Development of Low Iridium Catalyst Layer Structure for a Water Electrolysis Cell

Abstract

1.Introduction P2G is viewed as a solution essential for achieving carbon neutrality by 2050. It uses electrolysis to convert electricity from renewables into hydrogen, ready for storage and transportation. The key technology in the process is the water electrolyzer that converts energy into hydrogen without emitting CO2. A PEM water electrolyzer offers excellent adaptability to power fluctuations and high durability. PEM water electrolyzers use membrane electrode assemblies (MEAs) that integrate the electrolyte membrane and electrode. However, the MEA electrode relies on a large amount of iridium to ensure sufficient electrolytic efficiency. Iridium is one of the rarest precious metals and it is four to five times as costlyas platinum. Toshiba has developed a multilayer catalyst that uses a sputtering technology to deposit alternate layers of iridium oxide nanosheet films and void layers, which at low iridium loading achieved a performance superior to that of IrO2 powder anode. 2 ． Experimental The sputtering process was executed in a large chamber that can accommodate electrodes up to a maximum of 5 m2 in size. First, iridium together with pore-forming metal was sputtered onto a commercially available PTL substrate and then only pore-forming metal was sputtered, and this process was repeated to deposit multiple layers. Next, the pore-forming metal was removed to prepare the anode (Fig. 1). The loading of iridium metal was adjusted by the number of iridium layers. Then, a small electrode was cut from a large electrode and a 4 cm2 MEA was made by hot pressing of the anode, Nafion115®, and the Pt/C cathode. A conventional iridium oxide powder layer was sprayed on Nafion115 and a platinum-coated PTL substrate was prepared for reference. Water electrolysis performance of the MEAs was measured in a single-cell configuration by supplying water to the anode side and applying current to the cell under atmospheric pressure. 3 ． Results and discussion Figure 2 shows the water electrolysis IV curves at 80℃ for each type of anode. This novel catalyst structure successfully reduces the iridium to 0.15 mg/cm2, while maintaining the water electrolysis performance and durability. This superior performance is due to the homogeneity of the catalyst layer, which is achieved by a sputtering process enabling nanometer- thick coatings, and the high diffusivity of oxygen and water due to the multilayer structure of the iridium nanosheets and void layers. Our structure also has the advantage of reducing the amount of precious metals by dispensing with the need for Pt coating on the PTL. It was also found that there was no significant variation in water electrolysis performance within a 5 m2 electrode. Thus, the technology for production of large-scale electrodes has been developed. 600 cm2 MEA has also been tested. Figure 1