3D Imaging of Polymer Electrolyte Fuel Cell Electrodes

Abstract

The degradation of Polymer Electrolyte Fuel Cell (PEFC) electrodes was examined by electrochemical and 3D imaging methods. A potential cycling test was conducted with a commercial catalyst coated membrane (CCM) where the cathode side of the cell was subjected to a potential square wave of 0.6V and open circuit voltage. After 24000 cycles, the PEFC performance remains nearly unchanged except for minor current losses in the low current regime. According to hydrogen under potential desorption measurements the electrochemically active Pt area decreases to 25% of the initial value. Synchrotron-based X-Ray tomography of the CCMs yields thickness distributions of catalyst layers and the membrane. A bubble like structure of the cathode layer is observed. Focused Ion Beam/Scanning Electron Microscopy-serial sectioning was employed to determine the pore structure of the cathode. The porosity before and after degradation was almost unaltered by the cycling test. TEM lamellas of the pristine and degraded cathode catalyst layers show an increase of the Pt particle size from 2.2nm to 4.6nm. Under the given conditions, an increase of hydrogen permeability across the membrane and a loss of Pt surface area are the predominant degradation mechanisms.

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

“…32 Focused ion beam (FIB) milling combined with SEM can provide high-resolution 3D morphology of the catalyst layer. 26,33 However, FIB-SEM faces its own set of challenges. It is a destructive technique, and differential milling rates for different materials can be intrusive.…”

“…35 As an alternative to electron microscopy, microscale and nanoscale resolution X-ray computed tomography (CT) can be used to image fuel cell components. 26,36 Advances in diffraction-based X-ray optics using monocapillary condensers and Fresenel zone plate objectives has led to X-ray CT imaging at resolutions of 50 nm and better. Epting et al 36 previously demonstrated that nano-CT can be used to image the pore and solid structure of fuel cell catalyst layers, where the solid was the dense regions of Pt/C catalyst and Nafion ionomer.…”

“…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. 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 .…”

“…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. It is a destructive technique, and differential milling rates for different materials can be intrusive .…”

Advancements in cathode catalyst and cathode layer design for proton exchange membrane fuel cells

“…To better understand the morphologic structures and transport processes of CL, some ex-situ techniques have been explored, including mercury intrusion porosimetry 113 , focused ion beam scanning electron microscope (FIB-SEM) or scanning transmission electron microscope (STEM) 114 . Although high-resolution 3D morphology of CL can be obtained through FIB-SEM and STEM, there are some challenges for these methods, for instance, FIB-SEM itself is a destructive technique.…”