Evaluation of a sol–gel process for the synthesis of La1−xSrxMnO3+δ cathodic multilayers for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrical energy conversion devices with high efficiency and low pollution. In order to increase performances of SOFCs at intermediate temperature (700–800 °C) and to decrease materials cost, an alternative sol–gel synthesis method has been investigated to deposit La1−xSrxMnO3+δ (LSMx) as cathode thin films. Polycrystalline LSMx thin films were prepared by dip-coating using a polymeric solution. Lanthanum, strontium and manganese nitrates were used as raw materials. The viscosity of the solution was adjusted and the solution was deposited on polycrystalline ZrO2–8% Y2O3 ceramics. Prior to experiments, the substrate surface was eroded until a roughness of 20 nm and then cleaned with ethanol and dried. Film thicknesses were adjusted with the number of layers. Porosity and grain size of monolayers or multilayers were evaluated. Typical thickness of monolayer is 250 nm. A key parameter in the multilayer process was the intermediate calcination temperature (400, 700 or 1000 °C) of each further layer deposition. A correlation between this intermediate temperature and morphology, thickness and porosity was found; porosity is ranging from 3 to 40% and thickness can reach 1 micron for multilayers. Concerning electrochemical performances, the best results were obtained for LSM0.4 multilayers with an intermediate calcination temperature (called Ti) of 400 °C

Chemical composition of nanoporous layer formed by electrochemical etching of p-type GaAs

Abstract : We have performed a detailed characterization study of electrochemically etched p-type GaAs in a hydrofluoric acid-based electrolyte. The samples were investigated and characterized through cathodoluminescence (CL), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that after electrochemical etching, the porous layer showed a major decrease in the CL intensity and a change in chemical composition and in the crystalline phase. Contrary to previous reports on p-GaAs porosification, which stated that the formed layer is composed of porous GaAs, we report evidence that the porous layer is in fact mainly constituted of porous As2O3. Finally, a qualitative model is proposed to explain the porous As2O3 layer formation on p-GaAs substrate

Herstellung und Charakterisierung von einkristallinen InP Membranen für etliche Anwendungen

This work presents the fabrication process of single crystalline InP membranes in three steps - the electrochemical formation of an almost perfectly hexagonally close-packed array of currentline-oriented (curro) pores, the photo-electrochemical porosification of the bulk InP wafer back side, and the subsequent photochemical dissolution to open the curro-pore array. The photo-electrochemical and photochemical etching steps were investigated by in situ FFT-impedance spectroscopy (FFT-IS). The fitted FFT-IS data allowed to identify characteristic stages in the photo-electrochemical and in the photochemical step and to predict the morphology of the membrane surface after completion of the etching process. The membrane itself can be customized in a large variety starting with the thickness, over the membrane surface with at or cone-like pore openings, even up to a self-organized 3D-structured highly crysto-porous layer on the membrane back side with an ultra-high surface area. The pore shape can be tuned from elliptic to perfectly rectangular by post-etching under cathodic bias. It also results in a further reduction and homogenization of the pore wall width compared to the state after electrochemical etching of the curro-pore array and a gain in resistivity of the membrane due to the increased SCR coverage of the InP pore walls. In this work the fabrication of a multifunctional composite consisting of the piezoelectric InP membrane and a magnetic fillers in the form of nanowires and the usage of the InP membrane as anode in Li-ion batteries have been investigated. Multifunctional composites have been formed by galvanic metal deposition in InP pore arrays and membranes. Dense Ni and Fe-Co based nanowires were grown inside ultra-high aspect ratio InP membranes covered with a thin dielectric Al2O3 interlayer deposited by ALD prior to the galvanic deposition process. Without the dielectric interlayer only porous Ni nanowires, respectively tubular-like structures could be grown due to the preferential nucleation of Ni crystallites on the pore walls. The galvanic growth of Ni nanowires has been characterized by in situ FFT-IS. The fitted data allowed to separate the galvanic deposition into different stages and to consistently interpret them as the adsorption of boric acid on the growing Ni surface facilitating the deposition, the Ni deposition reaction itself, and the passivation of Ni surface by a diffusion-limited species. The magnetic characterization of the nanowires showed the easy magnetization direction of the Ni nanowires along the long nanowire axis z due to the ultra-high aspect ratio of the nanowires. In case of the Fe-Co based nanowires the easy magnetization direction is perpendicular to z. In both cases the coercivity exhibited a maximum in between H par z and H per z which could be explained by a combination of two magnetization reversal mechanisms being each energetically favorable under certain angles. Porous InP membranes as anode in Li-ion batteries are free from additives, binders, or other passive materials unlike all other InP anodes and exhibit a very high capacity per area. During lithiation the pore walls undergo drastic structural changes transforming InP into crystalline LiP and In3Li13 becoming visible as oscillations in the pore wall widths in the form of a regular sequence of bulges and necks, in anti-phase to neighboring pore walls. They are even maintained in the delithiated state, when nanocrystalline InP is formed. No delamination or large scale pulverization of the pore walls were observed indicating the capability of porous anodes to compensate for the high volume expansion during lithiation. The cycling performance of the porous InP anode was good without capacity losses in the first cycles, but a progressive degradation in the following cycles. The application of an Al2O3 coating of the pore walls was beneficial for the SEI layer formation. The combination of Al2O3 coating and a lower discharge voltage limit reduced the gravimetric capacity to less than 50 % of the uncoated anode, but seemed to be beneficial for the cycling stability.Diese Arbeit beschreibt den Herstellungsprozess von einkristallinen InP Membranen in drei Schritten - beginnend mit der elektrochemischen Herstellung von sogenannten "Strom-linien"- Poren ('curro' Poren) in einer beinahe perfekten, hexagonal-dichtest gepackten Anordnung, der fotoelektrochemischen Porosifizierung der Volumen InP Wafer-Rückseite und deren anschließende fotochemische Auflösung, notwendig zur vollständigen Öffnung der 'curro'-Poren. Die fotoelektrochemischen und fotochemischen Ätzschritte wurden zusätzlich per in situ FFT-Impedanz-Spektroskopie (FFT-IS) untersucht. Die gefitten FFT-IS Daten erlaubten die Identifikation von charakteristischen Etappen der fotoelektrochemischen und fotochemischen Ätzschritte, sowie die Voraussage der Oberflächenmorphologie der Membranen nach Beendigung der beiden Ätzschritte. Die Membran an sich kann in einer großen Vielfalt an die jeweilige Anwendung angepasst werden, beginnend bei der Membranendicke, über die Membranenoberfläche mit flachen oder konisch geformten Porenwänden; sogar eine selbstorganisierend entstandene, 3D-strukturierte, hoch 'crysto'-poröse Schicht auf der Membranenrückseite mit einer flächenmäßig ultra-hohen Ober-flächen ist möglich. Die Porenform der Membran kann eingestellt werden von elliptisch bis perfekt rechteckig mittels Nachätzung unter kathodischer Vorspannung. Dies resultiert außerdem in einer weiteren Verringerung und Homogenisierung der Porenwände im Vergleich zur nur elektrochemisch geätzten Membran. Daneben führt die kathodische Nachätzung zu einem Anstieg des elektrischen Widerstands aufgrund einer stark erhöhten Ausbreitung der Raumladungszone in den InP Porenwänden. In dieser Arbeit wurden die Herstellung von multifunktionalen Kompositen bestehend aus der piezoelektrischen InP Membran und eines magnetischen Füllers in Form von Nanodrähten sowie die Verwendung von InP Membranen als Anodenmaterial für Li-Ionen-Akkumulatoren untersucht. Die multifunktionalen Komposite wurden mittels galvanischer Abscheidung magnetischer Metalle in InP Porenarrays und InP Membranen hergestellt. Unporöse Ni und Fe-Co-basierende Nanodrähte konnten in InP Membranen mit einem ultra-hohen Aspektverhältnis hergestellt werden. Diese Membranen waren zuvor mit einer dünnen Schicht aus Al2O3 per ALD beschichten worden. Ohne diese dielektrische Zwischenschicht konnten nur poröse Ni Nanodrähte, bzw. tunnelartige Strukturen hergestellten werden mit einer bevorzugten Nukleation von Ni Kristalliten an den Porenwänden. Das galvanische Wachstum der Ni Nanodrähte wurde ebenfalls mittels in situ FFT-IS charakterisiert. Die gefitteten Daten erlaubten eine Aufteilung der galvanischen Abscheidung in einzelne Etappen, die konsistent interpretiert werden konnten als Absorption von Borsäure auf der wachsenden Ni-Oberfläche - was die Abscheidungsreaktion vereinfacht - als die Abscheidungsreaktion von Ni an sich und als Passivierung der Ni-Oberfläche durch eine diffusionslimitierte chemische Spezies. Die magnetische Charakterisierung der Nanodrähte zeigte, dass die einfach zu magnetisierende Richtung der Ni Nanodrähte entlang der langen Nanodrahtachse z liegt aufgrund des ultra-hohen Aspektverhältnisses der Nanodrähte. Im Fall der Fe-Co-basierenden Nanodrähte liegt die einfach magnetisierbare Richtung senkrecht zu z. In beiden Fällen wies die Koerzitivfeldstärke ein Maximum zwischen H par z und H per z auf, das durch eine Kombination von zwei Ummagnetierungsmechanismen erklärt werden konnte, die jeweils unter bestimmen Winkeln energetisch bevorzugt sind. Neben der Verwendung in multifunktionalen Kompositen können die InP Membranen auch als Anodenmaterial in Li-Ionen-Akkumulatoren eingesetzt werden, die ohne Additive, Bindemittel oder andere passive Materialien auskommen, im Gegensatz zu allen anderen InP Anoden. Die porösen InP Anoden wiesen eine hohe Kapazität pro Fläche auf. Während der Lithierung kommt es zu drastischen Strukturveränderungen der InP Porenwände. Hierbei wandelt sich InP in kristallines LiP und In3Li13 um, was sichtbar wurde durch eine regelmäßige Sequenz von Ausstülpungen und Einschnürungen der Porenwände, jeweils in Gegenphase zu den benachbarten Porenwänden. Diese regelmäßige Sequenz blieb sogar im delithierten Zustand erhalten, nachdem sich nanokristallines InP gebildet hatte. Es wurden keinerlei Ablösungen oder großflächige Pulverisierung der Porenwände beobachtet. Dies zeigt, dass diese poröse Anode in der Lage ist, der hohen Volumenexpansion während der Zyklierung standzuhalten. Die Zyklierfähigkeit der porösen InP Anode war gut, da keine Kapazitätsverluste in den ersten Zyklen auftraten. In den folgenden Zyklen kam es dann allerdings zu fortschreitenden Kapazitätsverlusten. Die Verwendung von Al2O3 Beschichtungen der Porenwände war vorteilhaft für die Ausprägung einer SEI-Schicht. Die Kombination einer Al2O3 Beschichtung mit einer niedrigerer niedrigeren Entlade-Grenzspannung führte zu einer geringeren gravimetrischen Kapazität von etwa 50 % in Bezug auf die unbeschichtete Anode. Allerdings erschien diese Kombination vorteilhaft in Bezug auf die Zyklenstabilität

Influence of the processing parameters of slurries for the deposit of nickelate thick films

Thick films cathodes for Solid Oxide Fuel Cells (SOFC) are prepared by dip-coating slurries made of several lanthanum nickelate oxide powders onto yttria stabilized zirconia (YSZ) substrates. The processing parameters for the slurries preparation and the multilayers coating have been optimized to obtain homogeneous, crack-free, thick and adherent films after heat treatment

Application of electro-active biofilms

The concept of an electro-active biofilm (EAB) has recently emerged from a few studies that discovered that certain bacteria which form biofilms on conductive materials can achieve a direct electrochemical connection with the electrode surface using it as electron exchanger, without the aid of mediators. This electro-catalytic property of biofilms has been clearly related to the presence of some specific strains that are able to exchange electrons with solid substrata (eg Geobacter sulfurreducens and Rhodoferax ferrireducens). EABs can be obtained principally from natural sites such as soils or seawater and freshwater sediments or from samples collected from a wide range of different microbially rich environments (sewage sludge, activated sludge, or industrial and domestic effluents). The capability of some microorganisms to connect their metabolisms directly in an external electrical power supply is very exciting and extensive research is in progress on exploring the possibilities of EABs applications. Indeed, the best known application is probably the microbial fuel cell technology that is capable of turning biomass into electrical energy. Nevertheless, EABs coated onto electrodes have recently become popular in other fields like bioremediation, biosynthesis processes, biosensor design, and biohydrogen production

Construction of photosensitised semiconductor cathodes

Recent studies suggest that the performance of dye-sensitised solar cells (DSC) has appeared to have reached a limit, therefore solar cells based on semiconductor materials, such as extremely thin absorber (ETA) solar cells and tandem solar cells are currently the subject of intense research in the framework of low-cost photovoltaic devices as sources of harvesting sunlight to generate electricity. Generally, semiconductor solar cells have been divided into two different types, namely anodic and cathodic type solar cells. Extensive research and development work has been focused on anodic semiconductor sensitised solar cells to date. In contrast, the cathodic semiconductor sensitised solar cells have received no attention which is very surprising. Developing the cathodic semiconductor sensitised solar cell concept is very important in the development of tandem solar cells as well as other new solar cell configurations. The main reason for the lack of research in this area was due to the rarity of p-type semiconductor materials, which made it difficult to find suitable materials to match the energy band edges for cathodic semiconductor sensitised solar cells (CSSC) as well as solid-state cathodic semiconductor solar cells (SS-CSSC). The primary aim of this thesis was to construct cathodic semiconductor sensitised solar cells as well as their solid-state analogues (SS-CSSC). The work conducted within this doctoral study presents state-of-art materials and thin film processing/preparation methods, their characterisation and developing CSSCs and SS-CSSCs employing such films in cascade configurations. No reports have been published in the literature on SS-CSSC to date. The first stage of this thesis is focused on optimising the morphology and the texture (porosity) of the CuI and NiO semiconductor photocathode, by the introduction of new deposition methods namely, pulsed-electrodeposition (PED) and Aerosol-Assisted Deposition (AAD) and Aerosol-Assisted Chemical Vapour Deposition (AACVD). The electrodes prepared by employing the methods mentioned above and controlling the deposition parameters systematically, we have achieved significant improvement in the film morphology and the texture of the deposited films. The resulting electrodes showed excellent improvement in the photoelectrochemical performance which made it suitable for application in construction of both CSSC and SS-CSSC. The photoelectrochemical performance of the electrodes can be seen clearly through the photocurrent density data. For the case of bare CuI, the PEC performance of electrode prepared by the AAD and PED compared against that of continuous-electrodeposition (ED) electrodes. The photocurrent density achieved for the electrodes prepared by AAD and PED was reported around 175 and 75 µAcm-2 respectively which are way higher than the ED case. At the second stage of this study, the work focused on fabrication and characterisation of the CSSCs. Cathodic sensitised PEC solar cells (CuI/Cu2S/(Eu2+/Eu3+) and NiO/Cu2S/(I3-/I-)) were fabricated by deposition of p-Cu2S on the texture controlled CuI and NiO photocathodes. The morphological properties of the photocathode, in particular layer thickness, particle size and film porosity, play an important role in the PEC performance of CSSCs. Optimisation of these parameters led to increased adsorption of the Cu2S light harvester on the photocathode s surface. As a result, the charge injection from Cu2S to the wide band gap photocathode material (CuI and NiO) was significantly improved. Due to this, the CSSC performance showed significant improvement as semiconductor sensitised cathodic solar cells (CSSC). The IPCE and photocurrent density of the CSSC achieved in this study was around (19 and 7 %) and (1 and 0.5 mAcm-2) for the CuI/Cu2S and NiO/Cu2S electrodes respectively. Finally, the SS-CSSC has been fabricated by employing n-Fe2O3 electron transport layer. The construction of SS-CSSC for the first time using the n-Fe2O3 electron transport layer (CuI/Cu2S/Fe2O3 and NiO/Cu2S/Fe2O3) allowed us to study the materials, optical and photoelectrochemical properties of this device. Under AM 1.5 illumination, the SS-CSSC shows a photocurrent density of 6 and 9 µAcm-2 for CuI/Cu2S/Fe2O3 and NiO/Cu2S/Fe2O3 solar cells, respectively. The results of this work indicated low performance for both SS-CSSC compared to CSSC results, due to the lack of adsorption between the absorber and Fe2O3 electrode. However, this study proved the concept of SS-CSSC based on semiconductor material, which is valuable for the future work of cathodic semiconductor sensitised solar cells as well as solid-state tandem solar cells

Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review

For several decades, the increasing productivity in many industrial domains has led to a significant and ever-increased interest to protective and hard coatings. In this context, titanium-aluminum nitrides were developed and are now widely used in a large range of applications, due to their high hardness, good thermal stability, and oxidation resistance. This chapter reviews the thermodynamical characteristics of the Ti-Al-N system by reporting the progress made in the description of the Ti-Al-N phase diagram and the main mechanical and chemical properties of Ti1−xAlxN-based coatings. As a metastable phase, the existence of the fcc-Ti1−xAlxN depends on particular process parameters, allowing stabilizing this desirable solid solution. The influence of process parameters, with a particular interest for chemical vapor deposition (CVD) methods, on morphology and crystallographic structure is then described. The structure of Ti1−xAlxN thin films depends also on the aluminum content as well as on the annealing temperature, due to the spinodal nature of the Ti-Al-N system. These changes of crystallographic structure can induce an improvement of the hardness, oxidation resistance, and wear behavior of these coatings. The main mechanical and chemical properties of physical vapor deposition (PVD) and CVD Ti1−xAlxN-based coatings are also described