Novel Non-Aqueous Symmetric Redox Materials for Redox Flow Battery Energy Storage Craig G. Armstrong This dissertation is submitted for the degree of Doctor of Philosophy January 2020 Department of Chemistry The search for ‘electrochemically promiscuous’ redox materials… - Craig Armstrong, 2017 ii Declaration This thesis has not been submitted in support of an application for another degree at this or any other university. It is the result of my own work and includes nothing that is the outcome of work done in collaboration except where specifically indicated. Many of the ideas in this thesis were the product of discussion with my supervisor Dr Kathryn E. Toghill. Dr Ross W. Hogue assisted in the acquisition of experimental results in chapters 4, 6 and 7. He is also credited for co-writing [3], of which Chapter 6 is based, and is a second author on [4]. Excerpts of this thesis have been published in the following academic publications [1–4]. [1] C.G. Armstrong, K.E. Toghill, Cobalt(II) complexes with azole-pyridine type ligands for non-aqueous redox-flow batteries: Tunable electrochemistry via structural modification, J. Power Sources. 349 (2017) 121–129. doi:10.1016/j.jpowsour.2017.03.034. [2] C.G. Armstrong, K.E. Toghill, Stability of molecular radicals in organic non- aqueous redox flow batteries: A mini review, Electrochem. Commun. 91 (2018) 19–24. doi:10.1016/j.elecom.2018.04.017. [3] R. Hogue, C. Armstrong, K. Toghill, Dithiolene Complexes of First Row Transition Metals for Symmetric Non-Aqueous Redox Flow Batteries, ChemSusChem. (2019) 1–11. doi:10.1002/cssc.201901702. [4] C.G. Armstrong, R.W. Hogue, K.E. Toghill, Application of the dianion croconate violet for symmetric organic non-aqueous redox flow battery electrolytes, J. Power Sources. 440 (2019) 227037. doi:10.1016/j.jpowsour.2019.227037. The research presented within this thesis was in part financially supported by the Engineering and Physical Sciences Research Council (EPSRC), however the author declares no conflicts of interest. Craig Gregory Armstrong. MSci Lancaster University, UK iii Acknowledgements First and foremost, I would like to thank my supervisor Dr Kathryn Toghill. I am very grateful for her offering me this PhD position in the first instance and mentoring me since then. Despite my initial lack of electrochemistry knowledge, I can now call myself an electrochemist and battery chemist, because of Kathryn’s tutoring. She has been an exceptional academic to work for as she deeply cares for her group members. In reflection, being Kathryn’s first PhD student has been a challenge as it was mostly on me to build our experiments from scratch, however, her continuing support has led to a very fruitful PhD with four publications. In addition, I have developed a resourcefulness and progressive attitude from Kathryn because of our limited resources, which I think is lost on some PhD students. I therefore consider Kathryn to be my role model both in academic and personal life. Thank you for all the opportunities I have had, such as the international conferences and working in Switzerland for three months. I would also like to thank other Toghill group members past and present for their assistance. Most noteworthy Dr Ross Hogue, of whom I learnt many synthetic chemistry tricks, and Dr Declan Bryans for sharing his battery knowledge. Many thanks should be directed to my family for financially supporting me through my university endeavours and their positivity towards my studies. I apologise for all those long car journeys to ferry me back and forth with all my junk to university, however I know they never minded too much. The last eight years has certainly been worth it all. Most importantly, I would like to thank my life partner Sapphire McNeil for keeping me alive while writing this heinous document. She continues to support all my big decisions in life, and I treasure her company. Sorry for every late night in the office or laboratory in completing my PhD. Finally, I would like to thank the members of my department and fellow PhD students for making my studies so enjoyable. Despite the endless building works, false alarms, power cuts and biblical flooding, I am very grateful to have worked in such an inclusive and nurturing department. I would also like to thank the university for funding my PhD as well as the RSC and EPSRC for additional funding. Thank you! -Craig iv Novel Redox Materials for applications in Symmetric Non-Aqueous Redox Flow Battery Energy Storage – Craig G. Armstrong – January 2020 Abstract Redox flow batteries are promising energy storage devices for grid-scale applications due to their decoupled power and capacity. The utilisation of non- aqueous electrolytes, as opposed to conventional water-based electrolytes, is a promising pathway for achieving techno-economic targets via advancements in energy density. Herein a selection of novel redox materials were explored for application as symmetric redox flow battery electrolytes whereby the same compound is used in both the battery cathode and anode reactions. Metal coordination compounds such as Co(II) complexes with tridentate azole-pyridine ligands demonstrated good long-term stability and cell potentials in excess of 1.5 V, however low solubility due to their large size is problematic. Smaller metal complexes with bidentate dithiolene ligands gave promising redox properties however instability of charged oxidation states causes rapid decomposition of the inorganic electrolyte. Similar instability was observed for a new symmetric redox material, croconate violet, which arises from high reactivity of radical states. The instability of the charged oxidation states of novel redox materials remains a challenge for the research field, as capacity loss has been reported in practically every novel non-aqueous redox material. Indeed, by developing the ferrocene- ferrocenium ion redox couple for non-aqueous cell characterisation, a noteworthy capacity loss over extended battery cycling experiments was observed. The present work therefore highlights the challenges with identifying suitable non- aqueous redox materials for application. v Contents Chapter 1 Introduction to Redox Flow Batteries .............................................. 1 1.1 Energy storage technologies .......................................................................................... 1 1.2 Development of the commercial redox flow battery ............................................ 4 1.3 Redox flow battery architecture ................................................................................... 7 1.3.1 Electrodes and current collectors........................................................................ 8 1.3.2 Membranes ................................................................................................................... 9 1.4 Electrolytes for redox flow batteries ....................................................................... 11 1.4.1 Aqueous electrolytes ............................................................................................. 11 1.4.2 Non-aqueous electrolytes .................................................................................... 14 1.5 Progress in non-aqueous redox flow batteries .................................................... 17 1.6 Thesis objectives and overview ................................................................................. 21 Chapter 2 Fundamental Electrochemistry Theory ........................................ 23 2.1 Equilibrium electrochemistry and thermodynamics ........................................ 23 2.1.1 Electrochemical equilibrium .............................................................................. 24 2.1.2 Electrode potentials ............................................................................................... 27 2.1.3 Thermodynamics of electrochemical reactions .......................................... 29 2.1.4 Practical reference electrodes............................................................................ 30 2.2 Dynamic electrochemistry ........................................................................................... 32 2.2.1 Mass transport ......................................................................................................... 33 2.2.2 Structure of the electric double layer .............................................................. 35 2.2.3 Electrode kinetics ................................................................................................... 37 2.2.4 Marcus theory .......................................................................................................... 43 2.2.5 The Cottrell equation and the Nernst diffusion layer ............................... 44 2.3 Electrochemical techniques......................................................................................... 46 2.3.1 Principals of voltammetry ................................................................................... 47 2.3.2 Hydrodynamic methods: the rotating disc electrode ............................... 52 2.3.3 Impedance ................................................................................................................. 54 2.3.4 Electrolysis ................................................................................................................ 56 2.4 The electrochemistry of redox flow batteries ...................................................... 57 2.4.1 Theoretical cell potential ....................................................................................