A Comparative Study on the Activity and Stability of Iridium-Based Co-Catalysts for Cell Reversal Tolerant PEMFC Anodes

Abstract

In fuel cell applications with long lifetime requirements, the management of stressing operating conditions—such as hydrogen starvation events—plays a pivotal role. Among other remedies, the incorporation of an OER-enhancing co-catalyst, is widely employed to improve the intrinsic stability of Pt/C-based anode catalyst layers in PEM fuel cells. The present study investigates several supported and unsupported Ir-based co-catalysts comprising different oxidation states of iridium: from metallic to oxidic character, both anhydrous rutile-type IrO2 and hydrated amorphous form. Utilizing a single-cell setup, cell reversal experiments were conducted initially after break-in of the MEA and after seven days of continuous operation under reductive H2 atmosphere at application-relevant conditions. The initial cell reversal tolerance was found to increase in the order metallic Ir < crystalline Ir oxide < amorphous Ir oxyhydroxide. By contrast, after continuous operation under H2 the order changes drastically to amorphous Ir oxyhydroxide ∼ metallic Ir < crystalline Ir oxide. This led us to conclude that the amorphous Ir oxyhydroxide is likely reduced to metallic Ir during continuous H2 operation, while IrO2 provides a reasonable trade-off between initial OER activity, high structural and chemical stability at high anode potentials during H2 starvation and low reducibility under prolonged H2 operation.