Mesoporous highly conductive carbon aerogel composites based on chitosan

Abstract

The invention relates to a carbon aerogel composite based on chitosan and recycled activated carbon, and to a method for its production.

Description

[0001] The invention relates to a carbon aerogel composite based on chitosan and recycled activated carbon and a process for its production.

[0002] Carbon aerogels (CA) are three-dimensional, open-pore, amorphous materials produced by carbonizing organic aerogels (e.g., biopolymers or biomass- and phenol-based materials). Unique properties of carbon aerogels, such as well-controlled porosity, large specific surface area, electrical conductivity, and low enveloping density, make them promising materials for applications as electrode materials in batteries. The electrical conductivity of amorphous carbon materials depends on their electronic structure, the proportion of graphitic structures or graphitic character, and heteroatoms. Starting materials such as chitin or chitosan, whose structures already contain heteroatoms (especially nitrogen), are particularly suitable for this purpose.

[0003] The enveloping density and electrical conductivity correlate well. Carbon aerogels with high density exhibit high electrical conductivity. This is due to the fact that there are more transport pathways in a dense network than in one with fewer connections. The pore structure of the material therefore also influences the electrical conductivity. Furthermore, the carbonization temperature and any subsequent activation (e.g., with CO2) affect the electrical conductivity of carbon aerogels. The electrical conductivity of chitosan-based carbon aerogel, which was carbonized at 800 °C for 1 h in N2 and activated for 1 h in CO2, is approximately 44 S / m. The electrical conductivity increases with increasing temperature.

[0004] Graphite is a common, commercially available material due to its low cost. The main disadvantage is that graphite production is an energy- and time-intensive process. Both natural graphite processing and the production of synthetic graphite cause CO2 emissions, as well as the release of NO. x and SO x Compounds, as well as particulate emissions. The addition of electrically conductive additives such as carbon black can significantly increase conductivity. Other additives such as graphene, carbon tubes, and reduced graphene oxide are used less frequently due to their high production costs. Recovered carbon black (rCB) made from scrap tires represents an alternative.

[0005] The main disadvantages of known carbon materials are their comparatively low capacity, their higher weight, and the impact of current technology on humans and the environment. The mining of natural graphite releases fine dust that is harmful to health, while synthetic graphite production requires high temperatures, resulting in a large carbon footprint.

[0006] The object of the present invention is to optimize the properties of carbon aerogels, in particular to improve their electrical conductivity.

[0007] To achieve this objective, the present invention provides a composite material comprising a chitosan aerogel. Due to their high pore volume, chitosan aerogels are expected to exhibit higher capacity than graphite in the application area of ​​lithium-ion batteries.

[0008] The invention relates to a conductive composite material which comprises, in particular consists of, a chitosan aerogel and recovered carbon black.

[0009] A first embodiment of the invention consists in a carbon aerogel composite based on chitosan and recycled activated carbon, which is characterized in that the activated carbon is free of ash, silicate compounds and zinc components and the composite has a conductivity, measured by the four-point method, of at least 20 S / m measured at a pressure of 3 MPa.

[0010] In a preferred embodiment of the invention, the aforementioned composite is characterized in that it contains 1 to 90 wt.% of activated carbon based on the amount of aerogel.

[0011] The chitosan-based aerogel composites according to the invention can be produced by various methods.

[0012] Particularly preferred is a process which is characterized in that (a) adding activated carbon to the chitosan polymer solution during the synthesis of the aerogel or (b) the activated carbon is mixed with the chitosan-based, optionally carbonized, aerogel and (c) carries out the carbonisation by methods known per se.

[0013] The aforementioned task was solved by combining a special carbon aerogel based on renewable raw materials, in this case the biopolymer chitosan, with recovered carbon black (rCB) from used tires. This combination can significantly improve electrical conductivity.

[0014] While conventional resorcinol-melamine-formaldehyde (RMF)-based carbon aerogels exhibit a microporous structure, the present carbon aerogels are primarily meso- and macroporosity. This is also evident from the reported values ​​for the pore volumes of the micro-, meso-, and macropores. Mesoporous carbon aerogels, including the chitosan aerogels described here, exhibited lower conductivity. The carbonization temperature has a significant influence on the electrical conductivity of these carbon aerogels. With increasing temperature between 600 °C and 2500 °C, the electrical conductivity increases. The addition of electrically conductive additives can significantly increase the conductivity. The specific surface areas obtained at the same carbonization temperature also differ (RMF-based carbon aerogel: 634 m 2 / g, chitosan-based CA: 445 m 2 / G).

[0015] In a preferred embodiment of the invention, the above-mentioned composite is characterized in that it is predominantly meso- and macroporous, wherein in particular the meso- and macropore volume measured with nitrogen sorption at 77 K is at least 0.1 cm 3 / g, in particular at least 0.4 cm 3 / g.

[0016] In a further preferred embodiment of the invention, the aforementioned composite is characterized in that it contains nitrogen in the structure determined by elemental analysis of at least 1.5 wt.%.

[0017] Particularly preferably within the meaning of the invention, the aforementioned composite is characterized in that the resulting aerogel or composite is spherical, rice-shaped or noodle-shaped, wherein the activated carbon is homogeneously distributed in the aerogel.

[0018] Chitosan-based carbon aerogels according to the invention contain nitrogen in the structure. The nitrogen present in the structure of the aerogels can be represented as nitro (-CN + -O -), amino (-C-NH2) nitrogen, or as graphitic, pyridinic, and / or pyrrolic nitrogen. Each configuration has different effects on the electronic transport properties. While the effect of nitro and amino nitrogen atoms can be neglected, structural nitrogen atoms, such as pyridinic and pyrrolic nitrogen, have a significant influence on the electrical conductivity of porous carbon. Structural nitrogen replaces carbon atoms within the carbon lattice and thus contributes p-electrons to the n-conjugated system. Thus, the various nitrogen dopings influence the electronic band structure and thus the overall conductivity of the carbon material. Furthermore, the amount of structural nitrogen atoms and the distribution of the nitrogen atoms are of great importance, which should be taken into account when correlating conductivities with nitrogen concentration.

[0019] The chitosan aerogel composites described here have a high electrical conductivity ( Fig. 1). The combination of chitosan aerogel with the additive rCB leads to higher electrical conductivity. This increases with the amount of additive.

[0020] Fig. Figure 1 (top) shows the electrical conductivity of the carbon aerogel mixture containing 10, 40, and 90 wt% carbon aerogel, as well as the electrical conductivities of pure recovered carbon black and pure carbon aerogel. The carbon aerogel used was carbonized at 800 °C for 1 h in a N2 atmosphere. Fig. 1 (bottom) shows the electrical conductivity of carbon aerogels obtained by carbonization at different temperatures.

[0021] Fig. Figure 2 shows the pore size distribution of carbon aerogels obtained by carbonization of chitosan aerogels (left: carbonization for 1 h, right: carbonization for 2 h, bottom: carbonization in N2 for 1 h and activation in CO2 for 1 h). The pore size distribution of RMF-based CA is shown for illustrative purposes only.

[0022] An overview of the results of the pore volumes (V P ), micropore volumes (V Micro ), BET surface areas (S BET ), micropore surfaces (S Micro ) and meso- and macropore surfaces (S Ext ) are provided in the following tables. V P / cm 3 / g V Micro / cm 3 / g V Rest / cm 3 / g S BET / m 2 / g S Micro / m 2 / g S Ext / m 2 / g Massenverlust / % t / h T / °C Total Micro Meso + Macro Total Micro Meso + Macro 1 700 0.604 0.108 0.496 381 264 117 78 1 800 0.626 0.132 0.494 445 325 120 80 1 900 1.057 0.227 0.830 760 565 195 84 1 1000 1.103 0.329 0.774 1104 825 279 88 V P / cm 3 / g V Micro / cm 3 / g V Rest / cm 3 / g S BET / m 2 / g S Micro / m 2 / g S Ext / m 2 / g Massenverlust / % t / h T / °C Total Micro Meso + Macro Total Micro Meso +Macro 2 700 0.808 0.081 0.727 401 185 216 79 2 800 0.723 0.170 0.553 552 422 130 81 2 900 1.627 0.348 1.279 1266 844 422 90 2 1000 2.057 0.399 1.658 1795 951 844 95 V P / cm 3 / g V Micro / cm 3 / g V Rest / cm 3 / g S BET / m 2 / g S Micro / m 2 / g S Ext / m 2 / g Massenverlust / % T / °C t / h Total Micro Meso + Macro Total Micro Meso + Macro 700 1 0.604 0.108 0.496 381 264 117 78 700 2 0.808 0.081 0.727 401 185 216 79 700 3 0.629 0.108 0.521 382 262 120 76 V P / cm 3 / g V Micro / cm 3 / g V Rest / cm 3 / g S BET / m 2 / g S Micro / m 2 / g S Ext / m 2 / g Massenverlust / % t(N2) / h t(CO2) / h T / °C Total Micro Meso + Macro Total Micro Meso + Macro 1 1 700 0.748 0.098 0.650 381 230 151 76 1 1 800 0.592 0.104 0.488 373 255 118 78 1 1 900 0.696 0.187 0.509 602 473 129 82

[0023] Fig. Figure 3 describes an elemental analysis of carbon aerogels obtained by carbonization of chitosan aerogels with and without CO2 activation. The nitrogen content of the carbon aerogels decreases with increasing carbonization temperature. The same trend is observed for the activation of the materials with CO2.

[0024] Fig. Figure 4 includes scanning electron micrographs of a chitosan aerogel (left) and the corresponding carbon aerogel obtained at a carbonization temperature of 800 °C (right).

[0025] The electron micrographs show that the fibrillar structure typical for polysaccharides could be achieved ( Fig. 4 left). This structure is also visible in the resulting carbon aerogel ( Fig. 4 right). This implies that a change in the chitosan aerogel structure must also affect the resulting carbon aerogel.

[0026] In contrast to RMF-based aerogels (only monoliths are possible), the manufacturing process for chitosan aerogels allows for more variable shapes, primarily due to the faster gelation of chitosan. Both beads, which can be produced using jet cutting, and noodle-like geometries through extrusion are possible. The former process, in particular, allows for the production of beads of variable size, which, depending on the subsequent application, may not require further processing (e.g., milling), since gelation, aging, drying, and especially carbonization result in significant shrinkage of the materials and simultaneous loss of mass.

[0027] Furthermore, the gelation of chitosan is not strictly speaking a sol-gel process as with R(M)F or silica aerogels, but rather a dissolution of polymers, which are subsequently made insoluble in a regeneration bath for gelation.

[0028] With RMF-based gels, only limited homogeneity of the composites can be achieved due to very rapid gelation and rapidly increasing viscosity. If an additive is added before the viscosity increases, sedimentation of rCB occurs in the solution. If the additive is added to an already highly viscous solution, agglomerates form, making it impossible to achieve a uniform distribution of rCB in the gel. In contrast, when rCB is added during the preparation of chitosan gels, sedimentation or agglomeration is considered unlikely due to the high viscosity of the solution even before gelation.

[0029] If chitosan is first processed into aerogels and then carbonized, unlike RMF-based materials, a further washing step after carbonization is also eliminated, as no salts need to be removed. This could potentially be considered an advantage, as this process could break a mechanical bond between the rCB and the CA. Determination of electrical conductivity

[0030] The electrical conductivity of the composites was investigated using the Loresta GX resistance measurement system (Mitsubishi Chemical Europe). This system is based on the 4-pin surface resistance measurement method based on IS K 7194:1994 (Testing Method For Resistivity Of Conductive Plastics With A Four-Point Probe Array). Current flows through the outer pins, resulting in a measurable potential difference between the two inner pins. This potential difference depends on the surface resistance of the sample material, which in turn is converted into the volume resistivity and thus the electrical conductivity. These calculations are implemented in the Loresta GX measurement system. The measurements were performed in the pressure range from 3 to 28 MPa. Example: Production of carbon aerogel composites • Preparation of the solution and gelation ◯ First, 8 L of distilled water were poured into a bucket. Then, 3 wt.% (240 g) of chitosan powder was dispersed in it using a KPG stirrer with a dissolver disc. Then, 3 wt.% glacial acetic acid (240 g) was added all at once and stirred for 2 h at 500-700 rpm. The solution was left to stand overnight to allow any air bubbles to float to the surface. ◯ The next day, the dissolver disc was set to 1 / 3 or 2 / 3 of the liquid fill level, and the solution was stirred at each level for 20 minutes at the same stirring speed. For quality control, a sample was then taken, and its viscosity was determined using a RotaVisc or rheometer. ◯ For the regeneration bath required for gelation, 10 L of water were placed in a bowl in which 2.5 wt% (250 g) of sodium hydroxide was dissolved using a KPG stirrer. ◯ The chitosan solution was filled into a jet cutter and injected into the regeneration bath. After each 1 L of chitosan was added, 20 g of sodium hydroxide was added to the regeneration bath before another liter of chitosan solution was processed in this manner. • Aging, washing, solvent exchange (if necessary) and drying ◯ The chitosan beads thus prepared were aged overnight in the sodium hydroxide solution. ◯ The pH of the solution was adjusted to 8-9 with acetic acid. ◯ The chitosan beads were first washed with water until the electrical conductivity of the liquid was achieved, which was in the range of pure water. ◯ If supercritical drying was necessary, the pore fluid was replaced with ethanol (at least 95%). ◯ After drying the chitosan beads, a colorless to slightly yellowish material was obtained. • Production of composites ◯ The composites were produced both directly during the preparation of the chitosan polymer solution and subsequently by mixing. ◯ Solution-phase preparation: The chitosan aerogels were synthesized as described. 10 to 90 wt. % of the recovered carbon black additive was added to the solution along with chitosan. ◯ Production by blending: After carbonization, the dried chitosan aerogels were ground in a vibratory mill at 30 Hz for 300 seconds. They were then blended with 10 to 90 wt. % of the recovered carbon black (rCB) additive. • Carbonization ◯ Chitosan, chitosan aerogels, and composites were carbonized in an inert gas atmosphere at 700 °C to 1000 °C for one to several hours. They can be activated thermally (N2, Ar) or chemically (e.g., CO2, K2CO3) as needed. • Crushing ◯ The carbon aerogels or composites are milled in a vibratory mill for 300 seconds at 30 Hz.

Claims

[1] Carbon aerogel composite based on chitosan and recycled activated carbon, characterized by that the activated carbon is free from ash, silicate compounds and zinc components and that the composite has a conductivity, measured using the four-point method, of at least 20 S / m measured at a pressure of 3 MPa. [2] Composite according to claim 1, characterized by that it contains 1 to 90 wt.% of activated carbon based on the amount of aerogel. [3] Composite according to claim 1 or 2, characterized by that it is predominantly meso- and macroporous, with the meso- and macropore volume measured with nitrogen sorption at 77 K being at least 0.1 cm 3 / g, in particular at least 0.4 cm 3 / g. [4] Composite according to one of claims 1 to 3, characterized by that it contains nitrogen in the structure determined by elemental analysis of at least 1.5 wt.%. [5] Composite according to one of claims 1 to 4, characterized by that the resulting aerogel or composite is spherical, rice or noodle-shaped, with the activated carbon being homogeneously distributed in the aerogel. [6] A process for producing a composite according to any one of claims 1 to 5, characterized by that one (a) the activated carbon is added to the chitosan polymer solution during the synthesis of the chitosan aerogel or (b) the activated carbon is mixed with the chitosan-based, optionally carbonized, aerogel and (c) carries out the carbonisation by methods known per se. [7] Method according to claim 6, characterized by that chitosan is used as a starting material for the production of aerogels.

Images