Self-assembled flower-like MnO2 grown on Fe-containing urea-formaldehyde resins based carbon as catalyst for oxygen reduction reaction in alkaline direct methanol fuel cells

Abstract

The urea-formaldehyde resins carbon (UFC) and Fe-containing UFC (Fe-UFC) have been prepared by direct carbonization of urea-formaldehyde resins (UF resins) without any surfactants. On this basis, N-doped materials of UFC/MnO2 and Fe-UFC/MnO2 composites were successfully synthesized by the situ redox method. UF resins played both roles of carbon source and nitrogen source. The specific surface areas are 265.7 m2·g−1 and 229.4 m2·g−1 for UFC/MnO2 and Fe-UFC/MnO2, respectively. Direct methanol fuel cells (DMFCs) were assembled with the prepared catalyst as cathode catalyst, polymer fiber membrane (PFM) as electrolyte film and PtRu/C as anode catalyst. DMFCs and cyclic voltammetry (CV) performance tests indicate that Fe-UFC/MnO2-based DMFC displays superior oxygen reduction reaction (ORR) catalytic activity than UFC/MnO2 owing to Fe-Nx existing and a large number of pyridine-N in Fe-UFC/MnO2. The lone pair electrons of pyridine-N can coordinate with transition metal elements and form Fe-Nx groups on Fe-UFC/MnO2, which can be used as the electrocatalytic active sites for ORR. On the other hand, the presence of iron on the prepared catalyst can reduce the band gap, facilitate the transfer from MnIII to MnIV, and provide free electrons for adsorbed O2 or active oxygen species, thus accelerating the conductivity and ORR catalytic efficiency.