Engineering topics

Karabacak, Tansel

Publications and source records attributed to Karabacak, Tansel.

5 records

DOE OSTI2021

Nanocolumnar Pt:Ni Alloy Thin Films by High Pressure Sputtering for Oxygen Reduction Reaction

Self-supported nanocolumnar Pt:Ni thin films (TFs) with varying Pt:Ni atomic ratios and Pt mass loadings were produced on a microporous layer (MPL)-like surface composed of carbon particles by high pressure sputtering and examined as oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. Cauliflower-like microstructures were observed from scanning electron microscopy imaging. Various Pt:Ni atomic ratios were obtained by simply changing the relative deposition power between Pt and Ni source and investigated by X-ray diffraction and quartz crystal microbalance analysis. Additionally, electrochemical characterization of the Pt:Ni-TF/MPL-like-layer/glassy-carbon samples was conducted through benchtop cyclic voltammetry and rotating disk electrode measurements. The electrochemically active surface area (ECSA) was found to be between 22–42 m 2 g -1 for different Pt:Ni atomic ratios. Lower Pt mass loadings exhibited a higher ECSA and the catalytic activity of all Pt:Ni ratios increased with the increase in Pt mass loading. The ORR activity of the Pt:Ni-TFs increased in the order of 3:1 < 1:1 < 1:3 with exhibiting a specific activity of 1781 μA cm -2 and mass activity of 0.66 A mg -1 for the Ni-rich film with 1:3 ratio. The catalytic performance of Pt:Ni-TFs were higher than traditional high surface area carbon supported Pt nanoparticles, elemental Pt nanorods, and Pt-Ni nanorods.

08 HYDROGEN↗

DOE OSTI2020

Microstructural Evolution and ORR Activity of Nanocolumnar Platinum Thin Films with Different Mass Loadings Grown by High Pressure Sputtering

Nanocolumnar platinum thin films (Pt-TFs) with different Pt mass loadings were grown by high pressure sputtering (HIPS) and investigated as oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cell applications. Mass loading was controlled by changing the sputter deposition time. A cauliflower-like columnar microstructure was achieved by depositing the Pt-TFs onto a microporous layer (MPL)-like surface composed of carbon particles in order to mimic catalyst-coated gas diffusion electrodes. Microstructural evolution of HIPS Pt-TFs and their ORR activity were investigated. Electrochemical characterization of the nanocolumnar Pt-TFs was performed by cyclic voltammetry and rotating disk electrode measurements on Pt-TF/MPL-like-layer/glassy-carbon samples in an aqueous perchloric acid electrolyte. The electrochemically active surface area increased from 18 to 39 m 2 g –1 as the Pt mass loading was decreased. Specific activity of the films was similar (~600 μA cm –2 ) for all Pt mass loadings, due to the similar nanoparticle sizes of ~5 nm as observed by transmission electron microscopy and X-ray diffraction. Furthermore, mass activity of the films increased from 0.11 to 0.26 A mg –1 as the Pt mass loading was decreased, which is an indication of the effective Pt utilization and better access through the catalyst layer at lower Pt mass loadings.

08 HYDROGEN↗

DOE OSTI2020

Oxygen Reduction Reaction Activity of Nanocolumnar Platinum Thin Films by High Pressure Sputtering

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 prepared by low pressure sputtering was also studied for comparison. Pt-TFs were deposited on a microporous layer (MPL)-like surface composed of carbon particles to mimic catalyst-coated gas diffusion electrodes. Electron microscopy imaging revealed that HIPS Pt-TFs developed a cauliflower-like columnar microstructure, which originated from a shadowing effect during HIPS. This shadowing effect is enhanced on the rough surface of the MPL-like carbon that leads to the nano-cauliflower formation. With this approach, we also aimed to relate the catalyst performance obtained by benchtop tests directly to membrane electrode assembly test results. The electrochemically active surface area of Pt-TFs increased from 10 to 19 m 2 /g with increasing sputter pressure. Specific activity of conventional high-density and nanocolumnar films were similar at ~600 μA/cm 2 , which is likely due to their similar crystal grain sizes, >5 nm. 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, which is consistent with the columnar microstructure of HIPS films providing a better catalyst utilization compared to conventional Pt-TF.

08 HYDROGEN↗

DOE OSTI2019

Enhanced hydrogen storage by a variable temperature process

We present a simple method of variable temperature process that can potentially enhance the hydrogen storage properties of a large variety of solid state materials. In this approach, hydrogen gas is first introduced at about room temperature, which is followed by a gradual increase to a preset maximum temperature value, T max . Using this approach, we investigated hydrogen absorption properties of vertically aligned arrays of magnesium nanotrees and nanoblades fabricated by glancing angle deposition (GLAD) technique, and conventional Mg thin film. Weight percentage (wt%) storage values were measured by quartz crystal microbalance (QCM). After exposing Mg samples to H 2 at 30 bar and 30 °C, dynamic absorption measurements were conducted as the temperature was increased from 30 °C to maximum values of T max = 100, 200, and 300 °C all within 150 min. QCM measurements revealed that variable temperature method results in significant improvements in hydrogen storage values over the ones obtained by conventional constant temperature process. At a low effective temperature T eff = 165 °C (T max = 300 °C), we achieved storage values of 6.19, 4.76, and 2.79 wt% for Mg nanotrees, nanoblades, and thin film, respectively.

08 HYDROGEN↗

DOE OSTI2018

Hydrogen storage properties of magnesium nanotrees investigated by a quartz crystal microbalance system

We report on enhanced low temperature hydrogen storage properties of magnesium “nanotrees” fabricated by glancing angle deposition (GLAD) method. The arrays of nanotrees and conventional thin films of elemental Mg have been deposited directly onto gold coated unpolished quartz crystal substrates. Mg nanotrees were about 15 μm in height, 10 μm by 1 μm in lateral size, and were composed of “nanoleaves” of about 20 nm in thickness, 2 μm length, and 1 μm width. Hydrogen absorption and desorption properties of Mg nanotrees and thin films were investigated using a quartz crystal microbalance (QCM) testing system that is capable of measuring weight changes with a nanogram sensitivity. QCM absorption tests were performed at temperatures 100, 200, and 300 °C under 30 bars of H 2 pressure. Measurements revealed that Mg nanotrees can absorb hydrogen at significantly higher weight percentage (wt%) and faster rates compared to conventional Mg films under similar conditions. Hydrogen storage of Mg thin film was observed to be at 0.02, 0.30 and 3.91 wt% (weight percentage), while it reached to 1.26, 3.75, and 5.86 wt% for nanotrees at temperatures 100, 200, and 300 °C, respectively, after 150 min. In addition, the results of desorption experiments show that Mg nanotrees can start to release hydrogen at temperatures as low as 100 °C at a rate of 0.11 wt% (vs. 0.01 wt% for thin film at the same temperature) with desorption rates reaching to 1.05 wt% at 200 °C (0.26 wt% for thin film) and 2.57 wt% at 300 °C (1.45 wt% for thin film), which are considerably lower desorption temperatures compared to previously reported values for bulk Mg (>300 °C). The enhancement in hydrogen absorption and desorption properties of Mg nanotrees is believed to originate from their thin and isolated nanoleaves that also have an improved oxidation resistance property.

08 HYDROGEN↗