Impact of Magnetic Graphene Nanofluidic Electrolytes for I3
-/I -
in Zinc/Iodide Redox Flow Batteries
Authors: Cao Yifan , Sadri Rad , Trifkovic Milana , Roberts Edward P.L. 
Published: August 2024

Redox flow batteries (RFBs) stand out as a highly promising technology to tackle the growing challenges of intermittent energy supply, known for their extended cycle life, low environmental impact, scalability, and design flexibility1
,
2. Among various RFBs, aqueous zinc-iodide redox flow batteries (ZIRFBs), have been gaining increasing attention, due to their low material costs, high solubility (ZnI2
~ 7M), rapid kinetics, and impressive theoretical energy density (322 Wh/L)3
,
4. However, the redox reactions of I3
-/I-
on the graphite felt cathode in ZIRFBs exhibit poor reversibility and electro-activity compared to the Zn/Zn2+
pair at the anode5. This discrepancy results in significant electrode polarization resistance, a major hindrance to achieving high energy efficiency and a primary obstacle to commercialize large-scale ZIRFBs. To address this issue, researchers typically enhance the cathode’s electrochemical activity or conductivity to promote the I3
-/I-
reaction, thereby reducing polarization resistance, by modifying the cathode with materials such as graphene quantum dots6, metal-organic frameworks5, and conductive polymers7.

Unlike electrode modifications, electrolyte nanofluids have been relatively underexplored in the RFBs field, despite several studies on carbon-based nanofluids in all vanadium RFBs and polysulfide/iodide RFBs. These nanofluidic electrolytes have demonstrated superior conductivity and higher surface area compared to carbon felt, thereby enhancing battery performance. In this study, a magnetic graphene composite was developed using a self-assembly method facilitated by electrostatic force. The graphene was prepared by electrochemical exfoliation of graphite, modified with poly(diallyldimethylammonium chloride) (PDDA) to provide a positively-charged.

Material characterization with X-Ray diffraction analysis (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM) confirmed the successful synthesis of the composite. Flow cell experiments with a ZIRFB using a 3M solution revealed a notable improvement in the charge-discharge energy efficiency upon the addition of 0.3 mg/ml PDDA-graphene, from 72% to 82% at a current density of 30 mA/cm2, and from 47% to 60% at a current density of 70 mA/cm2. Performance improvements were also observed compared to unmodified graphene.

This magnetic nanofluid holds promise for the easy separation of nanoparticles and electrolytes by a magnetic field, enabling the recovery and reuse of the nanoparticles. To endow nanoparticles with magnetism for cost-effective recovery, we further modified the PDDA-graphene with magnetite, using electrostatic self-assembly. The mass ratio of graphene to magnetite in the composite had a significant impact on the ZIRFB flow cell performance. At a mass ratio of 4:1, the nanofluid enhanced ZIRFB energy efficiency by 7.3% and 9.7% at current densities of 30 and 70 mA/cm2, respectively. Conversely, when the mass ratio was 1:1, adverse effects on battery performance were observed. In addition to the charge-discharge energy efficiency, the battery performance was also evaluated using polarization to determine the maximum power density, and electrochemical impedance spectroscope (EIS).

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