Platinum Nanorod Arrays As ORR Electrocatalyst for Polymer Electrolyte Membrane Fuel Cells

Abstract

Polymer electrolyte membrane fuel cell (PEMFC) is one of the most promising energy devices for stationary, portable, and transportation power-supply applications. PEMFCs have high energy conversion efficiency, low operating temperature, and are environmentally friendly compared to the other types of fuel cells. The dissolution of platinum (Pt) during the oxygen reduction reaction (ORR) on the cathode causes voltage losses and requires a high loading of Pt compared to the anode, and thus increases the cost of the fuel cell. In addition, carbon support used in conventional Pt/C catalyst present problems including oxidation of carbon, the formation of peroxide species causing degradation of the polymeric membrane, and separation from the ionomer over time leading to loss of catalyst effectiveness. While extensive efforts and significant progress have been made, there are still some challenges related to the fuel cell performance, durability, and cost that need to be addressed in order to have successful commercialization. In order to overcome these challenges, columnar Pt nanorods stand out as a good candidate for non-conventional catalysts due to their high surface area, high stability, and no need for a support material. For this purpose, vertical arrays of Pt nanorods (NRs) were grown onto microporous layer (MPL) side of a gas diffusion layer and glassy carbon (GC) substrates by glancing angle deposition (GLAD) magnetron sputtering to investigate their ORR properties for PEMFC cathode applications. The weight loading of Pt NRs were calculated using quartz crystal microbalance (QCM). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to determine the microstructure of NRs. Magnetron sputtering provides high-purity catalyst material, controlled and ultra-low loadings. It is also a scalable fabrication method with no toxic by-products. GLAD involves a modified substrate alignment geometry where incident flux of atoms approaches the substrate at oblique angles. ORR activity of Pt NRs was evaluated in aqueous acidic electrolyte using the rotating disk electrode technique. The electrochemically active surface area (ECSA) was measured for Pt NRs deposited on the flat GC and on the rough surface of MPL. The results indicate about an order of magnitude increase in ECSA for Pt NRs grown on GC compared to conventional high density Pt thin film on GC. The enhancement agrees well with the higher surface-to-volume ratio of NRs compared to a planar film. Isolated columnar structure and ECSA of NRs are further enhanced when it is deposited on MPL due to the “shadowing effect” involved in our growth process. We are currently developing a new methodology for performing more detailed electrochemical tests on samples of Pt NRs on MPL and will report ECSA enhancement and other relevant analysis during the presentation. Electrochemical stability of the Pt NRs against dissolution will be investigated by using inductively-coupled plasma-mass spectrometer (ICP-MS) capable of detecting trace concentrations (<ppb) of dissolved elements in solution. This work is supported by the U. S. Department of Energy (DOE) under contract number DE-EE0007652.