Demand for new and large energy storage systems is increasing for applications such as remote area power systems, wind turbine generators, load-levelling at electric power stations, as well as emergency back-up applications. The use of batteries as portable electrical power sources has also increased dramatically and to some extent technology has not been able to keep pace with demands. Longer cycle life and higher volumetric energy densities are needed for portable energy applications such as electric vehicles while load-levelling applications are more sensitive to cost.
Lithium and nickel metal hydride emerged more recently as excellent high energy density alternatives to conventional lead-acid and nickel-cadmium batteries although the relatively high cost of metal hydride has limited use to smaller-scale portable equipment and hybrid electric vehicles.
The redox flow cell appears to offer great promise as a low cost, high efficiency system for large-scale energy storage and the first generation vanadium redox battery (G1 VRB) is rapidly moving towards full commercialization in a wide range of stationary applications.
The VRB system employs vanadium redox couples in sulphuric acid in both half-cells and was pioneered by Prof. Maria Skyllas-Kazacos and her team at the University of New South Wales in Australia. The electrolyte used in vanadium redox batteries is a mixture of vanadium and sulphuric acid of acidity similar to lead-acid batteries.
Vanadium redox batteries are widely hailed as a strong contender for the alternative energy storage requirements. This technology could also potentially replace lead-acid batteries in UPS and other back-up uses, subject to competitive pricing of the vanadium-based electrolyte. The VRB market promises steady growth over the next few decades constrained mainly by price volatility of vanadium in steel alloying as its primary application. It follows that the successful commercialisation of VRB is contingent on access to low cost vanadium from sources decoupled from cyclical price and demand fluctuations of the construction industry globally.
The challenges with production of electrolyte are well described from the website of Cellenium Technologies: “Vanadium is commercially available as vanadium pentoxide (V₂O₅), or as ammonium vanadate (NH₄VO₃). In both these compounds the vanadium is in the oxidation state V⁵⁺. However, the electrolyte required for first filling vanadium regenerative fuel cells is acid vanadium sulfate with half the vanadium in the oxidation state V³⁺, and half in the state V⁴⁺. Unfortunately, vanadium pentoxide is only slightly soluble in sulfuric acid and water, and the methods used until now for preparing the acid vanadium electrolyte have been complex and costly chemical and electrochemical processes. The overall economics of vanadium fuel cells needs a better method of preparing the electrolyte from solid vanadium pentoxide.”