Remarkable Mass Activities for the Oxygen Evolution Reaction at Iridium Oxide Nanocatalysts Dispersed on Tin Oxides for Polymer Electrolyte Membrane Water Electrolysis

Abstract

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...

Effect of Electronic Conductivities of Iridium Oxide/Doped SnO₂ Oxygen-Evolving Catalysts on the Polarization Properties in Proton Exchange Membrane Water Electrolysis

“…In particular, the σapp, catalyst value of the IrOx/Sb-SnO2 catalyst was the highest, 8.1 × 10 -1 S cm -1 . As reported previously for Pt/Nb-SnO2 [33] and IrOx/M-SnO2 (M = Nb and Ta) [32], such an increase in the conductivity for IrOx/Sb-SnO2 is ascribed to the shrinkage of the depletion layer of the SnO2 support particles [33]. Thus, we successfully synthesized IrOx/M-SnO2 catalysts with similar microstructures but with a range of different of σapp, catalyst values.…”

“…V, which was similar to that for the conventional catalyst. supports [28,32,35]. Hence, the enhanced MAs of IrOx/M-SnO2 might be ascribed not only to a significant increase in the active surface area, by the use of IrOx nanoparticles, but also their interaction with the oxide supports.…”

“…Pt for the conventional catalyst) loaded on the electrode substrate. In order to remove oxygen gas bubbles effectively from the electrode surface, the flow rate of the electrolyte solution was adjusted at 160 cm s −1 [32]. These IrOx/M-SnO2 catalysts showed onset potentials for the OER from 1.35 to 1.40…”

“…The IrOx/M-SnO2 catalysts were prepared in the similar manner described in our previous paper [32]. Briefly, Sn0.96Nb0.04O2−δ, Sn0.975Ta0.025O2−δ, and Sn0.95Sb0.05O2−δ (projected composition, where δ is the mole fraction of oxygen deficiencies) with the fused-aggregate network structure were synthesized by the flame pyrolysis method [29].…”

“…It was found that an IrOx/Ta-SnO2 catalyst exhibited an apparent MA of 15 A mgIr −1 for the OER in 0.1 M HClO4 solution at 1.5 V vs. RHE and 80 °C, which suggests the possibility of reducing the loading of Ir in an anode catalyst to a level as low as 0.1 mgIr cm −2 at a voltage efficiency of 90% (Ecell = 1.65 V) operated at 1 A cm −2 , i.e. the anode potential of 1.5 V, cathode potential of −0.05 V, and the ohmic loss of the PEM of 0.10 V [32].…”