Annotation of the results obtained in 2023
Based on the results of experiments at all stages of the Project, a description of the mechanism of formation of metal oxide aerogel during thermal-oxidative decomposition of metal-carbonyl precursor in supercritical carbon dioxide saturated with oxygen is proposed. The solubility of the precursor has a significant influence on the ability to form an aerogel, which we demonstrated using the example of a set of iron carbonyls. It turned out that the specific amount of carbonyl groups per metal atom does not affect the solubility in supercritical carbon dioxide. Most likely, the spatial geometry of the precursor plays a major role. The characteristics of manganese and iron oxide aerogels obtained earlier in the Project were studied, such as their magnetic susceptibility and electrocatalytic activity in the reaction of oxygen reduction in an alkaline medium. Using a magnetometer, it was demonstrated that the initial samples of manganese and iron oxide aerogels do not have any ferromagnetic response. It has been shown that magnetic susceptibility appears in iron oxide aerogel after annealing in air at 600 ºC. Using the rotating disk electrode method, it was shown that dispersed manganese oxide aerogel mixed with carbon black in the oxygen reduction reaction in an alkaline medium demonstrates electrocatalytic activity comparable to the activity of a commercial catalyst containing platinum. Based on the results of the Project, it was concluded that to be effective in real applications, the resulting manganese and iron oxide aerogels need to be mechanically strengthened, for example using a carbon skeleton. A new method has been developed for the in situ synthesis of metal oxide aerogel inside a carbon aerogel using thermal decomposition of the corresponding metal carbonyl in an oxygen-enriched supercritical carbon dioxide environment. Using scanning and transmission electron microscopy and elemental analysis, it was shown that the carbonyl precursor of the metal (using the example of manganese and iron carbonyls) actually penetrates deep into the complex structure of the carbon aerogel obtained by the standard resorcinol-formaldehyde method, forming a united functional composite during thermal oxidation. At the same time, the specific surface area of such composites decreases by less than 30%, amounting to more than 600 m2/g. The density increases by less than 20%, amounting to less than 40 mg/ml. It has been shown that the total amount of the metal phase in the composite and the specific surface area of the material can be controlled by varying the loading of the carbonyl precursor. Thus, a new effective approach to creating composite materials based on carbon aerogels and metal oxides has been demonstrated. The possibility of intercalation of the tetrabutylammonium cation, as well as the simultaneous intercalation of tetrabutylammonium and lithium cations into graphite in a subcritical electrolyte at high pressure was shown. Intercalation of the tetrabutylammonium cation is found to be carried out at potentials of -2.9 V vs. Pd and below. It was found that if carbon dioxide is added to the system, cation intercalation occurs outside the electrolyte stability window. Electrochemical synthesis of graphene in a non-aqueous subcritical electrolyte based on carbon dioxide/acetonitrile by intercalation of tetrabutylammonium cation at a temperature of 40°C and a pressure of 300 bar made it possible to obtain large sheets of graphene (>1 μm) of small thickness (2 layers) with a relatively low content of oxygen atoms (C:O = 5.0) compared with a similar synthesis in a liquid electrolyte. The oxygen content in such sheets is also lower than it was during anionic intercalation in a subcritical electrolyte, studied earlier. The synthesis of graphene using the simultaneous intercalation of lithium and tetrabutylammonium cations led to a higher content of oxygen atoms in the carbon network (C:O = 2.0), and such intercalation of graphite was not highly efficient. Electrochemical synthesis in an aqueous electrolyte based on phthalic acid at 10 V at normal pressure made it possible to scalable synthesis of a multilayer carbon material with an optimal combination of properties: high conductivity and dispersibility. This has made it possible to significantly improve the specific electrode characteristics of lithium-ion cathode coatings based on NCM811 in batteries compared to carbon black used today. It has been shown that when scaled up, the reaction of hexachlorocyclophosphazene with sodium alkoxide of perfluorooctan-1-ol, proposed earlier in the Project, leads to low degrees of substitution of chlorine atoms with fluoroalkyl substituents. The main product was monosubstituted fluorinated cyclophosphazene. Changing such experimental parameters as the time and temperature of the reaction, replacing the solvent with toluene or chloroform, as well as additional purification of the starting substances by distillation under reduced pressure of perfluorooctan-1-ol in a stream of argon or the recrystallization of hexachlorocyclophosphazene from hexane does not lead to an increase in the degree of substitution of chlorine atoms. Fluorinated cyclophosphazenes with degrees of substitution of 4 and 5 were applied to the surface of cotton/polyester fabric in a supercritical carbon dioxide medium. Using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and studying the dynamics of the contact angle of a drying drop of water, the sufficient uniformity of the resulting coating was confirmed. It was established that the coating is formed mainly by a component with 5 substituted chlorine atoms, which indicates insufficient solubility of fluorinated cyclophosphazenes with a degree of substitution of 4 and lower in sc CO2. It has been shown that the application of fluorinated cyclophosphazenes also leads to hydrophobization of the fabric surface. An increase in the fire resistance (oxygen index) of the modified material was also demonstrated. The solubility of commercial flame-retardant agents trichloropropyl phosphate and triethyl phosphate in sc CO2 was studied using a high-pressure optical vessel. It has been shown that trichloropropyl phosphate and triethyl phosphate are completely soluble at 40 ºC and pressures less than 350 bar. These agents are applied to fabrics in an sc CO2 medium, and an increase in the fire resistance (limiting oxygen index) of modified materials is demonstrated. Thus, a set of methods has shown that the application of fluorinated cyclophosphazenes and some commercially available flame-retardant agents to textile material from solutions in sc CO2 can be an effective approach to creating fire-resistant coatings.

 

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Annotation of the results obtained in 2021
A method for creating metal oxides aerogels using iron carbonyl precursors differing in structure and content of carbonyl groups has been studied. It was shown that the specific amount of carbonyl groups per metal atom does not determine the precursor solubility in sc CO2. It was found that the ability of the precursor to form a monolithic aerogel as well as the grain size of the resulting aerogel correlate with the solubility of the precursor in sc CO2. A novel top-down approach to the graphene synthesis is proposed, which combines electrochemical ion intercalation and supercritical fluid intercalation. Potentiostatic intercalation of BF4 anion into highly-oriented pyrolytic graphite in a supercritical electrolyte based on a CO2/acetonitrile mixture in a specially designed high-pressure 3-electrode electrochemical cell has been performed. The intercalation stage of the anion inside the electrode is studied by means of Raman spectroscopy. The electrolyte composition is analyzed after the intercalation and high-pressure cell decompression by means of atomic force and transmission electron microscopy. As a result, few (2–4) layers graphene flakes with partial crystallinity have been detected in the electrolyte. Additionally, the intercalated graphite electrode has been ultrasonicated, which also resulted in the formation of few layer graphene stacks with lower lateral dimensions and higher thickness. It was found that decompression and possible intercalation of solvent appear to play a key role in graphene synthesis in a supercritical electrolyte. Electrochemical graphene oxide obtained at normal pressure in the tryptophan-based electrolyte was used as an additive to the electrolyte for aqueous zinc-ion batteries. The results of the phase-field model combined with experimental characterization revealed that the addition of this material effectively promotes the uniform distribution of the electric field and the Zn-ion concentration field, reduces the nucleation overpotential of Zn metal, and provides a more uniform deposition process on the metal electrode surface. As a result, an improved cyclability of the aqueous Zn-ion battery is achieved. In the reaction of hexachlorocyclophosphazene with fluorinated sodium alcoholate, new derivatives of cyclophosphazene containing fluoroalkyl substituents at phosphorus atoms were obtained. The obtained compounds were studied by a complex of physicochemical methods of structure analysis, including NMR spectroscopy, FTIR spectroscopy, gas chromatography-mass spectrometry, and elemental analysis. The correlating results of the methods indicate that 4- and 5-substituted fluorinated cyclophosphazene molecules predominate in the reaction product. Studies of the phase behavior of the obtained cyclophosphazenes in sc CO2 showed that these compounds are readily soluble in the sc fluid. Thus, the obtained fluorinated cyclophosphazenes can be used directly for the deposition of functional coatings from solutions in sc CO2, or may serve as "building blocks" for the development of the chemistry of cyclophosphazenes soluble in sc CO2 We address the effect of exposure to sc CO2 on the properties of water-containing Nafion membranes. A simple model is proposed to describe the pressure-induced reorganization of the phase-separated matrix. The model predicts that the cylindrical ionic pores in Nafion restructure under pressure applied: wider pores shrink in diameter to a larger degree than narrower ones. As a result, the pores ensemble becomes more monodisperse and narrower as a whole, which should determine detectable changes in the transport properties. The consistency of the model predictions with the main experimental observations was checked. The modified samples demonstrate the lower water reabsorption, the higher the pressure during the treatment. Moreover, both proton conductivity and relative crystallinity increase with increasing pressure, while methanol permeability decreases. DSC measurements reveal larger fraction of bound water for the modified films. Apparently, the bound water is sufficient to ensure high proton conductivity. The improved selectivity is explained by the narrower channels. The use of renewable energy sources requires the development of more efficient devices for storing it at the peak of production and then releasing it on time when consumption increases, such as vanadium flow batteries (VFB). In order to optimize the VFB properties, we proposed a composite based on a mesoporous polyolefin matrix and a polymer with internal microporosity PIM-1 as an ion-conducting membrane. The resulting hierarchical structure demonstrates improved selectivity, which allows the composite to be used for real applications in power sources.

 

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Annotation of the results obtained in 2022
The effect of temperature on the properties of the obtained materials in the previously proposed method for creating metal oxide aerogels in supercritical CO2 was studied using iron and manganese precursors. It was shown that an increase in the synthesis temperature leads to a decrease in grain size and a corresponding increase in the specific surface area of the obtained materials. It was possible to increase the efficiency of charge intercalation of graphite in an electrolyte with compressed CO2 by decreasing the intercalation potential. It was shown that during such intercalation no irreversible defects caused by anodic oxidation are introduced into the intercalated compound of graphite. The electrolyte composition was varied. Based on the analyzed data, the result was obtained on the optimal composition of the electrolyte for the most efficient intercalation. Data were obtained on the chemical composition of the particles contained in the electrolyte after intercalation and decompression, as well as those obtained with sonication of the expanded HOPG electrode. The oxygen content in graphene sheets and their sizes are comparable with the literature data on the production of graphene using liquid-phase splitting and using splitting in a supercritical medium. The conditions for the comparative process of graphite delamination in an aqueous electrolyte at normal pressure are selected, which make it possible to obtain a material with scalability, good dispersibility and yet retaining electronic conductivity. Testing of this material in working battery cells showed the possibility of achieving higher capacitive characteristics in comparison with commercial carbon black. The study of the reaction of hexachlorocyclophosphazene with fluorinated sodium alcoholate was performed to obtain new fluorinated cyclophosphazenes soluble in supercritical CO2. In particular, the possibilities of scaling up the proposed synthesis have been studied as well the first studies on the preparation of cross-linked fluorinated cyclophosphazenes to be dispersed in sc CO2 were performed. In addition, colloidal solutions of a commercially available phosphoric acid ester flame retardant in sc CO2 were studied. Furthermore, experiments were carried out on the use of sc CO2 as a medium for deposition of the flame-retardant coatings on textile materials. It was confirmed by SEM that the deposition of the flame-retardant agent from a supercritical medium occurs uniformly, and the measurement of the limiting oxygen index confirmed the imparting of flame-retardant properties to the modified textile material. A method for creating a composite membrane based on a polypropylene substrate and a chitosan coating deposited from an aqueous phase with compressed CO2 and then decorated with copper ions was proposed. It was shown that the presence of a chitosan coating stabilizes the deposition and leads to the formation of a homogeneous copper-containing nanofilm with good adhesion on the surface of polymer fibers.

 

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