In order to reduce the amount of noble metal catalysts for the oxygen evolution reaction (OER) in polymer electrolyte membrane water electrolysis (PEMWE) while maintaining high efficiency, we synthesized new catalysts: IrO x nanoparticles dispersed on Nb-SnO 2 and Ta-SnO 2 supports with fused-aggregate structures. IrO x nanoparticles with a uniform size of ca. 2 nm were highly dispersed on these supports. The OER activities were evaluated by linear sweep voltammetry (LSV) in 0.1 M HClO 4 at 80 • C using a channel flow electrode cell. The IrO x /Ta-SnO 2 catalysts exhibited an apparent mass activity (MA) of 15 A mg Ir -1 for the OER at 1.5 V vs. RHE, which was 32 times higher than that of a conventional catalyst (mixture of Pt black and IrO 2 powders). This suggests a possible reduction of the loading of the noble metal anode catalyst to a level as low as 0.1 mg cm −2 at a voltage efficiency of 90% at 1 A cm −2 . © The Author Renewable energy sources such as solar and wind powers are sustainable alternatives to fossil fuels, although the electricity generation from such sources is intermittent in nature. Thus, the conversion and storage of surplus renewable electricity to other forms of energy are required for leveling of their large output fluctuations. Water electrolysis makes it possible to produce high purity hydrogen from renewable electric power and thus to level the output fluctuations when combined with stationary fuel cells. Polymer electrolyte membrane water electrolysis (PEMWE) has received much attention because of advantages such as high energy conversion efficiency, even at high current densities, and a compact system with easy maintenance and start-up and shut-down, 1-3 compared to alkaline 4 and solid oxide electrolysis, 5 leading to the feasibility of on-site hydrogen production. Nevertheless, conventional PEMWE cells are very expensive due to their use of a large amount of noble metals such as Pt and/or Ir as electrocatalysts (2 mg Pt+Ir cm −2 or more in each electrode), in addition to their use of costly polymer electrolyte membranes. 3,6 To solve the catalyst problem, one possible approach is to use nano-sized catalysts highly dispersed on support materials in place of noble metal blacks with large particle sizes, typically over 50 nm. For the support material at the anode, high durability at the high potentials of the oxygen evolution reaction (OER) under acidic conditions is required. Carbon supports, which have been commonly used in polymer electrolyte fuel cells (PEFCs), cannot be used due to the severe corrosion at such high potentials. 7,8 Tin oxide, 9 titanium oxide, 10,11 titanium carbide, 12 and silicon carbide-silicon, 13 among others, have been examined as supports for noble metal catalysts for the OER. The OER activities of iridium and/or ruthenium oxide nanoparticle (or nanodendrite) catalysts supported on antimony-doped tin oxide (Sb-SnO 2 ), which exhibited a relatively high electronic conductivity, have been examined by several groups.14-19 However, to our knowledge, the micros...

We have developed IrOx/M-SnO2 (M = Nb, Ta, and Sb) anode catalysts, IrOx nanoparticles uniformly dispersed on M-SnO2 supports with fused-aggregate structures, which make it possible to evolve oxygen efficiently, even with a reduced amount of noble metal (Ir) in proton exchange membrane water electrolysis. Polarization properties of IrOx/M-SnO2 catalysts for the oxygen evolution reaction (OER) were examined at 80 °C in both 0.1 M HClO4 solution (half cell) and a single cell with a Nafion® membrane (thickness = 50 μm). While all catalysts exhibited similar OER activities in the half cell, the cell potential (Ecell) of the single cell was found to decrease with the increasing apparent conductivities (σapp, catalyst) of these catalysts: an Ecell of 1.61 V (voltage efficiency of 92%) at 1 A cm−2 was achieved in a single cell by the use of an IrOx/Sb-SnO2 anode (highest σapp, catalyst) with a low Ir-metal loading of 0.11 mg cm−2 and Pt supported on graphitized carbon black (Pt/GCB) as the cathode with 0.35 mg cm−2 of Pt loading. In addition to the reduction of the ohmic loss in the anode catalyst layer, the increased electronic conductivity contributed to decreasing the OER overpotential due to the effective utilization of the IrOx nanocatalysts on the M-SnO2 supports, which is an essential factor in improving the performance with low noble metal loadings.

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

We have developed IrOx/M-SnO2 (M = Nb, Ta, and Sb) anode catalysts, IrOx nanoparticles uniformly dispersed on M-SnO2 supports with fused-aggregate structures, which make it possible to evolve oxygen efficiently, even with a reduced amount of noble metal (Ir) in proton exchange membrane water electrolysis. Polarization properties of IrOx/M-SnO2 catalysts for the oxygen evolution reaction (OER) were examined at 80 °C in both 0.1 M HClO4 solution (half cell) and a single cell with a Nafion® membrane (thickness = 50 μm). While all catalysts exhibited similar OER activities in the half cell, the cell potential (Ecell) of the single cell was found to decrease with the increasing apparent conductivities (σapp, catalyst) of these catalysts: an Ecell of 1.61 V (voltage efficiency of 92%) at 1 A cm-2 was achieved in a single cell by the use of an IrOx/Sb-SnO2 anode (highest σapp, catalyst) with a low Ir-metal loading of 0.11 mgIr cm-2 and Pt supported on graphitized carbon black (Pt/GCB) as the cathode, with 0.35 mgPt cm−2. In addition to the reduction of the ohmic loss in the anode catalyst layer, the increased electronic conductivity contributed to decreasing the OER overpotential due to the effective utilization of the IrOx nanocatalysts on the M-SnO2 supports, which is an essential factor in improving the performance with low noble metal loadings.

No abstract

No abstract

No abstract