Metal-Oxide-Supported Ir-Decorated Electrocatalysts for Polymer Electrolyte Membrane Water Electrolysis

Abstract

Introduction Since polymer electrolyte membrane water electrolysis (PEMWE) is similar in cell structure to polymer electrolyte fuel cells (PEFC), technologies used for PEFCs can be applied to PEMWEs. However, Pt electrocatalysts supported on carbon black (Pt/C) typically used for PEFCs cannot be applied under a high potential up to 2.0V [1]. Here, we apply SnO2 and titanium as alternative support materials. SnO2 may be promising as an anode electrocatalyst support material since it has relatively high electronic conductivity and stability against a high potential, among various metal oxides [2 ,3 ]. The top surface of metallic titanium is naturally oxidized to form stable titanium oxide, so that titanium supports may exhibit high durability under the strongly-acidic PEMWE condition [4]. In this study, we prepare the iridium-decorated SnO2/VGCF electrocatalysts (IrO2/Nb-SnO2/VGCF). Furthermore, by making such Ti sheet acting as the gas diffusion layer (GDL) and current collector, we prepare a GDL-integrated PEMWE electrode consisting of Ir electrocatalyst and Ti-based catalyst support (Ir/Ti). Experimental Sn0.98Nb0.02O2/VGCF was prepared via an ammonia co-precipitation method. IrO2 nanoparticles were decorated on Sn0.98Nb0.02O2/VGCF by evaporating the aqueous solution mixture containing Sn0.98Nb0.02O2/VGCF and H2IrCl6.nH2O while stirring, followed by heat treatment at 440°C in air. The Ti sheet was etched with 1M NaOH solution at 60°C for 1 hour, followed by heat treatment at 400°C in 5%H2-N2 for 30 minutes. Ir nanoparticles were decorated on the NaOH-etched Ti sheet by the arc plasma deposition (APD). The microstructure of the IrO2/Nb-SnO2/VGCF and the Ir/Ti sheet was observed by SEM, STEM, and TEM. Electrochemical measurements were made for the half cell and the single cell. Results and discussion We prepared IrO2/Sn(Nb)/VGCF with IrO2 loading of 23.6wt.%. Figure 1 shows the micrograph of the IrO2/Nb-SnO2/VGCF electrocatalyst. The micrograph indicates that Ir (or IrO2) nanoparticles have particle size of 1 to 3nm in diameter. I-V characteristics and durability of the MEA made with the IrO2/Sn(Nb)O2/VGCF electrocatalyst were evaluated. The MEA exhibited low activation overvoltage, but still with high IR loss and poor durability Figure 2 shows the STEM image of the Ir/Ti sheet after 2000 pulses of the APD with the Ir loading of 0.086mg/cm2, showing Ir particles of 1 to 2 nm on the etched Ti-based material. I-V characteristics of the MEA made with the Ir/Ti with the oxidized Ti surfaces were evaluated. The Ir/Ti MEA exhibited higher performance than that of the IrO2/Sn(Nb)O2/VGCF electrocatalyst despite very low Ir loading, indicating that the Ir/Ti MEA could be of interest for PEMWE applications.