Ionic cross-linked nanocellulose / MXene composite membrane and preparation method thereof

Abstract

The invention provides an ionic cross-linked nano cellulose / MXene composite membrane and a preparation method of the ionic cross-linked nano cellulose / MXene composite membrane. The preparation method comprises the following steps: firstly, preparing nano-cellulose dispersion liquid by adopting a TEMPO oxidation method; then preparing the nanocellulose dispersion liquid and the MXene nanosheet dispersion liquid into a composite film according to a certain solute mass ratio by a vacuum-assisted self-assembly method; and finally, immersing the composite membrane into a metal ion solution to complete a cross-linking process to obtain the ionic cross-linked nano cellulose network anchored MXene composite proton exchange membrane. By changing the content of the nanocellulose, the physicochemical properties of the composite membrane can be optimized, regulated and controlled. Ionic crosslinking not only can effectively enhance the stability of the composite membrane, but also can construct a long-range ordered proton conduction channel between layers of the MXene nanosheet, so as to provide more conduction sites for proton conduction. The preparation method is mild, controllable, simple and efficient, the prepared composite membrane has excellent mechanical strength and proton conduction performance, and a new design direction is provided for design of the ion conduction membrane.

Description

Technical Field

[0001] The present invention belongs to the fields of nanocellulose functional materials and proton exchange membranes, and particularly relates to an ion-crosslinked nanocellulose / MXene composite membrane and a preparation method thereof. Background Art

[0002] Promoting the construction of the methanol fuel industry is an important measure to help achieve the diversified development of China's energy. A direct methanol fuel cell (DMFC) is a new type of energy conversion device that can directly convert the chemical energy in methanol into electrical energy. It has attracted much attention due to advantages such as rich fuel sources, high energy density, low pollution level, and wide application range. The proton exchange membrane (PEM) is the core component of DMFC, mainly responsible for functions such as transporting protons, isolating electrons, and isolating fuels. Its performance directly determines the power density and energy conversion efficiency of DMFC. Currently, the most widely commercially applied PEM is the perfluorosulfonic acid membrane (Nafion) produced by DuPont in the United States. However, the high cost, complex synthesis steps, high environmental pollution level, and high methanol permeability during the production of Nafion restrict its development and application in DMFC. Therefore, developing new proton-conducting materials to overcome the obstacles of Nafion membranes is a key step in promoting the development of DMFC and building China's energy system.

[0003] MXene (Ti 3 C 2 T x ) is a new type of two-dimensional transition metal carbonitride. Due to its high specific surface area, controllable interlayer spacing, and rich oxygen-containing functional groups, it has become a preferred material for ion transport media. However, directly assembling two-dimensional MXene nanosheets into MXene membranes is challenged by basic problems such as poor mechanical strength and insufficient morphological integrity. Therefore, introducing suitable spacers between MXene nanosheets to enhance the interaction force and film-forming property between the sheets is the key to constructing high-performance MXene ion-conducting membranes. Compared with synthetic polymers, nanocellulose is a nanoscale fiber extracted from natural cellulose, with advantages such as biodegradability, renewability, non-toxicity, and high strength. Existing research has proven that the membrane materials constructed by nanocellulose have excellent methanol-blocking properties. Therefore, regulating the assembly behavior of MXene nanosheets by means of the special physical and chemical structure of nanocellulose has very important technical, environmental, and social significance for constructing high-performance MXene composite proton exchange membranes for DMFC.

[0004] Chinese Patent Publication No. CN115041027A discloses a doubly regulated two-dimensional MXene composite membrane and a preparation method thereof, using sodium lignosulfonate to modify MXene nanosheets, and then combining with MoS 2It is prepared into a composite membrane by vacuum-assisted self-assembly, and the composite membrane has excellent permeability and selectivity.

[0005] Chinese Patent Publication No. CN112844065A discloses a preparation method of an MXene composite membrane. The MXene dispersion liquid and the HNTs dispersion liquid are prepared into a composite membrane by vacuum filtration self-assembly under alkaline conditions, and the composite membrane shows stronger water permeability. Summary of the Invention

[0006] In view of the application requirements of PEM in DMFC mentioned in the above background technology, the present invention proposes an ion-crosslinked nanocellulose / MXene composite membrane and its preparation method; in short, TEMPO-oxidized nanocellulose is used as a spacer for MXene nanosheets, and is assembled into a PEM material with excellent mechanical properties, proton conduction properties and fuel cell properties through vacuum-assisted self-assembly and ion crosslinking; first, a nanocellulose dispersion liquid is prepared by TEMPO-mediated oxidation method; then, the nanocellulose dispersion liquid and the MXene nanosheet dispersion liquid are assembled into a composite membrane according to different mass ratios by vacuum-assisted self-assembly; finally, the obtained composite membrane is immersed in CaCl 2 solution to complete the ion crosslinking process to obtain an ion-crosslinked nanocellulose / MXene composite proton exchange membrane. The specific steps are as follows: Step 1. Preparation of TEMPO-oxidized nanocellulose: Add a certain mass of softwood pulp into a reaction solution containing TEMPO and NaBr, and then obtain a TEMPO-oxidized nanocellulose dispersion liquid by dropping appropriate amounts of NaClO and NaOH solutions and washing and centrifuging. Step 2. Preparation of the nanocellulose / MXene composite membrane: Mix the nanocellulose dispersion liquid obtained in Step 1 and the MXene nanosheet dispersion liquid evenly according to different mass ratios; obtain the nanocellulose / MXene composite membrane by vacuum-assisted self-assembly of the uniformly mixed composite solution on a polyacrylonitrile substrate. Step 3. Preparation of the ion-crosslinked nanocellulose / MXene composite proton exchange membrane: Immerse the nanocellulose / MXene composite membrane obtained in Step 2 in CaCl 2 solution to complete the crosslinking process between Ca 2+ and nanocellulose to obtain an ion-crosslinked nanocellulose / MXene composite proton exchange membrane.

[0007] Furthermore, in the preparation method of the present invention: Preferably, in Step 1, the concentrations of the TEMPO and NaBr solutions are 0.1 × 10 -3 M, the concentration of NaClO is 5 mM, and the concentration of NaOH is 1 M.

[0008] Preferably, the mass ratio of the solutes of the nanocellulose solution and the MXene nanosheet dispersion in step 2 is 0.2 - 1:1.

[0009] Preferably, in step 3, the mass fraction of CaCl 2 is 2 wt%.

[0010] Preferably, in step 3, the immersion time of the nanocellulose / MXene composite membrane in the CaCl 2 solution is 24 h.

[0011] Compared with the MXene membrane prepared by the prior art, the preparation method provided by the present invention can use green and renewable nanocellulose materials to prepare MXene composite proton exchange membrane materials with high mechanical strength and flexibility; the ion crosslinking process constructs long-range ordered proton transport channels between the layers of MXene nanosheets, providing continuous transport sites for the rapid migration of protons and further enhancing the proton conduction ability of the composite membrane. The prepared MXene composite membrane is expected to exhibit excellent operating stability and power density in DMFCs, including but not limited to devices such as hydrogen-oxygen fuel cells, rechargeable batteries, and water electrolysis for hydrogen production. Description of the Drawings

[0012] Figure 1 are the surface and cross-sectional morphology diagrams of composite membrane 1 in the examples observed by scanning electron microscopy; Figure 2 are the surface and cross-sectional morphology diagrams of composite membrane 2 in the examples observed by scanning electron microscopy; Figure 3 are the surface and cross-sectional morphology diagrams of composite membrane 3 in the examples observed by scanning electron microscopy; Figure 4 are the surface and cross-sectional morphology diagrams of composite membrane 4 in the examples observed by scanning electron microscopy; Figure 5 are the surface and cross-sectional morphology diagrams of composite membrane 5 in the examples observed by scanning electron microscopy; Figure 6 is the comparison diagram of the proton conduction performance of composite membranes 1 - 5 in the examples; Detailed Embodiments The design idea of an ion-crosslinked nanocellulose / MXene composite membrane proposed by the present invention is as follows: aiming at the problems of poor mechanical strength and morphological stability of the original MXene membrane, TEMPO-oxidized nanocellulose is used as a spacer between MXene nanosheets to enhance the interaction between the sheets, and a nanocellulose / MXene composite proton exchange membrane with excellent mechanical strength, morphological stability and proton conduction ability is prepared by vacuum-assisted self-assembly and ion-crosslinking methods. Its preparation process mainly includes: preparation of TEMPO-oxidized nanocellulose, preparation of nanocellulose / MXene composite membrane, and preparation of ion-crosslinked nanocellulose / MXene composite proton exchange membrane. By controlling the mass of nanocellulose and MXene nanosheets, the thickness of the composite membrane can be effectively controlled; the ion-crosslinking process can not only enhance the morphological stability of the composite membrane, but also construct long-range ordered proton conduction channels inside the composite membrane. The preparation method of the present invention is simple, efficient, mild and controllable, which is helpful for expanding the preparation strategy of MXene composite membranes and realizing the high-value utilization of forest biomass materials.

[0013] The following further illustrates the present invention with reference to the accompanying drawings and specific examples, but the following examples are only for illustration and should not be considered as any limitation to the present invention.

[0014] Example 1 To prepare an ion-crosslinked nanocellulose / MXene composite proton exchange membrane, the steps are as follows: Step 1: Preparation of TEMPO-oxidized nanocellulose: First, add 1 g of softwood pulp into 100 mL of 0.1 × 10 -3 M TEMPO and NaBr solution and stir well; secondly, add NaClO solution dropwise into the above solution to initiate the oxidation reaction, and add 1 M NaOH solution dropwise to maintain the solution pH at 10; finally, after washing and centrifuging with deionized water, a TEMPO-oxidized nanocellulose dispersion is obtained.

[0015] Step 2: Preparation of nanocellulose / MXene composite membrane: The nanocellulose dispersion obtained in Step 1 and the MXene nanosheet dispersion are uniformly mixed according to the solute mass ratio of 0.2:1, and a nanocellulose / MXene composite membrane is obtained by vacuum filtration self-assembly.

[0016] Step 3: Preparation of ion-crosslinked nanocellulose / MXene composite proton exchange membrane: Immerse the nanocellulose / MXene composite membrane obtained in Step 2 into 2 wt% CaCl 2 solution for 24 h, and then wash the surface of the composite membrane with deionized water to remove the residual CaCl 2The solution was immersed in dilute sulfuric acid solution for acidification to obtain an ion-crosslinked nanocellulose / MXene composite proton exchange membrane, denoted as composite membrane 1.

[0017] Example 2 To prepare an ion-crosslinked nanocellulose / MXene composite proton exchange membrane, the preparation process was basically the same as that in Example 1, except that in step 2, the nanocellulose dispersion and the MXene nanosheet dispersion were uniformly mixed according to the solute mass ratio of 0.4:1, and a nanocellulose / MXene composite membrane was obtained by vacuum-assisted self-assembly. The finally obtained membrane was denoted as composite membrane 2.

[0018] Example 3 To prepare an ion-crosslinked nanocellulose / MXene composite proton exchange membrane, the preparation process was basically the same as that in Example 1, except that in step 2, the nanocellulose dispersion and the MXene nanosheet dispersion were uniformly mixed according to the solute mass ratio of 0.6:1, and a nanocellulose / MXene composite membrane was obtained by vacuum-assisted self-assembly. The finally obtained membrane was denoted as composite membrane 3.

[0019] Example 4 To prepare an ion-crosslinked nanocellulose / MXene composite proton exchange membrane, the preparation process was basically the same as that in Example 1, except that in step 2, the nanocellulose dispersion and the MXene nanosheet dispersion were uniformly mixed according to the solute mass ratio of 0.8:1, and a nanocellulose / MXene composite membrane was obtained by vacuum-assisted self-assembly. The finally obtained membrane was denoted as composite membrane 4.

[0020] Example 5 To prepare an ion-crosslinked nanocellulose / MXene composite proton exchange membrane, the preparation process was basically the same as that in Example 1, except that in step 2, the nanocellulose dispersion and the MXene nanosheet dispersion were uniformly mixed according to the solute mass ratio of 1:1, and a nanocellulose / MXene composite membrane was obtained by vacuum-assisted self-assembly. The finally obtained membrane was denoted as composite membrane 5.

[0021] Table 1 Preparation process conditions and properties of the obtained composite proton exchange membranes in Examples 1-5 In Table 1, A refers to the TEMPO-oxidized nanocellulose dispersion, and B refers to the MXene nanosheet dispersion.

[0022] By comparing Examples 1-5, it can be concluded that the water absorption rate and swelling rate of Composite Films 1-5 increase with the increase in the content of nanocellulose, which is due to the abundant hydrophilic groups in the molecular chains of TEMPO-oxidized nanocellulose; meanwhile, the mechanical strength of Composite Films 1-5 also increases with the increase in the content of nanocellulose, reaching up to 164.7 MPa at most, which is due to the excellent mechanical strength of the nanocellulose fibers themselves and the cross-linking of Ca 2+ ; in addition, by comparing Examples 1-5, it can be concluded that the proton conductivity of the composite film shows a trend of first increasing and then decreasing with the increase in the content of nanocellulose, with the highest being 44.7 mS cm -1 , which may be because the excessive nanocellulose leads to an increase in the thickness of the composite film, resulting in a decrease in the density of proton-conducting groups in the composite film, thus causing the proton conductivity to decrease.

[0023] In summary, the present invention uses MXene nanosheets as the main body, and regulates the assembly behavior of MXene nanosheets with the help of TEMPO-oxidized nanocellulose, and obtains a proton exchange membrane material with high stability through the methods of vacuum-assisted self-assembly and ionic cross-linking. By adjusting the mass ratio and dosage of nanocellulose and MXene nanosheets, the physical and chemical properties of the formed composite film can be flexibly regulated and optimized to better meet the requirements of energy devices such as fuel cells.

[0024] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many improvements and changes without departing from the purpose of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. An ion-crosslinked nanocellulose / MXene composite membrane and a method for preparing the same, characterized in that: The following steps are involved: Step 1: adding a certain mass of softwood pulp to a reaction solution containing TEMPO and NaBr, adding appropriate amounts of NaClO and NaOH solutions, washing and centrifuging to obtain a TEMPO-oxidized nanocellulose dispersion; Step 2: The nanocellulose dispersion obtained in step 1 and the MXene nanosheet dispersion are uniformly mixed according to different solute mass ratios, and then filtered by vacuum-assisted self-assembly to obtain a nanocellulose / MXene composite membrane; Step 3: Soak the composite membrane obtained in step 2 in a metal ion solution to complete the ion crosslinking process, then rinse the obtained composite membrane with deionized water to remove the metal ion solution on the surface, and soak it in a dilute acid solution for acidification to obtain an ion-crosslinked nanocellulose network-anchored MXene composite proton exchange membrane.

2. The method according to claim 1, characterized in that In step 1, the concentration of TEMPO and NaBr solution in the nanocellulose preparation process is 0.1-0.3 × 10 -3 M, the concentration of NaClO is 5 ~ 10 mM, and the concentration of NaOH is 1 ~ 3 M.

3. The method according to claim 1, characterized in that In step 2, the mass ratio of nanocellulose to MXene nanosheets is 0.2:1 to 1:

1.

4. The method according to claim 1, characterized in that In step 2, the mixing method of the nanocellulose dispersion and the MXene nanosheet dispersion includes ultrasound, stirring, and shaking.

5. The method according to claim 1, characterized in that In step 3, the metal ion solution is one of calcium chloride, copper chloride, zinc chloride, aluminum chloride and ferric chloride.

6. The method according to claim 1, characterized in that In step 3, the mass fraction of the metal ion solution is 2 to 5 wt%.

7. The method according to claim 1, characterized in that In step 3, the nanocellulose / MXene composite membrane is immersed in the metal ion solution for 12 to 24 hours.

8. The method according to claim 1, characterized in that: In step 3, the dilute acid solution is dilute hydrochloric acid or dilute sulfuric acid solution.

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