Defects of carbon as a factor determining electroactivities of oxygen reduction

Abstract

Carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) in alkaline media have attracted significant attentions and have been vigorously investigated to replace precious metal-based catalysts. To enhance ORR activities of the metal-free catalysts into the level close to those of precious metals, it is crucial to understand active sites of the carbon-based electrocatalysts. To test a null hypothesis that defects of carbon play an important role of ORR, we used carbon balls that were prepared from glucose solution via hydrothermal process. Defects generated through thermal decomposition of functional groups of as-synthesized carbon balls were controlled by temperature of thermal post-treatment in argon. Degree of defects was quantified by disorder/graphitic (D/G) ratios from Raman spectra and full widths at half maximum (FWHMs) of sp2 C1s peaks from x-ray photoelectron spectra (XPS). Higher defect densities were obtained in carbon balls annealed at higher temperatures between 400 oC and 1000 oC. Carbon balls of higher D/G ratios as well as higher FWHMs showed higher ORR activities. However, carbon ball annealed at 2300 oC was deviated from the defect-activity relation: its ORR activity decreased even if high D/G ratio was obtained. At such a high temperature, carbon ball was densified with generating highly developed crystallite phases and more amorphous phases. For comparison, carbon balls annealed at lower temperatures had no clear phase separation and more coarse structure. The densified structure does not allow a portion of defects to be exposed to electrolyte. Therefore, the carbon ball annealed at 2300 oC showed low ORR activity due to its closed defects. Temperature programmed desorption (TPD) confirmed that preferential adsorption of oxygen on defects is responsible for the ORR activity improvement. The role of defects was also confirmed by comparing ORR activities between bare carbon balls and defect-blocked carbon balls obtained by atomic layer deposition (ALD). Blocking defects of carbon balls with Al2O3 which disrupts electron conductivity would badly influence electroactivities of oxygen reduction reaction.