The features of partial and joint electroreduction of tungsten from Na2W2O7 and carbon from СО2 and Li2CO3 in equimolar NaCl–KCl melt at temperatures of 700–800 °С in different gaseous atmosphere (air, argon, carbon dioxide) were studied. Mechanisms and kinetic features of electrode reactions are determined.
The process of electroreduction of Na2W2O7 to metallic tungsten in chloride melts occurs in one stage in the potential range of -0.7 – -1.0 V against to the Pt reference electrode, process is electrochemically irreversible, and controlled by diffusion of the depolarizer to the cathode.
It was found that the process of reduction of Li2CO3 depends on the gaseous atmosphere above the melt. In air, it includes two electrochemical stages, with a preliminary chemical reaction of decomposition of lithium carbonate to CO2. Cyclic voltammograms shows two cathodic waves: the first corresponds to the reduction of CO2, which is formed as a result of thermal dissociation of the carbonate anion and the second wave is due to the directly reduction of the carbonate anion. It was established by XRD, SEM, and Raman spectroscopy that the cathode product of electrolysis at the potentials of both waves is amorphous high disordered carbon.
Based on electrochemical studies of the partial electrodeposition of carbon and tungsten from different systems two compositions of electrolytic baths has been chosen: (1) Na,K|Cl–Na2W2O7–СО2; (2) Na,K|Cl–Na2W2O7–Li2CO3–CO2. The features of the joint electroreduction of the components of the both system have been investigated. In the first system joint electroreduction of tungsten and carbon occurs at potentials -0.5 – -0.7 V against to the Pt reference electrode and at -1.5 – -1.7 V for the second system.
The carried out potentiostatic and galvanostatic electrolysis allowed choosing the required bath compositions and conditions. The physico-chemical properties of the products were studied by XRD, SEM, TEM, Raman spectroscopy, BET, DTG methods. From the first system obtained hexagonal WC/W2C/C composite with a particle size of 15–20 nm and composite of WC/C with a particle size of 10–15 nm with a specific surface area of 140 m2/g from the second. The electrolysis condition (current density, bath voltage, temperature, the ratio of the bath components) have been determined. The synthesis temperature is 700–800 °C. The current densities are 0.07 – 0.2 A/cm2. Bath composition: (1) Na,K|Cl (1:1) – Na2W2O7 (9,3 mass.%) – СО2 (1.5 MPa); (2) Na,K|Cl (1:1)–Na2W2O7 (16.5 mass.%) – Li2CO3 (4.1 mass.%) – CO2 (0.5 MPa). Replacing the traditional precursor of W (Na2WO4) with Na2W2O7 simplifies the composition of the electrochemical bath by eliminating sodium metaphosphate. The use of lithium carbonate as a source of C in the system increases the yield of the final product, reduces the CO2 pressure in the system and improves the characteristics of the products.
The carried out investigations show the use of tungsten carbide as electrode material for hydrogen production from acid electrolytic solutions to be worth-while. Tungsten monocarbide (with surface CNMs) obtained from the system Na,K|Cl–Na2W2O7–Li2CO3–CO2 has the best activity. The value of the Tafel slope for this carbide is -85 mV/dec, the exchange density of 7.0 ×10-4 A/cm2, the overpotential of hydrogen evolution -110 mV at a current of 10 mA/cm2. Surface carbon (5 mas.%) increase its electrocatalytic activity, viz increases the exchange current and decreases the process overpotential. The activity of tungsten carbide W2C is lower than WC.
