ORR Activity for Pt/Pd(111) Bimetallic Surface Prepared By Molecular Beam Epitaxy

Background

  From the view-points of reduction in usage of expensive Pt and enhancement in oxygen reduction reaction (ORR) activity [1], Pt-shell and Pd-core, core-shell type nano-particles attract much interest as cathode-electrode materials for polymer electrolyte fuel cells. Relation between topmost surface structures of the core-shell nano-particles and ORR activities is a key for developing highly-active, low-Pt-content catalysts. However, disscussion of ORR mechanism is complicated. In this study, we prepared well-defined Pt/Pd(111) bimetallic surfaces by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV) and performed EC measurements in an inert gas atomosphere.

Experimental

  The MBE apparatus and UHV-EC sample transfer system have been described earlier [2]. All sample fabrication processes were conducted in UHV. Pd(111) single crystal substrate was cleaned by Ar⁺ sputtering and annealing. Various-thick Pt was deposited onto the clean Pd(111) by an electron-beam evaporation method at 673K. The resulting surface structures were verified with low energy electron diffraction (LEED),   scanning tunelling microscope in UHV (UHV-STM). Then, the MBE-prepared Pt_xnm/Pd(111) surfaces were transferred without being exposed to air to the EC system set in an N₂-purged glove box. Cyclic voltammogram (CV) of the samples were recorded in N₂-purged 0.1M HClO₄, and, then, linear sweep voltammetry (LSV) was conducted by a rotating electrode (RDE) method at 1600 rpm after saturating the solution with O_2. EC degradation of the Pt_xnm/Pd(111)  was evaluated by applying potential cycles between 0.6V (3s) and 1.0V (3s) vs. RHE. The potential-cycled Pt_xnm/Pd(111) were re-transferred to the UHV-STM system to evaluate the potential-cycle-induced topmost surface structural changes.

Results and discussion

  From the results obtained by LEED and IRRAS for carbon monoxide adsorption, we deduce that 0.6nm-thick Pt almost covers the Pd(111) substrate surface. A UHV-STM image of the Pt_0.6nm/Pd(111) (inset in Fig. 1 (a)) clearly shows atomically-flat, wide terraces.

  Fig. 1 (a) shows CV curves of the Pt_0.6nm/Pd(111) (blue). CV curves for clean Pt(111) (black) and Pd(111) (grey) are also presented as references. EC charge of the H-related features (0.15V-0.3V ; Q_ab&de) for the Pt_0.6nm/Pd(111) markedly shirinks in comparison to that of clean Pd(111), suggesting that the topmost surface of the Pt_0.6nm/Pd(111) is epitaxial Pt(111).

  Fig. 1 (b) shows LSV curves for the respective surfaces. A half-wave potential of the as-prepared Pt_0.6nm/Pd(111) (blue) shifts positively relative to clean Pt(111), indicating ORR activity enhancement. The LSV curve of the as-prepared Pt_0.6nm/Pd(111) shifts to lower potentials after the 1000 potential cycles (red), revealing potential-cycle induced surface degradation. One might notice that the red LSV curve is almost identical to that for 0.15nm-thick-Pd deposited Pt(111) (Pd_0.15nm/Pt(111);green). The results obtained in this study suggest that the topmost surface of MBE-prepared Pt_0.6nm/Pd(111) is degraded through dissolution and precipitation of the substrate Pd atoms during the potential cycles.

Acknowledgement

  This work was supported by New Energy and Industrial Technology Development Organization (NEDO).

References

[1] J. Zhang et al. J. Phys. Chem. B, 108, 10955 (2004).

[2] Y. Iijima, T. Wadayama et al., J. Electroanal. Chem., 685, 79 (2012).