Electrochemical Synthesis of Tungsten Carbide in Molten Salts, Its Properties and Applications
Authors: Novoselova Inessa , Kuleshov Sergei , Fedoryshena Elena , Bykov Valeriy 
Published: February 2020

Currently, the most used material for electrocatalysis is platinum black. The use of this metal leads to the high costs of electrocatalytic devices. It is necessary to reduce Pt amount in the catalysts or to replace it by less expensive materials.

Tungsten carbides (WC and W2C) are very attractive materials for catalysis due to unique properties: high electronic conductivity, corrosion resistance and ability to withstand the action of catalytic poisons. Because of the relative cheapness of the production methods, these materials can be an alternative to expensive platinum. For application as catalyst WC need to be produced in the form of nanodispersed powders with high surface area.

In industry, WC is obtained by carbonizing of tungsten metal powders (or its oxide) with carbon black or carbon-containing gases. Commercial WC has micron sizes with a specific surface area of up to 2-5 m2/g. There are a number of non-traditional methods for production of nanosized WC. Among these methods, the high-temperature electrochemical synthesis (HTES) in molten salt is very promising one. Depending on the conditions of HTES, the ultradispersed powders and coatings of WC of high purity can be obtained; doping and modification of the product during the synthesis can be implemented. This method also allows to solve very important problem - electrochemical utilization of carbon dioxide with production a new chemical product.

The purpose of this work was to determine the systems and conditions for the electrolytic production of tungsten carbide in molten salts, to establish its chemical and phase composition, to study the morphological and structural features. Also the evaluation of possibility of WC application as an electrocatalyst in the reaction of hydrogen evolution was carried out.

The peculiarities of the partial and joint electroreduction of the synthesis components (tungsten and carbon from their oxygen-containing compounds) on platinum and glassy carbon cathodes were studied by cyclic voltammetry. Two compositions of electrolytic baths were used: (1) Na,К|Cl–Na2WO4–NaPO3; (2) Na,К|Cl–Na2W2O7. Carbon dioxide dissolved in the melt under excess pressure up to 17 atm was used as the carbon component of the synthesis. Mechanisms and kinetic features of electrode reactions were determined. The region of potentials and currents was chosen for the realization of electrochemical synthesis of carbides in the studied systems. The synthesis temperature was 700-750 °C. The cathode current density (ik) = 0.05 – 0.2 A/cm2. The highly dispersed powders were produced by electrolysis. The physico-chemical properties of the products were studied by XRD, SEM, TEM, Raman spectroscopy, BET, DTG methods. Electrocatalytic testing was carried out in a standard three-electrode cell. A graphite rod with a layer of catalytic inks was used as a working electrode. The catalytic ink was a suspension of carbide product in a 5 mas. % solution of Nafion. Non-working side part of an electrode was isolated. A platinum counter electrode and a silver chloride reference electrode were used.

It has been established by XRD method that depending on conditions and parameters of electrolysis, the composite mixtures of WC and W2C carbides with nano-dimensional carbon structures and platinum can be obtained at the cathode. The parameters of hexagonal α-WC crystal lattice (a = 2.9081 ± 0.005, c = 2.8211 ± 0.01) were calculated and the average particle size (5 – 10 nm) was established. The specific surface area of the powders that was 25-30 m2/g was determined by the BET method. According to the results of TEM, DTG and Raman spectroscopy, the produced WC powders contain up to 4 wt. % of free carbon, which has an amorphous structure and covers the WC nanocrystallites as a "coat". Different morphologies of WC powders were found by SEM, TEM methods: fibers and rods, layered particles, “curdled” conglomerates (Fig.1).

Electrocatalytic testing was carried out on different samples of composite mixtures: (1) WC: W2C; (2) WC: xPt; (3) WC: nC. Studies have shown that the overvoltage of hydrogen evolution (η) from 0.1 M solution of H2SO4 is in the range of 0.14 ÷ 0.5 V depending on the composition, morphology and conditions of synthesis. So for WC powders containing platinum (up to 60 mass %), the value of η is the smallest – 0.14 V. The electrodes consisting of a mixture of WC and W2C have the largest value of η.

It has been established that it is possible to produce the composite mixtures based on ultradispersed tungsten monocarbide of different composition by the HTES method. These products have the required properties for the application in electrocatalysis.

Fig 1. SEM images of electrolytic WC.

Figure 1