Fe₃O₄ Templated Pyrolyzed Fe−N−C Catalysts. Understanding the role of N‐Functions and Fe₃C on the ORR Activity and Mechanism

Abstract

Pyrolyzed non-precious metal catalysts have been proposed as an alternative to substitute the expensive and scarce noble metal catalysts in several conversion energy reactions. For the oxygen reduction reaction (ORR), the pyrolyzed catalyst MÀ NÀ C (M: Fe or Co) presents remarkable catalytic activity in acid and alkaline media. These pyrolyzed materials show a high heterogeneity of active sites being the most active in the MNx moieties. The activity and stability of these catalysts are also conditioned by other structural parameters such as the area, the N-doping, and by the presence of metal particles. In this study, we explore the use of Fe 3 O 4 nanoparticles as templates and as iron sources to synthesize FeÀ NÀ C. The best perform-ance for the ORR in acidic media was reached with the catalysts using nanoparticles covered by PANI and iron salts as the precursor, with an onset potential of 0.85 vs. RHE and a direct 4-electrons mechanism. We corroborated the use of the catalysts' redox potential as reactivity descriptors and discussed the detrimental role of the presence of Fe 3 C metallic particles in the mechanism. Based on the experimental results, we performed DFT calculations to explore the influence of N-doped species on the electronic density of the iron centers of FeN4 active sites, and we propose a theoretical model for increasing the activity based on the distance and ratio of N-doping to iron center.

Effect of Electrolyte Media on the Catalysis of Fe Phthalocyanine toward the Oxygen Reduction Reaction: Ab Initio Molecular Dynamics Simulations and Experimental Analyses

“…From this point of view, those results are in contrast with our observations using FePc as an electrocatalyst. Despite some similarities in the active site structure of the pyrolyzed catalyst and iron phthalocyanine, the anion influence is radically different because of a variation in the typology of the active sites (i.e., pyrolyzed catalysts are a composition of FeN 4+1 , FeN 4 , FeN 3, FeN 2+1 , FeN 2+2 , Fe 3 C, Fe 4 N, N pyridinic , and N pyrrolic − ) and differences in the carbon support, which has a porosity optimized for O 2 in the case of the pyrolyzed catalysts. − …”

Lignin-Derived Precious Metal-Free Electrocatalysts for Anion-Exchange Membrane Fuel Cell Application

“…75,76 At higher overpotentials (E < 0.8 V), higher slope values were obtained (−145, −154, and −117 mV dec −1 , respectively); the change of the slope can be attributed to a different ORR mechanism and potentialdependent surface concentration of Fe(II) at low overpotentials. 75,76 However, the cyclic voltammograms of the catalysts recorded in Ar-saturated 0.1 M KOH solution (Figure S9b) do not show any features between 0.8 and 0.65 V characteristic to the Fe(III)/(II) redox process, 75,77 possibly due to low content and the heterogeneous nature of Fe−N x centers. The small peaks at high overpotentials (E < 0.5 V) on Fe-containing catalysts can be attributed to the Fe(II)/(I) redox transition.…”

Unravelling the role of iron carbide in oxygen reduction catalysts for rechargeable zinc–air batteries: a comprehensive kinetics & mechanistic study

“…Fe-N x promotes the 4e − direct pathway whereas Fe 3 C directs the reduction via the stepwise pathway (2 + 2e ORR). 34,35 The N-doped carbon encapsulated Fe 3 C nanoparticles are active towards the oxygen evolution reaction (OER), whereas Fe-N x or N-C active sites are known for the ORR. Hence the dual active sites of Fe 3 C/Fe-N x composites act as a bifunctional catalyst in Zn-air batteries.…”