Investigation of Solvent Effects on the Dispersion of Carbon Agglomerates and Nafion Ionomer Particles in Catalyst Inks Using Ultra Small Angle X-Ray Scattering and Cryo-TEM
Authors: Yang Fan , Xin Le , Uzunoglu Aytekin , Stanciu Lia , Ilavsky Jan , Son Steven , Xie Jian 
Published: February 2020

The membrane electrode assembly (MEA) is the core component of polymer electrolyte membrane fuel cells (PEMFCs) and directly determines the performance and cost of PEMFCs. Recent research has been focused on developing high-performance, low-cost, and durable MEAs. The dispersion of the Pt/C aggregates and the ionomer particles in a solvent is the first and very critical step to achieve a fine structure of the catalyst layer (1-6). In order to prepare an appropriate ink, the Pt/C catalyst powder must be dispersed well in a solvent. Therefore, to verify whether a formulated catalyst ink disperses the Pt/C aggregates and the ionomer particles well is of great importance. In a catalyst ink, the solvent plays a critical role to disperse both Nafion ionomer and Pt/C catalyst particles. The polarity of the solvent is indicated by its dielectric constant. For instance, when the Nafion ionomer is mixed with a variety of organic solvents, the geometry and the micro-structure of both the Nafion ionomer and Pt/C particles tends to change (7). These changes are strongly associated with the surface energy of solid/liquid, to great extent, due to the dielectric constant (ε) of an organic solvent. Therefore, it is of great interest to study the effects of different organic solvents on the dispersion of both Nafion ionomer and Pt/C catalyst particles. Instead of a “trial-and-error” approach—measuring the MEA fuel cell performance of different ink formulation, designing an “ideal” ink guided by the knowledge of ionomer and catalyst particles in different solvents is a rational approach.

In 2010, our group developed a unique method of combined the ultra-small angle x-ray scattering (USAXS) and cryo-TEM to study the size and geometry of both the Pt/C aggregates and the ionomer particles in catalyst inks and the effects on dispersion in catalyst ink (8). USAXS can be used to measure the size and geometry of Nafion ionomer particles and Pt/C aggregates in liquid media without the issues of incident beam absorption by inks from using light scattering. With direct observation of particle dispersion from cryo-TEM imaging, by which the solid particles of Nafion ionomer and carbon aggregates in a liquid ink are frozen to lock their geometry and particle size distribution for TEM imaging, the USAXS fitting data can be well validated.

In this work, three different solvents with different dielectric constants, (1) ethanol (ETH), (2) glycerol (Glyc) and (3) mixture of isopropanol and water (1:4) were used to study the effect of different solvents on the dispersion of carbon particles in the ink systems. The USAXS results of carbon particles in different solvents are shown in figure 1. It can be seen that the size of carbon aggregate in solvent with medium dielectric constant (glycerol) decreases after adding Nafion. However, the carbon black aggregates sizes in solvents with higher and lower dielectric constant decrease after adding Nafion ionomer. This significant difference indicates that solvents have an important role in ink formulation.

From the results above, it is very clear that solvent plays an important role in ink formulation which affects the geometry and size of both Nafion and carbon particles, consequently, the structure of catalyst layer, which ultimately determines the performance of MEA. This also demonstrate that the unique method of USAXs combined with cryo-TEM is an effective means to study the ink formulation.

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Figure 1