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Tuesday, 15 November 2011

Looking into the Atomic World of Fuel-Cell Catalysts

Nov 15, 2011

Illustration of PEM fuel cell where atomic-scale alloying structure and surface distribution of trimetallic nanocatalyst was tuned by thermal treatment temperature and then studied using EMSL’s X-ray photoelectron spectrometer and catalytic reactor

Fuel cells hold the promise of green power, but a major challenge in making them practical for large-scale commercial application is overcoming the high cost of the platinum catalysts required for oxygen reduction reaction in the air cathode. Multimetallic alloy catalysts such as carbon-supported platinum-nickel-iron (PtNiFe/C) are being explored as means to both reduce the cost and increase the activity of catalysts. A research team from State University of New York at Binghamton, in collaboration with scientists from Pennsylvania State University and from EMSL, recently reported new insights into how the atomic-scale structure of nanoengineered trimetallic alloy catalysts can be tuned to affect the fuel-cell performance. This insight into the atomic world of structural tuning is key information for scientists who seek to engineer the catalysts to maximize their catalytic activity and stability (activity over time).

After synthesizing a PtNiFe/C catalyst nanoengineered for high electrocatalytic activity in oxygen reduction reaction, the research team treated the catalyst at different temperatures (e.g., 400–800°C) to further tune the catalytic activity. The catalytic activity and stability were then determined using a rotating disk electrode and a proton exchange membrane (PEM) fuel cell.

Both measurement methods showed that specific activity (rate of catalytic activity) increased with increasing treatment temperature, but the catalytic mass activity (activity per amount of catalyst) showed the opposite trend. A detailed X-ray absorption fine-structure (XAFS) spectrographic analysis of the atomic-scale coordination structures revealed increased hetero-atomic coordination with improved alloying structures for the trimetallic catalyst treated at the higher temperature, explaining the increased rate of catalytic activity. X-ray photoelectron spectroscopy (XPS) analysis further revealed a reduced surface concentration of platinum for the high-temperature-treated catalyst, explaining the decreased mass activity. The detailed understanding of such atomic-scale alloying structure and platinum surface distribution provides new information needed for nanoengineering design of low-cost, active, and robust alloy catalysts to enable green power devices.

Source: Pacific Northwest National Laboratory (PNNL)

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