Investigation on Effects of I/C Ratio on Dispersion Structure of PEFC Catalyst Ink By Scanning Electron Assisted Dielectric Microscopy

Sasabe, Takashi

To achieve high power density operation of polymer electrolyte fuel cells (PEFCs), it is required to realize higher performance catalyst layer with low oxygen transport resistance, high proton conductivity, and low Platinum loading. Because dispersion structure of catalyst ink strongly affects the catalyst layer structure, it is crucial to understand the dispersion mechanism of PEFC catalyst ink. We have reported that that solvent composition (ethanol concentration) of the catalyst ink strongly affect dispersion of the catalyst ink [1, 2], but effects of other components on the dispersion of the catalyst ink were not fully understood yet. In this study, effect of I/C ratio (amount of ionomer) on the dispersion of the catalyst ink was investigated.

The catalyst inks were fabricated by mixing platinum-supported carbon (TEC10V30E, Tanaka Kikinzoku), ionomer (DE1021, Sigma-Aldrich), and water/ethanol solvent. To investigate the effect of ionomer on dispersion of the catalyst ink, solvent compositon (ethanol/water = 20/80 wt%), amount of the platinum-supported carbon and solid content of the catalyst ink were kept constant., and the I/C ratio was changed from 0.25 to 1.25. Viscosity characteristics were measured at each shear rate in the range of 0.01 to 1000 1/s by a rotary rheometer (MCR302, Anton-Paar), and particle size distribution of the catalyst ink was measured by the laser diffraction type particle size distribution meter (LA-960V2, HORIBA) without any dilution. In addition, we have succeeded to directly observe the dispersion structure of the catalyst ink by using a scanning electron assisted dielectric microscopy (SE-ADM) for the first time. SE-ADM enabled observation of the catalyst ink with very little radiation damage and high-contrast imaging without staining or fixation at an 8-nm spatial resolution, and distribution of the platinum-supported carbon, ionomer, and solvent were clearly observed.

Figure (a) showed the effects of I/C ratio on aggregation size. The result clearly showed that the catalyst inks with too less (I/C=0.25) or too much (I/C=1.25) amount of ionomer were aggregated. Figure (b) showed the results of SE-ADM imaging. Because the dielectric constant of materials within the catalyst ink were different, the image clearly visualized distribution of the platinum-supported carbon, ionomer, and solvent, and the visualization results of the catalyst ink with different I/C ratio were consistent with the results of the particle size distribution measurement. Though the ionomer works as a surfactant within the catalyst ink, too much ionomer makes the dispersion of the catalyst ink worth, and tuning the amount of ionomer is important to realize the high performance catalyst layer.

Acknowledgement

This presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

[1] Kaname Iida et al. 2020 ECS Trans. 98, 497.

[2] Takashi Sasabe et al. 2021 ECS Trans. 104, 191.