Electrolytical Carbon Nanostructures from Molten Salts: Synthesis and Properties

Abstract

Carbon nanomaterials (CNMs) occupy a special place in nanoscience owing to exceptional thermal, electrical, chemical and mechanical properties. They are used in various areas of modern technologies: super-strong composite materials, supercapacitors, smart sensors, targeted delivery drugs, catalyst carriers, field emission devices, quantum wires, paints, for energy storing and conversion, for gas storing, in nanoelectronics, etc. Therefore, a great attention is given all over the world to the development of novel efficient method for the fabrication of CNMs, comprehensive investigation of their properties, and search for new applications. Today there are many methods for the synthesis of CNMs. In resent ten years, the method of high-temperature electrochemical synthesis (HTES) in molten salts has been rapidly developing. The essence of the method of HTES is the cathodic reduction of oxygen-containing carbon compounds dissolved in molten salts. There are several variants of HTES. The advantages of all variants of HTES are: (1) soft synthesis conditions, (2) simple hardware implementation, (3) the possibility of controlling the electrolysis conditions, (4) low energy consumption for electrolysis, (5) the use of cheap precursors, (6) easy doping during synthesis, (7) monodipersity of the product. An important advantage of HTES is the possibility of using carbon dioxide as a starting reagent. Global warming, caused by increase in the emission of greenhouse gases (CO2, CH4 and others), is recognized as a serious environmental problem of the mankind.The goal of this work is presentation of research results on the electrochemical transformation of carbon oxy-compounds (lithium carbonate and carbon dioxide) into nanoscale solid carbon of different structure and morphology (carbon nanotubes (CNTs), fibers (CNFs), graphene) in chloride-carbonate melts.On the basis of a voltammetric study, it has been found that direct electroreduction of CO2 dissolved in equimolar salt mixture Na,K|Cl under an excessive pressure (1 - 15 atmospheres) up to carbon at temperature 750 0C occurs in one stage at polarization rates above 0.2 Vs-1. The electrode process is controlled both by the charge transfer rate and by the rate of gas diffusion to the electrode. At polarization rates below 0.1 Vs-1, the electrode process occurs in two stages, and an Electrochemical-Chemical- Electrochemical mechanism of CO2 reduction has been suggested. The cathodic product is carbon films and powders of different structures and morphology. Typical morphologies of the electrolytic products obtained in the melts Na,K|Cl–CO2 are shown in Fig. 1.All the products obtained in the system Na,K|Cl–CO2 (10 atm.) contain CNTs, the majority of which are multi-walled and have a curved form (structural defects). Most often CNTs agglomerate into bundles, and more rarely are arranged as individual tubes. The outer diameter of CNTs varies from 5 to 250 nm, while the internal diameter from 2 to 140 nm. Almost all CNTs are filled partly with electrolyte salt. When the current density increases, the CNT diameter decreases (although every product obtained at the current densities used has CNTs of different diameters).It was found by voltammetry method that the process of reduction of Li2CO3 in Na,K|Cl melt depends on the gaseous atmosphere above the melt. In air, it includes two electrochemical stages. The preliminary chemical reaction of lithium carbonate dissociation (Li2CO3 ⇄ Li2O + CO2) leads to the formation of two electrochemically active particles: CO2 and LixCO3 2-x, which are reduced to elemental carbon at different potentials of -0.8 and -1.7 V respectively against Pt|O2/O2- reference electrode. Both processes are irreversible, and the electroreduction of LixCO3 2-x takes place with diffusion control of the delivery of the depolarizer to the electrode surface. Under of argon or carbon dioxide atmosphere over the melt, the process of lithium carbonate dissociation is suppressed; therefore, the deposition of carbon in this case occurs only from the cationized carbonate complex.X-ray diffraction, SEM and Raman spectroscopy revealed that the cathode product is a high disordered amorphous carbon. Agglomerated particles consist of degraded graphite structures with an approximate crystallite size of 30–40 nm.Figure 1. TEM micrographs (a÷e) and electron-beam images (d (inset in the upper left) and f) of carbon powders produced in the system Na,K|Cl–CO2 (10 atm.) at different current densities: (a ÷ e): i k = 13.5; 28; 42; 56; 72 mA/cm2 ; T = 750 0C; Figure 1