A team of researchers from Ewha Womans University in Korea have attached imidazolium sulfonate zwitterions (neutral molecules with a positive and a negative electrical charge at different locations within that molecule) to multiwalled carbon nanotubes to make metal-free electrocatalysts for the fuel cell oxygen reduction reaction. The new metal-free catalysts rival commercial precious metal catalysts in activity and fuel selectivity.
The researchers first functionalized the carbon nanotubes with allyl imidazolium groups, which were then co-polymerized with vinyl imidazolium salts to give the electrocatalysts. Testing results indicated that the imidazolium groups can polarize the nanotube π electrons, which leads to a positive charge on the nanotube surface. This polarization, along with the ability of the sulfonate group in the zwitterion to attract protons and keep them close to the electronically generated oxygen radical anion species, significantly increases the rate of the oxygen reduction reaction.
The zwitterionic MWCNTs functionalized with poly(vinylimidazolium sulfonate) have a more positive surface charge and exhibit a better electrocatalytic activity than the poly(vinylbutylimidazolium chloride)-functionalized MWCNTs. The IM-f-MWCNTs showed better fuel selectivity than the commercial Pt/C electrocatalyst.—Kim et al.
Further optimization should lead to the development of a series of cost-effective metal-free catalysts for the oxygen reduction reaction, they said.
Youn Soo Kim, Ju Yeon Shin, Hyun Ji Jeon, Areum Cha, Chongmok Lee, Sang-gi Lee (2012) Insight into the Origin of the Positive Effects of Imidazolium Salt on Electrocatalytic Activity: Ionic Carbon Nanotubes as Metal-Free Electrocatalysts for Oxygen Reduction Reaction, Chem. Asian J. doi: 10.1002/asia.201200800