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In this study, 1.6 M vanadium electrolytes in the oxidation forms V(III) and V(V) were prepared from V(IV) in sulfuric (4.7 M total sulphate), V(IV) in hydrochloric (6.1 M total chloride) acids, as well as from 1:1 mol mixture of V(III) and V(IV) (denoted as V3.5+) in hydrochloric (7.6 M total chloride) acid. These electrolyte solutions were investigated in terms of performance in vanadium redox flow battery (VRFB). The half-wave potentials of the V(III)/V(II) and V(V)/V(IV) couples, determined by cyclic voltammetry, and the electronic spectra of V(III) and V(IV) electrolyte samples, are discussed to reveal the effect of electrolyte matrix on charge-discharge behavior of a 40 cm2 cell operated with 1.6 M V3.5+ electrolytes in sulfuric and hydrochloric acids. Provided that the total vanadium concentration and the conductivity of electrolytes are comparable for both acids, respective energy efficiencies of 77% and 72–75% were attained at a current density of 50 mA∙cm−2. All electrolytes in the oxidation state V(V) were examined for chemical stability at room temperature and +45 °C by titrimetric determination of the molar ratio V(V):V(IV) and total vanadium concentration.

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A series of vanadium redox-flow battery (VRFB) electrolytes at 1.55 M vanadium and 4.5 M total sulfate concentration are prepared from vanadyl sulfate solution and tested under conditions of appearance of "power drop" effect (discharge at high current density from high state-of-charge). A correlation between the initial electrolyte composition, the thermal stability of catholyte, and the susceptibility of VRFB to exhibit a "power drop" effect is derived. The increase in total acidity to 3 M, expressed as concentration of sulfuric acid in precursor vanadyl sulfate solution, enables "power drop"-free operation of VRFB at least at 75 mA cm À2. Thermally-induced degradation of electrolyte is evaluated based on decrease in vanadium concentration in the electrolyte series after exposure to the temperature of 45 C and based on characterization of catholytes series using 51 V, 17 O, and 1 H nuclear magnetic resonance spectroscopy.