Localizing Phosphoric Acid in High-Temperature PEM Fuel Cell Catalyst Layers Using Cryostatic Focused Ion Beam Scanning Electron Microscopy

Abstract

Phosphoric acid serves as a proton conductor in high-temperature PEM fuel cells (HT-PEMFCs). The Membrane Electrode Assembly (MEA) consists of a phosphoric acid doped polybenzimidazole (PBI) membrane sandwiched between two gas diffusion electrodes (GDEs). During cell activation, phosphoric acid percolates from the membrane into the GDEs. Thereby, the acid should distribute homogeneously over the entire catalyst layer to ensure sufficient proton conductivity. Understanding phosphoric acid distribution in the porous GDE is essential to achieve high-performing HT-PEMFCs.The phosphoric acid distribution in HT-PEMFC GDEs was previously investigated using impedance spectroscopy combined with the distribution of relaxation times analyses1. This study shows the significant impact of the catalyst nanostructure on phosphoric acid distribution. Additionally, NMR experiments indicate the wetting of the nanometer-sized pores when a GDE comes in contact with liquid phosphoric acid2. Employing energy-dispersive X-ray spectroscopy (EDS), different types of gas diffusion layers were evaluated3. A gas diffusion layer with smaller pore sizes was identified to increase the acid migration out of the cell. Furthermore, a uniform phosphoric acid distribution in both catalyst layers was determined with electron probe micro-analysis (EPMA) after operating a HT-PEMFC cell for 12,000 h4.In this work, the phosphoric acid distribution within GDEs is determined employing cryostatic focused ion beam scanning electron microscopy (cryo FIB-SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). By combining these methods, the location of phosphoric acid can be directly linked to the morphology of the catalyst layer.In-house fabricated spray-coated GDEs were assembled to an MEA and heated up to operating temperature to simulate the cell activation. After five days, a typical activation period, the cell was disassembled. Thereby, the cathode GDE was separated from the membrane. The GDE was submerged in liquid nitrogen to prevent redistribution of phosphoric acid. This method allows handling phosphoric acid under UHV conditions.Phosphoric acid could be detected both on the surface and in the bulk of the catalyst layer (Figure 1). It was found that the acid invades mainly the nanometer-sized pores, not the larger, micrometer-sized pores. These experiments confirm earlier results, which predicted preferential wetting of smaller pore walls using NMR experiments1. The acid can reach the catalyst particles housed inside nanometer-sized pores and contribute to the electrochemically active surface area of the cell.References N. Bevilacqua et al., J. Power Sources Adv., 7, 100042 (2021).E. Zhang et al., Chem. Commun., 57, 2547–2550 (2021).A. Kannan, Q. Li, L. N. Cleemann, and J. O. Jensen, Fuel Cells, 18, 103–112 (2018).Y. Oono, A. Sounai, and M. Hori, J. Power Sources, 210, 366–373 (2012). Figure 1