BR102013031741A2 - nitrogen-doped activated carbon produced from the thermal regeneration of methylene blue saturated material and its application as a supercapacitor electrode - Google Patents

Abstract

Nitrogen-doped activated carbon produced from the thermal regeneration of methylene blue saturated material and its application as a supercapacitor electrode. The present invention relates to obtaining an electric double layer capacitor from the thermal regeneration of high surface area activated carbon saturated with methylene blue. The obtained material (nca) was characterized by several analysis techniques and methodologies, presenting a supeificial area of 1726 m2 g-1 and 3.82% nitrogen in its composition. Electrochemical studies (cyclic voltammetry, charge and discharge and stability cycle) have shown that nca has good capacitive behavior, with low equivalent resistance and good charge and discharge stability after 1000 research cycles. The determination of specific capacitance values for nca ranged from 227.52 to 108, 44 and g-1. The present invention demonstrates that a residue generated from the dye adsorption process can be treated and converted into a product with technological application in the areas of portable electronics, hybrid electric vehicles and stand-by energy storage systems.

Description

ACTIVATED COAL DUPTED WITH NITROGEN PRODUCED FROM

REGENERATION OF THE BLUE-SATURED MATERIAL

METHYLENE AND ITS APPLICATION AS ELECTRODE FOR

SUPERCAPACITORS

1. INTRODUCTION

With the technological advancement and the decrease of the world's fossil fuel reserves, the development of researches that allow obtaining materials with high cargo storage capacity and coming from renewable sources has become increasingly necessary. Renewable sources of energy such as sun and wind are often intermittent and therefore cannot meet the needs of casual use. In this sense, in recent years, studies have been conducted with the purpose of preparing new electrochemical capacitors for application in high power systems.

Dual-layer electrical supercapacitors or capacitors (CEDCs) appear as a promising alternative for the rapid storage of electrical charges, which can be used to replace or supplement batteries in applications requiring high power density. CEDCs are characterized as materials that behave as power supplies that have the ability to store energy from the separation of electronic and ionic charges at the interface between the material, which constitutes the electrode and the electrolyte present in solution.

Because they have excellent charge transfer rates, long life cycles and are comparable to secondary batteries, these materials have direct technological application in systems that require charge storage. These include backup systems, uninterruptible power supplies, portable electronics, hybrid electronic vehicles, standby systems and others.

Porous carbonaceous materials appear as good candidates for use as supercapacitor electrodes because of their high surface area, high conductivity and chemical stability, and structure that allows pore size control. However, their specific capacitance values are directly related to the required high specific surface area values. Thus, with the purpose of circumventing this requirement, studies report the insertion / addition of groups on the surface of these materials, which has attributed to these pseudo-capacitance characteristics. This phenomenon is based on faradaic redox reactions of electroactive species in solution, which leads to capacitive current production and electric double layer formation.

Different chemical groups or atoms may be inserted on the surface of carbonaceous materials such as phenols, quinones / hydroquinones, phosphorus, boron and nitrogen. In turn, a significant improvement in the pseudo-capacitance effect has been observed with the addition of nitrogen in the structure of these materials. This improvement is observed because nitrogen: (i) acts as an electron donor in the carbon network, thus causing a change of Fermi level to the valence layer electrons of the material; and (ii) allows the insertion of oxygen that contributes to pseudocapacitance.

Previous studies show that there are four forms of nitrogen-containing functional groups that may contribute to the capacitive performance of carbonaceous materials. X-ray photoelectron spectroscopy analyzes indicate that pyridic (398.5 eV), pyrrolic (400 eV), quaternary nitrogen (401.2 eV) and pyridine N-oxide (403) pyridine groups can be found on the surface of these materials. eV).

According to the literature, the effects of pseudocapacitance may be mainly related to the presence of pyridine and pyrrole groups. In turn, the presence of quaternary nitrogen and pyridine N-oxide are involved in the transport of electrons on the material, which provides greater retention capacity during the charge and discharge process.

In addition to having high surface area and chemical groups that allow faradaic redox reactions to occur, another important surface feature in CEDCs is the presence of mesopores. These pores facilitate the transfer of electrons, lowering the resistance and allowing quick charge and discharge of the double layer.

2. State of the art

Due to its promising application in cargo storage systems, CEDCs have been studied by several researchers around the world. Synthesis of nitrogen-doped materials has generally been performed in two ways: (i) post-treatment of carbonaceous material with ammonia, amines or urea, and (ii) in situ doping using various nitrogen-containing precursors such as melanin resin, polyaniline, polyacrylonitrile, among others, leading to a more homogeneous incorporation of nitrogen species on carbon.

The carbonaceous precursors used to support the addition of nitrogen groups are usually nanoscale graphite or graphene, materials produced by pyrolysis of high molecular weight polymers, carbonic templetes obtained by the nanomolding technique and activated carbon. Most of the routes for obtaining these materials require several preparation steps, requiring expensive reagents and often extreme experimental conditions (high values of: temperatures, preparation time, inert gas flow, etc.).

High temperature conditions and complex reagent mixtures are evidenced by analysis of some inventions. EP 2,508,475 A1 describes the production of nitrogen-doped porous carbon produced by the mixture of melanin and magnesium citrate in different mixture ratios. The experimental conditions of synthesis include temperature of 800 ° C, heating rate of 10 ° C min'1, residence time of 1 hour and use of high purity nitrogen gas (99.9999%), producing materials with surface area values from 600 to 2200 m2 g'1 and nitrogen composition ranging from 0.5 to 30%. The capacitance values calculated by the half area of the voltammogram in the fifth cycle with a sweep speed of 1 mV s'1 were values between 230 and 323 F g'1. The invention characterized by EP 0 881 650 BI discloses the obtaining of CEDCs by chemical treatment of petroleum coke with KOH for porosity development. The activation process was carried out at temperatures ranging from 500 to 1000 ° C at times between 1 and 3 hours, and different mass percentages of KOH (100 to 500), which provided materials with surface areas of 500 ° C. at 1500 m2 g'1. The obtained materials presented capacitance values ranging from 11.9 to 16.6 pF m. EP 0 490 317 A1 uses sewage sludge as a precursor, which has been charred at temperatures of 150 to 600 ° C. After this process, the materials are heat treated again at temperatures from 700 to 1100 ° C in the atmosphere, containing carbon dioxide, water vapor or oxygen, producing materials containing 1 to 5% nitrogen and 3 to 30% oxygen. in its composition, used as a catalyst in the decomposition of hydrazine. The invention described in WO 2013033847 A1 discloses as precursor eggshell which is made up of carbon and nitrogen from proteins. The preparation of the material for subsequent application as electrode requires exhaustive and successive experimental steps, including carbonization at 800 ° C for 2 hours, subsequent washing with KOH (20%) for 2 hours and HCl (2 M) for 15 hours. After that, one portion of the material was heat treated under normal atmosphere for 2 hours, the other portion was activated with KOH at 700 ° C for 2 hours in argon atmosphere and washed with HCl (2 M) and deionized water. The material produced without the presence of inert gas showed about 8% nitrogen in its composition, BET surface area of 221.2 m2 g'1 and a specific capacitance at a current density of 0.2 A g'1 of 297 Activated carbon (KOH) had a BET surface area of 1575 m2 g'1 and capacitance at a current density of 0.2 A g'1 of 203 F g'1 with a percentage of doping of carbon nanotubes with different nitrogen precursors has been reported by EP 2 543 631 Al. This invention used as nitrogen precursors 1 H-benzotriazole (BTA), lH-1,2, 3-triazole [4-5-b] pyridine (TAPy) and 5- (2-pyridyl) -1H-tetrazol (PyTZ), which were mixed with carbon nanotubes and then subjected to a pyrolysis step at 700 ° C. The materials are presented as catalysts for oxygen reduction and fuel cell application WO 2010020007Al describes the use of coffee waste and ZnCE as activating agent for obtaining CEDC. The materials obtained showed BET surface area values ranging from 800 to 1200 m2 g'1 and specific capacitance values at a current density of 0.1 A g "1 up to 368 F g" 1.

In addition to the patents described, there are several scientific articles in the relevant literature, which synthesized nitrogen-doped carbons. These works often present complex preparation conditions, requiring the execution of several experimental steps. Hou et al. (Carbon, v.64, p.141, 2013) used as precursor an aniline monomer that was distilled at low pressures and then stored in an argon atmosphere. The aniline monomer polymerization was performed in the presence of potassium persulfate with the precursor, occurring from an oxidation reaction under controlled hydrothermal conditions. After this process, the mixture was heated at 800 ° C for 2 hours in the presence of argon. The material obtained presented on its surface a composition of 5.9% nitrogen and a surface area of 352 m2 g'1, which gave a specific capacitance at a current density of 0.5 A g'1 of 239 F g'1.

Hu et al. (Carbon, v.48, p.3599, 2010) synthesized a material under conditions of high temperatures (600 to 900 ° C), low heating rates (approximately 1.5 ° C min'1) and long pyrolysis times ( 6 to 12 hours), and use complex mixtures of reagents (formaldehyde, phenol, carbon tetrachloride, ethylenediamine, among others) to obtain precursors. The porosity development stage was performed with high KOHrprecursor impregnation ratio (5: 1), temperature of 900 ° C and time of 6 hours, which provided materials with surface areas ranging from 384 to 2222 m2 g'1. and a maximum specific capacitance value of 274 F g'1 at a current density of 0,25 A g'1.

As reported in the inventions described above (EP 2508475 A1, EP 0881650 A1, WO 2013033847 A1, EP 2543631 A1 and WO 2013020007 A1) and works published in specialized journals (Hou et al., Carbon, v.64, p. 141, 2013; Hu et al., Carbon, v.48, p.3599, 2010), the preparation of nitrogen-doped carbonaceous materials requires the use of several reagents, often in high quantities (concentrations or ratios) and procedures with several steps, which are performed at high temperatures and high times. The innovative aspect of the invention described herein is that it employs a novel route for obtaining nitrogen-doped carbonaceous materials using an adsorption process residue. The residue consists of a high surface area activated carbon obtained from coconut shell and chemical activation with NaOH, saturated with methylene blue dye, which was pyrolised under mild conditions such as temperature (635 ° C). (C), time (0.5 h) and inert gas flow (100 mL min -1). Another innovative aspect is the use of a carbon source from a renewable resource for preparation of nitrogen-doped materials. The high surface area of the activated charcoal, initially obtained, makes it possible to achieve high values of adsorption capacity of dyes and organic compounds, which after a subsequent heat treatment process, provides a nitrogen-doped material. These characteristics make it possible to use this material as electrodes for electric double layer capacitors. The present invention meets the need to provide a correct and adequate disposal of tailings obtained in adsorption processes, since the reuse of saturated adsorbent material is unfeasible due to the fact that the adsorption desorption process is unfavorable. Thus, this sustainable synthesis route allows for materials with appreciable nitrogen content on their surfaces which, together with the high surface area values, promote a significant increase in their capacitive behaviors.

3. DESCRIPTION OF THE FIGURES

For a better understanding of the object of the present invention the following figures have been made: Figure 1. Structure of methylene blue (AM) dye;

Figure 2. SEM images of CA-AM (a) and NCA (b);

Figure 3. FT-IR spectra for AM (a), CAsat (b) and NCA (c);

Figure 4. XPS spectrum for CA and NCA (a), deconvoluted N1s spectrum (b).

Figure 5. Cyclic voltammograms at different sweep speeds (a) and charge and discharge curves at different NCA current densities (b).

Figure 6. Specific NCA capacitance as a function of the number of cycles analyzed.

4, DETAILED DESCRIPTION OF THE INVENTION Activated charcoal (CA) removal: CA was prepared using coconut shell (Cocos nucifera L.) as a precursor. Properly dried material with particle size between 250 and 425 pm was placed inside a horizontal stainless steel tube reactor and heated in a muffle furnace with a heating rate of 20 ° C min'1 from room temperature to 500 ° C under nitrogen flow of 100 cm min 'and kept at this temperature for 2 h. The obtained charcoal was mixed with 3: 1 NaOH (m: m NaOH / coal) and water in vertical stainless steel tube under magnetic stirring for 2 h. The material was dried at 130 ° C for 4 h and placed in the muffle furnace under 100 cm3 min -1 nitrogen flow and heated at a rate of 20 ° C min'1 to a temperature of 700 ° C where it was held for 1 min. , 5 h. After cooling, the mixture was properly washed with portions of 0.1 M HCl solution, followed by boiling water until the measured pH of the solution reached a value of –6.5. The material was filtered using 0.45 µm pore size membrane filters. The obtained CA was dried at 110 ° C for 24 h and properly stored for further analysis. - Preparation of Nitrogen-Activated Charcoal (NCA): NCA was prepared from saturation of CA with methylene blue (AM) dye (Figure 1), followed by a thermal regeneration process. Saturation was performed by contacting 1.250 g of CA in contact with 1.250 L of a 1000 mg L "1 concentration AM solution under magnetic stirring for 2.5 h. Then the mixture solution was membrane filtered. 0.45 pm and the saturated CA (CAsat) was oven dried for 24 h at 100 ° C. The thermal regeneration process was performed under conditions previously optimized by the response surface methodology (MSR). regeneration time, temperature and nitrogen flux used in the process were 0.5 h, 635 ° C and 100 mL min'1, respectively - BET surface area analysis and vortex distribution: the textural characteristics of the materials were determined according to From the 77 K N2 adsorption / desorption isotherms using a surface area analyzer, the surface area was determined by the Brunauer-Emmett-Teller (BET) equation, the total pore volume, Vr, was defined as the amount max a nitrogen adsorbed at relative pressure P / P ° = 0.99. The micropore volume, Vp, was determined using the DeBoer method and the mesopore volume, Vm, was calculated by the difference between the values of Vj and Vp. The pore diameter, Dp, was calculated by the ratio AVjJSbet · - Scanning Electron Microscoma Analysis (SEM): morphology was examined by SEM analysis. - Boehm Method: The amount of oxygenated acid groups was determined by applying the Boehm method. Amounts of 0.25 g of NCA were weighed and stirred with 25 mL of HCl (0.025 mol L'1), NaOH (0.025 mol L'1), Na2CC> 3 (0.0125 mol L "1) solutions. and NaHCC> 3 (0.025 mol L -1) for 24 h After this time, the solutions were membrane filtered and the filtrate was titrated with 0.025 mol L "1 HCl and / or NaOH solutions. This methodology considers that: HC1 neutralizes all basic groups; NaOH neutralizes carboxylic acids, lactonic acids and phenolic groups; Na2CO3 neutralizes carboxylic and lactonic acids and NaHCO3 only carboxylic acids. Phenolic groups were determined by difference. The sum of carboxylic, lactonic and phenolic acid groups gives the total amount of acid groups. - Determination όΗργυ. the surface pH of the material was determined from the zero loading point pH (pHPCz) methodology. · 0.1 g NCA quantities were brought into contact with 25 mL volumes of NaCl solution (0.01 mol L '1), which had their pH values adjusted within the range of 3 to 10, using 0.01 mol L "HCl and NaOH solutions. The solutions were stirred for 24 h and then membrane filtered. pore size 0.45 µm.

The pH values were measured and the pHpcz was determined considering the smallest variation between the values on a graph of initial versus final pH. - Fourier Transform Infrared Spectroscopy (FT1R) Analysis: The organic structures present in the AM, saturated charcoal (CAsat) and NAC dyes were analyzed by 4 cm resolution Fourier Transform Infrared (FTIR) spectra. "1 and an acquisition rate of 20 scans min'1 in the range between 4000 and 400 cm'1. - X-ray Photoelectron Spectroscopy Analysis (XPS): X-ray photoelectron spectra were determined from a Al K «monochrome source spectrometer operating at 1486.7 eV with a voltage of 15 kV and an emission current of 10 mA. - Electrochemical measurements: The electrochemical properties of the NCA were assessed from cyclic voltammetry (VC) analyzes. ) and galvanostatic charge and discharge measurements using a 6.0 mol L1 KOH solution as an electrolyte. An electrochemical cell with three electrodes and 6.0 mol L "1 electrolyte solution was used to evaluate electrochemical behavior of the material. The working electrode was prepared by mixing the material (90% by mass), carbon (5%) and polytetrafluoroethylene (5%) in water. The mixture was deposited on a 1 cm2 area nickel foam. The amount of active material per area of nickel foam was 7.0 mg cm. Platinum and Hg / HgO electrodes were used as counter electrode and reference electrode, respectively.

The textural characteristics of the NCA were determined and the data obtained are presented in Table 1.

The data in Table 1 show that the surface area of NCA is 1726 m2 g'1, which corresponds to approximately 61% of the initial area observed for CA, which is 2825 m g '. The smallest surface area of NCA is due to the natural pore destruction caused by the heat treatment process, in addition to the polymerization of part of the AM on the surface of the CA. Additionally, NCA has characteristics of microporous material, having 92.22% micropores and 7.78% mesopores, with average pore diameter size of 2.08 nm.

Table 1. Textural properties of NCA and CA.

The superficial morphological characteristics of CAsat and NCA were analyzed using scanning electron microscopy (SEM). SEM images show that CASat (Figure 2a) has a surface with no apparent cracks, which was observed for NCA (Figure 2b), demonstrating that some of the impregnated AM was removed on its surface. The determination of surface functional groups by the Boehm method showed that NCA is mainly composed of acid groups, which correspond to 73% of the total groups, which are described as: carboxylic (0.35 mmol g'1), phenolic ( 0.42 mmol g'1) and lactonic (0.11 mmol g'1). On the other hand, the basic groups add up to 0.35 mmol g'1, corresponding to 27% of the total groups. The acid characteristic of the material is confirmed by the pHPCZ determination, which presented a value of 5.10. FTIR analysis provides information on the surface chemical characteristics of materials. The FITR spectrum for AM (Figure 3a) shows characteristic bands in the region of 674 and 900 cm -1, which indicate axial deformations of C-H bonds in polynuclear aromatic rings. The bands at 1020 and 1250 cm -1 are attributed to axial deformations of C-N bonds of aliphatic amines. According to the CASat spectrum (Figure 3b), characteristic AM peaks can be observed, indicating the process of dye adsorption on the CA. After thermal regeneration process, the NCA spectrum (Figure 3c) presents less peaks and lower intensity when compared to CAsat, besides the disappearance of the characteristic bands of the AM. Additionally, the approximately 1020 cm'1 band, characteristic of nitrogen groups, indicates the insertion of nitrogen on the surface of the NCA.

XPS analyzes (Figure 4a) indicate the presence of chemical elements on the surface of CA and NCA. The photoelectron lines at 285, 401 and 532 eV correspond to Cls, Nls and Ols, respectively. According to this figure (4a), an increase in the signal corresponding to the nitrogen groups can be observed. Additionally, the elemental analysis obtained by the XPS analysis shows that the carbon, oxygen and nitrogen content is 86.24, 13.15 and 0.61% for CA, and 83.16, 12.31 and 3, 82% for the NCA, respectively. According to the results, an increase of approximately 3% of nitrogen in NCA can be observed when compared to CA, confirming the polymerization of a part of the AM on the material surface. The deconvoluted Nls signal spectra (Figure 4b) corresponding to binding energies of 399.0 eV and 400 eV indicating the CN = C and N- (C) 3 groups on the NCA surface and only N- (C ) 3 about CA. According to the literature, carbonaceous materials, which have nitrogen incorporated in their structure, have been shown to be a promising material to be used as a capacitor.

Electrochemical measurements of the NCA electrode were performed and the cyclic voltammograms obtained are shown in Figure 5a. It can be observed that, even with high sweep speed (100 mV s'1), the voltammograms presented rectangular geometric shape, indicating good electrochemical properties. Figure 5b shows the galvanostatic charge and discharge curves at different current densities for the NCA. The curves obtained are linear, sharp and with low ohmic falls, which indicates good capacitive behavior with low equivalent resistance. This feature may be related to the presence of mesopores in the NCA structure, which speeds up the transfer rate and may reduce the internal resistance of the electrode, indicating that the internal energy consumed by resistance is low and all energy is effectively stored by the system. The galvanostatic charge and discharge measurement technique is considered ideal for the measurement of pseudo capacitance. Specific capacitance was calculated from the galvanostatic charge and discharge curves using the following relation: where, Cesp (F g'1) is the specific capacitance, / (A) refers to the charge and discharge current, AV (V) represents the voltage variation within the discharge time range At (s) in (g) corresponds to the electrode NCA mass.

The values of Cesp obtained for current densities of 0.5; 1; 2; 4 and 8 A g'1 were 227.52; 185.11; 168.22; 122.22 and 108.44 F g "1, respectively. These high specific capacitance values may be related to the pseudocapacitance effect of nitrogen groups added to the large surface area of NCA. The stability cycle of NCA (Figure 6) after 1000 charge and discharge cycles had a capacitance value of 138 F g "1, corresponding to a decrease of approximately 17% of the initial value. This is an important parameter as it is an indication of the lifetime of a capacitor.

Claims (7)

1. NITROGEN-DUPLED ACTIVATED COAL PRODUCED FROM THE THERMAL REGENERATION OF METHYLENE BLUE-SATURED MATERIAL, characterized by the production of high surface area activated carbon produced from an agroindustrial tailing, the coconut shell (Cocos nucifera L.); and chemical activation with NaOH. NITROGEN-DUPLED ACTIVATED COAL PRODUCED FROM THE THERMAL REGENERATION OF METHYLENE BLUE-SATURED MATERIAL according to claim 1, characterized by the saturation of activated carbon with the methylene blue dye (nitrogen precursor). NITROGEN-DUPLED ACTIVATED COAL PRODUCED FROM THE THERMAL REGENERATION OF METHYLENE BLUE-SATURED MATERIAL according to claims 1 and 2, characterized by the heat treatment of the material saturated with the dye using. NITROGEN-DUCTED ACTIVATED CARBON FROM THERMAL REGENERATION OF METHYLENE BLUE-SATURED MATERIAL according to claims 1, 2 and 3, characterized in that it uses methylene blue dye as a precursor to nitrogen groups. 5. NITROGEN-DUPLED ACTIVATED COAL PRODUCED FROM THE THERMAL REGENERATION OF METHYLENE BLUE SATURATED MATERIAL, according to claims 1, 2, 3 and 4, characterized by obtaining a nitrogen-doped material with a high surface area (1726 m2 g "1) which can be used as adsorbent material, catalyst and supercapacitor electrode. ACTIVATED CARBON COVERED WITH NITROGEN PRODUCED FROM THE THERMAL REGENERATION OF METHYLENE BLUE SATURED MATERIAL, according to claims 4 and 5, characterized by obtaining a material consisting of 3.82% nitrogen, formed by the pyridine groups and Pyrrolic. NITROGEN-DUPLED ACTIVATED COAL PRODUCED FROM THE THERMAL REGENERATION OF METHYLENE BLUE-SATURED MATERIAL according to claims 4, 5 and 6, characterized by obtaining a double-layer electric capacitor with specific capacitance at a current density 0.5 A g'1, 227.52 F