A vanadium sulfate and chloride mixed electrolyte proved to
be stable with a vanadium content of up to 2.5 M over a temperature
range of −5 ∼ 50 ° C because of the formation of stable
VO 2 Cl(H 2 O) 2 at elevated temperatures and reduction in SO 4
2 −
concentrations. The vanadium redox couples in mixed electrolytes
demonstrated improved reaction kinetics while remaining
as highly reversible as the sulfate system. The VRBs with the
mixed electrolytes were successfully demonstrated with the
same high effi ciencies as those of the sulfate systems, but over
a much broader temperature window. Our battery tests, along
with thermodynamic calculations, concluded that there was no
concern of chlorine gas evolution during the battery operation.
The highly improved energy capacity and the excellent electrochemical
performance over a broader operation temperature
window would help to substantially reduce the cost of the VRB
technologies. With cell, stack, and system engineering and optimization,
this new RFB technology is of great potential to accelerate
the market penetration of the renewable energy sources to
the electrical grid.