Agglomerates in Polymer Electrolyte Fuel Cell Electrodes: Part I. Structural Characterization

Abstract

This two-part study characterizes the structural and transport properties of polymer electrolyte fuel cell (PEFC) cathode catalyst electrode. The agglomerates comprising the electrode are characterized with nano scale resolution X-ray computed tomography (nano-CT). A hybrid reconstruction technique is used to further refine the 3-D agglomerate microstructure and obtain 1-nm voxel resolution of the constituent catalyst, ionomer, carbon support and primary and secondary pore phases. Our analysis of nano-CT data shows that the electrode composition is heterogeneous and the agglomerates have structural heterogeneities in terms of size and composition. There is a strong correlation between the ionomer to carbon weight ratio (I/C) and agglomerate microstructure, suggesting that ionomer plays role in formation of larger agglomerates. With the hybrid reconstructions, we show that ionomer film thickness is spatially non-uniform inside the agglomerate and is strongly dependent on the I/C. Smaller agglomerate size and high I/C are shown to favor the ionomer impregnation. We demonstrate that the majority of the primary pores are smaller than 15 nm for I/C = 0.8 and higher I/C reduces the primary pore size. We also characterize the uniformity of the catalyst distribution and show that the majority of the catalyst is located away from the agglomerate surface.

Along-the-channel modeling and analysis of PEFCs at low stoichiometry: Development of a 1+2D model

“…Finally, the transport-limited region at 100% RH was used to estimate agglomerate properties. The best fit was obtained at δ agg =4 nm, which is within expected values [74].…”

Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes

“…We attribute R Pt /I to the ionomer/Pt interface based on literature findings using both experimental and modeling techniques. [32][33] Increasing the I:C ratio increases δ , 9 while the interfacial component, R Pt /I , determined by sulfonic group/Pt surface interactions, remains constant. Upon combining equations 3 and 4, R WE is given by…”

“…[4][5][6][7][8] Such non-uniformities impact local transport properties, subsequently affecting species transport and power output. [9][10][11] Gas-transport loses currently limit the high-current-density performance of PEFC electrodes. [12][13] Several studies established the presence of a high local gas-transport resistance near the Pt sites, which is attributed to the ionomer thin film and its associated Pt/ionomer interactions.…”

Influence of Structural Modification of Micro‐Porous Layer and Catalyst Layer on Performance and Water Management of PEM Fuel Cells: Hydrophobicity and Porosity

“…This could facilitate a more homogenous and rapid distribution of oxygen and the possibility for water to escape more easily than for a fully nanoporous material [24][25][26]. Attention has been given to modifications of catalyst layer wettability properties [11,[27][28][29], as well as pore structure by using different catalyst supported carbons [30][31][32][33], variation of ionomer content [34][35][36] and application of different pore formers [37,38]. Nevertheless, the role of catalyst layer structure on water management has not been yet fully understood, due to its complexity.…”