(554d) Novel High Energy Density Vanadium Bromide/Polyhalide Redox Flow Battery

The development of renewable energy technologies for electricity generation is regarded as top priorities by governments and environmental groups to combat global warming and environmental deterioration concerns associated with fossil fuels.  However, the intermittent nature of the renewable energy sources remains as one of the stumbling blocks towards its wide spread application as they create fluctuation in power output and thus grid stability and reliability.  Energy storage is considered as the key to unlock this power instability and reliability.  Among energy storage technologies developed to date, redox flow batteries are considered the best option to store electricity from medium to large scale applications.

Of all the redox flow batteries, the all vanadium redox flow battery developed by Maria Skyllas-Kazacos group has received the most attention due to its high energy efficiency of over 80% and long life cycle.  The vanadium redox Battery (VRB) employs a solution of vanadium in sulphuric acid in both half-cells with the V²⁺/V³⁺ redox couple in the negative half-cell and the VO²⁺/VO₂⁺ redox couple in the positive half-cell.  This eliminates the cross contamination problems of cross-contamination of the two half-cell electrolytes during long-term use leading to significant reduction in capital and maintenance costs as it minimizes waste disposal and provides a greater ease of operation.  During charging, V³⁺ is reduced to V²⁺ at the negative half cell while VO²⁺ is oxidized to VO₂⁺ at the positive half cell.  These reactions are reversed during cell discharge. 

However, the energy density in G1 VRB is limited to about 25 Wh/kg due to the low solubility limit of the V(II) and/or V(III) ions (2 M or less) in sulphuric acid supporting electrolyte at temperatures below 5 °C and the stability of the V(V) ions at temperatures above 40 °C.  Furthermore, significantly lower electrolyte concentrations are needed in many geographic locations where the climate is more extreme and temperatures go below zero degrees in winter. In such climates, vanadium sulphate concentrations as low as 1 M may be needed to avoid precipitation, further increasing the volume of the vanadium sulphate electrolyte needed for the VRB.  Hence improvement to the electrolyte solution is required to increase the energy density to an acceptable level. 

New vanadium bromide/polyhalide has been investigated in a static cell and the result shows it could deliver energy density of 50 Wh/kg using 2M V^+3.5 in 8M HBr and 2M HCl in mixtures of bromine complexing agents comprising N-ethyl-N-methyl pyrolidinium bromide (MEP) and N-ethyl-N-methyl morpholinium bromide  (MEM).  The energy density was observed to increase with cycle number due to the more utilization of bromide ions and higher energy density could be expected with higher concentration V/Br.