stability at the time of the charge-discharge cycling process.

[0086] In terms of performance and charge-discharge cyclability, the system based on the homemade catalyst is found to have a clear upper hand compared to a system consisting of the state-of-the-art ORR/OER catalyst combination of Pt/C + Ru02.

[0087] The synergistic effect between MnCo2O4 nanoparticles and N-doped porous graphene promotes better interface formation triple phase boundary (TPB). This benefits the system in terms of its bifunctional characteristics to perform as an effective electrocatalyst for facilitating both ORR and OER processes.

[0088] The established triple phase boundary (TPB) enhances the available reaction sites for gas and electrolyte solutions. Secondly, the electronic interaction between Co and Mn creates an appropriate adsorption site for O2 and OH’ ions.

[0089] The N-doped porous graphene controls the optimum hydrophilicity, and hydrophobicity which helps to better wettability of the electrocatalyst. [0090] The factors mentioned above collectively result in higher performance of electrocatalyst under the rotating disc electrode (RDE) condition and as an air cathode in ZABs.