A cobalt-based catalyst for electrocatalytic two-electron oxygen reduction and a preparation method thereof

Abstract

The present application relates to the field of electrocatalysis, and provides a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction and a preparation method thereof. Cobalt chloride hexahydrate, sodium dithionite and sodium citrate are used as reaction raw materials to synthesize Co@PBA precursor by a coprecipitation method; after washing and drying, the Co@PBA precursor is subjected to high-temperature calcination and then acid washing to obtain a cobalt-doped PBA derivative catalyst Co@PBA-750. The Co@PBA-750 precursor is synthesized by the coprecipitation method, and the cobalt-doped PBA derivative catalyst is prepared by high-temperature calcination and acid washing, which utilizes the unique metal-organic framework characteristics of PBA to in-situ convert into a nitrogen-doped carbon composite material with uniformly dispersed cobalt species during pyrolysis; the coprecipitation method with slow drop is used, and sodium citrate is used as a complexing agent and a morphology stabilizer to effectively control the nucleation and growth rate of the precursor and inhibit particle agglomeration, so that the Co@PBA-750 nanospheres with uniform size and good dispersion are synthesized.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalysis technology, and particularly relates to a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction and its preparation method. Background Technology

[0002] hydrogen peroxide ( Hydrogen peroxide, as an environmentally friendly green oxidant, is widely used in pulp bleaching, water treatment, sterilization and disinfection, and chemical synthesis. Currently, the industrial production of hydrogen peroxide mainly relies on the anthraquinone process. Although this process accounts for over 90% of the market share, it suffers from problems such as high energy consumption, cumbersome steps, high storage and transportation costs, and the generation of large amounts of hazardous waste. The electrochemical synthesis method utilizes the two-electron oxygen reduction reaction (…). It can achieve on-site, on-demand production of hydrogen peroxide under mild conditions, effectively avoiding the environmental pollution and safety risks of traditional processes, and has become a very promising alternative technology.

[0003] The process involves Adsorption and partial reduction on the catalyst surface generate key intermediates. It is then further converted into hydrogen peroxide. However, this reaction faces a four-electron oxygen reduction pathway (to produce... Intense competition from other sources often results in low selectivity and Faraday efficiency for hydrogen peroxide. Ideally... The catalyst needs to meet the requirements of The intermediate possesses moderate adsorption strength to maintain high activity, while also being suitable for... It has weak adsorption to prevent Bond breakage triggers This approach combines high conductivity, excellent mass transfer efficiency, good stability, and low cost.

[0004] Carbon-based materials, due to their abundant reserves, excellent electrical conductivity, and tunable structure, have become... Catalyst research is a hot topic. Through heteroatom doping, defect engineering, or surface functional group modification, the electronic structure of carbon materials can be effectively controlled, and the adsorption energy of key intermediates can be optimized, thereby improving the selectivity of hydrogen peroxide. Prussian blue analogues, as typical metal-organic framework materials, possess a general formula... Its three-dimensional open framework structure, abundant metal active sites, tunable morphology, and adjustable metal composition make it an ideal precursor for the preparation of carbon-based catalysts. During pyrolysis, PBAs can be transformed in situ into nitrogen-doped carbon composite materials with uniform dispersion of metal nanoparticles or metal nitrides, which is beneficial for exposing more active sites and promoting reaction mass transfer.

[0005] The transition metal cobalt is considered due to its moderate oxygen binding energy. Potential active sites for PBAs can be identified through cobalt doping. By controlling the metal composition of PBAs using a cobalt doping strategy, active sites can be designed and optimized at the atomic scale. However, existing PBA-derived catalysts are prone to particle agglomeration during co-precipitation, leading to uneven distribution of active sites and increased mass transfer resistance, thus limiting further improvement in catalytic performance. Therefore, developing a preparation strategy that can effectively suppress agglomeration while introducing synergistic active sites is of great significance.

[0006] Microbial fuel cells (MFCs) can use electricity generated from organic matter in wastewater to drive the ORR (Organic Reactive Oxygen Reduction) reaction at the cathode to produce hydrogen peroxide, turning waste into treasure. Coupled with the electro-Fenton reaction to construct an electro-Fenton-type MFC, this method can efficiently degrade organic pollutants (such as azo dyes) in wastewater using in-situ generated hydrogen peroxide. This avoids the problems of hydrogen peroxide storage, transportation, and secondary pollution, improving the safety, economy, and environmental friendliness of the entire process. Therefore, developing efficient and stable PBA-derived carbon-based catalysts and applying them to the MFC cathode for in-situ hydrogen peroxide production and pollutant degradation has significant research and application value. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction and its preparation method, aiming to solve the problems mentioned in the background art.

[0008] In a first aspect, the present invention provides a method for preparing a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction, using cobalt chloride hexahydrate, sodium prussiate of potash, and sodium citrate as reactants, and synthesizing a Co@PBA precursor by co-precipitation; after washing and drying the Co@PBA precursor, it is calcined at high temperature and then acid-washed to obtain a cobalt-doped PBA-derived catalyst Co@PBA-750, which is a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction. Sodium citrate acts as a complexing agent and morphology stabilizer during the co-precipitation stage, inhibiting precursor particle aggregation and promoting the formation of uniformly dispersed nanospheres. During the high-temperature calcination stage, the oxygen-containing functional groups generated by the decomposition of sodium citrate are in situ incorporated into the carbon framework, forming Co-NO synergistic active sites with the cobalt-nitrogen coordination centers.

[0009] Co@PBA-750 precursor was synthesized via co-precipitation, followed by high-temperature calcination and acid washing to obtain the cobalt-doped PBA-derived catalyst Co@PBA-750. On one hand, by first synthesizing the Co@PBA-750 precursor with a uniform nanosphere morphology, and then performing high-temperature pyrolysis to convert the organic components into a nitrogen-doped carbon framework while retaining its spherical structure, the morphology collapse that might occur with direct pyrolysis is avoided. On the other hand, electrocatalytic two-electron oxygen reduction catalysts, especially metal-nitrogen-carbon (MNC) catalysts, typically require high pyrolysis temperatures (≥800℃) to form sufficient graphitization and active sites, but excessively high temperatures may lead to metal particle agglomeration. This invention performs pyrolysis at 750℃ and uses acid washing to remove unstable metal species, preserving the nanosphere morphology and porous structure while ensuring catalytic activity.

[0010] Furthermore, the specific steps include: Step S1: Dissolve cobalt chloride hexahydrate in deionized water to prepare solution A; dissolve sodium ferrous sulfate in deionized water to prepare solution B; dissolve sodium citrate in deionized water to prepare solution C; Step S2: Under stirring conditions, solutions A and B are added dropwise to solution C to carry out a coprecipitation reaction. After the reaction is complete, the resulting precipitate is centrifuged, washed, and dried to obtain the Co@PBA precursor. Step S3: Place the Co@PBA precursor in a tube furnace and calcine it under an inert atmosphere with programmed temperature rise. After natural cooling, the calcined product is obtained. Step S4: After grinding the calcined product, add an acid solution for acid washing, then filter, wash and dry to obtain the cobalt-doped PBA-derived catalyst Co@PBA-750.

[0011] Furthermore, in step S1, the concentration of solution A is 0.2 mol / L, the concentration of solution B is 0.25 mol / L, and the concentration of solution C is 0.045 mol / L.

[0012] Furthermore, in step S2, the coprecipitation reaction is carried out in a constant temperature water bath at 60°C, with a stirring speed of 400 rpm and a reaction time of 6 hours. Solution A and solution B are added slowly through a constant pressure dropping funnel, with the dropping rate controlled at 2-3 drops per second. The resulting precipitate was washed by centrifugation with deionized water and anhydrous ethanol, respectively. The centrifugation conditions were 8000 rpm for 3-5 minutes, and the washing was repeated three times. The drying conditions were as follows: drying in a vacuum drying oven at 60°C for 24 hours.

[0013] Metal-organic frameworks (PBAs), as a class of materials, possess ordered pore structures, large specific surface areas, and tunable metal active sites. In step S2, a Co@PBA-750 precursor was synthesized using a co-precipitation method. By slowly adding cobalt salt solution and sodium prussiate of Taurine solution dropwise to a substrate containing sodium citrate, sodium citrate was used as a complexing agent and morphology stabilizer to effectively control the nucleation and growth rate of the precursor. Simultaneously, the citrate ions adsorbed on the particle surface generated steric hindrance and electrostatic repulsion, effectively inhibiting particle aggregation and promoting the formation of uniformly sized and well-dispersed Co@PBA-750 nanospheres. After co-precipitation, centrifugation, washing, and drying were performed to obtain a pure Co@PBA-750 precursor. Compared to a one-step direct mixing method, the slow dropwise addition method is beneficial for the ordered growth of precursor crystals, ensuring the integrity and dispersibility of the nanosphere morphology.

[0014] Furthermore, in step S3, the programmed temperature calcination is specifically as follows: the temperature is increased to 400°C at a heating rate of 2°C / min, held for 1 hour, and then increased to 750°C at a heating rate of 5°C / min, held for 2 hours.

[0015] A programmed temperature ramp calcination strategy was adopted, involving segmented heating: first, the temperature was increased to 400℃ at a rate of 2℃ / min to remove adsorbed water; then, it was held at 400℃ for 1 hour to allow the precursor to initially decompose and form a carbon coating protective layer; finally, the temperature was increased to 750℃ at a rate of 5℃ / min and held for 2 hours to achieve complete carbonization and the formation of active sites. This process gradually transformed the cyano groups in the Co@PBA-750 precursor into a nitrogen-doped carbon framework, while simultaneously cobalt ions formed a cohesive nitrogen atom. Active center. More importantly, it was discovered for the first time that oxygen-containing functional groups (-COOH, -OH) generated during the pyrolysis of sodium citrate are in situ incorporated into the carbon framework, interacting with... The center forms a Co-NO synergistic active site, further optimizing The adsorption energy of the intermediate directs the reaction pathway. The segmented heating method avoids the drastic decomposition and morphological damage of the precursor caused by rapid heating, ensuring that the nanosphere structure is completely preserved during pyrolysis.

[0016] Further, in step S4, the acid washing treatment specifically involves adding the ground calcined product to a 0.5M solvent at a solid-liquid ratio of 1g:50mL. The solution was stirred at 60°C for 6 hours.

[0017] Acid washing was used to further optimize catalyst performance, through 0.5M... Stirring at 60°C for 6 hours selectively removes unstable cobalt nanoparticles and non-catalytically active cobalt oxides formed during pyrolysis, while retaining active cobalt species coordinated with nitrogen and oxygen. Acid washing simultaneously introduces more defect sites and porous structures onto the nanosphere surface, facilitating the exposure of more active centers and improving the catalyst's performance. Selectivity and activity.

[0018] Secondly, the present invention provides a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction, which is prepared by a method for preparing a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction.

[0019] Thirdly, the present invention provides the application of a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction in hydrogen peroxide production in a gas diffusion electrode flow-type microbial fuel cell.

[0020] Furthermore, a gas diffusion electrode flow-type microbial fuel cell based on the cobalt-doped PBA-derived catalyst Co@PBA-750 was constructed. The specific steps are as follows: Step S5: Gas diffusion electrode flow microbial fuel cell uses a cubic acrylic plate as the cell body of MFCs, and a gas diffusion electrode is set next to the cathode. Step S6: In the acclimatization stage of the gas diffusion electrode flow-type microbial fuel cell, the anode uses a carbon brush as the electrode, and the carbon brush and sludge are added together into the anode chamber, and anolyte is added to ensure the activity of microorganisms in the sludge; the cathode uses carbon paper as the electrode, potassium ferricyanide as the cathode liquid, and the anode and cathode are connected to an external resistor to connect the circuit. The resistance value is reduced by replacing the resistor to screen and cultivate electrogenic microorganisms. Once the electrogenic microorganisms stably attach to the carbon brush and form a stable biofilm, the domestication stage ends. Step S7: After the acclimatization stage, peristaltic pumps are connected to both the cathode and anode to make the electrolytes of both the cathode and anode flow at a rate of 15 mL / min. The cobalt-doped PBA-derived catalyst Co@PBA-750 is attached to carbon paper as the cathode electrode.

[0021] Further, in step S6, the resistance values ​​of the resistors are 1000Ω, 820Ω, 510Ω, 200Ω, 100Ω, 51Ω, 24Ω, 10Ω, 5Ω and 1Ω respectively. When the resistor is changed to 1Ω, the acclimatization stage ends.

[0022] The present invention has the following beneficial effects: (1) Co@PBA-750 precursor was synthesized by co-precipitation method, and then cobalt-doped PBA derivative catalyst was prepared by high-temperature calcination and acid washing. Taking advantage of the unique metal-organic framework properties of PBA, it was converted in situ into nitrogen-doped carbon composite material with uniformly dispersed cobalt species during pyrolysis.

[0023] (2) By using a slow-drop co-precipitation method, sodium citrate was used as a complexing agent and morphology stabilizer to effectively control the nucleation and growth rate of the precursor and inhibit particle aggregation, thus synthesizing Co@PBA-750 nanospheres with uniform size and good dispersion.

[0024] (3) The in-situ oxygen doping function of sodium citrate during pyrolysis was discovered for the first time, and... Co-NO synergistic active sites are formed at the center. Through a segmented programmed temperature calcination strategy, the cyano groups in the precursor are gradually converted into a nitrogen-doped carbon framework, while oxygen doping defects are introduced. Acid washing selectively removes unstable cobalt species, introducing more defect sites and porous structures on the surface of the nanospheres. Furthermore, compared with the sample without sodium citrate (which exhibits severely agglomerated morphology),... The selectivity was <45%, demonstrating the morphological stability and performance-enhancing effects of sodium citrate.

[0025] (4) Applying Co-doped PBA-derived catalysts to gas diffusion electrode flow-type microbial fuel cells as cathode catalysts. Reaction generation They have developed a gas diffusion electrode flow-type microbial fuel cell that can be used for in-situ degradation of organic pollutants in wastewater. Attached Figure Description

[0026] Exemplary embodiments of the present invention can be more fully understood by referring to the following figures: Figure 1 This is a SEM image of the Co@PBA-750 catalyst prepared in Example 1 of this invention, showing a uniform nanosphere morphology.

[0027] Figure 2 This is a SEM image of the Co@PBA-750 (without sodium citrate) catalyst prepared in Comparative Example 1 of this invention.

[0028] Figure 3 Here is the elemental mapping diagram of the Co@PBA-750 catalyst prepared in Example 1 of this invention, wherein: Figure 3 The a and ad diagrams in the figure represent the distribution of C, N, O, and Co elements, respectively.

[0029] Figure 4 This is the elemental mapping diagram of the Co@PBA-750 (without sodium citrate) catalyst prepared in Comparative Example 1 of this invention, wherein: Figure 4 The a and ad diagrams are the distribution diagrams of C, N, O, and Co elements, respectively.

[0030] Figure 5This is the ORR polarization curve of the Co@PBA catalytic material of the present invention measured by RRDE at a rotating ring electrode speed of 1600 rpm and a scan rate of 5 mV / s.

[0031] Figure 6 This is an ORR polarization curve measured by RRDE for the Co@PBA-750 catalyst prepared in Example 1 of this invention and the Co@PBA-750 catalyst prepared in Comparative Example 1 (without sodium citrate). The rotation speed of the rotating ring electrode is 1600 rpm and the scan rate is 5 mV / s.

[0032] Figure 7 This is a line graph showing the H2O2 selectivity and the number of transferred electrons of the Co@PBA catalytic material of the present invention.

[0033] Figure 8 The Co@PBA-750 catalyst prepared in Example 1 of this invention and the Co@PBA-750 catalyst prepared in Comparative Example 1 (without sodium citrate) are... Line graph of selectivity and number of transferred electrons.

[0034] Figure 9 It is the Tafel slope of the Co@PBA catalytic material of this invention.

[0035] Figure 10 This is a KL diagram of the Co@PBA-750 catalyst material of the present invention at different rotation speeds.

[0036] Figure 11 This is a voltage change diagram during the microbial domestication process of the gas diffusion electrode type microbial fuel cell of the present invention.

[0037] Figure 12 The present invention relates to the assembly of Co@PBA-750 catalyst material into a gas diffusion electrode flow-type microbial fuel cell. The yield versus Faraday efficiency graph. Detailed Implementation

[0038] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0040] This invention provides a method for preparing a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction. Using cobalt chloride hexahydrate, sodium prussiate of potassium, and sodium citrate as reactants, a Co@PBA precursor is synthesized via co-precipitation. After washing and drying, the Co@PBA precursor is calcined at high temperature and then acid-washed to obtain the cobalt-doped PBA-derived catalyst Co@PBA-750, which is the cobalt-based catalyst for electrocatalytic two-electron oxygen reduction. Sodium citrate acts as a complexing agent and morphology stabilizer during the co-precipitation stage, inhibiting precursor particle aggregation and promoting the formation of uniformly dispersed nanospheres. During the high-temperature calcination stage, the oxygen-containing functional groups generated by the decomposition of sodium citrate are in situ incorporated into the carbon framework, forming Co-NO synergistic active sites with the cobalt-nitrogen coordination centers.

[0041] In some embodiments, the following steps are specifically included: Step S1: Dissolve cobalt chloride hexahydrate in deionized water to prepare solution A; dissolve sodium ferrous sulfate in deionized water to prepare solution B; dissolve sodium citrate in deionized water to prepare solution C; Step S2: Under stirring conditions, solutions A and B are added dropwise to solution C to carry out a coprecipitation reaction. After the reaction is complete, the resulting precipitate is centrifuged, washed, and dried to obtain the Co@PBA precursor. Step S3: Place the Co@PBA precursor in a tube furnace and calcine it under an inert atmosphere with programmed temperature rise. After natural cooling, the calcined product is obtained. Step S4: After grinding the calcined product, add an acid solution for acid washing, then filter, wash and dry to obtain the cobalt-doped PBA-derived catalyst Co@PBA-750.

[0042] In some embodiments, in step S1, the concentration of solution A is 0.2 mol / L, the concentration of solution B is 0.25 mol / L, and the concentration of solution C is 0.045 mol / L.

[0043] In some embodiments, in step S2, the coprecipitation reaction is carried out in a constant temperature water bath at 60°C, with a stirring speed of 400 rpm and a reaction time of 6 hours. Solution A and solution B are added slowly through a constant pressure dropping funnel, with the dropping rate controlled at 2-3 drops per second. The resulting precipitate was washed by centrifugation with deionized water and anhydrous ethanol, respectively. The centrifugation conditions were 8000 rpm for 3-5 minutes, and the washing was repeated three times. The drying conditions were as follows: drying in a vacuum drying oven at 60°C for 24 hours.

[0044] In some embodiments, in step S3, the programmed temperature rise calcination specifically involves: heating to 400°C at a heating rate of 2°C / min, holding at that temperature for 1 hour, and then heating to 750°C at a heating rate of 5°C / min, holding at that temperature for 2 hours.

[0045] In some embodiments, in step S4, the acid washing treatment specifically involves adding the ground calcined product to a 0.5M solvent at a solid-liquid ratio of 1g:50mL. The solution was stirred at 60°C for 6 hours.

[0046] In some embodiments, the present invention provides a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction, which is prepared by a method for preparing a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction.

[0047] In some embodiments, the present invention provides the application of a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction in hydrogen peroxide production in a gas diffusion electrode flow-type microbial fuel cell.

[0048] In some embodiments, a gas diffusion electrode flow-type microbial fuel cell based on the cobalt-doped PBA-derived catalyst Co@PBA-750 is constructed, and the specific steps are as follows: Step S5: Gas diffusion electrode flow microbial fuel cell uses a cubic acrylic plate as the cell body of MFCs, and a gas diffusion electrode is set next to the cathode. Step S6: In the acclimatization stage of the gas diffusion electrode flow-type microbial fuel cell, the anode uses a carbon brush as the electrode, and the carbon brush and sludge are added together into the anode chamber, and anolyte is added to ensure the activity of microorganisms in the sludge; the cathode uses carbon paper as the electrode, potassium ferricyanide as the cathode liquid, and the anode and cathode are connected to an external resistor to connect the circuit. The resistance value is reduced by replacing the resistor to screen and cultivate electrogenic microorganisms. Once the electrogenic microorganisms stably attach to the carbon brush and form a stable biofilm, the domestication stage ends. Step S7: After the acclimatization stage, peristaltic pumps are connected to both the cathode and anode to make the electrolytes of both the cathode and anode flow at a rate of 15 mL / min. The cobalt-doped PBA-derived catalyst Co@PBA-750 is attached to carbon paper as the cathode electrode.

[0049] In some embodiments, in step S6, the resistance values ​​of the resistors are 1000Ω, 820Ω, 510Ω, 200Ω, 100Ω, 51Ω, 24Ω, 10Ω, 5Ω and 1Ω in sequence. When the resistor is changed to 1Ω, the acclimatization stage ends.

[0050] Example 1: Preparation of cobalt-doped PBA-derived catalyst (1) Preparation of precursor solution: 2.3794g of... (Cobalt chloride hexahydrate) was dissolved in 50 mL of deionized water to prepare a 0.2 mol / L solution A, which was rose-red in color; 2.42 g of... (Sodium prussiatetracycline) was dissolved in 20 mL of deionized water to prepare a 0.25 mol / L solution B, which was pale yellow; 0.662 g of... Sodium citrate was dissolved in 50 mL of deionized water to prepare a 0.045 mol / L solution C, which was colorless and transparent. Solutions A, B and C were sonicated until completely dissolved. (2) Synthesis of Co@PBA-750 precursor: Under constant temperature water bath of 60℃ and magnetic stirring of 400rpm, solutions A and B were slowly added dropwise to solution C through a constant pressure dropping funnel. The solution gradually changed from colorless to light green and then to dark green. The dropping rate was controlled at 2~3 drops per second. After the addition was completed, stirring was continued for 6 hours. After the reaction was completed, the resulting precipitate was centrifuged at 8000rpm for 3 minutes, the supernatant was discarded, and the precipitate was washed with deionized water and anhydrous ethanol. The centrifugation and washing were repeated three times. The washed precipitate was placed in a vacuum drying oven at 60℃ and dried for 12 hours to obtain Co@PBA precursor powder, which was then ground evenly with an agate mortar. (3) High-temperature calcination treatment: The Co@PBA precursor powder was placed in a ceramic boat and put into a tube furnace for calcination under an argon atmosphere with a gas flow rate of 80 mL / min. The calcination program was as follows: the temperature was increased from 40℃ to 400℃ at 2℃ / min and held for 1 hour; the temperature was increased to 750℃ at 5℃ / min and held for 2 hours. After the program was completed, the product was naturally cooled to room temperature and the calcined product was taken out. The entire process was protected by an argon atmosphere. (4) Post-pickling treatment: After grinding the calcined product, add 0.5M of [acid washing agent]. The solution, with a solid-liquid ratio of 1 g: 50 mL, was stirred in a 60°C water bath for 6 hours. After acid washing, it was vacuum filtered, and the filter cake was washed with deionized water until neutral. It was then placed in a 60°C vacuum drying oven and dried for 12 hours to finally obtain the cobalt-doped PBA-derived catalyst Co@PBA-750.

[0051] Example 2: Structural characterization and morphological analysis of cobalt-doped PBA-derived catalysts The Co@PBA-750 catalyst prepared in Example 1 was characterized by scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown in the figure. The results show that the catalyst prepared after adding sodium citrate exhibits a uniformly dispersed nanosphere morphology, with a sphere diameter of approximately 200–500 nm. The boundaries between the spheres are clear, and the dispersion is good, with no obvious adhesion or agglomeration. The surface of the spheres is relatively smooth, and some spheres show fine nanoscale pore structures.

[0052] Morphological analysis conclusion: Sodium citrate acts as a complexing agent and morphological stabilizer in the co-precipitation stage, through the interaction of citrate ions with... Regulating free chelation The release rate of the precursor crystal nuclei is controlled, thereby controlling the nucleation and growth rate of the precursor crystals. Simultaneously, the citrate ions adsorbed on the particle surface generate steric hindrance and electrostatic repulsion effects, effectively inhibiting disordered aggregation between particles and successfully inducing the formation of uniformly sized and well-dispersed nanospheres. This morphological transformation provides the structural basis for the full exposure of Co-NO synergistic active sites during subsequent pyrolysis.

[0053] Comparative Example 1: The comparative catalyst was prepared according to the steps of Example 1, except that sodium citrate was not added, that is, in step S2, solutions A and B were directly added dropwise to 50 mL of deionized water (instead of solution C).

[0054] SEM images of the catalyst (without sodium citrate) are shown below. Figure 2 As shown, the results indicate that the comparative catalyst exhibits an irregular agglomerated morphology; the particles are severely adhered to each other, with blurred boundaries, making it almost impossible to distinguish individual primary particles, and the overall catalyst presents a densely packed state with a lack of obvious pore structure.

[0055] Morphology comparison analysis: The introduction of sodium citrate successfully transformed the morphology from severely agglomerated lumps to uniformly dispersed nanospheres. This transformation proves the key role of sodium citrate, namely, as a morphology guide in the co-precipitation stage, effectively preventing particle aggregation and laying the morphological foundation for the subsequent pyrolysis to form a three-dimensional network structure with high catalytic activity.

[0056] Example 3: SEM-mapping elemental distribution and EDS quantitative analysis To further investigate the effect of sodium citrate on the distribution of active components in the catalyst, elemental mapping and EDS characterization were performed on the cobalt-doped PBA-derived catalyst Co@PBA-750 prepared in Example 1 and the comparative catalyst (without sodium citrate) prepared in Comparative Example 1. The results are as follows: Figures 3-4 As shown in Table 1.

[0057] (a) Elemental distribution of the sample with added sodium citrate: The elemental distribution of the Co@PBA-750 catalyst prepared after adding sodium citrate is as follows: Figure 3 As shown, the results indicate that the four elements C, N, O, and Co exhibit a highly uniform co-distribution characteristic: Co element ( Figure 3 In the sample d), the distribution is dense, continuous, and uniform, with stable signal intensity, which highly matches the distribution areas of C and N elements. No obvious Co nanoparticle aggregation or local enrichment was observed, indicating that cobalt species are uniformly anchored in the nitrogen-doped carbon framework in an atomically dispersed manner; N element ( Figure 3 b) and C element ( Figure 3 The complete overlap of the distribution of a) indicates that nitrogen was successfully doped into the carbon framework; the O element ( Figure 3 c) is also uniformly distributed, highly overlapping with the distribution areas of Co and N elements, confirming that the oxygen-containing functional groups generated by the pyrolysis of sodium citrate were successfully doped in situ, and that they are compatible with... The centers together constitute the Co-NO synergistic active site.

[0058] (II) Elemental distribution of the sample without sodium citrate The elemental distribution of the catalyst without added sodium citrate is as follows: Figure 4 As shown, the results indicate that the Co element ( Figure 4 The distribution of d) in the data exhibits significant sparseness, sporadic nature, and unevenness. Specifically, the Co element signal is distributed in a scattered pattern, with weak signals appearing only in a few areas, while the Co signal is extremely weak or absent in most areas, indicating that cobalt species have failed to be effectively incorporated into the carbon framework; the N element ( Figure 4 In the distribution of N (b), the distribution is relatively continuous, but there is no significant overlap with the distribution area of ​​Co, indicating that N has not formed an effective coordination structure with Co; C element ( Figure 4 In the figure, a) represents the main carbon framework, which is continuously distributed but has local defects. This result indicates that in the absence of sodium citrate, the precursor undergoes severe aggregation during co-precipitation, leading to Co... 2+ The distribution is uneven, and after pyrolysis, it is impossible to form uniform Co-N coordination centers.

[0059] (III) Quantitative Analysis by EDS

[0060] EDS energy dispersive spectroscopy analysis was performed on the samples of Example 1 and Comparative Example 1, and the results are shown in Table 1.

[0061] Table 1 Comparison of EDS elemental composition of catalysts

[0062] As shown in Table 1: Co content comparison: The Co content in the control catalyst without sodium citrate was 0.00 wt%, while the Co content in the cobalt-doped PBA-derived catalyst Co@PBA-750 with added sodium citrate significantly increased to 3.33 wt%. This result indicates that sodium citrate, acting as a complexing agent during the co-precipitation stage, effectively stabilized the cobalt in the solution. This promotes the uniform incorporation of cobalt ions into the precursor; at the same time, during pyrolysis and acid washing, the presence of sodium citrate allows the Co species to be anchored in the carbon skeleton in a stable form, preventing them from being completely removed by acid washing.

[0063] Comparison of nitrogen (N) content: The N content of the control catalyst without sodium citrate was 3.32 wt%, while the N content in the cobalt-doped PBA-derived catalyst Co@PBA-750 after adding sodium citrate decreased to 0.72 wt%. Combined with the elemental mapping diagram, it can be seen that without sodium citrate, although the N content was high, its distribution was uneven, with a large amount of N existing in an uncoordinated free form; after adding sodium citrate, the N content was moderate and highly overlapped with the distribution of Co, indicating that most of the N participated in... The formation of coordination centers significantly improves the utilization efficiency of unit N.

[0064] O content comparison: After adding sodium citrate, the O content in the cobalt-doped PBA-derived catalyst Co@PBA-750 was 3.02 wt%, which remained at a high level. This proves that the oxygen-containing functional groups generated by the pyrolysis of sodium citrate were successfully incorporated into the carbon framework in situ. The centers together constitute the Co-NO synergistic active site.

[0065] Na content: The Na content in both the cobalt-doped PBA-derived catalyst Co@PBA-750 and the comparative catalyst was low (0.36-1.27 wt%), indicating that the acid washing treatment effectively removed most of the Na, and the residual Na did not affect the catalytic performance.

[0066] (IV) Comparative Analysis and Comprehensive Conclusions Will Figure 3 , Figure 4 A comprehensive comparison with Table 1 shows the results of comparing Example 1 and Comparative Example 1 in Table 2.

[0067] Table 2 Comparison results between Example 1 and Comparative Example 1

[0068] The comparative results of this embodiment fully demonstrate that the addition of sodium citrate not only modulates the macroscopic morphology of the catalyst, but more importantly, achieves uniform dispersion of the active component at the nanoscale. Sodium citrate, through complexation, enables... The uniform distribution during the co-precipitation stage, coupled with the in-situ introduction of oxygen-containing functional groups during pyrolysis, ensures atomically uniform dispersion of Co-NO synergistic active sites on the catalyst surface. This structural feature is the fundamental reason why the cobalt-doped PBA-derived catalyst Co@PBA-750 prepared in Example 1 achieves 98% hydrogen peroxide selectivity, while the comparative catalyst without added sodium citrate exhibits only 30-45% hydrogen peroxide selectivity due to uneven elemental distribution and insufficient exposure of active sites.

[0069] Comparative Example 2: The comparative catalyst was prepared according to the steps of Example 1, except that: in the high-temperature calcination stage, the temperature was raised to 550°C to obtain Co@PBA-550.

[0070] Comparative Example 3: The comparative catalyst was prepared according to the steps of Example 1, except that: in the high-temperature calcination stage, the temperature was raised to 650°C to prepare Co@PBA-650.

[0071] Comparative Example 4: The comparative catalyst was prepared according to the steps of Example 1, except that: in the high-temperature calcination stage, the temperature was raised to 850°C to prepare Co@PBA-850.

[0072] Example 4: Cobalt-doped PBA-derived catalysts Performance testing (1) Preparation of working electrode: 5 mg of cobalt-doped PBA-derived catalyst Co@PBA-750 prepared in Example 1, 420 μL of deionized water, 50 μL of Nafion (5 wt%) and 30 μL of anhydrous ethanol were mixed and ultrasonically dispersed for 30 minutes to prepare a uniform catalyst slurry; 8 μL of catalyst slurry was drop-coated onto the clean, dry and polished glassy carbon disk electrode surface of the rotating ring electrode (RRDE) and dried under an infrared lamp for later use. (2) Electrochemical testing: A three-electrode system was used, with the catalyst-supported RRDE as the working electrode, platinum wire as the counter electrode, and Hg / HgO electrode as the reference electrode. Cyclic voltammetry was performed in N2-saturated and O2-saturated 0.1M KOH electrolytes, respectively, and linear scan voltammetry was performed in O2-saturated 0.1M KOH electrolytes. Test conditions: scan rate 5mV / s, rotation speed 1600rpm, and ring electrode potential constant at 1.2V vs. RHE; (3) Selectivity and electron transfer number calculation: Based on RRDE test results, using the formula ; ; calculate Selectivity and electron transfer number, among which For disk current, Let N be the ring current and N be the collection factor (0.125). Test results show that within the potential range of -0.1 to 0.2 V vs. RHE, With a selectivity exceeding 90% and an electron transfer number approaching 2, the catalyst exhibits excellent performance. performance.

[0073] (4) RDE test: LSV curves were obtained by setting different speeds (speeds were set to 400, 625, 900, 1225, 1600, 2025 and 2500 rpm in sequence), and they were substituted into the Koutecky-Levich (KL) equation below to obtain the KL curve. Further calculations were performed to obtain the electron transfer number n. ; ; ; Among them, J k and J d Let F be the kinetic limit current density and the diffusion limit current density, respectively, and J be the measured current density. The angular velocity (rpm) of the disk is represented by ω (ω = 2πN, where N is the rotational speed); F corresponds to the Faraday constant (F = 96485 C / mol); k represents the electron transfer rate constant. This represents the diffusion coefficient of O2 (in electrolytes with pH = 5~15). ); n represents the number of electrons transferred; υ represents the kinematic viscosity of the electrolyte (in electrolytes with pH = 5~5, ); This indicates the volume concentration of O2 (in electrolytes with pH = 5~15). ).

[0074] Example 5: Application of cobalt-doped PBA-derived catalysts in microbial fuel cells (1) Acclimation of microbial fuel cells: cubic acrylic plates are used as the cell body of MFCs, and carbon brushes with dimensions of 4cm×3cm×8cm are used as anode electrodes. After pretreatment, they are placed in the anode chamber, and crushed sludge is added to the prepared anolyte and injected into the anode chamber to ensure the activity of microorganisms in the sludge. Carbon paper with strong hydrophobicity and carbon powder layer is used as cathode electrode, and potassium ferricyanide is used as cathode liquid. The anode and cathode chambers are separated by a proton exchange membrane and connected to an external resistor to connect the circuit. By changing the resistor with increasingly smaller resistance values ​​(1000Ω, 820Ω, 510Ω, 200Ω, 100Ω, 51Ω, 24Ω, 10Ω, 5Ω, 1Ω), electrogenic microorganisms are cultivated and screened, so that they can stably attach to the carbon brush and form a stable biofilm.

[0075] (2) Construction of a gas diffusion electrode flow-type MFC: The cobalt-doped PBA-derived catalyst Co@PBA-750 prepared in Example 1 was prepared into a catalyst slurry. 100 μL of the catalyst slurry was drop-coated onto a 1 cm × 1 cm carbon paper electrode, dried with an infrared lamp, and then used as the cathode electrode. Both the cathode and anode were connected to peristaltic pumps to make the electrolyte flow at a rate of 15 mL / min. A gas diffusion electrode was set up next to the cathode to improve the oxygen mass transfer efficiency.

[0076] (3) Yield determination: will produce The cathode electrolyte was added dropwise to In solution, the generation in the electrolyte was measured using a UV spectrophotometer at a wavelength of 318 nm. concentration.

[0077] Test results show that this microbial fuel cell Yield can reach , The Faraday efficiency can reach up to 96.07%. The performance comparison results of Example 1 and Comparative Examples 1-4 are shown in Table 3.

[0078] Table 3 Performance Comparison Results

[0079] The Faradaic efficiency of the Co@PBA-750 catalyst prepared in Example 1 is as follows: Figure 12 As shown, the Co@PBA-750 catalyst prepared in Example 1 exhibits excellent Faraday efficiency within the tested potential range, especially at a relatively low potential of 0.1V vs. RHE. The Faraday efficiency reached a maximum of 96.07%.

[0080] The LSV test results of the Co@PBA-750 catalyst prepared in Example 1 and the Co@PBA-550, Co@PBA-650, and Co@PBA-850 catalysts prepared in Comparative Examples 2, 3, and 4 are as follows: Figure 5 As shown, the results indicate that compared with Co@PBA-550 catalyst (Comparative Example 2), Co@PBA-650 catalyst (Comparative Example 3) and Co@PBA-850 (Comparative Example 4), Co@PBA-750 catalyst (Example 1) exhibits a more positive onset potential and higher disk current and ring current in terms of electrocatalytic activity, proving that 750℃ is the reason for the high activity of its Co-doped PBA-derived catalyst.

[0081] The LSV test results of Co@PBA-750 prepared in Example 1 and the comparative catalyst (without sodium citrate) prepared in Comparative Example 1 are as follows: Figure 6 As shown, the results indicate that, compared to Comparative Example 1 (without sodium citrate), Example 1 (with sodium citrate) exhibited a more positive onset potential and higher disk current and ring current in terms of electrocatalytic activity, demonstrating that the addition of sodium citrate enhances the catalyst's performance. Key factors for activity. Specifically, in Example 1, the disk current density reached -2.5 mA / cm² at a potential of 0.1 V vs. RHE. 2The ring current density was also significantly higher than that of Comparative Example 1, indicating that the addition of sodium citrate successfully constructed the Co-NO synergistic active site, effectively promoting the... Suppressed Side reactions.

[0082] The Co@PBA-750 prepared in Example 1 and the Co@PBA-550, Co@PBA-650, and Co@PBA-850 prepared in Comparative Examples 2, 3, and 4, respectively. After calculating selectivity and the number of transferred electrons, as follows: Figure 7 As shown in the figure, Co@PBA-750 of Example 1 exhibited higher ORR during the ORR process. The selectivity, and the fact that the corresponding number of transferred electrons is close to 2, indicates that this catalytic material can effectively promote the transfer of O2 via... Path generation rather than prioritizing experience H2O is generated. The Co@PBA-750 catalytic material prepared in Example 1 exhibits good... The reaction selectivity indicates that the active sites constructed on its surface can be optimized. The adsorption and desorption behavior of intermediates, thereby improving The generation efficiency. This result further demonstrates the electrocatalytic efficiency of this catalytic material. preparation It has good application potential in this area.

[0083] The Co@PBA-750 prepared in Example 1 and the Co@PBA-750 prepared in Comparative Example 1 (without sodium citrate) After calculating selectivity and the number of transferred electrons, as follows: Figure 8 As shown in the figure. It can be seen from the figure that the catalyst of Example 1, throughout the entire test potential range (-0.1V~0.4V vs. RHE), The selectivity of the catalyst remained consistently above 90%, with the number of transferred electrons remaining stable between 2.1 and 2.2; while the H2O2 selectivity of the comparative example catalyst was only 30%–45%, with the number of transferred electrons being approximately 3.2–3.6. This comparative result indicates that the introduction of sodium citrate effectively promoted the construction of Co-NO synergistic active sites and significantly improved the selectivity. Selectivity, inhibiting Side reactions.

[0084] The Tafel slopes of Co@PBA-750 prepared in Example 1 and Co@PBA-550, Co@PBA-650, and Co@PBA-850 prepared in Comparative Examples 2, 3, and 4 are as follows: Figure 9As shown. By fitting the low overpotential region of the polarization curve, its Tafel slope was found to be 78 mV / dec. The low Tafel slope indicates that the Co@PBA-750 catalyst... The reaction has rapid reaction kinetics, which is beneficial for improving... The efficiency of electrochemical synthesis.

[0085] Voltage changes during the microbial domestication process of the gas diffusion electrode type microbial fuel cell in Example 5 are as follows: Figure 10 As shown in the figure, as the external resistance gradually decreases from 1000Ω to 1Ω, the output voltage of the microbial fuel cell gradually stabilizes. After approximately 600 hours of acclimatization, the battery can stably output a current of about 25mA at an external resistance of 1Ω, indicating that the electrogenic microorganisms have successfully formed a stable biofilm on the carbon brush anode surface, laying a good foundation for the subsequent construction of a gas diffusion electrode flow-type MFC.

[0086] The Co@PBA-750 catalyst material prepared in Example 1 was assembled into a gas diffusion electrode flow-type microbial fuel cell. Productivity and Faraday efficiency, such as Figure 11 As shown in the figure. It can be seen from the figure that at a potential of 0.1V vs. RHE, The highest Faraday efficiency reached 96.07%, corresponding to Yield Within the potential range of 0–0.4 V vs. RHE, the yield initially increases with a negative potential shift and then plateaus, while the Faraday efficiency remains high in the low to medium potential region. These results demonstrate that the Co@PBA-750 catalyst exhibits excellent performance under actual microbial fuel cell operating conditions. Selectivity and production Stability.

[0087] Advantages of this invention: (1) Using Prussian blue analogues (PBAs) as precursors, Co@PBA-750 precursors are synthesized by co-precipitation, and then Co-doped PBA-derived catalysts are prepared by high-temperature calcination and acid washing. Utilizing the unique metal-organic framework properties of PBA, it is in-situ transformed into a nitrogen-doped carbon composite material with uniformly dispersed cobalt species during pyrolysis; (2) Using a slow-drop co-precipitation method, sodium citrate is used as a complexing agent and morphology stabilizer to effectively control the nucleation and growth rate of the precursor and inhibit particle aggregation, thus synthesizing uniformly sized and well-dispersed Co@PBA-750 nanospheres; (3) The in-situ oxygen doping function of sodium citrate during pyrolysis is discovered for the first time, and... Co-NO synergistic active sites are formed in the center. Through a segmented programmed temperature calcination strategy, the cyano groups in the precursor are gradually converted into nitrogen-doped carbon frameworks. At the same time, oxygen doping defects are introduced. Acid washing selectively removes unstable cobalt species, introducing more defect sites and pore structures on the surface of nanospheres. (4) By comparing the sample without sodium citrate (the morphology is severely agglomerated and blocky, EDS shows that the Co content is 0.00wt% and the H2O2 selectivity is <45%), the morphological stability and performance improvement effect of sodium citrate are demonstrated. (5) The Co-doped PBA-derived catalyst is applied to the gas diffusion electrode flow microbial fuel cell as a cathode catalyst. The reaction generates hydrogen peroxide, enabling the construction of a gas diffusion electrode flow-type microbial fuel cell, which can be used for in-situ degradation of organic pollutants in wastewater.

[0088] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction, characterized in that, Co@PBA precursor was synthesized by co-precipitation using cobalt chloride hexahydrate, sodium prussiate of potassium, and sodium citrate as reactants. After washing and drying, the Co@PBA precursor was calcined at high temperature and then acid-washed to obtain the cobalt-doped PBA-derived catalyst Co@PBA-750.

2. The preparation method according to claim 1, characterized in that, Specifically, the steps include: Step S1: Dissolve cobalt chloride hexahydrate in deionized water to prepare solution A; dissolve sodium ferrous sulfate in deionized water to prepare solution B; dissolve sodium citrate in deionized water to prepare solution C; Step S2: Under stirring conditions, solutions A and B are added dropwise to solution C to carry out a coprecipitation reaction. After the reaction is complete, the resulting precipitate is centrifuged, washed, and dried to obtain the Co@PBA precursor. Step S3: Place the Co@PBA precursor in a tube furnace and calcine it under an inert atmosphere with programmed temperature rise. After natural cooling, the calcined product is obtained. Step S4: After grinding the calcined product, add an acid solution for acid washing, then filter, wash and dry to obtain the cobalt-doped PBA-derived catalyst Co@PBA-750.

3. The preparation method according to claim 2, characterized in that, In step S1, the concentration of solution A is 0.2 mol / L, the concentration of solution B is 0.25 mol / L, and the concentration of solution C is 0.045 mol / L.

4. The preparation method according to claim 3, characterized in that, In step S2, the coprecipitation reaction was carried out in a constant temperature water bath at 60°C, with a stirring speed of 400 rpm and a reaction time of 6 hours. Solution A and solution B are added slowly through a constant pressure dropping funnel, with the dropping rate controlled at 2-3 drops per second. The resulting precipitate was washed by centrifugation with deionized water and anhydrous ethanol, respectively. The centrifugation conditions were 8000 rpm for 3-5 minutes, and the washing was repeated three times. The drying conditions were as follows: drying in a vacuum drying oven at 60°C for 24 hours.

5. The preparation method according to claim 4, characterized in that, In step S3, the programmed temperature rise calcination is specifically as follows: the temperature is increased to 400°C at a heating rate of 2°C / min, held for 1 hour, and then increased to 750°C at a heating rate of 5°C / min, held for 2 hours.

6. The preparation method according to claim 5, characterized in that, In step S4, the acid washing process specifically involves adding the ground calcined product to a 0.5M H2SO4 solution at a solid-liquid ratio of 1g:50mL and stirring at 60℃ for 6 hours.

7. A cobalt-based catalyst for the electrocatalytic reduction of two-electron oxygen, characterized in that, It is prepared by the method for preparing a cobalt-based catalyst for electrocatalytic two-electron oxygen reduction as described in any one of claims 1-6.

8. The application of the cobalt-based catalyst for electrocatalytic two-electron oxygen reduction according to claim 7 in hydrogen peroxide production in a gas diffusion electrode flow-type microbial fuel cell.

9. The application according to claim 8, characterized in that, The specific steps for constructing a gas diffusion electrode flow-type microbial fuel cell based on the cobalt-doped PBA-derived catalyst Co@PBA-750 are as follows: Step S5: Gas diffusion electrode flow microbial fuel cell uses a cubic acrylic plate as the cell body of MFCs, and a gas diffusion electrode is set next to the cathode. Step S6: In the acclimatization stage of the gas diffusion electrode flow-type microbial fuel cell, the anode uses a carbon brush as the electrode. The carbon brush and sludge are added together into the anode chamber, and anolyte is added to ensure the activity of microorganisms in the sludge. The cathode uses carbon paper as an electrode and potassium ferricyanide as the catholy solution. The anode and cathode are connected to an external resistor to form a circuit. The resistance value is reduced by replacing the resistor to screen and cultivate electrogenic microorganisms. Once the electrogenic microorganisms stably attach to the carbon brush and form a stable biofilm, the domestication stage ends. Step S7: After the acclimatization stage, peristaltic pumps are connected to both the cathode and anode to make the electrolytes of both the cathode and anode flow at a rate of 15 mL / min. The cobalt-doped PBA-derived catalyst Co@PBA-750 is attached to carbon paper as the cathode electrode.

10. The application according to claim 9, characterized in that, In step S6, the resistance values ​​of the resistors are 1000Ω, 820Ω, 510Ω, 200Ω, 100Ω, 51Ω, 24Ω, 10Ω, 5Ω and 1Ω respectively. When the resistor is changed to 1Ω, the acclimatization stage ends.

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