1. 1 Hydrogen Storage with Carbon Nanotubes Andrew Musser

2. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 2 Outline  The hydrogen economy  Storage options  What are carbon nanotubes?  Promising initial results  Simulations of storage  Recent experimental results  Prospects

3. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 3 The Hydrogen Economy  Most abundant element on Earth, almost entirely within water  Production of hydrogen: break down hydrocarbons or water  Efficient consumption: fuel cells

4. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 4 The Storage Problem  Highest chemical energy mass density of any chemical fuel: 142 MJ/kg 4 kg of H 2 compared to standard vehicle size  US Dept. of Energy baselines for lightweight, energy-efficient storage:  6.0 wt% and 0.20-0.70 eV/H 2 binding energy by 2010  9.0 wt% by 2015  Extremely poor volumetric mass density

5. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 5 Storage Options  One of the most promising to date: Carbon Nanotubes US Dept. of Energy, www.eere.energy.gov carbon nanotubes metal hydrides

6. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 6 (0,n) zig-zag (n,n) armchair What are Carbon Nanotubes?  Single-walled nanotubes (SWNT): rolling graphene  Multi-walled nanotubes (MWNT): concentric SWNTs

7. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 7 Chirality (n,m)  Physical and electronic properties vary widely with the vectors that determine rolling n m

8. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 8 Why CNTs?  Stable, lightweight, inexpensive  Large active surface area  Large internal volume if it can be accessed +

9. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 9 How Are They Produced?  Decomposition of hydrocarbons  soots  Arc discharge  soots and fibers  Laser ablation  catalytic control of nanotube type  Chemical vapor deposition  catalytic control of CNT diameter  Consistency between batches can be problematic Liu et al., Science 1999

10. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 10 How Are They Purified?  Removal of catalyst particles and hydrocarbon contaminants  acid treatment and UHV baking  Opens tube ends, acid damage to side walls  Limited ability to separate CNTs by diameter and/or chirality  Needed for future applications  Breakdown of fibers and bundles into individual CNTs  surfactants and intense sonication

11. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 11  First and simplest approach: Physisorption  Van der Waals interaction between H 2 and CNT wall  Internal or external  No energy barrier to overcome, but relatively weak binding  low temperatures  Negligible effect on CNT electronic and physical structure How Can They Store Hydrogen?

12. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 12 Dillon et al., Nature 1997 A Remarkable Capacity? The 1 st Observation of H 2 Storage in CNTs (1997)‏  Arc discharge soots containing 0.1-0.2% narrow SWNT bundles  Low H 2 pressure at low T  Mass spectrometry of desorbed gases upon reheating in UHV  Total soot storage capacity: 0.01 wt%, attributed to SWNTs  Unclear where in SWNT H 2 is stored  Extrapolated pure SWNT capacity: 5-10 wt%  Markedly lower capacity found in later studies

13. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 13 Nanotube Doping Improved Capacity with Alkali Metals (1999)‏  Large MWNTs produced by catalytic decomposition of hydrocarbons, purified to 90%  Tubes doped with Li or K via solid-state reactions  Alkali to carbon ratio: 1/15  Weight changes monitored during heating and cooling cycles in pure H 2 stream at ambient pressure Chen et al., Science 1999 3.2 3.1 3.0 2.9 2.8 5.1 4.9 4.7 4.5 Li-doped K-doped Sample Weight (mg) Temperature (K) +15% +14% 270 370 470 570 670 770 870

14. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 14 Nanotube Doping Improved Capacity with Alkali Metals (1999)‏  Li-doped: peak adsorption of 15-20 wt% at 673 K  stable in ambient conditions  K-doped: peak adsorption of 14 wt% at 298 K  highly unstable in ambient conditions  Storage attributed to tube exterior surface  Later studies suggested hydroxide and water formation 3.2 3.1 3.0 2.9 2.8 5.1 4.9 4.7 4.5 Li-doped K-doped Sample Weight (mg) Temperature (K) +15% +14% 270 370 470 570 670 770 870 Chen et al., Science 1999

15. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 15 Liu et al., Science 1999 Room-Temperature Capacity Storage with Bare SWNTs of Higher Purity (1999)‏  Arc discharge SWNT fibers of 50-60% purity in large scale  Relatively large SWNTs  High H 2 pressure at ambient temperature  Weight changes monitored  Impure capacity of 4.2 wt%  Storage attributed to tube surface and curvature  Markedly lower capacity found in later studies

16. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 16 A Theoretical Reexamination  Early experiments too variable and sample dependent  new focus on calculations and MD simulations  More reactive species on CNT surface could physisorb and hold H 2 more strongly, as in Chen et al.  Affinity of bare CNTs for H 2 is too weak for RT storage

17. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 17 Durgun et al., Phys Rev B 2008 Storage Inside and Outside  Simulated functionalization with light transition metals Sc, Ti and V on slightly larger SWNTs  Sufficient interior space allows functionalization of inner surface  Each metal atom, inside or outside, can physisorb up to 4 H 2  At high coverage ~8 wt% storage should be possible with excellent binding energy  Trade-off: H 2 binding energy versus clustering Ti

18. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 18 Raising the Affinity of Carbon Liu et al., J Phys Chem C 2009  Problem with transition metals is material self-weight  significantly heavier than carbon  Simulation of medium-sized SWNTs with Li adsorbates  Stable against clustering  Charge transfer from Li activates carbon atoms  the entire SWNT can physisorb H 2  At moderate Li coverage, 13.45 wt% storage capacity and binding energy close to benchmarks Li

19. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 19 H A New Approach: Chemisorption  Simulated systems difficult to achieve in practice: chirality selection, clustering and controlled functionalization  Bare SWNT simulations find chemisorption more favorable  A fully hydrogenated SWNT could store 7.8 wt% hydrogen  Stability of hydrogenated SWNTs increases with diameter  Large kinetic barrier to chemisorbtion: dissociation of H 2 Nikitin et al., Nano Lett 2008

20. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 20 Avoiding the Dissociation Barrier Hydrogen Storage in C–H Bonds (2008)‏  Hydrogen chemisorption studied on 2 types of high- purity CVD films of SWNT  Mean CNT diameters of 16Ǻ and 20Ǻ determined by AFM  To avoid dissociation barrier, charged films with beam of atomic H  H 2 cracked by W catalyst at high temperature 16Ǻ 20Ǻ Nikitin et al., Nano Lett 2008 500 1000 1500 Frequency (cm -1 ) Intensity (arb.) diameter (nm)

21. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 21 Avoiding the Dissociation Barrier Hydrogen Storage in C–H Bonds (2008)‏  C-H bond formation monitored by in situ XPS  Small-diameter film degrades above 30% hydrogenation  Large-diameter film stable up to ~100% hydrogenation C=C bonds C-H bonds Binding Energy C=C bonds C-H bonds Degradation Binding Energy 16Ǻ20Ǻ  ~7.0 wt% storage capacity, almost entirely on bundle surface  2 / 3 of H 2 recovered at 200-300 C Nikitin et al., Nano Lett 2008

22. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 22 Summary  Early studies yielded promising but extremely controversial results  Problems of inconsistent production, purification and characterization  Subsequent simulations suggest promise of physisorption on functionalized nanotubes  Offers possibility of utilizing interior space of CNTs  Systems difficult to synthesize  Chemisorption of atomic H can be thermodynamically favorable  Significant kinetic barrier of hydrogen dissociation must be overcome  High storage capacity through chemisorption shown to be feasible with some SWNTs

23. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 23 Prospects for CNT storage  Synthesis of functionalized SWNT systems to investigate the feasibility of storage through physisorption  Investigation of catalytic “spillover” mechanisms for a practical source of atomic hydrogen for chemisorption  Parallel studies with other carbon nanomaterials

24. "Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009 24 Thank you for your attention  Questions?  I would like to acknowledge Dr. Maria Loi for her guidance in reviewing the literature and preparing this presentation.