Adjusting the Alloying Degree of Pt₃Zn to Improve Acid Oxygen Reduction Activity and Stability

Abstract

Proton exchange membrane fuel cell (PEMFC) is a new type of energy device, a relatively excellent way to achieve carbon neutrality. However, due to the relatively slow reaction rate of oxygen reduction reaction (ORR) at the cathode, platinum (Pt) is the key material of the cathode catalyst. However, Pt is a kind of noble metal, and its high cost restricts the PEMFC commercialization process. At present, the main approach is to combine transition metals with Pt to prepare Pt-based alloys and to reduce the use of Pt. Pt-based alloys are excellent catalysts for ORR, improving both the activity and stability, and increasing the Pt utilization rate and the number of active sites of the catalyst. In this paper, by employing nitrogen-doped carbon material derived from a metal organic framework as a support, Pt/NC, Pt₃Zn/NC-L and Pt₃Zn/NC-H catalysts were successfully synthesized by impregnation, freeze-drying and simple heat treatment. The particle sizes were around 2 nm and uniformly supported on the carbon. Raman data shows that the defect degree was slightly reduced after loading metal, mainly because the nanoparticles would be anchored in the defect position, and the metal would help the graphitization of carbon at high temperature. The introduction of Zn into Pt caused the Pt lattice to be shrunk, which shortens the Pt-Pt bond length, and optimizes the combination of Pt and oxygen-containing intermediates, ultimately enhances the ORR activity. On the highly alloyed Pt₃Zn/NC-H catalyst, the half-wave potential was 0.903 V, which is a positive shift of 57 mV compared with commercial Pt/C, moreover, the mass activity and area specific activity at 0.9 V were 4.50 times and 3.33 times to those of commercial Pt/C, respectively. The 10000-cycle stability test was carried out in an O₂-saturated 0.1 mol·L^-1 HClO₄ solution at 0.6 ~ 1.0 V (vs. RHE). The mass activity and area specific activity of commercial Pt/C decreased by 25.00% and 23.80%, respectively, while Pt₃Zn/NC-H catalyst revealed excellent stability. Transmission electron microscopic (TEM) observation shows that after the stability test, the nanoparticles were well dispersed on the nitrogen-doped carbon support, however, the commercial Pt/C became a slight agglomeration. Small particle sized Pt-based catalysts, constructed with a metal-organic framework-derived nitrogen-doped carbon material as a carrier, can improve the electrocatalytic activity and stability toward ORR, providing new ideas for the design and construction of Pt-based oxygen reduction catalysts.

Keywords

metal organic framework / NC carrier / electrocatalyst / Pt₃Zn alloy / oxygen reduction reaction

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