Rheological and Impedance Measurements and Analysis of Catalyst Ink Prepared By Different Mixing Processes
Authors: Suzuki Takahiro , Kitamura Sakiko , Tsushima Shohji 
Published: December 2020

Fundamental understanding of the state of catalyst inks such as agglomerate size and interparticle interaction is important to control mass transfer in the fabrication process and to fabricate well-established catalyst layers of proton exchange membrane fuel cells (PEMFCs). The catalyst ink is a slurry which contains platinum-supported carbon and ionomer in a solvent. The state of the catalyst ink affects the dynamics of materials during the fabrication process(1) and the porous structure of the catalyst layer as a result. The difficulty to evaluate the catalyst inks comes from their opacity and high particle concentration. Therefore, optical approaches are extremely limited. In addition, the time-dependency of the slurry should be considered. In this study, both rheological measurement and impedance measurement were conducted to characterize catalyst inks. The rheological measurement can apply to opaque and high concentration slurries like catalyst inks(2). The impedance measurement as well has a great potential to evaluate catalyst inks. The objective of this study is to establish evaluation methods of catalyst inks by rheological and impedance measurements especially focusing on the agglomerate size in the slurry.
The rheological measurement was conducted by using a rheometer with a cone and plate system. Shear rate range was 300 to 3000 /s. The impedance measurement was conducted using an in-house electrochemical cell. The frequency range was 4 Hz to 8 MHz. The measurement temperature was 25°C. Carbon black (CB, Ketjenblack, Lion)–ionomer dispersion (pseudo-catalyst ink), and platinum-supported carbon (Pt/C, TEC10E50E, Tanaka Kikinzoku Kogyo)–ionomer dispersion (catalyst ink) were prepared. The volume fraction of carbon to the solvent (45wt.% 1-propanol aqueous solution) was approximately 2% and ionomer to carbon ratio (I/C) was 1.0. Two types of mixing tools were used. One was a centrifugal planetary mixer and the other was ultrasonic homogenizer. The slurries mixed only by the planetary mixer (one-step mixing) and mixed by the planetary mixer and the following ultrasonic homogenizer (two-step mixing) were measured.
Figure 1(a) shows viscosity curves obtained from the rheological measurement. Two-step mixing showed lower viscosity in both CB slurry and Pt/C slurry. Pt/C slurries showed non-Newtonian behavior, whereas CB slurries showed Newtonian. Figure 1(b) shows the dielectric property of the slurries, which was obtained by impedance measurement. Two CB slurries showed a substantial difference in high frequency. The dielectric properties of the two Pt/C slurries were almost consistent with the one-step mixed CB slurry. Agglomerate size in the slurries was evaluated from the rheological measurements using the Krieger-Dougherty equation(3), and it was evaluated as well from the impedance measurement using the Schwarz equation(4). The rheological measurement showed smaller agglomerate size in the two-step mixed slurries, although the agglomerate size was several-hundred nanometer in both one-step and two-step mixed slurries. However, the impedance measurement showed several micrometers in the one-step mixed slurry. The impedance measurement can evaluate the slurries under a free state, whereas the rheological measurement evaluates the slurries under shear force conditions. Therefore, the impedance measurement could have detected a flocculated structure which is formed by a weak attractive force.
Acknowledgments
A part of this work was financially supported by JSPS KAKENHI Grant Number 18K13702 and 18H01383. This study was supported by Osaka University Nanofabrication Platform [S-19-OS-0011] in Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
References

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G. Schwarz, The Journal of Physical Chemistry, 66, 2636 (1962).

Figure 1