Single-Step Synthesis of Halogenated Graphene through Electrochemical Exfoliation and Its Utilization as Electrodes for Zinc Bromine Redox Flow Battery

Abstract

We have demonstrated a single step synthesis of halogen functionalized graphenes (HGs) through electrochemical exfoliation of graphite in aqueous potassium halide solutions. Few layers of as-synthesized HGs are characterized using powder X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Electron paramagnetic spectroscopy (EPR), Brunauer-Emmett-Teller (BET) surface area analysis, Field emission scanning electron microscopy (FESEM), and Transmission electron microscopy (TEM). The degree of halogenation is found to vary between 2.32 and 0.26 atomic % in fluorinated graphene (FG) and iodinated graphene (IG) respectively, which could be attributed to the difference in reactivity of the halogen species generated during the exfoliation process. A suitable mechanism is proposed in accordance with the exfoliation phenomena and functionalization of halogens on the graphene sheets based on the experimental observation. As synthesized HGs are utilized as electrode materials for zinc bromine flow batteries (ZBB) and the electrocatalytic effect of HGs electrodes for the 2Br−/Br2 redox couple has been investigated in detail. Cyclic voltammetry (CV), and charge-discharge studies were carried out to study the electrochemical performance of the HGs. Among all HGs, FG has shown the superior electrocatalytic behavior for 2Br−/Br2 redox reaction. The anodic (11.2 mA cm−2) and cathodic (10.7 mA cm−2) peak current densities are higher for FG than that of other halogenated graphenes. ZBB flow cell fabricated with FG as bromine electrode exhibits enhanced electrochemical performance in terms of efficiency (81% of voltaic efficiency and 72% energy efficiency) and durability up to 350 cycles.