Thanks to their high efficiency and environmentally-friendly features in the process of electricity generation, fuel cells are becoming increasingly popular for the manufacture of fuel cell vehicles (FCVs), such as buses, forklifts, automobiles and airplanes. However, the expensive nature of manufacturing fuel cell catalysts impedes the mass-production and extensive application of FCVs.
Schematic illustration of the high-temperature sulfur-anchoring synthetic approach. Image Credit: LIU Xinyi, LIU Zige
Fuel cell catalysts are typically composed of platinum (Pt) or Pt alloys with transition metals finely coated onto the porous carbon supports. Pt is a perfect catalytic material as it can endure acidic environments and boost the rate of chemical reactions capably. But, it is costly and has inadequate resource reserves. Therefore, it is crucial to create and monitor new catalysts with high catalytic activity and low Pt quantity for fuel cell commercialization.
Scientists at the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) have recently demonstrated a sulfur-anchoring technique at high temperature, effectively synthesized small-sized Pt intermetallic nanoparticle (i-NPs) catalysts with high mass activity and ultralow Pt loading. Their findings have been reported in the journal Science published on October 22nd.
They also set up i-NPs libraries comprising 46 types of Pt nanoparticles (NPs) to monitor economical and durable electrode materials as well as examine structure-activity relations of i-NPs methodically.
I-NPs have drawn plenty of attention due to their exceptional atomically ordered properties and superior catalytic performance in a number of chemical reactions. However, unavoidable metal sintering at high temperatures is unwanted during the synthesis of i-NPs, which will result in larger crystallites.
Thus, it results in a reduced specific surface area and lower catalytic events of the materials, which ultimately decreases the utilization rate of Pt, leading to a significant rise in the cost of fuel cells.
The study team, headed by LIANG Haiwei, resourcefully utilized robust Pt-sulfur chemical interaction. They prepared Pt intermetallic on sulfur-doped carbon (S-C) supports so as to overpower NPs sintering at high temperatures, and they could acquire atomically ordered i-NPs with an average size of < 5 nm.
S-C supports revealed exceptional anti-sintering ability that the scientists obtained Pt NPs with the average diameter remaining <5 nm after annealing at a high temperature of nearly 1000˚C. But, severe Pt sintering was noticed after the same annealing process was performed on commercial carbon black supports.
To exploit the anti-sintering property, the scientists synthesized 46 types of small-sized Pt-based i-NPs on S-C supports and proven i-NPs libraries. Spectral features were measured, and the results confirmed the robust chemical interactions of Pt-S bonds.
Furthermore, the X-ray diffraction (XRD) outcomes revealed a high ordering degree and small size of i-NPs catalysts in libraries, corresponding with the statistical investigation of the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations.
Based on the i-NPs libraries, we can systematically study the relationship between structure and performance of catalysts and sufficient samples helped us screen out efficient catalysts which were expected to largely decrease the cost of fuel cell.
Haiwei LIANG, Study Lead, University of Science and Technology of China, Chinese Academy of Sciences
The scientists monitored i-NPs and applied them for proton-exchange membrane fuel cells (PEMFCs). These catalysts displayed superior electrocatalytic performance for oxygen reduction reaction (ORR), particularly in H2-air PEMFC. Although the Pt loading of i-NPs was 11.5 times lower than that of Pt/C cathode, the i-NPs’ catalyst cathodes displayed a similar capability to the Pt/C cathode.
This study offers a common method for the synthesis of Pt alloy catalysts employed in hydrogen fuel cells. This process raises the chances of decreasing the quantity of Pt used, thereby reducing 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 its transformation from laboratory to the public.
Haiwei LIANG, Study Lead, University of Science and Technology of China, Chinese Academy of Sciences
Journal Reference:
YANG, C-L., et al. (2021) Sulfur-anchoring synthesis of platinum intermetallic nanoparticle catalysts for fuel cells. Science. doi.org/10.1126/science.abj9980.