(630a) Optimization of Vanadium Redox Flow Battery: Ion-Exchange Membrane Screening and State of Charge Monitoring

Non‑stable and hardly predictable power of renewable sources demands an effective management on the power transmission level. Engineering development of new local energy storages forming a part of the modern transmission network and tailored to efficient and smart electricity is one of the major challenges.

Redox flow batteries are perspective devices mainly for the stationary storage of electric energy, due to their flexible installation and operation, versatile scalability, reduced self-discharge and high round-trip efficiencies. Among many possible combinations of redox couples, the so called all-vanadium redox flow battery (VRFB) offers the efficient energy storage of flexible capacity, predominantly for stationary applications as load levelling, smart-grids, peak shaving and back-up sources. The vanadium redox flow battery consist of two electrode spaces most often separated by ion-exchange membrane. Carbon felt is commonly used as electrodes / bipolar plates. The electrolytes are stored in external tanks and are pumped in to the battery stack where charging/discharging takes place. The usage of identical electro-active element on both sides of the cell (vanadium ions in four different oxidation states: 2+, 3+, 4+ and 5+) results in long-life operation without irreversible contamination of electrolytes. This concept was proposed by Skyllas-Kazacos [1] several decades ago. However up to this days there are relevant engineering problems preventing the wide utilization of the technology.

This work focus on the optimisation of VRFB in regards to ion-exchange membrane selection and online monitoring of the state of charge (SoC) of the battery. 

The ion-exchange membrane (e.g. Nafion) is one of the key components of the VRFB battery stack, it has a direct influence on the power and the efficiency of the system. The reduction of the membrane cost should bring a significant economic advantage for the VRFB against other competitive technologies. Several polymeric ion-exchange membranes were characterized with respect to the conductivity and permeability for vanadium ions across the membrane. The impact of membrane composition, method of its fabrication, thickness and ion-exchange capacity on the measured characteristics was observed. The selected membranes were tested in our laboratory VRFB (efficiency 83% at 56 mA/cm² with Nafion N115).

Our state of the charge monitoring system is based on the combination of three techniques: conductometry, open cell voltage measurements and spectrophotometry. The combination of these methods enables for the on-line SoC monitoring of the individual VRFB electrolytes without the necessity of current load interruptions which is the desired property for the VRFB management. At the same time, the monitoring system enables a closer insight to the key phenomena as vanadium and proton diffusion through the membrane, hydrogen evolution etc.

[1] M. Skyllas-Kazacos, M. Rychcik, R. G. Robins, A. G. Fane, and M. A. Green, Journal of The Electrochemical Society 1986, 133, 1057.