(Invited) In Situ Electron Microscopy Methods for Understanding Activity and Degradation in Fuel Cell Electrocatalysts

Abstract

The scanning transmission electron microscope (STEM) is a powerful tool for imaging materials at the atomic scale. While imaging and spectroscopic analysis is typically performed at room temperature within the vacuum of the microscope, the last decade has seen a rapid development of micro-electromechanical system (MEMS)-based holders enabling a wide variety of in situ experiments, including heating in the presence of a gas and potential cycling within a liquid.[1,2] This talk will briefly review the pioneering efforts aimed at in situ observations of electrochemical materials along with the most recent evolutions in experimental, software and holder design for electrochemical materials.The talk will then focus on the application of in situ methods to fuel cell materials. We will first address atomically-dispersed platinum group metal-free (PGM-free) electrocatalysts derived from metal organic frameworks (MOFs). A critical step in the synthesis of these ORR catalysts is the high temperature heat treatment (up to 1100oC) required to convert the Fe, Ni, or Co-doped MOFs into graphitic carbons containing the proposed individual metal-nitrogen active sites.[3,4] In situ pyrolysis of the catalyst precursors was performed in a low-voltage, aberration-corrected STEM coupled with electron energy loss spectroscopy (EELS) to provide additional insight into the interaction between the metal-nitrogen sites and the graphitic carbon support. These experiments were correlated with in situ X-ray absorption spectroscopy (XAS) to demonstrate that active sites form in a lower temperature regime (600-800oC), but higher annealing temperatures are required to enhance graphitization of the carbon support, leading to increased performance in fuel cell tests.Enclosed environmental cells allow for heating in the presence of a gas at pressures up to 1 atmosphere and can be used to closely replicate catalyst synthesis conditions, as will be demonstrated on Pt-based catalyst in which annealing was performed under 100% hydrogen. A few studies to date have also explored fuel cell electrocatalyst degradation in liquid cells.[5-7] Such studies demonstrate the exciting potential for in situ electrochemistry, but are hindered by limited spatial resolution due to imaging through thick liquid layers and the long cycling times required to observe meaningful degradation. We take a step back from the challenging in situ experiments and use identical location (IL)-STEM to compare catalyst degradation mechanisms observed in the aqueous three electrode cell relative to tests performed on membrane electrode assemblies (MEAs). Like previous in situ and IL-STEM studies, coalescence mechanisms dominate in an aqueous environment, while Ostwald ripening is dominant in MEA tests.[8-10] We will discuss how sample preparation and cycling parameters can be adjusted to more closely replicate losses observed in fuel cell tests.[11]