Manganese dioxide-nitrogen doped carbon quantum dot electrode slurry and preparation method and application thereof

Abstract

The invention discloses a preparation method of manganese dioxide-nitrogen doped carbon quantum dot electrode slurry. Performing hydrothermal reaction on the carbon source substance and the nitrogen source substance; carrying out solid-liquid separation on the hydrothermal product, and removing a precipitation product to obtain a nitrogen-doped carbon quantum dot suspension; dropwise adding a KMnO4 aqueous solution into the NCQD suspension, enabling MnO4 <-> to fully react with functional groups on the surface of the NCQD, and generating a reduction product MnO2 on the surface of the NCQD to obtain a MnO2-coated NCQD suspension; and adding a KOH aqueous solution into the MnO2 (at) NCQD slurry to obtain the MnO2 (at) NCQD electrode slurry. The prepared MnO2-coated NCQD slurry can give full play to the electric double-layer capacitance characteristic of the NCQD and the pseudocapacitance characteristic of MnO2. The reversible redox reaction of the permanganate ion / manganate ion couple can provide additional pseudocapacitance contribution, so that the specific capacity of the electrode slurry is further improved.

Description

Technical Field

[0001] This invention relates to the field of supercapacitors, and in particular to a manganese dioxide-nitrogen-doped carbon quantum dot electrode paste, its preparation method, and its application. Background Technology

[0002] Aqueous flow supercapacitors (CFS) offer high power characteristics, high safety, and low manufacturing costs. Composed of an electrode slurry reservoir and a reactor, their key feature is the use of a fluid electrode slurry to store electrical energy. This unique energy storage method allows for the separation of stored energy from output power, offering flexibility in layout and scalability, making it a promising candidate for smoothing and peak shaving in renewable energy sources such as solar and wind power. However, due to the use of activated carbon particles dispersed in an aqueous electrolyte as the electrode material, CFS suffers from low energy density and poor electrode slurry suspension stability. Using other materials with high specific capacitance, such as graphene nanosheets, to prepare the electrode slurry, or introducing redox-active substances, such as soluble organic molecules or metal ions with variable valence, into the electrode slurry, can improve the specific capacitance to some extent.

[0003] Compared to activated carbon particles, nitrogen-doped carbon quantum dots (NCQDs) exhibit higher suspension and dispersibility. Electrode slurries using nitrogen-doped carbon quantum dots as the active material have the advantages of high fluidity and low sedimentation, but their capacity still has room for further improvement. Manganese dioxide has the ability to undergo rapid and reversible redox reactions and is a commonly used pseudocapacitive electrode material in supercapacitors, but it suffers from poor electronic conductivity, resulting in poor high-current charge-discharge capability, i.e., low rate performance. Combining manganese dioxide with nitrogen-doped carbon quantum dots, which have high electronic conductivity, can significantly improve the specific capacity of the electrode slurry while maintaining its suspension and dispersion stability. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a manganese dioxide-nitrogen-doped carbon quantum dot (MnO2@NCQD) electrode slurry, its preparation method, and its application in flow supercapacitors. This invention solves the problem of further improving the specific capacity of NCQD electrode slurry while maintaining its suspension and dispersion stability.

[0005] To achieve the above objectives, the technical solution adopted by the present invention includes:

[0006] In a first aspect, the present invention provides a method for preparing a manganese dioxide-nitrogen-doped carbon quantum dot electrode paste, comprising the following steps:

[0007] (1) Dissolve carbon source and nitrogen source in water and carry out hydrothermal reaction; the hydrothermal reaction temperature is 100-200℃ and the time is 0.1-24h;

[0008] (2) The hydrothermal products were subjected to solid-liquid separation to remove the precipitated products and obtain nitrogen-doped carbon quantum dot suspension (NCQD).

[0009] (3) The NCQD quantum dots were purified by dialysis; the purified NCQD quantum dot suspension was dried to obtain NCQD powder;

[0010] (4) Disperse NCQD powder in water to obtain NCQD suspension; the content of NCQD in the suspension is 0.1-40 g / 100 ml;

[0011] (5) Dissolve KMnO4 in water to obtain an aqueous solution of KMnO4; the concentration of the aqueous solution of KMnO4 is 0.1~6.38g / 100ml;

[0012] (6) While stirring, add the KMnO4 aqueous solution dropwise to the NCQD suspension, so that the MnO 4- The KMnO4 aqueous solution reacts fully with the functional groups on the NCQD surface to generate the reduction product MnO2 on the NCQD surface, resulting in a MnO2@NCQD suspension. The amount of KMnO4 aqueous solution added is 10-200 ml / (100 ml NCQD suspension). The solution is either not concentrated or concentrated so that the content of MnO2@NCQD in the MnO2@NCQD slurry is 0.1-40 g / 100 ml.

[0013] (7) Add KOH aqueous solution to MnO2@NCQD slurry and stir evenly to obtain MnO2@NCQD electrode slurry; the content of MnO2@NCQD in MnO2@NCQD electrode slurry is 0.1~20g / 100ml and the content of KOH is 0.1~25g / 100ml.

[0014] Furthermore, in step (1), the temperature of the hydrothermal reaction is 140–190°C, preferably 160–180°C; and the time is 5–20 h, preferably 10–15 h.

[0015] Furthermore, in step (4), the content of NCQD in the obtained suspension is 5-30 g / 100 ml, preferably 5-15 g / 100 ml.

[0016] Furthermore, in step (5), the concentration of the obtained KMnO4 aqueous solution is 1 to 6.38 g / 100 ml, preferably 3 to 6.38 g / 100 ml.

[0017] Further, in step (6), the amount of KMnO4 aqueous solution added is 50-150 ml / (100 ml NCQD suspension), preferably 80-120 ml / (100 ml NCQD suspension).

[0018] Further, in step (6), the content of MnO2@NCQD in the MnO2@NCQD slurry is 1-30g / 100ml, preferably 10-20g / 100ml.

[0019] Further, in step (7), the content of MnO2@NCQD in the MnO2@NCQD electrode slurry is 1-15 g / 100 ml, preferably 5-10 g / 100 ml; the content of KOH is 10-20 g / 100 ml, preferably 15-20 g / 100 ml.

[0020] Furthermore, the carbon source is one or a combination of two or more of sucrose, glucose, and soluble starch;

[0021] The nitrogen source is one or a combination of two or more of urea, melamine, ammonium nitrate, and ammonia water;

[0022] The mass ratio of carbon source to nitrogen source is 95:5 to 60:40 (preferably 92:8 to 80:20, more preferably 90:10 to 85:15); the mass ratio of the total mass of carbon source and nitrogen source to water is 2:98 to 30:70 (preferably 3:97 to 15:85, more preferably 4:96 to 10:90).

[0023] Furthermore, in step (2), the centrifugation speed used for solid-liquid separation is 2000-12000 r / min (preferably 4000-10000 r / min, more preferably 6000-8000 r / min), and the centrifugation time is 0.1-5 h (preferably 1-4 h, more preferably 2-3 h).

[0024] Further, in step (3), the NCQD suspension is purified for 24-72 hours using a 100-500 Da molecular weight cutoff dialysis membrane, and the retentate that has not permeated through the membrane is collected.

[0025] In step (6), the concentration temperature is 50-95°C (preferably 60-90°C, more preferably 70-80°C).

[0026] Secondly, the present invention provides an electrode slurry obtained by the preparation method described above.

[0027] Thirdly, the present invention provides an application of the electrode paste as an electrode paste for a flow-type supercapacitor in a flow-type supercapacitor.

[0028] Furthermore, the electrode slurry is pumped into the positive and negative electrodes of the flow supercapacitor, respectively, and circulated between the positive electrode and the positive electrode slurry pool, and between the negative electrode and the negative electrode slurry pool, to form a flow supercapacitor.

[0029] Furthermore, the flow-type supercapacitor is charged using a constant voltage and current limiting method, and discharged using a constant current method.

[0030] Beneficial effects of the technical solution of this invention

[0031] (1) NCQD prepared by hydrothermal method has higher electronic conductivity than CQD prepared by the same method, and is suitable as an active material for use as an electrode active material of liquid flow supercapacitor.

[0032] (2) Under neutral conditions, by utilizing the strong oxidizing properties of KMnO4, NCQD is oxidized to improve its suspension properties. At the same time, KMnO4 is reduced to MnO2 with pseudocapacitive properties on the NCQD surface, forming MnO2@NCQD composite quantum dots, which can significantly improve the capacity of the electrode slurry.

[0033] (3) After adding KOH solution to the MnO2@NCQD slurry, the resulting MnO2@NCQD electrode slurry exhibits strong alkalinity. In this strong alkaline solution, permanganate ions are reduced to manganate ions during discharge, and manganate ions are oxidized to permanganate ions during charging. This reversible redox reaction contributes pseudocapacitance to the electrode slurry, further enhancing its specific capacity. Attached Figure Description

[0034] Figure 1 Microstructure image of MnO2@NCQD prepared in Example 1;

[0035] Figure 2 The charge-discharge performance of the MnO2@NCQD electrode slurry prepared in Example 1 and the NCQD electrode slurry prepared in Comparative Example 3 after assembling a flow supercapacitor is shown in the figure. Detailed Implementation

[0036] The present invention will now be described in detail with reference to the embodiments.

[0037] Unless otherwise specified, the raw materials used in the examples were commercially purchased and used directly without processing; the instruments and equipment used were based on the manufacturer's recommended operating parameters.

[0038] In this embodiment, the capacity of the flow capacitor was tested using a Biologic electrochemical workstation.

[0039] In this embodiment, a transmission electron microscope was used to observe the morphology of the atomic clusters.

[0040] Example 1

[0041] 1. Preparation of MnO2@NCQD electrode paste

[0042] (1) Preparation of NCQD

[0043] 4g of sucrose was dispersed in 60.0ml of deionized water and stirred for 30min. Then, 0.5g of urea was added to the mixture and stirred continuously at 80℃ for 30min until dissolved. The mixture was then added to a 100ml hydrothermal reactor, sealed, and placed in an oven at 180℃ for 12h of hydrothermal reaction. The reaction product was then centrifuged at 8000r / min for 20min to remove the precipitate, retaining the supernatant. The supernatant was purified using a 500Da dialysis membrane. This membrane allows unreacted carbon and nitrogen source molecules to pass freely while retaining NCQD. The dialysis process lasted 48h. The purified NCQD suspension (retentate) was then lyophilized to obtain NCQD powder. Transmission electron microscopy (TEM) results showed that the particle size of NCQD was 2–3nm. The nitrogen content in NCQD was determined to be 2.4% by an organic element analyzer. (2) Preparation of MnO2@NCQD

[0044] 1.0 g of NCQD powder was added to 10 ml of deionized water and ultrasonically dispersed to obtain an NCQD suspension. Then, 0.158 g of KMnO4 was added to 10 ml of deionized water and stirred thoroughly until completely dissolved to obtain a KMnO4 aqueous solution. While stirring, the KMnO4 aqueous solution was added dropwise to the NCQD suspension to disperse the KMnO4. - The MnO2@NCQD suspension is obtained by fully reacting with the functional groups on the NCQD surface to generate the reduction product MnO2. The morphology of MnO2@NCQD is as follows: Figure 1 As shown. The particle size of the prepared MnO2@NCQD particles is 3-4 nm. The MnO2@NCQD suspension was heated to 50-95℃ (75℃ in this case) while stirring, and then concentrated to 10 ml to obtain MnO2@NCQD slurry.

[0045] (3) Preparation of MnO2@NCQD electrode paste

[0046] Add 3.366g KOH to 10ml of deionized water and stir until completely dissolved to obtain a 6M KOH aqueous solution. Add the prepared 6M KOH aqueous solution to the MnO2@NCQD slurry prepared in step (2) and stir evenly to obtain a MnO2@NCQD electrode slurry with a KOH concentration of 3M.

[0047] 2. Construction and performance testing of flow supercapacitors

[0048] (1) Carbon felt is used as the current collector and electrode. The carbon felt has dimensions of 30×30×5mm (length×width×thickness). The carbon felt is supported by a polytetrafluoroethylene (PTFE) frame (square annular frame). The size and shape of the hollow area inside the frame match the size and shape of the carbon felt. Nafion membrane is used as the diaphragm. The structure of the liquid flow capacitor reactor is a sandwich structure of stainless steel end plate-graphite plate-electrode frame (with carbon felt in the hollow area inside)-diaphragm-electrode frame (with carbon felt in the hollow area inside)-graphite plate-stainless steel end plate. An annular rubber gasket is placed between the PTFE frame, diaphragm and graphite plate and fastened with bolts to ensure a tight seal between components and prevent slurry leakage. The electrode slurry is divided into two equal parts and pumped into the positive and negative electrode chambers of the reactor respectively using a transfer pump. Capacity testing is performed under flow conditions. The flow rate of the electrode slurry is 50ml / min.

[0049] (2) The capacity of the flow supercapacitor was tested using cyclic voltammetry, with a potential window of 0V to 0.6V and a potential scan rate of 2mV / s. At a scan rate of 2mV / s, the specific capacity of the electrode slurry reached 183F / g. The cyclic voltammetry test results are as follows: Figure 2 As shown. Under the same conditions, the specific capacity of the nitrogen-doped carbon quantum electrode paste (NCQD electrode paste) in Comparative Example 3 was 77 F / g. The specific capacity of the MnO2@NCQD electrode paste was 2.4 times that of the nitrogen-doped carbon quantum electrode paste.

[0050] Example 2

[0051] In Example 1, the nitrogen source urea was replaced with the same mass of melamine, and all other conditions remained unchanged. MnO2@NCQD electrode slurry was prepared following the steps of Example 1 (process and conditions were the same as in Example 1). The prepared MnO2@NCQD particles had a particle size of 3–5 nm. Capacity testing was conducted under the same conditions, and the specific capacity of the MnO2@NCQD electrode slurry reached 124 F / g at a scan rate of 2 mV / s, which is 1.6 times that of the NCQD electrode slurry.

[0052] Example 3

[0053] In Example 1, the sucrose was replaced with the same mass of glucose, and all other conditions were kept unchanged. MnO2@NCQD electrode slurry was prepared following the steps of Example 1 (process and conditions were the same as in Example 1). The prepared MnO2@NCQD particles had a particle size of 3–5 nm. Capacity testing was conducted under the same conditions, and the specific capacity of the MnO2@NCQD electrode slurry reached 162 F / g at a scan rate of 2 mV / s, which is 2.1 times that of the NCQD electrode slurry.

[0054] Example 4

[0055] The hydrothermal reaction temperature in Example 1 was changed to 160℃, and other conditions remained unchanged. MnO2@NCQD electrode slurry was prepared according to the steps of Example 1 (process and conditions were the same as in Example 1). The particle size of the prepared MnO2@NCQD particles was 2–4 nm. Capacity testing was conducted under the same conditions, and the specific capacity of the MnO2@NCQD electrode slurry reached 172 F / g at a scan rate of 2 mV / s, which is 2.2 times that of the NCQD electrode slurry.

[0056] Example 5

[0057] The hydrothermal reaction time in Example 1 was reduced to 10 h, and other conditions were not changed. MnO2@NCQD electrode slurry was prepared according to the steps of Example 1 (process and conditions were the same as in Example 1). The particle size of the prepared MnO2@NCQD particles was 2–4 nm. Capacity testing was performed under the same conditions. The specific capacity of the MnO2@NCQD electrode slurry reached 172 F / g at a scan rate of 2 mV / s, which is 2.2 times that of the NCQD electrode slurry.

[0058] Example 6

[0059] Using 3.5g of sucrose as the carbon source and 1.0g of urea as the nitrogen source, and without changing other conditions, MnO2@NCQD electrode slurry was prepared according to the steps of Example 1 (the process and conditions were the same as in Example 1). The particle size of the prepared MnO2@NCQD particles was 2.5–4 nm. Capacity testing was performed under the same conditions, and the specific capacity of the MnO2@NCQD electrode slurry reached 188 F / g at a scan rate of 2 mV / s, which is 2.44 times that of the NCQD electrode slurry.

[0060] Example 7

[0061] The amount of KMnO4 in Example 1 was changed to 0.12g, and other conditions remained unchanged. MnO2@NCQD electrode slurry was prepared according to the steps of Example 1 (process and conditions were the same as in Example 1). The particle size of the prepared MnO2@NCQD particles was 2–4 nm. Capacity testing was performed under the same conditions, and the specific capacity of the MnO2@NCQD electrode slurry reached 152 F / g at a scan rate of 2 mV / s, which is 1.97 times that of the NCQD electrode slurry.

[0062] Comparative Example 1

[0063] In Example 1, no carbon source was added and other conditions were not changed. The material was prepared according to the steps of Example 1 (the process and conditions were the same as in Example 1). The reaction product did not contain MnO2@NCQD and could not be used as an electrode paste.

[0064] Comparative Example 2

[0065] In Example 1, no nitrogen source was added, and other conditions were not changed. The material was prepared according to the steps of Example 1 (the process and conditions were the same as in Example 1) to obtain MnO2@CQD electrode slurry. The particle size of the prepared MnO2@CQD particles was 4-5 nm. Capacity testing was performed under the same conditions, and the specific capacity of the MnO2@CQD electrode slurry at a scan rate of 2 mV / s was 34 F / g.

[0066] Comparative Example 3

[0067] 4.45 g of sucrose was dispersed in 60.0 ml of deionized water and stirred for 30 min. Then, 0.05 g of urea was added to the mixture, and the mixture was stirred continuously at 80 °C for 30 min. The mixture was then transferred to a 100 ml hydrothermal reactor, sealed, and placed in an oven at 180 °C for hydrothermal reaction for 12 h. The reaction product was then centrifuged at 8000 rpm for 20 min to remove the precipitate, retaining the supernatant. The supernatant was purified using a 500 Da dialysis membrane to obtain NCQD powder.

[0068] 1.0 g of NCQD powder was added to 10 ml of deionized water and ultrasonically dispersed to obtain an NCQD solution. Then, 0.158 g of KMnO4 was added to 10 ml of deionized water and stirred thoroughly until completely dissolved to obtain a KMnO4 aqueous solution. While stirring, the KMnO4 aqueous solution was added dropwise to the NCQD suspension to disperse the KMnO4. - The MnO2@NCQD particles react fully with the functional groups on the NCQD surface to generate the reduction product MnO2, resulting in a MnO2@NCQD suspension. The particle size of the prepared MnO2@NCQD particles is 3–5 nm. The MnO2@NCQD suspension is heated to 50–95 °C with stirring, and then concentrated to 10 ml to obtain a MnO2@NCQD slurry.

[0069] 3.366 g of KOH was added to 10 ml of deionized water and stirred until completely dissolved to obtain a 6 M KOH aqueous solution. The prepared 6 M KOH aqueous solution was added to the MnO2@NCQD slurry and stirred until homogeneous, resulting in a 3 M KOH concentration MnO2@NCQD electrode slurry. The capacity of a room temperature flow supercapacitor using the MnO2@NCQD slurry was tested using cyclic voltammetry. At a scan rate of 2 mV / s, the specific capacity of the active material in the NCQD electrode slurry was 65 F / g. The nitrogen content in the NCQD prepared in this comparative example was too low, resulting in poor capacity performance of the MnO2@NCQD slurry. Therefore, the MnO2@NCQD slurry prepared in this comparative example is not suitable as an electrode slurry for flow supercapacitors.

[0070] Comparative Example 4

[0071] 2.25 g of sucrose was dispersed in 60.0 ml of deionized water and stirred for 30 min. Then, 2.25 g of urea was added to the mixture, and the mixture was stirred continuously at 80 °C for 30 min. The mixture was then transferred to a 100 ml hydrothermal reactor, sealed, and placed in an oven at 180 °C for hydrothermal reaction for 12 h. The reaction product was then centrifuged at 8000 rpm for 20 min to remove the precipitate, retaining the supernatant. The supernatant was purified using a 500 Da dialysis membrane to obtain NCQD powder.

[0072] 1.0 g of NCQD powder was added to 10 ml of deionized water and ultrasonically dispersed to obtain an NCQD solution. Then, 0.158 g of KMnO4 was added to 10 ml of deionized water and stirred thoroughly until completely dissolved to obtain a KMnO4 aqueous solution. While stirring, the KMnO4 aqueous solution was added dropwise to the NCQD suspension to disperse the KMnO4. - The MnO2@NCQD particles react fully with the functional groups on the NCQD surface to generate the reduction product MnO2, resulting in a MnO2@NCQD suspension. The particle size of the prepared MnO2@NCQD particles is 3–4 nm. The MnO2@NCQD suspension is heated to 50–95 °C with stirring, and then concentrated to 10 ml to obtain a MnO2@NCQD slurry.

[0073] 3.366 g of KOH was added to 10 ml of deionized water and stirred until completely dissolved to obtain a 6 M KOH aqueous solution. The prepared 6 M KOH aqueous solution was added to the MnO2@NCQD slurry and stirred until homogeneous, resulting in a 3 M KOH concentration MnO2@NCQD electrode slurry. The capacity of a room temperature flow supercapacitor using the MnO2@NCQD slurry was tested using cyclic voltammetry. At a scan rate of 2 mV / s, the specific capacity of the active material in the NCQD electrode slurry was 38 F / g. In this comparative example, the excessively high proportion of nitrogen source material in the raw materials resulted in a high nitrogen content and low specific surface area in the NCQD, further contributing to the low capacity of the MnO2@NCQD slurry. The MnO2@NCQD slurry prepared in this comparative example is not suitable as an electrode slurry for flow supercapacitors.

[0074] Comparative Example 5

[0075] In Example 1, MnO2 loading was not performed, and other conditions were not changed. Material preparation was carried out according to the steps of Example 1 (process and conditions were the same as in Example 1) to obtain the NCQD electrode slurry. The particle size of the prepared NCQD particles was 2–3 nm. Cyclic voltammetry test results are as follows: Figure 2As shown, at a scan rate of 2 mV / s, the specific capacity of the active material in the NCQD electrode slurry is 77 F / g.

[0076] Comparative Example 6

[0077] 4g of sucrose was dispersed in 60.0ml of deionized water and stirred for 30min. Then, 0.5g of urea was added to the mixture and stirred continuously at 80℃ for 30min until dissolved. The mixture was then added to a 100ml hydrothermal reactor, sealed, and placed in an oven at 180℃ for hydrothermal reaction for 12h. The reaction product was then centrifuged at 8000r / min for 20min to remove the precipitate, retaining the supernatant. The supernatant was purified using a 500Da dialysis membrane. This dialysis membrane allows unreacted carbon and nitrogen source molecules to pass freely while retaining NCQD. The dialysis process lasted for 48h. The purified NCQD suspension (retentate) was then lyophilized to obtain NCQD powder.

[0078] 1.0 g of NCQD powder was added to 10 ml of deionized water and ultrasonically dispersed to obtain an NCQD suspension. Then, 0.0005 g of KMnO4 was added to 10 ml of deionized water and stirred thoroughly until completely dissolved. While stirring, the KMnO4 aqueous solution was added dropwise to the NCQD suspension to disperse the KMnO4. - The MnO2@NCQD suspension was obtained by reacting fully with the functional groups on the NCQD surface to generate the reduction product MnO2. The particle size of the prepared MnO2@NCQD particles was 3-4 nm. The MnO2@NCQD suspension was heated to 50-95 °C with stirring, and then concentrated to 10 ml to obtain a MnO2@NCQD slurry. 3.366 g of KOH was added to 10 ml of deionized water and stirred until completely dissolved to obtain a 6 M KOH aqueous solution. The prepared 6 M KOH aqueous solution was added to the MnO2@NCQD slurry and stirred evenly to obtain a 3 M KOH concentration MnO2@NCQD electrode slurry. The capacity of a room temperature flow supercapacitor using the MnO2@NCQD slurry was tested using cyclic voltammetry. At a scan rate of 2 mV / s, the specific capacity of the active material in the NCQD electrode slurry was 95 F / g. The MnO2@NCQD slurry prepared in this comparison has a low specific capacity and is not suitable as an electrode slurry for flow supercapacitors.

[0079] Comparative Example 7

[0080] 4g of sucrose was dispersed in 60.0ml of deionized water and stirred for 30min. Then, 0.5g of urea was added to the mixture and stirred continuously at 80℃ for 30min until dissolved. The mixture was then added to a 100ml hydrothermal reactor, sealed, and placed in an oven at 180℃ for hydrothermal reaction for 12h. The reaction product was then centrifuged at 8000r / min for 20min to remove the precipitate, retaining the supernatant. The supernatant was purified using a 500Da dialysis membrane. This dialysis membrane allows unreacted carbon and nitrogen source molecules to pass freely while retaining NCQD. The dialysis process lasted for 48h. The purified NCQD suspension (retentate) was then lyophilized to obtain NCQD powder.

[0081] 1.0 g of NCQD powder was added to 10 ml of deionized water and ultrasonically dispersed to obtain an NCQD suspension. Then, 0.75 g of KMnO4 was added to 10 ml of deionized water and stirred thoroughly. Since the amount of KMnO4 added exceeded its solubility, the KMnO4 could not be completely dissolved. While stirring, the KMnO4 aqueous solution containing undissolved KMnO4 was added dropwise to the NCQD suspension to disperse the KMnO4. - The MnO2 particles react with the functional groups on the NCQD surface to generate the reduction product MnO2. Although undissolved KMnO4 gradually dissolves and participates in the reaction during this process, its uneven concentration results in significant differences in the size of the prepared MnO2@NCQD particles, with larger particles precipitating. Therefore, the MnO2@NCQD particles prepared using this comparative example are not suitable for preparing electrode slurries for flow supercapacitors.

Claims

1. A method for preparing a manganese dioxide-nitrogen-doped carbon quantum dot electrode paste, characterized in that: Includes the following steps: (1) Dissolve carbon source and nitrogen source in water and carry out hydrothermal reaction; the hydrothermal reaction temperature is 100-200℃ and the time is 0.1-24h; (2) The hydrothermal products were subjected to solid-liquid separation to remove the precipitated products and obtain nitrogen-doped carbon quantum dot suspension (NCQD). (3) The NCQD quantum dots were purified by dialysis; the purified NCQD quantum dot suspension was dried to obtain NCQD powder; (4) Disperse NCQD powder in water to obtain NCQD suspension; the content of NCQD in the suspension is 0.1-40 g / 100 ml; (5) Dissolve KMnO4 in water to obtain an aqueous solution of KMnO4; the concentration of the aqueous solution of KMnO4 is 0.1~6.38g / 100ml; (6) While stirring, add the KMnO4 aqueous solution dropwise to the NCQD suspension, so that the MnO 4- The KMnO4 aqueous solution reacts fully with the functional groups on the NCQD surface to generate the reduction product MnO2 on the NCQD surface, resulting in a MnO2@NCQD suspension. The amount of KMnO4 aqueous solution added is 10-200 ml / (100 ml NCQD suspension). The solution is either not concentrated or concentrated so that the content of MnO2@NCQD in the MnO2@NCQD slurry is 0.1-40 g / 100 ml. (7) Add KOH aqueous solution to MnO2@NCQD slurry and stir evenly to obtain MnO2@NCQD electrode slurry; the content of MnO2@NCQD in MnO2@NCQD electrode slurry is 0.1~20g / 100ml and the content of KOH is 0.1~25g / 100ml.

2. The preparation method according to claim 1, characterized in that: In step (1), the temperature of the hydrothermal reaction is 140–190°C, preferably 160–180°C; the time is 5–20 h, preferably 10–15 h. In step (4), the NCQD content in the obtained suspension is 5-30 g / 100 ml, preferably 5-15 g / 100 ml; In step (5), the concentration of the obtained KMnO4 aqueous solution is 1 to 6.38 g / 100 ml, preferably 3 to 6.38 g / 100 ml; In step (6), the amount of KMnO4 aqueous solution added is 50-150 ml / (100 ml NCQD suspension), preferably 80-120 ml / (100 ml NCQD suspension); In step (6), the content of MnO2@NCQD in the MnO2@NCQD slurry is 1-30g / 100ml, preferably 10-20g / 100ml; In step (7), the content of MnO2@NCQD in the MnO2@NCQD electrode paste is 1-15 g / 100 ml, preferably 5-10 g / 100 ml; the content of KOH is 10-20 g / 100 ml, preferably 15-20 g / 100 ml.

3. The preparation method according to claim 1, characterized in that: The carbon source is one or a combination of two or more of sucrose, glucose, and soluble starch. The nitrogen source is one or a combination of two or more of urea, melamine, ammonium nitrate, and ammonia water; The mass ratio of carbon source to nitrogen source is 95:5 to 60:40 (preferably 92:8 to 80:20, more preferably 90:10 to 85:15); the mass ratio of the total mass of carbon source and nitrogen source to water is 2:98 to 30:70 (preferably 3:97 to 15:85, more preferably 4:96 to 10:90).

4. The preparation method according to claim 1, characterized in that: In step (2), the centrifugation speed used for solid-liquid separation is 2000-12000 r / min (preferably 4000-10000 r / min, more preferably 6000-8000 r / min), and the centrifugation time is 0.1-5 h (preferably 1-4 h, more preferably 2-3 h).

5. The preparation method according to claim 1, characterized in that: In step (3), the NCQD suspension is purified for 24-72 hours using a 100-500 Da molecular weight cutoff dialysis membrane, and the retentate that does not permeate through the membrane is collected. In step (6), the concentration temperature is 50-95°C (preferably 60-90°C, more preferably 70-80°C).

6. An electrode slurry obtained by the preparation method according to any one of claims 1-5.

7. The application of the electrode paste of claim 6 as an electrode paste for a flow supercapacitor in a flow supercapacitor.

8. The application according to claim 7, characterized in that: Electrode slurries are pumped into the positive and negative electrodes of a flow-type supercapacitor, respectively, and circulated between the positive electrode and the positive electrode slurry pool, and between the negative electrode and the negative electrode slurry pool, thus forming a flow-type supercapacitor.

9. The application according to claim 8, characterized in that: The flow supercapacitor is charged using a constant voltage and current limiting method, and discharged using a constant current method.

Images