A novel polybenzimidazole membrane containing bulky naphthalene group for vanadium flow battery

Poly[2,2’-(1,4-naphthalene)-5,5′-bibenzimidazoles] (NPBIs) were successfully synthesized in Eaton's reagent and first investigated as dense proton exchange membranes for vanadium flow batteries. The basic properties of NPBI were characterized and compared with poly[2,2’-(4,4′-oxybis(1,4-phenylene))-5,5′-bibenzimidazoles] (OPBIs) and Nafion® 212. NPBI displayed superior solubility in common aprotic polar solvents and hence eliminated the processability issues of PBIs for large scale application. The amorphous structure of NPBI endowed the membranes with higher acid doping level (2.2) compared to OPBI membranes (2.0) in 3 M H2SO4. NPBI membranes thus exhibited lower area resistance than OPBI membranes. Both NPBI and OPBI membranes demonstrated ultra-high selectivity compared with Nafion® 212. The NPBI membrane was more stable than OPBI in oxidizing 30 wt% H2O2 solution, confirming its excellent oxidative stability. Assembled in batteries, 15 μm thickness NPBI membrane conferred an energy efficiency (EE) of 91% which is higher than Nafion® 212 (85%) and OPBI (88%) at 40 mA/cm2. In addition, during long-term cycling (around 900 cycles) in single-cells, both NPBI and OPBI membranes mainly suffered from creep deformation instead of chemical degradation. Combining the inexpensive initial materials, mild synthesis condition, excellent solubility, high chemical stability and impressive performance, the NPBI membranes hold the potential for large scale use in vanadium flow batteries or even broader fields, like fuel cells.