Density functional theory study of LaMnO3 and its competing oxides: an insight into a prospective alkaline fuel cell cathode Ehsan Aleem Ahmad Department of Chemistry Imperial College London A thesis submitted for the degree of Doctor of Philosophy October 2013 Declaration of Originality I hereby declare that this thesis is a presentation of my original research work and has not been submitted previously for a degree qualification or any other academic qualification at this University or any other institution of higher education. Wherever contributions of others are involved, every effort is made to indicate this, with due refer- ence to literature and acknowledgements of collaborative research and discussions. The research work presented in this thesis was conducted under the guidance of Professor Nicholas M. Harrison and Professor Anthony Kucernak, both at the Imperial College London, London. Ehsan Aleem Ahmad October 2013 3 Declaration of Copyright The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work 5 Abstract LaMnO3 is an inexpensive alternative to precious metals (e.g. platinum) as a catalyst for the oxygen reduction reaction (ORR) in alkaline fuel cells (AFCs). In fact, recent studies have shown that among a range of non-noble metal catalysts, LaMnO3 provides the highest catalytic activity. However, further development of this catalyst is limited by the fact that very little is known about LaMnO3 in the AFC environment. While it has been established that the bulk phase possesses an orthorhombic structure, it has not been possible to determine the structure of the surfaces or the sites active towards the ORR. In this work, therefore, periodic hybrid-exchange (B3LYP) density functional calculations are performed in order to understand the origins of the catalytic activity of LaMnO3. The long term goal is to suggest strategies for optimising the activity of LaMnO3 through control of its crystallite morphology. Initially, the phase stability of LaMnO3 with respect to its competing (La, Mn) oxides is determined by accurate calculation of the Gibbs formation energies of each compound (1.6% mean error). The accuracy achieved is higher than in previous litera- ture, validating the methodology adopted and the reliability of the chemical potentials determined to limit the stability of the bulk and surfaces of LaMnO3. Having deter- mined the ground state of each Mn oxide it was possible to simulate electron energy-loss spectroscopy (EELS) for Mn in different valence states and local environments. The simulated EELS demonstrated that it is possible to identify its oxidation state and local coordination (i.e. the surface structure) on LaMnO3 surface terminations, based on the shift and shape of predicted L3 edges, which correlate well with measured EEL spectra. Calculations of the low-index, stoichiometric and non-polar surfaces of LaMnO3 were then performed in order to predict the equilibrium crystal morphology. For each low energy surface the adsorption sites were also identified. The energetics of the surfaces are rationalised in terms of the cleavage of Jahn-Teller distorted Mn-O bonds, the compensation of undercoordination for ions in the terminating layer and relaxation 7 effects. Finally the adsorption sites identified are investigated by adsorption of molecular O2. The binding energies, adsorbate structure and charge transfer are analysed to predict the reactivity of each site. Results indicate that Jahn-Teller distortion and the coordination of Mn sites modulate the binding strength of O2. The main results presented are the crystallite morphology, the identification of sur- face reaction sites and the chemical characterisation of those sites. This is a theoretical characterisation of the LaMnO3 catalyst providing detailed atomistic information that has not been possible to deduce from experiment. 8 Acknowledgements First and foremost I would like to thank my supervisors, Prof. Nicholas Harrison and Prof. Anthony Kucernak, for providing me with the opportunity to study for a PhD. They gave me flexibility and freedom in my studies but were always there for guidance and support when needed. I owe gratitude to Dr. Giuseppe Mallia for his mentoring, patience and friendship over the past four years. I would also like to thank Dr. Leandro Liborio and Dr. Denis Kramer for many fruitful discussions on ab initio thermodynamics, and Dr. Vaso Tileli for providing her expertise on electron energy-loss spectroscopy. A special thanks goes to my colleagues and friends in Prof. Harrison's theory group; Romi, Monica, Fred, Ross and Ruth, for the many chats, scientific discussions and fun over lunch and coffee. I would also like to mention other friends in the Thomas Young Centre and outside of academia; Michele for training with me, Jing and Leo for the laughs, Evgeniy for being a joker and the table tennis, Luke for the football, Sophie for her patience, and Luca, Basmah and Jazz for being good neighbours. The last four years in London went by so quickly having you all around. Finally, my gratitude goes to my family, especially my mother, for their love, care and patience. My thanks also goes to the EPSRC and STFC for funding my PhD and I would like to acknowledge that this work made use of the high performance computing facilities of Imperial College London and - via membership of the UK's HPC Materials Chem- istry Consortium funded by EPSRC (EP/F067496) - of HECToR, the UK's national high-performance computing service, which is provided by UoE HPCx Ltd at the Uni- versity of Edinburgh, Cray Inc and NAG Ltd, and funded by the Office of Science and Technology through EPSRC's High End Computing Programme. 9 List of Publications • E. A. Ahmad, L. Liborio, D. Kramer, G. Mallia, A. R. Kucernak, and N. M. Har- rison, Thermodynamic stability of LaMnO3 and its competing oxides: A hybrid density functional study of an alkaline fuel cell catalyst, Phys. Rev. B 84 085137 2011. • E. A. Ahmad, G. Mallia, D. Kramer, V. Tileli, A. R. Kucernak, and N. M. Harri- son, Comment on \2D Atomic Mapping of Oxidation States in Transition Metal Oxides by Scanning Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy", Phys. Rev. Lett. 108 259701 2012. • E. A. Ahmad, G. Mallia, D. Kramer, A. R. Kucernak, and N. M. Harrison, The stability of LaMnO3 surfaces: a hybrid exchange density functional theory study of an alkaline fuel cell catalyst, J. Mater. Chem. A 1 37 2013 • E. A. Ahmad, G. Mallia, D. Kramer, A. R. Kucernak, and N. M. Harrison, O2 adsorption on orthorhombic LaMnO3: a hybrid exchange density functional study of an alkaline fuel cell catalyst In Preparation 11 Contents 1 Introduction 25 1.1 Background . 26 1.2 Perovskites . 32 1.3 LaMnO3 ................................... 34 1.3.1 Review of Experiments . 35 1.3.2 Review of Computations . 41 1.4 Competing Oxides . 44 1.4.1 Review of Experiments . 46 1.4.2 Review of Computations . 49 1.5 Outline . 51 1.5.1 Objectives . 51 1.5.2 Thesis Structure . 52 2 Theoretical Background and Techniques 55 2.1 Electronic Structure Methods . 55 2.2 The Periodic Model . 64 2.3 The CRYSTAL Program . 67 2.4 Geometry Optimisation . 69 2.5 Computational Methods Applied to LaMnO3 ............... 71 2.6 Computational Details . 73 3 The Thermodynamic Stability of LaMnO3 and its Competing Oxides 75 3.1 Introduction . 75 3.2 Ab Initio Thermodynamics . 76 3.2.1 Standard chemical potentials . 76 3.2.2 Phase Diagram . 83 13 CONTENTS 3.3 Ground States . 85 3.4 Formation Energies . 89 3.5 Phase Diagram of the La-Mn-O system . 90 3.6 Summary . 91 4 Oxidation State Mapping of Manganese 93 4.1 Introduction . 93 4.2 Electron Energy Loss Spectroscopy . 94 4.3 Mapping the Oxidation states of Mn in Mn3O4 .............. 96 4.4 Summary . 101 5 The Stability of Orthorhombic LaMnO3 Surfaces 103 5.1 Introduction . 103 5.1.1 Surface Simulation . 104 5.2 Surface Structure and Energetics . 106 5.2.1 The (110) surface . 108 5.2.2 The (001) surface . 110 5.2.3 The (101) surface . 110 5.2.4 The (100) surface . 112 5.3 The Jahn Teller Effect and Transition Metal Ion Undercoordination . 114 5.4 Electron Density Difference Maps . 116 5.5 Crystal Shape and Manganese Sites . 120 5.6 Summary . 122 6 Oxygen Adsorption on Orthorhombic LaMnO3 123 6.1 Introduction . 123 6.2 Binding Energies of Adsorbed Molecules . 124 6.3 Adsorption Modes of O2 .......................... 127 6.3.1 The (100) Surface . 128 6.3.2 The (001) Surface . 129 6.3.3 The (101) Surface . 131 6.3.4 The (110) Surface . 132 6.4 Energetics and Electronic Properties . 134 6.4.1 BSSE Correction . 134 6.4.2 Adsorption Sites . 137 6.5 Summary . 140 14 CONTENTS 7 Conclusions 143 A Basis Sets 147 A.1 Oxygen . 148 A.2 Manganese . 149 A.3 Lanthanum . 150 B Thermodynamic Equations 151 B.1 The Surface Formation Energy of a Non-stoichiometric Surface . 151 B.2 Change of variable for NS surface . 151 B.3 The Inequalities Plotted in the La-Mn-O Phase Diagram . 152 B.4 Inequality for Lanthanum Oxide on the Mn-O2 axis . 153 B.5 Phase Diagram Line Equations . 154 B.6 Oxygen Chemical Potential by Oxide Method . 154 C Permissions 157 Bibliography 162 15 List of Figures 1.1 The water splitting and hydrogen fuel cell reactions.