Layered porous biochar prepared from heavy bio-oil as well as preparation method and application of layered porous biochar

Abstract

The invention discloses layered porous biochar prepared from heavy bio-oil as well as a preparation method and application of the layered porous biochar. Heavy bio-oil is used as a carbon source, calcium salt is used as a template agent, K2CO3 is used as an activating agent, and the hierarchical porous biochar with high SSA and a micro-graphitized structure is successfully prepared through a hard template method. The obtained biochar has an excellent layered porous structure, and the obtained electrode has excellent capacitive performance and has great potential as an effective electrode material for preparing a supercapacitor. In addition, the obtained symmetric supercapacitor can keep 98% of high coulombic efficiency after being circulated for 60000 times. An effective way is provided for green utilization of the heavy bio-oil, and structural engineering of the high-performance supercapacitor carbon-based electrode material is achieved through a simple preparation technology.

Description

Technical Field

[0001] The present invention relates to the technical field of supercapacitors, and more specifically to a layered porous biochar prepared by using heavy bio-oil, and a preparation method and application thereof. Background Art

[0002] The continuous growth of global energy demand and the continuous development of technology have led to the rapid consumption of non-renewable resources such as coal, oil, and natural gas. Therefore, obtaining usable energy from renewable resources such as biomass energy and solar energy has received more and more attention. In order to solve the problem of energy shortage, it is necessary not only to develop new sustainable energy, but also to have efficient energy storage devices to avoid intermittent energy loss and limit the use of renewable energy.

[0003] Bio-oil is a product of biomass pyrolysis, which has the characteristics of high water content, high oxygen content, high viscosity, and low calorific value. Therefore, it is difficult to process and utilize it in depth, and the heavy components of bio-oil are prone to polymerization and cause coking problems. At present, the utilization of bio-oil is mostly catalytic reforming, catalytic pyrolysis, and hydrodeoxygenation. However, these methods have the problems of complex reaction conditions and high cost.

[0004] As a highly efficient energy storage device, supercapacitors have become one of the most important energy storage systems due to their advantages such as high output power, long cycle life, and short charging time. The performance of supercapacitors mainly depends on the physical and chemical properties of their electrode materials, including transition metal oxides, conductive polymers, and carbon-based materials. However, the first two electrode materials have disadvantages such as short cycle life and low specific capacitance. Therefore, carbon-based materials are widely used due to their acceptable capacitance values, stable chemical structures, and low costs. Biomass-derived porous biochar has been shown to be a potential electrode material due to its high specific surface area (SSA), low cost, low internal resistance, and excellent pore structure.

[0005] However, currently, the preparation of porous biochar derived from biomass mostly adopts template method and chemical method. Among them, the chemical method often brings metal pollution problems, so its use is limited. The hard template is limited by the inherent pore structure of the template, resulting in a narrow size distribution of the obtained biochar and a fixed pore structure. Although the soft template method can control the structure formed by the biochar, its process is cumbersome, costly, and not suitable for large-scale production. Therefore, it is still necessary to study the current preparation methods of biochar, improve the structural characteristics of the obtained biochar, and improve its performance as an electrode material. Summary of the invention

[0006] To this end, the technical problem to be solved by the present invention is how to obtain a hierarchical porous biochar with a micro-graphitized structure and improve the specific surface area (SSA) and electrochemical properties of the porous biochar. Based on this, the present invention provides a hierarchical porous biochar prepared using heavy bio-oil, and a preparation method and application thereof.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] The present invention provides a method for preparing layered porous biochar using heavy bio-oil, comprising the following steps:

[0009] (1) separating the bio-oil to obtain heavy bio-oil for future use;

[0010] (2) adding calcium salt to heavy bio-oil, stirring evenly and drying to obtain a carbonaceous precursor for later use;

[0011] (3) using a hard template method, pyrolyzing the carbonaceous precursor to obtain a carbon / CaO composite powder; after washing and drying, a layered porous biochar preparation can be obtained;

[0012] (4) The layered porous biochar preparation is mixed with K2CO3 for activation, and then washed and dried to obtain the activated layered porous biochar.

[0013] Preferably, in step (1), the bio-oil is pyrolysis oil of pine sawdust at 500°C.

[0014] Preferably, in step (1), the specific method of separation is: separating the bio-oil into heavy bio-oil and light bio-oil by a rotary evaporator under the conditions of 60° C. and −0.95 MPa.

[0015] Preferably, in step (2), the mixing mass ratio of the calcium salt to the heavy bio-oil is 1:1-3; the stirring temperature is 100-110° C., and the stirring time is 6-9 h; the drying temperature is 100-110° C., and the drying time is 24-36 h.

[0016] Preferably, in step (2), the mixing mass ratio of the calcium salt to the heavy bio-oil is 1:1; the stirring temperature is 110° C., and the stirring time is 6 h; the drying temperature is 105° C., and the drying time is 24 h.

[0017] Preferably, in step (2), the calcium salt is calcium citrate tetrahydrate (CCT), calcium acetate (CA) or calcium carbonate (CC).

[0018] Preferably, in step (2), the calcium salt is calcium citrate tetrahydrate (CCT).

[0019] Preferably, in step (3), the pyrolysis temperature is 700°C-800°C, the heating rate during pyrolysis is 10-15°C / min, and the pyrolysis reaction time is 1-3h.

[0020] Preferably, in step (3), the pyrolysis temperature is 700°C, the heating rate during pyrolysis is 15°C / min, and the pyrolysis reaction time is 2h.

[0021] Preferably, in step (4), the mixing mass ratio of the layered porous biochar preparation and K2CO3 is 1:1-1:3; the activation temperature is 800-900°C, the heating rate during activation is 10-20°C / min, and the activation reaction time is 1-2h.

[0022] Preferably, in step (4), the mixing mass ratio of the layered porous biochar preparation and K2CO3 is 1:2; the activation temperature is 800°C, the heating rate during activation is 10°C / min, and the activation reaction time is 1.5h.

[0023] Preferably, in steps (3) and (4), 1M HCl solution and deionized water are used for washing respectively; the drying temperature is 70-80° C., and the drying time is 10-12 h.

[0024] The invention relates to a layered porous biochar prepared by utilizing heavy bio-oil. The layered porous biochar is prepared by the preparation method.

[0025] The invention relates to application of layered porous biochar prepared by utilizing heavy bio-oil in preparing supercapacitors.

[0026] The technical solution of the present invention achieves the following beneficial technical effects:

[0027] The present invention uses heavy bio-oil as a carbon source, organic calcium salts (calcium citrate tetrahydrate (CCT), calcium acetate (CA)) or inorganic calcium salts (calcium carbonate, CC) as templates, and K2CO3 as an activator, and successfully prepares hierarchical porous biochar with high SSA and micro-graphitized structure through a hard template method. The calcium salt provides a rich pore structure for the pyrolysis of heavy bio-oil to convert it into a solid biochar product, and the retained calcium salt plays a key role in the subsequent activation process of constructing a carbon structure framework and adjusting its morphology. The prepared biochar is a weakly alkaline compound with low toxicity and corrosiveness, and the pore structure is further enriched after being activated by K2CO3.

[0028] The present invention uses organic salt CCT to assist template carbonization, which shows a synergistic effect with K2CO3 activation. The obtained biochar (CCT-700-1:2) has an excellent hierarchical porous structure with an SSA of 2213.09 m 2 g -1The CCT-700-1:2 electrode has excellent capacitance performance at 0.5A g -1 At the current density, the specific capacitance is 213.86F g -1 , has great potential as an effective electrode material for preparing supercapacitors. In addition, CCT-700-1:2 / / CCT-700-1:2 symmetric supercapacitors have a power density of 250W kg -1 It can provide 16.36W h kg -1 The invention provides an effective way for the green utilization of heavy bio-oil and realizes the structural engineering of high-performance supercapacitor carbon-based electrode materials through simple preparation technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Electron microscope images of the materials obtained in the embodiments of the present invention and the comparative examples;

[0030] In the figure: (a) SEM image of BC; (b) SEM image of BC-K2CO3; (c) SEM image of CC-700-1:2; (d) SEM image of CA-700-1:2; (e) SEM image of CCT-700-1:2; (f) TEM image of CCT-700-1:2;

[0031] Figure 2 N2 adsorption / desorption isotherms and pore size distribution curves of the materials obtained in the examples of the present invention and the comparative examples;

[0032] In the figure: (a) N2 adsorption / desorption isotherm of biochar sample; (b) pore size distribution curve of biochar sample; (c) N2 adsorption / desorption isotherm of activated biochar; (d) pore size distribution curve of activated biochar; in (a) and (c), the horizontal axis represents: the relative pressure between the real pressure and the actually measured saturated vapor pressure; the vertical axis represents: the adsorption amount; in (b) and (d), the horizontal axis represents: the pore diameter; the vertical axis represents: the pore volume;

[0033] Figure 3 Raman spectra and XRD spectra of the materials obtained in the embodiments of the present invention and the comparative examples;

[0034] In the figure: (a) Raman spectrum; (b) XRD spectrum; in (a), the horizontal axis represents: Raman shift; the vertical axis represents: Raman light intensity; in (b), the horizontal axis represents: 2θ diffraction angle; the vertical axis represents: diffraction intensity;

[0035] Figure 4 Electrochemical performance of the electrodes of the materials obtained in the examples and comparative examples of the present invention in a three-electrode system containing 6M KOH electrolyte;

[0036] In the figure: (a) 100mV s -1 CV curves at 1A g -1 GCD curve when (c) specific capacitance diagram; (d) Nyquist diagram; in (a), the horizontal axis represents voltage; the vertical axis represents current density; in (b), the horizontal axis represents time; the vertical axis represents voltage; in (c), the horizontal axis represents current density; the vertical axis represents specific capacitance; in (d), the horizontal axis represents impedance; the vertical axis represents reactance;

[0037] Figure 5 Electrochemical performance of symmetrical supercapacitor of electrode of material CCT-700-1:2 obtained in Example 1 of the present invention;

[0038] Among them: (a) CV curves at different scan rates; (b) GCD curves at different current densities; (c) 0.5~20Ag -1 C s curve; (d) Nyquist diagram of supercapacitor; (e) relationship between energy density and power density; (f) 5A g -1 The cycle performance of supercapacitors at different times; (a), the horizontal axis represents voltage; the vertical axis represents current density; (b), the horizontal axis represents time; the vertical axis represents voltage; (c), the horizontal axis represents current density; the vertical axis represents specific capacitance; (d), the horizontal axis represents impedance; the vertical axis represents reactance; (e), the horizontal axis represents power density; the vertical axis represents energy density; (f), the horizontal axis represents number of cycles; the vertical axis represents Capacitance retention: capacitance retention rate, Coulombic efficiency: Coulombic efficiency. DETAILED DESCRIPTION

[0039] Sources of raw materials used in the embodiments of the present invention:

[0040] Bio-oil was purchased from BTG Bioliqu ids (Netherlands). Bio-oil is a liquid product obtained by pyrolysis and condensation of pine sawdust at 500°C in a rotating cone pyrolysis reactor. Bio-oil has high carbon (53.31 wt.%), high oxygen (39.07 wt.%), and contains hydrogen (7.50 wt.%), small amounts of nitrogen (0.10 wt.%) and sulfur (0.02 wt.%) elements.

[0041] Activator (K2CO3), CCT, CA, CC and HCl were purchased from Nanjing Chemical Reagent Co., Ltd.

[0042] Example 1

[0043] A method for preparing layered porous biochar using heavy bio-oil comprises the following steps:

[0044] (1) Under the conditions of 60° C. and −0.95 MPa, bio-oil (pine sawdust pyrolysis oil from BTG Bioliqu ids) was separated into heavy bio-oil and light bio-oil by a rotary evaporator, and the heavy bio-oil was recovered for later use;

[0045] (2) adding 15 g of calcium salt (calcium citrate tetrahydrate (CCT)) to 15 g of heavy bio-oil, stirring at 110 °C for 6 h, and drying in an oven at 105 °C for 24 h to obtain a carbonaceous precursor for later use;

[0046] (3) Using the hard template method, the obtained carbonaceous precursor was evenly spread in a ceramic ark and placed in a tube furnace for reaction at a reaction temperature of 700°C, a heating rate of 15°C / min, and a reaction time of 2h. After pyrolysis, a carbon / CaO composite powder was obtained; it was washed with 1M HCl solution and deionized water to remove excess salt, and dried at 80°C for 12h to obtain a layered porous biochar preparation; named CCT-700.

[0047] (4) The layered porous biochar preparation was evenly mixed with K2CO3 in a mass ratio of 1:2, placed in a ceramic ark, and placed in a tube furnace for reaction. The reaction temperature was 800°C, the heating rate was 10°C / min, and the reaction time was 1.5h. After the activation reaction, it was washed with 1M HCl solution and deionized water respectively to remove excess salt. After drying at 80°C for 12h, the activated layered porous biochar was obtained; it was named CCT-700-1:2.

[0048] Example 2

[0049] This embodiment provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Embodiment 1 is that in step (4), the layered porous biochar preparation body and K2CO3 are evenly mixed in a mass ratio of 1:1, and the obtained activated layered porous biochar is named CCT-700-1:1.

[0050] The other steps and parameters are the same as those in Example 1.

[0051] Example 3

[0052] This embodiment provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Embodiment 1 is that in step (4), the layered porous biochar preparation body and K2CO3 are evenly mixed in a mass ratio of 1:3, and the obtained activated layered porous biochar is named CCT-700-1:3.

[0053] The other steps and parameters are the same as those in Example 1.

[0054] Example 4

[0055] This embodiment provides a method for preparing layered porous biochar using heavy bio-oil, which is different from Embodiment 1 in that the calcium salt in step (2) is calcium acetate (CA); the obtained layered porous biochar preparation is named CA-700; and the obtained activated layered porous biochar is named CA-700-1:2.

[0056] The other steps and parameters are the same as those in Example 1.

[0057] Example 5

[0058] This embodiment provides a method for preparing layered porous biochar using heavy bio-oil, which is different from Embodiment 1 in that the calcium salt in step (2) is calcium carbonate (CC); the obtained layered porous biochar preparation is named CC-700; and the obtained activated layered porous biochar is named CC-700-1:2.

[0059] The other steps and parameters are the same as those in Example 1.

[0060] Comparative Example 1

[0061] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 4 is that in step (4), the layered porous biochar preparation body and K2CO3 are evenly mixed in a mass ratio of 1:1, and the obtained activated layered porous biochar is named CA-700-1:1.

[0062] The other steps and parameters are the same as those in Example 4.

[0063] Comparative Example 2

[0064] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 4 is that in step (4), the layered porous biochar preparation body and K2CO3 are evenly mixed in a mass ratio of 1:3, and the obtained activated layered porous biochar is named CA-700-1:3.

[0065] The other steps and parameters are the same as those in Example 4.

[0066] Comparative Example 3

[0067] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 5 is that in step (4), the layered porous biochar preparation body and K2CO3 are evenly mixed in a mass ratio of 1:1, and the obtained activated layered porous biochar is named CC-700-1:1.

[0068] The other steps and parameters are the same as those in Example 5.

[0069] Comparative Example 4

[0070] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 5 is that in step (4), the layered porous biochar preparation body and K2CO3 are evenly mixed in a mass ratio of 1:3, and the obtained activated layered porous biochar is named CC-700-1:3.

[0071] The other steps and parameters are the same as those in Example 5.

[0072] Comparative Example 5

[0073] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 1 is that step (4) is not involved. The layered porous biochar obtained in this comparative example is named CCT-700.

[0074] The other steps and parameters are the same as those in Example 1.

[0075] Comparative Example 6

[0076] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 1 is that step (4) is not involved, and the reaction temperature of step (3) is 800°C. The layered porous biochar obtained in this comparative example is named CCT-800.

[0077] The other steps and parameters are the same as those in Example 1.

[0078] Comparative Example 7

[0079] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 4 is that step (4) is not involved. The layered porous biochar obtained in this comparative example is named CA-700.

[0080] The other steps and parameters are the same as those in Example 4.

[0081] Comparative Example 8

[0082] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 4 is that step (4) is not involved, and the reaction temperature of step (3) is 800°C. The layered porous biochar obtained in this comparative example is named CA-800.

[0083] The other steps and parameters are the same as those in Example 4.

[0084] Comparative Example 9

[0085] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 5 is that step (4) is not involved. The layered porous biochar obtained in this comparative example is named CC-700.

[0086] The other steps and parameters are the same as those in Example 5.

[0087] Comparative Example 10

[0088] This comparative example provides a method for preparing layered porous biochar using heavy bio-oil. The difference from Example 5 is that step (4) is not involved, and the reaction temperature of step (3) is 800°C. The layered porous biochar obtained in this comparative example is named CC-800.

[0089] The other steps and parameters are the same as those in Example 5.

[0090] Comparative Example 11

[0091] This comparative example provides a method for preparing biochar using heavy bio-oil, which is different from Example 1 in that the step (2) of adding calcium salt is not involved, and the heavy bio-oil obtained in step (1) is directly used for the pyrolysis treatment in step (3);

[0092] This comparative example prepared a heavy bio-oil at 700℃ and 15℃min -1 The biochar obtained by pyrolysis at a rising temperature rate for 2 h was named BC; it was then mixed with K2CO3 at a mass ratio of 1:1 for activation, and the resulting product was named BC-K2CO3.

[0093] The other steps and parameters are the same as those in Example 1.

[0094] Performance measurement:

[0095] 1. Preparation of biochar electrode

[0096] In order to prepare the working electrode, the carbon sample (80wt.%), conductive carbon black (10wt.%) and polyvinylidene fluoride (PVDF, 10wt.%) were fully ground in an agate mortar with 2μL of n-methyl-2-pyroketone (NMP) to obtain a black slurry. The black slurry was then applied to the prepared nickel foam, dried in a vacuum oven at 60℃ for 16h, and pressed at 5MPa for 1min to finally obtain the electrode.

[0097] 2. Characterization of biochar samples

[0098] The elemental analysis of biochar was performed using an elemental analyzer (EA, 2400 series II, Perkins-Elmer, Waltham, USA). The surface morphology of biochar samples was characterized by scanning electron microscopy (SEM, Regulus 8100, Hitachi, Tokyo, Japan) and field emission transmission electron microscopy (TEM, jeem-2100UHR, Jeol, Tokyo, Japan). The biochar samples were characterized by powder X-ray diffraction (XRD, Ultra IV, Rigaku, Tokyo, Japan) and laser Raman spectroscopy (Raman, DXR532, Thermo Scientific, Waltham, MA, USA). The specific surface area and pore size distribution of biochar samples were determined at 77 K using a self-adsorption specific surface area analyzer (BSD-PS, BeiShiDe Instruments, Beijing, China) with nitrogen as the adsorbent.

[0099] 3. Electrochemical performance test

[0100] The electrochemical properties of biochar-based single electrode and solid-state symmetric supercapacitors were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and constant current charge-discharge method (GCD). The electrochemical measurements were performed using an electrochemical workstation (CHI660E, Chenhua Instrument Co., Ltd., Shanghai, China). In a three-electrode system, the prepared biochar electrode, Hg / HgO electrode and platinum electrode were used as working electrode, reference electrode and counter electrode, respectively, to study the electrochemical properties of a single biochar electrode. In the electrochemical workstation, 6M KOH solution was used as the electrolyte to test CV, EIS and GCD. The EIS measurement used an AC voltage with an amplitude of 5mV and a frequency fluctuation range of 0.01-100000Hz. The electrochemical performance and cycle stability of the biochar-based symmetric supercapacitor were evaluated using the same electrochemical workstation.

[0101] Specific capacitance (C, Fg -1 ) is obtained from the GCD curve by formula (1):

[0102] C m =IΔt / mΔV (1)

[0103] In the formula, Cm(F -1 ) is the specific capacitance of the electrode material, I (A) is the constant discharge current, m (g) is the mass of biochar, and Δt (s) is the discharge time associated with the voltage change ΔV (V).

[0104] In a dual-electrode symmetrical supercapacitor system, the CV and GCD of the supercapacitor were tested on an electrochemical workstation CH I660E. According to the GCD curves at different currents, the specific capacitance of the supercapacitor was calculated by formula (2):

[0105] C s =2 IΔt / mΔV (2)

[0106] Where Cs(F -1 ) is the specific capacitance of the supercapacitor, I(A) is the discharge current, Δt(s) is the discharge time, and ΔV(V) is the effective discharge voltage. The calculation formulas for energy density and power density are as follows:

[0107] E=C s ΔV 2 / 8×3.6 (3)

[0108] P=E×3600 / Δt (4)

[0109] Where E(W h kg -1 ) is the energy density and P (W kg-1) is the power density.

[0110] Result analysis:

[0111] 1. Morphology and structure of biochar

[0112] The morphology and porous structure of biochar are closely related to the reaction site and mass transfer process, and play a vital role in the capacitive performance of biochar. The present invention uniformly mixes the calcium salt template with heavy bio-oil, removes the template by pyrolysis to obtain the target pore structure. Subsequently, the biochar sample is further activated with K2CO3 to enrich the porosity and obtain hierarchical porous biochar. Figure 1 As shown in a, the surface of untreated biochar (BC) is smooth and has almost no pore structure. After activation by K2CO3 ( Figure 1 b), there are evenly distributed mesoporous channels on the surface of BC-K2CO3. This indicates that K2CO3 has certain pore-forming and activation capabilities due to the volatilization of CO2 during the heating process. However, the pore size is too concentrated in a small range to form a hierarchical porous structure. Figure 1 ce shows the morphology of biochar samples prepared by different templates. The surface of biochar prepared by CC method is very smooth, mainly mesopores, with fewer micropores. The surface of biochar prepared by CA method is rough, with dense and small micropores, and relatively few mesopores. The surface of biochar prepared by CCT has many micropores and mesopores, which are interconnected to form a well-layered porous structure. In order to further study the microstructure of CCT-700-1:2, TEM imaging was performed, such as Figure 1f. The pore structure is well developed, with a high proportion of disordered micropores and mesopores. This pore structure is conducive to the diffusion and transport of electrolytes, giving it better rate capability.

[0113] In order to investigate the text parameters of biochar, N2 adsorption-desorption measurements were performed ( Figure 2 and Table 1). It can be seen that, except for BC, all biochars exhibit type IV adsorption isotherms with obvious hysteresis loops, indicating the presence of a large number of micropores / mesopores. BC has non-porous characteristics, with SSA, micropore volume, and total pore volume of 19.73 m 2 g -1 , 0cm 3 g -1 and 0.045cm 3 g -1 Compared with CCT-700, CA-700 and CC-700, the SSA and micropore volume of CCT-800, CA-800 and CC-800 all decrease with increasing temperature. This indicates that the calcium template is excessively decomposed at high temperature and the pore structure collapses. Figure 2 b It can be seen that the pore size of the unactivated biochar samples is mainly mesopores. In addition, the number of mesopores in CA-700 and CA-800 is much larger than that in other samples, with SSA of 385.15 m 2 g -1 and 353.49m 2 g -1 This may be because CA is an organic calcium and does not consume water during the reaction, resulting in partial evaporation of water to generate more mesopores.

[0114] Since the above biochars have higher SSA, the biochars prepared at 700 °C were selected for further activation with K2CO3. Figure 2 As shown in c, all biochars showed type IV adsorption isotherms after activation. After activation, due to the formation of a large number of micropores, the SSA was between 896 and 2213 m 2 g -1 Another phenomenon is that as the mass ratio of biochar to K2CO3 increases from 1:1 to 1:3, the SSA and micropore volume of the obtained activated carbon first increase and then decrease. As the proportion of the activator is too large, over-activation leads to the collapse of micropores and the formation of more mesopores. Therefore, the mass ratio of 1:2 is the best activation condition. Figure 2 It can be seen that the activated carbon contains a large number of micropores and mesopores, forming a hierarchical porous structure. Interestingly, the SSA of CA-700-1:2 (972.12m 2 g -1 ) is the lowest, even lower than the SSA of BC-K2CO3 (1115.92m 2 g-1 ), and CA-700 has the highest SSA before activation. CCT-700-1:2 has the highest SSA (2213.09m 2 g -1 ) and micropore volume (0.835 cm 3 g -1 ), the ratio of micropore volume to total pore volume (61.08%) was the highest, indicating that CCT-assisted template carbonization and K2CO3 activation had a synergistic effect. In summary, the hierarchical porous structure of biochar was successfully regulated by combining CCT hard template method and K2CO3 chemical activation method.

[0115] Table 1. Pore structure parameters of biochar samples

[0116]

[0117] Elemental analysis was performed to detect the amount of various elements on the activated biochar. As shown in Table 2, the C content of BC is very high (92.14%) because it is directly obtained from heavy bio-oil. After activation with K2CO3, the C content of BC-K2CO3 dropped to 84.72%, and the O content increased to 13.86%. This indicates that K2CO3 released a large amount of CO2 during the activation process, which increased its O content and consumed the C element. Similar phenomena also occurred in CCT-700-1:2, CA-700-1:2, and CC-700-1:2. It is worth noting that the O content of CA-700-1:2 is as high as 31.17%, indicating that CA can consume a large amount of C elements to create mesopores during the carbonization process.

[0118] Table 2. Elemental analysis of activated biochar

[0119]

[0120] In order to investigate the properties of graphite, Raman spectroscopy and XRD characterization were performed on the selected activated biochars ( Figure 3 ). The Raman spectrum of biochar shows two bands ( Figure 3 a), respectively at 1310cm -1 (d-band) and 1590cm -1 (g band). The d band represents the disorder and defects of the carbon layer, while the g band is the product of the crystal phase and graphite structure. The intensity ratio of the d band to the g band (I D / I G ) can be used to evaluate the graphitization degree of biochar. D / I G The lower the ratio, the higher the degree of graphitization. Compared with BC (3.681), the I of BC-K2CO3 D / I GThe value increased to 4.709, indicating that the degree of graphitization was significantly reduced after activation. D / I G The value (3.853) is close to that of BC and much smaller than that of CA-700-1:2 and CC-700-1:2. D / I G values ​​(4.592 and 4.726). It is worth noting that the reduction in the degree of graphitization may reduce the conductivity of fractionated porous carbon, further affecting its electrochemical performance. Therefore, it is very important to ensure the microstructure and SSA of biochar without reducing the degree of graphitization.

[0121] Depend on Figure 3 b shows that the activated carbon has two different characteristic diffraction peaks at 2θ values ​​of 23.4° and 43.3°, corresponding to the (002) and (100) crystal planes of graphite, respectively. These two diffraction peaks represent the degree of graphitization of the carbon material. Compared with BC, the diffraction peaks of these activated carbons are weaker (002) and wider, indicating a lower degree of graphitization. The peak intensity of CCT-700-1:2 is similar to that of BC, which is consistent with the results of Raman analysis. This proves that CCT-700-1:2 has a micrographite structure, with high SSA and hierarchical porous structure. The micrographite structure in biochar is conducive to electron transfer, making it more favorable as an electrode material for supercapacitors.

[0122] 2. Electrochemical performance

[0123] (1) Three-electrode system test

[0124] Five activated carbons with mature hierarchical porous structures, namely CCT-700-1:1, CCT-700-1:2, CCT-700-1:3, CC-700-1:2 and CA-700-1:2, were selected and their electrochemical properties were characterized by CV, GCD and EIS tests under a three-electrode environment. Figure 4 As shown in a, at a scan rate of 100 mV s -1 The CV curves of different samples showed similar quasi-rectangular curve shapes, indicating that the prepared biochar electrodes have ideal double-layer characteristics. Among them, CCT-700-1:2 has the largest integral area and the best capacitance performance. This is attributed to its highest SSA, because SSA is usually the determining factor of the specific capacitance of EDLC electrode materials. -1 The GCD curve is as follows Figure 4 As shown in b, it can be observed that the curves of CA-700-1:2 and CC-700-1:2 are symmetrical isosceles triangles, while the curve of CCT-700-1:2 is a nonlinear curve, indicating the existence of pseudo capacitance. Figure 4 c shows the specific capacitance (Cm ) and current density. As the current density increases, C m The capacitance of CCT-700-1:2 gradually decreases at different current densities. -1 When CCT-700-1:2 m Up to 213.86F g -1 When the current density is 20A g -1 When Cm is maintained at 149F g -1 , the capacitance retention rate is 69.7%, proving that it has good amplification and electrochemical performance.

[0125] The EIS of the sample is as follows Figure 4 d. It consists of three parts: (1) the semicircle in the high-frequency region represents the interfacial charge transfer resistance between the electrolyte and the electrode; (2) the oblique line in the medium-frequency region is related to the diffusion resistance; and (3) there is an almost vertical line in the low-frequency region, which reflects the capacitive property, generally indicating high conductivity and good wettability. The smaller the diameter of the semicircle line, the smaller the charge transfer resistance, indicating that the electrolyte ions diffuse quickly in the carbon skeleton and the electron conduction is good. Obviously, CCT-700-1:2 shows the closest vertical line in the low-frequency region, indicating that it has the highest conductivity. In addition, the diffusion resistance is the lowest in the medium-frequency region. Combining the above results, it can be concluded that CCT-700-1:2 has the best electrochemical performance.

[0126] (2) Dual-electrode system test

[0127] To evaluate the usability, a symmetric supercapacitor was assembled using two CCT-700-1:2 electrodes in 6 M KOH. Figure 5 a is the CV curve of CCT-700-1:2 at different scan rates. As the scan rate increases, they still maintain a quasi-rectangular shape. In addition, due to the low resistance of the electrode, only at a high scan rate (100 mV s -1 ) will cause slight distortion. Figure 5 b is 0.5~20A g -1 GCD curves of CCT-700-1:2 at different current densities. All curves are linear, indicating excellent capacitive performance and electrochemical reversibility. Calculated 0.5~20A g -1 C s , the results are as follows Figure 5 c. 0.5A g -1 Maximum C s The value is 235.1F g -1 When the current density is as high as 20A g -1 When C s The value is 177.6Fg-1 , the capacitance retention rate is 75.5%. This means that the supercapacitor prepared by CCT-700-1:2 has excellent amplification performance because it has a large SSA, a layered porous structure and low impedance, which is conducive to accelerating the transmission and diffusion of ions. As the most important characteristics of supercapacitors, energy density and power density are calculated based on the value of Cs. CCT-700-1:2 symmetrical supercapacitor has a power density of 125W kg -1 The maximum energy density is 8.16W hkg -1 .

[0128] exist Figure 5 In the Nyquist curve shown in d, the diameter of the small semicircle line in the high-frequency region indicates that the electrolyte ions diffuse rapidly in the carbon framework and the electron conduction is good. In the medium-frequency region, the slope of the curve is greater than 1, indicating that the ion diffusion resistance between the electrode surface and the electrolyte is low. An almost vertical line appears in the low-frequency region, indicating that the ion diffusion resistance is low, which means that the electrolyte can easily enter the pores of the electrode material. Therefore, it has good double-layer capacitance performance. This is due to the layered porous structure of CCT-700-1:2, which enables the rapid exchange of ions / charges in the electrochemical process, shortens the ion transfer path, and thus improves its capacitance performance. The relationship between the energy density and power density of CCT-700-1:2 is shown in Figure 2. Figure 5 e. CCT-700-1:2 has a power density of 250W kg -1 It can provide 16.36W h kg -1 High energy density, with a power density of 5000W kg -1 It can still reach 12.65W h kg -1 .

[0129] Long cycle life is another key factor in evaluating the electrochemical performance of SSCs. To evaluate the cycling stability of CCT-700-1:2, the -1 Continuous charge and discharge cycles were measured at current densities of Figure 5 As shown in Figure 5, after 60,000 cycles, the capacity still remains at 98.5%, with little capacitance decay. This indicates excellent cycling stability. The Coulombic efficiency, the ratio of the discharge capacitance to the charge capacitance, was calculated based on the GCD test. The results show that after 60,000 cycles, it still remains at around 98%.

[0130] In summary, the present invention successfully prepared hierarchical porous biochar with high SSA and micro-graphitized structure using heavy bio-oil as carbon source, CCT, CA or CC as template and K2CO3 as activator. Calcium salt plays a key role in the process of converting heavy bio-oil into solid biochar products, building carbon structure framework and regulating its morphology. In addition, among the three calcium salt templates, only CCT-assisted template carbonization showed synergistic effect with K2CO3 activation, which may be due to the fact that it is an organic salt. The obtained biochar (CCT-700-1:2) has an excellent hierarchical porous structure with an SSA of 2213.09 m 2 g -1 The CCT-700-1:2 electrode has excellent capacitance performance at 0.5A g -1 At the current density, the specific capacitance is 213.86F g -1 , has great potential as an effective electrode material for preparing supercapacitors. In addition, CCT-700-1:2 / / CCT-700-1:2 symmetric supercapacitors have a power density of 250W kg -1 It can provide 16.36W h kg -1 The invention provides an effective way for the green utilization of heavy bio-oil and realizes the structural engineering of high-performance supercapacitor carbon-based electrode materials through simple preparation technology.

[0131] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for preparing layered porous biochar using heavy bio-oil, characterized in that: The following steps are involved: (1) separating the bio-oil to obtain heavy bio-oil for future use; (2) adding calcium salt to heavy bio-oil, stirring evenly and drying to obtain a carbonaceous precursor for later use; (3) using a hard template method, pyrolyzing the carbonaceous precursor to obtain a carbon / CaO composite powder; after washing and drying, a layered porous biochar preparation can be obtained; (4) The layered porous biochar preparation is mixed with K2CO3 for activation, and then washed and dried to obtain the activated layered porous biochar.

2. The method according to claim 1, characterized in that: In the step (1), the bio-oil is pyrolysis oil of pine sawdust at 500°C.

3. The method according to claim 1, characterized in that: In the step (1), the specific separation method is: separating the bio-oil into heavy bio-oil and light bio-oil under the conditions of 60° C. and −0.95 MPa.

4. The method according to claim 1, characterized in that: In the step (2), the mixing mass ratio of the calcium salt to the heavy bio-oil is 1:1-3; the stirring temperature is 100-110° C., and the stirring time is 6-9 hours; the drying temperature is 100-110° C., and the drying time is 24-36 hours.

5. The method according to claim 4, characterized in that: In the step (2), the calcium salt is calcium citrate tetrahydrate, calcium acetate or calcium carbonate.

6. The method according to claim 1, characterized in that: In the step (3), the pyrolysis temperature is 700°C-800°C, the heating rate during pyrolysis is 10-15°C / min, and the pyrolysis reaction time is 1-3h.

7. The method according to claim 1, characterized in that: In the step (4), the mixing mass ratio of the layered porous biochar preparation and K2CO3 is 1:1-1:3; the activation temperature is 800-900°C, the heating rate during activation is 10-20°C / min, and the activation reaction time is 1-2h.

8. The method according to claim 1, characterized in that: In the steps (3) and (4), 1M HCl solution and deionized water are used for washing respectively; the drying temperature is 70-80° C. and the drying time is 10-12 h.

9. A layered porous biochar prepared using heavy bio-oil, characterized in that: The layered porous biochar is prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the hierarchical porous biochar prepared using heavy bio-oil as claimed in claim 9 in preparing supercapacitors.

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