Given that the formation of the neutral complex VO 2 Cl starts
at around 20 ° C (see Figure 2 d), reactions 7 to 9 should dominate
only in the elevated temperature zone ( > 20 ° C). On the
other hand, in the low temperature zone ( < 20 ° C), reactions 1
to 3 should still be the dominant reactions. The new reaction
mechanism at the elevated temperatures is schematically illustrated
in Figure 4 for charging and discharging, respectively
mixed system (containing 2.5 M SO 4
2 − and 6 M Cl − ) at ambient
temperature. Figure 5 b gives its columbic, voltage, and energy
efficiency as a function of cycling number. Stable performance
with an energy efficiency of 87% was observed during a course
of 20 days. The change of gas-phase pressures of the positive
and negative electrolytes containers was minimal, indicating
no signifi cant gas evolution in the system. Figure 5 c shows the
performance of this system at 0, 5, 40, and 50 ° C, respectively.
While higher at higher temperatures, an energy effi ciency over
80% was obtainable at 0 °
C. Again, negligible, if any, gas evolution
was observed, even during the operation at 50 ° C. Table 3
compares the performance of the VRB cells using 2.5 M and
3.0 M vanadium mixed electrolytes with that using 1.6 M
vanadium sulfate electrolyte. Overall, the cells using the mixed
sulfate-chloride electrolytes were successfully demonstrated at
much higher vanadium concentrations than the sulfate system
while still being highly efficient.