Sebastian Fiechter

Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells

Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by (57)Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH(3) at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN(4)-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (Fe(x)N, with x≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN(4)-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e(-) per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials.

Influence of Sulfur on the Pyrolysis of CoTMPP as Electrocatalyst for the Oxygen Reduction Reaction

This work presents the preparation and investigation of pyrolyzed cobalt-tetramethoxyphenylporphyrin (CoTMPP) supported by iron oxalate with and without sulfur as electrocatalysts for the oxygen reduction reaction (ORR) in acid media. A preparation method which needs no addition of carbon supports allows the structural investigation of the pyrolysis products by X-ray photoemission spectroscopy, Raman spectroscopy, and X-ray diffractometry without any interferences of a carbon support. Already with low metal loading, rotating ring disk electrode measurements reveal the high ORR activity and enhanced selectivity which are apparently caused by an increased number of catalytic centers and higher efficient ones due to a well developed porosity and a suitable molecular structure of the formed carbon. A thermogravimetric investigation of the pyrolysis process shows that the addition of sulfur to the precursor influences the carbonization of the porphyrin in a favorable way. It has been found that extended graphene layers present a particularly suitable matrix for highly active catalytic centers.

Influence of discharge parameters on the layer properties of reactive magnetron sputtered ZnO:Al films

Abstract Aluminum-doped ZnO layers have been prepared by reactive d.c. magnetron sputtering from Zn:Al (2 wt.%) targets onto unheated substrates (Si, glass, glassy carbon). In dependence on the O2 partial pressure in the argon sputtering gas there exists a narrow process window around a p O2 (p Ar + p O2 ) ratio of 5–10% which yields transparent, low-resistance layers. The discharge voltage dependence on the oxygen partial pressure is a sensitive indicator for the oxidation state of the target surface and can be used for the regulation of the deposition process. Lower O2 partial pressures yield metallic-like, opaque, but highly resistant layers. Higher oxygen partial pressures lead to transparent but highly resistant ZnO layers. Layers of lowest resistivity (5 × 10−4 Ω cm) and highest optical transmission (90%) have a stoichiometric ratio Zn:O of 1.0 and exhibit the largest grains (≈40 nm) as has been measured by Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). By comparing the metallurgical Al content (using RBS) in the films with the carrier concentration (Hall and conductivity measurements) we obtain an overall electrical activation of aluminiu in the best case of about 60%. We found an exponential dependence of the specific resistance on the ZnO crystallite size which explains the strong dependence of the sheet resistance on the oxygen partial pressure.

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

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.

Structural, optical and electrical properties of polycrystalline iron pyrite layers deposited by reactive d.c. magnetron sputtering

Thin iron disulphide pyrite (FeS2) films (5–100 nm) were prepared by reactive d.c. magnetron sputtering from an iron target in an Ar/H2S atmosphere. Optimum deposition was obtained with a sputtering power of 100 W in an 80% H2S gas mixture at a total pressure of 5 Pa. Iron pyrite films were successfully deposited on different glass substrates, at substrate temperatures up to 350°C. Higher temperatures led to sulphur loss of the films. X-ray diffraction measurements have shown that the films are polycrystalline with a grain size up to 50 nm. Rutherford backscattering analysis yielded an overall stoichiometry of FeS1.95 in the best films. From the optical transmission and reflection an optical absorption coefficient of 5×105 cm−1 for λ<560 nm was calculated. The films show p-type conductivity. A hole concentration upto 1×1020 cm−3 and a mobility up to 25 cm2V−1s−1 at room temperature was determined by Hall measurements. The conductivity type is also supported by UV photoelectron spectroscopy where the position of the Fermi level was found to be very close to the valence band edge.

Electrocatalysts for Oxygen Reduction Prepared by Plasma Treatment of Carbon-Supported Cobalt Tetramethoxyphenylporphyrin

during the pyrolysis of CoTMPP. Additionally, solid decomposition products of the oxalate metal and oxides form a framework embedded within the pyrolysis product which is removed by a subsequent acid treatment. Finally, a highly porous carbon matrix with embedded centers is obtained. With this procedure materials with high electrochemical activities toward the oxygen reduction have been achieved in rotating disk electrode RDE measurements in 0.5 M H2SO4 at 0.7 V normal hydrogen electrode NHE close to that of commercial 20% Pt/C E-TEK. However, even with this advanced technique scanning electron microscopy SEM images reveal particles of several micrometer dimension composed of that highly porous material. In technical applications e.g., gas diffusion electrodes in fuel cells this presumably leads to a lower efficiency due to long diffusion pathways for protons and gas molecules. Therefore, alternative synthesis techniques are desired which result in smaller particle sizes 50‐100 nm as it is state of the art for carbon supports which are utilized for commercial platinum catalysts Vulcan, Black Pearls ca. 20 nm. In the last decades plasma treatment of organic material was intensively investigated and is considered as a promising nanotechnological approach. Our contribution shows that the transfer of CoTMPP into highly electrochemical active CoN4 centers embedded in a carbon matrix can be reached by plasma treatment instead of conservative heat-treatment. In order to find optimal operating parameters CoTMPP on Black Pearls was processed by plasma treatment as well as by classical heat-treatment. The obtained products were compared in terms of structure and catalytic activity.

Size-related native defect engineering in high intensity ultrasonication of nanoparticles for photoelectrochemical water splitting

We report for the first time the demonstration of high intensity sonication treatment as a simple and effective way to fundamentally improve the performance of nanoparticles for photoelectrochemical (PEC) water splitting. The capability of making highly photoactive nanoparticles by high intensity sonication is highly appreciated to open up new opportunities in various areas, including PEC water splitting, dye-sensitized solar cells, and photocatalysis.

On the Influence of Sulphur on the Pyrolysis Process of FeTMPP-Cl-based Electro- Catalysts with Respect to Oxygen Reduction Reaction (ORR) in Acidic Media

Pyrolysis of chloroiron-tetramethoxyphenyl-porphyrin (FeTMPP-Cl) in the presence of iron oxalate (± sulphur) leads to the formation of higly porous and active catalysts for the oxygen reduction reaction (ORR). In order to clarify the influence of sulphur the pyrolysis process is analyzed by thermogravimetry (TG) and by high-temperature X-ray diffraction (HT-XRD). In the absence of sulphur iron carbide (FexC) is formed which catalyses the proceeding graphitisation of the pyrolysis products. As a result catalytic active centres are decomposed by this reaction. This can be avoided by the addition of sulphur because iron monosulphide (FeS; troilite) is formed instead of FexC. Furthermore, FeS can easily be removed in a successive etching step so that nearly all inactive by-products can be removed. The results are in accordance with the higher electrochemical performance of the sulphur containing catalysts.

Photosensitive oxide semiconductors for solar hydrogen fuel and water disinfection

Abstract Hydrogen is expected to become a commonly used energy carrier on the global scale in the near future. However, hydrogen as a fuel is environmentally friendly only when generated from water using renewable energy, such as solar energy. Therefore, intensive research aims to develop a new generation of solar materials, which may be used for the production of hydrogen fuel from water using solar energy. The highly promising candidates for solar energy conversion are photosensitive oxide semiconductors (POSs), particularly the TiO2-based semiconductors, which may be used for converting solar energy into the chemical energy required for hydrogen generation from water, as well as water purification (removal of microbial agents and toxic contaminants from water). The present work considers an R&D strategy for developing TiO2-based systems capable of converting solar energy into the chemical energy via water oxidation. The effect of surface versus bulk semiconducting properties on the performance of POSs is considered in terms of partial and total water oxidation. The progress requires modification of the key performance-related properties (KPPs) in order to enhance the light-induced reactivity of the POSs with water. The most recent approach in the development of POSs with enhanced performance is deposition of metallic islets of different size and shape in order to induce a plasmonic effect. The development of high-performance POSs can be achieved through a multidisciplinary approach. It is shown that defect disorder has a critical effect on the light-induced reactivity of POSs and the solar energy conversion. Therefore, defect engineering may be applied in the development of high-performance POSs. This work considers the hurdles in the development of high-performance POSs for specific applications and formulates the key questions that must be addressed to overcome these hurdles. The concepts developed for TiO2 may be expanded for other metal oxides.

Structural Investigation of Carbon Supported Ru-Se Based Catalysts using Anomalous Small Angle X-Ray Scattering

Currently the commercial applicability of proton exchange membrane fuel cells (PEMFC) is almost exclusively bound to the employment of expensive and rare platinum. However as, recently successfully demonstrated, selenium modified ruthenium based catalysts also exhibit high catalytic activity for the oxygen reduction reaction in fuel cells [1]. One advantage of ruthenium is that it has only one-tenth the costs of platinum. Furthermore, in direct methanol fuel cell (DMFC) the undesired crossover of methanol through the membrane from the anode space into the cathode compartment is still an unsolved issue. Therefore, state of the art platinum cathode catalysts suffer from an activity loss under these conditions due to methanol oxidation. As alternative, carbon supported ruthenium nano-particles modified with selenium were suggested featuring absolute methanol tolerance. Although intense studies of these catalysts were performed during the last decade, no definite conclusion with respect to the nature of the catalytically active sites and the constitution of the RuSe nano-particles could be drawn. We performed an Anomalous Small Angle X-ray Scattering (ASAXS) experiment to clarify the structural and chemical features of these catalytically active Semodified ruthenium nano-particles, such materials were prepared by thermolysis of Ru3(CO)12 in an organic solvent with and without the presence of dissolved selenium. The results are due to be discussed considering data from XRD, XPS, and EXAFS measurements, as well as TEM images [2,3]. Particularly, data about the selenium distribution over the catalysts surface are aspired, because catalytic efficiency depends strongly on the selenium content of the catalysts. Investigating RuSe electrocatalysts we will show that the ASAXS method represents a powerful tool to determine size distributions and volume fraction of structures on a nano scale domain. Furthermore, it becomes a sensitive method on chemical composition fluctuations by taking into account the so-called anomalous or resonant behaviour of the atomic scattering amplitude of an element, containing in the sample, near its x-ray absorption edge. We investigated with ASAXS a complete set of samples, including the final working RuSe catalyst supported on a commercial carbon black, and some intermediate preparation states like non-modified (selenium free) ruthenium nano-particles or the bare carbon support only. We will discuss scattering curves taken in the vicinity of the selenium and the ruthenium K absorption edge, respectively. Figure 1. shows the large Q range of two scattering curves taken at 5eV below the selenium K absorption edge. The sample which contains selenium (red circles) shows a small hump at Q ~ 7 nm while the sample without selenium (blue triangle) does not. Thus, selenium within the sample generates structural features clearly detectable and analysable by SAXS. Taken into account the energy dependences (the anomalous dispersion effect) of the scattering intensity of all measured samples, a structure model of the catalytically active metallic nano-particles has been deduced, suggesting a nearly spherical Ru particle, with a mean diameter of 2.3 nm decorated with Se aggregates. The selenium structure onto the ruthenium nanocrystallites features a diameter less than 0.5 nm. The real shape of these ruthenium supported selenium clusters is not yet finally clarified. However, it is suggested to represent a symmetric ring like observed for free selenium clusters. [4] We investigated the same systems with small angle neutron scattering (SANS) confirming the structure model obtained from ASAXS. The comparison of the volume size distribution of the particles of both methods will be discussed. We also confirmed that the selenium modified ruthenium nano-particles are extremely resistant against particle growth. Even after annealing at 800°C the average particle diameter of the ruthenium particles was found to remain below 2.5 nm.