Microstructural Design of PEFC Electrocatalyst Layer Using Mesoporous Carbon
Authors: Suzuki Takashi , Miyamoto Ryo , Yasutake Masahiro , Noda Zhiyun , Lyth Stephen Matthew , Nishihara Masamichi , Matsuda Junko , Sasaki Kazunari 
Published: December 2024

To utilize PEFCs for applications such as heavy duty vehicles (HDV), alternative electrocatalysts are desirable to be developed to achieve both high power output and high durability. Pt-Co alloy catalysts are widely known for their high activity1, and this research group has been developing Pt-Ta-Co catalysts by adding Ta2. Ta is stable in the cathode environment and has been shown to improve durability by depressing grain growth and direct contact between the Pt-based catalyst and the carbon support3. Mesoporous carbon (MC) is used as a support material for commercial FCVs (second-generation MIRAI) because the unique structure of mesopores can prevent a direct contact between the Pt-based catalyst and the ionomer for improving catalytic activity and durability5.
Figure 1 shows cross-sectional images of the electrocatalyst layer using (a) Ketjen black (KB), atypical carbon black used for PEFCs, and (b) MC as carbon support. The MEA fabrication processes for KB have been optimized and high cell performance has been obtained2. However, as shown in the SEM images, MC has a larger and non-uniform particle size compared to KB, so the MEA fabrication processes suitable for MC have to be analyzed and optimized. In this study, MEA fabrication processes for MC for improving cell performance are studied.
The Pt-Ta-Co catalysts were prepared using Pt (acac)2, Ta (OC2H5)5, and CO (NO3)2 as precursors, supported on MC (MH-18, TOYO TANSO, Japan), and then heat-treated. The microstructural observation of the prepared catalysts by STEM showed highly dispersed Pt-based particles of about 3 nm in diameter. MEAs were fabricated by spray-printing a slurry of electrocatalyst, pure water, ethanol, and Nafion ionomer onto a Nafion membrane. In this study, MEAs were fabricated by varying the weight ratio of Nafion ionomer to carbon (I/C) and the volume ratio of pure water to ethanol (W/E), and cell performance was evaluated.
Half-cell electrochemical measurements of the Pt-Ta-Co/MC electrocatalyst exhibited higher electrochemical active surface area (ECSA) and mass activity (MA) compared to a typical standard catalyst, Pt/KB (TEC10E50E, Tanaka Kikinzoku Kogyo, Japan). The effects of I/C and W/E on I-V characteristics and activation, ohmic, and concentration overvoltages of single cells were evaluated and discussed.

Acknowledgement

This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). An educational part for young scientists was supported by the Japan Scientist Technology Agency (JST) as a part of Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE), Grant Number JPMJAP2307.

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Figure 1