Impact of Carbon Support Structure on the Durability of PtCo Electrocatalysts

Abstract

Fuel cell research in the United States is undergoing a dramatic shift towards heavy duty applications, with a strong near-term focus on Class 8 long-haul trucks. New and improved materials and electrode designs will be required to meet the significantly increased lifetime (30,000 h) and efficiency (72%) targets.[1] At the electrocatalyst level, this means defining new metrics focusing on the end-of-life performance. Pt-alloy nanoparticles are attractive catalysts for the sluggish oxygen reduction reaction at the cathode, but demonstrate relatively poor durability due to the leaching of the transition metal upon contact with the acidic fuel cell environment and during fuel cell operation.[2] Electrocatalyst durability depends on a variety of parameters, which include average particle size, degree of mono-dispersity, and composition. The support structure also plays a key role in particle durability, as it both impacts the particle size distribution and accessibility of the catalyst particle. Building on previous studies on the impact of carbon support on catalyst durability for both Pt and Pt-alloy catalysts,[3,4] in this work we explore commercial PtCo nanoparticle catalysts with 32wt.% precious metal on five different carbon support structures, namely standard and graphitized forms of Ketjen and Vulcan carbons along with Acetelyne Black. These materials were incorporated into membrane-electrode assemblies (MEAs) and subjected to accelerated stress tests (AST) following the DOE catalyst durability protocol.[5]The pristine and AST-cycled MEAs were characterized using synchrotron-based small and wide angle X-ray scattering (SAXS/WAXS) and scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS) to determine average particle diameter and composition, as well as particle size distribution and size vs. composition variations. Changes in the thickness of the Pt-skin layer were also quantified by mapping individual particles using aberration-corrected STEM-EDS before and after cycling. Select samples were also studied by analytical electron tomography to explore the relationship between particle location within the support and retention of Co. The extensive physical characterization will be correlated with electrochemical performance to uncover the impact of particle size, composition, and support type on performance and durability.[6]