In the world literature, it is possible to meet increasing quantity of publications devoted to the composite materials, which have unique properties: high mechanical strength, high melting point, corrosion resistance, heat resistance, high microhardness, catalytic and electrocatalytic properties. First of all it is related to the new challenges for humanity: flights into space, overcoming supersonic speeds, protection of materials against new aggressive environments, conducting of energetically hard-to-execute reactions etc. Composite materials based on carbon fibers reinforced with carbides and double carbides can solve such problems and are already being actively implemented in science-intensive technological processes. A promising method for the synthesis of such materials is synthesis in molten salt systems by electrolysis and/or currentless transfer.

In our previous work, the results on the electrochemical synthesis in molten salts of refractory metal carbides (NbC, TaC, Mo₂C) in the form of coatings and crystals on carbon fibers were reported [1]. The synthesis was carried out by the currentless transfer process, which can be explained by the formation of refractory metal complexes in an intermediate oxidation state due to the metal interacts with its own salt and the subsequent disproportionation reaction on the substrate with the formation of metal carbide.

No splicing of carbon fibers with each other was observed, and the coating repeated the relief of the original carbon fiber, the coatings were uniform both in the cross section and the length of the entire fiber. The thickness of TaC and NbC coatings was about 50–250 nm depending on the synthesis time. Crystals of Mo₂C had a well-defined structure with the size approximately of 8-20 µm. Tantalum carbide TaC had a cubic modification with the border-centered crystal lattice. Phase of niobium carbide NbC had a cubic crystal lattice. Crystals of molybdenum carbide Mo₂C had a hexagonal structure.

Investigation of electrocatalytic properties has shown the highest activity of the composite material NbC/C in comparison with other carbides and Pt and Cu catalysts in the hydrogen peroxide decomposition reaction [2]. It was determined that reaction has a zero order for all refractory metal carbides and for Pt and Cu. The reaction rate increases in the next row: Mo₂C < TaC < Pt < Cu < NbC. The values of the activation energy on Mo₂C/C, Pt, TaC/C, Cu and NbC/C electrodes were calculated using the Arrhenius equation.

The properties of the electrical double layer in various electrolytes were studied because the kinetics of electrode process depends on the double layer structure.

The study of the electrical double layer properties of composite materials "refractory metal carbide-carbon fiber" was carried out by electrochemical impedance spectroscopy.

The electrolytes used in this study were sodium sulfate Na₂SO₄ and hydrogen peroxide H₂O₂. Capacitance curves were obtained from impedance spectroscopy measurements for both electrolytes and for each electrode (TaC/C, NbC/C and Mo₂C/C). Capacitance values of double layers were measured in µF for the Na₂SO₄. The values of double layers capacitance were extremely low and measured in nF in the H₂O₂ solution.

REFERENCES

[1] V.S. Dolmatov, S.A. Kuznetsov. 2015. Proceedings of 10^th International Conference on Molten Salt Chemistry and Technology and 5^th Asian Conference on Molten Salt Chemistry and Technology. Shenyang, China: P. 81-85.

[2] V.S. Dolmatov, S.A. Kuznetsov. 2021. J. Electrochem. Soc. 168: 122501.