© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim COMMUNICATION 487 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de FULL PAPER Adv. Energy Mater. 2012, 2, 487–493 1. Introduction Renewable energy technologies have attracted considerable scientiﬁc and social interest in recent years. Among the avail- able renewable energy sources, solar and wind are probably the most abundant and readily accessible, and are increas- ingly being recognized as essential components of future global energy production. [1] However, the variable nature of the power from these intermittent renewable sources makes their use and dispatch through the aging grid quite chal- lenging. One effective way to smooth out the intermittency is to use electrical energy storage. Redox ﬂow batteries (RFBs), one of the most promising electrical energy storage systems, are capable of storing large amounts of power or energy (up to multi-MW and -MWh, respectively) in a pair of reduced and oxidized species dissolving in two separate liquid electrolytes. The conversion between the electrical energy and chemical (or electrochemical) energy occurs as the electrolytes ﬂow though electrodes in a cell stack. Between the electrodes is an ionic conducting membrane or separator that keeps the two electrolytes from mixing while allowing the transport of the charge- carrying ions to complete the circuit. With the electrolyte and electro-active materials stored externally, the most prominent advantage of the RFBs is that their power and energy are not coupled as other bat- tery systems are. The decoupling of the power and energy requirements thus renders to RFBs considerable design lati- tude to optimize power acceptance and delivery capabilities while maintaining the same energy density for different energy storage applications. Unlike other con- ventional battery systems, such as Li-ion batteries, the electrodes in RFBs do not undergo physical and chemical changes during cycling; therefore they are free of repetitive mechanical or structural stresses, leading to a potentially longer service life, which could lower the capital cost of the battery system since the investment can be amortized over a longer time period. By storing the reactive materials separately, the RFBs are also an inherently safer energy storage system compared with other types of batteries. In a ﬂow battery, the ﬂowing electrolytes carry away the heat generated during operation, resulting in an unparalleled advantage over other battery systems in terms of thermal management, especially for large-scale energy storage. Other advantages include quick response, deep discharge ability, the capability to withstand ﬂuctuating power supply, and tolerance to over-charge and over-discharge. Invented in 1975 at the National Aeronautics and Space Administration (NASA), [2] the redox ﬂow battery has witnessed continuous development over the years. The ﬁrst true redox ﬂow battery used the Fe 2 + /Fe 3 + halide solution electrolyte in the posi- tive half-cell and the Cr 2 + /Cr 3 + halide solution electrolyte in the negative, which encountered a severe cross-contamination issue. In an effort to mitigate the challenge of the cross-contamination, two important redox ﬂow battery systems were invented and developed in the 1980s, namely GEN 2 Fe/Cr redox ﬂow bat- teries, which employ a mixed electrolyte as both positive and negative electrolyte, [3] and all-vanadium ﬂow batteries (VRBs), which enlist the same element—vanadium in this case—in both positive and negative electrolytes. [4–7] In addition, a number of other redox chemistries were reported, including V 2 + /V 3 + vs. Br - /ClBr 2 , [8–10] Br 2 /Br - vs. S/S 2 - , [11,12] Br - /Br 2 vs. Zn 2 + /Zn, [13,14] Ce 4 + /Ce 3 + vs. V 2 + /V 3 + , [15] Fe 3 + /Fe 2 + vs. Br 2 /Br - , [16] Mn 2 + /Mn 3 + vs. V 2 + /V 3 + , [17] Fe 3 + /Fe 2 + vs. Ti 2 + /Ti 4 + , [18] and others. [19] Wei Wang, Zimin Nie, Baowei Chen, Feng Chen, Qingtao Luo, Xiaoliang Wei, Guan-Guang Xia, Maria Skyllas-Kazacos, Liyu Li,* and Zhenguo Yang* A New Fe/V Redox Flow Battery Using a Sulfuric/Chloric Mixed-Acid Supporting Electrolyte A redox ﬂow battery using Fe 2 + /Fe 3 + and V 2 + /V 3 + redox couples in chloric/sul- furic mixed-acid supporting electrolyte is investigated for potential stationary energy storage applications. The Fe/V redox ﬂow cell using mixed reactant solutions operates within a voltage window of 0.5–1.35 V with a nearly 100% utilization ratio and demonstrates stable cycling over 100 cycles with energy efﬁciency >80% and no capacity fading at room temperature. A 25% improve- ment in the discharge energy density of the Fe/V cell is achieved compared with a previously reported Fe/V cell using a pure chloride acid supporting electrolyte. Stable performance is achieved in the temperature range between 0 and 50 °C as well as when using a microporous separator as the mem- brane. The improved electrochemical performance makes the Fe/V redox ﬂow battery a promising option as a stationary energy storage device to enable renewable integration and stabilization of the electric grid. DOI: 10.1002/aenm.201100527 Dr. W. Wang, Z. Nie, Dr. B. Chen, Dr. F. Chen, Dr. Q. Luo, Dr. X. Wei, Dr. G.-G. Xia, Dr. L. Li, Dr. Z. Yang Paciﬁc Northwest National Laboratory P. O. Box 999, Richland, WA 99354, USA E-mail: liyu.li@pnnl.gov; zgary.yang@pnnl.gov Prof. M. Skyllas-Kazacos School of Chemical Engineering University of New South Wales Sydney, NSW, Australia, 2052