Carbon-based charged adsorbent as well as preparation method and application thereof

Abstract

The invention discloses a carbon-based charged adsorbent. The adsorbent comprises a porous carbon-based material and lithium ions loaded on the porous carbon-based material. The invention also discloses a preparation method of the carbon-based charged adsorbent, which specifically comprises the following steps: carrying out negative constant-current lithium charging under a three-electrode system, taking the carbon-based material as a working electrode, taking the electrolyte as a salt solution containing lithium ions, reacting under the current of-100 to-300mA, taking the charged carbon-based material out of the electrolyte after the reaction, and drying to obtain the carbon-based charged adsorbent. And washing and drying to obtain the lithium-containing charged adsorbent. The invention finally discloses application of the carbon-based charged adsorbent in the aspect of capturing water in air.

Description

Technical Field

[0001] The present invention relates to a carbon-based charged adsorbent, and also relates to a preparation method of the carbon-based charged adsorbent and application of the carbon-based charged adsorbent as an adsorbent material for capturing water in the air. Background Art

[0002] With the intensification of global climate change and population growth, water shortage has become a global challenge. In arid and semi-arid areas, traditional water resource acquisition methods (such as groundwater extraction and seawater desalination) are difficult to meet the growing population demand. Therefore, atmospheric water harvesting (AWH) technology, as an emerging solution, extracts liquid water from the air through an adsorption-desorption cycle, providing a revolutionary approach for water-scarce areas. The adsorbent materials currently used in AWH technology, such as metal organic frameworks (MOF-801), have high specific surface areas and excellent adsorption capacity, but their preparation process is complex and costly (> $185 / kg), making them difficult to promote; while low-cost materials (such as silica gel and activated carbon) are easy to scale up, but their adsorption capacity is significantly reduced under low humidity conditions (<40% RH). For example, the water adsorption capacity of commercial activated carbon at 30% RH is less than 0.2 g / g, which is difficult to meet the actual needs of arid areas. In addition, the existing adsorption materials used in atmospheric water capture technology still have the problem of high regeneration energy consumption. The traditional thermal desorption technology usually requires a high temperature of 120-150°C and an energy consumption of up to 1200kWh / m 3 . Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to provide a carbon-based charged adsorbent, which, when used to capture water in the air, has good adsorption capacity even under low humidity (such as 30% RH), and the water adsorption amount can reach 0.5L / kg water capture amount. At the same time, it can also achieve low energy consumption regeneration and good cycle stability (it still has good water adsorption capacity after multiple cycles of regeneration); Another purpose of the present invention is to provide a method for preparing the above-mentioned carbon-based charged adsorbent and its application as an adsorbent material to capture water in the air.

[0004] Technical solution: The carbon-based charged adsorbent described in the present invention comprises a porous carbon-based material and lithium ions loaded on the porous carbon-based material; the lithium ions and the negative charges on the surface of the porous carbon-based material are adsorbed together through electrostatic interaction to form a stable double-layer structure.

[0005] Binding mechanism of lithium ions: In the electrochemical system, the porous carbon material is used as the working electrode (cathode). By applying a negative constant potential (relative to the Ag / AgCl reference electrode), the cathode obtains electrons, and the surface of the carbon material is negatively charged. Under the action of the external electric field, the lithium ions in the electrolyte migrate to the surface of the carbon-based material (cathode). At this time, the negative charge on the surface of the carbon-based material and the lithium ions are adsorbed together due to electrostatic interaction to form a stable double-layer structure. In addition, lithium ions can also form coordination bonds with oxygen-containing functional groups (such as hydroxyl, carboxyl, and carbonyl) on the surface of the carbon-based material to further enhance the stability of the load. Finally, the adsorption material after lithium charging is obtained.

[0006] Load calculation: The lithium ion load (or lithium charge) can be calculated by the amount of charge during the electrochemical charging process. According to Faraday's law, the amount of lithium ions loaded is proportional to the amount of charge transferred during the charging process.

[0007] Formula: Q=n×F

[0008] Q: the total amount of charge transferred during electrochemical charging, in coulombs (C); n: the number of moles of lithium ions;

[0009] F: Faraday constant, 96,485 C / mol (the amount of charge required per mole of electrons).

[0010] Calculation of lithium ion loading: n = Q / F

[0011] Assuming the mass of the porous carbon material is m (g), the loading amount of lithium ions (unit: mmol / g) is:

[0012] Load capacity = n / m = Q / (F×m)

[0013] In actual experiments: Use a constant potential mode (such as -0.5V to -1.0V) and record the total charge (Q) through a precision electrochemical workstation. According to the above formula, substitute the charge into the calculation formula to obtain the number of moles of lithium ions loaded on the unit mass of carbon-based materials.

[0014] Assuming that the charge recorded during the charging process is 50C and the mass of the carbon-based material used is 0.1g, the lithium ion loading is:

[0015] Loading amount = Q / (F×m) = 50 / (96,485×0.1)≈5.18 mmol / g.

[0016] Wherein, the porous carbon-based material is activated carbon cloth, graphene aerogel or biomass-based carbon material.

[0017] The preparation method of the above-mentioned carbon-based charged adsorbent is specifically as follows: negative constant current lithium charging is performed under a three-electrode system, wherein the carbon-based material is used as the working electrode, the electrolyte is a salt solution containing lithium ions, and the reaction is carried out at a current of -100 to -300 mA. After the reaction, the charged carbon-based material is taken out from the electrolyte, and the charged adsorbent containing lithium is obtained after washing and drying. Lithium ions enter the pores of the porous carbon-based material under the action of the electric field and combine with the active sites on the surface of the carbon material, thereby giving the carbon-based material charging ability and adsorption ability.

[0018] Wherein, the electrolyte is a LiCl aqueous solution, a Li2SO4 aqueous solution or a LiNO3 aqueous solution, and the concentration of the electrolyte is 2 to 3M.

[0019] Among them, the lithium charging reaction time is 1 to 2 hours.

[0020] Among them, in the three-electrode system, the reference electrode is a standard hydrogen electrode (SHE) or an Ag / AgCl electrode, and the auxiliary electrode is a platinum electrode.

[0021] Among them, after the lithium charging is completed, the carbon-based material after the lithium charging is taken out and repeatedly washed with deionized water until there is no residual salt in the washing liquid, that is, it is fully washed with deionized water to remove the residual electrolyte, and then dried. The drying temperature is 80-100°C and the drying time is 10-12h. The purpose of sufficient drying is to dry out the residual moisture to avoid affecting the subsequent water capture results.

[0022] The application of the above-mentioned carbon-based charged adsorbent in capturing water in the air is as follows: the prepared carbon-based charged adsorbent is exposed to the air, and the electrostatic interaction between lithium ions and water molecules is used to capture water vapor in the air to form a stable lithium-water complex. After adsorption saturation, the adsorbent is heated to 90-120°C by solar heating or photothermal heating to desorb and condense water molecules, and collect liquid water.

[0023] The present invention constructs a stable lithium ion storage network in the carbon-based adsorption material, in which lithium ions serve as active sites and form stable lithium-water complexes with water molecules through electrostatic interaction, significantly improving the water adsorption performance of the adsorbent; at the same time, the photothermal properties of the carbon-based material enable it to utilize photothermal conversion performance to desorb water molecules, thereby enabling rapid desorption of water at a relatively low heating temperature (the photothermal properties of the carbon-based material of the present invention enable it to efficiently absorb solar energy and rapidly desorb adsorbed water molecules), thereby reducing regeneration energy consumption.

[0024] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the preparation process of the carbon-based charged adsorbent of the present invention is simple and the preparation cost is low. When it is used to capture water in the air, it can significantly improve the adsorbent's ability to capture water vapor in the air, and under low humidity (such as 30% RH), the water capture capacity of the adsorbent is much higher than that of traditional adsorbent materials, and the water adsorption amount can reach 0.5L / kg. At the same time, it can also achieve low-energy regeneration, and has good cycle stability. It still has good water adsorption capacity after multiple cycles of regeneration. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The SEM images of the carbon-based charged adsorbents prepared in Example 1 and Example 3; wherein A is the SEM image of the carbon-based charged adsorbent prepared in Example 3; B is the SEM image of the carbon-based charged adsorbent prepared in Example 1; C to D are mapping images of the carbon-based charged adsorbent prepared in Example 1;

[0026] Figure 2 : are the BET curves of the carbon-based materials corresponding to Examples 1 to 3 before and after lithium charging; wherein, A and D are the BET curves of the carbon-based material (graphene aerogel) of Example 1 before and after lithium charging; B and E are the BET curves of the carbon-based material (biological lignin carbon) of Example 3 before and after lithium charging; C and F are the BET curves of the carbon-based material (conductive carbon cloth) of Example 2 before and after lithium charging;

[0027] Figure 3 The adsorption performance comparison of carbon-based charged adsorbents and MOF (ZIF-8 carbon material) made from graphene aerogel, conductive carbon cloth, bio-lignin carbon and common commercial activated carbon in Examples 1 to 3 is shown; wherein A is the adsorption kinetics of water molecules in the air of the above five materials at 25°C and relative humidity (RH=30%); B is a comparison of the adsorption capacity of water for the above five materials in air at 25°C and relative humidity of RH=40%, RH=30% and RH=20% respectively;

[0028] Figure 4 This is the lithium charging curve of graphene aerogel during the preparation of the carbon-based charged adsorbent in Example 1;

[0029] Figure 5 The (adsorption-desorption) curve of the carbon-based charged adsorbent prepared based on graphene aerogel in Example 1 at 25° C. and relative humidity (RH=30%) for 10 cycles;

[0030] Figure 6 The (adsorption-desorption) curve of the carbon-based charged adsorbent prepared based on the conductive carbon cloth in Example 2 at 25° C. and relative humidity (RH=30%) for 10 cycles;

[0031] Figure 7 This is the (adsorption-desorption) curve of the carbon-based charged adsorbent prepared based on bio-lignin carbon in Example 3 at 25° C. and relative humidity (RH=30%), with 10 cycles. DETAILED DESCRIPTION

[0032] Example 1

[0033] A method for preparing a carbon-based charged adsorbent based on graphene aerogel is specifically as follows: negative constant current lithium charging is performed under a three-electrode system, wherein 0.5 g of graphene aerogel is used as a working electrode, the reference electrode is an Ag / AgCl electrode, and the auxiliary electrode is a platinum electrode; 40 mL of a 2M LiCl aqueous solution is used as an electrolyte, and the graphene aerogel is charged with lithium for 1 hour at a current of -300 mV, the material after lithium charging is taken out, and it is repeatedly washed with deionized water until there is no residual salt in the washing liquid, and the washed material is placed in a vacuum drying oven and dried at 100°C for 10 hours to prepare a lithium-containing charged adsorbent, named C1.

[0034] The dried graphene aerogel adsorbent was placed in a humidity control box with the humidity set to 20%, 30% and 40% respectively, the temperature was 25°C, and adsorption was carried out for 30 minutes; its adsorption capacity for water in the air at different humidity was tested, such as Figure 3 As shown, the water adsorption capacities are 0.44 L / kg, 0.5 L / kg and 0.6 L / kg respectively.

[0035] After adsorption saturation at different humidity levels, the adsorbent was heated to 90°C using a solar lamp or an infrared lamp for 30 minutes, and the condensed water was collected. The desorption rate was as high as 99%.

[0036] pass Figure 5 It can be seen that after 10 cycles (adsorption-desorption) at 25°C and relative humidity (RH=30%), the graphene aerogel adsorbent still has good water adsorption performance and a high desorption rate (complete desorption), indicating that it has good cycle stability. 30 minutes is the adsorption time, and 30 minutes is the desorption time, which constitutes a cycle 1. A total of ten cycles were performed, as shown in the figure, and they were all stable.

[0037] Example 2

[0038] Activated carbon cloth (ACC-5092-10) was selected and cut into 2 cm×2 cm sheets; a 2 M LiCl aqueous solution was prepared as an electrolyte.

[0039] A method for preparing a carbon-based charged adsorbent based on activated carbon cloth is specifically as follows: negative constant current lithium charging is performed under a three-electrode system, wherein 0.5 g of activated carbon cloth is used as a working electrode, the reference electrode is a standard hydrogen electrode (SHE), and the auxiliary electrode is a platinum electrode; 40 mL of a 2M LiNO3 aqueous solution is used as an electrolyte, and the activated carbon cloth is charged with lithium for 2 hours at a current of -100 mV, the lithium-charged material is taken out, and it is repeatedly washed with deionized water until there is no residual salt in the washing liquid, and the washed material is placed in a vacuum drying oven and dried at 100°C for 10 hours to prepare a lithium-containing charged adsorbent named C3.

[0040] The dried activated carbon cloth adsorbent was placed in a humidity control box with the humidity set to 20%, 30% and 40% respectively, the temperature was 25°C, and adsorption was performed for 30 minutes; its adsorption capacity for water in the air at different humidity was tested, such as Figure 3 As shown, the water adsorption capacities are 0.35 L / kg, 0.41 L / kg and 0.45 L / kg respectively.

[0041] After adsorption saturation at different humidity levels, the adsorbent was heated to 90°C using a solar lamp or infrared lamp for 30 minutes, and condensed water was collected, which could reach 99% at different humidity levels.

[0042] pass Figure 6 It can be seen that after 10 cycles (adsorption-desorption) at 25°C and relative humidity (RH=30%), the activated carbon cloth adsorbent still has good adsorption performance for water, and the desorption rate is also very high (complete desorption), indicating that it has good cyclic stability performance.

[0043] Example 3

[0044] A method for preparing a carbon-based charged adsorbent based on lignin-based carbon material is specifically as follows: negative constant current lithium charging is performed under a three-electrode system, wherein 0.5 g of lignin-based carbon material is used as a working electrode, the reference electrode is an Ag / AgCl electrode, and the auxiliary electrode is a platinum electrode; 40 mL of a 3M Li2SO4 aqueous solution is used as an electrolyte, and the activated carbon cloth is charged with lithium for 1 hour at a current of -200 mV, the lithium-charged material is taken out, and it is repeatedly washed with deionized water until there is no residual salt in the washing liquid, and the washed material is placed in a vacuum drying oven and dried at 100°C for 10 hours to prepare a lithium-containing charged adsorbent named C2.

[0045] The dried adsorbent was placed in a humidity control box with the humidity set to 20%, 30% and 40% respectively, the temperature was 25°C, and adsorption was carried out for 30 minutes; its adsorption capacity for water in the air at different humidity was tested, such as Figure 3As shown, the water adsorption amounts are 0.5 L / kg, 0.46 L / kg and 0.38 L / kg, respectively.

[0046] After adsorption saturation at different humidity levels, the adsorbent was heated to 90°C using a solar lamp or infrared lamp for 30 minutes, and condensed water was collected. The corresponding desorption rates at different humidity levels could reach 99%.

[0047] pass Figure 7 It can be seen that after 10 cycles (adsorption-desorption) at 25°C and relative humidity (RH=30%), the adsorbent still has good water adsorption performance and a high desorption rate (complete desorption), indicating that it has good cyclic stability.

[0048] Comparative Example 1

[0049] A method for preparing a carbon-based charged adsorbent based on graphene aerogel is specifically as follows: negative constant current lithium charging is performed under a three-electrode system, wherein 0.5 g of graphene aerogel is used as a working electrode, a reference electrode is an Ag / AgCl electrode, and an auxiliary electrode is a platinum electrode; 40 mL of a 2M LiCl aqueous solution is used as an electrolyte, and the graphene aerogel is charged with lithium for 1 hour at a current of -500 mV, the material after lithium charging is taken out, and repeatedly washed with deionized water until there is no residual salt in the washing liquid, and the washed material is placed in a vacuum drying oven and dried at 100°C for 10 hours to prepare a lithium-containing charged adsorbent.

[0050] The dried graphene aerogel adsorbent was placed in a humidity control box, with the humidity set to 20%, 30% and 40%, the temperature set to 25°C, and adsorbed for 30 minutes; its adsorption capacity for water in the air at different humidity levels was tested, and the water adsorption was 0.14L / kg, 0.2lL / kg and 0.26L / kg respectively. This is because the cathode will have a strong hydrogen evolution reaction, which makes it impossible for lithium ions to be adsorbed on the surface of the cathode material.

[0051] pass Figure 1 It can be seen that the pores of the adsorbents obtained in Example 1 and Example 3 are obvious and evenly distributed; the presence of C and Li elements in the adsorbent can be seen from the mapping diagram.

[0052] pass Figure 2 It can be seen that the adsorbent prepared by the method of the present invention will not affect the adsorption capacity and pore distribution of the carbon-based material. Materials with large specific surface area and rich pores can provide more lithium ion adsorption sites, thereby significantly improving the adsorption performance. Figure 2 From the BET, we can see that the specific surface areas of graphene aerogel, lignin-based carbon, and conductive carbon cloth are 700 cm 3 / g, 265cm 3 / g, 160cm 3 / g, so it affects the loading amount of lithium ions, thereby affecting the water adsorption performance. In addition, the pore distribution will also affect the diffusion rate of lithium ions. Graphene aerogel has abundant mesopores and micropores, which ensures the rapid migration and adsorption of lithium ions; lignin-based carbon has more micropores, but insufficient mesopores, resulting in limited adsorption rate; conductive carbon cloth is mainly macropores, with fewer micropores and mesopores, and has the lowest adsorption performance. In addition, surface active sites will also enhance adsorption capacity. Graphene aerogel and lignin-based carbon are rich in hydrophilic functional groups, so their adsorption capacity for lithium ions is better than that of conductive carbon cloth.

[0053] pass Figure 3 As can be seen from Figure A, the adsorbent prepared by the method of the present invention can reach adsorption equilibrium in 20 to 30 minutes, while the ZIF-8 carbon material and the ordinary activated carbon material need 1 to 2 hours to reach adsorption equilibrium, indicating that the adsorbent prepared by the present invention can not only improve the adsorption capacity, but also improve the adsorption efficiency.

[0054] Figure 4 It is mainly divided into three stages: Initial stage (0 to several hundred seconds): The voltage drops rapidly to close to -2.0V (relative to Ag / AgCl). This is mainly due to the initial current driving the rapid migration of lithium ions to the surface of the graphene aerogel electrode to form a double electric layer. Intermediate stage (several hundred seconds to 2000 seconds): The voltage gradually rises and tends to be stable (about -1.0V). This stage mainly reflects that lithium ions enter the pores of the graphene aerogel and are further adsorbed. Late stage (2000 seconds to 3600 seconds): The voltage is close to stability, indicating that the adsorption sites of the graphene aerogel are gradually saturated and the system tends to dynamic equilibrium. It shows that under constant current (-300mA), as lithium ions gradually migrate and are adsorbed, the polarization voltage will gradually decrease and tend to be stable.

Claims

1. A carbon-based charged adsorbent, characterized in that: The adsorbent includes a porous carbon-based material and lithium ions loaded on the porous carbon-based material by electrostatic action; Lithium ions and the negative charges on the surface of porous carbon-based materials adsorb each other through electrostatic interaction to form a double-layer structure.

2. The carbon-based charged adsorbent according to claim 1, characterized in that: The loading amount of lithium ions on the porous carbon-based material is: loading amount = Q / (F×m); where Q is the total charge transferred during electrochemical charging; F is the Faraday constant, 96,485 C / mol; and m is the mass of the porous carbon-based material.

3. The carbon-based charged adsorbent according to claim 1, characterized in that: The porous carbon-based material is one of activated carbon cloth, graphene aerogel or biomass-based carbon material.

4. The method for preparing the carbon-based charged adsorbent according to claim 1, characterized in that: Specifically, negative constant current lithium charging is carried out in a three-electrode system, wherein a carbon-based material is used as a working electrode, the electrolyte is a salt solution containing lithium ions, and the reaction is carried out at a current of -100 to -300 mA. After the reaction, the charged carbon-based material is taken out from the electrolyte, and a charged adsorbent containing lithium is obtained after washing and drying.

5. The preparation method according to claim 4, characterized in that: The electrolyte is a LiCl aqueous solution, a Li2SO4 aqueous solution or a LiNO3 aqueous solution.

6. The preparation method according to claim 5, characterized in that: The concentration of the electrolyte is 2-3M.

7. The preparation method according to claim 4, characterized in that: The lithium charging reaction time is 1 to 2 hours.

8. The preparation method according to claim 4, characterized in that: In the three-electrode system, the reference electrode is a standard hydrogen electrode or an Ag / AgCl electrode, and the auxiliary electrode is a platinum electrode.

9. The preparation method according to claim 4, characterized in that: The drying temperature is 80-100°C and the drying time is 10-12h.

10. Use of the carbon-based charged adsorbent according to claim 1 in capturing water in the air.

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