Photon Factory Activity Report 2017 #35 (2018)

BL-3A, BL-4C/2016G588, 2017G126

Adsorbed Structures of Oxygen Species on Pt(111) during Oxygen ReductionReaction
1,*22Toshihiro KONDO Takuya MASUDA , and Kohei UOSAKI
1Ochanomizu University, 2-1-1, Ohtsuka, Bunkyo-ku, Tokyo 112-8610, Japan
2
National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan
1 Introduction
It is very important to clarify the absorbed structure ofoxygen species on the platinum electrode surface duringoxygen reduction reaction (ORR), which is a key reaction
at the cathode of the polymer electrolyte membrane fuelcell (PEMFC) and then, the potential-dependent
structures at the Pt single-crystal electrode/electrolyte
solution interfaces has been extensively studied as amodel system using various surface analysis techniques[1-6]. Because adsorbed oxygen species reacts
immediately, however, the adsorbed structure of oxygen
species has not been elucidated yet, so the ORR
mechanism is still unknown.
Using in situ surface x-ray scattering (SXS) techniquewith a spectro-electrochemical cell in the thin layer
configuration, we can control the electrolyte solution
thickness and therefore, i.e., we can control the reaction
rate by the cell configuration [7-9]. Using suchconfiguration, in this study, we tried to find the adsorbed
structure of oxygen species on Pt(111) single-crystal
electrode surface during ORR.
2 Experiment
After annealing and quenching, the Pt(111) singlecrystal disk was set to the spectro-electrocheical cell [7]and the SXS measurements were carried out in theelectrolyte solution, which was saturated by air bubbling.
The Pt(111) electrode made contact with the solution
while keeping the potential at 0.2 V (vs. RHE), where theelectrode surface was covered with underpotentiallydeposited hydrogen. The electrode potential was scannedin the positive or negative derection from 0.2 V, thenstopped at a certain potential at which the SXS
measurement was going to be carried out. After holding
the potential for more than 10 minutes, the SXS
measurement was carried out with the electrolyte
thickness of ca. 30 µm. In situ measurement was carriedout under an open air environment, which means oxygenis induced to the solution through the Mylar@ window.
All the data fitting with a theoretical equation was carriedout using a three-layered model.
3 Results and Discussion
Figure 1 shows a typical rod result. Based on thepotential dependent rod fitting with a theoretical equation,
ca. 0.2 ML oxygen species exists on the Pt(111) surface
during ORR. In ultra-high vacuum (UHV), the (2×2)
adsorbed structure was observed on the Pt(111)-(1×1)
surface and this adsorbed amount was similar to one

observed in UHV. The detailed potential-dependentadsorbed structure of oxygen species are now underinvestigation.

Fig. 1: The (00) rod measured at 0.4 V vs. RHE. Circles
and solid line represent the data points and best fitting
curve, respectively.
Acknowledgement
We wish to thank Dr. Tamura of the Japan Atomic
Energy Agency for his help with CTR fitting program.
References[1] K. Uosaki, Jpn. J. Appl. Phys. Part I 54, 030102(2015).
[2] C. A. Lucas et al., Faraday Discuss. 140, 41 (2008).
[3] D. V. Tripkovic et al., Faraday Discuss. 140, 25(2008).
[4] H. A. Hansen et al., Phys. Chem. Chem. Phys. 10,3722 (2008).
[5] N. M. Marković and P. N. Ross, Jr., Surf. Sci. Rep. 45,117 (2002).
[6] M. J. Weaver, Langmuir 14, 3932 (1998).
[7] T. Kondo et al., Electrochim. Acta 47, 3075 (2002).
[8] T. Kondo et al., Surf. Sci. 631, 96 (2015).
[9] T. Kondo et al., J. Phys. Chem. C 120, 16118 (2016).
* kondo.toshihiro@ocha.ac.jp