CN115547700A - Derivative plant-based porous carbon and preparation method and application thereof - Google Patents
Derivative plant-based porous carbon and preparation method and application thereof Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/22—Electrodes
- H01G11/30—Electrodes characterised by their material
- H01G11/32—Carbon-based
- H01G11/44—Raw materials therefor, e.g. resins or coal
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/22—Electrodes
- H01G11/24—Electrodes characterised by structural features of the materials making up or comprised in the electrodes, e.g. form, surface area or porosity; characterised by the structural features of powders or particles used therefor
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/22—Electrodes
- H01G11/26—Electrodes characterised by their structure, e.g. multi-layered, porosity or surface features
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/22—Electrodes
- H01G11/30—Electrodes characterised by their material
- H01G11/32—Carbon-based
- H01G11/34—Carbon-based characterised by carbonisation or activation of carbon
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
- H01G11/84—Processes for the manufacture of hybrid or EDL capacitors, or components thereof
- H01G11/86—Processes for the manufacture of hybrid or EDL capacitors, or components thereof specially adapted for electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/13—Energy storage using capacitors
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Abstract
The invention provides a derivative plant-based porous carbon and a preparation method and application thereof. The invention utilizes bean sprout biomass as an active carbon electrode material for preparing the supercapacitor, effectively improves the uniformity of the prepared active carbon, and further explores a proper green preparation method and an electrochemical performance optimization mechanism. Specifically, the derivative plants represented by the mung bean sprouts are used as the precursor of the activated carbon for the super capacitor, a series of related quality improvement modification work is developed from two aspects of pore structure adjustment and nitrogen atom doping on the basis, and reference is provided for high-value utilization of biomass and preparation of the activated carbon for the super capacitor, so that the method has good practical application value.
Description
Technical Field
The invention belongs to the technical field of preparation of biochar materials, and particularly relates to derived plant-based porous carbon and a preparation method and application thereof.
Background
The information in this background section is only for enhancement of understanding of the general background of the invention and is not necessarily to be construed as an admission or any form of suggestion that this information forms the prior art that is already known to a person of ordinary skill in the art.
With the increasing development of energy and environmental crisis, the super capacitor as a novel energy storage device receives wide attention due to high power density and long cycle life. Electrode materials are key factors influencing the performance of the supercapacitor, and generally, electrode materials for the supercapacitor mainly comprise carbon materials, conductive polymers, metal oxides and the like, wherein carbon materials with p-conjugated structures such as graphene, carbon nanotubes, ordered mesoporous carbon, activated carbon and the like are widely used for the electrode materials. Wherein, the plant is a good precursor of the active carbon for the super capacitor by the natural and ordered tissue and cell structure of the plant.
The plant usually contains structures with high crosslinking degree such as thin-wall tissues, transport tissues and mechanical tissues, and the shapes can be well maintained after simple pyrolysis, so that shapes which are easy to transport electrolyte ions such as thin sheets and hierarchical porous shapes can be formed. On one hand, due to the diversity of plant types and plant tissues, the uniformity of the formed plant-based activated carbon is poor, and the existence of non-structural impurities such as metal, alcohol extract and the like can increase the leakage current of the activated carbon, increase the impedance and block pore channels, thereby increasing the energy consumption in the preparation process. On the other hand, the large specific surface area of the activated carbon provides sufficient adsorption sites for electrolyte ions, but is limited by poor self-wettability and pore connectivity, and the pores of the porous carbon are difficult to be utilized efficiently. Therefore, the uniformity of the material is improved and the quality is improved by optimizing the biomass raw material or optimizing the process conditions on the basis of keeping the original good shape of the plant, so that the high-quality activated carbon electrode with higher specific capacity and stronger uniformity is obtained, and the method is a necessary way for large-scale commercial application of the plant-based activated carbon.
Disclosure of Invention
Aiming at the defects of the prior art, the inventor provides the derivative plant-based porous carbon and the preparation method and application thereof through long-term technical and practical exploration. The invention utilizes bean sprout biomass as an active carbon electrode material for preparing the supercapacitor, effectively improves the uniformity of the prepared active carbon, and further explores a proper green preparation method and an electrochemical performance optimization mechanism. The present invention has been completed based on the above results.
In order to achieve the technical purpose, the invention adopts the following technical scheme:
in a first aspect of the invention, the application of the derivative plants in preparing the porous carbon electrode material of the super capacitor is provided.
The derivative plants are bean sprout plants including but not limited to mung bean sprouts, soybean sprouts and the like. The biochar prepared by the bean sprout plant has better electrochemical performance than that of a bean plant due to the clean growth environment, short growth period of raw materials and natural micron-sized thin-walled tissues, air cavities, conduits and other structures, and meanwhile, the short growth period and the aquatic characteristic avoid the absorption of excessive impurity metal ions of the plant in soil, so that the content of the substances in the biochar is lower, and the energy consumption in the subsequent impurity removal process is lower.
In a second aspect of the invention, a preparation method of a derivative plant-based porous carbon material is provided, and the preparation method comprises the step of treating a derivative plant by using a carbonization-KOH activation method.
Wherein the derivative plant is bean sprout, which is also called sprout vegetable, and is edible sprout cultivated from seeds of various cereals, beans and trees, also called living vegetable. The bean sprouts include, but are not limited to, mung bean sprouts, soybean sprouts, etc., preferably mung bean sprouts.
Specifically, the preparation method comprises the following steps:
s1, putting bean sprouts in an inert atmosphere for high-temperature pyrolysis;
s2, adding an activating agent into the product obtained in the step S1 for heating and dipping treatment;
s3, placing the product obtained in the step S2 in inert gas for secondary heating and activating treatment;
and S4, carrying out acid washing, water washing and drying on the product obtained in the step S3 to obtain the catalyst.
Wherein, in the step S1, the bean sprouts may be bean sprout powder obtained through a simple pretreatment, and the pretreatment method includes: cleaning bean sprout, drying, pulverizing, and sieving.
Specifically, the sieving mesh number is 80 meshes, and powder smaller than 178 mu m is obtained for standby.
The inert atmosphere can be nitrogen, and the specific conditions of the high-temperature pyrolysis are as follows: pyrolyzing at 450-650 deg.C for 0.5-3h; the temperature rise rate is controlled to be 1-10 ℃ per minute -1 Preferably 5 ℃ min -1 (ii) a The nitrogen flow rate is 0.5-5 L.min -1 Preferably 1 L.min -1 。
In the step S2, the mass ratio of the activating agent to the product is 1-10, such as 1;
the mass ratio of the activator to the product is named impregnation ratio or alkali-carbon ratio.
The specific conditions of the heating and dipping treatment are as follows: soaking at 70-90 deg.C for 1-3h (preferably at 80 deg.C for 2 h);
in the step S3, the specific conditions of the secondary heating activation treatment include: heating to 300-400 deg.C (preferably 350 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C), maintaining the temperature for 10-50min (preferably 30 min), heating to 750-850 deg.C (preferably 800 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C), maintaining the temperature for 1-3h (preferably 2 h), and maintaining the temperature with nitrogen as shielding gas at a flow rate of 0.5-5 L.min -1 Preferably 1 L.min -1 。
In the step S4, the acid added in the acid washing step may be hydrochloric acid, preferably 1.0M hydrochloric acid, the water washing step is to repeatedly wash with water until the pH of the supernatant is neutral, and the drying step is to dry for 1 to 48 hours, preferably for 24 hours at 105 ℃, preferably by drying.
In order to further improve the uniformity of the finally prepared activated carbon, post-treatment removal is carried out on bean sprouts, and the method specifically comprises the steps of removing non-structural components of the bean sprouts and removing impurities of bean sprout biomass by acid washing; wherein, the method for removing the non-structural components of the bean sprouts can adopt ethanol or ethanol reflux assisted by ultrasound to treat the biomass raw material, wherein the reflux operation conditions comprise: refluxing bean sprout and ethanol solution at 70-90 deg.C for 0.5-2 hr, cooling to remove supernatant, and repeating for 2-3 times; preferably at 80 ℃ for 1h; the ethanol can be 60-80% (preferably 70%) by mass of ethanol solution, and the mass volume ratio of the bean sprouts to the ethanol solution is 1-5.
Further, ultrasonic crushing treatment is added in the ultrasonic-assisted ethanol reflux treatment process, and the ultrasonic power is controlled to be 100-500W, preferably 300W.
Acid washing and impurity removal of bean sprout biomass are mainly used for deeply removing metal ions in bean sprouts; specifically, the step of acid washing impurity removal comprises the step of placing the dried bean sprouts into a hydrochloric acid solution, wherein the acid washing conditions are as follows: treating at 60-80 deg.C for 1-3 hr, preferably at 70 deg.C for 2 hr, with hydrochloric acid concentration of 0.1-1mol L -1 Preferably 0.5mol L -1 The mass volume ratio of the bean sprouts to the hydrochloric acid solution is 1-5 (g/ml), preferably 2. Furthermore, ultrasonic treatment can be introduced in the pickling process, and the ultrasonic treatment is introduced to promote the mass transfer process, so that the pickling impurity removal efficiency is improved, the treatment time is shortened, and the leaching effect of metal elements is improved, so that the treatment time can be 0.5-2h, preferably 1h, and the ultrasonic power is 100-150W, preferably 120W in the ultrasonic pickling process.
The pretreatment removal of the post-content in the bean sprouts can be performed before the step S1, or the pretreatment removal of the post-content in the bean sprout powder can be performed by the bean sprout powder obtained after the simple pretreatment of the bean sprouts.
In order to further optimize the pore structure of the bean sprouts and improve the electrochemical performance of the activated carbon, H is preferably adopted 3 PO 4 -two-step KOH activation process. Specifically, the preparation method comprises the step of performing phosphoric acid impregnation before the step S1, wherein the phosphoric acid impregnation method comprises the following steps: mixing bean sprout with phosphoric acid, adding, stirring, and drying.
The specific phosphoric acid impregnation method comprises the following steps: mixing mung beans and phosphoric acid according to a mass ratio of 0.8-2 to 1, adding the mixture into water, controlling the dipping temperature of the phosphoric acid to be 80-140 ℃, preferably 120 ℃, and drying (drying for 8-12h, preferably 10h at 80-140 ℃) for later use. The invention discovers through research that H 3 PO 4 The product of the KOH activation method has a better hierarchical structure, and the carbonization temperature is effectively reduced by introducing phosphoric acid in the early stage of activation, so that the carbonized product has rich mesopores and certain micropores, and further reaction pore-forming of KOH is facilitated. KOH can create developed pores and more abundant micropores on the basis of phosphoric acid activation. The phosphoric acid activation method and the KOH activation are reasonably combined, so that the use amount of KOH is effectively reduced, and the product has higher specific capacitance.
Furthermore, due to the diversity of material components and structures, the optimization of parameters such as morphology, pore structure, conductivity, surface chemical composition and the like of the activated carbon of the plants is easy to be lost, and the plants are difficult to cooperate. On the basis of obtaining a proper specific surface area and a pore channel structure, nitrogen atom doping is beneficial to improving the conductivity and the wettability, a pseudo capacitor can be introduced through an oxidation-reduction reaction, the specific capacity is improved, and a nitrogen-containing functional group can promote the migration rate of electrons in a carbon skeleton, so that the conductivity of the porous carbon material is improved.
The invention adopts melamine with higher nitrogen content as nitrogen doping agent, KOH as activating agent, and the combination of carbon and nitrogen and the formation of pores are cooperated to occur at high temperature, so that the target sample has reasonable pore size distribution, and the carbon skeleton contains a plurality of doping nitrogen atom distributions. Compared with the common back-end doping, the synergistic process can simplify the production process and reduce the cost on the basis of realizing better electrochemical performance of the plant activated carbon, and lays a good foundation for large-scale application.
Therefore, the preparation method of the invention further comprises the step of mixing and calcining the porous carbon material prepared in the step S4 and melamine to obtain the nitrogen-doped activated carbon material.
Specifically, the preparation method comprises the following steps: mixing melamine, KOH and pickled bean sprout powder carbonized sample, heating to 300-400 deg.C (preferably 350 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C), maintaining at the temperature for 10-50min (preferably 30 min), heating to 750-850 deg.C (preferably 800 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C) and maintaining for 1-3h (preferably 2 h), with nitrogen as shielding gas and at a flow rate of 0.1-1 L.min -1 Preferably 0.6 L.min -1 And then carrying out acid washing, water washing and drying to obtain the catalyst.
Wherein the mass ratio of the melamine to the KOH and the carbonized bean sprout powder after acid washing is 1-50; further preferably 1.
In a fourth aspect of the present invention, there is provided a porous carbon electrode comprising the above-described derivatized plant-based porous carbon material.
In a fifth aspect of the present invention, there is provided a supercapacitor comprising the above-described derivatized plant-based porous carbon material or the above-described porous carbon electrode.
Compared with the prior art, one or more of the technical schemes have the following beneficial effects:
(1) Based on the theory of botany, the characteristic derivative plant bean sprouts with non-soil-borne and hierarchical porous structures are selected as precursors of the activated carbon for the super capacitor, so that the absorption of external metal elements can be reduced to the greatest extent, and the bean sprout-based activated carbon synthesized by the structural derivative with higher ratio of the parenchyma tissue to the transport tissue has a better pore structure.
(2) The biomass is pretreated, upgraded and modified from the angle of removing the post-content of the bean sprout cells, the initial structure composition of the biomass is optimized, and the uniformity of the prepared activated carbon is further improved.
(3) The method combines the characteristics of a phosphoric acid activation method and a KOH activation method to improve the problem of single pore structure of the activated carbon prepared by the single activation method, proves the superiority of phosphoric acid-KOH secondary activation pore-forming, and discloses the contribution of different activators in secondary activation pore-forming, internal surface area improvement and material electrochemical performance.
(4) Bean sprouts are used as raw materials, KOH and melamine are used as an activating agent and a nitrogen source material respectively, and the structural nitrogen-doped hierarchical porous carbon with excellent electrochemical performance is prepared. The method for cooperatively regulating and controlling the performance of the porous carbon by high-temperature activated pore forming and structural nitrogen doping is obtained, a mechanism for converting nitrogen-containing functional groups at high temperature and a mechanism for improving super-capacitive performance are disclosed, and an N-Q structure with a high proportion in the nitrogen-doped porous carbon is beneficial to improving the electric double layer capacitance performance and the cycle stability of an electrode, so that the method has a good practical application value.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are included to illustrate an exemplary embodiment of the invention and not to limit the invention.
FIG. 1 is a graph showing the effect of phosphoric acid impregnation ratio on specific surface area and pore volume in example 3 of the present invention.
FIG. 2 is a graph showing the effect of immersion temperature on specific surface area and pore volume in example 3 of the present invention.
FIG. 3 is a graph showing the effect of carbonization temperature on pore volume and specific surface area in example 3 of the present invention.
FIG. 4 is a graph showing the effect of carbonization time on surface area and pore volume in example 3 of the present invention.
FIG. 5 is a graph showing the effect of KOH impregnation on comparative surface area and pore volume in example 3 of the present invention.
FIG. 6 is a plot of the pore size distribution of HP and HPK samples of example 3 of the present invention.
FIG. 7 is SEM images of (a-c) HP and (d-f) HPK at different magnifications in example 3 of the invention.
FIG. 8 is an XRD spectrum (b) Raman spectrum of (a) HPK and HP in example 3 of the present invention.
Fig. 9 is a constant current charge and discharge curve (a) of HPK at different current densities (b) of HP at different current densities (c) of HPK and HP rate curves in example 3 of the present invention.
FIG. 10 shows cyclic voltammograms of (a) HP and (b) HPK in example 3 of the present invention.
FIG. 11 is a graph showing AC impedance curves of HP and HPK in example 3 of the present invention.
FIG. 12 shows the energy density and power density of HP and HPK in example 3 of the present invention.
FIG. 13 shows (a) MBSN in example 4 of the present invention 1:X The nitrogen adsorption/desorption curve (b) is a pore size distribution curve.
FIG. 14 shows (a-c) MBSN in example 4 of the present invention 1:3 And (d-f) MBSN 1:7 Enlargement at different magnifications.
FIG. 15 shows MBSN in example 4 of the present invention 1:X XRD (a) and Raman spectrum of (b).
FIG. 16 shows MBSN in example 4 of the present invention 1:X (ii) XPS Total Spectrum (a) of (b) N1 s.
FIG. 17 shows MBSN in example 4 of the present invention 1:X The constant current charging and discharging curve (b), the cyclic voltammetry curve (c), the multiplying factor curve (d) and the impedance frequency scanning curve.
FIG. 18 is the MBS in embodiment 4 of the invention 1:7 At 5A g -1 Cycling performance at current density.
FIG. 19 is a graph of power density and energy density in example 4 of the present invention.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The present invention is further illustrated by the following specific examples, which are provided for the purpose of illustration only and are not intended to be limiting. If the experimental specific conditions not noted in the examples, they are generally according to the conventional conditions, or according to the conditions recommended by the sales companies; the present invention is not particularly limited, and can be commercially obtained.
Example 1 preparation of derivatized plant-like porous carbon and electrochemical Performance study thereof
Preparation of porous carbon
The fresh mung bean sprouts and mung beans selected in the experiment are purchased in local supermarkets. Cleaning fresh mung bean sprout, soybean sprout and mung bean with clear water, drying in a forced air drying oven at 100 deg.C for 24 hr, pulverizing in a pulverizing machine, and sieving with 80 mesh sieve to obtain powder with particle size of less than 178 μm.
The preparation process of the active carbon mainly comprises the steps of carbonization, acid cleaning, impregnation, activation, acid cleaning and drying in sequence. Firstly, respectively placing the two obtained biomass powders in a horizontal tubular furnace, and pyrolyzing the two biomass powders for 2 hours at 600 ℃ in a nitrogen atmosphere, wherein the heating rate is 5 ℃ min -1 The flow rate of nitrogen is 1 L.min -1 . And (3) dipping: the preparation method comprises the following steps of (1) weighing a carbonized sample and an activating agent according to the required impregnation ratio (the mass ratio of the activating agent to the carbonized sample comprises 2. And (3) an activation process: the impregnated carbonized-like-activator mixture was placed in an atmospheric muffle furnace at 5 ℃ C. Min -1 The temperature is raised to 350 ℃ at the temperature raising rate, the temperature is kept for 30min at the temperature, and then the temperature is raised to 5 ℃ for min -1 The temperature is raised to 800 ℃ at the temperature raising rate and is kept for 2 hours, nitrogen is used as protective gas, and the flow is 1 L.min -1 . Acid washing process: and (3) placing the sample obtained after activation in a water bath, heating in a constant-temperature water bath at 80 ℃, simultaneously carrying out magnetic stirring, dropwise adding 1.0M hydrochloric acid until the pH is =2, and repeatedly washing with deionized water until the pH of the supernatant is =7 after carrying out magnetic stirring for 1 h. And finally, placing the activated carbon after the acid washing in a forced air drying oven, and drying for 24 hours at 105 ℃ to obtain the bean sprout/mung bean based activated carbon.
Preparation of super capacitor
According to the mass ratio of 1:1: and 8, weighing conductive graphite, a polytetrafluoroethylene solution and active carbon, then putting the materials into a beaker, adding 35mL of absolute ethyl alcohol, carrying out ultrasonic treatment for 30min, and then putting the materials into a hot air drying oven at 100 ℃ to be dried into paste. The porous carbon powder is evenly coated on a foamed nickel current collector with the diameter of 1.5cm, and about 10mg of porous carbon is coated on each current collector. Then, the electrode slices are placed in a vacuum drying oven to be dried for 12 hours at the temperature of 80 ℃. Taking out, and pressing with oil press under 15MPa for 1min. And (4) assembling the cut water system diaphragm and the electrode slice in 6M KOH solution to complete the assembly of the button type symmetrical super capacitor. After being placed for 12 hours, the electrochemical performance of the test piece can be tested. All electrochemical performance characterizations were tested by the CS310H electrochemical workstation.
The present example concludes as follows:
(1) The bean sprout-based activated carbon derived from mung beans has properties similar to those of mung bean-based activated carbon in all aspects but superior to those of mung bean-based activated carbon due to a more reasonable hierarchical porous structure.
(2) The mung bean sprout based activated carbon obtains the maximum specific surface area of 3314.382m at the impregnation ratio of 5 (named MBS-5) 2 g -1 Specific pore volume of 2.289cm 3 g -1 The pore diameter is mainly concentrated between 0.5 nm and 0.7nm, the peak value is 0.65nm, and the good pore diameter distribution is K in KOH electrolyte + And OH - The adsorption of ions provides an effective internal specific surface area.
(3) The mung bean sprout-based active carbon mostly comprises amorphous carbon, a small amount of graphitized crystals exist, and the mung bean sprout-based active carbon does not have a two-dimensional graphene structure; meanwhile, a large number of hydrophilic groups such as C = O, O = C-O and the like are contained, so that the wettability of the material is increased.
(4) The mung bean sprout-based active carbon electrode has good electrochemical characteristics, and the MBS-5 obtains the highest specific capacitance and mainly takes the electric double layer capacitance as the main part, and the specific capacitance is 0.5A g -1 The current density reaches 295 Fg -1 When the current density reaches from 0.1ag -1 Increased to 50 ag -1 Then, the capacity retention ratio reached 78.8%. At 5.0A g -1 After 5000 times of constant current charging and discharging, the capacity retention ratio of MBS-5Still 93.7% can be achieved.
Example 2 Soybean sprout precursor-based upgrading modification
Although the non-native growing environment of the bean sprouts avoids excessive absorption of metal ions from the outside, inherent metal ions still exist in the bean sprouts, and compared with plants with higher lignification degree, when the bean sprouts sprout, the metal ions are converted into free metal ions from phytic acid-metal-protein chelate state in the bean sprouts, so that the metal ions are easier to be absorbed by human bodies, and the structural strength of the bean sprouts is lower, so that the metal leaching is easier to realize in the acid pickling process.
In the embodiment, non-fruiting components of mung bean sprouts are removed, and the biomass raw material is treated by ethanol/ultrasonic-assisted ethanol reflux, wherein the reflux operation parameters are as follows: 20g of biomass was refluxed with 500ml of a 70% by mass ethanol solution at 80 ℃ for 1h, and the supernatant was cooled off and repeated twice. Ultrasonic-assisted ethanol reflux operation: refluxing 20g of biomass with 500ml of 70% ethanol solution at 80 deg.C for 1h with ultrasonication treatment at 300W, repeating the preparation process of the activated carbon twice in the same manner as in example 1, wherein the activated carbon prepared from bean sprouts subjected to ultrasonic-assisted ethanol extraction is named as MBSEU-5 (E expressed sensory ethanol and ultra expressed peptides, 5 expressed peptides), and the activated carbon prepared from bean sprouts subjected to ethanol extraction is named as MBSEU-5 (E expressed peptides)
Meanwhile, the present embodiment introduces an acid washing process for deeply removing metal ions in the bean sprouts. Acid washing is a simple and effective way for removing alkali metals and alkaline earth metals from biomass, and acid pretreatment can also remove most hemicellulose components in the substrate, deconstruct the compact structure of the substrate, change the surface morphology, and improve the porosity of the raw material.
In order to improve the leaching rate of metal ions and the acid washing efficiency, an ultrasonic pretreatment step is introduced, and bean sprouts can promote cell wall breakage, cell dissociation and intracellular substance dissolution in the ultrasonic treatment process. The organic combination of the two can shorten the reaction time, thereby improving the leaching rate of the metal ions in the biomass.
The pretreatment parameters were: placing dried 20g mung bean sprout raw material in 500ml 0.5mol L -1 In the hydrochloric acid solution, the pickling temperature is 70 ℃, the pickling time is 2 hours, the ultrasonic-pickling time is 1 hour, the ultrasonic power is 120w, and the preparation process of the rest of the activated carbon is the same as the above. Activated carbon prepared by subjecting bean sprouts to ultrasonic assisted hydrochloric acid pickling is named as MBSAU-5 (mung bean powder based transformed carbon, AU representations and technologies, 5 representations and speed), and activated carbon prepared by subjecting bean sprouts to hydrochloric acid pickling is named as MBSA-5 (A representations and technology).
The experimental conclusion of this example is as follows:
(1) The acid washing promotes the leaching of metals in the bean sprouts and simultaneously causes the hydrolysis of cell walls, so as to cause the dissociation of the original structure of the bean sprouts, the MBSA-5 also causes the collapse of mesopores when more micropores are generated, and the higher alkali-carbon ratio aggravates the ablation of the pore structure. The ethanol extraction promotes the dissolution of non-structural components in the bean sprouts, the number of the mesopores of the MBSE-5 is increased, the pore structure is optimized, and the rapid transportation of electrolyte ions is facilitated.
(4) The introduction of the ultrasonic wave promotes the mass transfer process, improves the depth of the pretreatment process and saves the pretreatment time. The ultrasonic wave promotes the leaching of metal elements, and the MBSAU-5 basically meets the requirements of GB/T37386-2019 document on the content of the metal elements. Unreasonable ultrasonic treatment time and power changes the ultrasonic effect from improving reaction efficiency to aggravating damage of the pretreatment process to the original shapes of the bean sprouts. Finally, MBSAU-5 and MBSEU-5 generate more pores with low accessibility, such as less than 0.55nm, and the excessive micropores narrow the mesoporous distribution and reduce the mesoporous number.
(6) The four pretreatment modes not only improve the degree of order of the material, but also increase the proportion of C = C and oxygen-containing functional groups, but have limited improvement on the graphitization of the material. Moreover, the oxygen-containing functional group improves the wettability of the material and increases the accessibility of the pore structure.
(7) MBSE-5 has the highest specific capacitance and mainly has the electric double layer capacitance of 0.5 ag -1 367 Fg is reached under the current density -1 When the current density reaches from 0.5 ag -1 Increased to 50 ag -1 When the capacitance retention rate is 82%,
(8) The content of metals in the bean sprouts is low, the decomposition temperature is not obviously improved after acid washing, the hydrolysis of hemicellulose and cellulose is promoted by acid washing, the pore passages of the biomass are dredged and exposed, the heat transfer and the overflow of micromolecular volatile substances are facilitated, and the maximum decomposition rate is obviously promoted. The non-structural component of the alcohol extract improves the reactivity among the three components, and is beneficial to the decomposition of structural substances, so that the stability of the bean sprout carbon skeleton after the ethanol extraction is improved.
Example 3H 3 PO 4 Preparation of porous carbon by-KOH two-step activation method and electrochemical properties
Green bean sprouts are used as raw materials in the experiment, and H is adopted 3 PO 4 The preparation method of the activated carbon comprises the steps of phosphoric acid impregnation, carbonization, KOH impregnation, KOH activation and the like. The phosphoric acid impregnation scheme is as follows: firstly, mixing mung bean sprout powder and analytically pure phosphoric acid according to the mass ratio of 0.8-2, adding 100ml of deionized water, stirring at room temperature for 1h for later use, and then placing the phosphoric acid-biomass mixture in a forced air drying oven to dry for 10h at the temperature of 80-140 ℃. Carbonizing: and (3) putting the dried sample into a tube furnace to be carbonized for 0.5-2h at the temperature of 450-600 ℃. Wherein the steps of pickling after carbonization/activation, KOH impregnation, KOH activation, drying and the like are the same as in example 1, wherein the impregnation ratio is selected from 1 3 PO 4 -KOH two-step activation method. Meanwhile, only phosphoric acid activation, not KOH activation, was performed for comparison.
In order to obtain the optimal preparation conditions of the bean sprout derived porous carbon, an orthogonal experiment is carried out by five experimental factors of carbonization temperature, carbonization time, phosphoric acid impregnation temperature, KOH impregnation ratio and carbonization time. Orthogonal experimental factors are shown in Table 1, and L (4) was designed according to the orthogonal experimental factors and levels shown in Table 1 5 ) There were 16 sets of experiments, the experimental arrangement is shown in table 2.
This example divides the above influencing factors into a phosphoric acid impregnation process and a "phosphoric acid-biomass polymer" thermal process according to the mechanisms of the two activation methods. The phosphoric acid impregnation process mainly comprises two influencing factors of phosphoric acid impregnation ratio and phosphoric acid impregnation temperature, and the thermal process mainly comprises three factors of carbonization temperature, KOH impregnation ratio and carbonization time.
TABLE 1 orthogonal test factors and levels
TABLE 2 orthogonal experimental design
Table 3 lists the response values of the specific surface area, pore volume, yield of the activated carbon sample at various factor levels. The properties of porous carbons under different preparation conditions vary widely.
The activated carbon samples prepared in experiment numbers 1-16 are sequentially marked as No.1-No.16. As can be seen from table 3, the bean sprout activated carbon prepared under most of the experimental conditions has a huge specific surface area: (>3200m 2 A pore volume of (a) and (b) is larger>1.7cm 3 g -1 ) The bean sprouts are an ideal biomass raw material for preparing the activated carbon with high specific surface area. The mung bean sprout-based activated carbon has good application potential in the aspects of adsorption, gas storage, super capacitor carbon electrode materials and the like.
TABLE 3 specific surface area and pore volume of activated carbon under different preparation conditions
Effect of phosphoric acid impregnation ratio
In fig. 1, as the phosphoric acid impregnation ratio increases, the specific surface area and the pore volume both show a tendency of increasing first and then decreasing, wherein the peak values of the specific surface area and the total pore volume both appear at the phosphoric acid impregnation ratio of 1.2, and a better hydrolysis effect is achieved while the original structure is not damaged when cell walls and other substances are hydrolyzed.
Influence of phosphoric acid impregnation temperature
The effect of the impregnation temperature on the impregnation process is mainly reflected in the promotion of the hydrolysis process, and in fig. 2, the specific surface area and pore volume of the sample are maximized at an impregnation temperature of 120 ℃. Within a certain range, the higher the impregnation temperature, the more severe the hydrolysis, but too high an impregnation temperature also causes an excessive hydrolysis problem, and therefore, the tendency of change is the same as the impregnation ratio, and both tend to increase first and then decrease.
Influence of carbonization temperature
In fig. 3, the specific surface area and the pore volume show a tendency of increasing first and then decreasing with an increase in the carbonization temperature within a certain range, and the specific surface area and the pore volume are maximum at 500 ℃. At 500 ℃, the cross-linking effect is best, the phosphate ester bond is not damaged too much due to over-temperature, and the shrinkage of the original structure of the plant is well prevented on the basis of creating a sufficient pore structure.
Influence of carbonization time
As can be seen from fig. 4, the extension of the carbonization time has little effect on the specific surface area and pore volume of the activated carbon, which are the greatest at a carbonization time of 2h. The extension of the carbonization time is beneficial to the full reaction of the phosphoric acid and the bean sprout biomass, so the carbonization time is selected to be 2h.
Influence of KOH impregnation ratio
In fig. 5, when the impregnation ratio is 4, the specific surface area and pore volume of the activated carbon are maximized. KOH has strong pore-forming capability, and compared with phosphoric acid activation, the KOH has more violent reaction and more micropores generated by activation. When the impregnation ratio is low, the KOH content is relatively low, the activated carbon is insufficiently activated, and the raw material is sufficiently activated as the impregnation ratio is increased, so that the specific surface area of the product is increased. When the impregnation ratio is more than 4, carbon atoms originally as a pore structure are reacted to cause collapse of pores, and the specific surface area and pore volume are decreased.
Compared with pure alkali activation, the introduction of phosphoric acid pre-activation generates initial pores, which is beneficial to the full contact of KOH and activated carbon, creates more micropores with higher accessibility on the basis of the original pores, avoids the problem of low accessibility of a single KOH pore-forming hole, reduces the carbonization temperature, shortens the carbonization time, and has a more reasonable pore structure.
The pore structure parameters for the preparation of activated carbon using the phosphoric acid process (HP) and the two-step activation process (HPK) using the preferred conditions described above are shown in Table 4 below:
TABLE 4 pore Structure parameters of phosphoric acid Process and two-step activation Process for the preparation of activated carbon
The total pore volume of HPK reaches 2.295cm 3 g -1 The specific surface area also reaches 3729.359m 2 g -1 The increase in specific surface area and pore volume is primarily attributed to the increase in the number of micropores, which is more evidenced by the increase in microporosity and the decrease in the average pore diameter of the micropores. As can be seen from FIG. 6, the micropores are also mainly distributed between 0.5 and 0.8nm, but the micropore part of the pore size distribution curve of HPK is generally on MBS-5, and the micropore volume rise is obvious. Meanwhile, the HP is mainly composed of micropores with the diameter less than 0.7nm and has little mesoporous content, which is found from a pore size distribution curve of the HP, and the fact that the micropores are mainly generated by phosphoric acid pre-activation under the condition is proved, after the phosphoric acid is subjected to pre-activation treatment, compared with a carbonized sample, the pore size distribution is more reasonable, in the subsequent alkali activation process, alkali is more fully contacted with activated carbon, and the use amount of KOH is reduced to a certain extent.
Micro-topography
As shown in FIGS. 7 (a-c), the HP surface is rough, the pores are mainly circular, and the pore size distribution is not uniform. Meanwhile, in fig. 7 (d-f), HP has an interconnected porous structure after KOH secondary activation, and compared with a sample only subjected to phosphoric acid activation treatment, the surface of the material has obvious and richer pores, and developed pore channels and hierarchical pore structures of the material can be clearly seen through the rich pores.
Microcrystalline structure
In fig. 8 (a), HPK has no sharp diffraction peaks at both diffraction angles of 23.5 ° and 44 °, demonstrating that it is mainly composed of amorphous carbon with poor crystallinity due to the generation of a large number of micropores during the KOH secondary activation, the presence of which prevents the parallel arrangement and bonding of crystallites, thereby preventing further growth of graphitic crystallites.
And HP has obvious diffraction peak at 25.8 degrees, compared with the standard 23.5 degree graphite diffraction peak, the characteristic diffraction peak has obvious right shift, according to the Bragg formula: 2d sin θ = λ, d 002 This indicates that the interlayer spacing in the activated carbon becomes small, and although having a certain degree of graphitization, the crystallinity is poor to be an amorphous structure. The graphite diffraction peak of HP in the (100) crystal plane of 2 θ =44 ° is weak, and carbon nanoflakes hardly exist. Phosphoric acid participates in the activation reaction of bean sprout biomass and also reacts with partial impurities in the bean sprouts, and the impurities are removed after acid washing, so that the uniformity of the activated carbon is contributed.
Raman spectra of the two materials are shown in FIG. 8 (b), and G peaks of HPK and HP are at 1346.99 and 1351.74cm, respectively -1 Nearby, and D peak is respectively distributed at 1598.22 and 1584.38cm -1 Nearby, I G /I D Values of 1.187 and 1.172, respectively, HPK has a higher I than HP G /I D Higher degree of order, less defects. The smaller full width at half maximum at the 2D peak of HP compared to HPK demonstrates the more lamellar graphitic structure of HP.
Surface functional group
TABLE 5 ratios of four elements in HP and HPK and carbon to oxygen atomic ratio
The carbon to oxygen atomic ratios of HP and HPK were 8.3 and 15.96, respectively. The higher carbon to oxygen atomic ratio of HPK can be attributed to the destruction of the functional groups containing oxygen and phosphorus on the surface of the HP by KOH activation, thereby increasing the carbon atom ratio.
At the same time, sp of the two materials 2 The proportion of C = C in each characteristic peak of C1s is the largest, and the high content of C = C is beneficial to promoting charge transfer and improving the conductivity of the material. In addition, the existence of hydrophilic functional groups such as C = O, O = C-O and the like in the HP sample and the HPK sample can effectively improve the wettability of the electrode material, increase the contactable area of ions and further improve the specific capacitance.
Constant current charge-discharge and multiplying power characteristics of activated carbon
In fig. 9 (a-b), the charging and discharging curves of HP and HPK are both in the shape of approximate triangle, which shows that the material is mainly based on the electric double layer capacitance, but the GCD curve of HP is slightly distorted, probably because the HP contains some functional groups containing nitrogen, oxygen and phosphorus, which can provide a part of pseudocapacitance for the material due to low-temperature activation and the use of phosphoric acid, whereas the GCD curve of HPK is more symmetrical, which proves that HPK contains less pseudocapacitance and higher electric double layer capacitance.
Meanwhile, in FIG. 9 (a), the charging and discharging time of HPK is significantly longer than that of HP, and it is proved that the specific capacitance of HP is lower at 0.5 Ag -1 Has a specific capacitance of only 83 Fg at a current density of (3) -1 And the HPK after the second KOH activation is 0.5A g -1 Specific time capacitance of 349 Fg -1 . It is noted that the current density is set to 0.1ag -1 Increased to 50 ag -1 The capacitance retention of HPK was 82.3%, respectively, in inverse HP, which was at 50 Ag -1 In time, the charging and discharging time is less than two seconds, the graph line is seriously deformed, and the charging and discharging voltage of the graph line is obviously beyond the range of the test voltage. This indicates that the current density at this time has far exceeded its rated current density and the capacitor has been in an unstable operating state. At a current density of 0.1ag -1 Increased to 20 ag -1 In this case, the capacitance retention of HP was 28%, and the attenuation was severe.
Cyclic voltammetric characteristics of activated carbon
As shown in fig. 10, when the cyclic voltammetry tests of HP and HPK at different scanning speeds are continued, it is found that the pseudocapacitance characteristic of HP is more obvious at each scanning speed, and the cyclic voltammetry curve of HP is more deformed at a large scanning speed, and the pseudocapacitance can be generated due to the relatively low carbonization activation temperature of the phosphoric acid activation method and the protection effect of phosphoric acid on carbon at a low temperature. The biomass raw material has certain nitrogen and oxygen elements, and can form some functional groups containing nitrogen and oxygen in the preparation process, and the functional groups are not easily damaged due to the protection effect of phosphoric acid at relatively low temperature. In addition, the use of phosphoric acid introduces some phosphorus-containing groups, and the phosphorus-containing functional groups also contribute to pseudocapacitance.
But the HPK can keep a better similar rectangular shape at each sweep speed, which shows that the specific capacitance of the KOH secondary activated porous carbon is mainly provided by an electric double layer capacitor, functional groups on the surface of the HP are damaged under the influence of high temperature and KOH in the secondary activation process, and the falling off of the functional groups containing nitrogen and phosphorus reduces the occupation ratio of pseudo capacitance, so that the EDLC performance of the material is good.
Impedance characteristics of activated carbon
For an ideal electric double layer capacitance, its Nyquist curve shows a curve with a slope close to 90 ° in the low frequency region. In the graph, the slope of the Nyquist curve of HPK is obviously larger than that of a sample prepared by only using phosphoric acid activation, the lower slope value of HP can be attributed to the pseudo-capacitance effect of a large number of functional groups on the surface of the sample, and the double-electric-layer capacitance characteristic of the material is effectively improved by secondary activation.
On one hand, some functional groups can be damaged under the combined action of high temperature and an activating agent, and the pseudocapacitance effect is weakened; on the other hand, the pore connectivity of the material effectively improved by KOH secondary activation is realized, and more pore channels are punched, so that the effective area for forming the double electric layers is greatly improved.
As shown in FIG. 11, while the ESR of HPK is 1.74 Ω, the equivalent series resistance is higher compared with MBS-5, and the carbon-oxygen atom of HPK is higher than MBS-5, it is known that the oxygen-containing functional group can effectively reduce the equivalent series resistance by improving the wettability of the material. The carbon-to-oxygen atomic ratio of HPK is 15.96 and larger than MBS-5, which means that the oxygen content is smaller, the wettability of HPK becomes poor, and the contact resistance becomes large.
Energy density and power density of activated carbon
The energy density and the power density at different current densities were calculated, and the results are shown in FIG. 12 when the power density was 50W kg -1 The energy density was 13.13Wh kg -1 (ii) a The power density is increased to 4658W kg -1 When the energy density is high, the energy density can still be maintained at 9.4Wh kg -1 . Therefore, the HPK electrode prepared by the method has good energy storage characteristics, and is a reliable and potential electrode material. And the energy density of HP is far lower than HPK due to lower specific capacitance and is 50W kg -1 When the energy density is 3.82Wh kg at most -1 。
In summary, in this embodiment, mung bean sprouts are used as raw materials and H is adopted 3 PO 4 The pore structure of the bean sprouts is optimized by a KOH two-step activation method, the influence of each influencing factor of a phosphoric acid impregnation process and a thermal process on the pore structure, the surface characteristic, the graphitization degree and the electrochemical performance of the activated carbon is researched by an orthogonal test, and the specific conclusion is as follows:
(1) The optimal preparation parameters are as follows: KOH alkali-carbon ratio (4. The specific surface area of the mung bean sprout activated carbon (named as HPK) prepared under the condition is as high as 3729.359m 2 g -1 Pore volume of 2.295cm 3 g -1 Wherein the pore diameter is mainly concentrated in 0.5-0.8nm and 2-4nm, and the number of HPK micropores is more. The specific capacitance of HPK in a two-electrode system (6M KOH electrolyte) was 349 Fg -1 (test Current 0.5 ag -1 ) The EDLC has good performance, and the current is 0.1A g -1 Increased to 50 ag -1 The capacity retention was 82.3%.
(2) The phosphoric acid activation method has simple preparation steps and lower activation temperature. The phosphoric acid has a certain protection effect on carbon bodies in the carbonization process, and elements such as nitrogen, oxygen and the like in the raw materials can be reserved in the form of functional groups. The use of phosphoric acid also introduces some phosphorus-containing functional groups, so that the activated product prepared by the method has obvious pseudocapacitance.
(3) Compared with phosphoric acid activation method and carbonization-KOH activation method, H 3 PO 4 The product of the KOH activation method has a better hierarchical structure, and the carbonization temperature is effectively reduced by introducing phosphoric acid in the early stage of activation, so that the carbonized product has rich mesopores and certain micropores, and further reaction pore-forming of KOH is facilitated. KOH can create developed pores and more abundant micropores on the basis of phosphoric acid activation. The phosphoric acid activation method and the KOH activation are reasonably combined, so that the use amount of KOH is effectively reduced, and the product has higher specific capacitance.
Example 4 preparation of Bean sprout-based porous carbon by synergistic activation and Nitrogen doping Process
In the experiment, melamine is selected as a nitrogen source, a mung bean sprout carbonized sample subjected to acid washing and drying is used as a raw material, and KOH is used as an activating agent to prepare the porous carbon material. Weighing 5g KOH and 1g pickled mung bean sprout powder carbonized sample each time, mixing with certain melamine, placing in a muffle furnace in atmosphere at 5 deg.C for 5 min -1 Heating to 350 deg.C at a heating rate for 0.5h, heating to 800 deg.C for 2h while using high-purity nitrogen as shielding gas at a gas flow rate of 0.6L min -1 . The obtained product is put into a magnetic stirring water bath kettle, and 0.1mol L of the product is firstly dripped -1 The hydrochloric acid is used until the solution is neutral, and then deionized water is used for repeatedly washing, and finally drying is carried out. This panel was labeled MBSN 1：X The lower standard number is the blending mass ratio of melamine and carbon. E.g. MBSN 1：7 Represents a sample prepared by mixing melamine and MBS-0 in a mass ratio of 1.
Effect of blending ratio on pore Structure and morphology of Material
Pore structure characterization
TABLE 6 MBSN 1:X Pore structure parameter of
The tendency of the pore structure as a function of the melamine blend ratio is shown in Table 6. In table 6, the pore volume and specific surface area of the micropores are substantially consistent with the variation tendency of the blend ratio, demonstrating that the specific surface area of the material is mainly contributed by the micropores. MBSN 1:x Specific surface of the sampleThe volume and micropore volume are both higher than MBS-5, which proves that the addition of melamine promotes the alkali activation process to generate more micropores. Wherein, MBSN 1:x The specific surface area and the total pore volume tend to be improved along with the blending ratio under the condition of lower blending ratio of melamine, which shows that melamine and KOH have a synergistic pore-forming effect obviously when the blending ratio is lower, wherein MBSN (melamine methyl cyanide) 1:5 And MBSN 1:7 Has the largest specific surface area and the largest total pore volume, and is 3558.184m 2 g -1 And 2.443cm 3 g -1 . When the blending ratio is greater than 1.
In fig. 13 (a), the nitrogen adsorption-desorption curves of all nitrogen-doped samples rise significantly in the low pressure region, and as the pressure continues to increase, the appearance of a hysteresis loop can be observed, and the tail of the curve rises slightly. As explained above, MBSN 1:X The method is mainly characterized in that micropores are used as main materials, and a certain amount of mesopores exist in four samples. At the same time, MBSN 1:7 The adsorption amount and the hysteresis loop area of the MBSN are both larger than other samples in the figure, and the MBSN is proved 1:7 Has larger pore volume and more mesoporous content. In FIG. 13 (b), micropores and mesopores increased in the range of 0.5 to 0.8nm, and MBSN increased in the range of 2 to 4nm for all the nitrogen-doped samples 1:7 The mesoporous distribution interval is the same as MBS-5, but the pore diameter distribution curve is higher than MBS-5, which proves that the MBSN is in the range of 2-4nm 1:7 More mesoporous structures are generated, and the more mesoporous number is beneficial to the transportation of electrolyte ions. This is similar to the MBSN shown in FIG. 13 (a) 1:7 The larger hysteresis loop area leads to a coincidence.
In summary, in the synergistic activation and nitrogen doping process, melamine mainly plays a role in promoting KOH activation at a low doping ratio, and the high doping ratio can intensify the activation process of KOH and simultaneously block the pore channels.
Characterization of microscopic morphology
In FIG. 14 (a), MBSN can be seen at low magnification 1:3 Having a plurality of broken particles on the surfaceThe particles and flocs are piled up, and the piled layer is obviously thicker and is not uniformly distributed. MBSN 1:3 The floc on the surface is derived from the high-temperature condensation process of melamine and is largely similar to alpha-C 3 N 4 The deposit of (2) covers the activated carbon surface.
In FIG. 14 (d), MBSN at low magnification 1:7 The tubular shape of the material is clearer, and due to the lower addition of melamine, the MBSN 1:7 No more obvious surface floc, and MBSN 1:3 Compared with the surface, the surface is smoother. The addition of melamine improves the intensity of the activation process, the parenchyma on the surface of the vascular structure is peeled off, the pores on the peeled parenchyma fragments are rich (in a frame in a picture 14 (e)), more tubular structures are exposed, the natural channels are beneficial to potassium vapor, melamine and some pyrolysis products entering the tube bundle to fully contact with the wall surface and react,
in FIG. 14 (f), MBSN is at 50k magnification 1：7 A deposit of melamine flocs on the surface can still be observed, but the effect of blocking the pores is less pronounced than in fig. 14 (c).
Influence of nitrogen doping on porous carbon Material composition
As shown in FIG. 15 (a), MBSN 1:X The wide peaks near 23.5 ° and 44 ° correspond to crystal face (002) and crystal face (100) of standard graphite respectively, and the weak diffraction intensities at two positions prove that the above samples have poor crystallinity, which proves that five samples in the figure mainly consist of amorphous carbon with low graphitization degree and do not have a single-layer graphite structure.
TABLE 7 MBSN 1:X I of (A) G /I D Value of
In Table 7, I of all nitrogen-doped samples G /I D The value is improved to a certain extent compared with the original value because of the MBSN 1:9 The addition amount of the melamine in the mixture is too small, so that the melamine I G /I D The value boosting is limited. As shown in FIG. 6-3 (b) and Table 6-2, the blending ratio increasedAdding, I G /I D The values show a tendency of increasing and then decreasing, which indicates that excessive melamine may have a retarding effect on graphitization when the blending amount exceeds the optimum value. In addition, when the optimal value is exceeded, non-conductive melamine polymer is generated in the material, and the improvement of the graphitization degree is also influenced.
This indicates that nitrogen doping can effectively improve defect structures in the carbon material. The 2D peak with low peak intensity indicates that few layers of graphite have few structures, consistent with the XRD conclusion.
Effect of Nitrogen doping on porous carbon Material composition
As shown in FIG. 16, MBSN 1:X Mainly consists of three elements of C, N and O, and the diffraction peak of N is more obvious along with the increase of the mixing ratio because of MBSN 1:9 The content of nitrogen in the product is less than 1%, and the analysis of nitrogen-containing functional groups is not carried out. In FIG. 6-5 (b), the high resolution N1s spectrum is decomposed into four peaks, 399.0eV (pyridine nitrogen N-6), 400.4-400.6 eV (pyrrole nitrogen N-5), 401.6eV (graphite nitrogen N-Q), and 402.3eV (pyridine nitrogen oxide N-X). The N-6 structure means that nitrogen atoms are in graphite sp 2 A pyridine-like nitrogen bound to two adjacent carbon atoms in the network. The N-5 structure refers to pyrrole-like nitrogen in which a nitrogen atom and a carbon atom are combined in a five-membered ring structure, and the irregular pentagonal structure of N-5 is a defect structure which is unstable. The N-Q structure means that nitrogen atoms are in graphite sp 2 And the graphite-like nitrogen is formed by hybridization and combination of three adjacent carbon atoms in the network. And the structure N-X means that the nitrogen atom is other than and sp 2 The carbon atoms in the network are bonded to oxygen atoms in addition to being bonded to oxygen atoms and have properties similar to oxygen-containing functional groups.
In Table 8, the content of N-5 in all samples is less than that of MBSN 1:7 This is because of the MBSN 1:7 The addition amount of the medium melamine is low, the doping effect is not obvious, and it is the defect that MBSN is caused 1:7 In (II) G /I D Lower value, sample MBSN 1:7 The abundance of N-Q is significantly higher than that of the other three samples. Compared with the structures of N-5 and N-6, N-Q is positioned in the carbon body, which shows that the carbon material is more stable, and the N-Q can synergistically improve the conductivity and the cycle performance of the carbon material, and the results show thatBenefit from the synergistic effect of activation and nitrogen doping, MBSN 1:7 The distribution of mesostructure nitrogen is relatively uniform, and nitrogen atoms are not only embedded into the edge of the carbon skeleton, but also firmly embedded into the carbon-based surface.
TABLE 8 MBSN 1:X Peak area ratio of each part of medium nitrogen
Electrochemical characterization
As shown in FIG. 17 (a), MBSN 1:7 The constant-current charging and discharging curve of the material shows an isosceles triangle shape and has good symmetry, which proves that the capacitance of the material is mainly formed by an electric double layer capacitor, in the graph (b) of FIG. 17, along with the increase of the scanning rate, the cyclic voltammetry characteristic curve still keeps a similar rectangular shape, when the scanning rate is increased to 200mV/s, the CV curve shows a fusiform shape, which proves that the MBSN curve shows 1:7 There is a small amount of pseudocapacitance due to the small amount of nitrogen-containing functional groups on the surface of the material.
In FIG. 17 (c), when the current is changed from 0.1ag -1 Increased to 50 ag -1 When, MBSN 1：7 Specific capacitance of 411Fg -1 Reduced to 306F g -1 The capacity retention is 74.45%, and the higher rate characteristic is due to interconnected channels brought by the increase of the internal surface area, high graphitization and a nitrogen-containing structure doped with a carbon skeleton.
In fig. 17 (d), in the low frequency region, the curves of the samples each appear as a straight line of approximately 90 °, which represents that the diffusion resistance is minimum and the electric double layer performance is good.
In FIG. 18, MBSN 1：7 At 5A g -1 As can be seen from fig. 18, the capacitance retention ratio after 5000 cycles at the current density slightly increases in the initial stage. This is because as the circulation proceeds, the temperature of the electrolyte increases, increasing the rate of ion transport. This in turn further promotes the faradaic reaction and hence a brief rise in specific capacitance. However, the pseudocapacitance provided by the nitrogen-containing functional groups is not stable. After a certain cycle, the surface functional groups are destroyed and the capacitance is provided only by the electric double layer capacitance. So that the specific capacitance drops again. The energy storage process of the double-layer capacitor only relates to a physical process and is more stable than the pseudocapacitor, so the capacitance retention rate of the material is kept stable after 1000 cycles. After 5000 cycles, the capacity retention rate of the material can still be 96%. Compared with 93.7 percent of MBS-5, the improvement is partial.
FIG. 19 shows MBSN 1:7 Energy density versus power density. The power density is 50W kg -1 The energy density was 14.36Wh kg -1 (ii) a The power density is increased to 12.47kW kg -1 The energy density was maintained at 11.44Wh kg -1 . In summary, the porous carbon electrode has excellent energy storage characteristics only by the synergistic effect of a proper pore structure and a reasonable graphitization degree. Thus, the MBSN prepared in this example 1:7 The electrode has good energy storage characteristics, and is a reliable and potential electrode material.
In MBSN 1:7 In the preparation process, KOH activation and nitrogen atom doping are carried out synchronously, with the rise of temperature, KOH continuously consumes carbon atoms, and then macropores and mesopores which are communicated with each other are gradually formed. Meanwhile, when the temperature exceeds 650 ℃, melamine is also decomposed into intermediate products such as cyano, amino, triazine ring and the like. In the continuous activation process, nitrogen atoms are embedded into the carbon skeleton by thermal conditions. When the temperature reaches 760 ℃, the liquid potassium metal changes to potassium vapor. At this time, the pore-forming reaction gradually changes from a liquid-solid reaction to a gas-solid reaction.
The potassium vapor continuously enters and escapes from the three-dimensional pore structure to generate new micropores, so that the internal surface area is further enlarged, and a sufficient reaction space is provided for the melamine. Meanwhile, some nitrogen-containing small molecules enter the newly formed micropores along with potassium vapor, so that nitrogen-containing groups enter deeper reaction spaces.
Therefore, it is considered that MBSN is synthesized 1:x Not only does potassium vapor act to form more micropores, but it also affects the uniformity of nitrogen atoms in the carbon skeleton. The synergistic preparation strategy can regulate and control the distribution of doped nitrogen atoms in a carbon skeleton, and promote the wettability and conductivity of the materialAnd (4) electrical property.
In summary, in the embodiment, the nitrogen-doped hierarchical porous carbon is prepared by using the mung bean sprouts as a carbon source and the melamine as a nitrogen source and using the KOH activation and simultaneous doping processes. The influence of the preparation process and the nitrogen source mixing ratio on the specific surface area, the pore structure, the electrochemical performance, the structure and the components of the material is explored. The specific research results are as follows:
(1) The selection of the precursor lays a good structural foundation for preparing the hierarchical porous structure and the ultrahigh specific surface area of the nitrogen-doped porous carbon. The melamine plays a certain synergistic effect in KOH pore-forming, and shows that the specific surface area and the micropore volume of the material can be effectively increased. Potassium simple substance generated by KOH during pore forming can effectively assist melamine to go deep into the carbon skeleton for doping, and potassium steam not only plays a role in forming more micropores, but also influences the uniformity of nitrogen atoms in the carbon skeleton.
(2) In the melamine mixing ratio of 1: and 9, the prepared porous carbon material has the optimal electrochemical performance when used as a symmetrical capacitor. At 0.1Ag -1 The specific capacitance of the capacitor can reach 411Fg under the current density -1 At 50A g -1 The specific capacitance of the capacitor can still reach 306 Fg under the current density -1 At 5 ag -1 After 5000 times of cyclic charge and discharge under the current density, the capacity retention rate can still reach 96 percent. The highest energy density can reach 14.36Wh kg -1 The output power density at this time was 50W kg -1 。
(3) The addition of melamine effectively improves the defect structure in the carbon material, but the improvement effect on the graphitization degree is not obvious, and the MBSN 1:7 In (II) G /I D The value was highest and the abundance of N-Q was significantly higher than the other three samples. Benefits from the synergistic effects of activation and nitrogen doping, MBSN 1:7 The distribution of mesostructure nitrogen is relatively uniform, and nitrogen atoms are not only embedded into the edge of the carbon skeleton, but also firmly embedded into the carbon-based surface.
In summary, the invention aims at the problem of poor uniformity of the plant-based activated carbon for the super capacitor, the plant raw materials are screened on the basis of the theory of botany, and finally, the derived plants represented by the mung bean sprouts are selected as the precursor of the activated carbon for the super capacitor, so that a series of related quality improvement and modification work is developed on the basis of two aspects of pore structure regulation and nitrogen atom doping. The main conclusions are as follows:
(1) The potential of the material is researched by selecting the plant species preferably, selecting a non-native mung bean sprout type 'derivative plant' (derived from other legumes) with a good structure and a rapid growth as a precursor, using KOH as an activating agent, and preparing activated carbon by a carbonization-alkali activation method, and comparing the activated carbon with activated carbon prepared by taking mung beans as a raw material.
As a result, it was found that, in the case of an alkali-carbon ratio of 5, the two materials each achieved the maximum specific surface area and pore volume, wherein the specific surface area and pore volume of MBS-5 were 3314.382m, respectively 2 g -1 And 2.2890cm 3 g -1 The active carbon material has pore size of 0.5-0.7nm and porosity of 50.74%, and is mainly microporous. The specific capacitance of (6M KOH electrolyte) MBS-5 in a two-electrode system is 295 Fg -1 (test Current 0.5A g -1 ) After 5000 times of charge-discharge tests, the retention rate of the capacitor reaches 93.7 percent, and the current is 0.1A g -1 Increased to 50 ag -1 The capacity retention was 78.8%.
Compared with mung beans, the mung bean sprout type 'derived plant' (derived from bean plants) mainly comprises transport tissues and parenchyma tissues, the vascular bundles of the transport tissues are accompanied by communicated air cavities with the diameter of a few micrometers, and gaps also exist among parenchyma cells, so that a better foundation is laid for the formation of a hierarchical porous structure. Because of the lack of a natural grading porous structure, the rationality of the pore structure and the electrochemical performance of the mung bean based activated carbon MBP-5 are inferior to those of MBS-5,
(2) The bean sprout raw material is respectively subjected to acid pickling pretreatment and ethanol solution extraction pretreatment from the aspects of metal removal and ethanol extraction content removal of plants, and ultrasound is introduced during the acid pickling pretreatment and the ethanol solution extraction pretreatment for carrying out synergistic enhanced heat and mass transfer.
Thermogravimetric analysis shows that the maximum decomposition rate of the biomass after the acid cleaning treatment is increased to 0.8%/DEG C from 0.6%/DEG C, the ethanol extraction promotes the maximum decomposition temperature to be increased to 340 ℃ from 310 ℃, the maximum decomposition rate is increased to 0.6%/DEG C from 0.54, and the introduction of the ultrasonic wave further enhances the pretreatment effect. The existence of ethanol extraction promotes the reaction activity among the three main components, and is beneficial to the decomposition of structural substances. After the acid washing promotes the removal of metal ions and the hydrolysis of cellulose and hemicellulose, the pore canals of the biomass are dredged and exposed, so that the heat transfer and the overflow of small-molecule volatile substances are facilitated, and the speed is obviously promoted.
After acid washing treatment, due to hydrolysis of substances such as hemicellulose and the like and leaching of other small molecular substances, the number of micropores of MBSA-50.55-0.7nm is remarkably increased, the microporosity is increased from 50.74% in MBS-5 to 58.6%, but the mesopore distribution is narrowed due to excessive micropores; meanwhile, the introduction of the ultrasonic wave increases the number of micropores with poor accessibility, such as less than 0.55nm, in the MBSAU-5, and thus the reduction of mesopores is aggravated.
After ethanol extraction, the dissolution of the content after ethanol extraction weakens the blocking effect of the content on the pore channel, the number of micropores and mesopores of the prepared MBSE-5 is improved, and the pore volume is increased to 2.489cm 3 g -1 . And the original appearance of the bean sprouts can be damaged when the mass transfer is promoted by the excessively high ultrasonic power, the average pore diameter of the finally prepared MBSEU-5 micropores is reduced to 0.5598nm from 0.6219nm of MBS-5, and the collapse of the mesopores is caused by excessive micropores. Wherein MBSE-5, MBSA-5, MBSAU-5 and MBSEU-5 are respectively 0.5 ag -1 At a current density of 357,303,286 and 262 Fg -1 . The metal ion content of the MBSAU-5 prepared by ultrasonic pickling meets GB/T37386-2019.
(3) Taking bean sprouts as raw materials and adopting H 3 PO 4 The KOH two-step activation method overcomes the defects of a single phosphoric acid method and KOH, the performance of the bean sprout-based activated carbon is further optimized, and the influence of an impregnation process and a thermal process on the phosphoric acid-bean sprout polymer compound is researched through an orthogonal test.
The optimal preparation process comprises the following steps: KOH alkali-carbon ratio (4. The specific surface area of HPK of the activated carbon prepared by the condition is as high as 3729.359m 2 g -1 Hole, holeThe volume is 2.295cm 3 g -1 Wherein the pore diameter is mainly concentrated in 0.5-0.8nm and 2-4nm, and the porous material has proper hierarchical pore structure distribution. The specific capacitance of HPK in a two-electrode system (6M KOH electrolyte) was 349 Fg -1 (test Current 0.5A g -1 ) The EDLC has good performance, and the current is 0.1A g -1 Increased to 50 ag -1 The capacity retention was 82.3%.
(4) The method comprises the steps of taking melamine as a nitrogen source and bean sprouts as a carbon source, and carrying out KOH activation-nitrogen doping synergistic modification on bean sprout activated carbon. Melamine plays a certain synergistic role in KOH pore-forming, and KOH activation is cooperated with nitrogen doping to prepare MBSN 1:7 Has a specific surface area of 3459.470m 2 g -1 Pore volume of 2.443cm 3 g -1 . Potassium simple substance generated by KOH during pore-forming can effectively assist melamine to go deep into the carbon skeleton for doping, and potassium steam not only plays a role in forming more micropores, but also influences the uniformity of nitrogen atoms in the carbon skeleton.
The addition of melamine effectively improves the defect structure in the carbon material, but the improvement effect on the graphitization degree is not obvious, and the MBSN 1:7 In (II) G /I D The value was highest and the abundance of N-Q was significantly higher than the other three samples. Benefits from the synergistic effects of activation and nitrogen doping, MBSN 1:7 The distribution of mesostructure nitrogen is relatively uniform, and nitrogen atoms are not only embedded into the edge of the carbon skeleton, but also firmly embedded into the carbon-based surface.
In a two-electrode system (6M KOH electrolyte), MBSN 1:7 Has a specific capacitance of 374 Fg -1 (test Current 0.5A g -1 ) The EDLC has good performance, and after 5000 times of charge and discharge tests, the capacitance retention rate reaches 96 percent, so that the EDLC has good cycle characteristics.
The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims (10)
1. The application of the derivative plants in preparing the porous carbon electrode material of the super capacitor;
the bean sprout is characterized in that the derivative plants are bean sprout plants, and the bean sprout plants comprise mung bean sprouts and soybean sprouts, and further comprise mung bean sprouts.
2. A preparation method of a derivative plant-based porous carbon material is characterized by comprising the steps of treating a derivative plant by a carbonization-KOH activation method;
further, the preparation method comprises the following steps:
s1, putting bean sprouts in an inert atmosphere for high-temperature pyrolysis;
s2, adding an activating agent into the product obtained in the step S1 for heating and dipping treatment;
s3, placing the product obtained in the step S2 in inert gas for secondary heating activation treatment;
and S4, carrying out acid washing, water washing and drying on the product obtained in the step S3 to obtain the catalyst.
3. The preparation method of claim 2, wherein in the step S1, the bean sprouts are mung bean sprout powder obtained through simple pretreatment, and the pretreatment method comprises: cleaning bean sprout, drying, pulverizing, and sieving;
specifically, the sieving mesh number is 80 meshes, and powder smaller than 178 mu m is obtained for later use;
the inert atmosphere is nitrogen, and the specific conditions of the high-temperature pyrolysis are as follows: pyrolyzing at 450-650 deg.C for 0.5-3h; the temperature rise rate is controlled to be 1-10 ℃ per minute -1 Preferably 5 ℃ min -1 (ii) a The nitrogen flow rate is 0.5-5 L.min -1 Preferably 1 L.min -1 。
4. The method according to claim 2, wherein in step S2, the mass ratio of the activating agent to the product is 1 to 10, and the activating agent is KOH;
the specific conditions of the heating and dipping treatment are as follows: soaking at 70-90 deg.C for 1-3 hr (preferably at 80 deg.C for 2 hr).
5. The method according to claim 2, wherein in step S3, the specific conditions of the secondary temperature-raising activation treatment include: heating to 300-400 deg.C (preferably 350 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C), maintaining the temperature for 10-50min (preferably 30 min), heating to 750-850 deg.C (preferably 800 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C), maintaining the temperature for 1-3h (preferably 2 h), and maintaining the temperature with nitrogen as shielding gas at a flow rate of 0.5-5 L.min -1 Preferably 1 L.min -1 。
6. The method according to claim 2, wherein in the step S4, the acid is hydrochloric acid, preferably 1.0M hydrochloric acid, and the washing step is repeated rinsing with water until the pH of the supernatant is neutral, and the drying step is performed at 100-110 ℃, preferably 105 ℃ for 1-48h, preferably 24h, preferably by drying.
7. The preparation method of claim 2, wherein the post-treatment removal of the bean sprouts comprises removing non-structural components of the bean sprouts and removing impurities from the bean sprout biomass by acid washing; the method for removing the non-structural components of the bean sprouts adopts ethanol or ultrasonic-assisted ethanol reflux to treat the biomass raw material, wherein the reflux operation conditions comprise: refluxing bean sprout and ethanol solution at 70-90 deg.C for 0.5-2 hr, cooling to remove supernatant, and repeating for 2-3 times; preferably at 80 ℃ for 1h; the ethanol can be 60-80% (preferably 70%) by mass of ethanol solution, and the mass volume ratio of the bean sprouts to the ethanol solution is 1-5;
further, ultrasonic crushing treatment is added in the ultrasonic-assisted ethanol reflux treatment process, and the ultrasonic power is controlled to be 100-500W, preferably 300W;
acid washing and impurity removal of bean sprout biomass are used for deeply removing metal ions in bean sprouts; specifically, the step of acid washing and impurity removing comprises the step of placing the dried bean sprouts into a hydrochloric acid solution, and the acid washing is specificThe conditions are as follows: treating at 60-80 deg.C for 1-3 hr, preferably at 70 deg.C for 2 hr, with hydrochloric acid concentration of 0.1-1mol L -1 Preferably 0.5mol L -1 The mass volume ratio of the bean sprouts to the hydrochloric acid solution is 1-5 (g/ml) to 10-80 (g/ml), preferably 2; furthermore, ultrasonic treatment is introduced in the acid washing process, wherein in the ultrasonic acid washing process, the treatment time is 0.5-2h, preferably 1h, and the ultrasonic power is 100-150W, preferably 120W;
the pretreatment removal of the postimpurities in the bean sprouts is performed before the step S1, or the bean sprouts are subjected to simple pretreatment to obtain bean sprout powder, and the postimpurities in the bean sprout powder are subjected to pretreatment removal.
8. The process according to claim 2, wherein H is used 3 PO 4 -a two-step KOH activation process, in particular, the preparation process comprises, before step S1, a phosphoric acid impregnation, the phosphoric acid impregnation process comprising: mixing bean sprouts and phosphoric acid, adding the mixture, stirring, and drying for later use;
the specific phosphoric acid impregnation method comprises the following steps: mixing mung beans and phosphoric acid according to a mass ratio of 0.8-2, adding the mixture into water, controlling the dipping temperature of the phosphoric acid to be 80-140 ℃, preferably 120 ℃, and drying (drying for 8-12h, preferably 10h at 80-140 ℃) for later use;
the preparation method also comprises the step of mixing and calcining the porous carbon material prepared in the step S4 and melamine to obtain a nitrogen-doped activated carbon material;
specifically, the preparation method comprises the following steps: mixing melamine, KOH and pickled bean sprout powder carbonized sample, heating to 300-400 deg.C (preferably 350 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C), maintaining the temperature for 10-50min (preferably 30 min), heating to 750-850 deg.C (preferably 800 deg.C) at a heating rate of 1-10 deg.C (preferably 5 deg.C), maintaining the temperature for 1-3h (preferably 2 h), and maintaining the temperature for 0.1-1 L.min with nitrogen as shielding gas at a flow rate of 0.1-1 L.min -1 Preferably 0.6 L.min -1 Then carrying out acid washing, water washing and drying to obtain the product;
wherein the mass ratio of the melamine to the KOH and the carbonized bean sprout powder after acid washing is 1-50; further preferably 1.
9. A porous carbon electrode, characterized in that the porous carbon electrode comprises a derivatized plant-based porous carbon material obtained by the preparation method according to any one of claims 2 to 8.
10. A supercapacitor, characterized in that the supercapacitor comprises a derived plant-based porous carbon material obtained by the preparation method according to any one of claims 2 to 8 or a porous carbon electrode according to claim 9.
Priority Applications (1)
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