Thin film fuel cells with vanadium oxide anodes: Strain and stoichiometry effects

Thin film solid oxide fuel cells incorporating vanadium oxide anodes having open circuit potential of 1 V with hydrogen fuel have been realized. The as-deposited anode stoichiometry was varied by choice of growth conditions of vanadium oxide and a striking correlation to fuel cell performance (open circuit potential and peak power density) is observed. Possible mechanisms leading to the experimental observations based on calculated thermodynamic phase stability under fuel cell operating environments, spectroscopic analysis of the anodes and strain-related arguments are presented

Internal stresses in ultrathin oxide films: influence on growth and reliability

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The role of internal stress on the growth and breakdown of barrier and porous anodic oxide films

Anodic oxidation, or anodizing, can be defined as the electrochemically-controlled growth of an oxide film on a metal. Owing to its simplicity, anodizing is used extensively in a wide range of applications. These include providing corrosion protection and an aesthetic finish to metallic surfaces, manufacturing nanoporous templates, and producing a dielectric medium in electrolytic capacitors. In all these cases, process instabilities which terminate the growth of a dense anodic oxide film, like breakdown and pore initiation, are of key importance. Internal stresses have often been suggested to be a key factor controlling these instabilities in anodic oxide films. However, no direct quantitative correlation between internal stress and either pore development or breakdown of anodic oxide films has ever been established. In this thesis, we measured the internal stress in anodic oxides in situ during their growth. By systematically varying the electrochemical growth conditions, and thereby the magnitude of the internal stress, we were able to unravel its influence on the development of porosity in anodic alumina films, and on the breakdown of anodic zirconia films. In the case of anodic alumina, energy-based stability analyses revealed that the internal stress is unlikely to be the controlling factor for pore initiation and pore spacing selection. Instead, these processes were found to be rather governed by an electrostatic energy induced surface instability. On the other hand, experimental evidence was provided that, contrary to pore initiation, pore growth in anodic alumina can be considered to be a stress-assisted viscous flow process. In this respect, our internal stress data allowed to quantify the viscosity of anodic alumina at room temperature in the presence of large ionic currents. Finally, internal stress was identified as a key factor as well for initiating breakdown in anodic zirconia films. The microstructural origin of the breakdown in that case was identified as a phase transformation allowing the zirconia to densify, the compressive internal stress in the growing anodic oxide film being the driving force for such a transformation.(FSA 3) -- UCL, 201

Stress-affected and stress-affecting instabilities during the growth of anodic oxide films

The objective of the current paper is to (re-)address the question whether internal stress is a fundamental parameter driving some generic cases of growth instabilities commonly encountered during the growth of anodic oxide films, namely breakdown and pore initiation. This has been done by unraveling possible correlations between a key electrochemical characteristic of the instability event and the internal stress evolution, the latter being measured in situ during the very same anodising experiment. As such, we have been able to make more conclusive statements as compared to the merely speculative arguments in the literature whether these instabilities have a mechanical origin or not. In the case of breakdown, the two well-documented types of breakdown events encountered during galvanostatic Zr anodising were both found to be stress-affected: instantaneous compressive internal stresses were identified as the driving force for both the densifying phase transformation responsible for type-I breakdown, as well as for the buckling-induced delamination events observed during type-II breakdown. Pore initiation in anodic Al2O3 on the other hand was found not be stress-affected. Instead, pore formation is rather believed to induce itself a modification in the mechanical behaviour, and was therefore classified as stress-affecting

Vanadium oxide electrodes for low temperature solid oxide fuel cells

Thin film solid oxide fuel cells (µ-SOFCs), which use electrodes and electrolyte films with thicknesses of the order of tens of nanometers, have demonstrated power densities above 0.5 W/cm2 at temperatures below 600°C. The decrease of the operating temperature of SOFCs opens up the possibility of using them as power sources in mobile applications, but also allows demonstrating new power source operational capabilities enabled by the use of non-traditional SOFC materials. In this respect, we have recently demonstrated that using vanadium oxide on the anode-side of µ-SOFCs allows storing energy to continue generating power for short time periods in the absence of fuel. In the present contribution, we explore in detail the influence of the stoichiometry of as-deposited thin film vanadium oxide anodes on the performance of µ-SOFCs. Vanadium oxide films were deposited by reactive magnetron sputtering without external heating. Different oxygen concentrations in the sputtering chamber allowed obtaining different stoichiometries in the as-deposited films, as indicated by x-ray photoelectron spectroscopy and x-ray diffraction. The stoichiometry was varied from V metal to V2O5. V and VO films were crystalline while other as-deposited vanadium oxide stoichiometries were amorphous. µ-SOFCs with yttria-stabilized zirconia electrolyte, porous platinum cathode, and anodes with different vanadium oxide stoichiometries were fabricated and tested at temperatures between 150 and 440°C. Composition effects on open circuit voltage and peak power performance will be considered in depth in connection with the electrocatalytic activity of the oxide electrodes

Stress-induced breakdown during galvanostatic anodising of zirconium

Although internal stress is frequently being suggested as a plausible reason for oxide breakdown during valve metal anodising. no direct quantitative evidence has been made available yet. In this work, we anodized sputtered zirconium thin films galvanostatically at room temperature in sulphuric acid until breakdown was observed, and simultaneously measured the internal stress evolution in the oxide in situ, using a high-resolution curvature setup It was found that the higher the magnitude of the observed internal compressive stress in the oxide, the smaller the oxide thickness at which breakdown occurred The moment of breakdown was identified from a slope change in the cell voltage evolution, indicative fora decrease in anodising efficiency The latter presumably occurs as a result of oxygen evolution, initiated by the relative increase of the cubic or tetragonal zirconia phase content relative to the monoclinic one. This was evidenced in turn by comparing electron diffractograms. taken in a transmission electron microscope, before and after breakdown The critical role of internal stress on oxide breakdown during zirconium anodising can therefore be associated with its promoting effect on the densifying phase transformation of monoclinic oxide. (C) 2010 Elsevier Ltd. All rights reserve