Master in Chemical Engineering Study of redox flow battery systems for residential applications A Master’s dissertation of Nuno Miguel Azevedo Dias Lima Delgado Developed within the course of dissertation held in VisBlue Aps Supervisor at FEUP: Prof. Adélio Mendes Supervisor at VisBlue: Dr. Luis Carlos Pérez Martínez Chemical Engineering Department July of 2017 CLASSIFIED DOCUMENT. USE ONLY FOR EVALUATION PURPOSES. Study of redox flow battery systems for residential applications Acknowledgements I would not be able to do this thesis in first place without Professor Adélio Mendes. Prof. Mendes not only presented me with the opportunity to do my thesis in VisBlue Aps, but was also very supportive with practical matter. He was always available to hear me and give important feedback about my work. A big thank you to Prof. Mendes for such support and opportunity. I would also like to thank Dr. Luis Martínez for being such amazing supervisor and most of all, friend, by guaranteeing from beginning that I had everything that I needed for my stay to be as pleasant as possible, from providing pillows and bed sheets to lab material and supporting with the rent, by having lunch with me most of the days at Statsbiblioteket, by being always available to hear me, either about work or personal matter, and by teaching me how to operate batteries To VisBlue Aps team, a big big big thank you! It has been a pleasure to work with such amazing and versatile team who accepted me not just as an internship student but also as a collaborator and friend: Thank you to Søren Bødker (CEO) for accepting my internship, teaching me that there is space for leadership and friendship in a company and for keeping everyone extra- motivated; to Professor Anders Bentien for being always available to help me with the thesis and for also making sure I was not missing anything in Denmark by helping me, for example, with the room rent along with Dr. Martínez; to Mads Hansen and Morten Madsen for helping me feeling integrated in the team from the first until the last day of my stay, for teaching me everything I learnt regarding construction, mechanic and electrical matter and for, besides co- workers, also being my friends; to Jakob Terp, which also contributed for me to feel integrated in the team. This is a team I truly believe in, not only for their dedication and relationship between each other, but also for how amazingly they deal with success and failure and how they push themselves further to improve as professionals and human beings. I had financial support from Erasmus+ program and without such support, I would never be able to be so financially comfortable. Thank you for the people in charge of such amazing program for this opportunity! I had an amazing support from my parents (Maria Helena Dias and Pedro Delgado) on my decision of doing this thesis in Aarhus, Denmark. So, I would like to say a big thank you to them for supporting me emotionally and making such financial efforts to make sure that I was not lacking anything in a very expensive country. To my girlfriend, Diana Gomes, thank you for being so supportive. Despite the distance between the two of us for nearly 5 months, without her support and trust my stay would have been much harder. Study of redox flow battery systems for residential applications I could not forget to thank Dr. André Monteiro for being always available to help me in whatever I needed, for being always honest and for the support provided during my internship in July 2016 at LEPABE. This also includes the whole team working at lab 202. To who else made this thesis possible, thank you. This work was supported by VisBlue, by Aarhus University and by research project POCI-01- 0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy (LEPABE) UID/EQU/00511/2013) funded by Fundo Europeu de Desenvolvimento Regional (FEDER) through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT (Fundação para a Ciência e a Tecnologia). Study of redox flow battery systems for residential applications Abstract Vanadium Redox Flow Batteries (VRFB) outstand other electrochemical energy storage devices due to their high cyclability, which can be as high as 10 000 cycles. Storage capacity fade is one of the most important factors that compromises the long-term operation, stability and thus cyclability of VRFB. In this work, a potentiometric titration method that couples potassium permanganate and ammonium iron (II) sulfate was developed, validated and optimized. The method was used to measure the concentration of vanadium ions with a maximum variation coefficient, when optimized, of 4.46 % and to estimate state of charge and detect electrolyte imbalance. Furthermore, two VRFB systems of technical relevance (48 V DC nominal potential), one with anion (FAP450) and one with cation exchange membranes, were studied. It was found that electrolyte either on negative or positive tank may independently limit the charge or discharge process of VRFB because of electrolyte imbalance at the battery. It was also concluded that vanadium ions and water crossover direction is dependent on the type of membrane and capacity fade magnitude is dependent on stack design, stack materials and operation parameters of the battery. Keywords: Vanadium redox flow battery (VRFB), potentiometric titration, performance limiting tank, capacity fade, mass transference Study of redox flow battery systems for residential applications Declaration I hereby declare, on my word of honour, that this work is original and that all non-original contributions were properly referenced with source identification. Study of redox flow battery systems for residential applications Index 1 Introduction ........................................................................................... 1 1.1 Energy storage ................................................................................... 1 1.2 Redox flow batteries ........................................................................... 4 1.3 Presentation of the company ................................................................. 5 1.4 Thesis objective ................................................................................. 5 2 Vanadium redox flow battery ..................................................................... 6 2.1 State of the art .................................................................................. 6 2.1.1 Standard potential ......................................................................................7 2.1.2 Equilibrium potential ...................................................................................9 2.1.3 Efficiencies ...............................................................................................9 2.2 Components and materials .................................................................. 10 2.2.1 Ion exchange membrane ............................................................................. 11 2.2.2 Electrodes .............................................................................................. 12 2.2.3 Electrolyte .............................................................................................. 12 3 Capacity fade and state of charge in VRFB ................................................... 14 3.1 Capacity fade factors ......................................................................... 14 3.1.1 Membrane crossover .................................................................................. 14 3.1.2 Hydrogen and oxygen evolution .................................................................... 15 2+ 3.1.3 V oxidation with oxygen air ........................................................................ 16 3.2 Methods to assess the state of charge and electrolyte imbalance .................. 16 3.2.1 Vanadium ions concentration ....................................................................... 17 3.2.2 Standard reduction potential ....................................................................... 18 3.2.3 Open circuit potential ................................................................................ 18 3.2.4 Conductivity ............................................................................................ 18 4 Methods and materials ............................................................................ 20 4.1 VRFB specifications ........................................................................... 20 4.2 Sample collecting ............................................................................. 22 i Study of redox flow battery systems for residential applications 4.3 Redox titrations ............................................................................... 22 4.4 Validation ....................................................................................... 25 5 Results and discussion ............................................................................ 26 5.1 Validation ....................................................................................... 26 5.2 VisBlue 6 ........................................................................................ 27 5.2.1 Standard operation .................................................................................... 27 5.2.2 Remixing operation ................................................................................... 30 5.3 VisBlue 8 .......................................................................................