Effect of Electrochemical Synthesis Conditions on the Composition, Structure, and Morphology of Tungsten Carbide Powders

Abstract

High-temperature electrochemical synthesis (HTES) in molten salts is highly promising among the up-to-date methods for the production of carbide powders. Ultrafine composite powders of tungsten carbides (WC|C, WC|C|Pt, W₂C|WC, and W₂C|W) were synthesized using the HTES method in electrolytic baths with different chemical compositions under various synthesis conditions (cathode current density, CO₂ pressure in the electrolyzer, temperature, and cathode material). Composite powders (up to 3 wt.% free carbon) with a WC particle size of 20–30 nm were prepared using Na, K|Cl (1 : 1)–Na₂W₂O₇ (6.4 wt.%)–CO₂ (1.25 MPa) and Na, K|Cl (1 : 1)–Na₂WO₄ (12.0 wt.%)–NaPO₃ (0.7 wt. %)–CO₂ (1.25 MPa) electrolytic baths at a temperature of 750°C. When the CO₂ pressure was reduced to 0.75 MPa, composite W₂C|WC powders formed at the cathode. The ratio of carbide phases in the composites depended on the initial concentration of tungsten salts in the electrolyte and on the CO₂ gas pressure in the electrolyzer. The addition of Li₂CO₃ (4.5 wt.%) to the electrolytic salt mixture decreased the tungsten carbide particles to 10 nm, changed their morphology, and increased the free carbon content in the composite up to 5 wt.%. The specific surface area of the powder increased by a factor of 4 to 7 (from 20–35 to 140 m²/g). The resulting products were modified with fine platinum particles through the use of platinum cathodes. The HTES method demonstrated its potential for producing tungsten carbide powders with the properties allowing their use as electrocatalysts in the hydrogen evolution reaction. For the WC|C composite powders synthesized in the Na, K|Cl–Na₂W₂O₇–Li₂CO₃–CO₂ system, the hydrogen evolution potential was –0.02 V relative to the normal hydrogen electrode, the overpotential η at a current density of 10 mA/cm² was –110 mV, the exchange current was 7.0 ⋅ 10^–4 A/cm², and the Tafel slope was –85 mV/dec.