Pt-Ni Nanoparticle-Supported Multiwalled Carbon Nanotube Prepared By One-Pot Pyrolysis Method in Room-Temperature Ionic Liquids
Authors: Yao Yu , Izumi Reiko , Sakamoto Tsuyoshi , Tsuda Tetsuya , Kuwabata Susumu 
Published: February 2020

Platinum nanoparticle-supported on carbon black (Pt/C) are commonly used as a standard electrode catalyst for low-temperature fuel cells due to their good catalytic activity to the oxygen reduction reaction (ORR). But still the electrode catalyst has several issues. For example, a large over potential exceeding ca. 300 mV is required to enhance the ORR rate, carbon corrosion of the catalyst proceeds during the operation, and Pt is one of expensive metals. Recent years Pt-based binary or ternary electrocatalysts with other metals such as Ru, Mo, and Cu are attracting attention because those can increase the catalytic activity and reduce the costs. Of these, there is increasing interest in the Pt-Ni alloys for their favorable catalytic behavior and lower cost.1 In addition to this, several research groups reported that room-temperature ionic liquid (RTIL) can increase the catalytic activity and the durability of the catalysts.2,3 In this research, based on our experience on the metal nanoparticle preparation in RTILs, we attempted to establish a novel preparation method for the Pt-Ni nanoparticle by a pyrolytic method of the metal ions in RTIL and an easy approach for supporting the resultant Pt-Ni onto multiwalled carbon nanotube with sp2structure leading to a higher carbon corrosion resistance.

N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide ([N1113][Tf2N]) was used as a reaction medium. A mixture of platinum acetylacetonate (Pt(acac)2) and Ni[Tf2N]2 was dissolved in the [N1113][Tf2N]. The concentration ratio of Pt(acac)2 to Ni[Tf2N]2 was controlled to be 1 : 1 and 3 : 1. Each IL solution was agitated at 300 °C for 1~24 hrs under N2 atmosphere. The characterization of the resulting Pt-Ni alloy nanoparticle was conducted by TEM. The composition of the nanoparticle was determined with an EDX and with an ICP. As to the fabrication of Pt-Ni nanoparticle-supported multiwalled carbon nanotube, before the pyrolytic reaction treatment, 2.5 mg of multiwalled carbon nanotube (MWCNT) was added to the RTIL solution with metal salts and dispersed by stirring overnight. After that, the solution was agitated at 300 °C for 4 or 6 hrs under N2 atmosphere and cooled down to room temperature. For comparison, Pt nanoparticle-supported multiwalled carbon nanotube was also prepared by a similar approach. The electrocatalytic activity was determined with a potentiostat/galvanostat. The detailed information was given in our previous article.4

Figure 1a and b shows TEM images of the RTIL solution after the heat treatment at 300 °C for 1 hour. Monodispersed nanoparticles in the RTIL were recognized. EDX analysis of the nanoparticle indicated that they have two component, i.e., Pt and Ni. The mean particle size was 1.47 nm. When Pt-Ni nanoparticle-supported MWCNT (PtNi-MWCNT) was prepared by a similar approach under several metal salt concentration conditions, the mean particle size was ca. 2.0 nm and the particle size was of little relevance to the ratio of Pt to Ni. As shown in Fig. 1c and d, most Pt-Ni nanoparticles are on the MWCNT uniformly, suggesting that PtNi-MWCNT can be easily fabricated by the one-pot pyrolytic process. Electrocatalytic activity of the PtNi-MWCNT for electrochemical ORR was investigated by using standard techniques. All the electrodes showed favorable catalytic abilities but the catalyst prepared in the solution with 5 mM Pt(acac)2 and 5 mM Ni[Tf2N]2(Sample 2) showed the best performance. The characteristics of the resulting PtNi-MWCNT are summarized in Table I along with the data on a commercially available catalysts (TEC30V10E). It is interesting to note that ECSA, mass activity, and Pt loading amount were improved by shorter agitation time, i.e., 4 hrs. At this moment, we do not have the idea to explain it, but we guess the nanoparticle produced under such condition has a different alloy phase or a surface composition. The most important thing is that our created PtNi-MWCNT produced by the one-pot pyrolytic process showed a better performance than the commercially available Pt catalyst. This approach will be one of the choices for creating next-generation electrocatalysts.

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Figure 1