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Dive into the research topics where Iris Herrmann-Geppert is active.

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Featured researches published by Iris Herrmann-Geppert.


Journal of the American Chemical Society | 2016

On an Easy Way To Prepare Metal–Nitrogen Doped Carbon with Exclusive Presence of MeN4-type Sites Active for the ORR

Ulrike I. Kramm; Iris Herrmann-Geppert; Jan Behrends; Klaus Lips; Sebastian Fiechter; Peter Bogdanoff

Today, most metal and nitrogen doped carbon catalysts for ORR reveal a heterogeneous composition. This can be reasoned by a nonoptimized precursor composition and various steps in the preparation process to get the required active material. The significant presence of inorganic metal species interferes with the assignment of descriptors related to the ORR activity and stability. In this work we present a simple and feasible way to reduce the contribution of inorganic metal species in some cases even down to zero. Such catalysts reveal the desired homogeneous composition of MeN4 (Me = metal) sites in the carbon that is accompanied by a significant enhancement in ORR activity. Among the work of other international groups, our iron-based catalyst comprises the highest density of FeN4 sites ever reported without interference of inorganic metal sites.


Journal of Materials Chemistry | 2014

Effect of iron-carbide formation on the number of active sites in Fe–N–C catalysts for the oxygen reduction reaction in acidic media

Ulrike I. Kramm; Iris Herrmann-Geppert; Sebastian Fiechter; Gerald Zehl; Ivo Zizak; Iris Dorbandt; Dieter Schmeißer; Peter Bogdanoff

In this work Fe–N–C catalysts were prepared by the oxalate-supported pyrolysis of FeTMPPCl or H2TMPP either in the presence or absence of sulfur. The well-known enhancing effect of sulfur-addition on the oxygen reduction activity was confirmed for these porphyrin precursors. The pyrolysis process was monitored in situ by high-temperature X-ray diffraction under synchrotron radiation (HT-XRD) and thermogravimetry coupled with mass-spectroscopy (TG-MS). It was found that the beneficial effect of sulfur could be attributed to the prevention of iron-carbide formation during the heat-treatment process. In the case of pyrolysis of the sulfur-free precursors an excessive iron-carbide formation leads to disintegration of FeN4-centers, hence limiting the number of ORR active sites on the final catalyst. Physical characterization of the catalysts by bulk elemental analysis, X-ray diffraction (XRD), Raman and 57Fe Mosbauer spectroscopy confirmed the outcome from HT-XRD and TG-MS. It could be shown that the avoidance of carbide formation during pyrolysis represents a promising way to enhance the density of ORR active sites on those catalysts. This can be done either by sulfur-addition or the performance of an intermediate acid leaching. As iron carbide is often found as a by-product in the preparation of Fe–N–C catalysts this work gives some general strategies for enhancing the density of active sites enabling higher current densities.


Journal of Materials Chemistry | 2014

α-Fe2O3 films for photoelectrochemical water oxidation – insights of key performance parameters

Lichao Jia; Karsten Harbauer; Peter Bogdanoff; Iris Herrmann-Geppert; Alejandra Ramírez; Roel van de Krol; Sebastian Fiechter

We report the deposition of ultra-thin α-Fe2O3 (hematite) films on fluorine-doped tin oxide (FTO) substrates using radio frequency (RF) sputtering, and the investigation of their photoelectrochemical (PEC) performance towards water oxidation. By varying the deposition pressure and time, the film microstructure and morphology could be optimized. The best hematite films having a thickness of about 50 nm exhibited a photocurrent density of 0.59 mA cm−2 at U = 1.23 V vs. RHE and 1.92 mA cm−2 at U = 1.85 V using a tungsten halogen lamp of 40 mW cm−2 light intensity in the wavelength range from 300 to 600 nm. These values are comparable or even higher than those ever measured hematite films (undoped and having no co-catalyst deposited on top of the electrode). Further measurements were explored to investigate the limiting factors in our films for possibly approaching their predicted PEC properties. A detailed analysis reveals that a slow water oxidation reaction and a trapping of charges on the surface, especially at the potential below 1.4 V, are obviously the reasons for the limited PEC performance.


220th ECS Meeting | 2012

Hematite (α-Fe2O3) Photoanodes for the Photooxidation of Water

Iris Herrmann-Geppert; Peter Bogdanoff; Linda Hepperle; Sebastian Fiechter

Nanostructured hematite films prepared by a sol-gel procedure were evaluated for the photo-induced oxygen evolution reaction (OER). Surface treatments such as plasma and post-annealing at 500 °C in oxygen gas flow were used in order to condition the electrode surface. Electrochemical measurements of the plasma treated samples revealed that the surface was reorganized in terms of enhanced catalytic properties and reduced recombination centers at the surface. Photocurrents in the milli-Ampere range were yielded at 0.92 V(NHE) in 1 M KOH. In contrast to the plasma samples, the post-annealing samples displayed higher bulk crystallinity resulting in a more enhanced photovoltage. XRD and TEM analysis confirmed the texturing of a beneficial highly oriented hematite film (110) parallel to the back contact. Nevertheless, lower photocurrents for this post-annealed sample compared to the plasma one indicated that the catalytic centers were not fully recovered at the surface.


Solar Hydrogen and Nanotechnology VIII | 2013

Cold gas spraying of semiconductor coatings for the photooxidation of water

Thomas Emmler; Henning Gutzmann; Philipp Hillebrand; Mauricio Schieda; R. Just; Frank Gärtner; Peter Bogdanoff; Iris Herrmann-Geppert; Thomas Klassen

This contribution shows the potential of cold gas spraying for the production of photoelectrodes employing photoelectrocatalysts for the water oxidation reaction. Conventional methods of coating usually employ sol-gel methods and calcination to obtain a good binding of the coating to the substrate. In cold gas spraying, particles are accelerated to high velocities by a pressurized gas. Nitrogen is used as process gas, preheated and then expanded in a De Laval type nozzle. On impact with the substrate the particles deform, break up and build an efficient interface to the back contact (as revealed, for example, by scanning electron microscopy). Cold gas spraying is a method for the direct bonding of particles to a substrate and does not require additives that have to be removed e.g. by a calcination step. Thereby it allows the direct fabrication of a working electrode ensemble. In our initial experiments, the state-of-the-art photocatalyst titanium dioxide (TiO2) was explored. The cold-gas-sprayed coatings revealed significantly higher activities for the oxygen evolution reaction (OER), as compared to films derived from wet-chemical processes. Due to the demand for photocatalysts with band gap suitable for visible light absorption, this approach was extended to the promising catalyst material hematite. In correlation with photoelectrochemical measurements, the operating parameters of the cold gas spray process are discussed in terms of their influence on the photocatalytic properties of the semiconductor.


Journal of Physical Chemistry C | 2011

Effect of an Ammonia Treatment on Structure, Composition, and Oxygen Reduction Reaction Activity of Fe–N–C Catalysts

Ulrike I. Kramm; Iris Herrmann-Geppert; Peter Bogdanoff; Sebastian Fiechter


Physical Chemistry Chemical Physics | 2013

Surface aspects of sol–gel derived hematite films for the photoelectrochemical oxidation of water

Iris Herrmann-Geppert; Peter Bogdanoff; Jörg Radnik; Steffen Fengler; Thomas Dittrich; Sebastian Fiechter


220th ECS Meeting | 2011

Improved Platinum Electrocatalyst for the Oxygen Reduction Reaction Using Nitrogen-Modified Carbon Support

Iris Herrmann-Geppert; Joerg Radnik; Ulrike I. Kramm; Sebastian Fiechter; Peter Bogdanoff


ECS Transactions | 2014

Cold Gas Sprayed TiO2-Based Electrodes for the Photo-Induced Water Oxidation

Iris Herrmann-Geppert; Peter Bogdanoff; Henning Gutzmann; Thomas Dittrich; Thomas Emmler; Regina Just; Mauricio Schieda; Frank Gärtner; Thomas Klassen


PRiME 2016/230th ECS Meeting (October 2-7, 2016) | 2016

Solar Water Splitting Photoelectrodes Structured By Laser Interference Lithography

Herman Kriegel; Mauricio Schieda; Iris Herrmann-Geppert; Dmitry L. Voronov; Deirdre Olynick; Thomas Klassen

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Thomas Klassen

Helmut Schmidt University

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Thomas Emmler

Free University of Berlin

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Ulrike I. Kramm

Technische Universität Darmstadt

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Frank Gärtner

Helmut Schmidt University

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Thomas Dittrich

National University of Colombia

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