Oxygen reducing activity of methanol-tolerant catalysts by high-temperature pyrolysis

This study demonstrates the feasibility of platinum-free catalysts, which exhibit not only a high activity in the oxygen reduction reaction (ORR) but also a high tolerance of methanol. A cobalt (II) tetramethoxyphenylporphyrin (CoTMPP) precursor was dispersed in N,N-dimethylmethanamide (DMF), which was ultrasonically stirred for 30 min to yield a homogeneous solution, and then filtered to remove the solvent. The CoTMPP precipitate was pyrolyzed at temperatures of 300, 500, 700 and 900 °C in N2 atmosphere. Raman spectra include strong peaks at 1330 and 1550 cm− 1, which are associated with the D- and G-peaks of pyrolyzed CoTMPP above 500 °C, revealing that the original porphyrin structure of CoTMPP yields a network structure of poly-aromatic hydrocarbons upon the pyrolysis. Pyrolyzed CoTMPP loaded on carbon blacks (CoTMPP/C) at 700 °C exhibits a higher ORR activity than other various pyrolysis temperatures. In a methanol-containing solution, pyrolyzed CoTMPP/C preferentially undergoes the ORR rather than the methanol oxidation reaction, and so exhibits a high tolerance of methanol. Pyrolyzed CoTMPP has the pyrrolic nitrogen, and part of the cobalt-containing nitrogen chelate is cleaved and bound to other atoms, forming Co―Nx―Cy (x + y = 4), which are responsible for the ORR activity and the high tolerance of methanol.

Research highlights
► We study pyrolyzed CoTMPP/C catalysts, which have not only the methanol-tolerant capability but also oxygen-reducing activity in the cathode of DMFC.
► The pyrolyzed temperatures strongly impact on the catalytic structures.
► At 700 °C, the original porphyrin structure of CoTMPP yields a network structure of poly-aromatic hydrocarbons, Co-Nx-Cy and pyrrolic N upon the pyrolysis, which are beneficial for the enhanced oxygen reduction reaction and the high methanol tolerance.