סמינר מחלקתי בזום - אלירן רפאל חמו, סטודנט תואר שלישי

:You are cordially invited to attend this seminar to be held on 10.11.2020 at 15:00

Stable and Active Catalyst supports Designed based on Transition Metal Carbides for Hydrogen Fuel Cells

Eliran Rephael Hamo, Ph.D. Student

Under the supervision of Dr. Brian Rosen

Polymer electrolyte membrane fuel cells (PEMFCs) are one of the most promising products of the 21st century for power. Their use encompasses portable applications, transportation, and a stationary grid-power mainly due to their low-temperature operation and quick start-up. However, the primary challenge is improving fuel cell durability to meet updated U.S. Department of Energy targets (e.g. 8000+ hours for portable applications). PEM fuel cell catalysts currently suffer from low durability, undermining their wide-scale deployment into the consumer and industrial markets. Platinum is still the most common metal used in PEMFCs as it provides among the highest activity for electrode reactions and lifetime stability. An effective way to decrease Pt loading is the adoption of supports to enhance both Pt dispersion and its utilization. Requirements for such support include factors such as surface area, conductivity, and electrochemical and mechanical stability. Carbon is currently the industrial standard for supporting the Pt catalyst particles, yet carbon-supported catalysts suffer from low durability. Corrosion of the carbon-based support was identified to be the major contributor to performance degradation as they suffer from corrosion via carbon oxidation to CO2 (at the cathode). This phenomenon exacerbates related issues such as Pt sintering or agglomeration. Therefore, there is a significant interest in exploring stable alternatives to replace carbon supports in PEM fuel cells.

Transition metal carbides (TMCs) have attracted significant attention over the last several years as a possible replacement for carbon-based catalyst supports in fuel cells. TMCs exhibit electronic structures similar to Pt-group metals and have been shown to enhance the catalytic activity of fuel cell reactions in part to their strong metal-support interaction (MSI). Despite these advantages over carbon supports, the large-scale deployment of TMC-based supports in fuel cells is still hindered by concerns of durability at the high potential on the cathode during start-up and shutdown operation. Molybdenum carbide in particular has been the center of attention as it imbues high activity for oxygen reduction, yet unprotected Mo2C will begin to oxidize just over 0.4V vs. RHE making them less practical for use as cathode catalysts support.

Here, we modify both the bulk and surface of Pt/Mo2C catalysts and apply them to room-temperature fuel cells which operate under both acidic (as cathode) and alkaline (as anode) environments.  The co-reduction carburization method enabled the low-temperature preparation of TMC alloy supports (e.g. Mo2C-TaC, Mo2C-W2C). By contrast, DC magnetron sputtering was used to modify the surface of the carbide catalysts with Ta-based phases. Bulk alloy formation such as Mo2C-TaC showed enhanced corrosion resistance in acidic fuel cells, yet this came at the expense of activity. By contrast, when the same bulk Mo2C-TaC alloys were employed in alkaline fuel cells (at the anode), increased durability was observed together with increased activity. Experimental and computational efforts by us have shown that durability was attributed to the oxygen binding energy (OBE) of the carbide while activity was attributed to enhanced metal-support interaction, which varied as a function of carbide composition. Despite the fact that bulk alloying with TaC diminished the performance of Pt/Mo2C in acidic fuel cells, the addition of a protective Ta layer to Pt/Mo2C by magnetron sputtering was shown to increase both activity and durability.  Engineering of the support (rather than the metal catalyst) by bulk and surface techniques should therefore be considered as a strategy to simultaneously improve activity and durability in energy conversion and storage systems.

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Topic: PHD Department Seminar
Time: Tuesday, November 10th, 15:00
https://us02web.zoom.us/j/85326142774?pwd=ZVRPODI3RTYyc2JFSmNZZi9mbzZPQT09