The Marriage of the FeN₄ Moiety and MXene Boosts Oxygen Reduction Catalysis: Fe 3d Electron Delocalization Matters

Abstract

Iron-nitrogen-carbon (Fe-N-C) is hitherto considered as one of the most satisfactory alternatives to platinum for the oxygen reduction reaction (ORR). Major efforts currently are devoted to the identification and maximization of carbon-enclosed FeN moieties, which act as catalytically active centers. However, fine-tuning of their intrinsic ORR activity remains a huge challenge. Herein, a twofold activity improvement of pristine Fe-N-C through introducing Ti C T MXene as a support is realized. A series of spectroscopy and magnetic measurements reveal that the marriage of FeN moiety and MXene can induce remarkable Fe 3d electron delocalization and spin-state transition of Fe(II) ions. The lower local electron density and higher spin state of the Fe(II) centers greatly favor the Fe electron transfer, and lead to an easier oxygen adsorption and reduction on active FeN sites, and thus an enhanced ORR activity. The optimized catalyst shows a two- and fivefold higher specific ORR activity than those of pristine catalyst and Pt/C, respectively, even exceeding most Fe-N-C catalysts ever reported. This work opens up a new pathway in the rational design of Fe-N-C catalysts, and reflects the critical influence of Fe 3d electron states in FeN moiety supported on MXene in ORR catalysis.

Two‐Dimensional Conjugated Aromatic Networks as High‐Site‐Density and Single‐Atom Electrocatalysts for the Oxygen Reduction Reaction

“…As shown in Figure 1f,t wo pairs of peaks at 721.3/ 715.0 and 800.7/785.5 eV were observed in the EELS spectrum of CAN-Pc(Fe/ Co), corresponding to the Fe L 2 /L 3 and Co L 2 /L 3 edges,r espectively.E lement mapping analysis also confirmed the homo- Theelectronic states of C, N, Fe,and Co were probed by X-ray photoelectron spectroscopy (XPS). [16] Theextent of electron delocalization on the Fe and Co atoms in the bimetallic CAN-Pc(Fe/Co) sample was explored further using X-ray absorption fine structure (XAFS) measurements.Inthe Co K-edge X-ray absorption near-edge structure (XANES) spectra (Supporting Information, Figure S17 a), both CAN-Pc(Fe/Co) and CAN-Pc(Co) showed similar proximate XANES curves to CoPc,b ut not to Co foil and Co oxides. [14] Thee nhanced intensities of the deconvoluted p-p*a nd metal-N peaks in the spectrum of CAN-Pc(M) relative to the corresponding peaks for FePc and CoPc can be attributed to the enhanced conjugation and stronger bonding between the central metal ions and phthalocyanine macrocycles in the former.…”

“…Notably,t he CAN-Pc(Fe/Co) presented the more electron transfer from central metal ions to phthalocyanine macrocycles than the CAN-Pc(Fe) and CAN-Pc(Co), as evidenced from its most positive charge state of either Fe or Co in all the samples. [16] Theextent of electron delocalization on the Fe and Co atoms in the bimetallic CAN-Pc(Fe/Co) sample was explored further using X-ray absorption fine structure (XAFS) measurements.Inthe Co K-edge X-ray absorption near-edge structure (XANES) spectra (Supporting Information, Figure S17 a), both CAN-Pc(Fe/Co) and CAN-Pc(Co) showed similar proximate XANES curves to CoPc,b ut not to Co foil and Co oxides. This suggests that CAN-Pc(Fe/Co) and CAN-Pc(Co) both possess aC o-N 4 structure like CoPc.C areful examination of the Co pre-edge region showed the high energy shift of the XANES edges increased in the order CoO < CoPc < CAN-Pc(Co) < CAN-Pc(Fe/Co) < Co 3 O 4 .T his is consistent with the XPS results,inwhich the bimetallic sample presented the most positive charge distribution (closer to the Co III state).…”

Surface Modified MXene‐Based Nanocomposites for Electrochemical Energy Conversion and Storage

“…While the freeze‐dried TiO 2 @Ti 3 C 2 T m MXene reported by Chen and co‐workers might belong to IWI products rather than assembly 30j. Guo and co‐workers recently obtained the iron phthalocyanine (FePc)@Ti 3 C 2 T m MXene hybrids by self‐assembled the FePc with Ti 3 C 2 T m MXene . As the FePc molecular possesses a conjugate rigid plane structure, the driven force for the complex assembly may mainly originate from the van der Waal's interactions, while not ruling out the existence of other secondary bindings.…”

2D Early Transition Metal Carbides (MXenes) for Catalysis

“…The catalyst delivered a potential of 1.64 V versus RHE at a current density of 10 mA cm −2 in 0.1 m KOH (Figure d), which was better than the benchmark IrO 2 catalysts. Guo and co‐workers reported a Ti 3 CT x MXene supported iron phthalocyanine (FePc) catalyst for enhanced activity of ORR . With the addition of Ti 3 C 2 T x , the half‐wave potential ( E 1/2 ) shifted to ≈0.88 V (Figure e).…”

“…f) TOFs of active FeN 4 sites in FePc and FePc/Ti 3 C 2 T x . Reproduced with permission . Copyright 2018, Wiley‐VCH.…”