(28a) Review on Vanadium Redox Flow Battery Technologies: Current Trends and Future Directions

Renewable energy technologies are becoming increasingly important to help mitigate the environmental impacts of energy production and to preserve the planet for future generations. In accordance with these objectives, several electrochemical energy storage technologies, including lithium-ion, sodium-sulfur and ZEBRA batteries have been proposed and deployed in field applications. For all these rechargeable batteries, coupled energy storage capacity and power rating is a common drawback limiting their widespread implementation. In contrast, redox flow batteries offer the advantage of decoupled power and energy ratings, and the ability to adjust them for specific operational requirements (especially for energy centric applications).

During the last 30 years all-vanadium redox flow batteries (VRFB) have evolved from pioneering academic research to optimized and reliable energy storage systems with high cycle stability and longevity. Furthermore, VRFB technology is intrinsically safe, and the risk of thermal runaway is readily avoided. VRFBs are designed to effectively store and release energy for more than 20 years. For a comprehensive analysis of the total cost of ownership, it is necessary to consider long-term operating costs and system efficiency since they will become more significant over the lifecycle of the system than initial capital investment for acquisition and installation. Therefore, it is critical to optimize all the components of the battery to enhance its efficiency, durability and cycle stability.

This review starts with discussing basic thermodynamic terms, which are voltage- (VE), charge- (CE) and energy efficiency (EE). The voltage efficiency is correlated with the internal resistance of a stack or single cell. On this basis, the influence of each component in the electrochemical energy converter is discussed. The overall resistance can be separated into fractions with electronic and ionic contributions arising from system components such as, current collector, bipolar plate, electrode, membrane and electrolyte. Additionally, the contributions of the electrochemistry (reaction overvoltage) and mass transport have to be considered, which affect the voltage efficiency as well. To minimize the electrochemical overvoltage both the carbon-based electrode material and the electrolyte can be optimized, which is accomplished by methods of activation and new electrolyte mixtures, respectively, resulting in reduced charge transfer resistance for the vanadium reactions. The recent state of the art is analyzed by a literature review, and new developing technologies are discussed. The majority of VRFB demonstrations have been based on the cells with carbon felt electrodes, which provide excellent conditions for electrolyte flow. However, in recent years, carbon papers are introduced as an alternative to carbon felt electrodes, which are thin with a high surface area. In this review, the benefits and shortcomings of both approaches are discussed. It should also be noted that regardless of electrode type almost all VRFB systems reported in the literature require a membrane to separate positive and negative electrolyte. However, membranes often contribute to total resistance with the highest percentage. To overcome this drawback, new types of electrolytes (immiscible) have been discussed in the recent literature which enable membrane-free flow cell designs. Overall, the objective of this review is to offer a summary of recent developments and best design practices in the field of VRFBs.