Nitrogen-doped carbon nanotube supported platinum-based electrocatalysts and preparation and use thereof

Abstract

The application discloses a nitrogen-doped carbon nanotube loaded Pt-based electrocatalyst and a preparation method and application thereof, the catalyst is composed of noble metal Pt as an active component and nitrogen-doped carbon nanotubes as a carrier; specifically, pretreated carbon nanotubes and melamine are taken as carbon sources and nitrogen sources respectively by using an amination calcination method, nitrogen-doped carbon nanotubes are prepared after high-temperature pyrolysis, and the catalyst is synthesized by using an impregnation reduction method, taking sodium borohydride as a reducing agent and reducing a Pt precursor under intense stirring at room temperature. The catalyst carrier prepared by the application has a large specific surface area, Pt nanoparticles are uniformly dispersed on the carrier, and due to the strong interaction between Pt and the N-C carrier, the catalyst has high ammonia electrocatalytic oxidation activity and has a good application prospect in an alkaline membrane direct ammonia fuel cell.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of catalysts and fuel cells, and particularly relates to a nitrogen-doped carbon nanotube supported Pt-based electrocatalyst, a preparation method thereof and application thereof in ammonia electrocatalytic oxidation. BACKGROUND

[0002] In today's era, with the increasing depletion of non-renewable energy and the increasingly serious environmental pollution problem, the development and utilization of new energy is very important in various fields, especially in the field of power generation. The new energy commonly used for power generation at present includes solar energy, wind energy, nuclear energy, hydrogen energy, etc. Among them, hydrogen energy is used in fuel cells for power generation, which has been hailed as the fourth generation of power generation technology after water power, thermal power and nuclear power. It can directly convert the chemical energy stored in fuel into electrical energy, and its power generation efficiency avoids the limitation of Carnot cycle and the energy loss caused by multiple conversion processes. Moreover, in theory, as long as the fuel is continuously supplied, the fuel cell can continuously generate electricity, so it has good application prospects.

[0003] However, the high-pressure storage and transportation of hydrogen energy, flammability and explosiveness, and supply network and many other problems are not conducive to its large-scale popularization and application. In order to overcome these problems, ammonia, alcohol, hydrocarbons and other hydrogen-containing compounds are undoubtedly a feasible solution for storage and transportation. Compared with other hydrogen storage methods, ammonia is a potential hydrogen storage carrier because it has the advantages of high energy density, high hydrogen content, easy liquefaction at room temperature, low production cost, mature transportation and storage infrastructure, etc. Therefore, fuel cells can use ammonia as a power source to realize the future "hydrogen economy" through "ammonia economy".

[0004] However, considering the impact on the environment, ammonia is also one of the toxic pollutants in water discharge, which can cause eutrophication of the ecosystem. In addition, it has been reported that ammonia also poses a potential threat to public safety. Therefore, the research on ammonia electro-oxidation is an important problem in the field of energy and environment. The main problem of ammonia electro-oxidation at low temperature is that the ammonia oxidation reaction AOR on the anode catalyst is slower and more complex than HOR, which requires a high overpotential, so a highly efficient catalyst is needed. In the past few decades, scientists have made a lot of efforts to develop effective catalysts, but still have not found a catalyst with high activity comparable to platinum and long-term stability under actual fuel cell operating conditions. Therefore, platinum or platinum alloy is still the only realistic choice. However, as a noble metal, platinum has a high cost which seriously hinders the large-scale commercialization of ammonia electro-oxidation, so it is necessary to find a suitable catalyst carrier to reduce the use of platinum and improve the catalytic activity of platinum as a catalyst.

[0005] Currently, the most common support for electrocatalysts for ammonia electro-oxidation is carbon-based materials, which have large surface area, high electrical conductivity, porous structure and low cost. However, the original carbon materials are relatively chemically inert, cannot well anchor the active metal phase, and the bonding sites on the surface of the carbon-based materials are insufficient, which causes poor dispersibility and stability of the metal nanoparticles. Therefore, there is an urgent need to design an active carbon-based material with a functionalized surface, adjustable porosity and electronic properties. SUMMARY

[0006] In view of the above problems, the present application is prepared by pyridine N doping treatment on carbon materials, and then loading Pt metal, so as to obtain a supported nano Pt-based catalyst with high activity and stability, which is expected to be applied in alkaline membrane direct ammonia fuel cell.

[0007] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0008] A nitrogen-doped carbon nanotube supported Pt-based electrocatalyst, which comprises Pt as an active component and nitrogen-doped carbon nanotubes as a support; wherein the proportion of Pt in the catalyst is 8.0-12.5 wt.%, and the proportion of N in the support is 1-28 wt.%.

[0009] The preparation method of the nitrogen-doped carbon nanotube supported Pt-based electrocatalyst comprises the following steps:

[0010] (1) Add concentrated nitric acid (68 wt%) to carbon nanotubes, and stir vigorously in an 80℃ oil bath for 16h, then cool to room temperature, filter, wash with deionized water until neutral, and dry to obtain pretreated carbon nanotubes CNTs;

[0011] (2) Mix and grind the pretreated carbon nanotubes and melamine together, then place them in a crucible, and then put them into a tube furnace for carbonization pyrolysis in a nitrogen atmosphere, and then slowly cool to room temperature to obtain nitrogen-doped carbon nanotubes, marked as NC-x;

[0012] (3) At room temperature, mix the nitrogen-doped carbon nanotubes, isopropyl alcohol and deionized water uniformly under ultrasonic stirring, then add chloroplatinic acid solution and continue to mix uniformly under ultrasonic stirring, then slowly add excess sodium borohydride solution under stirring, and react for about 3-4h, then filter and wash until no Cl is detected in the filtrate with AgNO3 solution - , and vacuum dry the product to obtain the nitrogen-doped carbon nanotube supported Pt-based electrocatalyst, marked as Pt / NC-x.

[0013] Further, the temperature of the carbonization pyrolysis in step (2) is 600-700℃, and the time is 1-3h.

[0014] Further, the concentration of the chloroplatinic acid solution in step (3) is 19.3 mmol / L; and the concentration of the sodium borohydride solution is 0.158 mol / L.

[0015] Further, in step (3), the mixture should be ultrasonically treated for several hours to ensure the completion of the reaction.

[0016] Further, the temperature for the vacuum drying in step (3) is 60-70 ℃, and the time is 12-14 h.

[0017] The nitrogen-doped carbon nanotube supported Pt-based electrocatalyst described above can be used in the electrocatalytic oxidation reaction of ammonia, and thus can be used in the anode reaction of an alkaline membrane direct ammonia fuel cell.

[0018] The present application has the following advantages:

[0019] (1) Different from the preparation methods of other N-doped carbon materials, the present application modifies the carbon material by pyridine N-doping in a relatively simple method (amination calcination), and then synthesizes a nitrogen-doped carbon supported Pt-based catalyst by a simple impregnation liquid phase reduction method. The prepared nitrogen-doped carbon support has a large specific surface area, and the anchored active metal Pt particles have a small particle size and are uniformly dispersed, which can increase the contact area of the catalyst active center and the reactants, and the strong interaction between the metal and the support also helps to improve the ammonia oxidation (AOR) activity.

[0020] (2) The Pt / N-CNTs catalyst prepared by the present application has a better catalytic performance than the commercial Pt / C catalyst when applied in the electrocatalytic oxidation reaction of ammonia.

[0021] (3) The required materials of the present application have a relatively low cost, and the preparation process is simple and easy to operate. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 XRD comparison chart of the Pt / C and Pt / NC-x catalysts prepared for the comparative examples and the examples;

[0023] Figure 2 TEM morphology comparison chart of the Pt / CNTs (A) and Pt / NC-3 (B) catalysts prepared for the examples;

[0024] Figure 3 BET comparison chart of the NC-x support and the Pt / NC-x catalysts prepared for the examples;

[0025] Figure 4 Cyclic voltammogram of the Pt / NC-x catalyst prepared for the example in an ammonia solution;

[0026] Figure 5Cyclic voltammograms of Pt / NC-3 catalyst prepared for the example in ammonia and base solution saturated with argon;

[0027] Figure 6 Structure-activity relationship of N in Pt / NC-x catalysts prepared for the example;

[0028] Figure 7 Power density plots of Pt / NC-3 catalyst prepared for the example in single cell compared with commercial Pt / C catalyst. DETAILED DESCRIPTION

[0029] In order to make the content described in the present application more convenient to understand, the technical solutions described in the present application are further described below in combination with specific embodiments, but the present application is not limited to this.

[0030] Example 10 Preparation of 10 wt.% Pt / NC-x catalysts

[0031] The carbon nanotubes were poured into a round-bottom flask, and an appropriate amount of concentrated nitric acid (68 wt%) was added. The mixture was stirred vigorously in an 80°C oil bath for 16 h, then cooled to room temperature, filtered, washed with deionized water until neutral, and dried in an oven at 110°C to obtain the pretreated CNTs.

[0032] 1 g of pretreated CNTs was ground with different amounts (0 g, 0.0976 g, 0.1699 g, 0.2182 g, 0.4563 g, and 0.5061 g, respectively) of melamine in a mortar to form a uniform mixture, then transferred to tin paper for wrapping, and the tin paper-wrapped mixture was transferred to a high-temperature-resistant quartz reaction tube in a tube furnace. Under the condition of flowing N2 atmosphere, the mixture was carbonized and pyrolyzed at 700°C for 2-3 h. After the tube furnace was slowly cooled to room temperature, nitrogen-doped carbon nanotubes were obtained. Finally, the support was named NC-x, where x represents the N content in the corresponding support, and the naming was gradually increased according to the N content, corresponding to CNTs, NC-1, NC-2, NC-3, NC-4, and NC-5 (the N contents were 0%, 1%, 2%, 3%, 4%, and 5%, respectively. 0%, 1.01%, 2.42%, 4.78%, 11.25%, and 27.45%)

[0033] About 0.0343 g of NaBH4crystals (20-40 times the molar mass of Pt metal) were weighed into 10 mL of 0.1 mol / l NaOH solution and mixed thoroughly by ultrasonic agitation to give a NaBH4solution. At room temperature, 50 mg of the NC-x support prepared above was weighed into a 200 mL beaker, 25 mL of deionized water and 25 mL of isopropanol were added, and the mixture was ultrasonically agitated for 30 min and then stirred vigorously using a magnetic stirrer for 30 min to form a uniform dispersion. 1.325 mL of a 19.3 mmol / L solution of chloroplatinic acid was added and stirred for 30 min to allow the precursor of the metal salt to be uniformly impregnated on the surface of the support. The prepared NaBH4solution was then slowly added dropwise to the above dispersion using a rubber-tipped dropper, and after the addition was complete, the mixture was ultrasonically agitated for a further 3 h. After the reaction was complete, the sample was collected by centrifugation and filtered and washed until no Cl - was detected in the filtrate using AgNO3solution, and then dried in a vacuum oven at 60 °C for more than 12 h to give 10 wt.% Pt / NC-x.

[0034] Preparation of Comparative Example 10 wt.% Pt / C (C is commercial XC-72) catalyst

[0035] About 0.0343 g of NaBH4crystals (20-40 times the molar mass of Pt metal) were weighed into 10 mL of 0.1 mol / l NaOH solution and mixed thoroughly by ultrasonic agitation to give a NaBH4solution. At room temperature, 50 mg of the NC-x support prepared above was weighed into a 200 mL beaker, 25 mL of deionized water and 25 mL of isopropanol were added, and the mixture was ultrasonically agitated for 30 min and then stirred vigorously using a magnetic stirrer for 30 min to form a uniform dispersion. 1.325 mL of a 19.3 mmol / L solution of chloroplatinic acid was added and stirred for 30 min to allow the precursor of the metal salt to be uniformly impregnated on the surface of the support. The prepared NaBH4solution was then slowly added dropwise to the above dispersion using a rubber-tipped dropper, and after the addition was complete, the mixture was ultrasonically agitated for a further 3 h. After the reaction was complete, the sample was collected by centrifugation and filtered and washed until no Cl - was detected in the filtrate using AgNO3solution, and then dried in a vacuum oven at 60 °C for more than 12 h to give 10 wt.% Pt / NC-x.

[0036] The loading of the metal Pt was measured by inductively coupled plasma mass spectrometry (ICP-MS).

[0037] Physical structure characterization test: XRD test was performed on X'pert Pro powder diffractometer (Panalytical, Netherlands) equipped with X'Celerator detector. Cu Kα radiation (λ = 0.154 06 nm) was used, the working voltage was 45 kV, the working current was 40 mA, and the test range was: 2θ = 10-90 °.

[0038] Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM-EDX) images were collected on a Philips TECNAI G2 F20 field emission electron microscope equipped with energy dispersive X-ray (EDX) and high-angle annular dark field (HAADF).

[0039] The specific surface area and pore structure of the catalyst sample were tested at a liquid nitrogen temperature of-196 ℃ by using a gas adsorption instrument (ASAP 2020) of Micrometrics, USA. Specifically, the sample was pretreated at 300 ℃ under vacuum for 4 h to remove the adsorbents on the surface of the sample, and then the pore structure and specific surface area of the precursor were calculated according to the mesopore analysis model (BJH) and the BET method, respectively.

[0040] The test method of the prepared catalyst in an alkaline ammonia solution is as follows:

[0041] 4.1-5.0 mg of the catalyst, 900-950 μL of isopropyl alcohol, 40.0-42.4 μL of ultrapure water and 10.1-11.2 μL of Nafion film solution were weighed in a 3 mL weighing bottle, and a uniform dispersion solution was obtained by ultrasonic oscillation under an ice water bath for more than 30 min. Then 9.8 μL of the dispersion solution was carefully dropped onto a glassy carbon electrode, and the thin film electrode was obtained by natural drying at room temperature.

[0042] All electrochemical tests were tested in a standard five-port electrolytic cell, with the catalyst-coated glassy carbon electrode as the working electrode, the graphite rod as the counter electrode, and the mercury / mercury oxide electrode as the reference electrode. The electrolyte system was 1 M KOH solution and 1 M KOH + 0.1 M NH3 solution (the electrolyte was prepared with high-purity reagents and ultrapure water). During the test, the electrode was first activated in the blank KOH solution, and then tested in the 0.1 M ammonia alkaline solution, with the scanning voltage range of-0.85 V~0.10 V and the scanning rate of 5 mV s -1Stability was tested at a constant voltage of 0.65 V. The entire electrolytic cell was maintained at an operating temperature of approximately 25 °C. The potential relative to the reversible hydrogen electrode (RHE) was calculated using the formula E(RHE) = E(Hg / HgO) + 0.0592 pH + 0.098 (25 °C). Before each test, all reference electrodes were calibrated to the RHE in the same electrolyte solution. For the preparation of the working electrode, the glassy carbon electrode surface was polished with alumina (50 nm) polishing powder before each electrochemical experiment. The noble metal loading on all catalysts was 10%, and the elemental atomic ratios were determined by ICP-OES measurement.

[0043] Figure 1 The XRD patterns of the Pt / C and Pt / NC-x catalysts prepared in the comparative examples and embodiments are shown in the figure. The figure shows that well-resolved diffraction peaks appear at 2θ = 39.8°, 46.4°, and 67.7°, which correspond to Pt's (111), (200), and (220) peaks, respectively, proving that Pt was successfully loaded onto the support.

[0044] Figure 2 The images show a TEM morphology comparison of the Pt / CNTs and Pt / NC-3 catalysts prepared for the examples. It is clearly shown that the Pt metal particles on the Pt / NC-3 catalyst are approximately 2.5 nm in size and uniformly dispersed on the support without agglomeration. Furthermore, compared with Pt / CNTs, the introduction of N creates defects in the carbon nanotubes. These defect sites on the support surface facilitate the attachment and nucleation of metal nanoparticles, promoting particle dispersion and preventing growth. The smaller particle size also contributes to improved catalytic activity. On the other hand, the increased number of defect sites also promotes the adsorption of reactants on the catalyst surface, which is beneficial for the catalytic reaction.

[0045] Table 1 and Figure 3 The figures show the structural information and BET comparison diagrams of the NC-x support and Pt / NC-x catalyst prepared in the examples. As can be seen from the figures, after loading Pt metal onto the support, the specific surface area of ​​the catalyst increases compared to the corresponding support, while the pore volume and pore size do not change significantly. This indicates that the Pt metal is loaded on the surface of the CNTs support rather than within the pores, which facilitates contact between the active metal component and reactant molecules, thus improving the catalytic performance of the catalyst.

[0046] Table 1-1 Structural Information of NC-x Carriers

[0047]

[0048] Table 1-2 Structural information of Pt / NC-x catalysts

[0049]

[0050] Figure 4 The cyclic voltammograms of different Pt / NC-x catalysts prepared for the examples in ammonia solution. As can be seen from the figure, the peak current density of Pt / C is 19 A g -1 Pt , the peak current density of Pt / NC-3 catalyst is 52 A g -1 Pt , and the electrocatalytic activity is obviously improved. At the same time, it can also be seen that with the gradual increase of N content, the AOR activity presents a volcano type change rule, which proves that it is not the more the N content, the better the activity. Combined with the specific surface area analysis, with the increase of N content, the specific surface area presents a gradually decreasing trend. This further proves that it is not the larger the specific surface area of the catalyst, the better the catalytic activity.

[0051] Figure 5 The cyclic voltammograms of the Pt / NC-3 catalyst prepared for the example in the ammonia and alkali solution saturated with argon. When the potential is scanned positively, the oxidation peak of NH3 appears in the potential range of 0.4-0.8 V vs . RHE, and the curve decreases exponentially at a higher high potential (0.6-0.8 V vs . RHE), which is due to the blocking of the active Pt surface by the strongly adsorbed reaction intermediates (such as N ad and NO ad ), and the generation of nitrogen oxygen species (such as NO and N2O), which reduces the selectivity of N2 generation; in the potential scanning process, the basic no peak current appears in the negative retrace process, indicating that the AOR is a completely irreversible reaction. The selectivity of N2 and the reaction kinetics in different potential ranges have an influence in the potential scanning process.

[0052] Figure 6 The structure-activity relationship diagram of N and activity in the Pt / NC-x catalyst prepared for the example. As shown in the figure, the AOR peak current density is positively correlated with the pyridine N species. From this, it can be inferred that when platinum is loaded on NC to prepare a catalyst, the pyridine N species plays a crucial role, which provides a brand-new strategy for the subsequent design of NC materials.

[0053] Figure 7 The power density curve comparison diagram of the Pt / NC-3 catalyst prepared for the example and the commercial Pt / C catalyst in a single cell. As shown in the figure, the power density of the Pt / NC-3 as an anode catalyst is higher than that of the commercial Pt / C.

[0054] The above only describes the preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.

Claims

1. A nitrogen-doped carbon nanotube supported Pt-based electrocatalyst for ammonia electrocatalytic oxidation comprising an active component and a support, characterized in that, The active component is Pt, and the carrier is nitrogen-doped carbon nanotubes; wherein the proportion of Pt in the catalyst is 10 wt%, and the proportion of N in the carrier is 3 wt%; The preparation of the nitrogen-doped carbon nanotube-supported Pt-based electrocatalyst comprises the following steps: 1) Pour the carbon nanotubes into a round-bottom flask, add concentrated nitric acid with a mass concentration of 68%, and stir vigorously in an 80°C oil bath for 16h. Then cool to room temperature, filter, wash with deionized water until neutral, and dry in an oven at 110°C. The pretreated carbon nanotubes are obtained; 2) Grind 1g of the pretreated carbon nanotubes and 0.1699g of melamine in a mortar to form a uniform mixture, then transfer it to tin paper for wrapping, and transfer the tin paper-wrapped mixture to a high-temperature-resistant quartz reaction tube in a tube furnace. Carbonize and pyrolyze under the condition of flowing N2 atmosphere at 700°C for 2-3h. Slowly cool the tube furnace to room temperature to obtain nitrogen-doped carbon nanotubes. The obtained nitrogen-doped carbon nanotubes contain pyridine nitrogen. 3) 0.0343 g of NaBH4crystal was weighed and dissolved in 10 mL of NaOH solution, the concentration of the NaOH solution used was 0.1 mol / L, and it was mixed thoroughly by ultrasonic stirring to obtain a NaBH4solution; at room temperature, 50 mg of the nitrogen-doped carbon nanotubes prepared above was weighed into a 200 mL beaker, 25 mL of deionized water and 25 mL of isopropyl alcohol were added, and after ultrasonic stirring for 30 min, it was stirred vigorously by a magnetic stirrer for 30 min to form a uniform dispersion liquid; 1.325 mL of chloroplatinic acid solution with a concentration of 19.3 mmol / L was added and stirred for 30 min to make the chloroplatinic acid uniformly impregnated on the surface of the nitrogen-doped carbon nanotubes to obtain a dispersion system, and then the prepared NaBH4solution was slowly added dropwise into the above dispersion system by using a rubber bulb dropper, and after the addition was completed, it was continuously stirred by ultrasonic stirring for 3 h, and after the reaction was completed, the sample was collected by centrifugation, and the sample was filtered and washed until no Cl - was detected in the filtrate by using an AgNO3solution, and then it was dried in a vacuum oven at 60°C for more than 12 h to obtain a nitrogen-doped carbon nanotube-supported Pt-based electrocatalyst.

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