Enhancing Alkaline Ethanol Oxidation on Ordered Intermetallic Pt3pb-Ptpb Core-Shell Nanoparticles Prepared by Converting Nanocrystalline Metals to Ordered Intermetallic Compounds
Authors: Jeevagan Arockiam John , Gunji Takao , Tanabe Toyokazu , Usui Sho , Tsuda Takashi , Kaneko Shingo , Saravanan Govindachetty , Abe Hideki , Tanaka Manabu , Matsumoto Futoshi 
Published: February 2020

1.
Introduction

Recently, interest in direct fuel cells (DFCs) has increased in the field of polymer electrolyte membrane (PEM) fuel cells in which small organic molecule (SOM) liquid fuels, such as methanol (MeOH), ethanol (EtOH), and formic acid (FA), were used as fuels.1 Ethanol is a particularly attractive fuel because it has a lower toxicity, it is biogenerated, and it does not release carbon that was previously sequestered underground as coal, petroleum, or natural gas into the atmosphere.2 However, the full potential of DFCs has not been realised due to the slow kinetics in the anode reactions.3 Literature reports have described much faster oxidation kinetics for organic fuels in alkaline media than in acidic media.4 One of the research and development challenges for alkaline-type polymer electrolyte fuel cells (PEFCs) using SOMs is the design of better alternatives to the Pt, Pd, and Pt-Ru alloys currently used as the anode catalysts.5 To improve the electrocatalytic activity, Pt- and Pd-based alloy electrocatalysts have been investigated in alkaline media.6 Although the aforementioned alloys are promising materials, problems are associated with the use of disordered alloys (and alloys in general) as catalysts for fuel cell applications, including the low utilisation of the surface for electrocatalysis, the surface segregation of metal atoms, and the partial poisoning by CO due to insufficient quantities of the bimetallic elements on the surface. A new approach that prevents the problems inherent in disordered alloy catalysts has been proposed for highly active electrocatalysts for fuel cell applications.7 In contrast with a disordered alloy, intermetallic compounds with definite compositions and structures, such as PtPb and PtBi, exhibit excellent electrocatalytic performance for oxidising FA in acidic solutions in their onset potential for oxidation and current density.8,9 Abruña and co-workers have examined the FA, MeOH and EtOH oxidation activities for a wide range of intermetallic compounds in acidic media and found a significant number of the compounds, such as PtPb, PtBi and Pt3Ti, exhibit enhanced catalytic activities compared with pure Pt.8, 9 Our previous study indicated that the PtPb and PtBi ordered intermetallic compounds exhibited higher electrocatalytic activity for MeOH and EtOH oxidation in alkaline aqueous solutions than other Pt, Pt-Ru alloy and Pt-based ordered intermetallic compounds.10 In this paper, to enhance the electrocatalytic activity of PtPb ordered intermetallic compounds toward MeOH and EtOH in alkaline aqueous solutions, carbon black (CB)-supported Pt3Pb-PtPb intermetallic compound core-shell NPs were prepared via a two-step synthesis in which Pt3Pb-PtPb intermetallic compound core-shell NPs were formed on the CB through the reaction of the CB-supported Pt NPs and a Pb precursor in the presence of a reducing agent under microwave irradiation (Fig.1). The possibility of developing high-performance DFCs using Pt3Pb-PtPb intermetallic compound core-shell NPs as anode catalysts for alkaline EtOH oxidation is demonstrated.

2
. Results and Discussion

We have successfully synthesised Pt3Pb-PtPb intermetallic compound core-shell NPs through a two-step synthesis in ethylene glycol under microwave irradiation. The pXRD, HX-PES and TEM/STEM characterisations demonstrated that pure PtPb NPs and Pt3Pb-PtPb intermetallic compound core-shell NPs could be prepared using one- and two-step syntheses, respectively, without requiring annealing. The Pt3Pb-PtPb intermetallic compound core-shell NPs exhibited higher catalytic activity for ethanol oxidation in alkaline aqueous solutions, enhanced tolerance for CO-poisoning and improved stability compared with commercial Pt NPs or pure PtPb NPs.

3
. References

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