Quentin Van Overmeere

Rational design of the electrode morphology for oxygen evolution – enhancing the performance for catalytic water oxidation

The fundamental understanding of the electrode/electrolyte interface is of pivotal importance for the efficient electrochemical conversion and storage of electrical energy. However, the reasons for the low rate of electrocatalytic oxygen evolution and issues of long-term material stability, which are central constraints for attaining desirable efficiency for sustainable technologies like water electrolysis or electrochemical CO2 reduction, are still not completely resolved. While a lot of attention has been directed towards the search for new materials with unique (electro)catalytic properties, experimental results accumulated during the last four decades and prediction from models suggest that RuO2 possesses superior activity for oxygen evolution under acidic conditions. Considering that RuO2 is a material of choice, we show that tailoring the surface morphology on the meso- and macroscale has great potential for the improvement of the efficiency of this gas evolving reaction. Advanced analytical tools have been utilized for the combined investigation of both activity and stability. Namely, the potential dependent frequencies of gas-bubble evolution, an indicator for the activity of the electrode, were acquired by scanning electrochemical microscopy (SECM), while the dissolution of RuO2 was monitored using a micro electrochemical scanning flow cell combined with an inductively coupled plasma mass spectrometer (SFC-ICP-MS). The obtained fundamental insights will aid improving the design and thus performance of electrode materials for water oxidation.

Energy storage in ultrathin solid oxide fuel cells

The power output of hydrogen fuel cells quickly decreases to zero if the fuel supply is interrupted. We demonstrate thin film solid oxide fuel cells with nanostructured vanadium oxide anodes that generate power for significantly longer time than reference porous platinum anode thin film solid oxide fuel cells when the fuel supply is interrupted. The charge storage mechanism was investigated quantitatively with likely identified contributions from the oxidation of the vanadium oxide anode, its hydrogen storage properties, and different oxygen concentration at the electrodes. Fuel cells capable of storing charge even for short periods of time could contribute to ultraminiaturization of power sources for mobile energy.

In situ detection of porosity initiation during aluminum thin film anodizing

High-resolution curvature measurements have been performed in situ during aluminum thin film anodizing in sulfuric acid. A well-defined transition in the rate of internal stress-induced curvature change is shown to allow for the accurate, real-time detection of porosity initiation. The validity of this in situ diagnostic tool was confirmed by a quantitative analysis of the spectral density distributions of the anodized surfaces. These were obtained by analyzing ex situ atomic force microscopy images of surfaces anodized for different times, and allowed to correlate the in situ detected transition in the rate of curvature change with the appearance of porosity.

Effect of Current Density on the Internal Stress Evolution during Galvanostatic Ti Thin Film Anodizing

A high resolution curvature measurement technique was used for investigating in situ the internal stresses developing during galvanostatic anodization of titanium thin films. The titanium electrodes were anodized in a 0.1 M H3PO4 electrolyte, with current densities ranging from 0.5 to 4.1 mA/cm(2). Two distinct stages were observed in the evolution of the cell voltage with time: a first, low efficiency growth stage and a second near 100% efficiency growth stage. The transition between both stages systematically occurred at 6.0 V, independent of current density. In terms of internal stresses, these two stages correspond to two distinct regimes as well. In the first stage, compressive internal stresses were observed on the order of several gigapascals, which increased with decreasing current density. In the second stage, a tensile instantaneous stress component was found with an average value of 237 MPa, independent of current density. For the first stage, oxygen evolution inside the oxide film was responsible for both the low efficiency growth and the large compressive stresses. The tensile instantaneous stress evolution in the second stage was rationalized in terms of the rate of net free volume generation at the metal/oxide interface.

Monolithic Integration of Nanoscale Solid Oxide Fuel Cell Membranes onto Polymer Scaffolds through Stress Control

Ultrathin fast-ion conducting oxide membranes are of broad interest to a range of energy conversion technologies. We demonstrate a low-temperature (<30 °C) process for controlling internal stress in an archetypal fast-ion conductor, crystalline Y2O3-doped ZrO2 (YDZ), which allows us to form stable suspended nanomembranes akin to those fabricated at high temperature (>550 °C). Such a low-temperature synthesis method then enables us to monolithically integrate the suspended oxide-ion conducting membranes onto polyimide (Kapton by DuPont), a polymer with vastly different physical properties than that of a ceramic. Integrated functional heterostructure solid oxide fuel cells operable below the glass transition temperature of the polymer are demonstrated. Our results describe a mechanistic low-temperature processing route for forming stable multifunctional membrane structures, applicable to the realization of various energy conversion and sensing devices and structural skins for miniature autonomous systems.

Strain Dependent Electronic Structure and Band Offset Tuning at Heterointerfaces of ASnO 3 (A=Ca, Sr, and Ba) and SrTiO 3

The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO3 (A=Ca, Sr, and Ba) thin films are characterized using in situ X-ray and ultraviolet photoelectron spectroscopies, with band gaps evaluated using spectroscopic ellipsometry. Scanning transmission electron microscopy with geometric phase analysis is used to resolve strain at atomic resolution. The VB electronic structure is strain state dependent in a manner that correlated with a directional change in Sn-O bond lengths with strain. However, VB offsets are found not to vary significantly with strain, which resulted in ascribing most of the difference in band alignment, due to a change in the band gaps with strain, to the conduction band edge. Our results reveal significant strain tuning of conduction band offsets using epitaxial buffer layers, with strain-induced offset differences as large as 0.6 eV possible for SrSnO3. Such large conduction band offset tunability through elastic strain control may provide a pathway to minimize the loss of charge confinement in 2-dimensional electron gases and enhance the performance of photoelectrochemical stannate-based devices.

Filamentous Phages Displaying Multivalent Peptide Motives With Specific Affinity To Anodic Alumina Surfaces

In this work, we report for the first time on the successful selection and identification of peptide motives that exhibit a specific affinity to anodic alumina surfaces when multivalently displayed on a filamentous phage. It was also demonstrated that, for a selected phage clone, a chemical functionalisation (biotinylation) of the bacteriophage does not deteriorate its specific affinity to anodic alumina. Moreover, such biotinylated bacteriophages, after being immobilised onto an anodic alumina surface, have been shown to allow for the quantitative detection of streptavidine using an ELISA protocol. These results are believed to pave the way for shifting the surface design of integrated biosensing devices from traditional, chemically modified synthetic surfaces, like silane-based self-assembled monolayers, towards molecular linkers based on genetically engineered polypeptides.

Interface energetics and atomic structure of epitaxial La1−xSrxCoO3 on Nb:SrTiO3

The energetics at oxide semiconductor/La1−xSrxCoO3 heterojunctions, including the respective alignment of the valence and conduction bands, govern charge transfer and have to be determined for the design of future La1−xSrxCoO3-based devices. In this letter, the electronic and atomic structures of epitaxial La1−xSrxCoO3 on Nb-doped strontium titanate are revealed by scanning transmission electron microscopy, electron energy loss spectroscopy, and in situ x-ray and ultra violet photoelectron spectroscopies. For LaCoO3, a valence band (VB) offset of 2.8 ± 0.1 eV is deduced. The large offset is attributed to the orbital contributions of the Co 3d states to the VB maximum of the LaCoO3 thin films, with no evidence of interface dipole contributions. The sensitivity of the valence band orbital character to spin state ordering and oxygen vacancies is assessed using density functional theory.