High-temperature electrochemical synthesis (HTES) in molten salts allows one to produce a wide range of interesting in the application terms chemicals: single-phase carbon nanomaterials of various structures and morphologies, as well as carbides, borides, and silicides of refractory metals [1]. A necessary condition for the successful implementation of the synthesis is a detailed study of the electrochemical behavior of the synthesis precursors - compounds of carbon and refractory metal.

This work presents the results of studies on the mechanism of carbon and tungsten electrodeposition reactions from their oxide compounds (СО₂; Li₂СО₃; Na₂WO₄: Na₂W₂O₇; Na₃WO₃F₃) in their partial and joint presence in a chloride melt. The possibility of controlling the mechanisms of electrode processes by changing the acid-base properties of melts is shown.

The research algorithm was as follows. At the first stage, a thermodynamic analysis of possible pathways for the electroreduction of precursors and the choice of optimal systems and conditions for the synthesis were carried out. The next step was a voltammetric study of the electrochemical behavior (partial and joint) of synthesis precursors and the establishment of the electrode processes mechanism. After analysis of the obtained results, electrolyzes were performed in potentio- and galvanostatic modes to obtain phases of carbon, tungsten (partial reduction of precursors) and tungsten carbides (joint reduction of precursors) of various compositions. Finally, the optimal synthesis conditions were determined.

Based on the analysis of thermodynamic calculations and published data, we have chosen CO₂ and Li₂CO₃ compounds as carbon precursors, and Na₂WO₄, Na₂W₂O₇, Na₃WO₃F₃ as tungsten precursors. An equimolar mixture of NaCl and KCl was chosen as the supporting electrolyte, since the cathodic decomposition potentials of NaCl and KCl are more negative than the potentials of carbon and tungsten evolution. In addition, these chlorides are cheap and highly water-soluble salts that simplify the washing of the final product. The optimum temperature is 750 °C.

For the synthesis of tungsten carbides, the following 7 compositions of electrolytic baths were chosen: (1) Na,K|Cl – Na₂WO₄ – CO₂; (2) Na,K|Cl – Na₂WO₄ – NaPO₃ - CO₂; (3) Na,K|Cl – Na₂WO₄ – MgCl₂ - CO₂; (4) Na,K|Cl – Na₂W₂O₇ – CO₂; (5) Na,K|Cl – Na₂W₂O₇ – Li₂CO₃; (6) Na,K|Cl – Na₂W₂O₇ – Li₂CO₃ – CO₂; (7) Na,K|Cl – Na₃WO₃F₃ – CO₂.

The partial electrochemical reduction of the synthesis precursors (СО₂; Li₂CO₃; Na₂WO₄; Na₂WO₄ – xMgCl₂; Na₂WO₄ – xNaPO₃; Na₂W₂O₇; Na₃WO₃F₃) in a chloride melt NaCl-KCl (1:1) was studied by cyclic voltammetry at different depolarizer concentrations, potential scan rates (0.02-10 Vs^-1), reverse potentials and gaseous media in the cell.

Based on the cycle voltammogram diagnostics and analysis of the products obtained by the potentiostatic electrolysis mode, the mechanisms of electrode reactions are proposed and the areas of potentials and current densities of the carbon and tungsten deposition are determined. The data obtained made it possible to purposefully select the concentrations of precursors for the electrochemical synthesis of tungsten carbides.

A voltammetric study of the joint deposition of tungsten and carbon was carried out. The synthesis of nano-sized powders of carbon, tungsten, tungsten carbides of various compositions and structures (C; W; W|W₂C|; W|W₂C|WC|; W₂C|WC; WC|C) has been implemented. The properties of electrolytic products have been studied by XRD, SEM, BET, and Raman spectroscopy. The influence of the synthesis parameters and conditions on their phase composition, morphological and structural features has been established.

The electroreduction of CO₂ and Li₂CO₃ to carbon in molten chlorides can be taken as the basis of high-temperature electrochemical synthesis of various carbonaceous nano-scaled inorganic materials: carbon films and powders of different structures and morphology (carbon nanofibres; nanotubes, graphene oxide, amorphous carbon). Joint electroreduction of CO₂ and Li₂CO₃ with various tungsten oxyanions makes it possible to obtain a whole range of composite electrolytic materials based on tungsten carbides. The morphology and properties of the obtained products can be changed cardinally depending on the nature of the chosen precursors and synthesis conditions. Tungsten carbides have an average particle size of ~ 10 – 30 nm and consist of hollow mesoporous spherical structures with a specific surface area of ~140 m²g^-1.

The electroreduction of carbon and tungsten anions in molten salts presented in this work providing a promising route for the synthesis of various carbonaceous materials with great application opportunity. It was shown that the use of obtained electrolytic nano-sized powder composites WC|C as cathode material for the hydrogen evolution in sulfuric acid solutions is to be promising.

[1] Shapoval VI, Malyshev VV, Novoselova IA, Kushkhov KhB (1995) Modern problems of HTES of transition metal compounds. Russ Chem Rev 64:125–132.