Bifuctional electrocatalyst, its process of preparation and application in metal air batteries

Abstract

The present invention relates to a solid-state rechargeable zinc-air battery featuring a novel bifunctional electrocatalyst, a dual-crosslinked polyacrylic acid hydrogel electrolyte, and a stannate-based additive for in situ zinc anode modification. The cathode comprises a gas diffusion layer coated with ruthenium-ruthenium oxide core-shell nanoparticles supported on nitrogen-doped graphene, providing enhanced bifunctional catalytic activity and stability. The anode consists of zinc metal modified in situ by a stannate-based additive to form a solid electrolyte interphase layer, effectively suppressing dendrite formation. The electrolyte membrane is a polyacrylic acid hydrogel, covalently and ionically cross-linked, and soaked in an aqueous solution containing potassium hydroxide, zinc acetate, and a stannate-based additive, resulting in improved mechanical strength, ionic conductivity, and battery safety. The integrated system delivers high power density, specific capacity, and robust cycling stability, offering a significant advancement in the field of solid-state zinc-air batteries.

Description

[0001] BIFUCTIONAL ELECTROCATALYST, ITS PROCESS OF PREPARATION AND APPLICATION IN METAL AIR BATTERIES

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to electrochemistry. More particularly, the present invention is about electrochemical energy storage technology, specifically to bifunctional electrocatalysts, separators, and their applications in rechargeable metal-air batteries, including solid-state zinc- air batteries.

[0004] BACKGROUND OF THE INVENTION

[0005] The field of electrochemical energy storage has seen growing interest in metal-air battery technologies, driven by the need for high energy density and cost-effective solutions. Rechargeable zinc-air batteries (ZABs) offer attractive theoretical energy densities and utilize abundant zinc resources, positioning them as promising alternatives to lithium-ion systems. Advancements in electrode materials, electrocatalysts, and solid-state electrolytes are necessary to address challenges associated with ZABs, such as sluggish oxygen reduction and oxygen evolution kinetics and the risk of electrolyte leakage.

[0006] Efforts to develop practical metal-air systems have focused on achieving efficient bifunctional catalysts that can catalyze both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), alongside robust solid-state separators to prevent electrolyte degradation and dendrite formation. Hydrogel-based electrolytes have emerged as a route to combine high ionic conductivity with mechanical flexibility, potentially enabling near solid-state operability and improved cycling stability. Nonetheless, integrating high-performance catalysts with durable separators in a single ZAB architecture remains an important objective.

[0007] US20180093893A1 describes a nitrogen and phosphorus-co-doped mesoporous carbon material (NPMC) exhibiting catalytic activity for both ORR and OER when employed as a battery electrode. However, the disclosed material demonstrates limited charge / discharge current densities and can suffer from carbon corrosion under prolonged cycling, reducing longterm activity.

[0008] WO2023223350 describes a three-dimensional nitrogen-doped porous graphene electrocatalyst, along with an all- solid- state rechargeable zinc-air battery employing this catalyst on the air cathode to achieve enhanced cycle stability and power density. However, the approach still faces challenges with catalyst particle aggregation and moderate interfacial impedance, which can limit rate capability and operational lifetime.

[0009] Despite progress in bifunctional catalyst design and hydrogel separator development, existing systems often exhibit insufficient electrochemical activity at practical current densities, combined with mechanical and interfacial instability under repeated cycling. In particular, preventing zinc dendrite penetration and maintaining a leak-free electrolyte interface are ongoing concerns that hinder reliable long-term operation.

[0010] A significant unmet need lies in integrating robust bifunctional catalysts with mechanically resilient electrolyte systems that collectively address oxygen electrocatalysis, dendrite suppression, and interface stability. In particular, there is a demand for catalyst-support combinations that resist particle aggregation and maintain high catalytic activity at low overpotentials, as well as separator architectures that prevent electrolyte leakage and inhibit zinc dendrites under harsh cycling conditions. Addressing these specific shortcomings is critical to advancing zinc-air batteries with the energy density, safety, and operational reliability required by next-generation energy storage markets.

[0011] OBJECTIVES OF THE INVENTION

[0012] The main objective of the present invention is to provide a hydrogel electrolyte-mediated in- situ Zn-anode modification and bifunctional electrocatalyst that can overcome nanoparticle aggregation, provide increased interfacial stability, and cyclic stability.

[0013] Another objective is to provide a bifunctional electrocatalyst that can maintain high catalytic activity at low overpotentials for oxygen electrocatalysis.

[0014] Yet another objective of the present invention is to provide catalyst support combinations with separator architectures that can prevent electrolyte leakage and inhibit zinc dendrites during cycling conditions.

[0015] ACRONYMS USED TO DESCRIBE THE INVENTION

[0016] N: Nitrogen

[0017] B: Boron

[0018] S: Sulfur F: Fluorine

[0019] Al: Aluminium

[0020] 3D: 3 dimensional

[0021] CNT: Carbon nanotube

[0022] NGr: N-doped graphene

[0023] PAA: poly aery lie acid c-PAA: cross-linked network structure of PAA hydrogel d-PAA: dually cross-linked, single network structure of PAA hydrogel

[0024] KZ: before using it as an electrolyte membrane, the as-prepared d-PAA hydrogel (0.53 mm thick) was soaked for 24 h in 6 M KOH, 0.2 M Zn acetate electrolyte, and the film.

[0025] KZS: before using it as an electrolyte membrane, the as-prepared d-PAA hydrogel (0.53 mm thick) was soaked for 24 h in 6 M KOH, 0.2 M Zn acetate electrolyte, and Sn electrolyte additive and film.

[0026] ADT: Accelerated Degradation Test

[0027] CA: Constant Applied Potential test

[0028] SUMMARY OF THE INVENTION

[0029] The present invention relates to a bifunctional electrocatalyst and its preparation process and application in solid-state rechargeable zinc-air batteries.

[0030] In an aspect, the present invention provides a bifunctional electrocatalyst comprising: a. transition metal Ml; b. transition metal oxide; and c. a support wherein the transition metal Ml (a) is ruthenium; the transition metal oxide (b) is ruthenium oxide; the support (c) is nitrogen-doped graphene.

[0031] In the bifunctional electrocatalyst of the invention, the ruthenium and ruthenium oxide form an active core- shell structure, and the nitrogen-doped graphene forms a sheet- like morphology. The ruthenium is generally encased by ruthenium oxide nanoparticles. The active core-shell of ruthenium-ruthenium oxide nanoparticles can be uniformly dispersed over the sheet-like nitrogen-doped graphene. In an embodiment, the active core- shell of ruthenium-ruthenium oxide nanoparticles has an average diameter of 2-5 nm.

[0032] In another aspect, the present invention provides a process for preparing the bifunctional electrocatalyst comprising: a) dispersing a nitrogen-doped graphene support material in a solvent followed by ultra-probe sonication to obtain a suspension; b) reacting the suspension of step a) with a ruthenium precursor and melamine followed by drying to obtain a mixture; c) annealing the mixture of step b) under an inert atmosphere to form ruthenium nanoparticles on the nitrogen-doped graphene; d) treating the annealed material of step c) with acid; e) washing, filtering, and drying the material of step d) to obtain pre-calcined bifunctional electrocatalyst; and f) calcining the pre-calcined electrocatalyst in air to partially oxidize the surface of the ruthenium nanoparticles, thereby forming bifunctional catalyst comprising ruthenium- ruthenium oxide active core-shell nanoparticles supported on nitrogen-doped graphene.

[0033] In yet another aspect, the present invention provides a solid-state rechargeable zinc-air battery comprising:

[0034] (a) a cathode comprising a gas diffusion layer coated with the bifunctional electrocatalyst of the present invention;

[0035] (b) an anode comprising a zinc metal, wherein the surface of the zinc metal is modified in situ by a stannate-based additive to form a solid electrolyte interphase layer; and

[0036] (c) an electrolyte membrane comprising a dual-crosslinked polyacrylic acid hydrogel covalently cross-linked with a multifunctional cross-linker and ionically cross-linked with multivalent metal ions, the hydrogel being soaked in an aqueous solution comprising potassium hydroxide, zinc acetate, and a stannate-based additive. In the zinc-air battery, the polyacrylic acid hydrogel can be dual-crosslinked by both covalent and ionic cross-linkers and can be prepared by a freeze-drying process to provide a porous network structure.

[0037] In a further aspect, the present invention provides a process for preparing the solid-state rechargeable zinc-air battery of the present invention, comprising:

[0038] (i) preparing the bifunctional electrocatalyst of the present invention;

[0039] (ii) polymerizing acrylic acid monomer with a covalent crosslinking agent to form a poly aery lie acid network, adding an ionic crosslinking agent and an initiator, curing the polymer, and freeze-drying to form a porous dual-doped crosslinked hydrogel;

[0040] (iii) soaking the hydrogel in an aqueous solution comprising potassium hydroxide, zinc acetate, and a stannate -based additive to obtain the hydrogel electrolyte membrane;

[0041] (iv) fabricating a cathode by coating the bifunctional electrocatalyst on a gas diffusion layer;

[0042] (v) taking a zinc metal as an anode and contacting with the hydrogel electrolyte membrane to form an in situ stannate-derived interphase layer on the zinc anode; and

[0043] (vi) assembling the cathode, the zinc anode, and the electrolyte membrane in a cell housing to provide the solid-state rechargeable zinc-air battery.

[0044] The stannate-based additive can be sodium stannate preferably at a concentration of about 0.005 M. The freeze-drying step in the above process can be carried out at 70-90 bar for 8-10 hours.

[0045] BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 (a to d) shows FESEM images and the TEM images of the bifunctional electrocatalyst.

[0047] Figure 2 (a to d) shows the comparative thermogravimetry (TG-DTA) profiles recorded for (a) NGr, (b) Ru / NGr, (c) Ru-RuO2 / NGr, and (d) Ru-RuO2 / rGO.

[0048] Figure 3 (a to k) shows Ground State Optimized Structures of the bifunctional electrocatalyst. Figure 4 shows (a) The ORR linear sweep voltammograms (LSVs), (b) the ORR Tafel plots,

[0049] (c) the rotating ring-disc electrode (RRDE) analysis of Pt / C and Ru-RuOi / NGr showing the electron transfer number (n-value) and % of H2O2 generated corresponding to the ORR process,

[0050] (d) the OER LSV profiles recorded in N2 saturated 1 M KOH electrolyte, (e) the OER Tafel plots, (f) the comparative LSV profiles of Ru-RuO2 / NGr, Pt / C, and Ru02 revealing the bifunctional electrocatalytic activity of the systems toward ORR / OER, (g) the bar diagram corresponding to the comparative potentials for OER measured at the current density of 10 mA cm-2(EjiomAcm-2), the halfwave-potential (E1 / 2) for ORR and the AE values (AE = EjioOER - E1 / 2 ORR) extracted from the ORR / OER LSV profiles, (h) the LSV profiles corresponding to ORR recorded before and after the durability analysis of Ru-RuO2 / NGr, (i) the OER chronoamperometric (CA) stability test performed in 1 M KOH.

[0051] Figure 5 shows (a) Comparative XRD patterns recorded after ADT and the CA test, (b) The TEM images of Ru-RuO2 / NGr after 5K ADT cycles and (c) FESEM image and corresponding elemental mapping of Ru-RuO2 / NGr after 5K ADT cycles.

[0052] Figure 6 shows (a) schematic representation of the synthesis of the c-PAA and d-PAA hydrogel electrolytes, (b) the representation of the steps involved during the synthesis of the d-PAA hydrogel electrolyte, (c and d) the FESEM image of the d-PAA hydrogel after the thermal drying, and after the freeze-drying respectively.

[0053] Figure 7 shows the schematic representation of the contact angle measured with different electrolytes: (a-a4) the K electrolyte displaying its hydrophobic nature over the Zn anode, (b- b4) the KZ electrolyte displaying slightly hydrophobic nature over the Zn anode, and (c-c4) the KZS electrolyte displaying its superhydrophilic nature over the Zn anode.

[0054] Figure 8 shows the contact angle images recorded on (a-c) Ru, RuCL / NGr an (d-f) Pt / C electrode with different electrolyte.

[0055] Figure 9 shows demonstration of the solid-state aqueous Zn-air battery: (a) comparative polarization plots, (b) specific capacity calculated, (c) comparative galvanostatic discharge profiles recorded at different current densities (mA cm'2), (d) comparative galvanostatic charge-discharge profiles recorded at 2 mA cm'2each cycle for 5 min. in rechargeable Zn-air battery (RZAB), (dl-d3) zoomed part of the region marked in (d), (e) cycling stability of the ZABs; (f-g) confocal 3D images recorded by staking and 3D construction of the Zn electrodes after the charge-discharge cycles: (f) without additive cycled Zn anode showing the growth of the dendrites, and (g) in presence of Sn-additive cycled Zn anode.

[0056] DETAILED DESCRIPTION OF THE INVENTION

[0057] Accordingly, in order to accomplish an objective, the present invention provides a bifunctional electrocatalyst and its preparation process and application in metal air batteries.

[0058] The present invention relates to a solid-state rechargeable zinc-air battery (ZAB) that integrates a novel bifunctional electrocatalyst, a dual-crosslinked polyacrylic acid hydrogel electrolyte, and a stannate-based additive for in situ zinc anode modification. This combination addresses key challenges in the field, including sluggish oxygen electrocatalysis, dendrite formation, and electrolyte leakage, thereby enabling high energy density, robust cycling stability, and enhanced safety.

[0059] In an embodiment, the present invention provides a bifunctional electrocatalyst comprising: a. transition metal Ml; b. transition metal oxide; and c. a support, wherein the transition metal Ml (a) is ruthenium; the transition metal oxide (b) is ruthenium oxide; the support (c) is nitrogen-doped graphene.

[0060] In the bifunctional electrocatalyst of the invention, the ruthenium and ruthenium oxide form an active core- shell structure, and the nitrogen-doped graphene forms a sheet- like morphology. The ruthenium is generally encased by ruthenium oxide nanoparticles. The active core- shell of ruthenium-ruthenium oxide nanoparticles is uniformly dispersed over the sheet-like nitrogen- doped graphene. In an embodiment, the active core-shell of ruthenium-ruthenium oxide (Ru- RuCh) nanoparticles has an average diameter of 2-5 nm. The nitrogen in support acts as a dopant, which induces charge and spin distribution near the carbon atoms due to the high electronegativity of the N, which makes the adsorption of the O and its dissociation easier. Uniform distribution of the transition metal and transition metal oxide nanoparticles with an average particle size of 2-5 nm over support and strong interaction with support enhances their intrinsic electrochemical activity and stability. This uniform distribution ensures a high density of exposed electrochemically active sites, while the core-shell structure provides a synergistic interface that optimizes the adsorption energies of oxygenated intermediates. This modulation lowers the energy barriers for oxygen adsorption and dissociation, thereby enhancing both the oxygen reduction reaction (ORR) during battery discharge and the oxygen evolution reaction (OER) during charging.

[0061] The dual role of the nitrogen (N)-doped graphene is to provide a conductive network for rapid electron transfer, and stabilize the Ru-RuCh nanoparticles by preventing their aggregation through strong interfacial interactions. The presence of twin boundary defects in the nanoparticles further increases the density of catalytically active sites, alters the d-band center of the metal, and improves intermediate binding energies.

[0062] Thus, the close interplay between the Ru-RuCh core-shell nanoparticles and the N-doped graphene support leads to an electrocatalyst with enhanced intrinsic activity, improved chargetransfer kinetics, high durability under alkaline conditions, and strong resistance against corrosion or particle agglomeration. These attributes make the catalyst a promising candidate as a bifunctional oxygen electrode in high-performance solid-state rechargeable zinc-air batteries.

[0063] In an embodiment, the present invention provides a process for preparing the bifunctional electrocatalyst comprising: a) dispersing a nitrogen-doped graphene support material in solvent followed by ultra-probe sonication to obtain a suspension; b) reacting the suspension of step a) with a ruthenium precursor and melamine followed by drying to obtain a mixture; c) annealing the mixture of step b) under an inert atmosphere to form ruthenium nanoparticles on the nitrogen-doped graphene; d) treating the annealed material of step c) with acid; e) washing, filtering, and drying the material of step d) to obtain pre-calcined bifunctional electrocatalyst; and f) calcining the pre-calcined electrocatalyst in air to partially oxidize the surface of the ruthenium nanoparticles, thereby forming bifunctional catalyst comprising rutheniumruthenium oxide active core-shell nanoparticles supported on nitrogen-doped graphene.

[0064] The process begins by dispersing nitrogen-doped graphene (NGr) in a solvent under constant stirring followed by ultra-probe sonication to obtain a homogeneous suspension. The constant stirring in step (a) is carried out for 1 to 6 hrs, preferably 1. The solvent in step (a) is selected from but not limited to water, distilled water, ethanol or deionized water, preferably the solvent is deionized water. The ultra-probe sonication in step (a) is carried out for 10-20 minutes with 5-10 seconds on and 25-30 sec off-ramp.

[0065] The drying in step (b) is carried in the temperature range of 60°C - 120°C, preferably 100. The ruthenium precursor is selected from but not limited to ruthenium acetate tetrahydrate (Ru (OAC2. 4HIO), Ruthenium chloride (RuCh-xEhO), Ruthenium ammonium chloride (RU(NH4)C1), preferably the ruthenium precursor is Ru (OAC2. 4H2O). The melamine acts as a nitrogen source and as a structure-directing agent, ensuring in- situ nitrogen doping of graphene during annealing and stabilizing the distribution of Ru species. The drying step removes solvent gently while preventing premature decomposition, thereby ensuring the uniformity of the precursor coating on graphene sheets.

[0066] Subsequently, the dried mixture is annealed in step (c) under an inert atmosphere, such as nitrogen or argon, at a temperature range of 800-900°C, preferably 850°C, for 2-3 hours. During this step, the ruthenium precursor is reduced to metallic Ru nanoparticles that nucleate and grow over the N-doped graphene sheets. Simultaneously, melamine decomposes to release nitrogen species that become incorporated into the graphene lattice, creating graphitic N- dopants. These dopants modulate the electronic structure of the support and create anchoring sites that strongly bind Ru nanoparticles, thereby preventing aggregation and enhancing longterm stability.

[0067] Following annealing, the resulting Ru / NGr material is treated with a strong acid in step (d) to remove unstable or excess metallic residues and expose clean active surfaces. The strong acid is selected from, but not limited to, hydrochloric acid (HC1), sulfuric acid (H2SO4), or perchloric acid (HCIO4), preferably H2SO4. The acid treatment improves the purity of the catalyst and promotes the formation of well-dispersed nanoscale Ru sites with high catalytic activity. The acid-treated material is subsequently washed, filtered, and dried in step (e) in the temperature range of 60 to 120°C, preferably 100°C, to obtain a pre-calcined electrocatalyst. Drying at this stage ensures removal of residual acid and moisture while preserving the porous morphology of the support.

[0068] Finally, the pre-calcined material undergoes calcination in step (f) in air at a temperature of 250-300°C for 2-5 hours, preferably 250°C for 3 hours. This mild oxidation treatment selectively and partially oxidizes the surface of the Ru nanoparticles, forming core-shell nanostructures where metallic Ru is encased in a thin layer of RuCE. The thickness of the RuCE shell is precisely controlled by this step to balance stability with catalytic activity. The thickness of the RuCE shell is in the range of 0.5 to 2 nm. The resulting Ru-RuCE / NGr catalyst combines the metallic conductivity of Ru with the oxide’s ability to adsorb oxygen intermediates, thereby creating synergistic active sites for both ORR and OER.

[0069] Thus, the overall process provides a robust strategy to prepare a bifunctional electrocatalyst featuring uniformly dispersed Ru-RuCE core-shell nanoparticles supported on N-doped graphene.

[0070] In an embodiment, the present invention provides an air battery system employing the bifunctional electrocatalyst of the invention. The system may be configured as a half-cell air battery, a full-cell zinc-air battery, or a solid-state rechargeable zinc-air battery.

[0071] In the half-cell configuration, the air battery comprises a working electrode coated with the bifunctional Ru-RuCE / NGr electrocatalyst, a reference electrode (HgHgO), a counter electrode (graphite rod), and an ion-conducting electrolyte (IM KOH) solution. This configuration allows for the direct evaluation of the catalyst’s bifunctional activity toward the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charge. The half-cell arrangement demonstrates the catalyst’s ability to reduce oxygen efficiently and evolve oxygen reversibly, reflecting its potential for practical energy devices.

[0072] In a full-cell configuration, the present invention discloses a zinc-air battery comprising a zinc- based anode, a cathode containing the bifunctional Ru-RuO2 / NGr electrocatalyst, and a dualdoped hydrogel electrolyte. The zinc anode serves as the oxidation site during discharge, while the cathode coated with the bifunctional electrocatalyst provides efficient ORR / OER activity. The hydrogel electrolyte not only conducts ions but also contributes to stable cycling performance through its porous structure and dopant-induced mechanical reinforcement. In an embodiment, the invention provides a solid-state rechargeable zinc-air battery comprising:

[0073] (a) a cathode comprising a gas diffusion layer coated with the bifunctional Ru-RuCE / NGr electrocatalyst of the present invention;

[0074] (b) an anode comprising a zinc metal, wherein the surface of the zinc metal is modified in situ by a stannate-based additive to form a solid electrolyte interphase layer; and

[0075] (c) an electrolyte membrane comprising a dual-crosslinked polyacrylic acid hydrogel covalently cross-linked with a multifunctional cross-linker and ionically cross-linked with multivalent metal ions, the hydrogel being soaked in an aqueous solution comprising potassium hydroxide, zinc acetate, and a stannate-based additive.

[0076] The cathode is formed by uniformly coating the bifunctional electrocatalyst on the gas diffusion layer (GDL), at a loading of 0.5-30 mg / cm2, ensuring continuous coverage while maintaining porosity for oxygen diffusion. The gas diffusion layer (GDL), is used as a substrate containing a micro porous layer over a carbon fiber. The Ru-RuCE / NGr electrocatalyst exhibits a charge density difference below 0.65, a structural feature that minimizes charge localization and provides highly active sites for oxygen adsorption and electron transfer. The presence of twinboundary defects within the nanoparticles further enhances bifunctional activity by tuning adsorption energies of oxygenated intermediates.

[0077] The anode comprises metallic zinc, which undergoes in situ surface modification when in contact with the hydrogel electrolyte containing stannate ions (sodium, potassium, chloride, or sulfate stannates, preferably sodium stannate at a concentration of about 0.005 to 5 Molar (M)). The metallic zinc can be zinc metal powder or zinc foil. This treatment produces a stable stannate-derived SEI layer, which effectively suppresses zinc dendrite formation, mitigates hydrogen evolution, and prevents corrosion, thereby extending the lifetime and safety of the battery.

[0078] The electrolyte membrane, prepared by freeze-drying, exhibits a porous three-dimensional network that ensures high ionic conductivity and facilitates uniform zinc ion transport. The dual-crosslinked hydrogel provides high mechanical strength from covalent linkages and elasticity and self-healing ability from ionic crosslinks. When soaked in KOH / Zn2+ / stannate electrolyte, the hydrogel demonstrates enhanced ionic conductivity, mechanical flexibility, and dendrite suppression, enabling stable cycling performance even under high current densities.

[0079] The solid-state zinc-air battery thus benefits from a combination of the cathode providing efficient and durable bifunctional oxygen catalysis due to the Ru-RuCh / NGr structure; the zinc anode with stannate additive achieving long-term cycling stability by suppressing dendritic growt; and the dual-doped hydrogel membrane ensuring robust ionic transport, leak-free operation, and superior safety compared to liquid electrolytes.

[0080] Collectively, these features yield a zinc-air battery that demonstrates high specific capacity, low overpotentials, stable charge-discharge cycling, and long operational lifetime.

[0081] In an embodiment, the present invention provides a process for preparing a solid-state rechargeable zinc-air battery incorporating the bifunctional electrocatalyst and dual-doped hydrogel electrolyte of the invention. The process is designed to yield a stable device with efficient oxygen catalysis, dendrite suppression, and high ionic conductivity.

[0082] In an embodiment, the process for preparing the solid-state rechargeable zinc-air battery, comprises steps of:

[0083] (i) preparing the bifunctional electrocatalyst of the present invention;

[0084] (ii) polymerizing acrylic acid monomer with a covalent crosslinking agent to form a polyacrylic acid network, adding an ionic crosslinking agent and an initiator, curing the polymer, and freeze-drying to form a porous dual-doped crosslinked hydrogel electrolyte;

[0085] (iii) soaking the hydrogel electrolyte in an aqueous solution comprising potassium hydroxide, zinc acetate, and an additive to form a hydrogel electrolyte membrane;

[0086] (iv) fabricating a cathode by coating the bifunctional electrocatalyst on a gas diffusion layer;

[0087] (v) taking a zinc metal as an anode and contacting with the hydrogel electrolyte membrane to form an in situ stannate-derived interphase layer on the zinc anode; and

[0088] (vi) assembling the cathode, the zinc anode, and the hydrogel electrolyte membrane in a cell housing to provide the solid-state rechargeable zinc-air battery. The process begins with the step (i) of preparation of the bifunctional electrocatalyst as described previously, wherein ruthenium-ruthenium oxide nanoparticles are deposited in a core-shell configuration on nitrogen-doped graphene. This material, when coated on a conductive substrate, forms the oxygen cathode with superior bifunctional activity.

[0089] In parallel, step (ii) involves forming a hydrogel electrolyte membrane. The addition of a crosslinking agent in step (ii) introduces dynamic coordination bonds, generating a dualcrosslinked hydrogel with enhanced mechanical strength and elasticity. The freeze-drying step in step (ii) can be carried out at 70-90 bar for 8-10 hours. The hydrogel is subsequently freeze- dried under controlled temperature and pressure conditions, producing a porous three- dimensional structure with interconnected channels that enable rapid ion transport. The dried hydrogel is then soaked in an aqueous electrolyte solution and an additive to form a hydrogel electrolyte membrane. This soaking step hydrates the network, allows penetration of electrolyte ions, and imparts the dual function of high ionic conductivity and in situ stabilization of the zinc anode surface. In an embodiment, the additives are selected from but not limited to stannate-based, ZnO, KF, K2CO3, dodecyl trimethyl ammonium bromide, sodium dodecyl sulfate, cetyltrimethyl ammonium bromide; preferably the additive is stannate-based. In an embodiment the stannate-based additives is selected from but not limited to from potassium stannate, sodium stannate, chloride stannate, or sulfate stannate; preferably the stannate-based additive is sodium stannate, at a concentration of about 0.005 to 5 M.

[0090] In the zinc-air battery, the polyacrylic acid hydrogel can be dual-crosslinked by both covalent and ionic cross-linkers and can be prepared by a freeze-drying process to provide a porous network structure. In an embodiment, the dual-doped polymer hydrogel electrolyte is synthesized by the free radical polymerization process by using the freeze-drying method. The dual-doped hydrogel electrolyte provides the porous morphology thereby improving the formation of active sites for better performance.

[0091] Thus, the integrated process results in a solid-state zinc-air battery with synergistic component functionality, addressing the persistent challenges of low catalytic efficiency, dendrite formation, and electrolyte leakage in conventional zinc-air systems, while paving the way for practical, high-performance, and sustainable energy storage devices.

[0092] In an embodiment, the present invention provides a process for synthesis of the dual-doped polymer hydrogel electrolyte comprising the steps of: a) dissolving the monomer and crosslinking agent in a solvent under constant stirring for 2-4 hrs followed by heating and allowing to cool; b) adding the ionic cross-linking agent to the above solution and stirring for about 12 hrs; c) transferring the solution to petri dish and degassing with N2 and the addition of initiator; d) sealing the petri dish with Teflon tap and heat in the oven; and e) freeze-drying the material to obtain said dual-doped polymer hydrogel electrolyte.

[0093] The constant stirring in step (a) is done for 2-4 hours at 30-50°C, and cooling at a temperature range of 30 to 25 to form a pre-polymer solution. The ionic crosslinker is then introduced with continued stirring in step b) for 11-12 hrs to ensure uniform incorporation. The solution is cast into molds, degassed to remove air bubbles, sealed, and polymerized in an oven at 70-80°C for 40-50 minutes. Finally, the hydrogel is subjected to freeze-drying at 70-90 bar for 8-10 hours, producing a porous three-dimensional network structure.

[0094] In an embodiment, the monomer is selected from but not limited to the acrylic acid, acrylamide, 2-acrylamide-2-methyl- 1 -propanesulfonic acid (AMPS), hyaluronic acid, and chitin / chitosan. The monomer more particularly selected is acrylic acid reacted with sulfate radicle and substrate the hydrogen atom, subsequently generating AA radicals, and initiating the polymerization to generate the polyacrylic acid (PAA) chain; preferably the monomer is acrylic acid. The solvent is selected from but not limited to water, deionized water, and distilled water; preferably the solvent is distilled water. In an embodiment, the covalent cross-linking agent is selected from but not limited to bis-N, N'-methylene bisacrylamide (MBAA), EDTA, ethylene glycol dimethacrylate (EGDMA), glutaraldehyde, sodium borate / boric acid; preferably the crosslinking agent is MBAA. The introduction of the bis-N, N'-methylene bisacrylamide (MBAA) into a PAA-based precursor, results in a covalently cross-linked network structure of PAA hydrogel (c-PAA) electrolyte. In an embodiment, the ionic cross-linking agent is selected from but not limited to iron (III) nitrate nonahydrate (Fe (NOsh- (FhOjg), ferric nitrate, multivalent metal ions like zinc (Zn), chromium (Cr), titanium (Ti), zirconium (Zr) and aluminium (Al) by using their corresponding precursors, preferably the ionic cross linking agent is Fe (NOsh- (FhOjg. The introduction of Fe (NOsh- (FhOjg as ionic crosslinking agent into a c-PAA-based precursor results in a dually cross-linked, single net- work structure of PAA hydrogel (d-PAA) electrolyte. The use of ionic crosslinking agents enhances the mechanical strength of the hydrogels, while the covalent crosslinkers improve their conductivity.

[0095] The polymerization is initiated by a free radical initiators selected from but not limited to ammonium persulphate (APS), potassium persulphate (KPS), 2,2’-azobis-(2-methyl propionamidine) dihydrochloride (V-50), azobisisobutyronitrile (AIBN), and benzoyl peroxide, preferably the initiator is APS.

[0096] The freeze-dried d-PAA shows a 3D porous network structure that allows faster ion transport and facilitates efficient charge-discharge processes. The d-PAA hydrogel exhibits a flexible network compared to pure c-PAA. The freeze-dried d-PAA hydrogel exhibits interconnected pores that facilitate rapid ion transport and improve charge-discharge kinetics.

[0097] The present invention addresses the key limitations of conventional zinc-air batteries, including poor bifunctional catalysis, dendrite-induced short-circuiting, and liquid electrolyte leakage, thereby providing a scalable and practical energy storage solution.

[0098] EXAMPLES:

[0099] Following examples are by way of illustration therefore should not be constructed to limited the scope of the invention.

[0100] Example 1: Synthesis of Ruthenium-Ruthenium Oxide (R11-R11O2) composite on the simultaneous nitrogen-doped graphene (NGr) and reduced graphene oxide (rGO) lOOmg of graphene oxide (GO) was dispersed in 30ml of distilled water via constant stirring for 1 h followed by 15 min ultra-probe sonication with 10 sec on and 30 sec off-ramps. Afterward, 2 mmol of ruthenium acetate tetrahydrate (Ru (OAC2. 4H2O) and 10 mmol of melamine were added to the GO suspension which was then kept for drying in air at 80 °C with constant stirring for 5-6 h. Later, the obtained material was annealed at 900 °C with a ramp rate of 10 °C / min. The annealing at 900 °C was continued for the next 3 h in an Ar atmosphere. The obtained material was treated with 0.5 M H2SO4 at 80 °C with constant stirring for 8 h, to remove the amorphous carbon and the excess / unreacted ruthenium; the sample was washed with deionized water several times followed by ethanol, which was subsequently fdtered and dried at 60 °C for 6 h in a vacuum oven. Finally, the obtained composite was again calcined at 250 °C for 2 h in an air atmosphere to partially oxidize the surface layer of Ru to RuCh and the obtained material is labeled as Ru-RuCh / NGr.

[0101] Example 2: Field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) analyses of the bifunctional electrocatalyst

[0102] For the morphological studies, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) analyses were performed and the corresponding images are presented in Figure 1. The FESEM image presented in Figure la shows the sheetslike morphology of NGr featured with relatively smooth surface characteristics. However, subsequent to the decoration of the Ru-RuCh nanoparticles, the structural morphology of NGr in Ru-RuCh / NGr appears to be changed from smooth to rough surface sheets, as shown in Figure lb. The uniform distribution of the elements indicates the role of N-doping in the graphene. The TEM images of Ru-RuCh / NGr (Figures 1c and Id) show a uniform distribution of 2-5 nm sized Ru-RuCh nanoparticles over NGr, featuring fine and close distribution of the Ru-RuCh nanoparticles without any markable indications of agglomerations.

[0103] Example 3: Thermogravimetric analysis (TGA) of the bifunctional electrocatalyst

[0104] Further, thermogravimetric analysis (TGA) was used to examine the loading of Ru-RuCh over NGr by ramping up the temperature up to 900 °C in an oxygen environment. The TGA and the derivative (DTA) plots of NGr, Ru / NGr, Ru-RuCh / rGO, and Ru-RuCh / NGr are displayed in Figure 2. As it comes to NGr in Figure 2a, a significant weight loss is seen at around 433 °C as a result of NGr being oxidized into gaseous CO2 and the NOXspecies and approximately 0 % impurity was observed. In contrast, the TG-DTA curve of the Ru / NGr in Figure 2b exhibits a three-stage weight loss process, with the initial loss of weight occurring through the evaporation of the physiosorbed water molecules. The second weight loss region, defined by the combustion reaction (loss of gaseous CO2 and the NOX) occurring at 320 °C, is followed by a subsequent weight loss characterized by a slight increase after 700 °C. This latter phase is indicative of the pyrolysis process leading to the formation of RuCh, where, the Ru undergoes conversion to RuCh in the O2 atmosphere. This transformation results in the residual product consisting of approximately 33 % RuCh supported on the NGr substrate. From this, actual loading of the Ru metal could be calculated using the equation,

[0105] % of Ru = (wt. % from TGA) xMolecular wei9ht ofRuwhich is25.07 o / o

[0106] Molecular weight of Ru02 As depicted in Figure 2c, a significant weight loss is seen at around 400 °C as a result of NGr from Ru-RuCh / NGr being oxidized into gaseous CO2 and the NOXspecies and approximately 72 % RUO2was observed. However, the TG-DTA profile of Ru-RuCh / rGO presented in Figure 2d shows that there is no weight loss up to 400 °C. Subsequently, a significant weight loss is seen at around 400 °C as a result of Ru-RuCh / rGO being oxidized into the RuO2species. From the residue content, approximately 29 % loading of RuO2was observed, confirming the favorable role played by the doped N in facilitating more effective loading of the Ru species in the case of Ru-RuCh / NGr.

[0107] Example 4: Ground state geometry of the bifunctional electrocatalyst

[0108] Based on morphological and physical characterization the ground state geometry of the complex were determined, the Ru (100) and RuCh (101) surfaces (refer to Figures 3a and 3b) were considered. A hetero surface is created by combining the two surfaces, and it is adsorbed on N-doped graphene (NGr) was optimized and fixed, resulting surfaces are given the nomenclature Ru-RuCh and Ru-RuCh / NGr, see Figure 3c and 3d. Ru-RuCh / NGr surface is observed to have electron accumulation on interface of surface (NGr) and Ru-RuCh nanoparticle, which is ascertained by Charge Density Difference (CDD) plots using isosurface value 0.005 which represents negative and positive charge accumulation (shown in purple and green, respectively) given in Figure 3e respectively, making them potential candidates for Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction (ORR). With calculated H2O adsorption energies of -0.57, 1.35, -0.88, and 0.29 eV, respectively,

[0109] Equation 1 shows that the adsorption of H2O on Ru / NGr, RuO2, Ru-RuO2, and Ru-RuO2 / NGr further establishes corroboration with d-band center values of -3.35, -6.40, -2.50, and -4.20 eV, respectively (see Figure 3f-3i) since a more positive band center indicates greater exothermic adsorption of H2O.The determination of the Gibbs Free Energy (AG) for the OER and ORR pathways is crucial in the design and optimization of electrocatalysts. OER and ORR are fundamentally complex processes because of the higher energy barrier and four-electron transfer reaction. The Potential Determining Step (PDS) for OER pathway has been evaluated by optimizing oxygenated intermediates *OH, *0, *OOH and *O2. Notably, experiments suggest the formation of *0 intermediate is the PDS which is 2.49 and 1.36 eV for Ru-RuO2 and Ru-RuCh / NGr respectively, provided in Figure 3j. The overpotential for Ru-RuCh and Ru- RuCb / NGr has been calculated using Equation:

[0110] 3 TJ — U Limiting ~ Equilibrium found to be 1.26 and 0.13 V, respectively.

[0111] Further, for ORR, formation of *OOH intermediate is the PDS which is 4.16 and 3.99 eV for Ru-RuCh and Ru-RuCh / NGr respectively, provided in Figure 3k. The overpotential for Ru- RuO2and Ru-RuCh / NGr was found to be 2.93 and 2.76 V, respectively.

[0112] Example 5: Electrochemical analysis of bifunctional electrocatalyst towards ORR activities

[0113] The electrochemical ORR activity of the synthesized samples was recorded at 10 mV s-1 of scan rate in 02-saturated 0.1 M KOH solution by using a rotating disc electrode (RDE) technique. The comparative ORR polarization curves of the synthesized catalysts and the state- of-the-art Pt / C are shown in Figure 4a. Ru-RuO2 / NGr shows a significantly improved ORR activity with an onset potential of 926 mV and El / 2 of 808 mV, which indicates the synergistic interaction between Ru-RuO2 and N dopant as well as quite good Ru-RuO2nano-particles dispersion over NGr aiding to its catalytic activity. The N doping in the graphene layer produces rich graphene edge defects, which force the edge sites to adsorb more oxygen molecules and thereby improving the ORR reaction kinetics. The Tafel slope extracted from the plot based on the Tafel equation:

[0114] (r| = a + b x log j, where a is constant, b is the Tafel slope, T] is the overpotential, and j is the current density) can be used as valuable performance indicator to grade the feasibility of the ORR reaction kinetics. As shown in Figure 4b, the low ORR Tafel slope value of Ru-RuO2 / NGr (82 mV / decade) indicates its high intrinsic activity towards ORR in comparison to NGr (103 mV / decade), Ru-RuO2 / rGO (108 mV / decade), and Ru / NGr (96 mV / decade). The observed activity trend is probably due to the close interplay between the interface of NGr surface and RU-RUO2nanoparticles altering the intermediate adsorption energies, and making it one of the potential candidates toward ORR electrocatalysis. Example 6: Electrochemical analysis of bifunctional electrocatalyst towards PER activities

[0115] At a scan rate of 10 mV / s, the electrochemical OER performance of the as-synthesized catalysts was evaluated in 1 M KOH electrolyte. The comparative linear sweep voltammetry (LSV) plots of the as-synthesized materials and the state-of-the-art RUO2 / VC (4: 1) are shown in Figure 4d. As can be seen, NGr exhibits minimal OER activity and it required 470 mV of overpotential to produce 10 mA cm'2current density. To achieve the same current density of 10 mA cm'2, the RUO2 / VC (4: 1) required a 300 mV overpotential, while Ru-RuO2 / NGr fared substantially better in the series by exhibiting an overpotential of 210 mV. Conversely, the system Ru-RuO2 / rGO exhibited an overpotential of 280 mV to attain an equivalent current density, indicating the beneficial influence of N doping in the substrate (NGr) as well as its interface with the Ru- RUO2by combining the electronic structure and lattice strain effects on enhancing the intrinsic activity of the catalyst towards OER. Ru-RuO2 / NGr Tafel slope value shows that the OER kinetics improves due to the interface between Ru and RuO2 over NGr in the composite material and resulting defects can boost the active sites’ kinetics. This interaction enhances the catalytic activity by providing abundant active sites and facilitating charge transfer. It provides appropriate binding energies for oxygenated intermediate species.

[0116] Example 7: Performance of the ORR and OER bi-functional activity of the catalysts

[0117] To evaluate the performance of the ORR and OER bi-functional activity of the catalysts, the difference between the voltage measured for OER at 10 mA cm'2and the El / 2 value of ORR (AE = Ej lOOER-El / 2 ORR) is often used (as shown in Figure 4f) as a practical method. A smaller AE value is the characteristic of an ideal bifunctional oxygen electro-catalyst. The calculated values of El / 2 for ORR, Ej@10mAcm'2for OER, and the difference between these two values for Ru-RuO2 / NGr and Pt / C / RuO2 / VC (standard pair) are shown in the bar diagram in Figure 4g. The Ru-RuO2 / NGr catalyst shows an AE of 0.63 V, which is lower than the standard pair system of Pt / C / RuO2 / VC (AE = 0.68 V). The robustness of the synthesized material was evaluated for ORR. The CV study was carried out for 5000 cycles at a scan rate of 100 mV s'1in a potential window of 0.60-1.0 V vs. RHE. To evaluate the performance change during the catalytic process, a comparative LSV has been performed before and after the ADT. The comparative LSV profiling of Ru-RuO2 / NGr and Pt / C, before and after 5000 cycles, is shown in Figures 4h, respectively. The difference in E1 / 2 is 26 mV for Pt / C and 20 mV for Ru-RuCh / NGr after ADT. This study confirms that the Ru-RuCh / NGr electrocatalyst has greater endurance than Pt / C given the lower shift of E1 / 2 in Ru-RuCh / NGr than Pt / C. The inclusion of the N-doped graphitic carbon support and its potent interaction with the catalytic active sites are responsible for the observed improved durability of Ru-RuCh / NGr. Further, the stability of Ru-RuCh / NGr towards OER has been evaluated through chronoamperometric (CA) analysis (Figure 4i). The CA was performed for 12 h at 10 mA cm'2, and result revealed~90 % retention of initial activity after 12 h, indicating greater stability of the material under the alkaline conditions, ie., 1 M KOH. The mixed oxidation state of Ru in Ru-RuO2 / NGr plays crucial role. By lowering Ru valence state and forming stable Ru-RuO2 structure, dissolution of Ru is suppressed, leading to increased durability.

[0118] Example 8: Phase and structure analysis of the cycled catalyst, i.e., post Accelerated Degradation Test (APT) and Constant Applied Potential (CA) analysis

[0119] For analyzing the phase and structure of the cycled catalyst, i.e., post ADT and CA analysis, the catalyst-coated GDL was collected, and ex-situ XRD analysis was performed. For comparison, the XRD analysis of the bare GDL and the GDL-coated catalysts were also performed before the ADT and CA analyses. Figure 5a represents the comparative XRD pattern, showing all the four samples share the same XRD peak patterns of the GDL except a few extra peaks present in the case of Ru-RuCh / NGr, the post- ADT, and the CA test samples. These extra peaks located at 38.3, 42.2, and 43.9° correspond to the (100), (002), and (101) lattice planes, respectively, of the metallic Ru nanocrystals which are well matched with JCPDS 00-006-0663. Also, the prominent peaks located at 28.0, 35.1, 40.0, 40.5, 54.3, and 69.4° are corresponding to the (100), (101), (002), (102), (110) and (103) planes of the tetragonal phase of the RUO2, which are well corroborated with JCPDS 00-040-1290, confirming the well stability of the Ru-RuCh / NGr. The TEM analysis was performed on the Ru-RuO2 / NGr to understand any morphological changes after the ADT. Figure 5b shows the TEM images of Ru-RuO2 / NGr after ADT indicating the intact Ru-Ru02 nanoparticles anchored over NGr even after 5000 ADT cycles. Figure 5c represents the FESEM image and corresponding mapping of the constituent elements of Ru-RuO2 / NGr, showing the uniform distribution of Ru, O, N, and C well retained after 5000 cycles of ADT.

[0120] Example 9: Dual-Doped Polyacrylic Acid Hydrogel (d-PAA) Electrolyte

[0121] For the synthesis of the hydrogel electrolyte, a free radical polymerization synthesis was used. The role of the initiator is to generate free radicals and initiate the polymerization. 4 g of acrylic acid (AA) and 6 mg of bis-N, N'-methylene bisacrylamide (MBAA, the crosslinking agent), were dissolved in 12 mL of deionized (DI) water under constant stirring for 3 h at 40 °C. After 3 h, the heating was stopped and the solution was cooled down to 25-35°C, and subsequently a measured amount of Fe (NO)s was added as the ionic crosslinker and further stirred for 12 h. The final solution was transferred into a Petri dish and degassed in an N2 atmosphere, and 12 mg of potassium persulfate (K2S2O8, initiator) was added. Following this, the petri dish was sealed with Teflon tap and placed in an oven for 40-50 min at 70 °C. Next, the polymerized gel was freeze-dried at 80 bars for 8-10 h, which plays an important role in the layer sheets formation. The obtained polymer was labelled as the dual -doped polyacrylic acid hydrogel (d- PAA). Before using it, the as-prepared d-PAA hydrogel (0.53 mm thick) was soaked for 24 h in 6 M KOH with 0.005 M sodium stannate and 0.2 M Zn (Ac)2.6H2O electrolyte mixture (Figure 6a and b).

[0122] Example 10: Covalent-crosslinked Polyacrylic Acid Hydrogel (c-PAA) Electrolyte

[0123] For the synthesis of c-PAA similar synthesis procedure as it was performed for d-PAA (Refer Example 9) without adding Fe (NO)s. Apart from that, the synthesis process of c-PAA was similar to that of the d-PAA synthesis.

[0124] Example 11: Morphological features of the obtained d-PAA after freeze-drying and thermal-drying

[0125] FESEM analysis was performed and the corresponding images are presented as Figure 6c, 6d. In Figure 7c, the FESEM image depicts dense morphology of the d-PAA due to the thermal drying process, whereas, the freeze-dried d-PAA shows a 3D porous network structure. The porous interconnected network structure allows faster ion transport, and facilitates efficient charge-discharge processes. In one of the studies, PAA-KOH composition was tuned for improved ionic conductivity and Al anode stability in the primary Al-air batteries.

[0126] Example 12: Contact angle study

[0127] The contact angle study was performed to know the electrode-electrolyte interfacial tension in the presence of various employed electrolyte compositions. The additive electrolyte includes stannate, which adsorbs on the pristine Zn surface and forms a solid electrolyte interface (SEI) layer to modify the interface and reduce the contact angles. A comparative contact angle study with and without the electrolyte additives shows the increased hydrophilicity in the presence of the additives which is an indication of reduced interfacial tension (Figure 7a-c). Figure 7a- a4 shows the contact angle data with 6 M KOH (K) electrolyte displaying its hydrophobic nature over the Zn anode. Figure 7b-b4 depicts the study with 6 M KOH and 0.2 M Zn acetate (KZ) electrolyte, displaying a slightly hydrophobic nature over the Zn anode, whereas Figure 7c-c4 shows the results based on 6 M KOH, 0.2 M Zn acetate and sodium stannate (KZS) electrolyte displaying its super hydrophilic nature over the Zn anode.

[0128] Further, to know the electrode-electrolyte interface, the contact angle of K, KZ and KZS electrolytes were recorded over Ru-RuOi / NGr and Pt / C electrodes as shown in Figure 8. Surprisingly, the Ru-RuOi / NGr electrode surface shows a KZS contact angle of 117.1° compared to K (127.4°) and KZ (135.6°) as shown in Figure 8a-c. Figure 8d-f depicts the Pt / C surface which shows the KZS, KZ, and K contact angles of 128.9°, 134.5° and 130.4° respectively. According to the obtained results, it can be inferred that the Ru-RuOi / NGr and Pt / C electrodes has poor wettability with the K electrolyte, whereas KZS electrolyte exhibits a well-balanced hydrophilic / hydrophobic characteristic.

[0129] Example 13: Solid-State Rechargeable Zn-Air Battery (RZAB) Performance Demonstration

[0130] For comparison, the Pt / C electrode-based ZAB was also fabricated and tested. A schematic illustration of the solid-state RZAB is shown in Figure 9. Figure 9a illustrates a comparative current (I) - voltage (V) polarization plots along with the corresponding power density profiles for the ZABs based on Ru-RuOi / NGr and Pt / C with KZS as the electrolyte. Clearly, the I-V plot corresponding to the Ru-RuOi / NGr system outperforms the counterpart system based on Pt / C in the entire region of the polarization, leading to a significantly higher peak power density of nearly 84.4 mW cm-2for the former compared to 44.5 mW cm-2for the later. The calculated specific capacity of the solid-state ZAB based on Pt / C is about 768 mAh / gzn compared to 750 mAh / gzn for the system based on Ru-RuOi / NGr as shown in Figure 9b. The galvanostatic discharge curve of battery in presence of KZS electrolyte recorded from the discharge current density of 1.0 to 10 mA cm-2(Figure 9c) shows higher stability and low voltage drop with increasing and decreasing the current density. Further, in order to verify the rechargeability of the system (z.e., RZAB), the long-term galvanostatic charge and discharge cycling of the battery has been performed and illustrated in Figures lOd. In this graph, the bifunctional Ru- RuOi / NGr catalyst-based batteries was operated for the charge- dis charge (CD) cycles at a current density of 2.0 mA cm-2, each cycle for 5 min, in the presence of both the KZS and KZ electrolyte systems. The KZS -based RZAB runs for 20 h, which is much stable than that of the system based on the KZ electrolyte. In the initial 1 h of the CD cycles, there is no voltage gap observed as shown in the zoomed part of Figure 9d. The KZS-based system depicts 0.43 V, whereas the KZ-based system shows 0.52 V (Figure 9dl), which has been increased after 5 h of cycling (0.54 and 0.60 V for KZS and KZ, respectively in Figure 9d2) and again slightly increased after the 17 h (0.66 V for KZS in Figure 9d3). These performance profiles thus indicate that the efficiency of the KZS battery is well maintained throughout the 100 chargedischarge cycles, whereas, the efficiency of the KZ battery is dropped at 32 charge-discharge cycles (Figure 9e). The confocal 3D image recorded by staking and 3D construction of the additive-free system shows a bulky growth pattern of the dendrites over the cycled Zn-anode as shown in Figure 9f. On the other hand, in the presence of the Sn-additive, the cycled Zn anode shows uniform surface as shown in Figure 9g. This observation was further supported with the FESEM analysis of the post-cycled Zn anodes.

[0131] Example 14: Comparison of the ORR and PER activities of the prior art reported electrocatalysts Vs Ru-RuO / NGr catalyst of the present invention

[0132] Table 1 below summarizes the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of various reported electrocatalysts from prior art and compares them with the Ru-RuO2 / NGr catalyst of the present invention.

[0133] Onset potential (EORR) and half-wave potential (El / 2) are indicators of ORR activity. EOER ( @ 10 mA cm2) represents the potential required for OER at a standard current density. AE (EOER - EORR) represents the overall bifunctional performance window: the smaller the gap, the more efficient the catalyst is for reversible ORR-OER processes.

[0134] In summary, the table highlights that the present invention offers a balanced improvement in both ORR and OER, with the lowest bifunctional overpotential window (AE), making it a superior electrocatalyst compared to the reported materials.

[0135] ADVANTAGES OF THE INVENTION

[0136] • The bifunctional electrocatalyst shows enhanced activity and stability towards the two oxygen reactions, oxygen reduction and evolution reactions (ORR and OER). • The introduction of the stannate additive to the electrolyte induced an in-situ Zn-anode modification, which subsequently improves the interfacial stability of the ZABs and hence the battery life cycles.

[0137] • The in-situ Zn anode surface modification assisted in achieving a high-rate cycle capability.

[0138] • The bifunctional electrocatalyst is responsible for increased interfacial stability, and cyclic stability, avoiding nanoparticle aggregation composite.

Claims

WE CLAIM:

1. A bifunctional electrocatalyst comprising: a. transition metal Ml; b. transition metal oxide; and c. support; wherein the transition metal Ml (a) is ruthenium; the transition metal oxide (b) is ruthenium oxide; the support (c) is nitrogen-doped graphene.

2. The electrocatalyst as claimed in claim 1 , wherein the ruthenium and ruthenium oxide form an active core-shell structure, and the nitrogen-doped graphene forms a sheet-like morphology.

3. The electrocatalyst as claimed in claim 1 wherein said ruthenium is encased by ruthenium oxide nanoparticles.

4. The electrocatalyst as claimed in claim 1, wherein, the active core-shell of rutheniumruthenium oxide nanoparticles is uniformly dispersed over the sheet-like nitrogen-doped graphene.

5. The electrocatalyst as claimed in claim 1 wherein the active core-shell of rutheniumruthenium oxide nanoparticles has an average diameter of 2-5 nm.

6. A process for preparing the bifunctional electrocatalyst as claimed in claim 1, comprising: a) dispersing a nitrogen-doped graphene support material in solvent followed by ultraprobe sonication to obtain a suspension; b) reacting the suspension of step a) with a ruthenium precursor and melamine followed by drying to obtain a mixture; c) annealing the mixture of step b) under an inert atmosphere to form ruthenium nanoparticles on the nitrogen-doped graphene; d) treating the annealed material of step c) with acid;e) washing, filtering, and drying the material of step d) to obtain pre-calcined bifunctional electrocatalyst; and f) calcining the pre-calcined electrocatalyst in air to partially oxidize the surface of the ruthenium nanoparticles, thereby forming bifunctional catalyst comprising rutheniumruthenium oxide active core-shell nanoparticles supported on nitrogen-doped graphene.

7. A solid-state rechargeable zinc-air battery comprising:(a) a cathode comprising a gas diffusion layer coated with the bifunctional electrocatalyst as claimed in claim 1 ;(b) an anode comprising a zinc metal, wherein the surface of the zinc metal is modified in situ by a stannate -based additive to form a solid electrolyte interphase layer; and(c) an electrolyte membrane comprising a dual-crosslinked polyacrylic acid hydrogel covalently cross-linked with a multifunctional cross-linker and ionically cross-linked with multivalent metal ions, the hydrogel being soaked in an aqueous solution comprising potassium hydroxide, zinc acetate, and a stannate-based additive.

8. The battery as claimed in claim 7, wherein the polyacrylic acid hydrogel is dualcrosslinked by both covalent and ionic cross-linkers and is prepared by a freeze-drying process to provide a porous network structure.

9. A process for preparing the solid-state rechargeable zinc-air battery as claimed in claim 7, comprising:(i) preparing the bifunctional electrocatalyst as claimed in claim 6;(ii) polymerizing acrylic acid monomer with a covalent crosslinking agent to form a poly aery lie acid network, adding an ionic crosslinking agent and an initiator, curing the polymer, and freeze-drying to form a porous dual-doped crosslinked hydrogel electrolyte;(iii) soaking the hydrogel in an aqueous solution comprising potassium hydroxide, zinc acetate, and a stannate-based additive to obtain the hydrogel electrolyte membrane;(iv) fabricating a cathode by coating the bifunctional electrocatalyst on a gas diffusion layer;(v) taking a zinc metal as an anode and contacting with the hydrogel electrolyte membrane to form an in situ stannate-derived interphase layer on the zinc anode; and(vi) assembling the cathode, the zinc anode, and the hydrogel electrolyte membrane in a cell housing to provide the solid-state rechargeable zinc-air battery.

10. The process of claim 9, wherein the stannate -based additive is sodium stannate at a concentration of about 0.005 M.

11. The process of claim 9, wherein the freeze-drying is carried out at 70-90 bar for 8-10 hours.

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