New all-iron redox flow battery for renewables storage

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From pv magazine International

Researchers at the Dornsife College of Letters, Arts and Sciences of the University of Southern California (USC) have demonstrated a new all-iron redox flow battery for renewable energy storage they claim is inexpensive, safe and eco-friendly as well as offering a long lifetime.

Presented in the study Improvements to the Coulombic Efficiency of the Iron Electrode for an All-Iron Redox-Flow Battery, published in the Journal of The Electrochemical Society, the device is said to have demonstrated coulombic efficiency – the ratio of charge extracted against charge put in over a cycle – of 97.9%, thanks to functional electrolyte additives, pH and elevated temperature.

The researchers say that level of efficiency is among the best recorded for charge and discharge of an iron electrode. “The coulombic efficiency during electrodeposition of iron was found to improve with increasing pH at all values of current density,” stated the USC group. “We have found that ascorbic acid has an important role in determining the coulombic efficiency.”

Hydrogen problem

The researchers claim to have addressed the problem of parasitic evolution of hydrogen at the iron electrode during battery charging, which causes a drastic reduction in performance of redox flow batteries and occurs as a significant side reaction in devices featuring acidic electrolytes. “Hydrogen evolution affects the repeated cycling of the redox-flow cell by rapidly changing the composition of the electrolyte in the system,” the researchers said.

Adsorption of ascorbic acid added to the electrolyte is said to have inhibited hydrogen evolution at zero pH and elevation of the pH value near the surface of the electrode raised coulombic efficiency during iron electrodeposition. Ascorbic acid was used as an electrolyte additive in iron plating to minimize the air oxidation of iron – which prevents the kinetics of hydrogen evolution – in combination with an increase in pH. The electrolyte solution was developed with iron(II) chloride and ammonium chloride. “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,” said the USC group.

Cadmium

The academics said hydrogen evolution could be further inhibited if a second metal, such as cadmium, which is not exactly an eco-friendly material, is co-deposited with iron and becomes immiscible with it. “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,” stated the group. “By staying on the surface, the second metal will continue to provide the suppression of hydrogen evolution during the entire course of electrodeposition.”