Redox flow batteries have been studied since the 1970s by NASA who have invented the first iron-chromium RFBs. Recently, many researchers have been endeavored to develop novel redox chemistries (aqueous organic RFBs, zinc-based RFBs, etc.) as alternatives to the well-developed all vanadium redox flow batteries (VRFBs), which are limited by the low energy density and high chemical cost. Polysulfide-bromine RFBs (PSBs) have been examined in the 1990s, and Regenesys Technologies (UK) Ltd. has launched the commercialization of PSBs considering the intrinsic low chemical cost and high energy density of polysulfide as negolyte. However, the limited cycle life and shelf life of polysulfide-based RFBs disappointed both industrial and academic research fields in later years.
In 2016, we have developed the first polysulfide-iodide redox flow batteries (PSIB) with one of the highest energy density and lowest chemical cost as a promising system for energy storage (Nano Energy, 2016, 30, 283−292.). However, similar challenge of fast capacity decay was encountered in PSIB’s development owing to active molecules crossover through commercial Nafion membrane, which brings us back to focus on solving the bottleneck regarding ion-selective membrane design with enhanced ionic selectivity. Several strategies have been employed for resolving crossover problem in RFBs for years, for instance, developing symmetric electrolytes under high power charging/discharging (i. e. decrease the discharge duration to drive higher utilization of active species before crossover); implementing solid-state electrolyte as the separator while compromising the power capability of aqueous cells and inducing compatibility concerns of electrolytes with solid-state electrolytes; and decorating redox active centres to enlarge the molecular size to forbid easy transportation, etc. However, none of the above can effectively address the cross-contamination problems, especially those with highly concentrated electrolytes employing small molecules.
In this work, we address this challenging problem pertaining to aqueous RFBs via developing a charge-reinforced ion-selective (CRIS) membrane. The simple and readily applicable strategy for CRIS in enhancing the stability of ARFBs relies on the high absorption ability of Ketjen black carbon and the high hydrophobicity of the infiltrated polyvinylidene fluoride (PVDF) polymer into the Nafion matrix. We have found that the developed CRIS membrane effectively repels the crossover of polysulfide and polyiodide, restrains membrane swelling, mitigates water/OH− transportation and facilitates the full conversion of electrochemical reactions in both posolyte and negolyte. Particularly, this approach can be universally applied in polychalcogenide and polyhalide-based redox flow batteries, both of which are promising candidates for ARFBs with tunable redox potential and high water solubility, but remain silent for years due to fast crossover. The demonstrated PSIB static cell revealed a low capacity decay rate (0.005% per day and 0.0004% per cycle) for 1200 cycles/2.9 months at 100% state of charge. The flow prototype system demonstrated 17.9 Ah l−1posolyte+negolyte over 3.1 months (500 cycles) with no detectable capacity decay. This unprecedented enhancement of the cycling stability of PSIB results in a transformative influence on the levelized cost of storage (LCOS) of PSIB. LCOS is a determining factor for the commercialization of emerging energy storage technologies, including investment cost, operation and management (O&M) cost, cycling stability, and shelf life, etc. The techno-economic analysis reveals that the CRIS-enabled polysulfide-based RFBs promise competitive LCOS for long-duration energy storage applications.
We hope that the results presented in our work (https://doi.org/10.1038/s41560-021-00804-x) in Nature Energy provide insights into ion-selective membrane design for long lifetime aqueous redox flow batteries.