Active Sites and Mechanism of Oxygen Reduction Reaction Electrocatalysis on Nitrogen‐Doped Carbon Materials

Abstract

The oxygen reduction reaction (ORR) is a core reaction for electrochemical energy technologies such as fuel cells and metal–air batteries. ORR catalysts have been limited to platinum, which meets the requirements of high activity and durability. Over the last few decades, a variety of materials have been tested as non‐Pt catalysts, from metal–organic complex molecules to metal‐free catalysts. In particular, nitrogen‐doped graphitic carbon materials, including N‐doped graphene and N‐doped carbon nanotubes, have been extensively studied. However, due to the lack of understanding of the reaction mechanism and conflicting knowledge of the catalytic active sites, carbon‐based catalysts are still under the development stage of achieving a performance similar to Pt‐based catalysts. In addition to the catalytic viewpoint, designing mass transport pathways is required for O2. Recently, the importance of pyridinic N for the creation of active sites for ORR and the requirement of hydrophobicity near the active sites have been reported. Based on the increased knowledge in controlling ORR performances, bottom‐up preparation of N‐doped carbon catalysts, using N‐containing conjugative molecules as the assemblies of the catalysts, is promising. Here, the recent understanding of the active sites and the mechanism of ORRs on N‐doped carbon catalysts are reviewed.

Gadolinium‐Induced Valence Structure Engineering for Enhanced Oxygen Electrocatalysis

“…The spectrum could be fitted with three different N species, which correspond to pyridinic‐N (398.1 eV), pyrrolic‐N (400.0 eV), and graphitic‐N (401.6 eV),55,56 respectively. Considering the size difference between N and C atoms, the introduction of N may cause the structure change of carbon materials, such as more structural defects 57. Raman spectroscopy was performed to elucidate the structural characteristics of NG‐based aerogels, including Gd 2 O 3 –Co/NG, Gd 2 O 3 /NG, Co/NG, and NG samples.…”

“…Considering the size difference between N and C atoms, the introduction of N may cause the structure change of carbon materials, such as more structural defects. [57] Raman spectroscopy was performed to elucidate the structural characteristics of NG-based aerogels, including size of sp 2 domains, which suggests the degree of disorder. As viewed in Figure 3f, the I D /I G value of Gd 2 O 3 -Co/NG is estimated to be about 1.08, which is better than that of Gd 2 O 3 /NG (I D /I G = 0.98), Co/NG (I D /I G = 0.91), NG (I D /I G = 0.91), and N-free rGO (I D /I G = 0.80).…”

3D Carbon Materials for Efficient Oxygen and Hydrogen Electrocatalysis

“…The early work in metal‐free catalysis of the ORR from Dai's group attributed the increased activity of the N‐doping of CNTs to the change in adsorption of O 2 from end‐on to side‐on, or parallel diatomic adsorption, and the subsequent weakening of the O‐O bond . A recent review from Singh et al on the active sites and mechanism of the ORR on N‐doped carbons sheds light on the desired structures for good activity. N can exist in the graphitic (three bonds to C), pyridinic (2 bonds to C in a six‐membered ring), or pyrrolic (2 bonds to C in a five‐membered ring) sites within a carbon matrix, and there was some debate in the literature until recently as to the active site, in part due to the difficulty in producing catalysts that do not have a mix of N environments and also inhomogeneities in the morphology and degree of graphitization in the carbon matrix.…”

Biomass Derived Graphene‐Like Carbons for Electrocatalytic Oxygen Reduction Reaction

“…Among those candidates, the N‐doped carbon material as a representative metal‐free catalyst is of particular interest . Alien nitrogen atom doped into the graphitic framework changes electronic density of carbon surface and thus favors the oxygen molecule adsorption and further reduction ,. Despite of the high activity of N‐doped carbons, their performances are still far from satisfactory ,,.…”

“…[29][30][31][32][33][34][35][36][37][38][39][40] Alien nitrogen atom doped into the graphitic framework changes electronic density of carbon surface and thus favors the oxygen molecule adsorption and further reduction. [29,[41][42][43] Despite of the high activity of N-doped carbons, their performances are still far from satisfactory. [34,39,[44][45][46][47][48][49] This is mainly because of their relatively low exposed nitrogen contents, low surface utilization, and unfavorable mass-transferring ability.…”