It is important to study the effect of Fe(III) on the positive electrolyte, in order to provide some practical guidance for the preparation and use of vanadium electrolyte. The effect of Fe(III) on the thermal stability and electrochemical behaviour of the positive electrolyte for the vanadium redox flow battery (VRFB) was investigated. When the Fe(III) concentration was above 0.0196 mol l ⁻¹ , the thermal stability of V(V) electrolyte was impaired, the diffusion coefficient of V(IV) species decreased from (2.06–3.33) × 10 ⁻⁶ cm ² s ⁻¹ to (1.78–2.88) × 10 ⁻⁶ cm ² s ⁻¹ , and the positive electrolyte exhibited a higher electrolyte resistance and a charge transfer resistance. Furthermore, Fe(III) could result in the side reaction and capacity fading, which would have a detrimental effect on battery application. With the increase of Fe(III), the collision probability of vanadium ions with Fe(III) and the competition with the redox reaction was aggravated, which would interfere with the electrode reaction, the diffusion of vanadium ions and the performance of VRFB. Therefore, this study provides some practical guidance that it is best to bring the impurity of Fe(III) below 0.0196 mol l ⁻¹ during the preparation and use of vanadium electrolyte.

The physicochemical and electrochemical performance of electrolytes prepared with different grades of V₂O₅ raw materials were investigated systematically for a vanadium redox flow battery. Physicochemical tests showed that the conductivity of electrolytes prepared with lower grades of V₂O₅ raw materials obviously decreased, while the viscosity increased. The results of electrochemical experiments showed that the electrochemical activity and reversibility of electrolytes decreased, and the solution resistance increased obviously, as the grade of V₂O₅ raw materials gradually decreased. In addition, the battery efficiency and charge–discharge capacity were negatively affected by impurities in the lower grade V₂O₅ raw materials, due to an increase of polarization on the charge–discharge voltage. Moreover, the performance of electrolytes was related to the total concentration of impurities in the electrolyte, and Na, K impurity ions were the main factors that adversely affected the electrochemical activity and reversibility, mass transfer, and capacity of the electrolytes. Based on the economic analysis, the impurities in V₂O₅ raw materials would not only reduce the performance of electrolytes, but also affect the production costs of electrolytes and the economic profits. Through this fundamental research, people can better understand the influence of V₂O₅ raw materials on electrolyte properties, and direct more attention to research how to effectively use lower grade V₂O₅ raw materials to reduce the costs of electrolyte preparation.

Glucose, sucrose, D(+)-xylose and α-lactose monohydrate are selected as additives relative to the negative electrolyte of Vanadium Redox Flow Battery (VRFB), with the aim of reducing vanadium permeation and improving electrochemical performance to mitigate capacity decay. The results of a charge–discharge test show that the cell with α-Lactose monohydrate in the negative electrolyte exhibits the best capacity retention. The capacity retention of a single cell employing 1 wt% α-Lactose monohydrate in the negative electrolyte was 71% after 30 cycles, which is 41.5% higher than 29.5% of the control group. Correspondingly, adding α-Lactose monohydrate into the negative electrolyte also significantly inhibits vanadium crossover and water transfer. Furthermore, the effects of additives on the performance of the negative electrolyte are studied by thermal stability experiments, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The stability experiments indicate that the introduction of 1 wt% α-Lactose monohydrate can elevate the stability of the negative electrolyte at low temperatures. The electrochemical measurements indicate that V(III) electrolyte with 1 wt% α-Lactose monohydrate obtains superior electrochemical activity and reversibility, which can be ascribed to the fact that the hydroxyl group carried by the additive provides more active sites for the redox reaction. Herein, the study provides a meaningful reference for mitigating the capacity decay of VRFB.