Experimental Observation of Redox-Induced Fe–N Switching Behavior as a Determinant Role for Oxygen Reduction Activity

Abstract

The commercialization of electrochemical energy conversion and storage devices relies largely upon the development of highly active catalysts based on abundant and inexpensive materials. Despite recent achievements in this respect, further progress is hindered by the poor understanding of the nature of active sites and reaction mechanisms. Herein, by characterizing representative iron-based catalysts under reactive conditions, we identify three Fe-N4-like catalytic centers with distinctly different Fe-N switching behaviors (Fe moving toward or away from the N4-plane) during the oxygen reduction reaction (ORR), and show that their ORR activities are essentially governed by the dynamic structure associated with the Fe(2+/3+) redox transition, rather than the static structure of the bare sites. Our findings reveal the structural origin of the enhanced catalytic activity of pyrolyzed Fe-based catalysts compared to nonpyrolyzed Fe-macrocycle compounds. More generally, the fundamental insights into the dynamic nature of transition-metal compounds during electron-transfer reactions will potentially guide rational design of these materials for broad applications.

Enhancement of oxygen reduction at Co-porphyrin catalyst by supporting onto hybrid multi-layered film of polypyrrole and polyoxometalate-modified gold nanoparticles

“…The appearance of the molybdate (PMo 12 ) redox transitions at more positive potentials relative to those characteristic of the Co(II)/(III) redox couple may explain higher activity toward oxygen reduction of the systems containing PMo 12 O 40 3− together with cobalt porphyrin in comparison to pure CoPPIX [33]. The similar electrocatalytic enhancement effect has been postulated for the pyrolyzed FeNx catalyst co-existing with metallic centers characterized by redox processes existing at the relatively more positive potentials [34]. Furthermore, it is reasonable to expect that gold nanostructures acting as the powerful hydrogen peroxide reduction catalyst induce the second step in the oxygen reduction mechanism: reductive decomposition of hydrogen peroxide.…”

Synthesis and Characterization of Carbon/Nitrogen/Iron Based Nanoparticles by Laser Pyrolysis as Non-Noble Metal Electrocatalysts for Oxygen Reduction

“…Furthermore, the nature of the active sites is still strongly debated. Different views conflict with each other and for example, some authors reported that iron species coordinated with nitrogen in various configuration referred as FeN x or FeN x C y are mainly responsible for the electrochemical activity [47][48][49], while others showed that iron-free electrocatalysts with C x N y sites have significant activity [50,51]. Furthermore, recently, the ORR activity of iron based (Fe 3 C) core-shell particles in which no nitrogen atoms are incorporated in the carbon shell was also reported [52].…”

Structural and mechanistic basis for the high activity of Fe–N–C catalysts toward oxygen reduction

“…5,[10][11][12][13][14][15][16][17][18] To date, three types of active sites have been shown to exhibit decent ORR activity in acidic media: metal-nitrogen moieties embedded in carbon (denoted as MN x C y ), 9,11,14,16,18,19 nitrogen-carbon moieties (denoted as N x C y ), 20,21 and nitrogen doped carbon encapsulating inorganic metal species (denoted as M@N x C y ). 5,[10][11][12][13][14][15][16][17][18] To date, three types of active sites have been shown to exhibit decent ORR activity in acidic media: metal-nitrogen moieties embedded in carbon (denoted as MN x C y ), 9,11,14,16,18,19 nitrogen-carbon moieties (denoted as N x C y ), 20,21 and nitrogen doped carbon encapsulating inorganic metal species (denoted as M@N x C y ).…”