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Friday, June 21

NOTE TIME CHANGE
Combinatorial Exploration of Catalysts for Oxygen Reduction

Jonathan Petrie, Army Research Laboratory, Adelphi, Maryland
Materials Science and Technology Division Seminar
10:00 AM — 11:00 AM, Chemical and Material Sciences Building (4100), Room C-301
Contact: Ho Nyung Lee (hnlee@ornl.gov), 865.574.9782

Abstract

Almost 200 years ago, room-temperature fuel cells were developed as continuous chemical storage media for electrical energy. While hydrogen was oxidized at the anode, oxygen was reduced at the cathode according to the oxygen reduction reaction (ORR):
4H+ + O2 + 4e- ? 2H20 ?E° = 1.229 V

Platinum (Pt) was used to catalyze this reaction. Today, the catalyst of choice for such fuel cells (e.g., PEM) remains platinum. Despite its high cost and susceptibility to hydrocarbon poisoning, no alternative material has yet overcome platinum's high ORR activity, good conductivity, and superior chemical/mechanical resistance.

We have developed a high-throughput method to investigate intriguing non-Pt ORR catalysts for use near room temperature in PEM fuel cells. Multiple element targets were simultaneously sputter deposited onto a single substrate to produce combinatorial thin film libraries covering a range of compositions. These films were initially tested using a pH–sensitive fluorescent dye in an oxygenated testing solution. Since protons are expended in the ORR reaction, likely regions of high ORR activity at low overpotentials are optically identified by local changes in fluorescence. Such regions correspond to unique compositions which were examined in further detail using more traditional voltammetry and a battery of other characterization steps.

One fruitful area of research includes alloys of palladium (Pd), a noble metal in the platinum group. While pure Pd is less catalytic than Pt, its reactivity is enhanced by modifying the electronic structure through ligand and/or strain effects. By using our combinatorial method to screen candidate Pd-containing materials that incorporate these effects, we have identified non-Pt catalysts with an ORR activity that may exceed that of platinum. Time permitting, other interesting catalysts (e.g., Pb-Ru pyrochlores) will be discussed. In all cases, continuous gradients of composition, strain, thickness, temperature, etc., were leveraged with a general understanding of ORR catalysis to more easily and quickly focus on promising materials.