State of Charge and Capacity Tracking in VRFB Systems

Abstract

Numerous battery technologies have been developed, each with their own advantages and disadvantages which are discussed herein. An emerging form is the vanadium redox flow battery (VRFB), which utilizes vanadium's two soluble redox couples to create a fully liquid state system based off a single metal. The VRFB electrolyte is prone to several degradation mechanisms which reduces its capacity over time. While state of charge (SOC) monitoring is commonly done via voltage, this is only able to detect symmetrical capacity losses across the full system. As the electrolyte displays brilliant color changes upon cycling, the SOC may be monitored via electrolyte absorbance. As the proposed method is able to monitor each electrolyte individually, asymmetrical capacity losses may be detected. The primary goal of this thesis is to a) develop a flow cell which allows for optical monitoring of a continuous flow of acidic electrolyte, b) develop a measurement system capable of monitoring an electrolyte's absorbance of a white light source, and c) determine the suitability and accuracy of this technique for SOC monitoring of the VRFB anolyte and catholyte. Testing is performed on a 2.5 kW, 40 cell VRFB stack with 8L and 40 L volumes of electrolyte. An automatic electrolyte re-balancing mechanism is constructed by adding a hydraulic shunt between the two tanks, which has successfully demonstrated the ability to recover symmetric capacity loss due to net water transport. For continual monitoring of asymmetric losses, an FD11a photodiode was used to monitor electrolyte absorbance in a custom-made flow cell with thicknesses of 1/4", 1/8", 1/16". Although experiments were unsuccessful due to the high absorbance of the electrolyte, results displayed promise should a smaller optical path/stronger light source be used.