HIGH-ENTROPY COMPOSITE GLYCERATE, METHODS FOR PREPARING THEREOF AND ELECTROCATALYSTS COMPRISING THE SAME

Abstract

Provided is a high-entropy composite glycerate represented by NiCrFeCoMn(C.sub.3H.sub.5O.sub.4).sub.n and an electrocatalyst thereof, wherein n is a positive integer from 1 to 3, and wherein each of the Ni, Cr, Fe, Co and Mn includes an atom percent of 5 to 35 based on the total amount of the Ni, Cr, Fe, Co and Mn. Each of the metals is homogenously distributed within the high-entropy composite glycerate, and the high-entropy composite glycerate can reduce an overpotential for oxygen evolution reaction by the synergistic effect resulting from the structure formed by the quinary-metal glycerate. The high-entropy composite glycerate is suitable for catalyzing oxygen evolution reaction, and therefore has a prospect for application. Methods for preparing the high-entropy composite glycerate are also provided.

Claims

1. A high-entropy composite glycerate represented by Formula (I) below:
NiCrFeCoMn(C.sub.3H.sub.5O.sub.4).sub.n   (I), wherein n is a positive integer from 1 to 3, and wherein each of the Ni, Cr, Fe, Co, and Mn has an atom percent between 5 and 35 based on a total amount of the Ni, Cr, Fe, Co, and Mn.

2. The high-entropy composite glycerate according to claim 1, wherein each of the Ni, Cr, Fe, Co, and Mn forms a metal glycerate with the glycerate anions, and the metal glycerate has layered structures.

3. The high-entropy composite glycerate according to claim 2, wherein the layered structures formed from the Ni, Cr, Fe, Co, and Mn with the glycerate anions are stacked upon each other and allow the high-entropy composite glycerate to have a particle structure.

4. The high-entropy composite glycerate according to claim 1, wherein each of the Ni, Cr, Fe, Co, and Mn is homogenously distributed within the high-entropy composite glycerate.

5. A method for preparing the high-entropy composite glycerate according to claim 1, comprising: performing a solvothermal reaction of a reaction solution comprising a precursor dissolved therein and glycerol, wherein the precursor comprises a Ni.sup.2+ metal salt, a Co.sup.2+ metal salt, a Cr.sup.3+ metal salt, a Mn.sup.2+ metal salt, and a Fe.sup.3+ metal salt; and separating the high-entropy composite glycerate from the reaction solution after the solvothermal reaction.

6. The method according to claim 5, wherein the reaction solution is prepared by dissolving the precursor in isopropanol and adding the glycerol into the isopropanol dissolved with the precursor, wherein a weight ratio of the glycerol to the isopropanol is from 1:2 to 1:5.

7. The method according to claim 5, wherein the solvothermal reaction is performed at a temperature of 140° C. to 200° C. for 6 to 16 hours.

8. The method according to claim 5, wherein a metal salt of each of the Ni.sup.2+ metal salt, the Co.sup.2+ metal salt, the Cr.sup.3+ metal salt, the Mn.sup.2+ metal salt, and the Fe.sup.3+ metal salt is independently selected from the group consisting of a metal nitrate, a metal halide, a metal acetate, and a metal sulfate, and the metal salts in the reaction solution have the same mole number.

9. The method according to claim 5, wherein a molar ratio of the precursor to the glycerol is from 1:10 to 1:20, and a molar ratio of each of the Ni.sup.2+ metal salt, the Co.sup.2+ metal salt, the Cr.sup.3+ metal salt, the Mn.sup.2+ metal salt, and the Fe.sup.3+ metal salt to the glycerol is from 0.01 to 1.

10. An electrocatalyst for electrolyzing water, comprising the high-entropy composite glycerate according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present disclosure can be more understood by reading the following descriptions of the embodiments, with reference made to one or more of the accompanying drawings below.

[0023] FIG. 1A illustrates the surface morphology of the high-entropy composite glycerate of an embodiment of the present disclosure examined by scanning electron microscopy (SEM).

[0024] FIG. 1B illustrates the surface morphology of the high-entropy composite glycerate of an embodiment of the present disclosure examined by transmission electron microscopy (TEM).

[0025] FIG. 2 illustrates the element mappings of the high-entropy composite glycerate of an embodiment of the present disclosure examined by dark field scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDS).

[0026] FIG. 3A illustrates the X-ray diffraction (XRD) patterns of the high-entropy composite glycerate of an embodiment of the present disclosure and the metal glycerates of comparative preparation examples.

[0027] FIG. 3B illustrates the XRD pattern of the high-entropy composite glycerate of an embodiment of the present disclosure.

[0028] FIGS. 4A to 4G illustrate the spectrograms of X-ray photoelectron spectroscopy (XPS) of the high-entropy composite glycerate of an embodiment of the present disclosure.

[0029] FIG. 5 illustrates the spectrogram of Fourier transform infrared spectrometer (FTIR) of the high-entropy composite glycerate of an embodiment of the present disclosure.

[0030] FIGS. 6A and 6B illustrate the linear sweep voltammetry (LSV) plots of electrodes prepared by the high-entropy composite glycerate of an embodiment of the present disclosure and the metal glycerates of comparative preparation examples.

[0031] FIGS. 7A and 7B illustrate the Tafel plots of electrodes prepared by the high-entropy composite glycerate of an embodiment of the present disclosure and the metal glycerates of comparative preparation examples.

[0032] FIGS. 8A and 8B illustrate the bar graph of an overpotential (η.sub.100) of electrodes prepared by the high-entropy composite glycerate of an embodiment of the present disclosure and the metal glycerates of comparative preparation examples at a current density of 100 mA/cm.sup.2.

[0033] FIG. 9A illustrates the surface morphology of the high-entropy composite glycerate of an embodiment of the present disclosure after oxygen evolution reaction, which is examined by scanning electron microscopy (SEM).

[0034] FIGS. 9B and 9C illustrate the surface morphology of the high-entropy composite glycerate of an embodiment of the present disclosure after oxygen evolution reaction, which is examined by transmission electron microscopy (TEM).

[0035] FIG. 10 illustrates the image of the high-entropy composite glycerate of an embodiment of the present disclosure examined by high-angle annular dark-field (HAADF) in combination with EDS.

[0036] FIGS. 11A to 11F illustrate the spectrograms of X-ray photoelectron spectroscopy (XPS) of the high-entropy composite glycerate of an embodiment of the present disclosure after oxygen evolution reaction.

[0037] FIG. 12 illustrates the bar graph of the ratio of O—H/O-M and the added higher oxidation-state fractions (HOFs) of the high-entropy composite glycerate of an embodiment of the present disclosure and the metal glycerates of comparative preparation examples.

[0038] FIG. 13 illustrates the ratio of O—H/O-M of the high-entropy composite glycerate of an embodiment of the present disclosure and the metal glycerates of comparative preparation examples and an overpotential (limo) at a current density of 100 mA/cm.sup.2 of the electrodes prepared thereby.

DETAILED DESCRIPTION

[0039] In the following descriptions of the embodiments of the present disclosure, reference is made to the accompanying drawings, which are shown to illustrate the embodiments in which the present disclosure may be practiced. These embodiments are provided to enable those skilled in the art to practice the present disclosure. It is understood that other embodiments may be used and that changes can be made to the embodiments without departing from the scope of the present disclosure. The following descriptions are therefore not to be considered as limiting the scope of the present disclosure.

[0040] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

[0041] Titles or subtitles may be used in this disclosure for the convenience of a reader, which shall have no influence on the scope of the present disclosure.

[0042] Unless otherwise defined herein, scientific and technical terminologies employed in the present disclosure have the meanings that are commonly understood and used by one of ordinary skill in the art. All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0043] As used herein, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements).

[0044] The present disclosure provides a high-entropy composite glycerate, which is represented by Formula (I) below:

NiCrFeCoMn(C.sub.3H.sub.5O.sub.4).sub.n   (I),

wherein n is a positive integer from 1 to 3, such as 1, 2, and 3. In some embodiments of the present disclosure, each of the Ni, Cr, Fe, Co, and Mn has an atom percent between 5 and 35, such as 5, 10, 15, 16, 19, 20, 22, 23, 25, 30, and 35, based on the total amount of the Ni, Cr, Fe, Co, and Mn.

[0045] For example, each of the Ni, Cr, Fe, Co, and Mn of the high-entropy composite glycerate is combined with a glycerate anion to form a metal glycerate. In some embodiments of the present disclosure, the metal glycerates formed from each of the Ni, Cr, Fe, Co, and Mn with the glycerate anions are represented by chemical formulae of (NiC.sub.6H.sub.12O.sub.8), (CrC.sub.9H.sub.18O.sub.12), (FeC.sub.9H.sub.18O.sub.12), (CoC.sub.6H.sub.12O.sub.8), and (MnC.sub.6H.sub.12O.sub.8), respectively.

[0046] In at least one embodiment of the present disclosure, the high-entropy composite glycerate comprises quinary-metal of Ni, Cr, Fe, Co, and Mn. Each of Ni, Cr, Fe, Co, and Mn with a glycerate anion forms a layered structure. That is to say, (NiC.sub.6H.sub.12O.sub.8), (CrC.sub.9H.sub.18O.sub.12), (FeC.sub.9H.sub.18O.sub.12), (CoC.sub.6H.sub.12O.sub.8), and (MnC.sub.6H.sub.12O.sub.8) formed from each of the metals with the glycerate anions are layered structures. For example, the metal glycerate may have a layered structure that is composed of stacked metal-oxygen sheets separated by glycerate anions. The layered structure is similar to anion-intercalated hydroxides, which provides interlayer spacing for the accommodation of reactants, and therefore, the open layered structure allows rapid transport of the reactants to the material and also gives increased catalytic active sites. Accordingly, the high-entropy composite glycerate of the present disclosure is composed of multiple elements, has high-entropy stabilization effect, and thus has an excellent performance of electrochemical OER.

[0047] In at least one embodiment of the present disclosure, the layered structures of the high-entropy composite glycerate are stacked upon each other, and allow the high-entropy composite glycerate to form a particle structure. In at least one embodiment of the present disclosure, the high-entropy composite glycerate particles can be prepared to have various particle sizes according to actual demand. In addition, the high-entropy composite glycerate particles having various particle sizes may exhibit good homogeneity.

[0048] In at least one embodiment of the present disclosure, each of the Ni, Cr, Fe, Co, and Mn of the high-entropy glycerate is homogenously distributed within the high-entropy composite glycerate.

[0049] The present disclosure further provides a method for preparing the aforementioned high-entropy composite glycerate, comprising preparing a reaction solution, performing a solvothermal reaction of the reaction solution, and separating the high-entropy composite glycerate from the reaction solution after the solvothermal reaction.

[0050] In at least one embodiment of the present disclosure, the reaction solution comprises a precursor dissolved therein and glycerol, and the precursor comprises a Ni.sup.2+ metal salt, a Co.sup.2+ metal salt, a Cr.sup.3+ metal salt, a Mn.sup.2+ metal salt, and a Fe.sup.3+ metal salt.

[0051] In at least one embodiment of the present disclosure, the preparation of the reaction solution comprises dissolving the precursor in the isopropanol first, and adding glycerol into the isopropanol dissolved with the precursor.

[0052] In at least one embodiment of the present disclosure, the glycerol to the isopropanol has a weight ratio from 1:2 to 1:5, such as 1:2, 1:3, 1:3.5, 1:4, and 1:5. In some embodiments of the present disclosure, the weight ratio of the glycerol to the isopropanol is 1:3.49.

[0053] In at least one embodiment of the present disclosure, the preparation of the reaction solution further comprises stirring the reaction solution to completely dissolve the precursor therein, thereby forming a homogeneous phase.

[0054] The term “solvothermal reaction” as used herein refers to a method for preparing a martial, in which precursors in a reaction solution is subjected to react with a solvent other than water under a given temperature and a pressure produced by the reaction solution in a hermetic container. In at least one embodiment of the present disclosure, the solvothermal reaction of the reaction solution is performed at a temperature of 140° C. to 200° C. For example, the solvothermal reaction for the reaction solution is performed at, but not limited to, a temperature of 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., and 200° C. In some embodiments of the present disclosure, the solvothermal reaction of the reaction solution is performed for a time period of 6 to 16 hours. For example, the solvothermal reaction for the reaction solution is performed for, but not limited to, a time period of 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and 16 hours. In yet embodiment of the present disclosure, the solvothermal reaction of the reaction solution is performed at a temperature of 150° C. for 10 hours.

[0055] In at least one embodiment of the present disclosure, the metal salts are selected from the group consisting of metal nitrates, metal halides, metal acetates, and metal sulfates, and the metal salts are present in the reaction solution in the same mole number.

[0056] In at least one embodiment of the present disclosure, a molar ratio of the precursor to the glycerol is from 1:10 to 1:20, such as 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, and 1:20, but is not limited thereto.

[0057] In at least one embodiment of the present disclosure, a molar ratio of each of the Ni.sup.2+ metal salt, the Co.sup.2+ metal salt, the Cr.sup.3+ metal salt, the Mn.sup.2+ metal salt, and the Fe.sup.3+ metal salt to the glycerol is from 0.01 to 1, such as 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.082, 0.09, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 1.00, but is not limited thereto.

[0058] Additionally, the present disclosure also provides an electrocatalyst for electrolyzing water, comprising the high-entropy composite glycerate of at least one embodiment of the present disclosure. In other words, the present disclosure provides a use of the aforementioned high-entropy composite glycerate as a catalyst in electrolyzing water, or in catalyzing the electrolyzation of water. For example, the present disclosure provides a use of the aforementioned high-entropy composite glycerate in catalyzing OER and/or HER.

[0059] Without further elaboration, it is believed that one skilled in the art can utilize the present disclosure to its fullest extent. The following embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

PREPARATION EXAMPLE 1

Preparation of the High-Entropy Composite Glycerate

[0060] Firstly, a precursor was dissolved in 34 mL of isopropanol (Macron, a purity of 99.5%) under continuously stirring, and the precursor included cobalt(II) nitrate hexahydrate (Co(NO.sub.3).sub.2.6H.sub.2O, J.T. Baker, a purity of 99%), chromium (III) nitrate nonahydrate (Cr(NO.sub.3).sub.3.9H.sub.2O, Alfa Aesar, a purity of 98.5%), iron (III) nitrate nonahydrate (Fe(NO.sub.3).sub.3.9H.sub.2O, J.T. Baker, a purity of 99%), manganese (II) nitrate hexahydrate (Mn(NO.sub.3).sub.2.6H.sub.2O, Alfa Aesar, a purity of 98.5%), and nickel (II) nitrate hexahydrate (Ni(NO.sub.3).sub.2.6H.sub.2O, Alfa Aesar, a purity of 98.5%). Each of the cobalt (II) nitrate hexahydrate, the chromium (III) nitrate nonahydrate, the iron (III) nitrate nonahydrate, the manganese (II) nitrate hexahydrate, and nickel (II) nitrate hexahydrate had a mole number of 0.5 mmole. Subsequently, 6 mL of glycerol (Macron, a purity of 99.5%) was added into the isopropanol dissolved with the precursor, and a reaction solution with a homogeneous phase was generated by continuously stirring the isopropanol added with the glycerol. The color of the reaction solution was dark brown.

[0061] Next, the reaction solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave, and heated at a temperature of 150° C. for 10 hours to perform a solvothermal reaction. After the solvothermal reaction, the autoclave was placed at a room temperature for cooling down to the room temperature naturally. Further, the precipitate was separated from the reaction solution through centrifugation, washed with absolute ethanol for several times, and dried at 60° C. for 24 hours in a vacuum oven, so as to obtain the high-entropy composite glycerate, which was referred as FeNiCoCrMn-G herein.

COMPARATIVE PREPARATION EXAMPLE 2

Preparation of a Binary-Metal Glycerate

[0062] The preparation of the binary-metal glycerate was the same as the process described in preparation example 1, except that the precursor included Fe(NO.sub.3).sub.3.9H.sub.2O and Ni(NO.sub.3).sub.2.6H.sub.2O, and each of Fe(NO.sub.3).sub.3.9H.sub.2O and Ni(NO.sub.3).sub.2.6H.sub.2O had a mole number of 0.5 mmole. The obtained product was FeNi-binary-metal glycerate, which was referred as FeNi-G herein.

COMPARATIVE PREPARATION EXAMPLE 3

Preparation of a Ternary-Metal Glycerate

[0063] The preparation of the ternary-metal glycerate was the same as the process described in preparation example 1, except that the precursor included Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, and Co(NO.sub.3).sub.2.6H.sub.2O, and each of Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, and Co(NO.sub.3).sub.2.6H.sub.2O had a mole number of 0.5 mmole. The obtained product was FeNiCo-ternary-metal glycerate, which was referred as FeNiCo-G herein.

COMPARATIVE PREPARATION EXAMPLE 4

Preparation of Quaternary-Metal Glycerate

[0064] The preparation of the quaternary-metal glycerate was the same as the process described in preparation example 1, except that the precursor included Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O, and Mn(NO.sub.3).sub.2.6H.sub.2O, and each of Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O and Mn(NO.sub.3).sub.2.6H.sub.2O had a mole number of 0.5 mmole. The obtained product was FeNiCoMn-quaternary-metal glycerate, which was referred as FeNiCoMn-G herein.

COMPARATIVE PREPARATION EXAMPLE 5

Preparation of Quaternary-Metal Glycerate

[0065] The preparation of the quaternary-metal glycerate was the same as the process described in preparation example 1, except that the precursor included Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O, and Cr(NO.sub.3).sub.3.9H.sub.2O, and each of Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O, and Cr(NO.sub.3).sub.3.9H.sub.2O had a mole number of 0.5 mmole. The obtained product was FeNiCoCr-quaternary-metal glycerate, which was referred as FeNiCoCr-G herein.

COMPARATIVE PREPARATION EXAMPLE 6

Preparation of Quaternary-Metal Glycerate

[0066] The preparation of the quaternary-metal glycerate was the same as the process described in preparation example 1, except that the precursor included Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, Cr(NO.sub.3).sub.3.9H.sub.2O, and Mn(NO.sub.3).sub.2.6H.sub.2O, and each of Fe(NO.sub.3).sub.3.9H.sub.2O, Ni(NO.sub.3).sub.2.6H.sub.2O, Cr(NO.sub.3).sub.3.9H.sub.2O, and Mn(NO.sub.3).sub.2.6H.sub.2O had a mole number of 0.5 mmole. The obtained product was FeNiCrMn-quaternary-metal glycerate, which was referred as FeNiCrMn-G herein.

COMPARATIVE PREPARATION EXAMPLE 7

Preparation of Quaternary-Metal Glycerate

[0067] The preparation of the quaternary-metal glycerate was the same as the process described in preparation example 1, except that the precursor included Fe(NO.sub.3).sub.3.9H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O, Cr(NO.sub.3).sub.3.9H.sub.2O, and Mn(NO.sub.3).sub.2.6H.sub.2O, and each of Fe(NO.sub.3).sub.3.9H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O, Cr(NO.sub.3).sub.3.9H.sub.2O, and Mn(NO.sub.3).sub.2.6H.sub.2O had a mole number of 0.5 mmole. The obtained product was FeCoCrMn-quaternary-metal glycerate, which was referred as FeCoCrMn-G herein.

COMPARATIVE PREPARATION EXAMPLE 8

Preparation of Quaternary-Metal Glycerate

[0068] The preparation of the quaternary-metal glycerate was the same as the process described in preparation example 1, except that the precursor included Ni(NO.sub.3).sub.2.6H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O, Cr(NO.sub.3).sub.3.9H.sub.2O, and Mn(NO.sub.3).sub.2.6H.sub.2O, and each of Ni(NO.sub.3).sub.2.6H.sub.2O, Co(NO.sub.3).sub.2.6H.sub.2O, Cr(NO.sub.3).sub.3.9H.sub.2O, and Mn(NO.sub.3).sub.2.6H.sub.2O had a mole number of 0.5 mmole. The obtained product was NiCoCrMn-quaternary-metal glycerate, which was referred as NiCoCrMn-G herein.

[0069] The high-entropy composite glycerate prepared from preparation example 1 and the metal glycerates prepared from comparative preparation examples 2-8 were analyzed as follows.

(1) Analysis of the Element Concentrations

[0070] The chemical compositions of the high-entropy composite glycerate and the binary-metal glycerates, ternary-metal glycerates and quanternary-metal glycerates were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS, Thermo-Element XR), and the results were shown in Table 1.

TABLE-US-00001 TABLE 1 The concentrations of the metal elements in preparation example 1 and comparative preparation examples 2-8 Fe Ni Co Cr Mn (atom (atom (atom (atom (atom Sample ID %) %) %) %) %) Preparation example 1 22.0 19.4 19.1 23.0 16.5 Comparative preparation 51.3 48.7 0 0 0 example 2 Comparative preparation 34.5 32.9 32.6 0 0 example 3 Comparative preparation 27.1 25.6 26.0 0 21.3 example 4 Comparative preparation 25.7 23.9 23.5 26.9 0 example 5 Comparative preparation 26.8 24 0 28.6 20.6 example 6 Comparative preparation 26.9 0 24.4 28.2 20.5 example 7 Comparative preparation 0 26.1 25.2 28.4 20.3 example 8

[0071] As shown in Table 1, the concentrations of each metal element were approximately equivalent to the ratio of the added precursors in preparation example 1 and comparative preparation examples 2-8.

(2) Surface Morphology

[0072] As shown in FIGS. 1A and 1B, the morphology of the high-entropy composite glycerate of preparation example 1 was examined by scanning electron microscopy (SEM, JEOL 6701F) and transmission electron microscopy (TEM, JEOL 2100F). It thus can be seen that the high-entropy composite glycerates of the present disclosure were microspheres with uniform sizes.

(3) Element Distribution

[0073] As shown in FIG. 2, the high-entropy composite glycerate of preparation example 1 was analyzed by dark field scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDS). In FIG. 2, it can be seen that each of the metal elements was homogenously distributed in the high-entropy composite glycerate microspheres of the present disclosure.

(4) Analysis of Crystalline Structure

[0074] The crystalline structures of the samples from preparation example 1 and comparative preparation examples 2-8 were analyzed by X-ray diffractometry (XRD, Rigaku) with a Cu Kα radiation source having a wavelength of 1.5418 Å, and the scanning angle was from 10° to 80° with a scanning rate of 2°/min. The analysis results were shown in FIGS. 3A and 3B. For example, FeNi-G showed diffraction peaks at 10.8°, 19.2°, 35.3°, and 60°, indicating that the metal glycerates are composed of stacked metal-oxygen sheets separated by bonded glycerate ions. As shown in FIG. 3A, by adding more elements, the diffraction peaks became broader. It is suggested that the metal glycerate including more metal elements would have a lower crystallinity.

(5) Analysis of Oxidation State

[0075] As shown in FIGS. 4A to 4G, the oxidation state of each metal element of the high-entropy composite glycerate of preparation example 1 was analyzed by X-ray photoelectron spectroscopy (XPS, Versaprobe PHI 5000). The Fe 2p spectrum of FIG. 4A showed broadened Fe 2p.sub.3/2 and Fe 2p.sub.1/2 peaks, belonging to Fe.sup.3+, center at 712.4 eV and 723.6 eV, respectively. A satellite peak of Fe.sup.3+ was located at 718.4 eV. In the Ni 2p spectrum as shown in FIG. 4B, the Ni 2p.sub.3/2 Ni.sup.2+ and Ni 2p.sub.1/2 Ni.sup.2+ peaks were located at 855.2 eV and 872.9 eV, respectively, and the Ni 2p.sub.3/2 Ni.sup.3+ and Ni 2p.sub.1/2 Ni.sup.3+ peaks were located at 857.2 eV and 875.3 eV, respectively. Two prominent shake-up satellite peaks at 861.4 eV and 880.0 eV were observed. That is to say, the high-entropy composite glycerate of the present disclosure has two oxidation states of Ni.sup.2+ and Ni.sup.3+. Two oxidation states of Co.sup.3+ and Co.sup.2+ can be seen in the Co 2p spectrum of FIG. 4C. The peaks at 780.6 eV and 796.4 eV corresponded to Co 2p.sub.3/2Co.sup.3+ and Co 2p.sub.1/2Co.sup.3+, respectively. The peaks at 782.3 eV and 798.2 eV belonged to Co 2p.sub.3/2Co.sup.2+ and Co 2p.sub.1/2Co.sup.2+, respectively. Two prominent shake-up satellite peaks can be found at 786.1 eV and 802.6 eV. In the Cr 2p spectrum as shown in FIG. 4D, the peaks of Cr 2p.sub.3/2Cr.sup.3+ and Cr 2p.sub.1/2Cr.sup.3+ were located at 576.5 eV and 586.3 eV, respectively. In the Mn 2p spectrum of FIG. 4E, the peaks at 641.3 eV and 653.2 eV belonged to Mn 2p.sub.3/2 and Mn 2p.sub.1/2, respectively. The Mn 2p.sub.3/2 peak can be deconvoluted into three peaks, which were Mn.sup.2+ at 640 eV, Mn.sup.3+ at 641.35 eV, and Mn.sup.4+ at 643 eV, respectively, and the peak at 647 eV was a satellite peak. In addition, the O 1s spectrum in FIG. 4F were deconvoluted into four components, which were assigned to the lattice oxygen O-M bonding at 529.6 eV, O—H bonding at 531.3 eV, C—O bonding at 532 eV, and physically absorbed water at 533.2 eV, respectively. For the C 1s spectrum as shown in FIG. 4G, the peaks at 284.8 eV, 286.2 eV, and 287.6 eV were attributed to the C—C bonding, C—O bonding, and C═O bonding, respectively.

(6) Molecular Structure

[0076] As shown in FIG. 5, the molecular structure of the high-entropy composite glycerate in preparation example 1 was analyzed by Fourier transform infrared spectrometer (FTIR, PerkinElmer Frontier). It was observed in FIG. 5 that the high-entropy composite glycerate had a glycerol moiety. For example, in the absorption spectrum of the high-entropy composite glycerate of the present disclosure, the broad IR absorption band centered at 3,400 cm.sup.−1 was attributed to the stretching vibrations of hydrogen bonded O—H groups; the absorption bands between 2,850 and 2,950 cm.sup.−1 were assigned to the C—H stretching vibrations; the high-intensity band at 1,580 cm.sup.−1 corresponded to the C═C stretching vibrations; the peak at 1,640 cm.sup.−1 was attributed to the vibrations of C═O bonding; the absorption band at 1,358 cm.sup.−1 was indexed to the C—H bending vibrations; the band at 1,100 cm.sup.−1 was attributed to the C—O stretching vibrations; and the peak at about 800 cm.sup.−1 was assigned to the out-of-plane C—H bending vibrations. Further, the prominent IR band centering at 630 cm.sup.−1 was associated with metal-oxygen (M-O; M refers to Fe, Ni, Co, Cr, or Mn) stretching vibrations. In view of the foregoing, as shown in the molecular structure of FIG. 5, the high-entropy composite glycerate prepared in preparation example 1 indeed formed the structure of multi-metal glycerate.

[0077] For evaluating the activity of the high-entropy composite glycerate in electrolyzing oxygen evolution reaction, working electrodes were prepared as follows. For example, the high-entropy composite glycerate of preparation example 1 and the metal glycerates of comparative preparation examples 2-8 were used as catalyst powder.

Preparation of Working Electrode

[0078] 5 mg of the catalyst powders and 40 μL of a 5 wt % perfluorosulfonic acid (PFSA) polymer solution (Nafion, DuPont, a purity of 5%) were dispersed in a mixture of water (350 μL) and alcohol (150 μL), and the mixture dispersed with catalyst powders and Nafion was conducted under ultrasonication for at least 1 hour to form a homogeneous catalyst slurry. Next, the catalyst slurry was drop-casted onto a Ni foam substrate to cover an area of 1 cm×1 cm, and was dried in a vacuum oven at 60° C. for 1 hour. The working electrode was obtained, and the active mass loading thereof was about 2.5 mg/cm.sup.2. Before being used, the working electrode was washed with a HCL solution having a molarity of 3 M for 10 minutes, and then sonicated in de-ionized water, acetone, and ethanol subsequently for several times to remove the surface oxides from the electrode.

[0079] The electrochemical measurements were performed at room temperature through an Autolab electrochemical workstation (Muti Autolab/M204) with a built-in electrochemical impedance spectroscopy (EIS) analyzer. The measurement was conducted with a standard three-electrode cell for evaluating the activity of the samples from preparation example 1 and comparative preparation examples 2-8 as electrocatalysts in electrolyzing oxygen evolution reaction. The electrolyte is 1 M of KOH (Honeywell, a purity of more than 99%), and a platinum foil and Ag/AgCl (3 M of KCl) were used as a counter electrode and a reference electrode, respectively. Before the measurement, the Ag/AgCl electrode was calibrated. The potential (E.sub.Ag/AgCl) was calibrated to the reversible hydrogen electrode (RHE) potential (E.sub.RHE) according to the following Equation (1). The pH value of the electrolyte was measured by a PHM 201 pH meter.

E.sub.RHE=E.sub.Ag/AgCl+0.1976+0.059×pH   (1)

[0080] Polarization curves were documented under the linear sweep voltammetry (LSV) mode with a sweep rate of 5 mV/s.sup.−1. All the curves were corrected with 80% of IR compensation. The LSV curves of the samples from preparation example 1 and comparative preparation examples 2-8 were recorded in FIGS. 6A and 6B, and the LSV curve of the baseline of Ni foam was also shown in these figures. The Tafle plots of FIGS. 7A and 7B were derived from the LSV curves of FIGS. 6A and 6B, respectively. AC impedance was measured in a frequency range from 10.sup.−1 to 10.sup.−5 Hz with an AC amplitude of 10 mV. In order to obtain the average values and error margins, three independent electrodes were prepared and tested for each sample (preparation example 1 and comparative preparation examples 2-8). The results were recorded in the following Table 2, and the data were represented by standard deviation based on three tests.

TABLE-US-00002 TABLE 2 Comparison of the catalytic performance of OER between preparationexample 1 and comparative preparation examples 2-8 η (mV) @ Tafel slope Sample 100 mA/cm.sup.−2 (mV/dec) Preparation example 1 278 ± 5 40 ± 2 Comparative preparation example 2 337 ± 4 75 ± 3 Comparative preparation example 3 328 ± 6 56 ± 4 Comparative preparation example 4 320 ± 5 55 ± 2 Comparative preparation example 5 298 ± 3 42 ± 2 Comparative preparation example 6 313 ± 8 50 ± 2 Comparative preparation example 7 299 ± 3 48 ± 1 Comparative preparation example 8 381 + 4 72 ± 3

[0081] Referring to the LSV plot as shown in FIG. 6A, the Tafel plot as shown in FIG. 7A, the bar graph of the overpotential as shown in FIG. 8A and the catalytic performance as shown in Table 2, by comparing the high-entropy composite glycerate (preparation example 1) with the binary-metal glycerate, the ternary-metal glycerate, and the quarternary-metal glycerate (comparative preparation examples 2-5), it was demonstrated that the addition of metal elements can improve the performance of OER. As shown in FIG. 8A and Table 2, the high-entropy composite glycerate prepared in preparation example 1 (FeNiCoCrMn-G) had the overpotential of 278 mV at a current density of 100 mA/cm.sup.2, which was the lowest overpotential, indicating that the high-entropy composite glycerate had the highest activity of electrocatalyzation. Accordingly, the order of the activity of electrocatalyzation from high to low was: FeNiCoCrMn-G (preparation example 1)>FeNiCoCr-G (comparative preparation example 5)>FeNiCoMn-G (comparative preparation example 4)>FeNiCo-G (comparative preparation example 3)>FeNi-G (comparative preparation example 2). The catalytic kinetics of the catalysts, i.e., the performance for catalyzing OER, were evaluated by the results shown in FIG. 7A and Table 2. The high-entropy composite glycerate prepared in preparation example 1 (FeNiCoCrMn-G) had the lowest Tafel slope (40 mV/dec), indicating that it exhibited the excellent performance for catalyzing OER. The order of the Tafel slope for catalyzing OER from low to high was: FeNiCoCr-G (comparative preparation example 5, 42 mV/dec)<FeNiCoMn-G (comparative preparation example 4, 55 mV/dec)<FeNiCo-G (comparative preparation example 3, 56 mV/dec)<FeNi-G (comparative preparation example 2, 75 mV/dec). In other words, in term of the kinetics of catalytic reaction, the order of the OER performance equaled to the order of the electrocatalytic activity. As such, the addition of Fe into mono-metal Ni glycerate (Ni-G) to form FeNi-G led to the modification of the electronic structure, and thus OER was improved. The electrocatalytic activity may be further increased by adding Co, another known OER active material, as a third element into FeNi-G to form FeNiCo-G, since Co had a higher electronegativity than Fe. Hence, Co was served as an electron acceptor, such that the Fe 2p peak in the XPS spectrum shifted positively, and the oxidation state of Fe.sup.2+ was disappeared, leaving only the oxidation state of Fe.sup.3+ in FeNiCo-G. Meanwhile, the electronic structure modification caused by the addition of Co led to a slight positive shift of the Ni 2p peak.

[0082] The Fe 2p peak may shift significantly by adding a fourth metal of Cr, while the Ni 2p peak may shift insignificantly by adding a fourth metal of Mn, indicating that the electrocatalystic activity was further increased, and the addition of Cr showed a higher degree of activity enhancement than the addition of Mn. Since the ionic radius of Cr is the largest among all the metal ions, the presence of Cr may cause compressive strain that was beneficial to OER reaction, which was attributed to weakened chemisorption at the active sites.

[0083] The addition of both of Cr and Mn to form FeNiCoCrMn-G may continue the positive peak shift, thereby enhancing the activity of OER. Therefore, the general trend is that the more the peak shift is, the lower the η.sub.100 is, indicating that the electronic structure is gradually modified toward improving the OER activity. However, it was observed that with the addition of metals, the existing metal ions may have higher oxidation states, indicating the variation of local coordination environment that influences the OER performance.

[0084] The synergistic effects resulted from the high-entropy configuration was demonstrated by comparing the high-entropy composite glycerate of the present disclosure (preparation example 1) with the quaternary-metal glycerates (comparative preparation examples 5-8), that is, by removing one metal from the high-entropy composite glycerate of the present disclosure. It was also demonstrated that the performance of the high-entropy composite glycerate of the present disclosure was high than other metal glycerates. Please refer to the LSV plot as shown in FIG. 6B, the Tafel plot as shown in FIG. 7B and the bar graph of the overpotential as shown in FIG. 8B, along with the catalytic performance as shown in Table 2. As mentioned above, the lower the overpotential is, the higher the electrocatalytic activity is, and the lower the Tafel slope is, the better the OER performance is. As shown in FIG. 8B and Table 2, the order of the electrocatalytic activity indicated by the overpotential from high to low was FeNiCoCrMn-G (preparation example 1, 278 eV)>FeNiCoCr-G (comparative preparation example 5, 298 eV)>FeCoCrMn-G (comparative preparation example 7, 299 eV)>FeNiCrMn-G (comparative preparation example 6, 313 eV)>FeNiCoMn-G (comparative preparation example 4, 337 eV)>NiCoCrMn-G (comparative preparation example 8, 381 eV). As shown in FIG. 7B and Table 2, the order of the OER performance indicated by the Tafel slope was FeNiCoCrMn-G (preparation example 1, 40 mV/dec)>FeNiCoCr-G (comparative preparation example 5, 42 mV/dec)>FeCoCrMn-G (comparative preparation example 7, 48 mV/dec)>FeNiCrMn-G (comparative preparation example 6, 50 mV/dec)>FeNiCoMn-G (comparative preparation example 4, 55 mV/dec)>NiCoCrMn-G (comparative preparation example 8, 72 mV/dec). In light of the foregoing, the high-entropy composite glycerate of the present disclosure had the excellent electrocatalytic activity and OER performance.

[0085] In addition, among the five kinds of quaternary-metal glycerates of comparative preparation examples 4-8, the performance of the quaternary-metal glycerate with the removal of Fe from the high-entropy composite glycerate of the present disclosure (FeNiCoCrMn-G) was the worst, indicating that Fe played a role in the OER activity of multiple metal glycerate. The performance of the quaternary-metal glycerate with the removal of Cr from FeNiCoCrMn-G was the second worst. It thus can be seen that in addition to the modification of the electronic structure, the strain effect also involved in the high-entropy glycerate.

[0086] After oxygen evolution reaction, the high-entropy composite glycerate of the present disclosure was examined by SEM and TEM. It can be seen in FIGS. 9A-9C that, after oxygen evolution reaction, the microsphere structure of the high-entropy composite glycerate was transformed into a thin, porous sheet-like structure. Moreover, as shown in FIG. 10, each of the metal elements was still homogenously distributed in the high-entropy composite glycerate after oxygen evolution reaction.

[0087] Furthermore, for comparing the change of the oxidation states of the metal elements in the high-entropy composite glycerate, please refer to FIGS. 11A-11F. As shown in FIG. 11A, the Fe 2p spectrum remained unchanged with the presence of only Fe.sup.3+. In FIGS. 11B and 11C, the intensities of the Ni.sup.3+ and Co.sup.3+ peaks were slightly increased. The Cr spectrum as shown in FIG. 11D indicated a positive peak shift. The Cr 2p.sub.3/2 peak and the Cr 2p.sub.1/2 peak, belonging to Cr.sup.3+, were located at 577.5 eV and 587.2 eV, respectively. It was observed in FIG. 11E that Mn.sup.2+ was partially oxidized to higher oxidation states, and the increased metal of 3.sup.+ oxidation state represented the formation of metal oxyhydroxides. Furthermore, the O 1s spectrum of FIG. 11F showed a clear transition. Before oxygen evolution reaction, the peak of the O—H bonding was 531.3 eV, which was the highest intensity. After oxygen evolution reaction, the intensity of O-M bonding increased to 529.6 eV. For example, the ratio of intensity of O-M and O—H (O-M/O—H) was approximately 1 after oxygen evolution reaction. With the results shown in FIG. 11F, it was demonstrated that the aforementioned metal oxyhydroxides were formed in the high-entropy composite glycerate of the present disclosure during oxygen evolution reaction. In view of the above, the properties of the glycerate and the synergetic effect of the high-entropy glycerate render the excellent activity of oxygen evolution reaction to the high-entropy composite glycerate of the present disclosure. Additionally, it is believed that the high-entropy composite glycerate of the present disclosure (FeNiCoCrMn-G) can be further modified through adjusting the concentration of the precursor and/or adding more than five kinds of metals, thereby further improving the activity of oxygen evolution reaction and water-splitting of the high-entropy composite glycerate.

[0088] Please refer to FIG. 12. In the bar graph of each sample, the left bars represented O—H/O-M, and the right bars represented the added higher oxidation-state fractions, i.e., HOFs. The HOF, (M.sup.3++M.sup.4+)/(M.sup.2++M.sup.3++M.sup.4+), of each metal M was analyzed by XPS. The added HOFs in all metals of each of the samples (i.e., preparation example 1 and comparative preparation examples 2-8) were showed in the right bars of FIG. 12. The added HOF of the high-entropy composite glycerate of the present disclosure was the highest, followed by the quaternary-metal glycerates, the ternary-metal glycerates, and the binary-metal glycerates. For the glycerates, the numbers of coordinate covalent bonding between the metals and ligands depended on the oxidation states of the metals. In other words, the metals having oxidation states of 3.sup.+ and 4.sup.+ may have three and four attached ligands, respectively. As shown in the left bars of FIG. 12, with the added HOFs increased, the ratio of O—H/O-M may increase as well. As shown in FIG. 13, it was demonstrated that the electrocatalytic activity may be enhanced with the increase of O—H as the coordination environment was changed. The resulting coordinate covalent bonding may render more freedoms to the glycerate anions to flip the coordination mode, and therefore facilitate OER. The exception was the metal glycerate without Fe, i.e., NiCoCrMn-G. Despite the fact that NiCoCrMn-G had a high ratio of O—H/O-M, the overpotential (η.sub.100) thereof was still pretty high due to removal of Fe.

[0089] Also, it was expected that crystalline distortion (expanding) may occur in the metal glycerate having higher oxidation states or more coordinate covalent bonding. As shown in FIG. 3A, the addition of Co into FeNi-G may decrease the peak intensity, and the addition of Mn into FeNiCo-G may further decrease the peak intensity, indicating the expanding of the interlayers in the layered structure of the metal glycerates. It was also demonstrated that more active sites can accelerate the charge transfer between the intermediates and the active sites.

[0090] Moreover, hydrogen evolution reaction (HER) of the high-entropy composite glycerate of preparation example 1 and the metal glycerates of comparative preparation examples 2-8 in the 1 M KOH electrolyte were evaluated and documented in the following Table 3. The data provided in Table 3 were represented by standard deviation based on three tests.

TABLE-US-00003 TABLE 3 Comparison of HER catalytic performance between preparation example 1 and comparative preparation examples 2-8 η (mV) @ Tafel slope 10 mA/cm.sup.−2 (mV/dec) Preparation example 1 210 ± 4 105 ± 3 Nickel foam 255 ± 4 132 ± 4 Comparative preparation example 2 224 ± 5 192 ± 6 Comparative preparation example 3 254 ± 5 137 ± 5 Comparative preparation example 4 251 ± 6 130 ± 4 Comparative preparation example 5 273 ± 3 116 ± 5 Comparative preparation example 6 241 ± 2 119 ± 3 Comparative preparation example 7 259 ± 3 122 ± 3 Comparative preparation example 8 268 + 8 113 + 4

[0091] As shown in Table 3, the high-entropy composite glycerate of the present disclosure had the excellent performance, which was indicated by the lowest overpotential of 210 mV at a current density of 10 mA/cm.sup.2 and the lowest Tafel slope of 105 mV/dec.

[0092] Further, in order to determine the stability and durability of the high-entropy composite glycerate of preparation example 1 of the present disclosure for catalyzing OER, the continuous CV cycling at a scan rate of 100 mV/s for 3,000 cycles was used. The activities of OER were recorded when the selected cycles were reached (1, 1,000, 2,000, and 3,000 cycles). After 1,000 cycles, the activities of OER for all samples were increased, except for FeNiCoCr-G. After 2,000 cycles, the binary-metal glycerates, ternary-metal glycerates, and quaternary-metal glycerates showed decreased activities of OER. Nevertheless, FeNiCoCrMn-G of preparation example 1 exhibited higher activity of OER after 3,000 cycles. It was demonstrated that the electrode prepared by the high-entropy composite glycerate of the present disclosure had excellent electrochemical stability, and exhibited the synergistic effect of high-entropy configuration. In addition, the long-term durability of the working electrodes was evaluated by chronopotentiometry at a current density of 10 mA/cm.sup.−2 and 100 mA/cm.sup.−2 for 36 hours. As a result, FeNiCoCrMn-G exhibited no significant change after 36 hours, indicating that FeNiCoCrMn-G had excellent durability. That is to say, the high-entropy composite glycerate of the present disclosure as an electrocatalyst is highly stable during oxygen evolution reaction.

[0093] On the other hands, the overall water-splitting performance of the high-entropy composite glycerate of the present disclosure in a high-entropy composite glycerate (HEG) electrolytic cell was examined by a two-electrode system. As a result, the cell voltage was 1.63 V at a current density of 10 mA/cm.sup.−2, indicating that the HEG electrolytic cell had good overall water-splitting performance. Moreover, the LSV curve of the high-entropy composite glycerate of the present disclosure in the HEG electrolytic cell showed no significant change after continuous testing for 24 hours, indicating the long-term durability. The above excellent performances of the cycling stability and durability were derived from the essence of the high-entropy composite entropy itself.

[0094] While several embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the applications for which this disclosure is used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, embodiments may be practiced otherwise than as described and claimed.