Low-platinum catalyst developed to boost fuel cell commercialization

Fuel cell bus in Beijing, China. (Image by N509FZ, Wikimedia Commons).

A team at the University of Science and Technology of China developed a new catalyst that requires small amounts of platinum and produces high catalytic activity for fuel cell commercialization.

In a paper published in the journal Science, the researchers explain that fuel cell catalysts are usually made of platinum or Pt alloys with transition metals thinly coated onto the porous carbon supports because the precious metal is an ideal catalytic material as it can withstand the acidic conditions and increase the rate of chemical reactions efficiently. However, since it is expensive and has insufficient resource reserves, its utilization is one of the factors hindering the mass production of fuel-cell vehicles.

Thinking about a solution to this issue, the group at USTC employed a sulphur-anchoring method of high-temperature to synthesize small-sized Pt intermetallic nanoparticle (i-NP) catalysts with ultralow Pt loading and high mass activity. Their experiment not only proved successful but also allowed them to establish i-NP libraries, including 46 types of Pt nanoparticles (NPs) to screen inexpensive and durable electrode materials as well as explore structure-activity relations of i-NPs systematically.

I-NPs have attracted wide attention because of their unique atomically ordered properties and excellent catalytic performance in many chemical reactions. However, inevitable metal sintering at high temperatures is undesired during the synthesis of i-NPs, as it will lead to larger crystallites. Thus, it results in a decreased specific surface area and lower catalytic activities of the materials, and eventually reduces the utilization rate of Pt, therefore greatly increasing the cost of fuel cells.

The research team led by Liang Haiwei, thus, utilized a strong Pt-sulfur chemical interaction. They prepared Pt intermetallics on sulphur-doped carbon (S-C) supports in order to suppress NPs sintering at high temperatures, and they were able to obtain atomically ordered i-NPs with an average size of <5 nm. S-C supports showed excellent anti-sintering ability, and the researchers obtained Pt NPs with the average diameter still <5 nm after annealing at high temperatures up to 1000°C. However, severe Pt sintering was observed after the same annealing process on commercial carbon black supports.

To take advantage of the anti-sintering property, the researchers synthesized 46 types of small-sized Pt-based i-NPs on S-C supports and established i-NP libraries. Spectral characterizations were measured, and the results verified the strong chemical interactions of Pt-S bonds. Moreover, the X-ray diffraction (XRD) results showed a high ordering degree and small size of i-NP catalysts in libraries, consistent with the statistical analysis of the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations.

“Based on the i-NP libraries, we can systematically study the relationship between structure and performance of catalysts,” Liang said in a media statement. “Sufficient samples helped us screen out efficient catalysts which were expected to largely decrease the cost of fuel cells.”

The researcher and his group screened i-NPs and applied them for proton-exchange membrane fuel cells (PEMFCs). These catalysts exhibited excellent electrocatalytic performance for oxygen reduction reaction (ORR).

In their view, this method raises hopes for reducing the quantity of Pt used, thereby decreasing the cost of fuel cells.

“By engineering the porous structures and surface functionalities of carbon supports, the efficiency of fuel cells can be further improved, thus accelerating their move from the laboratory to the public,” Liang said.