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

The performance of membrane electrode assembly (MEA) greatly depends on the structure of catalyst layers. Carbon particles with Pt nanoparticles sitting on its surface and Nafion ionomer network which bind the catalyst Pt/C particles together and provide the proton conduction path (proton transport to reaction sites) are two major components in a catalyst layer (1-3). A good catalyst/ionomer interface is important because such interface provides 3-phase reaction zone for oxygen reduction reaction (ORR). The ionomer coverage over carbon particles directly affects the catalyst utilization, consequently, the mass activity and the thickness of the ionomer layer over a carbon particle determines the O2 diffusion barrier. Hence, achieving the balanced catalyst/ionomer interface with appropriate H+conductivity and thin layer (low diffusion barrier) is critical step for a high performance MEA. The fabrication of MEA starts from the catalyst ink in which catalyst powder and Nafion ionomer are mixed in a solvent. It will be desired if the catalyst/ionomer interface can be formed in a catalyst ink. To study the formation of such an interface, the dispersion of catalyst powder and ionomer particles in an ink system needs to be studied to see if the particle size of carbon changes after Nafion ionomer addition—increased carbon particle size generally indicates the formation of the interface.
Ultra-small angle X-ray scattering (USAXS) characterization had been proved to be a great method to investigate the particle size of aggregate system (4). We have developed a unique method (5) to characterize the catalyst dispersion in inks using the combined USAXS and Cryo-TEM by which USAXS can provide the geometry and size distribution of different particles in a ink while cryo-TEM can confirm the USAXS results with direct observation of geometry and size distribution of these particles (Cryo-TEM is usually used to characterize biology samples by fast freezing the liquid samples to form a very thin ice to lock all particles within the liquid to keep their original structure and size of these particle without interruption). In this work, we studied the interaction between Nafion ionomer and two carbon blacks which are NH2 functionalized carbon black (XC72-NH2) and SO3H functionalized carbon black (XC72-SO3H). By comparing the particle size change of carbon black particles before and after addition of Nafion ionomer in two ink systems, we can study the interaction between Nafion and carbon black aggregates.
The USAXS results and corresponding Cryo-TEM images of four ink systems are shown in figure 1 and 2. The particle size from USAXS fitting is shown in Table 1. It can be seen in figure 1 and table 1 that the size of the aggregates of carbon black in NH2-CB ink system which is in the 2nd level of USAXS fitting increases a lot (87.1%) after adding Nafion ionomer. However, according to the observation from figure 2 and table 1, the size of the carbon black aggregates which is in the 2nd level of the USAXS fitting only decreased a little (13.5%). Nafion ionomer particles with negative charges (-SO3
-) on the surface attract the positively charged (-NH3
+) carbon particles via electrostatic forces to form larger aggregates. Opposite results can be found in XC72-SO3H ink system because XC72-SO3H particles with negative charges (-SO3
-) was repelled by the negatively charged (-SO3
-) Nafion ionomer particles.
From the results above, it is clearly that the charged groups on carbon can effectively affect the formation of catalyst/ionomer interface which can be used as an effective tool to build the interface. Understanding the interaction between Nafion ionomer and catalyst support will lead to the rational design a high performance MEA.
Reference
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Figure 1