Development of Proton Conduction Membranes in Application of Vanadium Flow Battery

Abstract

The polymeric hydrophilic/hydrophobic interactions into membrane formation were introduced. A general and straightforward strategy for preparing membranes with nanometer-scale pores was suggested by utilization of hydrophilic/hydrophobic interactions to generate phase separation and removal of polyion aggregates through water immersion. Poly(vinylidene fluoride)(PVDF) and sodium allyl sulfonate(SAS) serve as the membrane material and pore-generator, respectively, resulting in chemically stable and oxidation-resistant membranes with various potential applications. Following the same procedure invented to produce the laboratory-scale membranes, the scale-up process to manufacture large-area membranes was completed. The obtained membrane exhibited the conductivity of 3.5×10-2S·cm-1, thickness of 100 μm, bursting strength over 0.3 MPa and effective area of 800 mm×900 mm.In order to investigate whether this membrane is capable of assembling vanadium flow battery(VFB) stack, the permeation selectivity for H+/VO2+mixtures through the membranes, made from different pore-generator contents, were measured. Obviously,proton transports through membrane far faster than vanadium ion. Since the volume of H+is much smaller than that of VO2+in electrolyte, the difference in charge exclusion effect, which distinguishes from general ion exchange mechanism due to Donnan equilibrium effect, leads to the selectivity for H+/VO2+up to 306. Using this nano-porous membrane, a 15 k W VFB stack was fabricated to evaluate the membrane performance for application. Generally, the stack test indicates that the membrane provided feasible proton permeation and rejection of vanadium ion, with average columbic efficiency of 93% and energy efficiency of 72% during the period of over 700 charge/discharge cycles, which shows promising market potential for electricity energy conversion and storage processes.