Enhanced Oxygen Reduction Performance on PtNiN/C Catalysts
Authors: Song Neal , Chen Jingyi , Wang Jia X. 
Published: December 2020

The high cost of Pt-based catalysts and their sluggish kinetics for the oxygen reduction (ORR) are still problems hindering the wide application of proton exchange membrane fuel cells (PEMFC).1,2 Lots of efforts in developing low-PGM catalysts with high oxygen reduction activity and durability have been made in recent decades. Ni is one of the mostly studied and best alloying elements with Pt for promoting ORR performances in acidic media,3,4 which can induce favorable compressive strains from its slightly smaller atomic size and significantly enhance the activity of Pt and PGMs due to its similar electronic configurations.5-7 Nitriding PtNi through post-treatment with ammonia was found effective in improving the durability of the PtNi/C catalysts, for a Ni4N core encapsulated by a 2-4 monolayer-thick Pt shell was characterized in X-ray diffraction patterns (XRD) and electron energy loss spectroscopy (EELS) mapping.6

In this work, we reported a facile method for synthesis of PtNiN/C catalyst through annealing the mixture of Vulcan XC72R Carbon, Pt(acac)2 and Ni(acac)2 in ammonia gas flow at 500 °C for 2 h and then 300 °C for 1 h. For comparison, PtNi/C catalyst without nitriding was synthesized under pure hydrogen gas flow with the same condition. As is shown in Figure 1, the single diffraction peak locating between the Pt (111) and Ni4N (111) in the PtNiN/C catalyst indicates a uniform structure of Pt-monolayer-skin covering the PtNiN-core rather than separated Pt- and Ni-rich phases in the PtNi/C catalyst. This may result from the difference of the lattice parameters between Pt (3.924 Å) and Ni4N (3.745 Å) compared with Ni (3.524 Å), indicating the usefulness of introducing N atoms into the PtNi crystalline structures to significantly alleviate the lattice mismatch of the two metal atoms and the feasibility of utilizing Ni4N instead of Ni as a lattice contraction promoter.

Figure 2 shows the ORR polarization curves of the PtNi/C and PtNiN/C electrocatalysts measured at 10 mV s-1 in O2-saturated 0.1 M HClO4 solution with a rotation speed of 1600 rpm before and after 5,000 accelerated durability test (ADT) cycles. It can be noticed that the PtNiN/C showed much higher Pt mass activity than the PtNi/C electrocatalyst, which may result from the downshifted d-band center induced by the compressive strains from Ni4N on the Pt-skins leading to easier desorption of the -OH intermediates. Meanwhile, DFT calculations suggested that the presence of N atoms in the core facilitated the diffusion of Pt atoms to the defect sites at surface, which subsequently resulted in the enhancement of durability of the PtNiN/C demonstrating the effectiveness of introducing N atoms into PtNi lattices to form a stable crystalline structure.6 More detailed discussion on the structure and ORR performance of these catalysts will be presented at the meeting.

Acknowledgements

This research was supported by the U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy (EERE) and used instruments of the Center for Functional Nanomaterials at Brookhaven National Laboratory, both under contract DE-SC0012704.

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Figure 1