Chemical stability of poly(phenylene oxide)-based ionomers in an anion exchange-membrane fuel cell environment

In recent years, intense research interest has been focused towards the development of anion exchange membrane fuel cells (AEMFCs) due to their potential to circumvent the need for expensive platinum catalysts, tackling the high cost that impedes mass commercialization of fuel cells. However, AEMFCs are not yet practical due to the low chemical stability of the quaternary ammonium (QA) cationic groups during cell operation. Several functionalized polymers for anion exchange membranes (AEMs), including substituted poly(phenylene oxide) (PPO), have been proposed as suitable ionomeric materials, as they present good stability in strong alkaline solutions. However, while they perform well in ex situ stability tests in aqueous solutions, they still present limited performance during AEMFC operation. As the current density in the fuel cell increases, more water is consumed at the cathode side, reducing the hydration level and, in turn, increasing the nucleophilicity of OH- and its capability to attack the QA groups. Here, using our recently reported ex situ stability protocol that simulates the low-hydration environment of an AEMFC during operation, the alkaline stability of PPO-based anion exchange ionomers is measured and compared. Good agreement with previously studied QA molecules tested using the same protocol was found. Yet, the degradation processes in these ionomers are further accelerated compared to the small QA molecules as a consequence of the lower polarity of the polymer environment, which further increases the hydroxide reactivity. This study demonstrates the competence of this new ex situ stability protocol to test not only QA molecules, but also ionomers and membranes, showing alkaline stability results that are comparable to those obtained in real AEMFC tests.