Synthesis and Oxygen Evolution Performances of Iridium Oxide Electrocatalysts Supported on Tin Oxides for Polymer Electrolyte Membrane Water Electrolysis
Authors: Ohno Hideaki , Nohara Shinji , Kakinuma Katsuyoshi , Miyake Akiko , Deki Shigehito , Watanabe Masahiro , Uchida Hiroyuki 
Published: February 2020

Polymer electrolyte membrane water electrolysis (PEMWE) has received much attention as an attracting method to produce high-purity hydrogen with high energy conversion efficiency. However, conventional PEMWE cells are very expensive due to usage of a large amount of noble metal catalysts (a few mg cm–2). The aim of our research is the reduction of the amount of noble metal catalysts to 1/10 maintaining high-efficiency ≥ 90%. Although the amount of noble metal catalysts could be largely reduced by using nano-sized catalysts highly dispersed on support materials in place of conventional noble metal black (typically, ≥ 50 nm), carbon supports cannot be used at the anode due to the corrosion at high potentials of O2 evolution. Recently, our group succeeded in synthesizing Pt catalysts supported on corrosion-resistant M-SnO2 (M=Nb, Ta and Sb) with fused aggregated structures for polymer electrolyte fuel cells.1,2 In the present study, we newly synthesized IrOx nanoparticles dispersed on the M-SnO2(M=Nb, Ta and Sb) supports, and evaluated their oxygen evolution reaction (OER) activities for the PEMWE.
The M-SnO2 (M=Nb, Ta and Sb) supports were prepared by flame pyrolysis of organometallic salt solution.3 IrOx nanoparticles were dispersed on them by a colloidal method.4 The catalysts thus prepared were characterized by XRD, TEM, XPS, ICP-AES, and apparent electrical conductivity measurement. For electrochemical measurements, a channel flow electrode cell was used.5 A catalyst ink was pipetted on a planar Au substrate electrode embedded in the Teflon® cell, followed by Nafion®-coating and drying. The OER activities were examined by linear sweep voltammetry (LSV) at 10 mV s–1 in 0.1 M HClO4 at 80oC under ambient air atmosphere.
We observed XRD peaks assigned to rutile-type SnO2, but could not detect any other peaks attributed to Ir, IrO2 or impurities. As shown in Fig. 1, it was observed by TEM that nanoparticles with uniform size of ca. 2 nm were highly dispersed on the supports in all catalysts prepared. Because XPS of the catalysts indicated the presence of both Ir0 and Ir4+, we denote them as IrOx. By ICP-AES, the amount of Ir loaded in IrOx/M-SnO2 (M=Nb, Ta and Sb) catalysts were determined to be 11.3, 10.4 and 8.8 wt%, respectively. Thus, we successfully synthesized IrOx nanoparticle catalysts highly dispersed on the doped SnO2 supports with no large difference in the microstructure regardless of the kind of dopants.
The apparent electrical conductivities σapp for the three supports and the corresponding catalysts measured under a pressure of 19 MPa are shown in Fig. 2. Sb-SnO2 support exhibited the σapp much higher than those of Nb-SnO2 and Ta-SnO2. The values of σapp were found to increase up to two orders of magnitude by loading IrOx, Especially, the σapp of the IrOx/Sb-SnO2 catalyst was the highest 0.80 S cm–1. Such a phenomenon accords with that observed for Pt catalysts supported on doped SnO2, which may be related to a decrease in the electronic depletion region of SnO2.6

Figure 3 shows LSVs of these catalysts compared with a conventional catalyst (mixture of commercial IrO2 and Pt black, 50:50 in weight ratio). The onset potential for the OER current for our catalysts was ca. 1.38 V vs. RHE. The IrOx/Ta-SnO2 and IrOx/Sb-SnO2 catalysts exhibited higher mass activities (MAs) for the OER at 1.50 V than that of IrOx/Nb-SnO2. The MA
of 16 A mgIr
–1 at IrOx/Ta-SnO2 was 30 times higher than that of the conventional catalyst. Such high MAs of IrOx/Ta-SnO2 or IrOx/Sb-SnO2 catalysts enable PEMWE operation efficiency of 90% at 1 A cm‒2 with the noble metal anode catalyst as low as 0.1 mgPt+Ir cm‒2.

Acknowledgement

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

1) K. Kakinuma, Y. Chino, Y. Senoo, M. Uchida, T. Kamino, H. Uchida, S. Deki, and M. Watanabe, Electrochim. Acta, 110, 316 (2013).
2) Y. Senoo, K. Taniguchi, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, Electrochem. Commun.,
51, 37 (2015).
3) K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).
4) M. Watanabe, M. Uchida, and S. Motoo, J. Electroanal. Chem., 229, 395 (1987).
5) N. Wakabayashi, M. Takeichi, H. Uchida, and M. Watanabe, J. Phys. Chem. B, 109, 5836(2005).
6) Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, RSC Adv., 4, 32180 (2014).

Figure 1