Résumé
Iridium oxide nanocatalysts (IrOx) show promise to lower the material cost of proton-exchange membrane water electrolyzers (PEMWE) owing to their high activity towards the oxygen evolution reaction (OER), and their good resistance to acidic and oxidative conditions. However, enhanced OER activity often comes at the expense of stability. Therefore, only a comprehensive understanding of how the structural, morphological, and chemical characteristics of IrOx nanoparticles influence these two conflicting properties will enable to achieve a delicate balance between them.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 340◦C to 670◦C. We obtained a material library with various properties ranging from small amorphous IrOx NPs supported on carbon to larger self-supported crystalline IrO2 (Figure 1a), which was used to deconvolute the effects of the oxidation state, the structure and the size of IrOx NPs and to tune their OER activity and stability1,2. To analyze the structure, morphology, and oxidation state of Ir in these catalysts, we used transmission electron microscopy TEM (Figure 1b-j.), X-ray photoelectron spectroscopy XPS (Figure 1l-m.), thermogravimetric analysis (TGA) and both laboratory-based and synchrotron-based X-ray diffraction (XRD) techniques (Figure 1k.).We assessed their electrocatalytic activity towards the OER and determined their stability number3 (S-number) using on-line inductively coupled plasma mass spectrometry and electrochemistry. We observed the decrease of activity and the increase of the stability with an increasing annealing temperature2. We successfully correlated certain physico-chemical properties (electronic structure, morphology, and size) with either the OER activity or the stability of the catalysts. Specifically, we identified certain thermal annealing conditions that achieved an optimal balance between OER activity and stability, and we were able to obtain small NPs (< 7 nm) even at temperature above 500°C4, with a good particle size control.