Preparation method and application of high-capacity and high-magnification MXene electrode for targeted rapid elimination of active Ti-OH sites

Abstract

The invention relates to the technical field of supercapacitor negative electrode materials, and discloses a preparation method and application of a high-capacity and high-magnification MXene electrode capable of rapidly eliminating active Ti-OH sites in a targeted manner. The preparation method comprises the following steps: adding an acid solution and a salt solution into a single / few-layer Ti3C2MXene aqueous dispersion, and heating and stirring in an air atmosphere to obtain a reaction mixed solution; carrying out centrifugal washing on the reaction mixed solution by using deionized water until the supernate is neutral; and collecting centrifugal precipitate, adding deionized water for ultrasonic dispersion to obtain an MXene aqueous dispersion liquid with active Ti-OH sites eliminated, and preparing a high-capacity and high-magnification MXene electrode by using the MXene aqueous dispersion liquid. According to the method, the single / few-layer Ti3C2MXene aqueous dispersion is stirred through air contact and moderate heating, and the ionic intercalation of the salt solution and the hydrolysis inhibition of the acid solution are matched, so that the high-activity Ti-OH sites are fully oxidized, hydrolyzed and removed, and the mass specific capacity, the rate capability and the cycle performance of MXene are improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of supercapacitor negative electrode materials, in particular to a preparation method of a high-capacity high-rate MXene electrode with active Ti-OH site targeted rapid elimination and application thereof. BACKGROUND

[0002] Transition metal carbides / nitrides (MXene) is a new type of two-dimensional material, which is mainly obtained by etching A-site elements in the MAX phase of precursor layered ceramic. It was first successfully prepared by the Yury Gogotsi team of Drexel University in the United States in 2011 through a hydrofluoric acid etching method. MXene has the usual excellent electrical properties, mechanical properties and high specific surface area of two-dimensional materials, and also has some unique advantages, such as rich chemical composition and surface functional groups, excellent hydrophilicity and adjustable interlayer spacing. These characteristics make MXene quickly become a research hotspot in the fields of energy storage (such as supercapacitors, lithium / sodium / zinc ion batteries), electromagnetic shielding, catalysis, flexible electronics and biomedicine.

[0003] In recent years, researchers have developed various fluoride-free etching technologies (molten salt method, organic acid method) and direct synthesis technologies (such as chemical vapor deposition) of MXene, but the wet chemical etching of MAX phase in a fluorine-containing acid solution to obtain MXene is still the current mainstream preparation method. However, the MXene obtained by this method is extremely prone to degradation when stored in the form of an aqueous dispersion. To solve this problem, researchers have proposed various modification strategies. Yury et al. proposed a strategy of storing large sheet MXene aqueous solution in an argon gas sealed bottle at low temperature in 2017, or filtering into a thin film and then redispersing into an aqueous solution for use, thereby alleviating the degradation of MXene aqueous dispersion. In addition, methods such as edge / surface shielding and surface modification are used to protect MXene, for example, using polyanion salts (such as polyphosphate, polysilicate or polyborate) to protect the positively charged edges of MXene, and using antioxidants (such as ascorbic acid and sodium ascorbate) to provide electrons to inhibit the oxidation of active sites of MXene. However, by taking advantage of the degradation differences of different components of MXene in the aqueous dispersion, selective degradation and elimination of certain components can achieve multiple improvements in the capacity, rate and cycle performance of MXene, and this method is currently missing. This is mainly due to the fact that the degradation mechanism of MXene is still unclear, so that the corresponding scheme cannot be proposed.

[0004] CN202111398199.4 discloses a kind of porous nanosheet, combined with organic molecule intercalation and fast high temperature processing to improve the oxidation resistance of MXene, while shortening ion transmission path, improve the rate performance of MXene, but relatively energy-consuming and complex operation.CN202110805929.1 discloses a kind of MXene / graphene / carbon nanotube composite three-dimensional porous open structure, overall improves the exposure number of MXene active site, improves the specific capacity of MXene itself and the rate performance of composite electrode, but the specific capacity of electrode is reduced to some extent by composite material. SUMMARY

[0005] To overcome the shortcomings of the prior art, one of the purposes of the present application is to provide a preparation method of a high-capacity and high-rate MXene electrode that targets the rapid elimination of active Ti-OH sites. The second purpose of the present application is to provide an application of a high-capacity and high-rate MXene electrode that targets the rapid elimination of active Ti-OH sites. The present application targets the removal of low work function functional groups Ti-OH that do not provide electrochemical capacity and hinder the migration of cations through oxygen consumption reaction and controllable hydrolysis reaction. The resulting MXene significantly improves the mass specific capacity, rate performance and cycle performance when used as a supercapacitor electrode.

[0006] To this end, the first aspect of the present application provides a preparation method of a high-capacity and high-rate MXene electrode that targets the rapid elimination of active Ti-OH sites, comprising the following steps:

[0007] An acid solution and a salt solution are added to a single / few-layer Ti3C2 MXene water dispersion to obtain a mixed solution;

[0008] The mixed solution is heated and stirred under air atmosphere to obtain a reaction mixture;

[0009] The reaction mixture is centrifuged and washed with deionized water until the supernatant reaches neutral;

[0010] The centrifugal precipitate is collected and dispersed by ultrasonic with deionized water to obtain a MXene water dispersion that eliminates active Ti-OH sites;

[0011] The high-capacity and high-rate MXene electrode is prepared using the MXene water dispersion that eliminates active Ti-OH sites.

[0012] Further, the concentration of Ti3C2 MXene in the mixed solution is controlled to be 1.5 mg / mL to 2.5 mg / mL.

[0013] Further, the acid solution is one or more of dilute sulfuric acid, dilute hydrochloric acid and dilute hydrofluoric acid, and the concentration of acid in the mixed solution is controlled to be 0.5 mol / L to 2 mol / L.

[0014] Further, the salt solution is one or more of lithium sulfate, lithium chloride, sodium sulfate, and sodium chloride, and the concentration of the salt in the mixed solution is controlled to be 0.02 g / mL to 0.2 g / mL.

[0015] Further, the heating and stirring include: the heating temperature is 30°C to 60°C, and the stirring time is 0.5 h to 12 h.

[0016] Further, the centrifugal washing includes: the centrifugal speed is 3000 rpm to 8000 rpm, and the centrifugal time for each time is 3 min to 10 min, until the supernatant is neutral.

[0017] Further, the adding of deionized water for ultrasonic dispersion includes: under the condition of 0°C to 4°C and argon protection, ultrasonic treatment is performed at a power of 500 W to 800 W for 10 min to 60 min.

[0018] Further, the preparation method of the single / few-layer Ti3C2 MXene aqueous dispersion solution includes:

[0019] LiF is added to an 8 mol / L to 10 mol / L HCl solution and mixed for 5 min to 20 min until the LiF is completely dissolved, and Ti3AlC2 MAX powder is slowly added in 3 to 6 times under an ice water bath environment and stirred; the mixed system is incubated at 30°C to 60°C for 24 h to 48 h for etching reaction; after the reaction is completed, 1 mol / L to 3 mol / L hydrochloric acid solution is first used for centrifugal washing 1 to 3 times, the centrifugal speed is 2000 rpm to 4000 rpm, and the centrifugal time for each time is 3 min to 10 min; then deionized water is used for centrifugal washing until the supernatant is neutral, the centrifugal speed is started from 3000 rpm and gradually increased to 8000 rpm, and the centrifugal time for each time is 3 min to 10 min; deionized water is added to the obtained precipitate, ultrasonic treatment is performed at a power of 500 W to 800 W for 1 h to 3 h under the condition of 0°C to 4°C and argon protection, and a multi-layer Ti3C2 MXene dispersion solution is obtained; the multi-layer Ti3C2 MXene dispersion solution is subjected to low-speed centrifugation at 2000 rpm to 4000 rpm for 0.5 h to 2 h, and the upper liquid is taken to obtain the single / few-layer Ti3C2 MXene aqueous dispersion solution.

[0020] Further, in the etching reaction, the mass of LiF, the volume of HCl solution, and the mass ratio of Ti3AlC2 MAX powder are (1 to 2) g:(20 to 50) mL:1 g.

[0021] In a second aspect, the application provides a use of the high-capacity and high-rate MXene electrode for targeted rapid elimination of active Ti-OH sites, wherein the high-capacity and high-rate MXene electrode is prepared by the above preparation method and used as a negative electrode of a supercapacitor.

[0022] Compared with the prior art, the application has at least the following beneficial effects:

[0023] The preparation method of the high-capacity and high-rate MXene electrode provided by the application can fully oxidize and hydrolyze the high-activity Ti-OH sites by air contact, moderate temperature stirring of a single / few-layer Ti3C2 MXene water dispersion liquid, ion intercalation of a salt solution, and hydrolysis inhibition of the substrate by an acid solution, without the need for an inert atmosphere during the reaction process, with moderate reaction temperature and short reaction time, and high production capacity can be achieved. Targeted removal of the active Ti-OH sites can reduce the mass of the electrode non-capacitive contribution sites and the ion transmission resistance caused by the hydroxyl groups, thereby improving the mass specific capacity, rate performance, and cycle performance of the MXene. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the application, the drawings needed in the embodiment description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0025] Figure 1 O element fine spectrum of the MXene film prepared in the application example 1 and the comparative example 1.

[0026] Figure 2 Transmission electron microscope image of the MXene prepared in the application example 1, the comparative examples 1-3.

[0027] Figure 3 X-ray diffraction spectrum of the MXene film electrode prepared in the application example 1, the comparative examples 1-3.

[0028] Figure 4 CV test curve (scan rate 5 mV / s) of the three-electrode system supercapacitor assembled by the MXene film electrode prepared in the application example 1, the comparative examples 1-3.

[0029] Figure 5 Cycle stability of the three-electrode system supercapacitor assembled by the MXene film electrode prepared in the application example 1 and the comparative example 1 (scan rate 50 mV / s, capacity calculated from the CV integral area). DETAILED DESCRIPTION

[0030] In order to better understand the above technical solutions, the technical solutions of the embodiments of the present application will be described in detail below through the drawings and specific embodiments. It should be understood that the specific features in the embodiments of the present application and the specific features in the embodiments are detailed descriptions of the technical solutions of the embodiments of the present application, but not limitations of the technical solutions of the present application. In the case of no conflict, the technical features in the embodiments of the present application and the embodiments can be combined with each other.

[0031] In a first aspect of the embodiments of the present application, a preparation method of a high-capacity and high-rate MXene electrode targeting rapid elimination of active Ti-OH sites is provided, comprising the following steps:

[0032] An acid solution and a salt solution are added to a single / few-layer Ti3C2 MXene aqueous dispersion to obtain a mixed solution;

[0033] The mixed solution is heated and stirred under an air atmosphere to obtain a reaction mixture;

[0034] The reaction mixture is centrifugally washed with deionized water until the supernatant reaches neutrality;

[0035] The centrifugal precipitate is collected and dispersed by ultrasonic with deionized water to obtain a MXene aqueous dispersion in which active Ti-OH sites are eliminated;

[0036] The MXene aqueous dispersion in which active Ti-OH sites are eliminated is used to prepare a high-capacity and high-rate MXene electrode.

[0037] The preparation method of the high-capacity and high-rate MXene electrode provided by the embodiments of the present application uses air contact, moderate heating and stirring of the single / few-layer Ti3C2 MXene aqueous dispersion, and ion intercalation of the salt solution and hydrolysis inhibition of the substrate by the acid solution, so that the high-activity Ti-OH sites can be fully oxidized and removed. There is no need for inert atmosphere during the reaction process, the reaction temperature is moderate, the reaction time is short, and high productivity can be achieved.

[0038] In some embodiments, the concentration of Ti3C2 MXene in the mixed solution is controlled to be 1.5 mg / mL-2.5 mg / mL.

[0039] Specifically, when the concentration of Ti3C2 MXene is higher than 2.5 mg / mL, the layers are prone to stacking and the reaction with the acid solution is not sufficient, and when the concentration is lower than 1.5 mg / mL, the processing efficiency is too low and the Ti3C2 MXene is prone to be oxidized.

[0040] In some embodiments, the acid solution is one or more of dilute sulfuric acid, dilute hydrochloric acid, and dilute hydrofluoric acid, and the concentration of the acid in the mixed solution is controlled to be 0.5 mol / L-2 mol / L.

[0041] Specifically, dilute sulfuric acid, dilute hydrochloric acid, and dilute hydrofluoric acid are common inorganic acids, which do not attach ions to the surface of the material to shield the functional groups from contacting the solution, among which the anions can intercalate the MXene layers to prevent stacking, and the hydrogen ions can inhibit the hydrolysis reaction of the MXene matrix. Moreover, when the acid concentration is lower than 0.5 mol / L, the MXene solution stirred in an air atmosphere is prone to continuous hydrolysis, thereby destroying the two-dimensional structure, and not only targeting the removal of the high-activity Ti-OH functional groups; when the concentration reaches 2 mol / L, the hydrolysis reaction of the MXene is already sufficiently inhibited, and excessive increase in the concentration will increase the cost without further inhibiting the hydrolysis reaction.

[0042] In some embodiments, the salt solution is one or more of lithium sulfate, lithium chloride, sodium sulfate, and sodium chloride, and the concentration of the salt in the mixed solution is controlled to be 0.02 g / mL to 0.2 g / mL.

[0043] Specifically, lithium sulfate, lithium chloride, sodium sulfate, and sodium chloride belong to common inorganic salts, and the anions thereof can be used in combination with the acid solution to effectively intercalate the MXene layers to prevent stacking, and a concentration lower than 0.02 g / mL cannot guarantee the intercalation effect, which may result in insufficient contact of the Ti-OH functional groups at the stacking sites with air and slow oxidation and hydrolysis, and poor targeted elimination effect; a concentration higher than 0.2 g / mL has no additional benefit to promoting the reaction.

[0044] In some embodiments, the heating and stirring include: a heating temperature of 30°C to 60°C, and a stirring time of 0.5 h to 12 h.

[0045] Specifically, heating and stirring can promote sufficient contact of the MXene with oxygen, which is conducive to rapid oxidation of the high-activity Ti-OH sites, improves the reaction efficiency and shortens the reaction time, and this is also conducive to improving the cycle stability of the MXene electrode with the active Ti-OH sites being rapidly and targetedly eliminated at room temperature. However, a too high temperature may result in an excessively intense oxidation reaction, which is difficult to accurately control by controlling the time, and therefore the heating temperature is controlled to be within 60°C.

[0046] In some embodiments, the centrifugal washing includes: a centrifugal speed of 3000 rpm to 8000 rpm, and a centrifugal time of 3 min to 10 min each time, until the supernatant is neutral.

[0047] Specifically, after the centrifugal washing, the supernatant is neutral, and then the next step of dispersion is performed.

[0048] In some embodiments, the deionized water is added for ultrasonic dispersion, including: under the protection of argon at 0°C to 4°C, ultrasonic treatment is performed at a power of 500 W to 800 W for 10 min to 60 min.

[0049] Specifically, due to the introduction of positive hydrogen ions in the stirring process, a certain degree of self-assembly may occur between the surface negative MXene, thereby causing partial aggregation of MXene, and some stacking after deionized water centrifugal washing, so the power of ultrasonic does not need to be too high, the time does not need to be too long, and under the condition of 0℃-4℃ and argon protection, ultrasonic treatment at a power of 500W-800W for 10min-60min can be performed without long-time ultrasonic treatment as forced peeling in etching MXene.

[0050] In some embodiments, the preparation method of the single / few-layer Ti3C2 MXene aqueous dispersion solution comprises the following steps:

[0051] LiF is added into an 8mol / L-10mol / L HCl solution and mixed for 5min-20min until LiF is completely dissolved, and Ti3AlC2 MAX powder is slowly added in 3-6 times under ice water bath environment and stirred; the mixed system is incubated at 30℃-60℃ for 24h-48h for etching reaction; after the reaction is completed, 1mol / L-3mol / L hydrochloric acid solution is used for centrifugal washing for 1-3 times, the centrifugal speed is 2000rpm-4000rpm, and the centrifugal time is 3min-10min each time; deionized water is used for centrifugal washing until the supernatant is neutral, the centrifugal speed is started from 3000rpm and gradually increased to 8000rpm, and the centrifugal time is 3min-10min each time; deionized water is added to the obtained precipitate, ultrasonic treatment is performed at a power of 500W-800W for 1h-3h under the condition of 0℃-4℃ and argon protection, and a multi-layer Ti3C2 MXene dispersion solution is obtained; the multi-layer Ti3C2 MXene dispersion solution is subjected to low-speed centrifugation at 2000rpm-4000rpm for 0.5h-2h, and the single / few-layer Ti3C2 MXene aqueous dispersion solution is obtained by taking the upper liquid.

[0052] Specifically, the single / few-layer MXene aqueous dispersion solution is obtained by low-speed and long-time centrifugation after the multi-layer MXene dispersion solution is obtained by one-time etching, washing and dispersion, so that the complete layered structure of MXene itself is retained, and more contact sites are provided for subsequent reaction processes.

[0053] Preferably, in the above etching reaction, the mass of LiF, the volume of HCl solution and the mass ratio of Ti3AlC2 MAX powder are (1-2)g:(20-50)mL:1g.

[0054] Specifically, according to the reaction equation 2Ti3AlC2+6LiF+6HCl=2Ti3C2+Li3AlF6+AlCl3+3LiCl+3H2, the relative molecular mass of Ti3AlC2 MAX is 194.6 g / mol, 1 g of MAX corresponds to 5.14 mmol of Ti3AlC2, the relative molecular mass of LiF is 25.94 g / mol, and at least 0.4 g is required according to the molar ratio, but since the reaction is difficult to complete, and increasing the molar ratio of LiF helps to etch and delaminate MXene, the present application uses 1 g to 2 g of LiF, and the volume of the hydrochloric acid solution is related to its concentration, wherein the concentration of HCl participating in the chemical reaction is too high to be volatile and the volume of the reaction system is too small, and the chemical reaction is too slow when the concentration is too low, therefore the concentration of hydrochloric acid is 8 mol / L to 10 mol / L, and correspondingly, according to the molar amount required by the reaction equation, the volume is controlled at 20 mL to 50 mL, which is relatively suitable for promoting the etching degree of MAX.

[0055] In a second aspect of the present application, a high-capacity high-rate MXene electrode targeted at rapidly eliminating active Ti-OH sites is provided, and the high-capacity high-rate MXene electrode prepared by the above preparation method is used as a negative electrode of a supercapacitor. Preferably, in the high-capacity high-rate MXene electrode, the loading amount of Ti3C2 MXene is 0.5 mg / cm 2 -1.5 mg / cm 2 .

[0056] In the embodiments of the present application, by targetedly removing active Ti-OH sites, the mass of electrode non-capacitive contribution sites and the ion transmission resistance caused by hydroxyl groups can be reduced, thereby improving the mass specific capacity, rate performance and cycle performance of MXene.

[0057] In the embodiments and comparative examples of the present application, the specific sources of various materials are as follows:

[0058] Reagents / materials Commercial source Ti3AlC2 MAX Jilin Yi Yi Technology Co., Ltd. Hydrochloric acid National Pharmaceutical Group Lithium fluoride (LiF) Shanghai Aladdin Bio-Chem Technology Co., Ltd. Sulfuric acid National Pharmaceutical Group Hydrofluoric acid National Pharmaceutical Group Lithium sulfate Shanghai Aladdin Bio-Chem Technology Co., Ltd. Lithium chloride Shanghai Aladdin Bio-Chem Technology Co., Ltd.

[0059] In the embodiments and comparative examples of the present application, the electrochemical test method is as follows: a 1 mg / cm 2 loaded MXene film is suction filtered, cut into a square electrode piece with a size of 5 mm x 5 mm, and assembled into a three-electrode supercapacitor using a glassy carbon electrode, wherein activated carbon is used as a counter electrode, a silver / silver chloride electrode is used as a reference electrode, and 5 mol / L sulfuric acid is used as an electrolyte.

[0060] Embodiment 1

[0061] A preparation method of a high-capacity high-rate MXene electrode targeted at rapidly eliminating active Ti-OH sites is as follows:

[0062] (1) 1g LiF was added into 20 mL of 9 mol / L hydrochloric acid solution at room temperature and stirred for 20 minutes until completely dissolved. 1g of Ti3AlC2 MAX powder was slowly added in three portions in an ice water bath. The container was transferred to an oil bath pot and incubated at 40℃ for 48 hours. After the end of the incubation, the solution was centrifuged twice at 1 mol / L hydrochloric acid 3000 rpm for 5 minutes. The supernatant was discarded, and then the precipitate was washed with deionized water. The centrifugal speed was increased from 3000 rpm, 4000 rpm, 5000 rpm, 6000 rpm, 7000 rpm, 8000 rpm, and the supernatant was nearly neutral at this time. The bottom precipitate was poured out, 100 mL of deionized water was added, and argon was introduced into the solution under ice water bath conditions and ultrasonic treatment at 600W for 2h to obtain a multi-layer MXene dispersion solution;

[0063] (2) The multi-layer MXene dispersion solution was further centrifuged by dispensing into centrifuge tubes and centrifuging at 4000 rpm for 40 min. The supernatant was poured out to obtain a single / less layer MXene dispersion solution with good layered structure. 5 mL of the dispersion solution was vacuum filtered and dried at 40℃ for 12h to calculate the original sample dispersion solution concentration by weight;

[0064] (3) According to the original sample dispersion solution concentration, 50mg MXene-containing dispersion solution was taken, and appropriate amount of deionized water was added to make the concentration of MXene in the total solution 2mg / mL. Lithium sulfate solution was added to make the solute concentration in the total solution 0.1g / mL. Sulfuric acid solution was added to make the total solution sulfuric acid concentration 1mol / L. Incubation was carried out at 40℃ for 2h in air with stirring;

[0065] (4) After stirring, the supernatant was washed to neutral by centrifugation at 8000 rpm for 5 min with deionized water. 50 mL of deionized water was added to the bottom precipitate, and argon was introduced into the solution under ice water bath conditions and ultrasonic treatment at 600W for 20 min. The solution concentration was calculated and 1mg / cm 2 The loaded MXene electrode was subjected to electrochemical test.

[0066] Example 2

[0067] A preparation method of a high-capacity high-rate MXene electrode targeting rapid elimination of active Ti-OH sites, specifically as follows:

[0068] (1) The same as example 1;

[0069] (2) The same as example 1;

[0070] (3) According to the concentration of the original sample dispersion liquid, take a dispersion liquid containing 50 mg MXene, add appropriate amount of deionized water to make the concentration of MXene in the total solution 2 mg / mL, add lithium chloride solution to make the solute concentration in the total solution 0.07 g / mL, add sulfuric acid solution to make the concentration of sulfuric acid in the total solution 1 mol / L, 50℃ incubation, stirring in air for 1 h;

[0071] (4) Same as Example 1.

[0072] Example 3

[0073] A preparation method of a high-capacity high-rate MXene electrode targeting rapid elimination of active Ti-OH sites, specifically as follows:

[0074] (1) Same as Example 1;

[0075] (2) Same as Example 1;

[0076] (3) According to the concentration of the original sample dispersion liquid, take a dispersion liquid containing 50 mg MXene, add appropriate amount of deionized water to make the concentration of MXene in the total solution 2 mg / mL, add lithium sulfate solution to make the solute concentration in the total solution 0.1 g / mL, add hydrochloric acid solution to make the concentration of hydrochloric acid in the total solution 1 mol / L, 50℃ incubation, stirring in air for 6 h;

[0077] (4) Same as Example 1.

[0078] Example 4

[0079] A preparation method of a high-capacity high-rate MXene electrode targeting rapid elimination of active Ti-OH sites, specifically as follows:

[0080] (1) Same as Example 1;

[0081] (2) Same as Example 1;

[0082] (3) According to the concentration of the original sample dispersion liquid, take a dispersion liquid containing 50 mg MXene, add appropriate amount of deionized water to make the concentration of MXene in the total solution 2 mg / mL, add lithium sulfate solution to make the solute concentration in the total solution 0.1 g / mL, add hydrofluoric acid solution to make the concentration of HF in the total solution 2 mol / L, 40℃ incubation, stirring in air for 3 h;

[0083] (4) Same as Example 1.

[0084] Comparative Example 1

[0085] This comparative example does not perform subsequent treatment on the original sample dispersion liquid, and directly uses the MXene original sample dispersion liquid to obtain a MXene electrode by filtration and drying, and the specific experimental steps include:

[0086] (1) same as example 1;

[0087] (2) The multi-layer MXene dispersion liquid was further centrifuged in a centrifuge tube, centrifuged at 4000 rpm for 40 min, and the supernatant was poured out to obtain a single / less layer MXene dispersion liquid with good layered structure. 5 mL of the dispersion liquid was vacuum filtered, vacuum dried at 40°C for 12 h, and then weighed to calculate the solution concentration. A 1 mg / cm 2 The loaded MXene electrode was subjected to electrochemical test.

[0088] Comparative example 2

[0089] In this comparative example, no acidic solution was added, and the mixture was heated and stirred in air (under the same conditions as in example 1), and then centrifuged, washed, and filtered to obtain a MXene electrode. The specific experimental steps included:

[0090] (1) same as example 1;

[0091] (2) same as example 1;

[0092] (3) According to the concentration of the original sample dispersion liquid, a dispersion liquid containing 50 mg of MXene was taken, and an appropriate amount of deionized water was added to make the concentration of MXene in the total solution 2 mg / mL. Lithium sulfate solution was added to make the concentration of solute in the total solution 0.1 g / mL, and the mixture was incubated at 40°C and stirred in air for 2 h;

[0093] (4) After stirring, the mixture was centrifuged at 8000 rpm for 5 min with deionized water for 6 times, 50 mL of deionized water was added to the bottom precipitate, and the mixture was ultrasonicated in an argon gas bath at 600 W for 20 min. After calculating the solution concentration, the mixture was filtered to 1 mg / cm 2 The loaded MXene electrode was subjected to electrochemical test.

[0094] Comparative example 3

[0095] In this comparative example, no lithium sulfate salt solution was added, and the remaining steps were the same as in example 1. The specific experimental steps included:

[0096] (1) same as example 1;

[0097] (2) same as example 1;

[0098] (3) According to the concentration of the original sample dispersion liquid, a dispersion liquid containing 50 mg of MXene was taken, and an appropriate amount of deionized water was added to make the concentration of MXene in the total solution 2 mg / mL. Lithium sulfate solution was added to make the concentration of solute in the total solution 0.1 g / mL, and the mixture was incubated at 40°C and stirred in air for 2 h;

[0099] (4) same as example 1.

[0100] Test data

[0101] 1. Material surface element distribution and morphology test

[0102] The MXene film electrodes obtained from Examples 1-4 and Comparative Examples 1-3 were subjected to EDS test, and the element contents of Ti, C, O, F and Cl on the surface were obtained as shown in Table 1:

[0103] Table 1 Surface element content (atomic ratio) of materials of Examples 1-4 and Comparative Examples 1-3

[0104] Electrode material Ti C O F Cl Example 1 34.83％ 36.81％ 11.92％ 10.68％ 5.76％ Example 2 35.88％ 35.99％ 11.73％ 10.51％ 5.89％ Example 3 35.21％ 36.32％ 10.99％ 12.05％ 5.43％ Example 4 36.28％ 35.37％ 12.19％ 11.75％ 4.41％ Comparative Example 1 40.47％ 31.63％ 16.19％ 8.16％ 3.55％ Comparative Example 2 38.97％ 28.05％ 20.28％ 8.91％ 3.79％ Comparative Example 3 37.12％ 34.81％ 12.67％ 10.18％ 5.22％

[0105] As can be seen from Table 1, the Ti and O in the MXene electrode material prepared in Examples 1-4 are sharply reduced compared to Comparative Example 1 (original MXene), while the ratio of F and Cl elements to C element changes slightly compared to Comparative Example 1. It is speculated that the subsequent treatment has little effect on C and Ti-F and Ti-Cl, and the main Ti-OH sites are eliminated in the process, which is proved by the great difference in hydroxyl in XPS between Example 1 and Comparative Example 1 (see Figure 1 ). Comparative Example 2 is a reaction condition without adding acid solution to inhibit hydrolysis, exposed to air and heated, and the O content increases significantly. It is speculated that oxidation and further Ti-C bond hydrolysis occur at Ti-OH, and oxidation byproducts are produced (see Figure 2 ), and the XRD curve of Figure 3 also confirms this. The characteristic (002) peak of the two-dimensional structure almost disappears, and the characteristic diffraction peak of TiO2 appears. The transmission electron microscope images of Example 1 and Comparative Example 3 are similar to Comparative Example 1, indicating that the acidic condition can protect the two-dimensional structure of the substrate. In Comparative Example 3, no salt solution is added during stirring, and at this time, there is no cation intercalation support between the MXene layers. H + Self-assembly occurs with the negatively charged MXene on the surface, resulting in insufficient Ti-OH oxidation and hydrolysis reaction in part of the stacked layers, resulting in insufficient efficiency of targeted removal. However, the Ti-OH on the surface can also react effectively. Due to the inhibition of Ti-C bond hydrolysis in the acidic environment, the two-dimensional structure is also well maintained, and the (002) peak of MXene characteristics only has a small degree of shift compared to Comparative Examples 1 and 3 (see Figure 3 ).

[0106] 2. Rate performance and cycle performance at room temperature

[0107] The MXene electrodes prepared in Examples 1-4 and Comparative Examples 1-3 according to the present application were used as working electrodes, activated carbon electrodes were used as counter electrodes, silver / silver chloride electrodes were used as reference electrodes, Celgard3501 was used as a separator, and 5 mol / L sulfuric acid was used as an electrolyte to assemble a three-electrode system supercapacitor, and the electrochemical performance was detected.

[0108] The electrochemical performance of the supercapacitor of the three-electrode system was tested using a Shanghai Chenhua electrochemical workstation (CHI 660E). First, cyclic voltammetry (CV) was performed at a scan rate of 5 mV / s to discharge / charge for 10 cycles, and then CV curves at different scan rates were tested, with a voltage range of -0.6 V to 0.3 V. The mass specific capacity and cycle stability at different scan rates were measured as shown in Table 2:

[0109] Table 2 Electrochemical performance of MXene electrodes of Examples 1-4 and Comparative Examples 1-3

[0110]

[0111]

[0112] As can be seen from Table 2, the mass specific capacity of the MXene electrodes prepared in Examples 1-4 at a scan rate of 5 mV / s is greater than 500 F / g, which is more than 60% higher than that of the untreated Comparative Example 1 sample, which is consistent with the mass loss of about 40% after removal of Ti-OH in the original sample. The capacity retention rate of Examples 1-4 at 100 mV / s is significantly higher than that of Comparative Examples 1 and 2, and slightly better than that of Comparative Example 3. This is because the removal efficiency of Ti-OH in Comparative Example 3 is slightly lower than that in Example 1, and the hydroxyl functional group hinders the transmission of hydronium ions, resulting in a slightly poorer rate performance than Examples 1-4. The CV curve shape of Example 1 and Comparative Example 3 is similar to that of Comparative Example 1, but the mass specific capacity is higher, which indicates that the Ti-OH functional group does not contribute to the double-layer capacitance or pseudo-capacitance of the material itself, and the removal of Ti-OH does not affect the energy storage mechanism of MXene (see Figure 4 ).

[0113] Comparative Example 2 did not add an acid solution to inhibit hydrolysis, and the sample was heated under exposure to air, resulting in obvious oxidation and hydrolysis of the sample, producing oxidation byproduct particles (see Figure 2 ). Therefore, the mass specific capacity is low, and the capacity retention rate after cycling is extremely low, as shown in the CV curve at 5 mV / s (see Figure 4 ). The capacity change of Comparative Examples 1 and 3 at 50 mV / s for 15,000 cycles is shown in Figure 5 . The CV curve was integrated to calculate the capacity every 500 cycles, and the cycle retention rate of Example 1 was significantly better, which was mainly because the Ti-OH was removed in advance, and no oxidation occurred in the MXene electrode during the test, and the functional group remained unchanged; whereas in Comparative Example 1, there were more Ti-OH, which would slowly oxidize and produce hydrogen gas when exposed to air during the electrochemical test, leading to gradual failure of the material.

[0114] The high-capacity high-rate MXene electrode for targeted rapid elimination of active Ti-OH sites adds cations to the solution to intercalate the MXene layers, fully exposes the MXene layers to the reaction system, uses the oxygen consumption reaction to oxidize the active trivalent Ti-OH functional groups to tetravalent and remove them by hydrolysis, and further inhibits the hydrolysis of the matrix Ti-C by an acidic environment. In this way, after selectively removing the high-activity Ti-OH sites, the invalid mass can be greatly reduced, the mass specific capacity of the electrode is improved, and the rate performance and cycle performance of the MXene-based supercapacitor are also improved.

[0115] The above only is the preferred embodiment of the present application, and does not limit the present application, any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application. The above is only the preferred embodiment of the present application, it should be pointed out that, for ordinary skilled in the art, without departing from the technical principles of the present application, a number of improvements and variations can also be made, these improvements and variations should be regarded as the protection scope of the present application.

Claims

1. A method for preparing a high-capacity, high-rate MXene electrode that targets and rapidly eliminates active Ti-OH sites, characterized in that, Includes the following steps: An acid solution and a salt solution were added to a monolayer / few-layer Ti3C2MXene aqueous dispersion to obtain a mixed solution; The mixture is heated and stirred in air to obtain a reaction mixture; The reaction mixture was centrifuged and washed with deionized water until the supernatant was neutral. Collect the centrifuged precipitate, add deionized water and ultrasonically disperse to obtain an aqueous dispersion of MXene with active Ti-OH sites eliminated; The high-capacity, high-rate MXene electrode was prepared using the MXene aqueous dispersion that eliminates active Ti-OH sites.

2. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to claim 1, characterized in that, The concentration of Ti3C2 MXene in the mixture was controlled to be between 1.5 mg / mL and 2.5 mg / mL.

3. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to claim 1, characterized in that, The acid solution is one or more of dilute sulfuric acid, dilute hydrochloric acid, and dilute hydrofluoric acid, and the concentration of the acid in the mixture is controlled to be 0.5 mol / L to 2 mol / L.

4. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to claim 1, characterized in that, The salt solution is one or more of lithium sulfate, lithium chloride, sodium sulfate, and sodium chloride, and the concentration of the salt in the mixture is controlled to be 0.02 g / mL to 0.2 g / mL.

5. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to any one of claims 1-3, characterized in that, The heating and stirring process includes: a heating temperature of 30℃ to 60℃ and a stirring time of 0.5h to 12h.

6. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to claim 1, characterized in that, The centrifugation washing process involves centrifuging at a speed of 3000 rpm to 8000 rpm for 3 to 10 minutes each time, until the supernatant is neutral.

7. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to claim 1, characterized in that, The addition of deionized water for ultrasonic dispersion includes: ultrasonic treatment at a power of 500W-800W for 10min-60min under argon protection at 0℃~4℃.

8. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to claim 1, characterized in that, The method for preparing the monolayer / few-layer Ti3C2MXene aqueous dispersion includes: Add LiF to an 8 mol / L–10 mol / L HCl solution and mix for 5–20 minutes until the LiF is completely dissolved. Then, slowly add Ti3AlC2MAX powder in 3–6 portions while stirring. Incubate the mixture at 30–60°C for 24–48 hours for etching. After the reaction, wash the mixture 1–3 times with 1 mol / L–3 mol / L hydrochloric acid solution at 2000–4000 rpm for 3–10 minutes each time. Finally, wash with deionized water. Wash until the supernatant is neutral, centrifuge at 3000 rpm and gradually increase to 8000 rpm, with each centrifugation lasting 3 min to 10 min; add deionized water to the precipitate and sonicate at 500 W to 800 W for 1 h to 3 h under argon protection at 0 °C to 4 °C to obtain a multilayer Ti3C2MXene dispersion; centrifuge the multilayer Ti3C2MXene dispersion at 2000 rpm to 4000 rpm for 0.5 h to 2 h, and take the upper liquid to obtain the monolayer / few-layer Ti3C2MXene aqueous dispersion.

9. The method for preparing a high-capacity, high-rate MXene electrode with targeted and rapid elimination of active Ti-OH sites according to claim 8, characterized in that, In the etching reaction, the ratio of the mass of LiF, the volume of HCl solution, and the mass of Ti3AlC2MAX powder is (1-2) g : (20-50) mL : 1 g.

10. An application of a high-capacity, high-rate MXene electrode that targets and rapidly eliminates active Ti-OH sites, characterized in that... The high-capacity, high-rate MXene electrode prepared by the preparation method according to any one of claims 1-9 is used as the negative electrode of a supercapacitor.

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