Innovative Approach for Preparing a CNT-Supported Pt Nanoparticle Functional Electrocatalyst Using Protic Ionic Liquids

Due to their high conductivity and high physicochemical stability, carbon nanotubes (CNTs) have received a great deal of attention as a promising support material for Pt-based electrode catalysts for redox reactions (ORRs). However, to immobilize Pt nanoparticles (Pt NPs) on their inert surfaces, several CNT pretreatments, including the chemical generation of functional groups and polymer modifications, have been attempted. In this study, we propose a straightforward preparation method for Pt NPs supported on single- and multi-walled carbon nanotubes (SWCNTs and MWCNTs) at room temperature. The preparation method includes only two steps: the magnetron sputtering of Pt onto diethylmethylammonium-based protic ionic liquid (IL) and the mixing of the resultant Pt NP-dispersed protic IL with pristine CNTs. Zeta potential measurements reveal that the spontaneous immobilization of the Pt NPs on the CNT surface during the mixing is facilitated by electrostatic interactions between the Pt NPs negatively charged by anion adsorption and the CNTs positively charged by cation adsorption. The mass activity for the ORR of the Pt NP-modified SWCNTs (Pt-SWCNTs) and MWCNTs (Pt-MWCNTs) prepared using diethylmethylammonium trifluoromethanesulfonate as a medium is approximately 2.5 times higher than that of a commercially available electrocatalyst. This high performance is attributable to the small size (ca. 1.9 nm) of Pt NPs with a narrow size distribution and high dispersity on CNTs. In the case of Pt-SWCNTs, surprisingly, the ORR activity is slightly enhanced after 20,000 cycles of an accelerated degradation test because of an unexpected Pt NP shape change from spherical to nanorod-like along the grooves formed at the contacts of the CNTs in the SWCNT bundle. This shape variation and the improvement in catalytic activity will lead to the development of innovative strategies for maintaining electrocatalytic activity over a long period.