Experimental and theoretical modeling of Fe-, Co-, Cu-, Mn-based electrocatalysts for oxygen reduction

Abstract

Abstract:Experience gained during efforts towards optimization of noble-metal-free electrocatalysts for oxygen reduction is simultaneously used to understand the chemical and morphological necessities for inducing efficient multi-electron transfer catalysis. The analysis of many preparative experimental steps between the moderately performing metal porphyrines and the highly efficient transition metal-and sulfur-containing pyrolised catalyst material contributes to the following model of the catalyst: The metals function enclosed in nitrogen or graphitic environment where they are shielded against irreversible oxidation. The metals can be exchanged but are not identical in their efficiency. Higher efficiency is achieved, when the function of a binary reaction center is warranted. The carbonization of the environment is critical and provides intercalated metal centers and attached metal complexes in graphite environment for interaction with the nitrogen-chelated partner center in the simultaneously obtained graphene layers. Three alternatives for the binary catalytic center are presented and their relevance discussed on the basis of EXAFS, RAMAN, EPR, Mössbauer and X-ray spectroscopy. A parallel is drawn with the cytochrome oxidase oxygen reduction catalysis, which is proposed to proceed according to roughly the same mechanism.

Exploring Active Sites in Multi‐Heteroatom‐Doped Co‐Based Catalysts for Hydrogen Evolution Reactions

“…This effect is commonly known for M–N–C catalysts . When performed in an inert gas atmosphere, the increase in activity was basically correlated with an increase in BET surface area …”

Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

“…Recent ab initio investigations have discussed the formation of MeN x C y moieties in graphene sheets [27][28][29][30][31] . Binuclear Fe 2 N x C y sites have also been reported for complexes or proposed for Me-N-C catalysts 30,[32][33][34] . However, the spectroscopic fingerprint of this plethora of hypothetical sites has not yet been calculated nor compared to the spectroscopic response of Me-N-C catalysts free of metal-based crystallographic structures.…”

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

“…[32,37,[40][41][42][43][44] There is enormous research activity concerned with the development of NC catalysts for the ORR, and the methods used for their synthesis are numerous.T he synthesis of NC catalysts can be conveniently grouped into two categories, namely,m ethods which do not involve the use of any metal precursors and those which make use of metal precursors. [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] Proponents of the M-N x /C catalysts believe that the metal ion is involved in the active site,c ontrary to the theories that claim that the metal ions only facilitate the formation of active sites but do not themselves participate as active sites. This controversy arises because of the close similarity between NC catalysts produced using metal precursors and the M-N x /C catalysts,w here the presence of the metal (M = Co,F e, or Mn) is considered essential.…”