The effective gas diffusivity, porous structure, and tortuosity factor of catalyst layers used in proton exchange membrane fuel cells were evaluated using a microfluidic device. Sufficient gas transport properties of the catalyst layers are a key factor for achieving high-performance catalyst layers and fuel cells. In the present study, catalyst layers with different thicknesses and different carbon supports were evaluated. Stand-alone carbon black and multi-walled carbon nanotubes were blended into the catalyst layers as the support. The all-carbon-black-based catalyst layer contained some volume of isolated pores and some amount of microcracks, which depended on its thickness. The tortuosity factor was evaluated considering the effects of the isolated pores and microcracks. However, the tortuosity factor of the all-carbon-black-based catalyst layer was larger than the well-known Bruggeman-correlated tortuosity factor. When carbon nanotubes were blended into the catalyst layer, the tortuosity factor was drastically decreased to less than one-half that of the carbon-black-based catalyst layers. A change in the number of straight pores formed by the fibrous support and variation of the ionomer distribution can affect the tortuosity factor.
Gas transport properties in catalyst layers of polymer electrolyte fuel cells are key factors for improving cell performance. It is necessary to clarify the relationship between the gas transport properties and the microstructure in catalyst layers. In this study, the oxygen diffusivities in catalyst layers with different ionomer-to-carbon ratio (I/C) were evaluated using a microfluidic device, and tortuosity factors were obtained. The porous structures of catalyst layers were evaluated by the nitrogen physisorption method and observation using a field emission scanning electron microscope. It was revealed that the tortuosity factor of the catalyst layers with high and low I/C was larger than that of catalyst layers with middle I/C. This can be attributed to the segregation of ionomers in high-I/C catalyst layers and the agglomeration of carbon particles in low-I/C catalyst layers.
Catalyst layers of polymer electrolyte fuel cells are electrochemical reaction fields with complicated porous structures. Catalyst layers are composed of carbon particles supporting platinum catalyst (Pt/C), ionomers and micropores. Gas transport properties in catalyst layers are key factors for improving cell performance. It is necessary to clarify the relationship between the gas transport properties and the microstructure. The authors reported that ionomer in the catalyst layer affects the porous structure and resultant cell performance [1]. Therefore, the objective of this study is to investigate the effects of ionomer to carbon ratio (I/C) of the catalyst layer on the porous structure formation and gas transfer. In this study, gas transport properties in catalyst layers with different I/C were evaluated by the measurement of gas transfer through thin porous media operated under a micro-channel apparatus (Gas-TOUCA) [2]. The porous structures of catalyst layers were evaluated by the nitrogen adsorption method and the observation using a Field-emission scanning electron microscope (FE-SEM). Typical experimental results of the measurement of Gas-TOUCA are shown in Figure 1. The measurement was carried out for the catalyst layers under the four conditions of I/C 0.1, 0.3, 0.5 and 1.0. The effective diffusion coefficient in the catalyst layer with I/C 0.5 was the largest, and that in the catalyst layer with I/C 1.0 was the smallest of the four conditions. The tortuosity factor of the catalyst layers with I/C 0.1, 0.3 and 1.0 was approximately 3. However, the tortuosity factor of catalyst layers with I/C 0.5 was approximately 2. Segregation of ionomer was observed in the catalyst layers with I/C 0.5 and 1.0 by SEM. On the other hand, there was no ionomer segregation in the catalyst layers with I/C 0.1 and 0.3, and most of the ionomer could have adsorbed on the Pt/C particles. Ionomer works as a dispersant in electrode slurries [3]. Less amount of ionomer leads to large attractive interaction of particles in the slurry, whereas an excessive amount of ionomer leads to deposition to pores in the catalyst layer. Therefore, the experimental results indicated that there is an optimum I/C ratio for gas transport. References [1] T. Suzuki, S. Tsushima and S. Hirai, Int. J. Hydrogen Energy, 36, 12361 (2011). [2] T. Suzuki, Y. Nakata and S. Tsushima, ECS Trans., 92, 175 (2019). [3] T. Suzuki, S. Okada and S. Tsushima, ECS Trans., 86, 193 (2018). Acknowledgments This work was financially supported by JSPS KAKENHI Grant Number 18K13702 and 18H01383. A part of this study was supported by the Osaka University Nanofabrication Platform [F-19-OS-0016] and [S-19-OS-0011] in the Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Figure 1
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