Characterization of Catalyst Inks By Rheology and Microscopic Particle Properties
Authors: Suzuki Takahiro , Okada Shinya , Tsushima Shohji 
Published: February 2020

Fundamental understanding of the state of catalyst inks and fabrication process is important 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 dynamics of materials during the fabrication process of the catalyst layer(1), and then the resultant structure(2). Additionally, the state of the catalyst ink can have time dependence, which means the procedure of particle agglomeration and sedimentation inside the slurry. Therefore, quantitative evaluation of the catalyst ink is required. In this study, a rheological measurement was conducted to characterize catalyst inks used to fabricate PEMFC catalyst layers. The rheological measurement can apply to opaque and high viscosity slurries like catalyst inks. Ionomer dispersion, pseudo-catalyst ink and catalyst ink with various ionomer content were measured to clarify the relationship between the rheological behavior and the types and amount of particles and interparticle interactions in the slurry.
The rheological measurement was conducted by using a rheometer with a cone and plate system. Shear rate range was 200 to 3000 /s. The measurement temperature was 25°C. Downward measurement (from upper to lower shear rate) following the upward measurement was conducted. Ionomer dispersion (DE2020, Sigma-Aldrich), carbon black (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 2% and ionomer to carbon ratio (I/C) was a parameter from 0.3 to 2.0.
The viscosities of the slurries as a function of the shear rate are shown in Figure 1. The three types of the slurries showed quite different rheological properties with each other. The ionomer dispersions are Newtonian fluid in all I/C conditions from 0.3 to 2.0 (Fig. 1 (a)). On the other hand, pseudo-catalyst ink (Fig. 1 (b)) and catalyst ink (Fig. 1 (c)) were non-Newtonian fluid when I/C was low. The pseudo-catalyst ink showed a transition from non-Newtonian to Newtonian as I/C increased from 0.3 to 0.5. The ink also showed the transition from Newtonian to non-Newtonian as I/C increased from 1.0 to 2.0. The catalyst ink slightly showed non-Newtonian properties even in I/C 0.5 and 1.0. Additionally, the catalyst ink with I/C 0.3 and 0.5 showed relatively large hysteresis between the upward and downward measurements.
The rheological property of the slurries can be the results of the state difference of the particles in the slurries such as the particle agglomeration and/or ionomer adsorption. The measurements of particle size distribution by dynamic light scattering and zeta potential were applied to the slurries (The results are not shown here). The comprehensive discussion to understand the state of the catalyst ink will be presented.
Acknowledgments
This work was supported by JSPS KAKENHI Grant Number 16K18028.
References

T. Suzuki, M. Kobayashi, H. Tanaka, M. Hayase and S. Tsushima, ECS Trans., 69, 465 (2015).

W. K. Epting, J. Gelb and S. Litster, Adv. Funct. Mater., 22, 555 (2012).

Figure 1