Monitoring the Particle Size Distribution, Phase Quantification and Chemical Composition of Acid-Leached PtCo Electrocatalyst Under Start-up/Shut-Down Conditions Probed By in-Situ SAXS and WAXS Techniques
Authors: Janssen Marek , Quinson Jonathan , Drnec Jakub , Martens Isaac , Pittkowski Rebecca Katharina , Park Daseung , Singer Matthias , Weber Philipp , Arenz Matthias , Oezaslan Mehtap 
Published: September 2023

Large progress has been made towards commercialization of polymer electrolyte membrane fuel cells (PEMFCs) through the development of highly active and robust Pt-Co alloy catalysts for oxygen reduction reaction (ORR).[1,2] However, most ORR catalyst materials suffer from the extremely corrosive conditions occurring during start-up/shut-down (SUSD).[3] Consequently, understanding the degradation mechanisms during SUSD is highly crucial for designing new catalyst materials with enhanced ORR activity and long-term durability.

In this work, we show a comprehensive and complementary combination of different advanced techniques such as in-situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) as well as ex-situ scanning transmission electron microscopy in combination with electron energy loss spectroscopy (STEM-EELS) to provide deeper insights into the degradation processes of acid-leached Pt-Co catalyst under SUSD conditions.

Firstly, Pt-Co nanoparticles supported on high surface area carbon (HSAC) were synthesized using wet impregnation – freeze-drying – thermal annealing method.[4] Compared to our previous works [4-6], an acid-leaching (1 M H2SO4, 15 °C, 2 h) was applied to dissolve the less noble metal from the surface of alloy nanoparticles forming a Pt-enriched skin. STEM-EELS data has also confirmed the formation of the Pt-enriched skin. In addition, ex-situ WAXS data signifies that the acid-leached PtCo catalyst consists of two face-centered cubic (fcc) disordered crystal phases: Pt90Co10 phase with a crystallite size of 3.6 ± 0.4 nm and Pt70Co30 phase with 4.2 ± 0.4 nm size.

In terms of ORR performance, the acid-leached PtCo catalyst exhibits a 2.5- & 2-times higher mass activity (0.61 ± 0.12 A mgPt
-1) and a 3.5- and 1.3-times higher specific activity (665 ± 84 µA cmPt
-2) at 0.9 VRHE compared to commercial ~2 nm Pt/C (0.24 ± 0.05 A mgPt
-1, 187 ± 29 µA cmPt
-2) and heat-treated 4-5 nm Pt/HSAC (0.30 ± 0.04 A mgPt
-1, 297 ± 42 µA cmPt
-2), respectively. After SUSD experiments (800 cycles, 0.5 - 1.5 VRHE, 50 mV s-1, 0.1 M HClO4), the mass activity decreases to 0.21 ± 0.06 A mgPt
-1 (-65 %) and 0.15 ± 0.03 A mgPt
-1 (-38 %) for the acid-leached PtCo catalyst and commercial Pt/C, respectively.

To monitor the potential-dependent changes in particle/crystallite size, phase quantification and chemical composition of the acid-leached PtCo catalyst under the SUSD conditions, in-situ SAXS and WAXS experiments were simultaneously performed at the beamline ID31 at European Synchrotron Radiation Facility (ESRF).

In-situ WAXS analysis reveals that the lattice parameter of both Pt90Co10 and Pt70Co30 phases increases as a function of the SUSD cycle number, signifying the successive depletion of cobalt mainly from the Pt70Co30 phase. More precisely, the lattice parameter changed from 3.819 ± 0.004 Å (Pt70Co30) to 3.829 ± 0.004 Å (Pt76Co24) during the first 150 SUSD cycles. Only a slight change in lattice parameter for Pt90Co10 phase is observed during the SUSD protocol. Based on the Rietveld quantification, the amount of the Pt70Co30 phase decreases from 61 ± 2 wt.% to 47 ± 2 wt.% during the first 150 SUSD cycles. Afterwards, the Co depletion from this phase is less pronounced with additional cycle number. Furthermore, STEM-EELS investigations on acid-leached PtCo catalyst after first 150 SUSD cycles has confirmed the drastic Co depletion within the nanoparticles. After the SUSD protocol, the chemical composition of the acid-leached PtCo catalyst alters from Pt67±3Co33±3 to Pt81±3Co19±3 (-42 at.%) obtained from the bulk techniques like EDX and µ-XRF.

In addition, the Rietveld refinement reveals an increase in crystallite sizes for both Pt-Co crystal phases over the course of the SUSD protocol which is in excellent agreement to the in-situ SAXS and ex-situ TEM results (from 4.1 ± 1.0 nm to 5.2 ± 0.8 nm, +27 %). In detail, the initial larger Pt70Co30 phase shows only a small increase in crystallite size (from 4.3 ± 0.4 nm to 4.8 ± 0.4 nm, +12 %), whereas for the initial smaller Pt90Co10 phase a gradual increase from 3.2 ± 0.3 nm to 4.5 ± 0.4 nm (+41 %) is observed during the SUSD experiment.

Based on the in-situ SAXS/WAXS and the ex-situ STEM-EELS data, the Co depletion is the dominant degradation mechanism for the disordered fcc Pt70Co30 phase during the first 150 SUSD cycles. With increasing SUSD cycle number, the particle growth via Ostwald ripening and particle coalescence predominates. Our degradation model can be used to further improve the ORR performance and durability of acid-leached Pt-Co catalysts under the SUSD conditions.

Literature:

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[2] C.Wang et al.,Energies,2016,9,603-642.

[3] Y.Yu, et al.,J.Power Sources,2012,205,10-23.

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[5] D.J.Weber et al.,J.Mater.Chem.A,2021,9,15415-15431.

[6] P.Weber et al.,ACS Catal.,2022,12,6394-6408.