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International audience; The improvement of the electrochemical behavior of solid oxide cells (SOC) stronglydepends on optimizing the oxygen electrode architecture and the choice of electrochemicallyactive materials. Lanthanide-nickelates are considered as interesting candidates for the activefunctional layer in high- to intermediate temperature SOC, however thermally induced celldeterioration remains a challenge.Based on decomposition studies of praseodymium-nickelate (Pr2NiO4+δ, PNO) [1], theelectrochemical properties of one of its decomposition products, Pr6O11, were investigated byRakesh et al. [2]. For nanostructured Pr6O11 electrodes, deposited by means of electrostaticspray deposition (ESD) on top of GDC (Ce0.9Gd0.1O2−δ) electrolytes, exceptional low valuesof polarization resistance were found (0.02 Ω cm2at 600 °C). Combined with the observedchemical stability (10 days, 800 °C, air), the material was considered a potentially interestingSOC oxygen electrode material for intermediate temperature application.The focus of this work was put on the synthesis and electrochemical testing of Pr6O11-GDCcomposite electrodes by ESD and screen-printing (SP) with the aim to increase theelectrochemically active interface (triple-phase boundary, TPB) between electrolyte andelectrode material and to support ionic charge transfer. Specifically, the influence of electrodearchitecture was investigated by comparing double-layer (Pr6O11 topped by a LSM currentcollecting layer (CCL)) and triple-layer architectures (Pr6O11 on porous, screen-printed GDC,topped by a LSM-CCL), which were prepared based on previous work [3].In this talk, the impact of ESD parameters and sintering conditions on the Pr6O11 electrodenanostructure is presented, as well as the importance of a minimum CCL thickness. As aconsequence of different grain sizes and porosity at the Pr6O11-GDC interface, an emphasis isput on the development of polarization resistance values. Finally, the study of an “ex-situ”composite triple-layer architecture Pr6O11-GDC cell is discussed.

Electrochemical sensors are powerful and versatile analytical tools in different fields, for example in medical analysis and healthcare applications. Today’s requirements on them correlate with the advances made in electrode design and performance: the demand of portable point-of-care devices, of implantable sensors for monitoring application, as well as the analysis of small sample volumes, requires the design of small-scale sensors or arrays and thus, the fabrication of miniaturized electrodes.Electrode miniaturization, however, imposes a physical limitation linked to the reduction of the specific and of the electrochemically active electrode area. For amperometric sensors (e.g. biosensors), this implies a loss in faradaic current, resulting in a lower signal, a worse signal-to-noise ratio and potentially reduced detection limit and stability.In literature, the impact of electrode structure has been profoundly studied and interesting structuration approaches have been published. However, the majority involves complex, multi-step processes of time-consuming nature, which appear unsuited for the production of sensors on a large scale. Printing techniques however, well-established in the industrial production of low-cost solid-state sensors, are not designed to generate structured electrochemical interfaces and porous networks.In this work, commercial carbon ink was deposited on polymer substrates and subjected to a process-oriented supercritical CO2-treatment. In one step, electrode structuration and solvent extraction were achieved to increase the specific and electrochemically active surface. The process parameters temperature, exposure time and ink dilution were studied upon their influence on the structural generation. Physical and electrochemical techniques comprised optical and digital microscopy, nitrogen adsorption studies, Scanning Electron Microscopy, Cyclic Voltammetry, Electrochemical Impedance Spectroscopy and Chronoamperometry. The results confirmed the modulation of surface structure and bulk porosity of electrodes, resulting in an increased and characteristic electrochemical behaviour.The practical utility of the technique was confirmed by preparing a variety of sensors for the purpose of hydrogen peroxide detection. The structuration protocol was transfered to commercial, mediated carbon inks which allowed to create porous electrodes, but suggested a partial loss of mediator. Alternatively, the deposition of the enzyme horseradish peroxidase on structured carbon electrodes lead to increased sensor sensitivity, highlighting the interest for further in-depth studies and the application of the structuration technique specifically for the preparation of enzymatic biosensors., Les capteurs électrochimiques sont des outils analytiques polyvalents utilisés dans différents domaines, comme dans le diagnostic médical ou les applications liées à la santé. Les dispositifs médicaux point-of-care, portables et implantables, ainsi que l'analyse d'échantillons de faible volume, nécessitent la conception de capteurs ou de réseaux de capteurs à petite échelle et, par conséquent, la fabrication d'électrodes miniaturisées.La miniaturisation des électrodes impose cependant une limitation physique liée à la réduction de la surface spécifique et électroactive. Pour les capteurs ampérométriques (par exemple les biocapteurs), cela implique une perte de courant faradique, ce qui entraîne un signal plus faible, un rapport signal/bruit inférieur et potentiellement une limite de détection élevée ainsi qu'une stabilité réduite.L'influence de la structure des électrodes a été étudiée en détail dans la littérature et des approches de structuration intéressantes ont été publiées. Cependant la majorité de ces méthodes sont basées sur des processus complexes multi-étapes et très chronophages. De ce fait, ceux-ci semblent inadaptés à la production de capteurs à grande échelle. D'autre part, les techniques d'impression ou de sérigraphie sont très utilisées dans la production industrielle de capteurs à faible coût mais ne sont pas conçues pour générer des interfaces électrochimiques structurées et des réseaux poreux.Dans ce travail, une encre carbone commerciale a été déposée sur des substrats polymères et soumise à un traitement industrialisable utilisant le CO2 supercritique. En une seule étape, la structuration des électrodes et l'extraction de solvants ont été réalisées afin d'augmenter la surface spécifique et ainsi la surface électroactive. L'influence des para-mètres, température, temps d'exposition et dilution d'encre, sur la structuration de la surface à été étudiée. Des techniques de caractérisation physiques et électrochimiques comprenant la microscopie optique, les études d'adsorption d'azote, la microscopie électronique à balayage, la voltampérométrie cyclique, la spectroscopie d'impédance électrochimique et la chronoampérométrie ont été mises en oeuvre. Les résultats ont confirmé que la structure de surface et la porosité apparente des électrodes peuvent être modifiées et optimisées, ce qui a entraîné une nette amélioration des performances électrochimiques.Afin de confirmer l'influence de la structuration des électrodes sur les performances électrochimiques, différents capteurs ont été préparés dans le but de détecter le peroxyde d'hydrogène. Dans un premier temps, le protocole de structuration a été étendu à des encres carbone commerciales intégrant des médiateurs rédox. Les résultats obtenus confirment la création de porosité, mais suggèrent une perte partielle de médiateur pendant la structuration. L'adsorption d'enzyme peroxydase de raifort a entraînée une augmentation de la sensibilité pour les électrodes de carbone structurées. Ceci met en évidence l'intérêt de la méthode de structuration par CO2 supercritique notamment pour la fabrication de biocapteurs enzymatiques.

Publication in the conference proceedings of EUSIPCO, Rhodes, Greece, 1998

Textured regions in images can be defined as those regions containing a signal which has some measure of randomness. This thesis is concerned with the description of homogeneous texture in terms of a signal model and to develop a means of spatially separating regions of differing texture. A signal model is presented which is based on the assumption that a large class of textures can adequately be represented by their Fourier amplitude spectra only, with the phase spectra modelled by a random process. It is shown that, under mild restrictions, the above model leads to a stationary random process. Results indicate that this assumption is valid for those textures lacking significant local structure. A texture segmentation scheme is described which separates textured regions based on the assumption that each texture has a different distribution of signal energy within its amplitude spectrum. A set of bandpass quadrature filters are applied to the original signal and the envelope of the output of each filter taken. The filters are designed to have maximum mutual energy concentration in both the spatial and spatial frequency domains thus providing high spatial and class resolutions. The outputs of these filters are processed using a multi-resolution classifier which applies a clustering algorithm on the data at a low spatial resolution and then performs a boundary estimation operation in which processing is carried out over a range of spatial resolutions. Results demonstrate a high performance, in terms of the classification error, for a range of synthetic and natural textures