class="head no_bottom_margin" id="sec1title">IntroductionFlow batteries are of tremendous importance for their application in increasing the quality and stability of the electricity generated by renewable energies like wind or solar power (, ). However, research into flow battery systems based on zinc/bromine, iron/chromium, and all-vanadium redox pairs, to name but a few, has encountered numerous problems, such as the corrosion of bromine, poor kinetics of Cr2+/Cr3+ redox pair, relatively high cost, and low energy density of all-vanadium redox pairs, although these battery systems are currently at the demonstration stage (, ). These barriers have, on the one hand, hindered their further wide scale deployment, and on the other hand, accelerated research efforts into new flow battery chemistries (or the next-generation flow batteries, aqueous or non-aqueous redox flow batteries) (, ). Among the reported new systems, non-aqueous redox flow battery systems, having the features of wide electrochemical window, high energy density, inexpensive redox active materials, etc., are currently at the proof-of-concept stage. However, the low concentration and poor ion conductivity of organic-based electrolytes are the most critical issues to overcome (). Although aqueous flow battery systems, like TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-based flow battery and quinone-based flow battery, have been successfully demonstrated at the laboratory scale (the electrode area is normally less than 10 cm2), their relatively low performance at high current density (normally less than 100 mA cm−2, when the energy efficiency [EE] was above 80%) (, , , , , ) limits the quick response for energy conversion and increases the integration cost. In addition, some of the systems still have low open cell voltage (OCV) or low electrolyte concentration. The low working current density together with the low OCV will result in low power density of a flow battery, further leading to an increased stack size and an overall increased capital cost of a flow battery system. Currently, only a few membrane materials (such as perfluorinated ion-exchange membranes because of their high stability in critical medium, e.g., strongly acidic or alkaline conditions and highly oxidative medium) have been considered (, , href="#bib26" rid="bib26" class=" bibr popnode">Wang et al., 2011, href="#bib12" rid="bib12" class=" bibr popnode">Li et al., 2015, href="#bib13" rid="bib13" class=" bibr popnode">Li et al., 2016, href="#bib14" rid="bib14" class=" bibr popnode">Lin et al., 2015, href="#bib15" rid="bib15" class=" bibr popnode">Lin et al., 2016, href="#bib20" rid="bib20" class=" bibr popnode">Orita et al., 2016), which will definitely further result in cost issue for battery stacks (href="#bib4" rid="bib4" class=" bibr popnode">Chu et al., 2017). Besides, upscaling for practical application of these newly developed aqueous flow battery systems have been rarely reported, which may induce limitations on the energy storage system development to some extent.The alkaline zinc ferricyanide flow battery owns the features of low cost and high voltage together with two-electron-redox properties, resulting in high capacity (href="#bib19" rid="bib19" class=" bibr popnode">McBreen, 1984, href="#bib2" rid="bib2" class=" bibr popnode">Adams et al., 1979, href="#bib1" rid="bib1" class=" bibr popnode">Adams, 1979). The alkaline zinc ferricyanide flow battery was first reported by G. B. Adams et al. in 1981; however, further work on this type of flow battery has been broken off, owing to its very poor cycle life and the relatively low operating current density (35 mA cm−2) (href="#bib19" rid="bib19" class=" bibr popnode">McBreen, 1984). The poor cycle life is mainly due to the zinc dendrite under alkaline medium, where a cadmium-plated (Zn- or Cu-plated) iron substrate was employed for zinc stripping/plating, while the low operating current density could be due to the high resistance of a cation-conducting membrane and severe zinc dendrite derived from the metal electrode.Here we present a long cycle life alkaline zinc-iron flow battery with a very high performance. The battery employs Zn(OH)42−/Zn and Fe(CN)63−/Fe(CN)64− as the negative and positive redox couples, respectively, while a self-made, cost-effective polybenzimidazole (PBI) membrane and a 3D carbon felt electrode were combined. The PBI membrane carrying heterocyclic rings can guarantee fast transportation of hydroxyl ions after doping with a base solution (href="#bib11" rid="bib11" class=" bibr popnode">Li et al., 2003, href="#bib31" rid="bib31" class=" bibr popnode">Yuan et al., 2016a). Most importantly, the PBI membrane with ultra-high mechanical stability can resist the zinc dendrite very well, which ensures the cycling stability of the alkaline zinc-iron flow battery. In addition, a 3D porous carbon felt with high porosity and surface area, which serves as guidance for the zinc stripping/plating and suppresses zinc dendrite/accumulation effectively, provides the battery with excellent cycling stability and rate performance. Moreover, the concentration of Fe(CN)63−/Fe(CN)64− redox couple can reach 1 mol L−1 by optimizing the composition of electrolyte, which is much higher than the reported concentration of this redox couple (0.4 mol L−1) (href="#bib20" rid="bib20" class=" bibr popnode">Orita et al., 2016, href="#bib14" rid="bib14" class=" bibr popnode">Lin et al., 2015, href="#bib15" rid="bib15" class=" bibr popnode">Lin et al., 2016, href="#bib23" rid="bib23" class=" bibr popnode">Selverston et al., 2016, href="#bib24" rid="bib24" class=" bibr popnode">Selverston et al., 2017). The high concentration of active materials thus can afford the battery with high energy density. As a result, the proposed zinc-iron flow battery demonstrated an EE of 82.78% even at a high current density of 160 mA cm−2. A charge/discharge experiment of 500 cycles further confirmed the excellent stability of this system.
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