Catalyst-Integrated Porous Transport Layer for Polymer Electrolyte Membrane Water Electrolysis
Authors: Yasutake Masahiro , Noda Zhiyun , Matsuda Junko , Lyth Stephen Matthew , Nishihara Masamichi , Sasaki Kazunari 
Published: December 2024

Introduction

Widespread commercialization of polymer electrolyte membrane (PEM) water electrolyzers requires significant cost reduction. A large amount of platinum metal group (PGM) loading is a primary factor in increasing the electrolyzer cost1. In particular, Ir catalysts and Pt coating on the porous transport layer (PTL) are necessary for the anode. Our research group has developed the Ir catalyst-integrated PTL by increasing the surface area of Ti-based PTL with surface treatment and coating Ir on the PTL, as shown in Fig. 1 2–4. Ir catalysts directly deposited on the PTL also act as a protective coating to prevent Ti oxidation of the PTL. In other words, this alternative anode design has two kinds of oxygen evolution reaction (OER) sites: the catalyst layer and the Ir coating layer on PTL. Suppose the total amount of iridium in this alternative anode is the same as in the conventional structure, and the electrolysis performance is sufficient. In that case, this alternative anode design can reduce the PGM loading for the conventional platinum coating on PTL. Here, we evaluate the impact of this alternative anode design on the initial cell performance and durability.

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 Ir catalyst-integrated PTL was prepared by depositing iridium onto the NaOH-etched titanium sheets via arc plasma deposition. Electrocatalyst paste for the PEM electrolysis 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 Ir catalyst-integrated or Pt-coated PTLs were used for the alternative or conventional anode. The durability tests were performed at 1.7 V holding for 100 h.

Results and discussion

Demonstrating the OER activity of Ir coating on the surface-modified PTL is essential. We compared the electrolysis cell performance using the conventional Pt-coated PTL and the Ir catalyst-integrated PTL. The cell with Ir catalyst-integrated PTL has lower activation overvoltage than conventional Pt-coated PTL. This lower activation overvoltage suggests that the Ir catalyst on the PTL can contribute to the oxygen evaluation reaction. After the durability test, the activation overvoltage of the conventional anode increased, while that of the alternative anode hardly changed. Iridium oxide has one order of magnitude higher electronic conductivity than platinum oxide5. Therefore, relatively high electronic conduction between the catalyst layer and the Ir catalyst-integrated PTL could maintain the cell performance.

References

The International Renewable Energy Agency, Green Hydrogen Cost Reduction, (2020).

M. Yasutake, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, K. Ito, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 167, 124523 (2020).

M. Yasutake, Z. Noda, J. Matsuda, S. M. Lyth, M. Nishihara, K. Ito, A. Hayashi, and K. Sasaki, Int. J. Hydrogen Energy, 49, 169 (2024).

M. Yasutake, Z. Noda, J. Matsuda, S. M. Lyth, M. Nishihara, K. Ito, A. Hayashi, and K. Sasaki, J. Electrochem. Soc.,
170, 124507 (2023).

T. Nobuo, K. Nasu, A. Fujimori, and K. Shiratori, Electronic Conduction in Oxides, 2nd ed., p. 30, (1983).

Acknowledgments

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