Excavated Fe‐N‐C Sites for Enhanced Electrocatalytic Activity in the Oxygen Reduction Reaction

Abstract

Platinum (Pt) is the best electrocatalyst for the oxygen reduction reaction (ORR) in hydrogen fuel cells, but it is an extremely expensive resource. The successful development of a cost-effective non-Pt ORR electrocatalyst will be a breakthrough for the commercialization of hydrogen-air fuel cells. Ball milling has been used to incorporate metal and nitrogen precursors into micropores of carbon more effectively and in the direct nitrogen-doping of carbon under highly pressurized nitrogen gas in the process of the preparation of non-noble ORR catalysts. In this study, we first utilize ball milling to excavate the ORR active sites embedded in Fe-modified N-doped carbon nanofibers (Fe-N-CNFs) by pulverization. The facile ball-milling process resulted in a significant enhancement in the ORR activity and the selectivity of the Fe-N-CNFs owing to the higher exposure of the metal-based catalytically active sites. The degree of excavation of the Fe-based active sites in the Fe-N-CNFs for the ORR was investigated with cyclic voltammetry, X-ray photoelectron spectroscopy, and pore-size distribution analysis. We believe that this simple approach is useful to improve alternative ORR electrocatalysts up to the level necessary for practical applications.

On the Role of Metals in Nitrogen‐Doped Carbon Electrocatalysts for Oxygen Reduction

“…[48,52,129] Maldonado and Stevenson [129] performed analysis of the heterogeneous decomposition of hydrogen peroxide at Ndoped carbon nanofibers (N-CNFs) and observed that the rate of decomposition of hydrogen peroxide on N-CNFs was tremendously increased by about 100 fold relative to the nondoped CNFs.W eo bserved in ar ecent study that metalfree NC catalysts disproportionate hydrogen peroxide nearly as fast as Fe-N x /C catalysts and that decomposition of hydrogen peroxide is an important reaction step in the reduction of O 2 on this type of catalysts. [9,16,38,44,47,48,65,109,126,136,163,164] In Figure 12 c, the catalytic activity of am etal-free NC catalyst is seen to be extremely enhanced in the presence of transition metals. [9,16,38,44,47,48,65,109,126,136,163,164] In Figure 12 c, the catalytic activity of am etal-free NC catalyst is seen to be extremely enhanced in the presence of transition metals.…”

“…[69] unzipped MWCNTs to expose Fe, and by afurther treatment step with NH 3 ,they were able to transform them into av ery active and stable catalyst for O 2 reduction under both acidic and alkaline conditions.T op robe whether Fe was involved in the active sites,t he authors added CN À ions to the electrolyte and observed ad egeneration of activity,t hus suggesting that Fe was directly involved as part of the active site(s). [163] 5. [163] 5.…”

“…In related studies,i tw as demonstrated that metal impurities intercalated within the graphene lattice of CNTs were chemically accessible and participated in the redox chemistry of biomarkers by intercalation of molecules within the CNT lattice. [163] 5. Stability of M-N x /C and NC Catalysts M-N x /C catalysts are top contenders among non-platinum O 2 reduction catalysts.…”

“…[48] Ther esults reported in Figure 12 ba lso support the electroreduction of O 2 by the two electron transfer pathway on N-graphene,a se stimated from the diffusion-limited current on the basis of Levichs RDE Equation, and there are an umber of reports consistent with these examples. [9,16,38,44,47,48,65,109,126,136,163,164] In Figure 12 c, the catalytic activity of am etal-free NC catalyst is seen to be extremely enhanced in the presence of transition metals. [162] Catalytic enhancement has also been observed to increase with metal loading up to ac ertain point.…”

Hybrid of Fe@Fe3O4 core-shell nanoparticle and iron-nitrogen-doped carbon material as an efficient electrocatalyst for oxygen reduction reaction

“…Lee et al utilized a ball milling method to excavate the ORR active sites embedded in Fe-modified N-doped carbon nanofibers by pulverization. The facile ball-milling process resulted in a significant enhancement in the ORR activity and the selectivity of the obtained catalyst owing to the high exposure of the metalbased catalytically active sites [14]. In 2011, a highly active electrocatalyst (Fe-N-C) reported by Wu et al has shown a maximum power density of 0.55 W cm À2 and excellent stability of 700 h of long-term testing, which is one of the best active nonprecious metal catalysts to date [15].…”