Deconvoluting the through-Plane Degradation Mechanisms of the Pt-Co/C Cathode Catalyst Layer in Polymer Electrolyte Membrane Fuel Cells Under Load Cycling Conditions
Authors: Coats Matthew , Foster Jayson , Kuiken Dylan , Cheng Lei , Braaten Jonathan , Stuhmeier Bjorn Marcel , Berner Ulrich , Johnston Christina , Pylypenko Svitlana 
Published: December 2024

Polymer electrolyte membrane fuel cells (PEMFCs) are an important technology to reduce greenhouse gas emissions in heavy duty vehicles (HDVs). State-of-the-art cathode catalyst layers (CCLs) that drive the kinetically slow oxygen reduction reaction (ORR) are Pt-M (M = first row transition metal) nanoparticles (NPs) supported on a carbon support (Pt-M/C). Pt-M NPs improve initial ORR kinetics and lower nominal Pt content, however, their degradation under load cycling conditions is still poorly understood due to the multidimensional physiochemical problem driven by both thermodynamics and kinetics. Alloy leaching leads to lower kinetic performance of the catalyst and affects ionomer properties, particle growth lowers efficiency of the catalyst, and changes in the CCL structure effects long-term transport efficiency. These phenomena occur at different rates and length scales. Furthermore, the initial material can be quite heterogenous in both NP size and composition, which adds further complexity in understanding the aging phenomena in Pt-M/C.
This study focuses on understanding degradation mechanisms on commercially available membrane electrode assemblies (MEAs) using Pt-M/C CCLs to enable better understanding of potential pathways to more novel materials, including more complex ‘dealloyed’ bimetallic catalysts, and electrode design. Samples were extracted from aged MEAs tested using a voltage cycling protocol after 0 (BOL), 1000, 3000, 30000, and 108000 cycles. Scanning transmission electron microscopy and energy dispersive spectroscopy (STEM/EDS) studies have been instrumental to understanding localized heterogeneities in the catalyst aging phenomena in tested MEAs, aiding in identification of contributing degradation mechanisms. STEM/EDS provides both visualization of the initial and aged materials and allows to quantify composition and size changes throughout the entire CCL. We approach the imaging and mapping from a through-plane spatial perspective (ex-situ) including near the gas diffusion layer – CCL interface (GDL-CCL), middle of the catalyst layer (MID-CCL), and CCL-membrane (CCL-PEM) interface. Relative changes in the particle size and elemental ratios were used to quantify the losses of catalyst and compare to overall losses for each respective number of cycles. In the past, we have used the F/Pt ratio (average, in-plane, and through-plane) to quantify Pt (catalyst) loss relative to F (ionomer) for Pt/C CCLs and particle size analysis in the three locations in the CCL to track spatial particle growth. Here, we present additional parameters, including the F/Co and Pt/Co ratios, as metrics to track Co loss and Pt loss or growth. X-ray photoelectron spectroscopy (XPS) was also used to deconvolute both elemental and chemical changes as a function of number of cycles. However, the signal acquisition depth of XPS is limited up to 10 nm and can only be used to complement trends in the STEM/EDS quantification near the GDL-CCL interface. Nevertheless, a mechanistic approach involving both techniques highlights STEM/EDS and XPS as complementary characterization techniques for assessment of degradation of Pt-Co/C occurring at different length scales as a function of number of cycles.