Unveiling the high-activity origin of single-atom iron catalysts for oxygenreduction reaction for methanol fuel cells

Polymer electrolyte membrane fuel cells (PEMFCs) have been considered as promising solutions tomeet the future energy requirements due to highly efficient energy conversion and environmentallyfriendly features. Among PEMFCs, DMFCs have gained intensive attraction for the use of liquid fuel,which is convenient for storage and transportation [۱]. However, the commercialization of DMFCs ishampered by the expensive and scarce platinum-based catalysts, especially for the sluggish ORR in thecathode. Furthermore, their performance is severely limited by methanol crossover, which leads tocatalyst poisoning and a mixed potential [۲]. Therefore, extensive investigations have been devoted toexplore low-cost and methanol-tolerant catalysts for ORR. In particular, Fe-N-C catalysts are expectedto replace expensive Pt-based catalysts owing to their intrinsic nature of methanol tolerance andconsiderable ORR activity in acid medium. Experiments combined with density functional theory(DFT) calculations reveal the excellent ORR activity could be generated from Fe-Nx species formed bycoordination of Fe with nitrogen [۳]. Hence, atomically dispersed Fe-N-C catalysts with homogenousFe-Nx active sites have been intensively studied in pursuit of the maximum metal utilization and highelectrocatalytic performance. In recent works, H۲-O۲ PEMFCs using atomically dispersed Fe-N-Ccatalysts exhibit excellent performance with a highest power density over ۱ W cm-۲ [۴]. These worksdemonstrate the possibility of the application of atomically dispersed Fe-N-C catalysts in DMFCs onaccount of the same ORR process in the cathodes. Thus, the application of atomically dispersed Fe-NCcatalysts in DMFCs is highly expected, but has not been reported yet.Study on the pyrolysis procedure reveals that Fe atoms in FeN۴ are from Fe(acac)۳, while ZIF-۸ acts asstructural support and nitrogen sources. The as-prepared catalyst demonstrates a comparable ORRactivity with the commercial Pt/C possessing a half-wave potential (E۱/۲) of ۰.۷۵ V in KOH medium(Fig.۱), in combination with excellent methanol tolerance and stability. One of the highest peak powerdensities of ۵۷ mW cm−۲ in DMFC (۳ M CH۳OH) by adopting Fe-NC as cathode catalyst is obtained,which is ۲.۸-fold higher than that of the commercial Pt/C in the same operating condition. The current study highlights the advantages and prospect of atomically dispersed Fe-N-C catalysts in theapplication of DMFCs, and also enables this new fabrication strategy a promising approach towardsadvanced catalytic materials for energy conversion devices.

Keywords:

Oxygen reduction reaction , Methanol-tolerant catalysts , Direct methanol fuel cells