Flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst and preparation method and application thereof

Abstract

The present invention relates to a flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst, a preparation method thereof and an application. The rhodium (Rh) element is doped in β-Ni(OH)2, and the Rh-doped β-Ni(OH)2 presents a flower shape and is composed of petal-shaped flakes, all of which are β-type Ni(OH)2. It is prepared by a hydrothermal method in a hydrothermal reaction kettle. When used for methanol oxidation reaction, the anodic oxidation current density can reach 90 mA cm ‑2 , and the Tafel slope is only 40 mV dec ‑1 . The preparation method adopted in the present invention is simple, and the microscopic morphology, size uniformity and stability of the product are good, which is suitable for industrial production; doping with a trace amount of rhodium element significantly reduces the usage amount of precious metals in the preparation process compared with commercial Pd / C and Pt / C catalysts, greatly reducing the production cost, and at the same time reducing the poisoning effect of CO on the catalyst and improving the stability of the catalyst. The doping of rhodium element can inhibit the formation of γ-NiOOH, change the electronic structure of Ni in β-Ni(OH)2, increase the proportion of surface defects in β-Ni(OH)2, reduce the impedance of charge transfer, and thus further improve the catalytic activity.

Description

Technical Field

[0001] The invention belongs to the technical field of methanol electrocatalytic oxidation, and relates to a flower-shaped rhodium-doped beta-Ni(OH)2 methanol oxidation electrocatalyst, a preparation method and an application thereof. Background Art

[0002] Direct methanol fuel cell (DMFC) uses methanol as fuel. It has the advantages of high energy density, low operating temperature, easy storage and transportation, and environmental friendliness. It is considered to be a promising energy production device. Platinum-based materials have high catalytic activity in the methanol oxidation process and are therefore the most commonly used anode materials for DMFC. However, high costs and CO poisoning effects have become obstacles to its commercialization.

[0003] Ni(OH)2 has good stability in alkaline electrolytes and is not easily poisoned during methanol oxidation catalysis, so it is expected to become a substitute catalyst for precious metals in DMFC. Fleischmann (see document M.Fleischmann, K.Korinek, D.Pletcher, The oxidation of organic compounds at a nickel anode in alkaline solution, J.Electroanal.Chem., 31 (1971) 39-49) et al. pointed out that its catalytic effect is mainly through Ni(OH)2 and NiOOH in the cyclic voltammetry process of alkaline solution, and the high-valent NiOOH acts as an oxidant in the oxidation process of organic compounds. In addition, some studies believe that methanol is oxidized on the NiOOH surface, and NiOOH only serves as an active surface and does not participate in the oxidation reaction. Nickel hydroxide (Ni(OH)2) electrode will form four different phases in the cyclic voltammetry process, i.e. β-Ni(OH)2, α-Ni(OH)2, β-NiOOH and γ-NiOOH. Since the formation of γ-NiOOH causes electrode expansion and affects electrolyte distribution, β-NiOOH performs better than γ-NiOOH in the electrocatalytic process. In addition, α-Ni(OH)2 cannot exist stably in alkaline solution, so the crystal structure of β-Ni(OH)2 is more conducive to methanol oxidation catalyst.

[0004] Studies have shown that the methanol oxidation catalytic performance of nickel-based compounds can be significantly improved by doping with another element. For example, doping Mn in α-Ni(OH)2 not only reduces the starting potential of the methanol oxidation reaction, but also increases the anodic oxidation current density (see the literature Mn-doped Ni(OH)2 nanostructures as an efficient electrocatalyst for methanol oxidation in basic solution, Journal of The Electrochemical Society, 167 (2020) 104501); by regulating the doping amount of boron in NiCu2O4, its methanol oxidation current density can be increased to twice that of undoped (see the literature NKTR, HLR, NB, Facile synthesis of boron doped NiCu2O4 for efficient methanol oxidation: Selective towards value-added formate formation, Ceramics International, (2022)). Rhodium itself as a catalyst also exhibits excellent catalytic activity and anti-CO poisoning performance in the process of methanol catalytic oxidation. It can be inferred that the doping of rhodium will have a positive effect on the catalytic performance of β-Ni(OH)2.

[0005] Therefore, by designing β-Ni(OH)2 with a larger specific surface area and doping it with rhodium, it is expected to obtain a catalyst with better methanol oxidation catalytic activity. Based on the above theory and research foundation, the present invention adopts a hydrothermal method to prepare β-Ni(OH)2 composed of petal-shaped flakes, and achieves rhodium doping through an adsorption reduction method. Summary of the invention

[0006] Technical issues to be solved

[0007] In order to avoid the shortcomings of the prior art, the present invention proposes a flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst and a preparation method and application thereof, in order to reduce the production cost of methanol fuel cell catalysts and further improve the catalytic performance on the basis of existing nickel-based catalysts.

[0008] Technical Solution

[0009] A flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst, characterized in that rhodium Rh element is doped into β-Ni(OH)2, the Rh-doped β-Ni(OH)2 presents a flower shape, is composed of petal-shaped flakes, and all are β-type Ni(OH)2.

[0010] A method for preparing the flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst, characterized by the following steps:

[0011] Step 1: dissolving nickel sulfate and trisodium citrate in deionized water, then adding KOH aqueous solution, reacting by hydrothermal method, and obtaining flower-shaped β-Ni(OH)2 powder by centrifugal separation, washing and freeze drying;

[0012] Step 2: Disperse the obtained flower-shaped β-Ni(OH)2 powder in deionized water by ultrasonic vibration and magnetic stirring, add rhodium chloride and 1 mol / L sodium hypophosphite aqueous solution and stir evenly, let the mixed solution stand at room temperature for 5 to 6 days, and obtain flower-shaped rhodium-doped β-Ni(OH)2 powder through centrifugal separation, washing and freeze-drying.

[0013] The nickel sulfate and trisodium citrate are dissolved in deionized water and magnetically stirred at room temperature for 15 to 30 minutes to form a uniform solution.

[0014] The hydrothermal method is used to heat the mixture in a hydrothermal reactor at 110 to 130° C. in an oven for 15 to 17 hours and then cool the mixture.

[0015] The concentration of nickel sulfate in the aqueous solution for the hydrothermal reaction is 0.02 mol / L, the concentration of trisodium citrate is 0.75-1.25 mol / L, and the concentration of KOH is 0.25-0.45 mol / L.

[0016] In the final mixed solution of step 2, the concentration of β-Ni(OH)2 is 1.5 g / L, the concentration of rhodium chloride is 1-2 mmol / L, and the concentration of sodium hypophosphite is 0.07-0.1 mol / L.

[0017] An application of the flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst in a methanol oxidation reaction, characterized in that: when the flower-shaped rhodium-doped β-Ni(OH)2 electrocatalyst is used for methanol oxidation reaction, its anodic oxidation current density can reach 90 mA cm -2 , the Tafel slope is only 40mV dec -1 .

[0018] Beneficial Effects

[0019] The present invention proposes a flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst and a preparation method and application thereof. The β-Ni(OH)2 is doped with rhodium Rh element. The Rh-doped β-Ni(OH)2 presents a flower shape and is composed of petal-shaped flakes, all of which are β-type Ni(OH)2. The catalyst is prepared by a hydrothermal method in a hydrothermal reactor. When the catalyst is used for methanol oxidation, its anodic oxidation current density can reach 90 mA cm-2 , the Tafel slope is only 40mV dec -1 .

[0020] The beneficial effects of the present invention are as follows:

[0021] 1. The preparation method adopted by the present invention is simple, the microscopic morphology, size uniformity and stability of the product are good, and it is suitable for industrial production. The prepared β-Ni(OH)2 has a unique flower-like morphology, which gives it a higher specific surface area, and thus can provide more active sites for the catalytic reaction process.

[0022] 2. The present invention uses transition metal hydroxide Ni(OH)2 as the main component and uses a trace amount of rhodium for doping. Compared with commercial Pd / C and Pt / C catalysts, it significantly reduces the amount of precious metals used in the preparation process, greatly reduces production costs, and at the same time reduces the poisoning effect of CO on the catalyst and improves the stability of the catalyst.

[0023] 3. The flower-shaped rhodium-doped β-Ni(OH)2 electrocatalyst prepared by the present invention has high catalytic activity for methanol oxidation reaction, and its anodic oxidation current density can reach 90 mA cm -2 , the Tafel slope is only 40mV dec -1 .

[0024] 4. The doping of rhodium can inhibit the formation of γ-NiOOH, change the electronic structure of Ni in β-Ni(OH)2, increase the proportion of surface defects of β-Ni(OH)2, reduce the impedance of charge transfer of β-Ni(OH)2, and further improve the catalytic activity of β-Ni(OH)2. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a transmission electron microscope morphology image of the flower-shaped rhodium-doped β-Ni(OH)2 electrocatalyst prepared in Example 1 of the present invention;

[0026] Figure 2 The HADDF-STEM image of the flower-shaped rhodium-doped β-Ni(OH)2 electrocatalyst prepared in Example 1 and its energy spectrum component surface distribution analysis;

[0027] Figure 3 is an X-ray diffraction (XRD) analysis diagram of the flower-shaped undoped and rhodium-doped β-Ni(OH)2 electrocatalysts prepared in Example 1;

[0028] Figure 4 is a photoelectron spectroscopy (XPS) analysis diagram of the flower-shaped undoped and rhodium-doped β-Ni(OH)2 electrocatalysts prepared in Example 1;

[0029] Figure 51 is a cyclic voltammogram of the flower-shaped undoped and rhodium-doped β-Ni(OH)2 electrocatalysts prepared in Example 1 measured in an alkaline methanol solution.

[0030] Figure 6 It is a Tafel plot of the flower-shaped undoped and rhodium-doped β-Ni(OH)2 electrocatalysts prepared in Example 1 measured in an alkaline methanol solution. DETAILED DESCRIPTION

[0031] The present invention will now be further described with reference to the embodiments and the accompanying drawings:

[0032] Example 1

[0033] Dissolve 0.8mmol nickel sulfate hexahydrate and 50mmol trisodium citrate dihydrate in 32mL deionized water and stir evenly; then add 8mL 2mol / L KOH aqueous solution and stir for 20 minutes; pour the solution into a hydrothermal reactor and heat in an oven at 120℃ for 16 hours; then obtain flower-shaped β-Ni(OH)2 through centrifugation, washing and freeze-drying; disperse the obtained 20mg flower-shaped β-Ni(OH)2 in 14mL deionized water by magnetic stirring; then add 6mg rhodium chloride trihydrate and 1mL 1mol / L sodium hypophosphite aqueous solution dropwise to the dispersion and stir evenly; let the above mixture stand at room temperature for 6 days, and obtain flower-shaped rhodium-doped β-Ni(OH)2 through centrifugation, washing and freeze-drying. Its morphology and element distribution are shown in Figure 1 and Figure 2 .Depend on Figure 1 It can be seen that the prepared Rh-doped β-Ni(OH)2 presents a flower shape consisting of petal-like flakes. Figure 2 It can be seen that the Rh element is evenly distributed on the surface of the flower cluster. Figure 3 The XRD test results show that the crystal structure does not change significantly before and after Rh doping, and is all β-type Ni(OH)2. The phase related to Rh is not detected in XRD due to its small size. The chemical state analysis of its surface elements is shown in Figure 4 After Rh doping, the binding energy of Ni in β-Ni(OH)2 shifts positively (see Figure 4 (b)), indicating that Rh doping changes its electronic structure, which is beneficial to the improvement of catalytic performance. According to the O1s spectrum, compared with β-Ni(OH)2, Rh-doped β-Ni(OH)2 contains a larger proportion of defects, which can provide more adsorption sites for catalytic reactions (see Figure 4 (d)).

[0034] Example 2

[0035] 0.8mmol nickel sulfate hexahydrate and 30mmol trisodium citrate dihydrate are dissolved in 35mL deionized water and stirred evenly; then 5mL 2mol / L KOH aqueous solution is added and stirred for 20 minutes; the solution is poured into a hydrothermal reactor and heated in an oven at 110°C for 17 hours; then flower-shaped β-Ni(OH)2 is obtained by centrifugation, washing and freeze-drying; 20mg of the obtained flower-shaped β-Ni(OH)2 is dispersed in 13.5mL deionized water by magnetic stirring; then 8mg rhodium chloride trihydrate and 1.5mL 1mol / L sodium hypophosphite aqueous solution are added dropwise to the dispersion and stirred evenly; the above mixture is allowed to stand and react at room temperature for 5 days, and flower-shaped rhodium-doped β-Ni(OH)2 is obtained by centrifugation, washing and freeze-drying.

[0036] Example 3

[0037] 0.8mmol nickel sulfate hexahydrate and 40mmol trisodium citrate dihydrate are dissolved in 33mL deionized water and stirred evenly; then 7mL 2mol / L KOH aqueous solution is added and stirred for 20 minutes; the solution is poured into a hydrothermal reactor and heated in an oven at 120°C for 15 hours; then flower-shaped β-Ni(OH)2 is obtained by centrifugation, washing and freeze-drying; 20mg of the obtained flower-shaped β-Ni(OH)2 is dispersed in 14mL deionized water by magnetic stirring; then 4mg rhodium chloride trihydrate and 1mL 1mol / L sodium hypophosphite aqueous solution are added dropwise to the dispersion and stirred evenly; the above-mentioned mixed solution is allowed to stand and react at room temperature for 6 days, and flower-shaped rhodium-doped β-Ni(OH)2 is obtained by centrifugation, washing and freeze-drying.

[0038] The inventors used the flower-shaped rhodium-doped β-Ni(OH)2 catalyst obtained in Example 1 to test the methanol electrocatalytic oxidation reaction. The specific method was to evenly disperse 3 mg of the catalyst in 1 mL of isopropanol, drop 3 μL of the dispersion onto a glassy carbon electrode, cover it with a layer of Nafion film and dry it for electrochemical testing. The test results are shown in Figure 5 and Figure 6 .Depend on Figure 5 and Figure 6 It can be seen that the flower-shaped rhodium-doped β-Ni(OH)2 catalyst prepared by this method exhibits good methanol electrocatalytic oxidation performance in alkaline solution. Its anodic oxidation current density can reach 90mA cm -2 , the Tafel slope is only 40mV dec -1 , and has excellent stability, so it has good industrial application prospects.

Claims

1. A method for preparing a flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst, characterized in that: Rhodium Rh element is doped in β-Ni(OH)2, and the Rh-doped β-Ni(OH)2 presents a flower shape and is composed of petal-shaped flakes, all of which are β-type Ni(OH)2; the preparation steps are as follows: Step 1: dissolving nickel sulfate and trisodium citrate in deionized water, then adding KOH aqueous solution, reacting by hydrothermal method, and obtaining flower-shaped β-Ni(OH)2 powder by centrifugal separation, washing and freeze drying; Step 2: Disperse the obtained flower-shaped β-Ni(OH)2 powder in deionized water by ultrasonic vibration and magnetic stirring, add rhodium chloride and 1 mol / L sodium hypophosphite aqueous solution and stir evenly, let the mixed solution stand at room temperature for 5 to 6 days, and obtain flower-shaped rhodium-doped β-Ni(OH)2 powder through centrifugal separation, washing and freeze-drying.

2. The method according to claim 1, characterized in that: The nickel sulfate and trisodium citrate are dissolved in deionized water and magnetically stirred at room temperature for 15 to 30 minutes to form a uniform solution.

3. The method according to claim 1, characterized in that: The hydrothermal method is used to heat the mixture in a hydrothermal reactor at 110 to 130° C. in an oven for 15 to 17 hours and then cool the mixture.

4. The method according to claim 1, characterized in that: The concentration of nickel sulfate in the aqueous solution for the hydrothermal reaction is 0.02 mol / L, the concentration of trisodium citrate is 0.75-1.25 mol / L, and the concentration of KOH is 0.25-0.45 mol / L.

5. A flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst prepared by the method of any one of claims 1 to 4, characterized in that: Rhodium Rh element is doped into β-Ni(OH)2, and the Rh-doped β-Ni(OH)2 presents a flower shape and is composed of petal-shaped flakes, all of which are β-type Ni(OH)2.

6. An application of the flower-shaped rhodium-doped β-Ni(OH)2 methanol oxidation electrocatalyst according to claim 5 in methanol oxidation reaction, characterized in that: When the flower-shaped rhodium-doped β-Ni(OH)2 electrocatalyst is used for methanol oxidation, its anodic oxidation current density is 90 mA cm -2 , Tafel slope is 40mV dec -1 .

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