Oxygen Reduction Reaction Activity and Durability for Pt/TaNx Model Catalysts Fabricated in Ultra-High-Vacuum
Authors: Takahashi Shuntaro , Takahashi Naoki , Todoroki Naoto , Tanabe T , Wadayama Toshimasa 
Published: February 2020

Introduction

The relatively low activity and durability of Pt nanoparticles (NPs) supported carbon (Pt/C) catalyst limit wide commercialization of polymer electrolyte fuel cell (PEFC). To address this problem, Pt-shell/transition-metal-core type NPs have been investigated. Toyoda and co-workers have been demonstrated superior ORR activity of the Pt layers formed on TaB2 single crystal substrate [1]. They explain the activity enhancement to be caused by the electronic effect of Ta on Pt. Because nitride of 4-6th group metals, e.g., Hf, Ta, and W are thermodynamically stable, Ta-nitride can be expected to be highly durable core-material under PEFC operating condition. However, no study has been conducted for Pt-Ta-N ternary alloy NPs for ORR catalysts. In this study, Pt-shell/Ta nitride-core NPs (Pt/TaNx) model catalyst were fabricated on a highly-oriented pyrolytic graphite substrate (HOPG) through Arc-plasma depositions of TaNx NPs followed by electro-beam deposition of Pt in ultra-high vacuum: ORR activity and durability of the Pt/TaNxNPs are discussed.

Experimental

HOPG substrate was scraped by Scotch tape in air and cleaned by annealing for 30 min in ultra-high vacuum (UHV). Ta and TaNx core are synthesized by arc plasma deposition on the HOPG substrate at 873K or 1173K in UHV (Ta NPs) and N2 partial pressure of 0.1 Pa (TaNx NPs). Subsequently, Pt was deposited on the Ta or TaNx NPs at 873K by using electron-beam deposition method in UHV. The deposition amounts of Ta and Pt were estimated to be 0.7 and 1.9 μg/cm2, respectively. UHV-fabricated Pt/Ta and Pt/TaNx NPs samples were transferred from UHV to electrochemical (EC) setup without exposure to air. ORR properties of the samples are discussed by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in 0.1 M HClO4. Durability of the samples discussed on the bases of mass activity that estimated based on ik
values at 0.9V during applying square potential cycles (PCs) between 0.6 (3 s) and 1.0 V (3 s) in oxygen-saturated 0.1 M HClO4 at room temperature. Surface morphologies of the sample alloy NPs were observed by scanning tunneling microscopy (STM).

Results & Discussions

STM images of the samples were shown in Fig. 1(a). Monodispersed NPs with average diameter of ca. 5nm are formed on the substrate irrespective of the core Ta and TaNx preparation conditions. Fig.1(b) shows CV curves for the corresponding samples. Oxygen species adsorption charge of the Pt/TaNx-1173K is larger than that of Pt/Ta-873K and Pt/TaNx-873K, suggesting that high-temperature nitridationaffect the electronic effect of the core Ta on Pt. Fig. 1(c) shows changes in mass activity of the samples under applying the PCs. The Pt/TaNx-1173K exhibits a 2.4-fold higher initial mass activity compared with that of the commercial Pt/C [2] and is the highest among the samples tested in this study. Furthermore, the Pt/TaNx-1173K shows high ORR durability: the values estimated after 10k PCs were about ca. 80% of the initial one. The results obtained in this study clearly reveal that nitridization of transition-metal-cores is effective for improving ORR activity and durability of Pt-shell/transition-metal-core type NPs catalysts.

Acknowledgement

This study was supported by the new energy and industrial technology development organization (NEDO) of Japan and Grant-in-Aid for challenging exploratory research from the Japan society for the promotion of science (T. W.).

References

[1] E. Toyoda, R. Jinnouchi, T. Ohsuna, T. Hatanaka, T. Aizawa, S. Otani, et al., Angew. Chem. Int. Ed. Engl., 52 (2013) 4137–4140.
[2] M.K. Debe, Nature, 486 (2012) 43–51.

Figure 1