Oxygen Reduction Reaction Sites and Electron Conduction Mechanism of Zr-Oxide-Based Catalyst:Soft X-Ray SR-XPS and First-Principles Calculations
Authors: Mogi Masato , Kubobuchi Kei , Matsumoto Masashi , Asada Toshihiro , Takahashi Yohei , Arao Masazumi , Kamiguchi Kazuhiro , Ishihara Akimitsu , Uehara Naoki , Okada Yohei , Ota Ken-ichiro , Mizoguchi Teruyasu , Yaji Koichiro , Harasawa Ayumi , Imai Hideto 
Published: February 2020

Polymer electrolyte fuel cells (PEFCs) have expected as one of the key energy devices in future sustainable society. PEFCs, however, have difficult issues such as insufficient oxygen reduction reaction (ORR) activity and expensive Pt consumption. To overcome such difficulties, various non-platinum ORR catalysts, such as, carbide, nitride, and oxide were examined as alternative of Pt [1]. Among them, Zr oxide-based catalysts attract much attention. Such transition-metal-oxide catalysts show the high ORR onset potentials, but ORR current is very low. Thus, identifying the ORR sites, and designing surface structure to have effective ORR sites, is mail issues in developing oxide-base catalysts.
Previously, we reported that oxygen-vacancy sites could be ORR sites by surface-sensitive conversion-electron-yield x-ray absorption spectroscopy (CEY-XAS) [2]. But we still lack detailed information on preferable electronic structure of oxide-base ORR catalyst. Since carbon is deposited at catalysts surface during partial oxidation preparation processes, conventional laboratory XPS does not provide effective information, especially at valence band (V.B.) region. Thus, we conducted highly brilliant soft X-ray synchrotron radiation (SR)-XPS measurements. First-principles calculation was adopted to interpret the partial density of state (PDOS) of V. B.
ZrO2 based catalyst samples were prepared under O2 and H2/N2 atmosphere from Zr- phthalocyanine. The SR-XPS measurements were performed on BL-18A beam line at Photon Factory (PF) of High Energy Accelerator Research Organization (KEK). PDOS calculation was carried out by using the plane-wave basis projected augmented wave (PAW) method (VASP) [3].
Figure 1 (a) shows the soft x-ray SR-XPS spectra of ZrO2 reference. Each spectrum was obtained after subtracting the spectrum measured at 31 eV X-ray energy. We can see that peak intensity at 4 to 8 eV regions were increased as incident energy increased, suggesting that these peaks originate from ZrO2. As shown in Fig.1 (b), the SR-XPS spectra of ZrO2 catalyst also show similar trend. This indicates that we can truly observe V.B of catalysts even if a part of surface is covered with carbon.
Figure 2 shows the PDOS spectra calculated ZrO2 with oxygen vacancy. The Peak near 4eV can be originated from O 2p. This result suggest reduction of SR-XPS spectral intensity for catalysts compared with reference ZrO2comes from oxygen vacancy in the catalysts, and such O-vacancy should play a role in emergence of ORR activity. Detailed electronic structure consideration will be presented at Meeting.

Acknowledgement:

A part of this work was performed under the “Non-precious metal oxide-based cathode for PEFC Project” supported by the New Energy and Industrial Technology Development Organization (NEDO). SR-XPS measurements were performed with an approval of High Energy Accelerator Research Organization (Proposal No. 2013P102) .

References:

[1] A. Ishihara, Y. Shibata, S. Mitsushima, K. Ota, J.Electrochem. Soc., 155, B400-B406 (2008).
[2] H. Imai, M. Matsumoto, T. Miyazaki, S. Fujieda, A. Ishihara, M. Tamura, and K. Ota, Appl. Phys. Lett. 96, 191905 (2010).
[3] G. Kresse and J. Furthmuller, Phys. Rev. B, 54, 11169 (1996).