1,721,011 research outputs found
Using electrochemical impedance spectroscopy to measure microstructural degradation in nickel-impregnated scandia-stabilised-zirconia electrode
No full textGuidelines and numerical analysis for the rational design of 3D manufactured solid oxide fuel cell electrodes
No full textThe recent growth of 3D printing has opened the scope for designing microstructures for solid oxide fuel cell (SOFC) electrodes with improved power density and lifetime. This technique can introduce structural modifications of the electrode/electrolyte interface at a scale which is larger than the particle size but smaller than the cell size, such as the insertion of dense electrolyte pillars in the order of 5-100 um.
This study presents some guidelines and sets the minimum requirements for the rational design of 3D printed electrodes based on an electrochemical numerical model and approximated analytical solutions for functional layers with negligible electronic resistance. Results show that this structural modification enhances the power density only when the effective ionic conductivity factor keff of the composite electrode is smaller than 0.5. The maximum performance improvement can be straightforwardly predicted analytically as a function of keff only. A design study on a wide range of pillar shapes indicates that improvements in electrochemical performance are achieved by any chosen structural modification capable to provide ionic conduction farther from the electrolyte up to a characteristic thickness (typically ~10-40 um) without removing electrochemically active volume at the electrolyte interface. The best performance improvements are reached in the limit of thin ( ~80 um) pillars when the composite electrode domain is optimised for maximum three-phase boundary (TPB) density, pointing towards the design of scaffold electrodes with well-defined geometry and fractal structures
Cermet membrane reactors for oxygen separation with low silver content
No full textIn this contribution we first present our results for oxygen separation/membrane reactors with silver and doped ceria and our approach to manufacturing cermets with low metal content (silver < 10 vol%) and then we concentrate on our more recent results on the Sc-stabilized zirconia and silver.
Dense composites of silver and Sc-stabilized ZrO2 (Ag-ScSZ) are manufactured from ScSZ sub-micrometric particles coated with Ag via Tollens’ reaction. There is a significant reduction in level of silver, (11.9 vol %), required for percolation. This ensures a metallic conductivity of 186 S cm-1 and an oxygen flux of 0.014 umol cm-2 s-1 at 600°C for a 1-mm thick membrane when used as a pressure-driven separation membrane between air and argon. We measure and model the impedance of a non-percolating sample to show that oxygen transport in the silver droplets inside the composite is dominated by diffusion of neutral species and not by the charge transfer reaction at the interface between ScSZ and silver. The model establishes that oxygen transport takes place in both silver and ScSZ but it is still dominated by transport in the ionic conductor and that the surface of a separation membranes does not require further activation as the silver can reduce oxygen readily
Using 3D imaging and electrochemical modelling to design and optimise electrode performance
No full textSolid oxide fuel cells (SOFCs) are some of the most promising electrochemical devices in terms of clean and efficient energy production. At the SOFC anode, fuel is oxidised without an intermediate combustion process at the triple-phase boundaries (TPBs) between metal, ceramic and porous phase. These three different phases have a porous and complex microstructure, whose changes upon operation and degradation have yet to be fully related to bulk measurements, such as impedance spectroscopy (EIS) measurements, commonly used to assess cell performance.
In this work we use time-lapse 3D imaging in close association with electrochemical modelling to move towards intelligently designing electrode structures. A porous scandia stabilised zirconia (ScSZ) scaffold was fabricated using tape casting, impregnated with Ni and characterised with FIB-SEM in order to obtain 3D microstructural parameters such as phase fractions, phase connectivity and tortuosity, and the length and density of the TPBs. The 3D microstructural parameters were used as an input in a 1D electrochemical model to predict both the changes in EIS spectra upon microstructure degradation in a reduced atmosphere as well as the strategies of microstructure optimisation needed for improved performance.
Symmetrical cells were set up to compare experimental EIS spectra with the performance predicted by the electrochemical model. The degraded electrode morphology was characterised in 3D and both the model and the experimental results show a factor of three decrement in the TPB density related to the decline in performance during degradation. Following the model suggestion, new electrodes were fabricated and cycled to check for performance improvements. This approach enables the move towards intelligently designed structures with attributes tailored towards optimised performance
Model-based design and 3D characterization of a SOFC electrode microstructure
No full textThe characterization of SOFC performance and reliability has conventionally relied on bulk parameter measurements, such as fuel cell impedance and other electrochemical parameters. These bulk parameters, however, have increasingly been correlated with the porous microstructure of the electrodes but have yet to be fully linked to the degradation of the micro- and nanostructure of the electrodes during use.
In this work, a design led approach to electrode manufacture is implemented. A Ni-ScSZ scaffold was first produced using tape casting and characterised using FIB-SEM tomography in order to quantify the TPB density as well as the tortuosity of the phases. This initial microstructure was also used as an input in a physically-based electrochemical model to predict impedance. The electrode was then incorporated into a symmetrical cell and tested at 610°C to compare its performance to the one predicted by the physical model as well as to examine the degradation of the anode with time. The comparison allowed for a critical assessment of the assumptions of the electrochemical model and for the prediction of better performance with different phase fractions. This approach allows for a seminal pass at manufacturing electrodes with desired specific performance requirements using a predictive model
Physically-based interpretation of impedance spectra of solid oxide fuel cell anodes
No full textSolid oxide fuel cells (SOFCs) represent a promising technology for the sustainable production of electricity, whose electrochemical performance needs further improvement. Electrochemical impedance spectroscopy (EIS) allows the identification of the dynamic response of the different processes that contribute to the electrode resistance, but the interpretation of spectra is often a complex task.
In this contribution, we adopt a physically-based model for the deconvolution of EIS spectra of composite anodes made of nickel and scandia-stabilized zirconia. The model takes into account the electrochemical reaction (Butler-Volmer-type kinetics) as well as the transport of gases (Stefan-Maxwell model) and charges across the electrode thickness. The microstructural parameters required by the model are obtained from the tomographic reconstruction of the samples. The model is fitted and validated in samples with different Ni volume fractions in a wide range of temperature and hydrogen contents as shown in the Figure.
Model simulations indicate that the low-frequency feature of the spectra is mainly due to gas diffusion while the high-frequency arc is the contribution of the coupled ionic transport and electrochemical reaction. In addition, material-specific kinetic parameters are extracted and applied for the interpretation of EIS data obtained in nanostructured electrodes. The results of the study are used to identify the limiting factors of the anode and to guide the design of more efficient electrodes
Simulated impedance of diffusion processes in tomographically derived microstructures
No full textImpedance spectroscopy is a powerful and widely used technique for characterising processes in electrochemical devices, such as batteries and fuel cells. The performance of these devices is closely related to their 3D microstructures; however, the elements used for representing them are typically either zero dimensional (resistors, capacitors etc.) or occasionally 1D. The most commonly used 1D elements are Warburg diffusion elements, which are particularly useful as they have analytical solutions and so can be easily incorporated into standard EIS fitting algorithms. However, the transport processes that these elements are used to represent are inherently 3D, and so Warburg diffusion elements must capture transport phenomena with bulk parameters such as tortuosity factors and porosities.
Details of the geometry of porous electrodes have recently become routinely available using microtomography. This technique typically represents the geometry as an array of cuboid volume elements (voxels) that must be segmented from greyscale to a phase labelled volume. In 2016, the authors published an open-source software package, TauFactor, which allows for the rapid calculation of tortuosity factors from segmented tomography data. An updated version of the software is here presented to efficiently calculate diffusive impedance spectra in the frequency domain, directly from segmented tomography data.
Numerical results show that the diffusion impedance may significantly deviate from the Warburg analytical solution for structures showing an inhomogeneous distribution of pores. In particular, multiple peaks may appear in the high-frequency region in the complex plane, which may be misinterpreted as separate electrochemical processes in real impedance data (see fig.1 for a simple 2D example). Two classes of structures, namely with increasing or decreasing porosity distribution, can be identified from the analysis of diffusion impedance. Thus, the software can be used to overcome the limits of conventional equivalent circuit analysis in modelling transport phenomena in porous electrodes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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