Flow Battery Technologies
Flow batteries have been in existence for decades and several technologies have been developed. Most of them are based on metal ions, sometimes associated with halogens, such as : vanadium (the most widely used), iron, zinc, chromium… Some of these batteries can be hybrid, with half of the battery being based on a redox flow design and the other half with another design, such as a fuel cell. Various options are also possible, such as aqueous or organic electrolytes, the use of suspensions instead of solutions, membraneless systems, etc.
Using these chemistries has a number of consequences on the features and the process. The most conspicuous is that the pH in the electrolytes is usually extremely acidic. The electrolytes are highly corrosive since the goal is to dissolve metals as effectively as possible. Even if materials are carefully selected, this causes reliability, safety and environmental issues.
Once a chemistry is selected, the battery maker is limited by its constraints and eventually designs the system around a compromise. Halogens, for instance, are a way to increase the energy density and build a smaller system, but they are difficult to handle. Iron or zinc are cheap but sustaining the performance is challenging. Vanadium can be used on both sides of the battery, but even with this advantage the cost of the electrolyte and the difficulty in designing a reliable and inexpensive system have proved daunting. For these reasons, most experiments in redox flow batteries have yielded tantalizing results: the potential qualities are unquestionable, but what is actually delivered is below expectations.