Pt-based Thin Films as Efficient and Stable Catalysts for Oxygen Electroreduction

Research output: Book/ReportPh.D. thesis – Annual report year: 2016Research

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This thesis presents the fabrication and characterization of Pt-based thin film catalysts for Oxygen Reduction Reaction (ORR). Gadolinium and Yttrium have been used as alloying materials, in preparation for the replacement of the traditional but economically disadvantageous pure Pt catalysts at the cathode of Polymer Electrolyte Membrane Fuel Cells (PEMFCs).
Herein the fabrication method, which consists of co-sputtering of thin films, is presented in detail, explaining the challenges one must face in order to fabricate oxygen-free Pt-lanthanides and Pt-early transition metals alloys, and the proposed solutions.
The characterization of the catalysts focused mainly on the electrochemical testing using a Rotating Ring Disk Electrode (RRDE) setup, and includes X-ray Diffraction (XRD), X-ray Photoemission Spectroscopy (XPS), Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS).
The investigated films included pure Pt sputtered thin films, as well as PtxGd and PtxY thin film alloys of different compositions and thicknesses, with the aim of a model study to pursue more active and stable ORR catalysts. While the Pt and PtxGd films were deposited at DTU Physics, the PtxY alloys were fabricated at Chalmers University, which has been collaborating in the NACORR project.
When tested electrochemically, 50 nm thick Pt5Gd thin film catalysts exhibited a 4.5-fold enhancement in activity at 0.9 V vs. Reversible Hydrogen electrode (RHE) compared with polycrystalline Pt. This value increases to a 7-fold enhancement for 30 nm thick Pt3Y films. Moreover, pure Pt thin films showed an activity which was roughly double the one recorded for polycrystalline Pt, and this could be due to the different kind of surfaces generated by sputtering.
Both the Pt5Gd and Pt3Y films maintain over 80 % of the initial ORR activity when cycled 10000 times between 0.6 and 1.0 V vs. RHE in 0.1 M HClO4, and that is an indicator of the good stability of these catalysts. Investigation of the films through XRD showed that a metallic alloy structure is formed, matching the structure of polycrystalline samples. XPS and EDX analyses confirmed the composition of the alloys, proving good control of the co-deposition rates of the sputter chamber. With this techniques, it was possible to observe the formation of a thick, strained Pt overlayer, which is probably responsible for the activity enhancement.
A study of the thickness of Pt3Y alloys revealed that the thin film formation during sputtering happened by island growth, and showed that smooth films were obtained when the thickness was equal or above 27 nm.
A brief study was conducted at Stanford University, in collaboration with the Jaramillo group and SLAC, on Pt and Pt5Gd films deposited via evaporation. The results underlined the importance of an oxygen-free environment when dealing with Pt-lanthanides thin film fabrication.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages178
Publication statusPublished - 2016

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