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

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Hydrogen (H2) starvation, for example due to insufficient supply of H2 or nonuniform gas distribution, remains one of the major challenges for PEMFC applications.[1,2] The H2 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 H2 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 IrOx 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 IrOx, (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 IrOx species are obtained by the colloidal route. Using the RDE technique in 0.1 M HClO4, 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 IrOx catalysts. Especially, the Pt-Ir NPs prepared by the colloidal route exhibit 2.5 times increase in OER mass activity (normalized to the Ir content) at 1.5 VRHE compared to commercial IrOx.To gain fundamental insights into the reversibility and catalytically active states of these bifunctional Pt-Ir catalysts under working conditions, operando Quick-XANES experiments by jumping between hydrogen evolution reaction (HER) and OER within a few seconds were conducted. Very interestingly, these jumping experiments reveal that the bifunctionality of these Pt-Ir NPs is highly reversible for several minutes independent of the Pt:Ir ratio and synthesis route. However, the oxidation state of iridium during the OER is strongly influenced by the Pt:Ir ratio as well as the synthesis route. For both synthetic routes, using a Pt:Ir ratio of 3:1 results in a higher abundance of Ir4+ species during the OER compared to using a 1:1 ratio. During the OER the content of oxidized Pt and Ir species are lower for the larger NPs (3 – 4 nm) prepared by the wet-impregnation route compared to the colloidal ones (2 nm). In addition, we reveal the influence of electronic effects between alloyed and non-alloyed NPs during the electrochemical reactions, especially for OER. During the HER, independent from the synthetic route the Pt-Ir NPs show a reduction of Ptz+ species to metallic platinum within a few seconds, while the Ir4+ species are partially reduced to form Ir3+ species. These dynamic changes in oxidation state of both elements within Pt-Ir alloy NPs are highly reversible within seconds under the HER or OER conditions.Based on the operando Quick-XANES, our results provide new insights into the dynamic behavior of bifunctional Pt-Ir NPs during HOR and OER within a few seconds to design new cell reversal tolerant catalyst materials with low loading of the very costly and scarce iridium.