Mechanistic Insights into the Bifunctionality of Cell-Reversal Tolerant Pt-Ir Alloy Catalysts during HOR and OER Using Operando Quick-XANES

Hydrogen (H 2 ) starvation, for example due to insufficient supply of H 2 or nonuniform gas distribution, remains one of the major challenges for PEMFC applications.[1,2] The H 2 starvation in PEMFCs results in a rapid (several milliseconds to few seconds) increases in voltage at the anode.[3] In order to continuously provide protons and electrons to the cathode during the H 2 starvation, the carbon oxidation reaction (COR) commences as an alternative reaction at the anode.[4] To mitigate carbon corrosion and improve the long-term durability of PEMFCs, several material strategies can be utilized. One promising approach is the utilization of co-catalysts such as IrO x to accelerate the oxygen evolution reaction (OER) as an alternative source for protons and electrons.[3,5] The iridium can be (i) physically mixed into the platinum-based catalyst layer as IrO x , (ii) added to the same carbon support material as Pt nanoparticles (NPs) or (iii) alloyed with Pt NPs. Although several studies showed an increase in cell reversal tolerance using Pt-Ir alloys instead of physical mixing [6-9], no operando spectroscopic studies have so far been conducted to understand the surface oxidation states and electron transfer processes between the two metals and metal oxides during the cell reversal event. As this is a very dynamic process occurring within several milliseconds to few seconds, our approach is to use operando Quick-X-ray absorption near edge structure (Quick-XANES). This allows us to fundamentally understand the dynamics of structural and electronic interactions of both metals during the hydrogen oxidation reaction (HOR) and OER. In this work, alloyed Pt-Ir NPs with Pt:Ir ratios of 1:1 and 3:1 were prepared by two different synthetic routes (colloidal and wet-impregnation).[10] Very interestingly, the oxidation states of platinum and iridium as well as the particle size strongly vary depending on the synthesis route. More precisely, the wet-impregnation enables preparing Pt-Ir NPs of 3 – 4 nm size, while ~2 nm Pt-Ir NPs with amorphous IrO x species are obtained by the colloidal route. Using the RDE technique in 0.1 M HClO 4 , all Pt-Ir catalysts show activities close the diffusion limiting current, indicating high HOR kinetics. Despite the different oxidation states and particle sizes, the Pt-Ir NPs show considerable activity for the OER compared to pure commercial Pt/C and IrO x catalysts. Especially, the Pt-Ir NPs prepared by the colloidal route exhibit 2.5 t