Oxygen evolution reaction (OER) catalysts for polymer-electrolyte water electrolysis (PEWE) suffered from their low surface area because carbon support cannot be used for OER operating at high potential. Connecting metal nanoparticles enable catalysts to conduct electrons through nanoparticle networks without any support materials while keeping their high surface area. Thus, connected nanoparticle catalysts (Fig. 1 (A)) are promising for OER. Previously, we have developed connected Pt-Fe nanoparticle catalysts with hollow capsule structure for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).^[1]

In this study, we proposed Ru and Ir nanoparticle catalysts for OER in water electrolysis because Ru and Ir have high OER activity. Ru and Ir nanoparticle catalysts were synthesized as follows. First, Ru and Ir nanoparticles were synthesized on silica template (PDDA/SiO₂) via polyol method using tetraethylene glycol as reducing agent and Ru(III) acetylacetonate or Ir(III) acetylacetonate as the metallic precursors (M/PDDA/SiO₂, M is Ru or Ir). Then, these nanoparticles were treated in supercritical ethanol at 330ºC for 90 min (Ru) or 60 min (Ir) (sc-M/SiO₂). Dissolution of SiO₂ in 3 M NaOH solution at 85ºC for 3 h made porous hollow structure (M capsule). OER performance were evaluated by electrochemical measurement in 0.1 M HClO₄ solution.

Fig. 1 (B) shows TEM image of Ir capsule. Capsule and networks structure of each catalysts were observed by TEM images. All Ru catalysts inactivated for OER at the first CV cycle. OER activity of Ru/PDDA/SiO₂ did not improve by heat oxidation treatment in air at 400ºC. Fig. 1 (C) shows OER curves of Ir catalysts. All Ir catalysts showed stable OER. Among them, Ir/PDDA/SiO₂ had the highest OER activity. Mass activity at 1.48 V of Ir/PDDA/SiO₂ was higher than RuO₂^[2], IrO₂^[2] and Ir/C­^[3] in previous reports as shown in Fig.1 (D). Heat treatment of Ir/PDDA/SiO₂ at 200ºC further improved.

In conclusion, we successfully synthesized connected Ru and Ir nanoparticle catalysts. Among them, Ir/PDDA/SiO₂ showed the highest OER activity and the activity was further improved by heat oxidation treatment. The results showed that connected Ir nanoparticle catalysts are promising as carbon-free catalysts for water electrolysis.

[1] T. Tamaki, H. Kuroki, T. Yamaguchi et al., Energy Environ. Sci., 8, 3545-3549 (2015).

[2] Y. Lee et al., J. Phys. Chem. Lett., 3, 399-404 (2012).

[3] HN. Nong et al., Chem. Sci., 5, 2955-2963 (2014).