Structural and Compositional Tuning of Platinum Alloy Nanowires for Enhanced Oxygen Reduction Reaction

Polymer electrolyte fuel cells (PEFCs) have gained considerable attention due to their high energy density, high conversion efficiency, and wide operating temperatures. The main challenges encountered in the commercialization of PEFCs are the sluggish kinetics and low durability of the cathodic oxygen reduction reaction (ORR) catalyst. The conventional electrocatalyst, platinum (Pt) nanoparticles (2~5nm) dispersed on carbon support, suffer insufficient catalytic activity and low durability owing to enlargement in particle size due to particle coalescence or Ostwald ripening during PEFC operations. In this scenario, the facile fabrication of advanced functional catalysts with improving ORR activity and stability is significant for PEFCs. Among the various catalysts reported so far in the literature, Pt is an effective metal for catalyzing the ORR reaction due to its intrinsic catalytic performance and stability, however, it's scarcity and high cost limit its large-scale practical application. Compared to nanoparticle counterparts, the anisotropic one-dimensional nanostructure has been explored for higher activity due to its high flexibility, outstanding conductivity, and thermal stability.1–3 Several strategies have been adopted in the literature to enhance the catalytic activity and the Pt utilization efficiency. One aspect is the atomic scale tuning of the Pt chemical environment through proper alloying with bimetallic or trimetallic elements. In addition, it is essential to construct favorable structures in alloy systems with maximum exposed active sites. In this work, we synthesized a unique nanostructure of Pt alloy nanowires (PtNiCo NW) through a solvothermal method by reducing the Pt, Ni, and Co precursors in a solvent mixture with reducing agents and surfactants (oleyl amine with W(CO)6, glucose and CTAB). The atomic scale tailoring of the Pt chemical environment of the catalyst was achieved by varying the alloy composition. The TEM observation indicates that structural changes occurred by changing the reaction conditions of the catalysts; initially, smooth nanowires are formed with a diameter of 2 nm (Fig. 1a) followed by the formation of the rough surface due to an increase in the reduction of Ni over the Pt NWs over time. A porous nanostructure of Pt with Ni (Fig 1b) was fabricated through an acid treatment process. The electrochemical surface area and ORR activities of the PtNiCo NWs on carbon support were estimated from rotating disk electrode measurements in 0.1 M HClO4 electrolyte solution. Compared with the smooth PtNiCo NW catalyst, the porous PtNiCo NW catalyst with a rough surface exhibited a higher mass activity of 1.6 ± 0.1 Amg-Pt−1 and an excellent specific activity of 3.7 ± 0.2 mAcm-Pt−2. The higher specific activity would arise from the increased roughness on the Pt surface through the formation of porous structure and the optimized surface-active sites on the catalyst surface through composition tuning. Acknowledgment This presentation is based on the results obtained from the project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References M. Li et al, Science, 2016, 354 (6318), 1414–1419. X. Tian et al, Science, 2019, 336 (6467), 850–856. K. Jiang et al, Sci. Adv., 2017, 3(2), e160170. Fig 1. TEM images of Pt-based nanowires having (a) smooth and (b) rough-porous structures. Figure 1