Impact of Magnetic Graphene Nanofluidic Electrolytes for I3 -/I - in Zinc/Iodide Redox Flow Batteries

Abstract

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). Vinco, J. H., Domingos, A. E. E. da C., Espinosa, D. C. R., Tenório, J. A. S. & Baltazar, M. dos P. G. Unfolding the Vanadium Redox Flow Batteries: An indeep perspective on its components and current operation challenges. J. Energy Storage 43, 103180 (2021).Lourenssen, K., Williams, J., Ahmadpour, F., Clemmer, R. & Tasnim, S. Vanadium redox flow batteries: A comprehensive review. J. Energy Storage 25, (2019).Yang, Y., Liang, S. & Zhou, J. Progress and prospect of the zinc–iodine battery. Curr. Opin. Electrochem. 30, 100761 (2021).Khor, A. et al. Review of zinc-based hybrid flow batteries: From fundamentals to applications. Mater. Today Energy 8, 80–108 (2018).Li, B. et al. Metal-Organic Frameworks as Highly Active Electrocatalysts for High-Energy Density, Aqueous Zinc-Polyiodide Redox Flow Batteries. Nano Lett. 16, 4335–4340 (2016).Liu, Y. et al. Stable static zinc-iodine redox battery constructed with graphene quantum dots coated graphite felt. J. Power Sources 520, 230861 (2022).Yang, J., Song, Y., Liu, Q. & Tang, A. High-capacity zinc-iodine flow batteries enabled by a polymer-polyiodide complex cathode. J. Mater. Chem. A 9, 16093–16098 (2021).