Scientists at the Dornsife College of Letters, Arts, and Sciences of USC have demonstrated a safe, inexpensive, eco-friendly, long-life, all-iron redox flow battery with a coulombic efficiency of 97.9% thanks to pH, functional electrolyte additives, and elevated temperature. coulombic efficiency is the ratio of charge extracted against charge put in over a charge cycle.
The high level of efficiency achieved in this study is among the best recorded for charge and discharge of an iron electrode. The researchers said:
The coulombic efficiency during electrodeposition of iron was found to improve with increasing pH at all values of current density. We have found that ascorbic acid has an important role in determining the coulombic efficiency.
Another problem of redox flow battery that has been solved in this study is that of the parasitic evolution of hydrogen during charging at the iron electrode.
The team also addressed the issue of parasitic evolution of hydrogen in redox flow batteries. It’s something that happens at the iron electrode during battery charging as a side reaction in devices featuring acidic electrolytes. It drastically reduces the performance of such batteries. The researchers explained:
Hydrogen evolution affects the repeated cycling of the redox-flow cell by rapidly changing the composition of the electrolyte in the system.
They managed to inhibit hydrogen evolution by bringing the pH level to zero (except for near the surface of the electrode) and adding ascorbic acid to the electrolyte. At the surface of the electrode, the pH value was elevated to raise the coulombic efficiency during iron electrodeposition.
They used ascorbic acid as an electrolyte additive in iron plating then increased the pH. The combination minimizes the air oxidation of iron and prevents the kinetics of hydrogen evolution. The electrolyte solution was developed with ammonium chloride and iron(II) chloride. The USC team said:
While the two compounds are well known individually, it’s the first time they’ve been combined to prove their potential for large scale energy storage.
A second metal, such as cadmium, could be used to further inhibit hydrogen evolution. The cadmium would have to be co-deposited with the iron and become immiscible with it. The team explained:
This will ensure that the second metal remains on the surface at all times and does not diffuse into the bulk of the iron electrode. By staying on the surface, the second metal will continue to provide the suppression of hydrogen evolution during the entire course of electrodeposition.
However, cadmium isn’t a very eco-friendly material.
The scientist also discovered that as the device’s operating temperature rose, so did the battery efficiency. They said:
We observed a coulombic efficiency of 97.9% at 60 degrees Celsius compared to 91.8% at 25 degrees Celsius.
The group believes that the future of large-scale electrical energy storage could benefit from their research. They concluded:
With the fundamental insights gained in this study and the improvements in coulombic efficiency demonstrated, we believe that the all-iron redox flow battery based on iron chloride will continue to present an attractive pathway for large scale electrical energy storage and the authors believe that the future will benefit from research on approaches to achieving 100% coulombic efficiency.
Recently, there have been many (and will be many more) studies on batteries because such technology is essential in our battle against climate change. Some other exciting new devices include graphene-lithium batteries that charge 18 times faster, potassium batteries that clean themselves while charging, and batteries made of seawater instead of heavy metals.
