The performance of all vanadium redox flow batteries at below-ambient temperatures

• Two VFB single cells performance at 0 and 20 °C was compared.
• The efficiencies and capacity of a stack were studied in controlled environments.
• Vanadium crossover reduced, benefitted the coulombic efficiency at low temperature.
• Operating a VFB at < 0 °C will result in significant losses in efficiency.

Temperature is a key parameter influencing the operation of the VFB (all vanadium redox flow battery). The electrochemical kinetics of both positive and negative vanadium redox couples were studied using CV (cyclic voltammetry). The CV results showed that the anodic peak current for the VO2+VO2+/VO2+ couple and the cathodic peak current for the V3+/V2+ decreased with temperature. The peak potential difference ΔEp for VO2+VO2+/VO2+ couple varied slightly with decrease in temperature while that for V3+/V2+ increased sharply from 276 mV at 30 °C to 481 mV at −10 °C, indicating that this reaction become more irreversible at the low temperature. Two VFB single cells operating at 0 and 20 °C were cycled and their performance was compared. While the low temperature reduced vanadium crossover and benefitted the coulombic efficiency, a concomitant lowering in the rate of proton transport resulted in an increase in ohmic over-potential and hence a lower voltage efficiency. The efficiencies and capacity of a VFB stack were monitored in controlled environments. The static resistance of the stack varied slightly between 26.5 and 29.0 mΩ at 5 and 10 °C, but increased to 38.1 mΩ on average at −2 °C.