流れ　2023年10月号　目次

―　特集テーマ：ASME-JSME-KSME流体工学国際会議　― 巻頭言 （朴，松田，洪） 横風に対する鉄道車両の耐力評価方法と安全な鉄道運行のためのその応用 三須 弥生（JR東日本） 乱流の中の秩序 後藤 晋（大阪大学），本告 遊太郎（大阪大学） LES/粒子シミュレーションによる化学反応を伴う乱流平面噴流の数値解析 シン ジャボオ（名古屋大学）,　渡邉 智昭（名古屋大学大学院）,　長田 孝二（京都大学大学院） 液体ロケットインデューサにおけるキャビテーション不安定現象発生時の翼端漏れ渦キャビテーションの非定常特性 田村 浩紀（東北大学），奈倉 悠人（東北大学），川崎 聡（宇宙航空研究開発機構），伊賀 由佳（東北大学） Water Condensation in PEMFCs at Nano-scale: Insights through Lattice DFT simulations Clint John Cortes OTIC（東京大学），Masazumi ARAO（FC-Cubic），Masashi MATSUMOTO（FC-Cubic），Hideto IMAI（FC-Cubic），Ikuya KINEFUCHI（東京大学） Reconstruction of Fluid Stress Field from Flow Birefringence using Physics-Informed Convolutional Encoder-Decoder (PICED) Daichi IGARASHI（東京農工大学），Shun MIYATAKE（東京農工大学），Jingzu YEE（東京農工大学），Yoshiyuki TAGAWA（東京農工大学） Determination of Permeability in the Volume Penalisation Method with a Smooth Mask Function 辻元 大智（大阪大学），中尾 祐太（大阪大学），辻 拓也（大阪大学），田中 敏嗣（大阪大学），鷲野 公彰（大阪大学） 　｜　リンク一覧にもどる　｜

 

Water Condensation in PEMFCs at Nano-scale: Insights through Lattice DFT Simulations

Clint John Cortes OTIC 東京大学， Ikuya KINEFUCHI 東京大学， Masazumi ARAO FC-Cubic， Masashi MATSUMOTO FC-Cubic， Hideto IMAI FC-Cubic

Abstract

In Polymer Electrolyte Membrane Fuel Cells (PEMFCs), Cathode Catalyst Layers (CCLs) play a crucial role in facilitating the oxygen reduction reaction through a network of Platinum (Pt) nanoparticles and ionomer films. Ideally, for optimal catalytic performance, all Pt nanoparticles should be connected to the ionomer network. In recent years, High Surface Carbon (HSC) supports like Ketjenblack have been widely used, where Pt nanoparticles are deposited both on the exterior surface of HSC and within its highly porous interior structure ^[1]. However, since the ionomer cannot penetrate micro-pores, Pt nanoparticles deposited inside HSCs remain disconnected from the ionomer network. Nevertheless, the use of HSCs has significantly improved PEMFC performance. This seemingly counter-intuitive result can be explained by water condensation. Condensed water acts as a conductive pathway for protons, forming water bridges that link interior Pt nanoparticles to the exterior Pt-ionomer network ^[2]. Investigating water condensation inside the micro-pores of a CCL particle is challenging due to the system's extremely small scale. Visualizing nano-scale water condensation through experiments is tediously difficult while remaining too large for molecular-based simulations.

In this study, we employed a coarse-grained numerical simulation using reconstructed structural data from an actual CCL particle sourced from a commercial PEMFC, specifically the Toyota MIRAI Gen 2 model. Firstly, the porous structure of the CCL particle is reconstructed from segmented images (Pt nanoparticles, carbon support, and void/pores) taken by Transmission Electron Microscopy (TEM) with a resolution of 0.18nm, as shown in Figure 1 (a) – (b).

The water condensation is reproduced using the lattice Density Functional Theory (DFT) simulation, a computationally efficient approach to study phase changes in complex porous structures ^[3]. To validate the parameters used in our lattice DFT simulations, we compared the isotherm curve obtained from our simulation results with experimental data. A sample of the resulting water distribution (after parametric analysis) is shown in Figure 1 (c).

The resulting water distribution obtained from the simulation serves as crucial input to investigate Pt nanoparticle activation under varying relative humidity (RH) conditions. Specifically, we imposed criteria for the activation of Pt nanoparticles based on the presence of continuous paths of condensed water (known as 'water bridges') that connect interior Pt nanoparticles to the outer surface of the CCL particle, as shown in Figure 2. In Figure 1 (c), activated Pt nanoparticles are colored red, inactive ones are colored yellow.

We demonstrated in this study that lattice DFT simulation can be a powerful yet less expensive tool to reproduce water condensation in CCL particles of PEMFCs, and at the same time ascertain Pt activation behavior at different RH conditions. Although there is still room for further investigation on this topic, this study can help shed light on optimizing Pt nanoparticle utilization for enhanced fuel cell performance.

Acknowledgment

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan (Grant number JPNP20003).

Key words

polymer electrolyte membrane fuel cell, cathode catalyst layer, lattice density functional theory, capillary condensation of water, porous media

References

[1]	Islam, M.N., et al., "Designing fuel cell catalyst support for superior catalytic activity and low mass-transport resistance," Nature Communications 13 (2022), 6157.
[2]	Ramaswamy, N., et al., "Carbon support microstructure impact on high current density transport resistances in PEMFC cathode," Journal of The Electrochemical Society 167 (2020), 064515.
[3]	Monson, P., "Mean field kinetic theory for a lattice gas model of fluids confined in porous materials," Journal of Chemical Physics 128 (2008), 084701.

Figures

Fig. 1  (a) Segmented TEM images of a PEMFC cathode catalyst layer particle used in Toyota MIRAI Gen 2 model. (b) 3D Reconstructed porous structure from TEM images. (c) Distribution of condensed water (blue) obtained from lattice DFT simulation, with corresponding activated Pt (red) and inactive Pt (yellow).

Fig. 2  Schematic of activation of Pt nanoparticles based on possible proton pathway.