Recently, preparation of carbon materials by molten salt electrolysis has been demonstrated as a promising way of carbon capture, utilization and storage. This paper discusses the results of studies on the mechanism of carbon electrodeposition reactions in molten carbonate salts, especially voltammetric investigations. A CV study of the electroreduction of Li2CO3 against the background electrolyte NaCl–KCl was carried out. It was found that the electroreduction of Li2CO3 to carbon occurs through the stage of a preliminary chemical reaction of acid-base type with forming of electrochemically active particles – CO2 and LixCO32-x at potentials of -0.8 and -1.7 V (against Pb|PbCl2 reference electrode), respectively. Both processes are irreversible, and the electroreduction of LixCO32-x takes place with diffusion control of the delivery of the depolarizer to the electrode surface. Electrolysis of melts have been carried out under potentiostatic conditions in an air atmosphere at different potentials (E = -1.0 and -1.8 V) at the temperature 750°C. The product yield was 40–60%. Compositions, morphological and structural peculiarities of carbon products have been studied by XRD analyses, SEM and Raman spectroscopy. XRD study shows that a symmetrical halo occurs at 2θ ~21 and 43°, suggesting that obtained carbon materials are amorphous. The ratio of the intensities of D and G bands (ID/IG) on Raman spectra was > 1 that indicates the extent of defects and impurities in the electrolytic carbon or the presence of ultrathin carbon. Agglomerated particles consist of degraded graphite structures with an approximate crystallite size of 30–40 nm. Their general appearance resembles the model of "crumpled paper".
The electroreduction of Li2CO3 to carbon in molten salts presented in this study is environmentally benign and affordable, providing a promising route for the production carbon nanomaterials. Also, this work highlights the possibility of using Li2CO3 as a precursor of high-temperature electrochemical synthesis of various carbonaceous materials, in particular, tungsten carbides.
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