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Structural modifications of the Pt/HSAC electrocatalyst induced by intermediate characterizations followed by IL-TEM
The morphological changes of the Pt/HSAC electrocatalyst induced by the intermediate characterizations were investigated by identical location transmission electron microscopy (IL-TEM). The comparison of the TEM images recorded for the Pt/HSAC samples aged in AST 0 or AST 1 conditions allowed a direct quantification of the effect of the intermediate characterizations. The AST 3 explored the influence of COad stripping voltammograms, and the AST 4 investigated whether this effect is due to the electrochemically reducing potential or to the CO atmosphere. Recently, a possible limitation of the IL-TEM technique was discussed by Schlögl et al. [66]. The authors argued that, at temperature close to T = 330 K, the dissolution of the gold TEM grid initiates at E 1.3 V vs. RHE. However, since this upper potential limit was never exceeded during the ASTs, this phenomenon is very unlikely to have occurred here.
Effect of the number of intermediate characterizations
AST 0 and AST 1, respectively. In both case, well-known degradation mechanisms appear: (i) the migration of Pt nanocrystallites, resulting in an increased degree of agglomeration (white rectangles), (ii) the electrochemical Ostwald ripening, that is the preferential dissolution of the smallest Pt nanocrystallites over time (production of Ptz+ ions) and the increase of the mean particle size due to redeposition of these ions (green circles), (iii) the thinning/shrinking/collapsing of the carbon particles due to electrochemical corrosion (white arrows) or the structural re-arrangement of the carbon particles (red line).
It is clear from these micrographs that the extent of degradation of the Pt/HSAC nanoparticles increases with an increase in the number of intermediate characterizations. These observations are quantified in Figure III – 19 and in Figure III – 20 where the particle size distributions, the surface-averaged mean particle size and the density of “isolated” particles are displayed, respectively. The Pt nanoparticles increase in size more significantly, and there is a more pronounced decrease in the fraction of the smallest Pt nanocrystallites when intermediate characterizations are implemented in the AST: + 38 % in size for AST 1 vs. + 32 % in size for AST 0. This may be explained by the redeposition of Ptz+ ions produced by electrochemical Ostwald ripening during the negative-going potential sweep in cyclic voltammetry experiments.
Surface-averaged mean particle size ( d S ) and density of isolated (non-agglomerated and spherical) Pt nanoparticles before (filled symbols) and after (open symbols) different ASTs.
Furthermore, Figure III – 21 illustrates that the changes of the HSAC structure are more severe when intermediate characterizations are included in the AST. For both AST 0 (left-hand side) and AST 1 (right-hand side), the borders of the primary carbon particles are modified, but the brighter contrast of IL-TEM micrographs recorded after AST 1 suggests more pronounced corrosion of the HSAC in AST1. Further evidence is provided in Figure III – 22: the greater depreciation of the D1-band intensity in Raman spectra following AST1 provides evidence of the harsher corrosion of the organized domains of the HSAC support, in agreement with previous reports [67, 68]. Considering the above, it is not surprising that Pt nanoparticles detach from the carbon support (blue arrows) during AST 1 (Figure III – 18) and not in AST 0 (Figure III – 17). It is also possible to provide evidences of this phenomenon because certain detached Pt nanoparticles are trapped in the Lacey carbon membrane of the TEM grid. The detachment of the Pt nanoparticles likely accounts for the 13% difference in the ECSA loss monitored between AST 0 and AST 1.
Effect of the nature of intermediate characterizations
Finally, the effect of the nature of the intermediate characterizations (cyclic and COad stripping voltammograms) used during ASTs was investigated. For that purpose, AST 1 that includes four intermediate CVs and four COad intermediate stripping voltammograms, AST 3 that includes four COad intermediate stripping voltammograms and AST 4 that includes four intermediate CVs and four “pseudo” COad stripping voltammograms were compared.
Figure III – 23 shows the IL-TEM images recorded before/after AST 3. In these experimental conditions, crystallite migration/agglomeration appears to be the predominant degradation mechanisms (see inset). Indeed, the mean diameter of the isolated Pt nanoparticles is only slightly altered ( aged = 2.29 nm vs. fresh = 2.06 nm in Figure d N d N III – 19) and the population of small Pt nanoparticles (between 1 nm and 1.5 nm) is preserved (Figure III – 20). As proposed in our former study [69], this can be explained by considering that CO molecules partially reduce the COsurf species present on the carbon support, thereby facilitating their movement. A striking result is the absence of Pt nanocrystallite growth in these AST conditions: this suggests that CO molecules inhibit the dissolution of Pt into the electrolyte or the redeposition of Ptz+ ions onto larger Pt crystallites. Note however that, in the last scenario, a decrease in the mean Pt/HSAC crystallite growth would have been observed.
or flat rafts between individual Pt nanocrystallites (Figure III – 24 inset). Since similar variations of the density of isolated Pt nanoparticles and of the mean Pt nanoparticle size were observed during AST 4 as in AST 1 (Figure III – 19 and Figure III – 20), this confirms that CO molecules prevent the dissolution of Pt nanoparticles into Ptz+ ions. The interested reader is referred to Ref. [69] for a more detailed discussion.
Figure III – 24. IL-TEM images of the Pt/HSAC electrocatalyst after AST 4 (96 h polarization at E = 1.0 V vs. RHE and T = 330 K in 0.1 M H2SO4 + CVs + “pseudo” COad stripping voltammograms).
3.6 Influence of the nature of the carbon support, the gas atmosphere and the potential limits used in AST Under steady-state PEMFC operation, the potential of a PEMFC cathode varies between 0.60 V and 1.0 V vs. RHE (depending on the current density), while under start-up/shutdown conditions, the electrode potential may reach up to 1.5 V vs. the RHE [20, 70, 71]. The degradation of the carbon support may also be different in different gas atmospheres.
To investigate the effect of these parameters, different ASTs were performed on HSAC, Vulcan TKK and RG carbon supports under neutral (argon) and oxidative (oxygen) atmosphere. Two distinct protocols were used: a load-cycle protocol with a square potential 88 ramp between 0.60 V – 1.00 V and a start-up/shutdown protocol with a square potential ramp between 1.00 V –1.50 V vs. RHE, which mimic the potential range experienced during steady-state and start-up/shutdown cycles, respectively. These ASTs are based on the Fuel Cell Commercialization Conference of Japan (FCCJ) protocols [20].
Table of contents :
Chapter I. Introduction
1.1 Fuel cell technologies and industrial overview
1.1.1 Energy issues of today’s society
1.1.2 Fuel cell technology: operating principle, applications and types of fuel cells
1.1.3 Fuel cell industrialization and market
1.2 PEMFC – Proton Exchange Membrane Fuel Cell
1.2.1 Single cell: electrochemical reactions and membrane electrode assembly (MEA)
1.2.2 Proton Exchange Membrane (PEM)
1.2.3 Catalytic layers
1.2.4 Gas diffusion layers (GDLs)
1.3 From single cell to stack and fuel cell system
1.4 Degradation mechanisms of MEAs during PEMFC operation
1.4.1 PEM and ionomer degradation during PEMFC operation
1.4.2 Pt nanoparticles degradation during PEMFC operation
1.4.3 Carbon support degradation during PEMFC operation
References
Chapter II. Experimental Section
2.1 Electrocatalysts
2.2 Electrochemical measurements
2.3 Preparation of the catalytic inks and of the porous catalytic layers
2.3.1. Preparation of the catalytic inks
2.3.2. Preparation of the porous catalytic layers
2.4 Electrochemical characterization
2.5 Accelerated stress tests (ASTs)
2.6 PEMFC testing
2.7 Ex situ preparation of the fresh/aged MEAs cathode electrocatalysts
2.8 Characterization of the fresh and aged electrocatalysts
2.8.1 Physical techniques…
2.8.2 Chemical techniques
2.8.3 Electrochemical techniques
References
Chapter III. From Pt/C electrocatalysts suspensions to accelerated stress tests
3.1 Introductory note to Raman spectroscopy of carbon materials and accelerated stress tests protocols
3.2 Structural properties of carbon supports
3.3 Preparation of catalysts’ suspensions
3.3.1 Definition of the initial surface state and effect of Nafion® on the carbon support
3.4 Effect of storage of the ink under air atmosphere on the carbon support
3.4.1 Effect of short-term storage of the ink
3.4.2 Effect of long-term storage of the ink
3.5 Role of intermediate characterizations, nature of the carbon support, gas atmosphere and potential limits on ECSA and Q/HQ redox couple during ASTs
3.5.1 Electrochemical evidences on the influence of intermediate characterizations on ECSA and Q/HQ redox couple evolution
3.5.2 Structural modifications of the Pt/HSAC electrocatalyst induced by intermediate characterizations followed by IL-TEM
3.6 Influence of the nature of the carbon support, the gas atmosphere and the potential limits used in ASTs
3.6.1 Electrochemical results
3.6.2 Structural modifications of the carbon supports after the ASTs
3.6.3 Structural modifications of the Pt nanoparticles after the ASTs
3.7 Conclusion
References
Chapter IV. Carbon corrosion in PEMFCs : from model experiments to real-life operation in MEAs
4.1 Introduction
4.2 Results and Discussion
4.2.1 Electrochemical characterization
4.2.2 Physical characterization
4.2.3 Chemical characterization
4.2.4 Comparison with MEA operated on site during 12,860h
4.2.5 Degradation mechanism of HSAC supports
4.3 Conclusion
References
Chapter V. Carbon corrosion induce by membrane failure: the weak link of PEMFC longterm performance
5.1 Introduction
5.2 Results and Discussion
5.2.1 110 cell system operating performance
5.2.2 In situ evidences of performance heterogeneities at the stack and the MEA level
5.2.3 Linking degradation of electrical performance to membrane failures
5.2.4 Linking the degradation of the cathode catalytic layer and of the electrical performance to the membrane failures
5.3 Discussion
5.4 Conclusion
References
Chapter VI. Conclusion and Outlook
