A method for preparing and applying an Fe-N / C oxygen reduction catalyst

Abstract

This invention relates to the field of fuel cell technology, and in particular to a method for preparing and applying a Fe-N / C oxygen reduction catalyst. The preparation method includes the following steps: reacting 2-methylimidazole with a non-noble metal source in a solvent to obtain a precursor; subjecting the precursor to a first heat treatment under an inert atmosphere to obtain a first intermediate product; acid-washing the first intermediate product, mixing it with ammonium fluoride, and subjecting it to a second heat treatment under an inert atmosphere to obtain a second intermediate product; and acid-washing the second intermediate product to obtain the Fe-N / C oxygen reduction catalyst. This invention successfully prepares a Fe-N / C oxygen reduction catalyst with high active site exposure, a hierarchical porous structure, and excellent stability through synergistic modification by in-situ vapor-phase etching with ammonium fluoride and gradient heat treatment.

Description

Technical Field

[0001] This invention relates to the field of fuel cell technology, and in particular to a method for preparing and applying an Fe-N / C oxygen reduction catalyst. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) are considered crucial energy conversion devices for future sustainable energy systems due to their high energy conversion efficiency and environmental friendliness. However, their cathode oxygen reduction reaction (ORR) kinetics are relatively slow, currently relying primarily on platinum (Pt)-based noble metal catalysts to enhance the reaction rate. The high cost and resource scarcity of Pt-based catalysts limit the large-scale commercial application of PEMFCs. Therefore, developing low-cost, highly active, and highly stable non-platinum-based oxygen reduction catalysts has become a research hotspot in this field.

[0003] Among numerous non-precious metal catalysts, iron-nitrogen-doped carbon materials (Fe-N / C) stand out due to their unique Fe-N composition. x Fe-N / C catalysts are considered one of the most promising alternatives to Pt-based catalysts due to their active site structure, good conductivity, and relatively low preparation cost. However, existing technologies for preparing Fe-N / C catalysts still suffer from the following main drawbacks: First, the utilization rate of active sites is low. In conventional high-temperature carbonization processes, Fe-N... x The active sites are easily covered by amorphous carbon layers or graphitic carbon layers, resulting in a large number of active sites being buried inside the carbon matrix or under the surface carbon layer, unable to fully contact oxygen molecules in the electrolyte. Although the total number of active sites in the catalyst is large, the proportion of effective sites actually participating in the catalytic reaction is low, limiting the overall catalytic activity. Second, the pore structure is simple, resulting in poor mass transfer performance. Existing preparation methods usually cannot precisely control the pore structure of the catalyst. Most Fe-N / C catalysts are mainly microporous, lacking sufficient mesopores or macropores. The simple microporous structure is not conducive to the rapid diffusion of oxygen molecules into the active sites inside the catalyst, nor is it conducive to the timely desorption of reaction products. As a result, mass transfer polarization is prone to occur in the high current density region, reducing the limiting current density and the overall battery performance. Third, it is difficult to balance catalytic activity and long-term stability. In order to improve catalytic activity, existing technologies often use high pyrolysis temperatures to promote the formation of active sites. However, high-temperature treatment can easily lead to disordered reconstruction or even collapse of the carbon skeleton, or cause the agglomeration of metal species, thereby impairing the long-term operational stability of the catalyst.

[0004] Therefore, providing a Fe-N / C oxygen reduction catalyst and its preparation method that can effectively improve the exposure of active sites, optimize pore structure to improve mass transfer performance, and simultaneously achieve high activity and high stability is of great significance for promoting the practical application of non-precious metal catalysts in the field of fuel cells. Summary of the Invention

[0005] The purpose of this invention is to address the problems existing in the prior art by providing a method for preparing Fe-N / C oxygen reduction catalysts and their applications. Through the synergistic modification of in-situ vapor-phase etching with ammonium fluoride and gradient thermal treatment, active sites are exposed, pore structure is optimized, and the stability of the catalyst is improved.

[0006] To achieve the above objectives, the present invention provides a method for preparing a Fe-N / C oxygen reduction catalyst, comprising the following steps: S1. 2-Methylimidazole is reacted with a non-precious metal source in a solvent to obtain the precursor; S2. The precursor is subjected to a first heat treatment under an inert atmosphere to obtain a first intermediate product; S3. After acid washing of the first intermediate product, it is mixed with ammonium fluoride and subjected to a second heat treatment under an inert atmosphere to obtain the second intermediate product; S4. The second intermediate product is acid-washed to obtain the Fe-N / C oxygen reduction catalyst.

[0007] In an optional embodiment, in S1, the non-precious metal source includes iron acetylacetone and zinc nitrate hexahydrate; the mass ratio of iron acetylacetone to zinc nitrate hexahydrate is (1.5-3.5):(7-9).

[0008] In an optional embodiment, in S1, the solvent is selected from methanol; the ratio of the mass of the 2-methylimidazole, the mass of the non-precious metal source, and the volume of the solvent is (16.5-18.5) g : (8.5-12.5) g : (350-550) mL.

[0009] In an optional embodiment, in S1, the reaction temperature is 20-30°C and the time is 3-5 hours.

[0010] In an optional embodiment, S1 specifically includes: adding 2-methylimidazole to methanol and sonicating it to fully dissolve it, obtaining solution A; adding a non-precious metal source to methanol and stirring until completely dissolved, obtaining solution B. Under stirring conditions, solution A is added dropwise to solution B, and then the reaction is carried out under continuous stirring to obtain a precursor suspension. The precursor suspension is centrifuged to separate the solid product, which is then washed with ethanol and dried to obtain the precursor.

[0011] In one optional embodiment, the centrifugation speed is 6000-8000 r / min and the time is 1-3 min; the ethanol washing is performed ≥2 times; and the drying temperature is 50-70℃ and the time is 12-24 h.

[0012] In an optional embodiment, in S2, the inert atmosphere is selected from nitrogen or argon; the heating rate of the first heat treatment is 5-10℃ / min, the target temperature is 600-900℃, and the holding time is 1-3h.

[0013] In an optional embodiment, in S3, the pickling reagent is a sulfuric acid solution with a concentration of 0.3-0.7 mol / L; the volume ratio of the pickling reagent to the mass of the first intermediate product is (50-100) mL: (0.5-1.5) g; the pickling temperature is 20-30℃ and the time is 6-12 h.

[0014] In an optional embodiment, in step S3, the first intermediate product is acid-washed, then washed with water and dried sequentially, before being mixed with ammonium fluoride. The drying temperature is 50-70°C, and the time is 12-24 hours. The mass ratio of the first intermediate product to ammonium fluoride is 1:(0.5-2). Here, the mass ratio of the first intermediate product to ammonium fluoride refers to the mass ratio of the dried first intermediate product to ammonium fluoride.

[0015] In an optional embodiment, in S3, the inert atmosphere is selected from nitrogen or argon; the heating rate of the second heat treatment is 5-10℃ / min, the target temperature is 900-1100℃, and the holding time is 1-3h.

[0016] In an optional embodiment, the pickling reagent in S4 is a sulfuric acid solution with a concentration of 0.3-0.7 mol / L; the volume ratio of the pickling reagent to the mass of the second intermediate product is (50-100) mL: (0.5-1.5) g; the pickling temperature is 20-30°C, and the pickling time is 6-12 h.

[0017] In an optional embodiment, in step S4, the second intermediate product is acid-washed, then sequentially washed with water and dried to obtain the Fe-N / C oxygen reduction catalyst. The drying temperature is 50-70°C, and the time is 12-24 hours.

[0018] The present invention also provides an Fe-N / C oxygen reduction catalyst, which is prepared by the aforementioned preparation method.

[0019] The present invention also provides the application of the Fe-N / C oxygen reduction catalyst in the preparation of fuel cell cathode catalysts.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) In the second heat treatment process of the present invention, the HF gas generated by the high-temperature decomposition of ammonium fluoride can selectively chemically corrode the amorphous carbon and unstable metal species in the carbon matrix, thereby effectively removing the excess carbon layer covering the surface of the active site and directionally exposing the embedded Fe-N x Active sites. Compared to existing technologies where active sites are easily covered by carbon layers, resulting in a large number of sites being unable to participate in the catalytic reaction, this invention exposes more active sites to the reaction interface, effectively improving the utilization rate of active sites and thus enhancing the apparent catalytic activity of the catalyst.

[0021] (2) This invention utilizes the chemical corrosion effect of HF gas generated by the decomposition of ammonium fluoride and the pore-forming effect of NH3 gas. The synergistic effect of these two factors allows the catalyst to retain its original microporous structure while introducing a large number of mesopores. This multi-level pore structure with coexistence of micropores and mesopores facilitates the rapid diffusion of oxygen molecules into the active sites inside the catalyst and promotes the timely desorption of reaction products, effectively reducing mass transfer polarization. Therefore, the catalyst prepared in this invention exhibits a higher H2-air peak power density in PEMFCs, and its overall mass transfer performance is superior to that of Fe-N / C catalysts with a single pore structure in the prior art.

[0022] (3) This invention employs a gradient heat treatment process: the first heat treatment is carried out at a relatively low temperature to achieve pre-carbonization of the precursor and form a preliminary stable carbon skeleton; the second heat treatment is carried out at a higher temperature, utilizing the atmosphere generated by the in-situ decomposition of ammonium fluoride to complete etching, doping, and further graphitization. By separating and controlling the carbonization process from the etching process, the catalyst structure collapse caused by excessive etching or disordered reconstruction of the carbon skeleton in a single high-temperature treatment is avoided. At the same time, the acid washing step effectively removes unstable metal species, further improving the stability of the catalyst. Therefore, the Fe-N / C catalyst prepared by this invention maintains high oxygen reduction catalytic activity while exhibiting excellent electrochemical stability and long-term fuel cell operating stability, overcoming the shortcomings of existing technologies where activity and stability are difficult to achieve simultaneously.

[0023] (4) The preparation method provided by this invention uses readily available raw materials, involves simple steps, and requires no complex equipment. By adjusting the proportion of ammonium fluoride, the etching and doping levels can be easily controlled to meet different performance requirements. This process is suitable for industrial-scale production and has good application prospects. Attached Figure Description

[0024] Figure 1 These are TEM characterization images of the Fe-N / C oxygen reduction catalysts in Example 1 and Comparative Example 1 of this invention at a 200 nm scale. Figure 1 In this context, 'a' represents Comparative Example 1 and 'b' represents Example 1. Figure 2This is an AC-STEM characterization image of the Fe-N / C oxygen reduction catalyst in Example 1 of this invention on a 5 nm scale. Figure 3 This is a characterization diagram of the specific surface area and pore structure of the Fe-N / C oxygen reduction catalyst in Example 1 of this invention; Figure 3 In the diagram, 'a' represents the specific surface area curve, and 'b' represents the pore structure distribution diagram. Figure 4 These are SCV polarization curves of the Fe-N / C oxygen reduction catalysts prepared in Examples 1-3 and Comparative Example 1 of this invention; Figure 5 This is a comparison of the SCV curves of the Fe-N / C catalysts prepared in Example 1 and Comparative Example 1 before and after 50,000 ADT cycles. Figure 5 In this context, 'a' represents Comparative Example 1 and 'b' represents Example 1. Figure 6 This is a comparison chart of the hydrogen fuel cell performance of the catalysts in Example 1 and Comparative Example 1 of the present invention; Figure 7 This is a comparison chart of the hydrogen fuel cell stability of the catalysts in Example 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0025] The following embodiments are provided to better understand the present invention and are not limited to the described embodiments. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0026] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0027] Example 1 This embodiment provides a method for preparing a Fe-N / C oxygen reduction catalyst, including the following steps: 17.6 g of 2-methylimidazole was added to 200 mL of methanol and sonicated until fully dissolved to obtain solution A. 2.4 g of ferric acetylacetone and 8 g of zinc nitrate hexahydrate were added to 250 mL of methanol and stirred at 400 rpm until completely dissolved to obtain solution B. Under stirring at 400 rpm, solution A was added dropwise to solution B, and the reaction was carried out under continuous stirring at 25 °C for 4 h to obtain a precursor suspension. The precursor suspension was centrifuged (7000 rpm, 2 min) to separate the solid product, which was then washed three times with ethanol and dried at 50 °C for 12 h to obtain the precursor.

[0028] The precursor was heated to 750°C at a heating rate of 5°C / min under a nitrogen atmosphere for the first heat treatment, and held at that temperature for 2 hours to obtain the first intermediate product.

[0029] The first intermediate product was immersed in a 0.5 mol / L sulfuric acid solution (the volume ratio of the sulfuric acid solution to the mass of the first intermediate product was 75 mL: 1 g), acid-washed at 25 °C for 6 h, washed with water after acid washing, and dried at 50 °C for 18 h. Then, the dried first intermediate product was mixed with ammonium fluoride at a mass ratio of 1:1, and heated to 1000 °C at a heating rate of 5 °C / min under a nitrogen atmosphere for a second heat treatment, which was held at this temperature for 2 h to obtain the second intermediate product.

[0030] The second intermediate product was soaked in a 0.5 mol / L sulfuric acid solution (the volume ratio of the sulfuric acid solution to the mass of the second intermediate product was 75 mL: 1 g), acid-washed at 25 °C for 6 h, washed with water after acid washing, and dried at 50 °C for 18 h to obtain the Fe-N / C oxygen reduction catalyst.

[0031] Example 2 This embodiment is basically the same as Example 1, except that the mass ratio of the dried first intermediate product to ammonium fluoride is modified to 1:2.

[0032] Example 3 This embodiment is basically the same as Example 1, except that the mass ratio of the dried first intermediate product to ammonium fluoride is modified to 1:0.5.

[0033] Comparative Example 1 The only difference between this comparative example and Example 1 is that the addition of ammonium fluoride is omitted.

[0034] Comparative Example 2 In this comparative example, the precursor prepared in Example 1 was directly mixed with ammonium fluoride at a mass ratio of 1:1, and then heated to 1000°C at a heating rate of 5°C / min under a nitrogen atmosphere for heat treatment and held at that temperature for 2 hours.

[0035] The results showed that the product was almost completely volatilized, with a yield close to 0. This indicates that if the precursor is directly subjected to high-temperature treatment with ammonium fluoride without a first heat treatment, the carbon skeleton will not exist stably. Therefore, a first heat treatment is necessary to form a preliminary stable carbon structure.

[0036] Experimental Example 1 The Fe-N / C oxygen reduction catalysts prepared in Example 1 and Comparative Example 1 were characterized by transmission electron microscopy (TEM). The TEM images of the Fe-N / C oxygen reduction catalysts in Example 1 and Comparative Example 1 under a 200 nm scale were obtained, as shown below. Figure 1 As shown; Figure 1 In the figures, 'a' represents Comparative Example 1 and 'b' represents Example 1. The Fe-N / C oxygen reduction catalyst of Example 1 was further characterized by aberration-corrected scanning transmission electron microscopy (AC-STEM), yielding the AC-STEM characterization image of the Fe-N / C oxygen reduction catalyst in Example 1 at a 5 nm scale, as shown below. Figure 2 As shown.

[0037] Furthermore, the specific surface area and pore structure of the Fe-N / C oxygen reduction catalyst in Example 1 were characterized, resulting in the specific surface area and pore structure characterization diagrams of the Fe-N / C oxygen reduction catalyst in Example 1, as shown below. Figure 3 As shown; Figure 3 In the diagram, 'a' represents the specific surface area curve, and 'b' represents the pore structure distribution diagram.

[0038] from Figure 1 and Figure 2 It can be seen that the catalyst structure of Example 1 has changed significantly, such as the dodecahedral structure being etched and destroyed; at the same time, the metallic Fe in the catalyst exhibits an atomic-level distribution, corresponding to the bright spots in the figure. Figure 3 This indicates that the catalyst possesses a typical microporous-mesoporous hierarchical structure. The above results demonstrate that the Fe-N / C oxygen reduction catalyst prepared by the method of this invention has a hierarchical porous structure, which is beneficial for reactant mass transfer and ensures sufficient exposure of active sites.

[0039] Experimental Example 2 The Fe-N / C oxygen reduction catalysts prepared in Examples 1-3 and Comparative Example 1 were subjected to stepped cyclic voltammetry (SCV) testing. The test conditions were as follows: at room temperature, using a three-electrode system in an O2-saturated 0.5 mol / L H2SO4 electrolyte, SCV scanning was performed on the working electrode with a step voltage of 0.03 V, a waiting time of 30 s for each potential, and a rotating disk electrode (RDE) speed of 900 r / min. The oxygen reduction reaction (ORR) performance of the catalysts was evaluated by comparing the obtained polarization curves. The SCV polarization curves of the Fe-N / C oxygen reduction catalysts prepared in Examples 1-3 and Comparative Example 1 are shown below. Figure 4 As shown.

[0040] from Figure 4 It can be seen that the half-wave potentials of Examples 1, 2 and 3 reached 0.85V, 0.845V and 0.82V respectively, which are all higher than 0.815V of Comparative Example 1, indicating that the catalyst prepared by the present invention has excellent electrochemical performance.

[0041] Experimental Example 3 Accelerated durability testing (ADT) was performed on the Fe-N / C oxygen reduction catalysts prepared in Example 1 and Comparative Example 1, respectively. The tests were conducted in an O2-saturated 0.5 mol / L H2SO4 electrolyte, with a potential cycling range of 0.60 V to 0.80 V and a scan rate of 100 mV·s. -1 The catalyst was cycled a total of 50,000 times. After ADT, a stepped cyclic voltammetry (SCV) test was performed again. The SCV test conditions were: room temperature, three-electrode system, O2-saturated 0.5 mol / L H2SO4 electrolyte, step voltage of 0.03 V, waiting time of 30 s for each potential, and RDE speed of 900 r / min. The stability of the catalyst was evaluated by comparing the SCV curves before and after ADT. A comparison of the SCV curves of the Fe-N / C catalysts prepared in Example 1 and Comparative Example 1 before and after 50,000 ADT cycles is shown in the figure. Figure 5 As shown; Figure 5 In the text, 'a' represents Comparative Example 1 and 'b' represents Example 1.

[0042] from Figure 5 As can be seen from 'a', after 50,000 ADT cycles, the half-wave potential of Comparative Example 1 shifts by 25mV. Figure 5 As shown in b, under the same conditions, the limiting current density retention rate of Example 1 is 98.8%, and the half-wave potential shift is only 10 mV. These results indicate that the Fe-N / C catalyst prepared in this invention exhibits superior electrochemical stability.

[0043] Experiment Example 4 The Fe-N / C oxygen reduction catalyst prepared in Example 1 and the Fe-N / C catalyst prepared in Comparative Example 1 were used as cathode catalysts in hydrogen fuel cells, and their voltage-current density curves and power-current density curves were tested. Test conditions: temperature 80℃, humidity 100%, pressure 200 kPa, and with air introduced, the anode gas to cathode gas flow rate ratio was 0.7:1.7 (L·min). -1 The performance comparison chart of the hydrogen fuel cells of the catalysts in Example 1 and Comparative Example 1 is obtained, as shown in the figure. Figure 6 As shown.

[0044] from Figure 6 It can be seen that the peak power density of the hydrogen fuel cell prepared by the catalyst in Example 1 reaches 550 mW·cm⁻¹. -2 This is higher than the 410 mW·cm⁻¹ of the battery prepared with the catalyst in Comparative Example 1. -2 This indicates that the catalyst of the present invention has better power output performance under actual fuel cell operating conditions.

[0045] Experimental Example 5 The Fe-N / C oxygen reduction catalyst prepared in Example 1 and the Fe-N / C catalyst prepared in Comparative Example 1 were respectively assembled into hydrogen fuel cells, and accelerated durability testing (ADT) was conducted. Test conditions: temperature 80℃, humidity 100%, pressure 200 kPa, and with air introduced, the anode gas to cathode gas flow rate ratio was 0.2:0.2 (L·min). -1 The stability comparison chart of the hydrogen fuel cells of the catalysts in Example 1 and Comparative Example 1 is obtained, as shown in the figure. Figure 7 As shown.

[0046] from Figure 7 As can be seen, the hydrogen fuel cell prepared with the catalyst in Example 1 maintained a current density retention rate of 77.2% after 100 hours of operation at a constant voltage of 0.6V; while the current density of the battery in Comparative Example 1 decreased to 71.2% after only 35 hours of operation. These results indicate that the catalyst prepared in this invention has superior stability during long-term operation of fuel cells.

[0047] In summary, this invention successfully prepared a Fe-N / C oxygen reduction catalyst with high active site exposure, hierarchical porous structure, and excellent stability through synergistic modification by in-situ vapor-phase etching of ammonium fluoride and gradient thermal treatment.

[0048] Finally, it should be noted that the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a Fe-N / C oxygen reduction catalyst, characterized in that, Includes the following steps: S1. 2-Methylimidazole is reacted with a non-precious metal source in a solvent to obtain the precursor; S2. The precursor is subjected to a first heat treatment under an inert atmosphere to obtain a first intermediate product; S3. After acid washing of the first intermediate product, it is mixed with ammonium fluoride and subjected to a second heat treatment under an inert atmosphere to obtain the second intermediate product; S4. The second intermediate product is acid-washed to obtain the Fe-N / C oxygen reduction catalyst.

2. The preparation method according to claim 1, characterized in that, In S1, the non-precious metal source includes iron acetylacetone and zinc nitrate hexahydrate; the mass ratio of iron acetylacetone to zinc nitrate hexahydrate is (1.5-3.5):(7-9).

3. The preparation method according to claim 1, characterized in that, In S1, the solvent is selected from methanol; the ratio of the mass of the 2-methylimidazole, the mass of the non-precious metal source, and the volume of the solvent is (16.5-18.5) g : (8.5-12.5) g : (350-550) mL.

4. The preparation method according to claim 1, characterized in that, In S1, the reaction temperature is 20-30℃ and the time is 3-5h.

5. The preparation method according to claim 1, characterized in that, In S2, the heating rate of the first heat treatment is 5-10℃ / min, the target temperature is 600-900℃, and the holding time is 1-3h.

6. The preparation method according to claim 1, characterized in that, In S3, the pickling reagent is a sulfuric acid solution with a concentration of 0.3-0.7 mol / L; the pickling temperature is 20-30℃ and the time is 6-12 h; the mass ratio of the first intermediate product to ammonium fluoride is 1:(0.5-2).

7. The preparation method according to claim 1, characterized in that, In S3, the heating rate of the second heat treatment is 5-10℃ / min, the target temperature is 900-1100℃, and the holding time is 1-3h.

8. The preparation method according to claim 1, characterized in that, The pickling reagent in S4 is a sulfuric acid solution with a concentration of 0.3-0.7 mol / L; the pickling temperature is 20-30℃ and the pickling time is 6-12 h.

9. A Fe-N / C oxygen reduction catalyst, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. The application of the Fe-N / C oxygen reduction catalyst according to claim 9 in the preparation of fuel cell cathode catalyst.

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