The Effect of Carbon Support Structure on Ionomer Distributions in PEM Fuel Cell Catalyst Layers

Ionomer thin films form the proton-conducting path within the catalyst layers (CL) of polymer electrolyte membrane fuel cells (PEMFCs) and the web-like ionomer network provides support for the porous CL structure.  The recast ionomer is assumed to be relatively uniform in thickness, structure, and chemistry across the entire CL thickness, but there are numerous factors that can significantly influence the homogeneity and dispersion of the ionomer as well as its stability during electrochemical aging.  These factors include (1) the type of carbon support material used, including the extent of graphitization (or lack thereof) and the presence of surface oxide functional groups, (2) the loading, dispersion, and size/shape of Pt-based electrocatalysts, and (3) the CL pore structure.  Recently, extensive studies have been conducted to identify the optimum conditions for examining ionomer thin films using electron microscopy and spectroscopy techniques¹ and molecular dynamics (MD) simulations have been performed to identify the structure and adhesion of ionomer thin films on carbon and Pt surfaces.² The current work will expand on this microscopy research to apply specific microscopy methods, including high-resolution imaging and spectroscopy coupled with electron tomography, to identify the nature of ionomer film dispersions within CLs prepared with different carbon supports having varying degrees of graphitization (high surface area carbon, Vulcan, and fully graphitized low surface area carbon) and Pt loadings.  Additionally, changes to these ionomer films within the different CLs during accelerated stress testing (AST), specifically the AST for carbon corrosion (1.2V hold for 100-400 hours), will be analyzed and compared in terms of changes to the ionomer distributions on the changing carbon surfaces.  Microscopy observations will be coupled with MD simulations to better understand the correlations between ionomer dispersion and structure and the materials comprising the CLs.

References:

1.     D.A. Cullen, et al., “Imaging and Microanalysis of Thin Ionomer Layers by Scanning Transmission Electron Microscopy,” in preparation.

2.     Q. He, et al., “Structure of the Ionomer Film in Catalyst Layers of Proton Exchange Membrane Fuel Cells,” Journal of Physical Chemistry C 117 25305-25316 (2013).

Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) through a user proposal at ORNL’s Center for Nanophase Materials Sciences, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.