CN113998697B - Preparation method of leaf-based nitrogen-doped porous carbon and application of leaf-based nitrogen-doped porous carbon in oxygen reduction electrocatalysis in full pH range - Google Patents
Preparation method of leaf-based nitrogen-doped porous carbon and application of leaf-based nitrogen-doped porous carbon in oxygen reduction electrocatalysis in full pH range Download PDFInfo
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Abstract
The invention discloses a preparation method of leaf-based nitrogen-doped porous carbon and application of the leaf-based nitrogen-doped porous carbon in oxygen reduction electrocatalysis in a full pH range. Cleaning and drying the leaf-based biomass, and grinding; weighing a certain amount of biomass powder material, soaking the biomass powder material in an acetic acid-sodium acetate solution containing hemicellulase, standing at constant temperature for a period of time for hydrolysis, filtering, washing, drying, pre-carbonizing in an inert atmosphere, cooling to room temperature, grinding into powder, weighing with a nitrogen source and an activating agent according to a mass ratio, performing high-energy vacuum mechanical force ball milling, carbonizing a ball-milling mixture at high temperature in the inert atmosphere, pickling, filtering, washing until filtrate is neutral, and drying to obtain the nitrogen-doped porous carbon material. The invention utilizes an enzyme co-high-energy vacuum mechanical force ball milling method to carry out modification treatment on biomass, and the prepared nitrogen-doped porous carbon material has large specific surface area, obvious hierarchical pore structure and rich nitrogen content, and has excellent ORR electrocatalytic activity in a full pH range (0-13).
Description
Technical Field
The invention belongs to the field of inorganic nano carbon materials and electrocatalysis, relates to nitrogen-doped porous carbon, and in particular relates to a preparation method of leaf-based nitrogen-doped porous carbon and application of leaf-based nitrogen-doped porous carbon in oxygen reduction electrocatalysis in a full pH range.
Background
Energy is the material basis for human survival and development. Today's society has an increasing demand for energy. Therefore, the development of new efficient, renewable clean energy sources is the focus of current research. The fuel cell is environment-friendly and efficient, is different from the traditional non-renewable energy source, does not need to burn in the material/energy circulation process, does not generate toxic and harmful gas, can directly convert chemical energy of anode fuel into electric energy under relatively mild conditions, and has high energy conversion efficiency.
The oxygen reduction reaction (ORR, oxygen reduction reaction) is not only the core reaction of fuel cells, but is also critical in other energy conversion and storage technologies such as metal-air cells. ORR is a complex, multiple electron gain and loss reaction. Thus, the choice of electrocatalyst is particularly important for the kinetically retarded ORR. Noble metals such as platinum (Pt) and rhodium (Rh) have better electrocatalytic activity on ORR, but have the defects of lack of noble metal reserves, high price and poor stability, and are extremely easy to be polluted by trace CO and H in the use process 2 S, etc., which have hindered the progress of fuel cell technology development and commercialization. Therefore, the novel high-efficiency non-noble metal ORR electrocatalyst with excellent research performance and good stability has profound significance.
The carbon material has excellent physicochemical properties, and can be used as an electrode material of an energy conversion and storage device. The biomass-based carbon material has wide sources and contains rich inorganic elements and special pore canal structures, so that the biomass can be used as a high-quality carbon precursor. Modification of carbon materials with heteroatoms, such as sulfur (S), boron (B), phosphorus (P), and nitrogen (N), can increase the wettability and conductivity of the surface of the carbon material, increasing the number of reactive sites, and thereby improving the electronic and chemical properties of the carbon material and its internal structure. Since N and C are in the same period in the periodic table of elements and have close radii and many similar physical properties, the nitrogen-doped carbon material exhibits optimal ORR electrocatalytic activity.
Currently, most of the successfully prepared biomass-based carbon materials have better ORR activity in alkaline electrolytes and are plagued in neutral and acidic. For practical application considerations to fuel cells (the electrolyte of a biofuel cell needs to be near neutral; proton exchange membrane fuel cells typically use an acidic electrolyte), it is important to develop a high performance electrocatalyst suitable for the full pH range.
Disclosure of Invention
The invention aims at providing a preparation method of leaf-based nitrogen-doped porous carbon, which is low in cost, and the prepared porous carbon is excellent in electrochemical performance and good in stability.
The second purpose of the invention is to provide the application of the leaf-based nitrogen-doped porous carbon prepared by the preparation method in oxygen reduction electrocatalysis in the full pH range.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
in one aspect, the invention provides a method for preparing leaf-based nitrogen-doped porous carbon, comprising the following steps:
step one: cleaning, drying and grinding the leaf-based biomass to obtain a biomass powder material; weighing a certain amount of biomass powder material, soaking the biomass powder material in an acetic acid-sodium acetate buffer solution containing hemicellulase, standing for a period of time at constant temperature for enzymolysis, filtering, washing with water, and drying to obtain a material A;
step two: pre-carbonizing the material A in an inert gas atmosphere, cooling to room temperature, and grinding into powder to obtain a material B;
step three: respectively weighing a material B, a nitrogen source, potassium bicarbonate serving as an activating agent and potassium hydroxide according to a mass ratio, placing the materials in a vacuum ball milling tank, and performing vacuum high-energy mechanical ball milling after vacuumizing in the tank; carbonizing the ball-milling mixture at high temperature in an inert gas atmosphere to obtain a material C;
step four: and (3) acid washing the material C, washing the material C with water until the filtrate is neutral, and drying to obtain the nitrogen-doped porous carbon material.
Preferably, in step one, the leaf-based biomass is cypress leaves.
Preferably, the enzyme activity of the hemicellulase in the first step is 100-500U/g; the pH value of the buffer solution acetic acid-sodium acetate is 4.0-6.0; the enzymolysis temperature is 40-70 ℃ and the enzymolysis time is 12-48 h.
Preferably, the pre-carbonization heating rate in the second step is 5 ℃/min, the temperature is maintained for 2 hours after the temperature is raised to 400 ℃, then the temperature is reduced to 300 ℃ at 5 ℃/min, and the temperature is cooled to the room temperature.
Preferably, the high temperature carbonization process in the third step is: heating to 300 ℃ at a speed of 5 ℃/min for 2 hours, then heating to 900 ℃ at a speed of 5 ℃/min for 2 hours, then cooling to 400 ℃ at a speed of 5 ℃/min, and naturally cooling to room temperature.
Preferably, the nitrogen source in the third step is one of melamine, dicyandiamide and urea.
Preferably, material B is mixed with a nitrogen source, potassium bicarbonate (KHCO 3 ) And potassium hydroxide (KOH) at a mass ratio of 1:3:3:1.
Preferably, the conditions for vacuum high energy mechanical force ball milling are: the rotation speed was 300rpm for 30min, wherein 15min was clockwise and 15min was counterclockwise.
Preferably, the acid washing in the fourth step is carried out by using a 1mol/L dilute hydrochloric acid solution at 60℃and 300rpm for 3 hours.
The invention also provides application of the nitrogen-doped porous carbon material prepared by the method in oxygen reduction electrocatalysis in a full pH range.
The nitrogen-doped porous carbon material can be used for oxygen reduction electrocatalytic reactions under alkaline, neutral and acidic conditions. The method comprises the following steps: 3mg of the obtained nitrogen-doped porous carbon-based material was weighed in a centrifuge tube, 170. Mu.L of deionized water, 70. Mu.L of isopropyl alcohol and 10. Mu.L of 5% perfluorosulfonic acid-polytetrafluoroethylene copolymer (Nafion) solution were then added, and an even slurry was formed by ultrasonic treatment for 60 minutes, and 8. Mu.L of the mixed slurry was removed and dispersed on the surface of a polished Rotating Disk Electrode (RDE), and dried at room temperature to obtain a working electrode. The prepared working electrode, a 3.5mol/L silver/silver chloride reference electrode and a platinum wire counter electrode form a three-electrode system, and the three-electrode system is characterized by alkalinity (0.1 mol/L KOH, pH=13), neutrality (0.1 mol/L PBS, pH=7) and acidity (0.5 mol/LH) 2 SO 4 Ph=0) electrolyte, the test was performed with an electrochemical workstation.
Compared with the prior art, the invention has the following beneficial effects:
(1) The carbon precursor used for preparing the nitrogen-doped porous carbon material is cypress branches and leaves with wide distribution, low cost and rich hemicellulose content;
(2) The invention utilizes novel and green feasible enzyme to carry out modification treatment on biomass by combining with a high-energy vacuum mechanical force ball milling method, the method not only can regulate and control the surface pore structure of cypress branches and leaves, but also can effectively improve the doping efficiency of nitrogen atoms in a carbon framework, and the finally prepared nitrogen-doped porous carbon material has large specific surface area, obvious hierarchical pore structure and rich nitrogen content, and promotes mass transfer in the oxygen reduction reaction process under the full pH range;
(3) The invention successfully synthesizes the biomass-based nitrogen-doped carbon material with excellent ORR electrocatalytic activity in the full pH range (0-13), not only effectively advances the commercialization process of the fuel cell, but also has simple and feasible experimental scheme, and accords with the main points of cleanness, environmental protection and resource recycling;
(4) The preparation method for regulating and controlling the pore structure of the biomass-based nitrogen-doped carbon material provided by the invention has the advantages of simplicity, high innovation, great exploration and development space, and the prepared nitrogen-doped porous carbon material has high catalytic activity, and the oxygen reduction catalytic performance in alkaline, neutral and acid electrolytes is obviously superior to that of other carbon-based materials of the same class. In a 0.1mol/L KOH solution, the initial potential is approximately 1.01V (relative to the reversible hydrogen electrode RHE), the half-wave potential is approximately 0.85 (relative to the RHE), and the limiting current density is approximately-5.48 mA/cm 2 The method comprises the steps of carrying out a first treatment on the surface of the In a 0.1mol/L PBS solution, the initial potential is approximately 0.91 (relative to RHE), the half-wave potential is approximately 0.73 (relative to RHE), and the limiting current density is approximately-5.55 mA/cm 2 The method comprises the steps of carrying out a first treatment on the surface of the At 0.5mol/L H 2 SO 4 In the solution, the initial potential is approximately 0.88 (relative to RHE), the half-wave potential is approximately 0.73 (relative to RHE), and the limiting current density is approximately-5.62 mA/cm 2 . The ORR performance of the material under alkaline, neutral and acidic conditions was comparable to commercial 20% pt/C. In addition, the material also has excellent methanol tolerance and long-term stability, and has wide application prospect.
Drawings
FIG. 1 is a Transmission Electron Microscope (TEM) image of the nitrogen-doped porous carbon material prepared in example 1;
FIG. 2 is a High Resolution Transmission Electron Micrograph (HRTEM) of the nitrogen-doped porous carbon material prepared in example 1;
FIG. 3 is a nitrogen isothermal (77K) adsorption/desorption curve of the nitrogen-doped porous carbon materials prepared in example 1 and comparative examples 1-3;
FIG. 4 is a graph showing pore size distribution curves calculated based on a Density Functional Theory (DFT) model for the nitrogen-doped porous carbon materials prepared in example 1 and comparative examples 1-3;
FIG. 5 is an N-peak spectrum of X-ray photoelectron spectroscopy (XPS) of the nitrogen-doped porous carbon material prepared in example 1;
FIG. 6 is a Linear Sweep Voltammogram (LSV) at 1600rpm in an oxygen saturated 0.1mol/L KOH solution of 20% Pt/C for the nitrogen-doped porous carbon materials prepared in example 1, comparative examples 1-3 and comparative example 4;
FIG. 7 is a Linear Sweep Voltammogram (LSV) at 1600rpm in oxygen saturated 0.1mol/L PBS solution of the nitrogen doped porous carbon material prepared in example 1, comparative examples 1-3, and 20% Pt/C in comparative example 4;
FIG. 8 shows 0.5mol/L H of the nitrogen-doped porous carbon materials prepared in example 1, comparative examples 1-3 and 20% Pt/C in comparative example 4 saturated with oxygen 2 SO 4 Linear Sweep Voltammogram (LSV) at 1600rpm in solution;
FIG. 9 is a methanol tolerance test of the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 in an oxygen saturated 0.1mol/L KOH solution;
FIG. 10 is a methanol tolerance test of the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 in oxygen saturated 0.1mol/L PBS solution;
FIG. 11 shows the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 at 0.5mol/L H saturated with oxygen 2 SO 4 Methanol tolerance test in solution;
FIG. 12 is a stability test of the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 in an oxygen saturated 0.1mol/L KOH solution;
FIG. 13 is a stability test of the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 in an oxygen saturated 0.1mol/L PBS solution;
FIG. 14 shows the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 at 0.5mol/L H saturated with oxygen 2 SO 4 Stability test in solution.
Detailed Description
The invention will be described in further detail with reference to the drawings and the specific examples.
Example 1
Fully cleaning collected cypress leaves with deionized water, drying at 80 ℃, and slightly grinding with a mortar. 3.0g of the treated cypress leaves are weighed, soaked in acetic acid-sodium acetate buffer solution (pH=4.8) containing hemicellulase (enzyme activity 200U/g), then placed in a constant temperature water bath at 50 ℃ for 24 hours, after enzymolysis is completed, the samples are filtered, washed to be neutral, and dried at 80 ℃. Putting the dried sample in N 2 Heating to 400 ℃ at a speed of 5 ℃/min under the atmosphere, carbonizing for 2 hours, cooling to 300 ℃ at a speed of 5 ℃/min, and cooling to room temperature to obtain the biochar. Grinding the carbonized biochar into powder by an agate mortar for 10min, and bagging for later use.
0.5g of biochar, 1.5g of melamine, 0.5g of potassium hydroxide and 1.5g of potassium bicarbonate are weighed and placed in a vacuum ball milling tank, the tank is vacuumized for 10min to enable the tank to be in a vacuum state, and then high-energy vacuum mechanical force ball milling is carried out for 30min at 300rpm, wherein the time is 15min clockwise and 15min anticlockwise. The ball-milled mixture was placed in a tube furnace at N 2 Heating to 300 ℃ at a speed of 5 ℃/min under protection for 2 hours, heating to 900 ℃ at a speed of 5 ℃/min for 2 hours, cooling to 400 ℃ at a speed of 5 ℃/min, and naturally cooling to room temperature to obtain the nitrogen-doped porous carbon material which is not pickled. And (3) pickling the material with 1mol/L dilute hydrochloric acid solution at 60 ℃ and 300rpm for 2 hours, filtering, washing with water until the filtrate is neutral, and drying to obtain the nitrogen-doped porous carbon material.
3mg of the nitrogen-doped porous carbon material of example 1 was weighed into a centrifuge tube and removed by pipetting gun 170Mu L of deionized water, 70 mu L of isopropanol solution and 10 mu L of 5% perfluorosulfonic acid-polytetrafluoroethylene copolymer (Nafion) solution are subjected to ultrasonic treatment for 60min to form uniform dispersion, 8 mu L of mixed slurry is sucked by a pipette and dripped on the surface of a polished Rotary Disk Electrode (RDE), and the mixture is naturally dried at room temperature to obtain the working electrode. The prepared working electrode, a 3.5mol/L silver/silver chloride reference electrode and a platinum wire counter electrode form a three-electrode system, and the three-electrode system is characterized by alkalinity (0.1 mol/L KOH, pH=13), neutrality (0.1 mol/L PBS, pH=7) and acidity (0.5 mol/LH) 2 SO 4 Ph=0) electrolyte, the test was performed with an electrochemical workstation. At O 2 Or N 2 The saturated electrolyte was scanned for cyclic voltammograms and linear voltammograms, respectively, at a scan rate of 5mV/s and 10mV/s, respectively.
Comparative example 1
Unlike example 1, the following is: the treated cypress leaves were immersed in an acetic acid-sodium acetate buffer solution (ph=4.8) containing cellulase (enzyme activity 200U/g).
The method for manufacturing a working electrode using the nitrogen-doped porous carbon material prepared in comparative example 1 as an oxygen reduction electrocatalyst was the same as that of example 1.
Comparative example 2
Unlike example 1, the following is: the treated cypress leaves were immersed in an acetic acid-sodium acetate buffer solution (ph=4.8).
The method for manufacturing a working electrode using the nitrogen-doped porous carbon material prepared in comparative example 2 as an oxygen reduction electrocatalyst was the same as that of example 1.
Comparative example 3
Unlike example 1, the following is: 0.5g of charcoal and 1.5g of melamine, 0.5g of potassium hydroxide and 1.5g of potassium bicarbonate were weighed into an agate mortar and manually ground for 30min (15 min clockwise, 15min anticlockwise).
The method for manufacturing a working electrode using the nitrogen-doped porous carbon material prepared in comparative example 3 as an oxygen reduction electrocatalyst was the same as that of example 1.
Comparative example 4
1mg of 20% Pt/C was weighed into a centrifuge tube and removedAnd (3) taking 170 mu L of deionized water, 70 mu L of isopropanol solution and 10 mu L of 5% perfluorosulfonic acid-polytetrafluoroethylene copolymer (Nafion) solution by a liquid gun, performing ultrasonic treatment for 60min to form uniform dispersion, sucking 8 mu L of mixed slurry drop on the surface of a polished Rotary Disk Electrode (RDE) by the liquid gun, and naturally airing at room temperature to obtain the working electrode. The prepared working electrode, a 3.5mol/L silver/silver chloride reference electrode and a platinum wire counter electrode form a three-electrode system, and the three-electrode system is characterized by alkalinity (0.1 mol/LKOH, pH=13), neutrality (0.1 mol/L PBS, pH=7) and acidity (0.5 mol/L H) 2 SO 4 Ph=0) electrolyte, the test was performed with an electrochemical workstation. At O 2 Or N 2 The saturated electrolyte was scanned for cyclic voltammograms and linear voltammograms, respectively, at a scan rate of 5mV/s and 10mV/s, respectively.
(1) Transmission Electron Microscope (TEM) and High Resolution Transmission Electron Microscope (HRTEM) testing
The prepared nitrogen-doped porous carbon material (example 1) was observed under a transmission electron microscope (fig. 1) and a high resolution transmission electron microscope (fig. 2). The biomass cell wall mainly comprises cellulose, hemicellulose and lignin, which are tightly combined to form a natural three-dimensional reticular fiber structure. Wherein cellulose and lignin act as a backbone, hemicellulose acts as a filler, hemicellulose is a heteropolymer composed of several different types of monosaccharides (e.g. pentasaccharide and hexasaccharide), accounting for about 35% of biomass. Hemicellulose has loose structure, low polymerization degree and higher water solubility, and is easy to hydrolyze when contacted with hemicellulase. The cypress leaves are treated by the hemicellulase and high-energy vacuum mechanical force ball milling method, so that the surface pore size distribution of the cypress leaves can be regulated and controlled, and the effective doping rate of nitrogen atoms can be promoted. As shown in fig. 1, the carbon material prepared by the hemicellulase in combination with the high-energy vacuum mechanical force ball milling method is in an irregular, light and thin gauze-like form, and the surface of the carbon material contains rich nano-pore structures (micropores/mesopores), which is helpful for increasing the specific surface area and disorder degree of the carbon-based material. As shown in fig. 2, the nitrogen-doped carbon material not only has higher surface roughness and rich layered porous structure, but also contains obvious lattice fringes, which indicates that the carbon framework material has higher graphitization degree. The abundant pore structure and high graphitization degree are beneficial to the mass transfer and electron conduction of the nitrogen-doped carbon material, which is important for the ORR electrocatalytic activity of the carbon material.
(2) Nitrogen isothermal adsorption and desorption test
Specific surface area and pore structure tests were performed on example 1, comparative example 2 and comparative example 3, respectively, using a nitrogen physical adsorption and desorption instrument. As shown in FIG. 3, each of example 1, comparative example 2 and comparative example 3 has a microporous and mesoporous structure, and according to analysis, the corresponding specific surface areas thereof were 1616.77m, respectively 2 /g、1423.44m 2 /g、1301.29m 2 /g and 1083.96m 2 /g; the total pore volume is 1.12cm respectively 3 /g、1.11cm 3 /g、0.91cm 3 Per g and 0.88cm 3 And/g. In combination with the pore size distribution curve calculated based on the DFT model (fig. 4), the enzyme does have an effect on the pore structure of the carbon-based material in conjunction with the vacuum mechanical force ball milling method. The micropore specific surface areas of example 1, comparative example 2 and comparative example 3 were 1015.70m, respectively, calculated based on the DFT model 2 /g、877.29m 2 /g、798.05m 2 /g and 576.62m 2 Per gram, the micropore volume is 0.40cm respectively 3 /g、0.35cm 3 /g、0.31cm 3 Per g and 0.23cm 3 Per g, the corresponding specific surface areas of the mesopores are 491.34m respectively 2 /g、439.78m 2 /g、410.05m 2 /g and 418.80m 2 Per g, mesoporous volume of 0.49cm 3 /g、0.46cm 3 /g、0.41cm 3 Per g and 0.47cm 3 And/g. From this, the pore structure of example 1 (hemicellulase) and comparative example 1 (cellulase) are both richer than comparative example 2, which did not use enzyme-assisted treatment, demonstrating that the use of enzyme-assisted treatment can indeed have an impact on biomass pore distribution. The larger specific surface area of example 1 than example 2 further demonstrates that the selection of the appropriate enzyme species is more conducive to pore structure construction, thereby enhancing the ORR performance of the carbon material. In addition, example 1 (high energy vacuum mechanical force ball milling) has a larger specific surface area and pore volume than comparative example 3 (manual milling), since the high energy vacuum mechanical force ball milling method allows the activated pore formers to be more uniformly doped with the carbon-based material in the later stageWhen carbonizing at high temperature, the pore-forming effect is better, which is beneficial to improving the specific surface area of the carbon-based material and increasing the mass transfer efficiency in ORR.
(3) X-ray photoelectron spectroscopy (XPS) test
X-ray photoelectron spectroscopy (XPS) test was performed on example 1, comparative example 2 and comparative example 3. The results show that the atomic percentages of total nitrogen content of example 1, comparative example 2 and comparative example 3 are 4.13%, 3.54%, 2% and 2.52%, respectively, which indicates that the nitrogen atoms have been successfully doped into the carbon framework, and that the nitrogen content of example 1 (hemicellulases in conjunction with vacuum mechanical force ball milling) is highest, indicating that its ORR electrocatalytic activity is better than other materials. The nitrogen content of example 1 is richer than that of comparative example 3, which shows that the high-energy vacuum mechanical force ball milling method is used for treating the biomass to enable the nitrogen source to be doped more uniformly, and the doping efficiency of nitrogen atoms is improved. FIG. 5 is a narrow sweep peak-splitting curve of nitrogen peaks for example 1, which can be largely divided into pyridine nitrogen, pyrrole nitrogen, graphite nitrogen and nitric oxide. Of these, pyridine nitrogen and graphite nitrogen play a major role in ORR electrocatalytic activity. According to the analysis, the total relative amounts of pyridine nitrogen and graphite nitrogen of example 1, comparative example 2 and comparative example 3 were 61.62%, 51.91%, 46.91% and 44.54%, respectively, and example 1 was not only the highest in total nitrogen content, but also the ORR critical active site content.
(4) Oxygen reduction electrocatalysis performance test
Working electrodes were prepared in examples 1, comparative example 2, comparative example 3 and comparative example 4, respectively, and a three-electrode system was formed with a silver/silver chloride reference electrode and a platinum wire counter electrode, in alkaline (0.1 mol/L KOH), neutral (0.1 mol/L PBS) and acidic (0.5 mol/L H) 2 SO 4 ) A Linear Sweep Voltammogram (LSV) test was performed in the electrolyte at a sweep rate of 10mV/s. FIGS. 6, 7 and 8 are linear sweep voltammograms at 1600rpm in alkaline, neutral and acidic electrolytes, respectively.
Under alkaline conditions, the initial potentials of example 1, comparative example 2, comparative example 3 and comparative example 4 were 1.01V, 0.99V, 1.00V and 1.00V (relative to the reversible hydrogen electrode RHE), respectively, and the half-wave potentials were 0.85V, 0.84V and 0, respectively.80V (relative to RHE), limiting current densities of-5.48 mA/cm, respectively 2 、-5.11mA/cm 2 、-4.50mA/cm 2 、-4.94mA/cm 2 And-5.30 mA/cm 2 The method comprises the steps of carrying out a first treatment on the surface of the Under neutral conditions, the initial potentials of example 1, comparative example 2, comparative example 3 and comparative example 4 were 0.91V, 0.89V, 0.87V and 0.94V (with respect to the reversible hydrogen electrode RHE), respectively, the half-wave potentials were 0.73V, 0.63V, 0.65V, 0.62V and 0.65V (with respect to the RHE), respectively, and the limiting current densities were-5.55 mA/cm, respectively 2 、-5.25mA/cm 2 、-5.02mA/cm 2 、-4.80mA/cm 2 And-5.38 mA/cm 2 The method comprises the steps of carrying out a first treatment on the surface of the Under acidic conditions, the initial potentials of example 1, comparative example 2, comparative example 3 and comparative example 4 were 0.88V, 0.83V and 0.88V (with respect to the reversible hydrogen electrode RHE), respectively, the half-wave potentials were 0.73V, 0.71V, 0.66V and 0.62V (with respect to the RHE), respectively, and the limiting current densities were-5.62 mA/cm, respectively 2 、-5.07mA/cm 2 、-4.60mA/cm 2 、-4.75mA/cm 2 And-5.30 mA/cm 2 (comparative example 4:0.2v vs RHE). The ORR electrocatalytic activity of example 1 is comparable to, and even better than, comparative example 4, regardless of the alkaline, neutral and acidic electrolytes.
In addition, in order to meet the practical application requirements of the oxygen reduction electrocatalyst, the methanol resistance test and the long-term stability test were performed on example 1 and comparative example 4. FIGS. 9-11 are methanol tolerance tests of the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 in oxygen-saturated alkaline, neutral and acidic electrolytes, respectively, showing that the current of comparative example 4 is drastically attenuated under alkaline, neutral or acidic conditions, while example 1 is hardly affected by methanol, and the current is not significantly changed, indicating that example 1 has excellent methanol resistance after injection of methanol into the electrolyte. FIGS. 12 to 14 are, respectively, 0.1mol/L KOH, 0.1mol/L PBS and 0.5mol/L H saturated with oxygen for the nitrogen-doped porous carbon material prepared in example 1 and 20% Pt/C of comparative example 4 2 SO 4 Stability test in solution example 1 electricity in alkaline, neutral and acid electrolytes after continuous operation for up to 12 hoursThe current retention was 92%, 95% and 96%, respectively, whereas the current retention in the three electrolytes of comparative example 4 was 77%, 61% and 58%, respectively, example 1 had better long-term stability. Therefore, the nitrogen-doped carbon material prepared by the method has outstanding oxygen reduction performance in all aspects, and is expected to become a substitute material for a commercial platinum-based ORR electrocatalyst.
As is evident from a comparison of example 1, comparative example 2 and comparative example 3 prepared according to the present invention, the enzyme co-high energy vacuum mechanical force ball milling treatment is closely related to the pore structure and nitrogen atom doping content of the carbon-based material. The method can not only effectively adjust the pore diameter structure of the cypress leaves, form rich nano pore structures on the surface of the carbon precursor and increase the specific surface area of the cypress leaves, promote the substance transfer and transportation in the ORR process, but also remarkably improve the doping efficiency of nitrogen atoms and increase the effective active sites on the surface of the carbon material, thereby promoting the ORR catalytic activity of the carbon-based material in the full pH range. The invention provides a novel green preparation method capable of improving and enhancing the pore structure of a carbon-based material and the doping efficiency of hetero atoms, breaks through the prior technical barriers, points out the direction of the oxygen reduction electrocatalyst in a new optimized full pH range, and has high innovation and wide application prospect.
Claims (6)
1. The preparation method of the leaf-based nitrogen-doped porous carbon is characterized by comprising the following steps of:
step one: cleaning and drying leaf-based biomass cypress leaves, and grinding to obtain biomass powder materials; weighing biomass powder material, soaking the biomass powder material in acetic acid-sodium acetate buffer solution containing hemicellulase, standing for a period of time at constant temperature for enzymolysis, filtering, washing with water, and drying to obtain a material A; the enzyme activity of the hemicellulase is 100-500U/g; the pH value of the acetic acid-sodium acetate buffer solution is 4.0-6.0; the enzymolysis temperature is 40-70 ℃ and the enzymolysis time is 12-48 h;
step two: pre-carbonizing the material A in an inert gas atmosphere, cooling to room temperature, and grinding into powder to obtain a material B;
step three: respectively weighing a material B, a nitrogen source, potassium bicarbonate serving as an activating agent and potassium hydroxide according to a mass ratio of 1:3:3:1, placing the materials in a vacuum ball milling tank, and performing vacuum high-energy mechanical force ball milling after the tank is vacuumized; carbonizing the ball-milling mixture at high temperature in an inert gas atmosphere to obtain a material C; the conditions of the vacuum high-energy mechanical force ball milling are as follows: the rotation speed is 300rpm, the time is 30min, wherein the rotation speed is 15min clockwise and 15min anticlockwise;
step four: and (3) acid washing the material C, washing the material C with water until the filtrate is neutral, and drying to obtain the nitrogen-doped porous carbon material.
2. The method for preparing the leaf-based nitrogen-doped porous carbon according to claim 1, wherein the pre-carbonization heating rate in the second step is 5 ℃/min, the temperature is raised to 400 ℃ and maintained for 2 hours, then the temperature is reduced to 300 ℃ at 5 ℃/min, and the temperature is cooled to room temperature.
3. The method for preparing the leaf-based nitrogen-doped porous carbon according to claim 1, wherein the high-temperature carbonization process in the third step is as follows: heating to 300 ℃ at a speed of 5 ℃/min for 2 hours, then heating to 900 ℃ at a speed of 5 ℃/min for 2 hours, then cooling to 400 ℃ at a speed of 5 ℃/min, and naturally cooling to room temperature.
4. The method for preparing leaf-based nitrogen-doped porous carbon according to claim 1, wherein the nitrogen source in the third step is one of melamine, dicyandiamide and urea.
5. The method for preparing leaf-based nitrogen-doped porous carbon according to claim 1, wherein the acid washing in the fourth step is carried out by adopting 1mol/L dilute hydrochloric acid solution at 60 ℃ and 300rpm for 3 hours.
6. Use of the nitrogen-doped porous carbon material prepared according to the method of claim 1 in oxygen reduction electrocatalysis over the full pH range.
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