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Tuneable Graphite Intercalates for Hydrogen Storage Tuneable graphite intercalates for hydrogen storage Arthur Lovell A Thesis presented for the degree of Doctor of Philosophy Condensed Matter and Materials Physics Department of Physics and Astronomy University College London England September 2007 Declaration I, Arthur Lovell, con¯rm that the work presented in this thesis is my own. Where information has been derived from other sources, I con¯rm that this has been indicated in the thesis. The work in this thesis is based on research carried out at the Condensed Matter and Materials Physics Group, the Department of Physics and Astronomy, University College London, England. No part of this thesis has been submitted elsewhere for any other degree or quali¯cation. Copyright °c 2007 by Arthur Lovell. \The copyright of this thesis rests with the author. No quotations from it should be published without the author's prior written consent and information derived from it should be acknowledged". ii Tuneable graphite intercalates for hydrogen storage Arthur Lovell Submitted for the degree of Doctor of Philosophy September 2007 Abstract The development of hydrogen as an energy transfer mechanism is of great importance to alleviate environmental damage and economic destabilisation caused by over-reliance on oil, as long as the hydrogen can be generated re- newably. To be suitable for road transport applications, safe and compact hydrogen storage systems need to be developed, the primary technological motivation for this PhD project which investigates hydrogen absorbed into graphite intercalation compounds (GICs), to gain a fundamental physical un- derstanding of the sorption processes to improve such materials' capacity for hydrogen storage. Literature searching has led to a principal investigation, primarily using neutron scattering and thermogravimetry, of potassium and calcium-GICs with hydrogen. Inelastic neutron scattering on hydrogenated KC24 has shown hydrogen sorption in this system to be quantitatively di®er- ent from its analogues RbC24 and CsC24. A consistent model of the H2 sites and dynamics has been proposed. Time-resolved structural data on the hy- driding phase transition in KC8Hx have been obtained. A calcium-ammonia intercalate has shown most promise for hydrogen storage, with uptake of 3.2 wt.% H2 at 77 K and 2 bar, a signi¯cant amount of the 6 wt.% target set by the US DoE. It is concluded that available internal volume and donor charge in GICs are critical parameters for optimising hydrogen uptake. Dedicated to Jungmin Acknowledgments There are many people due thanks for their contributions to this work, and they broadly fall into two categories: academic colleagues, and friends. It is a great privilege for me that so many are members of both camps. Primarily, I thank Neal Skipper for his equally ¯ne attributes of proli¯c in- spiration, relaxed supervision and unfailing good (not to mention dry) humour. Despite an ever-expanding panoply of research interests, he has always been available for discussion, advice, encouragement, expenses claims, and sitting in instrument cabins until the small hours. I thank Steve Bennington for his second supervision and CASE sponsorship, collaboration on di®raction re¯nement and on writing the two papers (so far). Many thanks also to Felix Fernandez-Alonso for, as they say, useful discussions, and for entitling me to include the CASTEP work and the siting model, not to mention being local contact on IRIS. And thanks to Mark Ellerby for advice, help and the occasional pat on the back! At ISIS I should acknowledge Ron Smith, Mark Adams, Stewart Parker and Timmy Ramirez-Cuesta, instrument scientists; Alex Hannon for assistance with pycnometry; Andy Church and the sample environment sta®; Keith Ref- son, Dickon Champion and the User O±ce. The list in London is long, but here goes: John Dumper and MAPS workshop; John the glassblower; Cather- ine Jones, Muna Dar and Denise Ottley; Tony Harker, Peter Storey and the Graduate School for conference funding; Tony again for discussions and sup- port; Sian Fogden for more pycnometry; Bernard Bristoll, Ra¯d Jawad, Mar- v vi tin Palmer and Mark Sterling for various lendings and borrowings, and Mike Cresswell for radiation badges. Steve Leake and Chris Howard made the earlier ammoniated-Ca-GIC sam- ples, and allowed me to use an SEM image. Both helped with running the IGA, and Chris assisted with sample synthesis on various occasions. Helen Walker I thank for providing me with the thesis template and, with Ross Springell, convincing me to write up in LATEX. She paid the penalty by having to read my TEXnical queries night and day. I have proven that a minimal command of the coding can produce reasonable results; for de¯ciencies blame neither them, nor LATEX, but me. I also have to thank Ross for many fruitful discussions, usually about 21 Down. Among other friends, colleagues and fellow-travellers, I'm grateful to Tom Weller, Emily Milner, Helen Thompson, Cecilia Gejke, Peter Lock, Jonathan Wasse, Zeynep Kurban, Tom Headen, Emma Wilson, Steve Hussey, Clare Leavey, Helen Hanlon, Jenny Riesz and Kat Stockton (not least for getting a Science paper from her Masters project eventually!), and all o±ce, lab and tea mates past and present. I apologise to anybody omitted from this list, and thank them too. I wish ¯nally to say a big thank-you to my family, particularly my parents, Martin and Kate, for the unstinting support I've always received and hopefully never under-appreciated, and Jungmin Kim, whose love, faith and encourage- ment have more than outweighed her ability to distract me, and to whom I dedicate this thesis. This work was funded by EPSRC and CCLRC. Thesis information This thesis was typeset using a LATEX template originating with M. Imran and modi¯ed by H. Walker (2006). The image ¯les were made in a variety of environments down to and including MS Paint. The 3D structure renderings were made using Accelrys Materials Studio. The neutron instrument ¯gures were hand-drawn by the author, scanned and digitally manipulated before being included. I hope I can be forgiven for resisting the urge to start each chapter with an apposite quotation. The total number of words in the document is 56,324 in chapter body text, or 61,775 including titles, image captions and prefaces. vii Contents Declaration ii Abstract iii Acknowledgments v Thesis information vii 1 Introduction 1 1.1 Thesis motivation . 1 2 Background 4 2.1 The motivation for hydrogen storage . 4 2.1.1 The end of the oil era . 4 2.1.2 Anthropogenic climate change . 5 2.1.3 Hydrogen as energy carrier . 11 2.1.4 Hydrogen fuel cells . 13 2.1.5 New hydrogen synthesis pathways . 15 2.1.6 The hydrogen economy . 16 2.1.7 Hydrogen storage . 17 2.2 Hydrogen storage methods . 19 2.2.1 Gas compression . 20 2.2.2 Gas liquefaction . 22 viii Contents ix 2.2.3 Metal hydrides . 23 2.2.4 Physisorption systems . 25 3 Graphite intercalation compounds and hydrogen 28 3.1 Graphite intercalates . 28 3.1.1 Structure and properties of graphite . 28 3.1.2 Intercalation into graphite . 30 3.2 GIC synthesis . 38 3.2.1 Graphite morphologies . 41 3.2.2 Papyex exfoliated graphite properties . 44 3.3 Ternary GICs with hydrogen . 46 3.3.1 Structural properties of KC8 and KC24 .......... 48 3.3.2 Hydrogen in KC8 ...................... 49 3.3.3 Hydrogen in KC24 ...................... 51 3.4 Tuneability of GICs for hydrogen storage . 58 3.4.1 Project plan . 64 4 Theory of experimental techniques 67 4.1 Neutron scattering . 67 4.1.1 Introduction . 67 4.1.2 The Q vector . 70 4.1.3 The di®erential cross-sections . 72 4.1.4 Scattering from a ¯xed nucleus . 73 4.1.5 Scattering from a general system of particles . 74 4.1.6 Coherent and incoherent scattering . 75 4.1.7 Neutron di®raction . 76 4.1.8 Scattering from crystals . 78 4.1.9 Inelastic neutron scattering . 81 4.1.10 Experimental techniques . 84 4.1.11 Neutron instruments . 87 Contents x 4.1.12 Di®raction data re¯nement . 90 4.2 Gravimetric analysis . 91 4.2.1 Introduction . 91 4.2.2 The Intelligent Gravimetric Analyser . 93 4.2.3 Buoyancy correction . 94 4.2.4 IGA Density determination . 101 4.2.5 Adsorption theory . 102 4.2.6 Isotherm types . 104 4.2.7 The Langmuir model . 106 4.2.8 The BET isotherm . 108 4.2.9 Isotherm analysis using the IGA . 109 5 Experimental Work 112 5.1 Synthesis of GICs . 112 5.1.1 Preparation of Graphite . 113 5.1.2 Synthesis of KC8 and KC24 ................ 116 5.1.3 CaC6 synthesis by Li alloy co-intercalation . 122 5.1.4 Ca-GIC using a metal-ammonia solution . 125 5.2 Thermogravimetry using the IGA . 130 5.2.1 Introduction . 130 5.2.2 Sample loading . 131 5.2.3 Measuring uptake using the IGA . 137 5.3 Neutron di®raction on Polaris . 138 5.3.1 General experimental details . 141 5.3.2 Polaris data analysis . 144 5.3.3 Polaris experiment on deuterium and ammonia in KC24 . 146 5.3.4 Polaris experiment on hydrogen in KC8 .......... 148 5.3.5 Polaris experiment on hydrogen in CaC6 ......... 149 5.4 Inelastic neutron scattering on IRIS . 151 Contents xi 5.4.1 General experimental details . 153 5.4.2 First experiment: INS on H2 and ND3 in KC24 ...... 154 5.4.3 Second experiment: INS on low coverage H2 in KC24 .. 157 5.4.4 IRIS data analysis . 158 5.5 Inelastic neutron scattering on TOSCA . 159 5.5.1 Medium energy INS on H2 in KC24 ...........
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