Background
Aerogels are three-dimensional materials with an open-cell foam structure of high relative surface area and nano-pore size. The most common aerogel is a silica aerogel, which is made up of bonded silicon and oxygen atoms joined in long chains that then form randomly linked beads with air pockets between them.
Silica aerogels are among the lowest density solids known and have many potentially useful properties. However, the high production costs associated with the manufacture of silica aerogels have limited widespread commercial applications, which are currently limited to high-value military and aerospace projects. Another type of aerogel is a carbon-based aerogel, which consists of a network of aggregated carbon nanoparticles. Carbon aerogels have similar properties to silica aerogels, but tend to have excellent mechanical integrity. Pure carbon aerogels and polymer-compounded carbon aerogels also have electrical conductivity, which depends on density and is generally at a value of 10-8to 10-1S/cm range. Carbon aerogels are extremely strong in the infrared regionOnly 0.3% of the radiation between 250nm and 14.3um is reflected. Furthermore, since such solids conduct heat only through narrow atomic chains, the thermal conductivity of carbon aerogels can be lower than or equal to that of air. These particular characteristics of carbon aerogels make them useful in a number of industrial applications including desalination, thermal and/or acoustic insulation, solar energy collection, catalyst support, and the like.
The graphene is represented by sp2Planar sheets of bonded carbon atoms, which are monoatomic thick, are densely packed in a honeycomb lattice. Graphene is a two-dimensional building material for carbon materials of all other dimensions. It can be wrapped into a fullerene family, rolled into one dimension like nanotubes or stacked into three dimensional graphite. Graphene has excellent in-plane mechanical, structural, thermal and electrical properties similar to carbon nanotubes. Previous studies have used polymers to enhance the structural and mechanical properties of graphene materials, in which the polymers used (e.g., polyvinyl alcohol, PVA) are not connected to graphene or Graphene Oxide (GO) sheet structures, and the resulting polymer network is separate from the actual carbon backbone structure.
Disclosure of Invention
The invention provides a preparation method of a novel composite carbon-based aerogel, the aerogel uses pure graphene oxide or graphene, and is combined with an inherent conductive polymer through a covalent bond, so that the conductivity is greatly improved, and large-scale industrial production is possible to realize.
The carbon atoms in the aerogel of the present invention are arranged in a sheet-like nanostructure and are creatively connected together by an Intrinsically Conductive Polymer (ICP) having high conductivity. The method includes using graphite oxide as a raw material, forming a sol-gel dispersion of graphite oxide in a liquid, and drying the dispersion to form a graphene oxide aerogel. The method also includes reducing the graphene oxide in the aerogel to graphene. The method further includes generating new conductive polymers in situ during the formation of the network using graphene oxide.
The technical scheme adopted by the invention is as follows:
a highly conductive polymer-carbon-based composite aerogel comprising carbon atoms arranged in a lamellar nanostructure, and a polymer covalently bonded to the carbon atoms; the polymer comprises a compound having at least one carboxyl group and a polymer having at least one alcohol group.
Further, the polymer-carbon-based composite aerogel is a polymer-graphene oxide composite aerogel or a polymer-graphene composite aerogel.
Further, the compound having at least one carboxyl group is 4-carboxybenzaldehyde, and the polymer having at least one alcohol group is one of the following: polyvinyl alcohol, a polymer obtained by modifying a hydroxyl group of polyparaphenylene, and a polymer obtained by modifying a hydroxyl group of polyparaxylylene.
A method for preparing the highly conductive polymer-carbon-based composite aerogel, comprising the steps of:
1) dispersing graphite oxide in a liquid to form a sol-gel dispersion of graphite oxide;
2) preparing a polymer precursor using a compound having at least one carboxyl group and a polymer having at least one alcohol group;
3) combining the prepared sol-gel dispersion of graphite oxide with the polymer precursor to form a hydrogel;
4) and drying the hydrogel to form the polymer-graphene oxide composite aerogel.
Further, the method also comprises the step of reducing the graphene oxide into graphene so as to form the polymer-graphene composite aerogel.
Further, the mass ratio of the polymer to the graphite oxide is in a range of 0.1: 1 to 5:1 range; the content of graphite oxide in the sol-gel dispersion of graphite oxide is 0.1mg/mL to 25 mg/mL.
Further, in the step 2), a compound with at least one carboxyl group and a polymer with at least one alcohol group are adopted, in a solution with the volume ratio of water to an organic solvent of 0: 1-99.9: 1 and the temperature range of 40-95 ℃, hydriodic acid is used as a catalyst to form a polymer precursor, the dehydrogenation reaction is carried out under the action of the hydriodic acid, and meanwhile, a conjugated bond is formed in a main polymer chain.
Further, the compound having at least one carboxyl group is 4-carboxybenzaldehyde, and the polymer having at least one alcohol group is one of the following: polyvinyl alcohol, a polymer obtained by modifying a hydroxyl group of polyparaphenylene, and a polymer obtained by modifying a hydroxyl group of polyparaxylylene.
Further, step 3) adds a polymer precursor to the sol-gel dispersion of graphite oxide, maintaining the solution at between 50 ℃ and 87 ℃ by mechanical impact with sodium carbonate as catalyst to achieve complete gelation.
Further, step 4) performs the drying using a supercritical method.
The invention has the following advantages and beneficial effects:
1) graphene oxide is firstly bonded to a conductive polymer (ICP) through a covalent bond.
2) The material preparation process is simple, the polymer precursors are combined in the final material using a specific catalyst, two progressive reagent addition sequences are required: i.e. combining a polymer having at least one alcohol group (including but not limited to polyvinyl alcohol, PVA) and a compound having at least one carboxyl functional group (including but not limited to 4-carboxybenzaldehyde, CBA) to form a new polymer (PVA + CBA); ii: bonding the new polymer to the two-dimensional graphene oxide.
3) The material obtained by the invention has good structural integrity. It should be noted that in this material, the polymer is not a separate structural network, but is directly bonded to the GO sheet.
4) Polymers with different chain lengths can be used as precursors to modulate the reaction with graphene oxide lamellae and in this way obtain aerogel products with different densities, different relative surface areas, different pore sizes, different mechanical properties and different conductivities. These properties can also be tuned by changing the polymer/GO or changing the size of the polymer.
5) The obtained carbon-based aerogel with a lamellar nanocarbon structure has a higher surface area to volume ratio and a significantly enhanced electrical conductivity (about 128S/cm) than similar materials obtained by other methods.
6) In the prior art, polymers which can be dissolved in a graphene oxide solution are mainly selected, and the purpose of adding the polymers is to enhance the strength and porosity of the formed graphene aerogel; the resulting graphene aerogel conductivity enhancement is due to the reduction of graphene oxide to graphene. Unlike it, the main innovation of the present invention is: a) the preparation method of the novel polymer-graphene composite aerogel is provided, wherein the composite polymer is formed by combining two substances under the action of a catalyst, has good conductivity, and has a functional group structure which can be connected with a graphene/graphene oxide lamellar structure through covalent bonds, so that the prepared graphene/graphene oxide aerogel material has excellent conductivity. The selection of these two substances, and the recombination of the formed new polymer with graphene oxide, are important innovations in the present invention. b) The conductivity of the three-dimensional graphene aerogel is enhanced not by reducing graphene oxide into graphene but by using a novel conductive composite polymer which is connected with a graphene/graphene oxide lamellar structure.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, the present invention shall be described in further detail with reference to the following detailed description and accompanying drawings.
According to exemplary embodiments of the present invention, the inventors developed carbon-based aerogels with lamellar nanostructures, including graphene oxide aerogels and graphene aerogels. This carbon-based aerogel has a lamellar nanostructure, specifically including: (i) carbon atoms are ordered in a sheet-like nanostructure in a low-dimensional space (nanometer or lower), (ii) the sheet-like structure is not arranged in a specific structure in a high-dimensional space (up to micrometer), (iii) a compound having at least one carboxyl functional group and a polymer having at least one alcohol group are used to form a macroscopic structure of the carbon-based aerogel. Offer potential improvements over existing carbon aerogel technologies for a variety of applications, including sensors, photocatalytic materials, thermoelectric devices, thermal shields, electrically conductive composites, and electrochemical applications, such as porous electrodes for batteries, fuel cells, and supercapacitors, among others. The preparation of these materials can be achieved by forming new sol-gel precursors of composite polymers and graphite oxide, followed by Supercritical Fluid Extraction (SFE) drying. Graphite oxide is prepared based on graphite oxide flakes by the Hummers method, and graphite oxide is then added to the liquid to form a suspension. The solids content of the graphite oxide suspension may vary from about 0.1mg/mL to about 25 mg/mL. In one embodiment, the graphite oxide layer is dispersed in a liquid using a mechanical process, such as an ultrasonic process, to cause the graphite oxide to swell in the liquid to form a sol-gel.
The preparation of the polymer precursor adopts a compound with at least one carboxyl functional group and a polymer with at least one alcohol group, and the polymer precursor is formed by stirring in a solution of water and ethanol (or other organic solvents) with the volume ratio of 0: 1-99.9: 1v/v and the temperature range of 40-95 ℃ by taking hydroiodic acid as a catalyst. Such as, but not limited to, 4-carboxybenzaldehyde (CAS: 619-66-9), and the latter, such as, but not limited to, polyvinyl alcohol (CAS: 9002-89-5). 4-carboxybenzaldehyde (according to the proportions chosen) is first dissolved in ethanol and stirred vigorously at 55 ℃ for at least 30 minutes. When completely dissolved, the corresponding polyvinyl alcohol was added to the solution and stirred at 80 ℃ for at least 4 hours. The use of hydroiodic acid allows the dehydrogenation of two precursor polymer molecules while forming conjugated bonds in the main polymer chain. In the examples presented here, polyvinyl alcohol chains are converted into polyacetylene chains. This critical stage allows the formation of an entirely new conductive polymer (in this case poly (4-formylbenzoyl peroxide) acetylene).
Gel formation is achieved by adding the polymer solution and mechanical impact, such as vigorous stirring or ultrasound, and sodium carbonate as a catalyst, by which the graphite oxide in the liquid is exfoliated. The solution was kept between 50 ℃ and 87 ℃ to achieve complete gelation, and then the resulting hydrogel was dried to produce a graphene oxide aerogel. The most suitable drying technique includes drying using supercritical points, using a pressure of 7-24MPa, at a temperature of 35-100 ℃. The graphene oxide in the aerogel may then be reduced to graphene by, for example, a thermal treatment process (e.g., vacuum heating or heating in an inert atmosphere such as argon or nitrogen) or heating in a reducing atmosphere. In any case, the temperature is generally maintained above 200 ℃. The typical heating period depends on the sample thickness, but at least 3 hours are required to convert all of the graphene oxide aerogel into graphene aerogel. In general, polymers having the above characteristics (i.e., composite polymers formed by reacting a compound having at least one carboxyl group and a polymer having at least one alcohol group) that are soluble in the liquid used to disperse the graphite oxide can be used. Water-soluble or alcohol-soluble polymers are preferred because of the good ability of these liquids to produce graphite oxide dispersions. The weight ratio of polymer to graphite oxide is typically in the range of about 0.1: 1 to about 5:1, in the above range. Other ratios may also be suitable depending on the application and the desired chemical/mechanical properties. Any heat treatment of the aerogel should be carried out under conditions that do not cause decomposition of the polymer compound, typically in the range of 120 ° to 450 ℃.
The invention is further described (illustrated but not limited) by the following examples.
Example 1:
dried graphite oxide was obtained by oxidizing graphite flakes by the Hummers method, and then the dried graphite oxide was added to a 99.9% ethanol solution and lightly sonicated to form a 7.5mg/mL dispersion. Pouring the dispersion intoIn a 100mL beaker, the prepared polymer precursor was combined in a mass ratio of 10: 1. In this step, it is important to maintain a relatively high concentration of the two solutions in order to allow the graphene oxide sheets to aggregate more readily with the polymer and to avoid dispersion of the individual sheets in the dispersion medium. And then reacting the obtained solution in a water bath at the temperature of not higher than 85 ℃ for about 60 minutes to obtain the tough graphene oxide wet gel. The wet gel has a network formed inside, but all the pores are filled with the dispersion medium. Then using supercritical CO2The wet gel is dried to extract the dispersion medium. Placing the wet gel into an extraction chamber, and extracting with liquid CO under 7.5MPa and 50 deg.C2Fill for at least 30 minutes of reaction to give aerogel in cylindrical shape as shown in figure 1 (right).
The color of the obtained graphene oxide aerogel sample ranges from dark brown to black. SEM images of the graphene oxide aerogels shown in fig. 2 and 3 show good mechanical integrity with a foam-like structure. The SEM image of fig. 2 shows a highly nanoporous network of graphene oxide sheets, with the size of the platelets varying from a few microns to tens of microns. Graphene oxide aerogels are electrical semiconductors, with modest electron charging observed during Scanning Electron Microscope (SEM) imaging. In fig. 3, relatively large pore sizes can also be seen, in the range of tens of microns, indicating a hierarchical structure of the pore dispersion. The samples prepared in this way show an increase in conductivity (up to 128s/cm) at a distance of 6-7mm, measured with two electrodes of the four-probe method.
Example 2:
another graphene oxide aerogel was prepared in the same manner as in example 1, except that the polymer solution was added to the dispersion at a ratio of 2.5: 1. The resulting polymer-graphene oxide composite aerogel, as shown in fig. 1 (in), has a more compact, non-brittle structure than other aerogels.
Example 3:
after the graphene oxide composite aerogel is obtained, the graphene oxide composite aerogel can also be subjected to heat treatment, and the graphene oxide aerogel is reduced into graphene aerogel. The graphene oxide aerogel is subjected to heat treatment in a nitrogen inert atmosphere, wherein the temperature is slowly increased from room temperature to 250 ℃, and the temperature is kept for 5 hours. The obtained heat-treated graphene composite aerogel is completely blackened after heat treatment, but the porous carbon network of the heat-treated graphene composite aerogel is remained, so that the graphene oxide can be reduced while the conjugated graphene structure of the graphene oxide is remained.
Example 4:
in this example, another graphene oxide aerogel was prepared in the same manner, using a polymer in which the compound having at least one carboxyl group was still 4-carboxybenzaldehyde, and the polymer having at least one alcohol group was obtained by modifying a poly-p-phenylene (PPP) group with a hydroxyl group. PPP can be modified to introduce hydroxyl (OH) groups and then this new precursor used for the same procedure as in example 1. The new precursor combines with CBA to form a composite polymer and finally with GO. Since PPP chains are typically short, GO aerogels with a compact monolayer structure can be obtained using this modified polymer.
Example 5:
in this example, another graphene oxide aerogel was prepared in the same manner, and the polymer used in the method was a polymer in which the compound having at least one carboxyl group was still 4-carboxybenzaldehyde and the polymer having at least one alcohol group was a polymer obtained by modifying a hydroxyl group of parylene (PPX). PPX can be modified to introduce hydroxyl (OH) groups (OH substituted for one H atom) and then this new precursor used in the same procedure as in example 1. In the preparation process of the modified PPX polymer, the alpha, alpha' -disubstituted p-xylyl group is dehydrogenated mediated by strong base to obtain poly (p-phenylenevinylene) (PPV), wherein the oxygen-containing bond is used for combining CBA to obtain a new composite polymer, and the last step is to combine the composite polymer with GO. The structure of the resulting polymer-GO composite aerogel is related to the length of the polymer backbone.
The above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and a person skilled in the art can modify the technical solution of the present invention or substitute the same without departing from the principle and scope of the present invention, and the scope of the present invention should be determined by the claims.