One-Pot Preparation of Pt Nanoparticle-Supported Graphene Nanoplatelets By Ionic Liquid-Pyrolysis Method
Authors: Xiao Qingning , Yao Yu , Tsuda Tetsuya , Kuwabata Susumu 
Published: December 2020

sp2-hybridized carbon materials, such as carbon nanotube and graphene nanoplatelet (GNP), have attracted a great deal of attention for their unique physicochemical properties. The carbon materials emerged as the promising candidates to replace carbon blacks as polymer membrane electrolyte fuel cell (PEMFC) catalyst supports, because those can prevent carbon oxidation and Pt nanoparticle catalyst agglomeration during start and stop cycling of the PEMFC.1 However, the problem is how to anchor Pt nanoparticles over the smooth chemical inert surface of these carbon materials. It has a big risk leading to a decrease in the desired stability of GNP by the commonly used method of grafting functional groups onto the destroyed honeycomb structure.2 Recently, based on the ionic liquid (IL)-based two-step method,3 we established a simple and mass-production available one-pot pyrolysis method with ionic liquid (IL one-pot process) for preparing Pt metal and PtNi alloy nanoparticle-supported multi-walled carbon nanotube composite electrocatalysts for oxygen reduction reaction (ORR).4 In the present study, this method was applied to two kinds of GNPs with different thicknesses, 20~30 layers (GNP-20) and 3 layers (GNP-3). The impacts of IL and GNP species on the catalytic performance to ORR were examined in detail.

Two types of ILs, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide ([N1,1,1,3][Tf2N]) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][Tf2N]) were used as reaction media. These ILs were purified by an appropriate pretreatment process prior to use. 2.5 g L-1 of platinum(II) acetylacetonate was mixed with 1.25 g L-1 GNP-20 in the ILs. The Pt nanoparticle supported GNP-20 composite materials (Pt/GNP-20s) were prepared by agitating the IL mixtures at 573 K for 4 hrs under N2 atmosphere. Similarly, Pt/GNP-3s were prepared using GNP-3 instead of GNP-20. The resultant composites were washed with acetonitrile and water several times and dried in vacuum overnight. The final products were characterized by TEM, ICP, and XRD. Their electrocatalytic performances were evaluated by electrochemical measurements.3,4

Figure 1 shows TEM images of Pt nanoparticle supported GNPs prepared by the IL one-pot process method. Compared to the specimens prepared in [N1,1,1,3][Tf2N], those in [C4mim][Tf2N] have smaller Pt nanoparticle size and show better Pt nanoparticle dispersion, if carbon supports are the same. These results seem to be related to the molecular structures of organic cations in reaction media. [C4mim][Tf2N] contains a larger cation with an aromatic ring, which can form a π-π stacking structure on the GNPs,5 would confine the nucleation points for Pt nanoparticle deposition at the anchor points. Difference in the GNP species affected Pt nanoparticle size on the Pt/GNP-20s and Pt/GNP-3s. For example, when [N1,1,1,3] [Tf2N] was used, mean particle sizes for the Pt nanoparticle on GNP-20 and GNP-3 were 3.8 nm and 3.3 nm, respectively. This result is attributed to a difference in specific surface area, i.e., Pt nanoparticle growth is suppressed on the GNP-3 having a larger surface area and more nucleation points. As given in Table I, the similar tendency was also recognized in [C4mim][Tf2N].

Electrocatalytic performances for the obtained Pt/GNP-20s and Pt/GNP-3s were evaluated by commonly-used electrochemical approaches in N2 and O2 saturated HClO4 solutions along with a commercial catalyst (TEC10V30E). All the electrochemical data are summarized in Table I. The Pt/GNP-3 prepared in [C4mim][Tf2N] shows the highest mass activity (439.34 A g-1) among all Pt nanoparticle supported GNPs because of the smallest mean particle size. Considering oxygen accessibility, Pt nanoparticle density per unit area of the basal plane on GNPs may also affect the electrocatalytic performances. Interestingly, after 15000 cycle durability test, all the Pt/GNPs showed higher mass activity retention rates than TEC10V30E. This should be due to the chemically stable surface structure at the basal plane on the GNP.

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