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526 S. Xiao et al. / Electrochimica Acta 187 (2016) 525–534 [26]. Detailed studies have been conducted on the solubility of vanadyl sulfate in concentrated sulfuric acid solutions based on the solubility [27,28]. Great efforts have been directed to prevent or delay the precipitation in the electrolyte of the VRFB. It is found that high sulfuric acid concentration can dramatically enhance the stability of V(V) solution. But it also takes negative impact on the solubility of the V(II), V(III), and V(IV) ions [22]. Some organic or inorganic chemicals can be used as additives to stabilize the vanadium ions [29–34]. These additives could improve the stability of the electrolyte in a relatively wide range of temperatures (-5 C– 40C). Yang et al. proposed another strategy employing chloride [35] or a sulfate-chloride mixed acid system [24] as the supporting electrolytes instead of the pure sulfuric acid solutions, which shows better thermal stability and solubility. As an energy storage device, VRFB is usually used in different climates areas. The environmental temperature can affect the properties of electrolyte and electrode kinetics pronouncedly, which thereby influences the battery performance. Unfortunately, there have been few specific reports dealing with the temperature influences on the electrolyte properties. In this paper, we focused on temperature effect on the physicochemical and electrochemical properties of the different vanadium ion solutions (V(II), V(III), V3.5+, V(IV) and V(V)) from -35 C to 50 C. In order to establish a scientific and reasonable research method, the electrolyte with a moderate composition of 1.5 M vanadium in 3.875 M total sulphate was selected in this series of study. The static stability, viscosity, conductivity, cyclic voltammetry and electrochemical impedance spectroscopy of the electrolytes were investigated and compared. These features are used to evaluate and predict battery perfor- mance varying with the temperature, and then can further be used to determine the electrolyte composition referring to the environment. 2. Experimental 2.1. Preparation of electrolyte The initial V3.5+ electrolyte (V(III)/V(IV) = 1:1) was prepared by electrolytic dissolution of the suitable weight of V2O5 in the H2SO4 supporting electrolyte. Then, the V3.5+ electrolyte was injected into a vanadium redox flow battery (VRFB) and charged at a constant current density of 80 mA cm2. The detail parameters of the VRFB single-cell was described previously [36,37]. By precisely control- ling the charged electric quantity, four kinds of pure vanadium electrolyte with single valence could be obtained, namely the violet electrolyte of V(II), the green electrolyte of V(III), the blue electrolyte of V(IV) and the yellow electrolyte of V(V). In this paper, V(II), V(III), V3.5+, V(IV) and V(V) are used to denote V2+, V3+, V3.5+, V4+ Fig. 1. (a) Photographs of five types of vanadium electrolytes. The corresponding vanadium species changes during precharge and charge-discharge process is also shown in the figure; (b) Reaction equations of the vanadium electrolytes in positive and negative half-cell during different status of the VRFB; (c) Ionic compositions of the five types of vanadium electrolytes.PDF Image | Broad temperature adaptability vanadium redox flow battery
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