In our previous studis, we have developed the ways to synthesize Pt nanoparticles by magnetron sputtering onto ionic liquid (IL) and to adsorb them on carbon materials by mixing the Pt-dispersed IL and the carbon powder on heating [1-3]. Furthermore, it was found that the prepared carbon-supported Pt nanoparticles possess distinct electrocatalytic behavior toward the oxygen reduction with the four-electron reaction. Furthermore, its durability is a little better than the commertially available carbon-supported Pt catalyst [4,5]. Several analyses of the prepared catalyst revealed that IL used certainly existed between Pt nanoparticles and the surface of the carbon supporter as an adhesive agent. It was then speculated that the prevention of direct touch between Pt and carbon suppress the oxidation of carbon catalyzed by Pt, resulting in the better durability. In order to know such the mechanism in detail, we began the present study using the protonic ionic liquids (PILs).

PILs have been investigated as electrolytes in which free protons are available as ionic carriers [6]. Watanabe et al. have reported that calcination of PILs result in graphitization of the liquids with high yeild [7-8]. This fascinate feature let us use PILs as the adhesive agents for preparation of the Pt nanoparticle-carbon material. Two PILs were employed in this study; [DPA][HSO₄] and [DEMA][HSO₄] were used in this study. Since [DPA][HSO₄], which is graphitizaed with very high yeild, is solid at room temperature, this cannot be used as a medium for preparation of Pt nanoparticles by sputtering. Therefore, we first prepared Pt nanoparticle-dispersed [DEMA][HSO₄] and some amount of [DPA][HSO₄] and carbon powder (Vulcan) were added before heating at 473 K.

Fig. 1 shows the transmission electron microscopy (TEM) images of [DPA][HSO₄]/[DEMA][HSO₄]-Pt/carbon catalyt, showing that Pt nanoparticles are homogeneously adsorbed on the carbon surfaces. The amount of Pt loading was determined as ca. 18–23% by inductively coupled plasma mass spectrometry (ICP-MS). In Fig. 2, changes in the electrochemical srface area (ECSA) as a function of cycle test number are given for the [DPA][HSO₄]/[DEMA][HSO₄]-Pt/carbon and other catalysts. While the commercially available TEC10V30E had a higher initial ECSA value of 51.3 m²g^-1, it decreased rapidly to 15.5 m²g^-1 (30.2%) after 15000 cycles. In contrast, for the three catalysts prepared by using ILs, their ESCA increased during the initial several thousand cycles and then decreased very slowly. Among them the [DPA][HSO₄]/[DEMA][HSO₄]-Pt/carbon exhibited the highest durablity. The several spectroscopic analyses taken after the cycle tests revealed that polymerization of [DPA], allowing us to speculate that generation of conducting poly(diphenyl amine) during the cycle tests improves the durability of the [DPA][HSO₄]/[DEMA][HSO₄]-Pt/carbon against the cycle tests.

[1] T. Tsuda, T. Kurihara, Y. Hoshino, T. Kiyama, K. Okazaki, T. Torimoto and S. Kuwabata, Electrochem., 2009, 77, 693–695.

[2] T. Tsuda, K. Yoshii, T. Torimoto and S. Kuwabata, J. Power Sources, 2010, 195, 5980–5985.

[3] S. Kuwabata, T. Tsuda and T. Torimoto, J. Phys. Chem. Lett., 2010, 3177–3188.

[4] K. Yoshii, T. Tsuda, T. Arimura, A. Imanishi, T. Torimoto and S. Kuwabata, RSC Adv., 2012, 2, 8262–8264.

[5] K. Yoshii, K. Yamaji, T. Tsuda, R. Izumi, T. Torimoto, and S. Kuwabata, J. Mater. Chem. A, 2016, 4, 12152–12157.

[6] A. Noda, Md. Abu Bin H. Susan, S. Mitsushima, K. Hayamizu and M. Watanabe, J. Phys. Chem. B, 2003, 107, 4024–4033.

[7] S.-Y. Lee, A. Ogawa, M. Kanno, H. Nakamoto, T. Yasuda and M. Watanabe, J. Am. Chem. Soc., 2010, 132, 9764-9773.

[8] S. Zang, M. S. Miran, A. Ikoma, K. Dokko and M. Watanabe, J. Am. Chem. Soc., 2014, 136, 1690–1693.