A high-energy-density zinc/iodine-bromide redox flow battery (ZIBB) has recently been developed by Prof. LU Yi-Chun, Assistant Professor of the Department of Mechanical and Automation Engineering, and her research team. ZIBB achieved the highest reported energy density for aqueous redox flow batteries to date. The breakthrough was published in the renowned journal Energy & Environmental Science in early 2017, and was recently featured by the magazine Chemistry World, published by The Royal Society of Chemistry, the United Kingdom.

Bromide ion (Br-): The Key to Releasing Energy Density

The aqueous Redox Flow Battery (RFB) is a device that generates electricity by electron transfer between two electrolytes. The RFB is safe and environment-friendly, with high design flexibility and a long life (several decades), and it appears to have high commercialization potential. With the introduction of Bromide ions (Br-), Professor Lu's research group boosted the energy density of ZIBB to as high as 101 Wh L-1, achieving the highest reported energy density to date, i.e. an improvement of at least 20% in capacity relative to a control system.

In zinc/iodine RFBs, highly soluble zine iodide is the major active material in the electrolyte, with iodide ions (I-) and zinc (Zn) being the electrochemical active ingredients at the positive and negative electrodes respectively. Free iodine (I2) is present in the battery, and iodide ions (I-) act as the stabilizing agent to form triiodide ions (I3-), thereby stabilizing the cycle life of the battery (efficiency as high as 95% over 50 cycles). However, the power of the iodide ions (I-) in contributing battery capacity is wasted as they are "trapped" as a stabilizing agent.    
 
The group therefore pioneered the innovation by which bromide ions are introduced as a replacement for the "trapped" iodide ions (I-), i.e. forming iodine bromide ions (I2Br-) by reacting bromide ions (Br-) with iodine (I2). The process allows a stable cycle life in the battery without sacrificing energy capacity. 

The Potential of ZIBB

This new energy storage system with high energy density and a stable cycle life has potential in the growing market for electric cars. "The price of electric cars would be significantly lowered if this type of battery was adopted, with much longer driven mileage. Moreover, this type of battery is much safer in a regular crash. Some work in commercializing the technology is underway. The technology is promising when applied to higher-performing, lower-cost, and larger-scale energy storage systems." Professor LU commented.

Their experiment also proved that the technology is able to boost the theoretical energy density of all electrochemical energy storage systems that involve iodine by one third. In the future, the team is going to research further into low-cost and high-performance electrodes and membrane materials to optimize the technology.