Catalyst-Layer Ionomer Imaging of Fuel Cells
Authors: Guetaz Laure , Lopez-Haro Miguel , Escribano Sylvie , Morin Arnaud , Gebel Gerard , Cullen David A. , More Karren L. , Borup Rod L 
Published: February 2020

Investigation of membrane/electrode assembly (MEA) microstructure has become an essential step to optimize the MEA manufacturing processes or to study the degradation of the different MEA components. Transmission electron microscopy (TEM) is a tool of choice as it provides direct imaging of the different components. TEM is then widely used for analyzing catalyst nanoparticle structures and distribution, as well as their chemical composition. The carbon support can also be imaged; for example, the degree of graphitization can be investigated. However, the ionomer network inside the electrode is more difficult to be imaged due to the fact that the ionomer mainly forms an ultrathin layer surrounding the carbon support. Moreover, these two components, having similar density, present no difference in contrast. The presence of the ultrathin ionomer layer can only be revealed in some favorable zones on high resolution TEM images1. The entire ionomer network inside the catalyst layer has never been imaged, even though this microstructural parameter plays a crucial role in ionic conduction through the catalyst layer. In this work, we show how the advanced TEM techniques of electron tomography and X-ray energy dispersion spectroscopy (EDS) elemental mapping provide new possibilities for imaging this ionomer network.

Electron tomography in HAADF-STEM (high annular dark field – scanning transmission electron microscopy) mode was successfully used to image the 3D morphology of the ultrathin ionomer layer surrounding the carbon particles2. For this experiment, the ionomer contrast was enhanced by selectively staining the ionic domains with Cs+ions.  In order to avoid high contrast of the Pt nanoparticles (compared to the ionomer contrast), a model active layer consisting of ionomer and carbon black without Pt nanoparticles was made using usual electrode manufacturing process. The influence of the ionomer/carbon black ratio introduced in the ink was studied by preparing two samples with two different ionomer/carbon black ratios equal to 0.5 and 0.2 w/w. Figure 1 shows the 3D-rendered volume extracted from the reconstructed tomogram for the two samples. The 3D repartition of the ultrathin ionomer layer (in blue) surrounding the carbon black (in grey) is clearly revealed. The quantitative tomogram data analyses showed that doubling the amount of ionomer in the catalyst layer does not change the mean thickness of the ionomer layer, measured around 7 nm, but leads to a twofold increase in its degree of carbon particle coverage. These electron tomography analyzes are of great interest for finding the optimum manufacturing process that will lead to the maximum carbon coverage without increasing the ionomer layer thickness. Indeed, the optimized ionomer network structure have to ensure both ionic contact with a maximum of Pt nanoparticles and the connectivity of the ionic conduction paths to the membrane without inhibiting the gas diffusivity.

On the other hand, recent developments of high performance EDS detectors offer new possibilities to acquire chemical elemental maps in a very short time (few seconds or minutes). Fluorine being one of the main elements of the ionomer, ionomer distribution within the catalyst layer can be visualized by acquiring fluorine EDS elemental maps in a MEA ultramicrotomed TEM sample (Figure 2). One limitation of this technique is that a high EDS signal requires a high electron dose, especially when high spatial resolution is required. Unfortunately, the ionomer is highly sensitive to electron beam radiation damage and a continuous F loss happens during the TEM observation. Measurement of the F loss under different acquisition conditions has shown that F loss can be minimized by controlling the electron dose and by cooling the specimen during analysis3. Using these conditions, it is now possible to obtain quantitative data on the active layer ionomer content that in some electrodes can reveal distribution heterogeneity. But more importantly, these analyses allowing a quantitative comparison of the ionomer content in different electrodes offer the possibility to study the active layer ionomer evolution after MEA ageing tests. 

1K. L. More,  R. Borup, K. S. Reeves, ECS Trans 3, 717-733 (2006).

2 M. Lopez-Haro, L. Guetaz, T. Printemps, A. Morin, S. Escribano, P.H. Jouneau, P. Bayle-Guillemaud, F. Chandezon, G. Gebel, Nat Commun, 5, 5229 (2014).

3 D. A. Cullen, R. Koestner, R. S. Kukreja, Z. Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H. M. Meyer, C. M. Parish, K. L. More, J. Electrochem. Soc., 161 (10) F1111-F1117 (2014).

Figure 1