Operando and Dynamic XAS Studies of Metal-Support Interactions of Iridium-Based Catalysts for Oxygen Evolution Reaction (OER) in Proton Exchange Membrane Water Electrolysis (PEMWE)

Abstract

Iridium (Ir) is an important electrocatalyst material for the oxygen evolution reaction (OER) in acidic media. However, high loadings of very costly and scarce Ir are needed to achieve sufficient performance in proton exchange membrane water electrolysis (PEMWE). [1] To establish a large-scale green hydrogen economy based on PEMWE in the near future, the Ir loading needs to be further reduced. In the last decades, promising Ir-based catalyst strategies has been developed to improve the OER activity by controlling the size of Ir nanoparticles (NPs) [2] and tuning the electronic interactions of the Ir NPs with the catalyst support material, referred to as strong metal support interaction (SMSI). The latter has frequently been observed for heterogeneous catalysts (e.g. Au NPs supported on titanium oxide [3]), while the SMSI for Ir-based OER electrocatalysts is poorly understood to date.This study focuses on operando and dynamic X-ray absorption spectroscopy (XAS) analysis of Ir NPs deposited on three support materials: graphitized carbon (C), titanium oxide (TiO2) and antimony-doped tin oxide (ATO). The surfactant-free Ir NPs with controlled size of around 1 nm were obtained by a colloidal approach in low-boiling point alcohols. [2, 4] Afterwards, the Ir NPs were homogeneously distributed on different support materials with similar loadings and particle sizes confirmed by TEM and XRD. Solute components from the aged catalyst materials (Ir and/or support material) in the electrolyte solution were determined by ICP-OES, while the solid catalyst powders were analysed by TGA and µ-XRF. Our first/initial electrochemical results by recording cyclic voltammetry (CV) profiles show that after the immobilization the Ir NPs obtained from the same batch are strongly oxidized on TiO2, while a metallic character of the Ir is still observed on the C and ATO. The OER mass activity of 120±8 A/gIr for Ir/C is 2-times higher than for Ir/TiO2 (60±6 A/gIr) and 1.2-times higher than for Ir/ATO (95±4 A/gIr) at 1.5 VRHE,iR-free and 25 °C obtained from rotating disc electrode (RDE) setup.Operando and dynamic Ir LIII edge XANES measurements during the cycling from 0.06 to different upper potentials (0.8, 1.0, 1.2, 1.4 and 1.6 VRHE) at 2 mV/s were performed in acidic media at the SuperXAS (X10DA) beamline, Switzerland. All measurements were carried out using a home-made spectro-electrochemical flow-cell. Principal component analysis (PCA) of Quick-XANES data confirmed the tendencies of the formation of irreversible Irz+ species on TiO2 even at low upper potentials, while ATO and C tend to retain the metallic character of the Ir NPs. From the PCA, two independent components were identified to describe the changes in the series of XANES data. The compositions of these components were then analyzed using linear combination fitting (LCF).For 60 wt.% Ir/C, the cycling up to 1.4 VRHE shows two components with 50:25:25 wt.% of Ir0:Ir3+:Ir4+ and 42:0:58 wt.% of Ir0:Ir3+:Ir4+ obtained from the PCA and LCF analysis. During the cycling, the ratios of these components change gradually. In spite of the strong hysteresis behavior, we observed that the ratio between both components reverses to the initial state. In other words, only the component with 42:0:58 wt.% of Ir0:Ir3+:Ir4+ is present at 1.4 VRHE, while the other component is dominant at lower anodic potential.In the same experiment, the analysis of our data for 60 wt% Ir/ATO using PCA indicates two independent components with 74:11:15 wt.% of Ir0:Ir3+:Ir4+ and 70:0:30 wt.% of Ir0:Ir3+:Ir4+. The cycling up to 1.4 VRHE shows similar behavior like 60 wt.% Ir/C.Although the same batch of colloidal Ir NPs were used for the immobilization, the PCA and LCF analysis for 60 wt.% Ir/TiO2 reveal two components with high content of Ir4+ species, more precisely 36:64 wt.% of Ir0:Ir4+ and 2:98 wt.% of Ir0:Ir4+. The cycling up to 1.4 VRHE shows mainly a reversible behavior due to high content of Ir4+ species on TiO2. With increasing anodic potential, the content of metallic Ir further decreases and the main component with almost only Ir4+ species is stable at 1.4 VRHE. In the backward direction, the reduction to metallic Ir takes place to small extent. Therefore, the component with 36:64 wt.% of Ir0:Ir4+ is mainly present at 0.06 VRHE.In summary, our PCA and LCF analysis of the operando and dynamic Quick-XANES data provide further insights into the electronic structure of Ir species and the mechanism of the complex metal-support interaction for Ir NPs immobilized on C, ATO and TiO2.