Complex Degradation Mechanisms Accessible to Anion Exchange Membrane Ionomers on Model Catalysts, NiO and IrO 2

Abstract

For anion exchange membrane (AEM) electrolysis to be cost-and performance-competitive to proton exchange membrane (PEM) electrolysis, evaluating and improving the stability of the ionomer at the ionomer−catalyst interface will be key to this emerging technology. Theoretical calculations of molecular fragments of the ionomers detailed the complex degradation mechanisms accessible to four different classes of ionomers (Nafion, Sustainion, Versogen, and quaternary ammo-nium types�ETFE, Gen 2, and Georgia Tech) on model catalysts of platinum group metal IrO 2 and earth-abundant NiO. These mechanisms may occur during the making of the ionomer-catalyst ink or in the alkaline environment of AEM electrolysis or are energetically accessible at the electrochemical potentials of electrolysis. We identified diverse degradations such as (H)SO 4 production, water formation, oxidation to an alcohol, and deprotonation, leading to ionomer instability and competing with the oxygen evolution reaction (OER). Theory predicted that the weakly bound, intact cations of Sustainion's methyl imidazolium on NiO and Versogen's piperidinium on NiO combinations to be particularly stable and active for OER; these findings were validated by half-cell rotating disk electrode tests, where following break-in, their performance increased by 7−8 times. IrO 2 may be stable and maintain OER activity, but site access remains limited due to the strong binding and reactivity of the ionomer at the high potentials of electrolysis (at 1.4 V, Nafion's SO 3 splits into SO 2 + O; at 0.6 V, double deprotonation of Versogen can occur; at 1.5 V, ring oxidation of Sustainion to an alcohol initiates).