Synchrotron-Based In Situ Characterization of Accelerated Degradation for Pt and Pt Monolayer Electrocatalysts

One critical issue facing the commercialization of low-temperature fuel cells is the gradual decline in performance during operation, mainly caused by the loss of the electrochemical surface area of carbon-supported Pt nanoparticles at the cathode.^1,2  The major reasons for the degradation of the cathodic catalyst layer are the dissolution of Pt and the corrosion of carbon under certain operating conditions, especially during potential cycling.  Different cycling places various loads on fuel cells.  A detailed understanding of degradation mechanisms of Pt is therefore critical in designing durable materials for the oxygen reduction reaction (ORR). 

In situ X-ray absorption spectroscopy (XAS) techniques offer a unique ability to investigate both electronic and structural properties on electrocatalysts; with specially designed in situ electrochemical cell it is possible to measure XAS of electrocatalysts under electrochemical potential controls.³ The information obtained is essential for elucidating the atomic arrangement, the oxidation degree, and the effects of metal-oxide substrates on electronic properties of catalysts, in relation to their catalytic activity.

The present work explores the detailed changes in structural and electronic properties of Pt and Pt ML electrocatalysts during potential cycling by combining XAS and XRD measurements.  The in situ cell allows us to measure XAS and XRD of the same sample at the same electrochemical potentials. Fig. 1a shows in situ XANES spectra for Pt L3-edge of carbon-supported Pt nanoparticles at potentials from 0.41 to 1.51 V.  The intensity of the white line considerably increases at the higher potential as a consequence of depleting Pt’s d-band due to the Pt oxide formation.⁴ In situ XRD patterns of the (220) reflection from the same specimen measured immediately after the XAS at electrochemical potentials are shown in Fig. 1b.  Broadening of the peak is fairly small; the particle size changed from 2.7 nm to 2.3 nm by applying potentials from 0.41 to 1.51 V, indicating that the thickness of the oxide formed is approximately monolayer-thick or less.  Although the potential to 1.51 V significantly increases the white line due to oxidation, the oxide is confined only to the top surface layer on the nanoparticles.   Exact mechanisms for Pt oxidation/degradation with elevating potentials are still unclear, despite many models involving a “place-exchange” process^5,6 have been proposed; a combination of in situ XAS and XRD is able to provide such the fundamental insights of the nano-scale states of catalysts under the electrochemical conditions.

Pt monolayer electrocatalysts offer a dramatically reduced Pt content while affording considerable possibilities for enhancing their catalytic activity and stability.⁷  In XAS tests, we demonstrated that Pt oxidation of Pt monolayer electrocatalysts is considerably hampered compared with Pt/C catalysts.  Plausible origins of the enhanced stability will be discussed at the meeting.

References

1.  H.A. Gasteiger, et al., Appl. Catal. B Environ. 56 (2005) 9.

2.  K. Sasaki, et al., in Polymer Electrolyte Fuel Cell Durability, F.N. Büchi, et al, Eds, Springer, (2009), p.7.

3.  K. Sasaki, et al., Electrochim. Acta, 55 (2010) 2645.

4.  S. Mukerjee, et al., J. Electrochem. Soc., 142 (1995) 1409;

5.  B. E. Conway, Prog. Surf. Sci., 49 (1995) 331.

6.  G. Jerkiewicz, et al., Electrochim. Acta,  49 (2004) 1451

7.  R. R. Adzic, et al., Top. Catal., 46 (2007) 249.

Acknowledgement

This work is supported by the US Department of Energy (DOE), Division of Chemical Sciences, Geosciences and Biosciences Division, under the contract no. DE-AC02-98CH10886.

Figure Caption

Fig. 1 (a) In situ XANES of Pt L3 edge from a Pt/C catalyst in in 1 M HClO₄ from 0.41 to 1.51 V.  (c)  In situ XRD patterns of the (2 2 0) reflection from a Pt/C catalyst in in 1 M HClO₄ at 0.41 and 1.51 V (the wavelength: 1.0 Å).