A PEM fuel cell converts hydrogen and oxygen into water and, as part of the process, produces electricity. Hydrogen is oxidized when electrons are released and hydrogen ions are formed; the released electrons supply current for an electric motor. Oxygen is reduced by gaining electrons, and in reaction with hydrogen ions, water, the only byproduct of a fuel cell reaction, is produced. Platinum electrocatalysts speed up these oxidation and reduction reactions (ORR).

In reactions during the stop-and-go driving of an electric car, however, the platinum dissolves, which reduces its efficiency as a catalyst—a major impediment for vehicle-application of fuel cells.

Under lab conditions that imitate the environment of a fuel cell, the Brookhaven researchers added gold clusters to the platinum electrocatalyst, which kept it intact during an accelerated stability test. This test is conducted under conditions similar to those encountered in stop-and-go driving in an electric car. The research is reported in the 12 January 2007 edition of the journal Science.

Fuel cells are expected to become a major source of clean energy, with particularly important applications in transportation. Despite many advances, however, existing fuel-cell technology still has drawbacks, including loss of platinum cathode electrocatalysts, which can be as much as 45 percent over five days, as shown in our accelerated stability test under potential cycling conditions. Using a new technique that we developed to deposit gold atoms on platinum, our team was able to show promise in helping to resolve this problem. The next step is to duplicate results in real fuel cells.

—Radoslav Adzic, co-author

In the unique method developed at Brookhaven, the researchers displaced a single layer of copper with gold on carbon-supported platinum nanoparticles. After being subjected to several sweeps of 1.2 volts, the gold monolayer transformed into three-dimensional clusters. Using x-rays as probes at Brookhaven’s National Synchrotron Light Source, a scanning transmission microscope at Brookhaven’s Center for Functional Nanomaterials, and electrochemical techniques in the laboratory, the scientists were able to verify the reduced oxidation of platinum and to determine the structure of the resulting platinum electrocatalyst with gold clusters, which helped them to gain an understanding of the effects of the gold clusters.

In the Brookhaven experiment, the platinum electrocatalyst remained stable with potential cycling between 0.6 and 1.1 volts in more than 30,000 oxidation-reduction cycles, imitating the conditions of stop-and-go driving.

The gold clusters protected the platinum from being oxidized. Our team’s research raises promising possibilities for synthesizing improved platinum-based catalysts and for stabilizing platinum and platinum-group metals under cycling oxidation/reduction conditions.

—Radoslav Adzic

This research is funded through the US Department of Energy’s Hydrogen Program.

In a separate study published online in Science Express, researchers enhanced the performance of platinum electrocatalysts in fuel cells. The slow rate of the oxygen reduction reaction (ORR) in PEM fuel cell is a major limiter for automotive applications.

The team from Argonne National laboratory, the University of Liverpool, Lawrence Berkeley National Laboratory and the University of South Carolina developed a Pt₃Ni(111) catalyst that is 10-fold more active for the oxygen reduction reaction than the corresponding Pt(111) surface, and 90-fold more active than current state-of-the-art Pt/C catalysts.

The Pt3Ni(111) surface has an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. Under operating conditions relevant to fuel cells, its near-surface layer exhibits a highly structured compositional oscillation in the outermost and third layers are Pt rich and the second atomic layer is Ni rich. The weak interaction between the Pt surface atoms and non-reactive oxygenated species increases the number of active sites for O₂ adsorption.

Resources:

• “Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters”; J. Zhang, K. Sasaki, E. Sutter, R. R. Adzic; Science 12 January 2007: Vol. 315. no. 5809, pp. 220 - 222 DOI: 10.1126/science.1134569

• “Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability”; Vojislav R. Stamenkovic, Ben Fowler, Bongjin Simon Mun, Guofeng Wang, Philip N. Ross, Christopher A. Lucas, Nenad M. Markovic; Science Express DOI: 10.1126/science.1135941