Hybrid Anode Design of Polymer Electrolyte Membrane Water Electrolysis for the Reduction of Total Platinum Group Metal Loading

Abstract

Introduction Reducing capital expenditure of polymer electrolyte membrane water electrolysis (PEMWE) is essential for reducing the production cost of green hydrogen. The amount of platinum group metal (PGM) loading significantly impacts the capital expenditure. Especially iridium catalyst and platinum coating on the porous transport layer of the anode have to be reduced. Our research group has developed the catalyst-integrated porous transport electrode (PTE), where iridium catalysts were directly deposited on porous transport layer (PTL) acting as an electronic conductive coating on the PTL1 , 2. This structure could reduce total PGM loading of PEMWE cells due to being virtually free of Pt coating on the PTL, typically 1 to 5 mg cm-2 3. However, the catalyst-integrated PTE has an issue of the reduction of activation overvoltage in increasing iridium catalyst loading because the mass activity of iridium catalysts in the catalyst-integrated PTE decreases with increasing iridium loading. The hybrid anode consisting of both the conventional catalyst layer and the catalyst-integrated PTE as the PTL could reduce activation overvoltage effectively, and consequently the capital expenditure significantly with less Ir and Pt loadings. Experimental Titanium microfiber sheets with nominal porosity of ca. 70% (Nikko Techno, Ltd., Osaka, Japan) were used as the substrates acting as catalyst supports and PTLs. To increase the surface area of the titanium PTL, chemical etching with NaOH solution was performed. First, titanium sheets were etched in an aqueous 1 M NaOH solution at 60 °C for 1 h. After that, the etched titanium sheets were washed under ultra-sonication in 0.01M HNO3 solution for 30 min and then washed in deionized water at room temperature for 10 min. Heat treatment was then performed at 400 °C in 5% H2-N2 gas for 30 min. The catalyst-integrated PTE was prepared by depositing iridium onto the NaOH-etched titanium sheets via arc plasma deposition. Electrocatalyst paste for the PEMWE cathodes was prepared by dispersing the Pt/C (Pt 46.5 wt.%, TEC10E50E, Tanaka Kikinzoku Kogyo Co., Tokyo, Japan), 99.5% ethanol, deionized water, and 5% Nafion solution. IrO2 electrocatalyst paste for the PEMWE anodes was prepared by dispersing the IrO2 catalyst (IrO2 (IV) TYPE II, Tokuriki Honten Co. Ltd., Tokyo, Japan), 99.5% ethanol, deionized water, and 5% Nafion solution. Using a spray printing system (Nordson, USA), these anode and cathode electrocatalyst pastes were spray-printed onto the electrolyte membrane (Nafion 212, E. I. du Pont de Nemours and Co., Wilmington, USA). The hybrid anodes were prepared by using the catalyst-integrated PTEs instead of Pt-coated PTLs. Results and discussion The conventional anode with 0.1 mg cm-2 of IrO2, the catalyst-integrated PTE with 0.344 mg cm-2 of Ir loading, and the hybrid anode consisting of this conventional IrO2-based catalyst and the catalyst-integrated PTE were prepared and evaluated. Figure 1 shows (a) I-V characteristics, (b) activation overvoltage, and (c) ohmic overvoltage of cells with three types of anodes. Figure 1(a) shows that the hybrid design is effective for significant performance improvement over the individual electrode design with the IrO2 layer or the PTE. Electrolysis voltage of the cell with the hybrid anode at 10 A cm-2 was around 2.2 V. The activation overvoltage with the hybrid anode was lower than that with the conventional IrO2-based anode only and the catalyst-integrated PTE only, indicating that the iridium catalysts in the catalyst-integrated PTE are active even in the hybrid anode, as shown in Fig. 1(b). The ohmic overvoltage with the catalyst-integrated PTE was higher than that with the conventional anode, but the hybrid anode exhibited a comparable value to the conventional one. This may be because of the introduction of the intermediate catalyst layer consisting of ionomers and IrO2 between the catalyst-integrated PTE and the electrolyte membrane, which is a mixed conductor of ions and electrons, especially reducing the proton transport resistance between the membrane and the PTE. Thus, hybrid anodes with catalyst-integrated electrodes instead of Pt-coated PTLs can potentially improve the performance of PEM electrolyzers and reduce the total PGM loading. References M. Yasutake, D. Kawachino, Z. Noda, J. Matsuda, K. Ito, A. Hayashi, and K. Sasaki, ECS Trans., 92(8), 833 (2019).M. Yasutake, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, K. Ito, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 167, 124523 (2020).The International Renewable Energy Agency, Green Hydrogen Cost Reduction, (2020). Figure 1