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 IrO_x 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 IrO_x 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 80^oC 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 IrO_x. By ICP-AES, the amount of Ir loaded in IrO_x/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 IrO_x 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 σ_app 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 σ_app much higher than those of Nb-SnO₂ and Ta-SnO₂. The values of σ_app were found to increase up to two orders of magnitude by loading IrO_x, Especially, the σ_app of the IrO_x/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 IrO_x/Ta-SnO₂ and IrO_x/Sb-SnO₂ catalysts exhibited higher mass activities (MAs) for the OER at 1.50 V than that of IrO_x/Nb-SnO₂. The MA of 16 A mg_Ir^–1 at IrO_x/Ta-SnO₂ was 30 times higher than that of the conventional catalyst. Such high MAs of IrO_x/Ta-SnO₂ or IrO_x/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 mg_Pt+Ir cm^‒2.

Acknowledgement

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

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