Performances of Polymer Electrolyte Membrane Water Electrolysis Cells with IrOx/Doped SnO2 Oxygen Evolution Catalysts
Authors: Ohno Hideaki , Nohara Shinji , Kakinuma Katsuyoshi , Miyake Akiko , Deki Shigehito , Uchida Hiroyuki 
Published: February 2020

Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology which enables a production of high-purity hydrogen with high energy conversion efficiency. However, conventional PEMWE cells are very expensive because noble metal catalysts (a few mg cm–2) are used. The aim of our research is the reduction of the amount of noble metal catalysts to 1/10 maintaining high-efficiency ≥ 90%. We reported that IrOx nanoparticles dispersed on M-doped SnO2 (M=Nb, Ta and Sb) supports with fused aggregated structures exhibited much higher mass activities for the oxygen evolution reaction (OER) than a conventional catalyst (mixture of commercial IrO2 and Pt black, 50:50 in weight ratio) in 0.1 M HClO4 electrolyte at 80 oC.1 In the present research, we examined performances of membrane-electrode assemblies (MEAs) with the IrOx/SnO2 based anode catalysts.
Uniform size IrOx nanoparticles (ca. 2 nm) highly dispersed on M-doped SnO2 (M=Nb, Ta and Sb) supports were prepared in the same manner as reported previously.1 The amounts of Ir loaded were quantified to be 11.3, 10.4 and 11.0 wt% for IrOx/M-SnO2 (M=Nb, Ta and Sb), respectively. The IrOx/M-SnO2, Nafion® binder (the volume ratio of Nafion binder to oxide support = 0.67), ethanol and pure water were mixed by using a planetary ball mill. The anode catalyst ink thus obtained was sprayed onto the Nafion® membrane with the thickness of 50 μm, followed by hot-pressing. The Ir loading at the anode was ca. 0.1 mgIr cm−2. Commercial Pt/graphitized carbon black (GCB) catalyst was used as the cathode with the Pt loadings of ca. 0.35 mgPt cm−2. The Japan Automobile Research Institute (JARI) standard cell with a geometric area of 25 cm2 was used for PEMWE measurements. Before the measurements, pure water at 80 oC was supplied to the cell, at least, for 12 h, and the ohmic resistance of the cell was then recorded by AC milli-ohmmeter. The I-V curves of the cells were measured at 80 oC.
Figure 1 shows I-V curves of the cells with the IrOx/M-SnO2 anode catalysts. The data of catalyst loading, ohmic resistance and cell potential at 1 A cm−2 are summarized in Table 1. The cell with IrOx/Ta-SnO2 catalyst showed a lower cell potential at 1 A cm-2 and ohmic resistance than that with IrOx/Nb-SnO2 catalyst, probably due to an increase in the apparent electrical conductivities σapp of the catalyst. However, the overpotential of the cell with IrOx/Ta-SnO2 catalyst was fairly larger than the conventional cell with Pt black + IrO2. We have already reported that Pt nanoparticle contributed to improving electric conductivities for SnO2 supports, which may be related to a decrease in the electronic depletion region of SnO2.4 Then, we synthesized Pt-loaded IrOx/Ta-SnO2 catalyst by the colloidal method. The use of Pt-IrOx/Ta-SnO2 catalyst significantly reduced both ohmic resistance and overpotential, achieving to the cell potential of 1.63 V at 1 A cm−2. The improved cell performance can be mainly ascribed to increases in the σapp (as shown in Fig. 2) and utilization efficiency of the catalyst. In addition, it was found that IrOx/Sb-SnO2 catalyst prepared very recently exhibited ca. 7 times higher σapp than Pt-IrOx/Ta-SnO2 catalyst. The evaluation of cell performance with IrOx/Sb-SnO2catalyst is now in progress.

Acknowledgement

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

References

1) H. Ohno, S. Nohara, K. Kakinuma, A. Miyake, S. Deki, M. Watanabe, and H. Uchida, 229th
ECS
Meeting,abstract 1422 (San Diego, May 30, 2016).
2) K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).
3) M. Watanabe, M. Uchida, and S. Motoo, J. Electroanal. Chem., 229, 395 (1987).
4) Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, RSC Adv., 4, 32180 (2014).

Figure 1