Electrochemical Characterization of Free-Standing Platinum Nanoelectrode Array Using Atomic Layer Deposition for Polymer Electrolyte Fuel Cells

Abstract

For polymer electrolyte fuel cells (PEFCs) to achieve broad commercialization several technological hurdles have to be resolved. These include the high cost associated with the use of Pt as electrocatalyst in the electrodes, and durability problems due to carbon corrosion. PEFCs’ conventional carbon-supported platinum (Pt/C) electrodes use 0.25 mg/cm2 of Pt loading, whereas the set target for 2020 by the U.S. Department of Energy (DOE) is 0.125 mg/cm2. Non-conventional thin-film electrodes prepared by atomic layer deposition (ALD) are a promising alternative to conventional designs1-2. In the ALD process, monolayers of Pt are deposited onto the substrate by dissociative chemisorption in a self-assembling reaction. In this work, we propose an ALD thermal exposure mode to deposit Pt nanostructures onto a sacrificial anodized aluminum oxide (AAO) substrate using the trimethylcyclopentadienylmethylplatinum (IV) and oxygen gas as precursors. Thermal exposure mode provides additional time for the reactions in the ALD chamber and allows higher penetration of the precursor into the substrate. After the deposition the ALD electrodes were hot-pressed onto Nafion XL membrane and sacrificial AAO template was etched. We have demonstrated the feasibility of long free-standing structures (6-8 μm), as shown by Figure 1a. The ALD cathode electrodes were electrochemically characterized within a fuel cell hardware where the cell had custom-made anode gas diffusion electrodes (GDE) with a loading of 0.2 mg/cm2. Polarization curves, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were performed at dry and wet conditions to assess activity and water management of these electrodes. A cyclic voltammogram is shown in Fig 1c, measured at a sweep rate of 40mV/s, 100% RH at 35° C and an inset to the figure shows a Nyquist plot of EIS under 60° C, 100%RH, H2/N2 at 0.8V, with a 5 mV perturbation measured between a range of frequencies from 200 MHz to 100 mHz. In this presentation we will compare various ALD electrode fabrication conditions, ionomer presence and provide detailed electrochemical analysis (electrochemical surface area, polarization curves, cyclic voltammetry) of these nanoelectrode arrays. Acknowledgements This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrustructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959. CNS is part of Harvard University. We thank Mr. Jake Berliner for helping in ALD electrodes characterization and Mr. Berney Peng for SEM sample preparation. Fabrication of the structures was carried out in part at the Tufts Micro and Nano Fabrication Facility.