Carbon-supported platinum group metals (PGMs) are used as oxygen reduction reaction (ORR) electrocatalysts at cathodes in polymer electrolyte fuel cells (PEFCs). Since platinum reserves are limited, Pt-Ni alloy electrocatalysts for the ORR have been intensively studied. One of the highly active Pt-Ni alloy electrocatalysts is a carbon-supported Pt-Ni alloy nanoframe[1]. This electrocatalyst exhibits much higher catalytic ORR activity than commercial Pt/C catalysts and almost no activity loss after 10,000 potential cycles. In the practical point of view, further improvement of the durability and reactivity is required. The use of nitrogen-doped carbon as the catalyst support might improve the catalytic activity as well as durability because it is expected that nitrogen doping in carbon supports may modulate the electronic structure of the catalyst and/or give strong chemical bonding between the support and the catalyst[2].

Herein, we report electrocatlytic activity and durability of Pt-Ni alloy nanoframes on nitrogen-doped carbon supports to understand effects of the nitrogen content on the catalytic activity and durability. We used carbon black (Vulcan), N-doped carbon and graphitic carbon nitride (g-C₃N₄) were used as catalyst supports. Pt-Ni alloy nanoframes [1] and N-doped carbon supports were prepared according to the literature[3]. For the preparation of g-C₃N₄, melamine was heated at 600 °C under air for 2 hours. Linear sweep voltammetry of Pt-Ni alloy nanoframes on supports was carried out in 0.1 M HClO₄ using a three electrode setup under oxygen.

TEM observations of Pt-Ni alloy nanoframes on supports suggested that the density of Pt-Ni nanoframes in a unit volume clearly increased with increase in the nitrogen content in the supports. Although the ORR activity of Pt-Ni nanoframes on N-doped carbon supports also depended on the nitrogen content, the optimum value of nitrogen content was 9%. The electronic interaction between N-species and Pt-Ni nanoframes will be the key to understand the activation mechanism.

This work was supported by NEDO.

[1] C. Chen et al., Science 343, 1339-1343 (2014); N. Becknell et al., J. Am. Chem. Soc. 137, 15817-15824 (2015).

[2] Y. Zhou et al., Energy Environ. Sci. 3, 1437-1446 (2010).

[3] Y. Zhang et al., Sci. Rep. 3, 2771. (2013).