To develop highly active and durable cathode electro-catalysts for polymer electrolyte fuel cell (PEFCs), core-shell type nanoparticles (NPs) have been actively studied. It is well known that oxygen reduction reaction (ORR) properties of the core-shell NPs greatly depend on the core-materials (transition metal elements (M) and/or Pt-M alloys). Particularly, deactivation of ORR under the PEFC operating conditions (low pH, electrochemical potential fluctuations) through elution of M is serious issue. Therefore, developments of highly-stable core materials under the operating conditions have been needed for practical applications of the core-shell type NPs. Ham et al. reported that early transition metal carbides, such as TaC_x and HfC_x, show high thermal stability and high corrosion resistance under the PEFC operating condition [1]. In this study, we prepared Pt/TaC_x NPs by using arc-plasma deposition (APD) of TaC_x followed by electron-beam deposition of Pt in ultra-high vacuum (UHV; ~10^-8Pa) and investigated the ORR properties (initial activity and electrochemical stability) of the Pt/TaC_x NPs.

Experimental

Highly-oriented pyrolytic graphite (HOPG) was used as a substrate. First, HOPG was scraped by Scotch tape in air and cleaned by annealing for 30 min in UHV. TaC_x NPs are synthesized by using APD (Advanced RICO; APS-1) of Ta at substrate temperatures of 1173K or 1273K under CH₄ partial pressure of 0.05 Pa. Subsequently, Pt was deposited on the TaC_x NPs/HOPG at 873K by electron-beam deposition in UHV. As a reference, Pt/Ta NPs was fabricated through APD of Ta followed by the electron-beam deposition of Pt at 873K in UHV. The prepared Pt/TaC_x and Pt/Ta NPs samples were transferred from UHV to electrochemical (EC) setup without air exposure to avoid sample oxidation and contamination. Cyclic voltammetry (CV) was performed in N₂-purged 0.1 M HClO₄. After that, linear sweep voltammetry (LSV) for ORR was conducted by using rotating disc electrode (RDE) method in O₂-saturated 0.1 M HClO₄. ORR activity was evaluated as j_k values at 0.9 V vs. RHE: the j_k value was estimated from Koutecky-Levich plots and electrochemical surface area (ECSA) of Pt. The electrochemical stabilities of the samples were discussed based on change in the j_k values during applying square-wave potential cycles (PCs) between 0.6 (3 s) and 1.0 V (3 s) in O₂-saturated 0.1 M HClO₄ at a room temperature. Crystal structures, surface morphologies, and micro-structures of the prepared NPs samples were observed by XRD, scanning tunneling microscopy (STM), and scanning transmission electron microscope (STEM) combined with energy dispersive X-ray spectrometry (EDS), respectively.

Results and Discussion

STM images of the Pt/Ta and Pt/TaC_x (1173K & 1273K) NPs were shown in Fig. 1(a). The NPs with average diameter of ca. 6 nm are synthesized on the HOPG substrate, irrespective of the Ta deposition conditions (with or without CH₄ partial pressure during APD). Fig. 1(b) shows CVs for the corresponding NPs. Hydrogen (Q_H; 0.05 ~ 0.35 V) and OH-related (Q_OH; 0.6 ~ 1.0 V) charges for both the Pt/TaC_x NPs are higher than that for the Pt/Ta NPs. The evaluated specific ORR activities (j_k / ECSA) of the samples are summarized in Fig. 1(c). Filled and hatched bars are the activity before and after 10k PCs of each samples, respectively. One might notice that both Pt/TaC_x NPs exhibited much higher activity than the Pt/Ta NPs even after the 10k PCs. Especially, the Pt/TaC_x - 1173K shows highest ORR activity and durability among the samples: the retention rate after 10k PCs was ca. 85% of the initial activity. The results clearly reveal that, as for the core materials, tantalum carbide is effective for enhancing ORR activity as well as durability of the core-shell type Pt-based NPs.

Acknowledgement

This study was supported by the new energy and industrial technology development organization (NEDO) of Japan.

References

[1] D.J.Ham et al., Energies, 2(2009),873.

Fig. 1 (a)STM images (100 nm × 100 nm), (b)CVs of the Pt/TaC_x model catalysts prepared under CH₄ partial pressure of 0.05 Pa at substrate temperature of 1173K(orange) and 1273K(green). The Pt/Ta model catalyst (black) prepared in UHV at the substrate temperature of 873K is shown as a reference. (c) Specific ORR activity of the samples