BIFUNCTIONAL ELECTROCATALYST FOR ALL-SOLID-STATE RECHARGEABLE ZINC-AIR BATTERY

Abstract

The present invention discloses 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.

Claims

1. A bifunctional electrocatalyst, comprising: a) a manganese-cobalt-based bimetallic spinel oxide (MnCo.sub.2O.sub.4), and b) N-doped 3D porous entangled graphene (NEGF); wherein the MnCo.sub.2O.sub.4 is uniformly distributed over the self-assembled N-doped 3D porous entangled graphene; and said MnCo.sub.2O.sub.4/NEGF electrocatalyst is three-dimensional and porous.

2. The bifunctional electrocatalyst as claimed in claim 1, wherein the MnCo.sub.2O.sub.4 is present in the range of 60-70 wt. % and the NEGF is present in the range of 30-40 wt. % of total wt. % of the electrocatalyst.

3. The bifunctional electrocatalyst as claimed in claim 1, wherein the MnCo.sub.2O.sub.4 is spherical in shape with a size in the range of 30 to 60 nm.

4. The bifunctional electrocatalyst as claimed in claim 1, wherein the pore size is in the range of 2 to 16 nm and a BET surface area in the range of 300-320 m.sup.2g.sup.1.

5. A process for the synthesis of bifunctional electrocatalyst (MnCo.sub.2O.sub.4/NEGF) as claimed in claim1, via solvothermal process, comprising the steps of: (i) preparing dispersed graphene oxide (GO) via improved Hummer's method, in water and ammonia solution (30% v/v) to obtain viscous graphene oxide solution; (ii) adding Co.sup.2+ and Mn.sup.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; (iv) freeze-drying the mixture of step (iii) under high vacuum pressure to obtain the desired bifunctional electrocatalyst.

6. The process as claimed in claim 5, wherein the heating of step (iii) is done at a temperature in a range of 150 to 200 degree C. for a time period of 10 to 15 hr.

7. The process as claimed in claim 5, wherein the freeze drying of step (iv) is done at a temperature in a range of minus 50 to minus60 degree C. for a time period of 8 to 12 hr.

8. An all-solid-state rechargeable zinc-air battery (ZAB) comprising; a) MnCo.sub.2O.sub.4/NEGF electrocatalyst as claimed in claim 1 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.

9. The all-solid-state rechargeable zinc-air battery (ZAB) as claimed in claim 8, wherein the anode material is Zinc material; and wherein the electrolyte material is selected from polyvinyl alcohol (PVA), potassium hydroxide (KOH) and a combination of PVA-KOH.

10. The all-solid-state rechargeable zinc-air battery (ZAB) as claimed in claim 8, wherein the catalyst slurry is brush-coated over a gas diffusion layer (GDL) and dried at 60 C. for 12 h to achieve a catalyst loading of 1.0 mg cm.sup.2 with electrode area of 1.0 cm.sup.2.

Description

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0026] FIG. 1: shows (a) the FESEM images of MnCo.sub.2O.sub.4/NEGF, displaying the porous architecture of the entangled 3D graphene sheets; (b) magnified FESEM image of MnCo.sub.2O.sub.4/NEGF; (c) 3D micro-CT images of MnCo.sub.2O.sub.4/NEGF, showing the porous structure of 2D sheets are connected; (d) TEM image of MnCo.sub.2O.sub.4/NEGF, shows the uniform distribution of MnCo.sub.2O.sub.4 over the N-doped of 3D graphene; (e) HRTEM image of MnCo.sub.2O.sub.4/NEGF, clearly shows the d-spacing for MnCo.sub.2O.sub.4, Inset showing the crystal nature of MnCo.sub.2O.sub.4. (f-k) elemental mapping for Co, Mn, N, O and C respectively.

[0027] FIG. 2: (a) shows the comparative pore size distribution of NEGF, MnCo.sub.2O.sub.4, and MnCo.sub.2O.sub.4/NEGF materials; (b) Comparative BET adsorption and desorption isotherm of NEGF, and MnCo.sub.2O.sub.4/NEGF, showing type-IV isotherm.

[0028] FIG. 3: (a) XPS analysis of MnCo.sub.2O.sub.4/NEGF; (a) comparative survey scan spectra of NEGF, Co.sub.3O.sub.4/NEGF, and MnCo.sub.2O.sub.4/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 MnCo.sub.2O.sub.4/NEGF; (c) deconvolutes Mn spectra, shows two peaks corresponding two oxidation state of Mn, +2 and +3; (d) deconvoluted N1s spectra, confirm the presence of four types of nitrogen;

[0029] FIG. 4: Electrocatalytic RDE performance analysis of NEGF, Co.sub.3O.sub.4/NEGF, Mn.sub.3O.sub.4/NEGF, and MnCo.sub.2O.sub.4/NEGF towards ORR and OER in comparison to the state-of-the-art (Pt/C) and RuO.sub.2 catalyst respectively; (a) comparable LSV profiles for NEGF, Co.sub.3O.sub.4/NEGF, MnCo.sub.2O.sub.4/NEGF, and Pt/C in O.sub.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, Co.sub.3O.sub.4/NEGF, MnCo.sub.2O.sub.4/NEGF, and Pt/C in O.sub.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, Co.sub.3O.sub.4/NEGF, Mn.sub.3O.sub.4/NEGF, and MnCo.sub.2O.sub.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, Co.sub.3O.sub.4/NEGF, Mn.sub.3O.sub.4/NEGF, and MnCo.sub.2O.sub.4/NEGF; (e) Tafel plot analysis for NEGF, Co.sub.3O.sub.4/NEGF, Mn.sub.3O.sub.4/NEGF, and MnCo.sub.2O.sub.4/NEGF for ORR activity; (f) Tafel plot analysis for NEGF, Co.sub.3O.sub.4/NEGF, Mn.sub.3O.sub.4/NEGF, and MnCo.sub.2O.sub.4/NEGF for OER activity.

[0030] FIG. 5. FIG. 5a and the inset image show the cross-sectional FESEM image of the bare GDL. GDL coated with MnCo.sub.2O.sub.4/NEGF (FIG. 6b). The inset of Figure FIG. 5b gives better clarity of the surface containing the 3D self-assembled structure of the coated layer of MnCo.sub.2O.sub.4/NEGF. FIGS. 5c and d show the 3D tomogram cross-section images of the bare GDL and the MnCo.sub.2O.sub.4/NEGF-coated GDL, respectively. The MnCo.sub.2O.sub.4/NEGF-coated surface of the GDL shows a water contact angle of 109.2 (FIG. 6f). The CA data corresponding to the base GDL is presented in FIG. 6e.

[0031] FIG. 6: All-solid-state rechargeable zinc-air battery (ZAB) performance evaluation for MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 as the air electrodes: (a) polarization plots recorded on the ZABs fabricated by employing MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 as the air electrodes; (b) comparative impedance plot recorded for ZAB set-up constructed with MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 as the air electrodes; (c) galvanostatic charge-discharge plot for MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2, shows the higher potential window for Pt/C+RuO.sub.2 compared to MnCo.sub.2O.sub.4/NEGF; (d) the galvanostatic charge-discharge cycling curves at 10 mA cm.sup.2, shows in case of Pt/C+RuO.sub.2, asymmetric charge-discharge plateau (e) galvanostatic discharge capacity of the battery at the various current density of 5, 10,20, 30 mA cm.sup.2.

[0032] FIG. 7 provides schematic illustration of the stages involved in the stepwise synthesis of MnCo.sub.2O.sub.4/NEGF as an ORR/OER bifunctional electrocatalyst, and demonstration of its application as the air-electrode for the Solid-State Rechargeable Zn-Air Battery.

[0033] FIG. 8 shows illustrative of claimed all solid-state rechargeable Zn-air battery set up.

DETAILED DESCRIPTION OF THE INVENTION

[0034] 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.

[0035] 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).

[0036] In a preferred embodiment, the present invention relates to MnCo.sub.2O.sub.4/3D NGr electrocatalyst which represents the self-assembly structure of nitrogen-doped three-dimensionally oriented graphene. The MnCo.sub.2O.sub.4 is uniformly distributed over the N-doped 3D graphene.

[0037] In another embodiment, the electrocatalysts (MnCo.sub.2O.sub.4/3D NGr) as air cathode material is prepared by solvothermal process comprising; [0038] (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; [0039] (ii) adding Co.sup.2+ and Mn.sup.2+ metal salts to the viscous graphene oxide solution of step (i) in 2:1 ratio at constant stirring followed by probe sonication; [0040] (iii) transferring the solution of step (ii) to a Teflon-lined autoclave and heating followed by cooling and washing to remove excess ammonia; [0041] (iv) freeze-drying the mixture of step (iii) under high vacuum pressure to obtain the desired catalyst.

[0042] 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. 1a and 1b).

[0043] In still another embodiment, the pore size of MnCo.sub.2O.sub.4/NEGF catalytic material ranges between 2-16 nm; has a BET surface area in the range of 300-320 m.sup.2g.sup.1.

[0044] The XRD pattern (FIG. 2b) of MnCo.sub.2O.sub.4/NEGF discloses a series of peaks at 2=18.3, 30.2, 35.6, 37.0, 43.2, 53.8, 57.2, 62.7 and 74.0, which are ascribed to (111), (220), (311), (400), (422), (511), (440) and (533) diffraction peaks corresponding to the spinel structure MnCo.sub.2O.sub.4. After incorporating spherical shaped MnCo.sub.2O.sub.4 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.

[0045] The extent of the defects to the graphitic nature of the employed conducting support is measured by calculating the I.sub.D/I.sub.G 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.sup.1 corresponds to the graphitic lattice vibration mode with the A.sub.1g symmetry, while the G-band peak appeared at 1590 cm.sup.1 corresponds to the E.sub.2g symmetry graphitic lattice vibration mode. Raman spectra of NEGF, and MnCo.sub.2O.sub.4/NEGF catalyst with the I.sub.D/I.sub.G values of 1.25, and 1.31, respectively. The increased I.sub.D/I.sub.G value from GO (1.0) to NEGF catalyst clearly indicates 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 are helpful for metal oxides nucleation. The defective sites are higher in MnCo.sub.2O.sub.4/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 stand out to assist the system towards catalytic activity enhancement. The total loading of the spinel oxide active site, which suppresses the BET surface area in MnCo.sub.2O.sub.4/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 MnCo.sub.2O.sub.4/NEGF, indicating the MnCo.sub.2O.sub.4 loading of 45 wt. % over the nitrogen-doped carbon. The observed higher loading of MnCo.sub.2O.sub.4 nanoparticles suppresses the overall surface area of the prepared MnCo.sub.2O.sub.4/NEGF catalyst to 300 m2gm1 m2 g.sup.1. The achieved higher loading of MnCo.sub.2O.sub.4 (45%) over conducting support maintains the overall conductivity and active sites density of the catalyst required for better electrochemical activity.

[0046] 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.sup.1 under O.sub.2 atmosphere to maintain the working electrode rotation at 1600 RPM. The comparative LSV profile (FIG. 4a) evidences the superior ORR performance achieved by MnCo.sub.2O.sub.4/NEGF compared to the control samples, i.e., NEGF (0.86 V), Co.sub.3O.sub.4/NEGF (0.89 V), and Mn.sub.3O.sub.4/NEGF (0.85 V). In addition, the ORR performance of MnCo.sub.2O.sub.4/NEGF (0.93 V) is 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, Co.sub.3O.sub.4/NEGF, Mn.sub.3O.sub.4/NEGF, MnCo.sub.2O.sub.4/NEGF, and RuO.sub.2 in 1 M KOH at a scan rate of 10 mV sec.sup.1 under N.sub.2 atmosphere. The LSVs recorded for MnCo.sub.2O.sub.4/NEGF (FIG. 4b) showed better electrochemical OER activity compared to NEGF, Co.sub.3O.sub.4/NEGF, and Mn.sub.3O.sub.4/NEGF. The superior performance of MnCo.sub.2O.sub.4/NEGF catalyst towards both oxygen reactions (ORR/OER) is observed in LSV analysis. The overall bifunctional activity (ORR-OER) of MnCo.sub.2O.sub.4/NEGF is found to be 0.82 V which is comparable or better than previously reported various bifunctional electrocatalysts (Table 1). The observed higher bifunctional oxygen reaction activity of the prepared catalyst is 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-US-00001 TABLE 1 Comparison of the bifunctional oxygen activity of the non-noble metal- based electrocatalysts and electrocatalyst of present invention. Half Wave Potential Ej@10 mA Bifunctional E (V) cm2 activity E Electrocatalysts vs. RHE (V vs. RHE) (mV) References Co.sub.3O.sub.4/NPGC 0.84 1.68 0.84 Dr. Ge Li et al., 2016 Co/NC-800 0.74 1.60 0.86 Qian Lu et al., 2019 Co.sub.3O.sub.4/CNW 0.76 1.57 0.83 Siyang Liu et al., 2015 CoMn.sub.2O.sub.4/NGr 0.80 1.66 0.86 Moni Prabu et al., 2014 Manganese - 0.88 1.68 0.80 Yongye Liang Cobalt Oxide et al., 2012 and Graphene MnCo.sub.2O.sub.4 - 0.78 1.65 0.87 Li Xu et al., graphene 2012 MnCo.sub.2O.sub.4/ 0.81 1.63 0.82 Present Work NEGF

[0047] From table 1, it is evident that lower E.sub.1/2 value means better activity; higher Ej higher means the improved limiting current; and lower E value means the better bifunctional activity for catalysts of present application.

[0048] In another embodiment, the MnCo.sub.2O.sub.4/NEGF catalyst of the present invention is stable up to 5000 cycles evidenced by the cyclic durability study (FIG. 5a-c).

[0049] In another preferred embodiment, the present invention relates to all-solid state rechargeable zinc air battery (ZAB) comprising; [0050] a) MnCo.sub.2O.sub.4/NEGF electrocatalyst coated on gas diffusion layer (GDL) in an air-cathode; [0051] b) an anode; and [0052] 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.

[0053] In still another embodiment, the anode material is Zinc material, which is more abundant, cheap, and non-toxic.

[0054] In yet another embodiment, the electrolyte material 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.

[0055] In still another embodiment, the gel electrolyte for all-solid-state rechargeable zinc-air battery (ZAB) is prepared by the process comprising: [0056] (i) 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 [0057] (ii) adding a base to the above solution drop wise at the same temperature and storing in the refrigerator to obtain the desired product.

[0058] In yet another embodiment, the catalyst slurry of MnCo.sub.2O.sub.4/NEGF for coating on to the GDL electrode is prepared by the process comprising: [0059] (i) adding MnCo.sub.2O.sub.4NEGF to the mixture of IPA and water (1:4) and sonicating; [0060] (ii) adding 10 wt % Fumion solution to the dispersion of step (i) and sonicating until complete dispersion is obtained; and [0061] (iii) coating the catalyst slurry over the gas diffusion layer (GDL) and drying to achieve a catalyst loading of 1.0 mg cm.sup.2.

[0062] FIG. 7 depicts a simplified illustration of the stages involved in the stepwise synthesis of MnCo.sub.2O.sub.4/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.sup.2+ and Mn.sup.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 (MnCo.sub.2O.sub.4) 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 CoMn spinel oxide nanoparticles (MnCo.sub.2O.sub.4/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.

[0063] 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 MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 coated electrodes. The comparative steady-state cell polarization leads to the maximum power density (P.sub.max) of 110 and 200 mW cm.sup.2 for the ZABs based on Pt/C+RuO.sub.2 and MnCo.sub.2O.sub.4/NEGF, respectively. The cathode catalysts show superior performance for the prepared catalyst (MnCo.sub.2O.sub.4/NEGF) compared to Pt/C+RuO.sub.2, which is ascribed to be better interface formation in the former catalyst. Galvanostatic charge/discharge curve measured at 10 mA cm.sup.2 is shown in FIG. 6c. The observed difference between the charging and discharging voltages of ZAB on MnCo.sub.2O.sub.4/NEGF during the initial process was 0.84 V which was lower than 0.91 V of the Pt/C+RuO.sub.2. After 50 h of continuous charge-discharge cycles, a nominal voltage difference increased by 0.10 V on ZAB consisting of MnCo.sub.2O.sub.4/NEGF compared to 1.1 V after 15 h cycle operation on Pt/C+RuO.sub.2 ZAB. Moreover, the magnified image shows (FIG. 6d) that in case of MnCo.sub.2O.sub.4/NEGF, charge-discharge voltage plateau are more symmetric but in the case of Pt/C+RuO.sub.2 deficient asymmetric charge-discharge curve is observed. This feature reveals the better bifunctional activity at ZAB air cathode interface in case of MnCo.sub.2O.sub.4/NEGF compared to Pt/C+RuO.sub.2.

[0064] The application of MnCo.sub.2O.sub.4/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 microstructure of the resulting electrodes (FIG. 6a-d). FIG. 6a 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 MnCo.sub.2O.sub.4/NEGF, a thick layer with 3D structure (indicated by the dotted yellow lines) is observed (FIG. 6b). The inset of FIG. 6b gives better clarity of the surface of the GDL containing the 3D self-assembled structure of the coated layer of MnCo.sub.2O.sub.4/NEGF. This 3D microstructured 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 yellow lines in FIGS. 6c and 6d, i.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. FIGS. 6c and 6d show the 3D tomogram cross-section images of the bare GDL and the MnCo.sub.2O.sub.4/NEGF-coated GDL, respectively. The tomography image in FIG. 6c shows the two distinct phases of OCF and GDF (marked with the dotted yellow lines) of the GDL as already indicated in the FESEM image of the corresponding sample presented in FIG. 6a. On the other hand, in the case of the 3D CT image of the catalyst-coated GDL (FIG. 6d), the 3D microstructure formation of the layer of MnCo.sub.2O.sub.4/NEGF is clearly evident and is demarcated with the dotted yellow line.

[0065] The 3D porous morphology of the MnCo.sub.2O.sub.4/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 MnCo.sub.2O.sub.4/NEGF-coated surface of the GDL shows a water contact angle of 109.2 (FIG. 6f). CA data corresponding to base GDL is presented in FIG. 6e. From these results, it is readily inferred that while aqueous electrolyte hardly wet bare GDL, GDL based on MnCo.sub.2O.sub.4/NEGF coating possesses balanced hydrophilic/hydrophobic characteristic, which is expected to result in optimum wettability at interface.

[0066] 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.sup.2 were recorded for MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 catalysts for 1 h. When the current density was increased from 5.0 to 10.0 mA cm.sup.2, the charge voltage with the MnCo.sub.2O.sub.4/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.sub.2. Even at 30.0 mA cm.sup.2, the former has a charge voltage of 1.10 V, which is about 210 mV higher than the Pt/C+RuO.sub.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 cm2. However, the ZAB with Pt/C+RuO.sub.2 catalyst are 1.05, 0.14, 0.15, and 0.60 V, respectively. Even in the case of Pt/C+RuO.sub.2 at a higher current density of 10 mA cm.sup.2 sudden drop of potential is observed. The catalyst (MnCo.sub.2O.sub.4/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.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 at 10 mA cm.sup.2 catalyst has a discharge time of about 48 h and 40 h, respectively. Hence, in the longer run, the MnCo.sub.2O.sub.4/NEGF catalyst is observed to outperform the Pt/C+RuO.sub.2 system both in terms of performance and long-term durability under a realistic ZAB system.

[0067] In a nutshell, the present invention provides electrode material consisting of manganese-cobalt-based bimetallic spinel oxide (MnCo.sub.2O.sub.4)-supported nitrogen-doped entangled graphene (MnCo.sub.2O.sub.4/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.sub.2O.sub.4/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 202 mW cm2, which is significantly improved as compared to one based on Pt/C+RuO2 standard catalyst pair(124 mWcm2). Solid-state device displaying an initial chargedischarge voltage gap of only 0.7 V at 10 mA cm2 showed only small increment of 86 mV after 50 h.

EXAMPLES

[0068] 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.

[0069] Materials: Graphite, potassium permanganate (KMnO.sub.4), manganese acetate tetrahydrate [Mn(OAC).sub.2.Math.4H.sub.2O], cobalt acetate tetrahydrate [Co(OAc).sub.2.Math.4H.sub.2O], ammonium hydroxide (NH.sub.4OH), zinc acetate and potassium hydroxides were purchased from Sigma-Aldrich. Sulphuric acid (H.sub.2SO.sub.4) and phosphoric acid (H.sub.3PO.sub.4) were acquired from Thomas Baker. All the chemicals were used as such without any further purification.

Example 1

[0070] (a) Synthesis of Graphene Oxide (GO): An improved Hummer's method was employed to synthesize graphene oxide (GO). Firstly, (1:6) graphite powder and KMnO.sub.4 were well mixed using a mortar and pestle. The resulting solid mixture was slowly added to the round bottom flask containing a mixture of H.sub.3PO.sub.4:H.sub.2SO.sub.4 (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% H.sub.2O.sub.2 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 HCl 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.

[0071] (b) Synthesis of MnCo.sub.2O.sub.4 Supported N-doped entangled 3D Graphene (MnCo.sub.2O.sub.4/NEGF): The as-prepared GO (example 1a) 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).sub.2.Math.4H.sub.2O and Co(OAc).sub.2.Math.4H.sub.2O was added to the solution with a 1:2 ratio, and kept stirring for another 6 h 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 10 h 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 MnCo.sub.2O.sub.4/NEGF. For comparison, the controlled samples such as N-doped entangled graphene (NEGF), Mn.sub.3O.sub.4 supported N-doped entangled 3D graphene (Mn.sub.3O.sub.4/NEGF), and Co.sub.3O.sub.4 supported N-doped entangled 3D graphene (Co.sub.3O.sub.4/NEGF) was also synthesized. The NEGF, Mn.sub.3O.sub.4/NEGF, Co.sub.3O.sub.4/NEGF was prepared by using the same methods without adding any metal precursor and graphene oxide, with the addition Co(OAc).sub.2.Math.4H.sub.2O, Mn(OAc).sub.2.Math.4H.sub.2O respectively, keeping all the other parameters as such.

[0072] (c) Preparation of physically mixed composite of MnCo2O4 and N-doped Entangled 3D Graphene (MnCo2O4@NEGF): To prepare the physically mixed composite of MnCo2O.sub.4 and NEGF, 100 mg of the as-prepared NEGF and 50 mg of MnCo2O.sub.4 were mixed with the help of a mortar and pestle.

Example 2: Physical Characterization

[0073] a) Field emission scanning electron microscopy (FESEM) analysis: FIG. 1(a) shows the FESEM image of the MnCo.sub.2O.sub.4/NEGF, which represents the self-assembly structure of nitrogen-doped three-dimensionally oriented graphene. The magnified image of MnCo.sub.2O.sub.4/3D NGr shown in FIG. 1(b) indicates the interconnected two-dimensional nitrogen-doped graphene. FIG. 1 (c) depicts 3D micro-CT images of MnCo2O4/NEGF, showing the porous structure of 2D sheets are connected.

[0074] b) Transmission electron microscopy (TEM) imaging: Transmission electron microscopy (TEM) imaging was performed to visualize the distribution of MnCo.sub.2O.sub.4 nanoparticles over 3D NEGF support (FIG. 1d). The TEM analysis shows that the spherical-shaped MnCo.sub.2O.sub.4 nanocrystals are uniformly distributed over individual sheets of N-doped graphene. The controlled distribution of the metal oxide nanoparticles is credited to the doped-N in the graphene sheets, which generates asymmetric carbon centers helping in the creation of homogeneous nucleation sites for growth of metal oxide nanoparticles. A fraction of metal oxide nanoparticles are distributed at the inner surface of 3D graphene, which are 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 is distributed mostly in the range of 30-60 nm. FIG. 1(e) shows the high-resolution transmission electron microscopy (HRTEM), elucidating that the metal oxides are crystalline in nature. The metal oxide nanoparticles are having lattice fringe widths of d-spacing 0.25 and 0.21 nm, which is ascribed to the (311) and (211) facets suggesting the formation of cubic MnCo.sub.2O.sub.4 spinel phase. The selected area electron diffraction (SAED) pattern shown in FIG. 1(f-k), is the elemental mapping of the MnCo.sub.2O.sub.4/3D NGr catalyst. Elemental mapping exhibits presence and distribution of Co, Mn, O, C, and N, which is 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.

[0075] c) Pore size: FIG. 2(a) shows the comparative pore size distribution of NEGF, MnCo.sub.2O.sub.4, and MnCo.sub.2O.sub.4/NEGF materials where the pores are distributed in the region of 2-20 nm for NEGF and 2-16 nm for MnCo.sub.2O.sub.4/NEGF. However, MnCo.sub.2O.sub.4 showed a significantly lower pore size distribution. The significantly suppressed pore size in the case of CoMn.sub.2O.sub.4/NEGF catalyst is in the range of 16-20 nm is mostly due to the agglomerated nonporous structure of spinel oxides (MnCo.sub.2O.sub.4). The Type-IV isotherms were seen in both NEGF and MnCo.sub.2O.sub.4/NEGF, FIG. 2(b). Moreover, the higher BET surface area of NEGF (450 m.sup.2 g.sup.1) confirms the highly porous nature of NEGF as observed in the FESEM and CT-tomography image analysis. A reduction in BET surface area of MnCo.sub.2O.sub.4/NEGF to 300 m.sup.2g.sup.1 showed that some of the metal oxide species are lying in the microspores obscuring porous surface. The large specific surface area of catalyst is beneficial towards establishment of effective TPB in catalysis process suitable for fabrication of air electrodes of rechargeable ZAB.

[0076] d) X-ray diffraction (XRD) analysis: X-ray diffraction (XRD) analysis of NEGF displays the broad diffraction peaks at 2 values of 26 and 43 corresponding to the (002) and (100) graphitic diffraction planes, respectively. The absence of any metallic peaks in the spectra suggests the higher purity level of the prepared nitrogen-doped 3D graphene. The XRD pattern of Co.sub.3O.sub.4/NEGF showed a comparatively intense peak at 2 values of 35 corresponds to (311) plan for Co.sub.3O.sub.4. However, after the incorporation of Mn into the spinel structure of Co.sub.3O.sub.4, the resulting MnCo.sub.2O.sub.4/NEGF showed almost similar peaks intensity with a small shift in the peak position. The XRD pattern of MnCo.sub.2O.sub.4/NEGF confirmed a series of peaks at 2=18.3, 30.2, 35.6, 37.0, 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 MnCo.sub.2O.sub.4 over NEGF, a graphitic (002) plane shift towards lower diffraction angle compared to NEGF has been observed. This is ascribed due to incorporation of spherical MnCo.sub.2O.sub.4 nanoparticles between graphene layers which increased d-spacing of nitrogen-doped graphene sheets.

[0077] e) X-ray photoelectron spectroscopy (XPS): X-ray photoelectron spectroscopy (XPS) measurements have been employed in FIG. 3 (a-d). The survey scan spectra of NEGF, Co.sub.3O.sub.4/NEGF, and MnCo.sub.2O.sub.4NEGF shown in the figure confirm the presence of Mn, Co, O, N, and C in the respective materials. The characteristic Co 2p XPS peaks corresponding to Co.sub.3O.sub.4/NEGF and MnCo.sub.2O.sub.4/NEGF appear at the binding energy (B.E.) value of 784.2 eV and 795.5 eV and 783.5 eV and 796.5 eV, respectively. The characteristic peak separation (15.84 eV) between two peaks remains the same for spinel oxides. However, the shift in the binding energy after incorporating Mn into the spinel oxides Co.sub.3O.sub.4 evidenced the formation of bimetallic (MnCo.sub.2O.sub.4) spinel oxides. The observed negative shift in the binding energy of MnCo.sub.2O.sub.4 compared to Co.sub.3O.sub.4 might be due to the charge transfer from Co to Mn. Furthermore, deconvoluted XPS spectra of Co 2p in MnCo.sub.2O.sub.4/NEGF showed two doublet peaks at the B.E. values of 783.1 and 798.8 eV with a band separation of 15.7 eV pointing towards the existence of the +2 and +3 oxidation states of Co. In addition, the deconvoluted Mn spectra shows the two spin-spin coupling peaks at the B.E. Values of 783.1 and 798.8 eV corresponding to the Mn 2p.sub.3/2 and Mn 2p.sub.1/2 states of Mn also confirm the existence of the +2 and +3 oxidation states. Moreover, the deconvoluted N 1s spectra of the MnCo.sub.2O.sub.4/NEGF displays the peaks at pyridinic-N at 398.6 eV and the pyrrolic-N at 399.7 eV as the major moieties along with smaller proportions from the graphitic-N at 400.5 eV and NH.sub.4.sup.+ at 405.5 eV. The presence of nitrogen doping in the conducting support is mostly responsible for improving surface wettability of electrocatalysts, thereby enhancing electrocatalytic activity.

[0078] f) Wettability of the Electrocatalyst: Hydrophilicity and Hydrophobicity property is an important aspect to maintain the effective electrochemical triple phase boundary (TPB) during electrochemical process. The contact angle (CA) measurement was performed in FIG. 3e to check the surface wettability of MnCo.sub.2O.sub.4/EGF and MnCo.sub.2O.sub.4/NEGF catalysts. The lower contact angle value of 24 for MnCo.sub.2O.sub.4/EGF confirmed the higher hydrophilicity of the catalyst, which can 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 MnCo.sub.2O.sub.4/NEGF reached to the value of 42. Optimum contact angle value implies that appropriate hydrophilicity/hydrophobicity of catalytic material is more conducive to form the gas-liquid-solid TPBs during the electrochemical reaction.

Example 3: Electrochemical Half-cell Studies

[0079] (a) Rotating Disk Electrode Study: 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, we included the ORR activity of 20% Pt/C and the OER activity of RuO2. The catalyst slurry was prepared by mixing the catalyst (5 mg) in 1 mL isopropyl alcohol-water (3:2) solution and 40 L of Nafion solution (5 wt %, Sigma-Aldrich) using approximately 1 h water-bath sonication. After that, 10 L of the catalyst slurry was drop-coated on the surface of the working electrode, which was polished with 0.3 m 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:

[0080] (b)-1: Preparation of the Gel Electrolytes: 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 is formed, 4 mL of 9 M KOH solution is added dropwise and the mixture is 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.

[0081] (b)-2: Assembly and test of solid-state ZAB device: 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.sup.2 (electrode area=1.0 cm.sup.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 steady-state 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.

[0082] (c) Electrocatalytic RDE performance analysis: 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.sup.1 under O.sub.2 atmosphere to maintain the working electrode rotation at 1600 RPM. The comparative LSV profile (FIG. 4a) evidences the superior ORR performance achieved by MnCo.sub.2O.sub.4/NEGF compared to the control samples, i.e., NEGF (0.86 V), Co.sub.3O.sub.4NEGF (0.89 V), and Mn.sub.3O.sub.4NEGF (0.85 V). In addition, the ORR performance of MnCo.sub.2O.sub.4/NEGF (0.93 V) is close to state-of-the-art catalysts (Pt/C), showing its higher catalytic potential (0.99 V). Similarly, the OER activity has also been measured for NEGF, Co.sub.3O.sub.4/NEGF, Mn.sub.3O.sub.4/NEGF, MnCo.sub.2O.sub.4/NEGF, and RuO.sub.2 in 1 M KOH at a scan rate of 10 mV sec1 under N.sub.2 atmosphere (FIG. 4b). LSVs recorded for MnCo.sub.2O.sub.4NEGF showed better electrochemical OER activity compared to NEGF, Co.sub.3O.sub.4NEGF, and Mn.sub.3O.sub.4/NEGF. The superior performance of MnCo.sub.2O.sub.4NEGF catalyst towards both oxygen reactions (ORR/OER) is observed in LSV analysis. Moreover, differences in OER potential (E.sub.j@10 mA cm.sup.2) and ORR half-wave potential (E.sub.1/2) are generally used to evaluate the performance of bifunctional catalyst. Overall bifunctional activity (ORR-OER) of MnCo.sub.2O.sub.4/NEGF is 0.82 V (FIG. 4c), which is comparable or better than previously reported various bifunctional electrocatalyst. Observed higher bifunctional oxygen reaction activity of prepared catalyst is 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

[0083] a) Preparation of Gel Electrolytes: 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.

[0084] b) All-solid-state ZAB Device: All-solid-state rechargeable ZAB was assembled by utilizing Zn powder as the anode, MnCo.sub.2O.sub.4/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. 11. For the preparation of catalyst slurry, MnCo.sub.2O.sub.4/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 cm2 (electrode area=1.0 cm.sup.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 MZn (Ac).sub.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.

[0085] c) Characterization of ZAB Assembly: The application of MnCo.sub.2O.sub.4/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 microstructure of the resulting electrodes (FIG. 6). FIG. 6a 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 MnCo.sub.2O.sub.4/NEGF, a thick layer with 3D structure (indicated by the dotted yellow lines) is observed (FIG. 6b). The inset of Figure FIG. 6b gives better clarity of the surface containing the 3D self-assembled structure of the coated layer of MnCo.sub.2O.sub.4/NEGF. Compared to the highly porous nature of the MnCo.sub.2O.sub.4/NEGF layer on the GDL, the catalyst layer of Pt/C+RuO.sub.2 is found to be significantly less porous. Figure FIGS. 6c and d show the 3D tomogram cross-section images of the bare GDL and the MnCo.sub.2O.sub.4/NEGF-coated GDL, respectively.

[0086] d) Electrochemical performance of all-solid-state rechargeable ZAB device: FIG. 7(a-e) shows the performance of all-solid-state rechargeable ZAB with the open-circuit voltage (OCV) values of 1.31 and 1.20 V, respectively, for MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 coated electrodes. The comparative steady-state cell polarization leads to the maximum power density (P.sub.max) of 110 and 200 mW cm.sup.2 for the ZABs based on Pt/C+RuO.sub.2 and MnCo.sub.2O.sub.4/NEGF, respectively. The cathode catalysts show superior performance for the prepared catalyst (MnCo.sub.2O.sub.4/NEGF) compared to Pt/C+RuO.sub.2, which is ascribed to be better interface formation in the former catalyst. The performance of the fabricated all-solid-state ZAB is 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 are significantly different for MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2, 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+RuO2 catalyst. Furthermore, the galvanostatic charge/discharge curve measured at 10 mA cm.sup.2 is shown in FIG. 7c.

TABLE-US-00002 TABLE 2 Comparison of the performance of the solid-state ZAB systems based on the non-precious metal-based electrocatalysts. Power density Electro- OCV (mW catalyst Electrolyte (V) cm2) Stability References Co.sub.3O.sub.4-x PVA-KOH 1.46 94.1 50 cycles Dongxiao Ji HoNPs@HP (Solid) for 18 et al., 2019 NCS h @ 3 mA cm2 Co/CoO/ PVA-KOH 1.32 28 36 h @ 2 Xingyu Cui NWC (Solid) mA cm2 et al., 2021 CoN.sub.4/NG PVA-KOH 28 6 h @ 1 Liu Yang1 et (Solid) mA cm2 al., 2018 MnOx-GCC PVA-KOH 1.42 18 30 h @ A. Sumboja (Solid) 0.7 mA et al., 2017 cm2 CoSx/ PVA-KOH 1.34 16 h @ 1 Qian Lu et CoNC-800 (Solid) mA cm2 al., 2019 MnCo.sub.2O.sub.4/ PVA-KOH 1.31 202 51 h Present NEGF (Solid) work

[0087] The observed difference between the charging and discharging voltages of ZAB on MnCo.sub.2O.sub.4/NEGF during the initial process was 0.84 V which was lower than 0.91 V of the Pt/C+RuO.sub.2. After 50 h of continuous charge-discharge cycles, a nominal voltage difference increased by 0.10 V on ZAB consisting of MnCo.sub.2O.sub.4/NEGF compared to 1.1 V after 15 h cycle operation on Pt/C+RuO2 ZAB. The higher stability of MnCo.sub.2O.sub.4/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 shows (FIG. 7d)NEGF-based that in the case of MnCo.sub.2O.sub.4/NEGF, the charge-discharge voltage plateau are more symmetric but in the case of Pt/C+RuO.sub.2 deficient asymmetric charge-discharge curve. This feature reveals the better bifunctional activity at the ZAB air cathode interface in the case of MnCo.sub.2O.sub.4/NEGF compared to Pt/C+RuO.sub.2.

[0088] The ORR process is 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 microstructure on ORR kinetics at ZAB air cathode (FIG. 7e). So, the discharge curve at various current densities 5, 10, 20, and 30 mA cm.sup.2 were recorded for MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 catalysts for 1 h. When the current density was increased from 5.0 to 10.0 mA cm.sup.2, the charge voltage with the MnCo.sub.2O.sub.4/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.sub.2. Even at 30.0 mA cm.sup.2, the former has a charge voltage of 1.10 V, which is about 210 mV higher than the Pt/C+RuO.sub.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 cm2. However, the ZAB with Pt/C+RuO2 catalyst are 1.05, 0.14, 0.15, and 0.60 V, respectively. Even in the case of Pt/C+RuO.sub.2 at a higher current density of 10 mA cm.sup.2 sudden drop of potential is observed. The catalyst (MnCo.sub.2O.sub.4/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. This demonstrates that these kinds of air cathode have outstanding application potential under high current density. Furthermore, the galvanostatic discharge curve recorded for MnCo.sub.2O.sub.4/NEGF and Pt/C+RuO.sub.2 at 10 mA cm.sup.2 catalyst has a discharge time of about 48 h and 40 h, respectively. Hence, in the longer run, the MnCo.sub.2O.sub.4/NEGF catalyst is expected to outperform the Pt/C+RuO.sub.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 is attributed to the suitable air cathode interface design, sufficient active sites, and efficient mass transfer properties of MnCo.sub.2O.sub.4/NEGF.

Advantages of the Invention

[0089] 3D porous architecture of N-doped graphene supported MnCo.sub.2O.sub.4 nanosphere viz. MnCo.sub.2O.sub.4/NEGF as an air cathode in all-solid-state Zinc-air batteries (ZABs). [0090] Features like porous 3D architecture of the catalyst, balanced hydrophilic/hydrophobic characteristics, and optimal ORR/OER activity are found to be favorably helping the system as an air-electrode for the rechargeable ZAB application. [0091] 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. [0092] In terms of activity of MnCo.sub.2O.sub.4/NEGF toward said reactions, overpotential values are found closely comparable to respective state of art systems(Pt/C for ORR & RuO.sub.2 for OER). [0093] The demonstration of a solid-state rechargeable ZAB device with MnCo.sub.2O.sub.4/NEGF as the air electrode delivered a maximum peak power density of 200 mWcm.sup.2, with good stability at the time of the charge-discharge cycling process. [0094] 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+RuO2. [0095] The synergistic effect between MnCo.sub.2O.sub.4 nanoparticles and N-doped porous graphene promotes better interface formation (TPB). This benefits the system in terms of its bifunctional characteristics to perform as an effective electrocatalyst for facilitating both ORR and OER processes. [0096] 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 O.sub.2 and OH.sup. ions. [0097] The N-doped porous graphene controls the optimum hydrophilicity, and hydrophobicity which helps to better wettability of the electrocatalyst. [0098] The factors mentioned above collectively result in higher performance of electrocatalyst under the RDE condition and as an air cathode in ZABs.