WO2017207593A1

WO2017207593A1 - A method for manufacturing microporous carbon particles

Info

Publication number: WO2017207593A1
Application number: PCT/EP2017/063075
Authority: WO
Inventors: Jaan Leis; Maike KÄÄRIK; Mati Arulepp; Anti Perkson
Original assignee: Oü Skeleton Technologies Group
Priority date: 2016-05-31
Filing date: 2017-05-31
Publication date: 2017-12-07

Abstract

This invention describes the novel principles of physical activation of initially porous carbon materials. More particularly, it describes the gas-phase activation of microporous carbon materials. A method is characterised by steps of selecting nanoporous carbon material with predetermined surface area and pore size, and activation of selected nanoporous carbon by heating and interacting carbon with water vapour at the temperature range from 500 º C to 900 ºC.

Description

A method for manufacturing microporous carbon particles

FIELD OF THE INVENTION

The present invention relates to the modification of porous carbon materials. More particularly, it relates to the gas-phase modification of microporous carbon materials. The invention also relates to the application of these materials for the energy storage in ultracapacitors and related devices.

BACKGROUND OF THE INVENTION

Physical activation of carbon materials in general terms is a method for creating the pores and a suitable pore size distribution in carbon. For energy storage purpose, the main goal of physical activation is to increase the surface area and also to enhance the access to the surface. Physical activation of carbon can be done by any chemical reagent, which is able to oxidize and etch the carbon. Most common gaseous reagents for the physical activation are oxygen, carbon dioxide, water vapour and steam. Physical activation can be done also in liquid phase for instance in hot nitric acid. Physical activation can be performed also by oxidising reagents trapped in the pores from liquid phase and thereafter heated up in inert atmosphere (e.g. argon) to the temperature suitable for oxidising carbon by the reagent.

The state of art methods for the physical activation of initially porous carbon have several deficiencies: 1 ) The gas-phase activation is unselective, i.e., the operator can not essentially control, which part of the carbon material (e.g. carbon particle) is etched. Most probably the etching starts from the surface of carbon, which increases the porosity of carbon and enhances the access of the inner carbon layers by oxidizing reagent. In fact, at the same time when inner carbon layers start to oxidize, the etching of outer layers also continues, that results in inhomogeneous porosity and a wide pore size distribution. Since, high energy density of carbon electrodes used in energy storage device such as e.g. ultracapacitor is achieved when the size of pores in carbon electrode is close to the size of the electrolyte ion, the wide pore size distribution is usually undesired feature of the electrode carbon.

2) Gas-phase physical activation of porous carbon commonly yields carbon with undesirably low bulk density, which is caused by the uncontrolled oxidation process as described in previous section. Electrodes from low-density carbon also have a low geometric density and consequently they produce low energy and power density of energy storage device built from such electrodes.

3) Although, the better distribution of pore sizes is achieved by the etching with reagents preliminary trapped in pores (in this way the oxidizing agent is evenly distributed in carbon and the oxidizing is performed more or less at the same time throughout all the material), the drawback of this method is that the reaction environment contains a very limited amount of oxidizing reagent, and therefore, the activation process also is very short that enables only slight modifications of pore size distribution.

This invention describes a method and principles of physical activation of initially porous carbon, which enable overcome the above-described deficiencies and additionally give a significant improvement in the energy and power density of resultant activated carbon materials. SUMMARY OF THE INVENTION

This invention describes the novel principles of physical activation of initially porous carbon materials. More particularly, it describes the gas-phase activation of microporous carbon materials. The invention also relates to the application of these materials for the energy storage in ultracapacitors and related devices. Additionally, this invention defines the nanoporous carbon with a range of suitable pore sizes for physical post-activation with oxidizing reagents that significantly enhances the electrical double-layer capacitance of nanoporous carbon.

Here, the term microporous carbon distinguishes the carbon material, where the size of pores is predominantly less than 2 nanometres. Nanopore is defined as a pore with a size of less or equal to 1 nanometre, and nanoporous carbon therefore distinguishes the material, which contains significant fraction of nanopores.

Typical nanoporous carbon material for instance can be produced from metal or metalloid carbides. This can be done by extracting the non-carbon atoms from carbide crystal lattices by means of deep vacuum, supercritical environment or chemical reagents such as gaseous chlorine or hydrogen chloride or other similar halogen containing compounds. We claim that porous carbon with a certain suitable surface area and pore size distribution can be physically activated in the way that results in superior electrochemical characteristics including high double layer capacitance and low temperature dependence of the carbon electrode composed of such physically activated carbon.

We claim that the method for activating porous carbon material comprising the steps of manufacturing the carbon material from the metal carbide by halogenation said metal carbide with halogen gas at chosen temperature in the range of 500-1000 °C, thereafter the resulting carbon powder is treated with hydrogen gas at least at 800 °C for at least a period required to dehalogenate said carbon material until neutral reaction for acidity of the exiting gaseous products), resulting carbide derived carbon material is heated and interacted with an inert gas/water vapour mixture at the temperature range from 500 °C to 1000 °C for at least 10 minutes to 50 minutes, thereafter for complete activation of the carbon surface the reactor is purged with an inert gas at least for 30 or more minutes at least at temperature of 900 °C, thereafter said reactor is slowly cooled to room temperature in an inert gas atmosphere.

In another embodiment, it is also possible prior to treatment with water vapour at 500 to 900 °C to saturate the nanopores of said porous carbon by water, that can be done by boiling the carbon in water until it precipitates, or it can be done by sucking water into the vacuumed carbon, or it can be done by saturating the carbon in water vapour at the ambient temperature but not exceeding 500 °C.

The effect of physical activation on the double layer characteristics of porous carbons according to this invention is demonstrated based on nanoporous carbide-derived carbon materials (also known as CDC), particularly, a series of CDC with originally different surface area and different average pore size distribution. Table 1 describes a series of CDC materials (TiC-1 to TiC-6), which were synthesised from titanium carbide powder by using chlorination method of metal carbides.

Precursor Chlorination Sa Vm Vt Vm/Vt APS

CDC Temp. (°C) (m²/g) (cc/g) (cc/g) (%) (A)

TiC-6 1000 1541 0.623 0.752 82.8 9.76

TiC-5 900 1393 0.581 0.654 88.8 9.39 TiC-4 800 1302 0.554 0.602 92.0 9.25

TiC-3 700 1 195 0.51 1 0.549 93.1 9.19

TiC-2 600 1073 0.460 0.491 93.7 9.15

TiC-1 500 1020 0.439 0.465 94.4 9.12

Table 1 . BET surface areas (Sa), total pore volumes (Vt), volume of micropores (Vm) and average pore diameters (APS=2^*Vt/Sa) of CDC materials used for physical activation

Table 2. BET surface areas and pore size distribution, presented as a ratio of volumes of smaller and larger than 1 nm pores, of initial and post-activated for 30 and 45 minutes CDC materials

Physical activation of the nanoporous CDC materials TiC-1 to TiC-6, described in Table 1 , was carried out with water vapour at 900°C by using the argon as carrier gas for water. Activation procedure was carried out by using two options: 1 ) for 30 minutes, and 2) for 45 minutes. Porosity characteristics of precursor CDC materials and of resulting activated carbon samples are presented in Table 2, where SBET designates the specific surface area calculated according to the BET theory and (Vp<inm / Vp>inm) designates the ratio of pore volume with a size below 1 nm and above 1 nm, whereby the pore volumes are calculated according to the BJH (Barret-Joyner- Halenda) theory. Specific capacitance and internal resistance of post-activated CDC materials in 1 .8M triethylmethylammonium tetrafluoroborate (TEMA) in acetonitrile (ACN) confirm that the electrochemical properties, particularly specific capacitance of post-activated porous carbon significantly depend on the pore size distribution of precursor carbon for physical activation. It is also clear that there exists an optimum pore structure of precursor carbon, e.g., described by the specific surface area and/or average pore size and/or relative fractions of pore volumes, which after physical activation results in a best performance of carbon regarding its double layer capacitance in composition of ultracapacitor electrode. Nanoporous precursor carbon materials and physically activated carbon samples derived from these precursor carbon materials were characterised by using symmetrically configured 2-electrode test-cells. The Fig 1 represents the influence of physical activation on gravimetric and volumetric capacitance of nanoporous CDC electrodes.

Data presented in Fig 1 confirm that increasing the activation time increases gravimetric capacitance, while decreasing the volumetric capacitance. It was revealed that most effective is the activation if nanoporous carbon initially has an average pore size of less than 0.92 nm and at least two-fold nanopore volume compared to the rest of pore volume.

Additionally, it is known and reconfirmed by the data presented hereby that pore structure of nanoporous precursor carbon of CDC type in terms of surface area and average pore size distribution can be influenced by the synthesis conditions applied in synthesis of CDC from metal or metalloid carbide. If synthesis is made by means of high-temperature treatment of carbide with chlorine gas, by the methods known from the prior art, the control of pore structure can be done by the temperature of chlorination reaction. General rule is that the higher the temperature of chlorination, the larger is average pore size of the resultant CDC as it can be seen from the data in Table 1 . It is thus obvious from the data presented hereby that there also exists an optimum temperature of chlorination for metal or metalloid carbides, which produces nanoporous precursor CDC, which after physical activation gives a maximum electrochemical performance, e.g., in terms of electrical double layer capacitance. Suitable nanoporous precursor CDC-s can be produced, e.g., from silicon carbide (SiC) or titanium carbide (TiC) or from the mixtures of various carbides, which include said SiC or TiC. Based on these results we claim that porous carbon with a certain predetermined surface area and pore size distribution can be physically activated in the way that results in superior electrochemical characteristics including high double layer capacitance and low temperature dependence of the carbon electrode composed of such physically activated carbon. Physical activation thereby may be treatment of the carbon material, e.g. nanoporous carbon powder, with a water vapour at a temperature, where water vapour can oxidize the carbon. The temperature of said treatment may be 900°C.

We also claim that if the precursor for the H2O-based post-activation according to this invention is TiC-derived CDC, the carbon made by chlorination-treatment at temperatures 600-800 °C must be chosen to yield the electrode material with highest capacitance and energy density.

Furthermore, we claim that H2O-based post-activation of nanoporous carbon according to this invention increases the volume of effective micropores for the adsorption of electrolyte ions and for the electric double-layer formation, wherein the pore size of said effective micropores is in the range of 0.8-0.9 nm.

BRIEF DESCRIPTION OF THE TABLES AND FIGURES

TABLE 1 is a table showing the porosity parameters of nanoporous carbons physically activated with water vapour for 30 and 45 minutes. TABLE 2 is a table showing the porosity parameters of nanoporous carbons used to demonstrate the effect of surface area and pore size distribution of carbon on the physical activation according to this invention.

Fig. 1 is a graph representing gravimetric and volumetric capacitance of physically activated nanoporous CDC materials in 1 .8M trimethylethylammonium tetrafluroborate in acetonitrile (TEMA/CAN). Activation time, t (act), is noted in figure.

Fig. 2 is a graph representing specific gravimetric capacitance vs. chlorination temperature of different physically activated CDC carbons tested at +25 °C. Activation time is noted on figure.

Fig. 3 is a graph representing specific gravimetric capacitance vs. chlorination temperature of different physically activated CDC carbons tested at -30 °C. Activation time is noted on figure. Fig. 4 is a graph representing specific capacitance of 45-minutes physically activated nanoporous CDC tested at different temperatures. The maximum double layer capacitance (Cmax) is observed at approximately 20 °C, because at higher testing temperatures the Brownian movement increases the ionic mobility and increases the rate of desorption of electrolyte ions at the porous carbon. At lower temperatures °C value decreases because of the increasing viscosity of electrolyte.

Fig. 5 is a graph representing specific resistance vs. chlorination temperature of different physically activated CDC carbons tested at -30 °C and at +25 °C. Activation time and testing temperature are noted in figure. Best Mode for Carrying Out the Invention

A method for manufacturing microporous carbon particles with increased capacity of electrical charge from the electrolyte of ultracapacitor comprises the steps where for manufacturing microporous carbon particles are selected nanoporous carbon material with predetermined surface area and pore size for example a carbon powder with average pore size of less than 0.92 nm and has at least two-fold nanopore volume compared to the rest of pore volume. In the next step the selected nanoporous carbon powder is activated in the reactor by heating and interacting with water vapour or a mixture of carrier gas and water vapour at the temperature range from 500 °C to 900 °C at least for 10 or more minutes. Before treatment the nanoporous carbon material with water vapour at 500 °C to 900 °C said nanoporous carbon material can be saturated by water in the way of:

- by boiling the carbon in water until it precipitates, or

- -by sucking water into the vacuumed carbon, or

- - by saturating the carbon in water vapour at the ambient temperature but not exceeding 500 °C.

After heating and interacting selected nanoporous carbon material in the reactor with water vapour or a mixture of carrier gas and water vapour for complete activation of the carbon surface the reactor is flushed with an inert gas at least for 30 or more minutes at least at temperature of 500 °C to 900 °C, thereafter the reactor is cooled to room temperature in an inert gas atmosphere and the activated nanoporous carbon material is removed from the reactor for the further processing. The inert gas used for flushing and cooling down the reactor is an argon. The carrier gas used in the mixture of the carrier gas and water vapour is inert gas, for example argon. For forming an inert gas and water vapour mixture the inert gas flow is passed with a flow rate of at least 1 l/min through distilled water heated up to 75-80 °C. After activation of the nanoporous carbon material the volume of micropores having a size in the range of 0.8-0.9 nm, which is an effective pore size range for adsorbing electrolyte ions and therefore yields a maximum energy density for the EDL electrodes made of this carbon powder.

A selected nanoporous carbon powder with predetermined surface area and pore size is manufactured from the metal carbide by halogenation said metal carbide with halogen, for example chlorine, at chosen temperature in the range of 500-1000 °C, thereafter the resulting carbon powder is treated with hydrogen gas at least at 800 °C to dehalogenate said carbon material. The metal carbide used is selected from titanium carbide (TiC) or silicon carbide (SiC). EXAMPLES OF THE INVENTION

A following examples and figures (example 3) describes the experiment setups and important observations, which is bases of this invention.

Example 1 . Description of the method used to synthesise of nanoporous CDC for physical activation. Titanium carbide (H.C. Starck, 0<4μηη, 50 g) powder was placed in the horizontal tubular quartz reactor and was treated with chlorine gas (AGA, 2.8) for 2 to 4 hours (depending on reaction temperature) at temperature in the range of 500-1000 °C (for the precise synthesis temperatures see in Table 1 ). A flow-rate of chlorine was 1 .5 l/min. By-produced TiCU was removed in the flow of excess chlorine and neutralised in alkali solution. After that the reactor was flushed with Argon (2 l/min) at 1000°C for 1 h to remove the excess of chlorine and residues of gaseous by-products from carbon. During heat-up and cooling the reactor was purged with argon (0.5 l/min). After cooling the reactor down to the room-temperature, the resulting carbon powder was moved into another horizontal quartz stationary bed reactor and treated with hydrogen gas at 800°C (1 l/min) for 1 hour to dechlorinate deeply the carbon material. During heating and cooling, the reactor was flushed with a slow stream of Argon (0.3 l/min). Characteristics of the products are presented in Table 1 . Example 2. Description of the method used for physical activation of nanoporous carbon:

A nanoporous CDC powder of Example 1 (5g) was placed in a quartz reaction vessel and loaded into tubular horizontal quartz reactor heated by a tube furnace. Thereupon the reactor was flushed with argon to remove air and the furnace was heated up to 900°C. The argon flow was then passed with a flow rate of 1 .5 l/min through distilled water heated up to 75-80°C and the resultant argon/water vapour mixture was let to interact with a carbon at 900°C for 30 minutes or 45 minutes, as noticed in Table 2. After that the reactor was flushed with argon for 30 more minutes at 900°C to complete the activation of a carbon surface and then slowly cooled to room temperature in argon atmosphere.

Characterization of carbon materials of this invention.

A porous structure of carbon materials was characterized by nitrogen adsorption methods. The low-temperature nitrogen adsorption experiments were done by using the Gemini 2375 device (Micromeritics). Specific surface of carbon samples was calculated according to the Brunauer-Emmet-Teller (BET) theory below relative nitrogen pressure (P/Po) of 0.2. A total pore volume was calculated at relative nitrogen pressure (P/Po) of 0.95.

Specific surface areas (SBET) according to the BET theory, micropore volumes according to benzene (W_s) and total pore volumes according to nitrogen (Vtot) of inventive examples are presented in Tables 1 -3.

Two-electrode test cells were assembled from activated carbon electrodes, whereas positive and negative electrodes were of the same material, density, shape and thickness. The electrodes were interleaved with porous separator from Nippon Kodoshi. The visible surface area of one electrode was 12.9cm² and the electrolyte was 1 .8M Et3MeNBF₄ in acetonitrile. The electrode package was placed in the sealed aluminium housing.

In Fig 4, it is seen that the maximum double layer capacitance (Cmax) of physically activated nanoporous CDC is reached at approximately 20°C, because at higher testing temperatures the Brownian movement increases the ionic mobility and increases the rate of desorption of electrolyte ions at the porous carbon. At lower temperatures C value decreases because of the increasing viscosity of electrolyte. Specific resistance in Fig 5 is expressed per visible surface area of one electrode in the cell. The resistance increases with decreasing of the testing temperature of the EDLC. This tendency is caused by an increase of viscosity of the electrolyte solution at lower temperatures. However, the physically activated CDC materials, obtained at lower chlorination temperature, are more nanoporous, characterised by lower surface area and smaller APS values, which hinders the ionic movement in the nanopores of the carbon. Therefore, it is obvious that the lower chlorination temperature, the higher internal resistance of the cell. However, the prolonged physical activation, e.g., examples of H2O-treated TiC derived samples for 45 minutes, decreases the nanopore content and increases APS value of carbons, which therefore lowers the internal resistance of the ultracapacitor cell.

Claims

1 . A method for manufacturing microporous carbon particles with increased capacity of electrical charge from the electrolyte of ultracapacitor, characterised by steps of:

- selecting nanoporous carbon material with predetermined surface area and pore size, and

- activation of selected nanoporous carbon, wherein

- nanoporous carbon material is heated and interacted in the reactor with water vapour or a mixture of carrier gas and water vapour at the temperature range from 500 °C to 900 °C at least for 10 or more minutes, thereafter

- for complete activation of the carbon surface the reactor is flushed with an inert gas at least for 30 or more minutes at least at temperature of 500 °C to 900 °C, thereafter

- said reactor is cooled to room temperature in an inert gas atmosphere.

2. The method for activating nanoporous carbon material according to claim 1 characterised by that before treatment the nanoporous carbon material with water vapour at 500 °C to 900 °C said nanoporous carbon material is saturated by water.

3. The method for activating nanoporous carbon material according to claim 2 characterised by that saturation of nanoporous carbon material can be done by boiling the carbon in water until it precipitates.

4. The method for activating nanoporous carbon material according to claim 2 characterised by that saturation of nanoporous carbon material can be done by sucking water into the vacuumed carbon.

5. The method for activating nanoporous carbon material according to claim 2 characterised by that saturation of nanoporous carbon material can be done by saturating the carbon in water vapour at the ambient temperature but not exceeding 500 °C.

6. The method for activating nanoporous carbon material according to claim 1 characterised by that the carrier gas in the mixture of the carrier gas and water vapour is inert gas.

7. The method for activating nanoporous carbon material according to claim 1 characterised by that the activation of the nanoporous carbon material increases the volume of micropores having a size in the range of 0.8-0.9 nm.

8. The method for activating nanoporous carbon material according to claim 6 characterised by that for forming an inert gas and water vapour mixture the inert gas flow is passed with a flow rate of at least 1 l/min through distilled water heated up to 75-80 °C.

9. The method for activating nanoporous carbon material according to claim 8 characterised by that the inert gas in the mixture of the inert gas and water vapour is argon.

10. The method for activating nanoporous carbon material according to claim 1 characterised by that the nanoporous carbon material with predetermined surface area and pore size is a carbon powder with average pore size of less than 0.92 nm and has at least two-fold nanopore volume compared to the rest of pore volume.

1 1 . The method for activating nanoporous carbon material according to claim 1 , characterised by that the nanoporous carbon powder with predetermined surface area and pore size is manufactured from the metal carbide by halogenation said metal carbide with halogen at chosen temperature in the range of 500-1000 °C, thereafter the resulting carbon powder is treated with hydrogen gas at least at 800 °C to dehalogenate said carbon material.

12. The method for activating nanoporous carbon material according to claim 1 1 characterised by that the metal carbide is a titanium carbide (TiC).

13. The method for activating nanoporous carbon material according to claim 1 1 characterised by that the metal carbide is a silicon carbide (SiC).

14. The method for activating nanoporous carbon material according to claim 1 1 characterised by that the halogen gas is chlorine.

15. The method for activating nanoporous carbon material according to claim 1 characterised by that the inert gas is an argon.