Introduction: Particle Size of Catalytic Inks
Catalytic inks are the key component when balancing cost, performance, and durability of proton exchange membrane fuel cells (PEMFC’s). Scaling up PEMFC production requires careful control of the ink to produce uniform electrode layers that use as little precious metal catalyst as possible. Particle size and dispersion critically impact the behavior of the ink and resulting performance of the electrode layers, affecting important parameters such as ink viscosity, ionomer distribution and morphology, catalyst utilization, interaction between the catalyst and ionomer, and the homogeneity and continuity of the electrode layer.1
Particle size is a crucial but difficult parameter to measure in catalytic inks. No single technique can provide a complete characterization because the inks contain a complex mixture of particles with different size scales. The active catalysts are Pt metal group nanoparticles with an optimal size of 2 to 5 nm. However, these nanoparticles are not independent but are rather dispersed onto carbon support particles. The carbon support particles have a primary particle size of 20 to 40 nm but are typically found in the form of larger agglomerates in the sub µm to µm size range. Ionomers, dispersed into aggregates with different shapes (rods or coils) and sizes ranging from 70 nm to 2.5 µm, are mixed with the carbon-supported catalysts to form the ink.
The catalyst and ionomer particles are dispersed in a solvent and mixed to optimize aggregate size and the contact between the ionomer and the catalyst particles. In general, smaller ionomer aggregates, from 200 to 400 nm in size, are beneficial for better H2/air performance2. The carbon-supported catalyst, however, can be under- or over-dispersed. When under-dispersed, the carbon remains highly agglomerated; the ionomer only coats the exterior of the agglomerates and the interior Pt catalyst are not accessible to protons and are therefore under-utilized. When over-dispersed, agglomerates break apart and Pt particles separate from the carbon support, which prevents them from being active in oxygen reduction reactions. The ideal dispersion produces small agglomerates of carbon-supported catalyst particles that promote uniform distribution of the ionomer on the agglomerate and better catalyst utilization.3
X-ray diffraction (XRD), laser diffraction (LD), and dynamic light scattering (DLS) are three characterization techniques with proven ability to scale-up in support of mass production. These three techniques each probe a different size regime and, when combined, can provide a comprehensive overview of the particles in the catalytic ink mixture.
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