Ionomer degradation in catalyst layers of anion exchange membrane fuel cells

Abstract

Anion exchange membrane fuel cells (AEMFCs) that operate at high pH, offer the advantage of enabling the use of abundant 3d-transition metal-based electrocatalysts. While they have shown remarkable improvement in performance, their long-term durability remains insufficient for practical applications with the alkaline polymer electrolytes (APEs) being the limiting factor. The stability of APEs is generally evaluated in concentrated alkaline solutions, which overlooks/oversimplifies the complex electrochemical environment of the catalyst layer in membrane electrode assembly (MEA) devices. Herein, we report a study of the degradation of the membrane and ionomer independently under realistic H₂–air (CO₂ free) fuel cell operation, using proton nuclear magnetic resonance (¹H-NMR), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). While the membrane degradation was minimal after the AEMFC stability test, the ionomer in the catalyst layers degraded approximately 20% to 30% with the cathode being more severely affected than the anode. The ionomer degradation decreased the catalyst utilization and significantly increased the ionic resistance, leading to significant performance degradation in the AEMFC stability test. These findings emphasize the importance of ionomer stability and the need to consider the electrochemical environments of MEAs when evaluating the stability of APEs.