Redox-flow | Secondary batteries: Redox flow battery—Vanadium Redox

Recent extreme climate events around the world have accelerated the push for renewable energy to replace fossil fuels as our principal power source for the grid of the future. Long duration energy storage is now the critical issue for the world's future energy supply and redox flow cells offer the best combination of energy efficiency, capital cost and life-cycle costs compared with other technologies. The vanadium flow battery (VFB) has already been acknowledged world-wide as one of the most attractive and versatile energy-storage technologies available for the world's growing demand for the storage of renewable energy, for back-up power and grid stabilization. By 2022, around 25 companies around the world have installed more than 150 medium-to-large scale VFB systems in Europe, Asia, North America and Australasia and vanadium mining and electrolyte production are being scaled up to meet the growing demand from the VFB market. Having the same vanadium in sulfuric acid electrolyte in both half-cells, the VFB system eliminates problems of cross-contamination, allowing very long life and simple electrolyte recycling, in addition to high energy efficiencies and the lowest levelized cost for storage capacities above 4 h. While it offers many advantages for large-scale energy-storage applications compared with other storage technologies, its main drawback has been the low specific energy of between 15 and 25 kW kg−1. In 2001, a second-generation VFB that employs a vanadium bromide electrolyte in each half-cell (G2 V/Br) was proposed. This was followed by the mixed-acid electrolyte VFB (G3 VFB) which has a higher specific energy and wider temperature range that make it more suited to very hot climates. Despite their higher energy densities however, both the G2 V/Br and the G3 mixed acid VFB systems require additional control measures that add to their cost.