HIGH ELECTRIC CONDUCTION AND THE METHOD OF MANUFACTURING ACTIVATED CARBONS, IN PARTICULAR THE

METHOD OF MANUFACTURING ELECTRODES

The present invention relates to activated carbons with a high nitrogen content and a high electric conduction and the method of manufacturing activated carbons, in particular the method of manufacturing electrodes in batteries.

Recently, considerable hope has been associated with the zinc-air battery, which consists of a set of replaceable zinc electrodes serving as the anode and a porous membrane made of active carbon serving as a barrier between the electrolyte and atmospheric air at the same time. In the carbon cathode, the reaction of reduction of oxygen from the air takes place. When the battery is being charged, the oxygen is liberated to the atmosphere. Such batteries are very promising in terms of the amount of stored energy, as one kilogram of the battery mass can accumulate 3 times more energy, compared to competitive energy storage and recognised to be the best lithium-ion batteries.

Battery efficiency depends on, but is not limited to, properties of the carbon cathode. Active carbons are the cathode material, which is capable of efficient reduction of oxygen from the air. As regards other potentially useful cathode materials, active carbons are relatively inexpensive and easy to obtain. The advantage of active carbons is the possibility of increasing the surface area, shaping the porous structure and modifying chemically the surface to provide the active carbon with properties of an oxygen reduction catalyst. These properties are usually demonstrated by active carbons, the surface of which is modified by the introduction of nanocrystallites of catalytically active metals, such as platinum and palladium. An alternative solution is to introduce nanocrystallites of metal oxides, which also feature catalytic properties, i.e. those capable of oxygen reduction. Examples of such oxides are simple oxides, e.g. tin(IV), titanium(IV) oxide and complex oxides included in the group of perovskite oxides, e.g. LaMnO ₃ . Carbon materials enriched with the aforementioned catalytic phases have demonstrated a series of useful operational features, such as low polarisation at considerable current loads. However, preferable operational parameters can be only obtained at a very high content of metals / metal oxides in the carbon cathode material, e.g. for a platinum content of 30%-wt Such a material has, however, a limited practical application due to the very high cost of platinum. In addition, the cathode material with such a high platinum content would have to be subjected to a metal recovery process due to economic and environmental reasons. Obtaining effective cathode materials based on the active carbon and catalytic metal oxides is also possible when the content of metal oxides is within the range of 30% to 40% by weight. The application of metal oxides limits costs of production of cathode materials; it does not, however, solve environmental problems, namely the disposal of used metal-air batteries/packs. Oxide and carbon cathode materials are a potential source of metal ions, which may be liberated from the battery and released into the natural environment.

The introduction of nanocrystallites of metals / metal oxides into the active carbon structure requires the application of complex methods with considerable manufacturing cost and using raw materials, which pose high harmfulness, such as carcinogenic solvents used in the Reverse Micelle Method.

The patent application no. US20050025970 discloses a method of production of cellulose particles to be applied in electrodes in lithium-ion and zinc-air batteries. The patent application no. US20130252082 discloses a carbon material, which is applied in electrical devices, such as batteries. The material is characterised by a specific area of over 200 m ² /g, and contains 1-6% or 6-20% of nitrogen. Porosity ranges from 0.1 to 0.6 cm ³ /g. As the precursor for manufacturing the material, a polymer containing nitrogen compounds was used.

The publication "Nitrogen-doped porous carbon/Co ₃ O ₄ nanocomposites as anode materials for lithium-ion batteries", ACS Appl Mater Interfaces, 2014 28;6(10):7117-25, describes the application of natural polymers, such as chitin obtained from marine arthropods, to produce the carbon material with nitrogen doping. The cell containing the material is characterised by a capacity of 1223 mAh/g at a current density of 100 mA/g and capacity of 1060 mAh/g after 100 cycles.

The publication "Naturally inspired nitrogen-doped porous carbon", J. Mater. Chem. A, 2013, 1, 2639-2645, discloses a carbon material obtained from shrimp shells, doped with nitrogen, for the application in lithium-ion batteries. The volume of pores is over 0.6 cm ³ /g and the amount of nitrogen exceeds 5%.

The publication "Hierarchically porous carbons with optimized nitrogen doping as highly active electrocatalysts for oxygen reduction", Nature Communication 5, 4973, describes a method of production of porous carbon structures for the application in batteries. Aromatic polymers and silicon colloids serve as carbon precursors.

The patent application no. P.404374 presents a method of combined use of mixed soluble forms of cellulose, urea and powders of water-insoluble carbonates to produce mezoporous active carbons with a high content of nitrogen, from the mixed precursor, the basic components of which are the precursor of carbon phase, the nitrogen carrier and the hard carbonate porophor, wherein distilled water and the nitrogen carrier are added to the methylcellulose. Later, the obtained gel mass is mixed with the porophor and held in heat under an atmosphere of low chemical reactivity. The obtained carbon materials is subjected to acid, filtered off, washed and dried. A urea solution is added to the gel mass as the nitrogen carrier. For the purpose of the porophor, a water solution of calcium carbonate is used. Holding in heat takes place under nitrogen at a temperature of 600-800°C for at least 1 hour.

In the patent application no. P.396955, the method of production of non- porous active carbons with a high nitrogen content consists in expansion of chitosan, which men undergoes depolymerisation and protonation. Later, it is agitated with a solution of carbonates and held in heat in an atmosphere of low chemical reactivity, and then subjected to acid, filtered off, washed and dried. For the expansion, distilled water is used. Depolymerisation and protonation are performed using a water solution of HC1. After depolymerisation, a water solution of sodium carbonate is added. Holding in heat takes place under nitrogen at a temperature of 600-800°C for at least 1 hour. After holding, HC1 or HNO3 is used.

The essence of the invention are activated carbons with a high nitrogen content and a high electric conduction, produced from natural polymers with a high nitrogen content as the carbon precursors. Chitin is preferably used as the carbon precursor. In another embodiment, chitosan is preferably used at the carbon precursor. According to the invention, the carbon contains from 0% to 20% of organic conductive additives and 5% to 15% of nitrogen. It is preferable to use nanostructured forms of carbon like graphene flakes and/or carbon nanotubes and/or graphitised soot, with a specific surface area greater than 100 m ² /g, as the conductive additive. According to the invention, the essence of the method of manufacturing activated carbons with a high nitrogen content and a high electric conduction is that a natural polymer with a high nitrogen content, preferably chitin and/or chitosan, is mixed with water and then with a water solution of strong acid, and then with an inorganic porophor in an amount from 10% to 50% relative to the mass of the natural polymer and from 0% to 20% of the conductive organic additive, and possibly up to 50% relative to the mass of the used natural polymer of the water solution of the low-particle compound containing nitrogen. For the inorganic porophor, salts of carbonic acid, in particular salts of sodium carbonate and/or potassium carbonate and/or calcium carbonate are used. Water- soluble carbonates are used in the form of a 20% water solution. Water-insoluble carbonates are used in the form of powder with particles, the size of which is less than 30 nm. The mixture may be dried at a temperature up to 120°C and carbonised at high temperatures without oxygen access at a temperature up to 1,000°C . It is preferable when urea is used as the low-particle compound containing nitrogen. The most preferable carbonisation temperature is the value between 600-90°0C . Afterwards, the mixture is cooled down to 50°C in oxygen-free atmosphere and mixed with a water solution of strong non- oxidising acid until the inorganic porophor is removed. This is preferably hydrochloric acid with a concentration of 1 mole in the ratio of 10 ml of the acid to 1 g of carbon. The carbon is separated from the liquid, washed with water with pH = 7 to remove the water-soluble substances and dried.

The cathode material, obtained from the activated carbon according to the invention, contains no metals or their derivatives and requires no application of harmful reagents. The method features a low cost of production, common availability of raw materials as well easy recycling and disposal. Active carbons are produced using biological raw materials obtained from renewable sources. The object of the invention is presented in the embodiments. Embodiment 1

First, distilled water in an amount of 100 cm ³ is added to 100 g of powdered raw chitosan poured in a beaker. Next, a solution of hydrochloric acid (HC1) in a concentration of 1 mole in an amount of 30 cm ³ is added. By the action of the acid, chitosan expands as a result of protonation of amino groups and water binding. Once mechanically agitated, gel is produced. Afterwards, a water solution of sodium carbonate in percent concentration of 17 %-wt. in an amount of 200 cm ³ is added. Once mechanically agitated, carbon nanotubes are added as the substance, which increases electric conduction of the final carbon material. Once agitated, the mass so obtained is placed inside a heating device providing controllable heating within the temperature range 20-900°C in a controllable nitrogen atmosphere. The heat-up rate is 10°C /min until the predefined carbonisation temperature of 800°C is reached. Once the intended temperature is reached, the obtained carbon is held in the heating device for 1 hour to be cooled down to 50°C , kept under a constant flow of nitrogen. The carbon material obtained after carbonisation is subjected for 24 hours to a water solution of strong hydrochloric acid (HC1) in a concentration of 1 mole in the ratio of 6 cm ³ of the acid solution to 1 gram of carbon. Once the acid is added, a rapid reaction takes place and CO ₂ gas is liberated. Then, using a Btlchner funnel, the carbon is filtered off and washed with distilled water at the same time to obtain a filtrate with a pH = 7. In order to evaporate excess water, the prepared carbon is dried at 50°C . The obtained carbon is characterised by a considerable surface area up to approx. 1,000 nrVg, a high total volume of pores of approx. 0.2 cm ³ /g and a high nitrogen weight content of 5-6%. The obtained carbon may be used to manufacture gas diffusion electrodes capable of reduction of oxygen taken from the air in the environment of 6-mole solution of sodium hydroxide at a temperature of 60°C . The obtained gas diffusion electrodes are characterised by low cathode polarisation; the cathode potential relative to the HgO/Hg electrode is not lower than -100 mV for a current load of 200 mA/cm ² with the electrode washed by a stream of air or oxygen. The obtained electrodes can withstand current loads exceeding 300 mA/cm ² .

Embodiment 2

First, distilled water in an amount of 100 cm ³ is added to 100 g of powdered raw chitosan poured in a beaker. Next, a solution of hydrochloric acid (HC1) in a concentration of 2 moles in an amount of 30 cm ³ is added. By the action of the acid, chitosan expands as a result of protonation of amino groups and water binding. Once mechanically agitated, gel is produced. Then, powdered calcium carbonate in an amount of 50 g and with a particle size being less than 30 urn is added. A water solution of urea in an amount of 30 g per 50 g of water is added to the gel containing all components. Once mechanically agitated, 10 g of graphene flakes are added as the substance, which increases the electric conduction of the final carbon material. Once agitated, the mass so obtained is dried in an electric drier at a temperature up to 120°C and placed inside a heating device providing controllable holding in heat within the temperature range 20-900°C in a controllable nitrogen atmosphere. The heat-up rate is 10 °C/min until the predefined carbonisation temperature of 900°C is reached. Once the intended temperature is reached, the obtained carbon is held in the heating device for 1 hour to be cooled down to 50°C, kept under a constant flow of nitrogen. The carbon material obtained after carbonisation is subjected for 24 hours to a water solution of strong hydrochloric acid (HC1) in a concentration of 1 mole in the ratio of 6 cm ³ of the acid solution to 1 gram of carbon. Once the acid is added, a rapid reaction takes place and CO ₂ gas is liberated. Then, using a Buchner funnel, the carbon is filtered off and washed with distilled water at the same time to obtain a filtrate with pH = 7. In order to evaporate excess water, the prepared carbon is dried at 50°C. The obtained carbon is characterised by a considerable surface area up to approx. 1,000 m ² /g, high total volume of pores of approx. 0.2 cm ³ /g and a nitrogen weight content of 13%.

The obtained carbon may be used to manufacture gas diffusion electrodes capable of reduction of oxygen taken from the air in the environment of a 6- mole solution of sodium hydroxide at a temperature of 60°C . The obtained gas diffusion electrodes (cathodes for oxygen reduction) are characterised by low cathode polarisation; the cathode potential relative to the HgO/Hg electrode is not lower than -100 mV for a current load of 200 mA/cm ² with the electrode being washed by a stream of air or oxygen. The obtained electrodes can withstand current loads exceeding 300 mA/cm ² .

Embodiment 3

First, distilled water in an amount of 100 cm ³ is added to 100 g of powdered raw chitin poured in a beaker. Next, a solution of hydrochloric acid (HC1) in a concentration of 1 mole in an amount of 30 cm ³ is added. By the action of the acid, chitin expands as a result of protonation of amino groups and water binding. Once mechanically agitated, gel is produced. Afterwards, a water solution of potassium carbonate in a percent concentration of 25 %-wt. in an amount of 200 cm ³ is added. Once mechanically agitated, graphitised soot is added as the substance, which increases the electric conduction of the final carbon material. Once agitated, the mass so obtained is placed inside a heating device providing controllable heating within the temperature range 20-900°C in a controllable nitrogen atmosphere. The heat-up rate is 10°C /min until the predefined carbonisation temperature of 800°C is reached. Once the intended temperature is reached, the obtained carbon is held in the heating device for 1 hour to be cooled down to 50°C , kept under a constant flow of nitrogen. The carbon material obtained after carbonisation is subjected for 24 hours to a water solution of strong hydrochloric acid (HC1) in a concentration of 1 mole in the ratio of 6 cm ³ of the acid solution to 1 gram of carbon. Once the acid is added, a rapid reaction takes place and CO ₂ gas is liberated. Then, using a Bilchner funnel, the carbon is filtered off and washed with distilled water at the same time to obtain a filtrate with pH = 7. In order to evaporate excess water, the prepared carbon is dried at 50°C. The obtained carbon is characterised by a considerable surface area up to approx. 1,000 m ² /g, high total volume of pores of approx. 0.2 cmVg and high nitrogen weight content of 5-6%. The obtained carbon may be used to manufacture gas diffusion electrodes capable of reduction of oxygen taken from the air in the environment of a 6-mole solution of sodium hydroxide at a temperature of 60°C . The obtained gas diffusion electrodes are characterised by low cathode polarisation; the cathode potential relative to the HgO/Hg electrode is not lower than -100 mV for a current load of 200 mA/cm ² with the electrode being washed by a stream of air or oxygen. The obtained electrodes can withstand current loads exceeding 300 mA/cm ² .

Embodiment 4

First, distilled water in an amount of 100 cm ³ is added to 100 g of powdered raw chitin poured in a beaker. Next, a solution of hydrochloric acid (HC1) in a concentration of 2 moles in an amount of 30 cm ³ is added. By the action of the acid, chitin expands as a result of protonation of amino groups and water binding. Once mechanically agitated, gel is produced. Then, powdered calcium carbonate in an amount of 50 g and with a particle size being less than 30 nm is added. A water solution of urea in an amount of 30 g per 50 g of water is added to the gel containing all components. Once mechanically agitated, 10 g of graphene flakes are added as the substance, which increases electric conduction of the final carbon material. Once agitated, the mass so obtained is dried in an electric drier at a temperature up to 120°C and placed inside a heating device providing controllable holding in heat within the temperature range of 20-900 °C in a controllable nitrogen atmosphere. The heat-up rate is 10°C/min until the predefined carbonisation temperature of 800°C is reached. Once the intended temperature is reached, the obtained carbon is held in the heating device for 1 hour to be cooled down to 50°C, kept under a constant flow of nitrogen. The carbon material obtained after carbonisation is subjected for 24 hours to a water solution of strong hydrochloric acid (HC1) in a concentration of 1 mole in the ratio of 6 cm ³ of the acid solution to 1 gram of carbon. Once the acid is added, a rapid reaction takes place and CO ₂ gas is liberated. Then, using a Buchner funnel, the carbon is filtered off and washed with distilled water at the same time to obtain a filtrate with pH = 7. In order to evaporate excess water, the prepared carbon is dried at 50 °C. The obtained carbon is characterised by a considerable surface area up to approx. 1,000 m ² /g, high total volume of pores of approx. 0.2 cm ³ /g and a nitrogen weight content of 13%. The obtained carbon may be used to manufacture gas diffusion electrodes capable of reduction of oxygen taken from the air in the environment of a 6-mole solution of sodium hydroxide at a temperature of 60°C.

The obtained gas diffusion electrodes (cathodes for oxygen reduction) are characterised by low cathode polarisation; the cathode potential relative to the HgO/Hg electrode is not lower than -100 mV for a current load of 200 mA/cm ² with the electrode being washed by a stream of air or oxygen. The obtained electrodes can withstand current loads exceeding 300 mA/cm ² .