(Invited) Enhanced Oxygen Reduction Reaction Activity on Pt-Monolayer-Shell Pdir/Ni-Core Catalyst
Authors: Song Liang , Vukmirovic Miomir B , Adzic Radoslav R. 
Published: February 2020

Proton exchange membrane fuel cell (PEMFC) has gained lots of attention as a most promising alternative to internal combustion engines for transportation due to its high energy generation efficiency and benign impacts on the environment. Despite its simplicity, viability and low operation temperatures, many technical difficulties and challenges are still hindering its widespread commercial application, among which the high cost of catalysts [1] and their low reaction rates toward oxygen reduction reaction (ORR) [2, 3] on the cathode side of the fuel cells are the most significant.
An effective way of reducing Pt content is through designing a core-shell-structured catalyst where core is made of less expensive but corrosion stable noble metals or alloys while monolayer thick shell is made of Pt. In addition, the catalytic properties (performance) of the Pt monolayer catalysts can be tuned by its interaction with substrate (core) [4].
Palladium, a Pt-group metal (PGM), and its alloy nanoparticles have been proved to be suitable substrates for Pt monolayer ORR catalysts because of their beneficial influence on Pt monolayer ORR activity, exemplified by PtMLPd/C whose ORR activity is higher compared to Pt/C(TKK 46.6%) as a result of decreased Pt-OH interaction [2,5]. Alloying Pd with Ir enhances the durability of the Pd core due to the high standard redox potential of Ir (Ir3+/Ir = 1.16V). In addition, the ORR activity of PtML on PdIr alloy nanoparticles increases due to significant decrease of the d-band center caused by contraction of Pt lattice imposed by the substrate [6]. Furthermore, nickel can be utilized as a subcore of PdIr nanoparticles to further lower the PGM loading for its similar electronic properties and capability of alloying with these two elements [6, 7].
We examined ORR activity of catalysts consist of Pt-monolayer shell PdIr- and PdIr/Ni-cores. IrPd nanoparticles were prepared by directly reducing Pd (PdCl2) and Ir ((NH4)2IrCl6) precursors (molar ratio=2:1, 1:2) from aqueous solution with NaBH4 as reductant and citric acid as a complexing agent. PdIr/Ni-subcore nanoparticles were synthesized by galvanic displacement of the surface atoms of the NaBH4-reduced Ni nanoparticles with Ir and Pd atoms to form PdIr ‘shells’. Both of the resulting nanoparticles were washed with de-ionized water, filtered, and dried in vacuum. Pt monolayer shell was deposited on the nanoparticles through galvanic displacement of a Cu monolayer, underpotentially deposited onto the core substrates, by Pt [4].
The diameters of nanoparticles are 2.2nm for PdIr (Fig. 1c and f) and 2.8-3.7nm for the PdIr/Ni (Fig. 1 a, b, d and e). The rotation disk electrode technique was used to examine the ORR activity of the PtML/PdIr and PtML/PdIr/Ni catalysts (Fig. 2). Their activities were compared to the ORR activities of Pt/C (46.6% TKK). The PtML/Pd2Ir/Ni catalysts showed better ORR activity than commercial Pt/C(46.6% TKK). The half-wave potentials and initial Pt mass activities of PrMLPd2Ir/Ni/C and PtMLPd2Ir/C catalysts are 889mV, 2.50 A/mgPt and 887mV, 1.86 A/mgPt respectively, which are much higher than those of PtMLPd/C (883mV, 0.25 A/mgPt). Moreover, after 5,000 cycles of the accelerated durability test, the PtMLPd2Ir/Ni/C catalyst remained a relatively high Pt mass activity of 2.15 A/mgPt much higher than that of TKK Pt/C which drops by 40% to 0.15 A/mgPt.

Acknowledgment

This manuscript has been authored by employees 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.

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Figure 1