Surface Doping Treatment of Octahedral Ptni Alloy Nanoparticle Catalysts for Durable Fuel Cell Cathodes

Abstract

Proton exchange membrane fuel cells (PEMFCs) convert chemical energy into electricity and uses hydrogen and oxygen as fuel. The sluggish kinetics of the oxygen reduction reaction (ORR) and harsh environment pose challenges on the development of cheap, active and stable catalysts. Shape-controlled bi- or tri-metallic Pt alloy nanoparticles, i.e. octahedral PtNi nanoparticles, have been reported to achieve extremely high mass activity in rotating disk electrode (RDE) experiments and for this reason are considered as a highly promising class of ORR catalysts.1 Despite the high mass activity, the low electrochemical surface area (ECSA), often below 50 m2/g, is considered not optimal for low-loaded PGM cathodes and possible to hinder their performance at higher current density in membrane electrode assemblies (MEA).2 A second challenge is their morphological stability under electrochemical testing. Ni loss and other degradation processes often result in shape changes into more “rounded” or concave structures with loss of the (111) facet and lower activity (Figure 1).3 Introduction of a third metal such as Co, Rh and Mo have been shown to have beneficial effects and is considered a promising step toward shape-stable PtNiX octahedral nanoparticles. In particular, an exceptionally high initial mass activity has been reported for Mo surface doped PtNi catalyst, which also showed good stability in an accelerated test in RDE.4 In this contribution we will show our recent efforts in improving the activity and especially the durability and morphological stability of PtNi based octahedral nanoparticle catalysts. A surface treatment involving the introduction of Mo is applied to octahedral PtNi/C. Activity and stability are evaluated from RDE measurements following a protocol developed inside the INSPIRE project.5 Degradation is monitored by in operando wide angle X-ray scattering (WAXS) performed at European Synchrotron Radiation Facility (ESRF). By this techniques is possible to follow the change in lattice parameter a during different stability tests. The results are correlated to Ni leaching from the crystalline alloy phase. P. Strasser, Science, 2015, 349, 379-380. A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 2016, 7, 1127-1137. L. Gan, C. H. Cui, M. Heggen, F. Dionigi, S. Rudi and P. Strasser, Science, 2014, 346, 1502-1506. X. Q. Huang, Z. P. Zhao, L. Cao, Y. Chen, E. B. Zhu, Z. Y. Lin, M. F. Li, A. M. Yan, A. Zettl, Y. M. Wang, X. F. Duan, T. Mueller and Y. Huang, Science, 2015, 348, 1230-1234. S. Martens, L. Asen, G. Ercolano, F. Dionigi, C. Zalitis, A. Hawkins, A. Martinez Bonastre, L. Seidl, A. C. Knoll, J. Sharman, P. Strasser, D. Jones and O. Schneider, J Power Sources, 2018, submitted. The project leading to this application has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 700127. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme and Hydrogen Europe and N.ERGHY. Figure 1. Life cycle of an octahedral PtNi nanoparticle showing a degradation pathway to loss of octahedral shape.3 Figure 1