An electrochemical impedance spectroscopy study of ZnI2 redox flow batteries

This thesis work focuses on the zinc-iodine redox flow battery system with ananalysis of the electrochemical changes which occur throughout charge and dis-charge. A literature review of energy storage techniques was first carried out, fo-cusing on flow batteries and the current state of art. The zinc-iodine system waschosen as the chemistry of focus due to its performance characteristics, promisingnewly published research on the subject, and its environmentally-safe chemistry.A ZnI2redox flow cell was first designed and characterized with scanning electronmicroscopy (SEM) and charge-discharge analysis in order to verify its performance.The impedance response of the cell was then captured at various charge levels. Sepa-rate impedance cells were then used to effectively analyze the impedance responses ofthe anolyte and catholyte separately. The half cell responses after charge/dischargewere determined to understand the electrochemical effects on each electrolyte as wellas the system as a whole.Upon cycling of the full cell, internal ohmic resistance, charge-transfer resistance,and double layer capacitance decreased significantly. The ohmic resistance changewas attributed to the differences in solution resistance of the anolyte. During charge,an increase in Zn+2ion mobility and migration of K+ions, together led to loweringof solution resistance. Charge-transfer resistance was lowered due to the increase inavailable deposition sites and ease of this reduction reaction on the anode surface asZn+2was reduced to metallic zinc. The full cell was then cycled at various levels ofcharge while impedance cells were used to measure the impedance response of thehalf cells. The impedance responses of the catholyte and anolyte displayed differentelectrochemical effects. While the solution resistance of the catholyte increased withcapacity due to the loss of free charge carriers, the charge-transfer resistance anddouble-layer capacitance decreased with state of charge (SoC) due to the depositionprocess of Zn. With a better understanding of the response of the individual halfcells, electrolyte or cell material improvements could be recommende