WO2022130351A1

WO2022130351A1 - Process for obtaining high surface area mesoporous carbons from biomass waste for energy storage

Info

Publication number: WO2022130351A1
Application number: PCT/IB2021/061987
Authority: WO
Inventors: Akshay Jain; Mahi SINGH
Original assignee: Cancrie Inc
Current assignee: Cancrie Inc
Priority date: 2020-12-19
Filing date: 2021-12-18
Publication date: 2022-06-23
Anticipated expiration: 2023-06-19

Abstract

The embodiments herein relate to a process of obtaining high surface area mesoporous carbons for energy storage and to serve as a superior material for supercapacitors, hybrid capacitors, redox flow batteries, lead acid batteries, etc. The mesoporous carbons obtained by present invention has high capacitance characteristics, high thermal stability, high specific surface area and provides higher strength to electrodes. The process of present invention leads to significant increase in pore volume and pore area, thus significantly improving the capacity and life span of energy storage devices. The present invention utilizes pre-treatment with oxygen and use of single or dual stage chemical activators.

Description

PROCESS FOR OBTAINING HIGH SURFACE AREA MESOPOROUS

CARBONS FROM BIOMASS WASTE FOR ENERGY STORAGE

BACKGROUND

Technical Field of Invention

[001] The embodiments herein generally relate to porous carbons and particularly to mesoporous carbons. The embodiments herein more particularly relate to a method of synthesizing high performance mesoporous carbons having higher surface area for use as energy storage materials.

Description of Related Art

[002] There is a strong need to move towards development of sustainable and renewable energy resources because of diminishing conventional resources such as fossil fuels. Considerable amount of research has been done in this direction such as production of biofuels, solar energy, use of biomass for energy production in the form of biofuels, etc.

[003] In recent years, waste-to-energy/-resource conversion has received considerable attention in response to a substantial increase in the utilization of natural resources and the amount of solid wastes generated, caused by rapid population growth. For example, the total amount of municipal solid waste (MSW) generated globally was 2000 million tons. Utilization of MSW for producing new resources, especially for environmental remediation, is attractive because it (i) offers solutions for solid waste management, (ii) reduces the cost of raw materials required for production of valuable chemicals and commercial products and (iii) addresses global environmental issues in the context of sustainability. Products such as porous carbon obtained from such wastes are being explored for a wide range of applications such as removal of heavy metals from contaminated water, removal of contaminants from flue gas, CO2 capture, hydrogen storage, heterogeneous catalysis, photocatalysis, bio-imaging, drug delivery, and energy storage.

[004] For this, a variety of batteries, capacitors, and hybrid capacitors are used as energy storage material. However, these suffer the drawbacks for large-scale storage, such as low power density, limited life span and low energy density. [005] Electrode quality plays a major role in overall performance of any energy storage device. Due to eco-friendly nature and low cost, activated carbon has been the preferred choice of electrode material. Activated carbons are versatile adsorbents due to their high surface area and therefore find applications in many fields such as separation of environmental contaminants and purification of gases, resource recovery, catalysis, etc.

[006] Activated carbon or activated charcoal, is a form of carbon that is processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions. Enhancement of the porosity of these activated carbons is highly desirable. This is usually achieved by employing different chemical activating agents, optimization of the activation conditions such as temperature, ramp rate and gas flow rate and pre-treatment methods including soaking or stirring. Oxygenated Functional Groups (carboxylic, lactonic and phenolic) content is an important attribute of the precursor which governs chemical activation and can thus predict the level of porosity in the activated carbons. Carbons with high micro porosity (pore size lesser than 2nm) are used extensively for adsorption of small sized pollutants or molecules; however, the treatment of systems containing large molecules (high molecular weight compounds, dyes, etc.) requires adsorbents with high mesopore content (pore size from 2nm to 50 nm). In addition, applications such as catalyst supports, battery electrodes, capacitors and gas storage also require high surface areas and high mesoporous content in the adsorbent matrix.

[007] Primarily, templating methods or activation processes with suitable activating agents are used to produce mesoporous carbons. Zhuang et al. showed that using commercial tri-block copolymer F127 as a structure-directing agent and tetraethyl orthosilicate as a template can yield 100% mesoporosity with high specific surface are as up to 2580 m2/g.

[008] Despite the ability to generate high mesopore surface as by hard templating, the process has some inherent drawbacks. Removal of the sacrificial component requires appropriate treatment procedures; in particular, hydrofluoric acid (which is a hazardous chemical) is used for the removal of silica templates. The carbonaceous material that constitutes the matrix may have structural defects formed during template removal. In the case of soft template synthesis, a constraint is imposed in terms of the availability of the right combination of carbon-yielding and pore- forming components. Furthermore, the use of specific precursors as sacrificial and matrix compounds increases the cost of production.

[009] However, in the existing state of the art, the limitation is the high ZnC12 requirement for the generation of mesoporosity. However, it is not possible to eliminate this step since the use of lower ZnC12 amounts give activated carbons with substantially lower mesoporosity, compared to the commercial product. But, at the same time, upto 90-95% recovery of ZnC12 has already been achieved. Recent works have shown that reactive carbonaceous precursors (biomass or carbohydrates) yield activated carbons with increased porosity when activated with NaOH and KOH but mainly microporous in nature. Therefore, as emphasized by Libra et al., there is a need to develop suitable hydrochars as precursors for activated carbon synthesis. OFG (Oxygenated functional groups - carboxylic, lactonic and phenolic) content in the precursor is an important indicator of reactivity which governs the chemical activation and thus can be used as a predictor of porosity in the activated carbons.

[0010] WIPO patent application no. WO2016072932A1 mentions activated carbon, hydrochar and processes for making the same. A biomass-oxidizing agent mixture is subjected to a hydrothermal carbonization process to form a hydrochar having an increased oxygenated functional group content compared to that of the biomass.

[0011] Chinese patent application no. CN 101759181 provides a method for producing activated carbon for super capacitors and relates to activated carbon. The method comprises steaming the materials and phosphoric acid, holding and carbonizing, dewatering, drying, pulverizing and packaging separately to obtain the finished product of activated carbon.

[0012] Another Chinese patent application no. CN 102205963 provides a method for preparing activated carbon for a biomass-based super capacitor, and relates to a new method for preparing activated carbon with high specific surface area and high specific capacitance by the steps of hydrolysis of biomass with concentrated acid, in- situ polycondensation and carbonization of saccharic acid solution and activation under certain conditions.

[0013] US patent no. US6057262 provides a process for the manufacture of activated carbon in the form of a powder, as granules or as extrudates. The process includes treating a biomass feedstock, such as woods, coconut shells, fruit pits, peats, lignites and all ranks of coal with a processing agent and an activation agent. The processing agent may be a natural or synthetic monomer, oligomer, polymer or mixtures thereof capable of interacting or co -polymerizing with the biomass feedstock. The activation agent may be, for example, phosphoric acid, zinc chloride or mixtures thereof.

[0014] Another Chinese patent application no. CN-110364369 provides method of preparation high-performance shredded coconut meat active carbon. This method separates coconut husk first and obtains shredded coconut meat, and hydro-thermal activates under certain conditions, and finally high temperature carbonization obtains shredded coconut meat active carbon in tube furnace.

[0015] But the existing state of the art suffers several drawbacks such as high cost, low capacitance, and use of hazardous chemicals (with reference to templated carbons).

[0016] In the view of foregoing, there is an urgent need to develop a cost-effective, low-cost process to obtain high performance mesoporous carbons with optimum capacitance. There is also a need to provide a method of synthesizing improved high surface area mesoporous carbons for energy storage which utilizes low temperature conditions, natural waste materials and simple equipment.

[0017] The above mentioned shortcomings, disadvantages and problems are addressed herein, as detailed below.

SUMMARY OF THE INVENTION

[0018] Thus, the primary object of the embodiments herein is to provide a process for obtaining high performance mesoporous carbons from biomass wastes.

[0019] Another object of the embodiments herein is to provide a process for obtaining high performance mesoporous carbons for use as energy storage in order to serve as a superior material for supercapacitors, hybrid capacitors, redox flow batteries, etc.

[0020] Yet another object of the embodiments herein is to provide a process of synthesizing high surface area mesoporous carbons prepared by oxygen pre-treating the raw materials obtained from natural waste resources.

[0021] Yet another object of the embodiments herein is to provide an eco-friendly and cost-effective process of producing high performance mesoporous carbons for energy storage. [0022] Yet another object of the embodiments herein is to provide a process of synthesizing high surface area mesoporous carbons utilizing the waste from natural resources.

[0023] Yet another object of the embodiments herein is to provide a process of synthesizing high surface area mesoporous carbons having an increased oxygen functional group (OFGs) for increased activity.

[0024] According to an embodiment herein, a process for obtaining high surface area mesoporous carbons for energy storage is provided. The process comprises pre- treating a raw material to obtain a char (101). The char is then activated using a chemical activator to obtain mesoporous carbon (102). The mesoporous carbon is collected (103). The mesoporous carbon is washed (104).

[0025] According to an embodiment herein, the chemical activator is Zinc Chloride solution.

[0026] According to an embodiment herein, the raw material is a biomass waste comprising saw dust, sugarcane bagasse, palm kernel shells, almond shells, coconut shell, coconut husks, municipal sludge, chicken poop, human hair, and a lignin powder or a combination thereof.

[0027] According to an embodiment herein, the activation of char is done in a furnace by mixing the char with the chemical activator in a ratio ranging from 2:1 to 8:1, more particularly in a ratio 3:1 to 5:1.

[0028] According to an embodiment herein, the activation of char is done in a furnace by heating a dried mixture of char and the first chemical activator at a temperature of 350 to 600°C increasing at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/ min in the presence of nitrogen gas (N2) for time period ranging from 1 to 8 hours, more particularly for 2 to 5 hours.

[0029] According to an embodiment herein, the flow rate of nitrogen gas (N2) ranges from 200 to 2000 mL/min, more particularly at a flow rate of 500-1000 mL/ min.

[0030] According to an embodiment herein, the mesoporous carbon is collected by cooling down the furnace to room temperature in the presence of N2 at a flow rate ranging from 100 to 500 mL/min., more particularly at 200-300 mL/min.

[0031] According to an embodiment herein, the washing is done with hydrochloric acid solution. [0032] According to an embodiment herein, the pre-treatment of raw material is done in presence of oxygen (O2) at a flow rate ranging from 200 to 1000 mL/min., more particularly at 500 to 1000 mL/min with an increasing temperature of up to 50°C to 350°C at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/minute for a time period ranging from 1 to 8 hours, more particularly for 2 to 5 hours . The pre-treatment of raw material is done in presence of oxygen (O2) preferably at 80°C to 220°C.

[0033] According to an embodiment herein, the first chemical activator is recycled.

[0034] According to an embodiment herein, the pre-treatment provides greater mesopore volume and wider pore size distribution in the range of 30 Å to 50 Å of the carbons.

[0035] According to an embodiment herein, the mesoporous surface areas is increased by two folds.

[0036] According to another embodiment herein, a mesoporous carbon for use as energy storage material is provided. The mesoporous carbon has a mesopore volume and a pore size distribution in the range of 30 Å to 50 Å of the carbons.

[0037] According to another embodiment herein, a mesoporous carbon for use as energy storage having increased mesopore area is provided. The mesopore area of the carbon is increased by two folds as compared to the conventional carbons.

[0038] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which: [0040] FIG. 1 is a flow chart showing the various steps involved in the process of obtaining high surface area mesoporous carbon, according to an embodiment herein.

[0041] FIG. 1a shows images of activated carbon obtained from lignin powder as starting material, according to an embodiment herein.

[0042] FIG. 1b shows images of activated carbon obtained from coconut shell as starting material, according to an embodiment herein.

[0043] FIG. 1c shows images of activated carbon obtained from human hair as starting material, according to an embodiment herein.

[0044] FIG. 2 shows schematic representation of process of pre-treating raw biomass waste with oxygen to obtain oxygenated char, according to an embodiment herein.

[0045] FIG. 3 shows schematic representation of chemical activation process to obtain mesoporous carbon, according to an embodiment herein.

[0046] FIG. 4 shows FE-SEM image of mesoporous carbon obtained from coconut- shell, according to an embodiment herein.

[0047] FIG. 5 shows a graph of nitrogen adsorption-desorption isotherms of mesoporous carbon prepared from coconut shell that underwent oxygen pre- treatment compared to carbon prepared without any pre-treatment of biomass, according to an embodiment herein.

[0048] FIG. 6 shows a graph of pore size distribution of a mesoporous carbon prepared from coconut shell that underwent oxygen pre-treatment compared to carbon prepared without any pre-treatment, according to an embodiment herein.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] In the following detailed description, a reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. The embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.

[0050] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such a process or method. Similarly, one or more steps of method or components preceded by "comprises... a" does not, without more constraints, preclude the existence of other, steps or components. Appearances of the phrase "in a preferred embodiment”, “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but not necessarily do, all refer to the same embodiment.

[0051] The various embodiments herein provide a process for obtaining high performance mesoporous carbons for energy storage to serve as a superior material for supercapacitors, hybrid capacitors and redox flow batteries. The present invention uses coconut shell, human hairs, and lignin as raw materials. The present invention provides a process of synthesising high performance mesoporous carbons having enhanced oxygen functional groups providing enhanced activity.

[0052] FIG. 1 is a flow chart showing the various steps involved in the process of obtaining high surface area mesoporous carbons, according to an embodiment herein. With respect to FIG. 1, the process comprises pre-treating a raw material to obtain a char (101). The char is then activated using a first chemical activator to obtain mesoporous carbon (102). The mesoporous carbon is collected (103). The mesoporous carbon is washed (104). The chemical activator is Zinc Chloride solution. The raw material is a biomass waste comprising saw dust, sugarcane bagasse, palm kernel shells, almond shells, coconut shell (Cocos nucifera), coconut husks, municipal sludge, chicken poop, human hair, and a lignin powder or a combination thereof. The activation of char is done in a furnace by mixing the char with a first chemical activator in a ratio ranging from 2:1 to 8:1, more particularly in a ratio3:l to 5:1. The activation of char is done in a furnace by heating a dried mixture of char and the first chemical activator at a temperature of 350 to 600°C increasing at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/ min in the presence of nitrogen gas (N2) for time period ranging from 1 to 8 hours, more particularly for 2 to 5 hours. The flow rate of nitrogen gas (N₂) is maintained at a range from 200 to 2000 mL/min., more particularly at a flow rate of 500-1000 mL/ min. The mesoporous carbon is collected by cooling down the furnace to room temperature in the presence of N₂ at a flow rate ranging from 100 to 500 mL/min., more particularly at 200-300 mL/min. The washing is done with hydrochloric acid solution. The pre-treatment of raw material is done in presence of oxygen (O2) at a flow rate ranging from 200 to 1000 mL/min . , more particularly at 500 to 1000 mL/min with an increasing temperature of up to 50°C to 350°C increasing at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/minute for a time period ranging from 1 to 8 hours, more particularly for 2 to 5 hours, or preferably at 80°C to 220°C. The first chemical activator is recycled.

[0053] According to an embodiment, the raw material is pre-treated to obtain a char. The pre-treatment of raw material is done in presence of oxygen (O2) at a flow rate ranging from 200 to 1000 mL/min . , more particularly at 500-1000 mL/min at temperature from 50°C to 350°C increasing at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/minute for a time period ranging from 1 to 8 hours, more particularly for 2 to 5 hours, or preferably at 80°C to 220°C. The pre-treatment is done more preferably at 100°C to 150°C.

[0054] According to an embodiment, the step of pre-treatment leads to surface modification of raw material forming a char with increased Oxygenated Function Group (OFG) content which is favourable for the chemical activation and thus has a strong advantageous effect in the preparation of the mesopore areas of the resulting carbons as compared to the raw biomass waste. The raw material is pre-treated under controlled conditions in the presence of oxygen to obtain char with high OFG (oxygenated functional groups) content and lower crystallinity that facilitates the production of mesoporous carbon with high-surface area. The oxygen pre-treatment provides greater mesopore volume and wider pore size distribution in the range of 30 Å to 50 Å when compared with the conventional direct soaking method. According to an embodiment of the present invention, the pre-treatment of the raw biomass waste in the presence of oxygen decreases the crystallinity of the char.

[0055] According to an embodiment herein, a significant increase in the mesopore area is further achieved by the use of chemical activating agents with the oxygen pre- treatment. Oxygen pre-treatment creates more Oxygenated Functional Group content (OFG) that promotes a facile chemical activation and thus paves way for developing synthesis protocols for char that optimize the use of natural resources, chemical reagents and energy.

[0056] According to an embodiment herein, the raw biomass waste comprises the waste products obtained from natural resources. Thus, the present invention utilizes the waste material in generating mesoporous carbons which is utilized in energy storage. Hence, in an embodiment herein, the raw material is biomass waste, wherein the biomass waste comprises saw dust, sugarcane bagasse, palm kernel shells, almond shells, coconut shells (Cocos nucifera), coconut husks, municipal sludge, chicken poop, human hairs and commercially available lignin powder or a combination thereof. The char is obtained from raw materials with reduced or low ash content and no metal content.

[0057] According to an embodiment herein, the coconut shells are crushed after trimming the fibres using a commercial laboratory blender (Waring) and then ground and sieved into coarse granules (10-20 mesh). The shell granules, hairs and lignin powder are dried at 80°C to 120°C for 15 to 40 hours.

[0058] In an embodiment herein, the char is activated by chemical activation employing zinc chloride as one of the chemical activators, and then the resultant mesoporous carbon is dried out in specific reaction conditions.

[0059] According to another embodiment herein, a successive activation of char by zinc chloride (ZnCl₂) followed by potassium hydroxide (KOH) results in optimal etching of surface which in turn yields higher surface area mesoporous carbons.

[0060] According to an embodiment herein, the heat generated in the process of activation is recovered to recycle the ZnC12 by membrane distillation. The activated carbon is washed to remove ZnC12 from first activation. According to another embodiment herein, potassium oxide (KOH) is used as a second chemical activator. The resultant mesoporous carbon again activated, and the KOH is removed after the successive activation, thereby releasing waste of ZnC12 and KOH. ZnC12 and KOH are separately recycled from water stream and the rejects released out of the process are easily disposed by mixing them together for precipitation. The precipitates are disposed of.

[0061] According to an embodiment herein, the chemical activation is mainly done at a temperature of 350°C to 1100°C. According to a preferred embodiment herein, the chemical activation is done at a temperature of 450°C to 900°C. According to a more preferred embodiment herein, the chemical activation is done at a temperature of 500°C to 800°C.

[0062] According to an embodiment herein, the char is activated by using a chemical activator to obtain mesoporous carbon. The chemical activator is Zinc Chloride solution, according to an embodiment herein. The activation of char is done in a furnace by mixing the char with a chemical activator in a ratio ranging from 2:1 to 8:1, more particularly in a ratio 3:1 to 5:1. According to an embodiment herein, the activation of char is done in a furnace by heating a dried mixture of char and the chemical activator at a temperature of 350°C to 600°C increasing at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/ min in the presence of nitrogen gas (N2) for time period ranging from 1 to 8 hours, more particularly for 2 to 5 h. According to an embodiment herein, the flow rate of nitrogen gas (N₂) is maintained at a range from 200 to 2000 mL/min., more particularly at a flow rate of 500-1000 mL/ min. The activation is preferably done at a temperature of 450-550°C.

[0063] According to another embodiment herein, a successive base activation of the mesoporous carbon is carried out with a second chemical activator to obtain a high surface area mesoporous carbon. The second chemical activator is KOH solution, according to an embodiment. The second chemical activator is mixed with the mesoporous carbon in a ratio ranging from 0.1:1 to 2:1, more particularly in a ratio 0.5:1 to 1:1. The successive base activation of mesoporous carbon is done in a furnace by heating a dried mixture of mesoporous carbon and the second chemical activator at temperature of 700-900°C increasing at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/ min in the presence of nitrogen gas (N₂) for time period ranging from 1 to 8 hours, more particularly for 2 to 5 h. According to an embodiment herein, the flow rate of nitrogen gas (N₂) ranges from 200 to 2000 mL/min., more particularly at a flow rate of 500-1000 mL/ min. The successive activation is preferably done at a temperature of 750-850°C.

[0064] According to an embodiment herein, the mesoporous carbon after activation and drying is ground in the size range of 1 micron to 100 microns for different performances and applications. In an embodiment, addition of Graphene and Carbon Nanotubes in certain weight ratios from 0.2% to 2% increases the conductivity of the mesoporous carbon mixture which in turn is favourable to be used as an electrode material.

[0065] According to an embodiment herein, addition of hair based activated carbon in certain weight ratios from 1% to 20% increases the nitrogen functionalities of the mesoporous carbon mixture which in turn is favourable to be used as an electrode material. In an embodiment, addition of hairs increases the nitrogen functionalities of the mesoporous carbon mixture, preferably from 0.2%-5%, and more preferably from 0.5%-3%.

[0066] FIG. 1a shows images of activated carbon obtained from lignin powder as starting material, according to an embodiment herein.

[0067] FIG. 1b shows images of activated carbon obtained from coconut shell as starting material, according to an embodiment herein.

[0068] FIG. 1c shows images of activated carbon obtained from human hair as starting material, according to an embodiment herein.

[0069] According to an embodiment herein, the prepared activated carbon may be used in a lead acid battery. In this aspect, the lead acid battery containing prepared activated carbon showed 7% improved performance in charge acceptance in comparison to conventional lead acid batteries which is expected to increase up to 20%. The lead acid battery containing prepared mesoporous carbon in this aspect further showed three times better vibration and shock absorption capacity in comparison to conventional lead acid batteries. The lead acid battery containing prepared mesoporous carbon in this aspect leads to 20 times improved GHG emission savings and is expected to achieve up to 60% more savings on warranty returns in comparison to conventional lead acid batteries.

[0070] According to an embodiment herein, the lead acid battery containing prepared activated carbon showed enhanced cohesive strength, charge acceptance and longer life cycle.

[0071] The invention is illustrated below with non-limiting examples. The reagents and reaction conditions mentioned such as specific high temperature and ratio of the raw materials are exemplary and should not be construed as being limiting to the values provided in the below examples.

EXAMPLE 1 [0072] Pre-treatment of raw biomass waste with oxygen to obtain char: FIG. 2 shows schematic representation of process of pre-treating raw biomass waste with oxygen to obtain oxygenated char, according to an embodiment herein. With respect to FIG. 2, the raw biomass waste comprising of coconut shell granules or lignin powder or human hair or a combination thereof was loaded on to SS316 trays inside a SS316 baffle box which was then placed in a muffle furnace. The temperature was ramped up to 250 C at a rate of 6°C/ min in the presence of O₂ precursor at a flow rate of 500 mL/minute for a time of 3 hours. The furnace was cooled to room temperature to obtain oxygen pre-treated char (O-char). Finally, the char was collected.

EXAMPLE 2

[0073] Chemical activation and drying of the char to obtain mesoporous carbon: FIG. 3 shows schematic representation chemical activation process to obtain mesoporous carbon, according to an embodiment herein. With respect to FIG. 3, the oxygen pre-treated char was mixed with chemical activator zinc chloride (ZnCl₂) solution (800 mL water in precursor equivalent to 250 grams of material) with a ZnCl₂: char ratio of 4:1 and dried at 105°C for 12 hours. The mixture was loaded on to stainless steel (SS316) trays inside a SS316 baffle box which was then placed in a high temperature muffle furnace connected to scrubber and gas exhaust. The temperature was ramped to 500°C at a rate of 6°C/ min in the presence of N₂ (passed using N₂ cylinder) at a flow rate of 500 mL/minute for a time of 3 hours. The furnace was cooled to room temperature in the presence of N₂ at a flow rate of 200 mL/min. The baffle box was opened, and product was collected.

EXAMPLE 3

[0074] Washing of mesoporous carbon: The mesoporous carbon collected above was stirred for 30 minutes in 2500 mL hydrochloric acid (about 0.1 mol/L) (first wash). Mesoporous carbon was separated using meshes of size 200μm and 325μm. Then the activated carbon was washed with 2500 mL distilled water (second wash). This first and second wash of water was used to recover the majority of zinc chloride from the solution by membrane distillation technique. Later mesoporous carbon was separated and washed with abundant distilled water until a pH of 6 was obtained for the rinse. Finally, the mesoporous carbon was dried at 105°C for 24 hours. EXAMPLE 4

[0075] Recycling of zinc chloride (ZnCl₂): Zinc chloride was recycled or recovered from the water from first and second wash in above step using membrane distillation technique.

EXAMPLE 5

[0076] Successive base activation and drying of char to obtain high surface area mesoporous carbon: The mesoporous carbon obtained from above procedure was successively activated with base potassium hydroxide (KOH). The mesoporous carbon was mixed with KOH solution (800 mL water in carbon equivalent to 250 g raw material) with a KOH: carbon ratio of 1:1 and dried at 105°C for 12 hours. The mixture was loaded on to stainless steel (SS316) trays inside a SS316 baffle box which was then placed in a muffle furnace. The temperature was ramped up to 800°C at a rate of 6°C/ minute in the presence of nitrogen (N₂) at a flow rate of 500 mL/minute for a time of 3 hours. The furnace was cooled to room temperature in the presence of N₂ at a flow rate of 200 mL/minute.

EXAMPLE 6

[0077] Washing of mesoporous carbon after successive base activation: The mesoporous carbon prepared after successive activation with KOH was stirred for 30 minutes in 2500mL hydrochloric acid (about 0.1 mol/L) (first wash). Activated carbon was separated using meshes of size 200μm and 325μm. Then the carbon was washed with 2500 mL distilled water (second wash). The activated carbon was separated and washed with abundant distilled water until a pH of 6 was obtained for the rinse. Finally, the activated carbon was dried at 105°C for 24 hours and used for analysis. This entire process is done by using meshes of size 200 and 325.

[0078] The KOH waste stream produced here and ZnCl₂ waste stream produced above after activation is mixed together to undergo precipitation and their disposal.

EXPERIMENTAL DETAILS

[0079] Adsorption-desorption isotherms, Brunauer-Emmett-Teller (BET) surface area (At) and pore volume of the adsorbents were obtained by using a gas sorption analyzer (Nova-3000 Series, Quantachrome). Adsorption-desorption isotherms were obtained with N₂ at 77 Kelvin after degassing the carbons at 150°C under N₂ atmosphere for 12 hours. The surface area (At) was calculated by applying the Brunauer- Emmett-Teller (BET) model to the isotherm data points of the adsorption branch in the relative pressure range p/p₀ < 0.3. Field Emission Scanning Electron Microscopy (JEOL JSM-6700F) was used to study surface morphology. The NLDFT (Nonlocal Density Functional Theory) equilibrium model method was used for obtaining pore size distribution for slit pores. In addition, the mesoporous surface area (Am_e) and microporous volume (V_mi) was determined using the t-plot method in which the total pore volume (V_t) was estimated to be the liquid volume of nitrogen at a relative pressure of about 0.98. The mesoporous volume (Vm_e) was then calculated by subtracting the microporous volume (Vm_i) from the total pore volume (Vt). The mesoporous area (Am_e) was then calculated by subtracting the micropore area from the BET surface area (A_t).

[0080] Experimental process for soaked sample (without any pre-treatment): The coconut shell granules is mixed with ZnC12 solution (800 mL water in precursor equivalent to 250 g raw material) with a ZnC12: char ratio of 4:1 and dried at 105°C for 12 h. The mixture is loaded on to SS316 trays inside a SS316 baffle box which is then placed in a muffle furnace. The temperature is ramped to 500°C at a rate of 6°C/ min in the presence of N₂ at a flow rate of 500 mL/ min and held for 3 h. The furnace is cooled to room temperature in the presence of N₂ at a flow rate of 200 mL/min. The baffle box is opened, and product is collected.

[0081] The mesoporous carbon collected above is stirred for 30 min in 2500 mL hydrochloric acid (about 0.1 mol/L) (first wash). Carbon was separated using meshes of size 200 and 325. Then the carbon was washed with 2500 mL distilled water (second wash). This first and second wash of water is used to recover the majority of Zinc Chloride from the solution by Membrane Distillation. Later carbon was separated and washed with abundant distilled water until a pH of 6 was obtained for the rinse. Finally, the mesoporous carbon was dried at 105°C for 24 h and used for analysis. This entire process is done by using meshes of size 200 and 325.

[0082] Table 1 illustrates the characteristics of mesoporous carbon prepared from biomass that underwent oxygen pre-treatment compared to carbon prepared without any pre-treatment of biomass, according to an embodiment herein. With respect to Table 1, the activated carbon prepared from biomass that underwent oxygen pre- treatment have high BET surface area, mesoporous area, mesoporous volume and mesoporosity compared to activated carbon prepared without any pre-treatment of biomass. The results substantiate the inference that the oxygen pre-treatment process of biomass improves the effectiveness of the activating agent and resulted in a higher mesopore area and a higher mesoporosity. The high OFG content on the char followed by zinc chloride activation facilitates formation of significantly high mesoporosity compared to carbons prepared without oxygen pre-treatment. The significant enhancement in both mesopore surface area and volume is attributed to the improved chemical activation resulting from increased OFG formation on the char.

Table 1: Characteristics of activated carbon

[0083] It is found that the mesoporous area is increased two folds as compared to the carbon prepared without any pre-treatment.

[0084] Field Emission Scanning Electron Microscopy (JEOL JSM-6700F) was used to study surface morphology of prepared carbons. FIG. 4 shows FE-SEM image of mesoporous carbon obtained from coconut- shell, according to an embodiment herein. With respect to FIG. 4, the surface morphology of mesoporous carbon obtained from coconut-shell biomass waste shows coarseness which is an indication of creation of pores and higher surface area. [0085] FIG. 5 shows a graph of nitrogen adsorption-desorption isotherms of mesoporous carbon prepared from coconut shell that underwent oxygen pre- treatment compared to carbon prepared without any pre-treatment of biomass, according to an embodiment herein. With respect to FIG. 5, the continuous lines represent adsorption and symbols represent desorption. Hysteresis confirms the presence of mesopores. It is evident from the figure that the volume adsorbed is higher for carbon prepared from biomass that underwent oxygen pre-treatment compared to carbon prepared without any pre-treatment of biomass. This demonstrates the importance of oxygen pre-treatment of the biomass prior to chemical activation.

[0086] FIG. 6 shows a graph of pore size distribution of mesoporous carbon prepared from coconut shell that underwent oxygen pre-treatment compared to carbon prepared without any pre-treatment of biomass, according to an embodiment herein. With respect to FIG. 6, the results demonstrate that the mesoporous carbon prepared from biomass that underwent oxygen pre-treatment provide better mesoporosity and greater mesopore volume in the range of 30 to 50 angstrom (A) when compared with carbon prepared without any pre-treatment of biomass. This is attributed to the presence of more oxygen functional groups (OFGs) compared to raw biomass. Higher mesopore volume and wider pore size distribution of mesoporous carbon prepared is due to better dehydration of biomass which eases the breakdown of glycosidic linkages within the biomass and thus results in increased mesoporosity.

[0087] The embodiments herein provide a process for obtaining high performance mesoporous carbons for energy storage to serve as a superior material for supercapacitors, hybrid capacitors and redox flow batteries, using biomass waste as raw material. Hence the present invention utilizes the biomass waste which is available in abundance and converts it into mesoporous carbon which is further used in energy storage applications such as lead acid batteries, etc.

[0088] The process of the present invention is eco-friendly and cost effective, thereby the products obtained are also eco-friendly and cost effective. The mesoporous carbons obtained by the process of the present invention has high capacitance characteristics, high thermal stability, and high specific surface area with enhanced mesoporosity. The present invention is easier to commercialize as it requires no special machineries. The present invention uses readily available machines and there is no wastewater produced as a by-product of pre-treatment. The present invention is much safer as there is no high pressure involved.

[0089] The mesoporous carbons obtained by present invention has high capacitance characteristics, high thermal stability and high specific surface area with a significant increase in mesopore, thus significantly improving the capacity and life span of supercapacitors, hybrid capacitors, redox flow batteries, etc.

[0090] It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the claims.

Claims

CLAIMS: We claim:

1. A process for obtaining high surface area mesoporous carbons for energy storage from biomass waste comprises: pre-treating a raw material to obtain a char (101); activating the char using a chemical activator to obtain mesoporous carbon (102), wherein the chemical activator is Zinc Chloride solution; collecting the mesoporous carbon (103); and washing the mesoporous carbon (104).

2. The process as claimed in claim 1, wherein the raw material is a biomass waste comprising saw dust, sugarcane bagasse, palm kernel shells, almond shells, coconut shells, coconut husks, municipal sludge, chicken poop, human hair, and a lignin powder or a combination thereof.

3. The process as claimed in claim 1, wherein the activation of char is done in a furnace by mixing the char with the chemical activator in a ratio ranging from 2:1 to 8:1, more particularly in a ratio3:l to 5:1.

4. The process as claimed in claim 1, wherein the activation of char is done in a furnace by heating a dried mixture of char and the chemical activator at a temperature of 350 to 600°C increasing at a rate ranging from 1 to 15 °C/ min, more particularly at a rate of 5 to 10°C/ min in the presence of nitrogen gas (N2) for time period ranging from 1 to 8 hours, more particularly for 2 to 5 hours.

5. The process as claimed in claim 4, wherein the nitrogen gas (N2) is passed at a flow rate ranging from 200 to 2000 mL/min, more particularly at 500-1000 mL/ min.

6. The process as claimed in claim 1, wherein the mesoporous carbon is collected by cooling down the furnace to room temperature in the presence of N2 at a flow rate ranging from 100 to 500 mL/min, more particularly at 200-300 mL/min.

7. The process as claimed in claim 1, wherein the washing is done with hydrochloric acid solution.

8. The process as claimed in claim 1, wherein the pre-treatment of raw material is done in presence of oxygen (O2) at a flow rate ranging from 200 to 1000 mL/min, more particularly at 500 to 1000 mL/min.

9. The process as claimed in claim 1, wherein the pre-treatment is done at a temperature of 50°C to 350°C, or preferably at 80°C to 220°C, increasing at a rate ranging from 1 to 15 °C/ min, or more particularly increasing at a rate of 5 to 10°C/minute.

10. The process as claimed in claim 1, wherein the pre-treatment is done for a time period ranging from 1 to 8 hours, or more particularly for 2 to 5 hours.

11. The process as claimed in claim 1, wherein the first chemical activator is recycled.

12. The process as claimed in claim 1, wherein the pre-treatment provides greater mesopore volume and wider pore size distribution in the range of 30 Å to 50 Å of the carbons.

13. The process as claimed in claim 1, wherein the mesoporous surface areas is increased two folds.

14. A mesoporous carbon for use as energy storage material having a mesopore volume and a pore size distribution in the range of 30 Å to 50 Å of the carbons.

15. A mesoporous carbon for use as energy storage having increased mesopore area, wherein the mesopore area is increased by two folds.