Be an Engineer – Save the World Fig. 7.40 - Microsoft Clip Art.
nanotubes for fuelcells
Arunai Engg. college
→It comprises any technological developments onthe nanometer scale, usually 0.1 to 100 nm.→ One nanometer equals one thousandth of amicrometer or one millionth of a millimeter.→ It is also referred as microscopictechnology.
CNT is a tubular form of carbon with diameter assmall as 1 nm.Length: few nm to microns.CNT is configurationally equivalent to a twodimensional graphene sheet rolled into a tube (singlewall vs. multiwalled). CNT exhibits extraordinary mechanical properties: Young’s modulus over 1 Tera Pascal, as stiff as diamond, and tensile strength ~ 200 GPa. CNT can be metallic or semiconducting, depending on (m-n)/3 is an integer (metallic) or not (semicon).
History of nanotubes 1991 carbon nanotubes discovered“graphitic carbon needles ranging from 4 nm– 30 nm and up to 1 micron in length” ( Sumino Iijima) Just wait, the next century is going to be incredible. We are about to be able to build things that work at the smallest possible length scales, atom by atom . These little nanothings will revolutionize our industries and our lives.”
• The strongest and most flexible molecular material because of C-C covalent bonding and seamless hexagonal network architecture• Strength to weight ratio ~500 times greater than Al, steel, titanium; one order of magnitude improvement over graphite/epoxy• Maximum strain ~10%; much higher than any material• Thermal conductivity ~ 3000 W/mK in the axial direction with small values in the radial direction• Very high current carrying capacity• Excellent field emitter; high aspect ratio and small tip radius of curvature are ideal for field emission• Other chemical groups can be attached to the tip or sidewall (called ‘functionalization’)
A fuel cell is an electromechanical energyconversion device which produces electricitywith an oxidant and a fuel source.Net reaction H2 + 1/2O2 → H2O
Fuel cells have been around since the middle of the nineteenth century, but their use has been limited to the space industry . Recently, companies have been looking for a more efficient, reusable energy source, and fuel cells are a likely candidate. Two factors inhibiting the use of fuel cells in consumer application are efficiency and size.
1839- First fuel cell designed by Sir William Robert Grove. 1889-The term fuel cell was coined by Ludwig Mond and Charles Langer. 1913- Dr. Francis Thomas Bacon created the first alkaline fuel cell which he termed the “Bacon Cell”. 1960’s- NASA uses fuel cells to power their manned space missions .
The automotive industry hopes to utilize fuel cells as either a sole power source or in conjunction with fossil fuels or ethanol in hybrid vehicles. All major automobile manufacturers from GM to Honda have a prototype fuel cell car fully developed in testing on city roads . Research focus: ◦ Safe storage of hydrogen ◦ Reduction of size of fuel cells
(i) Are the carbon materials appropriate for solid state hydrogen storage?(ii) If this were to be true, what type of carbon materials or what type of treatments for the existing carbon materials are suitable to achieve desirable levels of solid state hydrogen storage?(iii) What are the stumbling blocks in achieving the desirable solid state hydrogen storage?(iv) Where does the lacuna lie? Is it in our theoretical foundation of the postulate or is it in our inability to experimentally realize the desired levels of storage?
Why carbon materials for solid state hydrogen storage? Coordination number is variable/expandable Promote new morphologies Covalent character retention Variable hybridization possible Geometrical possibilities/size considerations Meta-stable state Similar to biological architectures “Haeckelites” Boron and nitrogen doped graphitic arrangements promise important applications.
Objectives Necessity of active sites Heteroatom containing carbon materials - appropriate candidates? Gradation of the carbon materials containing various heteroatoms Geometrical positions of the heteroatoms 12
Energy profile for hydrogen interaction with heteroatom substituted CNT clusters 10 TS I N P 8 S C Energy (eV) 6 TS II 4 TS I 2 0 + H2 Reaction coordinate Ea I Ea II H1-H2 X-H C-H1* C-H2* Substitution (eV) (eV) (Å) (Å) (Å) (Å) CNT 10.02 - 0.71 - - - N CNT 3.84 4.58 1.45 1.11 1.70 1.94 P CNT 3.81 3.99 1.51 1.61 1.27 2.33 S CNT 3.65 4.85 1.50 1.75 1.24 2.40 Ea = E (transition state) – E (reactant) * Shortest C-H bond distance 13
Energy profile of boron substituted CNT clusters Alternative position Ea I Ea II H1-H2 X-H C-H1∗ C-H2* Substitution (eV) (eV) (Å) (Å) (Å) (Å) CNT 10.02 - 0.71 - - - 2B CNT (adjacent) 2.22 2.98 1.95 1.31 2.59 2.72 2B CNT (alternate) 1.5 2.33 2.95 1.47 1.47 2.34 Ea = E (transition state) – E (reactant) * Shortest C-H bond distance Adjacent position 14
Outcome Substituted heteroatom acts as an active centre for hydrogen activation For the effective hydrogenation and hydrogen storage, the heteroatoms should be incorporated geometrically and chemically into the carbon network
Boron substituted carbon nanotubes- synthesis,characterization and hydrogen absorption activity
Boron containing carbon nanotubes prepared using alumina membrane Alumina membrane Borane (BH3.THF) (0.2μm pore size) in THF Divinyl benzene Stirred 273 K Polymerization at RT 3h Polymer /Alumina Carbonization 1173K Ar,4h Carbon / Alumina 48% HF 24h Carbon nanotubes (BCNT1) 0°C n B THF n + BH3:THF solvent n B N2 atm using hydroborane polymers n 18
Preparation of boron containing carbon nanomaterials using zeolite and pillared clay Tubular furnace H-Y Zeolite or pillared clay in Mass flow meter quartz boat After carbonization Conc. H2SO4 treated wit 48% HF to Acetylene gas remove the templateAr gas Ice bath BCNT 2 (Zeolite) Magnetic stirrer NaBH4 + THF BCNT 3 (Clay)Chemical vapor deposition of borane gas + acetylene mixture over template 19
Hydrogen absorption activity of boron containing carbon nanomaterials at 1 atm Amount of hydrogen absorbed (cm3/g) at 1 atm & at various Carbon Surface temperatures (°C) area nanomaterial (m2/g) -196 25 100 150 BC 11.9 3.63 0.6 3.63 4.68 PBC 429.9 73 - 2.90 3.02 BCNT1 523 127 - 16.5 14.8 BCNT2 62.3 3.22 - 2.38 4.73 BCNT3 32.7 1.09 - 1.7 - 20
Hydrogen storage capacity of boron containing carbon nanotubesBoron containing carbon nanotubes prepared with polymer precursor,show different boron chemical environments and structural morphology.This configuration has a bearing on hydrogen sorption characteristics. 21
Conclusions Theoretical studies have shown that the effective hydrogenation of CNTs is possible with activation centers and the heteroatom containing CNTs are able to activate the hydrogen in a facile manner compared to pure CNTs. For effective hydrogenation and hydrogen storage heteroatom should be incorporated geometrically and chemically into the carbon network. Nitrogen containing CNTs are amenable for hydrogen absorption than other carbon materials. However, these active sites should be made catalytic in nature by various preparation methods and surface engineering so that necessary hydrogen storage may be achieved. Boron containing carbon nanotubes have been produced successfully by template synthesis method. For boron atoms two different environments in the carbon nanotubes have been prepared and the maximum hydrogen storage capacity of 2 Wt % has been realised. This configuration has a bearing in hydrogen sorption characteristics. The heteroatom substitution in the carbon nanotubes opens up another avenue in the search for materials for hydrogen storage. 22