High-performance PBI membranes for flow batteries: from the transport mechanism to the pilot plant

Abstract

Flow batteries are promising large-scale energy storage technologies for smart grids and broad applications of renewable energies. Ion conductive membranes (ICMs) are the crucial components in flow batteries to resist electrolyte crossover and selectively transport charge carriers. An ICM with high stability and ion conductivity in a wide pH range is essential for different energy storage devices. Here, in this work, we report that polybenzimidazole (PBI) membranes have amphoteric ion transport characteristics and display an ultrastrong Donnan effect, which can highly resist protons in hydrochloric acid and hydroxides in potassium hydroxide solution. Acid swelling can weaken the Donnan effect, enhance the Grotthuss mechanism, promote the proton/hydroxide transport and dramatically increase the ion conductivity. As a result, PBI membranes demonstrate attractive performances in both acidic and alkaline flow batteries. A 3 kW vanadium flow battery (VFB) stack equipped with acid-doped PBI membranes showed an energy efficiency of 80% at a current density of 200 mA cm⁻² and a stable efficiency for 500 cycles at the design power (3 kW). A 1 kW alkaline Zn–Fe flow stack achieved a CE of 99% and an EE of ∼88% at a current density of 80 mA cm⁻². This paper provides an iconic case from ion transport mechanism clarification to the pilot plant scale and a deep understanding of the ion transport mechanism in ICMs to further guide the membrane design for wide pH-range flow batteries.