Non-Perflourinated Ion Conducting Membranes with Internal Crosslinking Network for Vanadium Flow Batteries (VFBs)

Abstract

Vanadium flow batteries (VFBs) have become one of the most promising technologies for large-scale energy storage due to its nature of high safety, long cycle-life and environment-friendly. As one of the key parts, a membrane mainly plays a role in separating positive electrolyte and negative electrolyte and transferring protons to complete current circuit. Currently, the common used membranes for VFBs are perflourinated ion exchange membrane (such as Nafion 115). However, the Nafion 115 could not meet VFBs large-scale application due to its high cost and low ion selectivity. Therefore, the non-perflourinated ion exchange membranes are used as a separator because of its high ion conductivity and high selectivity as well as low cost. However, the chemical stability of the non-perflourinated ion exchange membranes cannot satisfy long-time cycling in VFBs due to the introduction of ion exchange groups. To further improve the chemical stability of non-perflourinated ion exchange membranes, the anion exchange membrane with internal crosslinking structure is designed and fabricated (Figure 1-a). The internal crosslinking structure ensures high stability of a membrane due to decreasing membrane swelling. The energy efficiency (EE) of a single cell with the optimized membrane maintained around 80% and cycled more than 1500 charge-discharge process at the current density of 140 mA/cm2, which exhibited the longest cycling life under VFBs operating conditions among the reported non-perflourinated ion exchange membrane, as shown in Figure 1-a’. Even though the ion exchange membranes with internal crosslinking structure possess long cycle life in VFB, the chemical stability of the fabricated anion ion exchange membranes still cannot meet VFB practical application. To address this problem, the advanced charged sponge-like porous membranes with crosslinking structure are designed and fabricated via combining crosslinking structure with porous ion conducting membrane, showing in Figure 1-b. The sponge-like porous structure could inhibit vanadium ions transferring though membrane via the multi-layered barriers, thus improve membrane selectivity. The internal crosslinking structure will ensure excellently chemical stability under strong acid and oxidized environment as well. In Figure 1-b’, the single cell assembled with the optimized porous membrane exhibits a CE of more than 99% and an EE of over 80% at the current density of 120 mA/cm2 and cycles more than 6000 charge-discharge process, which exhibits a promising prospect in large-scale VFB application. In addition, the performance of fabricated porous membranes could be tuned via designing internal crosslinking structure (Figure 1 c and d), like constructing flexible crosslinking structure or decorating internal crosslinking structure with hydrophilic groups within membrane, which shows higher ion conductivity than initial membrane. Figure 1 non-perflourinated ion conducting membranes with internal crosslinking network Figure 1