Ru-Core Ir-Shell Catalysts Deposited on a Surface-Modified Porous Transport Layer for Polymer Electrolyte Membrane Water Electrolysis Anode
Authors: Yasutake Masahiro , Noda Zhiyun , Matsuda Junko , Lyth Stephen Matthew , Nishihara Masamichi , Sasaki Kazunari 
Published: December 2024

Introduction

The capital expenditure of electrolyzers significantly impacts the cost of green hydrogen. Regarding polymer electrolyte membrane water electrolysis, platinum group metal (PGM) loading is the major factor in increasing the electrolyzer cost. In particular, anode components have to use a large amount of PGM materials for their severe acidic and high potential condition. Iridium catalysts for oxygen evolution reactions (OER) and platinum coatings to prevent Ti oxidation of the porous transport layer (PTL) should be reduced to meet the future PGM loading requirements 1. Our research group has developed the Ir catalyst deposited PTL 2–4. In this electrode, Ir protects the porous transport layer from Ti oxidation and acts as a catalyst for the oxygen evolution reaction, leading to Pt loading reduction on PTL. However, Ir loading reduction is also certainly important due to their high material cost. Here, we develop a Ru-core Ir-shell catalyst deposited PTL to enhance the OER activity of the catalyst deposited on the PTL. Applying binary catalysts 5 on PTLs has a potential to reduce Ir loading by improving catalytic activity 3.

Experimental

Titanium microfiber sheets with a nominal porosity of ca. 70% (Nikko Techno, Ltd., Osaka, Japan) were used as the substrates acting as catalyst supports and PTLs. Chemical etching with NaOH solution was performed to increase the surface area of the titanium PTL. 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-deposited PTL was prepared by depositing the catalyst onto the NaOH-etched titanium sheets via arc plasma deposition. Iridium and ruthenium were deposited onto the NaOH-etched titanium sheets at room temperature via arc plasma deposition (APD-S, Advanced RIKO, Inc., Yokohama, Japan). The Ru-core Ir-shell catalyst structure was prepared by Ru deposition followed by Ir deposition. The total PGM loading was fixed at 0.17 mg cm-2, and the Ru/Ir ratio was varied to evaluate the effect of their ratio. The PTEs deposited with the Ru-core Ir-shell catalysts were applied as PEMWE anodes. MEAs were prepared by hot-pressing the PTLs with the cathode catalyst-coated electrolyte membranes at 140 °C and 0.3 MPa for 180 s.

Results and discussion

Figure 1 shows the Ru-core Ir-shell catalyst deposited on the surface-modified PTL 3. The core-shell catalysts with Ru/Ir ratios of (a) 3:1 and (b) 1:1 have Ir shell thicknesses of 3-4 nm and 4-6 nm, respectively. Therefore, it is found that the thickness of the Ir shell can be controlled by varying the loading ratio of Ir and Ru in the physical vapor deposition method. The cell with the Ru-core Ir-shell catalyst deposited on PTL exhibited higher mass activity for the Ir amount and higher electrolysis performance than the cell with only Ir deposited PTL. However, the cell durability decreased while thinner Ir shells had higher activity. Therefore, it is necessary to use more stable oxide core materials to improve durability.

References

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Acknowledgments

This study was supported by the Iwatani Naoji Foundation. An initial part of this study was supported by the Center of Innovation (COI) Program Grant Number JPMJCE1318 by the Japan Science and Technology Agency (JST), and the Fukuoka Strategy Conference for Hydrogen Energy.

Figure 1