Patent Application
20250230555
Aspects of the present disclosure are described in Mohamed, M. J. S., Slimani, Y., Gondal, M. A., Almessiere, M. A., Baykal, A., Hassan, M., Khan, A. Z. and Roy, A., “Role of vanadium ions substitution on spinel MnCo2O4 towards enhanced electrocatalytic activity for hydrogen generation” Sci Rep; 2023; 13, 2120, incorporated herein by reference in its entirety.
Support provided by the King Fahd University of Petroleum and Minerals (KFUPM) is gratefully acknowledged.
The present disclosure is directed towards electrocatalysts for water splitting systems and, more particularly, to vanadium-doped manganese cobalt spinel oxide based electrocatalysts for generating hydrogen.
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
The growing global energy demand and depletion of fossil fuels associated with toxic pollutants and greenhouse gas emissions has stimulated interest in developing a renewable energy technology that may protect the environment and provide enough energy to ensure socio-economic development. Hydrogen is one of the most sustainable and low-cost technologies for large-scale clean energy production. Electrocatalytic water splitting provides a sustained energy source for generating hydrogen/oxygen at a large scale with high purity, high efficiency, and zero pollutants. Several alloys and materials oxides are used for developing water splitting electrodes. However, there remains a need for the development of a suitable catalyst with low cost, natural abundance, excellent electrochemical activity, ease of preparation, and long-term stability for both HER and OER for electrocatalytic water splitting (EWS).
Certain oxides with transition metals, such as spinel structures (AB2O4), have high electronic conductivity and exhibit significant electrochemical activity in the hydrogen/oxygen evolution reaction (HER/OER) due to their chemical and physical characteristics. A selection of parameters such as temperature, preparation method, substitution ions, and pH of the precursor solution can affect the catalytic activity of spinels. Accordingly, adjusting the phase of the spinel oxide samples may improve the hydrogen evolution functioning of the sample. The cobalt-based spinel oxides (MnCo2O4, NiCo2O4, ZnCo2O4, CuCo2O4) have demonstrated significant performance in electrocatalytic OER. Moreover, MnCo2O4 has also been investigated as a potential applicant for energy storage applications, including batteries and supercapacitors, in terms of large capacitance, long cycling stability, and high operating voltage. These electrochemical properties are attributed to the multiple oxidation states (+2, +3, +4) of Mn and Co in magnetic spinel oxides. MnCo2O4 has an inverse spinel structure in which Mn2+ ions and the Co2+ ions have occupied the octahedral (Oh, B) sites and are evenly distributed over the Oh and tetrahedral (Td, A) sites. Cation substitution into MnCo2O4 spinel by doping with another element may significantly alter its electrical and magnetic features because the electron transfer distance between B and B is short, which enhances the electron transfer and the electrical conductivity. Therefore, altering the MnCo2O4 spinel through doping has potential to improve electrocatalytic activity.
Although several methods have been developed in the past to generate hydrogen, there still exists a need to develop a method for the generation of hydrogen that may circumvent the drawbacks of the prior art. It is one object of the present disclosure to provide a doped MnCo2O4 spinel electrocatalyst.
In an exemplary embodiment, a method of generating hydrogen is disclosed. The method includes applying a potential of −0.1 volts (V) to −1.0 volt (V) to an electrochemical cell, and the electrochemical cell is at least partially submerged in an aqueous solution. Further, on the application of the potential, the aqueous solution is reduced, thereby forming hydrogen. The electrochemical cell includes an electrocatalyst and a counter electrode. The electrocatalyst includes a substrate and vanadium-doped manganese spinel oxide microspheres (MnVxCo2-xO4) particles. The value of x is ≤0.4, the MnVxCo2-xO4 particles have a spherical shape, the MnVxCo2-xO4 particles have an average diameter of less than 100 nanometers (nm), and the MnVxCo2-xO4 particles are dispersed on the substrate to form the electrocatalyst.
In some embodiments, the MnVxCo2-xO4 particles have an average diameter of 5 nm to 30 nm.
In some embodiments, the MnVxCo2-xO4 particles include 25 weight percentage (wt. %) to 35 wt. % 0, 1 wt. % to 10 wt. % V, 20 wt. % to 30 wt. % Mn, and 40 wt. % to 50 wt. % Co, based on a total weight of the MnVxCo2-xO4 particles.
In some embodiments, the MnVxCo2-xO4 particles are aggregated, forming microspheres.
In some embodiments, the microspheres have an average diameter of 2 micrometers (μm) to 10 μm.
In some embodiments, the MnVxCo2-xO4 particles include less than 5 wt. % MnO2, based on a total weight of the MnVxCo2-xO4 particles.
In some embodiments, the MnVxCo2-xO4 particles have a cubic crystal structure.
In some embodiments, the MnVxCo2-xO4 particles have a crystallite size of 16 nm to 23 nm.
In some embodiments, the MnVxCo2-xO4 particles have a maximum magnetization value greater than 2.0 electromagnetic units per unit mass (emu/g) at 300 Kelvin (K).
In some embodiments, the MnVxCo2-xO4 particles have a maximum magnetization value greater than 12 emu/g at 10 K.
In some embodiments, the electrocatalyst has an overpotential of less than 250 millivolts (mV) at −10 milliampere per square centimeter (mA/cm2).
In some embodiments, the electrocatalyst includes MnV0.3Co1.7O4 particles and has an overpotential of 80 mV to 90 mV at −10 mA/cm2.
In some embodiments, the electrocatalyst has a Tafel slope of less than 120 mV per decade (mV/dec).
In some embodiments, the electrocatalyst includes MnV0.3Co1.7O4 particles and has an electrochemical active surface area (ECSA) of greater than 10 cm2.
In some embodiments, the substrate is glassy carbon.
In some embodiments, the aqueous solution further includes an acid.
In another exemplary embodiment, a method of preparing the electrocatalyst is described. The method includes mixing urea, a cobalt salt, a manganese salt, and a vanadium salt to form a mixture. The method further includes heating the mixture in an autoclave for at least 6 hours at 150° C. to 250° C. to form the MnVxCo2-xO4 particles, and further coating the MnVxCo2-xO4 particles on the substrate.
The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.
A more complete appreciation of this disclosure and many of the attendant advantages thereof may 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:
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 invention will now be described more fully hereinafter with reference to the accompanying drawings wherever applicable, in that some, but not all, embodiments of the disclosure are shown.
Where a numerical limit or range is stated herein, the endpoints are included. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.
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.
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.
The use of the terms “include,” “includes”, “including,” “have,” “has,” or “having” should be generally understood as open-ended and non-limiting unless specifically stated otherwise.
As used herein, “particle size” and “pore size” may be thought of as the lengths or longest dimensions of a particle and a pore opening, respectively.
As used herein, the term “electrode” refers to an electrical conductor used to contact a non-metallic part of a circuit, e.g., a semiconductor, an electrolyte, a vacuum, or air.
As used herein, the term “current density” refers to the amount of electric current traveling per unit cross-section area.
As used herein, the term “Tafel slope” refers to the relationship between the overpotential and the logarithmic current density.
As used herein, the term “electrochemical cell” refers to a device capable of generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions.
As used herein, the term “water splitting” refers to the chemical reaction in which water is broken down into oxygen and hydrogen as shown below.
2H2O→2H2+O2
As used herein, the term “overpotential” “refers to the difference in potential that exists between a thermodynamically determined reduction potential of a half-reaction and the potential at which the redox event is experimentally observed. The term is directly associated with a cell's voltage efficacy. In an electrolytic cell, the occurrence of overpotential implies that the cell needs more energy than that thermodynamically expected to drive a reaction. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally measured by determining the potential at which a given current density is reached.
The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material.
As used herein, the terms “electrocatalyst” and “catalyst” are used interchangeably and refer to the catalyst of the invention.
Aspects of the present disclosure are directed to a method for generating hydrogen using a vanadium-doped MnCo spinel oxide-based electrocatalyst (also referred to as an electrocatalyst or catalyst herein). The electrocatalyst is represented by MnVxCo2-xO4 (where x≤0.4) and is evaluated for its potential in hydrogen evolution reaction (HER). The results indicate that the electrocatalyst of the present disclosure demonstrated improved HER performance and outstanding electrocatalytic stability.
The first aspect of the present disclosure is directed to an electrocatalyst including spinel particles. Spinels are any of a class of minerals of general formulation AB2X4 which crystallize in the cubic crystal system, with the X anions arranged in a cubic close-packed lattice and the A cations occupy tetrahedral holes, and the B cation occupy octahedral holes. In an embodiment, A is selected from the group consisting of Mg, Zn, Fe, Mn, Cu, Ni, Ti, or Be. In an embodiment, B is selected from the group consisting of Al, Fe, Mn, Cr, and Co. In an embodiment, X is O or S. In a preferred embodiment, the electrocatalyst includes MnCo2O4. To alter the structure and electrochemical performance of the MnCo2O4, it can be doped with at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Fe, Co, Ru, and Rh. In a preferred embodiment, the electrocatalyst includes vanadium (V)-doped Mn(manganese) Co(cobalt) oxide particles, also referred to as particles. The particles are represented by the formula MnVxCo2-xO4, where x≤0.4. In an embodiment, x may be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, or 0.4. In a preferred embodiment, x=0.3. In an embodiment, the particles include at least one of Mn(II), Mn(III), Mn(IV), Mn(V), Mn(VI), and Mn(VII). In an embodiment, the particles include at least one of Co(I), Co(II), Co(III), and Co(IV). In an embodiment, the particles include at least one of V(II), V(III), V(IV), and V(V).
The particles may have many shapes, such as cones, cuboidal, pyramidical, cylindrical, wires, crystals, rectangles, triangles, pentagons, hexagons, prisms, disks, cubes, ribbons, blocks, beads, discs, barrels, granules, whiskers, flakes, foils, powders, boxes, stars, flowers, etc., and mixtures thereof. In a specific embodiment, the particles are spherical with an average diameter of fewer than 100 nanometers (nm). In some embodiments, the particles have an average diameter in the range of 1-90 nm, preferably 2-80 nm, preferably 3-70 nm, preferably 4-60 nm, preferably 5-50 nm, preferably 5-40 nm, and preferably 5-30 nm. In a preferred embodiment, the particles have an average diameter of 10-30 nm. In some embodiments, the particles are aggregated to form microspheres having an average diameter of 2-10 μm, preferably 3-9 μm, 4-8 μm, 5-7 μm or about 6 μm. In an embodiment, the particles have cubic, hexagonal, or monoclinic crystal structure. In a preferred embodiment, the particles have a cubic crystal structure. Wherein a portion, depending on the value of x, of the Co atoms in the cubic crystal structure are replaced with V atoms, thereby forming defects. Although, different oxides may be present, the MnVxCo2-xO4 particles include less than 5 wt. % MnO2, preferably less than 4 wt. %, 3 wt. %, 2 wt. %, or 1 wt. % based on the total weight of the MnVxCo2-xO4 particles.
The particles are crystalline with crystallite size in the range of 16-23 nm, preferably 16-22.5 nm, 17-22 nm, 18-21 nm, or 19-20 nm. The value of “x” affects the crystallite size of the MnVxCo2-xO4 particles, where x≤0.4. For example, when “x”=0.2, the particles have a crystallite size in the range of 16-23 nm, preferably 17-23 nm, preferably 18-23 nm, preferably 19-23 nm, preferably 20-22.5 nm, and preferably 22.4 nm. When “x”=0.3, the particles have a crystallite size in the range of 16-20 nm, preferably 16.5-19 nm, preferably 16.7-18 nm, preferably 17-17.5 nm, preferably 17.2 nm. Similarly, when “x”=0.4, the particles have a crystallite size in the range of 16-20 nm, preferably 16-19 nm, preferably 16-18 nm, and preferably 16 nm.
The particles include 25-35 wt. %, preferably 26-34 wt. %, preferably 27-32 wt. %, preferably 28-30 wt. %, preferably 28.1 to 29.5 wt. % oxygen (O); 1-10 wt. %, preferably 2-9 wt. %, preferably 3 to 8.75 wt. %, preferably 4-8.5 wt. %, preferably 4.5 to 8.5 wt. % of Vanadium (V); 20-30 wt. %, preferably 20-28 wt. %, preferably 20-25 wt. %, preferably 20-23 wt. %, preferably 20.5 to 22.5 wt. % of Manganese (Mn), and 40-50 wt. %, preferably 40-48 wt. %, preferably 40-46 wt. %, preferably 40-45 wt. %, preferably 41-45 wt. % of Cobalt (Co), based on the total weight of the MnVxCo2-xO4 particles. In an embodiment, the particles include 28.22 wt. % oxygen, 4.54 wt. % vanadium, 22.35 wt. % manganese, and 44.89 wt. % cobalt, based on the total weight of the MnVxCo2-xO4 particles. In some embodiments, the particles include 28.43 wt. % oxygen, 8.36 wt. % vanadium, 20.85 wt. % manganese, and 42.36 wt. % cobalt, based on the total weight of the MnVxCo2-xO4 particles. In certain other embodiments, the particles include 29.4 wt. % oxygen, 7.8 wt. % vanadium, 18.54 wt. % manganese, and 43.71 wt. % cobalt, based on the total weight of the MnVxCo2-xO4 particles.
The MnVxCo2-xO4 particles have a maximum magnetization value greater than 2.0 emu/g, preferably 2.0-3.5 emu/g, 2.2-3.2 emu/g, 2.4-3.0 emu/g, or 2.6-2.8 emu/g at 300 K and a maximum magnetization value greater than 12 emu/g, preferably 12-20 emu/g, 13-19 emu/g, 14-18 emu/g, or 15-17 emu/g at 10 K. The magnetic susceptibilities may be measured with a laboratory magnetometer such as a vibrating sample magnetometer, a superconducting quantum interference device, inductive pickup coils, a pulsed field extraction magnetometer, a torque magnetometer, a faraday force magnetometer, and an optical magnetometer. The magnetization values continuously increase with vanadium substitution.
The particles are dispersed on the substrate to form the electrocatalyst. The particles may form a continuous layer on the substrate. The particles cover at least 50%, preferably 55%, preferably 60%, preferably 65%, preferably 70%, preferably 75%, preferably 80%, preferably 85%, preferably 90%, and preferably >95% of the substrate. The substrate is an electrically conductive material such as, but not limited to, glassy carbon, graphite, gold, platinum, silver, iron, copper, aluminum, and the like. In a preferred embodiment, the substrate is glassy carbon. The substrate may have a thickness in a range of about 10 micrometers (μm) to 140 μm, for example, ranging from about 20 μm to about 120 μm, from about 50 μm to about 100 μm, from about 70 μm to about 95 μm, or from about 85 μm to about 90 μm, including all ranges and sub-ranges therebetween.
A method of generating hydrogen is described. The method includes applying a potential of −0.1 to −1.0 V, preferably −0.2 to −0.9 V, −0.3 to −0.8 V, −0.4 to −0.7 V, and −0.5 to −0.6 V to an electrochemical cell. The electrochemical cell includes the above described electrocatalyst (anode) and a counter electrode (cathode). The counter electrode may contain an electrically-conductive material such as platinum, platinum-iridium alloy, iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloy, and some other electrically-conductive material, where an “electrically-conductive material” as defined here is a substance with an electrical resistivity of at most 10−6 ohms meter (Ω·m), preferably at most 10−7 Ω·m, more preferably at most 10−8 Ω·m at a temperature of 20-25° C. In a preferred embodiment, the counter electrode includes any material which is capable of undergoing oxygen evolution reaction (OER). The counter electrode is complementary to the electrocatalyst of the present disclosure which undergoes the hydrogen evolution reaction (HER) in water splitting.
The form of the counter electrode may be generally relevant only in that it needs to supply sufficient current to the electrolyte solution to support the current required for the electrochemical reaction of interest. The material of the counter electrode 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. The counter electrode should preferably not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable electrode contamination. In some embodiments, the counter electrode is a platinum electrode, more specifically, a platinum wire. In a specific embodiment, the electrocatalyst forms the working electrode (anode), and a platinum wire forms the counter electrode for hydrogen generation.
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, or a brush. 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. The counter electrode should preferably not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable electrode contamination.
In one embodiment, the electrochemical cell further includes a reference electrode in contact with the electrolyte solution. A reference electrode is an electrode that has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each relevant species of the redox reaction. A reference electrode may enable a potentiostat to deliver a stable voltage to the working or counter electrodes. The reference electrode may be RHE, a standard hydrogen electrode (SHE), a normal hydrogen electrode (NHE), a saturated calomel electrode (SCE), a Cu—Cu(II) sulfate electrode (CSE), a silver chloride electrode (Ag/AgCl), a pH-electrode, a palladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), a mercury-mercurous sulfate electrode, or some other type of electrode. In a preferred embodiment, the reference electrode is Ag/AgCl.
The electrochemical cell is at least partially submerged in an aqueous solution, preferably 50%, preferably 60%, or more preferably at least 70%, 80%, or 100%. The aqueous solution includes water and an acid. The water may be tap water, distilled water, bidistilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. Preferably, at 25° C., the water has an electrical conductivity at less than 10 micro siemens per centimeter (μS·cm−1), preferably less than 2 μS·cm−1. The acid is at least one selected from sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, boric acid, perchloric acid, carbonic acid, acetic acid, formic acid, hydrobromic acid, sulfamic acid, hypochlorous acid, sulfurous acid, hydroiodic acid, hypophosphorous acid, phosphorous acid, fumaric acid, iodic acid, chromic acid, chloric acid, hydrogen cyanide, nitroxyl, oxalic acid, etc. In a preferred embodiment, the acid is sulfuric acid. The concentration of the acid in the aqueous solution is in a range of 0.1 to 5 M, preferably 0.2-3 M, preferably 0.3-1 M, preferably 0.4 M-0.7 M, preferably 0.5 M. On applying the potential water in the aqueous solution is reduced thereby forming hydrogen.
The electrocatalyst of the present disclosure may also be used in water-splitting reactions. In some embodiments, the electrocatalyst may also be used in the field of batteries, fuel cells, photochemical cells, water splitting cells, electronics, water purification, hydrogen sensors, semiconductors (such as field-effect transistors), magnetic semiconductors, capacitors, data storage devices, biosensors (such as redox protein sensors), photovoltaics, liquid crystal screens, plasma screens, touch screens, OLEDs, antistatic deposits, optical coatings, reflective coverings, anti-reflection coatings, and/or reaction catalysis.
The electrocatalyst when including MnVxCo2-xO4 particles, where x≤0.4, have an overpotential of less than 250 millivolts (mV) at −10 mA/cm2, preferably 75-250 mV, 100-225 mV, 125-200 mV, or 150-175 mV and a Tafel slope of less than 120 mV per decade, preferably 80-120, 85-115, 90-110, 95-105 mV per decade. In a specific embodiment, when the electrocatalyst includes the MnVxCo2-xO4 particles, where x is 0.3, it has an overpotential of 80-90 mV, preferably 82-88, preferably 84-86, preferably 85.9 mV at −10 mA/cm2, and a Tafel slope of 80-90, preferably 82-88, preferably 83-85, preferably 84 mV per decade.
In some embodiments, the electrocatalyst has an electrochemical active surface area (ECSA) of 7-12 cm2, preferably 8-11 cm2, or 9-10 cm2. In a preferred embodiment, when the electrocatalyst includes MnVxCo2-xO4 particles, where x is 0.3, the ECSA is greater than 10 cm2, preferably between 10-15, 11-13, preferably 11.4 cm2.
While not wishing to be bound to a single theory, it is thought that the improved HER performance of the MnVxCo2-xO4 particles, is attributed to the V doping which provides a unique morphology, increases the electrochemical surface area, and results in many defective sites, which are advantageous for the rapid transfer of charges.
At step 52, the method 50 includes mixing urea, a cobalt salt, a manganese salt, and a vanadium salt to form a mixture. The mixing may be carried out manually or with the help of a stirrer. Suitable examples of the cobalt salt include Co(NO3)2, CoCl2, CoBr2, CoIe, CoF2, CoS, CoSO4, cobalt(II) acetate, tris(ethylenediamine)cobalt(III) chloride ([Co(en)3]Cl3), [Co(NH3)6]Cl3, tris(triphenylphosphine)cobalt(I) chloride ((P(CH5)3)3CoCl), Co2O3 (cobalt(III) oxide), Co2O (cobalt(II) oxide), CoFe2O4, or some other cobalt salt or cobalt-containing compound. Preferably, the cobalt has a +2 oxidation state, though in an alternative embodiment, cobalt having a different oxidation state, such as +3, may be used. In one embodiment, the cobalt of the cobalt salt consists essentially of cobalt in a +2 oxidation state. As defined here, the cobalt “consisting essentially of cobalt in a +2-oxidation state” means that at least 95 wt. %, preferably at least 99.wt %, more preferably at least 99.5 wt. % of the cobalt, has a +2 oxidation state relative to the total weight of the cobalt. Preferably, the cobalt salt may be in any hydration state; for instance, Co(NO3)2 includes both Co(NO3)2 and Co(NO3)2·6H2O. In a preferred embodiment, the cobalt salt is Co(NO3)2·6H2O. Suitable examples of manganese salt include manganese nitrate, manganese sulfate, MnX2 (where X is F, Cl, Br, or I), manganese hydroxide, manganese molybdate, manganese phosphate, and similar inorganic compounds. Preferably, the manganese salt may be in any hydration state; for instance, Mn(NO3)2 includes both Mn(NO3)2 and Mn(NO3)2·6H2O. In a preferred embodiment, the manganese salt is Mn(NO3)2·6H2O. The vanadium salt is vanadium halide, preferably vanadium chloride (VCl3). Optionally, other vanadium salts, such as vanadium pentoxide, vanadyl sulfate, and vanadyl acetylacetone, may be used as well, alone or in combination with other known vanadium salts. Stoichiometric amounts of urea, the cobalt salt, the manganese salt, and the vanadium salt may be mixed in a solvent. The solvent may be organic or inorganic. In a preferred embodiment, the solvent is inorganic, preferably water.
At step 54, the method 50 includes heating the mixture in an autoclave for at least 6 hours at 150-250° C. to form the MnVxCo2-xO4 particles. The MnVxCo2-xO4 particles are produced by any well-known process known to the skilled artisan in the relevant field. Examples of suitable processes for producing the metal oxide of the present disclosure include, but are not limited to, hydrothermal, sol-gel, solid-state reaction, high energy ball miffing, co-sedimentation, and the like. As outlined in the working examples, hydrothermal reaction is adopted to produce the metal oxide composite of the present disclosure, in which the hydrothermal reaction is conducted at a temperature between 25-300° C., such as at the temperature of 25, 50, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240 or 250° C., preferably 180° C. for 1-15 hours, preferably 1,2,3,4,5,6,7,8,9,10,11,12,13,14, preferably 12 hours. In an embodiment, the hydrothermal reaction occurs at 180° C. for 12 hours to form the MnVxCo2-xO4 particles. In some embodiments, the heating may be carried out via any other heating appliances such as ovens, microwaves, hot plates, heating mantles and tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces, and hot-air guns. The MnVxCo2-xO4 particles may be washed further to remove unreacted reactants/impurities and dried in an oven.
At step 56, the method 50 includes coating the MnVxCo2-xO4 particles on the substrate. The substrate may be coated by any conventional techniques known in the art, for example, drop-casting, dipping, spraying, coating, etc.
The following details of the examples demonstrate a method for generating hydrogen as described herein. 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.
A one-step hydrothermal process is used to manufacture the MnVxCo2-xO4 microspheres, labeled as Vx—MnCo(x≤0.4) MCs. Different amounts of the vanadium is doped into the MnVxCo2-xO4, where x=0.0, 0.1, 0.2, 0.3, and 0.4. Stoichiometric amount of Co(NO3)2·6H2O, Mn(NO3)2·6H2O, VCl3, and urea (CH4N2O) were used as initial materials. The stoichiometric quantities of metal salts were dissolved in 20 milliliters (ml) of de-ionized water (DI H2O) by stirring at room temperature (RT), and then 1.2 g of CH4N2O was dissolved in 30 ml of DI water. Both mixtures were stirred and sonicated for 30 minutes. The solution was poured into an autoclave made of stainless steel and heated for 12 hours (h) at 180° C. Finally, the solid product was washed with warm DI H2O, filtered, and dried in an oven.
For the working electrode preparation, 80 microlitres (μL) of 5% Nafion solution and 4 mg of catalyst were dissolved in a mixture of DI H2O and ethanol in 1 ml (4:1 in volume ratio). Ultrasonication was utilized for 30 minutes to guarantee that the solution was fully dispersed. Then, 5 μL catalyst ink solution was deposited on a polished GC electrode and dried at 80° C. for 2 h to achieve the catalyst loading of 0.285 mg/cm2. The crystal structure of the synthesized MCs was analyzed via a Rigaku Benchtop Miniflex X-ray diffractometer (XRD) (manufactured by Rigaku, Japan). The surface morphology and chemical composition were investigated with scanning electron microscopy (SEM) and FEI with the Titan ST model high-resolution transmission electron microscopy (HRTEM) and transmission electron microscopy (TEM) along with energy dispersive X-ray spectroscopy (EDX).
The linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) were performed in a 0.5 molar (M) sulphuric acid (H2SO4) solution with a typical three-electrode system using an AutoLab PGSTAT302N electrochemical workstation. Glassy carbon (GC) electrodes coated with Vx—MnCo MCs were used as working electrodes, a saturated Ag/AgCl electrode was employed as the reference electrode, and the Pt wire was utilized as the counter electrode. Nernst equation was used to calibrate all the potentials to reversible hydrogen electrode potential (ERHE), and ERHE was used to calibrate all the potentials for hydrogen evolution reaction (HER) measurements using the following equation,
where E°Ag/AgCl is the standard electrode potential equivalent to 0.198 V at 25° C. The scan rate for LSV measurements was 5 mV·s−1. The electrochemical chronopotentiometry was used to measure the stability of the Vx—MnCo MCs at a fixed current of −10 mA/cm2. EIS measurements were measured by the same potentiostat (PGSTAT302N Autolab), equipped with a frequency module analyzer (FRA2).
The phase purity analysis of Vx—MnCo MCs was investigated through powdered XRD analysis and refined by Match! Software, as depicted in
The surface morphology and shape of Vx—MnCo MCs were investigated by scanning electron microscopy (SEM) analysis with different magnifications, as depicted in
The chemical composition and oxidation state of Vx—MnCo MCs (x=0.3) magnetic nanoparticles were determined using XPS analysis. A comprehensive survey scan was conducted to identify the sample elements, as presented in
Catalysis and magnetism may be different manifestations of some more fundamental atomic properties. However, the current chief applications of magnetism to catalysis are in structural studies of catalytic solids. On the other hand, the variation in magnetic parameters is a result of the inclusion of vanadium ions that may cause inhomogeneity and intrinsic pinning of the domain walls. Due to the dissimilarity in ionic radii of vanadium and cobalt ions, a change in the exchange interactions may be caused by the local crystal fields. Such an effect may generate energy barriers that may considerably affect the magnetization reversal process at low temperatures. At low temperatures (10 K), the vanadium composition dependence of the magnetic parameters like coercive field (Hc), remanent magnetization (Mr), and magnetization Mmax achieved at 70 kOe is presented in
The magnetization values continuously increase with vanadium substitution. However, the variation in the Hc value showed an anomaly at x>0.2. The Hc increases first with increasing vanadium content (x) up to 0.2 but then drops with additional rising vanadium content (x>0.2), as shown in
For non-substituted MnCo2O4 MCs, the cations are distributed among Td and Oh sites as follows: (Co3+)A[Co2+Mn3+]BO4. The Td sites are occupied by Co3+ ions (spin S=2 and g=2) with a magnetic moment of about closeμ(A)=4.9μB. Conversely, the Oh sites are occupied by Co2+ and Mn3+ ions with high spin state S=3/2 and 2, respectively. Hence, the magnetic moment of B site is equal to, and may be expressed as,
Accordingly, the orbital angular momentum of the magnetic ions situated at B sites is completely quenched. Upon vanadium doping, the cations are distributed as (Co3+)A[Co2-x2+Vx2+Mn3+]BO4, where vanadium ions occupy Oh sites once they substitute some ions of Co2+. In this configuration, the three ions residing in the B site display magnetic moments of μ(Co2+)=3.87μB,μ(V2+)=1.73μB, and μ(Mn2+)=4.9μB. Substituting some magnetic Co2+ ions residing in B sites with some V2+ ions with lower magnetic moments may reduce the magnetic moment of B sites. Theoretically, the net magnetic moment of the whole system may decrease with increasing x content since the magnetic moment of A sites is constant. However, in the present disclosure, an increment in magnetization with vanadium substitution has been observed. This indicates that other factors govern the magnetic behavior of present samples.
Furthermore, all products showed maximum magnetizations peak at a temperature noted as TP (lower than the Curie temperature) in the MZFC(T) curves, and afterward, it falls to shallow values. The appearance of a peak around TP is ascribed to either the spin-glass state or the blocking phenomenon caused by the finite sizes and surficial effects in Vx—MnCo(x≤0.4) MCs. Moreover, the presence of such a peak is explained based on the Hopkinson effect, a competing effect between the applied magnetic field and the magneto-crystalline anisotropy that varies with the temperature. Like what was reported in the above M-H part, the impact of domain wall pinning may also influence the shape of MZFC(T) curves. Indeed, by heating the specimen with a low magnetic field application, the domain walls' mobility are raised. This means that the domain wall pinning effect is diminished with the increase in temperature. Hence, the walls are moved easily toward the direction of the applied magnetic field, which provoke a small increment in the magnetization. Above TC, the product is demagnetized; hence, the magnetization drops to almost zero at TC. On the other hand, the thermal agitation tends to increase as the temperature increases, leading to a reduction in magnetization. Nevertheless, at T<TC, the rotation of easy magnetization may be more dominant than the thermal agitations, leading to a global rise in magnetization. Consequently, a maximum peak is detected in the MZFC(T) curves below TC. Furthermore, it was evident that TC shifts progressively towards higher temperatures with increasing ‘x.’ This is correlated with the slight increment of crystallite/grain size as the vanadium content increases. The variations in the critical temperatures of TSP and TP as a function of vanadium content are shown in
The electrochemical performance of bare and various concentrations of Vx—MnCo(x≤0.4) MCs towards HER was assessed in 0.5 M H2SO4 electrolyte with a three-electrode configuration. The LSV measurements were carried out to investigate the electrocatalytic activity of representative electrodes.
Electron spectroscopy for chemical analysis (ESCA) is an activity parameter relating to electrocatalytic performance, such as double layer capacitance (Cdl) and the specific capacitance (Cs). To confirm the assumption mentioned above for higher activity of Vx—MnCo(x=0.3) MCs, the ECSA was measured for bare and Vx—MnCo MCs electrodes. The following equation was utilized to calculate the ECSA.
Cdl values were derived using the non-Faradaic potential region in CV curves, and Cs were assumed to be 35 μF cm−2.
where Qs is the surface charge density was derived by integrating the charge of each CV curve over the whole potential range, the half value of the charge was obtained as the Qs values, and F is the Faraday constant (96,485 C mol−1). According to the Qs values, the calculated number of active sites for all the prepared samples of Vx—MnCo(x≤0.4) MCs were 1.31×10−7 mol cm−2, 1.35×10−7 mol cm−2, 1.77×10−7 mol cm−2, 2.41×10−7 mol cm−2, and 1.82×10−7 mol cm−2, respectively. This result confirms that the electrocatalyst Vx—MnCo(x=0.3) MCs has more active sites for adsorbing the hydrogen molecules responsible for HER.
The kinetics of the electrocatalytic process was investigated using Tafel analysis, which relates the connection between potential and current density. The corresponding Tafel equation, as in the following equation,
where Tafel constant, a; the Tafel slope, b; the current density, j; overpotential, η.
The intense electrocatalytic activity presented by the small semicircle generally shows fast electron transfer at the catalyst electrolyte interface. As illustrated in
Furthermore, no significant changes in the structure and composition were observed. This proves that Vx—MnCo(x=0.3) MCs electrocatalysts are highly stable. These findings demonstrate that Vx—MnCo(x=0.3) MCs is a high-performance HER electrocatalyst suitable for future hydrogen fuel cell technology. The overpotential comparison of electrocatalytic HER performance of Vx—MnCo(x=0.3) MCs with other HER catalysts reported in the literature is listed in Table 3.
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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 invention may be practiced otherwise than as specifically described herein.
