Alkali-stable polybenzimidazole anion exchange membranes tethered with N,N-dimethylpiperidinium cations for dilute aqueous KOH fed water electrolyzers

Polybenzimidazole (PBI) is currently considered as a membrane material for alkaline water electrolyzers (AWEs), and has to be fed with highly concentrated aqueous KOH electrolytes in order to ensure sufficient electrolyte uptake and conductivity. However, the harsh operating conditions significantly limit the lifetime of PBI membranes. In response, we here report on the synthesis and performance of a series of PBI membranes tethered with alkali-stable mono-piperidinium (monoPip) and bis-piperidinium (bisPip) side groups, respectively, that enables the use of more dilute KOH concentrations. The electrolyte uptake of these membranes was inversely proportional to the electrolyte concentration, which was in stark contrast to pristine PBI membranes. The high electrolyte uptake at low concentrations by the present membranes enables operation of AEMWE systems fed with dilute electrolytes, which significantly decreased membrane degradation. After immersion in 2 M aqueous KOH at 80 °C for up to 6 months, no degradation was detected by ¹H NMR spectroscopy in the monoPip series of AEMs, and a mere 7% ionic loss by Hofmann elimination in the bisPip series. Membranes tethered with bisPip groups produced the best AEMWE performance, and a sample with a hydroxide ion exchange capacity of 2.4 meq./g reached a high current density of 358 mA/cm² at 2 V with demonstrated stability over 100 h, using 2 M aqueous KOH and only simple nickel foam electrodes. This is comparable to the performance reported for Zirfon diaphragms and pristine PBI membranes operating with much higher concentrations of KOH in the range of 5-7 M. The low KOH concentration of the present membranes brings important advantages for the material stability in the cell, as well as for the balance of plant, and the results provide useful insights into the molecular design of AEMs for dilute electrolyte-fed AEMWE systems.