Carbon Nanotubes seminar PDF

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final seminar report

“Carbon Nanotubes Produced from Ambient CO2 for Li-ion Battery Anode”....

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Table of Contents

Chapter 1 : Carbon Nanotubes

1

Chapter 2 : Synthesis of Carbon Nanotubes

9

Chapter 3 : Carbon Nanotubes in Li-ion Batteries

14

Chapter 4 : Conclusion

20

Chapter 5 : References

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Chapter 1: Carbon Nanotubes

0

A carbon nanotube is a tube-shaped material, made of carbon, having a diameter measuring on the nanometer scale. A nanometer is one-billionth of a meter, or about 10,000 times smaller than a human hair. CNT are unique because the bonding between the atoms is very strong and the tubes can have extreme aspect ratios. A carbon nanotube can be as thin as a few nanometers yet be as long as hundreds of microns. To put this into perspective, if your hair had the same aspect ratio, a single strand would be over 40 meters long. Carbon nanotubes have many structures, differing in length, thickness, and number of layers. The characteristics of nanotubes can be different depending on how the graphene sheet has rolled up to form the tube causing it to act either metallic or as a semiconductor. There are many different types of carbon nanotubes, but they are normally categorized as either single-walled (SWNT) or multi-walled nanotubes (MWNT). A single-walled carbon nanotube is just like a regular straw. It has only one layer, or wall. Multiwalled carbon nanotubes are a collection of nested tubes of continuously increasing diameters. They can range from one outer and one inner tube (a double-walled nanotube) to as many as 100 tubes (walls) or more. Each tube is held at a certain distance from either of its neighboring tubes by interatomic forces.

Fig. Carbon Nanotubes

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Properties of Carbon Nanotubes

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Carbon nanotubes have a range of electric, thermal, and structural properties that can change based on the physical design of the nanotube.

Electrical properties •

If the nanotube structure is armchair then the electrical properties are metallic

•

If the nanotube structure is chiral then the electrical properties can be either semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor

•

Metallic nanotubes can carry an electrical current density of 4×109 A/cm2 which is more than 1,000 times greater than metals such as copper

Fig. Different types of SWCNTs

Thermal Properties 3

•

All nanotubes are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis.

•

It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per Kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 watts per meter per K.

•

The temperature stability of carbon nanotubes is estimated to be up to 2800oC in vacuum and about 750oC in air.

Structural Properities The structural design has a direct effect on the nanotube’s electrical properties. When n − m is a multiple of 3, then the nanotube is described as "metallic" (highly conducting), otherwise the nanotube is a semiconductor. The Armchair design is always metallic while other designs can make the nanotube a semiconductor. Material

Young's Modulus

Tensile Strength

Elongation at

(TPa)

(GPa)

Break (%)

E

SWNT

~1 (from 1 to 5)

13-53

16

Armchair SWNT

0.94T

126.2T

23.1

Zigzag SWNT

0.94T

94.5T

15.6-17.5

Chiral SWNT

0.92

MWNT

0.8-0.9E

150

Stainless Steel

~0.2

~0.65-1

15-50

Kevlar

~0.15

~3.5

~2

KevlarT

0.25

29.6

Types of Carbon Nanotubes Single-walled carbon nanotube structure

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Single-walled carbon nanotubes can be formed in three different designs: Armchair, Chiral, and Zigzag. The design depends on the way the graphene is wrapped into a cylinder. For example, imagine rolling a sheet of paper from its corner, which can be considered one design, and a different design can be formed by rolling the paper from its edge. A single-walled nanotube’s structure is represented by a pair of indices (n,m) called the chiral vector. (Shown Below)

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Multi-walled carbon nanotube structure

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There are two structural models of multi-walled nanotubes. In the Russian Doll model, a carbon nanotube contains another nanotube inside it (the inner nanotube has a smaller diameter than the outer nanotube). In the Parchment model, a single graphene sheet is rolled around itself multiple times, resembling a rolled up scroll of paper. Multi-walled carbon nanotubes have similar properties to single walled nanotubes, yet the outer walls on multi-walled nanotubes can protect the inner carbon nanotubes from chemical interactions with outside materials. Multi-walled nanotubes also have a higher tensile strength than single-walled nanotubes.

Fig. Multi-Walled Carbon Nanotube Structure

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Application areas of Carbon Nanotubes • Field Emitters/Emission • Molecular electronics: CNT based non volatile RAM • CNT based transistors • Energy Storage • CNT based fibers and fabrics • CNT based ceramics • Biomedical applications

Today we will be focusing on Lithium Intercalation 

The basic principle of rechargeable lithium batteries is electrochemical intercalation and deintercalation of lithium in both electrodes.



An ideal battery has a high-energy capacity, fast charging time and a long cycle time.



The capacity is determined by the lithium saturation concentration of the electrode materials.



For Li, this is the highest in nanotubes if all the interstitial sites (inter-shell van der Waals spaces, inter-tube channels and inner cores) are accessible for Li intercalation.



SWNTs have shown to possess both highly reversible and irreversible capacities. Because of the large observed voltage hysteresis.



Li-intercalation in nanotubes is still unsuitable for battery application. This feature can potentially be reduced or eliminated by processing, i.e. cutting, the nanotubes to short segments

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Health Hazards •

According to scientists at the National Institute of Standards and Technology, carbon nanotubes shorter than about 200 nanometers readily enter into human lung cells similar to the way asbestos does, and may pose an increased risk to health.

•

Carbon nanotubes along with the majority of nanotechnology, are an unexplored matter, and many of the possible health hazards are still unknown.

Chapter 2: Synthesis of Carbon Nanotubes 9

Traditional Methods : •

Arc discharge

•

Laser ablation

•

Chemical vapor deposition (CVD)

Comparison of traditional methods

Method

How

Arc discharge

Chemical

Laser ablation

method

vapour deposition

(vaporization)

Connect two graphite

Place substrate in oven,

Blast graphite with intense

o

rods to a power

heat to 600 C, and slowly

laser pulses; use the laser

supply, place them a

add a carbon-bearing gas

pulses rather than

few millimetres

such as methane. As gas

electricity to generate

apart, and throw the

decomposes it frees up

carbon gas from which the

switch. At 100 amps,

carbon atoms, which

NTs form; try various

carbon vaporises and

recombine in the form of

conditions until hit on one

forms a hot plasma.

NTs

that produces prodigious amounts of SWNTs

Typical

30 to 90%

20 to 100 %

Up to 70%

Short tubes with

Long tubes with diameters

Long bundles of tubes (5-

diameters of 0.6 - 1.4

ranging from 0.6-4 nm

20 microns), with

yield

SWNT

individual diameter from

nm

1-2 nm.

Method

M-WNT

Arc discharge

Chemical

Laser ablation

method

vapour deposition

(vaporization)

Short tubes with

Long tubes with diameter

Not very much interest in

inner diameter of 1-3

ranging from 10-240 nm

this technique, as it is too 10

nm and outer

expensive, but MWNT

diameter of

synthesis is possible.

approximately 10 nm

Merits

Can easily produce

Easiest to scale up to

Primarily SWNTs, with

SWNT, MWNTs.

industrial production; long

good diameter control and

SWNTs have few

length, simple process,

few defects. The reaction

structural defects;

SWNT diameter

product is quite pure.

MWNTs without

controllable, quite pure

catalyst, not too expensive, open air synthesis possible

Demerit s

Tubes tend to be

NTs are usually MWNTs

Costly technique, because

short with random

and often riddled with

it requires expensive lasers

sizes and directions;

defects

and high power

often needs a lot of

requirement, but is

purification

improving

A New Approach: Solar Carbon Capture Process

Solar Thermal Electrochemical Photo (STEP) Carbon capture Process is a method to capture and remove atmospheric carbon dioxide, using both the visible part of sunlight and the thermal energy of the sunlight. Traditional solar cells/panels convert only up to 40% of the sun’s thermal 11

heat to solar energy. The remaining 60% of the sunlight is untapped as it relates to the production of solar energy. This technique converts sunlight into electricity at 39% efficiency. STEP uses an electrolysis cell consisting of molten lithium carbonate (Li2CO3) as the electrolyte. Using the thermal energy of the sunlight, the cell is heated to a temperature above the melting point of lithium carbonate. Atmospheric carbon dioxide is then bubbled through the cell. The CO2 reacts with the lithium carbonate, and depending on the reaction temperature attained, either solid carbon is deposited at the cathode or carbon monoxide is produced.

Fig. Molten Lithium Carbonate Electrolysis System

This conversion of carbon dioxide into solid carbon is facilitated by the visible rays of the sun that drive the reaction, when the visible rays are converted to electricity through photovoltaic techniques. Thus, STEP uses both the visible part of the sunlight and the thermal characteristic of it to capture and split atmospheric carbon dioxide into solidified carbon, at temperatures below

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900ºC, or carbon monoxide, at temperatures above 900ºC, which may be constructively used for a variety of industrial applications. STEP process can be used to maximize the use of sun’s energy to form staple chemicals such as iron, bleach, aluminum, etc., and remove, convert and use carbon dioxide, a greenhouse gas to form energetic carbon rich materials and hydrogen. This process also reduces the need to cool solar cells/panels, as the deleterious heat generated from the solar cells is directed to the electrolysis reaction chamber and is also used as a secondary source of energy in the chemical conversion process.

Applications: Capturing CO2 industrially with an earlier unattained solar efficiency of 34%50%; thus using solar energy with an efficiency as high as 50% to capture and prevent release of CO2 into the atmosphere. Extracting iron from iron ores, like magnetite or hematite (Fe3O4 or Fe2O3) without releasing carbon dioxide into atmosphere. This presents a sustainable route to convert carbon dioxide into materials relevant to both gridscale and portable storage systems.

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Fig. Solar Thermal Electrochemical Photo Process

Chapter 3: Carbon Nanotubes in Li-ion Batteries

A commercial Li-ion battery technology Li-ion cell design and components : Lithium-ion batteries are composed of three parts: anode, cathode, and electrolyte. 14

The cathode, typically a lithium metal oxide, acts as the positive terminal of the battery (during discharge) and the anode, commercially composed of graphitic carbon, acts as the negative terminal. The cathode reacts according to the following half reaction: LiMO2 ↔ Li1−xMO2 + xLi+ + xe− The anode reacts according to the following half reaction: xLi+ + xe− + 6C ↔ LixC6

Fig. Schematic of lithium ion cell

While charging and discharging, Li+ ions move between the anode and cathode via the electrolyte, which is typically a lithium salt such as LiPF6 dissolved in organic solvent such as ethylene carbonate. Importantly, the electrolyte does not enable the conduction of free electrons; instead, the electrons that complete the half reaction move via an external wire.

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Commercially, the most common cathode material has been lithium cobalt oxide since its introduction by Sony in the early 1990s, due to its high energy density. Lithium manganese oxide is also common place in cathodes where higher current density is a concern. It is now possible to characterize the reactions in terms of charging and discharging. When charging, a voltage is applied across the anode and cathode that drives the half reactions in the forward (left to right) direction. Lithium ions are then formed from the lithium metal oxide in the cathode, diffuse across the electrolyte, and are finally inserted into the carbon/graphite anode. During the ion formation, the metal in the lithium metal oxide is reduced, producing a free electron to maintain charge neutrality. The electrons that are freed in the ion formation are subsequently driven across a wire that connects the two electrodes to finally provide the necessary electrons for the insertion half-reaction to take place. The voltage necessary to accomplish this is determined by the particular metal in the lithium metal oxide and by the material that the anode is composed of, as well as the electrolyte itself. When discharging, the reaction naturally tends in the reverse (right to left) direction, and the potential difference between the two electrodes is used to power devices. And so during discharge, electrons move from the anode to the cathode, positive current originates from the cathode, and so the cathode acts as the positive terminal.

General materials used as anodes: Graphite Metal and Alloy

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A New Approach : Carbon nanotubes as alternative anode materials As an allotrope of graphite, carbon nanotubes (CNTs) have been approved to be a good anode material for lithium batteries due to their 

Unique structure (one-dimensional cylindrical tubule of graphite sheet),



High conductivity (106 S m−1 at 300 K for single-walled CNTs (SWCNTs) and >105 S m−1 for multi-walled nanotubes (MWCNTs)) ,



Low density,



High rigidity (Young's modulus of the order of 1 TPa),



High tensile strength (up to 60 GPa) .

SWCNTs can have reversible capacities anywhere from 300 to 600 mAhg−1; this means it can be significantly higher than the capacity of graphite (320 mAhg−1), a widely used battery electrode material. Furthermore, mechanical and chemical treatments to the SWCNTs can further increase the reversible capacities up to 1000 mAhg−1. To enhance the charge capacity of the lithium ion batteries and to reduce the irreversible capacity, a practical route could be to synthesize hybrid composite materials with CNTs as a critical component.

Advantages carbon nanotubes have over graphite

Carbon nanotubes offer a means of raising the capacity of lithium battery significantly, without being susceptible to pulverization. Their morphology makes them uniquely suited to replace graphite as the de facto anodic material in commercial lithium ion batteries. As previously stated, desirable properties such as their high tensile strength, high conductivity, and relative inertness make CNTs good candidates for this purpose. 17

The advantages of such an approach are many. First and foremost, this enables the anode to take advantage of the high lithium capacity that metals have to offer without the problem of pulverization. This is because the highly conductive CNTs act as glue matrix for the metallic nanoparticles. When the nanoparticles suddenly alloy themselves with lithium and increase in size, the anode is able to remain structurally intact because the highly conductive CNTs act as a flexible wire mesh, allowing the metal particles to remain attached to the anode's current collector. The CNTs are then able to transport the electrons to and from the metal nanoparticles when they are alloying and dealloying. Second, the carbon nanotubes themselves are able to store any additional lithium that is not alloyed with the metallic nanoparticles.

Limitations of carbon nanotubes as anodes for lithium ion battery

Unfortunately, carbon nanotubes are a relatively recent discovery, and their production methods have yet to be refined enough for production of CNTs with desired structures such as diameters, number of layers, length, degree of defects, and electronic property, which are important factors and need to be considered for development of CNT based anodes.

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At present, another issue with carbon nanotubes is their irreversible lithium ion capacity. This is when upon the first charge, more lithium ions are inserted into the carbon nanotubes than ever come out. In effect, a fraction of the lithium ions are consumed instead of stored. Although this happens for graphitic carbon as well, the problem is more pronounced in carbon nanotubes. A related problem for CNT based anodes is the lack of a voltage plateau while the battery is discharging, as can be seen in Fig. Unlike graphitic anodes, CNT anodes typically have broad changes in voltage as the cell discharges. This can make them difficult to use in most electronics which require a stable voltage source. Moreover, this means that the increased specific capacity (often cited in units of mAhg−1) does not necessarily imply increased specific energy (J g−1). However the problems of high irreversible capacity and lack of a stable voltage as the battery discharges are both morphology dependent. In particular, CNTs decorated with metal nanoparticles and core/shell composite anodes of CNT and other materials can have much flatter discharge curves than anodes made of just CNTs.

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Fig. shows a schematic representing typical discharge curves of a graphite and CNT based anode. The CNT based anode curve does not represen...