Degradation of polybenzimidazole in alkaline solution with first-principles Modeling
Tipaporn Patniboon, Heine Anton Hansen
Abstract
Membrane-based polybenzimidazole (mPBI) emerges as an exciting electrolyte membrane for alkaline fuel cells and water electrolyzers due to its useful ion conductivity range after being doped with aqueous KOH. However, the polymer degradation at highly alkaline concentrations limits its practical use. Herein, the density functional theory (DFT) calculations are used to study the degradation mechanism of mPBI molecule in an alkaline solution. The pristine mPBI molecule deprotonates to form an ionized molecule in an alkaline solution, with the ionized form being predominant at high pH. The nucleophilic hydroxide at the C2 position initiates the degradation, whereas the formation of the fully deprotonated ionic form suppresses the hydroxide ion attack. The degradation reaction then proceeds by ring-opening and chain scission reactions. The ring-opening reaction is preferred with an ancillary hydroxide ion or water molecule during the proton transfer process. The rate-determining state is the transition state involving the amide cleavage during the chain scission. Combining implicit-explicit solvation models is found to stabilize intermediate and transition states, lowering the energy barrier. With one or two explicit water molecules, the free energy barrier agrees well with experimental polymer lifetimes. An increase in KOH concentration increases the degradation rate, agreeing with experiments.