Increased Ir–Ir Interaction in Iridium Oxide during the Oxygen Evolution Reaction at High Potentials Probed by Operando Spectroscopy

Abstract

The structure of IrO2 during the oxygen evolution reaction (OER) was studied by operando X-ray absorption spectroscopy (XAS) at the Ir L3-edge to gain insight into the processes that occur during the electrocatalytic reaction at the anode during water electrolysis. For this purpose, calcined and uncalcined IrO2 nanoparticles were tested in an operando spectroelectrochemical cell. In situ XAS under different applied potentials uncovered strong structural changes when changing the potential. Modulation excitation spectroscopy combined with XAS enhanced the information on the dynamic changes significantly. Principal component analysis (PCA) of the resulting spectra as well as FEFF9 calculations uncovered that both the Ir L3-edge energy and the white line intensity changed due to the formation of oxygen vacancies and lower oxidation state of iridium at higher potentials, respectively. The deconvoluted spectra and their components lead to two different OER modes. It was observed that at higher OER potentials, the well-known OER mechanisms need to be modified, which is also associated with the stabilization of the catalyst, as confirmed by in situ inductively coupled plasma mass spectrometry (ICP-MS). At these elevated OER potentials above 1.5 V, stronger Ir–Ir interactions were observed. They were more dominant in the calcined IrO2 samples than in the uncalcined ones. The stronger Ir–Ir interaction upon vacancy formation is also supported by theoretical studies. We propose that this may be a crucial factor in the increased dissolution stability of the IrO2 catalyst after calcination. The results presented here provide additional insights into the OER in acid media and demonstrate a powerful technique for quantifying the differences in mechanisms on different OER electrocatalysts. Furthermore, insights into the OER at a fundamental level are provided, which will contribute to further understanding of the reaction mechanisms in water electrolysis.

On the Durability of Iridium‐Based Electrocatalysts toward the Oxygen Evolution Reaction under Acid Environment

“…It was reported in a more recent study that the OER mechanisms on IrO 2 are different at higher potentials, which was associated with the catalyst durability. [ 42 ] Specifically, the calcinated IrO 2 exhibits much stronger Ir‐Ir interaction, that is, more significant structure change, at high OER potentials due to the removal of lattice oxygen, but shows better stability as compared with the uncalcined IrO 2 . Such counterintuitive results could be ascribed to the periodic IrOIr structure of calcined IrO 2 that benefits the formation of IrIr after oxygen removal, and the increased IrIr interaction subsequently stabilized the catalyst.…”

Sacrificial Cu Layer Mediated the Formation of an Active and Stable Supported Iridium Oxygen Evolution Reaction Electrocatalyst

“…An efficient catalyst would therefore be required to sufficiently increase the reaction rate of a complex four-electron process on the anode side, i.e., the OER. Iridium and its oxides are most often used as OER catalysts, as they present the best trade-off between activity and stability. , However, for efficient electrochemical conversion, high loadings of this very rare metal are required . Even though the costs of the expensive catalysts in relatively small systems present less than 10% of the total cost, loadings of iridium will need to be reduced from approximately 2 mg Ir cm –2 currently used to only 0.05 mg Ir cm –2 for the TW scale-up of the technology, not only because of the price of noble metals but mostly due to the future supply constraints. , Therefore, the development of new catalyst materials for the OER is demanded.…”

Inter‐relationships between Oxygen Evolution and Iridium Dissolution Mechanisms

“…The overall electrochemically active volume of the catalyst was nonetheless significantly lower than in hydrous iridium oxide, which explains the higher stability of rutile. The active involvement of oxygen from the rutile lattice was recently corroborated in a dynamic OER operation through observation of an increased Ir‐Ir interaction in IrO 2 by in‐operando XAS measurements [72] . After the release of oxygen from the lattice, the rearrangement of the structure leads to a decreased distance between the Ir atoms, which was proposed to be the origin of the higher stability of rutile.…”