Halogens as Positive Electrode Active Species for Flow Batteries and Regenerative Fuel Cells

Flow batteries offer solutions to a number of the growing concerns regarding world energy, such as increasing the viability of renewable energy sources via load balancing. However, issues regarding the redox couples employed, including high costs, poor solubilities/energy densities, and durability of battery materials are still hampering widespread adoption of this technology. Flow batteries with a positive half-cell consisting of a halogen based redox couple ( $$\hbox {Cl}^-/\hbox {Cl}_2$$ , $$\hbox {Br}^-/\hbox {Br}_2$$ , $$\hbox {I}^-/\hbox {I}_2$$ ) offer several advantages over other alternatives, such as being relatively inexpensive, highly soluble, and exhibiting faster kinetics than many other electroactive redox couples. This paper aims to provide a comprehensive comparative review of the thermodynamic and kinetic properties of relevant halogen and polyhalide redox couples, and recent advances in electrode and membrane materials for various halogen-based flow batteries and regenerative hydrogen fuel cells using halogens instead of oxygen.