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The preparation of nano-structured carbon materials by electrolysis of molten lithium chloride at graphite electrodes
Journal of Electroanalytical Chemistry 647 (2010) 150–158
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The preparation of nano-structured carbon materials by electrolysis of molten lithium chloride at graphite electrodes C. Schwandt *, A.T. Dimitrov 1, D.J. Fray Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Received 20 April 2010 Received in revised form 9 June 2010 Accepted 14 June 2010 Available online 19 June 2010 Keywords: Carbon nanotubes Carbon nano-particles Lithium chloride Graphite Molten salt electrochemistry
a b s t r a c t The molten salt electrolytic method for the preparation of nano-structured carbon materials has been subjected to a systematic investigation. It has been confirmed that the electrolysis of molten lithium chloride in the presence of a graphite cathode generates a carbonaceous product that contains nanostructured constituents like particles, fibres and tubes. It has furthermore been found that the precise composition of the product depends critically on a number of process parameters, namely, type of graphite material, reaction temperature, electrochemical polarisation regime, and reaction time. After careful optimisation of these parameters, it has become possible to achieve a content of nanotubes in the carbonaceous product of approximately 80%. This exceeds by far the results of all previous studies using this approach. The nanotubes synthesised are multi-walled with varying diameters and highly curved, and they occur in the form of aggregates of considerable size. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The preparation of nano-structured carbon materials of high aspect ratio like tubes and fibres is an area of huge interest to academia and industry. Carbon nanotubes, both single-walled and multiwalled, have attracted particular attention because they are endowed with a unique combination of physical and chemical properties. They possess excellent mechanical strength and stiffness, good thermal conductivity, and electronic conductivities varying from semiconducting to metallic. They also provide high chemical stability, and may serve as the host material for hydrogen or lithium storage. It has hitherto not been possible to produce carbon nanotubes in large quantities at high yields and low cost, and this prevents wider utilisation of the material despite huge existing demands. Comprehensive surveys covering the synthesis, properties, and application of carbon nanotubes are found in the abundant literature on the subject [1–6]. In one group of preparation methods, carbon nanotubes are synthesised from individual carbon atoms or small aggregates. Evaporation methods are based on carbon volatilisation from a source material through high-energy input from an electric arc, laser beam or electron beam, while pyrolytic methods involve thermal decomposition of hydrocarbons in the presence of hydrogen. * Corresponding author. Tel.: +44 (0)1223 334343; fax: +44 (0)1223 334567. E-mail address: [email protected] (C. Schwandt). 1 On leave from: Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University, 1000 Skopje, Republic of Macedonia. 1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.06.008
Chemical vapour deposition relies on thermal decomposition of gaseous hydrocarbons in the presence of supported nano-sized transition metal-based catalysts, and gas phase catalysis involves decomposition of gaseous hydrocarbons in the presence of volatile transition metal-based catalysts like ferrocene. In all cases the product is grown on a cold substrate. The above methods suffer from one or more of several disadvantages; they require sophisticated equipment and skilled operators; they are of low throughput, yield and/or selectivity; they are hazardous. This renders these methods expensive and scale-up problematic. In the other group of preparation methods, carbon nanotubes are created from pre-existing graphene layers of a graphitic feed material. One technique is based on intercalation of graphite with potassium metal and sonification of the resulting intercalation compound in an aqueous solvent, which leads to exfoliation of graphene layers and formation of multi-walled nanotubes via a scrolling mechanism [7]. This method should be less expensive than the aforementioned ones, but the use of a highly reactive metal in combination with ultrasound may be a critical issue. The other technique relies on electrolysis of a molten alkali metal chloride, with the reduction of the metal cation taking place at a graphite cathode. This method is considered inexpensive and nonhazardous, and will be discussed in greater detail below. The molten salt electrolytic method of preparing nano-structured carbon materials was originally discovered by Kroto and co-workers [8]. In this approach, an electrolytic cell is employed that contains a molten salt electrolyte of, typically, lithium chloride or sodium chloride, and two electrodes of graphite. The cell
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is polarised such that alkali metal ions of the electrolyte are reduced at the graphite cathode and chloride ions are oxidised at the anode. The cathodic reaction leads to the formation of a variety of carbon-based species, which separate from the cathode and assemble in the molten salt and may be recovered with water and organic solvents after cooling. Kroto et al demonstrated the feasibility of the method for the synthesis of pure carbon nanotubes [8,9] as well as partially metal-filled carbon nanotubes [10–12]. However, the carbonaceous product was always found to be of very heterogeneous composition, comprising various nano-structured constituents like particles, fibres, and curved multi-walled tubes of varying size, as well as significant amounts of relatively large graphite fragments that had become detached from the cathode without undergoing a reaction with the molten salt. Tubes were typically a minor fraction of the product and heavily entangled with other constituents. Fray and co-workers investigated the electrolytic preparation method further. It was realised that the cathodic reaction involves the intercalation of the alkali metal into the graphite electrode and the destabilisation of the layered graphite microstructure, which instigates the formation of carbon-based nano-structured species [13,14]. A series of fundamental studies into the underlying reaction steps were performed [15,16], first steps toward the design of a scaled-up cell were reported [17], and a systematic study into the application of sodium chloride as the molten salt electrolyte was carried out showing that tubes may constitute up to 25% of the carbonaceous product [18]. Chen and Fray furthermore provided a very comprehensive review covering the research activities up to the year 2003 [19]. Kaptay and coworkers also investigated the electrolytic preparation method. Carbonaceous products were prepared by using various molten salt electrolytes [20–22], and fundamental studies likewise suggested alkali metal intercalation into graphite as an essential step of the process [23–25]. The above-mentioned research groups have undertaken most of the research work in the field, and there are only very few additional studies. A French group reported the preparation of single-walled carbon nanotubes by way of this approach [26]; a Chinese group repeated work on the preparation of tin-filled carbon nanotubes but did not quantify composition of the product [27]; and a Russian group identified the presence of carbon nanotubes in the carbon-based waste products of an industrial lithium production cell [28]. In summary, it is obvious that the molten salt electrolytic method of preparing nano-structured carbon materials is comparatively simple and inexpensive, and should also be the most straightforward one to scale-up. However, the approach has not yet received much attention, and this is probably a consequence of the inferior product quality attained to date. In view of the relatively small amount of research work dedicated to the molten salt electrolytic preparation of carbon nanotubes from graphite, it has been considered that the full potential of this method may not as yet have been reached. Thus far, sodium chloride is the molten salt electrolyte that has been examined most thoroughly even though it possesses a rather limited temperature window for process optimisation. The melting point of sodium chloride of 801°C invariably determines the lower limit, and above the boiling point of sodium metal of 883°C the yield of the reaction decreases dramatically [14]. In contrast, there is no in-depth study on lithium chloride as the electrolyte even though it offers a much wider applicable temperature range from 605 to beyond 1300°C. The present article now provides the first systematic study into the electrolysis of lithium chloride in the presence of a graphite cathode with the aim of producing nano-structured carbon materials with high percentages of nanotubes. Parameters of interest have been the type of graphite, the reaction temperature, the electrochemical polarisation regime, and the reaction time. Product
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composition as a function of process parameters was assessed by means of electron microscopy.
2. Experimental The electrolysis experiments were performed in a tubular stainless steel reactor, which was positioned vertically inside a programmable electrical furnace. The upper end of the reactor was closed with a stainless steel plate, which provided feedthroughs for the electrode leads and a thermocouple as well as steel pipes for gas circulation. The interior of the reactor was continuously purged with argon that was dried over self-indicating calcium sulphate. A graphite crucible with inner height of 13 cm and inner diameter of 5.5 cm (EC4, Tokai Carbon UK) was used to contain the molten salt electrolyte. The crucible was filled with approximately 200 g of nominally anhydrous lithium chloride (213233, Aldrich), and was heated in an argon atmosphere and with a ramp rate of 120°C/h to the desired target temperature. In the electrolysis experiments, a graphite rod of 6.5 mm in diameter, and of specification detailed below, was used as the cathode, and the graphite crucible was employed as the anode. The surface area of the graphite rod cathode facing the molten salt electrolyte was 2.4 cm2. This was achieved by a tubular alumina sheath that covered tightly the upper part of the rod and only exposed the lower part to the melt. The graphite rod rested in the centre of an alumina tile, which was placed on the base of the graphite crucible and avoided short circuit between the electrodes. The rod was slightly tilted in order to ensure salt access to the bottom surface. Both the graphite rod cathode and the graphite crucible anode were contacted with nickel wire of 2 mm in diameter and connected to the electrochemical equipment. In some experiments, a quasi-reference electrode was employed. This consisted of a molybdenum wire of 0.5 mm in diameter positioned inside an open-ended alumina sheath. The correct functioning of this type of quasi-reference electrode in lithium chloride melts had been demonstrated in earlier work [15,16]. Electrolysis experiments were performed with either a two-terminal cell or a three-terminal cell. In the two-terminal mode, a constant current was imposed between the graphite rod cathode and the graphite crucible anode. The current was provided by a manually-operated auto-ranging power supply (LS 30-10, Wayne Kerr or AP 20-80, Farnell), and the voltage recorded as a function of time. In the three-terminal mode, a constant polarisation potential was applied to the graphite rod cathode versus the molybdenum quasi-reference, with the graphite crucible serving as the anode. The same equipment was used, and the current monitored with time. Each experiment was commenced approximately 30 min after reaching target temperature. Fig. 1 presents a schematic of the experimental set-up. Four different types of electro-carb graphite materials (EC4, EC5, EC15, and EC17, Tokai Carbon UK) were employed as the cathode. Table 1 compiles quantitative information on density and grain size, as provided by the supplier. It is seen that the four types of graphite possess considerably different properties. EC4 graphite is of relatively low density and composed of relatively large grains; EC17 graphite is of high density and consists of small grains; EC5 and EC15 graphites have intermediate properties. Figs. 2 and 3 show scanning electron microscopic images of EC4 and EC17 graphites, respectively. The micrographs confirm the difference in grain size and reveal the flake-type shape of the grains. The reaction temperatures ranged from 625 to 1200°C, with increments of 25, 50 or 100°C. The constant currents in two-terminal experiments were varied between 1.2 and 12.0 A, with increments of 1.2 A, corresponding to cathodic current densities of 0.5
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Fig. 1. Schematic of the experimental set-up. Two-terminal cell: the graphite rod is used as the cathode and the graphite crucible is used as the anode. Three-terminal cell: the same polarity is applied as in the two-terminal cell and the molybdenum wire is used as the quasi-reference electrode.
Fig. 2. Scanning electron microscopic image of a fracture surface of EC4 graphite, showing comparatively large flake-shaped graphite grains.
Table 1 Properties of the electro-carb graphite materials used as the cathode in the electrolysis experiments; data as provided by the supplier. Material
Density [g/cm3]
Average grain size [lm]
Maximum grain size [lm]
EC4 EC5 EC15 EC17
1.75 1.85 1.83 1.85
13 11 7 2
100 100 25 10
Fig. 3. Scanning electron microscopic image of a fracture surface of EC17 graphite, showing comparatively small flake-shaped graphite grains.
to 5.0 A/cm2 when referred to the initial geometric surface of the graphite rod cathode contacting the molten salt electrolyte. The constant polarisation potentials in three-terminal experiments were varied between -2.0 and -3.0 V versus the quasi-reference, and were chosen so as to establish appropriate cathodic current densities. The reaction times were between 1 and 40 min, with 10 min being used in the majority of cases. In total, more than 70 experiments were completed, and in particularly relevant instances two or three experiments were carried out under nominally identical conditions. Upon termination of an electrolysis experiment, the furnace was switched off and the electrolytic cell was allowed to cool.
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The solidified salt was removed with tap water, and the carbonaceous reaction product was collected by way of filtration with a glass frit in a mild vacuum. The retrieved material was rinsed with copious amounts of distilled water until no chloride ions were detected in the filtrate when tested with silver nitrate solution, and then dried in ambient air at around 150°C. This procedure was found to be much simpler and faster than the previously reported ones relying on the use of organic solvents. Each carbonaceous product was investigated by means of scanning electron microscopy using a high-resolution field emission gun system (JSM-6340F, JEOL). The composition of each sample was assessed through visual inspection of numerous micrographs. It is acknowledged that an objective and quantitative analysis of micrographs is difficult to achieve in this manner but, owing to the lack of alternatives, this approach is commonplace and has been used in all foregoing studies. It is believed that the quantitative comparison between the samples of this study is fairly reliable, whilst it is supposed that comparisons with the results from other studies are less accurate. Selected samples were additionally examined by way of transmission electron microscopy (200CX, JEOL).
3. Results 3.1. Orientational experiments An initial set of experiments was conducted in order to assess the appropriateness of the various types of graphite, EC4, EC5, EC15 and EC17, with respect to the generation of nano-structured carbonaceous products when employed as the cathode in the electrolysis of molten lithium chloride. The experiments were carried out in the two-terminal cell under current control, applying reaction temperatures of 700, 800 and 900°C, cathodic current densities of 2.0 and 3.0 A/cm2, and reaction times of 10 min. The carbonaceous products were retrieved, washed and dried, and then subjected to scanning electron microscopic analysis. The initial experiments demonstrated that the reaction of lithium ions at the graphite cathode generates carbonaceous products that can comprise up to four main types of constituents. Three of these constituents are nano-structured and may be classified as particles, fibres and tubes; the fourth constituent is macroscopic
Fig. 4. Scanning electron microscopic image of a carbonaceous product that contains all of the typical constituents, i.e., nano-particles, nano-fibres, nanotubes and macroscopic pieces, in the given area. Experimental conditions: cathode EC4 graphite; reaction temperature 800°C; initial cathodic current density 2.0 A/cm2; reaction time 10 min.
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pieces. Fig. 4 presents a scanning electron microscopic image of a sample that displays all of these constituents within a relatively small area. Nano-particles are characterised by their fairly equiaxed shapes and facetted surfaces. Nano-fibres are considerably bent and sometimes kinked and display rather abrupt changes in contrast along their lengths. Chains of particles and fibres are often difficult to differentiate. Nanotubes are identified by their smooth curvature and the more gradual change in contrast. A differentiation of tubes and fibres is also possible on the basis that tubes, unlike fibres, may be filled with metals like tin, as will be demonstrated elsewhere [29]. The final constituent seen on the image is pieces of relatively large size and with sharp edges. Comparison with the micrographs of the graphite feed material indicates that these are unreacted graphite grains. A more detailed microscopic analysis will follow in a later chapter. Composition of the carbonaceous product in terms of quantity and ratio of the above constituents was found to depend critically upon the type of graphite employed as the cathode, and was very reproducible for nominally identical processing conditions. When using EC17 graphite, the overwhelming nano-structured constituent in the carbonaceous product was particles, whilst constituents of high aspect ratio were vastly absent, and a considerable amount of large graphite pieces was also found. This result was rather independent of temperature and current density. When using EC4, EC5 and EC15 graphites, the carbonaceous products contained all of the four constituents mentioned above. However, for EC5 and EC15 graphites, only very small contents of nanotubes of less than 5% were observed in each case. By contrast, for EC4 graphite, product composition was very strongly dependent on the experimental conditions, and contents of tubes of up to 20% were present in some of the samples. This suggested EC4 graphite as the most promising candidate material for further investigation. 3.2. Optimisation experiments Prompted by the results gained in the orientational studies, a more systematic set of experiments was conducted with lithium chloride as the molten salt electrolyte and EC4 graphite as the cathode. Experiments were performed in the two-terminal cell under current control, and reaction temperature, cathodic current density and reaction time were varied, with the aim of maximising the content of nanotubes in the carbonaceous product. The first process parameter to be investigated was reaction temperature, and this was varied between 625 and 1200°C. Cathodic current density was 2.0 A/cm2; and reaction time was 10 min. The experiments revealed that reaction temperature is of paramount importance. A maximum content of nanotubes in the carbonaceous product of around 50% was found at the temperature of 775°C, the remainder was mainly nano-particles and large graphite pieces. When increasing the temperature, the amount of tubes decreased rapidly; at 800°C the content dropped to around 20%, and at 850°C it fell to close to 0%. Under these conditions nano-particles were the prevailing product, and this remained unchanged when raising temperature further. When decreasing the temperature, the amount of tubes again decreased; at 750°C the content dropped to around 15%, and at 700°C it was at around 5%. Under these conditions, nano-particles were again the main product. Upon lowering the temperature further, a moderate rise in the content of tubes would seem to occur at temperatures of around and below 650°C. However, these results were difficult to obtain and reproduce, as experimenting with a molten salt only marginally above the melting point is inherently intricate. In order to verify that the optimum temperature for nanotube formation is indeed at 775°C, additional experiments were carried out at five and ten degrees above and below this value, but no noticeable improvement was achieved.
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Cathodic current density was investigated by varying values between 0.5 and 5.0 A/cm2. Reaction temperature was 775°C, reaction time was 10 min at 2.0 A/cm2, and was adjusted for all other current densities such that the total charge passed was 2880 C in each case. The experiments showed that the maximum content of nanotubes in the carbonaceous product occurs at 2.0 A/cm2. This maximum seems to be relatively shallow, as current densities of 1.5 and 2.5 A/cm2 led to materials of similar appearance albeit somewhat reduced nanotube contents. At low current densities of 1.0 and 0.5 A/cm2, little product was formed and nano-particles and very irregularly shaped nano-fibres were the main constituents. At high current densities of 3.0 A/cm2 and beyond, the amount of large graphite pieces increased steeply and finally these became the prevailing constituent. Reaction time was examined by varying values between 1 and 20 min. Reaction temperature was 775°C, and cathodic current density was 2.0 A/cm2. For a short reaction time of 1 min, only very little carbonaceous product was formed. As reaction time was prolonged, the amount of product increased. Up to a reaction time of around 4 min, the content of nanotubes in the carbonaceous product remained at a level of approximately 80%. On extending reaction time further, the total amount of product continued to rise, but the fraction of tubes declined slowly while that of large graphite pieces grew. Under the given conditions, reactions typically came to an end after around 15 to 16 min of polarisation when the graphite material protruding the alumina sheath had been consumed completely. Following the optimisation of process parameters in the twoterminal cell, further experiments were conducted in the three-terminal cell. In these, the potential of the graphite rod cathode was varied between -2.0 and -3.0 V against the molybdenum quasi-reference electrode. Reaction temperature was 775°C, and reaction times were between 4 and 10 min. Examination of the carbonaceous products revealed that the optimum polarisation potential for the formation of nanotubes is at -2.5 V. This is in full agreement with the results obtained with the two-terminal cell, because the cathodic current density established in the three-terminal cell at -2.5 V was very close to 2.0 A/cm2, which had been identified as the optimum cathodic current density. Time-dependent experiments performed under optimised conditions in the three-terminal cell demonstrated that nanotube contents as high as 80% may be maintained for reaction times up to around 6 min. Table 2 compiles the process parameters that were found to be most appropriate for the preparation of carbon nanotubes within the range of conditions explored.
of the currents applied were selected such that the current densities at the graphite rod cathode at the beginning of a run were between 0.5 and 5.0 A/cm2. Corresponding currents ranged from 1.2 to 12.0 A, and the voltages necessary to sustain these currents were between 2.3 and 11.7 V, depending on temperature and current. Accordingly, the overall resistance of the cell was on the order of 1 to 2 X, and included contributions from the potential drops across the electrolyte, the electrodes, the graphite components and the nickel leads. Fig. 5 presents several voltage versus time curves that were recorded in experiments performed with EC4 graphite rod cathodes at the temperature of 775°C and initial cathodic current densities between 1.0 and 3.0 A/cm2. The voltages remain fairly constant throughout the major part of each run and, for high applied currents, increase in the final stage. Electrolysis experiments in the three-terminal cell were performed under the condition of constant polarisation potential. The magnitudes of the polarisation potentials applied were between -2.0 and -3.0 V versus the molybdenum quasi-reference. The resulting currents and current densities were of the same order of magnitude as those in the two-terminal runs. Fig. 6 presents a current versus time curve that was monitored during an experiment performed with an EC4 graphite rod cathode at 775°C and a polarisation potential of -2.5 V. After an initial peak, the current remains fairly constant throughout the first 5 min of the run. The current then becomes noticeably noisier throughout the remainder of the run. Fig. 7 compares the macroscopic appearances of graphite rod cathodes before and after electrolysis. Processing was carried out in the two-terminal cell with EC4 graphite at 775°C, a cathodic current density of 2.0 A/cm2 and different polarisation times. The sample that was exposed to the molten salt in the absence of cathodic polarisation underwent no visual changes. The sample that was polarised for 4 min shows that part of the graphite has been eroded and that erosion has occurred quite smoothly and uniformly. The sample polarised for 10 min displays a comparatively rough surface and significant macroscopic cracks. After 16 min of polarisation the graphite tip exposed to the molten salt had been completely consumed. The image shows typical cases, whilst the precise extent of erosion was a function of the process parameters
3.3. Electrolysis curves and anode erosion Electrolysis experiments in the two-terminal cell were performed under the condition of constant current. The magnitudes Table 2 Optimum process parameters for the preparation of carbon nanotubes from graphite by way of the electrolytic method using molten lithium chloride, as identified within the range of conditions investigated in the present study. Process parameter
Optimum value
Microstructure of graphite: Density Average grain size
1.75 g/cm3 13 lm
Reaction temperature
775°C
Polarisation regime: Cathodic current density Cathodic polarisation potential
2.0 A/cm2 2.5 V vs Mo
Reaction time
4–6 min
Fig. 5. Voltage versus time curves of electrolysis experiments conducted in the two-terminal cell. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current densities between 1.0 and 3.0 A/cm2; reaction times between 20 and 6.67 min.
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Fig. 8. Scanning electron microscopic image of a carbonaceous product that contains nano-particles as the main constituent. Experimental conditions: cathode EC4 graphite; reaction temperature 700°C; initial cathodic current density 2.0 A/ cm2; reaction time 10 min. Fig. 6. Current versus time curve of an electrolysis experiment conducted in the three-terminal cell. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; cathodic polarisation potential -2.5 V versus molybdenum; reaction time 10 min.
Fig. 7. Optical images of graphite cathodes before and after electrolysis experiments. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 2.0 A/cm2; reaction times (a) 0 min, (b) 4 min, (c) 10 min, (d) 16 min.
applied. Expectedly, the amount of carbonaceous product generated in an electrolysis experiment depended on the degree of erosion, and varied between a few milligrams and several hundreds of milligrams. 3.4. Microscopic characterisation of the carbonaceous products The carbonaceous products synthesised in the various electrolysis experiments were found to consist of a number of carbonbased constituents, namely, nano-particles, nano-fibres, nanotubes and macroscopic pieces of unreacted graphite, and their quantities and ratios were dependent on the experimental conditions applied. In the following, a representative selection of electron microscopic images is given. All images show typical features of products that were obtained from EC4 graphite under different experimental conditions. Fig. 8 presents a scanning electron microscopic image of nanoparticles. The particles possess a fairly uniform size distribution with diameters between 100 and 200 nm. As discussed, nano-particles are the prevailing reaction product under the vast majority of experimental conditions. Fig. 9a and b presents micrographs of predominantly nanotubes. The tubes are of varying diameter, with an average of around 40 to 60 nm and a maximum of around 150 nm. This order of magnitude compares well with the results from
Fig. 9. Scanning electron microscopic images of a carbonaceous product that contains nanotubes as the main constituent. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 2.0 A/cm2; reaction time 4 min. (a) and (b) show different areas of the same sample.
the majority of the foregoing studies using lithium chloride as the molten salt electrolyte [9,13,14,20]. The average length of the tubes is difficult to determine because it is almost always impossible to locate on the micrographs the positions of both ends of the same tube. This suggests that the tubes are in fact rather long,
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Fig. 10. Transmission electron microscopic image of a carbonaceous product that contains nanotubes as the main constituent. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; cathodic polarisation potential -2.5 V versus molybdenum; reaction time 6 min.
probably in excess of several micrometres. Fig. 10 is a transmission electron microscopic image showing a small section of a sample of predominantly tubes. Both the smooth surfaces and the hollow interiors are clearly discernible. As stated earlier, nanotubes are the desired product but dominate only within a very small window of processing conditions. Fig. 11 presents a scanning electron microscopic image of a mixture of nano-particles and large graphite pieces, and Fig. 12 shows a micrograph of a mixture of nanotubes and graphite pieces. Graphite fragments are undesired because they render the carbonaceous product heterogeneous in composition and lower the yield of nano-structured material. The most notable result of the present study is that, after careful optimisation of all relevant process parameters, it has become possible to synthesise carbonaceous products that contain a large percentage of nanotubes. Fig. 13 presents a low magnification scanning electron microscopic image of one such product. The micrograph shows agglomerates that consist virtually exclusively of tubes and are many micrometres wide. These results are unprecedented for the molten salt electrolytic method of carbon nanotube production.
Fig. 11. Scanning electron microscopic image of a carbonaceous product that contains nano-particles and large graphite pieces in the given area. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 3.0 A/cm2; reaction time 10 min.
Fig. 12. Scanning electron microscopic image of a carbonaceous product that contains nanotubes and large graphite pieces in the given area. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 3.0 A/cm2; reaction time 10 min.
4. Discussion The molten salt electrolytic method for the production of carbon nanotubes from graphite is a conversion process that comprises a complex sequence of reaction steps. As mentioned, it is widely believed that the cathodic process involves the intercalation of lithium from the molten salt electrolyte into the graphite electrode. The lithium is inserted into the spaces between the individual graphene layers where it may form intercalation compounds, and exerts mechanical stress on the host lattice. This leads to disintegration of the graphite structure such that individual graphene layers or stacks thereof become detached. These then rearrange chemically to form nanotubes, or other nano-structured entities, which assemble in the molten salt. The anodic process is the formation of chlorine gas. The onset potential of this reaction is significantly lower than the thermodynamic decomposition potential of lithium chloride. This is a consequence of the chemical interaction between lithium and carbon during intercalation which allows the lithium to be reduced at activities below unity [15,16]. þ
Cathodic half cell reaction : Li þ e þ xC ¼ LiCx 1 Anodic half cell reaction : Cl ¼ Cl2 þ e 2
ð1Þ ð2Þ
Fig. 13. Scanning electron microscopic image of a carbonaceous product that contains large agglomerates of nanotubes. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; cathodic polarisation potential -2.5 V versus molybdenum; reaction time 6 min.
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The exact cathodic reaction mechanism is unknown. One possibility is exfoliation, i.e., a mechanism in which the graphene layers peel off of suitably orientated graphite surfaces once a sufficient quantity of lithium has been incorporated into the grain boundary. The other possibility is extrusion, i.e., a mechanism in which graphene layers are sheared out of graphite grains as lithium is being incorporated. It is evident that a rigorous determination of the reaction mechanism would be difficult, if not impossible. Consequently, in the present study, the optimisation of the system has been performed in an empirical manner by assessing a wide range of experimental conditions. Nevertheless, it will be attempted in the following to rationalise the impact of the key process parameters. 4.1. Density and grain size of graphite cathode In the experimental programme, graphite of type EC4 has been identified as the only feed material capable of yielding a carbonaceous product with nanotubes as the majority constituent. By contrast, EC17 graphite could be converted only into nano-particles and also provided the highest amount of large graphite pieces in the product. EC5 and EC15 graphites showed intermediate behaviours. Notably, EC4 graphite possesses a much lower density than the other three types of graphite, and is relatively soft and flexible. It is therefore reasonable to assume that EC4 graphite is capable of accommodating relatively large quantities of lithium before undergoing disintegration, and that this is beneficial for the controlled break-down of the graphite structure and the formation of nanotubes. Furthermore, EC4 graphite consists of relatively large grains, whereas EC17 graphite possesses the smallest grains of the three materials with high density. It may hence be assumed that large grain size is likewise favourable for nanotube formation. 4.2. Reaction temperature The reason for the critical importance of reaction temperature with respect to the formation of nanotubes may be twofold. Firstly, the intercalation of lithium into the graphite structure and the concomitant release of graphene sheets from the graphite grains take place under the influence of lattice vibrations; and optimum lattice dynamics may only occur within a narrow temperature range. Secondly, the overall reaction pathway from graphite to nano-structured entities is highly complex and consists of several steps with potentially different activation energies; and the particular reaction sequence leading to tubes may only proceed within a small temperature interval. 4.3. Cathodic current density The occurrence of an optimum current density for nanotube formation implies the existence of an optimum intercalation rate of lithium into graphite. It is assumed that a very low intercalation rate allows the lithium to diffuse far into the graphite bulk without building up the optimum concentration for nanotube formation, and that very high intercalation rates cause excessive lithium accumulation in the surface-near region provoking damage of the graphene layers as well as macroscopic fracturing of the graphite body and release of large graphite fragments. 4.4. Reaction time Optimum reaction times were identified as those allowing the graphite to be eroded to a significant extent while maintaining a smooth and uniform erosion pattern. Short reaction times do not yield a significant amount of nano-structured carbonaceous product, and it is assumed that the degree of intercalation is insufficient
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to instigate substantial disintegration of the graphite. Long reaction times entail a deterioration of the carbonaceous product, especially because of the rising content of large graphite pieces, and it is supposed that this is a consequence of the creation of uneven surfaces on the eroding graphite cathode that facilitate detachment of graphite fragments. In the experiments performed in the two-terminal cell under current control, it has been observed that the applied voltage necessary to drive the pre-determined current remains fairly constant over a few minutes. This suggests that the decrease in macroscopic surface area, caused by advancing erosion, is approximately balanced by the increase in microscopic surface area, originating from enhanced surface roughness. This is an important finding because otherwise experiments under current control would not be meaningful. However, this type of process control is invariably compromised at long reaction times when substantial changes of the surface area are inevitable. It is noted that a small fraction of experiments carried out in the two-terminal cell were terminated early because of a quite abrupt stoppage of the cell reactions. In such a case, even the maximum output voltage of the potentiostat was insufficient to maintain the current, which essentially caused shut-down of the cell. This was observed occasionally when experiments were extended significantly beyond 10 min and only rarely for shorter runs. It is supposed that this may be due to the blockage of one of the electrodes. One possibility would be the formation of a continuous and badly wetting film of lithium metal around the cathode; the other would be the anode effect caused by the build-up of an insulating and well-adherent film of chlorine gas bubbles at the anode. 4.5. Type of polarisation Polarisation experiments performed in the three-terminal cell were found to provide carbonaceous products with high contents of carbon nanotubes for somewhat longer reaction times than experiments carried out in the two-terminal cell. This demonstrates that cathode potential control is superior to current control. The reason is thought to be that the optimum polarisation potential in the three-terminal cell may be maintained throughout the entire run, whereas the optimum current density in a two-terminal cell cannot be sustained constantly. However, the progressive release of large graphite pieces toward the end of a run was still found to occur. 5. Conclusions An extensive experimental study has been performed into the preparation of nano-structured carbon materials by way of electrolysis of molten lithium chloride with graphite electrodes. The combined application of lithium chloride as the molten salt electrolyte and graphite of type EC4 as the cathode has been identified as the key prerequisite for the synthesis of carbonaceous products with high contents of carbon nanotubes. Hitherto, sodium chloride has been considered an equally suitable electrolyte, and the properties of the graphite feed material had not been recognised as relevant at all. A detailed investigation of the system of lithium chloride and EC4 graphite has provided an optimised set of process parameters for carbon nanotube formation. The optimum reaction temperature is at around 775°C, with deviations from this value of as little as 25°C ensuing a massive decline in the nanotube content. All earlier studies on lithium chloride failed to identify the paramount importance of temperature. Cathodic current density should be at around 2.0 A/cm2; corresponding to a cathode potential of around -2.5 V versus a molybdenum quasi-reference. Optimum
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reaction time is between 4 and 6 min. Application of these conditions has rendered possible the synthesis of carbonaceous products with nanotube contents as high as 80%. It is presently endeavoured to implement improved process control and maintain carbon nanotube production under optimum conditions over extended reaction times [30]. Overall, the present study has led to a dramatic improvement of the molten salt electrolytic method of producing carbon nanotubes. Future work will focus on a variety of aspects. The separation of the nano-structured components from residual salt and unreacted graphite needs to be optimised. In this context, the suitability of a recently developed extraction technique using ethyl acetate [31] will be ascertained. The performance of the nanotubes prepared will be evaluated in chemical and structural applications. Because of the comparatively low formation temperature, it is surmised that the nanotubes possess a rather large density of defects. This may be beneficial in chemical applications that involve bonding and transport processes, but may be adverse to structural applications that demand very good mechanical properties. Acknowledgements The authors gratefully acknowledge financial support of this study by the Department of Materials Science and Metallurgy of the University of Cambridge through the J.E.O. Mayne Fund. ATD wishes to thank The Royal Society for a Visiting Scientist Fellowship. References [1] Th.W. Ebbesen (Ed.), Carbon nanotubes: Preparation and Properties, CRC Press, Boca Raton, FL, 1997. [2] P.J.F. Harris, Carbon Nanotubes and Related Structures, Cambridge University Press, 1999. [3] M.S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.), Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Springer, Berlin, 2001. [4] S. Reich, C. Thomsen, J. Maultzsch, Carbon Nanotubes: Basic Concepts and Physical Properties, Wiley-VCH, Weinheim, 2004.
[5] M. Meyyappan (Ed.), Carbon Nanotubes: Science and Applications, CRC Press, Boca Raton, FL, 2005. [6] M.J. O’Connell (Ed.), Carbon Nanotubes: Properties and Applications, CRC Press, Boca Raton, FL, 2006. [7] L.M. Viculis, J.J. Mack, R.B. Kaner, Science 299 (2003) 1361. [8] W.K. Hsu, J.P. Hare, M. Terrones, H.W. Kroto, D.R.M. Walton, P.J.F. Harris, Nature 377 (1995) 687. [9] W.K. Hsu, M. Terrones, J.P. Hare, H. Terrones, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 262 (1996) 161. [10] W.K. Hsu, M. Terrones, H. Terrones, N. Grobert, A.I. Kirkland, J.P. Hare, K. Prassides, P.D. Townsend, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 284 (1998) 177. [11] W.K. Hsu, J. Li, H. Terrones, M. Terrones, N. Grobert, Y.Q. Zhu, S. Trasobares, J.P. Hare, C.J. Pickett, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 301 (1999) 159. [12] W.K. Hsu, S. Trasobares, H. Terrones, M. Terrones, N. Grobert, Y.Q. Zhu, W.Z. Li, R. Escudero, J.P. Hare, H.W. Kroto, D.R.M. Walton, Chem. Mater. 11 (1999) 1747. [13] G.Z. Chen, X. Fan, A. Luget, M.S.P. Shaffer, D.J. Fray, A.H. Windle, J. Electroanal. Chem. 446 (1998) 1. [14] G.Z. Chen, I. Kinloch, M.S.P. Shaffer, D.J. Fray, A.H. Windle, High Temp. Mater. Process. 2 (1998) 459. [15] Q. Xu, C. Schwandt, G.Z. Chen, D.J. Fray, J. Electroanal. Chem. 530 (2002) 16. [16] Q. Xu, C. Schwandt, D.J. Fray, J. Electroanal. Chem. 562 (2004) 15. [17] A.T. Dimitrov, G.Z. Chen, I.A. Kinloch, D.J. Fray, Electrochim. Acta 48 (2002) 91. [18] I.A. Kinloch, G.Z. Chen, J. Howes, C. Boothroyd, C. Singh, D.J. Fray, A.H. Windle, Carbon 41 (2003) 1127. [19] G.Z. Chen, D.J. Fray, J. Min. Met. 39 B (2003) 309. [20] G. Kaptay, I. Sytchev, J. Miklósi, P. Nagy, P. Póczik, K. Papp, E. Kálmán, in: R.W. Berg, H.A. Hjuler (Eds.), Progress in Molten Salt Chemistry 1, Elsevier, Amsterdam, 2000, p. 257. [21] J. Miklósi, P. Póczik, I. Sytchev, K. Papp, G. Kaptay, P. Nagy, E. Kálmán, Appl. Phys. A72 (2001) S189. [22] M.S. Yaghmaee, Zs. Demeter, J. Sytchev, J. Lakatos, G. Kaptay, J. Min. Met. 39 B (2003) 343. [23] N. Borisenko, J. Sytchev, G. Kaptay, J. Min. Met. 39 B (2003) 269. [24] J. Sytchev, N.V. Borisenko, G. Kaptay, Kh.B. Kushkhov, Russ. J. Electrochem. 41 (2005) 956. [25] J. Sytchev, G. Kaptay, Electrochim. Acta 54 (2009) 6725. [26] J.B. Bai, A.-L. Hamon, A. Marraud, B. Jouffrey, V. Zymla, Chem. Phys. Lett. 365 (2002) 184. [27] H. Huang, W.K. Zhang, M.C. Li, Y.P. Gan, C.A. Ma, X.B. Zhang, Trans. Nonferr. Metal. Soc. 14 (2004) 441. [28] N.I. Alekseev, O.V. Arapov, I.M. Belozerov, Yu.G. Osipov, K.N. Semenov, S.V. Polovtsev, N.A. Charykov, S.G. Izotova, Russ. J. Phys. Chem. 79 (2005) S178. [29] R. Das Gupta, C. Schwandt, D.J. Fray, to be published. [30] C. Schwandt, A.T. Dimitrov, D.J. Fray, to be published. [31] T. Gábor, F.H. Kármán, J. Sytchev, E. Kálmán, G. Kaptay, Carbon 47 (2009) 1189.
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
The preparation of nano-structured carbon materials by electrolysis of molten lithium chloride at graphite electrodes C. Schwandt *, A.T. Dimitrov 1, D.J. Fray Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Received 20 April 2010 Received in revised form 9 June 2010 Accepted 14 June 2010 Available online 19 June 2010 Keywords: Carbon nanotubes Carbon nano-particles Lithium chloride Graphite Molten salt electrochemistry
a b s t r a c t The molten salt electrolytic method for the preparation of nano-structured carbon materials has been subjected to a systematic investigation. It has been confirmed that the electrolysis of molten lithium chloride in the presence of a graphite cathode generates a carbonaceous product that contains nanostructured constituents like particles, fibres and tubes. It has furthermore been found that the precise composition of the product depends critically on a number of process parameters, namely, type of graphite material, reaction temperature, electrochemical polarisation regime, and reaction time. After careful optimisation of these parameters, it has become possible to achieve a content of nanotubes in the carbonaceous product of approximately 80%. This exceeds by far the results of all previous studies using this approach. The nanotubes synthesised are multi-walled with varying diameters and highly curved, and they occur in the form of aggregates of considerable size. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The preparation of nano-structured carbon materials of high aspect ratio like tubes and fibres is an area of huge interest to academia and industry. Carbon nanotubes, both single-walled and multiwalled, have attracted particular attention because they are endowed with a unique combination of physical and chemical properties. They possess excellent mechanical strength and stiffness, good thermal conductivity, and electronic conductivities varying from semiconducting to metallic. They also provide high chemical stability, and may serve as the host material for hydrogen or lithium storage. It has hitherto not been possible to produce carbon nanotubes in large quantities at high yields and low cost, and this prevents wider utilisation of the material despite huge existing demands. Comprehensive surveys covering the synthesis, properties, and application of carbon nanotubes are found in the abundant literature on the subject [1–6]. In one group of preparation methods, carbon nanotubes are synthesised from individual carbon atoms or small aggregates. Evaporation methods are based on carbon volatilisation from a source material through high-energy input from an electric arc, laser beam or electron beam, while pyrolytic methods involve thermal decomposition of hydrocarbons in the presence of hydrogen. * Corresponding author. Tel.: +44 (0)1223 334343; fax: +44 (0)1223 334567. E-mail address: [email protected] (C. Schwandt). 1 On leave from: Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University, 1000 Skopje, Republic of Macedonia. 1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.06.008
Chemical vapour deposition relies on thermal decomposition of gaseous hydrocarbons in the presence of supported nano-sized transition metal-based catalysts, and gas phase catalysis involves decomposition of gaseous hydrocarbons in the presence of volatile transition metal-based catalysts like ferrocene. In all cases the product is grown on a cold substrate. The above methods suffer from one or more of several disadvantages; they require sophisticated equipment and skilled operators; they are of low throughput, yield and/or selectivity; they are hazardous. This renders these methods expensive and scale-up problematic. In the other group of preparation methods, carbon nanotubes are created from pre-existing graphene layers of a graphitic feed material. One technique is based on intercalation of graphite with potassium metal and sonification of the resulting intercalation compound in an aqueous solvent, which leads to exfoliation of graphene layers and formation of multi-walled nanotubes via a scrolling mechanism [7]. This method should be less expensive than the aforementioned ones, but the use of a highly reactive metal in combination with ultrasound may be a critical issue. The other technique relies on electrolysis of a molten alkali metal chloride, with the reduction of the metal cation taking place at a graphite cathode. This method is considered inexpensive and nonhazardous, and will be discussed in greater detail below. The molten salt electrolytic method of preparing nano-structured carbon materials was originally discovered by Kroto and co-workers [8]. In this approach, an electrolytic cell is employed that contains a molten salt electrolyte of, typically, lithium chloride or sodium chloride, and two electrodes of graphite. The cell
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is polarised such that alkali metal ions of the electrolyte are reduced at the graphite cathode and chloride ions are oxidised at the anode. The cathodic reaction leads to the formation of a variety of carbon-based species, which separate from the cathode and assemble in the molten salt and may be recovered with water and organic solvents after cooling. Kroto et al demonstrated the feasibility of the method for the synthesis of pure carbon nanotubes [8,9] as well as partially metal-filled carbon nanotubes [10–12]. However, the carbonaceous product was always found to be of very heterogeneous composition, comprising various nano-structured constituents like particles, fibres, and curved multi-walled tubes of varying size, as well as significant amounts of relatively large graphite fragments that had become detached from the cathode without undergoing a reaction with the molten salt. Tubes were typically a minor fraction of the product and heavily entangled with other constituents. Fray and co-workers investigated the electrolytic preparation method further. It was realised that the cathodic reaction involves the intercalation of the alkali metal into the graphite electrode and the destabilisation of the layered graphite microstructure, which instigates the formation of carbon-based nano-structured species [13,14]. A series of fundamental studies into the underlying reaction steps were performed [15,16], first steps toward the design of a scaled-up cell were reported [17], and a systematic study into the application of sodium chloride as the molten salt electrolyte was carried out showing that tubes may constitute up to 25% of the carbonaceous product [18]. Chen and Fray furthermore provided a very comprehensive review covering the research activities up to the year 2003 [19]. Kaptay and coworkers also investigated the electrolytic preparation method. Carbonaceous products were prepared by using various molten salt electrolytes [20–22], and fundamental studies likewise suggested alkali metal intercalation into graphite as an essential step of the process [23–25]. The above-mentioned research groups have undertaken most of the research work in the field, and there are only very few additional studies. A French group reported the preparation of single-walled carbon nanotubes by way of this approach [26]; a Chinese group repeated work on the preparation of tin-filled carbon nanotubes but did not quantify composition of the product [27]; and a Russian group identified the presence of carbon nanotubes in the carbon-based waste products of an industrial lithium production cell [28]. In summary, it is obvious that the molten salt electrolytic method of preparing nano-structured carbon materials is comparatively simple and inexpensive, and should also be the most straightforward one to scale-up. However, the approach has not yet received much attention, and this is probably a consequence of the inferior product quality attained to date. In view of the relatively small amount of research work dedicated to the molten salt electrolytic preparation of carbon nanotubes from graphite, it has been considered that the full potential of this method may not as yet have been reached. Thus far, sodium chloride is the molten salt electrolyte that has been examined most thoroughly even though it possesses a rather limited temperature window for process optimisation. The melting point of sodium chloride of 801°C invariably determines the lower limit, and above the boiling point of sodium metal of 883°C the yield of the reaction decreases dramatically [14]. In contrast, there is no in-depth study on lithium chloride as the electrolyte even though it offers a much wider applicable temperature range from 605 to beyond 1300°C. The present article now provides the first systematic study into the electrolysis of lithium chloride in the presence of a graphite cathode with the aim of producing nano-structured carbon materials with high percentages of nanotubes. Parameters of interest have been the type of graphite, the reaction temperature, the electrochemical polarisation regime, and the reaction time. Product
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composition as a function of process parameters was assessed by means of electron microscopy.
2. Experimental The electrolysis experiments were performed in a tubular stainless steel reactor, which was positioned vertically inside a programmable electrical furnace. The upper end of the reactor was closed with a stainless steel plate, which provided feedthroughs for the electrode leads and a thermocouple as well as steel pipes for gas circulation. The interior of the reactor was continuously purged with argon that was dried over self-indicating calcium sulphate. A graphite crucible with inner height of 13 cm and inner diameter of 5.5 cm (EC4, Tokai Carbon UK) was used to contain the molten salt electrolyte. The crucible was filled with approximately 200 g of nominally anhydrous lithium chloride (213233, Aldrich), and was heated in an argon atmosphere and with a ramp rate of 120°C/h to the desired target temperature. In the electrolysis experiments, a graphite rod of 6.5 mm in diameter, and of specification detailed below, was used as the cathode, and the graphite crucible was employed as the anode. The surface area of the graphite rod cathode facing the molten salt electrolyte was 2.4 cm2. This was achieved by a tubular alumina sheath that covered tightly the upper part of the rod and only exposed the lower part to the melt. The graphite rod rested in the centre of an alumina tile, which was placed on the base of the graphite crucible and avoided short circuit between the electrodes. The rod was slightly tilted in order to ensure salt access to the bottom surface. Both the graphite rod cathode and the graphite crucible anode were contacted with nickel wire of 2 mm in diameter and connected to the electrochemical equipment. In some experiments, a quasi-reference electrode was employed. This consisted of a molybdenum wire of 0.5 mm in diameter positioned inside an open-ended alumina sheath. The correct functioning of this type of quasi-reference electrode in lithium chloride melts had been demonstrated in earlier work [15,16]. Electrolysis experiments were performed with either a two-terminal cell or a three-terminal cell. In the two-terminal mode, a constant current was imposed between the graphite rod cathode and the graphite crucible anode. The current was provided by a manually-operated auto-ranging power supply (LS 30-10, Wayne Kerr or AP 20-80, Farnell), and the voltage recorded as a function of time. In the three-terminal mode, a constant polarisation potential was applied to the graphite rod cathode versus the molybdenum quasi-reference, with the graphite crucible serving as the anode. The same equipment was used, and the current monitored with time. Each experiment was commenced approximately 30 min after reaching target temperature. Fig. 1 presents a schematic of the experimental set-up. Four different types of electro-carb graphite materials (EC4, EC5, EC15, and EC17, Tokai Carbon UK) were employed as the cathode. Table 1 compiles quantitative information on density and grain size, as provided by the supplier. It is seen that the four types of graphite possess considerably different properties. EC4 graphite is of relatively low density and composed of relatively large grains; EC17 graphite is of high density and consists of small grains; EC5 and EC15 graphites have intermediate properties. Figs. 2 and 3 show scanning electron microscopic images of EC4 and EC17 graphites, respectively. The micrographs confirm the difference in grain size and reveal the flake-type shape of the grains. The reaction temperatures ranged from 625 to 1200°C, with increments of 25, 50 or 100°C. The constant currents in two-terminal experiments were varied between 1.2 and 12.0 A, with increments of 1.2 A, corresponding to cathodic current densities of 0.5
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Fig. 1. Schematic of the experimental set-up. Two-terminal cell: the graphite rod is used as the cathode and the graphite crucible is used as the anode. Three-terminal cell: the same polarity is applied as in the two-terminal cell and the molybdenum wire is used as the quasi-reference electrode.
Fig. 2. Scanning electron microscopic image of a fracture surface of EC4 graphite, showing comparatively large flake-shaped graphite grains.
Table 1 Properties of the electro-carb graphite materials used as the cathode in the electrolysis experiments; data as provided by the supplier. Material
Density [g/cm3]
Average grain size [lm]
Maximum grain size [lm]
EC4 EC5 EC15 EC17
1.75 1.85 1.83 1.85
13 11 7 2
100 100 25 10
Fig. 3. Scanning electron microscopic image of a fracture surface of EC17 graphite, showing comparatively small flake-shaped graphite grains.
to 5.0 A/cm2 when referred to the initial geometric surface of the graphite rod cathode contacting the molten salt electrolyte. The constant polarisation potentials in three-terminal experiments were varied between -2.0 and -3.0 V versus the quasi-reference, and were chosen so as to establish appropriate cathodic current densities. The reaction times were between 1 and 40 min, with 10 min being used in the majority of cases. In total, more than 70 experiments were completed, and in particularly relevant instances two or three experiments were carried out under nominally identical conditions. Upon termination of an electrolysis experiment, the furnace was switched off and the electrolytic cell was allowed to cool.
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The solidified salt was removed with tap water, and the carbonaceous reaction product was collected by way of filtration with a glass frit in a mild vacuum. The retrieved material was rinsed with copious amounts of distilled water until no chloride ions were detected in the filtrate when tested with silver nitrate solution, and then dried in ambient air at around 150°C. This procedure was found to be much simpler and faster than the previously reported ones relying on the use of organic solvents. Each carbonaceous product was investigated by means of scanning electron microscopy using a high-resolution field emission gun system (JSM-6340F, JEOL). The composition of each sample was assessed through visual inspection of numerous micrographs. It is acknowledged that an objective and quantitative analysis of micrographs is difficult to achieve in this manner but, owing to the lack of alternatives, this approach is commonplace and has been used in all foregoing studies. It is believed that the quantitative comparison between the samples of this study is fairly reliable, whilst it is supposed that comparisons with the results from other studies are less accurate. Selected samples were additionally examined by way of transmission electron microscopy (200CX, JEOL).
3. Results 3.1. Orientational experiments An initial set of experiments was conducted in order to assess the appropriateness of the various types of graphite, EC4, EC5, EC15 and EC17, with respect to the generation of nano-structured carbonaceous products when employed as the cathode in the electrolysis of molten lithium chloride. The experiments were carried out in the two-terminal cell under current control, applying reaction temperatures of 700, 800 and 900°C, cathodic current densities of 2.0 and 3.0 A/cm2, and reaction times of 10 min. The carbonaceous products were retrieved, washed and dried, and then subjected to scanning electron microscopic analysis. The initial experiments demonstrated that the reaction of lithium ions at the graphite cathode generates carbonaceous products that can comprise up to four main types of constituents. Three of these constituents are nano-structured and may be classified as particles, fibres and tubes; the fourth constituent is macroscopic
Fig. 4. Scanning electron microscopic image of a carbonaceous product that contains all of the typical constituents, i.e., nano-particles, nano-fibres, nanotubes and macroscopic pieces, in the given area. Experimental conditions: cathode EC4 graphite; reaction temperature 800°C; initial cathodic current density 2.0 A/cm2; reaction time 10 min.
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pieces. Fig. 4 presents a scanning electron microscopic image of a sample that displays all of these constituents within a relatively small area. Nano-particles are characterised by their fairly equiaxed shapes and facetted surfaces. Nano-fibres are considerably bent and sometimes kinked and display rather abrupt changes in contrast along their lengths. Chains of particles and fibres are often difficult to differentiate. Nanotubes are identified by their smooth curvature and the more gradual change in contrast. A differentiation of tubes and fibres is also possible on the basis that tubes, unlike fibres, may be filled with metals like tin, as will be demonstrated elsewhere [29]. The final constituent seen on the image is pieces of relatively large size and with sharp edges. Comparison with the micrographs of the graphite feed material indicates that these are unreacted graphite grains. A more detailed microscopic analysis will follow in a later chapter. Composition of the carbonaceous product in terms of quantity and ratio of the above constituents was found to depend critically upon the type of graphite employed as the cathode, and was very reproducible for nominally identical processing conditions. When using EC17 graphite, the overwhelming nano-structured constituent in the carbonaceous product was particles, whilst constituents of high aspect ratio were vastly absent, and a considerable amount of large graphite pieces was also found. This result was rather independent of temperature and current density. When using EC4, EC5 and EC15 graphites, the carbonaceous products contained all of the four constituents mentioned above. However, for EC5 and EC15 graphites, only very small contents of nanotubes of less than 5% were observed in each case. By contrast, for EC4 graphite, product composition was very strongly dependent on the experimental conditions, and contents of tubes of up to 20% were present in some of the samples. This suggested EC4 graphite as the most promising candidate material for further investigation. 3.2. Optimisation experiments Prompted by the results gained in the orientational studies, a more systematic set of experiments was conducted with lithium chloride as the molten salt electrolyte and EC4 graphite as the cathode. Experiments were performed in the two-terminal cell under current control, and reaction temperature, cathodic current density and reaction time were varied, with the aim of maximising the content of nanotubes in the carbonaceous product. The first process parameter to be investigated was reaction temperature, and this was varied between 625 and 1200°C. Cathodic current density was 2.0 A/cm2; and reaction time was 10 min. The experiments revealed that reaction temperature is of paramount importance. A maximum content of nanotubes in the carbonaceous product of around 50% was found at the temperature of 775°C, the remainder was mainly nano-particles and large graphite pieces. When increasing the temperature, the amount of tubes decreased rapidly; at 800°C the content dropped to around 20%, and at 850°C it fell to close to 0%. Under these conditions nano-particles were the prevailing product, and this remained unchanged when raising temperature further. When decreasing the temperature, the amount of tubes again decreased; at 750°C the content dropped to around 15%, and at 700°C it was at around 5%. Under these conditions, nano-particles were again the main product. Upon lowering the temperature further, a moderate rise in the content of tubes would seem to occur at temperatures of around and below 650°C. However, these results were difficult to obtain and reproduce, as experimenting with a molten salt only marginally above the melting point is inherently intricate. In order to verify that the optimum temperature for nanotube formation is indeed at 775°C, additional experiments were carried out at five and ten degrees above and below this value, but no noticeable improvement was achieved.
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Cathodic current density was investigated by varying values between 0.5 and 5.0 A/cm2. Reaction temperature was 775°C, reaction time was 10 min at 2.0 A/cm2, and was adjusted for all other current densities such that the total charge passed was 2880 C in each case. The experiments showed that the maximum content of nanotubes in the carbonaceous product occurs at 2.0 A/cm2. This maximum seems to be relatively shallow, as current densities of 1.5 and 2.5 A/cm2 led to materials of similar appearance albeit somewhat reduced nanotube contents. At low current densities of 1.0 and 0.5 A/cm2, little product was formed and nano-particles and very irregularly shaped nano-fibres were the main constituents. At high current densities of 3.0 A/cm2 and beyond, the amount of large graphite pieces increased steeply and finally these became the prevailing constituent. Reaction time was examined by varying values between 1 and 20 min. Reaction temperature was 775°C, and cathodic current density was 2.0 A/cm2. For a short reaction time of 1 min, only very little carbonaceous product was formed. As reaction time was prolonged, the amount of product increased. Up to a reaction time of around 4 min, the content of nanotubes in the carbonaceous product remained at a level of approximately 80%. On extending reaction time further, the total amount of product continued to rise, but the fraction of tubes declined slowly while that of large graphite pieces grew. Under the given conditions, reactions typically came to an end after around 15 to 16 min of polarisation when the graphite material protruding the alumina sheath had been consumed completely. Following the optimisation of process parameters in the twoterminal cell, further experiments were conducted in the three-terminal cell. In these, the potential of the graphite rod cathode was varied between -2.0 and -3.0 V against the molybdenum quasi-reference electrode. Reaction temperature was 775°C, and reaction times were between 4 and 10 min. Examination of the carbonaceous products revealed that the optimum polarisation potential for the formation of nanotubes is at -2.5 V. This is in full agreement with the results obtained with the two-terminal cell, because the cathodic current density established in the three-terminal cell at -2.5 V was very close to 2.0 A/cm2, which had been identified as the optimum cathodic current density. Time-dependent experiments performed under optimised conditions in the three-terminal cell demonstrated that nanotube contents as high as 80% may be maintained for reaction times up to around 6 min. Table 2 compiles the process parameters that were found to be most appropriate for the preparation of carbon nanotubes within the range of conditions explored.
of the currents applied were selected such that the current densities at the graphite rod cathode at the beginning of a run were between 0.5 and 5.0 A/cm2. Corresponding currents ranged from 1.2 to 12.0 A, and the voltages necessary to sustain these currents were between 2.3 and 11.7 V, depending on temperature and current. Accordingly, the overall resistance of the cell was on the order of 1 to 2 X, and included contributions from the potential drops across the electrolyte, the electrodes, the graphite components and the nickel leads. Fig. 5 presents several voltage versus time curves that were recorded in experiments performed with EC4 graphite rod cathodes at the temperature of 775°C and initial cathodic current densities between 1.0 and 3.0 A/cm2. The voltages remain fairly constant throughout the major part of each run and, for high applied currents, increase in the final stage. Electrolysis experiments in the three-terminal cell were performed under the condition of constant polarisation potential. The magnitudes of the polarisation potentials applied were between -2.0 and -3.0 V versus the molybdenum quasi-reference. The resulting currents and current densities were of the same order of magnitude as those in the two-terminal runs. Fig. 6 presents a current versus time curve that was monitored during an experiment performed with an EC4 graphite rod cathode at 775°C and a polarisation potential of -2.5 V. After an initial peak, the current remains fairly constant throughout the first 5 min of the run. The current then becomes noticeably noisier throughout the remainder of the run. Fig. 7 compares the macroscopic appearances of graphite rod cathodes before and after electrolysis. Processing was carried out in the two-terminal cell with EC4 graphite at 775°C, a cathodic current density of 2.0 A/cm2 and different polarisation times. The sample that was exposed to the molten salt in the absence of cathodic polarisation underwent no visual changes. The sample that was polarised for 4 min shows that part of the graphite has been eroded and that erosion has occurred quite smoothly and uniformly. The sample polarised for 10 min displays a comparatively rough surface and significant macroscopic cracks. After 16 min of polarisation the graphite tip exposed to the molten salt had been completely consumed. The image shows typical cases, whilst the precise extent of erosion was a function of the process parameters
3.3. Electrolysis curves and anode erosion Electrolysis experiments in the two-terminal cell were performed under the condition of constant current. The magnitudes Table 2 Optimum process parameters for the preparation of carbon nanotubes from graphite by way of the electrolytic method using molten lithium chloride, as identified within the range of conditions investigated in the present study. Process parameter
Optimum value
Microstructure of graphite: Density Average grain size
1.75 g/cm3 13 lm
Reaction temperature
775°C
Polarisation regime: Cathodic current density Cathodic polarisation potential
2.0 A/cm2 2.5 V vs Mo
Reaction time
4–6 min
Fig. 5. Voltage versus time curves of electrolysis experiments conducted in the two-terminal cell. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current densities between 1.0 and 3.0 A/cm2; reaction times between 20 and 6.67 min.
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Fig. 8. Scanning electron microscopic image of a carbonaceous product that contains nano-particles as the main constituent. Experimental conditions: cathode EC4 graphite; reaction temperature 700°C; initial cathodic current density 2.0 A/ cm2; reaction time 10 min. Fig. 6. Current versus time curve of an electrolysis experiment conducted in the three-terminal cell. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; cathodic polarisation potential -2.5 V versus molybdenum; reaction time 10 min.
Fig. 7. Optical images of graphite cathodes before and after electrolysis experiments. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 2.0 A/cm2; reaction times (a) 0 min, (b) 4 min, (c) 10 min, (d) 16 min.
applied. Expectedly, the amount of carbonaceous product generated in an electrolysis experiment depended on the degree of erosion, and varied between a few milligrams and several hundreds of milligrams. 3.4. Microscopic characterisation of the carbonaceous products The carbonaceous products synthesised in the various electrolysis experiments were found to consist of a number of carbonbased constituents, namely, nano-particles, nano-fibres, nanotubes and macroscopic pieces of unreacted graphite, and their quantities and ratios were dependent on the experimental conditions applied. In the following, a representative selection of electron microscopic images is given. All images show typical features of products that were obtained from EC4 graphite under different experimental conditions. Fig. 8 presents a scanning electron microscopic image of nanoparticles. The particles possess a fairly uniform size distribution with diameters between 100 and 200 nm. As discussed, nano-particles are the prevailing reaction product under the vast majority of experimental conditions. Fig. 9a and b presents micrographs of predominantly nanotubes. The tubes are of varying diameter, with an average of around 40 to 60 nm and a maximum of around 150 nm. This order of magnitude compares well with the results from
Fig. 9. Scanning electron microscopic images of a carbonaceous product that contains nanotubes as the main constituent. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 2.0 A/cm2; reaction time 4 min. (a) and (b) show different areas of the same sample.
the majority of the foregoing studies using lithium chloride as the molten salt electrolyte [9,13,14,20]. The average length of the tubes is difficult to determine because it is almost always impossible to locate on the micrographs the positions of both ends of the same tube. This suggests that the tubes are in fact rather long,
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Fig. 10. Transmission electron microscopic image of a carbonaceous product that contains nanotubes as the main constituent. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; cathodic polarisation potential -2.5 V versus molybdenum; reaction time 6 min.
probably in excess of several micrometres. Fig. 10 is a transmission electron microscopic image showing a small section of a sample of predominantly tubes. Both the smooth surfaces and the hollow interiors are clearly discernible. As stated earlier, nanotubes are the desired product but dominate only within a very small window of processing conditions. Fig. 11 presents a scanning electron microscopic image of a mixture of nano-particles and large graphite pieces, and Fig. 12 shows a micrograph of a mixture of nanotubes and graphite pieces. Graphite fragments are undesired because they render the carbonaceous product heterogeneous in composition and lower the yield of nano-structured material. The most notable result of the present study is that, after careful optimisation of all relevant process parameters, it has become possible to synthesise carbonaceous products that contain a large percentage of nanotubes. Fig. 13 presents a low magnification scanning electron microscopic image of one such product. The micrograph shows agglomerates that consist virtually exclusively of tubes and are many micrometres wide. These results are unprecedented for the molten salt electrolytic method of carbon nanotube production.
Fig. 11. Scanning electron microscopic image of a carbonaceous product that contains nano-particles and large graphite pieces in the given area. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 3.0 A/cm2; reaction time 10 min.
Fig. 12. Scanning electron microscopic image of a carbonaceous product that contains nanotubes and large graphite pieces in the given area. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; initial cathodic current density 3.0 A/cm2; reaction time 10 min.
4. Discussion The molten salt electrolytic method for the production of carbon nanotubes from graphite is a conversion process that comprises a complex sequence of reaction steps. As mentioned, it is widely believed that the cathodic process involves the intercalation of lithium from the molten salt electrolyte into the graphite electrode. The lithium is inserted into the spaces between the individual graphene layers where it may form intercalation compounds, and exerts mechanical stress on the host lattice. This leads to disintegration of the graphite structure such that individual graphene layers or stacks thereof become detached. These then rearrange chemically to form nanotubes, or other nano-structured entities, which assemble in the molten salt. The anodic process is the formation of chlorine gas. The onset potential of this reaction is significantly lower than the thermodynamic decomposition potential of lithium chloride. This is a consequence of the chemical interaction between lithium and carbon during intercalation which allows the lithium to be reduced at activities below unity [15,16]. þ
Cathodic half cell reaction : Li þ e þ xC ¼ LiCx 1 Anodic half cell reaction : Cl ¼ Cl2 þ e 2
ð1Þ ð2Þ
Fig. 13. Scanning electron microscopic image of a carbonaceous product that contains large agglomerates of nanotubes. Experimental conditions: cathode EC4 graphite; reaction temperature 775°C; cathodic polarisation potential -2.5 V versus molybdenum; reaction time 6 min.
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The exact cathodic reaction mechanism is unknown. One possibility is exfoliation, i.e., a mechanism in which the graphene layers peel off of suitably orientated graphite surfaces once a sufficient quantity of lithium has been incorporated into the grain boundary. The other possibility is extrusion, i.e., a mechanism in which graphene layers are sheared out of graphite grains as lithium is being incorporated. It is evident that a rigorous determination of the reaction mechanism would be difficult, if not impossible. Consequently, in the present study, the optimisation of the system has been performed in an empirical manner by assessing a wide range of experimental conditions. Nevertheless, it will be attempted in the following to rationalise the impact of the key process parameters. 4.1. Density and grain size of graphite cathode In the experimental programme, graphite of type EC4 has been identified as the only feed material capable of yielding a carbonaceous product with nanotubes as the majority constituent. By contrast, EC17 graphite could be converted only into nano-particles and also provided the highest amount of large graphite pieces in the product. EC5 and EC15 graphites showed intermediate behaviours. Notably, EC4 graphite possesses a much lower density than the other three types of graphite, and is relatively soft and flexible. It is therefore reasonable to assume that EC4 graphite is capable of accommodating relatively large quantities of lithium before undergoing disintegration, and that this is beneficial for the controlled break-down of the graphite structure and the formation of nanotubes. Furthermore, EC4 graphite consists of relatively large grains, whereas EC17 graphite possesses the smallest grains of the three materials with high density. It may hence be assumed that large grain size is likewise favourable for nanotube formation. 4.2. Reaction temperature The reason for the critical importance of reaction temperature with respect to the formation of nanotubes may be twofold. Firstly, the intercalation of lithium into the graphite structure and the concomitant release of graphene sheets from the graphite grains take place under the influence of lattice vibrations; and optimum lattice dynamics may only occur within a narrow temperature range. Secondly, the overall reaction pathway from graphite to nano-structured entities is highly complex and consists of several steps with potentially different activation energies; and the particular reaction sequence leading to tubes may only proceed within a small temperature interval. 4.3. Cathodic current density The occurrence of an optimum current density for nanotube formation implies the existence of an optimum intercalation rate of lithium into graphite. It is assumed that a very low intercalation rate allows the lithium to diffuse far into the graphite bulk without building up the optimum concentration for nanotube formation, and that very high intercalation rates cause excessive lithium accumulation in the surface-near region provoking damage of the graphene layers as well as macroscopic fracturing of the graphite body and release of large graphite fragments. 4.4. Reaction time Optimum reaction times were identified as those allowing the graphite to be eroded to a significant extent while maintaining a smooth and uniform erosion pattern. Short reaction times do not yield a significant amount of nano-structured carbonaceous product, and it is assumed that the degree of intercalation is insufficient
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to instigate substantial disintegration of the graphite. Long reaction times entail a deterioration of the carbonaceous product, especially because of the rising content of large graphite pieces, and it is supposed that this is a consequence of the creation of uneven surfaces on the eroding graphite cathode that facilitate detachment of graphite fragments. In the experiments performed in the two-terminal cell under current control, it has been observed that the applied voltage necessary to drive the pre-determined current remains fairly constant over a few minutes. This suggests that the decrease in macroscopic surface area, caused by advancing erosion, is approximately balanced by the increase in microscopic surface area, originating from enhanced surface roughness. This is an important finding because otherwise experiments under current control would not be meaningful. However, this type of process control is invariably compromised at long reaction times when substantial changes of the surface area are inevitable. It is noted that a small fraction of experiments carried out in the two-terminal cell were terminated early because of a quite abrupt stoppage of the cell reactions. In such a case, even the maximum output voltage of the potentiostat was insufficient to maintain the current, which essentially caused shut-down of the cell. This was observed occasionally when experiments were extended significantly beyond 10 min and only rarely for shorter runs. It is supposed that this may be due to the blockage of one of the electrodes. One possibility would be the formation of a continuous and badly wetting film of lithium metal around the cathode; the other would be the anode effect caused by the build-up of an insulating and well-adherent film of chlorine gas bubbles at the anode. 4.5. Type of polarisation Polarisation experiments performed in the three-terminal cell were found to provide carbonaceous products with high contents of carbon nanotubes for somewhat longer reaction times than experiments carried out in the two-terminal cell. This demonstrates that cathode potential control is superior to current control. The reason is thought to be that the optimum polarisation potential in the three-terminal cell may be maintained throughout the entire run, whereas the optimum current density in a two-terminal cell cannot be sustained constantly. However, the progressive release of large graphite pieces toward the end of a run was still found to occur. 5. Conclusions An extensive experimental study has been performed into the preparation of nano-structured carbon materials by way of electrolysis of molten lithium chloride with graphite electrodes. The combined application of lithium chloride as the molten salt electrolyte and graphite of type EC4 as the cathode has been identified as the key prerequisite for the synthesis of carbonaceous products with high contents of carbon nanotubes. Hitherto, sodium chloride has been considered an equally suitable electrolyte, and the properties of the graphite feed material had not been recognised as relevant at all. A detailed investigation of the system of lithium chloride and EC4 graphite has provided an optimised set of process parameters for carbon nanotube formation. The optimum reaction temperature is at around 775°C, with deviations from this value of as little as 25°C ensuing a massive decline in the nanotube content. All earlier studies on lithium chloride failed to identify the paramount importance of temperature. Cathodic current density should be at around 2.0 A/cm2; corresponding to a cathode potential of around -2.5 V versus a molybdenum quasi-reference. Optimum
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reaction time is between 4 and 6 min. Application of these conditions has rendered possible the synthesis of carbonaceous products with nanotube contents as high as 80%. It is presently endeavoured to implement improved process control and maintain carbon nanotube production under optimum conditions over extended reaction times [30]. Overall, the present study has led to a dramatic improvement of the molten salt electrolytic method of producing carbon nanotubes. Future work will focus on a variety of aspects. The separation of the nano-structured components from residual salt and unreacted graphite needs to be optimised. In this context, the suitability of a recently developed extraction technique using ethyl acetate [31] will be ascertained. The performance of the nanotubes prepared will be evaluated in chemical and structural applications. Because of the comparatively low formation temperature, it is surmised that the nanotubes possess a rather large density of defects. This may be beneficial in chemical applications that involve bonding and transport processes, but may be adverse to structural applications that demand very good mechanical properties. Acknowledgements The authors gratefully acknowledge financial support of this study by the Department of Materials Science and Metallurgy of the University of Cambridge through the J.E.O. Mayne Fund. ATD wishes to thank The Royal Society for a Visiting Scientist Fellowship. References [1] Th.W. Ebbesen (Ed.), Carbon nanotubes: Preparation and Properties, CRC Press, Boca Raton, FL, 1997. [2] P.J.F. Harris, Carbon Nanotubes and Related Structures, Cambridge University Press, 1999. [3] M.S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.), Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Springer, Berlin, 2001. [4] S. Reich, C. Thomsen, J. Maultzsch, Carbon Nanotubes: Basic Concepts and Physical Properties, Wiley-VCH, Weinheim, 2004.
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