DESC:FIELD OF THE INVENTION
The present invention pertains to a bifunctional electrode. More specifically, the present invention pertains to a bifunctional electrode comprising a nanoelectrocatalyst for water splitting reaction and method of preparation thereof. The bifunction electrode is utilized for water splitting reaction in an electrolyzer.

BACKGROUND OF THE INVENTION
Alkaline water electrolysis is being used to generate clean energy i.e., H2 by using Pt or Ir based electrocatalyst which are costly materials. The other electrocatalysts used for water oxidaton in alkaline electrolyzer are Ni-based catalysts. However, their catalytic performance remains far from satisfactory for the cost competitiveness of hydrogen production. To replace the costly material with earth abundant transition material and development of efficient electrocatalyst is the need of hour.

Further, alkaline water electrocatalyst uses 30 weight % KOH solution, which is highly corrosive, can corrode the electrocatalyst material and leaching of the active material may happen which will affect the stability of the electrocatalyst.

In literature, the evaluation of H2 generation in water splitting reactions are done with a three or two electrode system with electrolyte solution in beaker, wherein the results obtained for the evaluation of H2 production for the electrodes are unreliable, not accurate. Hence there is a requirement of an electrolyzer for water splitting reaction to obtain evaluation results that are reliable and scalable.

Therefore, there is requirement of an electrocatalyst for water splitting reaction which is inexpensive and can be used as an efficient alternative to Pt or Ir based electrocatalyst.

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

The present invention provides a bifunctional electrode comprising a nanoelectrocatalyst for water splitting reaction; wherein the nanoelectrocatalyst is a graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles containing 1 to 10 weight % of graphene; and wherein the nanoelectrocatalyst is deposited over a current collector substrate.

In an embodiment the present invention provides a process for the preparation of a bifunctional electrode as defined above, comprising the steps of:
i. preparing a cobalt ferrite [CoFe2O4-d] nanoparticle;
ii. dispersing the cobalt ferrite [CoFe2O4-d] nanoparticle with 1 to 10 weight % of reduced graphene oxide in a solvent to obtain a dispersion containing graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles; and
iii. depositing the dispersion containing graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles over a current collector substrate.

OBJECTIVES OF THE INVENTION
The main objective of the present invention is to provide a cost effective bifunctional electrode for water splitting reaction.

Another objective of the present invention is to provide a process for the preparation of the bifunctional electrode for water splitting reaction.

Another objective of the present invention is to provide a nanoelectrocatalyst for water splitting reaction, wherein the nanoelectrocatalyst is graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles.

Another objective of the present invention is to provide a process for the preparation of the nanoelectrocatalyst for water splitting reaction.

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

Figure 1 depicts X-ray diffraction (XRD) image of as prepared [CoFe2O4-d] nanoparticles.
Figure 2 depicts scanning electron microscopy (SEM) images of [CoFe2O4-d] nanoparticles.
Figure 3 depicts energy dispersive X-ray spectroscopy (EDS) of [CoFe2O4-d] nanoparticles.
Figure 4 depicts X-ray photoelectron spectroscopy (XPS) spectra of (a) O, (b) Co and (c) Fe in [CoFe2O4-d] nanoparticles samples.
Figure 5 depicts Raman spectra of [CoFe2O4-d] nanoparticles.

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

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

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

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

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference.

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

Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described herein. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such features of the invention, and steps of the process that are referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such features or steps.

The present invention provides a bifunctional electrode comprising a nanoelectrocatalyst for water splitting reaction.

In an embodiment of the present invention, the nanoelectrocatalyst comprises a graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles. The nanoelectrocatalyst is deposited over a current collector substrate.

In an embodiment of the present invention, the graphene is in an amount of 1 to 10 % of the total weight of the nanoelectrocatalyst.

In an embodiment of the present invention, cobalt ferrite [CoFe2O4-d] nanoparticles have a particle size in a range of 50 to 250 nm.

In an embodiment of the present invention, the current collector substrate is selected from a group consisting of Ni sheet, Cu sheet, Ti sheet, stainless steel sheet, Ni foam, Cu foam, Ti foam, and stainless steel foam; wherein the current collector substrate is conductive.

In an embodiment the present invention provides a process for the preparation of a bifunctional electrode as defined above comprising the steps of:
i. preparing a cobalt ferrite [CoFe2O4-d] nanoparticle;
ii. dispersing the cobalt ferrite [CoFe2O4-d] nanoparticle with 1 to 10 weight % of reduced graphene oxide in a solvent to obtain a dispersion containing graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles; and
iii. depositing the dispersion containing graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles over a current collector substrate.

In another embodiment of the present invention, the cobalt ferrite [CoFe2O4-d] nanoparticles are prepared by a process selected from sol-gel, precipitation, hydrothermal, and co-precipitation.

In an embodiment of the present invention, the cobalt ferrite nanoparticle are prepared by a process comprising the steps of:
i. preparing a solution of a size controlling agent and adding an Fe precursor and a Co precursor to form a reaction mixture;
ii. stirring the reaction mixture to obtain a solution;
iii. adding hot NH4OH solution having a temperature of 55 to 65 ? to the solution of step ii) under continuous stirring to obtain a product containing cobalt ferrite [CoFe2O4-d] nanoparticles;
iv. cooling the product and separating the CoFe2-dO4 nanoparticles from the product; and
v. drying the [CoFe2O4-d] nanoparticles and calcining the [CoFe2O4-d] nanoparticles.

In another embodiment of the present invention, the precursor of Fe used for the preparation of the cobalt ferrite nanoparticle is selected from a group consisting of Fe(NO3)3, FeSO4, FeCl3, Fe(NO3)2, and FeCl2, Fe2(SO4)3.

In another embodiment of the present invention, the precursor of Co used for the preparation of the cobalt ferrite nanoparticle is selected from a group consisting of cobalt chloride, cobalt nitrate, and cobalt sulphate; wherein Co is present in various oxidation states.

In an embodiment of the present invention, the concentration of Fe precursor is twice the concentration of Co precursor.

In an embodiment of the present invention, the reaction mixture is stirred at a temperature in a range of 25 to 30 ?.

In an embodiment of the present invention, the solution is stirred at 55 to 65 ? for 1 to 5 hours.

In an embodiment of the present invention, the [CoFe2O4-d] nanoparticles are separated through centrifugation.

In an embodiment of the present invention, the [CoFe2O4-d] nanoparticles are calcined at a temperature of 350 to 650 ? for 3 to 8 hours at a heating rate of 2 to 8 ?/min.

In an embodiment of the present invention, the nanoelectrocatalyst is cost effective and can be used as efficient alternative to Pt or Ir based electrocatalyst.

In another embodiment of the present invention, the size of the cobalt ferrite nanoparticle is controlled by a size controlling agent selected from a group consisting of sodium dodecyl sulfate, polyethylene glycol, polyvinyl pyrrolidone, and sodium deoxycholate surfactant. The size of the cobalt ferrite nanoparticle is further controlled by and encapsulating with graphene.

In another embodiment of the present invention, the solvent for dispersing the graphene encapsulated cobalt ferrite nanoparticle is selected from a group consisting of water ethanol, acetone, and isopropanol. In an embodiment of the present invention the dispersion is prepared by bath sonication for 30 minutes at room temperature.

In an embodiment of the present invention, the graphene encapsulated cobalt ferrite nanoparticles are deposited on Ni-foam by dip coating technique.

In another embodiment of the present invention, the deposition processes include different types of elements based on sulphides, phosphites, selenides, nitrides etc.

In another embodiment of the present invention, the graphene encapsulated cobalt ferrite nanoparticle deposited over the current collector substrate are calcined at a temperature in a range of 300 to 700 ?.

In an embodiment of the present invention, the bifunctional electrode comprising a nanoelectrocatalyst is used for water splitting reaction in an anion exchange membrane (AEM) electrolyzer.

In an embodiment of the present invention, the anion exchange membrane (AEM) electrolyzer comprises:
i. an assembly of electrodes containing an anode and a cathode, wherein the anode and the cathode comprises a nanoelectrocatalyst;
ii. an electrolyte; and
iii. a membrane or a diaphragm.

In an embodiment of the present invention the anode and the cathode are connected to two stainless steel plates as positive and negative terminal to an applied voltage, wherein the stainless steel plates are fixed by nut and bolt.

In an embodiment of the present invention, the electrolyte is a KOH solution consisting of 5 to 9 weight % of KOH.

In an embodiment of the present invention, the electrolyte flows continuously by a peristatic pump, wherein the electrolyte inlet through cathode and the outlet is used to collect the H2 gas.

In an embodiment of the present invention, the operational parameter which is the applied voltage in which the anion exchange membrane electrolyzer works is in the range of 1.8 to 2.4 V at room temperature. In an embodiment of the present invention, the anion exchange membrane electrolyzer produces a hydrogen evolution volume of 80 to 110 liter per hour.

In an embodiment of the present invention, the membrane or the diaphragm used is an anion exchange membrane for easy transport of anions i.e., OH- ions through the anion exchange membrane.

In some embodiments of the present invention, the anion exchange membrane is soaked in water and KOH solution for 0.5 to 12 hours to increase the hydrophilicity facilitating the easy transport of the anions through the anion exchange membrane.

In an embodiment of the present invention, the defect density in nanoelectrocatalyst is controlled by two-way approach including firstly the selection of iron precursor to create a mismatch between lattice of Co and Fe and creating more defects, and secondly controlling the calcination temperature and rate of calcination; wherein controlled calcination controls the defect site without affecting the lattice distortion.

In an embodiment of the present invention, the cobalt ferrite nanoparticles embedded in graphene enhance the flow of active species with low resistivity for water splitting reaction.

In another embodiment of the present invention, the cobalt ferrite nanoparticle has enhanced charge transfer of the active species, that in turn reduces the charge transfer resistance and increases the activity.

In an embodiment of the present invention, the nanoelectrocatalyst is active at a concentration of KOH less than 10 weight % with activity comparable to the high pH electrolyte solution in water splitting reaction.

In an embodiment of the present invention, the results obtained from the electrolyzer are reliable and can be scaled up.

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

Materials- Iron (III) chloride hexahydrate (FeCl3.6H2O), Cobalt (II) chloride hexahydrate (CoCl2.6H2O), sodium deoxycholate (DOC), Ammonia solution, DI water, were used in the synthesis process.

Example 1:
Co-precipitation method was used for the preparation of cobalt ferrite [CoFe2-dO4] nanoparticles. 5ml of 1M sodium deoxycholate (DOC) solution was prepared and in that 1.58g FeCl3.6H2O and 0.98g CoCl2.6H2O was added to form a reaction mixture. The reaction mixture was stirred at room temperature to form a solution. Then the solution temperature was increased to 60 ? and to this metal chloride solution a hot NH4OH solution (excess amount) was added under continuous stirring until the colour changed to green. After some time when the slurry turned brown. The stirring continued for 3 hours at 60 ? to obtain a product. After that the product was cooled down to RT, the [CoFe2O4-d] nanoparticles were separated through centrifugation.

The prepared [CoFe2O4-d] nanoparticles were dried and then calcined in N2 atmosphere at 550 ? for 5 hours at a heating rate of 5 ?/min. Then product cooled down to RT.

Reduced graphene oxide is prepared by taking graphite as a precursor, followed by oxidation and mild reduction. The active electrocatalyst is prepared by adding 1-10 weight % of reduced graphene oxide to the cobalt ferrite [CoFe2O4-d] nanoparticle and dispersing the materials in water for electrochemical evaluation.

In an embodiment of the present invention, 1 to 10 weight % of reduced graphene oxide is added to 10 mg of cobalt ferrite [CoFe2O4-d] nanoparticle and dispersed in 5 ml of water.

Characterization: The structural parameters obtained from X-Ray diffraction (XRD) patterns, X-photoelectron spectroscopy (XPS), Raman Spectroscopy. The morphology of the grains was determined by Field emission scanning electron microscopy (FESEM) images and the compositions were examined by energy dispersive spectroscopy (EDS) in the FESEM.

The XRD pattern of as prepared [CoFe2O4-d] ferrite nanoparticles is shown in Figure 1. All the obtained peaks recorded can be termed to a single phase spinel structure. Few additional peaks corresponds to the monometallic oxide phases are observed in the experiment. Based on the peak central positions obtained the two highly intense peak at (311) and (400) towards the low angle (2O - 35.5and 44.9) and other high intensity peak at (440) towards high angle (2O - 62.6) defined as the ferrite structure. The other peaks obtained at 2O - 30.1, 43.1, 57 correspond to the (220), (511) crystallographic planes of CoFe2-dO4, respectively.

A typical FESEM image of the prepared [CoFe2O4-d] ferrite nanoparticles is shown in Figure 2. The elemental distribution examined by energy dispersive X-ray mapping coupled with FESEM. As seen from the images, grains are in spherical type of structure, uniformly distributed and they are homogeneous without any agglomeration. The quantitative EDS analysis shown in Figure 3 was used to determine the composition of [CoFe2O4-d] ferrite particles. The prepared [CoFe2O4-d] nanoparticles were calcined in N2 atmosphere with slow heating rate to uniform growth and no agglomeration. The inert atmosphere provides the removal of surfactants in a controlled manner. As the temperature increases the chances of agglomeration, the calcination is performed at 550 K to get the desired product.

The electronic structure of [CoFe2O4-d] was examined by XPS analysis as shown in Figure 4. The O 1s peak can be deconvoluted into 3 peaks corresponds to the metal oxygen bond, oxygen defects and surface adsorbed oxygen. It can be seen in figure 4a that the peak centered at 531.52 eV correspond to the presence of abundant amount of oxygen defect in the electrocatalyst. It can be observed from figures 4(b) and (c), that the high-resolution Co 2p and Fe 2p XPS spectra of developed electrocatalyst sample is associated with two spin–orbit doublets characteristics of Co 2p3/2 and Co 2p1/2, and Fe 2p3/2 and Fe 2p1/2, respectively along with two shakeup satellite peaks. The Co 2P3/2 peak and Fe 2P3/2 peak can be deconvoluted to find out the concentration of Co2+ and Fe3+ ions in the octahedral and tetrahedral site. By further deconvolution and analyzing the results, it is found that the concentration of Co2+ and Fe3+ ions in octahedral site is more compared to the tetrahedral site which confirms the presence of oxygen defects in the structure.

Further to verify the XPS analysis Raman spectral studies were also performed. Figure 5 shows the Raman spectra of graphene encapsulated [CoFe2O4-d] spinel ferrite nanoparticles. Figure 5a shows the Raman spectra of reduced graphene oxide with the Stokes phonon energy shift creates two main peaks in the Raman spectrum, i.e. G band at around 1580 cm-1, and a second-order overtone of a different in- plane vibration, D band at around 1350 cm-1. Figure 5b shows the Raman spectra of [CoFe2O4-d] spinel ferrite nanoparticles. Group theory analysis predicts the following optical phonon distribution: 5T1u + A1g + Eg + 3T2g, in which, the 5T1u modes are IR active, whereas the other five (A1g + Eg + 3T2g) modes are Raman active composed to the motion of O ions and both A-site and B-site ions in the spinel structure. Furthermore, the A1g mode is associated to symmetric stretching of the oxygen anion, the Eg mode is associated to symmetric bending of the oxygen anion, and the T2g mode is due to asymmetric stretching of the oxygen anion with respect to the tetrahedral and octahedral cations. It can be seen from figure 5 that as synthesized [CoFe2O4-d] spinel ferrite nanoparticles shows Raman modes at ~209, ~272, ~380, ~480 and 577 cm-1. Raman modes at around 480 cm-1 shows a shoulder like feature at the lower wavenumber side (~474 cm-1). These bands were demonstrating the asymmetric stretching of the oxygen anion of the Fe–O and M–O bonds. The lower frequency Raman modes (~198, ~297, ~461, and ~558 cm-1) were assigned to the T2g and Eg Raman modes, corresponds to the vibration of the spinel structure as reported. Here, the shift of octahedral site especially the Eg mode at 272 cm-1 provides clarity to the lattice distortion arising from oxygen defects. The evolution of defect O is mainly attributed to the presence of unsaturated low valence metal centers i.e. Co2+ which is mostly present in the octahedral sites of [CoFe2O4-d].
,CLAIMS:1. A bifunctional electrode comprising a nanoelectrocatalyst for water splitting reaction; wherein the nanoelectrocatalyst is a graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles containing 1 to 10 weight % of graphene; and wherein the nanoelectrocatalyst is deposited over a current collector substrate.

2. The bifunctional electrode as claimed in claim 1, wherein the cobalt ferrite [CoFe2O4-d] nanoparticles have a particle size in a range of 50 to 250 nm.

3. The bifunctional electrode as claimed in claim 1, wherein the current collector substrate is selected from a group consisting of Ni sheet, Cu sheet, Ti sheet, stainless steel sheet, Ni foam, Cu foam, Ti foam, and stainless steel foam; wherein the current collector substrate is conductive.

4. The bifunctional electrode as claimed in claim 1, wherein, the bifunctional electrode works in an anion exchange membrane electrolyzer at an applied voltage in the range of 1.8 to 2.4 V in a temperature range of 25 to 30 ?; wherein the anion exchange membrane electrolyzer contains a KOH solution consisting of 5 to 9 weight % of KOH as electrolyte; and wherein the anion exchange membrane electrolyzer produces a hydrogen evolution volume of 80 to 110 liter per hour.

5. A process for the preparation of a bifunctional electrode as defined in claim 1, comprising the steps of:
i. preparing a cobalt ferrite [CoFe2O4-d] nanoparticle;
ii. dispersing the cobalt ferrite [CoFe2O4-d] nanoparticle with 1 to 10 weight % of reduced graphene oxide in a solvent to obtain a dispersion containing graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles; and
iii. depositing the dispersion containing graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticles over a current collector substrate.

6. The process as claimed in claim 5, wherein the cobalt ferrite [CoFe2O4-d] nanoparticle are prepared by a process comprising the steps of:
i. preparing a solution of a size controlling agent and adding an Fe precursor and a Co precursor to form a reaction mixture;
ii. stirring the reaction mixture to obtain a solution;
iii. adding a hot NH4OH solution having a temperature of 55 to 65 ? to the solution of step ii) under continuous stirring to obtain a product containing cobalt ferrite [CoFe2O4-d] nanoparticles;
iv. cooling the product and separating the [CoFe2O4-d] nanoparticles from the product; and
v. drying the [CoFe2O4-d] nanoparticles and calcining the [CoFe2O4-d] nanoparticles.

7. The process as claimed in claim 6, wherein the Fe precursor is selected from a group consisting of Fe(NO3)3, FeSO4, FeCl3, Fe(NO3)2, and FeCl2, Fe2(SO4)3; wherein the Co precursor is selected from a group consisting of cobalt chloride, cobalt nitrate, and cobalt sulphate; wherein the size controlling agent is selected from a group consisting of sodium dodecyl sulfate, polyethylene glycol, polyvinyl pyrrolidone, and sodium deoxycholate surfactant.

8. The process as claimed in claim 6, wherein the concentration of Fe precursor is twice the concentration of Co precursor; wherein the reaction mixture is stirred at a temperature in a range of 25 to 30 ?; wherein the solution is stirred at 55 to 65 ? for 1 to 5 hours; and wherein the CoFe2-dO4 nanoparticles are calcined at a temperature of 350 to 650 ? for 3 to 8 hours at a heating rate of 2 to 8 ?/min.

9. The process as claimed in claim 5, wherein the solvent is selected from a group consisting of water, ethanol, acetone, and isopropanol.

10. The process as claimed in claim 5, wherein the graphene encapsulated cobalt ferrite [CoFe2O4-d] nanoparticle deposited over the current collector substrate are calcined at a temperature in a range of 300 to 700 ?.