Electrochemical Sensor Development for Fluoride Molten Salt Redox Control THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Nikolas W. Shay Graduate Program in Mechanical Engineering The Ohio State University 2017 Master's Examination Committee: Jinsuo Zhang, Co-advisor Marat Khafizov, Co-advisor Copyrighted by Nikolas William Shay 2017 Abstract Investment in nuclear technology is experiencing a revitalization as nuclear power becomes uniquely poised to take the burden left by phasing out fossil fuels to meet climate change goals. The United States Department of Energy is investing in research and development of the Fluoride salt-cooled High-temperature Reactor (FHR) with the ultimate goal of a 2030 deployment. One challenge presented by this reactor is corrosion of the reactor’s structural materials by the molten salt due to foreign and generated impurities. These impurities will shift the reduction-oxidation (redox) potential of the salt beyond the equilibrium potential of candidate structural materials, causing accelerated corrosion. This issue demands control of the molten salt’s redox condition in order to prevent unacceptable levels of corrosion. Research has been conducted on methods for redox control and electrochemical measurement techniques. The limited research that has been conducted related to measurement apparatus either lack certain characteristics specific to application for the FHR reactor or appropriate comparison to demonstrate first rate performance. The primary issue presented by an electrochemical sensor for this application is the selection of an appropriate reference electrode. This report investigates candidate reference electrodes with the purpose of identifying a leading candidate and proposing a holistic electrochemical sensor design which possesses high performance, durability, and ii ease of use. Candidate reference electrodes are the platinum quasi-reference electrode, dynamic reference electrode, gold/sodium alloy reference electrode, and nickel/nickel(II) reference utilizing a boron nitride sheath. Electrochemical tests show that cyclic voltammetry is a precise technique to measure the concentration of a redox agent. Experiments also show that a quasi-reference electrode is the best suited reference for this application when paired with dynamic operational techniques. This choice of reference allows a full electrochemical sensor to be designed for placement in a molten salt forced convection loop. The design proposed here utilizes a stainless steel or nickel alloy housing, containing a boron nitride cylinder which serves to locate and electrically insulate the three-electrode cell. This proposed design is uniquely suited to meet the demands of redox control in an industrial molten salt application such as an FHR based on its high performance, durability, and ease of use. iii Acknowledgments Special acknowledgment is necessary for my advisor Dr. Jinsuo Zhang for being an excellent mentor, both academically and professionally, as well as being an irreproachable advocate for his students. Gratitude is expressed to Dr. Marot Khafizov for his generosity and service to a number of department students, including myself, during a critical transition period. Thanks are also expressed to Dr. Shaoqiang Guo for his collaboration and significant mentoring of electrochemical theory. I am also very grateful to Evan Wu and Ryan Chesser for sharing their advice and experience related to experimental electrochemistry. This study is funded by the NEUP-IRP program (Project number: IRP-14-7476, leading PI: Professor Farzad Rahnema). Professor Mike Short from the Massachusetts Institute of Technology generously donated the EuF3 which was used to perform this study’s experiments. iv Vita 2016................................................................B.S. Mechanical Engineering, The Ohio State University 2016-present ...................................................Graduate Research Assistant, Department of Mechanical, Aerospace, and Nuclear Engineering, The Ohio State University Fields of Study Major Field: Mechanical Engineering v Table of Contents Abstract ............................................................................................................................... ii Acknowledgments.............................................................................................................. iv Vita ...................................................................................................................................... v Table of Contents ............................................................................................................... vi List of Tables ................................................................................................................... viii List of Figures .................................................................................................................... ix Chapter 1: Introduction ...................................................................................................... 1 Molten Salt Nuclear Reactors ......................................................................................... 1 Corrosion and Materials .................................................................................................. 3 Redox Control of Molten Systems .................................................................................. 6 Electrochemical Techniques ......................................................................................... 11 Cyclic Voltammetry ...................................................................................................... 14 Reference Electrodes ..................................................................................................... 17 Project Proposal............................................................................................................. 22 Chapter 2: Methodologies ................................................................................................. 25 vi Experimental Methods .................................................................................................. 25 Sensor Design ................................................................................................................ 30 Chapter 3: Results and Discussion .................................................................................... 32 Quasi-reference ............................................................................................................. 32 Dynamic Reference ....................................................................................................... 45 Ni/Ni2+ with Boron Nitride Compartment .................................................................... 50 Gold Alloy Reference.................................................................................................... 53 Thermodynamic Study .................................................................................................. 59 Sensor Design ................................................................................................................ 63 Chapter 5: Conclusions ..................................................................................................... 68 Reference Electrode Selection ...................................................................................... 68 Sensor Design ................................................................................................................ 69 Sensor Applications....................................................................................................... 70 References ......................................................................................................................... 72 Appendix A: Cyclic Voltammetry Data ........................................................................... 76 Appendix B: Chronopotentiometry Data .......................................................................... 81 vii List of Tables Table 1: Summary of potassium reduction potential from full sweep cyclic voltammetry. ........................................................................................................................................... 37 Table 2: Summary of species concentration calculation from cyclic voltammetry measurements. ................................................................................................................... 43 Table 3: Summary of chronopotentiometry measurements with comparison to full sweep voltammetry. ..................................................................................................................... 50 Table 4: Summary of potassium equilibrium potential as found from chronopotentiometry measurements. ................................................................................................................... 60 Table 5: Summary of apparent standard potential as found from cyclic voltammetry data. ........................................................................................................................................... 60 viii List of Figures Figure 1: Applied potential profile for cyclic voltammetry. ............................................. 15 Figure 2: Operational schematic of the three-electrode cell. ............................................ 16 Figure 3: Boron nitride enclosed nickel/nickel(II) reference. ..........................................