Stainless steel made to rust: a robust water-splitting catalyst with benchmark characteristics

The oxygen evolution reaction (OER) is known as the efficiency-limiting step for the electrochemical cleavage of water mainly due to the large overpotentials commonly used materials on the anode side cause. Since Ni–Fe oxides reduce overpotentials occurring in the OER dramatically they are regarded as anode materials of choice for the electrocatalytically driven water-splitting reaction. We herewith show that a straightforward surface modification carried out with AISI 304, a general purpose austenitic stainless steel, very likely, based upon a dissolution mechanism, to result in the formation of an ultra-thin layer consisting of Ni, Fe oxide with a purity >99%. The Ni enriched thin layer firmly attached to the steel substrate is responsible for the unusual highly efficient anodic conversion of water into oxygen as demonstrated by the low overpotential of 212 mV at 12 mA cm−2 current density in 1 M KOH, 269.2 mV at 10 mA cm−2 current density in 0.1 M KOH respectively. The Ni, Fe-oxide layer formed on the steel creates a stable outer sphere, and the surface oxidized steel samples proved to be inert against longer operating times (>150 ks) in alkaline medium. In addition Faradaic efficiency measurements performed through chronopotentiometry revealed a charge to oxygen conversion close to 100%, thus underpinning the conclusion that no “inner oxidation” based on further oxidation of the metal matrix below the oxide layer occurs. These key figures achieved with an almost unrivalled-inexpensive and unrivalled-accessible material, are among the best ever presented activity characteristics for the anodic water-splitting reaction at pH 13

Electrochemical AFM and STM Studies of Redox Active Oligomers, Polymers and Drugs at Graphitic Materials

Carbon nanotubes (CNTs) and graphene (G) are the two (semi-)conducting allotropes of carbon offering a very high surface to bulk ratio. When combined with electrochemistry and using the principles of molecular self-assembling, many new applications of CNTs and graphene have become feasible, e.g. in the field of molecular electronics, as energy storage materials and for drug delivery. In this work scanning probe microscopy techniques (STM, AFM and EC-AFM) are used to understand the structure of self-assembled organic and electroactive molecules, oligomers and polymers on the surface of CNTs and graphene, and to rationalize their function as supramolecular system in the macroscopic world. It was found that there is a strong tendency for self-assembling in solution of aromatic electrophores with HOMO/LUMO levels close to the semiconducting CNT frontier orbitals. The tunneling currents through the aromatic guest molecules on CNT are much higher than for the same molecule on highly oriented pyrolytic graphite (HOPG). In the 1st chapter of this thesis the different microscopic techniques used with special emphasis on the electrochemical atomic force microscopy (EC-AFM), a relatively new combination technique which plays an important role in this work, are presented. It is followed by the structural analysis of stiff, semi-flexible and flexible oligo-viologen on CNTs. Using a large set of molecules with well defined small structural differences allowed to study how the superstructure (guests@CNT) is determined by the tiny changes in the guest structure. Mostly helical super structures of guests around the CNT host were observed. The guest’s rod length, its side chain length and its flexibility translate unambiguously into the corresponding STM images. A non-linear, star shaped oligo-viologen cannot wrap CNT without overlapping star branches, as expected from model considerations. In collaboration with a Korean research group we were able to build an n-doped FET using a reduced rigid oligo-viologen@CNT. Along with the oligomer the formation of stiff poly-viologens@CNT and poy-imides@CNT are studied. As compared to the oligomers@CNT, the polymers@CNT have less conformational freedom when wrapping a CNT. Thus, exclusively double to multi stranded helical wrapping was observed. An interesting new phenomenon was discovered with stiff poly-viologen or poly-naphthaline tetracarboxylic acid diimide with purely sp2 configured atoms, i.e. an outer, large diameter helical structure of “the guest” polymer and a CNT “host” sitting inside the spiral. The spiral diameter was simulated using PM7 calculations. The CNT can be moved within the large spiral by voltage pulse application. Subsequently, the conformation of flexible poly-viologens and poly-TEMPO on host materials such as CNTs and vapor grown carbon fibers (VGCF) was studies. Again, helical wrapping is observed, but the diameter adapts here to the CNT diameter. Monomer subunit resolution was achieved in case of polyTEMPO. The practical importance in energy storage is discussed in the corresponding original paper. The next chapter of thesis focuses on AFM imaging of a new battery material, i.e. poly-ferrocene on graphene oxide (PVF@GO) and on reduced graphene oxide (PVF@rGO), as well as poly-viologen as PV@GO and PV@rGO. A highlight is definitely the visualization of the so-called ion-breathing, i.e. the reversible counter ion movement from solution into the battery material composite upon electrochemical reduction/oxidation. To the best of my knowledge, this phenomenon is for the first time visualized here by a combination of electrochemistry and AFM. STM analysis of electrically conductive rGO allows for subunit resolution of polyviologen@rGO sitting in partially 2D crystalline structure on rGO. In the last chapter, my publication on the drug delivery system doxorubicin at carbon nanotubes is described. Dox@CNT is already used as drug delivery system in animal tests, but little is known on the structure of the drug on the carrier, and a reductive release trigger has so far not been identified. Rich structural variations of the drug on the CNT (helical strands of monomers and dimers) were found. It is possible to get the drug loading efficiency from STM image analysis. Reductive release of Dox@CNT was also unknown so far. Electron injection into Dox@CNT from an electrode or from the biological reducing agent glutathione (GSH) leads to irreversible release of Dox. Experimental results are in excellent agreement with semi-empirical simulations

Das Recht auf berufliche Bildung in der Arbeitsfoerderung: d. Spannungsverhaeltnis zwischen Rechtsanspruechen d. Einzelnen u. staatl. Gewaehrung von Rechtspositionen

SIGLEAvailable from Bibliothek des Instituts fuer Weltwirtschaft, ZBW, Duesternbrook Weg 120, D-24105 Kiel A 162274 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Cosmological aspects of sound speed parameterizations in fractal universe

In the framework of fractal universe, the unified models of dark energy and dark matter are being presented with the background of homogenous and isotropic FLRW geometry. The aspects of fractal cosmology helps in better understanding of the universe in different dimensions. Relationship between the squared speed of the sound and the equation of state parameter is the key feature of these models. We have used constant as well as variable forms of speed of sound and express it as a function of equation of state parameter. By utilizing the four different forms of speed of sound, we construct the energy densities and pressures for these models and then various cosmological parameters like hubble parameter, EoS parameter, deceleration parameter and Om- diagnostic are investigated. Graphical analysis of these parameters show that in most of the cases EoS parameters and trajectories of Om-diagnostic corresponds to the quintessence like nature of the universe and the deceleration parameters represent accelerated and decelerated phase. In the end, we remark that cosmological analysis of these models indicates that these models correspond to different well known dark energy models

High Performance Poly(viologen)–Graphene Nanocomposite Battery Materials with Puff Paste Architecture

Four linear poly­(viologens) (PV1, PV2: phenylic, PV3: benzylic, and PV4: aliphatic) in tight molecular contact with reduced graphene oxide (rGO), that is, PV@rGO, were prepared and used as anodic battery materials. These composites show exceptionally high, areal, volumetric, and current densities, for example, PV1@rGO composites (with 15 wt % rGO, corresponding to 137 mAh g^–1) show 13.3 mAh cm^–2 at 460 μm and 288 mAh cm^–3 with 98% Coulombic efficiency at current densities up to 1000 A g^–1, better than any reported organic materials. These remarkable performances are based on (i) molecular self-assembling of PVs on individual GO sheets yielding colloidal PV@GO and (ii) efficient GO/rGO transformation electrocatalyzed by PVs. Ion breathing during charging/discharging was studied by electrochemical quartz crystal microbalance and electrochemical atomic force microscopy revealing an absolute reversible and strongly anisotropic thickness oscillation of PV1@rGO at a right angle to the macroscopic current collector. It is proposed that such stress-free breathing is the key property for good cyclability of the battery material. The anisotropy is related to a puff paste architecture of rGO sheets parallel to the macroscopic current collector. A thin graphite sheet electrode with an areal capacity of 1.23 mAh cm^–2 is stable over 200 bending cycles, making the material applicable for wearable electronics. The polymer acts as a lubricant between the rGO layers if shearing forces are active

Ordered Topographically Patterned Silicon by Insect-Inspired Capillary Submicron Stamping

Insect-inspired capillary submicron stamping and subsequent surface-limited metal-assisted chemical etching (MACE) with ammonium bifluoride as a HF source are employed for the high-throughput production of ordered topographically patterned silicon (tpSi). Insect feet often possess hairy contact elements through which adhesive secretion is deployed. Thus, arrays of adhesive secretion drops remain as footprints on contact surfaces. Stamps for insect-inspired capillary submicron stamping having surfaces topographically patterned with contact elements mimic the functional principles of such insect feet. They contain spongy continuous nanopore networks penetrating the entire stamps. Any ink (organic or aqueous) may be supplied from the backside of the nanoporous stamps to the contact elements. We generated ordered arrays of submicron AgNO₃ dots extending square millimeters on Si by manual stamping with cycle times of a few seconds under ambient conditions; at higher load, ordered holey AgNO₃ films were obtained. Surface-limited MACE correspondingly yielded either macroporous tpSi or Si pillar arrays. Inkjet printing of polymer solutions onto the tpSi yielded patterns of polymer blots conformally covering the tpSi. Such blot patterns could potentially represent a starting point for the development of persistent and scratch-resistant identity labels or quick response codes on silicon surfaces

A unique ferredoxin acts as a player in the low-iron response of photosynthetic organisms

Iron chronically limits aquatic photosynthesis, especially in marine environments, and the correct perception and maintenance of iron homeostasis in photosynthetic bacteria, including cyanobacteria, is therefore of global significance. Multiple adaptive mechanisms, responsive promoters, and posttranscriptional regulators have been identified, which allow cyanobacteria to respond to changing iron concentrations. However, many factors remain unclear, in particular, how iron status is perceived within the cell. Here we describe a cyanobacterial ferredoxin (Fed2), with a unique C-terminal extension, that acts as a player in iron perception. Fed2 homologs are highly conserved in photosynthetic organisms from cyanobacteria to higher plants, and, although they belong to the plant type ferredoxin family of [2Fe-2S] photosynthetic electron carriers, they are not involved in photosynthetic electron transport. As deletion of fed2 appears lethal, we developed a C-terminal truncation system to attenuate protein function. Disturbed Fed2 function resulted in decreased chlorophyll accumulation, and this was exaggerated in iron-depleted medium, where different truncations led to either exaggerated or weaker responses to low iron. Despite this, iron concentrations remained the same, or were elevated in all truncation mutants. Further analysis established that, when Fed2 function was perturbed, the classical iron limitation marker IsiA failed to accumulate at transcript and protein levels. By contrast, abundance of IsiB, which shares an operon with isiA, was unaffected by loss of Fed2 function, pinpointing the site of Fed2 action in iron perception to the level of posttranscriptional regulation

A unique ferredoxin acts as a player in the low-iron response of photosynthetic organisms

Schorsch M, Kramer M, Goss T, et al. A unique ferredoxin acts as a player in the low-iron response of photosynthetic organisms. Proceedings of the National Academy of Sciences. 2018;115(51):E12111-E12120.Iron chronically limits aquatic photosynthesis, especially in marine environments, and the correct perception and maintenance of iron homeostasis in photosynthetic bacteria, including cyanobacteria, is therefore of global significance. Multiple adaptive mechanisms, responsive promoters, and posttranscriptional regulators have been identified, which allow cyanobacteria to respond to changing iron concentrations. However, many factors remain unclear, in particular, how iron status is perceived within the cell. Here we describe a cyanobacterial ferredoxin (Fed2), with a unique C-terminal extension, that acts as a player in iron perception. Fed2 homologs are highly conserved in photosynthetic organisms from cyanobacteria to higher plants, and, although they belong to the plant type ferredoxin family of [2Fe-2S] photosynthetic electron carriers, they are not involved in photosynthetic electron transport. As deletion offed2appears lethal, we developed a C-terminal truncation system to attenuate protein function. Disturbed Fed2 function resulted in decreased chlorophyll accumulation, and this was exaggerated in iron-depleted medium, where different truncations led to either exaggerated or weaker responses to low iron. Despite this, iron concentrations remained the same, or were elevated in all truncation mutants. Further analysis established that, when Fed2 function was perturbed, the classical iron limitation marker IsiA failed to accumulate at transcript and protein levels. By contrast, abundance of IsiB, which shares an operon withisiA, was unaffected by loss of Fed2 function, pinpointing the site of Fed2 action in iron perception to the level of posttranscriptional regulation