Electrolytic synthesis of carbon nanotubes from carbon dioxide in molten salts and their characterization

doi:10.1016/j.physe.2007.10.069

Physica E: Low-dimensional Systems and Nanostructures

Volume 40, Issue 7, May 2008, Pages 2231-2237

https://doi.org/10.1016/j.physe.2007.10.069 Get rights and content

Abstract

Carbon nanotubes (CNTs) were synthesized from CO₂ dissolved in molten salts using the novel electrolytic method developed by the authors. The electrolysis were carried out under current and potential controls. To establish the actual current and potential ranges, the electroreduction of carbon dioxide dissolved in the halide melts under an excess pressure up to 15 bar was studied by cyclic voltammetry on glassy-carbon (GC) electrode at a temperature of 550 °C. The electrochemical–chemical–electrochemical mechanism of CO₂ electroreduction was offered for explanation of the obtained results. The structure, morphology, and electronic properties of the CNTs obtained were studied using SEM, TEM, X-ray and electron diffraction analysis, Raman and ESR spectroscopy. It was found that the majority of the CNTs are multi-walled (MWCNTs), have curved form, and most often agglomerate into bundles. Almost all CNTs are filled partly with electrolyte salt. Except MWCNTs the cathode product contains carbon nanofibers, nanographite, and amorphous carbon. The dependences of CNT's yield, their diameter, and structure peculiarities against the electrolysis regimes were established.

Introduction

There are a number of known methods for synthesis of carbon nanotubes (CNTs) [1]. In contrast to other areas of CNT research, the total number of published articles on electrolytic CNT accounts for only a very small fraction of total CNT publications. In 1995 Hsu et al. [2] developed a new electrolytic method for CNT production, which was described in detail in Ref. [3]. The main point of this method is electrowinning of alkali (Li, Na, K) or alkaline-earth (Mg, Ca) metals from their chloride salts on a graphite cathode followed by the formation of carbon tubes by the interaction of the metal being deposited with the cathode. Unlike many other cases of electrolytic production of materials where the product is deposited from the solution onto the electrode, in the mentioned method CNTs are prepared by converting solid graphite (electrode) into CNTs that enter the solution phase. The carbon cathode erodes during the electrolysis and the electrolytic products are a mixture of CNTs and a large proportion of carbon nanoparticles of different structures originating from the graphite cathode. The CNTs produced are usually multi-walled (MWCNTs). The advantages of electrolytic method are: apparatus simplicity; possibility to control synthesis process by the electrolysis modes, low energy consumption for electrolysis; use of cheap raw materials, possibility of control of product structures and morphologies as well as of carbon phases doping in one step (while synthesis) by means of optimization of electrolysis conditions and electrolytic bath composition [3]. But the described method has several challenges: (i) cracking and destruction of graphite cathode during electrolysis, (ii) accumulation of electrolysis products: chlorine gas (anode), alkaline metal (cathode), and carbon nanomaterials (cathode) in bath. This creates serious problems for stable and continuous performance of electrolysis.

A novel electrolytic synthesis method for carbon nanomaterial generation from ionic melts is being developed in the Institute of General and Inorganic Chemistry [4], [5], [6]. Carbon dioxide, which is introduced in chloride melt under an excessive pressure, is the precursor of nanoallotropes of carbon. The process of cathodic reduction of carbon dioxide to elemental carbon on metallic electrodes is the basis of the proposed method. So, in this method, a new condensed carbon phase is generated on the cathode from a liquid molten salt phase which contains dissolved carbonic acid gas, by electrochemical reactions. Because of the low solubility of CO₂ gas in the chloride melts the only possible way to raise the rate of the cathode process is to generate an excessive gas pressure. All the advantages of electrolytic syntheses method pointed above may be completely attributed to the developing one. It was shown that over a wide current density and potential ranges the only cathodic product is carbon, and the product of anodic reaction is oxygen, i.e. an environment-friendly reaction: CO₂→C+O₂ is realized electrochemically. Using graphite anodes, which react with isolated oxygen, producing carbon dioxide, this electrolysis opens real perspective for creation of continuous technology. The first publication concerning this method appeared in 2004. It is still in progress.

The principal possibility of CNTs synthesis by this method was shown in Ref. [6]. In this paper an attempt to make detail characterization of produced carbon materials by SEM, transmission electron microscopy (TEM), the electron-diffraction method, X-ray phase analysis, Raman and ESR spectroscopy, and differentiation of product properties with electrolysis conditions was done.

Section snippets

Experiment

The principal scheme of high-temperature electrochemical cell, electrochemical apparatus, chemicals, and peculiarities of cathodic discharge of carbon dioxide on platinum and golden electrodes in the melt Na,K|Cl (molar ratio 0.5:0.5) were described by the authors elsewhere [6], [7]. Ternary mixture of earth metal chlorides (NaCl:KCl:CsCl, molar ratio 0.3:0.245:0.455, melting point 480 °C) was used as base electrolyte. Cyclic voltammetry (CV) study of CO₂ discharge (cathodic and anodic) was

Electrochemical measurements

CV study gives information about current and potential regions of electrochemical reactions occurring at electrodes, mechanism, and kinetics peculiarities of electrode processes. There are two waves with limiting currents and half-wave potentials, E_1/2: (A) E_1/2=−0.73 (−0.43) V, and (B) E_1/2=−0.98 (−0.68) V vs. platinum–oxygen, Pt, $\frac{1}{2}$ O₂|O²⁻ (platinum-carbonate, Pt, CO₂+ $\frac{1}{2}$ O₂| CO₃²⁻) reference electrodes are observed on the forward cathodic sweep (Fig. 1(a)). In this part of voltammetric curve, the

Conclusion

Electroreduction of carbon dioxide dissolved in molten salts to carbon can be taken as the basis of high-temperature electrochemical synthesis (HTES) of various nanoscaled carbon particles of different structures and morphologies. The possibility to produce MWCNTs by this method with content up to 40 vol% in the cathode product has been shown. The MWCNTs have curved form and most often agglomerate into bundles. Almost all CNTs are filled partly with the electrolyte salt.

It was found by the

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Electrolytic synthesis of carbon nanotubes from carbon dioxide in molten salts and their characterization

Abstract

Introduction

Section snippets

Experiment

Electrochemical measurements

Conclusion

J. Magn. Magn. Mater.

Rev. Adv. Mater. Sci.

Carbon Nanotubes: Synthesis, Structure, Properties and Applications

Nature

J. Min. Metall.