(Invited) Advanced Microscopy Techniques for Studying the Durability of Fuel Cells
Authors: Guetaz Laure , Escribano Sylvie 
Published: December 2020

Durability is still one of the main aspects for which significant progress is expected to allow a larger deployment of fuel cell vehicles. It is therefore crucial to further study the degradation mechanisms of the MEA components and to quantify them in relation to the fuel cell operation conditions. The different electron microscopy techniques (Fig. 1), including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), have shown their essential contribution to offer deep insight into several degradation mechanisms and will likely continue to have an impact on future efforts to improve durability.
TEM has been widely used to characterize Pt based PEMFC electro-catalysts and provides information on the size distribution of nanoparticles (NPs), their morphology and crystallographic structure. In addition, microscopes equipped with a Cs probe corrector and associated with electron energy loss spectroscopy (EELS) or X-ray energy dispersion spectroscopy (EDS) allow analyzing the chemical composition of each atomic column. Therefore, TEM characterization has been widely used to develop new high-performance catalysts with well-controlled chemistry/morphology and to study their stability.
The main Pt NP degradation mechanism that occurs during fuel cell operation is the electrochemical Ostwald ripening leading to a decrease in the number of NPs smaller than 4 nm associated with an increase in the average NP size (usually from 2-3 nm to 4-5 nm). If the cathode potential becomes higher than 0.9 - 1V, the greater dissolution of the small NPs produces a large amount of Pt ions that migrate toward the membrane where they are reduced by the H2 crossing over to form membrane Pt precipitates. Similar results are observed for Pt alloy NPs but in this case, the electrochemical Ostwald ripening mechanism leads to an increase in the thickness of the Pt shell surrounding the alloy core (typically from 0.6 to 1 nm)1 that affects their activity. Indeed, the negative standard potential of the non-noble elements (Co, Ni...) prevents the reduction of their ions within the MEA and only Pt is redeposited on the larger NPs. Ions of the non-noble elements migrate into the electrolyte and lower its proton conductivity. The membrane contamination can be detected by SEM/EDS analyses performed on MEA cross-section but taking care during sample preparation to prevent altering the ions distribution. This contamination can be limited by avoiding using Pt alloy NPs smaller than 4 nm. .
More recently, electron tomography analyses have revealed that when high surface area carbon are used, many Pt NPs are located inside the carbon and that during ageing tests, these interior NPs are less affected by the coarsening mechanism2,3. It is assumed that the confinement of the NPs inside small pores increases their interspacing and thus hampering the electrochemical Ostwald ripening mechanism. The development of carbon support with a well-controlled porosity allowing an optimal NP confinement seems to be a very promising research direction for which electron tomographic analyses will be essential.
On the other hand, the carbon support must have a high oxidation resistance to prevent structural collapse of the porous cathode catalyst layer if high potential values are accidentally reached as can locally happen during the start-up/shut-down phases. This severe cathode degradation by carbon corrosion is highlighted on MEA cross-section SEM images. Today FIB-SEM analyses providing 3D images of the porous structure offer the possibility to go further by measuring the evolution of the porosity.
Ionomer is also an important MEA component whose degradation dramatically limits PEMFC durability. MEA cross-section SEM images can reveal the physical degradation of the membrane such as thinning. In the electrode, however, the study of ionomer degradation remains challenging because high resolution 3D images are required. For this purpose, electron tomography experiments are still under development.
Many degradation studies used to concern MEA aged during accelerated stress test specific for each component. In our laboratory, we are more focused on identifying degradation phenomena that occur in stacks operating in or near real-world conditions. For these studies, we couple electron microscopy analyses with in-situ local measurements using segmented devices to select the regions of interest and to relate local degradation to local performance and conditions1.

Acknowledgement
: this project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation program under grant agreement No. 779565 (ID-Fast) and previously No. 621216 (SecondAct).

References:

Chandesris et al., JES, 165 (6), F3290 (2018)

B. T. Sneed et al., ACS Appl. Mater. Interfaces, 9, 29839 (2017).

E. Padgett et al., JES, 166 (4), F198 (2019)