State of the art of all-Vanadium Redox Flow Battery: A Research Opportunities M.R. Mohamed 1 , H. Ahmad 2 , and M.N. Abu Seman 2 1 Sustainable Energy & Power Electronics Research (SUPER) Group, 2 Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Kuantan, Pahang, MALAYSIA. Abstract This paper deals with the state of the art of redox flow battery (RFB) focusing on vanadium-based electrolytes. A broad review on energy storage technologies is first presented to bring RFBs system into perspective. Subsequently, discussions are focusing on vanadium-based RFB in regards to justify the motivation factors of chosen V-RFB as a system to be studied. Research potential and challenges for V-RFB system are discussed in detail. Keywords: redox flow battery, all-vanadium redox flow batteries, energy storage, hybrid electric vehicle 1. Introduction Over the last few years, many types of RFB have been considered; including bromide-polysulfide, vanadium-vanadium, vanadium-bromine, iron- chromium, zinc-bromine, zinc-cerium, and soluble lead RFBs. This paper anyhow would focusing on vanadium- based system as scope of the study while details review of the RFBs has been provided by Ponce et al. (Ponce- de-León et al., 2006). A concise summary of the energy storage technologies is first reviewed in intention of bringing V-RFB into perspectives. Research potential and challenges for V-RFB system are discussed. 2. Overview of Energy Storage Technologies In general, the energy storage technology can be grouped in 3 main categories i.e. mechanical storage systems, electrical storage systems, and electrochemical storage system. In mechanical storage system, while it has been installed for many applications, safety is the major obstacle for variety of applications. Flywheel technologies for example, even though it has been around for more than a century, this technology is only considered significant as energy storage for various applications due to the improvement in materials, magnetic bearing control and power electronics technologies (Hebner et al., 2002) in 1980s. Although it exhibits many advantages such as environmentally friendly, free from depth of discharge effects, could deliver high power and energy density, yet the system remains under review for larger commercial use due to safety of flywheel tensile strength that would cause wheel-flying apart. Further durability flywheel management and better storage is still in research to overcome the limitation. Alternatively, in electrical storage system, despite the fact that it operates at very high efficiency, the high power density but lower energy density delimits its applications. Ultra-capacitor for instance, could deliver very high power density, rating at 10 – 100 s kW, but it could only deliver a very low energy capacity (< 1 kW h) (Chen et al., 2009). The same with superconducting magnetic energy storage (SMES), high cost and environmental issues associated with strong magnetic field are major concerns for technology development. Current technology rating at 1 – 10 MW for small SMES (storage time in seconds) whereas larger scale of SMES rating at 10 – 100 MW (storage time in minutes) (Koshizuka et al., 2003). Therefore, the system is not intended to replace the other energy storage, but rather complimenting in whole energy storage system (Blanc, 2009). Electrochemical storage energy has become the oldest and most established energy storage devices which not only provide energy flexibility and environmental benefits, but also offer a number of important operating benefits to the load (Chen et al., 2009). It main advantages are ability to response for quick load demand, generally very high energy efficiency (average of 80 %) and simplicity for modularity (Kondoh et al., 2000). This has become very attractive features for various applications. While conventional lead acid has matured and proven to be cheaper (~ $200 / kW h) and most reliable energy storage for many years, its applications has been delimited by short cycle life (around 1000 cycles), low