Degradation mechanisms of Pt-based/C nanoparticles in model and real PEMFC conditions
The degradation of Pt-based nanoparticles observed during long-term PEMFC operation, and the associated loss of electrochemically active surface area (ECSA), is highly detrimental for the fuel cell performance. The major degradation mechanisms of Pt/C nanoparticles are well-established: (i) Pt nanoparticle agglomeration, possibly followed by their coalescence or detachment from the carbon support, 1-3 (ii) electrochemical Ostwald ripening i.e. the preferential dissolution of the smallest Pt crystallites, yielding the formation of Ptz+ ions and their redeposition onto larger particles, following the Gibbs–Thomson effect 1, 4, 5 and (iii) the chemical reduction of Ptz+ in the PEM via H2 crossing-over from the anode to the cathode 1, 4, 5. Alloying Pt with transition metals is not durable either, regardless of the PEMFC operating conditions 6-10. This can detrimentally affect the operation of the membrane and the ionomer, since “dissolved” metal cations inhibit the mass-transport of both the protons and oxygen molecules 11 which notably affects the ionomer and membrane 12, 13. With Pt3Co/C nanoparticles, Co atoms located in the near-surface layer are rapidly dissolved during the first hours of operation yielding the so-called “Pt-skeleton” structure. However, the formation of a “Pt-skeleton” structure does not ensure stability in the PEMFC environment. The Gibbs-Thompson-induced dynamic dissolution of the smallest Pt-Co/C particles yields Co2+/Ptz+ ions; because of different standard potentials, only Ptz+ ions can redeposit electrochemically onto larger-sized particles at the cathode potential (Ecathode 0.6 - 0.7 V vs. RHE). In the end, the structure of the Pt-Co/C nanoparticles in the long-term is determined by a balance between Co surface segregation and formation of oxygenated species from water splitting. When the PEMFC is operated at high current density (low cathode potential, below the onset of surface oxide formation from water), a steady-state is reached between the rate of Co dissolution at the surface and Co surface segregation. Consequently, Co and Pt atoms remain homogeneously distributed within the Pt-Co/C particles and the thickness of the Pt-shell is maintained to a small value not detectable by atomic-resolution high-angle annular dark field scanning transmission electron microscopy (STEM-HAADF). When the PEMFC is operated at low current density (high cathode potential), the formation of surface oxides from water and the resulting “place-exchange” mechanism enhance the rate of diffusion of Co atoms to the surface. Consequently, the fresh Pt3Co/C particles form core/shell particles with thick Pt-shells and Co content < 5 at% and, ultimately, “hollow” Pt nanoparticles. The formation of the “hollow” nanoparticles is attributed to the highly oxidizing environment of a PEMFC cathode, which accelerates Co interdiffusion from the bulk to the surface of the nanoparticles where it leaches out (nanoscale Kirkendall effect). Interestingly, the ORR activity of these hollow nanoparticles is 1.5-fold that of the pristine Pt3Co/C and 3-fold that of state-of-the-art “bulk” Pt/C nanoparticles, thereby offering a new route to the synthesis of more active and durable PEMFC electrocatalysts.
Figure 1. Structural modification of Pt-Co/C nanoparticles aged in a PEMFC environment. ADF images and chemical maps of individual nanoparticles are displayed in (a) for the fresh catalyst and (b-c) for the aged one. The electron energy loss spectroscopic maps provide direct evidence of the core-shell nanostructure of the fresh Pt3Co/C nanoparticles, resulting from acid-leaching and indicate that some aged nanoparticles maintain their core-shell nanostructure, while others display a “hollow” nanostructure. The Pt N3 signal is shown in red, and the Co L2,3 signal is shown in green.
The carbon substrate that supports the PtM nanoparticles should (from the thermodynamics view point) not be stable either in PEMFC environment. The first studies dealing with the degradation of carbon supports in fuel cells electrodes date from the 1970s - 80s (Binder 14, Kinoshita and Bett 15-18). At that time, research was focused on PAFCs which work at temperatures of 423 < T < 493 K. Since PEMFC operate at lower temperatures (T < 373 K), carbon corrosion was believed to be a negligible phenomenon in PEMFCs. This assertion was proven incorrect 19, 20. Driven by the durability targets for automotive PEMFCs, the understanding of the degradation mechanisms of carbon supports has become a major research topic. Having investigated the degradation mechanisms of a series of high surface area carbons (HSAC) used as supports in simulated and real PEMFC operating conditions 21-26, we have shown that the mechanism and the kinetics of the electrochemical corrosion of carbon (COR) depend on the presence/absence of Nafion ionomer, the upper potential limit (Figure 2), and the nature and the number of intermediate characterisations during accelerated stress tests. Raman spectroscopy showed that the COR is strongly structure-sensitive and proceeds more rapidly on disordered domains of the HSAC (amorphous carbon and defective graphite crystallites) than on graphitic domains. The coverage with carbon surface oxides was investigated with X-ray photoelectron spectroscopy and bridged to the intensity of the quinone/hydroquinone (Q/HQ) peak monitored by cyclic voltammetry. Finally, analyses realized on membrane electrode assemblies operated for 12,860h disclosed a perfect agreement between model and real PEMFC operating conditions, and confirmed the structural dependency of the COR kinetics.
Figure 2. a) Spectres Raman normalisés pour les électrocatalyseurs Pt/HSAC neufs et vieillis. Inset : zoom sur les bandes D1, G, D2 et carbonyle, b) Variation de la taille des cristallites de carbone dans le plan (La) après 96 heures de polarisation. Electrolyte: 0,1 M H2SO4; T = 330 K.
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.
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