Doped extra-high surface area activated carbon produced from hemp biomass and method for producing supercapacitor electrode from this activated carbon

Abstract

The invention relates to heteroatom-doped activated carbon with an exceptionally high surface area, produced from hemp biomass; the method of producing this activated carbon; and the method of producing a supercapacitor electrode from said activated carbon.

Description

[0001] DOPED EXTRA-HIGH SURFACE AREA ACTIVATED CARBON PRODUCED FROM HEMP BIOMASS AND METHOD FOR PRODUCING SUPERCAPACITOR ELECTRODE FROM THIS ACTIVATED CARBON

[0002] TECHNICAL FIELD

[0003] The invention relates to heteroatom-doped activated carbon with a very high surface area produced from hemp biomass and the method for producing this activated carbon.

[0004] BACKGROUND

[0005] In recent years, with the increasing population, advancements in technology, and rising living standards, the need for electricity storage has been steadily growing. While energy storage studies are continuing rapidly worldwide, the aim of these studies is to store energy more easily and quickly, increase storage capacity, and extend the duration for which stored energy can be used. Additionally, the longevity of the storage units is one of the most critical aspects being studied. In this context, supercapacitors form one of the most important groups of electrochemical energy storage systems designed to store and deliver energy rapidly. Due to their significant applications, various studies are being conducted to improve supercapacitors. Their operational stability, safety, long service life, ease of use, high cycle life, high power density, low cost, high electrical resistance, and large surface area make supercapacitors preferred in many areas. Due to their versatile applicability, they are widely used in urban transportation, high-density charge / discharge applications, emergency energy storage units, portable electronic devices, industrial power applications, and some hybrid systems. Carbon-based materials have long been used as electrode materials in supercapacitors due to their favorable surface and electrical properties. Among carbon materials, activated carbon is the most preferred electrode material due to its high surface area and low cost. The performance of carbon electrodes is directly related to the physical (surface area and pore structure) and chemical (functional groups on the surface) properties of the material used. Numerous studies have been conducted in the literature on the use of carbon-based materials derived from various sources as electrode materials for supercapacitors. These studies typically focus on pore size, the type and amount of functional groups on the surface, and how these features are specifically modified according to the intended application of the capacitor. Currently, the high cost of producing commercial supercapacitors limits their practical application. As a result, the use of electrodes made from activated carbon prepared from natural raw materials such as biomass waste has increased in recent years. However, due to the abundance and diversity of biomass waste and the differences in the physical and chemical properties of the activated carbon derived from each, further research in this area is important. Carbon materials used in supercapacitor electrodes are typically derived from fossil-based sources (e.g., coal, wood). Given the depletion of these reserves and their environmental impact, the development of carbon materials from renewable sources has gained importance. Using biomass-based materials as renewable carbon sources aligns with today’s environmental policies and sustainable energy approaches, making them increasingly significant. Therefore, research on using activated carbon derived from different biomasses as electrode materials has been growing. In the literature, a nitrogen, sulfur, and phosphorus-doped carbon sample with a 2583 m2 / g BET surface area was produced from walnut shell biomass — a frequently used biomass for activated carbon synthesis — and the specific capacitance of the electrode material developed with this sample was found to be 207 F / g (at 0.5 A / g current density) in a symmetric supercapacitor cell configuration (Guo et al., 2021). Activated carbon is a versatile material used in water and air purification, chemical processes, the food and pharmaceutical industries, energy, and more. As of 2022, the global activated carbon market is valued at approximately $4.5 billion, with around 6 million tons exported. The top ten exporters by value are China, USA, India, Belgium, Netherlands, Germany, Philippines, Sri Lanka, Japan, and Indonesia. Patents for various types of activated carbon are managed by a limited number of companies. However, in Turkiye, factories have recently begun commercial production of activated carbon due to growing recognition of its importance and increased research. For example, in 2015, a factory established in Istanbul by Nano Science Chemistry Technology Co. began producing activated carbon from coconut shells, wood, and coal. In Malatya, a factory that uses apricot kernels for production was completed this year, and around 10 more are in the setup phase. Globally, in commercial activated carbon production, only coconut shells (a renewable source) are used, accounting for 10%. The remaining activated carbon is produced from fossil-based sources: 35% from wood, 28% from bituminous coal, 14% from lignite, and 10% from peat. Supercapacitors made from activated carbon derived only from coconut shells have surface areas between 1500-2000 m2 / g and capacitance values between 100-120 F / g, depending on the production method.

[0006] OBJECTIVE OF THE INVENTION

[0007] Although there are studies on the use of activated carbon obtained from hemp fibers, either directly or doped with various atoms, as electrode materials in supercapacitors, no prior research has been conducted where hemp shavings are triple-doped (nitrogen, sulfur, phosphorus) to enhance their properties. In this invention, instead of hemp fibers, hemp shavings (bark), which make up approximately 60-70% of the hemp stalk, were used to produce activated carbon. This allows for the reuse of waste hemp material.

[0008] Furthermore, after thermal processing, only 30% of the mass of hemp fiber becomes biochar, compared to 36% of the shavings, making the latter more efficient. By using hemp shavings, not only is waste material repurposed, but higher-yield activated carbon can also be produced. The activated carbon doped with nitrogen, sulfur, and phosphorus derived from hemp shavings has a BET surface area of 3297 m2 / g and a specific capacitance value of 189 F / g (at 0.5 A / g current density) in a symmetric cell configuration. By using this local raw material for the first time in the literature, a higher-surface-area activated carbon has been obtained, and a high- performance supercapacitor electrode has been developed. Hemp is a woody, annual plant from the Cannabinaceae family. It can grow in all climates, requires very little water, and grows rapidly without the need for pesticides, making it an environmentally friendly product. While hemp has many industrial uses (paper, textile, construction, food, medicine, dyes, oils, ink, fuel), these applications generate significant amounts of waste. During fiber production, the bark (shavings), which constitutes about 60-70% of the stalk, is considered waste (Ranalli, 1999). Recycling these biomass wastes is vital.

[0009] Using hemp waste for activated carbon production is a suitable method, offering the benefit of reusing waste materials (Rosas et al., 2008; 2009; Yang et al., 2012; Wang et al., 2013). Hemp is rich in cellulose (74%) and hemicellulose (18%), making it easier to activate carbon. Its unique hierarchical pore structure, formed during growth and responsible for transporting water and nutrients, can remain intact after carbonization. Porous hemp-based carbon is crucial for rapid mass transport in electrochemical applications (Jiang et al., 2019). DETAILED DESCRIPTION OF THE INVENTION

[0010] The invention relates to activated carbon produced from hemp shavings and doped with nitrogen, sulfur, and phosphorus. The internal surface area of this activated carbon is 3297 m2 / g. Supercapacitor electrodes were prepared from this activated carbon, identifying one of the applications of the invention.

[0011] Activated Carbon Synthesis

[0012] During activated carbon production, some of the carbon-containing material is oxidized and removed along with non-carbon components. Chemical agents deform the structured carbon layers to create pores during activation. Impurities such as tar are intentionally removed, and closed pores are opened. During carbonization, aromatic structures are broken to expand existing pores.

[0013] For this invention, carbonization (pyrolysis) was followed by chemical activation. Ground hemp shavings were heated at 500°C for 1 hour in a nitrogen atmosphere. The resulting biochar was chemically activated with KOH and reheated at 800°C for 1 hour under nitrogen. The amorphous content and microstructure were analyzed with X-ray diffraction (XRD), morphology with FE-ESEM, chemical composition with EDS, surface area and pore distribution with BET analysis, and structure with FTIR spectroscopy.

[0014] Doping of Activated Carbon

[0015] Heteroatom doping was done in a single step. Urea (CH4N2O) for nitrogen, potassium sulfate (K2SO4) for sulfur, and potassium phosphate (KH2PO4) for phosphorus were used. The activated carbon and dopants (1 :3 molar ratio) were stirred at 70°C for 1 hour and then heat-treated in a nitrogen atmosphere. The resulting material’s properties were analyzed as described above. The specific surface area was determined to be 3297 m2 / g.

[0016] Supercapacitor Fabrication

[0017] To enable electrode use, the doped activated carbon was mixed with conductive carbon black, surfactant Triton X-100 (Ci4H22O(C2H4O)n), and binder PVDF. The mixture was spray-coated onto a stainless-steel current collector with 1 cm2area and dried at 110°C for 24 hours.

[0018] A 6.0 M KOH solution was used as the electrolyte. Two symmetric electrodes were placed facing each other with an electrolyte-soaked paper separator in between, then sealed with parafilm for electrochemical testing. Additionally, standard CR2032- type coin cell supercapacitor devices were also prepared.

Claims

CLAIMS1 . A doped activated carbon produced from hemp biomass, characterized in that comprising hemp shavings doped with nitrogen, sulfur, and phosphorus.

2. The nitrogen mentioned in Claim 1 , characterized in that it is urea (CH4N2O).

3. The sulfur mentioned in Claim 1 , characterized in that it is potassium sulfate (K2SO4).

4. The phosphorus mentioned in Claim 1 , characterized in that it is potassium phosphate (KH2PO4).

5. A supercapacitor electrode composed of a material as defined in any one of the preceding claims.

6. A method for producing doped activated carbon from hemp biomass, characterized by comprising the steps below:- Grinding the hemp shavings,- Heating at 500 °C for 1 hour in a nitrogen gas atmosphere in a tube furnace, Applying chemical activation to the resulting biochar using KOH and heating at 800 °C for 1 hour in a nitrogen gas atmosphere,- Doping the obtained activated carbon by mixing it with urea (CH4N2O) for nitrogen (N), potassium sulfate (K2SO4) for sulfur (S), and potassium phosphate (KH2PO4) for phosphorus (P) at a 1 :3 molar ratio, heating the mixture at 70 °C for 1 hour, and subjecting it to thermal treatment in a nitrogen atmosphere.

7. An electrode coating material produced from hemp biomass, characterized in that it comprises nitrogen, sulfur, and phosphorus doped activated carbon obtained from hemp shavings, a conductivity enhancer (carbon black), the surfactant Triton X-100 (Ci4H22O(C2H4O)n), and the binder polyvinylidene fluoride (PVDF).