A method for preparing a single or multiple halogen fully doped high-efficiency Fe-N-C catalyst by stepwise heating

Abstract

This invention discloses a method for preparing highly efficient Fe-N-C catalysts fully doped with single or multiple halogens through stepwise heating. First, N-C catalysts synthesized from a ZIF-8 complex precursor rich in pyridine N and encapsulated in o-phenanthroline are used as nitrogen-doped carbon supports (with a specific surface area greater than 800 m²). 2 g ‑1 Fe-N-C catalysts with fully doped single or multiple halogens are prepared by thoroughly mixing iron salts (such as ferrous chloride, ferric chloride, ferrous acetate, ferrous sulfate, etc.), ammonium halide salts (ammonium fluoride, ammonium chloride, ammonium bromide, and ammonium iodide) and N-C support, and heating in two steps. This method can ensure sufficient halogen doping, simplify the steps of synthesizing halogen-doped Fe-N-C catalysts, and also achieve co-doping of multiple halogens in Fe-N-C catalysts. In addition, the ammonia and hydrogen halide gases from the decomposition of ammonium salts can also etch carbon, thereby obtaining more carbon defects and mesopores.

Description

Technical Field

[0001] This invention belongs to the field of nanocatalytic materials technology, specifically relating to a method and application for preparing highly efficient Fe-NC catalysts with single or multiple fully halogen-doped catalysts through stepwise heating. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) have shown promising application prospects in fields such as new energy vehicles. However, the high cost of platinum (Pt)-based noble metal catalysts used in the oxygen reduction reaction (ORR) at the cathode severely limits their commercialization. Therefore, developing inexpensive and abundant MNC catalysts has become a current research focus. Among them, Fe-NC catalysts are highly anticipated due to their relatively good activity, but their activity still cannot meet the 2025 targets set by the U.S. Department of Energy (DOE). In recent years, research has found that increasing the Fe loading, heteroatom doping, adjusting the pyridine nitrogen content, and constructing hierarchical porous structures can improve the performance of Fe-NC catalysts, but none of these catalysts have met the DOE's 2025 targets.

[0003] Furthermore, these preparation methods often involve acid etching to dissolve the iron nanoparticles, or synthesizing ZIF-8 using a template followed by template etching, which are quite cumbersome. Therefore, developing an environmentally friendly and simple process for preparing hybrid catalysts is of great significance for the development of ORR catalysts, and this method also requires the preparation of highly active Fe-NC catalysts for widespread application. Summary of the Invention

[0004] The main objective of this invention is to provide a method for preparing highly efficient Fe-NC catalysts fully doped with single or multiple halogens through stepwise heating.

[0005] The technical solution adopted by this invention to solve its technical problem is:

[0006] A method for preparing highly efficient Fe-NC catalysts fully doped with single or multiple halogens through stepwise heating includes the following steps:

[0007] Step (1): Prepare ZIF-8 with a rhombic dodecahedral morphology;

[0008] Step (2): Disperse a certain proportion of ZIF-8 and o-phenanthroline in an ethanol / water mixed solution to form ZIF-8@Phen precursor; the mass ratio of ZIF-8 to o-phenanthroline is (10:1)-(10:5).

[0009] Step (3): The ZIF-8@Phen precursor was transferred into a ceramic boat and calcined at 800-1000℃ for 60-120 min in an inert gas environment to obtain the NC support;

[0010] Step (4): Mix the iron salt, ammonium halide salt and NC support evenly and place them in a ceramic boat. The mass ratio of iron salt, ammonium halide salt and NC support is (2-20):(200-500):(50-200). First, heat the mixture at a slow heating rate to 100 ℃-500 ℃ for 2 h-4 h in an inert atmosphere with a low flow rate. Then, continue to heat the mixture at 800 ℃-1000 ℃ for 1 h-3 h and then allow it to cool naturally. Finally, Fe-NC catalysts with single or multiple halogens fully doped are obtained.

[0011] Preferably, step (1) includes the following steps: dissolving zinc nitrate hexahydrate and 2-methylimidazole in a molar ratio of 1:(7-9) in methanol solution; then mixing and stirring the two solutions for 12 h - 24 h; then centrifuging and washing the mixed solution multiple times; finally drying the precipitate in a vacuum drying oven at 70 ºC to obtain white ZIF-8 powder with a size of 80 nm - 120 nm.

[0012] Preferably, in step (2), the volume ratio of ethanol to water in the ethanol / water mixed solution is (1.5-2.5):1.

[0013] Preferably, in step (2), 0.5 g – 2.5 g ZIF-8 powder is uniformly dispersed in a mixed solution of 18-22 mL anhydrous ethanol and 8-12 mL ultrapure water, and then 100 mg – 600 mg o-phenanthroline is added and stirred for 8 h – 20 h; then the solvent is evaporated by stirring and heating at 65-75 ºC; finally, it is thoroughly dried in a vacuum drying oven at 60-80 ºC to obtain a white ZIF-8@Phen complex precursor.

[0014] Preferably, in step (3), the inert gas includes argon or nitrogen;

[0015] Preferably, in step (4), the gas flow rate range for heating to 100 ℃ - 500 ℃ in the first step is 5-20 sccm and the heating rate is 1-3°C / min; the heating rate from 100 ℃ - 500 ℃ to 800 ℃ - 1000 ℃ is 5-20°C / min and the gas flow rate range is 20-50 sccm.

[0016] Preferably, in step (4), the ammonium halide salt can be one or more of ammonium fluoride, ammonium chloride, ammonium bromide and ammonium iodide, but the total amount remains unchanged.

[0017] Preferably, in step (4): the iron salt is an iron salt with a vaporization temperature of less than 800 °C, preferably including one of the iron salts such as ferric chloride, ferrous chloride, ferrous acetate, and ferrous sulfate.

[0018] Preferably, the iron salt is an iron salt with a vaporization temperature below 800 °C. It can be ferrous chloride, ferric chloride, ferrous sulfate, ferrous acetate, etc.

[0019] Another object of the present invention is to provide Fe-NC catalysts fully doped with one or more halogens prepared by the method.

[0020] The Fe-NC catalyst prepared by this invention, fully doped with single or multiple halogens, has an iron loading of 0.5-2 wt%, a halogen doping amount of 0.2-1 wt%, and a specific surface area of ​​900-1300 m². 2 g -1 The size is 70-100 nm.

[0021] The beneficial effects of this invention are:

[0022] This invention uses ZIF-8, encapsulated in phenanthroline rich in pyridine nitrogen, as a precursor to synthesize an NC support via one-step thermal evaporation in an inert gas. Then, iron salts, ammonium halide salts, and the NC support are used in a two-step heating method to construct Fe-NC catalysts fully doped with single or multiple halogens. This method ensures sufficient halogen doping, simplifies the synthesis of halogen-doped Fe-NC catalysts, and allows for the co-doping of multiple halogens in the Fe-NC catalyst. Furthermore, the ammonia and hydrogen halide gases from the decomposition of ammonium salts can etch carbon, resulting in more carbon defects and mesopores. All of these factors contribute to the high activity of the Fe-NC hybrid catalyst prepared by this method.

[0023] (1) The method of the present invention can dope one or more halogens into Fe-NC catalyst.

[0024] (2) The present invention proposes a stepwise heating method. In the first step of the heating process, controlling a lower flow rate and a slower heating rate can slowly decompose ammonium halide salts for continuous doping, thereby achieving the purpose of fully doping one or more halogens into the Fe-NC catalyst.

[0025] (3) The ammonium halide salt used in the method of the present invention can not only introduce halogen elements, but also the ammonia gas and hydrogen halide gas decomposed by the ammonium salt can etch carbon, thereby obtaining more carbon defects and mesopores.

[0026] (4) The present invention has a wide range of applications and is applicable to a variety of iron salts.

[0027] (5) The Fe-NC catalysts doped with single or multiple halogens prepared by the method of the present invention exhibit excellent performance in oxygen reduction catalysis, with half-wave potentials all above 0.8 V; among which Fe-NC BrCl It exhibits the highest performance in proton exchange membrane fuel cells, achieving a peak power of 1.33 W / cm² in a hydrogen-oxygen cell at an absolute pressure of 150 kPa. -2 . Attached Figure Description

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] Figure 1 This is the Fe-NC catalyst (Fe-NC) co-doped with Br and Cl prepared in Example 1 of the present invention. BrCl A schematic diagram of the synthesis of ).

[0030] Figure 2 These are SEM and TME images of the NC carrier prepared in Example 1 of this invention.

[0031] Figure 3 It is the Fe-NC prepared in Example 1 of this invention. X Aberration-corrected electron micrograph of the catalyst.

[0032] Figure 4 It is Fe-NC X Activity curve of the catalyst in the oxygen reduction electrocatalytic reaction.

[0033] Figure 5 It is Fe-NC X Activity diagram of the catalyst in a proton exchange membrane fuel cell. Detailed Implementation

[0034] Example 1

[0035] Preparation of ZIF-8:

[0036] (1) Dissolve 6.57 g of 2-methylimidazole and 2.97 g of zinc nitrate hexahydrate (molar ratio of 8:1) in 100 mL of methanol solution, disperse by ultrasonication first, and then stir to form a homogeneous solution.

[0037] (2) The two homogeneous solutions were mixed at 600 rpm to form a white solution, and the mixture was stirred at room temperature for 12 h.

[0038] (3) The material obtained in step (2) is centrifuged at a speed of 12000 pm, washed 5 times with methanol solution, and then dried at 70°C overnight to prepare ZIF-8.

[0039] Preparation of ZIF-8@Phen precursor:

[0040] (1) Disperse 1.0 g ZIF-8 in a solution of 20 mL anhydrous ethanol and 10 mL ultrapure water by ultrasonication for 2 h until it is uniformly dispersed.

[0041] (2) Add 250 mg of o-phenanthroline, disperse by ultrasonication for 20 min, and stir at 600 rpm for 12 h.

[0042] (3) Heat and stir at 70 °C to evaporate most of the solvent, then place in a vacuum drying oven at 70 °C to dry thoroughly.

[0043] The ZIF-8@Phen precursor was obtained.

[0044] Synthesis of NC vectors:

[0045] (1) Place 300 mg of ZIF-8@Phen precursor powder in a porcelain boat.

[0046] (2) Calcine in an argon atmosphere at 1000 °C for 60 min (heating rate of 10 °C / min), then calcine with the furnace.

[0047] The NC vector was prepared by cooling to room temperature.

[0048] Synthesis of Br and Cl fully doped Fe-NC catalysts:

[0049] (1) Grind 6 mg of ferrous chloride tetrahydrate, 150 mg of ammonium bromide, 150 mg of ammonium chloride and 100 mg of NC carrier thoroughly in an agate mortar, and then place it in a porcelain boat.

[0050] (2) In an argon atmosphere, the catalyst was first heat-treated at 400 °C for 2 h (heating rate of 2 °C / min, gas flow rate of 10 sccm). During this stage, ammonium bromide and ammonium chloride began to decompose slowly and gradually doped onto the NC support. Subsequently, the temperature was increased to 900 °C for 1 h at a gas flow rate of 40 sccm and a heating rate of 10 °C / min. During this stage, ferrous chloride vaporized and deposited onto the NC support. The catalyst was then cooled with the furnace to obtain a Fe-NC catalyst fully doped with Br and Cl (Fe-NC). BrCl ).

[0051] Example 2

[0052] Similar to Example 1, except that NH4Br and NH4Cl are replaced with NH4F.

[0053] Example 3

[0054] Similar to Example 1, except that NH4Br and NH4Cl are replaced with NH4Br and NH4I.

[0055] Example 4

[0056] Similar to Example 1, except that NH4Br and NH4Cl are replaced with NH4Br and NH4F.

[0057] Example 5

[0058] Similar to Example 1, except that NH4Br and NH4Cl are replaced with NH4Br, NH4Cl and NH4I.

[0059] Example 6

[0060] Similar to Example 1, except that NH4Br and NH4Cl are replaced with NH4Br, NH4Cl, NH4I and NH4F.

[0061] Example 7

[0062] Similar to Example 1, except that FeCl2·4H2O is replaced with FeSO4·4H2O.

[0063] Example 8

[0064] Similar to Example 1, except that FeCl2·4H2O is replaced with Fe(AC)2·4H2O.

[0065] Oxygen reduction electrocatalytic test:

[0066] Take the Fe-NC prepared in Example 1 BrCl catalyst.

[0067] (1) Before preparing the working electrode, the catalyst needs to be prepared into a uniform ink. Weigh 3 mg of the catalyst and add it to a mixed solution containing 300 μL isopropanol, 200 μL ultrapure water and 5 μL 5% Nafion solution, and sonicate for 3 h to prepare a uniform catalyst ink.

[0068] (2) In this work, all electrochemical measurements were performed on a CHI 760e electrochemical workstation with a standard three-electrode system. Measurements were taken on a ring-disk electrode (RRDE, area 0.2475 cm²). 2A catalyst-supported electrode was used as the working electrode, a graphite rod as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Before electrochemical testing, the electrode was sequentially polished 200 times each with Al₂O₃ abrasive powder of particle sizes of 5 μm, 1 μm, and 0.3 μm, using a figure-eight polishing method to ensure even force distribution and a smooth surface. After polishing with each particle size, the electrode was ultrasonically cleaned 3-5 times with ultrapure water to remove the abrasive powder. Ethanol can be used during the cleaning process to remove adsorbed organic matter from the electrode surface.

[0069] (3) Then, 25 µL of catalyst ink was dropped onto the rotating disk electrode (RRDE, disk area: 0.2475 cm²). 2 Pt ring area: 0.1866 cm² 2 The catalyst was then applied to a container and the solvent was evaporated under an infrared lamp. Finally, the catalyst loading was 0.6 mg cm⁻¹. -2 By using the transformation equation E RHE = E SCE +0.2415 +0.059 pH, the potential in this work is referred to as the reversible hydrogen electrode (RHE) potential.

[0070] (4) Before measuring the ORR polarization curve, the catalyst on the working electrode must be electrochemically cleaned by cyclic voltammetry. First, in a 0.1 M H₂SO₄ solution saturated with N₂, at 100 mV s⁻¹ -1 The scan rate was maintained between 0.03 V and 1.30 V (vs. RHE) until the CV curves completely overlapped to ensure catalyst cleanliness. Then, the gas was converted to O2, and O2 was continuously introduced for at least 20 min to ensure O2 saturation in the electrolyte. The rotator was set to an appropriate speed and turned on, with the speed set to 900 rpm, followed by a scan rate of 10 mV / s. -1 LSV curves were acquired at a scan rate in the potential range of 0.3 V - 1.1 V (vs. RHE). The half-wave potential of the oxygen reduction reaction was obtained from the LSV curves, and the kinetic current of the catalyst was calculated using the Koutecky-Levich (KL) equation. Additionally, the yield of hydrogen peroxide was obtained from the ring current, allowing for the calculation of the number of transferred electrons. All electrode potential data were 80% IR compensated.

[0071] (5) The results show that the MNC hybrid catalyst supported by noble metal alloy nanoparticles has excellent oxygen reduction reaction (ORR) activity, with a half-wave potential as high as 0.84 V (vs. RHE) and a mass activity as high as 4.302 A g at 0.85 V (vs. RHE). -1 .

[0072] Proton exchange membrane fuel cell testing

[0073] Take the Fe-NC prepared in Example 1 BrCl catalyst.

[0074] (1) Before testing, the catalyst needs to be prepared into a uniform ink. Weigh 7.875 mg of the catalyst and add it to a mixed solution containing 2 mL of isopropanol, 0.8 mL of ultrapure water and 170 μL of 5% Nafion solution. Sonicate for 3 h to form a uniform catalyst ink.

[0075] (2) Then, the MEA was prepared using the CCS (catalyst coated substrate) method, in which catalyst ink was coated onto the gas diffusion layer. The MEA-grade sheets were then fabricated sequentially as a gasket, gas diffusion layer, anode catalyst, proton exchange membrane, cathode catalyst, and gas diffusion layer by hot pressing. The MEA-grade sheets were then assembled into a fuel cell. A steady-state H2-O2 test was conducted at an open-circuit voltage of -0.25V (temperature 80 ℃; humidity 100% RH; H2 flow rate 0.3 L / min). -1 The O2 flow rate is 0.4 L / min. -1 (Absolute pressure is 150 kPa).

[0076] See results Figure 5 The peak power density of the fuel cell reached a maximum of 1.33 W / cm². -2 It shows great promise for practical application.

Claims

1. A method for preparing highly efficient Fe-NC catalysts fully doped with single or multiple halogens through stepwise heating, comprising the following steps: Step (1): Prepare ZIF-8 with a rhombic dodecahedral morphology; Step (2): Disperse a certain proportion of ZIF-8 and o-phenanthroline in an ethanol / water mixed solution to form ZIF-8@Phen complex; the mass ratio of ZIF-8 to o-phenanthroline is (10:1)-(10:5). Step (3): The ZIF-8@Phen precursor was transferred into a ceramic boat and calcined at 800-1000 °C for 60-120 min in an inert atmosphere to obtain the NC support. Step (4): Mix the iron salt, ammonium halide salt and NC support evenly and place them in a ceramic boat. The mass ratio of iron salt, ammonium halide salt and NC support is (2-20):(200-500):(50-200). The ammonium halide salt is one or more of ammonium fluoride, ammonium chloride, ammonium bromide and ammonium iodide. First, heat the mixture in an inert atmosphere at a low flow rate to 100℃-500℃ for 2-4 hours, and then continue to heat it to 800℃-1000℃ for 1-3 hours, and then let it cool naturally. Finally, Fe-NC catalysts with single or multiple halogens fully doped are obtained.

2. The method according to claim 1, characterized in that, Step (1) includes the following steps: Zinc nitrate hexahydrate and 2-methylimidazole, in a molar ratio of 1:(7-9), are dissolved in methanol solution respectively; then, the two solutions are mixed and stirred for 12 h-24 h; then, the mixed solution is centrifuged and washed multiple times; finally, the precipitate is dried in a vacuum drying oven at 70 ºC to obtain white ZIF-8 powder with a size of 80 nm-120 nm.

3. The method according to claim 1, characterized in that, In step (2), 0.5 g – 2.5 g ZIF-8 powder was uniformly dispersed in a mixed solution of 18-22 mL anhydrous ethanol and 8-12 mL ultrapure water, and then 100 mg – 600 mg o-phenanthroline was added and stirred for 8 h – 20 h. The solvent was then evaporated by stirring and heating at 65-75 ºC. Finally, the mixture was thoroughly dried in a vacuum drying oven at 60-80 ºC to obtain a white ZIF-8@Phen complex precursor.

4. The method according to claim 1, characterized in that, In step (4), 2 mg - 20 mg of iron salt, 200 mg - 500 mg of ammonium halide salt and 50 mg - 200 mg of NC carrier are thoroughly ground in an agate mortar.

5. The method according to claim 1, characterized in that... In step (4), the gas flow rate is 5-20 sccm when the temperature is raised to 100 ℃ - 500 ℃ in the first step; the heating rate is 1-3°C / min; the heating rate from 100 ℃ - 500 ℃ to 800 ℃ - 1000 ℃ is 5-20°C / min and the gas flow rate is 20-50 sccm.

6. The method according to claim 4, characterized in that... The iron salts mentioned are iron salts with a vaporization temperature below 800 ℃.

7. The Fe-NC catalyst fully doped with one or more halogens prepared by the method according to any one of claims 1-6.

8. The Fe-NC catalyst fully doped with one or more halogens according to claim 7, characterized in that: The iron loading is 0.5-2 wt%, the halogen doping is 0.2-1 wt%, and the specific surface area is 900-1300 m². 2 g -1 The size is 70-100 nm.

9. The application of the Fe-NC catalyst with single or multiple halogen fully doped according to claim 7 in electrocatalytic oxygen reduction reaction and in proton exchange membrane fuel cells.

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