Hierarchical oxygen reduction reaction electrocatalysts based on FeSn0.5 species embedded in carbon nitride-graphene based supports

Mesoporous carbon nitride supported 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(ii) as a selective and durable electrocatalyst for the production of hydrogen peroxide via two-electron oxygen reduction

Stability and efficiency of CoTMPP@MCN along with the general strategy for tuning the active site.

Metal (Mn, Fe, Co, Ni, Cu, and Zn) Phthalocyanine-Immobilized Mesoporous Carbon Nitride Materials as Durable Electrode Modifiers for the Oxygen Reduction Reaction

In the search for alternative sources to replace fossil fuels, carbon nitride materials can be used in a variety of ways. In the present work, porosity is introduced to the carbon nitride material using mesoporous silica material, MCM-41, as a hard template, and a mesoporous carbon nitride (MCN) material is synthesized. Further, the MCN is modified by immobilizing metal phthalocyanine (MPc, where M = Mn, Fe, Co, Ni, Cu, and Zn). The resulting MPc-incorporated MCN materials (MPc@MCN) were tested for the electrocatalytic oxygen reduction reaction (ORR) in acidic and basic media. Detailed studies reveal that the FePc@MCN and CoPc@MCN materials exhibit higher ORR activity than the other composites in 0.1 M KOH. FePc@MCN follows a direct four-electron oxygen reduction mechanism and shows ORR onset potential (vs RHE) at 0.93 V (in 0.1 M KOH), which is very close to the onset potential exhibited by the state-of-the-art material, Pt-C (1.0 V), and higher than several similar composites of MPc with carbon supports tested in similar environments. Besides, due to the inherent property of coordination through nitrogen present on the MCN, FePc@MCN shows excellent stability even after 3000 cyclic voltammetry (CV) cycles. FePc@MCN was found to have a better methanol tolerance in comparison to Pt-C in basic medium. CoPc@MCN shows a highly selective two-electron reduction reaction in both acidic and basic media at lower overpotential than many of the reported catalysts for the two-electron oxygen reduction. Therefore, these materials (FePc@MCN and CoPc@MCN) can be used as suitable alternatives to replace Pt and other expensive materials in ORR and related applications.

The Challenge of Achieving a High Density of Fe-Based Active Sites in a Highly Graphitic Carbon Matrix

As one of the most promising platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR), Fe–N–C catalysts with a high density of FeNx moieties integrated in a highly graphitic carbon matrix with a proper porous structure have attracted extensive attention to combine the high activity, high stability and high accessibility of active sites. Herein, we investigated a ZnCl2/NaCl eutectic salts-assisted ionothermal carbonization method (ICM) to synthesize Fe–N–C catalysts with tailored porous structure, high specific surface area and a high degree of graphitization. However, it was found to be challenging to anchor a high density of FeNx sites onto highly graphitized carbon. Iron precursors with preexisting Fe–N coordination were required to form FeNx sites in the nitrogen-doped carbon with a high degree of graphitization, while individual Fe and N precursors led to a Fe–N–C catalyst with poor-ORR activity. This provides valuable insights into the synthesis-structure relationship. Moreover, the FeNx moieties were identified as the major active sites in acidic conditions, while both FeNx sites and Fe2O3 were found to be active in alkaline medium.