|Fig. 1: Schematic of a vanadium redox-flow battery.|
While huge effort has been made on exploring and developing new energy sources, such as wind, sunlight, tides, and geothermal heat, it is also very important to investigate energy storage technologies, which can improve the stability of the new energy sources and make them easier to couple with traditional energy sources, like coal, oil, and natural gases. Energy storage technologies reversibly convert electricity to other forms to mitigate the fluctuation of electricity generation and consumption, and to overcome the mismatch between them.  Among the various large-scale energy storage technologies, redox-flow batteries are very promising and vanadium redox-flow batteries are the most developed and the most close to commercialization. [2,3]
As the schematic shown in Fig. 1, a vanadium redox-flow battery has two chambers, a positive chamber and a negative chamber, separated by an ion-exchange membrane. These two chambers are circulated with electrolytes containing active species of vanadium in different valence states, VO2+/VO2+ in the positive electrolyte and V2+/V3+ in the negative electrolyte. During discharge process, VO2+ is reduced to VO2+ at the positive electrode and V2+ is oxidized to V3+ at the negative electrode, as shown in Equation(1) and (2). The reactions proceed in the opposite direction during charge process. The active species are normally dissolved in a strong acid, and the protons transport across the ion-exchange membrane to balance the charge. The standard voltage produced by the vanadium redox-flow battery system is 1.25 V. [1-3]
|Positive Electrode: VO2+ + H2O - e- → VO2+ + 2 H+ (E0 = 0.99 V vs. SHE)||(1)|
|Negative Electrode: V3+ + e- → V2+ (E0 = -0.26 V vs. SHE)||(2)|
The same as other redox-flow batteries, vanadium redox-flow batteries have high energy efficiency, short response time, long cycle life, and independently tunable power rating and energy capacity. [3,4] Additionally, because the active species in positive electrolyte and negative electrolyte are all vanadium, though in different valence state, the vanadium redox-flow batteries do not have the issue of cross-mixing of positive and negative electrolytes. 
One disadvantage of vanadium redox-flow batteries is the low volumetric energy storage capacity, limited by the solubilities of the active species in the electrolyte. The cost of vanadium may be acceptable, because it is a relatively abundant material, which exists naturally in ~65 different minerals and fossil fuel deposits.  However, the system requires the using of expensive ion-exchange membrane, which can contribute more than 40% of the overall battery cost. 
Since the vanadium redox-flow batteries invented by the M. Skyllas-Kazacos group at University of New South Wales in 1980s, more than 20 large-scale demonstrations have been built in different countries, including Australia, Thailand, Japan, USA, and China. [1,6,7] One recent example is a 260 kW system installed by Dalian Institute of Chemical Physics and Rongke Power in 2010 in China. At the same time, they are also building a 5 MW system at a 30-50 MW wind farm for output power stabilization.  Despite those practical applications, commercialized vanadium redox-flow battery is still not available. [1,2]
Aiming to eventually promote the vanadium redox-flow batteries to commercial application, studies are carried out on the following aspects: (1) robust ion-exchange membranes with high proton conductivity, good selectivity, and especially low cost;  (2) three-dimensional electrodes with large surface area, good chemical stability in strong acid, and high catalytic activity;  and (3) additives or other approaches to stabilize the active vanadium species in electrolytes with high concentrations. 
Vanadium redox-flow battery is promising as an energy storage technology. I believe it would not take too long to overcome the limit and realize the commercialization of this technology.
© Xing Xie. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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