_xCo_2-xO₄particles 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 MnV_xCo_2-xO₄particles.

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 MnV_xCo_2-xO₄particles, 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 MnV_xCo_2-xO₄particles. 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 MnV_xCo_2-xO₄particles. 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 MnV_xCo_2-xO₄particles. 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 MnV_xCo_2-xO₄particles.

The MnV_xCo_2-xO₄particles 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 10⁻⁶ohms meter (Ω·m), preferably at most 10⁻⁷Ω·m, more preferably at most 10⁻⁸Ω·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 MnV_xCo_2-xO₄particles, 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 MnV_xCo_2-xO₄particles, 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 MnV_xCo_2-xO₄particles, 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 MnV_xCo_2-xO₄particles, 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.

FIG. 1 illustrates a flow chart of a method 50 of preparing an electrocatalyst. 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.

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, CoI_e, 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)₂includes both Co(NO₃)₂and 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)₂includes both Mn(NO₃)₂and 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 MnV_xCo_2-xO₄particles. The MnV_xCo_2-xO₄particles 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 MnV_xCo_2-xO₄particles. 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 MnV_xCo_2-xO₄particles may be washed further to remove unreacted reactants/impurities and dried in an oven.

At step 56, the method 50 includes coating the MnV_xCo_2-xO₄particles on the substrate. The substrate may be coated by any conventional techniques known in the art, for example, drop-casting, dipping, spraying, coating, etc.

Examples

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.

Example 1: Synthesis of Mn Spinel Oxide Microspheres (V_x—MnCo MCs) and Electrode Preparation

A one-step hydrothermal process is used to manufacture the MnV_xCo_2-xO₄microspheres, labeled as V_x—MnCo(x≤0.4) MCs. Different amounts of the vanadium is doped into the MnV_xCo_2-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).

Example 2: Electrochemical Characterization Techniques

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 V_x—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 (E_RHE), and E_RHEwas used to calibrate all the potentials for hydrogen evolution reaction (HER) measurements using the following equation,

E
RHE

=

E

app
⁢
1

+

0.059
pH

+

E

Ag
/
AgCl

o

where E°_Ag/AgClis 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 V_x—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).

Example 3: Structural Characterization

The phase purity analysis of V_x—MnCo MCs was investigated through powdered XRD analysis and refined by Match! Software, as depicted in FIG. 2. The XRD pattern disclosed the prominent dominant peaks of cubic spinel oxide structure with relatively low-intensity peaks associated with the trace of MnO₂. All diffraction peaks in FIG. 2 matched well with the standard international centre for diffraction data of MnCo₂O₄. Rietveld analysis of experimental XRD pattern data revealed the lattice parameters, cell volume, reliability factors, and crystallite size, is shown in Table 1. The lattice parameters increased with increasing the ‘x’ due to the expansion in the lattice due to a mismatch in ionic radii. The crystallite sizes were calculated between 16.0 nm and 22.4 nm, implementing Scherrer's formula for the most intense peak.

TABLE 1

Refined cell parameters of V_x—MnCo (x ≤ 0.4) MCs.

x
a = b = c (Å)
V (Å3)
D_XRD(nm) ± 0.10
χ²(chi²)
R_Bragg

0.0
8.1041
532.25
19.3
1.2
21.1

0.1
8.1051
532.43
21.1
0.8
27.1

0.2
8.1183
535.05
22.4
0.5
29.5

0.3
8.1192
535.22
17.2
1.3
30.3

0.4
8.1278
536.92
16.0
1.3
29.3

The surface morphology and shape of V_x—MnCo MCs were investigated by scanning electron microscopy (SEM) analysis with different magnifications, as depicted in FIGS. 3A-3E. The images exhibited the different sizes of spherical particles. The high magnification images revealed that the spherical particles comprised highly agglomerated small cubic particles, giving rise to the spherical particles with a rough surface. To further confirm the morphology and structure of V_x—MnCo MCs (x=0.3), TEM and HR-TEM were employed, as can be seen in FIG. 4A and FIG. 4B, respectively. The assembling of spherical particles can be observed in FIG. 4A, and HR-TEM approved the spinel phase of the V_x—MnCo MCs (x=0.3) sample (FIG. 4B). Following the XRD results, the well-defined D-spacing of 0.28 and 0.18 nm are assigned to the (220) and (331) facets of V_xMnCo MCs. The presence of Co, Mn, V, and O elements in V_xMnCo(x=0.2 and 0.4) MCs was confirmed by the EDX analysis, as illustrated in FIG. 4A and FIG. 4B. The V content was increased, and Mn and Co concentration was suppressed in the EDX spectra from x=0.2 (FIG. 5A) and x=0.4 (FIG. 5B), indicating that V was successfully doped into MnCo MCs and is consistent with XRD and TEM analysis.

The chemical composition and oxidation state of V_x—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 FIG. 6A. In addition, the deconvoluted characteristic peaks of each element are depicted in FIGS. 6B-6F, respectively. The core level spectrum of the C 1s state was observed at 284.8 electron volts (eV), further deconvoluted into three characteristic peaks, as depicted in FIG. 6B. The peaks observed at binding energies of 284.8 eV, 286.3 eV, and 288.3 eV are assigned to C—C, C—O—C, and O—C═O bonds, respectively. FIG. 6C shows the core level peaks of Co at a binding energy of 779.7 eV and 794.7 eV, which correspond to Co 2p_3/2and Co 2p_1/2, respectively. The deconvolution of the core peak of Co 2p_3/2introduced two characteristic peaks at 779.5 eV and 780.9 eV, corresponding to Co²⁺ and Co³⁺ states, respectively. Similarly, the Co 2p_1/2spectrum was deconvoluted into two peaks at 794.5 eV and 795.9 eV, indicating the presence of Co atoms at two different geometrical positions in the spinal structure. In addition, two satellite peaks of Co 2p_3/2and Co 2p_1/2spectra were also observed at 789.0 eV and 804.0 eV, respectively. Mn 2p_3/2and Mn 2p_1/2core spectra were observed at 641.6 eV and 653.2 eV, respectively, as depicted in FIG. 6D. Due to deconvolution, the core spectra of Mn were split into characteristic peaks associated with Mn²⁺ and Mn³⁺ states, respectively. The spin-orbit doublet of core level spectra of V 2p_3/2and V 2p_1/2were detected at 516.5 eV and 524.1 eV, respectively, as depicted in FIG. 6E. Furthermore, the deconvolution of the V 2p core spectrum introduces two peaks in each case, indicating the presence of V³⁺ and V⁵⁺ ionic states. FIG. 6F depicts the deconvoluted spectrum of O 1s species exhibiting characteristic peaks at 529.7 eV and 531.1 eV, respectively. The high-intensity peak at 529.7 eV represents the metal oxide bonding, whereas the low-intensity broad peak centered around 531.2 eV is associated with the adsorbed hydroxyl groups and unreacted carbonates.

FIG. 7A and FIG. 7B depicts the variations of magnetization against the applied magnetic field (M-H) for different V_x—MnCo(x≤0.4) MCs registered under H=±70 kOe and at temperatures of 300 Kelvin (K) and 10 K, respectively. M-H results measured at room temperature showed almost linear curves for all samples, indicating their paramagnetic behavior at 300 K. An apparent hysteresis loop-like feature may be observed in the M-H curves at low magnetic fields measured at 10 K, as pointed out by the enlarged view in the inset of FIG. 7B. The opened hysteresis loops at 10 K for all prepared samples reflect the transformation to a ferrimagnetic state at lower temperatures. In spinel oxides, three main magnetic interactions exist among the tetrahedral (A) and octahedral (B) sites via the intermediate O²⁻ ions through super-exchange interactions. For instance, the weak super-exchange interactions Co²⁺—O²⁻—Co³⁺—O²—Co²⁺ maintained an anti-ferromagnetism effect in Co₃O₄composition. Substituting some Co³⁺ ions with Mn³⁺ ions to form a MnCo₂O₄sample may affect the magnetic exchange interactions. Indeed, the inclusion of Mn³⁺ ions having a larger size than Co³⁺ ions may distort the oxygen octahedron and hence frustrate the tetrahedral site, provoking an insufficient magnetic ordering in the MnCo₂O₄(x=0.0 in this case) MCs. Accordingly, the manifestation of ferrimagnetism in MnCo₂O₄MCs at lower temperatures may be ascribed to the canting effect of antiferromagnetically ordered spins via structural distortions and breakdown of the equilibrium among the Co²⁺ sublattices' antiparallel magnetization caused by the inclusion of Mn³⁺ ions.

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 (H_c), remanent magnetization (M_r), and magnetization M_maxachieved at 70 kOe is presented in FIGS. 8A-8C, respectively. The room temperature (300 K) magnetization M_maxachieved at 70 kOe was also presented in FIG. 8C. There is a continuous increase in the magnetization M_maxwith increasing vanadium content in comparison to the parent sample at both 300 and 10 K. For different prepared compositions, M_max(at 70 kOe) increased as the temperature decreased from 300 K down to 10 K. For example, M_max(at 70 kOe) is equal to 2.0 emu/g and 3.2 emu/g at 300 K for samples with x=0.0 and 0.4, respectively, and increased to about 11.5 and 20.0 emu/g at 10 K. As the temperature decreases, M_max(at 70 kOe) increases since the thermal fluctuations contributing to demagnetization with be lowered at lower temperatures. Hence, M_max(at 70 kOe) and M_rand H_crise. In other words, the thermal agitation may decrease as the temperature decreases. Hence, the easy magnetization rotation may be more dominant than the thermal agitations, increasing global magnetization magnitude. As the composition content (x) rises, it is found that M_max(at 70 kOe) and M_rvalues increase simultaneously at low temperatures (T=10 K). For example, M_max(at ±70 kOe) is about 2.3 emu/g at 300 K and 11.5 emu/g at 10 K for the non-doped sample (x=0.0) and attained about 3.2 emu/g at 300 K and 20.0 emu/g at 10 K for the product with x=0.4. M_rmeasured at 10 K is about 0.8 emu/g for the non-doped sample (x=0.0) and attained about 1.58 emu/g for the product with x=0.4. For the intermediate compositions, M, values are about 13.1 emu/g, 14.9, and 17.3 emu/g for samples with x=0.1, 0.2, and 0.3, as shown in FIG. 8B, respectively.

The magnetization values continuously increase with vanadium substitution. However, the variation in the H_cvalue showed an anomaly at x>0.2. The H_cincreases first with increasing vanadium content (x) up to 0.2 but then drops with additional rising vanadium content (x>0.2), as shown in FIG. 8A. The coercivity H_cdramatically depends on the crystallite size. According to XRD results, it is found that the crystallite size increases with increasing x content up to 0.2 but falls with further increasing x content. At 10 K, H_cis about 1212.1 Oe for the non-doped sample (x=0.0), which progressively increases up to 1371.6 and 1390.3 Oe for the products with x=0.1 and 0.2, respectively, and starts to slightly decrease with the further increase in vanadium content (H_c=1230.2 Oe for x=0.3 and H_c=1214.5 Oe for x=0.4). The registered H_cvalues are non-negligible, indicating the complex magnetic features of the present samples at low temperatures. The variations in M_max(at 70 kOe) is due to the distribution of cations in different sites and, hence, the variation in the net magnetic moment of the whole system.

For non-substituted MnCo₂O₄MCs, 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)=4.9μ_B. 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,

μ
⁡
(
B
)

=

(

μ
Co

2
+

)

2

+

(

μ

Mn

2
+

)

2

=

(
3.87
)

2

+

(
4.9
)

2

=

6.24

μ
B

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[Co_2-x²⁺V_x²⁺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⁺)=3.87μ_B,μ(V²⁺)=1.73μ_B, and μ(Mn²⁺)=4.9μ_B. 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.

FIG. 9 depicts the temperature (10 K≤T≤325 K) dependence of magnetization (M-T) for different V_x—MnCo(x≤0.4) MCs carried out under FC and ZFC conditions. A dc-magnetic field of about H_app=100 Oe was applied. M-T curves measured under ZFC and FC conditions were noted as M_ZFC(T) and M_FC(T) curves, respectively. The general shape of M_ZFC(T) curves is practically comparable for different products. The marked transformation in the slope of both M_ZFC(T) and M_FC(T) plots indicates the ferrimagnetic Curie temperature T_c. These magnetic transitions at T_cemphasize the ferrimagnetic behavior of the prepared products. For V_x—MnCo(x≤0.4) MCs, T_cvalues are lesser than those registered for bulk MnCo₂O₄MCs (Tc˜185 K), which is mainly attributed to the effects of finite size. The V_x—MnCo(x≤0.4) MCs display a distinctive separation between the M_ZFC(T) and M_FC(T) curves at a specific temperature noted as T_SP<T_C, as can be seen from FIG. 9. The M_FC(T) plot declines progressively with the rising temperature and merges more than the M_ZFC(T) plot. The thermal irreversibility of magnetization is usually detected under Curie temperature. This is because of high magneto crystalline anisotropy in prepared samples.

Furthermore, all products showed maximum magnetizations peak at a temperature noted as T_P(lower than the Curie temperature) in the M_ZFC(T) curves, and afterward, it falls to shallow values. The appearance of a peak around T_Pis ascribed to either the spin-glass state or the blocking phenomenon caused by the finite sizes and surficial effects in V_x—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 M_ZFC(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 T_C, the product is demagnetized; hence, the magnetization drops to almost zero at T_C. On the other hand, the thermal agitation tends to increase as the temperature increases, leading to a reduction in magnetization. Nevertheless, at T<T_C, 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 M_ZFC(T) curves below T_C. Furthermore, it was evident that T_Cshifts 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 T_SPand T_Pas a function of vanadium content are shown in FIG. 10. It is noticed that T_Pand T_SPshift gradually towards higher temperatures with increasing “x”. For instance, T_Pis about 27.9 K for non-doped MCs, which has enhanced steadily from 28.7, 29.3, 29.7, to 30.0 K for x=0.1, 0.2, 0.3, and 0.4 compositions, respectively.

Example 4: Electrochemical Performance

The electrochemical performance of bare and various concentrations of V_x—MnCo(x≤0.4) MCs towards HER was assessed in 0.5 M H₂SO₄electrolyte with a three-electrode configuration. The LSV measurements were carried out to investigate the electrocatalytic activity of representative electrodes. FIG. 11A depicts the LSV acquired by scanning the samples within 0 to −0.5 V versus RHE at a 10 mV/s scan rate. V_x—MnCo(x=0.3) MCs electrocatalyst has the lowest onset potential (−78.9 mV), making it the most efficient catalyst among V_x—MnCo MCs. The designated V_x—MnCo(x=0.3) MCs exhibit remarkable HER activity in the overpotential of merely 85.9 mV to offer 10 mA cm⁻²current density among other different compositions of V_x—MnCo MCs. These results revealed that V_x—MnCo(x=0.3) MCs performed very well as an HER electrocatalyst, which may be attributed to the deprotonation of water molecules liberating a significant number of H species the V_x—MnCo MCs readily absorb. Further, it establishes chemical connections with nearby electrons or adsorbed hydrogen (H_ads), and both processes coincide, forming hydrogen gas (H₂) molecules. The remaining samples exhibited relatively low performance, possibly due to the fewer available active sites for the adsorption of hydrogen species and reduced electrochemical surface area. V doping achieves enhanced HER because adding V increases the electrochemical surface area and results in many defective sites, which are advantageous for the rapid transfer of charges. FIG. 11B illustrates a comparison plot of normalized overpotential values at 10 mA/cm²for several samples. The lowest overpotential value of 85.9 mV exhibited by the V_x—MnCo(x=0.3) MCs electrocatalyst demonstrates the effectiveness of the composite in catalysing HER, as illustrated in Table 2.

TABLE 2

HER parameters for different catalysts of

V_x—MnCo (x ≤ 0.4) MCs electrodes.

Onset
Overpotential
Tafel

potential
(mV) at −10
slope
DLC
ESCA

x
(mV)
mA/cm²
(mV/dec)
(μF/cm²)
(cm²)

0.0
141.5
268.0
131
192.5
5.5

0.1
129.5
211.4
115
250.7
7.2

0.2
114.6
169.7
103
291.5
8.3

0.3
78.9
85.9
84
397.8
11.4

0.4
104.6
109.0
96
346.5
9.9

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 V_x—MnCo(x=0.3) MCs, the ECSA was measured for bare and V_x—MnCo MCs electrodes. The following equation was utilized to calculate the ECSA.

ECSA
+

Cdl
/
Cs

Cdl values were derived using the non-Faradaic potential region in CV curves, and Cs were assumed to be 35 μF cm⁻². FIG. 11C depicts the CV curves obtained for the V_x—MnCo(x=0.3) MCs electrocatalyst at scan speeds ranging from 10 to 120 mV/s in the potential range of −0.25 V to −0.35 V versus RHE. FIG. 11D illustrates the linear fit achieved by fitting the current density at a potential of −0.30 V versus RHE against several scan rates. The slope of these straight lines (FIG. 11D) was used to determine the value of Cdl. The ESCA and Cdl values for all the prepared samples of V_x—MnCo MCs have been displayed in Table 2. The electrocatalyst V_x—MnCo(x=0.3) MCs exhibited outstanding performance with an ECSA value of 11.4 cm². To further examine the catalysts' number of active sites (N), the following equation was utilized to calculate:

N
=

Qs
/
F

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 V_x—MnCo(x≤0.4) MCs were 1.31×10⁻⁷mol cm⁻², 1.35×10