Carbon-Free Connected Ir-Ru Nanoparticle Catalysts for Polymer-Electrolyte Water Electrolysis
Authors: Sugita Yoshiyuki , Tamaki Takanori , Kuroki Hidenori , Yamaguchi Takeo 
Published: December 2020

Oxygen evolution reaction (OER) catalysts for polymer-electrolyte water electrolysis (PEWE) requires a high Ir loading of 1–4 mg cm−2 because a carbon support cannot be used for a PEWE anode where a highly oxidizing potential is applied and most supports including carbon are corroded. Connected nanoparticle catalysts showing electron conductivity and a high surface area without any conducting supports due to the formation of electron-conducting networks. Thus, the connected nanoparticles catalysts (Fig. 1 (A)) are promising for OER. Previously, we have developed connected Ir nanoparticle catalysts coated onto silica with high density for OER in PEWE.[1]

In this study, we proposed connected Ir-Ru nanoparticle catalysts for OER in PEWE because the alloy of Ir and Ru shows high OER activity in the research of nanoparticle catalysts on conducting supports [2–3]. The catalysts were synthesized as follows. First, connected Ir-Ru nanoparticles were synthesized onto silica template (PDDA/SiO2) with high density via polyol method using tetraethylene glycol as the reducing agent, and Ru(III) acetylacetonate and Ir(III) acetylacetonate as the metallic precursors (connected Ir-Ru/SiO2). Then, silica was coated onto connected Ir-Ru/SiO2 using tetraethyl orthosilicate and ammonia aqueous in ethanol by stirring for 6 hours at room temperature. After silica coating, these nanoparticles were treated in supercritical ethanol at 280–450 ºC for 60–90 min. Dissolution of SiO2 in 3 M NaOH solution at 85ºC for 3 h made porous hollow structure (connected Ir-Ru capsule). OER performance was evaluated by electrochemical measurement in 0.1 M HClO4 solution. Membrane electrode assemblies (MEAs) were prepared by CCM method using connected nanoparticle catalysts as the anode catalyst, Pt/C as the cathode catalyst and Nafion® membrane as the electrolyte membrane as shown previously. The Ir loading of the MEAs was below 1 mg cm−2, while the Pt loading was 0.3 mg cm−2. Water electrolysis and electrochemical impedance spectroscopy (EIS) were performed at 80 °C with circulating water in the anode flow channel.
Fig. 1 (B) shows TEM image of connected Ir-Ru capsule. Capsule and networks structure of each catalyst were observed by TEM images. Fig. 1 (C) shows OER curves of connected Ir and Ir-Ru catalysts. All synthesized catalysts showed higher OER activity than commercial catalyst. Among them, connected Ir-Ru/SiO2 had the highest OER activity, although both Ir-Ru catalysts showed lower onset potential than Ir catalysts. Water electrolysis performance of the MEA using connected Ir-Ru/SiO2 was higher than that of the MEA using connected Ir/SiO2, as shown in Fig 1 (D).
In conclusion, we successfully synthesized connected Ir and Ir-Ru nanoparticle catalysts. Among them, connected Ir-Ru/SiO2 showed the highest OER activity and water electrolysis performance. The results showed that connected Ir-Ru nanoparticle catalysts are promising as carbon-free catalysts for water electrolysis.
Acknowledgement
Part of this presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
[1] Y. Sugita, T. Tamaki, H. Kuroki and T. Yamaguchi, Nanoscale Adv., 2, 171 (2020).
[2] T. Audichon, B. Guenot, S. Baranton, M. Cretin, C. Lamy and C. Coutanceau, Appl. Catal. B Environ., 200, 493 (2017).
[3] R. V. Genova-Koleva, F. Alcaide, G. Álvarez, P. L. Cabot, H.-J. Grande, M. V. Martínez-Huerta and O. Miguel, J. Energy Chem., 34, 227 (2019).

Figure 1