Abstract
Self-supported nanocolumnar Pt-Ni alloy thin films (TFs) with different Pt:Ni compositions and Pt mass loadings were fabricated by high pressure sputtering (HIPS) on a microporous layer (MPL)-like surface composed of carbon particles in order to mimic the catalyst-coated gas diffusion layer (gas diffusion electrode) in a membrane electrode assembly and investigated as oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. HIPS is a simple physical vapor deposition method that is scalable and easily applicable to industrial sputter deposition systems. At high working gas pressures, columnar and less-dense structures are formed because of angular distribution of sputtered atoms that leads to a shadowing effect. Cauliflower-like microstructures were observed from scanning electron microscopy imaging. X-ray diffraction and quartz crystal microbalance analysis revealed that by simply changing the relative deposition power between Pt and Ni source, different Pt:Ni compositions can be achieved. Benchtop cyclic voltammetry and rotating disk electrode measurements were performed for electrochemical characterization of the Pt:Ni-TF/MPL-like-layer/glassy-carbon samples in an aqueous perchloric acid electrolyte. The electrochemically active surface area (ECSA) ranged between 22-42 m2/g for varying Pt:Ni compositions. Lower Pt mass loadings showed a higher ECSA likely due to smaller nanocauliflower diameters, while the ORR activity of all compositions increased as the Pt mass loading is increased. The catalytic performance of the Pt:Ni-TFs increased in the order of 3:1 < 1:1 < 1:3 with the 1:3 films exhibiting a specific activity of 1781 µA/cm2 and mass activity of 0.66 A/mg, indicating efficient catalyst utilization. The Pt:Ni-TFs were found to exhibit higher ORR activity than traditional high surface area carbon supported Pt nanoparticles, elemental Pt nanorods, and Pt-Ni nanorods.
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