WATER ELECTROLYSIS USING IRON-DOPED ZIF-8 CATALYSTS

Abstract

A method of oxygen evolution comprising contacting a working electrode including an iron-doped zeolitic imidazolate framework-8 (Fe-ZIF-8), a counter electrode, and a reference electrode with an aqueous electrolytic solution, applying a potential, and forming oxygen at the working electrode. The iron-doped ZIF-8 includes iron in an amount of 5 to 55 weight percent relative to the combined weight of iron and zinc in the iron-doped ZIF-8. The Fe-ZIF-8 structures are nanoparticles with a longest dimension of 0.5 to 5 m. The working electrode has an overpotential of 180 to 190 mV vs RHE at a current density of 10 mA/cm.sup.2.

Claims

1: A method of oxygen evolution, comprising: contacting a working electrode comprising an iron-doped zeolitic imidazolate framework-8, a counter electrode, and a reference electrode with an aqueous electrolytic solution, wherein the iron-doped zeolitic imidazolate framework-8 comprises iron in an amount of 5 to 55 percent by weight based on the total weight of iron and zinc in the iron-doped zeolitic imidazolate framework-8, wherein the iron-doped zeolitic imidazolate framework-8 is in the form of nanoparticles with a longest dimension of 0.5 to 5 m, applying a potential; and forming oxygen at the working electrode, wherein the working electrode has an overpotential of 180 to 190 mV vs RHE at a current density of 10 mA/cm.sup.2.

2: The method of claim 1, further comprising: forming the iron-doped zeolitic imidazolate framework-8 by: dissolving 2-methylimidazole in a polar solvent to form a first solution; dissolving a zinc salt and an iron salt in a polar solvent to form a second solution; mixing the first solution and the second solution to form a precipitate; collecting and washing the precipitate; and drying the precipitate at a temperature of 50 to 70 C. for 8 to 24 hours to form the iron-doped zeolitic imidazolate framework-8.

3: The method of claim 1, wherein the working electrode is a nickel foam substrate coated with a mixture comprising the iron-doped zeolitic imidazolate framework-8 and a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer.

4: The method of claim 3, further comprising: forming the working electrode by: dispersing the iron-doped zeolitic imidazolate framework-8, the sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, and a polar solvent in water to form a mixture; sonicating the mixture for 10 to 30 minutes; and coating the mixture onto the nickel foam substrate.

5: The method of claim 4, wherein 50 to 150 L of the mixture is coated onto the nickel foam substrate.

6: The method of claim 4, wherein the coating includes spray coating the mixture onto the nickel foam substrate.

7: The method of claim 1, wherein the iron-doped zeolitic imidazolate framework-8 comprises iron in an amount of 5 to 15 percent by weight based on the total weight of iron and zinc in the iron-doped zeolitic imidazolate framework-8 and the iron-doped zeolitic imidazolate framework-8 is in the form of nanoparticles with a longest dimension of 0.1 to 1 m.

8: The method of claim 1, wherein the working electrode has an overpotential of 260 to 280 mV vs RHE at a current density of 55 mA/cm.sup.2.

9: The method of claim 1, wherein the working electrode has an overpotential of 460 to 480 mV vs RHE at a current density of 150 mA/cm.sup.2.

10: The method of claim 1, wherein the working electrode is stable for 20 to 28 hours at a current density of 50 mA/cm.sup.2.

11: The method of claim 1, wherein the working electrode has a double-layer capacitance of 8 to 12 mF/cm.sup.2.

12: The method of claim 1, wherein the working electrode has a charge transfer resistance of 0.2 to 0.6 /cm.sup.2.

13: The method of claim 1, wherein a working area of the working electrode is 0.1 to 2 cm.sup.2.

14: The method of claim 1, wherein the counter electrode is a platinum wire.

15: The method of claim 1, wherein the reference electrode is a mercury/mercury oxide (Hg/HgO) electrode.

16: The method of claim 1, wherein the aqueous electrolytic solution comprises potassium hydroxide.

17: The method of claim 1, wherein the iron-doped zeolitic imidazolate framework-8 comprises iron in an amount of 5 to 15 percent by weight based on the total weight of iron and zinc in the iron-doped zeolitic imidazolate framework-8.

18: The method of claim 1, further comprising: producing hydrogen at the working electrode.

19: The method of claim 1, further comprising: producing formate at the working electrode.

20: The method of claim 1, further comprising: producing a hydrogenated product at the working electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

[0032] FIG. 1A is a schematic flow chart of a method of oxygen evolution, according to certain embodiments.

[0033] FIG. 1B is a schematic flow chart of a process of making an iron-doped zeolitic imidazolate framework-8 (also referred to as Fe-doped ZIF-8 and Fe-ZIF-8), according to certain embodiments.

[0034] FIG. 1C is a schematic flow chart of a process of making a working electrode, according to certain embodiments.

[0035] FIG. 1D is a schematic diagram for the synthesis of the Fe-ZIF-8, according to certain embodiments.

[0036] FIG. 2 depicts X-ray diffraction (XRD) patterns of ZIF-8 and 10-50 wt. % Fe-doped ZIF-8, according to certain embodiments.

[0037] FIG. 3A depicts a scanning electron microscope (SEM) image of 10 wt. % Fe-doped ZIF-8 captured at a scale of 10 micrometers (m) with a magnification of 2500, according to certain embodiments.

[0038] FIG. 3B depicts an SEM image of 20 wt. % Fe-doped ZIF-8 captured at a scale of 10 m with a magnification of 2500, according to certain embodiments.

[0039] FIG. 3C shows an SEM image of 30 wt. % Fe-doped ZIF-8 captured at a scale of 10 m with a magnification of 2500, according to certain embodiments.

[0040] FIG. 3D depicts an energy dispersive X-ray spectroscopy (EDS) analysis of 10 wt. % Fe-doped ZIF-8, according to certain embodiments.

[0041] FIG. 3E depicts an EDS analysis of 20 wt. % Fe-doped ZIF-8, according to certain embodiments.

[0042] FIG. 3F depicts an EDS analysis of 30 wt. % Fe-doped ZIF-8, according to certain embodiments.

[0043] FIG. 4A depicts linear sweep voltammetry (LSV) analysis for nickel foam (NF), ZIF-8/NF, and Fe-ZIF-8/NF electrodes in 1.0 M KOH solution at a scan rate of 5 mV s.sup.1, according to certain embodiments.

[0044] FIG. 4B depicts catalytic stability of Fe-doped ZIF-8/NF at 0.3 V (vs RHE) for 24 h, according to certain embodiments.

[0045] FIG. 5A depicts cyclic voltammograms (CVs) of ZIF-8/NF in 1.0 M KOH electrolyte at scan rates of 25-150 mV s.sup.1, according to certain embodiments.

[0046] FIG. 5B depicts CVs of 10 wt. % Fe-doped ZIF-8/NF in 1.0 M KOH electrolyte at scan rates of 25-150 mV s.sup.1, according to certain embodiments.

[0047] FIG. 5C depicts CVs of 20 wt. % Fe-doped ZIF-8/NF in 1.0 M KOH electrolyte at scan rates of 25-150 mV s.sup.1, according to certain embodiments.

[0048] FIG. 5D depicts CVs of 30 wt. % Fe-doped ZIF-8/NF in 1.0 M KOH electrolyte at scan rates of 25-150 mV s.sup.1, according to certain embodiments.

[0049] FIG. 5E depicts double layer capacitance (C.sub.dl) slopes for ZIF-8/NF and 10-30 wt. % Fe-doped ZIF-8/NF at scan rate of 25-150 mV s.sup.1, according to certain embodiments.

[0050] FIG. 5F depicts Nyquist plots for NF, ZIF-8/NF, 10 wt. % Fe-doped ZIF-8/NF, 20 wt. % Fe-doped ZIF-8/NF, and 30 wt. % Fe-doped ZIF-8/NF, according to certain embodiments.

DETAILED DESCRIPTION

[0051] In the following description, it is understood that other embodiments may be utilized, and structural and operational changes may be made without departure from the scope of the present embodiments disclosed herein.

[0052] When describing the present disclosure, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise. Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings wherever applicable, in which some, but not all embodiments of the disclosure are shown. In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words a, an, and the like generally carry a meaning of one or more, unless stated otherwise. Within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

[0053] Furthermore, the terms approximately, approximate, about, and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.

[0054] Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the disclosure are shown.

[0055] As used herein, the words about, approximately, or substantially similar may be used when describing magnitude and or position to indicate that the value and/or position described is within a reasonable expected range of values and/or positions. For example, a numeric value may have a value that is +/0.1% of the stated value (or range of values), +/1% of the slated value (or range of values), +/2% of the stated value (or range of values), +/5% of the slated value (or range of values), +/10% of the staled value (or range of values), +/15% of the stated value (or range of values), or +/20% of the stated value (or range of values). Within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

[0056] The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material.

[0057] In addition, the present disclosure is intended to include all isotopes of atoms occurring in the present compounds and complexes. Isotopes include those atoms having the same atomic number but different mass numbers. By way of general example, and without limitation, isotopes of hydrogen include deuterium and tritium.

[0058] As used herein, the term electrolytic cell refers to a device that facilitates a chemical reaction by applying an external electric current. The current drives a non-spontaneous reaction that would not occur under standard conditions. The external energy source is a voltage applied between the cell's electrodes (preferably at least 2 electrodes), an anode and a cathode, which are immersed in an electrolyte solution.

[0059] As used herein, the term water splitting refers to the chemical reaction in which water is broken down into oxygen and hydrogen: 2H.sub.2O.fwdarw.2H.sub.2+O.sub.2

[0060] As used herein, the term current density refers to the amount of electric current traveling per unit cross-section area.

[0061] As used herein, the term zeolitic material refers to a material having a crystalline structure or three-dimensional framework of, but not necessarily the elemental composition of, a zeolite. Zeolites are porous silicate or aluminosilicate minerals that occur in nature. Elementary building units of zeolites are SiO.sub.4 (and AlO.sub.4, if appropriate)tetrahedra. Adjacent tetrahedra are linked at their corners via a common oxygen atom, which results in an inorganic macromolecule with a three-dimensional framework (frequently referred to as the zeolite framework). The three-dimensional framework of a zeolite also includes channels, channel intersections, and/or cages having dimensions in the range of 0.1-10 nanometers (nm), preferably 0.2-5 nm, and more preferably 0.2-2 nm. Water molecules may be present inside these channels, channel intersections, and/or cages. Zeolites that are devoid of aluminum may be referred to as all-silica zeolites or aluminum-free zeolites. Some zeolites that are substantially free of, but not devoid of, aluminum are called high-silica zeolites. Sometimes, the term zeolite is used to refer exclusively to aluminosilicate materials, excluding aluminum-free zeolites or all-silica zeolites.

[0062] Aspects of the present disclosure are directed to a method of oxygen evolution using an iron-doped (Fe-doped) ZIF-8-based electrocatalyst. The Fe-doped ZIF-8 electrocatalyst with varying molar ratios of Zn/Fe (Fe-ZIF-8) was prepared and the electrocatalytic performance of the catalysts was evaluated for electrochemical water splitting. The catalytic activity of undoped ZIF-8 for the oxygen evolution reaction (OER) is lower than that of Fe-ZIF-8, which demonstrates electrocatalytic OER with a reduced overpotential. The results indicate that the catalytic activity of undoped ZIF-8 was found be lower compared to that of the Fe-ZIF-8 with a reduced overpotential, suggesting that the Fe-ZIF-8 exhibits potential as a catalyst for electrocatalytic water splitting for OER and hydrogen evolution reaction (HER).

[0063] FIG. 1A illustrates a schematic flow chart of a method 50 of oxygen evolution. The order in which the method 50 is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method 50. Additionally, individual steps may be removed or skipped from the method 50 without departing from the spirit and scope of the present disclosure.

[0064] At step 52, the method 50 includes contacting a working electrode including an iron-doped zeolitic imidazolate framework-8 (Fe-ZIF-8), a counter electrode, and a reference electrode with an aqueous electrolytic solution. The working electrode refers to an electrode in an electrochemical cell/device/sensor on which the electrochemical reaction of interest is occurring. The Fe-ZIF-8 includes iron in an amount of 5 to 55 percent by weight (wt. %), preferably 9 to 51 wt. %, preferably 10 to 30 wt. %, preferably about 10 wt. %, preferably about 20 wt. %, and preferably about 30 wt. % based on a total weight of iron and zinc in the Fe-ZIF-8. The Fe-ZIF-8 is in the form of nanoparticles with the longest dimension of 0.5 to 5 m, preferably 1 to 4.5 m, preferably 1.5 to 4 m, preferably 2 to 3.5 m, and preferably 2.5 to 3 m. In a specific embodiment, the Fe-ZIF-8 includes iron in an amount of 5 to 15 percent by weight (wt. %), preferably 7 to 13 wt. %, more preferably 9 to 10 wt. %, and yet more preferably about 9.5 wt. % based on a total weight of iron and zinc in the Fe-ZIF-8, and the Fe-ZIF-8 is in the form of nanoparticles with the longest dimension of 0.1 to 1 m, preferably 0.2 to 0.8 m, and preferably 0.4 to 0.6 m. The weight percentage of zinc and iron in the Fe-ZIF-8 is in the range of 1:1 to 10:1, preferably 2:1 to 9:1, preferably 3:1 to 8:1, preferably 4:1 to 7:1, and preferably 5:1 to 6:1.

[0065] The working electrode further includes a nickel foam (NF) substrate coated with a mixture including the Fe-ZIF-8 and a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer. Optionally, polyphenylsulfone (PPSU), sulfated poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (PFA), Flemion, Aciplex, Solef 6000, combinations thereof, and the like may be used in combination with or instead of sulfonated tetrafluoroethylene-based fluoropolymer-copolymer. The working area of the working electrode is 0.1 to 2 cm.sup.2, preferably 0.5 to 1.5 cm.sup.2, more preferably 0.9 to 1.1 cm.sup.2, and yet more preferably about 1 cm.sup.2. In an embodiment, the nickel foam substrate may optionally include metals in addition to nickel, such as zinc, aluminium, and the like, or alloys thereof. In other embodiments, the nickel foam substrate may be a carbon foam, a carbon paper, an aluminum foil, or any other conductive material known in the art as a support and/or substrate for electrocatalysis. The nickel foam substrate may be a porous material. In an embodiment, the average pore size, or largest diameter, of the NF substrate is 50 to 500 micrometers (m), preferably 100 to 400 m, and preferably 200 to 300 m. In an embodiment, the substrate has a thickness of 0.1 to 10 mm, preferably 0.5 to 8 mm, preferably 1 to 5 mm, and preferably 2 to 3 mm. In an embodiment, the pores of the nickel foam substrate have a cubical, conical, cuboidal, pyramidical, cylindrical shape, or any other pore shape known in the art. in some embodiments, the pores may have a spherical shape.

[0066] As used herein, the term reference electrode refers to an electrode with a stable and well-known electrode potential. In some embodiments, the reference electrode is a mercury/mercury oxide (Hg/HgO) electrode. In some embodiments, the reference electrode may include, but is not limited to, a standard hydrogen electrode (SHE), a calomel electrode (saturated calomel electrode, SCE), a copper/copper sulfate electrode (Cu/CuSO.sub.4), a standard calomel electrode (SCE), a Luggin capillary electrode, and any other reference electrode known in the art. As used herein, the term counter electrode or auxiliary electrode refers to an electrode used in an electrochemical cell for voltametric analysis and/or other reactions in which an electric current is expected to flow. An outer surface of the counter electrode may include an inert, electrically conducting chemical substance, such as platinum, gold, or carbon. The carbon may be in the form of graphite or glassy carbon. In an embodiment, the counter electrode is a platinum wire. In one embodiment, the counter electrode may be a wire, a rod, a cylinder, a tube, a scroll, a sheet, a piece of foil, a woven mesh, a perforated sheet, a brush, and the like. The counter electrode material should thus be sufficiently inert to withstand the chemical conditions in the electrolyte solution, such as acidic or basic pH values, without substantially degrading during the electrochemical reaction. In addition, the counter electrode should preferably not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable electrode contamination.

[0067] In some embodiments, the reference electrode and the counter electrode may be connected through electrical interconnects that allow for the passage of current between the electrodes when a potential is applied. In an embodiment, the reference electrode and the counter-electrode can have the same or different dimensions. The reference electrode and the counter-electrode may be arranged as obvious to a person of ordinary skill in the art.

[0068] The aqueous electrolytic solution may include water and an inorganic base. The base may be selected from a group comprising of an alkaline earth metal hydroxide, such as beryllium hydroxide (Be(OH).sub.2), magnesium hydroxide (Mg(OH).sub.2), strontium hydroxide (Sr(OH).sub.2), and calcium hydroxide (Ca(OH).sub.2), and/or an alkali metal hydroxide, such as lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), rubidium hydroxide (RbOH), and cesium hydroxide (CsOH). In some embodiments, the aqueous electrolytic solution includes potassium hydroxide as an electrolyte. The concentration of the inorganic base, preferably KOH, may be in the range of 0.1-5 M, preferably 1-3 M, more preferably 1-2 M, and yet more preferably about 1 M.

[0069] At step 54, the method 50 includes applying a potential. In an embodiment, the potential is applied to the working electrode. In an embodiment, the applied potential ranges from 1.0 to 3 V versus RHE, preferably-0.5 to 2.5 V vs RHE, preferably 0 to 2 V vs RHE, and preferably 0.5 to 1.5 V vs RHE. In some embodiments, the potential is applied at a scan rate of 1 to 5000 mV/s, preferably 5 to 2000 mV/s, preferably 10 to 1000 mV/s, preferably 20 to 500 mV/s, preferably 50 to 250 mV/s, and preferably 100 to 150 mV/s.

[0070] At step 56, the method 50 includes forming oxygen at the working electrode. The working electrode has an overpotential of 180 to 190 mV vs RHE, preferably 181 to 189 mV vs RHE, preferably 182 to 186 mV vs RHE, more preferably 183 to 185 mV vs RHE, and yet more preferably about 184 mV vs RHE at a current density of 10 mA/cm.sup.2. Overpotential in electrolysis refers to the additional energy required than thermodynamically expected to drive a reaction. To make the process commercially viable, the overpotential losses during water electrolysis should be reduced, and the exchange current density, which measures the reaction rate at equilibrium potential, should be improved. The electrocatalyst of the present disclosure produces oxygen from water efficiently at a low overpotential and a high exchange current density. The working electrode has an overpotential of 184 mV vs RHE at a current density of 10 mA/cm.sup.2, which is smaller than IrO.sub.2 (270 mV vs RHE at a current density of mA/cm.sup.2). In some embodiments, the working electrode has an overpotential of 260 to 280 mV vs RHE, preferably 263 to 277 mV vs RHE, preferably 265 to 275 mV vs RHE, more preferably 267 to 273 mV vs RHE, and yet more preferably about 270 mV vs RHE at a current density of 55 mA/cm.sup.2. In some embodiments, the working electrode has an overpotential of 460 to 480 mV vs RHE, preferably 463 to 477 mV vs RHE, preferably 465 to 475 mV vs RHE, more preferably 467 to 473 mV vs RHE, and yet more preferably about 470 mV vs RHE at a current density of 150 mA/cm.sup.2. In some embodiments, the working electrode is stable for 20 to 28 hours, preferably 21 to 27 hours, preferably 22 to 26 hours, more preferably 23 to 25 hours, and yet more preferably about 24 hours at a current density of 50 mA/cm.sup.2. In some embodiments, the working electrode has a double-layer capacitance of 8 to 12 mF/cm.sup.2, preferably 8.5 to 11.5 10 mF/cm.sup.2, more preferably 9 to 11 mF/cm.sup.2, and yet more preferably about 10 mF/cm.sup.2. In some embodiments the working electrode has a charge transfer resistance of 0.2 to 0.6 /cm.sup.2, preferably 0.25 to 0.55 /cm.sup.2, preferably 0.3 to 0.5 /cm.sup.2, more preferably 0.35 to 0.45 /cm.sup.2, and yet more preferably about 0.4 /cm.sup.2. In some embodiments, the method 50 includes producing hydrogen at the working electrode. In some embodiments, the method 50 also includes producing formate at the working electrode. In some embodiments, the method 50 also includes producing a hydrogenated product at the working electrode. In other embodiments, the method 50 includes producing any product and/or by-product from water splitting known in the art.

[0071] FIG. 1B illustrates a schematic flow chart of a process 70 of making the iron-doped zeolitic imidazolate framework-8. The order in which the process 70 is described is not intended to be construed as a limitation, and any number of the described process steps can be combined in any order to implement the process 70. Additionally, individual steps may be removed or skipped from the process 70 without departing from the spirit and scope of the present disclosure.

[0072] At step 72, the process 70 includes dissolving a 2-methylimidazole in a polar solvent to form a first solution. The polar solvent may include, but is not limited to, water, ethanol, methanol, acetone, dimethyl sulfoxide (DMSO), hydrochloric acid, ammonia, combinations thereof, and the like. In a preferred embodiment, the polar solvent is methanol.

[0073] At step 74, the process 70 includes dissolving a zinc salt and an iron salt in a polar solvent to form a second solution. Suitable examples of zinc salts include, but are not limited to, zinc sulfate, zinc chloride, zinc nitrate, zinc acetate, zinc carbonate, zinc oxide, zinc phosphate, zinc bromide, their hydrate salts, combinations thereof, and the like. In a preferred embodiment, the zinc salt is a zinc nitrate hexahydrate salt. Suitable examples of iron salts include iron(II) acetate, iron(II) bromide, iron(II) carbonate, iron(II) chloride, iron(II) chromite, iron(II) citrate, iron(II) cyanide, iron(II) fluoride, iron(II) fumarate, iron(II) gluconate, iron(II) hydride, iron(II) hydroxide, iron(II) iodide, iron(II) lactate, iron(II) molybdate, iron(II) nitrate, iron(II) oxalate, iron(III) chloride, iron(III) thiocyanate, ferric sulfate, ferrous fumarate, ferrous gluconate, ferrous succinate, iron(III) hydroxide, iron(III) nitrate, their hydrate salts, combinations thereof, and the like. In a preferred embodiment, the iron salt is an iron(II) sulfate heptahydrate. The polar solvent may include, but is not limited to, water, ethanol, methanol, acetone, dimethyl sulfoxide (DMSO), hydrochloric acid, ammonia, combinations thereof, and the like. In a preferred embodiment, the polar solvent is methanol.

[0074] At step 76, the process 70 includes mixing the first solution and the second solution to form a precipitate. In some embodiments, the first solution and the second solution are mixed by stirring, swirling, sonicating, or a combination thereof may be employed to form the metal solution.

[0075] At step 78, the process 70 includes collecting and washing the precipitate. The precipitate is collected by centrifugation, filtration, or any other separation means known in the art. The precipitate may be washed with a polar solvent such as water, ethanol, methanol, acetone, dimethyl sulfoxide (DMSO), hydrochloric acid, ammonia, a combination thereof, and the like. In a preferred embodiment, the precipitate is washed with methanol.

[0076] At step 80, the process 70 includes drying the precipitate at a temperature of 50 to 70 C., preferably 52 to 68 C., preferably 55 to 65 C., more preferably 58 to 62 C., and yet more preferably about 60 C. for 8 to 24 hours, preferably 9 to 20 hours, preferably 10 to 14 hours, more preferably 11 to 13 hours, and yet more preferably about 12 hours to form the iron-doped zeolitic imidazolate framework-8. In some embodiments, the drying can be performed by using heating appliances such as ovens, microwaves, autoclaves, hot plates, heating mantles and tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces, hot-air guns, and the like. In a preferred embodiment, the precipitate is dried at 60 C. in an oven, for example, a convection oven.

[0077] FIG. 1C illustrates a schematic flow chart of a process 90 of making the working electrode. The order in which process 90 is described is not intended to be construed as a limitation, and any number of the described process steps can be combined in any order to implement process 90. Additionally, individual steps may be removed or skipped from the process 90 without departing from the spirit and scope of the present disclosure.

[0078] At step 92, the process 90 includes dispersing the iron-doped zeolitic imidazolate framework-8, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, and a polar solvent in water to form a mixture. Optionally, polyphenylsulfone (PPSU), sulfated poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (PFA), Flemion, Aciplex, Solef 6000, and the like may be used in combination with and/or instead of the sulfonated tetrafluoroethylene-based fluoropolymer-copolymer.

[0079] At step 94, the process 90 includes sonicating the mixture for 10 to 30 minutes, preferably 12 to 28 minutes, preferably 15 to 25 minutes, more preferably 18 to 22 minutes, and yet more preferably about 20 minutes. The sonication is carried out to mix and/or agitate the mixture. In some embodiments, other modes of agitation known to those of ordinary skill in the art, for example, via stirring, swirling, mixing, or a combination thereof, may be employed to form the mixture.

[0080] At step 96, the process 90 includes coating the mixture onto the nickel foam substrate. In some embodiments, the mixture may be coated onto the nickel foam substrate using techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), spin coating, dip coating, electrophoretic deposition (EPD), Langmuir Blodgett (LB) technique, drop casting, sol-gel process, layer-by-layer (LbL) assembly, inkjet printing, spray coating, ultrasonic spray deposition, and the like. In some embodiments, the mixture is spray-coated onto the nickel foam substrate.

EXAMPLES

[0081] The following examples describe and demonstrate a method of oxygen evolution using an iron-doped zeolitic imidazolate framework-8 (Fe-doped ZIF-8) with varying molar ratios of Zn/Fe (Fe-ZIF-8) in water-splitting reactions. The examples are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.

Example 1: Chemicals

[0082] The chemical compounds utilized in this study were procured from Sigma Aldrich, US, including zinc nitrate hexahydrate (Zn(NO.sub.3).sub.2.Math.6H.sub.2O) (99.95%), iron(II) sulfate heptahydrate (FeSO.sub.4.Math.7H.sub.2O, Merck, Germany (99.5%), 2-methylimidazole (C.sub.4H.sub.6N.sub.2, Aldrich, USA (99.0%), and potassium hydroxide (99.5%). The procurement of 99.8% methanol (CH.sub.3OH) was carried out from Sharlu, located in Sharjah, United Arab Emirates.

Example 2: Synthesis of ZIF-8 and Fe-ZIF-8

[0083] A simple precipitation approach was employed to prepare ZIF-8 and various loaded iron ZIF-8. Briefly, a solution was prepared by dissolving 20 mmol of 2-methylimidazole in 50 mL of methanol. The solution was subsequently introduced into a 50 mL methanol solution comprising 5 mmol of FeSO.sub.4.Math.7H.sub.2O and Zn(NO.sub.3).sub.2.Math.6H.sub.2O in different proportions (0%, 10%, 20%, 30%, 40%, and 50%). The resulting mixture was vigorously agitated for 1 hour. Subsequently, the ZIF-8 (0% Fe loading) and Fe-ZIF-8 materials were collected via centrifugation. The prepared materials were washed several times with methanol. The products obtained were subjected to overnight drying at a temperature of 60 C., producing a white ZIF-8 powder and a yellow Fe-ZIF-8 powder.

Example 3: Electrocatalysts Preparation and Activity Analysis

[0084] 10 mg of the ZIF-8 or Fe-ZIF-8 catalyst was dispersed in a 1 mL mixture of 750 L isopropanol, 200 L DI water, and 50 L Nafion (5%). The mixture was sonicated for 20 minutes. Then, 100 L of the suspension was spray-coated onto 1 cm.sup.2 conductive nickel foam and dried at room temperature.

[0085] FIG. 1D depicts the synthesis process of producing the iron-doped ZIF-8. The schematic figure shows a solvothermal method using co-precipitation protocols at room temperature. Various molar ratios of the iron and zinc precursor was mixed with an organic linker to get a brownish precipitate. The brownish solid was separated and washed several times before characterization and analysis.

[0086] X-ray diffraction (XRD) patterns of simulated ZIF-8, parent ZIF-8, and Fe-ZIF-8 are shown in FIG. 2. The XRD patterns obtained for ZIF-8 exhibit high concordance with the simulated XRD patterns. The XRD patterns of Fe-ZIF-8 demonstrated the presence of characteristic peaks of ZIF-8, and no characteristic peaks of iron oxide were detected. The magnitude of the diffractive phenomena exhibited a decline in correspondence with a rise in the quantity of iron assimilated. An absence of diffraction patterns of ZIF-8 was noted upon exceeding an Fe ratio above 30 wt. %, suggesting that a large presence of iron results in a structural failure of the ZIF-8 framework.

[0087] FIG. 3A-FIG. 3F displays the morphology (FIG. 3A-FIG. 3C) and elemental composition (FIG. 3D-FIG. 3F) of the prepared materials. The scanning electron microscopy (SEM) images of the Fe-ZIF-8 are depicted in FIG. 3A-FIG. 3C. The observed data indicate that increased Fe loading results in increased particle sizes. This phenomenon may account for the collapse of the ZIF-8 structure beyond a loading of 30 wt. % iron. The EDS analysis in FIG. 3D-FIG. 3F indicates a slight decrease in weight percent compared to the theoretical values. The 10-Fe-ZIF-8 sample exhibits an actual weight of 9.5 wt. % compared to its theoretical value of 10%, 20-Fe-ZIF-8 and 30-Fe-ZIF-8 show iron percentages of 17.5% and 23.7% for 20 wt. % and 30 wt. %, respectively.

[0088] FIG. 4A displays linear sweep voltammetry (LSV) analysis for NF, ZIF-8/NF, and Fe-ZIF-8/NF electrodes. Using a common three-electrode cell, the OER performance of the electrocatalysts was examined to see how well they worked in an aqueous electrolyte containing 1.0 M KOH. Unless otherwise specified, the recorded current was normalized to the geometric area of the NF electrode to calculate current density, and the current-potential curves were corrected for iR drop. Linear sweep voltammograms (LSVs), which also depict the potentiostatic stability, are shown in FIG. 4A. OER activities were evaluated and compared using the same measuring circumstances throughout both processes. The 10% Fe-doped ZIF-8/NF electrode demonstrates an OER performance that is higher than the 20% Fe-doped ZIF-8/NF electrode, 30% Fe-doped ZIF-8/NF electrode, the ZIF-8/NF electrode, and the pristine NF electrode. The overpotentials for generating the standard current density of 10 mA cm.sup.2 are 184 mV vs RHE, 181 mV vs RHE, 180 mV vs RHE, 370 mV vs RHE, and 310 mV vs RHE for the ZIF-8/NF electrode, the 10% Fe-doped ZIF-8/NF electrode, the 20% Fe-doped ZIF-8/NF electrode, the 30% Fe-doped ZIF-8/NF electrode, and the pristine NF electrode, respectively. The difference in the overpotential for water splitting increased with increasing current density. For instance, the overpotential at 150 mA cm.sup.2 for the ZIF-8/NF electrode, the 10% Fe-doped ZIF-8/NF electrode, the 20% Fe-doped ZIF-8/NF electrode, the 30% Fe-doped ZIF-8/NF electrode, and the pristine NF electrode is 770, 470, 510, 520 and 820 mV vs RHE. The electrochemical stability of the 10% Fe-doped ZIF-8/NF electrode is evaluated under identical experimental conditions in basic electrolyte. Potentiostatic (current-time profile) measurements were carried out at 50 mA cm.sup.2 for 24 hours of continuous oxygen production. The results, shown in FIG. 4B, reveal good stability of the electrode during the time period.

[0089] An electrical double-layer capacitance (C.sub.dl) analysis is performed to collect information regarding the physiochemical properties and electrochemical active surface area (ECSA) of the electrodes. The Car is determined by recording CVs at various scan rates (speeds) ranging from 25 to 150 mV s.sup.1 and comparing the results. The findings are depicted in FIGS. 5A-5E.

[0090] FIG. 5E displays capacitive current vs scan rate for the ZIF-8/NF and Fe-doped ZIF8/NF electrodes. It was determined that the Cal values for the 10% Fe-doped ZIF-8/NF electrode, the 20% Fe-doped ZIF-8/NF electrode, the 30% Fe-doped ZIF-8/NF electrode, and the ZIF-8/NF electrode were 10, 7.0, 0.2, and 1.0 mF cm.sup.2, respectively. It is possible to attribute a greater C.sub.dl to a higher ECSA, which may translate into improved electrocatalytic performance. Electrochemical impedance spectroscopy, also known as EIS, is a technique used to analyze electrical resistance and interfacial charge transfer resistance (R.sub.ct) that is active between the surface of an electrode and H.sub.2O/OH.sup.. The measurements were carried out with a sinusoidal perturbation of overpotential of 300 mV vs RHE and with frequencies ranging from 10.sup.5 to 0.01 Hz. A Randle circuit was chosen to fit the EIS data due to an accuracy fit. The applied potential influences the R.sub.ct. As the potential was increased, the R.sub.ct value declined for all three electrodes. FIG. 5F depicts a comparison between the Nyquist plots of the NF electrode, the ZIF-8/NF electrode, the 10% Fe-doped ZIF-8/NF electrode, 20% Fe-doped ZIF-8/NF electrode, and the 30% Fe-doped ZIF-8/NF electrode measurements taken at an overpotential of 300 mV vs RHE. The R.sub.ct values were recorded as 4.2, 2.2, 0.4, 0.9, and 2.4 /cm.sup.2, respectively.

TABLE-US-00001 TABLE 1 OER comparative performances using ZIF-based electrocatalysts. References Electrocatalyst Electrolyte @ 10 mA cm.sup.2 1 ZIF-67@NPC-2 0.1M KOH 410 2 CoOx-ZIF 0.1M NaOH 318 3 Co-ZIF-9 0.1M KOH 510 @ 1 mA cm.sup.2 4 Ti@TiO.sub.2/CdS/ZIF-67 0.5M NaOH 287 5 ZIF-67@POM 1.0M KOH 490 6 Mn-ZIF-67 1.0M KOH 302 Present Fe-ZIF-8/NF 1.0M KOH 184 Work [1] Wang H. et al., ZIF-67 incorporated with carbon derived from pomelo peels: a highly efficient bifunctional catalyst for oxygen reduction/evolution reactions, Appl Catal B, 2017, 55-67; [2] Dou S. et al., Atomic-scale CoOx species in metal-organic frameworks for oxygen evolution reaction, Adv Funct Mater, 2017, 170-546; [3] Wang S. et al., Water oxidation electrocatalysis by a zeolitic imidazolate framework, Nanoscale, 2014, 9930-4; [4] Zhang T. et al., In-situ growth of ultrathin ZIF-67 nanosheets on conductive Ti@ TiO.sub.2/CdS substrate for high-efficient electrochemical catalysis, Electrochim Acta, 2016, 623-9; [5] Wang Y. et al., Core-shell-type ZIF-8@ZIF-67@ POM hybrids as efficient electrocatalysts for the oxygen evolution reaction, Inorg Chem Front, 2019, 2514-20; and [6] Selvasundarasekar S. S. et al., Effective Formation of a Mn-ZIF-67 Nanofibrous Network via Electrospinning: An Active Electrocatalyst for OER in Alkaline Medium. ACS Appl Mater Interfaces, 2022, 46581-94, which are incorporated herein by reference in their entireties.

[0091] A co-precipitation approach was utilized to produce an iron-doped zeolitic imidazolate framework-8 (Fe-ZIF-8), which was subsequently employed as an electrocatalyst for water oxidation. The Fe-ZIF-8 electrocatalyst showed good electrochemical activity, stability, and recyclability for an oxygen evolution reaction (OER). The electrochemical performance exhibited by these materials presents a promising opportunity for using iron-doped ZIF-8 catalysts in OERs. The catalysts of the present disclosure have demonstrated high activity, which can be advantageous in the advancement of efficient OER catalysts to promote clean energy production on a global scale. Furthermore, the method of the present disclosure was devised to augment the electrical properties of metal-doped zeolitic imidazolate framework-8 (ZIF-8) and generate advanced oxygen evolution reaction (OER) catalysts that exhibit good efficacy, economic feasibility, and ecological sustainability. The methodology also expands the scope of potential future designs for noble metal-free OER electrocatalysts. The present disclosure indicates that Fe-ZIF-8, a zeolite-imidazole framework doped with iron, exhibits potential as a catalyst for electrocatalytic water splitting, specifically for oxygen and hydrogen evolution reactions.

[0092] Numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the disclosure may be practiced otherwise than as specifically described herein.