Modification of graphite-based composite bipolar plates by “nanoglue” strategy for vanadium redox flow battery

Abstract

All-vanadium redox flow battery (VRFB) has been considered as a promising candidate for the construction of renewable energy storage system. Expanded graphite possesses immense potential for use as typical bipolar plates in VRFB stacks. Nevertheless, the pure expanded graphite bipolar plates suffer from severe swelling in electrolyte, resulting in the losses of mechanical stability and electrical conductivity, thus leading to the efficiency decay within several cycles. Herein, we present a “nanoglue” strategy for tuning the structure/surface properties of expanded graphite by employing polyvinylidene fluoride (PVDF) polymer as structural sealant. Such PVDF “nanoglue” on expanded graphite results in the fine-repairment toward the surface microcracks and cross-section edges, which is beneficial to suppress the electrolyte permeation and improve the anti-swelling capacity. Moreover, it has been found that the PVDF “nanoglue” can improve the flexibility, allowing for the fabrication of ultrathin bipolar plates (0.67 mm) with low electrical resistivity. Benefiting from these integrated characteristics, the VRFB employing the as-fabricated composite bipolar plates delivers excellent cyclic efficiencies (voltage efficiency, coulombic efficiency, and energy efficiency) and ultralow ohmic voltage loss of less than 1.1 mV (< 0.1% of the VRFB rated voltage of 1.25 V) at a high current density of 200 mA cm⁻².