Improving Electrochemical Activity of Rhombic Dodecahedral Pt₃(Ni,X) Nanoparticles Using Transition Metal Additions

Abstract

Large scale use of polymer electrolyte membrane fuel cell (PEMFC) technology remains in its infancy owing to high costs, performance and durability challenges. Novel platinum-based electrocatalysts have been developed to improve catalytic activity and overall durability in acidic environments. Of note is the synthesis of binary Pt3Ni in a rhombic dodecahedral shape with a Pt skin.[1] These nanoparticles demonstrate very high specific/mass activities and are comparably durable over 10,000 potential cycles. The objective of this work is to further improve on the Pt3Ni rhombic dodecahedral nanoframes using ternary additions Pt3(Ni,X), where X denotes an additional transition metal. Previous work[2] has been able to recreate the Pt3Ni rhombic dodecahedron under different synthesis conditions. These nanoparticles were used in combination with nitrogen-doped carbon support to examine the effects of surface nitrogen as anchoring sites. Examination using XPS, electrochemical CO stripping, and SEIRAS shows that the nitrogen sites alter the d-band center of surface platinum in addition to creating a more homogeneous distribution of the nanoparticles. The addition of a ternary element allows for additional fine tuning of the d-band center of platinum near the particle surface. This leads to an increase in overall activity over binary systems when examining electrocatalyst volcano plots. In addition, ternary elements could lead to a reduction in the rate of nickel leeching over repeated electrochemical cycles. DFT studies[3] examined the adsorption energy of ternary Pt3(M,X) where M is another transition metal different than X. The model arranged the ternary surfaces into intermetallic, Pt skin, and Pt skin with subsurface intermetallic layers. From this work, Rankin recommended the investigation of Pt skin ternary systems using Pd, Rh, Ru, Ir, and Os; as well as a Pt-Ni-Cu system with a subsurface intermetallic layer. Ternary Pt3(Ni,X) nanoparticle systems have been investigated previously but have not been directly synthesized into a rhombic dodecahedral shape. Thus, the scope of this work is to investigate Pt3(Ni,X) systems where X includes Cu, Pd, Rh, Ru, Ir, and Os. Using similar synthesis methods,[2] rhombic dodecahedral nanoparticles will be created with a Pt skin. Examination will follow using Scanning Tunneling Electron Microscopy (STEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), electrochemical CO stripping, and thin film rotating disk electrode (RDE) measurement. Successful candidates will be added with modified carbon supports (nitrogen doping, additions of polybenzimidazole (PBI), modifications of pore structure) with the goal of improving local mass transport at the support surface. Further annealing may also be investigated to create ordered intermetallic layers under the Pt skin which have been shown to improve activity. This work could lead to significant improvements into electrocatalyst activity and durability which are necessary to implement PEMFC technology onto a global scale. References [1] C. Chen et al., Science, 343, 1339-1343 (2014). [2] M.Kato et al., ACS Omega, 3, 8, 9052–9059 (2018). [3] R. Rankin et al., The Journal of Physical Chemistry C, 123, 21, 13236-13245 (2019).