Pdpt Alloy Nanocubes As Electrocatalysts for Oxygen Reduction Reaction in Acid Media

Platinum-based catalysts are the best electrocatalysts for oxygen reduction reaction (ORR) in fuel cells.¹ However, because of the high price and limited supply of Pt, numerous researches have focused on finding a way to reduce its amount in the catalysts. One way to decrease Pt loading is to replace it partially by other metals. Pd is a promising substitute due to the similar properties to the Pt (same group of the periodic table, similar atomic size and crystalline structure).²

In this work, PdPt alloy nanocubes with different metal ratios were synthesized in the presence of polyvinylpyrrolidone.³ The surface morphology of the PdPt samples was characterized by transmission electron microscopy (TEM). TEM images showed that PdPt nanoparticles were cubic-shaped and the average size of the cubes was about 8–10 nm. TEM image for sample Pd₅₄Pt₄₆ is shown in Fig. 1a.

Electrochemical measurements were performed to explore the electrocatalytic properties of the PdPt alloy nanocubes toward the ORR in 0.5 M H₂SO₄ using the rotating disk electrode (RDE) method. CO stripping and cyclic voltammetry (CV) were used for cleaning and characterizing the surface of catalysts. The CV responses showed improvement in surface cleanness after CO oxidation compared with those CVs initially measured. 

Comparison of polarization curves received from RDE measurements at 1900 rpm are presented in Fig. 1b. The half-wave potentials (E_1/2) for O₂ reduction on PdPt alloys were found to be higher than that of Pd nanocubes and were comparable with that of Pt nanocubes. The onset potential for Pd₃₆Pt₆₄ was the highest. The RDE data were analyzed using the Koutecky-Levich (K-L) equation and from the slopes the number of transferred electrons (n) was found. The value of n was close to four for all the catalysts studied.

For better comparison of the kinetic parameters of all the electrodes the Tafel plots were constructed from the RDE data (Fig. 1c). Two regions with distinct slope values were observed. In the low overpotential region the slope value was close to -60 mV dec^‒1 and at high overpotentials the Tafel slope value was over -130 mV dec^‒1. All the alloyed catalysts showed enhanced electrocatalytic activity for ORR as compared to the monometallic cubic Pd nanoparticles. From the alloyed catalysts Pd₃₆Pt₆₄ exhibited the highest specific activity, which was only slightly lower than that of cubic Pt nanoparticles.

Work is in progress to study the electroreduction of oxygen on PdPt alloy nanocubes in 0.1 M KOH solution using the RDE method. Single-waved polarization curves with well-defined current plateaus were observed. The E_1/2 values for PdPt alloys in alkaline solution were comparable. The K-L analysis showed that O₂ reduction on these alloys proceeds via 4-electron pathway in 0.1 M KOH. The Tafel analysis revealed that the mechanism of O₂ reduction on PdPt alloy nanocubes in alkaline media is similar to that in acid media.

The PdPt alloy nanocubes exhibit good electrocatalytic activity for the electroreduction of oxygen in both electrolytes. This indicates a great promise of these materials as cathode catalysts for low-temperature fuel cells.

References

1. H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner, Appl. Catal. B: Environ., 56, 9 (2005).

2. E. Antolini, Energy Environ. Sci., 2, 915 (2009).

3. K. Jukk, N. Kongi, K. Tammeveski, J. Solla-Gullón, and J. M. Feliu, Electrochem. Commun., 56, 11 (2015).