A composite bifunctional electrocatalyst, a process of preparation and applications thereof

Abstract

The present invention relates to a bifunctional electrocatalyst, its process for the preparation and applications thereof. More particularly, the invention relates to a bifunctional electrocatalysts / composite for use in electrolysis processes particularly in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), wherein the electrocatalyst comprises of M1-Nitrogen dual doped reduced graphene oxide (rGO) and co-encapsulated graphitic carbon nanotube (CNT) with another metal M2, the bifunctional electrocatalysts / composite is useful as electrode for rechargeable as well as primary Zn-air batteries.

Description

[0001] A COMPOSITE BIFUNCTIONAL ELECTROCATALYST, A PROCESS FOR THE PREPARATION AND APPLICATIONS THEREOF

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a composite bifunctional electrocatalyst comprises of Ml- Nitrogen dual doped reduced graphene oxide (rGO) and co-encapsulated graphitic carbon nanotube (CNT) with another metal M2. Particularly, present invention relates to a process for the preparation of the composite bifunctional electrocatalyst. More particularly, the present invention relates to the composite bifunctional electrocatalysts for use in electrolysis processes particularly in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The bifunctional electrocatalysts / composite is useful as electrode for rechargeable as well as primary Zn-air batteries.

[0004] BACKGROUND AND PRIOR ART OF THE INVENTION

[0005] In order to satisfy global energy demands and address environmental problems arising from large emission of greenhouse gases from fossil fuel-based energy sources, new electrochemical power source or energy conversion and storage systems are required. Amongst most promising electrochemical power source or energy conversion and storage systems, air- metal battery such as air-zinc battery or Zn-air batteries (ZABs) are feasible contenders in energy market. ZABs have renewed interest due to its high theoretical energy density (1084Whkg-1), low cost, natural abundance of Zn reserves, safety, and environmental friendliness. Even though ZABs are being consider as alternatives to expensive lithium-ion batteries (LIBs), they explored in present days are not rechargeable in nature.

[0006] Electrochemical power source is apparatus in which electrical energy can be generated through electrochemical reaction. The apparatus includes air-metal battery such as air-zinc battery, air-aluminium battery, and so on. The air-metal battery employs negative electrode formed of metal converted into metal oxide during discharge, and positive electrode as air positive electrode film, which is transmissive film including water molecules, and in contact with oxygen in air to generate hydroxyl ions. Such an air-metal battery has various advantages in comparison with a conventional hydrogen fuel cell. In particular, since a fuel such as zinc is plentifully provided as metal or oxide thereof, supply of energy provided from air-metal battery is not visually exhausted. In addition, while conventional hydrogen fuel cells require recharge, air-metal battery can be electrically recharged and can transmit higher output voltage (1 to 2.7 volts) than conventional fuel cells.

[0007] The air-metal secondary battery is discharged through the reaction 2Zn + O2 — > 2ZnO (Anode: Zn + 4(OH)“ Zn(OH)42-+ 2e“; Cathode: O2 + 2H2O + 4e“ 4OH“ ) and charged through the reaction 2ZnO 2Zn + O2 (Anode: Zn(OH)42’ + 2e“ — > Zn + 4(OH)“; Cathode: 4OH“ —> 02 + 2H2O + 4e ) to supply electrical energy to the outside. Then, when air-metal secondary battery arrives at discharge limit to stop supply of electrical energy to outside, battery can be electrically charged to be reused.

[0008] If ZABs can be operated in electrically rechargeable manner, it will essentially broaden its commercial scope for various applications. However, integrating features like rechargeability is not an easy task. To achieve rechargeability, the development activity focused on these strategies: develop an air electrode based on a suitable bifunctional electrocatalyst which can facilitate both oxygen reduction reaction (ORR, under discharge process) and oxygen evolution reaction (OER, under charging process). -The energy efficiency of rechargeable Zn-air batteries (RZABs) depends largely on the cathodic reaction. However, ORR kinetics of the cathode of such systems suffers from complex 4e“ / 4H+transfer processes (O2 + 2H2O + 4e“ — 4OH ). Also, as for OER required higher overpotential during charging process. The sluggish nature of the oxygen redox kinetics could be effectively overcome with the help of an electrocatalysts, such as precious metals platinum (Pt) supported over carbon (Pt / C) for ORR and Ru02 and IrO2 for OER, they are often considered as benchmark catalyst. However, the high cost, low abundance and poor durability of precious metal restrict the widespread usage of such catalysts and this inhibits the commercial prospects of electrically RZABs. Therefore, the development of cost- effective and durable catalysts based on transition metals for ORR and OER has been the focus of research since a long time.

[0009] There have been recent reports of new types of bi-functional catalysts such as CoP, CoO, NGM-Co, and Mn-N-C are facilitating both ORR and OER. There are few reports on the presence of p-block metal (B, Al, Ge, Sn, Pb) based electrocatalysts exhibiting great advantages over the other electrocatalysts based on various performance indicators. In particular the Al, N-co-doped graphene (Al, N-rGO) as well as the transition-metal Fe, N- co-doped graphene (Fe, N-rGO) are representative of improving ORR and OER activity at least in two ways: (i) improve the accessibility of active sites and (ii) increase the intrinsic activity of accessible active sites. The Al, N-rGO can create more active sites by breaking the atomic symmetry through Al and N heteroatoms. In contrast, Fe, N-rGO has a high spin density that enhances its electrocatalytic activity. Al, N-rGO is active for ORR with good stability. Whereas, Fe, N-rGO is exhibiting good ORR activity but poor stability at higher overpotential due to reduction of Fe. However, majority of currently available catalyst only exhibit a single catalytic function (ORR or OER), making them impractical for use in rechargeable Zn-air battery applications. Developing an inexpensive electrocatalyst with bifunctional activity (ORR and OER) and durability remains a challenge.

[0010] The Al is a very imperative p-block metal with a proven track record in the catalyst field, due to the presence of empty p-orbital in aluminium, it is acting as electron deficient type of dopant and create more accessible active site. Also, the N doping acting as electron rich type of dopant and create more accessible active site for metal coordination. Aluminium, and nitrogen dual-doped graphene (Al, N-rGO) could be used as support material as well as ORR and OER electrocatalysts. The Al, N-rGO, with its negatively charged nature of the catalyst surface originating from the N centre, have the potential to promote the available active sites for coordination of active metal (M) particle for uniform distribution during synthesis process and act as good support material (N-rGO). The M-Al, N-rGO active catalyst surface promote the available active site for adsorption of OH* which is a key intermediate for ORR and OER process. During the reduction process of graphene oxide, there is a possibility of restacking or agglomeration of graphene layers by Van-der- Waals interaction. This would lead to the limited active surface area for the reactions. If the graphene sheets are stacked, it would be difficult for the precursor ions to reach the interior layers. However, the ions could only build on the top or bottom surface of the graphene sheets, which would result in a lower active surface area because the stacked sheets could not be fully utilized.

[0011] References may be made to Journal “Small Methods 2021, 5, 2000827”, wherein binder- free bifunctional oxygen electrodes have been reported. Al, N-dual doping for liquid state battery has been reported and demonstrated. References may be made to Journal “Adv. Mater. Technol. 2022, 7, 2100673”, which elaborated in situ preparation and interface engineering for Flexible zinc-air batteries (FZABs) has been discussed.

[0012] References may be made to Journal “Journal of Colloid and Interface Science, Volume 552, 15 September 2019, Pages 744-751” wherein, synthesis first g-C N has been disclosed and then in second step final material g-C N used as C and N sources, and cobalt phthalocyanine (CoPc) as the Co- and N-sources. Haocheng Qi et al., Nanoscale, 2019, 11, 21943-21952 discloses achievement of Co / Co4N@N-CNTs / rGO through pyrolyzing core-shell structured ZIF-8@ZIF-67-modified GO (ZIF-8@ZIF-67 / GO) precursor. However, none of these articles report effective electrocatalysts which can act as bifunctional for ORR and OER reactions.

[0013] Hence, there remains an urgent need to develop improved, efficient, cost-effective, and environmentally friendly catalyst materials that are simple to scale up and competitive electrocatalysts to replace expensive noble metal catalysts available. Present invention addresses this requirement and stands out as versatile replacement for expensive catalysts based on noble metals for electrochemical systems involving ORR and OER as ratedetermining electrode process.

[0014] OBJECTS OF THE INVENTION

[0015] Main object of the present invention is to provide an improved, efficient, cost-effective and environmentally friendly as well as non-toxic composite bifunctional electrocatalyst.

[0016] Another objective of the present invention is to provide a process for the preparation of the composite bifunctional electrocatalyst.

[0017] Yet another objective of the present invention is to provide a composite bifunctional electrocatalyst that is robust and simple to scale up.

[0018] Yet another objective of the present invention is to provide a composite bifunctional electrocatalyst, wherein the composite provides a greater surface area, higher micro as well as meso porosity as well as improved electrocatalytically activity.

[0019] Yet another objective of the present invention is to provide a composite bifunctional electrocatalyst for use in electrolysis processes, particularly in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Yet another objective of the present invention is to provide a composite bifunctional electrocatalyst which is highly stable, i.e., said electrocatalyst is stable approximately upto 5000 cycles with minimal change in electrocatalysis.

[0020] SUMMARY OF THE INVENTION

[0021] Accordingly, present invention provides a composite bifunctional electrocatalyst comprising: i. p-block element M 1 ; ii. transition metal M2; iii. nitrogen; iv. reduced graphene oxide (rGO), and v. carbon nanotubes; wherein the reduced graphene oxide (rGO) is a dual doped reduced graphene oxide with said nitrogen and p-block element Ml, and; wherein the transition metal M2 is encapsulated inside the carbon nanotubes.

[0022] In an embodiment of the present invention, the p-block element Ml is in the range of 0.25- 4.5 wt. %; the transition metal M2 is in the range of 9.5-30 wt. %; the nitrogen is in the range of 10-15 wt. %; the reduced graphene oxide and the carbon nanotubes together is in the range of 60-85 wt.% such that the total wt% does not exceeds 100%.

[0023] In yet another embodiment of the present invention, the concentration of the Ml in the composite bifunctional electrocatalyst ranges from 0.25-4.5 wt. % specifically 0.5 wt. % or 1 wt. % or 2 wt. % or 3 wt. % or 3.9 wt. % or 1.31 wt. % or 0.31 wt. % or 2.31 wt. % or 4 wt. %.

[0024] In yet another embodiment of the present invention, the concentration of the M2 in the composite bifunctional electrocatalyst ranges from 9.5-25.5 wt. % specifically 10 wt. % or 12.5 wt. % or 15 wt. % or 16 wt. % or 17.5 wt. % or 18 wt. % or 21 wt. % or 23 wt. % or 25 wt. %.

[0025] In yet another embodiment of the present invention, the concentration of the N or nitrogen in the composite bifunctional electrocatalyst ranges from 10-15 wt. % specifically 10 wt. % or 11 wt. % or 11.4 wt. % or 11.5 wt. % or 12 wt. % or 13 wt. % or 14 wt. % or 15 wt. %.

[0026] In yet another embodiment of the present invention, the concentration of the graphene oxide and CNT in the composite bifunctional electrocatalyst ranges from 60-85 wt. % specifically, 60 wt. % or 65 wt. % or 70 wt. % or 72 wt. % or 74 wt. % or 75 wt. % or 80 wt. % or 82 wt. % or 83 wt. %.

[0027] In another embodiment of the present invention, the p-block element Ml is selected from the group consisting of aluminum, indium, boron, and sulphur; and the transition metal M2 is selected from the group consisting of cobalt, iron and nickel.

[0028] In yet another embodiment of the present invention, the transition metal M2 is distributed or located at the tip of carbon nanotubes as well as over the reduced graphene oxide sheets; and wherein said carbon nanotubes are separated by the reduced graphene sheets.

[0029] In yet another embodiment of the present invention, the composite bifunctional electrocatalysts comprises mesoporous as well as microporous metal particles with a BET surface area is in the range of 210 to 220 m2 / g wherein particle size of said microporous metal particles is in range of 1-2.99 nm, and particle size of said mesoporous metal particles is in range of 3-10.

[0030] In yet another embodiment of the present invention, the weight percent of Ml-N and dualdoped graphene oxide O'GO) is 1.31+1 Wt. % and 70-75+2 Wt. %.

[0031] In yet another embodiment of the present invention, the weight percent of M2-N and dualdoped graphene oxide O'GO) is 20+2 Wt. % and 70-75+2 Wt. %.

[0032] In yet another embodiment of the present invention, the ratio of carbon nanotube (CNT) and graphene to M2 is 23+2 Wt. % and 70+2 Wt. %.

[0033] In yet another embodiment, present invention provides a process for the preparation of the composite bifunctional electrocatalyst comprises the steps of: a) dispersing, stirring and sonicating graphene oxide (GO) in water for a period in the range of 15-20 mins at a temperature in the range of 25-30°C to obtain a graphene oxide solution; b) reacting the graphene oxide (GO) solution as obtained in step (a) with melamine acting as reducing and nitrogen doping agent to obtain a mixture; and c) annealing the mixture as obtained in step (b) in presence of a salt of Ml and a salt of M2 and gradually increasing the temperature from 600 to 900°C with a ramping rate of 10°C / min for a period in the range of 2 to 4h under inert conditions to obtain the bifunctional electrocatalyst.

[0034] In yet another embodiment of the present invention, salt of Ml is selected from Ml nitrate, Ml sulfate, Ml hydroquinone, Ml isopropoxide, and sodium based salt of Ml , and salt of M2 is selected from M2 acetate, M2 chloride, M2 nitrate, M2 acetylacetonate and M2 sulfate.

[0035] In yet another embodiment, present invention provides a half-cell comprises of:

[0036] A. a working electrode coated with bifunctional electrocatalyst as claimed in claim 1;

[0037] B. reference electrode;

[0038] C. counter electrode; and

[0039] D. ion-conducting solution or an electrolyte which is in contact with said working electrode; wherein the ion-conducting solution or the electrolyte comprises at least 1% of an ionic liquid.

[0040] In yet another embodiment, present invention provides a full cell comprising of: i. an anode; ii. bifunctional electrocatalyst as claimed in claim 1 as cathode; and iii. an electrolyte which is in contact with the bifunctional electrocatalyst of the cathode, and optionally with the anode; wherein the electrolyte comprises at least 1% of an ionic liquid.

[0041] BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 represents FE-SEM analysis: Co / CNT at different temperature (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C, and (e) 900 °C.

[0043] Figure 2 represents FE-SEM analysis: (a)N-rGO, (b) Al / N-rGO, (c)Al, Co / CNT, and (d), Al, Co / rGO. Figure 3 represents E-SEM-EDS spectrum for (a) Al, N-rGO and (b) Al, Co / N-rGCNT.

[0044] Figure 4 represents HR-TEM analysis: HR-TEM image of (a)N / rGO, (b) Al, N / rGO, (c) Al, Co / rGO (d), Co / N-rGCNT, (e) Al, Co / N-CNT (f) Al, Co / N-rGCNT.

[0045] Figure 5 represents Field emission scanning electron microscopy (FE-SEM) analysis of Al, Co / N-rGCNT: (a) shows Co encapsulated carbon nanotube and graphene sheets, and (b) High magnification image and respective elemental mapping show Co, Al, O, N, and C.

[0046] Figure 6 represents High-resolution transmission electron microscopy (HR-TEM): (a) shows Co encapsulated carbon nanotube over graphene sheet of Al, Co / N-rGCNT and (b), (c) High magnification image of Al, Co / N-rGCNT (inset (b) SAED pattern), (d), (e), and (f) shows d-spacing image (g) Al, Co / N-rGCNT respective elemental mapping of Al, Co, N, O, and C shows the uniform distribution and most of the Co is present on the tip of CNTs.

[0047] Figure 7 represents High-resolution transmission electron microscopy (HR-TEM): elemental mapping overlay image of Al, Co / N-rGCNT shows Al, N, O, and C randomly distribution and most of the Co is present on the tip of CNTs.

[0048] Figure 8 represents the Comparative analysis of: (a) XRD and (b) Raman spectras.

[0049] Figure 9 represents the XRD analysis: Co / CNT at different temperature (a) 500°C, (b) 600°C, (c) 700°C, (d) 800°C, and (e) 900°C.

[0050] Figure 10 represents Comparative XPS analysis: (a) Comparative Al 2p spectra of Al / N- rGO, Al, Co / rGO, Al, Co / N-CNT, and Al, Co / N-rGCNT, (b) Comparative Co 2p spectra of Co / N-rGCNT, Al, Co / rGO, Al, Co / N-CNT, and Al, Co / N-rGCNT, (c) deconvoluted N Is spectra of Al, Co / N-rGCNT (d) deconvoluted Nls spectra of N-rGO, (e) deconvoluted Nls spectra of Al / N-rGO, and (f) deconvoluted Nls spectra of Co / N-rGCNT.

[0051] Figure 11 represents comparative analysis of: (a) ORR linear sweep voltammogram (LSV) performed in 02 saturated 0.1M KOH electrolyte, respective potential.

[0052] Figure 12 represents comparative analysis of: (a) N-rGO, Al / N-rGO, Co / N-rGCNT, Al, Co / rGO, Al, Co / N-CNT, Al, Co / N-rGCNT, Pt / C, and RuO2 revealing bifunctional electrocatalytic activity toward ORR / OER and (b) show the LSV profiles recorded before and after the durability analysis of Al, Co / N-rGCNT.

[0053] Figure 13 represents transmission electron microscopy (TEM) after ADT: (a) shows Co encapsulated carbon nanotube in between graphene sheets of Al, Co / N-rGCNT and (b) High magnification image of Al, Co / N-rGCNT, and (c-i) HAADF-STEM EDX elemental mapping of Al, Co / N-rGCNT respective elemental mapping of Al, Co, N, O, and C shows the uniform distribution.

[0054] Figure 14 represents (a) polarization plots of the ZABs made with Al, Co / N-rGCNT, and Pt / C as the air catalysts, (b) Comparative galvanostatic discharge profile (c) 20 mA cm'2: in Zn-air Battery, (c) specific capacity calculated from the data of normalized with the mass of the consumed Zn, (d) Comparative galvanostatic discharge profile @ different current density (mA cm'2): in Zn-air Battery, (e) Comparative galvanostatic charge-discharge profile @ 5mA cm'2each cycle for 5 min: in rechargeable Zn-air Battery, (f) Zoom part of fig. (e), (g-j) Demonstration of the flexible ZAB: (g) digital photograph of flexible ZAB connected with multimeter show OCP 2.91 V (h) before LED connection, (i) digital photographs of LED connected with ZAB, and (j) digital photographs of LED connected with ZAB recorded after 5h.

[0055] Figure 15 represents the Galvanostatic charge-discharge profile with a 2 mg / cm’2Al, Co / N- rGCNT loading @ 5mA cm'2each cycle for 5 min: in rechargeable Zn-air Battery.

[0056] Figure 16 (a) Schematically illustration of an anion-exchange membrane fuel cell (AEMFC) assembly and (b) polarization plots of the AEMFC made with Al, Co / N-rGCNT, as the cathode catalysts.

[0057] Figure 17 represents the steps involved during the synthesis of the Al-Co / N-rGCNT electrocatalyst.

[0058] Figure 18A and 18B represents the representative drawings of electrochemical device containing said composite bifunctional electrocatalyst. DETAILED DESCRIPTION OF THE INVENTION

[0059] The terms “composite” or “bifunctional electrocatalyst” or “composite bifunctional electrocatalyst” or “Al, Co / N-rGCNT” are used herein interchangeably with same meaning throughout the specification.

[0060] The present invention discloses a composite bifunctional electrocatalyst for use in electrolysis processes, particularly in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in electrochemical power source or energy conversion and storage systems.

[0061] The present invention provides a composite bifunctional electrocatalyst comprising: i. p-block element M 1 ; ii. transition metal M2; iii. nitrogen; iv. reduced graphene oxide (rGO), and v. carbon nanotubes; wherein the reduced graphene oxide (rGO) is a dual doped reduced graphene oxide with said nitrogen and p-block element Ml, and; wherein the transition metal M2 is encapsulated inside the carbon nanotubes.

[0062] Present invention provides a process for the preparation of a composite bifunctional electrocatalyst comprising the steps of: a) dispersing, stirring and sonicating graphene oxide (GO) in distilled water; b) reacting graphene oxide (GO) with melamine as reducing and nitrogen doping agent at temperature around 400-500 °C; and c) annealing mixture from step b) in presence of a salt of Ml and a salt of M2 and high temperature of around 600-900°C to obtain the said composite bifunctional electrocatalyst. wherein Ml is selected from Al (Aluminium), In (Indium), B (Boron), and S (Sulphur), and wherein M2 is selected from Co (Cobalt), Fe (Iron) and Ni (Nickel).

[0063] The composite bifunctional electrocatalyst of the present invention comprises i. a reduced graphene sheets separated by CNT ; ii. M2 is mostly present close to a tip of CNT, and also, present on surface of graphene sheets; iii. Ml and M2 are together have a bimetallic electronic interaction; and iv. Ml and Nitrogen dual doping is done on graphene sheets.

[0064] The carbon nanotubes are separated by the reduced graphene sheets.

[0065] The present invention relates to half fuel cell and / or full fuel cell comprising cathode, anode, electrolyte, and air electrode; wherein the air electrode comprising said bifunctional electrocatalysts / composite as coating layer.

[0066] A composite bifunctional electrocatalyst comprises Al-N dual-doped graphene oxide (rGO) with Cobalt encapsulated carbon nanotube (CNT).

[0067] The Ml metal is aluminum and said M2 metal is Cobalt.

[0068] The concentration of aluminum and cobalt in said bifunctional electrocatalyst is 0.42 wt % and 29.6 wt %, respectively. The salt of Ml is aluminium nitrate nonahydrate (Al (NOsh- 9H2O), and the salt of M2 is cobalt acetate tetrahydrate (Co(OAc)2.

[0069] The present invention relates to a half-cell comprises of:

[0070] A. a working electrode coated with said bifunctional electrocatalyst,

[0071] B. reference electrode,

[0072] C. counter electrode, and

[0073] D. ion-conducting solution or an electrolyte which is in contact with said working electrode; wherein the electrolyte or ion-conducting solution comprises at least 1% of an ionic liquid.

[0074] A process for the preparation of a composite bifunctional electrocatalyst, wherein the process comprises the steps of: a) dispersing, stirring and sonicating graphene oxide (rGO) in distilled water; b) adding cobalt acetate tetrahydrate (Co(OAc)2. 4H2O), aluminium nitrate nonahydrate (Al (NOsh- 9H2O), and melamine (C3H6N3) to the solution from step a); c) drying the solution from step b) under continuous agitation; d) grinding mixture of step c); and e) annealing ground mixture from step d) gradually from 600°C to 900°C. A half cell containing a conductive electrode and a surrounding conductive electrolyte separated by a double layer, comprising a composite bifunctional electrocatalyst comprising a Ml-N (Nitrogen) dual-doped graphene oxide (rGO) with encapsulated carbon nanotube (CNT) with another metal M2, wherein Ml is selected from Al (Aluminium), In (Indium), B (Boron), and S (Sulphur), and wherein M2 is selected from Co (Cobalt), Fe (Iron) and Ni (Nickel).

[0075] A fuel cell containing a conductive electrode and surrounding a conductive electrolyte, comprising a composite bifunctional electrocatalyst comprising a Ml-N (Nitrogen) dualdoped graphene oxide (rGO) with encapsulated carbon nanotube (CNT) with another metal M2, wherein Ml may be selected from Al (Aluminium), In (Indium), B (Boron), and S (Sulphur), and wherein M2 may be selected from Co (Cobalt), Fe (Iron) and Ni (Nickel).

[0076] Considering the advantages of Al and its limited studies towards electrocatalysis, the present invention reports and exemplifies herein the development of a composite bifunctional electrocatalyst consisting of cobalt encapsulated carbon nanotubes with the aluminium, nitrogen-dual-doped reduced graphene (Al, Co / N-rGCNT). The preparation method involved a simple and scalable pyrolysis method at 900 °C for 3 h in the inert atmosphere. Melamine is used as a reducing as well as a nitrogen doping agent. The prepared Al, Co / N-rGCNT catalyst has shown a significant ORR (Eo = 975 mV vs RHE) and OER (rpo mA cm-2= 390 mV vs RHE) activity and durability in alkaline medium due to the favourable morphologies of Al, Co / N-rGCNT. The performance can be comparable to commercial Pt / C and RuOi Furthermore, the developed catalyst (Al, Co / N-rGCNT) served as an air electrode for rechargeable zinc-air battery (RZAB) and the cell could be successfully demonstrated in single-cell mode. RZAB demonstrate successfully that the catalyst is capable of maintaining its activity towards ORR and OER based on single electrode studies, suggesting that Co, Al-based catalyst is viable option for environmental friendly and cost-effective energy applications.

[0077] This disclosure provides improved alkaline membrane fuel cell performance and Rechargeable Zinc Air Batteries (RZAB) that is an electrochemical device having increased energy densities and rechargeability performance at decreased cost. In certain aspects, the present disclosure improves RZAB that operate using a cathode that includes at least one bifunctional electrocatalyst. In certain aspects, the methods of the disclosure can decrease operating and capital costs while maintaining or even improving energy density and rechargeability. In addition, without being bound to a particular theory, it is believed that bifunctional electrocatalyst and the ionic liquid function as a bi-functional co-catalyst system, demonstrating a strong synergistic effect. In certain embodiments, the RZABs of the disclosure exhibit significantly higher oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance.

[0078] A Rechargeable Zinc Air Batteries (RZAB) full cell as an electrochemical device comprises: a) an anode; b) a cathode comprising said bifunctional electrocatalyst; and c) an electrolyte is in contact with the composite bifunctional electrocatalyst of the cathode, and optionally with the anode, wherein the electrolyte comprises at least 1% (e.g., at least 10%, at least 20%, at least 30% or at least 50%) of an ionic liquid.

[0079] In another embodiment, a RZAB half-cell as an electrochemical device, comprises: a. a working electrode coated with said bifunctional electrocatalyst, b. reference electrode, c. counter electrode, and d. ion-conducting solution or an electrolyte which is in contact with said working electrode; wherein the electrolyte or ion-conducting solution comprises at least 1% (e.g., at least 10%, at least 20%, at least 30% or at least 50%) of an ionic liquid.

[0080] The working electrode is made of simple metal free carbon, glassy carbon, and graphitic carbon and so on.

[0081] The reference electrode is made of mercury / mercurous oxide (Hg / HgO), which is utilized as a reference electrode in alkaline solution such as for development of an alkaline hydrogen electrode.

[0082] The counter electrode is made of graphite rod, which is immersed in the ion-conducting solution / electrolyte as a counter electrode. The disclosure also provides methods of generating an electric potential, comprising: i. providing a RZAB or an electrochemical device as described herein; ii. allowing oxygen to contact the cathode; iii. allowing the metal of the anode to be oxidized to metal ions; and iv. allowing the oxygen to be reduced at a surface of the composite bifunctional electrocatalyst to form one or more metal oxides with the metal ions, thereby generating the electrical potential between the anode and the cathode.

[0083] Also described, is an electronic material comprising at least one composite bifunctional electrocatalyst and an electrolyte in contact (e.g., in direct contact) with the composite bifunctional electrocatalyst, the electrolyte comprising at least 1% (e.g., at least 10%, at least 20%, at least 30% or at least 50%) of an ionic liquid.

[0084] The electrolyte is selected from potassium hydroxide (KOH) and zinc acetate.

[0085] The ionic solution with said electrolyte has concentration in the range of 1-8 M.

[0086] The anode is selected from zinc metal based metal wherein said zinc is pristine / pure zinc, porous zinc or oxides of zinc.

[0087] Specifically, the dimension of 0.5 to 50 cm2in particular, 1*1 cm of Zn foil having thickness of 0.1 to 3 mm preferably, 0.4 to 0.8 mm (or 0.55 mm) is used as anode in said battery. It is submitted that said dimension and thickness is not limited to said values, and the same can be varied as per requirement e.g. pilot or industrial scale use, etc. The maximum dimensions and thickness of Zn anode in Zn-air batteries depends on the type and application of the battery. Zn-air batteries are used for various applications / devices, such as hearing aids, medical devices, pagers, electric vehicles, and grid-scale energy storage. Therefore, the optimal dimensions and thickness of Zn anode may differ for different industrial applications.

[0088] In another embodiment, the cathode is selected from simple metal free carbon, glassy carbon, and graphitic carbon and so on. The cathode comprises at least a coating of said Al, Co / N-rGCNT. In an embodiment, the cathode comprises at least a coating of Al, Co / N-rGCNT on gas diffusion layer (GDL) in the range of 0.5 mg / cm2to 2 mg / cm2

[0089] Specifically, the cathode comprises at least a coating of Al, Co / N-rGCNT on gas diffusion layer (GDL) in the range of 1 mg / cm2.

[0090] Inventors of the present application submits that the graphene oxide layers separated by cobalt encapsulated carbon nanotubes improves the accessibility of active sites as well as the mass transport properties of the material. As an added advantage, Al, N-dual-doped graphene cobalt encapsulated carbon nanotubes can favourable tune the intrinsic activity towards ORR and OER, which is proved in the present application. Also, the possibility of establishing effective interaction between cobalt encapsulated CNT and Al, N-dual-doped graphene can be taken as a way to overcome low electrical conductivity and stability of the material by doping of heteroatoms and layers separated cobalt encapsulated carbon nanotubes.

[0091] In a nutshell, the inventors of present application report the synthesis of bifunctional electrocatalyst for RZAB (rechargeable zinc air battery) and alkaline membrane fuel cell (AMFC) via annealing of graphene oxide (GO), cobalt acetate, aluminium nitrate, and melamine. During the synthesis of Al, Co / N-rGCNT via. annealing, the in-situ formation of carbon nanotube (CNT) occurs, where cobalt and melamine plays a key role. The developed material is characterized with the layer separated rGO with Co-encapsulated CNTs. During the annealing treatment, GO is converted into Al and N co-doped reduced graphene oxide (Al, Co / N-rGCNT), melamine and cobalt play an important role in the CNT formation. Cobalt encapsulated carbon nanotubes separated aluminium, nitrogen-dualdoped reduced graphene (Al, Co / N-rGCNT) has displayed an onset potential of 975 mV vs RHE and half-wave potential of 825 mV vs RHE for ORR in 0.1M KOH solution. The overpotential exhibited by Al, Co / N-rGCNT for OER is 390 mV in IM KOH solution. The difference between the potential of OER and ORR (AE) is 0.82 V for Al, Co / N-rGCNT. The values are close to most of the reported Pt-free transition metal-based electrocatalyst. The integrated Al, Co / N-rGCNT-cathode containing the primary battery exhibited an open circuit potential of -1.35 V and peak power density of 106.3 mW cm-2, which is close to the system based on Pt / C (-1.50 V, 120.8 mW cm-2). The Al, Co / N-rGCNT -based device showed a flat discharge profile at the discharge rate of 20 mA / cm2with a specific capacity of 737 mAh / gzn compared to 696 mAh / gzn for the system based on Pt / C. Furthermore, the developed catalyst (Al, Co / N-rGCNT) served as an air electrode for RZAB fabrication and the cell could be successfully demonstrated in single-cell mode. Additionally, the improvised homemade battery is able to continue operating a LED light on mode condition for more than 5 hours. As a result, we have demonstrated the feasibility of building and demonstrating a reliable rechargeable zinc-air battery using a catalytic system based on earth- abundant elements.

[0092] EXAMPLES

[0093] The following examples are given as a way of illustration only and should not be construed to limit the scope of the present invention.

[0094] Example 1

[0095] Synthesis of Aluminium, Nitrogen-co-doped graphene with cobalt encapsulated carbon nanotube (Al, Co / N-rGCNT)

[0096] For the synthesis of Al, Co / N-rGCNT, 100 mg of Graphene oxide (GO) was properly dispersed in 30 mL of distilled water at a temperature in the range of 25-30°C with stirring for Ih followed by 15 min ultra-probe sonication. Subsequently, 2 mmol of cobalt acetate tetrahydrate (Co(OAc)2. 4HiO), 0.15 mmol of aluminium nitrate nonahydrate (Al (NOs)3. 9H1O), and 10 mmol of melamine (C3H6N3) were added into the GO solution, which is then kept for drying in air at 80°C with constant stirring for 5 h. The obtained mixture then ground well in mortar and pestle, resulted in ash-colored powder. Further, the powder annealed at 900°C with a ramping rate of 10°C / min for 3h under inert conditions. The obtained material was kept for acid treatment to remove the unsupported cobalt and aluminium with 0.5M H2SO4 at 80°C with constant stirring for 8 h. After cooling down to room temperature, the sample was washed with deionized water and ethanol several times via filtration. Obtained filtrate dried at 60°C for 6 h in a vacuum oven. Finally, the composite was again calcined at 900°C for 1 h in Ar atmosphere, obtained material was named as Al, Co / N-rGCNT.

[0097] Comparative Example 1 Synthesis of N-doped reduced Graphene (N-rGO)

[0098] The N-rGO was prepared similar to the above procedure of Example 1 without adding any cobalt acetate and aluminium nitrate.

[0099] Comparative Example 2

[0100] Synthesis of Al, N-dual-doped reduced Graphene (Al, N-rGO)

[0101] Al, N-dual-doped reduced graphene oxide was prepared by adding aluminium nitrate with melamine without adding cobalt acetate. Apart from that synthesis process of Al, N-rGO is same as of example 1 synthesis.

[0102] Comparative Example 3

[0103] Synthesis of N-doped Cobalt encapsulated Graphene (Co / N-rGCNT)

[0104] Co / N-rGCNT was prepared by adding cobalt acetate with melamine without adding any aluminium nitrate. Apart from that synthesis process of Co / N-rGCNT is similar to that of example 1 synthesis.

[0105] Comparative Example 4

[0106] Synthesis of Al-doped Cobalt encapsulated Graphene (Al, Co / rGO)

[0107] Al, Co / rGO was obtained by similar synthesis procedure as performed for Al, Co / N- rGCNT without adding melamine. Apart from that synthesis process of Al, Co / rGO is similar to that of example 1 synthesis.

[0108] Comparative Example 5

[0109] Synthesis of Aluminium Cobalt N-doped CNT (Al, Co / N-CNT)

[0110] Synthesis of Al, Co / N-CNT involved dispersion of aluminium nitrate nonahydrate (Al (NO3)3. 9H2O), Cobalt acetate tetrahydrate (Co(OAc)2. 4H2O) and melamine (C3H6N3). By adding Al and Co salt along with melamine. Remaining synthesis process of Al, Co / N- CNT is similar to that of example 1 synthesis. Comparative Example 6

[0111] Synthesis of Cobalt encapsulated N-doped CNT (Co / N-CNT-X)

[0112] To understand the effect of annealing temperature on the CNT formation, the dispersion of cobalt acetate tetrahydrate (Co(OAc)2. 4HiO) and melamine (C3H6N3) anneals at different temperatures and obtained Co / N-CNT-X (X= 500 °C, 600 °C, 700 °C, 800 °C, 900 °C).

[0113] Example 2

[0114] Zinc-Air Battery (ZAB) Fabrication and Testing

[0115] ZAB was fabricated by using Al, Co / N-rGCNT, and Zn foil as the cathode and anode electrodes, respectively, by employing an in-house-built electrochemical cell. The catalyst slurry for the coating over GDL was prepared by sonicating Al, Co / N-rGCNT for 60 min in isopropyl alcohol, Nafion, and DI water solution. To accomplish a catalyst loading of 1.0 and 2.0 mg / cm2(electrode area = 1.0 cm2), the resulting catalyst slurry was brush- coated on a carbon gas diffusion layer (GDL) and dried at 60 °C for 12 h. The as-obtained cathodes and the Zn anode were cut into 1*4 cm2areas, and with the help of Teflon tape, only 1*1 areas were exposed. Finally, the ZAB was assembled by pairing the anode and cathode using 6.0 M KOH and 0.2 M Zn acetate, where only 1*1 cm2of the active electrodes were allowed to merge. The ZAB setup was subsequently tested at 25-30°C using a multichannel SP-300 model Bio-Logic potentiostat / galvanostatic. The battery was analyzed under continuous oxygen purging by a potential scan rate of 5mV s’1, and steadystate polarization of the battery was measured with the same setup.

[0116] Example 3: Characterization and Electrochemical analysis of the bifunctional electrocatalysts and other materials prepared in examples 1-7

[0117] Figure 17 illustrates the synthesis process of Al, Co / N-rGCNT. The layer-separated structure of Al, N-dual-doped reduced graphene with cobalt encapsulated carbon nanotubes was obtained by pyrolysis method and its application as the bifunctional air electrode in a rechargeable zinc-air battery (RZAB). During the synthesis process, Co2+and Al3+ions adsorbed over the GO due to the interactions with the functional groups. During the annealing, melamine was converted into carbon nitride at 500 °C and started the formation of carbon nanotubes (CNTs) at 600 °C in which cobalt acted as the nucleation site (Figure la, b). With increasing the annealing temperature at 700-900 °C, CNT formation was increasing and the well-defined nanotube structure attained at 900 C (Figure le-e) with simultaneous doping of aluminium and nitrogen. The growth of Al, N-CNT was mainly dependent on the melamine and cobalt amount which was confirmed by preparing controls samples and further the morphological analysis. For the morphological study, scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high- resolution transmission electron microscopy (HR-TEM) analysis was performed. As depicted in Figure 2a, b the FESEM images of N-rGO and Al, N-rGO show the thin nano sheets -like morphology. In contrast, the product obtained in the presence of melamine, cobalt, and the absence of graphene oxide forms the Al, Co / N-CNT (Figure 2c). Whereas, in the absence of melamine the formed Al, Co / rGO has an aggregated nanoparticles without any CNT formation as revealed in Figure 2d. Figure 3 represents the E-SEM-EDS profile for Al, Co / N-rGCNT, depicting the 18:1 atomic ratio of Co:Al. The HRTEM analysis confirms that the formation of CNT dependent on the presence of both cobalt and melamine during the annealing process (Figure 4a-f). The FESEM depicted in Figure 5a, b indicated that the as- synthesized Al, Co / N-rGCNT shows intercalated reduced graphene oxide sheets with bamboo-like CNT morphology. In Figure 5a, b the blue part indicates CNT and red arrow indicates graphene sheets. The careful observation reveals that the growth of nanotube is assisted in the presence of cobalt and melamine. It implies that cobalt not only acts as a metal precursor but also plays a sacrificial role in providing the nucleation site for the growth of bamboo-like carbon nanotubes. The respective elemental mapping of Co, Al, O, N, and C shows the uniform distribution.

[0118] As seen in Figure 6a, HR-TEM image of Al, Co / N-rGCNT show the cobalt nanoparticle encapsulated 20-60 nm thick carbon nanotube (Co / N-CNT) as well as Co / N-CNT separated thin graphene sheet anchored few cobalt nanoparticles. Figure 6b, c shows that the CNT formation starts from the Co nanoparticles, and in Figure 6b inset reveals selected area electron diffraction (SAED) pattern signifies the crystallinity of the material. Figure 6d-f shows d-spacing values corresponding to graphitic carbon from CNT and metallic cobalt nanoparticle at the CNT tip. The d-spacing of 0.318 nm corresponds to the (002) plane of graphitic carbon and 0.174 nm and 0.201 nm correspond to (200) and (111) planes of metallic cobalt, respectively. Figure 6g shows the Al, Co / N-rGCNT image, and the corresponding elemental mapping of Al, N, O, and C shows the randomly distribution, while Co is distributed at the tip of CNT as well as over GO sheet. Figure 7 elemental mapping overlay image of Al, Co / N-rGCNT shows Al, N, O, and C randomly distribution and most of the Co is present on tip of CNT, which confirms that Co acts as nucleation sites for CNT growth.

[0119] To understand the ratio of Al and Co in Al, Co / N-rGCNT inductive coupled plasma (ICP) analysis is carried out. For the analysis, initially material is calcined in air for carbon burning then the remaining sample is digested in freshly prepared aqua Regia with heating at 50 °C. The 0.42 Wt % and 29.6 Wt % of Al and Co are respectively present in Al, Co / N- rGCNT, it well matches with TGA Wt %.

[0120] After morphological confirmation, the phase and crystallinity analysis is accessed using XRD analysis. XRD profile presented in Figure 8 shows a peak centered at 29 equal to 26.2°, corresponding to the (002) plane of graphitic carbon in all samples, whereas 29 equal to 44.0 o, 51.2°, and 75.3° peaks corresponding to (111), (200), and (220) planes of metallic cobalt (JCPDS No. 01-089-4307), respectively. There are no Al-related crystalline diffraction peaks in the XRD pattern of Al / N-rGO. Also observed in FESEM and TEM analysis clean graphene sheet surface of Al / N-rGO (Figure 2b and 4b), confirming that Al could present in doped form in graphene sheet. The peaks below 30° in Al, Co / N-CNT could be due to the unreacted melamine, because it is which is acting as a nitrogen doping agent in the presence of graphene. The temperature dependent CNT formation study shows at 600 °C all peaks corresponding to Co / melamine, with increasing temperature 600-700 °C. Whereas with increasing the annealing temperature at 700-900 °C, graphitic as well as metallic cobalt peak intensity increases (Figure 9).

[0121] To know the defect and in situ dual-doping of N and Al into the rGO during the annealing process, Raman analysis is performed. Figure 8b reveals the comparative Raman spectra of Al, Co / N-rGCNT, Al, Co / N-CNT, Al, Co / rGO, Al / N-rGO and N-rGO show two prominent peaks corresponding to the D and G band at 1341.0, 1351.2, 1351.2, 1371.6, and 1351.2 cm1and 1596.1, 1596.1, 1616.5, 1616.5, and 1606.3 cm1respectively in the mentioned samples. The ID / IG ratio of Al, Co / N-rGCNT, Al, Co / N-CNT, Al, Co / rGO, Al / N-rGO, and N-rGO are 1.20, 1.19, 1.20, 1.18, and 1.19, respectively. Prominent peaks below 750 cm1correspond to the metal-metal or metal-ligand region. The Raman peaks below 750 cm1are attributed to the Co-Co stretching mode. A decrease in intensity of the 2D band is generally observed with doping. For the fully Raman resonant 2D peak, the intensity dependence on doping is caused by electron-electron scattering, which decrease as charge concentration decreases. A broad 2D peak at 2660.9 cm1in N-rGO (green color) disappears after Al-doping in Al / N-rGO (purple color) in Figure 8b which confirms the doping of aluminium. Intense 2D peak at 2676.0 cm-1is observe in Al, Co / N-CNT due to CNT formation, again intensity of this peak is decreases in Al, Co / N-rGCNT in presence of Al, N-dual doped reduced graphene, which is indicating the effect of doping. For understanding the metal loading in the materials, thermogravimetry analysis (TGA) is performed. The TGA profile of Al, Co / N-rGCNT, Al, Co / N-CNT, Al / N-rGO, and N-rGO showing approximately 30, 40, 17, and 0 % active material loading over graphene sheet and encapsulated CNTs, respectively. The Co nanoparticles located at the graphene sheet or the edge are easier to remove during acid treatment and the Co nanoparticles located in the center section of N-CNTs are difficult to remove, thus Wt.% of active material is high in the case of Al, Co / N-CNT then Al, Co / N-rGCNT.

[0122] Information about surface area and porosity is calculated from the BET analysis. N2 adsorption-desorption isotherm for Al, Co / N-rGCNT, Al, Co / rGO, Al, Co / N-CNT, and Co / N-rGCNT are almost identical, with type IV adsorption isotherms. At low N2 partial pressure, the Al, Co / N-rGCNT, Al, Co / rGO, and Co / N-rGCNT shows a steep increase in the micropore region, a well-defined hysteresis loop at 0.45 to 0.85 relative pressure, and increased at pressures above 0.95, suggested micropores and mesopores are coexist. All material shows micro as well as meso-porous structure. A difference in such porosity could facilitate the mass-transfer activity in ZAB. The higher BET surface area of Al, Co / N- rGCNT (215.2 m2 / g) compared to Co / N-CNT (123.8 m2 / g) and lower than that of Al, Co / rGO (498.7 m2 / g) and Co / N-rGCNT (303.6 m2 / g) most likely to the fact that minor pores probably collapsed during process of CNT formation and Al doping at high temperature.

[0123] The elemental composition of the synthesized materials was analysed through XPS analysis. The survey scan spectrum shows that Co, Al, O, N, and C are present in Al, Co / N- CNT, and Al, Co / N-rGCNT. In Figure lOd, Nls deconvoluted spectra in N-rGO shows the three different types of peaks corresponding to pyridinic-N (398.4 eV), graphitic-N (400.8 eV), and oxidised-N (403.3 eV) respectively. Whereas in Figure lOe shows one extra peak for Al-N (399.3) in Al / N-rGO, which confirms the formation of Al, N dualdoped reduced graphene oxide (Al, N-rGO). Figure 10c and f shows respective deconvoluted N Is spectra in Al, Co / N-rGCNT, and Co / N-rGCNT and (Table 1).

[0124] Table 1

[0125] As compared to Al / N-rGO and Co / N-rGCNT, the N Is spectra correspond to Al-N / Co-N in Al, Co / N-rGCNT shifted towards positive binding energy (399.8 eV) due to bimetallic electronic interaction with nitrogen (Co-N-Al). In addition, the pyridinic-N and graphitic - N peak intensity is increased in Al, Co / N-rGCNT, which enhances the surface wettability and diffusion limiting properties of the material. The deconvoluted Al 2p spectra in Al, Co / N-rGCNT shows the Al-N peak at 74.4 eV, it is confirmed that Al is present in the form of doping, and the surface oxide Al-0 peak is observed at 75.4 eV due to aluminium being prone to oxide formation. From the thermodynamic standpoint, Al-0 interactions are more favorable than Al-N interaction. As compared to Al / N-rGO, the Al 2p spectra in Al, Co / N- rGCNT, Al, Co / N-CNT, and Al, Co / rGO show a positive shift in the binding energy (Figure 10a). The deconvoluted Co 2p spectrum and peaks present at 780.7 eV and 795.6 eV corresponds to Co 2p3 / 2 and Co 2pi / 2 respectively. The deconvoluted Co 2p3 / 2 shows four different peaks corresponding to metallic Co, Co-N, Co-O, and satellite peaks at 778.5 eV, 780.5 eV, 783.3 eV, and 785.6 eV respectively. Similarly, Co 2pi / 2 also shows the four peaks at 793.6 eV (metallic Co), 796.0 eV (Co-N), 799.3 (Co-O), and 803.0 eV (satellite), respectively. In addition to our experimental study, we sought to provide computational evidence supporting our claim of Co and Al, N interaction through the use of dual-doped aluminium nitrogen. Our objective was to investigate the interplay between surface texture and electronic factors that promote ORR performance. The improved electron transport during the reaction is the electronic interaction between the active site metal nanoparticle and the conducting support. Our research group has recently reported on the favorable electronic interaction between doped-N and Co and Al and doped-N due to their synergetic effect. This result was obtained through DFT calculations on the interactions between doped-N, Al, and doped-N, Co. To optimize the graphene sheet, a single layer of graphene with 50 atoms was used and pyridinic N doping was created. Then, Al was co-doped in the ortho, meta, and para positions of the doped N on the sheet. Based on the formation energies, it was concluded that Al dual-doped at the ortho position of N is energetically favourable compared to other configurations, and therefore, this configuration was selected for further calculations. To determine the most favorable adsorption site for cobalt, it was placed in 10 different positions. The most energetically favourable position was found to be above the carbon adjacent to N and Al. It was observed that the placement of cobalt at position 4 gave the same final configuration as positions 1 and 3. Based on the lowest adsorption energy, the result from position 4 was selected for further adsorption of O2. Various sites on the surface were tested for O2 activation by placing an oxygen molecule, resulting in three final configurations. Among the 13 initial configurations, nine led to oxygen adsorbing above Co and Al, with an O2 bond activation of 1.46 A. While configuration (II) exhibited the best O2 activation with one oxygen coordinating to Co and the other to carbon, it was not the energetically most favourable. The site with the least activation for O2 was observed when oxygen adsorbed above Co alone. Considering the adsorption energy and probability of formation, the site where oxygen coordinates to both Al and Co is the most favourable site for further reaction. Therefore, the optimal adsorption site for O2 is above Co and Al with an 0-0 activation of 1.46 A. Compares the charge distribution of the O2-AI, Co / N-rG configuration with the Al, Co / N-rG configuration. In the former, Al loses more charge compared to Co on the surface. However, N on the graphene surface experiences a gain in charge. With the presence of O2, further loss of charge is observed, with approximately 0.14 e“ charge lost from Al and approximately 0.25 e“ from Co.

[0126] Half-cell study: After the physical characterizations, the bifunctional (ORR / OER) activity of synthesized materials is performed in alkaline conditions. The half-cell study is performed by using a catalyst-coated glassy carbon electrode (GCE) as a working electrode, Hg / HgO and graphite rod are used as the reference and counter electrodes, respectively. The ORR performance of state-of-the-art Pt / C, N-rGO, Al / N-rGO, Co / N-rGCNT, Al, Co / rGO, Al, Co / N-CNT, and Al, Co / N-rGCNT are performed in O2 saturated 0.1 M KOH electrolyte. The comparative ORR linear sweep voltammograms (LSVs) of the various samples. The N-rGO exhibits poor ORR activity with an onset potential of 845 mV and a half-wave potential (E1 / 2) of 686 mV. After simultaneous doping of aluminium and nitrogen in rGO, the kinetics of the material is improved by 120 mV, yielding an onset potential of 965 mV and E1 / 2 of 799 mV for Al / N-rGO. It is found that with the addition of cobalt with melamine and GO, the onset potential and Ei / 2of Co / N-rGCNT are 953 mV and 809 mV, respectively.

[0127] Al, Co / rGO shows an onset potential of 895 mV and E1 / 2 of 785 mV. The Al, Co / N-CNT sample has an onset potential of 920 mV and E1 / 2 of 790 mV. Al, Co / N-rGCNT shows promising electrochemical activity towards the ORR with an onset potential of 975 mV and E1 / 2 of 825 mV. Which indicates the synergetic interaction between Al, N-dual-doped reduced graphene and Co encapsulated CNT and further promotes the ORR. Pt / C shows the onset potential of 985 mV with 841 mV as the E1 / 2 value. The Tafel slope value verifies the better reaction kinetics of Al, Co / N-rGCNT (80 mV / decade) in comparison to N-rGO (111 mV / decade), Al / N-rGO (132 mV / decade), Co / N-rGCNT (86 mV / decade), Al, Co / rGO (105 mV / decade), and Al, Co / N-CNT (61 mV / decade). Pt / C shows the Tafel slope of 62 mV / decade, which are in accordance with the literature reports. To understand charge transfer resistance (RCT) of the material, electrochemical impedance spectroscopy (EIS) is performed at 0.80 V vs. RHE and the resulting Nyquist plot is examined. Among the synthesized materials, the lowest RCT value observed for Al, Co / N-rGCNT (49.2 Q), as compared to N-rGO (52.9 Q) and for Al / N-rGO, Co / N-rGCNT, Al, Co / rGO, and Al, Co / N- CNT are 50.0 Q, 47.4 Q, 50.7 Q, and 49.5 Q respectively. This indicates fast charge transfer occurs in Al, Co / N-rGCNT during ORR. The lowest RCT value eventually improves ORR kinetics and subsequently catalytic activity. RCT value exhibited by Pt / C (17.4 Q) is much lower than other samples.

[0128] The SCN’ ions interaction is carried out in order to accurately determine the influence of Co-species. SCN’ ions, which can target the metal centers and inhibit catalytic activity, are very reactive with transition metal species. In Al, Co / N-rGCNT shows no significant ORR activity losses. This points to a minimal impact of SCN’ ions attack on the aluminium nitrogen dual-doped Co-encapsulated reduced graphene oxide. Due to strong co-ordination of Co and Al with N enhance the stability of catalyst. It is also investigated how Al, N dual- doping affected the catalytic activity. The rotating ring-disc electrode (RRDE) experiment has been performed to understand the mechanism of ORR kinetics, anticipated 4-electron (4e ) / 2-electron (2e ) reduction process has been calculated. The electron transfer number (n) and the H2O2 amount formed during ORR. Based on the applied potential, H2O2 percentile composition is measured. At a potential of 0.6 V, the Al, Co / N-rGCNT and Pt / C are produced 11% and 1% of the H2O2 amount, respectively. Furthermore, the electron transfer number (n) has been deduced, and RRDE data indicated that the ‘n’ values of Al, Co / N-rGCNT, and Pt / C in the low potential region are respectively 3.8 and 4.0. Al which catalyses the reaction of peroxide disproportionation in Al, Co / N-rGCNT, is believed to be the cause of the observed trend in activity and lower H2O2%. The bifunctional activity of the prepared samples is further analyzed. The comparative OER linear sweep voltammograms (LSVs) of the various samples in N2 saturated 1 M KOH electrolyte at a scan rate of 10 mV s’1. The N-rGO displays negligible OER activity and it requires an overpotential of 470 mV to reach the current density of 10 mA cm’2. The RUO2 / VC requires an overpotential of 300 mV to maintain the same current density of 10 mA cm’2. Al, Co / N- rGCNT shows promising electrochemical activity towards the OER with an overpotential of 390 mV at a current density of 10 mA / cm2. Tafel slope verifies better OER reaction kinetics of Al, Co / N-rGCNT (102 mV / decade) compared to other samples. For N-rGO (90 mV / decade), Al / N-rGO (112 mV / decade), Co / N-rGCNT (104 mV / decade), Al, Co / rGO (116 mV / decade), and Al, Co / N-CNT (89 mV / decade). With the aid of Nyquist plots, the RCT of the material is analyzed using electrochemical impedance spectroscopy performed at 1.56 V. Co / N-rGCNT, Al, Co / rGO, and N-rGO shows the RCT values of 74.3 Q, 254.2 □, and 49.5 Q, respectively. The low RCT values observed for Al, Co / N-rGCNT (48.8 Q) indicates the fast charge transfer during the gas evolution reaction. The activity enhancement could be due to the electron deficient Al accepts the electrons from N and make it more electronegative, electronegative N would attract electrons from adjacent C atom and make it more positive center, thus facilitating the adsorption of OH* intermediate and enhances the reaction kinetics. Al and N doping helps for better OER reaction kinetics, which is confirmed by the low RCT values of 39.6 Q, and 42.7 Q for N-rGO and Al / N-rGO, respectively.

[0129] For rechargeable Zin-air battery, the ORR and OER bi-functional activity of the catalyst is crucial to obtain the high performance with good reversibility during charge-discharge process. Therefore, the difference between potential of OER and ORR (AE= Ej ioOER- E1 / 2ORR) is an important parameter to assess the catalyst bifunctionality. A small AE value is characteristic of an ideal bifunctional oxygen electrocatalyst. Al, Co / N-rGCNT shows AE of 0.82 V, which is lower than other catalysts prepared in this study (Figure 12a). The obtained AE, V, specific capacity and energy density values for Al, Co / N-rGCNT is comparable to reported / conventional bi-functional catalysts (Tables 2 and 3).

[0130] Table 2: Comparison of ORR and OER activities of reported systems vs the in-house (conventional) systems (Here, If onset potential and half wave potential values are higher or better, and EOER and AE values are lower, then the ORR and OER activities of such system is efficient / improved / superior, which is case of presently disclosed Al, Co / N- rGCNT catalyst based system).

[0131] Table 3. Performance comparison of ZAB devices with Al, Co / N-rGCNT and the commercial Pt / C catalyst

[0132] The electrochemical active surface area (ECSA) is evaluated using the double layer capacitance (Cdi) due to the direct relationship (ECSA = Cdl / Cs). For this, CV is taken in the potential window of 1.02 V to 1.12 V at different scan rates of 10, 20, 40, 60, 80, and 100 mV s’1. The scan-rate dependent CV plots for N-rGO, Co / N-rGCNT, Al / N-rGO, Al, Co / N-CNT, Al, Co / rGO, Al, Co / N-rGCNT, and Pt / C are shown in Figures 16a-g. The current at 1.07 V is plotted against the scan rates and the obtained slope is corresponding to the Cdi value. The measured Cdi value are 7.7, 5.3, 2.0, 3.2, 0.52 and 6.4 mF cm’2for N- rGO, Co / N-rGCNT, Al / N-rGO, Al, Co / N-CNT, Al, Co / rGO, and Al, Co / N-rGCNT, respectively. The higher Cdi value for Al, Co / N-rGCNT suggested better active site exploration. As discussed in introduction, in situ formed CNTs separate the rGO sheets during the formation of Al, Co / N-rGCNT and prevent re- stacking and enhance the accessible center to make easy path for ions to reach the interior layers that can boost catalytic activity and double layer capacitance by maximum utilization of the active centers.

[0133] The durability of the synthesized material is assessed by accelerated durability test (ADT) analysis in O2 saturated 0.1 M KOH solution. The CV analysis is performed in a Potential window of 0.6-1.0 V (vs. RHE), at a scan rate of 100 mV s-1for 5000 cycles with O2 saturation. As the operation is performed under activated conditions, ADT should provide information about material stability. A comparative LSV has been verified before and after ADT to investigate the performance change during the catalytic process. Figure 12b show the comparative LSVs analysis of Al, Co / N-rGCNT and Pt / C, before and after 5000 cycles. After 5000 CV cycles, the half-wave potential (E1 / 2) is decreased by 30 and 19 mV for Pt / C and Al, Co / N-rGCNT, respectively. The low shift of E1 / 2 in Al, Co / N-rGCNT in comparison to Pt / C implies better durability of Al, Co / N-rGCNT electrocatalyst than Pt / C. Observed better durability of Al, Co / N-rGCNT attributes to incorporating Al, N-doped graphitic carbon support, and its strong interaction with the catalytic active sites.

[0134] For analyzing the surface-texture of cycled catalyst, the inventors have done TEM analysis. Figure 13a and b, show TEM images of cycled catalyst reveals similar morphology of Al, Co / N-rGCNT, and Figure 13c-i represents HAADF-STEM EDX elemental mapping of cycled Al, Co / N-rGCNT for respective elemental mapping of Al, Co, N, O, and C which shows the uniform after 5000 cycles. The catalyst's comparable activity and long-time durability are further reflected in the performance of the zinc-air battery demonstration in a liquid state. The primary Zn-air battery is tested in 6M KOH containing 0.2 M Zn acetate solution at room temperature. As shown in Figure 16, gas diffusion layer (GDL) coated Al, Co / N-rGCNT electrode is employed as cathode and polished Zn foil as an anode in a two-electrode system with an active area of 1 cm2. Similarly, to know the comparative performance of Pt / C based system, GDL coated Pt / C is also employed as cathode under similar conditions. Figure 14a shows the polarization plot for the integrated Al, Co / N- rGCNT-cathode containing primary ZAB battery exhibited an open circuit potential of -1.35 V and peak power density of 106.3 mW cm-2, which is close to the system based on Pt / C (-1.50 V, 120.8 mW cm-2). Afterward, the Galvanostatic discharge of Al, Co / N- rGCNT and Pt / C based systems are recorded at 20 mA cm-2(Figure 14b). The discharge time for the ZABs based on Al, Co / N-rGCNT, and Pt / C is found to be nearly 18.7 h and 17.6 h, respectively. As a result of side reactions that are taking place over the Zn foil surface, the voltage drop concerning the operation time is expected to occur. The calculated specific capacity of the liquid state ZAB based on Al, Co / N-rGCNT is about 737 mAh / gzn at a discharge current density of a 20 mA cm-2, compared to 696 mAh / gzn for the system based on Pt / C and is shown in Figure 14c and Table 2. The Al, Co / N-rGCNT -based device showed a flat discharge profile at the discharge rate of 20 mA / cm2without much voltage drop, attributes the consistent battery performance. Furthermore, the Galvanostatic discharge curve of Al, Co / N-rGCNT and Pt / C recorded at 2 mA cm-2to 25 mA cm-2discharge current density (Figure 14d) shows that at a higher current density Al, Co / N- rGCNT shows higher stability and low voltage drop compared to Pt / C. Further, the Galvanostatic charge and discharge curve of the designed catalyst has been performed and is illustrated in Figure 14e. The Al, Co / N-rGCNT bi-functional catalyst-based battery operated for charge discharge cycles at a current density of 5 mA cm-2for 41h, each cycle for 5 minutes. The performance is better than that of the system based on Pt / C, which runs for only 30h under similar conditions. Also, with increasing Al, Co / N-rGCNT catalyst loading (2 mg cm'2), battery operated for charge discharge cycles at a current density of 5 mA cm'2for 87 h, each cycle for 5 min shown in Figure 15. It is also checked that the light- up of LED using the fabricated homemade flexible ZAB. A series connection of two ZABs had an OCP of 2.91 V, which each run LED light for more than 5 hours without much fading brightness of LED light. The enhanced performance of homemade device indicate role of the prepared Al, Co / N-rGCNT catalyst. It’s layer- separated arrangement, exposure active sites and Al, N-dual-doping enhance the stability of material shows long lasting performance. This result indicates the suitability of using Al, Co / N-rGCNT -based electrodes for secondary Zn-air battery applications. As a result of using Al, Co / N-rGCNT as the cathode catalyst and taking into account its high ORR activity, we were able to fabricate an anion-exchange membrane fuel cell (AEMFC) that's efficiency was assessed against the performance of a system based on Pt / C. The Al, Co / N-rGCNT and Pt / C have been used as the cathode and anode catalysts, respectively, to test the performance in a single cell that has been constructed. The commercially available FuMA-TechFAA membrane was used as anion exchange electrolyte membrane. Figure 16b shows two 2*2 cm2membrane electrode assemblies (MEAs) based on Pt / C as an anode and Al, Co / N- rGCNT as the cathode catalysts to produce single cells with comparative I-V polarisation plot. The Al, Co / N-rGCNT catalyst was used to fabricate the MEA, which had an open circuit voltage (OCV) of 0.94 V and a maximum power density of 77.6 mW cm2atl80 mA cm2current density. Although, the MEA made with Pt / C had an OCV of 0.97 V with a maximum power density of 102.6 mW cm2achieved at 208 mA cm2current density, (Figure 16b).

[0135] ADVANTAGES OF THE INVENTION

[0136] • Present invention provides improved, efficient, cost-effective, environmentally friendly and non-toxic composite bifunctional electrocatalyst.

[0137] • It also provides composite bifunctional electrocatalyst that is robust and simple to scale up.

[0138] • The present invention provides composite bifunctional electrocatalysts which provides a greater surface area, higher micro as well as meso porosity (of around 1- 10 nm) and improved electrocatalytically activity.

[0139] • The present invention provides composite bifunctional electrocatalyst for use in electrolysis processes, particularly in oxygen reduction reaction (ORR) & oxygen evolution reaction (OER).

[0140] • Present invention provides composite bifunctional electrocatalyst which is highly stable approx, upto 5000 cycles with minimal change in electrocatalysis.

Claims

We Claim1. A composite bifunctional electrocatalyst comprising: i. a p-block element M 1 ; ii. a transition metal M2; iii. nitrogen; iv. a reduced graphene oxide (rGO), and v. a carbon nanotubes; wherein the reduced graphene oxide (rGO) is a dual doped reduced graphene oxide with said nitrogen and the p-block element Ml, and; wherein the transition metal M2 is encapsulated inside the carbon nanotubes.

2. The composite bifunctional electrocatalyst as claimed in claim 1, wherein the p- block element Ml is in the range of 0.25-4.5 wt. %; the transition metal M2 is in the range of 9.5-30 wt. %; the nitrogen is in the range of 10-15 wt. %; the reduced graphene oxide and the carbon nanotubes together are in the range of 60-85 wt.%; wherein the total wt% does not exceed 100%.

3. The composite bifunctional electrocatalyst as claimed in claim 1, wherein the p- block element Ml is selected from the group consisting of aluminum, indium, boron, and sulphur; and the transition metal M2 is selected from the group consisting of cobalt, iron and nickel.

4. The composite bifunctional electrocatalyst as claimed in claim 1, wherein the transition metal M2 is distributed or located at the tip of carbon nanotubes as well as over the reduced graphene oxide sheets; and wherein said carbon nanotubes are separated by the reduced graphene sheets.

5. The composite bifunctional electrocatalyst as claimed in claim 1, wherein the composite bifunctional electrocatalysts comprises mesoporous as well as microporous metal particles with a BET surface area is in the range of 210 to 220 m2 / g.

6. A process for the preparation of the composite bifunctional electrocatalyst as claimed in claim 1, wherein said process comprising the steps of: a) dispersing, stirring and sonicating graphene oxide (GO) in water for a period in the range of 15-20 mins at a temperature in the range of 25-30°C to obtain a graphene oxide solution;b) reacting the graphene oxide (GO) solution as obtained in step (a) with melamine acting as reducing and nitrogen doping agent to obtain a mixture; and c) annealing the mixture as obtained in step (b) in presence of a salt of Ml and a salt of M2 and gradually increasing the temperature from 600 to 900°C with a ramping rate of 10°C / min for a period in the range of 2 to 4h under inert conditions to obtain the bifunctional electrocatalyst.

7. The process as claimed in claim 6, wherein the salt of Ml is selected from Ml nitrate, Ml sulfate, Ml hydroquinone, Ml isopropoxide, and sodium-based salt of Ml, and the salt of M2 is selected from M2 acetate, M2 chloride, M2 nitrate, M2 acetylacetonate and M2 sulfate.

8. A half-cell comprising:A. a working electrode coated with bifunctional electrocatalyst as claimed in claim 1;B. a reference electrode;C. a counter electrode; andD. an ion-conducting solution or an electrolyte which is in contact with said working electrode; wherein the ion-conducting solution or the electrolyte comprises at least 1% of an ionic liquid.

9. A full cell comprising: i. an anode; ii. a bifunctional electrocatalyst as claimed in claim 1 as cathode; and iii. an electrolyte which is in contact with the bifunctional electrocatalyst of the cathode, and optionally with the anode; wherein the electrolyte comprises at least 1% of an ionic liquid.

Images