Global warming is mainly caused by CO2 emissions, and must be substantially reduced to prevent climate change. Since around 20 percent of world-wide CO2 emissions are due to transportation on road, it is imperative to drive the automobiles by clean energy. Due to the high power density, high conversion efficiency, low environmental impact and low temperature operation, fuel cells have been considered as the most promising alternative to traditional internal combustion engine. However, the unaffordable Pt loadings as electrocatalysts, especially at cathode where oxygen reduction reaction (ORR) happens, has greatly hindered the wide-spread use of fuel cell technique.
Recently, Prof. Shaojun Guo at Peking University and co-workers from Soochow University and Brookhaven National Laboratory published their significant results in ORR electrocatalysis for fuel cells in Science magazine. They synthesized the distinctly hexagonal PtPb/Pt core/shell nanoplates with intermetallic (001) PtPb as the core and pure (110) Pt as the shell. When applied as electrocatalyst for ORR, the PtPb nanoplate exhibited significant activity and stability enhancement compared to the commercial Pt/C. Density functional theory (DFT) calculation results reveal that the impressively electrocatalytic performance are attributed to large tensile strain from  direction and small compressive strain from  direction. This work was published in Science: http://science.sciencemag.org/content/354/6318/1410.
Figure 1. The structural characterization of the PtPb nanocatalysts.
Previous theoretical and experimental studies indicate that the ORR on Pt can be greatly enhanced by properly tuning the surface strain. Generally, a compressive Pt surface is favorable for the ORR because the compressive strain can lower the d band center, thus weakening the adsorption strength between Pt and oxygen species. To gain better understanding in surface strain effect, Prof. Guo and co-workers designed a core-shell structure with a large tensile strained Pt(110) surface (Figure 1). Electrochemical evaluations showed such novel nanocatalysts exhibited much higher activity than the benchmark Pt/C catalyst, reaching a mass activity of 7.8 mA/cm2 and specific activity of 4.3 A/mg (Figure 2a,b). DFT studies revealed the bridge sites on Pt(110) surface under tensile strain acted as the most favorable site for ORR (Figure 2c,d). This is the first report on revealing the beneficial role of large tensile strain on ORR enhancement, which opened a new avenue for the structural design of future highly efficient ORR electrocatalysts.
Figure 2. The electrochemical evaluations, proposed active sites and oxygen binding energy determined by DFT calculations
This work is financially supported by National Natural Science Foundation of China, the National Key Research and Development Program of China, start-up funding from Peking University and Young Thousand Talented Program.