Polymer electrolyte fuel cells (PEFCs) play a critical role in clean and sustainable energy conversion technology in the near future. However, the widespread application of PEFCs is currently limited by their high loadings of costly and rare platinum group metals (PGM) and insufficient long-term durability. In the last 20 years, Pt-M alloys (M = Cu, Co, Ni) have been emerged as one of the promising catalyst materials to accelerate the sluggish kinetics of oxygen reduction reaction (ORR).[1-2] The superior ORR activity originates from ligand effects and geometric effects (lattice mismatch).[3]

In our work, we present a novel catalyst concept to efficiently modify Pt_xCo_1-x (1<x<0) alloy nanoparticles for the ORR in acidic and alkaline media. The chemical composition and structure of the Pt-Co nanoparticles are controlled by variation of the synthetic parameters during the industrial relevant wet impregnation method.

Our catalyst concept is based on the surface rearrangement of the Pt-Co alloy nanoparticles to form core-shell or inverse core-shell structures by tuning the electrochemical parameters. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and CO chemisorption were used to understand the different stages of the surface rearrangement of Pt-Co alloy nanoparticles. In fact, 12 to 30 – fold increases in Pt surface area-based specific activity and 6 to 11 – fold higher Pt mass-based activities are achieved with our catalyst concept compared to commercial Pt/C. The simple switching between core-shell and inverse core-shell nanoparticles creates an efficient bifunctional catalyst to tune the reactivity and selectivity for the ORR in acidic and alkaline media. The observed activity improvement allows to reduce the Pt loading by a factor of at least 6, thus exceeding the DOE targets from ~0.4 to below 0.1 mg cm^-2 of PGM loading for ORR.[4]

Based on our results, we present a novel Pt-Co catalyst concept to boost the electroreduction of oxygen for alkaline and acidic PEFCs.

References:

[1] T. Toda, H. Igarashi and M. Watanabe, Journal of The Electrochemical Society, 1998, 145, 4185-4188.

[2] M. Oezaslan, F. Hasché and P. Strasser, The Journal of Physical Chemistry Letters, 2013, 4, 3273-3291.

[3] B. Hammer and J. K. Norskov, Advances in Catalysis, 2000, 45, 71-129.

[4] https://www.energy.gov/eere/fuelcells/downloads/fuel-cell-technologies-office-multi-year-research-development-and-22.