Oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs) are important electrocatalytic processes for energy storage and conversion (e.g., in fuel cells and metal-air batteries), but tend to exhibit slow reaction kinetics and require high loadings of expensive precious metal catalysts (e.g., Pt or Ir) for which less-expensive alternatives are desirable. We have developed mesoporous carbon electrocatalysts based on the pyrolysis of an inexpensive nitrogenous organic precursor (melamine) and earth-abundant transition metals (iron and nickel) within surfactant-directed mesoporous silica, yielding high mesoporosity (surface areas of ~700 m2/g) with uniform pore dimensions, which are adjustable over the range of 2-6 nm. In particular, the use of melamine results in nitrogen-doped carbon materials (surface-nitrogen contents of ~3 atom %) with graphitic properties that provide a combination of high electrical conductivities (2 S/cm) and high thermal and chemical stabilities. Under alkaline conditions, Fe,N-doped mesoporous carbon catalysts exhibit ORR activities that are slightly higher than commercially available activated-carbon-supported platinum (Pt/C) catalysts and much higher than Ni,N-doped mesoporous carbon catalysts. By comparison, Ni,N-doped mesoporous carbon catalysts exhibit OER activities under alkaline conditions which are similar to commercially available activated-carbon-supported iridium catalysts and superior to Fe,N-doped mesoporous carbon catalysts. Under acidic conditions, Fe,N-doped mesoporous carbon catalysts exhibit ORR activities that are slightly lower than Pt/C catalysts, and are higher than Ni,N-doped mesoporous carbon catalysts. The significantly different ORR activities between the Fe,N- and Ni,N-doped mesoporous carbon catalysts, despite their comparable surface compositions and BET surface areas, indicate the important role of the transition metal in reducing oxygen. Systematic analyses of these catalysts to determine optimal surface compositions for ORR and OER activities suggest a way to improve electrocatalytic activities of precious-metal-free ORR or OER catalysts.

Effects of transition metals on oxygen reduction and oxygen evolution electrocatalytic activities of N-doped mesoporous carbon catalysts

247th National Meeting (2014)

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