Oxidized black phosphorus nanosheets/sulfonated poly (ether ether ketone) composite membrane for vanadium redox flow battery

A novel oxidized black phosphorus nanosheet (O-bPn) is proposed to improve ion selectivity of sulfonated poly (ether ether ketone) (SPEEK) membrane for vanadium redox flow battery (VRFB). O-bPn possesses a unique two-dimensional (2D) puckered lattice structure, which could function as an effective blocker for vanadium ion permeability. Moreover, diverse oxygen-containing groups on O-bPn surface introduce additional acidic proton carriers and also form hydrogen bonds with –SO3H in polymer matrix, promoting proton transport. The incorporation of O-bPn into the SPEEK matrix increases proton conductivity, and markedly reduces vanadium crossover, achieving a nearly 1.8-fold increase of ion selectivity than the pristine SPEEK membrane. With an optimal O-bPn content of 1.5 wt%, the composite membrane (S/O-bPn-1.5%) exhibits a 7% higher coulombic efficiency (CE, 98.3% vs. 92.1%) and 10% higher energy efficiency (EE, 86.6% vs. 78.9%) at 100 mA/cm2, and retains a higher capacity (30.1% vs. 7.8%) after 100 cycles as compared with Nafion212 membrane. The EE value of the S/O-bPn-1.5% is also at the top level of the previously reported SPEEK-based composite membranes. These superior performances of the S/O-bPn membrane demonstrate that O-bPn is a promising 2D nanofiller for highly ion-selective composite membrane for VRFB.