Pd@PtML Nanoparticles on Carbon-Supported WNi Substrates for the Oxygen Reduction Reaction
Authors: Song Liang , Nagamori Kiyotaka , Igarashi Hiroshi , Vukmirovic Miomir B , Sasaki Kotaro , Adzic Radoslav R. 
Published: February 2020

Several problems such as the high cost of Pt-based catalysts and their sluggish kinetics of the oxygen reduction reaction (ORR) are still hindering the broad application of proton exchange membrane fuel cells (PEMFC) [1,2]. The loss of activity of the catalysts is also a major issue. Most of the transition metals in Pt-based catalysts slowly dissolve in a strong acidic solution environment under high operating potentials, and their catalytic activity and efficiency are reduced [3]. Meanwhile corrosion of carbon causes a decrease in electrochemical surface area (ECSA) [4, 5], resulting in degradation of catalytic activity towards the ORR.
In this work, we synthesized a platinum-monolayer (PtML) shell on palladium (Pd) core nanocatalyst supported by tungsten-nickel (WNi) deposited on Vulcan XC72R carbon black (Pd@PtML/WNi/C), which shows outstanding activity and durability toward the ORR. W tends to segregate to the surface of WNi substrate during a thermal treatment process in H2, which was then oxidized into tungsten trioxide (WO3) thin layer when the WNi/C powder was collected and transferred in air. This thin WO3 layer may protect the WNi and carbon substrates and is conductive in an acid solution [6, 7], thereby providing a suitable surface for further deposition of Pd nanoparticles (10 – 15 nm). The Pd nanoparticles were deposited on the WNi substrate by purging a CO gas in an ethanol solution containing PdCl2 and the WNi/C power. Electrochemical measurements were carried out after a PtML shell was placed onto the Pd nanoparticles through galvanic displacement of an underpotentially deposited Cu monolayer [8].
The STEM image and EDS mapping of each element in the Pd@PtML/WNi/C nanoparticles are shown in Fig. 1. W and Ni atoms were spread uniformly on the carbon support while Pt monolayer deposited by Cu-UPD method covered the Pd nanoparticle surfaces. The rotation disk electrode technique was used to examine the ORR activity of the Pd@PtML/WNi/C catalyst (Fig.2). The Pt and mass activity of the Pd@PtML/WNi/C catalyst were calculated to be 1.95 A/mgPt and 0.21 A/mgPGM, which are much higher than that of the commercial TKK 46.6% Pt/C catalyst (not shown). No discernible decrease in Pt mass activity and half-wave potentials were noticed after cycling for 5,000 and 30,000 times, indicating the superb durability of the Pd@PtML/WNi/C catalyst compared with that of the TKK Pt/C. More detailed results on RDE and MEA tests of the Pd@PtML/WNi/C catalyst will be discussed at the meeting.
Acknowledgment
This manuscript has been authored by employees/guest of Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. This work was also performed in collaboration with N.E. Chemcat Corporation under contract no. NF-17-33.

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Figure 1