2D and 3D Characterization of PtNi Nanowire Electrode Composition and Structure

Extended surface Pt or Pt-M (M = Ni or Co) catalysts are a viable alternative to supported nanoparticle catalysts for the oxygen reduction reaction (ORR) at the cathode in proton exchange membrane fuel cells (PEMFCs). The activity and durability of these catalysts in membrane electrode assemblies (MEAs) are greatly dependent on the surface and bulk properties of the catalyst material, integration with ionomer, and the three-dimensional structure of the electrode, necessitating extensive characterization of the catalyst in the electrode at multiple scales. In this work, extended surface PtNi nanowire-based electrocatalysts derived by spontaneous galvanic displacement, and post-treated to obtain high specific and mass activities, are characterized by X-ray spectroscopies to assess catalyst composition and to determine the extent of Pt-Ni alloying. Transmission X-ray microscopy (TXM) is used to generate two- and three-dimensional images of catalyst layers with varied compositions within the electrode to show the distribution of nanowires and to study Ni dissolution and redeposition within the electrode. These techniques are complemented by electron microscopy, which is used to confirm leaching of nickel from the nanowires and its redeposition within the electrode. This process is shown to have detrimental effect on the MEA performance, which can be at least in part recovered by soaking electrodes in acid. The combination of these techniques provides unprecedented detail of the evolution of the nanowire-based extended surface catalyst and electrode structure guiding the targeted design of high performance electrodes based on this class of materials.