Electron Microscopy Study of Degradation Mechanisms in Platinum Group Metal-Free Catalysts

Abstract

Commercial low-temperature polymer electrolyte membrane fuel cells (PEMFCs) rely on expensive platinum-based catalysts to drive the oxygen reduction reaction at the cathode. In order to overcome cost barriers limiting market penetration in both transportation and stationary applications, significant efforts are being made to develop suitable platinum group metal-free (PGM-free) catalysts. The latest classes of PGM-free catalysts show very promising mass activity, but the poor durability of these carbon-based materials is still a significant challenge that must to be addressed. In this effort, advanced analytical electron microscopy methods were employed to study the degradation pathways of PGM-free catalysts which were submitted to accelerated stress tests. Catalysts provided by four different suppliers were tested concurrently in a specially designed segmented fuel cell. Polarization curves were recorded both before and after a durability stress test which involved 10k square wave cycles between 0.95 – 0.60 V in air. Analytical electron microscopy performed across a wide length scale (sub-nanometer to micron) will be presented which links the observed performance losses with changes to the atomically dispersed active sites, carbon support, and ionomer distribution. The advantages and disadvantages of different catalyst precursors (metal organic frameworks, cyanamide/polyaniline, etc.) will also be discussed from a durability standpoint. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat). Microscopy was performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility