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  • Author: Harald Natter x
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An overview is presented about the preparation of nanocrystalline metals by pulsed electrodeposition out of aqueous electrolytes and ionic liquids. Appropriate selection of the bath chemistry and the plating parameters provide the flexibility to control the crystallite size. For selected examples we demonstrate how these electrochemical methods can be used for the preparation of nanostructured bulk metals and nanoparticle for fuel cell applications. Electrochemical analytical methods like cyclic voltammetry and chronoamperometry are used to fundamentally understand the deposition mechanisms, the effect of bath additives on the micro- and nanostructure of the deposits and to characterize the catalytic activity of the electrocatalysts. Some improved physical properties of electrodeposited nanocrystalline metals are discussed, namely thermal stability of n-Fe, magnetisation of n-Ni and hardness of n-Al.

The kinetics of the electrocrystallization of nanostructured gold is investigated on both rotating disc and stationary glassy carbon electrodes. A possible mechanism of gold deposition is presented. A general expression of the current-time transient for the superposition of adsorption, three-dimensional nucleation and mixed charge transfer/diffusion controlled growth based on existing theories is reported for the kinetics on stationary electrodes. The experimentally recorded current-time transients are analyzed in terms of this model. Kinetic parameters are determined and their consequence is discussed.

The mechanical properties and strain rate sensitivity of nanocrystalline nickel was studied as a function of grain size at different temperatures in tensile tests and with a nanoindenter in order to examine the different deformation mechanisms of nanocrystalline materials. The effect of lateral boundaries and hydrogen on the nucleation of dislocations was studied in detail. For the first time it was possible to observe the reduction of the dislocation nucleation stress on a nanoscale. In addition the experiments yielded, depending on temperature and strain rate, the strain rate sensitivity, the activation volume and the creep exponents as a function of stress and grain size. From the creep experiments the transition between grain boundary sliding and dislocation climb as a function of temperature was obtained. The strain rate jump tests gave extremely small activation volumes, nearly a factor of 100 smaller than in conventional nickel as a function of grain size. To help in understanding this behaviour the strain rate sensitivity of single grains was tested with a nanoindenter. The results clearly showed that the primary interaction of dislocations with grain boundaries is the reason for the observed strong rate effects and small activation volumes.

Abstract

The stepwise oxidation of vanadium ions in electrolytes, as used in all vanadium redox flow batteries (VRFB), is studied offline by a combination of potentiometric titration and simultaneous UV/Vis/NIR spectroscopy. Eight different total vanadium concentrations between 0.2 mol L−1 and 1.6 mol L−1 have been investigated. The analyte (titrand, V2+ solution) is the anolyte (V2+/V3+ side) of a fully charged laboratory vanadium redox flow battery (VRFB). Absorption maxima are observed at λ = 850 nm for V2+ and at λ = 400 nm for V3+, the corresponding absorption coefficients are determined. In the former case an extrapolation procedure is necessary because during transfer from the VRFB to the titration cell, oxidation to V3+ by ambient oxygen cannot completely be avoided. Based on the knowledge of the absorption coefficients, via simultaneous photometry of V2+ and V3+, the state-of-charge of the anolyte of a VRFB can be determined. In the catholyte (V4+/V5+ side) of a VRFB the formation of an intermediate mixed valence VIV–VV complex at large vanadium concentration prevents a simple photometric SOC determination.

Abstract

By means of in-situ UV/Vis/NIR spectrometry, separately both in the anolyte as well as in the catholyte of a vanadium redox flow battery (single cell) partial state-of-charge values are determined online. The UV/Vis/NIR spectroscopic experimental set-up is calibrated using the state-of-charge value determined from measurements of the open-circuit-voltage (OCV) in the pristine state of the battery which is related to Nernst’s equation taking into account also H+ formation/consumption during the V4+/V5+ redox process. The comparison of both partial state-of-charge values indicates a possible imbalance of the battery, which can occur after long-term operation.

Abstract

Industrial chlor-alkali electrolysis represents one of the most energy- and resource-intensive technological applications of electrocatalysis. Improving process efficiency becomes a critical issue for the sustainable development and for alleviating the energy and environmental crisis. Rational design in the morphology of RuO2-based anodic electrocatalytic coatings and the control in the coating microstructure can contribute to massive energy saving compared to the current commercial Ru0.3Ti0.7O2 coating. This review covers recent developments in the anodic coatings. Performance enhancement for RuO2-based anodic coatings is achieved by using alternative preparation routes of sol-gel and electrodeposition. The target control in the coating surface morphologies and the increase in the utilization of active Ru species are demonstrated.

Atomic force acoustic microscopy (AFAM) is a near-field technique, where the vibration behavior of a micro-fabricated elastic cantilever beam in contact with a sample surface is sensitive to its local elastic properties. The resolution of this technique is given by the contact radius a c of the atomic force microscope sensor-tip on the sample surface. Taking into account only the Hertzian forces, a c depends on the static load applied by the cantilever, on the elastic constants of the tip and the sample and on the geometry of the contacting bodies. The shape of the sensor tip used in atomic force acoustic microscopy is between a sphere and a flat punch. Hence a c extends from just below 10nm to a few tens of nanometers. In this review, we give an overview of the AFAM technique, present data on the indentation moduli of nanocrystalline nickel, and discuss some of the error sources in quantitative AFAM. The AFAM indentation moduli measured are comparable to the values obtained by nanoindentation and lower than the indentation moduli calculated from ultrasonic velocity measurements. There seems to be a decrease of the indentation modulus with decreasing grain size for grain sizes below 30nm. The data are discussed taking into account X-ray diffraction and electron back-scattering data revealing some texture and macro-strain due to internal stresses in the samples investigated.