Marco Buongiorno Nardelli. NC State University and Oak Ridge National Laboratory

Transcription

1 Marco Buongiorno Nardelli NC State University and Oak Ridge National Laboratory

2 Energy can be neither created nor destroyed, but can only be transformed from one form to another. Energy and materials - a continual and mutually enriching relationship Materials produce energy or enable energy to be transferred into useful forms. Energy makes possible the production of a broad range of materials for society Materials store and deliver energy the batteries, wires and switches, hydrogen, and biofuels that convert energy from other forms.

3 Energy landscape Harnessing Materials for Energy

4 Design of nanoscale materials (mostly carbon-based) as media for Energy and Environmental applications Harnessing Energy through H generation and storage H 2 O dissociation on defective carbon substrates Thermal decomposition of methane (CH 4 ) H 2 storage in/on Carbon nanostructures Addressing the carbon footprint through CO/CO 2 sequestration /activation Sequestration and selective oxidation of CO on graphitic edges CO 2 activation on metal oxide interfaces Solar energy utilization First principles design of novel organic photovoltaic materials Energy distribution through electrical storage Polymeric composites for ultra-high-power capacitors

5 Electronic structure calculations based on Density Functional Theory Multi-scale approach: transition state search through Nudge Elastic Band, Rational Function Optimization, Metadynamics First principles thermodynamics, equilibrium and rate constants within Transition State Theory from Density Functional Perturbation Theory Optical absorption spectra from Time Dependent DFT Exact excitation energies from self energy corrections within the GW approximation Analysis of chemical bonds through Wannier Functions transformations Codes: quantum-espresso, CPMD, WanT, ParFuMS, YAMBO

6 Design of nanoscale materials (mostly carbon-based) as media for Energy and Environmental applications Harnessing Energy through H generation and storage H 2 O dissociation on defective carbon substrates Thermal decomposition of methane (CH 4 ) H 2 storage in/on Carbon nanostructures Addressing the carbon footprint through CO/CO 2 sequestration /activation Sequestration and selective oxidation of CO on graphitic edges CO 2 activation on metal oxide interfaces Solar energy utilization First principles design of novel organic photovoltaic materials Energy distribution through electrical storage Polymeric composites for ultra-high-power capacitors

7 Hydrogen water cycle. Sustainability is maintained if renewable energy is used to split water and the energy released is used only for work and heat. Basic operation of a fuel cell. The net input of the fuel cell is hydrogen and oxygen Its net output is water, electricity, and heat.

8 Catalysis is the essential technology for chemical transformation Carbon substrates regarded as media for energy conversion and storage carbon catalysis Defects in carbon nanostructures: Detrimental effect to hydrogen storage OR Catalytic role in the dissociation of small molecules Why not use defective carbon substrates as nano-scale catalysts rather than fuel tanks? Graphitic nano-carbon: simple and easy to model Flexible - can explore different geometries (graphene walls, nanotubes, nanohorns, other fullerenes) Can modify reactivity in various ways: vacancies, doping, addition of functional groups, etc.

9 Catalytic potential of a mono-vacancy site Formation energy of a graphene single vacancy (GSV) is E f = 170±10 kcal/mol H 2 O dissociation in free space: H 2 O H 2 + 1/2O 2, ΔH (0 K) = 116 kcal/mol, E A = 131 kcal/mol Not isogyric reaction: spin is not conserved, oxygen moves from the spin-singlet surface (H 2 O) to a spin-triplet state (O atom). Fundamental limitation: breaking a water molecule requires an excess energy of 45 kcal/mol to excite the molecule to a higher spin state prior to the dissociation. The direct thermolysis of H 2 O is effective at T 2500 K Possible solutions to overcome these high activation barriers (E A ) are: photo-excitation, enzymatic and biochemical reactions, electrolysis, etc. Defects in graphitic materials, however, have the potential to dissociate water, since the formation energy is comparable to reaction enthalpy.

10 H 2 O H 2 +O Defects in graphitic materials have the the potential to dissociate water (formation energy comparable to reaction enthalpy) and they have been observed in native carbon nanotubes - they exists! (Iijima et al. Nature, 2004) Complex reaction mechanism with many intermediate states - spin singlet NEB + Metadynamics + Rational Function Optimization

11 E= 0 E=- 8 E=- 11 E=- 41 I IM1 IM2 IM3 Energies in kcal/mol C 3 C 3 IM4 E=- 65 E=- 76 E=- 87 E=- 39 K E F

12 H 2 O H 2 +O Potential energy surface for water dissociation on a vacancy in graphene - low energy barriers wrt the direct thermal splitting due to the ability of the carbon substrate to keep the reaction on the spin singlet surface

13 C3 N-I C3 Reconstructed CC bond ~ 1.65 Å System less reactive one dangling bond atom C 3 Direct dissociation process with a very low energy barrier: E A = 23 kcal/mol N-TST N-F

14 C3 N-I C3 N-IM2 All analogs of the planar stable states are present N-IM2 is the precursor of all the other possible intermediate states N-E is still the global minimum of the PES N-I N-IM2: E A = 21 kcal/mol N-IM2 N-K: E A = 24 kcal/mol N-K N-F*: E A = 38 kcal/mol N-E N-F* : E A = 48 kcal/mol Curvature improves energetics for the H 2 O dissociation reaction

15 Ideal-gas equilibrium yields (fraction of the water that is converted to hydrogen at equilibrium in % mole) for the dissociation of water on a single vacancy in a nanotube and a graphene sheet as compared with the direct thermolysis of the water. (Santiso, Kostov, George, Gubbins and MBN, PRL 2005) Defective carbon substrate can produce hydrogen from water at temperatures at least a factor of 2 lower for hydrogen yields comparable to the free space reaction

16 Methods to remove O atom Chemical and thermal purification methods come with the side effect of damaging the carbon network and preferential desorption of CO and CO 2. hν Photodesorption of O atom by a resonant Auger process with excitation energy E (O 1s O 2p) 530 ev. Oxygen desorbs spontaneously with no damage to the carbon network. (Tomanek et al, PRB, 2004) Alternative Multiphoton vibrational excitation: Infrared laser pulses can be adapted to selectively excite and break the C-O bond. The corresponding C-O stretch is in the µm region, which is well accessible to high-intensity CO and CO 2 lasers.

17 Design of nanoscale materials (mostly carbon-based) as media for Energy and Environmental applications Harnessing Energy through H generation and storage H 2 O dissociation on defective carbon substrates Thermal decomposition of methane (CH 4 ) H 2 storage in/on Carbon nanostructures Addressing the carbon footprint through CO/CO 2 sequestration /activation Sequestration and selective oxidation of CO on graphitic edges CO 2 activation on metal oxide interfaces Solar energy utilization First principles design of novel organic photovoltaic materials Energy distribution through electrical storage Polymeric composites for ultra-high-power capacitors

18 Major challenges lie ahead in the discovery of efficient biomass conversion catalysts, as well as in the discovery of catalysts for conversion of CO 2 and possibly water into liquid fuels. CO 2 is a product both of carbon-containing fuel combustion and a number of the processes for making the fuels. It would be advantageous to dispose of the CO 2 in environmentally safe ways or, perhaps even better, to convert it into fuels or chemicals. Economical processes and catalysts for such conversions are lacking; the discovery of such catalysts and the creation of technology for CO 2 conversion is a massive challenge.

19 Reduce CO 2 emission by efficient CO management Adsorption of CO on carbonaceous material is relevant in the coal gasification process CO sequestration and selective oxidation on graphitic edges CO disproportionation (Boudouard reaction: 2CO C solid + CO 2 accretion of the graphitic edge important implications for the controlled growth of graphene sheets for electronics applications selective oxidation of CO: application to purification of syngas CO+H 2, H 2 is preserved: 2CO+H 2 C solid + CO 2 + H 2

20 + 6CO O C kcal/mol + 6CO - 6CO 2 + 6CO kcal/mol + 6CO - 6CO kcal/mol kcal/mol CO Forms Hexagonal rings on the edge Reduction of oxygenated edge through CO 2

21 R TS P Presence of dangling bonds makes the energy barrier (~ 1.5 ev) high Concerted process: unsaturated bonds exposed to CO molecules CO adsorbs to the edge exothermically Chemisorption of CO to the open edge is more favorable than CO readsorption to form CO 2 at low temperature

22 + 3H kcal/mol + 6CO - 3H kcal/mol + 6CO + 3H 2-6CO kcal/mol + 6CO - 3H kcal/mol Selective oxidation of CO is possible Graphene edge could serve as Gas Purifier

23 + 3H 2 + 6CO - 3H kcal/mol kcal/mol - H 2 O+CO kcal/mol - H 2 O+2CO kcal/mol Selective oxidation of CO is possible at the edge At low temperature H 2 is preserved and yield of H 2 O is low

24 Design of nanoscale materials (mostly carbon-based) as media for Energy and Environmental applications Harnessing Energy through H generation and storage H 2 O dissociation on defective carbon substrates Thermal decomposition of methane (CH 4 ) H 2 storage in/on Carbon nanostructures Addressing the carbon footprint through CO/CO 2 sequestration /activation Sequestration and selective oxidation of CO on graphitic edges CO 2 activation on metal oxide interfaces Solar energy utilization First principles design of novel organic photovoltaic materials Energy distribution through electrical storage Polymeric composites for ultra-high-power capacitors

25 I PVs: based on crystalline Si pn junctions medium high efficiency (25%) II PVs: multi-junctions high efficiency (30 %) IIIa PVs: novel approaches using exotic concepts (hot carriers, multi-e-h pairs, thermophotonics high cost - high efficiency - still in infancy IIIb: Organics low efficiency but very low cost Inexpensive Good optical absorption coefficients Compatible with plastic substrates Easy fabrication (low temperature printing) (Shaheen,MRS Bulletin, (2005))

26 Glass Cathode Donor + - Acceptor Anode Substrate

27 exciton generation exciton diffusion LUMO splitting LUMO+1 Absorption: Eg LUMO Eg HOMO splitting HOMO HOMO-1 charge collection exciton dissociation and charge transfer Efficiency: (1-R) η A η ED η CT η CC (S. R. Forrest, MRS Bulletin, (2005))

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29 naphthalene anthracene tetracene pentacene hexacene DFT and Time Dependent DFT approach based on linear response theory to compute the absorption spectra (Rocca, Baroni et al., JCP, (2008))

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31 Need to engineer higher carrier mobility by maximizing orbital overlap for enhanced hopping or achieve true band transport Morphology is crucial to achieve better carrier mobility and device performance crystalline structure herringbone cofacial Critical parameter in both hopping and band-like transport is the transfer integral t = (E L +1[ H ] E L[ H 1] ) 2 where E L+1[H] and E L[H-1] are the energies of LUMO and LUMO+1 [HOMO and HOMO-1] HOMO and LUMO splittings

32 Acenes interact strongly with metallic structures stabilization of flat lying configuration that in turn, stabilizes the cofacial packing in thin molecular films High resolution STM images [size=70 nm 2 and 10 nm 2 (insert)] showing the molecular structure of the multilayer pentacene film on Au(110) (P. Guaino et al., Appl. Phys. Lett. (2004))

33 LUMO LUMO HOMO HOMO The higher the acene, the more stable is the secondary minimum Peculiar state crossing character of the HOMO and LUMO states

34 Red shift of the absorption spectrum increased efficiency at higher wavelengths HOMO and LUMO splittings increase with decreasing distance, improving the degree of overlap and thus the transfer integral t = (E L +1[ H ] E L[ H 1] ) 2

35 Design of nanoscale materials (mostly carbon-based) as media for Energy and Environmental applications Harnessing Energy through H generation and storage H 2 O dissociation on defective carbon substrates Thermal decomposition of methane (CH 4 ) H 2 storage in/on Carbon nanostructures Addressing the carbon footprint through CO/CO 2 sequestration /activation Sequestration and selective oxidation of CO on graphitic edges CO 2 activation on metal oxide interfaces Solar energy utilization First principles design of novel organic photovoltaic materials Energy distribution through electrical storage Polymeric composites for ultra-high-power capacitors

36 Capacitors provide fast energy release: electric vehicles, fast switches, etc. Polypropylene has energy density up to 4 J/cc. Highest among currently used polymers. Ultra high energy density was discovered in PVDF (Polyvinylidene Fluoride) copolymers ~ 17J/cc (Zhang et al., Science 2006) Electric field induced phase transition greatly influenced by the degree of co-polymerization and disorder Search for optimal conditions to maximize the energy storage capacity of these materials Design of polymer-crystalline composites

37 PVDF (polyvinylidene fluoride) exists in two phases : non -polar (α) and polar (β) ξ = E dd α α β (Zhang et al., Science 2006) β High electric fields transform the ground state α phase to the polar β phase Phase transformation accompanied by Large change in polarization Volume change 4.5 % In ordered material, the phase transitions are abrupt and do not explain experimental data

38 PVDF-CTFE PVDF-TeFE Phase stability influenced by the concentrations of co-polymers (PVDF-CTFE or PVDF-TeFE) PVDF-CTFE Critical field changes with the concentrations of co-polymers (PVDF-CTFE or PVDF-TeFE)

39 Theoretical data still do not explain completely the experimental results (abrupt vs. smooth transition) Assume a disordered P(VDF-CTFE) or P(VDF-TeFE): nano domains of varying concentrations of CTFE or TeFE - nanodomains of varying concentration reversibly flip between non-polar and polar states Displacement field as a function of the electric field: shaded areas are the energy densities ξ = E dd Energy density can be as high as 25 J/cm 3, if disorder is reduced. Excellent agreement with experiments for a distribution of domains with disorder ~8% (Ranjan, Yu, MBN and Bernholc, PRL, 2007)

40 Design of nanoscale materials (mostly carbon-based) as media for Energy and Environmental applications Harnessing Energy through H generation and storage H 2 O dissociation on defective carbon substrates Thermal decomposition of methane (CH 4 ) H 2 storage in/on Carbon nanostructures Addressing the carbon footprint through CO/CO 2 sequestration /activation Sequestration and selective oxidation of CO on graphitic edges CO 2 activation on metal oxide interfaces Solar energy utilization First principles design of novel organic photovoltaic materials Energy distribution through electrical storage Polymeric composites for ultra-high-power capacitors

41 Credits: L. Huang, S. Paul, M. Nuñéz, E. Santiso, M. Kostov, A. George, V. Ranjan, L. Yu, K. Gubbins, J. Bernholc $$$: NCCS-ORNL, NSF, DOE, ONR, NCSU-Kenan Center, NCSU-CHiPS Codes: quantum-espresso, WanT, CPMD