(Invited) Enhanced Activities for the Oxygen Reduction Reaction at Pt-Skin Layers on Pt-Co Alloy Single-Crystal Electrodes

Development of highly active cathode catalysts for the oxygen reduction reaction (ORR) is one of the most important approaches to achieving high efficiency and low cost for polymer electrolyte fuel cells (PEFCs). Pt-based alloy catalysts, such as Pt-Co or Pt-Ni, have attracted considerable attention as alternative electrocatalysts to pure Pt in PEFCs, and have been reported to exhibit high ORR activity by us and numerous other researchers.¹ In practical fuel cells, Pt nanoparticles dispersed on carbon supports are employed in catalyst electrodes; the surfaces usually consist of low-index crystal facets such as (111) and (100). Recently, we have established a facile method for preparation of Pt-Co alloy single crystals with desired Co contents by using only a micro-torch, without any crucible or UHV system.² In the present study, we have evaluated the kinetically-controlled ORR current densities, j_k at Pt skin layers formed on Pt-Co alloy (111), (110) and (100) single-crystal surfaces in air-saturated 0.1 M HClO₄solution as a function of Co content by using the rotating disk electrode (RDE) method.

  The preparation of Pt-Co alloy single-crystal electrodes with desired Co contents has been described in the literature.² Prior to electrochemical measurements, well-defined (1 × 1) surfaces of polished crystal electrodes were freshly prepared by heating to 1223 K with an infrared lamp in H₂, and subsequently by cooling in H₂ for the (111) and (100) planes or 1% CO/He streams for the (110) plane.³ This heat treatment resulted in the formation of a pure Pt skin layer on Pt-Co(111) alloy surfaces, confirmed by in-situ scanning tunneling microscopy³and low energy ion scattering spectroscopy.

  Fig. 1 shows cyclic voltammograms of Pt_100-xCo_x(111) electrodes at various Co contents, x in N₂-saturated 0.1 M HClO₄ solution. As compared to the CV of pure Pt(111), dramatic changes were observed in those for the Pt-skin/Pt-Co(111) electrodes, demonstrating the alloying effects on the hydrogen underpotential deposition (HUPD) and surface oxidation process.³ The HUPD wave shifted to lower electrode potentials, and the HUPD charge, Q_HUPD decreased linearly as x increased.³ It should be emphasized that the decrease of Q_HUPDfor the Pt-Co alloy electrodes was ascribed to the modified electronic structure of the Pt skin layer by the underlying Pt-Co alloy, not to a decrease in the number of surface Pt sites.

  In contrast, a characteristic butterfly-like wave due to the formation of adsorbed OH in the surface oxidation region shifted to higher electrode potentials, and the spike peak became broader with increasing x. However, it should be noted that the surface oxidation charges, Q_ox up to 0.9 V for the Pt_100-xCo_x(111) electrodes were nearly constant and exhibited the same as that for pure Pt(111).

  Fig. 2 shows RDE polarization curves of the Pt_100-xCo_x(111) electrodes at 1500 rpm in air-saturated 0.1 M HClO₄ solution. The RDE polarization curves exhibited onset potentials of ORR current at the Pt_100-xCo_x(111) electrodes that were shifted to higher electrode potentials, compared to that at pure Pt(111). Fig. 3 shows the j_k values at 0.9 V as a function of the Co content, derived from the RDE measurements. The j_k values increased remarkably as x increased, and exhibited a maximum j_k value of 3.02 ± 0.33 mA cm^-2 at around x = 25%, which was more than 20 times larger than that for pure Pt(111). It should be emphasized that the enhancement factor of 20 at our Pt-skin-layer on Pt₇₅Co₂₅(111) was much higher (more than twice) than those previously reported for Pt-based alloy (111) single crystal electrodes.⁴

  In contrast, the j_k values of Pt skin layers on Pt₇₆Co₂₄(100) and Pt₇₄Co₂₆(110) at 0.9 V were only twice as large as those of the pure Pt(100) and (110) surfaces, respectively. However, for all of the three single-crystal planes, the Q_oxvalues up to 0.9 V were nearly the same between the Pt-skin/Pt-Co alloy and pure Pt electrodes. Thus, the enhanced ORR activities at Pt-skin layers on Pt-Co alloy single-crystal electrodes cannot be explained by the conventional view of “suppression of OH-poisoning”.

 

Acknowledgment

  Financial supports from the “HiPer-FC Project” of NEDO, Japan and a Grant-in-Aid No. 25410007 for Scientific Research of JSPS, Japan are gratefully acknowledged.

References

(1) T. Toda, et al., J. Electrochem. Soc., 146, 3750 (1999). (2) M. Wakisaka, et al., Electrochem. Commun., 13, 317 (2011). (3) M. Wakisaka, et al., Electrochem. Commun., 18, 55 (2012). (4) V. R. Stamenkovic, et al., Science, 315, 493(2007).; T. Wadayama, et al., J. Phys. Chem. C, 115, 18589(2011).