(Invited) Understanding PEMFC by Advanced Electron Microscopy

Abstract

A thorough knowledge of the atomic structure and composition of catalyst nanoparticles is paramount to the development of advanced materials for proton exchange membrane fuel cells (PEMFC), one of the most promising energy conversion devices for automotive and stationary applications. Pt and Pt-based alloys nanoparticles (NPs) are currently used as the catalyst to promote the kinetics of the hydrogen oxidation and oxygen reduction reactions in the anode and cathode of the fuel cell, respectively. Yet, the durability of the catalysts remains a major issue for their commercialization. In addition, it is essential to characterize the ionomer distribution and coverage over the catalyst NPs and the carbon support, as the ionomer/catalyst interface plays a critical role in the ORR and MEA fuel cell performance.Ideally, voltage cycling of the membrane electrode assemblies would be performed in-situ within a TEM, but these experiments are highly challenging, since separate gas phase reactants need to be present simultaneously, in particular hydrogen and oxygen Instead, a very close approximation to a true in-situ experiment is done via potential cycling in a half-cell in the presence of liquid electrolyte, as opposed to solid electrolyte in a single cell. This allows us to observe the exact same location before and after cycling through a technique called identical location TEM/STEM (IL-TEM/STEM). Under these conditions, various electron microscopy techniques, such as bright-field TEM and HAADF STEM coupled with EDS and EELS can be performed. To better acquire and understand TEM/STEM-based experimental images, in particular the selection of appropriate operating conditions, and interpretation/validation of the results, it is critical to perform multislice TEM/STEM image simulations to calculate the interaction of an electron beam with a material system.In this talk, the focus is to understand the behavior of Pt and Pt-alloy NPs during the various stages of fuel cell cycling. For this purpose a set-up was developed to simulate the effect of voltage cycling on the cathode side of the fuel cell. In this set up, catalyst NPs supported on carbon nanotubes and amorphous carbon were observed by advanced transmission electron microscopy, before and after cycling. In particular, we studied Pt and Pt-alloy NPs and carbon support degradation mechanisms as a function of cycling using for the first time a combination of 2D IL TEM and 3D IL TEM. This new approach reveals many unpredictable features of morphology changes/behavior of Pt NPs and carbon support, after experiencing the long durability test. These new findings provide an insight which great surpasses what have been observed using solely 2D IL TEM.The experiments show particle migration in conjunction with carbon corrosion during the initial cycles, whereas the appearance of single atoms and atomic clusters on the surface of the carbon support appear after additional voltage cycling as a result of surface dissolution of NPs. For the case of alloyed NPs, the experiments show a heterogeneous deposition of Pt on the NPs. Moreover, we demonstrate a novel approach to distinguish the ionomer phase from the carbon support in PEMFCs, by employing the carbon signal in STEM-EELS, using two types of detectors. In this fashion, not only the ionomer, but also the carbon support, can be mapped in high spatial resolution, despite some damage caused by the electron beam.