Durability Assessment of α-MnO2 Nanorods for Alkaline ORR under Application-Relevant Conditions

Abstract This study investigates the durability of α-MnO2, one of the most cost-effective catalysts for the oxygen reduction reaction (ORR) in alkaline media. A comprehensive set of various characterization techniques, including identical location transmission electron microscopy (IL-TEM), differential electrochemical mass spectrometry (DEMS), rotating ring-disk electrode (RRDE), and online inductively coupled plasma mass spectrometry (ICP-MS) coupled to both flow cell and gas diffusion electrode (GDE) half-cell, was employed. In addition, accelerated stress test (AST) measurements in the GDE half-cell followed by post mortem X-ray photoelectron spectroscopy (XPS), synchrotron X-ray diffraction (XRD), scanning electron microscopy (SEM), and TEM analysis were performed. Three electrolyte and temperature combinations ranging from standard lab conditions to application-relevant conditions were systematically investigated. The results reveal that Mn dissolution occurs under oxygen-free conditions due to oxidation-state transitions of the catalyst, whereas under oxygen, it is further accelerated by degradation processes associated with peroxide formation as a byproduct of the ORR, particularly at higher ORR overpotentials. Both increasing KOH concentration and temperature significantly enhance Mn dissolution due to accelerated Mn reduction and dissolution kinetics. Despite the increased dissolution, the condition of 4.0 M KOH at 25 °C exhibits the highest ORR activity after 5000 AST cycles. This apparent discrepancy is attributed to surface roughening of the catalyst, leading to an increased electrochemically active surface area. Overall, the results demonstrate that α-MnO2 exhibits promising initial ORR activity but undergoes substantial Mn dissolution under operating conditions. These findings provide mitigation strategies to limit catalyst degradation and improve operational durability.