## 1 Introduction  
Concern over environmental degradation and climate change associated with fossil fuel based electricity generation has prompted most countries to restructure their electricity generation and distribution in particular, towards increasing electricity generation from renewable energy sources. Renewable energy is intermittent in nature and thus the electricity generated from this source is not dispatchable, leading to unpredictable matching between supply and demand. Therefore energy storage is needed to prevent blackouts caused by unbalanced supply and demand. Besides, energy storage can also assist the utility planner to bridge the gap between the drops in the rate of electricity generated by renewable energy sources due to cloud cover over photovoltaic panel or reduced wind speed and the ramp up rate of gas powered peaking plants which normally take about 15 minutes to ramp up their power output. There are many types of energy storage technologies including pumped hydro, compressed air, fly-wheels and electrochemical systems such as fuel cells and redox flow batteries. Among these energy storage technologies, redox flow battery is considered the best option for medium to large scale storage owing to an excellent combination of energy efficiency, capital cost and life cycle costs without specific site requirement [1]. Redox flow battery uses two soluble redox couples as electroactive materials to store energy via oxidation and reduction reactions. In a typical set-up, the redox flow battery consists of 2 electrolyte reservoirs from which the electrolytes are circulated by pumps through an electrochemical cell stack comprising of a number of cells connected in series or parallel to enable reaction taking place at inert electrodes. Typically, each cell comprises of anode, cathode and an ion exchange membrane separator to allow diffusion of ions across the membrane while preventing the cross-mixing of the electrolyte solutions from these 2 reservoirs. Figure 1 shows the diagram of a vanadium redox flow battery (VRB). During charging, by applying an external power source at the terminals, the discharged form of the redox species in each half cell electrolyte is converted to the charged form. During discharge, electron flows between the redox species thus converting the chemical energy into electricity. Consequently, the concentration of the active redox species and electrolyte volume will determine the energy capacity of the system while the number of cells in the stack and electrode area will determine the system power. The redox flow battery therefore acts more like a regenerative fuel cell than a conventional battery [3]. Figure 1: Schematic of a Vanadium redox flow battery (Adapted from [2]). Of all the redox flow batteries developed to date, only the all vanadium redox flow battery developed at the University of New South Wales [3,4] has received the most attention due to its high energy efficiency of over 80% in large installations and a long cycle life. The major issues encountered by other redox flow batteries such as iron/chromium, iron/titanium, polysulfide bromine (also known as Regenesys) redox flow batteries include cross contamination of electrolytes due to different redox couple species used in each half-cell and the lack of development of an ideal membrane. The all vanadium flow battery uses the same vanadium element in both half cells which avoids the problems of cross contamination of the two half-cell electrolytes during long term usage. This helps in reducing the capital and maintenance costs, minimizes waste disposal and provides a greater ease of operation. Other attractive features include the ability to use underground electrolyte storage tanks leading to minimal footprint and temperature fluctuations in extreme climates. Furthermore, the redox flow battery can be recharged either conventionally or with mechanical refueling at a suitable refueling station, making it attractive for electric vehicles applications [2].