Imaging optimization of FIB cross sections for reliable porosity analysis of PEMFC cathode catalyst layers.

Focused ion beam-scanning electron microscopy (FIB-SEM) was used to investigate the effects of acceleration voltage, beam current, and cryogenic conditions on the cross-sectional preparation of proton exchange membrane fuel cell (PEMFC) cathode catalyst layers. Milling at 30 kV and 0.26 nA effectively minimized curtaining artifacts and produced an apparent cross-sectional pore morphology similar to that obtained by cryogenic milling, indicating that optimized room-temperature milling can provide cross sections suitable for consistent two-dimensional (2D) pore-area analysis under the conditions examined.Representative FIB-SEM cross-sectional images of PEMFC cathode catalyst layers prepared under different FIB milling conditions: (a-c) 5 kV at room temperature, (d-f) 30 kV at room temperature, and (g-i) 30 kV under cryogenic conditions (- 150 °C). The indicated voltage and current values represent the FIB acceleration voltage and ion beam current used for milling, respectively. The red dashed box in (f) highlights the optimized room-temperature milling condition (30 kV, 0.26 nA) used for comparison with the corresponding cryogenic condition in (i).