ORR Activity for Pt-Ni Nanoparticles on HOPG Prepared by Arc Plasma Deposition

Introduction

  Pt-M alloy nanoparticles (NPs) (M = Fe, Ni, Co, etc.) have been widely studied for practical applications as highly-active oxygen reduction reaction (ORR) catalysts in polymer electrolyte fuel cells. Pt-M NPs are generally synthesized through chemical reduction of the related metal complex in liquid-phase and used on carbon black (carbon-supported Pt-M). Recently, to improve electrochemical stability [1], carbon-support-less catalysts, e.g., nanostructured thin film (NSTF) of Pt-M, are proposed. In this study, we have synthesized Pt-Ni NPs on highly oriented pyrolytic graphite (HOPG) substrate (APD-Pt-Ni/HOPG) by using arc plasma deposition (APD) and evaluated the ORR activity.

Experimental

  HOPG substrate was scraped by Scotch tape in air and cleaned by annealing at 773 K for 30 min in ultra-high vacuum (UHV; ~10^－8). Pt-Ni NPs were synthesized by APD on the HOPG at 773K. Pt and Ni were synchronously deposited from respective arc plasma sources (ALVAC ARL-300). The deposited mass of Pt and Ni were estimated by quartz-crystal-microbalance installed in APD chamber. To control Pt-Ni NPs alloy compositions, arc voltages of Pt and Ni are changed independently from 100 to 180V. The Pt-Ni NPs compositions were evaluated by XPS and their microscopic structures were observed by HAADF-STEM. The Pt-Ni/HOPG samples were transferred from UHV to electrochemical (EC) setup without exposure to air: cyclic voltammetry (CV) was conducted in N₂-purged 0.1M HClO₄. After that, linear sweep voltammetry (LSV) was applied by using rotational disc electrode (RDE) method. The ORR activity was evaluated from j_kvalues at 0.9 V estimated from Koutecky-Levich Plots.

Results&Discussions

  By controlling Pt and Ni arc voltages, we synthesized Pt-Ni NPs with Pt:Ni compositions of 82:18, 47:53 and 17:83. Then, electrochemical dealloying [2] of the APD synthesized three samples was conducted in N₂-purged 0.1M HClO₄ by applying potential cycles between 0.05 and 1.0 V (sweep rates: 500mV/s). Fig.1(a) shows CV curves for the APD-Pt-Ni/HOPG (Pt_xNi_y) _­after electrochemical dealloying. Although dealloyed-Pt₈₂Ni₁₈ and Pt₄₇Ni₅₃ show lower- and higher-potential-shifts of hydrogen- and hydroxyl-related regions relative to APD-Pt/HOPG (Pt₁₀₀), the Pt₁₇Ni₈₃ exhibits marked extension in both regions. Fig.1(b) summarizes ORR activities: the dealloyed Pt₁₇Ni₈₃ shows highest ORR activity among the evaluated samples: its mass activity is 10 times higher than Pt/C catalyst [3]. A HAADF-STEM image of dealloyed-Pt₁₇Ni₈₃was shown in Fig.1(c). The image clearly shows that 2 to 3 nm nanoparticles are piled up on the substrate with thickness of ca. 20 nm. Alloy composition of the dealloyed Pt₁₇Ni₈₃ is estimated to be Pt:Ni = 73:17 by EDX. These results suggest that the nanoparticle-layered ultrathin film of Pt-Ni could be one of the highly-active and cost-effective ORR catalysts.

Acknowledgement

  This study was supported by NEDO of Japan.         

References

[1] D.F. van der Vliet, C. Wang, D. Tripkovic, D. Strmcnik, X.F. Zhang, M.K. Debe, R.T. Atanasoski, N.M. Markovic, V.R. Stamenkovic, Nat. Mater., 11 (2012) 1051-1058.

[2] S. Koh, P. Strasser, J. Am. Chem. Soc., 129 (2007) 12624-12625.

[3] M.K. Debe, Nature, 486 (2012) 43-51.