Activated carbon obtained from lignin for electric double layer capacitor

Abstract

The present invention relates to activated carbon obtained from lignin. The activated carbon has a BET specific surface area in the range of from 1200-1600 m2 / g and a total pore volume in the range of from 0.6-0.8 cm3 / g, and wherein the activated carbon adsorbs less than 1.1 wt% 1,1,1,2 tetra-fluoroethane based on the total weight of the activated carbon when scanning from 300 to 290°C using gravimetric adsorption capacity scanning. The invention further relates to a method for producing such an activated carbon, an electrode material for an electric double layer capacitor (EDLC) comprising such an activated carbon, and use of such an activated carbon in an EDLC.

Description

[0001] ACTIVATED CARBON OBTAINED FROM LIGNIN FOR ELECTRIC DOUBLE LAYER CAPACITOR

[0002] Technical field

[0003] The present invention relates to an activated carbon obtained from lignin and a method for manufacturing activated carbon from lignin. It also relates to an electric double layer capacitor comprising said activated carbon.

[0004] Background

[0005] Activated carbon is a form of porous carbon with high surface area, typically higher than 500 m2 / g. The surface area may also be even higher, such as at least 1000 m2 / g. The unique structure of activated carbon involving a large number of micropores allows it to exhibit excellent adsorptive properties. Due to its high surface area, electrical conductivity and stability, activated carbon is also suitable for use as an electrode material in an electric double layer capacitor (EDLC). As an electrode material, activated carbon typically exhibits high capacitance, low resistance as well as a fast charge-discharge capacity, all features that are required in an EDLC electrode.

[0006] Activated carbon can be obtained from both biobased starting materials, such as bamboo and coconut shells, and from oil-based starting materials, such as coal and petroleum pitch. The activation process can either be physical or chemical. Activated carbon can be produced using lignin as a starting material. Lignin is an aromatic polymer, which is a major constituent in e.g. wood and one of the most abundant carbon sources on earth. In recent years, with development and commercialization of technologies to extract lignin in a highly purified, solid and particularized form from the pulp-making process, it has attracted significant attention as a possible renewable substitute to primarily aromatic chemical precursors currently sourced from the petrochemical industry.

[0007] Today, the most commercially relevant source of lignin is kraft lignin. This lignin is obtained from hardwood or softwood through the kraft process. The lignin can be separated from alkaline black liquor using for example membrane- or ultrafiltration. LignoBoost is one common separation process and is described in for example W02006031175 A1 . In the LignoBoost process, lignin is precipitated from alkaline black liquor through reducing the pH level, usually by adding carbon dioxide, and then filtered off. The lignin filter cake is in the next step re-slurried under acidic conditions, commonly using sulfuric acid, and washed. The precipitated washed lignin can be used as it is or further dried.

[0008] When activated carbon is to be used as the electrode material in an EDLC, the high surface area of the activated carbon is important for high charge storage. To enable fast charging and discharging of the EDLC, it is important that ions in the electrolyte can diffuse in and out of the pores of the activated carbon electrodes.

[0009] Thus, there is a need for improved grades of activated carbon that can be used in electrodes of EDLCs.

[0010] Summary of the invention

[0011] It is an object of the present invention to provide activated carbon obtained from lignin.

[0012] It is a further object of the present invention to provide activated carbon from lignin, where the activated carbon can be used in the electrodes of an EDLC, and where diffusion of ions in and out of the pores of the activated carbon is facilitated.

[0013] The above-mentioned objects, as well as other objects as will be realized by the person skilled in the art in light of the present invention, are achieved by the various aspects of the present invention.

[0014] According to a first aspect, the present invention is directed to an activated carbon obtained from lignin, wherein the activated carbon has a BET specific surface area in the range of from 1200-1600 m2 / g and a total pore volume in the range of from 0.6-0.8 cm3 / g, and wherein the activated carbon adsorbs less than 1.1 wt% 1 ,1 ,1 ,2 tetra-fluoroethane based on the total weight of the activated carbon when scanning from 300 to 290°C using gravimetric adsorption capacity scanning.

[0015] It has surprisingly been found that the amount of high-energy pores, i.e. pores that strongly adsorb a broad range of species, is low in activated carbon obtained from lignin by a method according to a second aspect of the present invention. The activated carbon according to the first aspect of the invention is useful as electrode material in EDLCs, since in such applications it is important that ions can easily diffuse in and out of the pores of the activated carbon. Diffusion can be hindered if interactions between the activated carbon and the ions in the electrolyte are too strong, such as the case may be if there is a large number of high-energy pores present in the activated carbon.

[0016] The activated carbon according to the first aspect may be produced by the method according to the second aspect of the present invention.

[0017] According to a second aspect, the present invention is directed to a method for producing activated carbon. The method according to the second aspect comprises the steps of: a) providing lignin; b) subjecting the lignin to a first heat treatment at one or more temperatures in the range of from 500 to 700°C in an inert atmosphere during a time period in the range of from 60 to 240 minutes, so as to obtain a carbon material; and c) activating the obtained carbon material at one or more temperatures in the range of from 975 to 1050°C in an atmosphere comprising at least 10 vol% steam during a time period in the range of from 60 to 180 minutes, so as to obtain an activated carbon material.

[0018] By carrying out the activation in steam at a relatively high temperature, a low degree of high-energy pores in the activated carbon is obtained. If the activation is carried out at a relatively lower temperature, the amount of high-energy pores in the activated carbon increases and the obtained activated carbon is less suitable for use in EDLC applications. It is believed that when the activation is carried out at a higher temperature, the reaction between the carbon and the oxygen in the steam are to a higher extent completed, thus decreasing the number of high-energy pores.

[0019] According to a third aspect, the present invention is directed to an electrode material for an electric double layer capacitor comprising the activated carbon according to the first aspect or the activated carbon obtainable by the second aspect. As discussed above, due to the low number of high-energy pores present in the activated carbon of the present invention, it is suitable for use in EDLCs since diffusion of ions in and out of the pores enables fast charging and discharging. According to a fourth aspect, the present invention is directed to use of the activated carbon according to the first aspect or the activated carbon obtainable by the second aspect in an electric double layer capacitor.

[0020] Brief description of the drawings

[0021] Figure 1 shows adsorption of 1 ,1 ,1 ,2 tetra-fluoroethane in activated carbon samples between 300°C and 250°C measured using gravimetric adsorption capacity scanning.

[0022] Detailed description

[0023] The activated carbon in the present invention is obtained from lignin. It is intended throughout the present disclosure that the term "lignin" refers to any kind of lignin which may be used as the carbon precursor for making activated carbon. Examples of said lignin are, but are not limited to, lignin obtained from vegetable raw material such as wood, e.g. softwood lignin, hardwood lignin, and lignin from annular plants. Also, the lignin can be chemically modified.

[0024] Preferably, the lignin has been purified or isolated before being used in the method according to the present invention. The lignin may be isolated from black liquor and optionally be further purified before being used in the method according to the present invention. The purification is typically such that the purity of the lignin is at least 90%, preferably at least 95%, more preferably at least 98%, based on the dry weight of the lignin material. Thus, the lignin material used according to the method of the present invention preferably contains less than 10%, preferably less than 5%, more preferably less than 2% impurities, such as cellulose, carbohydrates and inorganic compounds, based on the dry weight of the lignin material.

[0025] The lignin used in method according to the present invention may be obtained through different extraction methods such as an organosolv process or a kraft process. The lignin may also be obtained from processes such as steam explosion or acidic pre-treatment followed by enzymatic hydrolysis. Preferably, the lignin used in the method according to the present invention is kraft lignin, i.e. lignin obtained through the kraft process. The kraft lignin may be obtained from hardwood or softwood. The lignin may be obtained by the process disclosed in W02006031175 A1 commonly referred to as the LignoBoost process. Typically, this process involves the steps of precipitation of lignin from alkaline black liquor by acidification; separation of the precipitated lignin; and re-slurrying the lignin under acidic conditions at least once. The obtained lignin may be dried and pulverized and thus provided as solid particles.

[0026] In some embodiments, the lignin is provided in the form of agglomerated lignin. The agglomerated lignin has an average particle size in the range of from 0.8 to 2.0 mm.

[0027] In the context of the present invention, the average particle size is defined as the volume average particle size (Dv5o). This value refers to the maximum particle size below which 50% of the volume of the sample exists. The particle size is in the context of the present invention taken to be the diameter of the particle. The average particle size may be determined using for example laser diffraction. In the context of the present invention, the diameter of a particle is the equivalent spherical diameter of the particle, if the particle is not spherical. The equivalent spherical diameter is the diameter of a sphere of equivalent volume.

[0028] The term “agglomerated lignin” as used herein refers to macroscopic particles in turn comprising clustered smaller particles of lignin. The agglomerated lignin is obtained by a compaction process. By providing the lignin in agglomerated form, a more compact and hard material is achieved. Hard agglomerates are advantageous during subsequent processing as they can resist physical impact during processing.

[0029] The agglomerated lignin having an average particle size in the range of from 0.8 to 2.0 mm may be produced by a method comprising the steps of: i) providing lignin in the form of a powder; ii) compacting the lignin powder to obtain compacted lignin; iii) crushing the compacted lignin so as to obtain agglomerated lignin having an average particle size in the range of from 0.8 to 2.0 mm.

[0030] The method is further described in WO2021250604 A1 , and is briefly discussed below.

[0031] The particle size distribution of the lignin in the form of a powder may be such that at least 80 wt-% of the particles have a diameter of less than 0.2 mm. The lignin in the form of a powder preferably has a moisture content of less than 45 wt%, or less than 25 wt%, or less than 10 wt%, or less than 8 wt%.

[0032] The compaction of the lignin is preferably carried out by roll compaction. Lignin powder is usually fed through a hopper and conveyed by means of a horizontal or vertical feeding screw into a compaction zone where the material is compacted into flakes by compaction rollers with a defined gap. By controlling the feeding screw speed and the pressure development in the compaction zone, flakes with uniform density can be obtained. As the powder is dragged between the rollers, it enters what is termed as the nip area where the density of the material is increased, and the powder is converted into a flake or ribbon. The specific press force exerted during the compaction may vary depending on the equipment used for compaction, but may be in the range of from 1 to 100 kN / cm. Equipment suitable for carrying out the compaction is known in the art.

[0033] In the crushing step, the compacted lignin is subjected to crushing or grinding, such as by means of rotary granulator, cage mill, beater mill, hammer mill or crusher mill and / or combinations thereof. After crushing, the crushed material is preferably subjected to a sieving step, to remove fine material. In addition, large material may be removed and / or recirculated back to the crushing step.

[0034] Providing the lignin in the form of agglomerated lignin is advantageous as the lignin is less prone to melting / swelling and dimensional changes during the subsequent heat treatments. In addition, dusting during handling of lignin powders is decreased by compacting lignin powder to agglomerates, thus avoiding problems such as explosions that can be caused by lignin dust during the processing. It is particularly advantageous to provide kraft lignin in the form of agglomerated lignin.

[0035] The method according to the present invention may comprise a further step of preheating the lignin or the agglomerated lignin at one or more temperatures in the range of from 140 to 300°C for a time period of at least 30 minutes in an oxidative atmosphere. The pre-heating step is carried out prior to the first heat treatment. After the pre-heating step, thermally stabilized lignin or thermally stabilized agglomerated lignin is obtained. The pre-heating may also be referred to as thermal stabilization. The pre-heating is carried out such that the lignin or agglomerated lignin is heated at one or more temperatures in the range of from 140 to 300°C, preferably from 180 to 250°C. The pre-heating is carried out for at least 30 minutes, i.e. the residence time of the lignin inside the equipment used for the pre-heating is at least 30 minutes. In one embodiment, the pre-heating is carried out for at least 1 hour, or at least 1 .5 hours. Preferably, the pre-heating is carried out for less than 12 hours, or less than 6 hours. The pre-heating may be carried out at the same temperature throughout the entire pre-heating stage or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. More preferably, the pre-heating is carried out such that the lignin is first heated at a temperature in the range of from 140 to 175°C for a period of at least 15 minutes and subsequently heated at a temperature in the range of from 175 to 250°C for at least 15 minutes.

[0036] The step of pre-heating can be carried out continuously or in batch mode. The preheating can be carried out using methods and equipment as known in the art. For example, the pre-heating may be carried out in a rotary kiln, moving bed furnace or rotary hearth furnace.

[0037] Oxidative species that can react so as to cross-link lignin are present in the oxidative atmosphere. The pre-heating may be carried out for example in the presence of oxygen, iodine, ozone, nitrogen dioxide, nitrobenzene, hydrogen peroxide and peracetic acid. Preferably, the pre-heating is carried out in air. Alternatively, any suitable oxidative species may be supplied in a nitrogen atmosphere. The carbon yield is typically improved by carrying out a pre-heating step. The lignin may be cross-linked by pre-heating in an oxidative atmosphere. This increases the stability of the lignin during the subsequent heat treatment. The thermal stability of the lignin is improved by carrying out a pre-heating step, such that a thermally stabilized (i.e. cross-linked) lignin is obtained. In particular, the processability of the lignin in terms of avoiding melting / swelling and retaining shape and dimension during any subsequent heat treatment is improved by the pre-heating step.

[0038] The activated carbon of the present invention is obtained from lignin by heat treatment and subsequent activation. The provided lignin or agglomerated lignin is subjected to a first heat treatment at one or more temperatures in the range of from 500 to 700°C in an inert atmosphere so as to obtain a carbon material. The heat treatment is carried out for a time period in the range of from 60 to 240 minutes, or from 60 to 180 minutes, or from 60 to 120 minutes, i.e. the residence time of the lignin inside the equipment used for the first heat treatment is in the specified range.

[0039] During the first heat treatment, the lignin is heated for a sufficient time such that the carbon content of the material increases and lignin is converted to a carbon material. The content of volatile material is reduced during the first heat treatment. The obtained carbon material has a somewhat porous structure, but the pores are typically small. The carbon content in the carbon enriched material obtained after the first heat treatment is at least 80 wt%. The obtained carbon enriched material may be referred to as char.

[0040] Lignin will typically melt / swell during the first heat treatment. In embodiments where lignin is provided in the form of a powder (i.e. not as agglomerates), the lignin may melt and fuse into a lignin cake. The lignin cake is preferably crushed prior to the activation step.

[0041] The thermal processability of lignin is improved by both providing lignin as agglomerates, and by carrying out a pre-heating step. Thus, the melting / swelling behaviour is significantly reduced if lignin is provided as agglomerated lignin and / or if a pre-heating step is carried out.

[0042] The first heat treatment may be carried out at the same temperature throughout the entire heat treatment or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. The first heat treatment may comprise a temperature ramp from a starting temperature to a target temperature. The heating rate may be 1-100 °C / min. For example, the heat treatment may involve several intermediate temperatures, with temperature ramps in between them, before reaching the target temperature needed for conversion of the lignin to a carbon material. The first heat treatment is carried out under inert atmosphere, preferably nitrogen atmosphere.

[0043] The term “inert atmosphere” as used herein, refers to an atmosphere that is depleted of oxygen. Preferably, the content of oxygen in the inert atmosphere is less than 1 vol%, such as less than 0.1 vol%. Preferably, a nitrogen atmosphere is used in all steps of the method according to the present invention where an inert atmosphere is required.

[0044] The obtained carbon material may be crushed to obtain an average particle size in the range of from 0.1 to 4.0 mm. By crushing the carbon material prior to activation, the activation process is more efficient since interaction between steam and the carbon material is facilitated due to the small size of the crushed material. It is easier for the steam to penetrate into the carbon material if the material has a small particle size. Any suitable equipment may be used in the crushing step, for example a cage mill, beater mill, hammer mill, flake crusher, crusher mill, ball mill, roller mill, or cut mill and / or combinations thereof. After crushing, the crushed carbon material may also be subjected to a sieving step to remove fine material. In addition, large pieces of material may be removed and / or recirculated back to the crushing step. The crushing step is preferably carried out in embodiments where the lignin is provided in the form of a powder which means that the lignin may melt and fuse during the first heat treatment. In embodiments where the lignin is provided as agglomerated lignin, crushing is not required since the agglomerated lignin is already in a size suitable for the activation step.

[0045] Activation of the carbon enriched material is carried out at one or more temperatures in the range of from 975 to 1050°C in an atmosphere comprising at least 10 vol%, or at least 20 vol%, or at least 30 vol% steam during a time period in the range of from 60 to 180 minutes, so as to obtain activated carbon. The steam, i.e. water in gas phase, is added to an inert atmosphere, such that the remainder of the volume is an inert gas, such as nitrogen. In some embodiments, activation is carried out in an atmosphere comprising from 20 to 30 vol% steam. In one embodiment, the activation is carried out in an atmosphere comprising 30 vol% steam and 70 vol% nitrogen.

[0046] In some embodiments, activation is carried out in an atmosphere comprising from 5 to 20 vol% carbon dioxide and at least 10 vol% steam. The remainder is an inert gas, such as nitrogen. For example, activation may be carried out in an atmosphere comprising 10 vol% steam, 5 vol% carbon dioxide and 85 vol% nitrogen.

[0047] The term “activated carbon” as used herein refers to a carbon material with a large number of small low-volume pores that increases the specific surface area of the material. The porous structure of the activated carbon is obtained through an activation process. In the present invention, a physical activation process with steam is carried out. In the activation process, the pores in the carbon material are enlarged due to a chemical reaction between carbon and steam such that carbon is removed from the walls of the pores, thus enlarging the pores. As a result, the specific surface area is enlarged. Depending on the conditions during the activation process, the properties of the obtained activated carbon can be tailored towards different applications.

[0048] The carbon content increases further during the activation step, and the activated carbon of the present invention comprises at least 90 wt% carbon, such as at least 95 wt% carbon, or at least 98 wt% carbon.

[0049] The activation is carried out for a time period in the range of from 60 to 180 minutes, or from 60 to 150 minutes, or from 60 to 120 minutes, i.e. the residence time of the carbon material inside the equipment used for the activation is in the specified range. If the time period is too short, the specific surface area of the obtained activated carbon may be lower than desired.

[0050] The temperature used during the activation is preferably from 980 to 1050°C, or from 980 to 1025°C. In one embodiment, the temperature used is 1000°C. If a temperature lower than 975°C is used, the amount of high-energy pores increases. If the temperature is too low, i.e. lower than 975°C, the specific surface area of the obtained activated carbon may be lower than desired. A high specific surface area is needed to ensure a high storage capacity when the activated carbon is used in an EDLC. For fast charging and discharging, it is also important that ions can easily diffuse in and out of the pores.

[0051] In some embodiments the activated carbon is milled. Milling may be carried out using any suitable equipment, such as using a cutting mill, blade mixer, ball-mill, impact mill, hammer mill and / or jet-mill. Milling may also be referred to as for example pulverization or grinding. Depending on the intended use of the activated carbon, the average particle size after milling may be less than 20 pm, such as in the range of from 1 to 20 pm, or from 1 to 10 pm. The obtained powdered activated carbon may be used as is, or the powder may be combined with a binder to form granulated activated carbon. By carrying out the activation of the carbon material obtained from lignin according to the second aspect of the present invention, activated carbon with a low amount of high-energy pores is obtained. The term “high-energy pores” as used herein, refers to pores in the activated carbon that can adsorb also compounds that are typically difficult to adsorb. Such compounds typically do not have functional groups and / or ionic groups on the surface, and therefore can not easily bind to or interact with the activated carbon. The presence of high-energy pores is in the present invention evaluated by the adsorption of 1 ,1 ,1 ,2 tetra-fluoroethane at high temperatures.

[0052] 1 ,1 ,1 ,2 tetra-fluoroethane serves as a model compound for species that are typically difficult to adsorb. Adsorption is typically more difficult at higher temperatures than at lower temperatures. Thus adsorption, also only to a low extent, at high temperatures is indicative of the presence of high-energy pores. The high-energy pores will enable strong adhesion of a broad range of species, which is not always desired. For example, in an EDLC it is instead important that species present in the electrolyte are not strongly adhered to the activated carbon electrodes.

[0053] Adsorption of 1 ,1 ,1 ,2 tetra-fluoroethane is measured using gravimetric adsorption capacity scanning (GACS), also known as gravimetric adsorption energy distribution (GAED). In GACS, the sample to be evaluated is placed on a scale in a chamber filled with 1 ,1 ,1 ,2 tetra-fluoroethane. The adsorption of 1 ,1 ,1 ,2 tetra-fluoroethane results in an increase in weight of the sample. During the measurement the temperature in the chamber is gradually decreased from 300°C to 100°C while the change in weight is measured. The amount of adsorbed 1 ,1 ,1 ,2 tetra-fluoroethane increases during the measurement as more 1 ,1 ,1 ,2 tetra-fluoroethane is adsorbed during the scan as the temperature is decreased. The adsorption of 1 ,1 ,1 ,2 tetra- fluoroethane is cumulative, i.e. no desorption occurs during the measurement.

[0054] To evaluate the presence of high-energy pores, only the adsorption at high temperature is of interest, for example the adsorption when scanning between 300°C and 290°C, or between 300°C and 275°C, or between 300°C and 250°C. At lower temperatures, adsorption is easier and the activated carbon will adsorb 1 ,1 ,1 ,2 tetra-fluoroethane to a higher extent and the difference between activated carbons with and without high-energy pores is not as pronounced. At high temperatures the presence of high-energy pores will have an impact on the adsorption and therefore the adsorption between e.g. 300°C and 290°C or 300°C and 250°C can be used to indicate the presence of high-energy pores in the activated carbon.

[0055] The activated carbon according to the first aspect of the invention adsorbs less than 1 .1 wt%, preferably less than 1 .0 wt%, 1 ,1 ,1 ,2 tetra-fluoroethane based on the total weight of the activated carbon when scanning from 300 to 290°C using GACS. This means that the increase in weight is less than 1 .1 wt% when decreasing the temperature from 300°C to 290°C, and that the increase is due to adsorption of 1 ,1 ,1 ,2 tetra-fluoroethane.

[0056] The activated carbon according to the first aspect of the invention may adsorb less than 1 .5 wt%, preferably less than 1 .3 wt%, 1 ,1 ,1 ,2 tetra-fluoroethane based on the total weight of the activated carbon when scanning from 300 to 275°C using GACS.

[0057] The activated carbon according to the first aspect of the invention may adsorb less than 2.2 wt%, preferably less than 1 .9 wt%, 1 ,1 ,1 ,2 tetra-fluoroethane based on the total weight of the activated carbon when scanning from 300 to 250°C using GACS.

[0058] The activated carbon according to the first aspect of the present invention further has a BET specific surface area in the range of from 1200 to 1600 m2 / g, or from 1400 to 1600 m2 / g. The specific surface area in the context of the present application is the BET specific surface area measured using nitrogen gas at 77 K. A sufficiently high specific surface area is required in order for the activated carbon to have good capacitive properties.

[0059] The activated carbon according to the first aspect of the present invention further has a total pore volume in the range of from 0.6-0.8 cm3 / g. The total pore volume includes both mesopores and micropores, and is measured using adsorption of nitrogen gas.

[0060] Both the specific surface area and the pore volume increases with the temperature used during the activation step. In addition, a high temperature is beneficial when it comes to minimizing the amount of high-energy pores. In the present invention, it was surprisingly found that by careful selection of the activation temperature, activated carbon with a low number of high-energy pores as well as high values of specific surface area and total pore volume could be obtained. The resulting activated carbon is suitable to use in an EDLC due to its fast charging and discharging properties and excellent charge storage.

[0061] The activated carbon according to the first aspect preferably has an iodine number in the range of from 1400 to 1700 mg / g activated carbon. The iodine number is indicative of the porosity of the activated carbon. It is determined by measuring the amount of iodine that the activated carbon adsorbs from a water solution comprising iodine.

[0062] The activated carbon according to the first aspect preferably has a butane activity in the range of from 27 to 35 g / g activated carbon. The butane activity is indicative of the volume of micropores in the activated carbon. Butane activity is determined by passing butane gas through a bed of activated carbon and measuring the mass gain that is due to adsorption of butane.

[0063] The activated carbon according to the first aspect preferably has a sulfur content in the range of from 0.5 to 1 .0 wt%, based on the total weight of activated carbon. The sulfur in the activated carbon may originate from the lignin from which the activated carbon is obtained. Sulfur-containing chemicals are used during the kraft process, and some of the sulfur remains in the lignin obtained by isolation from such a process. The capacity of the EDLC may be increased due to the presence of sulfur in the activated carbon. The amount of sulfur present in the activated carbon can be determined using elemental analysis (LECO analysis).

[0064] The activated carbon according to the first aspect preferably has a pore size distribution such that the activated carbon comprises, based on the total pore volume, at least 60% micropores and less than 40% mesopores. This means that out of the total pore volume, at least 60% of that volume is in the form of micropores and less than 40% is in the form of mesopores.

[0065] The activated carbon may comprise at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% micropores. The activated carbon may comprise from 60 to 99%, or from 70 to 99%, or from 80 to 99%, or from 90 to 99%, micropores. The activated carbon may comprise less than 40%, or less than 30%, or less than 20%, or less than 10%, or less than 5% mesopores. The activated carbon may comprise from 1 to 40%, or from 1 to 30%, or from 1 to 20%, or from 1 to 10%, or from 1 to 5%, mesopores.

[0066] When used as an electrode in an EDLC, it may be advantageous to have a larger amount of mesopores since that will facilitate ion transport in the EDLC. Therefore, the activated carbon may comprise from 10 to 40%, or from 10 to 30%, or from 10 to 20%, or from 15 to 40%, or from 15 to 30% mesopores. In such embodiments, the activated carbon may comprise from 60 to 90%, or from 70 to 90%, or from 80 to 90%, or from 60 to 85%, or from 70 to 85% micropores. The amount of mesopores typically increases with an increased degree of activation, which in turn may be obtained by increasing the activation time and / or temperature. However, when the degree of activation increases, the yield decreases. Thus, there is a tradeoff between a reasonable yield and the desired pore size distribution.

[0067] The activated carbon according to the first aspect is obtainable with the method according to the second aspect, as further outlined above.

[0068] According to a third aspect, the present invention is directed to an electrode material for an electric double layer capacitor comprising the activated carbon according to the first aspect or the activated carbon obtainable by the second aspect. Electrodes for an EDLC may be formed by combining the activated carbon with any suitable binder in a suitable solvent to form a slurry which is then coated on a metal foil. An EDLC is then formed by assembling the electrodes in a cell including a separator and an electrolyte. Ions in the electrolyte will diffuse towards an electrode in the EDLC depending on their charge and on the applied voltage, such that double layers form along the surface of the activated carbon electrodes. When high-energy pores are present in the EDLC, ions in the electrolyte may become trapped in the electrode and reduce its functionality.

[0069] According to a fourth aspect, the present invention is directed to use of the activated carbon according to the first aspect or the activated carbon obtainable by the second aspect in an electric double layer capacitor.

[0070] Examples

[0071] Example 1 - comparative Lignin powder (softwood kraft lignin obtained from LignoBoost process) was washed in sulfuric acid, dried, and then subjected to a first heat treatment in a Pusher furnace in a nitrogen atmosphere. The sample was first heated at 370°C for 1 hour, then at 570°C for 1 hour. The lignin powder melted during heating and fused into a black char cake. The cake was crushed using a cut-mill equipped with a 4 mm sieve, then the fines were removed using a 0.1 mm sieve. The crushed carbon material was activated at 950°C for 120 minutes in an atmosphere of 30 vol% steam and 70 vol% nitrogen and activated carbon was obtained. The yield was 60%.

[0072] Example 2

[0073] The procedure of the comparative example 1 was repeated except that the temperature during activation was increased to 1000°C. The yield was 43%.

[0074] Example 3 - measurements

[0075] The materials of example 1 and example 2 were further analyzed. The specific surface area was measured using the BET method with nitrogen gas.

[0076] The pore volume was evaluated using adsorption of nitrogen gas. The butane activity was evaluated by measuring the mass increase due to adsorption of butane gas. The iodine number was evaluated by measuring the adsorption of iodine from a water solution comprising iodine.

[0077] The measured values of BET specific surface area, pore volume, butane activity and iodine number are shown in table 1 .

[0078] Table 1: measurements

[0079] GACS was used to determine the amount of high-energy pores in the activated carbon. Activated carbon samples were placed on a scale inside a chamber. The sample was heated to 300°C under nitrogen purge. At 300°C, the heating supply was turned off and the purge gas surrounding the sample was changed to pure 1 ,1 , 1 ,2 tetra-fluoroethane (R134a). R134a was introduced at the bottom of the chamber and removed at the top of the chamber. The sample and the surrounding purged chamber were allowed to cool by passive heat loss to the ambient from 300°C to 100°C, with the sample temperature and weight being data-logged every thirty seconds. The complete data set consists of the weight loss data during the heating of the sample in nitrogen from 100°C to 300°C and the weight gain data as the sample cools in R134a from 300°C to 100°C. An increase in weight indicated adsorption of 1 ,1 ,1 ,2 tetra-fluoroethane. The scans for the material in example 1 and example 2 are shown in figure 1 . Only the part of the scan between 300°C and 250°C is shown in the figure.

[0080] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

Claims1 . An activated carbon obtained from lignin, wherein the activated carbon has a BET specific surface area in the range of from 1200-1600 m2 / g and a total pore volume in the range of from 0.6-0.8 cm3 / g, and wherein the activated carbon adsorbs less than 1.1 wt% 1 ,1 ,1 ,2 tetra-fluoroethane based on the total weight of the activated carbon when scanning from 300 to 290°C using gravimetric adsorption capacity scanning.

2. The activated carbon according to claim 1 , wherein the activated carbon has an iodine number in the range of from 1400 to 1700 mg / g activated carbon.

3. The activated carbon according to anyone of claims 1 or 2, wherein the activated carbon has a butane activity in the range of from 27 to 35 g / g activated carbon.

4. The activated carbon according to anyone of the preceding claims, wherein the activated carbon comprises, based on the total pore volume, at least 60% micropores and less than 40% mesopores.

5. The activated carbon according to anyone of the preceding claims, wherein the activated carbon comprises in the range of from 0.5 to 1 .0 wt% sulfur, based on the total weight of the activated carbon.

6. The activated carbon according to anyone of the preceding claims, wherein the activated carbon adsorbs less than 2.2 wt% 1 ,1 ,1 ,2 tetra-fluoroethane based on the total weight of the activated carbon when scanning from 300 to 250°C using gravimetric adsorption capacity scanning.

7. A method for manufacturing activated carbon, the method comprising the steps of: a) providing lignin; b) subjecting the lignin to a first heat treatment at one or more temperatures in the range of from 500 to 700°C in an inert atmosphere during a time period in the range of from 60 to 240 minutes, so as to obtain a carbon material; andc) activating the obtained carbon material at one or more temperatures in the range of from 975 to 1050°C in an atmosphere comprising at least 10 vol% steam during a time period in the range of from 60 to 180 minutes, so as to obtain an activated carbon material.

8. The method according to claim 7, wherein the lignin provided in step a) is kraft lignin.

9. The method according to any one of claims 7 or 8, wherein the method comprises an additional step of crushing the carbon material to obtain an average particle size in the range of from 0.1 to 4.0 mm.

10. The method according to any one of claims 7-9, wherein the lignin provided in step a) is agglomerated lignin having an average particle size in the range of from 0.8 to 2.0 mm, and wherein the agglomerated lignin is produced by a method comprising the steps of: i) providing lignin in the form of a powder; ii) compacting the lignin powder to obtain compacted lignin; iii) crushing the compacted lignin so as to obtain agglomerated lignin having an average particle size in the range of from 0.8 to 2.0 mm.11 . The method according to any one of claims 7-10, wherein the method comprises an additional step of pre-heating the lignin or agglomerated lignin to a temperature in the range of from 140 to 300°C for a period of at least 30 minutes so as to obtain a thermally stabilized lignin or a thermally stabilized agglomerated lignin.

12. The method according to claims 7-11 , wherein the activation is carried out in an atmosphere comprising 20-30 vol% steam.

13. The method according to claims 7-12, wherein the method further comprises a step of milling the activated carbon.

14. An electrode material for an electric double layer capacitor comprising the activated carbon according to any one of claims 1-6 or the activated carbon obtainable by any one of claims 7-13.

15. Use of the activated carbon according to any one of claims 1-6 or the activated carbon obtainable by any one of claims 7-13 in an electric double layer capacitor.

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