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PEMFC heterogeneities

Durability of PEMFC materials: Heterogeneities of ageing in real PEMFC conditions

Although we are not making any PEMFC testing ourselves in the EIP team, we have extensively worked since ca. 2005 (and still do) on the topic of the durability of their membrane-electrode assembly. For that purpose, we have tied strong links with an industrial partner (Axane) and colleagues of the LMOPS team within LEPMI (Corine Bas, Gilles de Moor and Lionel Flandin, specialized in the membrane degradation and diagnostics) and of the LEMTA in Univ. Lorraine Nancy (Jérome Dillet, Gaël Maranzana and Olivier Lottin). In this frame, we essentially expertised used MEAs after their operation in model or on-site conditions, using a broad set of physicochemical and electrochemical analyses.
A clear and important input of the collaborative work performed within the consortium cited above, is that PEMFC do not age in a homogeneous manner at the scale of the MEA and the stack. We unveiled that the extent of degradation is strongly correlated to the fluidic within the MEA, and therefore depend on the geometry of the bipolar plates. In consequence, the inlet and outlet do not degrade similarly, whatever the PEMFC operating conditions (e.g. stationary vs. start/stop) (Figure 1) 10-13. A second level of heterogeneity was found within the catalysts layer thickness (Figure 2), and a third one between the channel and lands of the bipolar plates (for a start-up/shut-down accelerated stress test) (Figure 3).


Figure 1. Schematic representation of the mass transport fluxes within a PEMFC MEA operating at “high current”, i.e., with an heterogeneous distribution of the current density along the gas channel (j > javerage at the inlet and j < javerage at the outlet, as a result of the larger air mass-transport hindrance of the air depleted oxidant in the flooded cathode outlet region). j and javerage stand for the localized or average current density, respectively. Reproduced from 10 with permission from Wiley.


 



Figure 2. (a, b) X-EDS analyses performed locally on ten different positions in the cathode CL imaged in (c). (a) The Pt:Co atomic ratio and (b) the Pt and Co weight percent [the contribution of carbon to the X-EDS signal was not considered for the calculation in (b)]. Reproduced from 10 with permission from Wiley.

Figure 3. SEM images in back-scattered mode of (A) the top view of the aged MEA and the cross-sections of the two distinct regions: (B) under a channel and (C) under a land. The upper electrodes in (B) and (C) are the cathodes. Reproduced from 12 with permission from Elsevier.


 
A last level of heterogeneity comes from the MEA constitutive materials, and in particular from the PtM/C electrocatalysts used at the cathode. The less noble materials corrode first, should it be for the transition metal alloyed to Pt 14, 15 or for disorganized carbon (vs. graphitized one) 16, 17.
Finally, in complement to the analysis of used MEAs, we conduct tailored accelerated stress-test (AST) at the lab scale, and notably use the identical-location transmission electron microscopy (ILTEM) technique. Using this technique enabled us to unveil the role of the atmosphere (and electrolyte composition) during the AST on the PtM/C materials degradation (Figure 4). Using an home-made “dry-cell” operating with a Nafion membrane as electrolyte and comparing the results obtained in this and those in classical liquid electrolyte, we demonstrated that (i) an excess of liquid water emphasizes the corrosion of carbon and the dissolution of platinum and (ii) Co plays the role of “sacrificial anode” and mitigates the dissolution of Pt (Figure 5) 18, 19.


Figure 4. IL-TEM images of Pt/C catalysts before and after 800 potential cycles between 0.05 and 1.23 V vs. RHE in 0.1 M HClO4 solutions containing Ar, CO or O2 (T = 20°C). Reproduced from 20 with permission from Elsevier.

 




Figure 5. (a) IL-TEM images of Pt3Co/C nanoparticles before and after aging in dry cell (Nafion 115 electrolyte) and (b) corresponding particle size distribution histograms. The (non-comprehensive) markers on the figure highlight representative examples of aging phenomena that occurred during the AST. Reproduced from 19 with permission from Elsevier.

 
Acknowledgements

The authors thank Oseo-Anvar for funding (H2E project). Some of this work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” n° AN-10-LABX-44-01.

References

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mise à jour le 5 février 2016

Univ. Grenoble Alpes