Preparation method of iron monatomic-loaded coffee ground derived carbon oxygen reduction catalyst

Abstract

The invention discloses a preparation method of an iron monatomic-loaded coffee ground derived carbon oxygen reduction catalyst, and belongs to the technical field of fuel cell and metal air cell electro-catalysis materials. The method comprises the following steps: cleaning and drying coffee grounds, mixing and dipping the coffee grounds with a zinc nitrate solution, and drying to obtain an activated precursor; performing high-temperature carbonization in an inert atmosphere to form a zinc-containing intermediate; removing zinc species through acid pickling to obtain micro / mesoporous synergistic porous nitrogen-doped carbon; and then dipping with ferric nitrate and carrying out secondary heat treatment, so that iron is anchored on the carbon carrier in a monatomic form to obtain the iron monatomic catalyst. By utilizing the synergistic effect of mild activation of zinc nitrate and gas-phase gas release pore-forming, the specific surface area and the defect / nitrogen-containing site density are remarkably improved, iron agglomeration is inhibited, a high-density monatomic active center is realized, and the catalyst shows excellent ORR activity and stability in an alkaline medium and can be used for an air electrode of a zinc air battery and a cathode of an alkaline fuel battery.

Description

A method for preparing a coffee grounds-derived carbon-oxygen reduction catalyst supported on iron single atoms Technical Field

[0001] This invention relates to the preparation technology of biomass-derived carbon-based electrocatalytic materials, and more particularly to a method for preparing a catalyst for oxygen reduction reaction (ORR) using zinc nitrate-activated coffee grounds-derived porous carbon loaded with iron single atoms, belonging to the field of fuel cell and zinc-air battery technology. Background Technology

[0002] The ever-increasing demand for energy and the growing severity of environmental problems have fueled greater interest in electrochemical processes within sustainable energy systems. Technologies such as fuel cells, metal-air batteries, and water splitting offer innovative solutions to address energy demands and environmental challenges.

[0003] Proton exchange membrane fuel cells and zinc-air batteries, as novel, safe, reliable, green, and sustainable devices, are of great significance for achieving efficient energy conversion and storage. Metal-air batteries and fuel cells generate electricity through electrochemical reactions at electrodes, which need to exhibit good performance in the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). ORR occurs at the cathode electrode of the battery, and its low kinetics and high overpotential necessitate the use of electrocatalysts to achieve efficient energy conversion. Platinum is widely used due to its excellent ORR electrocatalyst performance, but its high price and low global reserves have become a bottleneck for fuel cell development. Research on non-precious metal catalysts with high activity, stability, and selectivity has gradually shifted towards metal-free carbon catalysts. In recent years, research on synthesizing ORR catalysts using biomass resources has made progress. Biomass is the most abundant green carbon source in nature, with a short regeneration cycle and low cost. Among many biomass resources, coffee beans are one of the world's largest-produced natural products. Given the high global coffee consumption, the annual production of coffee beans is close to 7 million tons, while also generating a large amount of coffee grounds. Therefore, utilizing waste coffee grounds—a low-cost, renewable, and stable source of biomass waste—to prepare high-performance ORR electrocatalysts not only helps reduce the cost of catalytic materials and the environmental burden, but also aligns with the green circular concept of "treating waste with waste," and has significant research and industrialization implications. Summary of the Invention

[0004] This invention provides a method for preparing a highly dispersed iron single-atom oxygen reduction catalyst by mildly activating coffee grounds with zinc nitrate to construct a hierarchical porous carbon support and then obtaining it through ferric nitrate impregnation and secondary heat treatment. This method addresses the problems of insufficient pore structure and anchoring sites in existing biomass carbon supports, easy agglomeration of single atoms, and unsafe or environmentally unfriendly preparation processes. The coffee grounds-derived catalyst with iron single atoms supported in this invention achieves a half-wave potential of 0.89V in the oxygen reduction reaction under alkaline conditions, demonstrating performance comparable to commercial Pt / C catalysts and exhibiting excellent catalytic activity. Compared to traditional commercial Pt / C catalysts, this catalyst significantly reduces dependence on precious metals, lowers costs, and provides a new paradigm for high-efficiency non-precious metal oxygen reduction catalysts.

[0005] A method for preparing a high-performance oxygen reduction catalyst derived from coffee grounds and supported on iron single atoms, comprising the following specific steps:

[0006] (1) Coffee grounds pretreatment: The coffee grounds are washed with deionized water, filtered and dried to obtain pretreated coffee grounds;

[0007] (2) Zinc nitrate activation: Zinc nitrate is dissolved in deionized water to obtain a zinc nitrate solution. The pretreated coffee grounds are mixed with the zinc nitrate solution, impregnated, and then dried to obtain an activated precursor containing zinc nitrate.

[0008] (3) Carbonization to form pores: The activated precursor is carbonized in an inert atmosphere to obtain a zinc-containing porous carbon intermediate;

[0009] (4) Zinc removal treatment: The zinc-containing porous carbon intermediate is contacted with an acid solution to remove zinc species, washed until neutral and dried to obtain a porous coffee grounds-derived carbon carrier;

[0010] (5) Introducing iron single atoms: After impregnating the porous coffee grounds-derived carbon support with ferric nitrate solution and drying it, a second heat treatment is performed under an inert atmosphere to anchor iron in the form of single atoms on the carbon support, thereby obtaining an iron single-atom oxygen reduction catalyst.

[0011] In step (2), zinc nitrate is zinc nitrate hexahydrate Zn(NO3)2·6H2O.

[0012] In step (2), the mass ratio of zinc nitrate hexahydrate to pretreated coffee grounds is (1-5):1; the soaking time is 12 h; and the drying temperature is 60℃.

[0013] In step (3), the carbonization temperature is 700-900℃, preferably 900℃, and the holding time is 2 h; the heating rate is 5℃ / min; the inert gas is argon, and the flow rate is 100-150 mL / min.

[0014] The acid solution mentioned in step (4) is hydrochloric acid with a concentration of 6 mol / L; the acid washing temperature is room temperature and the acid washing time is 6 h.

[0015] The ferric nitrate mentioned in step (5) is ferric nitrate nonahydrate Fe(NO3)3·9H2O. The mass ratio of ferric nitrate nonahydrate to the porous coffee grounds-derived carbon support is 1:20. Preferably, Fe(NO3)3·9H2O is dissolved in methanol, and the porous coffee grounds-derived carbon support is dispersed in n-hexane.

[0016] Ferric nitrate nonahydrate Fe(NO3)3·9H2O was impregnated with the porous coffee grounds-derived carbon carrier in a solution and then dried.

[0017] The secondary heat treatment temperature in step (5) is 600℃, and the temperature is maintained for 1 h.

[0018] The catalyst obtained by this invention is used in the fields of fuel cells and zinc-air batteries for oxygen reduction.

[0019] The core of this approach lies in utilizing the synergistic effects of zinc nitrate's melting and wetting, oxidation / dehydration, and gas release during the heating process to construct a microporous-mesoporous synergistic porous structure in situ during the carbonization of coffee grounds, while retaining / inducing abundant defects and nitrogen- or oxygen-containing coordination sites. Subsequently, iron nitrate is used to introduce an iron source, and through a secondary heat treatment at a lower temperature, iron atoms are stably anchored as single-atom active centers, inhibiting the formation of metal nanoparticles.

[0020] The beneficial effects of this invention are:

[0021] (1) Using coffee grounds solid waste as raw material, the source is wide and the cost is low, which meets the needs of resource utilization and green manufacturing;

[0022] (2) Zinc nitrate activation is milder and safer than strong alkali activation, and zinc species can be recovered or removed through acid washing, and the process is controllable;

[0023] (3) Zinc nitrate activation-carbonization can obtain a hierarchical porous structure and a high specific surface area, which is beneficial to expose active sites and improve mass transfer;

[0024] (4) The nitrogen / oxygen groups and activation-induced defect sites in coffee grounds can serve as coordination anchors to achieve high dispersion and anti-agglomeration of iron single atoms, thereby improving ORR activity and durability.

[0025] (5) The obtained catalyst can be used in devices such as the air electrode of zinc-air battery and the cathode of alkaline fuel cell, and has the potential to replace precious metal catalysts.

[0026] (6) The catalyst of this invention achieves an oxygen reduction half-wave potential of 0.89 V vs. RHE in alkaline medium (0.1 M KOH) and a power density of 140 mW·cm⁻¹ in an alkaline medium (6 M KOH) zinc-air battery. -2This represents a breakthrough in the catalytic activity of biomass-derived catalysts. Attached Figure Description

[0027] Figure 1 is a schematic diagram of the preparation process of zinc nitrate activated coffee grounds supported iron single-atom catalyst (CZN-Fe) in Example 1;

[0028] Figure 2 shows the HAADF-STEM low-magnification morphology image (a) of the CZN-Fe catalyst in Example 1, the STEM-EDS distribution of Fe and N elements (b, c), and the high-magnification HAADF-STEM characterization of Fe single-atom sites (d).

[0029] Figure 3 shows the SEM image (a) and TEM image (b) of the CZN-Fe catalyst in Example 1;

[0030] Figure 4 shows the ORR half-wave potential of the CZN-Fe catalyst before and after 10,000 cycles in Example 1;

[0031] Figure 5 shows the test results of the 6 mol / L KOH zinc-air battery assembled in Example 1 using commercial Pt / C and CZN-Fe as air cathode catalysts respectively: schematic diagram of the device (a), open circuit voltage curve (b), charge-discharge curve (c), discharge curve and corresponding power density curve (d), rate curve (e), and capacity curve (f).

[0032] Figure 6 shows a comparison of TEM images of samples with different primary carbonization temperatures in Example 2: CZN-Fe-700, CZN-Fe-800, and CZN-Fe (900℃, comparison).

[0033] Figure 7 shows the linear sweep voltammetry (LSV) curves of CZN-Fe-700, CZN-Fe-800, and CZN-Fe (comparison) in 0.1 mol / L KOH in Example 2;

[0034] Figure 8 shows a comparison of TEM images of samples with different zinc nitrate to coffee grounds mass ratios in Example 3: CZN-Fe-1:1 and CZN-Fe (3:1, comparison).

[0035] Figure 9 shows the LSV curves of CZN-Fe-1:1 and CZN-Fe (3:1, comparison) in 0.1 mol / L KOH in Example 3. Detailed Implementation

[0036] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0037] Example 1: Preparation of zinc nitrate-activated coffee grounds supported iron single-atom catalyst

[0038] (1) Coffee grounds pretreatment: Take 10 g of coffee grounds (coffee residue / coffee grounds), wash with deionized water 3 times to remove soluble impurities, filter and dry at 60℃ for 12 h to obtain dried coffee grounds;

[0039] (2) Activation of zinc nitrate: Weigh 30 g of zinc nitrate hexahydrate Zn(NO3)2·6H2O (mass ratio of dried coffee grounds to 3:1), dissolve it in deionized water to prepare a zinc nitrate solution (e.g., about 50 mL); add dried coffee grounds to the solution, stir thoroughly and soak for 12 h; then evaporate and dry at 80 °C to obtain an activated precursor containing zinc nitrate;

[0040] (3) Carbonization and pore formation: The activated precursor is spread in a ceramic boat, placed in a tube furnace, heated to 900°C at 5°C / min under argon protection (100 mL / min) and held for 2 h, and then cooled naturally to obtain a zinc-containing porous carbon intermediate.

[0041] (4) Zinc removal treatment: The intermediate was added to 6 mol / L hydrochloric acid solution and stirred at room temperature for 4 h. After filtration, it was washed with deionized water until the pH of the filtrate was ≈7. It was then dried at 60℃ to obtain a porous coffee grounds-derived carbon support activated with zinc nitrate.

[0042] (5) Introducing iron single atoms: 100 mg of carbon support was impregnated and 100 μL of Fe(NO3)3·9H2O methanol solution with a concentration of 50 mg / mL was added to 10 mL of n-hexane. After stirring at 400 rpm for two hours, the mixture was washed with ethanol by centrifugation and dried. Then, the iron single atoms were anchored by heating to 600℃ at 5℃ / min under argon protection and holding for 1 h. After cooling, the iron single atom oxygen reduction catalyst CZN-Fe was obtained.

[0043] The synthesis flow chart of the CZN-Fe catalyst in this example is shown in Figure 1.

[0044] Figure 2 shows the HAADF-STEM low-magnification morphology image of the CZN-Fe catalyst in this example, the STEM-EDS elemental distribution of N and Fe, and the high-magnification single-atom characterization of Fe. It can be seen that N and Fe are uniformly distributed, and Fe exists in the form of single atoms.

[0045] The SEM and TEM images of the CZN-Fe catalyst in this example are shown in Figure 3. It can be seen that using zinc nitrate as an activator for the activation-carbonization of coffee grounds can obtain a hierarchical porous structure and a high specific surface area, which is beneficial for exposing active sites and improving mass transfer.

[0046] The ORR half-wave potential of the CZN-Fe catalyst before and after 10,000 cycles is shown in Figure 4. It can be seen that the ORR half-wave potential of CZN-Fe before cycling is 0.89 V, which reaches a relatively high level in the field of biomass ORR electrocatalysis. After 10,000 cycles, the half-wave potential only shifts negatively by 10 mV, indicating that the catalyst not only has high ORR catalytic activity, but also good cycling stability.

[0047] Figure 5 shows the test results of a zinc-air battery using CZN-Fe catalyst and a commercial Pt / C air cathode. Figure 5(a) is a schematic diagram of the device; Figure 5(b) is the open-circuit voltage curve; Figure 5(c) is the charge-discharge curve; Figure 5(d) shows the discharge curve and corresponding power density curve; Figure 5(e) is the rate curve; and Figure 5(f) is the capacity curve. As shown in Figure 5, the battery using CZN-Fe as the air cathode catalyst exhibits higher power density, stable open-circuit voltage, and discharge capacity and cycle stability comparable to Pt / C, demonstrating good application prospects. The peak power of CZN-Fe is 140 mW / cm². -2 .

[0048] Example 2: Effects of different carbonization temperatures on pore structure and single-atom loading

[0049] The procedure was followed as in Example 1, except that the carbonization temperatures in step (3) were set to 700℃ and 800℃ (held for 2 h), respectively, while all other conditions remained unchanged. By lowering the carbonization temperature, the degree of graphitization of the carbon framework, the evolution of the pore structure, and the retention of oxygen-containing / defect sites on the surface can be controlled, thereby affecting the loading of iron single atoms and the ORR activity. The catalysts obtained in this example were named CZN-Fe-700 and CZN-Fe-800.

[0050] Figure 6 shows TEM images of the CZN-Fe-700 and CZN-Fe-800 catalysts in this example and the CZN-Fe catalyst in Example 1. Figure 6 shows that the activation effect improves and the pore structure becomes more developed with increasing temperature.

[0051] Using CZN-Fe-700 and CZN-Fe-800 catalysts as working electrodes, platinum sheet electrode as counter electrode, and Hg / HgO as reference electrode, alkaline electrocatalytic oxygen reduction was carried out in a 0.1 mol / L potassium hydroxide solution system.

[0052] Linear sweep voltammetry (LSV): LSV involves applying a voltage to the working electrode, scanning from an initial potential at a constant rate of change to a predetermined termination potential, and recording the current-potential curve. The half-wave potential can be obtained from the voltammetric curve. The half-wave potential is the potential at which the current equals half the diffusion current. This value is independent of the concentration of the reduced ions and reflects the reducing power of the catalyst. Therefore, it is an important method for investigating the ORR activity of catalysts.

[0053] The LSV curves of CZN-Fe-700, CZN-Fe-800 in this example and CZN-Fe in Example 1 are shown in Figure 7. The half-wave potentials are 0.88V, 0.88V, and 0.89V, respectively. This indicates that zinc nitrate has a universally applicable and relatively superior activation effect at high temperatures, and the subsequently anchored iron single atoms all exhibit high catalytic activity.

[0054] Example 3: Effect of different zinc nitrate dosages (mass ratios) on pore structure and iron single-atom loading

[0055] The procedure was followed as in Example 1, except that the mass ratio of zinc nitrate to coffee grounds in step (2) was adjusted to 1:1, while other conditions remained unchanged. By adjusting the amount of zinc nitrate, the gas release and pore-forming intensity during the carbonization process can be altered, thereby regulating the micropore / mesopore morphology and ratio, as well as the density of defect sites, to achieve control over the anchoring density and dispersibility of iron single atoms. The catalyst obtained in this example was named CZN-Fe-1:1.

[0056] Figure 8 shows TEM images of the CZN-Fe 1:1 catalyst in this example and the CZN-Fe catalyst in Example 1 (for comparison). Figure 8 shows that as the proportion of activator increases, the activation effect improves, the etching becomes more complete, the lamellar structure becomes thinner, and the pore structure becomes more developed. (A pore structure collapses when the mass ratio of activator to coffee grounds reaches 5:1).

[0057] The LSV curves of the CZN-Fe-1:1 catalyst in this example and the CZN-Fe (comparison) in Example 1 are shown in Figure 9. CZN-Fe-1:1 exhibits almost no ORR activity, while CZN-Fe (comparison) has a half-wave potential of 0.89 V. This indicates that the ultra-high specific surface area of ​​the carbon support is crucial for exposing active sites when fully activated.

[0058] The specific embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing a high-performance oxygen reduction catalyst derived from coffee grounds and supported on iron single atoms, characterized in that, The specific steps are as follows: (1) Coffee grounds pretreatment: coffee grounds are washed with deionized water, filtered and dried to obtain pretreated coffee grounds; (2) Zinc nitrate activation: zinc nitrate is dissolved in deionized water to obtain zinc nitrate solution, the pretreated coffee grounds are mixed with zinc nitrate solution and impregnated and then dried to obtain an activated precursor containing zinc nitrate; (3) Carbonization and pore formation: the activated precursor is carbonized under an inert atmosphere to obtain a zinc-containing porous carbon intermediate; (4) Zinc removal treatment: the zinc-containing porous carbon intermediate is contacted with an acid solution to remove zinc species, washed until neutral and dried to obtain a porous coffee grounds-derived carbon support; (5) Introduction of iron single atoms: the porous coffee grounds-derived carbon support is impregnated with iron nitrate solution and dried, and then subjected to a second heat treatment under an inert atmosphere to anchor iron in the form of single atoms on the carbon support to obtain an iron single-atom oxygen reduction catalyst.

2. The method according to claim 1, characterized in that, In step (2), zinc nitrate is zinc nitrate hexahydrate Zn(NO3)2·6H2O.

3. The method according to claim 1, characterized in that, In step (2), the mass ratio of zinc nitrate hexahydrate to pretreated coffee grounds is (1-5):1; the soaking time is 12 h; and the drying temperature is 60℃.

4. The method according to claim 1, characterized in that, In step (3), the carbonization temperature is 700-900℃, preferably 900℃, and the holding time is 2 h; the heating rate is 5℃ / min; the inert gas is argon, and the flow rate is 100-150 mL / min.

5. The method according to claim 1, characterized in that, The acid solution mentioned in step (4) is hydrochloric acid with a concentration of 6 mol / L; the acid washing temperature is room temperature and the acid washing time is 6 h.

6. The method according to claim 1, characterized in that, The ferric nitrate mentioned in step (5) is ferric nitrate nonahydrate Fe(NO3)3·9H2O. The mass ratio of ferric nitrate nonahydrate to the porous coffee grounds-derived carbon support is 1:

20. Preferably, Fe(NO3)3·9H2O is dissolved in methanol, and the porous coffee grounds-derived carbon support is dispersed in n-hexane.

7. The method according to claim 1, characterized in that, Ferric nitrate nonahydrate Fe(NO3)3·9H2O was impregnated with the porous coffee grounds-derived carbon carrier by solution and then dried.

8. The method according to claim 1, characterized in that, The secondary heat treatment temperature in step (5) is 600℃, and the temperature is maintained for 1 h.

9. The catalyst prepared according to any one of claims 1-8.

10. The application of the catalyst prepared according to any one of claims 1-8 in the field of fuel cell and zinc-air battery technology for oxygen reduction.

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