Oxygen reduction at thin dense La0.52Sr0.48Co0.18Fe0.82O3- δ electrodes: Part I: Reaction model and faradaic impedance

The faradaic impedance of oxygen reduction has been simulated for thin dense two-dimensional $${\text{La}}_{{0.52}} {\text{Sr}}_{{0.48}} {\text{Co}}_{{0.18}} {\text{Fe}}_{{0.82}} {\text{O}}_{{3 - \delta }}$$ electrodes in air at 600°C. The reaction model accounts for the defect chemistry of the ceramic films and includes bulk and surface pathways. It was demonstrated that the contribution of the surface pathway to the reaction was negligible due to the small length of triple phase boundary gas/electrode/electrolyte. The diffusion of oxygen in the bulk of $${\text{La}}_{{0.52}} {\text{Sr}}_{{0.48}} {\text{Co}}_{{0.18}} {\text{Fe}}_{{0.82}} {\text{O}}_{{3 - \delta }}$$ (LSCF) can be evidenced by measuring the polarization resistance as a function of the electrode thickness that ranged between 10 and 800nm. When recorded as a function of the electrode potential and thickness, the frequency response exhibited features that were specific to the rate-determining steps of the reaction. The oxygen reduction mechanism and kinetics can therefore be identified by means of impedance spectroscopy. The faradaic impedances calculated for realistic values of the rate constants exhibited a noteworthy large faradaic capacitanc

Oxygen reduction at thin dense La0.52Sr0.48Co0.18Fe0.82O3- δ electrodes: Part II: Experimental assessment of the reaction kinetics

The mechanism and kinetics of oxygen reduction at thin dense two-dimensional $${\text{La}}_{{0.52}} {\text{Sr}}_{{0.48}} {\text{Co}}_{{0.18}} {\text{Fe}}_{{0.82}} {\text{O}}_{{3 - \delta }}$$ (LSCF) electrodes have been investigated in air between 500 and 700 °C with electrochemical impedance spectroscopy and steady-state voltammetry. Dense and geometrically well-defined LSCF films with various thicknesses ranging between 16 and 766nm have been prepared on cerium gadolinium oxide substrates by pulsed laser deposition and structured with photolithography. The current collection was ensured by a porous LSCF layer. A good agreement was found between the experimental data and the impedance of the reaction model calculated with state-space modelling for various electrode potentials and thicknesses. It was evidenced that oxygen adsorption, incorporation into the LSCF and bulk diffusion are rate-determining while charge transfer at the electrode/electrolyte interface remains at quasi-equilibrium. The 16 and 60nm thin dense LSCF electrodes appear to be more active towards oxygen reduction than thicker layers and porous films at 600 and 700°

3D microstructure effects in Ni-YSZ anodes : prediction of effective transport properties and optimization of redox stability

This study investigates the influence of microstructure on the effective ionic and electrical conductivities of Ni-YSZ (yttria-stabilized zirconia) anodes. Fine, medium, and coarse microstructures are exposed to redox cycling at 950 ºC. FIB (focused ion beam)-tomography and image analysis are used to quantify the effective (connected) volume fraction (Φeff), constriction factor (β), and tortuosity (τ). The effective conductivity (σeff) is described as the product of intrinsic conductivity (σ0) and the so-called microstructure-factor (M): σeff = σ0 x M. Two different methods are used to evaluate the M-factor: (1) by prediction using a recently established relationship, Mpred = ε β^0.36/τ^5.17, and (2) by numerical simulation that provides conductivity, from which the simulated M-factor can be deduced (Msim). Both methods give complementary and consistent information about the effective transport properties and the redox degradation mechanism. The initial microstructure has a strong influence on effective conductivities and their degradation. Finer anodes have higher initial conductivities but undergo more intensive Ni coarsening. Coarser anodes have a more stable Ni phase but exhibit lower YSZ stability due to lower sintering activity. Consequently, in order to improve redox stability, it is proposed to use mixtures of fine and coarse powders in different proportions for functional anode and current collector layers

Fuel Cell Modeling and Simulations

Fundamental and phenomenological models for cells, stacks, and complete systems of PEFC and SOFC are reviewed and their predictive power is assessed by comparing model simulations against experiments. Computationally efficient models suited for engineering design include the (1+1) dimensionality approach, which decouples the membrane in-plane and through-plane processes, and the volume-averaged-method (VAM) that considers only the lumped effect of pre-selected system components. The former model was shown to capture the measured lateral current density inhomogeneities in a PEFC and the latter was used for the optimization of commercial SOFC systems. State Space Modeling (SSM) was used to identify the main reaction pathways in SOFC and, in conjunction with the implementation of geometrically well- defined electrodes, has opened a new direction for the understanding of electrochemical reactions. Furthermore, SSM has advanced the understanding of the COpoisoning- induced anode impedance in PEFC. Detailed numerical models such as the Lattice Boltzmann (LB) method for transport in porous media and the full 3-D Computational Fluid Dynamics (CFD) Navier-Stokes simulations are addressed. These models contain all components of the relevant physics and they can improve the understanding of the related phenomena, a necessary condition for the development of both appropriate simplified models as well as reliable technologies. Within the LB framework, a technique for the characterization and computer- reconstruction of the porous electrode structure was developed using advanced pattern recognition algorithms. In CFD modeling, 3-D simulations were used to investigate SOFC with internal methane steam reforming and have exemplified the significance of porous and novel fractal channel distributors for the fuel and oxidant delivery, as well as for the cooling of PEFC. As importantly, the novel concept has been put forth of functionally designed, fractal-shaped fuel cells, showing promise of significant performance improvements over the conventional rectangular shaped units. Thermo-economic modeling for the optimization of PEFC is finally addressed

A thermally self-sustained micro-power plant with integrated micro-solid oxide fuel cells, micro-reformer and functional micro-fluidic carrier

Low temperature micro-solid oxide fuel cell (micro-SOFC) systems are an attractive alternative power source for small-size portable electronic devices due to their high energy efficiency and density. Here, we report a thermally self-sustainable reformer – micro-SOFC assembly. The device consists of a micro-reformer bonded to a silicon chip containing 30 micro-SOFC membranes and a functional glass carrier with gas channels and screen-printed heaters for start-up. Thermal independence of the device from the externally powered heater is achieved by this exothermic reforming reaction above 470 °C. The reforming reaction and the fuel gas flow rate of the n-butane/air gas mixture controls the operation temperature and gas composition on the micro-SOFC membrane. In the temperature range between 505 °C and 570 °C, the gas composition after the micro-reformer consists of 12 vol% to 28 vol% H2. An open-circuit voltage of 1.0 V and maximum power density of 47 mW/cm2 at 565 °C is achieved with the on-chip produced hydrogen at the micro-SOFC membranes

Micro-solid oxide fuel cells running on reformed hydrocarbon fuels

Micro‐solid oxide fuel cell (micro‐SOFC) systems are predicted to have a high energy density and specific energy and are potential power sources for portable electronic devices. A micro‐SOFC system is under development in the frame of the ONEBAT project [1‐3]. In this presentation, we report on the fabrication and characterization of a sub‐system assembly consisting of a startup heater and a micro‐reformer bonded to a Si chip with electrochemically‐active micro‐SOFC membranes. A functional carrier including fluidic channels for gas feed and integrated heaters was bonded to a microreformer with an overall size of 12.7 mm x 12.7 mm x 1.9 mm [4‐7]. As a catalyst, a foam‐like material made of ceria‐zirconia nanoparticles doped with rhodium was used to fill the 58.5 mm3 reformer cavity. This micro‐reformer allows for high methane and butane conversion of > 90 % with a hydrogen selectivity of > 80 % at 550 °C in the reformer [7, 8]. A silicon chip with 30 free‐standing micro‐SOFC membranes (390 μm x 390 μm) with a thickness of less than 500 nm was bonded to the carrier‐reformer assembly described above. The micro‐SOFC membrane consisted of an yttria‐ stabilized zirconia thin film electrolyte. Both Pt‐based and ceramic‐based electrode materials were tested regarding the thermal stability and carbon poisoning at temperatures below 600 °C. The functional‐carrier mirco‐reformer micro‐SOFC assembly was electrochemically tested with hydrocarbon fuel between 300 °C and 600 °C. The fuel cell performance and the microstructural evolution of the anode are discussed as well

Micro-solid oxide fuel cells as power supply for small portable electronic equipment

Micro-solid oxide fuel cell (SOFC) systems are anticipated for powering small, portable electronic devices, such as laptop, personal digital assistant (PDA), medical and industrial accessories. It is predicted that micro-SOFC systems have a 2-4 higher energy density than Li-ion batteries [1]. However, literature mainly focuses on the fabrication and characterization of thin films and membranes for micro-SOFC systems [2-12]; the entire system approach is not yet studied in detail. We will therefore discuss in this paper the entire approach from the fabrication of thin films and membranes up to the complete system, including fuel processing, thermal management and integration

Oxygen reduction at thin dense La0.52Sr0.48Co0.18Fe0.82O3– δ electrodes: Part I: Reaction model and faradaic impedance

ISSN:1385-3449ISSN:1573-866

Post-buckling design of thin-film electrolytes in micro-solid oxide fuel cells

The buckling behavior of a thin-film electrolyte of a micro-solid oxide fuel cell is investigated based on experimental measurements, analytical estimations and numerical simulations. An energy minimization procedure is applied in combination with the Rayleigh-Ritz method to represent the buckling modes, evaluate the buckling amplitude and determine the threshold values for instability transitions in the system. The residual stresses in the film deposited on a silicon substrate are evaluated based on wafer curvature whereby nanoindentations tests are applied to estimate the Young's modulus of the deposited film. The partial release of residual stresses in the film during free etching of the substrate is estimated by a new method combining pre-etching optical measurements with posteriori stress analysis. The energy interpretation of the obtained deflection shape is discussed. Comparisons between simulation results and experimental data show the efficiency of this method to predict various buckling stages of free-standing thin films. A post-buckling design space for thin-film electrolyte fabrication is presented by applying a stress-based failure criterion

Microstructural aspects of Ti6Al4V degradation in H2O2-containing phosphate buffered saline

Ti6Al4V surfaces were exposed to simulated inflammation conditions in H2O2-containing phosphate buffered saline with and without FeCl3. Scanning electron microscopy analysis revealed significantly different degradation modes for the α and β phases. While the α grains are covered by a ca. 400 nm thick protective nanostructured oxide layer, the attack of the β phase generates a porous microstructure with microscaled cracks and a low polarization resistance. The β phase is postulated to be sensitive to H2O2 reduction products and less able to generate a passive oxide film. The presence of FeCl3 enhances the cathodic activity and the β phase degradation