Synthesis and structural characterization of carbon powder by electrolytic reduction of molten Li₂Co₃-Na₂Co₃-K₂CO₃

A new method was proposed for the preparation of carbon powders based on the electrochemical reduction of a fused eutectic mixture of lithium-sodium-potassium carbonates at 450°C. Transmission electron microscopy observations have revealed the presence of three forms of carbon in the powder samples depending on the deposition potential values: amorphous carbon, graphite, and fibers. The presence of graphite and amorphous carbon was confirmed by X-ray diffraction measurements. The d₀₀₂ values were rather constant whatever the sample and close to 0.34 nm. Moreover, surface analyses by X-ray photoelectron spectroscopy revealed the presence of nearly metallic and ionic lithium belonging to lithium oxides. The specific areas were measured by the Brunauer-Emmett-Teller (BET) method. Whatever the deposit potential values, the total BET specific surface area is much higher than that corresponding to external one because of the pore walls contribution. It has been shown that, for carbon powder dried at 400°C under vacuum, the specific surface area decreased as the deposition potential became more cathodic: from about 850 m² g^-1 at -2.4 V vs. CO₂-O₂ to about 500 m² g^-1 at -6.0 V vs. CO₂-O₂. With increasing negative values of the potential, the nucleation and the growth of carbon competes with the formation of nearly metallic lithium and/or lithium oxides. The presence of the latter induces the closure of some nanopores.