{"content"=>"The Marriage of the FeN Moiety and MXene Boosts Oxygen Reduction Catalysis: Fe 3d Electron Delocalization Matters.", "sub"=>{"content"=>"4"}} (original) (raw)
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ChemElectroChem, 2019
A model for a non-precious metal catalyst for the oxygen reduction reaction (ORR) in aqueous media has been prepared by functionalizing a commercial Vulcan XC-72 carbon support with a terpyridine-based nitrogenous ligand. The terpyridine ligand geometry allows the formation of active catalytic sites by selectively embedding a N 3 /C structural motif into the carbon support confirmed by using thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) measurements. Room-temperature metal-ligand coordination results in the desired FeÀ N 3 /C moieties on the surface. This model system was used to demonstrate the catalytic activity of the surfaces containing mainly FeÀ N 3 sites for the ORR in acidic and basic media. Importantly, we demonstrate that the system could be prepared under mild reaction conditions, does not require hightemperature treatments, and shows catalytic activity for the ORR. Interestingly, when the system was pyrolyzed in an N 2 atmosphere at 700°C the resulting activity declined. The nonheat-treated FeÀ N 3 /C surface demonstrates comparable activity in acidic electrolyte medium when compared to most literature catalysts that are typically heat treated to produce four nitrogen atoms coordinated to one iron center (FeÀ N 2 + 2 /C). Interestingly, despite the fact that many systems reported so far in the literature exhibit enhanced activity after heat treatment, our system showed an increase in activity when the material was not pyrolyzed.
ChemElectroChem
A model for a non-precious metal catalyst for the oxygen reduction reaction (ORR) in aqueous media has been prepared by functionalizing a commercial Vulcan XC-72 carbon support with a terpyridine-based nitrogenous ligand. The terpyridine ligand geometry allows the formation of active catalytic sites by selectively embedding a N 3 /C structural motif into the carbon support confirmed by using thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) measurements. Room-temperature metal-ligand coordination results in the desired FeÀ N 3 /C moieties on the surface. This model system was used to demonstrate the catalytic activity of the surfaces containing mainly FeÀ N 3 sites for the ORR in acidic and basic media. Importantly, we demonstrate that the system could be prepared under mild reaction conditions, does not require hightemperature treatments, and shows catalytic activity for the ORR. Interestingly, when the system was pyrolyzed in an N 2 atmosphere at 700°C the resulting activity declined. The nonheat-treated FeÀ N 3 /C surface demonstrates comparable activity in acidic electrolyte medium when compared to most literature catalysts that are typically heat treated to produce four nitrogen atoms coordinated to one iron center (FeÀ N 2 + 2 /C). Interestingly, despite the fact that many systems reported so far in the literature exhibit enhanced activity after heat treatment, our system showed an increase in activity when the material was not pyrolyzed.
ChemSusChem, 2017
Structures and morphologies of Fe-N-C catalysts have been believed crucial for the number of active sites and local bonding structures, therefore governing overall catalyst performance for the oxygen reduction reaction (ORR). The relevant knowledge is still lacking for the rational catalyst design. Through combining different nitrogen/carbon precursors, including polyaniline (PANI), dicyandiamide (DCDA), and melamine (MLMN), we aim to tune catalyst morphology and structure for facilitating the ORR. Instead of commonly studied single precursors, multiple precursors during the synthesis provide a new opportunity to promote catalyst activity and stability via a likely synergistic effect. The best performing Fe-N-C catalyst derived from PANI+DCDA is superior to individual PANI or DCDA-derived one. In particular, when compared to extensively explored PANI-derived catalysts, the binary precursors are able to achieve improved half-wave potential of 0.83 V and enhanced electrochemical stability in challenging acidic media, indicating significantly increased active site number and strengthened local bonding structures. Multiple key factors associated with the observed promotion are elucidated including the optimal pore size distribution, highest electrochemically active surface area, presence of dominant amorphous carbon, and thick graphitic carbon layers with more pyridinic nitrogen edge sites likely bonded with atomic iron.
Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts
Angewandte Chemie (International ed. in English), 2017
Fe-N-C catalysts with high O2 reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H2 O2 reduction, a key intermediate during indirect O2 reduction and a source of deactivation in fuel cells. Catalysts comprising different relative contents of FeNx Cy moieties and Fe particles encapsulated in N-doped carbon layers (0-100 %) show that both types of sites are active, although moderately, toward H2 O2 reduction. In contrast, N-doped carbons free of Fe and Fe particles exposed to the electrolyte are inactive. When catalyzing the ORR, FeNx Cy moieties are more selective than Fe particles encapsulated in N-doped carbon. These novel insights offer rational approaches for more selective and therefore more durable Fe-N-C catalysts.
A MnOx enhanced atomically dispersed iron–nitrogen–carbon catalyst for the oxygen reduction reaction
Journal of materials chemistry. A, Materials for energy and sustainability, 2022
Cost-effective and highly efficient Fe-N-C single-atom catalysts (SACs) have been considered to be one of the most promising potential Pt substitutes for the cathodic oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Nevertheless, they are subject to severe oxidative corrosion originating from the Fenton reaction, leading to poor long-time durability of PEMFCs. Herein, we propose a MnO x engineered Fe-N-C SAC (Mn-Fe-N-C SAC) to reduce and even eliminate the stability issue, as MnO x accelerates the degradation of the H 2 O 2 by-product via a disproportionation reaction to weaken the Fenton reaction. As a result, the Mn-Fe-N-C SAC shows an ultralow H 2 O 2 yield and a negligible half-wave potential shift after 10 000 continuous potential cycles, demonstrating excellent ORR stability. Besides, the Mn-Fe-N-C SAC also shows an improved ORR activity compared to the common Fe-N-C SAC. Results show that the MnO x interacts with the Fe-N x site, possibly forming Fe-Mn or Fe-O-Mn bonds, and enhances the intrinsic activity of single iron sites. This work provides a method to overcome the stability problem of Fe-N-C SACs while still yielding excellent catalytic activity, thus showing great promise for application in PEMFCs.
Journal of Physical Chemistry B, 2002
Three catalysts for the electroreduction of oxygen have been prepared by pyrolyzing between 400 and 1000°C two iron precursors (Fe acetate or Fe porphyrin) adsorbed on a synthetic carbon made from the pyrolysis of PTCDA (perylene tetracarboxylic dianhydride) in a H 2 /NH 3 /Ar atmosphere. One Fe loading (0.2 wt %) has been used for the catalyst made from the salt precursor. Two Fe loadings (0.2 and 2.0 wt %) have been used for the catalyst made from the porphyrin precursor. These three catalysts have been analyzed by ToF SIMS and RDE (or GDE) in order to find correlations between ions detected by ToF SIMS and the catalytic activity. These correlations provide information about the number and the structure of the catalytic sites, which are active in these materials. By following the variation of FeN x C y + ions, it is found that (i) two different catalytic sites exist simultaneously in all catalysts made with the Fe salt or the Fe porphyrin; (ii) one site, named FeN 4 /C, is at the origin of three families of FeN x C y + ions: FeN 4 C y + , FeN 3 C y + , and FeN 1 C y +. The most representative ion of that site is FeN 4 C 8 +. The other site, labeled FeN 2 /C, is at the origin of the family of FeN 2 C y + ions. The most representative ion of that site is FeN 2 C 4 + ; (iii) the abundance of FeN 2 /C goes through a maximum for catalysts pyrolyzed between 700 and 900°C. When Fe acetate is the Fe precursor, FeN 2 /C may represent up to 80% of the catalytic sites, while this falls to a maximum of about 50% when Fe porphyrin is the precursor; (iv) FeN 2 /C is more electrocatalytically active than FeN 4 /C; (v) at high porphyrin loading (2.0 wt % Fe), the catalytic sites bound to the carbon support are covered with a porous layer of pyrolyzed Fe porphyrin.
Nano Energy, 2016
Compared to extensively studied oxygen reduction reaction (ORR) catalysis in alkaline media, development of highly active and stable nonprecious metal catalysts (NPMCs) to replace Pt in acidic electrolytes remains grand challenges. Among currently studied catalysts, the Fe-N-C formulation holds the greatest promise for the ORR in acid. Here, we report a new highly active and stable Fe-N-C catalyst featured with well-dispersed atomic Fe in porous carbon matrix, which was prepared through one single thermal conversion from Fe-doped ZIF-8, a metal-organic framework (MOF) containing Zn 2+ and well-defined Fe-N4 coordination. Unlike other Fe-N-C catalyst preparation, no additional tedious post-treatments such as acid leaching and the second heating treatment are required in this work. Notably, an O2-free environment for preparing the Fedoped ZIF-8 precursor is found to be crucial for yielding uniform Fe distribution into highly porous N-doped carbon matrix. The resulting new Fe-N-C catalyst exhibited exceptionally improved ORR
Electrochimica Acta, 2006
Fe-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cell conditions have been prepared by adsorbing two Fe precursors on various commercial and developmental carbon supports. The resulting materials have been pyrolyzed at 900 • C in an atmosphere rich in NH 3. The Fe precursors were: iron acetate (FeAc) and iron tetramethoxy phenylporphyrin chloride (ClFeTMPP). The nominal Fe content was 2000 ppm (0.2 wt.%). The carbon supports were HS300, Printex XE-2, Norit SX-Ultra, Ketjenblack, EC-600JD, Acetylene Black, Vulcan XC-72R, Black Pearls 2000, and two developmental carbon black powders, RC1 and RC2 from Sid Richardson Carbon Corporation. The catalyst activity for ORR has been analyzed in fuel cell tests at 80 • C as well as by cyclic voltammetry in O 2 saturated H 2 SO 4 at pH 1 and 25 • C, while their selectivity was determined by rotating ring-disk electrode in the same electrolyte. A large effect of the carbon support was found on the activity and on the selectivity of the catalysts made with both Fe precursors. The most important parameter in both cases is the nitrogen content of the catalyst surface. High nitrogen content improves both activity towards ORR and selectivity towards the reduction of oxygen to water (4e − reaction). A possible interpretation of the activity and selectivity results is to explain them in terms of two Fe-based catalytic sites: FeN 2 /C and FeN 4 /C. Increasing the relative amount of FeN 2 /C improves both activity and selectivity of the catalysts towards the 4e − reaction, while most of the peroxide formation may be attributed to FeN 4 /C. When FeAc is used as Fe precursor, iron oxide and/or hydroxide are also formed. The latter materials have low catalytic activity for ORR and reduce O 2 mainly to H 2 O 2 .