Deciphering the Roles of Chemistry and Structure in Iridium Oxide Oxygen Evolution Catalysts through the Study of Annealed Iridium Oxide Nanoparticles
Authors: Clauss Delphine , Martin Vincent , Drnec Jakub , Mirolo Marta , Dubau Laetitia , Maillard Frederic 
Published: February 2024

The development of proton exchange membrane (PEM) water electrolysis, which enables the production of de-carbonized hydrogen, is hindered by the low efficiency of the oxygen evolution reaction (OER). This is especially true in acidic environment where only noble metals (mainly Ru and Ir) can be used as electrocatalysts. Iridium oxide nanocatalysts (IrOx) are promising due to their high OER activity and good resistance to acidic and oxidative conditions. However, finding a compromise between their electrocatalytic activity and stability remains challenging. Moreover, it is not yet clear which parameter (the material structure1 or the initial iridium oxidation state2) controls these two properties.

To address these challenges, we synthesized IrOx nanoparticles (NPs) supported on carbon (IrOx/C), through a modified polyol process, and annealed them under air, at temperatures ranging from 200◦C to 850◦C. This allows us to deconvolute the effects of the oxidation state, the structure and the size of IrOx NPs and to tune their activity and stability3. To analyze the structure, morphology, and oxidation state of Ir in these catalysts, we used transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and both laboratory-based and synchrotron-based X-ray diffraction (XRD) techniques.

We observed in situ the crystallization of IrOx NPs, leading to crystallite size growth and change in Ir electronic state (see Figure 1a). Additionally, we investigated the morphological changes of IrOx NPs with respect to annealing temperature (refer to Figure 1b-f). We assessed their electrocatalytic activity towards the OER and determined their stability number4 (S-number) using inductively coupled plasma mass spectrometry and electrochemistry. We observed that increasing the annealing temperature led to an increase in particle size and degree of crystallinity, but did not result in significant changes in the iridium oxidation state (refer to Figure 1g). We observed the evolution of the electrochemical activity of the nanoparticles with the annealing temperature (see Figure 1h). Our results show a correlation between the changes in electronic structure, morphology, and size, and the evolution of OER activity and stability of the IrOx NPs. Specifically, we identified certain conditions that achieved an optimal balance between activity and stability.

(1) Elmaalouf, M.; Odziomek, M.; Duran, S.; Gayrard, M.; Bahri, M.; Tard, C.; Zitolo, A.; Lassalle-Kaiser, B.; Piquemal, J.-Y.; Ersen, O.; Boissière, C.; Sanchez, C.; Giraud, M.; Faustini, M.; Peron, J. Nat.
Commun.
2021, 12 (1), 3935. https://doi.org/10.1038/s41467-021-24181-x.

(2) Claudel, F.; Dubau, L.; Berthomé, G.; Sola-Hernandez, L.; Beauger, C.; Piccolo, L.; Maillard, F. ACS Catal.
2019, 9 (5), 4688–4698. https://doi.org/10.1021/acscatal.9b00280.

(3) Geiger, S.; Kasian, O.; Shrestha, B. R.; Mingers, A. M.; Mayrhofer, K. J. J.; Cherevko, S. J. Electrochem. Soc.
2016, 163 (11), F3132. https://doi.org/10.1149/2.0181611jes.

(4) Geiger, S.; Kasian, O.; Ledendecker, M.; Pizzutilo, E.; Mingers, A. M.; Fu, W. T.; Diaz-Morales, O.; Li, Z.; Oellers, T.; Fruchter, L.; Ludwig, A.; Mayrhofer, K. J. J.; Koper, M. T. M.; Cherevko, S. Nat. Catal.
2018, 1 (7), 508–515. https://doi.org/10.1038/s41929-018-0085-6.

Acknowledgments: This work was supported by the French National Research Agency in the frame of the MATHYLDE project (grant number n° ANR-22-PEHY-0001).

Figure 1 – a. X-ray diffractograms obtained on IrOx/C annealed at 290, 340, 380, 420, 470, 520, 570, 620, 670 and 850°C under air. Each measurement lasted 13 minutes. The hashes correspond to tetragonal IrO2 (PDF card 00-043-1019) and the stars to metallic Ir (PDF card 00-006-0598). b-f. TEM images of IrOx/C NPs as-synthesized and calcined at 290, 380, 420, 570 and 850 °C. g. X-ray photoelectron spectra showing the Ir 4f levels of IrOx/C NPs as-synthesized and calcined at 380, 420, 520 and 850 °C. The dotted lines indicates the theoretical position of the peaks for Ir(IV) and Ir(III). h. Cyclic voltammograms measured in 0.1 M HClO4 at 50 mV s−1 indicating the change in electrochemical properties of IrOx/C NPs with annealing temperature.

Figure 1