Redox flow batteries present an attractive alternative to lithium-ion in the stationary storage segment, thanks to potentially longer lifetimes and capability for daily 100% discharge without loss of performance.
Many commercial projects featuring different types of flow batteries are already underway for both large-scale and residential energy storage. However, questions are being raised about the materials used in some of the batteries, with the toxicity of vanadium used in most of the large-scale projects a particular sticking point.
Zinc-based flow batteries present a less toxic option, and a zinc-bromine chemistry has already seen some commercial development. Zinc-iron (Zn-Fe) is another option widely explored by researchers, with the abundance of iron making it a particularly attractive option. However, dendrite growth and the related performance loss and short-circuiting, have held this chemistry back so far.
Scientists led by the University of Calicut in India fabricated a zinc-iron redox flow battery that demonstrated discharge voltage of approximately 1.34 V at 25 mA cm−2, with a coulombic efficiency (CE) of 92%, voltage efficiency (VE) of 85% and energy efficiency (EE) of ~78% for 30 charge-discharge cycles. Most importantly, the battery showed no performance degradation over those initial 30 cycles, and zero dendrite growth. The battery is described in full in the paper A dendrite free Zn-Fe hybrid redox flow battery for renewable energy storage, published in Energy Storage.
Anion exchange membrane
Though it would have to retain these characteristics for a lot more than 30 cycles, the results are encouraging. The group credits the suppression of dendrites to its use of an anion-exchange membrane (AEM) that it designed and fabricated using guanidine carbonate, formaldehyde, melamine and polymer binding agent.
They note that when this was replaced with a porous PVC membrane as a control experiment dendrite formation could be observed even on the battery’s first charge. “Absence of dendrite growth might be due to the fact that, only the chloride ions shuttles between two electrolyte solutions through AEM and this probably promotes uniform zinc deposition and prevents the dendrite growth,” the group states.
They expect to follow up with further details on the fabrication of the AEM and its role in the battery’s performance, and the group remains convinced that its approach can prove useful for the stationary storage of renewable energy, among other applications. “We have successfully demonstrated working of a high efficiency and stable Zn-Fe hybrid redox flow battery with no dendrite growth during zinc deposition by optimizing charge-discharge conditions and employing an anion exchange membrane as separator,” their paper concludes.
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