Electron microscopy to study membrane electrode assembly ( MEA ) materials and structure degradation

Abstract

The mass production of proton exchange membrane fuel cell (PEMFC) for stationary or mobile applications requires long‐term durability and minimization of the voltage loss observed upon operation. The extent of PEMFC performance degradation depends on the type of materials composing the membrane electrode assembly (MEA), their manufacturing technique, their structure, and the operating conditions of the cell. Recently, electron microscopy (EM) has become a technique of choice to achieve a better understanding of the physical/chemical origin of the MEA degradation as a function of operation time. Unlike most other physical, chemical, and electrochemical in situ or ex situ techniques (global techniques), EM enables the identification of local changes to the various components of the MEA. For example, defect formation at the micrometer scale in the proton exchange membrane (PEM) and the active layer (observed by scanning electron microscopy), change of porosity and redistribution of the electrocatalyst at the nanometer scale (observed by field‐emission gun scanning electron microscopy (FEG‐SEM) and transmission electron microscopy (TEM)), or structure/composition degradation of electrocatalyst nanoparticles at the atomic scale (observed by high‐resolution transmission electron microscopy (HRTEM)) can be derived from EM. A combination of the conclusions derived from local EM observations with those derived from other global characterization techniques yields the determination of MEA degradation mechanisms and provides some important routes to improve the PEM fuel cell performance and durability.

Hybrid approach combining multiple characterization techniques and simulations for microstructural analysis of proton exchange membrane fuel cell electrodes

“…Besides the standard porosimetry methods, techniques that can capture the internal microstructure are critical in gaining insight into the three-dimensional distribution of the electrode components. The two-dimensional electron microscopy techniques, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), are popular tools for characterization of PEMFC electrodes [9]. SEM is commonly applied to resolve macro structures while TEM [10][11][12][13][14], with its high resolution (<1 nm), serves as a powerful tool to characterize size, shape, distribution and crystalline structure of C supported nano-scale catalyst particles in the electrode.…”

“…The two-dimensional electron microscopy images are the representations of threedimensional nanostructures. To be able to investigate the whole electrode with TEM, 100-150 nm thick slices need to be extracted from the electrode [5,9]. However, slicing limits the spatial integrity between consecutive images and this method suffers from not being fully representative of the three-dimensional microstructure, i.e.…”

Resolving Electrode Morphology’s Impact on Platinum Group Metal-Free Cathode Performance Using Nano-CT of 3D Hierarchical Pore and Ionomer Distribution

“…28,29 However, the transmission imaging requires the sample to be very thin, on the order of 100 nm. 24,27,30,31 X-ray-computed laminography (XCL) has also been combined with X-ray absorption fine structure (XAFS) to analyze the chemical states of Pt in the fresh and cycled catalyst layer along with 3D morphological characterization. 32 Focused ion beam (FIB) milling combined with SEM can provide high-resolution 3D morphology of the catalyst layer.…”

“…In addition to porosimetry, the catalyst layer structure is commonly evaluated by scanning electron microscope (SEM) or transmission electron microscope (TEM) imaging . Although SEM imaging offers high resolution, it only provides external information from the exposed surface in a 2D image. − TEM and related methods, such as scanning TEM (STEM), offer even higher resolution and can be extended to 3D tomography.…”

“…Although SEM imaging offers high resolution, it only provides external information from the exposed surface in a 2D image. − TEM and related methods, such as scanning TEM (STEM), offer even higher resolution and can be extended to 3D tomography. STEM imaging in conjunction with energy-dispersive X-ray spectroscopy (EDS or EDX) had been used for chemical mapping of the materials. , However, the transmission imaging requires the sample to be very thin, on the order of 100 nm. ,,, X-ray-computed laminography (XCL) has also been combined with X-ray absorption fine structure (XAFS) to analyze the chemical states of Pt in the fresh and cycled catalyst layer along with 3D morphological characterization . Focused ion beam (FIB) milling combined with SEM can provide high-resolution 3D morphology of the catalyst layer. , However, FIB-SEM faces its own set of challenges.…”