Self-Supported Nanocolumnar Platinum Thin Film Catalysts for Oxygen Reduction Reaction

Nanocolumnar platinum thin films (Pt-TFs) were produced by high pressure sputtering (HIPS) and investigated as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells. Conventional high-density Pt-TF by low pressure sputtering was also studied for comparison. Pt-TFs were deposited on a microporous layer (MPL)-like surface composed of carbon particles in order to mimic the catalyst coated gas diffusion layer in a membrane electrode assembly (MEA). Scanning electron microscopy and transmission electron microscopy images revealed that HIPS Pt-TFs developed a cauliflower-like columnar microstructures, which originated from a shadowing effect. At high working gas pressure conditions of HIPS, sputtered Pt atoms go through several gas phase collisions, lose their directionality, and can arrive at the substrate surface at oblique angles that results in the formation of columnar islands that shadow the gaps among them from incident flux of atoms. This shadowing effect is believed to be enhanced on the rough surface of MPL-like carbon particles that leads to the formation of nano-cauliflower structures. Electrochemical characterization of the samples was performed by cyclic voltammetry and rotating disk electrode measurements on Pt-TF/MPL-like-layer/glassy-carbon-insert samples in aqueous perchloric acid electrolyte. With this approach, we also aimed to relate the catalyst performance obtained by benchtop tests directly to MEA results. Electrochemically active surface area (ECSA) of PT-TFs were found to be ranging from 10 m2/g to 19 m2/g with increasing sputter pressure, which was determined from the charge for hydrogen adsorption and desorption in the cyclic voltammograms. In addition, carbon monoxide (CO)-stripping of Pt-TF coated gas diffusion electrodes were performed in a fuel cell station and similar ECSA values were recorded (ranging from 11 to 15 m2/g). This indicated that successful mimicking of a gas diffusion electrode in an MEA can be achieved by utilizing an MPL-like substrate surface for benchtop tests in aqueous electrolyte environment as opposed to directly depositing Pt-TFs on glassy carbon inserts. Specific activity (SA) values for conventional high-density and nanocolumnar films were similar that was approximately ~600 μA/cm2, which is believed to correspond to grain sizes of >5 nm as obtained from TEM and XRD analysis. On the other hand, mass-specific (MA) activity values increased from 0.06 A/mgPt for conventional Pt-TF to 0.13 A/mgPt for HIPS Pt-TFs. Higher MA agrees with the columnar microstructure of HIPS thin films that increases the surface to volume ratio of the Pt layer that provides a better catalyst utilization compared to conventional Pt-TFs.