Return of the Iron Age

Abstract

In this issue of Joule, Liu and coworkers present the use of the well-known ferrocyanide complex as an active electrode material for an aqueous organic/ organometallic redox flow battery. The ammonium cation was used as counter cation in place of the former-utilized sodium and potassium ions. Interestingly, by this simple cation exchange, remarkable capacity and cycle stability were achieved. A battery system is presented comprising ammonium ferrous hexacyanide and an organo-functionalized viologen compound.An increasing amount of energy is produced by renewables such as wind, hydro, or solar power to significantly decrease CO 2 emissions derived from the utilization of fossil fuels (i.e., coal, gas). It is expected that by 2040 over 40% of the total global energy will be produced by renewable sources. 1 Due to the nonlinear production possibilities of the renewables, the demand for innovative and ''green'' energy storage sys-tems is rising continuously as the capacities of conventional energy storage such as hydro power, thermal energy, pressurized air, or flywheel-based systems are strongly limited due to geological or physical limitations. 2 As a result, innovative new storage concepts are currently the focus of research.Redox-flow batteries (RFBs) represent an alternative approach toward con-ventional static chemical energy storage systems. This battery concept makes it possible to scale the capacity of the battery independently from the power, which is produced, once the battery has been charged. This unique characteristic is enabled by the construction principle of these electrochemical storage systems: two tanks contain an organic or aqueous solution of redox-active organic or inorganic compounds that exhibit reversible reduction and oxidation processes. The electrolyte solutions are pumped into two half-cells that are separated by a membrane and connected to an electrode each, which enables charging and current collection. The energy stored correlates to the volume of the tanks/amount of electrolyte, whereas the power scales with the size of the electrochemical cell/the cell stacks. Numerous systems have been reported utilizing redox-active metal salts (including the most prominent example, the vanadium RFB), organic compounds, as well as polymeric redox systems as active materials. 3,4 If this battery concept is considered under ecological points of view, multiple requirements must be met when investigating novel electrode materials. Preferably, new electrolytes feature low toxicity, high solubility, high stability upon cycling, and a low materials price. 5 Additionally, the desired electrolyte materials should be already commercially available or accessible through straightforward synthetic routes, enabling large-scale synthesis of the required compounds. Therefore, electrolytes comprising iron-containing compounds Figure 1. Redox Flow Battery Constructed by Liu and Coworkers Illustration of the redox flow battery that was reported by Liu and coworkers 10 to investigate the cycle stability of the synthe...