Institut fur Physikalische Chemie TU Dresden Simulations of the Hydrogen storage capacities of carbon materials. von Dipl.-Chem. Lyuben Zhechkov 2007 Institut fUr Physikalische Chemie Fakultat Mathematik und Naturwissenschaften Technische Universitat Dresden Simulations of the Hydrogen storage capacities of carbon materials. Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften (Doctor rerum naturalium) vorgelegt von Dipl.-Chem. Lyuben Zhechkov geboren in Sofia, Bulgaria Dresden 2007 Eingereicht am 05. Juni 2007 1. Gutachter: Prof. Dr. Gotthard Seifert 2. Gutachter: Prof. Dr. Tzonka Mineva 3. Gutachter: Prof. Dr. Barbara Kirchner Verteidigt am 23. Oktober 2007 1 The most exciting phrase to hear in science, the one that heralds new discoveries, is not EurekaT (I found it!) but That's funny Isaac Asimov (1920 - 1992) 11 Acknowledgement This work has been realised in the department of physical chemistry and elec­ trochemistry in the Technical University of Dresden directed by Prof. Dr. Gothard Seifert and under the supervision of Dr. Thomas Heine. I would like to express my gratitude to the Head of the department Prof. Dr. Gotthard Seifert who gave me the opportunity to accomplish this inter ­ esting work To my supervisor Dr. Thomas Heine with whom I had the honour to work and who has introduced and guided me through the topic of my thesis. This work would not be possible without the help of my colleagues and friends. I would like to specially thank to: Dr. Serguei Patchkovskii and Dr. Serguei Yurchenko for their advices, great ideas and the program code which was used in the computational simu­ lations. Dr. Tzonka Mineva for refereeing this work and for her collaboration and help. The referees and the members of the commission who accepted to judge this work. Knut Vietze for the support and the fruitful discussions of physics, pro ­ graming and German beer. Dr. Boris Naidenov, Dr. Byan Jonev, Dr. Sandrine Hazebroucq, Dr. Andrey Enyashin for the nice time spent in Germany. To Nina who shares my life for several years and whose patience and support deserve a lot more than just to be thankful... This work was supported by the Deutsche Forschungsgemeinschaft, the Stiftung Energieforschung Baden-Warttemberg, and the National Sciences and Engineering Research Council of Canada. 1V Abbreviations B3LYP Becke 3-Parameter (Exchange) and Lee, Yang, and Parr (correlation; density functional theory) BSSE Basis Set Superposition Error cc-pVTZ Correlation Consistent Polarised Valence Ttriple Zeta CCSD(T) Coupled-Cluster with Single and Double and Perturbative Triple excitations CIG C60 Intercalated in Graphite CNT Carbon NanoTubes CNF Carbon NanoFibers DC Disperssion Corrected DFT Density Functional Theory DFTB Density Functional based Tight Binding DOS Density Of States FCVT Freedom car and Vehicle Technologies GGA Generalized Gradient Approximation Equilibrium constant LJ Lenard-Jones MOF Metalo Organic Frameworks MP2 M0ller-Plesset perturbation theory second order PES Potential Energy Surface KSCED Kohn-Sham of Constrained Electronic Densities PAH PolyAromatic Hydrocarbons PW91 Perdew-Wang 91 (The exchange component of Perdew and Wang ’s 1991 functional) SWCNT Single Walled Carbon NanoTube ZPE Zero-Point Energy Vi Contents Introduction 3 I Dihydrogen physisorption 11 1 Methodology of calculations 13 1.0.1 Fugacity.............................................................................. 15 1.1 Approximations and inaccuracies of the model ......................... 17 1.1.1 Interaction potential ....................................................... 17 1.1.2 The ideal gas approximation ........................................... 19 1.1.3 Neglect of the coupling of the guest motion and host Vibrations .......................................................................... 19 1.1.4 Neglect of the internal degrees of freedom of the guest . 20 1.1.5 Model of the host structure ........................................... 23 1.1.6 Grid density ....................................................................... 24 2 Interaction potential 25 2.1 Computational details and models.............................................. 25 2.2 Extrapolation to graphene surface.............................................. 31 2.3 Many H2-PAH interactions .......................................................... 36 2.4 H2 — Csp 2 Potential ....................................................................... 37 2.5 Empiric entropy estimation ....................................................... 40 II Carbon host systems 43 3 Physisorption on graphene 45 3.1 Mono- and double layer models ................................................. 45 vii viii CONTENTS 3.2 Real gas physisorption ................................................................ 49 4 Graphene spacers 59 4.1 Layer-layer separation ................................................................. 60 4.2 CIG storage capacities ................................................................. 61 4.3 Hydrogen diffusion ....................................................................... 66 5 SWCNTs and Carbon foams 69 5.1 SWCNT and bundles .................................................................... 70 5.1.1 Isolated SWCNT.............................................................. 71 5.1.2 Bundles of nanotubes ....................................................... 76 5.2 Carbon foams ................................................................................ 80 6 Graphite and fullerene modifications 87 6.1 Dislocations in graphite ............................................................. 87 6.2 C28 —fullerites................................................................................ 90 Summary 95 Introduction 1 Introduction 3 As hydrogen (protium) consists of a single proton and a single electron, it is nature ’s simplest, lightest atom. The unique chemical and physical properties of the element ’s neutral, cationic, and anionic siblings, rely para ­ doxically on this “simplicity ”. Since hydrogen has only one occupied electron shell it exhibits three common oxidation states: +1, 0, and —1 (correspon ­ ding to Is0, Is1, and Is2 electronic configurations, respectively). The relative change in the number of electrons surrounding the nucleus, associated with the transformations H° —► H 1 and H° —► H+I (le, 100 %), is thus the largest among the chemical elements of the periodic table; so are the relative changes of many chemical and physical key properties of these three unitary species. Therefore, hydrogen has key role in the universe and life supporting processes. In its 15-billion-year domination in the universe hydrogen is far- and-away the most abundant element in the cosmos, of which it makes 75 % of the weight composition. 1 1 The hydrogen fusion has crucial importance for the evolution of the stars and is the only source of remarkable amount of life-giving solar rays which provide our own planet and our plant world with energy in the form heat and light. As one of the elements present in the water molecule and the overwhelming majority of the organic compounds, hydro ­ gen is pivotal to life. Furthermore it is one of the most important elements in the chemistry of the energy-giving materials, such as the light fractions of hydrocarbons (mineral oils and methane gas). Hydrocarbons are technologically the most commonly used energy car­ riers. However, in a combustion engine only 30 % of the produced energy is due to the 2 H + O —► H20 reaction 1. The rest is the so called “dirty ” energy which results in C02 production. At the same time, hydrogen ’s usage as fuel suggests many advantages over the hydrocarbons — it is non-toxic, renewable, clean to use, and packs much more energy per bond. The inevitable socio-economic effect of recurring fuel crises, the threat of the early end of the fossil fuels era in the coming 50 years, the increasing pollution of our environment, and the problem of an anthropogenic-induced climate change force the utilisation of clean and sustainable energy resources. Projections to the middle of the next century indicate that unabated global energy trends would lead to an annual global energy demand about four times xExept for the natural gas where the produced energy is about 50 %. 4 Introduction the present level. Thus, a demand for clean and renewable energy sources has resulted in increased worldwide attention to the possibilities for a hydrogen- based economy as a long-term solution. Hydrogen is undoubtedly one of the key alternatives to replace petroleum products as a clean energy car­ rier for both transportation and stationary applications. Due to the unique electronic structure of its atom, hydrogen possesses the largest amount of chemical energy per bond (142 MJmol-1 kg_1)J 1 This is about three times higher energy than the hydrocarbons (47 MJ mol-1 kg—1); t 1 therefore it is an excellent candidate for energy source. Furthermore, its usage promotes new technologies based on more efficient energy consumption (e.g. fuel cells based on electrochemical burning of H2 and 02)2. However, technological difficulties — subject to this work — yet prevent the usage of H2 as widely spread energy carrier. The interest in hydrogen as energy carrier experi ­ enced a renaissance in the late 1960s and in the 1970s, and has grown even more dramatically since 1990. Partly catalysed by the deuterium cold fu­ sion controversy; 1 1 many advances in hydrogen production and utilisation technologies have been made during the