TEM Study of the Ionomer/Catalyst Interface and 3D Pore Structure of Catalyst Layer for PEMFC
Authors: Yu Kang , Xin Le , Godoy Andres , Xie Jian , Ferreira Paulo Jorge 
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are promising energy conversion devices due to their high efficiency, high energy density and low operating temperatures [1]. One of the major obstacles for high performance PEMFCs is that the sluggish oxygen reduction reaction (ORR) and the ORR activity in a membrane electrode assembly (MEA) are dependent of the structure of the ionomer/catalyst interface in catalyst layers. Coverage of the ionomer over the surface of catalyst particles and the thin ionomer film over the catalyst particles, as well as the porosimetry of the catalyst layer are critical factors to be considered in the fabrication of MEAs. Clearly, the determination of ionomer coverage over the surface of the catalyst particles, the morphology of ionomer film over the surface of catalyst particles and porosimetry of catalyst layer of MEAs are of paramount significance for developing high performance and low cost MEAs. Transmission electron microscopy (TEM) is a powerful tool to provide direct observations of the structures.

In this work, we present the TEM analysis of MEAs for (1) ionomer coverage over carbon particles, (2) the thickness of ionomer film over carbon particles and (3) the porosimetry of catalyst layer using different TEM techniques. MEAs with different functional groups (which were chemically grafted on the surface of carbon support particles via diazonium reactions) were fabricated using (1) NH3
+- functionalized carbon black (CB) and (2) SO3
- functionalized CB. These two types of MEAs were analyzed in terms of ionomer/catalyst interface and porosimetry. In order to prepare electron transparent TEM samples, different methods were applied, including normal/partial embedding ultra-microtomy and cryo-microtomy. Figure 1 show scanning transmission electron microscopy (STEM) images of catalyst layers (CLs) of two MEAs, as well as energy-dispersive X-ray spectroscopy (EDS) mapping of the same area. Combining these results, it is evident that the fluorine in the NH3
+- functionalized carbon CL is uniformly distributed over the entire region. Meanwhile, the SO3
- functionalized carbon CL (as circled in Figure 1c) shows non-uniform distribution; the F signal is much weaker in some areas than others or even disappeared. These maps indicate that the NH3
+- functionalized carbon CL has better ionomer coverage than that of the SO3
- functionalized carbon CL. On the other hand, the X-ray spectra of both samples are shown in Figure 2. The C/F ratio in NH3
+- functionalized carbon CL is 216 while it is only 16 in SO3
- functionalized carbon CL. Such a large discrepancy in the C/F ratio between two different CLs are mainly due to the CL’s intrinsic property - the ionomer coverage difference - as these two TEM samples were prepared using the same method. As these two MEAs are prepared with the same catalyst support/ionomer ratio, higher C/F signal ratio in NH3
+- functionalized carbon CL indicates more complete coverage and thinner ionomer layer. An overview of catalyst layers in both CLs is shown by the TEM bright-field images in Figure 3. The thickness of catalyst layer made of NH3
+- functionalized CB is approximately 6.3μm, while it is only approximately 1.4 μm of that made of SO3
- functionalized CB. It suggests that the carbon support aggregates are much smaller in the catalyst layer made of NH3
+- functionalized CB, based on the fact that two MEAs are synthesized with same amounts of carbon supports and ionomer. Furthermore, 3D tomography TEM and reconstruction of both MEAs based on focused ion beam (FIB) supports this conclusion. Highly porous catalyst layer of NH3
+- functionalized CB facilitates mass transport for both reactants (H2 and O2) to reach reactive sites and water dissipated out.

In summary, TEM characterization shows that NH3
+- functionalized CB catalyst layer has better ionomer coverage, thinner ionomer layer, as well as better porosimetry than SO3
- functionalized CB catalyst layer. In consistency with the TEM characterizations, the NH3
+- functionalized CB MEA provide better electrochemical performance.

Reference

[1] Kocha, S. S. Principles of MEA preparation. In Handbook of Fuel Cells − Fundamentals, Technology and Applications, ed. 1; Vielstich, W., Lamm, A., Gasteiger, H. A., Eds.; Wiley: Chichester, UK, 2003; Vol. 3, 538

Figure 1