A feasibility study of scaling-up the electrolytic production of carbonnanotubes in molten salts

Aleksandar T. Dimitrov 1, George Z. Chen *, Ian A. Kinloch, Derek J. Fray *

Department of Material Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK

Received 18 July 2002; received in revised form 28 August 2002

Abstract

The feasibility of scaling-up the electrolytic production of carbon nanotubes in molten salts has been investigated with the aid of

electron microscopy (TEM and SEM). Using molten LiCl as the electrolyte and commercial graphite as both cathode and anode

materials, carbon nanomaterials, including nanotubes, were prepared by constant voltage electrolysis. The cell was more than 20

times as large as that used in previous work. The nanotube concentration in the final product increased with cell voltage (including

iR drop) from 1 vol.% at 4.0 V to 35 vol.% at 8.4 V. Under desired conditions, the charge and energy consumption for the cathode

erosion was 0.28 Ah/g and 4.1 Wh/g, of which 60�/70 wt.% were for producing nanomaterials (nanotubes: �/30 vol.%). When

adding 1 wt.% SnCl2 to the electrolyte, partial and fully filled nanotubes were obtained with the nanomaterials containing up to 20

wt.% Sn. Preliminary results from applying the product as the electrode in lithium ion batteries are reported.

# 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Carbon nanotubes; Molten salts; Electrolysis; Scaling up; Electron microscopy

1. Introduction

Carbon nanotubes (CNTs) are strong, stiff, and

electrically conductive. They possess large aspect ratios

and surface areas, and can be either multi- or single

walled structures [1]. Practical uses of CNTs have

become more promising in recent years, such as fillers

in polymer composites, field emitters for flat panel

displays, electrodes for supercapacitors and elements

in nano-electronics [2�/4]. CNTs are typically produced

in the gas phase, by the evaporation of pure carbon

using a high-powered energy source (e.g. electric arc,

laser or solar heat) or by the catalytic decomposition of

gaseous hydrocarbons over a transition metal (e.g.

acetylene over Fe) [5�/7]. However, in 1995, Hsu et al.

discovered that CNTs could also be produced in molten

LiCl by electrolysis using high purity carbon electrodes,

presenting the first example of producing CNTs in a

condensed phase [8]. In their experiments, the carbon

cathode underwent erosion and nanometre sized pro-

ducts, including multiwalled CNTs were found in the

molten salt (electrolyte). Subsequently they conducted a

more detailed study and concluded that for CNT

production, the immersion of cathode in the electrolyte

needed to be shallow (B/3 cm), the current moderate

(B/5 A) and the voltage low (B/5 V) [9]. The discovery

of Hsu et al. stimulated further work in this laboratory

under various conditions in different electrolytes (LiCl,

NaCl and KCl) and, particularly, suitable CNT pre-

paration temperatures for each electrolyte were estab-

lished [10,11]. It was concluded that CNTs were not to

form at temperatures higher than the boiling points of

the alkali metals [10,11]. In 2000, Kaptay et al. reported

that, on graphite cathode, the electro-deposition of Li,

Na, K, Mg and Ca from the respective molten chlorides

all led to the formation of CNTs, but not of Sn and Ni

[12].

Hsu et al. [13] reported later that the electrolytic

method could also be used to make carbon coated metal

nanowires by adding a low melting temperature metal or

* Corresponding authors. Tel.: �/44-1223-762965; fax: �/44-122-

3334567

E-mail addresses: gzc20@hermes.cam.ac.uk (G.Z. Chen),

djf25@hermes.cam.ac.uk (D.J. Fray).1 Academic Visitor from the Faculty of Technology and

Metallurgy, University ‘St.Cyril and Methodius’, 16 Rudjer

Boskovic, 1000 Skopje, R Macedonia.

Electrochimica Acta 48 (2002) 91�/102

www.elsevier.com/locate/electacta

0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 5 9 5 - 9

salt in small concentrations to the electrolyte. For

example, SnCl2 added to LiCl resulted in the production

of nanotubes filled with Sn [13] and this is an advantage

of the electrolytic method over some of the otherproduction methods since the filled nanotubes are

produced in situ.

While all results from this and other laboratories

suggest that the electrochemical formation of CNTs in

molten salts is related with the erosion of the cathode,

such behaviour of a carbon cathode is, as was first

pointed out by Fray [10,14], common in the Hall-

Heroult cell where the intercalation of alkali metals,particularly sodium, into the carbon cathode is known

to have caused cathode expansion, cracking and erosion.

Based on the facts that CNTs were collectable from the

solidified salt phase and also the CNT yield depended

strongly on electrolysis current and temperature [8�/12],

it was further proposed [11,15] that the alkali metal ion

(M�) intercalates, under the influence of a sufficiently

negative electrode potential, into the graphite latticewhere it is reduced in situ. The alkali metal then expands

the lattice and more strain is put into the lattice as the

amount of metal increases, until the lattice fragments.

The carbon based fragments may then enter the molten

salt and, without the protection of the graphite lattice,

undergo through an inter- and/or intra-fragment re-

combination process, leading to the formation of

various carbon nanoparticles and nanotubes in theelectrolyte [11,12]. Alternatively, as it was initially

postulated in this laboratory, the intercalation of alkali

metal could lead to the extrusion of carbon. Instead of

forming CNTs in electrolyte via the re-combination of

carbon fragments, the outcome of extrusion may be the

direct formation of CNTs or their pre-cursors on the

surface of the cathode. Interestingly, this prediction had

not been experimentally confirmed in this and otherlaboratories [8�/14] until recently when Kaptay et al.

reported their observation of carbon micro-tubes on the

surface of a graphite cathode [16]. Nevertheless, the two

proposed post-intercalation processes can be considered

essentially the same if the extruded CNTs or their pre-

cursors can fall into and grow further in the electrolyte

or the inter- and/or intra-fragment re-combination may

occur on the cathode’s surface.As reported in literature, the majority of researchers

performed constant current electrolysis in their prepara-

tions of CNTs in molten salts and, even when voltage

control was used, results were mainly expressed in terms

of current [9]. However, in constant current electrolysis,

because of erosion, the surface area of cathode changes

with time and so does the current density. This is

obviously undesirable as the electrode reaction can besignificantly altered by current density variation, mak-

ing it difficult to correlate electrochemical parameters

with the products. In a two-electrode cell, a relatively

constant cathode potential, and hence the current

density, can be achieved if a constant cell voltage is

applied between the cathode and an anode of much

larger surface area. Following this approach, constant

voltage electrolysis was carried out in molten NaCl in

this laboratory and the results exhibited a maximum

CNT yield against cell voltages that was varied between

3 and 9 V [15]. This observation supports strongly the

intercalation mechanism in which the speed of alkali

metal intercalation, which can be correlated with the

electrode potential (and hence the current density), plays

a key role in determining the composition and structure

of fragmented or extruded products [11,15].

It should be pointed out that, up till now, all reports

on the formation of CNTs in molten salts have

described electrolysis in relatively small cells. In addi-

tion, the electrolysis times given in these reports are

usually short, which is due to firstly the complete

erosion of the cathode and secondly the saturation of

the salt by electrolysis products, particularly the alkali

or alkaline earth metal. Therefore, it would be both

scientifically interesting and practically important to

investigate the electrochemical method at increased

volume and time scales. In this work, a relatively large

two-electrode cell, as shown in Figs. 1 and 2, was used

for electrolysis. To compensate cathode erosion, step-

by-step feeding of the cathode into the electrolyte was

also applied during electrolysis. In addition, instead of

current, the cell voltage was kept constant in each

experiment and correlated with the CNT concentration

in the produced carbon nanomaterials that was esti-

mated from electron microscopic analysis. The reported

result of producing tin filled CNTs by adding SnCl2 to

the electrolyte [13] was reproduced at an increased scale.

Furthermore, the extraction of the produced carbonac-

eous materials from the solidified salt phase was

investigated. Finally, the preliminary results of using

the extracted carbonaceous product, containing about

Fig. 1. A photograph graphite showing the crucible (left, large) used in

this work and that (right small) used previously by Chen et al. [10,11].

Each scale on the ruler on the far right is 1 cm.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/10292

30 vol.% CNTs, as the lithium intercalation electrode in

lithium ion batteries are reported.

2. Experimental

Electrolysis was carried out in a cylindrical graphite

cell (external: 15.0 cm height and 9.0 cm diameter;

internal: 14.0 cm height and 7.0 cm diameter, Graphite

Technologies, high density grade) which also acted as

the anode during electrolysis. This cell had a volume

significantly larger than those used previously in thislaboratory [10,11] (see Fig. 1). A graphite rod (6.5 mm

diameter, Graphite Technologies, EC4 grade, density:

1.75 g/cm3) was used as the cathode and was fed, step-

by-step, into the electrolyte after electrolytic consump-

tion. Molten LiCl or LiCl�/SnCl2 (99:1 by weight, both

from Aldrich, ACS grade) was used as the electrolyte at

700 8C (Table 1). As was observed previously, the

electrolysis continuously produced alkali metal (Li)that could float on the surface of the molten salt,

providing a short circuit between anode and cathode. To

prevent this, the cathode was surrounded with an

alumina sheath (external and internal diameters: 1.15

and 0.8 cm) whose lower end was placed about 1.5 cm

underneath the surface of the molten salt. The cell and

electrode arrangement is illustrated in Fig. 2. The initial

depth of the molten salt was greater than 5.0 cm, and thecathode’s exposed length (outside the alumina sheath)

was 1.0 cm, giving rising to a surface area ratio of

anode-to-cathode to be greater than 60.

The graphite cell was placed in a sealable Inconel tube

reactor held in a vertical 1600 8C Lenton furnace. The

experimental set-up was described in detail previously

[10,11] and is also schematically shown in Fig. 2. Thesalt was always thermally pre-dried inside the Inconel

tube at 250 8C, for at least 2 h and then melted under an

argon flow (100 cm3/min). The electrolysis was con-

trolled by a Farnell LS30-10 auto ranging Power supply

and carried out at constant voltages (4.0�/8.4 V). The

cathode was inserted as the electrolysis progressed to

replace the cathode that had been eroded. The cathode

insertion length was the same as the initial one and thetime of electrolysis was the same as the complete erosion

time that was determined separately at different vol-

tages. The number of cathode insertions was up to 20,

equivalent to a total cathode length of 20 cm or 11.6 g of

graphite being consumed (experiment 8, Table 1). In

addition, due to the increase of cell volume, the amount

of the electrolyte (LiCl) used in this work was up to 250

g, which is more than 20 times as much as that used inprevious work [11]. However, it should be noted that,

because the cell also functioned as the anode, the use of

a larger cell increased the cathode-to-anode distance

from less than 1.0 cm in previous work [10,11] to over

3.0 cm, leading to greater iR drops across the electrolyte

(voltage losses due to the electrolyte resistance).

After having progressed for a predetermined period,

counted by the number of cathode insertions, theelectrolysis was terminated. The cathode was then

removed from the reactor and the electrolyte left to

solidify overnight under a slow flow of argon (B/10 cm3/

min). The carbonaceous materials produced were har-

vested, in principle, using the same methodology as

previously reported [8�/11], i.e. (i) dissolving the solidi-

fied electrolyte in water; and (ii) extracting the carbo-

naceous materials by adding toluene to the aqueousmixture, resulting in the carbonaceous materials aggre-

gating at the water�/toluene interface. In this work,

however, additional washing steps were applied between

the dissolution and toluene extraction, aiming at remov-

ing the electrolyte and related impurities that might have

been trapped in the carbonaceous materials.

Complete dissolution of the carbon containing soli-

dified electrolyte (100�/250 g) was achieved within anhour by one or two additions of 250�/300 ml water to

and mechanical stirring inside the cell (the solubility of

LiCl in water is 81 and 107 g at 25 and 80 8C,

respectively). The black aqueous mixture in the cell

was poured into a large beaker to which more water

(500�/2000 ml) was added. The solution was further

stirred for another hour, and then left to stand still

overnight, giving time for the carbonaceous material tofully precipitate. Afterwards, the clear top solution was

decanted. This washing procedure might be repeated

once or twice. To improve the washing efficiency, in

some cases, hot water (80 8C) was used and mechanical

Fig. 2. A schematic diagram of the experimental set-up for molten salt

electrolysis.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/102 93

stirring was carried out while placing the beaker in an

ultrasonic bath.

After washing and decanting most of the water, a

small amount of the carbonaceous material was dried

and analysed directly and the rest was transferred into a

large separation flask (1.0 l) containing mixed water and

toluene (10:1, vol.), followed by thorough shaking. Theflask was then allowed to stand still for phase separa-

tion. As reported previously, after phase separation, the

carbonaceous materials were found to aggregate at the

water/toluene and toluene/glass wall interfaces [10].

Once separated, the water phase was drained. The

carbonaceous materials and toluene were transferred

into a storage bottle (with lid). The toluene could be re-

collected by the conventional evaporation method, andre-used in the next extraction. However, for most

samples, some toluene was kept in the storage bottle

with the carbonaceous materials. This was because it

was much easier to simply shake the bottle to tempora-

rily and uniformly suspend the carbonaceous materials

and take a small amount of the suspension for further

analysis, e.g. by electron microscopy which was per-

formed on JEOL 200CX (TEM, 200 kV) and JEOL6340F (SEM, 5 to 20 kV, capable of energy dispersive

X-ray analysis). The CNT concentration in the collected

carbonaceous materials was estimated from SEM

images of relatively low magnifications [8�/11].

Some of the samples were sent to AEA Technologies

and made into the lithium intercalation electrodes whose

capacity and rechargeablity were examined in a testing

lithium ion battery by the standard method developedby AEA Technologies.

3. Results and discussion

3.1. Electrolysis

In previous work, electrolytic preparation of CNTs in

molten salts was conducted using naked graphite rod

cathodes and constant current [8�/13,16]. Consequently,

due to its lower density than the corresponding molten

chloride salt, the electrolytically formed alkali or alka-

line earth metal floated on the electrolyte’s surface and

provided a short circuit between the anode and cathode.

While such a problem might not be very serious for

batch preparation, it will certainly prevent further or

continuous operations from using the same molten salt.

In this work, in order to avoid the floating alkali metal

(Li) shorting the electrodes, it was decided to use an

alumina sheath to surround the upper portion of the

cathode in contact with the electrolyte, as shown in Fig.

2. In addition, as discussed in Section 1, constant

voltage electrolysis was applied. Because such experi-

mental arrangements have never been applied in pre-

vious work, it is worth reporting firstly the cell

performance in the batch operation mode.

In this work, all electrolysis experiments were con-

ducted at 700 8C which is in the optimal temperature

range for CNT preparation in molten LiCl as deter-

mined in previous work [10,11]. Fig. 3 plots the currents

measured at different times and voltages when electro-

lysing a fresh bath of pure molten LiCl for the first

cathode insertion (Fig. 3a), and that at 7.6 V for a

number of consecutive insertions (Fig. 3b). For the first

insertion (Fig. 3a), apart from the strong dependence of

the current on the applied cell voltage, the plots show

clearly two processes: the current drops relatively

rapidly in a short period soon after applying the voltage

and then declines slowly with time, either continuously

at low voltages (Fig. 3a) or passing through one or more

peaks at high voltages (Fig. 3b). It is worth mentioning

that, when a high voltage (�/7.0 V) was applied,

intermittent cracking noises were heard from the reactor

during electrolysis. Simultaneously, the current was seen

to rise and fall on the display of the power supply. In

some cases, the rise and fall of the current were as sharp

as a spike (not shown in Fig. 3 which were recorded

manually) and, in others, took a relatively longer time,

forming the broad current peaks as shown in Fig. 3b. On

Table 1

Experimental conditions

Experiment

number

Electrolytea Cell voltage

(V)

Initial current density

(A/cm2)

Number of 1.0 cm

insertions

CNT concentration in product

95 vol.%

Composition Mass (g)

1 LiCl 100 4.0 1.0 6 1

2 LiCl 100 5.2 1.5 6 5

3 LiCl 100 6.5 2.0 6 10

4 LiCl 100 7.6 2.5 6 25

5 LiCl 100 8.4 3.0 6 35

6 LiCl�1%SnCl2 100 8.4 3.0 6 25

7 LiCl 250 8.4 3.0 15 35

8 LiCl�1%SnCl2 250 8.4 3.0 20 25

a Electrolysis was carried out at 700 8C under argon (100 cm3/min).

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/10294

the basis of a previous report of a cracked graphite

cathode resulted from electrolysis in NaCl [11], it is

believed that the noises came from the cracking of the

cathode caused by fast intercalation and in situ reduc-

tion of Li� ion. Apparently, if the cracking was small

and quick, leading to the detachment of broken pieces, a

current spike could form; if the cracking was large and

slow, allowing the broken pieces to hang onto the

cathode for a moment before falling into the electrolyte,

the effect would then be a temporary increase in the

surface area of the cathode and hence the relatively slow

current rise and fall.

When terminating the electrolysis at the end of the

initial current drop and removing the cathode from the

reactor, it was observed that most of the cathode

underneath the alumina sheath disappeared, particularly

in high voltage experiments. After the initial fast erosion

stage, however, it took a much longer time for the small

remaining portion of the cathode to erode completely,

which is reflected by the slowly declining section of the

current�/time plots in Fig. 3a. This phenomenon may be

further explained as follow. The decrease of current with

time is generally associated with the decrease of the

cathode’s surface area due to erosion. According to the

intercalation mechanism, the erosion results from the

fast intercalation of Li� ion into and its reduction inside

the graphite lattice [11]. What has not been mentioned in

the Li� ion intercalation mechanism is the highly likely

simultaneous formation of liquid Li metal at thecathode. The formed liquid Li metal may have two

negative effects on the intercalation process. It reduces

the current efficiency for the erosion of the cathode and

more importantly, if accumulated quickly, covers up the

surface of the cathode, preventing further intercalation

of Li� ion. At the early stage of electrolysis, the formed

Li metal can be removed from the cathode surface via

dissolution into the molten salt. Also, the erosion of thecathode, particularly via cracking, not only disturbs the

accumulation of Li metal, but also allows the renewal of

the cathode’s surface. However, when the electrolyte

near the cathode becomes saturated, Li metal accumu-

lates and gradually covers up the cathode’s surface. The

fact that, in this work, the remaining portion of the

cathode had a surface facing downward could have also

helped to entrap the Li metal. The gradual cover up ofthe cathode surface by Li metal slows the overall

intercalation of Li� ion and hence the speed of erosion,

leading to the slowly declining current.

The increase of current with increasing cell voltage,

see Fig. 3a, is not unusual. However, the explanation

may be slightly more complicated, considering all the

possible processes at the cathode. The first contribution

comes, as expected, from the potential determined speedof electron transfer for the reduction of Li� ion to the

metal, either on the surface or within the lattice of the

graphite cathode. Secondly, it has been theoretically and

experimentally confirmed, in a separate programme of

this laboratory focusing on the mechanism of lithium

intercalation into graphite [17], that the speed of Li�

ion intercalation also increases with the cathodic shift of

the electrode potential. Thirdly, the formed Li metal,either dissolved in or floated on the surface of the

electrolyte, cannot only undergo re-oxidation at the

anode but also provide paths for electronic conduction,

both shall reduce the current efficiency. Apparently, in

terms of improving current efficiency for CNT prepara-

tion, the formation of Li metal on the surface of the

cathode is a negative factor but unfortunately inevitable

due to the very negative potential required to enable fastLi� ion intercalation.

3.2. Feeding cathode

For scaling up the electrolytic production of CNTs, in

addition to an enlarged reactor (electrolytic cell), it

would be more desirable if the production may be

conducted continuously. This should then include con-

tinuous feed of reactants into and removal of productsfrom the reactor. In this work, the reactants were the

cathode and molten LiCl and the products included

carbonaceous materials and liquid lithium metal from

Fig. 3. Current�/time plots of electrolysis in molten LiCl at 700 8Cunder argon using a graphite rod cathode and a graphite crucible

anode: (a) at different constant voltages for the first cathode insertion

in fresh electrolyte; and (b) at 7.6 V for consecutive insertions in the

same electrolyte. Cathode diameter, 0.65 cm; insertion depth, 1.0 cm;

anode surface area, 150 cm2; anode-to-cathode separation, �/3.2 cm.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/102 95

the cathode, and chlorine gas from the anode. Except

for chlorine gas that escapes from the reactor naturally,

continuous removal of the other two products requires

sophisticated mechanism and devices for handling the

corrosive high temperature molten materials. Obviously,

the experimental set-up used in this work was not

suitable for continuous removal of the molten materials.

While ongoing efforts are being made in this direction,

as a first attempt, this work focused on feeding the

cathode.Because the aim of feeding the cathode is to compen-

sate the loss of the cathode due to erosion, it is necessary

to first determine the erosion rate of the cathode. As

discussed in previous reports [8�/11], the cathode erosion

rate can be influenced by a number of parameters,

including geometry of cathode, cathode insertion depth,

melt type and composition, working temperature and

most importantly current density (cathode potential). In

this work, the insertion depth and cell voltage were

varied but all the other parameters were fixed (see

Section 2). It should be pointed out that although it was

hoped to maintain a relatively stable cathodic current

density by constant voltage electrolysis with a large

anode-to-cathode surface area ratio, strictly speaking,

variation of the cathode current density could still occur

to a certain degree. For example, because of the total

current varies with time due to the erosion decreasing

the cathode surface area, the iR drop of the cell would

decrease. Therefore, the actual cathode potential would

also become more negative with time, leading to an

increase in the potential driven current density. Never-

theless, the influence of cell voltage on the erosion rate

could still be, although to a lesser quantitative degree,

investigated by visual inspection and comparison of the

cathodes after different times of electrolysis. Fig. 4 plots

the cell voltage or the initial cathode current density

against the electrolysis time when complete cathode

erosion was achieved. Interestingly, it can be seen that

the complete cathode erosion time decreases almost

linearly with increasing the cell voltage or the initial

current density. This inverse dependence of erosion rate

on voltage or initial current density is in fact expected

according to the intercalation mechanism that predicts

the speed of Li� ion intercalation and hence the cathodeerosion to increase with increasing the cathode current

density [11].

Based on the data in Fig. 3a and Fig. 4, the charge

and energy consumption per gram of eroded cathode

(density 1.75 g/cm3) were evaluated and found to

undergo through a maximum at an intermediate voltage

(6.5 V). At the high voltage end (8.4 V), the consump-

tion was 0.28 Ah/g for charge and 4.1 Wh/g for energy.As will be discussed later, it was found that, on average,

about 60�/70% of the eroded graphite were converted

into carbon nanomaterials of which 25�/35% were

CNTs. If the same charge consumption is used to reduce

the Li� ion (disregarding the product being Li metal or

Li2C2 or other intercalation compounds) with 90�/100%

current efficiency, it can be calculated that the number

of reduced Li� ions is about a quarter of and the sameas that of the carbon atoms in the carbon nanomaterials

and CNTs, respectively. Interestingly, these findings

may be compared favourably with the intercalation

mechanism proposed in Ref. [11] in which it was

assumed that the formation of CNTs resulted from the

decomposition of the metastable species MiCx that were

generated by fast intercalation and in situ reduction of

M� (the alkali or alkaline earth metal ion). It wasfurther assumed that the values of i and x in MiCx ,

correlated as x ]/i ]/2, were crucial for CNT formation.

It should be pointed out that although simple atomic

ratios between electrolytically reduced lithium ions and

produced carbon nanomaterials (Li:C�/1:4) or CNTs

(Li:C�/1:1) are unprecedentedly reported in literature,

such correlations may not be very surprising considering

the following facts. First of all, these findings haveresulted from experiments in which electrolysis was

conducted in fresh molten LiCl and terminated before

too much accumulation of lithium metal in and above

the electrolyte, and also the alumina sheath functioned

effectively to further reduce electronic conduction

through lithium metal that were accumulated in the

upper region of the electrolyte. Secondly, recent chron-

oamperometric studies in molten LiCl [17] revealed thatthe current response of a graphite rod electrode to a

potential step in the range of 0.8�/�/1.8 V vs. Li�/Li was

dominated by the contribution from lithium intercala-

tion into graphite. Finally, Hsu et al. [18] reported that

the thermal decomposition of the metal carbide, Al4C3,

indeed led to the formation of graphite fragments and

also CNTs if a transition metal catalyst was present.

Having determined the electrochemical parameters inrelevance with cathode erosion, particularly the com-

plete cathode erosion time, experiments were then

carried out to feed the cathode into the electrolyte via

step-by-step manual insertion. In these experiments, the

Fig. 4. Plots of the complete cathode erosion time against the cell

voltage and initial current density of electrolysis. Experimental

conditions were the same as in Fig. 3a.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/10296

interval between consecutive insertions was determined

according to Fig. 4. For example, the interval was 2.5

min at 8.4 V and 5 min at 7.6 V. During the electrolysis

after each insertion, it was observed that, considering

experimental error (e.g. the inaccuracy in insertion

depth resulting from manual operation), the current

always started at the same high values and ended at the

same low values, although in some cases the current

decreased continuously and in other cases passed

through one or more peaks. Typical current�/time plots

recorded in one of these experiments are presented in

Fig. 3b. The insertion depth was kept at 1.0 cm in most

experiments. This is because firstly the result was more

repeatable at this insertion depth, and secondly, at 2.0 or

3.0 cm, much larger portions of the cathode cracked off

at the early stage of electrolysis. Also, with further

increasing the insertion depth, the cathode surface area

increased, resulting in, at high voltages, greater currents

than the limit of the power supply.

It can be seen in Fig. 3b that, except for the peaks, the

currents recorded after each insertion are very similar to

each other, demonstrating the step-by-step cathode

insertion to be a feasible method for scaling up the

electrolytic production of CNTs. After some systematic

trials with feeding the cathode (Table 1, Experiments 1�/

6), it was found that the CNT concentration in the

product increased with the cell voltage (see Table 1 and

Fig. 5) until the power consumption of the cell reached

the limit of the power supply. At the highest voltage

achieved (8.4 V), the CNT concentration increased to

30�/40 vol.%. More importantly, the concentration of

CNTs in the product was found to be fairly constant

when the insertion number increased from 6 to 20,

which are equivalent to 3.5�/11.6 g of graphite (density

1.75 g/cm3) being consumed.

On the basis of previous work, if the anode and

cathode were both naked, one would have expected that,

under constant voltage and with carrying out each

insertion and electrolysis, the electrolyte must have

been saturated very quickly with Li metal which then

floated to the upper portion of the electrolyte. There-

fore, the initially decreasing current would soon re-

increase with time due to electronic conduction via the

floated Li metal. This is clearly not the case for thecurrent�/time plots in Fig. 3, demonstrating the effec-

tiveness of using the alumina sheath to reduce the

influence of floated Li metal.

It should be mentioned that, in this work, in order to

achieve the same current density as reported previously

[8�/11], the applied voltage must be higher. For example,

the average current density observed at 5.2 V in

Experiment 2 was about 1.3 A/cm2 which is less thanthat observed at 5 V by Hsu et al. [9]. This discrepancy

could be attributed to the relatively large cell resistance

(see Section 2). Therefore, the real potential differences

between the anode and cathode in this work must have

been lower than the applied voltages, meaning that the

plot shown in Fig. 5 might show a maximum if the

voltage could have been increased further [11,15,17].

3.3. Adding SnCl2to the electrolyte

It was first reported by Hsu et al. [13] that adding a

small amount of SnCl2 into molten LiCl resulted in the

electrolytic production of CNTs that were individually

filled with tin metal, demonstrating the advantage of the

electrolytic CNT production method over other CNT

production methods in terms of functionalising intern-

ally CNTs with foreign materials. (Hsu et al. viewedthese tin filled CNTs as tin nanowires enclosed by a

carbon layer.) Therefore, it would be interesting and

desirable to repeat the electrolytic production of tin

filled CNTs at enlarged scales. This was carried out

successfully as described in Table 1 as Experiment 8.

The experimental conditions were the same as that of

Experiment 7 although more cathode insertions were

applied. It was noticed that although Sn2� ions can bereduced at much more positive potentials than that for

Li� ion reduction, very similar current�/time plots were

recorded from Experiments 7 and 8. The CNT concen-

trations in the final products from the two experiments

were not very different either. However, high resolution

SEM images of the CNTs from Experiment 8 revealed

clearly the inclusion of the heavier element, Sn, in the

CNT cavities and EDX analysis at a lower magnifica-tion confirmed the product contained about 20 wt.% tin

(see Section 3.5).

3.4. Product processing

The chloride salts of alkali and alkaline earth elements

have relatively large solubility in water. Following the

suggestion of a previous report [11], the solidified LiCl,100�/250 g, contained in the cell was effectively dissolved

into water by one or two additions of 250�/300 ml water

to the cell and stirring mechanically within an hour (it

Fig. 5. Dependence of CNT concentration in the extracted and

washed final products on cell voltage. Experimental conditions are

given in Table 1.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/102 97

would take days for the salt to dissolve in still water).

After dissolution, it was found that there were often

relatively large graphite pieces at the bottom of the

crucible. These large pieces, apparently resulting fromthe cracking of the cathode during electrolysis, were

more often observed in experiments of higher cell

voltages. However, the reproducibility of these larger

pieces was poor, particularly in terms of total mass, size

and shape. In one experiment, the whole bottom section

(intact cylinder, 0.5 cm average length) of the cathode

was collected. These large pieces were manually picked

out and weighed after further leaching in water for atleast 24 h and drying. The total mass of the large pieces

was higher for experiments of higher cell voltages but

was generally less than 30 wt.% of the total amount of

eroded cathode.

The carbonaceous materials in the black aqueous

suspension from the cell could be directly extracted by

adding toluene [8�/11]. However, under the scanning

electron microscope, it was found that such collectedsamples contained too much electrolyte and related

impurities. Apparently upon drying, the impurities ‘re-

crystallised’ into very fine structures on the nanometre

scales, as shown in Fig. 6. Unexpectedly, these fine

structures, although suspected to contain LiCl and Li2O,

did not cause much charging problems under the

electron beam, making it difficult to find and examine

the carbon nanomaterials. It was therefore decided toadd washing steps before using toluene for extraction. A

general description of the washing procedure is given in

Section 2. The effectiveness of different combinations of

the washing parameters was compared, including total

volume of water (up to 4 l), water temperature (25 and

80 8C), number of repetitive washings (up to eight

times), and with or without using the ultrasonic bath. It

was found that mechanical stirring, with hot water andsonication, was more effective for removing the LiCl salt

and related impurities from the carbonaceous materials.

After washing, the carbonaceous materials were

found to still contain some micrometer sized or larger

particles. However, very little of these micro-particles

were observed in samples that were obtained after

sonication and toluene extraction, suggesting the latter

to be useful steps for separating nanotubes and nano-particles from the relatively larger particles in the

carbonaceous materials. The mass ratio of the product

(nanomaterials after washing and toluene extraction)

over all carbon materials collected from the salt could be

greater than 60 wt.%. In particular, the total masses of

toluene extracted carbonaceous materials in Experi-

ments 7 and 8 reached about 6 and 8 g (both were 69

wt.% of the total mass of eroded cathode), respectively.In a couple of cases, the carbonaceous materials were

dried directly inside the storage bottle, to remove the

solvent (mainly toluene and water), by heating the bottle

to 80 8C and kept at this temperature overnight. It was

then observed that the carbonaceous materials con-

tracted into a sponge-like cylinder whose diameter was

about two-thirds of that of the bottle. The cylinder

could withstand light touch but crashed into smaller

sponge-like pieces and powder when pressed harder.This phenomenon suggests that there existed some kind

of weak interaction between the electrolytic CNTs and

nanoparticles.

3.5. Electron microscopic analyses

In this work, electron microscopy, together with

energy dispersive X-ray analysis, was used mainly for

two purposes: identifying and evaluating different con-stituents and the concentration of CNTs in the products

as described in previous reports [8�/11]. In general, the

products obtained from the solidified electrolyte, by

Fig. 6. Typical SEM images of: (a) CNTs and nanoparticles on top of

re-dried electrolyte; and (b) the nanostructure formed by the re-dried

electrolyte. Note that the fine structure in (b) is also visible in the

background of (a). The two images were taken from the same sample

but located at different regions of the sample holder. The sample was

obtained from solidified LiCl via cold water dissolution and toluene

extraction. No additional washing was applied to the sample.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/10298

dissolution in cold water and extraction with toluene,

were found to contain too many electrolyte related

impurities and some micrometer sized carbon particles,

see Figs. 6 and 7a, that made the microscopic observa-

tion of CNTs difficult. In particular, the impurities were

found, under SEM, to form unique nanostructures that

could be easily confused with CNTs, see Fig. 6b. EDX

analysis revealed that, as shown in Table 2, these fine

structures were composed of mostly oxygen, chlorine

and carbon (lithium was outside the range of detectable

elements). Therefore, these fine structures might have

resulted from not only LiCl, but also other compounds,

e.g. Li2O or Li2CO3 or some carboxylic groups in the

fine structures. Nevertheless, after the washing steps

described in Section 2, no or little re-crystallised

electrolyte and less micrometer sized materials were

observed in the extracted products. The micrometer

particles are believed to have been removed by sonica-

tion (breaking up some relatively weak micrometer sized

structures) and toluene separation (microparticles were

too heavy to be suspended at the water�/toluene inter-

face).

The morphologies of the properly washed products

obtained from pure LiCl were very similar to those

reported previously [8�/11]. Typically, the observed

nanotubes were curved and multiwalled (Figs. 7�/9)

and their sizes ranged between 10 and 100 mm in length

and 20�/40 nm in diameter. In addition to nanotubes,

the other main constituent in the product was aggre-

gated carbon nanoparticles, as seen in Fig. 7b and Fig.

8a. Under TEM, many of these tubes and particles were

found to contain a foreign material. For nanotubes, the

foreign material was more often seen at the tips and the

bent portions, however, in a few cases, much greater

portions of the nanotubes were filled (Fig. 8b). These

fillings could be either lithium or lithium carbide as

suggested previously [8�/11].

Fig. 9a and b show respectively the low and high

magnification SEM images of the final products and the

CNTs obtained from Experiment 8 in which 1 wt.%

SnCl2 was added to the LiCl melt. The low magnifica-

tion image (Fig. 9a) shows very similar microscopic

features as those products obtained from a pure LiCl

melt. However, as shown in Table 2, EDX analysis in

the same area of Fig. 9a revealed that the product

contained about 20 wt.% tin. In addition, a significant

amount of oxygen was detected and suspected to have a

number of origins, e.g. the carboxylic groups on the

surface of the carbon nanoparticles and nanotubes, the

re-oxidation products of metals (Li and Sn) that was not

enclosed in the carbon structures, and water molecules

that were trapped by hydrogen bonds in between the

carbon particles and nanotubes. It is also notable that

the sample contained very little chlorine, demonstrating

the LiCl salt was effectively removed during the washing

procedure. At a higher magnification, the SEM image

(Fig. 9b) certainly gives more accurate information.

Unlike those CNTs obtained from a pure LiCl melt, all

the CNTs shown in Fig. 9b have a brighter core filling

up completely or most of the central cavity of the CNTs.

While the brightness of the central core in comparison

with the dark wall indicates that the core contained a

Fig. 7. Typical SEM images of: (a) a sample obtained after the

washing procedure, showing both micrometer (labelled with ‘m’) and

nanometre sized carbon particles; and (b) the enlarged image of the

boxed region in (a) showing aggregated carbon nanoparticles with a

few CNTs.

Table 2

Typical results of EDX analyses of samples similar to those in Fig. 6b

and Fig. 9a

Elements Fig. 6b sample (95 wt.%)a Fig. 9a sample (95 wt.%)a

C 9 47

Cl 41 Under detection limit

O 50 31

Sn N/A 22

a Errors were estimated from at least three analyses of different but

similar areas, and found to be different for different elements, but

usually higher for lighter elements.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/102 99

significantly heavier (or high atomic number) element

than the wall, EDX analysis further confirmed that the

nanotubes were indeed composed of tin and carbon. It is

unusual that such contrast shown in Fig. 9b can be

directly detected by the secondary electron beam, but

the observation may be understandable considering that

the carbon walls of the CNTs were of only a fewnanometers in thickness.

3.6. Preliminary tests in lithium ion batteries

Some of the final products (washed, containing 30

vol.% CNTs) from this work were tested as the negativeelectrode in a testing lithium ion battery by AEA

Technologies. The obtained results, via galvanostatic

charge�/discharge, showed that the material had an

initial charge�/discharge capacity about 15% higher

than the standard carbon used by AEA Technologies,

but the recycling stability of this high capacity was not

as satisfactory. Incidentally, promising results of apply-

ing multiwalled CNTs in lithium ion batteries wererecently reported by Mukhophdhyay et al. [19], parti-

cularly showing the intercalation of lithium in the CNT

electrode to be highly reversible. The intercalation

mechanism in the CNT electrode was confirmed to

involve ‘staging transition’, usually observed during Li

intercalation into graphite electrodes. Therefore, con-

sidering that the electrolytic CNTs can be prepared

considerably cheaper at an enlarged scale, it is worth-while to continue the research in using this material for

lithium ion battery application. One of the future tasks

in this research is to develop an effective method to

separate the CNTs from the other materials, and it is

believed that once a purer electrolytic material becomes

available, better understanding and hence more satisfac-

tory results are foreseeable.

4. Conclusions

In summary, the feasibility of scaling up the electro-

lytic production of CNTs in molten LiCl has been

investigated and successfully demonstrated. Four major

changes were introduced in this work. (1) The reactor

(cell) volume (538.51 cm3) was more than 20 times aslarge as that (25.12 cm3) used in previous work in this

laboratory [11]. This change increased significantly the

reactor’s capacity for inputting reactants or raw materi-

als (molten LiCl and graphite cathode, tested to 250 and

11.6 g, respectively) and accommodating products

(carbon nanomaterials, tested to 8 g, and unreacted Li

metal). (2) Instead of constant current electrolysis as in

most previously reported cases, constant voltage elec-trolysis was employed in this work with the cell voltage

being varied between 4.0 and 8.4 V. To complement this

change, the anode-to-cathode surface area ratio was

maintained over 60 so that the cathode potential, and

hence the cathode current density, could remain rela-

tively stable during electrolysis in which the cathode was

continuously eroded. (3) The electrolytic method was

tested under a semi-continuous operation mode byreplacing the eroded cathode via a simple step-by-step

cathode insertion mechanism during electrolysis. The

insertion was guided through an alumina sheath (tube)

Fig. 8. TEM images of: (a) aggregated CNTs and nanoparticles; and (b) partially filled CNTs produced in molten LiCl. Electrolysis conditions are

the same as Experiment 7 given in Table 1.

A.T. Dimitrov et al. / Electrochimica Acta 48 (2002) 91�/102100

that not only made the insertion convenient but also

protected the cathode from shorting with the anode

when large quantity of Li metal formed and floated in

the upper portion of the molten LiCl. The insertion

frequency was determined by the complete cathode

erosion time that was determined separately for every

cell voltage. (4) In addition to using water to dissolve,

under stirring, the solidified electrolyte that contained

the carbonaceous product, a washing procedure, com-

bining water leaching under stirring and the use of hot

water (80 8C) and an ultrasonic bath, was developed for

improving the quality of the final product.

With these changes, previously reported electrolytic

productions of CNTs in pure molten LiCl [9,11] and tin

filled CNTs in molten LiCl containing 1 wt.% SnCl2 [13]

were successfully repeated, except for that the cell

voltages were notably higher, which is a common

phenomenon when scaling up an electrolytic method

and reasonably attributable to the increase of the anode-

to-cathode separation. In connection with this, the CNT

concentration in the final product was found to increase

with cell voltage (including iR drop) from 1 vol.% at 4.0V to 35 vol.% at 8.4 V, although it is expected that the

CNT production will decrease at higher voltages. The

charge and energy consumptions per gram of graphite

eroded from the cathode were calculated from the

electrolysis data and found to pass through maximums

at 6.5 V in the voltage range of 4.0�/8.4 V (including iR

drop), and to be 0.28 Ah/g and 4.1 Wh/g at 8.4 V. At

this voltage, of the total mass of the eroded graphitecathode, 60�/70 wt.%, as determined by weighing and

confirmed by SEM, were the desired carbon nanoma-

terials in which the CNT concentration was between 25

and 35 vol.% (estimated from SEM analysis). Finally,

some of the electrolytically produced carbon nanoma-

terials, containing about 30 vol.% CNTs, were tested as

the electrode in a lithium ion battery, showing a charge�/

discharge capacity that was 15% higher than thestandard carbon material, but the charge�/discharge

cycle life needs to be improved.

Acknowledgements

The authors gratefully acknowledge the financial

support from AEA Technologies. GZC thanks Darwin

College Cambridge for the 2000 Schlumberger Inter-

disciplinary Research Fellowship.

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