Method for producing composite electrode material based on carbon from sugar sorghum cake and transition metal sulfides for supercapacitors

Abstract

FIELD: electrochemical energy storage systems.SUBSTANCE: composite electrode materials for supercapacitors based on carbon from biomass and transition metal compounds. A method for producing a composite electrode material for supercapacitors which involves obtaining a carbon base from sugar sorghum cake and introducing a transition metal compound into it. Before introducing the metal compound, the carbon base is subjected to hydrothermal nitriding in the presence of urea, followed by carbonization in an inert atmosphere to obtain nitrided carbon. The introduction of the transition metal compound is carried out by in situ hydrothermal synthesis, in which an aqueous solution of ammonium molybdate salt, cobalt (II) nitrate, iron (II) sulfate or nickel (II) nitrate, and thiourea reacts with the resulting nitrated carbon to form a metal sulfide on its surface.EFFECT: range of highly efficient sorghum-based composite materials for supercapacitors is expanded by creating a material with a different active phase composition, improved contact between components, and greater stability.3 cl, 2 dwg, 4 ex

Description

[0001] The present invention relates to the field of electrochemical energy storage systems, specifically to composite electrode materials for supercapacitors based on biomass-derived carbon and transition metal compounds. Supercapacitors, as key elements of energy storage systems, require the development of highly efficient and inexpensive electrode materials. One of the most promising areas is the creation of hybrid (composite) materials that combine the high electronic conductivity and surface area of ​​a carbon matrix with the high Faradaic activity of transition metal compounds such as Mo, Co, Ni, Fe, and other sulfides.

[0002] The most common approach is to use carbon materials derived from biomass (e.g., coconut shells, bamboo). However, these sources are either imported or have a limited raw material base. In particular, sugarcane, which is widely studied internationally, cannot be cultivated on an industrial scale in the Russian Federation due to unsuitable climatic conditions. This creates a dependence on foreign raw materials.

[0003] Carbon materials for supercapacitors obtained from sorghum biomass are known. For example, the work [Fuming Zhang, Hongchao Lang, Jinggao Wub, Jing Huang, Sorghum-derived porous carbon for outstanding green supercapacitors, New J. Chem., 2024, 48, 332-341, DOI: 10.1039 / d3nj04881k] describes porous carbon synthesized by direct carbonization and chemical activation of sorghum grains with KOH. In the work [Xiaonan Xu, Yongpeng Cui, Jing Shi, Wei Liu, Shougang Chen, Xin Wang, Huanlei Wang, Sorghum core-derived carbon sheets as electrodes for a lithium-ion capacitor, RSC Adv., 2017, 7, 17178-17183, DOI: 10.1039 / c7ra02279d] sheet carbon was synthesized by carbonization of sorghum core at 1300°C in a nitrogen atmosphere followed by activation with air at 300°C. The resulting material has a specific surface area of ​​1122 m 2 / g and used in lithium-ion capacitor, demonstrating high energy density (124.8 Wh / kg) and capacity retention of 66% after 5000 cycles. In the work [Minjun Kim, Hyunsoo Lim, Xingtao Xu, Md Shahriar A. Hossain, Jongbeom Na, Nur Nadia Awaludin, Jagrat Shah, Lok Kumar Shrestha, Katsuhiko Ariga, Ashok Kumar Nanjundan, Darren J. Martin, Joseph G. Shapter, Yusuke Yamauchi, Sorghum biomass-derived porous carbon electrodes for capacitive deionization and energy storage, Microporous and Mesoporous Materials, 2021, 312, 110757, https: / / doi.org / 10.1016 / j.micromeso.2020.110757] porous carbon materials from sorghum stems activated with KOH at different temperatures (600, 800, 900°C) were studied. The material activated at 800°C showed a specific surface area of ​​1347 m 2 / g and a specific capacitance of 257 F / g at 1 mV / s in a three-electrode system with a KOH electrolyte. The study also demonstrated the application of these materials for capacitive deionization. A drawback of these approaches is the limited specific energy capacity, typical of materials operating primarily through the electric double layer. Furthermore, these methods utilize valuable plant parts (cores, stems, and grains) rather than processing waste, which reduces economic and environmental efficiency.

[0004] A closer solution is composite materials based on carbon from sorghum and transition metal compounds. In particular, [Min Fu, Zitong Zhu, Zhihao Zhang, Qingru Zhuang, Wei Chen, Qingyun Liu, Microwave deposition synthesis of Ni(OH)2 / sorghum stalk biomass carbon electrode materials for supercapacitors, Journal of Alloys and Compounds, 2020, 846, 156376, https: / / doi.org / 10.1016 / j.jallcom.2020.156376] disclose a method for producing a Ni(OH)2 / carbon composite from sorghum stalks, where nickel hydroxide is deposited on a carbon matrix using microwave synthesis. This solution is aimed at increasing the specific capacitance due to faradaic reactions. The disadvantages of this solution are:

[0005] 1. Limited choice of electrochemically active component (hydroxides), which may limit the specific capacity and long-term stability.

[0006] 2. The microwave deposition method used may result in uneven distribution of the metal phase and weak interaction with the carbon matrix.

[0007] 3. The carbon matrix is ​​not subjected to targeted heteroatomic modification (e.g. nitriding) to specifically improve its electrical conductivity and wettability with electrolyte.

[0008] The technical result of the claimed invention is to expand the arsenal of highly efficient sorghum-based composite materials for supercapacitors by creating a material with a different active phase composition, improved contact between components and higher stability.

[0009] The technical result consists in creating a composite material based on transition metal sulfides, tightly associated with a nitrided carbon matrix, which provides a synergistic combination of faradaic and capacitive capacity, high values ​​of specific surface area, outstanding cyclic stability (up to 72% after 3500 cycles for a composite with CoS) and the possibility of using agricultural processing waste.

[0010] To achieve the technical result, the following is carried out: 1) modification of the carbon carrier by hydrothermal nitriding followed by carbonization, which allows obtaining a carbon material with increased electrical conductivity and a developed surface favorable for further functionalization; 2) formation of a composite structure by deposition of transition metal sulfides by hydrothermal synthesis directly on the surface of the nitrided carbon, ensuring uniform distribution of the active phase and durability.

[0011] The technical result is achieved in that the method for producing a composite electrode material for supercapacitors consists of obtaining a carbon base from sugar sorghum cake and introducing a transition metal compound into it, wherein the carbon base, before introducing the metal compound, is subjected to hydrothermal nitriding in the presence of thiourea, followed by carbonization in an inert atmosphere to obtain nitrided carbon; the introduction of the transition metal compound is carried out by hydrothermal synthesis in situ, in which an aqueous solution of a transition metal salt selected from the group: ammonium molybdate, cobalt (II) nitrate, iron (II) sulfate, nickel (II) nitrate, and thiourea react with the obtained nitrided carbon to form a sulfide of the corresponding metal on its surface. Moreover, the hydrothermal synthesis is carried out at a temperature of 205-215 °C for 19-21 hours.Obtaining a carbon base from sugar sorghum cake involves treating it at least twice with an aqueous solution of alkali at a temperature of 85-95°C with intermediate washing to a neutral pH.

[0012] The combination of the capacitive properties of the carbon matrix and the faradaic activity of metal sulfides produces a material with improved specific characteristics and high cycling stability. The technology allows the use of various transition metal salts and current collector types, ensuring its versatility and potential for scalability.

[0013] Brief description of drawings

[0014] Fig. 1 - SEM images of samples based on nitrided carbon and metal sulfides:

[0015] Fig. 1a shows the surface morphology of the nitrided carbon (NC) obtained according to Example 2;

[0016] Fig. 1b shows the surface morphology of the cobalt sulfide (NC_CoS)-based composite material obtained according to Example 3;

[0017] Fig. 1c shows the Raman spectra of nitrided carbon (NC) compared with cobalt sulfide (NC_CoS) composite material.

[0018] Fig. 2 shows the stability for electrodes formed according to Example 4:

[0019] Fig. 2a shows the stability of NC(Ni) at 0.5 A / g;

[0020] Fig. 2b shows the stability of NC_CoS(Ni) at 1.5 A / g.

[0021] Examples of the invention

[0022] Example 1. Preparation of carbon base from sweet sorghum cake

[0023] Russian variety sweet sorghum cake is washed with distilled water to remove water-soluble impurities and sugars. The washed raw material is dried at a temperature of 105 ± 5°C to a constant weight. The dried cake is ground to a powder with a particle size of less than 200 μm. To remove lignocellulosic components and form the primary pore structure, alkaline treatment is performed: 20 g of powder is poured into 100 ml of a 5% aqueous potassium hydroxide (KOH) solution and heated at 90 ± 3°C for 4 hours with stirring. After cooling, the mixture is filtered, and the precipitate is washed with deionized water to a neutral pH (7.0 ± 0.5). The KOH treatment procedure followed by washing is repeated one more time to ensure completeness of the process. The finally washed product is dried at 105±3°C for 6 hours.

[0024] Example 2. Obtaining nitrided carbon (NC) and its properties

[0025] 2.0 g of the carbon powder prepared in Example 1 are dispersed in 40 ml of a 0.66 M aqueous urea solution in a Teflon-lined autoclave reactor. The closed autoclave is placed in a drying oven and maintained at a temperature of 160 ± 3 °C for 6 hours to perform hydrothermal nitriding. After cooling, the resulting material is filtered and successively washed with ethanol and deionized water. The washed product is dried at 10 ± 3 °C and then carbonized in a nitrogen stream. Carbonization is carried out in a tubular furnace by heating the sample to a temperature of 800 ± 10 °C at a rate of 5 °C / min and maintaining it at this final temperature for 2 hours.

[0026] The obtained nitrided carbon (NC) is characterized using Raman spectroscopy (Fig. 1c), in the spectrum in addition to the main D- and G-bands and their overtones (~2800 cm -1 ), a distinct peak was identified at 824 cm -1This signal can be associated with several phenomena characteristic of functionalized biomass-derived carbon materials. The presence of heteroatoms (mainly oxygen) in the carbon matrix, whose vibrations (e.g., deformation vibrations of COC bonds in epoxy or ester groups) can manifest themselves in this region. Vibrations associated with the boundaries of nanoscale graphite-like domains or specific lattice defects. Overtone or combination modes of the low-order carbon framework. In SEM images (Fig. 1a), nitrided carbon (NC) has a structure formed mainly of long carbon filaments with a specific surface area of ​​about 2668 m 2 / g and contains nitrogen atoms in surface functional groups.

[0027] Example 3. Obtaining a composite material based on cobalt sulfide (NC_CoS) and its properties

[0028] 0.1 g of nitrided carbon (NC), obtained in Example 2, is dispersed in a mixture of 20 ml of an aqueous solution of cobalt nitrate Co(NO3)2 6H2O with a concentration of 0.0101 M and 20 ml of a 0.04 M solution of thiourea (CH4N2S). The salt concentration is calculated so that the mass fraction of cobalt in the target composite is approximately 30% of the NC mass. The mixture is stirred on a magnetic stirrer for 60 minutes, then transferred to a Teflon liner of an autoclave. Unlike the prototype, which uses fast (3 min) microwave synthesis, the claimed method carries out hydrothermal treatment at 210 ± 3 ° C for 20 hours, which ensures the complete reaction and the formation of a strong bond between the sulfide phase and the carbon matrix. After synthesis, the autoclave is cooled, the resulting precipitate is separated by centrifugation (1000 rpm, 15 min), washed with ethanol and water, and dried at 105±3°C. The resulting composite is designated NC_CoS.

[0029] The obtained NC_CoS composite material is characterized by Raman spectroscopy (Fig. 1c), the presence of a set of clear peaks at 670, 502 and 475 cm -1 in the spectrum is direct evidence of the formation of a crystalline phase. These peaks may correspond to cobalt oxides (for example, Co3O4, which is characterized by bands at ~690, ~520, and ~480 cm -1 or cobalt sulfides. In the SEM images (Fig. 1b), the NC_CoS composite material has a structure formed by long carbon filaments with uniformly distributed CoS particles. The specific surface area of ​​the NC_CoS composite is about 1,251 m 2 / G.

[0030] Similar composite materials were obtained using other transition metal salts: ammonium molybdate, iron (II) sulfate, nickel (II) nitrate.

[0031] Example 4. Formation of an electrode

[0032] To form the electrodes, use the powder obtained in Examples 2 and 3 and the polymer binder (polyvinylidene fluoride (PVDF)) in a ratio of 90:10. A 0.020 g sample of the powder and polyvinylidene fluoride are dissolved in N-methylpyrrolidone (C5H9NO). The resulting solution is thoroughly stirred with a glass rod until the PVDF is completely dissolved. The resulting mass is then applied to a nickel foam substrate measuring 30 x 10 mm. 2 Before application, the nickel foam substrate is pre-treated with a 10% HCl solution and rinsed with bidistilled water. After application, the substrate and material are dried at room temperature (20±5°C) for 12 hours.

[0033] An electrode prepared by applying a paste of the material obtained in Example 2 and a polyvinylidene fluoride binder (10 wt.%) to nickel foam in a three-electrode cell with 1 M KOH electrolyte exhibits a specific capacitance of 26 F / g at a current density of 0.5 A / g. After 3500 charge-discharge cycles, the material retains about 56% (Fig. 2a) of its initial capacity.

[0034] An electrode prepared by applying a paste of the material obtained in Example 3 and a polyvinylidene fluoride binder (10 wt.%) to nickel foam in a three-electrode cell with 1 M KOH electrolyte exhibits a specific capacity of 60 F / g at a current density of 1.5 A / g. After 3500 charge-discharge cycles, the material retains approximately 72% (Fig. 2b) of the initial capacity, confirming the high cyclic stability achieved due to the synergy between the nitrided carbon matrix and the metal sulfide, as well as the use of a more stable class of compounds (sulfides) compared to the hydroxides used in the prototype.

[0035] Similar electrodes were obtained using other composite powders based on transition metals: NC_MoS, NC_NiS, NC_FeS.

[0036] The technical and economic advantage lies in the use of sweet sorghum cake, a byproduct of this crop's processing. Sweet sorghum is a drought-resistant, high-yielding crop adapted to Russia's agroclimatic conditions, making it a strategically important domestic raw material. The use of cake (a byproduct) further enhances the economic and environmental efficiency of the method. This development is consistent with the goals of import substitution and the transition to a closed-loop economy, as declared in strategic documents on Russian agriculture and technological development.

Claims

1. A method for producing a composite electrode material for supercapacitors, comprising obtaining a carbon base from sugar sorghum cake and introducing a transition metal compound into it, characterized in that the carbon base, before introducing the metal compound, is subjected to hydrothermal nitriding in the presence of urea, followed by carbonization in an inert atmosphere to obtain nitrided carbon; the introduction of the transition metal compound is carried out by hydrothermal synthesis in situ, in which an aqueous solution of a transition metal salt selected from the group: ammonium molybdate, cobalt (II) nitrate, iron (II) sulfate, nickel (II) nitrate, and thiourea interact with the obtained nitrided carbon to form a sulfide of the corresponding metal on its surface.

2. The method according to paragraph 1, characterized in that the hydrothermal synthesis is carried out at a temperature of 205-215°C for 19-21 hours.

3. The method according to paragraph 1, characterized in that obtaining a carbon base from sugar sorghum cake includes its at least twofold treatment with an aqueous solution of alkali at a temperature of 85-95°C with intermediate washing to a neutral pH.

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