Browsing by Author "Muhler, Martin"

Now showing 1 - 9 of 9
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    Highly Active Metal-free Nitrogen-Containing Carbon Catalysts for Oxygen Reduction Synthesized by ThermalTreatment of Polypyridine-Carbon Black Mixtures
    (Electrochemistry communications, 2011) Xia, Wei; Masa, Justus; Bron, Michael; Schuhmann, Wolfgang; Muhler, Martin
    A straight-forward method for the synthesis of metal-free catalysts for oxygen reduction by thermal treatment of a mixture of poly(3,5-pyridine) with carbon black in helium is reported. The catalyst was characterized by X-ray diffraction and photoelectron spectroscopy, cyclic voltammetry and rotating disk electrode measurements. The new catalyst exhibited remarkable activity similar to Pt-based catalysts in alkaline media.
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    Highly Concentrated Aqueous Dispersions of Graphene Exfoliated by Sodium Taurodeoxycholate: Dispersion Behavior and Potential Application as a Catalyst Support for the Oxygen-Reduction Reaction
    (Chemistry–A European Journal, 2012) Sun, Zhenyu; Masa, Justus; Liu, Zhimin; Schuhmann, Wolfgang; Muhler, Martin
    A high-yielding exfoliation of graphene at high concentrations in aqueous solutions is critical for both fundamental study and future applications. Herein, we demonstrate the formation of stable aqueous dispersions of pristine graphene by using the surfactant sodium taurodeoxycholate under tip sonication at concentrations of up to 7.1 mg mL−1. TEM showed that about 8 % of the graphene flakes consisted of monolayers and 82 % of the flakes consisted of less than five layers. The dispersions were stable regardless of freezing (−20 °C) or heat treatment (80 °C) for 24 h. The concentration could be significantly improved to about 12 mg mL−1 by vacuum-evaporation of the dispersions at ambient temperature. The as-prepared graphene dispersions were readily cast into conductive films and were also processed to prepare Pt/graphene nanocomposites that were used as highly active electrocatalysts for the oxygen-reduction reaction.
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    Metal-free Catalysts for Oxygen Reduction in Alkaline Electrolytes: Influence of the Presence of Co, Fe, Mn and Ni Inclusions
    (Electrochimica Acta, 2014) Masa, Justus; Zhao, Anqi; Xia, Wei; Muhler, Martin; Schuhmann, Wolfgang
    Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800 °C for 2 h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650 °C for 2 h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1 M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH− in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst.
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    MOF-Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions
    (Chemistry–A European Journal, 2017) Aijaz, Arshad; Masa, Justus; Antoni, Hendrik; Muhler, Martin
    Developing high-performance non-precious metal catalysts (NPMCs) for the oxygen-reduction reaction (ORR) is of critical importance for sustainable energy conversion. We report a novel NPMC consisting of iron carbide (Fe3C) nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized by a new metal-organic framework (MOF)-templated assembly approach. The electrocatalyst exhibits excellent ORR activity in 0.1 m KOH (0.89 V at −1 mA cm−2) and in 0.5 m H2SO4 (0.73 V at −1 mA cm−2) with a hydrogen peroxide yield of below 1 % in both electrolytes. Due to encapsulation of the Fe3C nanoparticles inside porous b-NCNTs, the reported NPMC retains its high ORR activity after around 70 hours in both alkaline and acidic media.
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    MoSSe@reduced Graphene Oxide Nanocomposite Heterostructures as Efficient and Stable Electrocatalysts for the Hydrogen Evolution Reaction
    (Nano Energy, 2016) Konkena, Bharathi; Masa, Justus; Xia, Wei; Muhler, Martin; Schuhmann, Wolfgang
    Non-noble metal based materials efficiently catalyzing the hydrogen evolution reaction (HER) are reported based on a novel strategy where electrocatalytically active ultrathin molybdenum sulphoselenide sheets are incorporated into electrically conducting reduced graphene oxide sheets via a self-assembly approach. By taking advantage of the electrostatic attraction between the two oppositely charged nanosheets, MoSSe@rGO composite materials are obtained exhibiting superior electrocatalytic activity and stability for the HER allowing a current density of 5mAcm−2 at a low overpotential of only 135mV. These findings pave the way to novel electrocatalysts based on composites of MoSSe and reduced graphene oxide towards the design of ultra-light, mechanically robust and electrically conductive electrode materials for electrocatalytic water splitting.
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    N-doped Carbon Synthesized from N-containing Polymers as Metal-free Catalysts for the Oxygen Reduction under Alkaline Conditions
    (Electrochimica Acta, 2013) Zhao, Anqi; Masa, Justus; Muhler, Martin; Schuhmann, Wolfgang; Xia, Wei
    Nitrogen-doped carbon materials were synthesized and used as metal-free electrocatalysts for the oxygen reduction reaction (ORR) under alkaline conditions. The synthesis was achieved by thermal treatment of nitrogen-containing polymers diluted in different carbon materials. Polypyrrole, polyaniline and polyacrylonitrile were used as N precursors. Carbon black and two types of commercial carbon nanotubes were used as carbon matrices. The obtained N contents were in the range of 1–1.8 wt.%. Different N species including pyridinic, pyrrolic and quaternary N were quantitatively determined by X-ray photoelectron spectroscopy. The ORR activities were evaluated in 0.1 M KOH. Rotating disc electrode studies revealed the presence of multiple active centers in all the samples. The sample obtained using polypyrrole and small diameter nanotubes (ca. 15 nm) had the highest onset potential at −0.07 V vs. Ag/AgCl/3 M KCl, which also showed a significantly higher electrochemical stability than the sample from carbon black and polypyrrole. The ORR activity was not correlated to the total nitrogen amount, but to the amount of pyridinic and quaternary N species. For the onset potential and the (Npyridinic + Nquaternary)/Ntotal ratio a quasi-linear relation was found, which points to the substantial role of pyridinic- and quaternary-N species in ORR catalysis.
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    On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction
    (Angewandte Chemie International Edition, 2015) Masa, Justus; Xia, Wei; Muhler, Martin; Schuhmann, Wolfgang
    The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-Nx/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-Nx/C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR.
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    Perovskite-based Bifunctional Electrocatalysts for Oxygen Evolution and Oxygen Reduction in Alkaline Electrolytes
    (Electrochimica acta, 2016) Elumeeva, Karina; Masa, Justus; Muhler, Martin; Schuhmann, Wolfgang
    Due to the high cost of precious metal-based electrocatalysts for oxygen reduction and oxygen evolution, the development of alternative low cost and efficient catalysts is of high importance for energy storage and conversion technologies. Although non-precious catalysts that can efficiently catalyze oxygen reduction and oxygen evolution have been developed, electrocatalysts with high bifunctional activity for both oxygen evolution and reduction are needed. Perovskites based on modified lanthanum cobaltite possess significant activity for the oxygen evolution reaction. We describe the synthesis of a bifunctional oxygen electrode with simultaneous activity for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media by direct growth of nitrogen-doped carbon nanotubes on the surface of a perovskite containing Co and Fe by means of chemical vapor deposition. The difference in the overvoltage between ORR (at 1mA/cm2) and OER (at 10mA/cm2) was below 880mV in 0.1M KOH. The formation of H2O2 during the ORR was reduced by at least three fold when using the bifunctional catalyst as compared to the non-modified perovskite. Long-term durability tests indicate stable performance for at least 37h during the OER and 23h during the ORR.
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    Trace metal residues promote the activity of supposedly metal-free nitrogen-modified carbon catalysts for the oxygen reduction reaction
    (Electrochemistry communications, 2013) Masa, Justus; Zhao, Anqi; Xia, Wei; Muhler, Martin; Schuhmann, Wolfgang
    We show in this study that the presence of trace metal residues in some supposedly metal-free catalysts for oxygen reduction, at concentrations which are difficult to detect using conventional methods such as XPS and EDX, can profoundly promote the ORR activity of the catalysts.