Active sites and factors influencing them for efficient oxygen reduction reaction in metal-N coordinated pyrolyzed and non-pyrolyzed catalysts: a review

Abstract

This review outlines the fundamentals of active sites in biomimetic oxidase and M–N/C catalysts, responsible for carrying out oxygen reduction reaction at a very high TOF.

Self‐Sacrificial Template Synthesis of a Nitrogen‐Doped Microstructured Carbon Tube as Electrocatalyst for Oxygen Reduction

“…The observed value 21 T is between the values expected for pure carbide and for the nitride. It may be that the carbide is doped with N in C‐PANI‐MIL‐2 which leads the field reducing from 26 T to 21 T. Herein, together with the absence of Fe 3 C phase in XRD patterns (Figure d), the pure Fe 3 C coated by graphitic layers as active sites doesn't seem to fit catalysts in this work . Concerning with sextet‐2, it may represent γ‐Fe 2 O 3 , because γ‐Fe 2 O 3 is represented by the broad sextet with IS of 0.32 mm s −1 and hyperfine magnetic field of 487 kOe.…”

“…Since Fe is chelated with four pyridinic‐N in FeN 2+2 , the doublet may be assigned to FeN 2+2 more possibly. However, the turn over frequence (TOF) of the Fe‐N 2+2 /C site was proved similar in alkaline and acidic electrolytes . When the C‐PANI‐MIL‐2 shows different ORR performance in the alkaline and acidic electrolytes, more characterization such as XAFS should be conducted to recognize whether it's FeN 2+2 or FeN 4 .…”

“…From the results of t‐plot micropore analysis method (Table S1), a highest micropore area of 746 m 2 g −1 in C‐PANI‐MIL‐2 is also confirmed. Since the microporous structure may hold active sites for O 2 adsorption and reduction, C‐PANI‐MIL‐2 catalyst, with a superior micropore structure and BET surface area, may show a greater ORR behavior than other C‐PANI‐MIL catalysts accordingly. On the other hand, the meso‐ and macro‐pores of C‐PANI‐MIL‐2, which also appeared in SEM and TEM images of Figure , not only guarantee rapid diffusion of the reactant across the surface of catalyst, but also afford extra electrode‐O 2 ‐electrolyte triple‐phase interface to accelerate ORR.…”

Phase Diversity of Nickel Phosphides in Oxygen Reduction Catalysis

“…The cost, which is triggered by the exclusive use of noble metal catalysts (predominantly platinum and its alloys) particularly at the cathode side due to low reaction rates and high overpotential of the cathodic oxygen reduction reaction (ORR), is considered as the primary hurdle for fuel cells commercialization. [4,6,7,12,25] Among numerous types of NPMCs, transition metal-based nitrogendoped carbon (MÀN/C) catalysts for ORR, obtained through heat treatment of either iron and cobalt N 4 -macrocyclic metal complexes, [34][35][36][37] or simple precursor composites consisting of metal salts, a nitrogen source, and a carbon precursor, [38][39][40][41][42][43][44][45][46][47][48] have been extensively studied in recent years. [28][29][30][31][32][33][34] Therefore, exploring low-cost and efficient alternative catalysts for ORR is highly demanded.…”

Nitrogen-Doped Hollow Carbon Spheres with Embedded Co Nanoparticles as Active Non-Noble-Metal Electrocatalysts for the Oxygen Reduction Reaction

“…In order to solve these problems, heteroatom-doped carbon catalysts used in the ORR process have been developed in recent years [8][9][10][11][12]. Nitrogen doping has been extensively studied for its similar size to carbon, while the N atom has one electron more than carbon in the external shell [13,14].…”