Introduction
In recent years, world’s energy consumption has been increasing at a faster rate due to growing population, modern lifestyle and rapid industrialization1. Carbon materials provide solutions for producing, transmitting and storing energy and help in developing cheaper, cleaner and more energy-efficient technologies. Carbon with its 1s², 2s², 2px¹, 2py¹, 2pz⁰ electronic configuration can undergo sp³, sp² and sp hybridizations and forms structures like diamond, graphite, fullerenes, carbon nanotubes (CNTs), graphene etc. Taking appropriate starting material and selecting processing routes2,several modifications of carbon (carbon fibers, carbon foam, meso-carbon-micro beads, carbon-carbon composites etc.) can be made. This paper reviews energy production, transmitting and storage systems using carbon materials.
Energy Production
Non–Chargeable batteries
Electrolyte for Leclanche battery3 is composed of a mixture of carbon black, powdered graphite, manganese dioxide and an acid electrolyte. Carbon black is made by partial combustion of hydrocarbon gas (acetylene)or aromatic oils. Carbon black has lower electrical conductivity than graphite but retains more electrolytes. Thus, graphite and carbon black are used in electrolyte, depending upon high conductivity (camera flash light)and low conductivity (torch) applications. In batteries used in automobiles, carbon black along with short carbon fibers has been tried to increase electrical conductivity and maximizing surface areas.
Re-chargeable Batteries
Li-ion re-chargeable battery4, is key component of today’s portable entertainment, computing and telecommunication gadgets as it provides a renewable and clean source of energy. In Li-ion batteries, Li+ ions shuttle between guest–host type arrangements acting as cathode and anode, preventing reduction of Li+-ion to poisonous lithium metal5. Main challenge in design of these batteries is availability of suitable electrode material that should not degrade over many discharge-recharge cycles. Other requirements for use as anodic materials in Li-ion battery are high capacity, excellent reversibility and high degree of lithiation. Carbon materials6-8 (graphite, carbon fibers, meso-carbon micro-beads, pitch coke etc.) has been tested as anodic material. These carbons can be categorized in soft carbon (pitch based carbon), hard carbon (polymer based carbon) and graphite carbon. In early Li- ion batteries, pitch coke, obtained by thermal treatment of pitch at 800 to 1000°C, was used as anode, but now the use of graphite has been widely accepted due to its low cost, high lithiation capacity and high energy-density. Soft carbon although has high energy density but suffer from irreversibility. Recent work9 revealed that soft carbons could be made suitable for Li-ion batteries.
Fuel Cells
Fuel cell converts chemical energy into electrical energy in a cleaner and an energy-efficient way. In fuel cell, oxygen and hydrogen (H2) react together and produce energy and water as by- product. Fuel cell is powered by H2, which is produced either from natural gas or fossil fuels. Although, fuel cell was successfully developed way back in 1960, but its commercial application is yet to be seen. Main problem is the availability of established H2 storage material for continuous and consistent supply of H2 and scarce availability of appropriate fuel cell components.
Success of two highly developed fuel cell technologies, polymer electrolyte fuel cell (PEFC) and phosphoric acid fuel cell (PAFC), is mainly due to use of various components made of carbon-based materials (bipolar plates, catalyst- supports, gas-diffusion system, current collector plate), which are very effective. Specific requirements of carbon materials used in fuel cell may differ by their type but making right choice of carbon material for a given application is quite important.
Bipolar plates of fuel cells fabricated from graphite/ polymer facilitate electrical conductivity. Gas diffusion layer made from woven carbon cloth or carbon paper is hydrophobic and remains unaffected from water produced during reactions. In fuel cell, electro-catalyst used is platinum metal dispersed on carbon support. The use of carbon support reduces precious catalyst requirement to <0.5 mg/cm2 as compared to >2 mg/cm2, if platinum is directly applied to support without any significant impact on performance and lifetime. Current collector plate of fuel cell made of graphite minimizes gas diffusion.
Graphite helps in increasing electrical conductivity. Use of exfoliated graphite shows better electrical conductivity and performance. A membrane11 prepared from catalyst-filled CNTs and used in fuel cell for electrocatalyzing oxygen reduction and methanol oxidation has been found to be very effective.
Many automobile giants (Chevrolet, Chrysler, Ford, Honda, Toyota) are working on development of prototype fuel cell cars. Honda has launched a petroleum free fuel cell car12. Use of fuel cell for power generation in spacecraft has also been reported13.
Energy Storage
Hydrogen Storage
H2, a cleanest fuel, on combustion, produces energy and water vapours14. Energy density of H2 (38 KWh/ kg), being higher than gasoline (14 KWh/kg), makes H2 very attractive transportation fuel. Main problem associated with H2 technology is the availability of safe and practically possible H2 storage device, which could easily load and unload H2 to provide continuous and consistent supply to fuel cell.
Carbon materials have high surface area, cavities, micro-pores where H2 can be adsorbed. Since, kinetic diameter of H2 (0.4059 nm) is little higher than graphite (0.3355 nm), no H2 intercalation in graphite is reported. Activated carbon has also been tested for H2 storage but found to be ineffective due to a very small percentage of surface interacts with H2 molecules at ambient temperature and pressure. However, CNT are perennial candidate for H2 storage15. CNTs are produced by arc-discharge, laser ablation and chemical vapor deposition methods16. There are two variations of CNTs, single walled nanotubes (SWNT) and multi- walled nanotubes (MWNT). SWNT are composed of individual graphene sheets rolled into seamless hollow cylinder (diam, 1-20 nm). MWNT by contrast are made up of a few tens of cylindrical graphitic sheets. CNTs17 provide large external surface and internal hollow cavity for gas storage by chemisorption or physiosorption. H2 can be adsorbed on outer wall of CNTs by H-C bonding or inside cavity as H-H bonds. However, adsorption inside the cavity is not stable18,19. In case of MW NTs,
H2 can be adsorbed between layers of CNTs, however, presence of H2 between tubes increase radius of CNTs and make them unstable. In case of bundled CNTs, H2 can also rest in places between CNT bundles20. Research results of H2 storage in CNTs are quite diversified. H2 adsorption21 (7%) at 0.67 bar and 327°C surpasses minimum requirement of 6.5wt% recommended by US Department of Energy (DOE). Amount of H2 adsorption or desorption can be increased up to 4.2 wt% from 3.3 % at ambient temperature and pressure19. Main drawback for commercialization of CNTs based H2 storage technology is high cost of CNTs (~ 50- 600 $/g) depending upon purity.
Another promising carbon material for storing H2 is carbon nano-fibres22, which consists of stacked graphite layers (diam, 5-100 mm; length, 5-100 mm), and is capable of adsorbing H2 up to 50%wt at ambient temperature and high pressure. These carbon fibers are produced by interaction of metal–catalyst nano-particles with hydrocarbon vapor at high temperature.
Fullerene, another potential carbon material for H2 storage, is third allotrope of carbon containing only carbon atoms - unlike diamond and graphite - arranged in soccer ball shape. Most stable fullerenes are C₆₀ and C₇₀ composed of 12 pentagons, and 20 and 25 hexagons respectively. C₆₀ can form compounds like C₆₀H₂₄, C₆₀H₃₆ and C₆₀H₄₈. Further addition of H2 atoms to fullerene ruptures fullerene cage. C₆₀H₄₈ represent 6.3 wt% of H2 adsorption. Fullerene can achieve H2 adsorption up to 6 wt% at 180°C and ~25 bar23. But, desorption of H2 needs high temperatures, at least 225°C due to very high strength of C-H bond. If desorption temperature could be somehow lowered down, it can be a good material for H2 storage.
Mechanical Energy Storage
Besides H2 storage, CNTs have also been reported to be useful for mechanical energy storage. CNTs24 behave like spring on applying 25 Kbar pressure while steel looses its spring characteristics even at 20 Kbar pressure. Thus property of CNTs can be used for designing energy absorbing composite materials, as base isolation for smart structural systems.
Energy Transmission
CNTs, depending upon their chirality (armchair & zigzag) and diameter, can act as conductor or semiconductor to produce portable, high-speed robust electronic devices like flat panel displays, portable cell phones, computer chips etc. Similarly, fullerenes are promising material for making electronic textiles, molecular transistors, electromagnetic interference (EMI) shielding etc. A variety of carbon materials (CNTs, fullerenes, and carbon foam) have sp2 hybridization and have delocalized electrons moving along the molecules producing conductivity. Intermolecular versatility of CNTs is very useful for making nano-circuits for electronic devices. Incorporation of pentagon and heptagon among hexagonal network develops kinks in SWNTs that create different conduction environment for electrons movement resulting nanotubes wires that behave like conductor on one side while semi-conductor on other side25.
Nano-Electronic Devices
One of the interesting applications of CNTs in electronics is field emitter for making flat panel displays. Korean electronic giant ‘SAMSUNG’ made world’s first prototype of flat panel display containing CNTs 26. This display works even at a very low voltage (<1V/μm) and uses a bundle of CNTs mounted on a support. When electric field is applied on CNTs, a strong electric field is developed and electrons start emitting through free end of carbon tubes, because of their sharpness. Such devices are very useful for making flat-panel displays and are brighter than cathode rays tube (CRT) based displays. A CNT based data storage device has been developed and shows fast writing and reading speeds27. Carbon Nano Electronics Company28 (CNCL) has developed a conductive textile made of surface grafting of C60-conjugate polymers for several electronic applications like intelligent chem/bio electronic sensing and wearable computing system, miniature electronic gadgets, miniature power sources etc. CNCL has also developed antistatic coating for electronic devices from nano-particles of fullerene–polyaniline conjugate. Fullerenes–polyaniline based CNTs are also very effective as electromagnetic interference (EMI) shield coating for laptop, cellular phones, electro- chemical switches, remote control, microwave etc. Graphene has a great potential for the development of single electron transistor, composite materials etc29,30. Graphene-based microprocessors are expected to be developed in the years to come31.
Heat Sinks
Oak Ridge National Laboratory (ORNL) has developed carbon foam32, a lightweight, highly thermal conducting material, under the brand name PocofoamTM. There is a great future for highly thermal conducting material for cooling computer chips, making light car radiators etc. Carbon foam anode exhibits better ion discharge capacity to electrode compared to graphite fiber anodes33.
Conclusions
Carbon materials for environmentally benign energyrelated technologies have shown potential for production, transmission, and energy storage devices. However, further development is needed in production of advance carbon materials (carbon nanotubes and fullerenes) at low cost and temperature and development of conversion process of low value carbon rich refinery/ petrochemical streams into advance carbon materials.
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