TauFactor: An open-source application for calculating tortuosity factors from tomographic data

TauFactor is a MatLab application for efficiently calculating the tortuosity factor, as well as volume fractions, surface areas and triple phase boundary densities, from image based microstructural data. The tortuosity factor quantifies the apparent decrease in diffusive transport resulting from convolutions of the flow paths through porous media. TauFactor was originally developed to improve the understanding of electrode microstructures for batteries and fuel cells; however, the tortuosity factor has been of interest to a wide range of disciplines for over a century, including geoscience, biology and optics. It is still common practice to use correlations, such as that developed by Bruggeman, to approximate the tortuosity factor, but in recent years the increasing availability of 3D imaging techniques has spurred interest in calculating this quantity more directly. This tool provides a fast and accurate computational platform applicable to the big datasets (>10^8 voxels) typical of modern tomography, without requiring high computational power

Validation of a physically-based solid oxide fuel cell anode model combining 3D tomography and impedance spectroscopy

This study presents a physically-based model for the simulation of impedance spectra in solid oxide fuel cell (SOFC) composite anodes. The model takes into account the charge transport and the charge-transfer reaction at the three-phase boundary distributed along the anode thickness, as well as the phenomena at the electrode/electrolyte interface and the multicomponent gas diffusion in the test rig. The model is calibrated with experimental impedance spectra of cermet anodes made of nickel and scandia-stabilized zirconia and satisfactorily validated in electrodes with different microstructural properties, quantified through focused ion beam SEM tomography. Besides providing the material-specific kinetic parameters of the electrochemical hydrogen oxidation, this study shows that the correlation between electrode microstructure and electrochemical performance can be successfully addressed by combining physically-based modelling, impedance spectroscopy and 3D tomography. This approach overcomes the limits of phenomenological equivalent circuits and is suitable for the interpretation of experimental data and for the optimisation of the electrode microstructure

Safety assessment of scenarios triggered by accidental seawater immersion of lithium batteries in innovative naval applications

The request of lithium-ion batteries in naval applications features an increasing trend due to the need of high performance energy storage devices. Beside the hazard of runaway when the battery is crushed or overcharged, the naval environment poses additional safety issues due to the possible immersion with seawater of the battery room following accidental flooding. In such a case, seawater electrolysis may generate hydrogen and chlorine, thus giving rise to an explosive and toxic mixture in a confined environment. In this study, a quantitative safety assessment of the possible accidental scenarios induced by seawater electrolysis in battery rooms is performed. The analysis is based on i) the deterministic evaluation of the hazardous gases development according to a three-dimensional physically-based electrochemical model and ii) the discussion of possible prevention and mitigation measures. The dynamics of gas release and the time required to produce an explosive atmosphere are evaluated as a function of the level of seawater for a reference case study. The outcomes of the study support the safety assessment of accidental scenarios induced by seawater electrolysis in naval applications and provide indications on the effectiveness of hazard reduction measures. More specifically, the study reveals that a proper electric insulation of the surface of battery terminals represents a sensible mitigation strategy to reduce the severity of explosions in addition to the ventilation of the battery room

Guidelines for the rational design and engineering of 3D manufactured solid oxide fuel cell composite electrodes

The growth of 3D printing has opened the scope for designing microstructures for solid oxide fuel cell s (SOFCs) with improved power density and lifeti me. This technique can introduce structural modifications at a scale larger than particle size but smaller than cell size, such as by inserting electrolyte pillars of ~5 - 100 µ m. This study sets the minimum requirements for the rational design of 3D printed electrodes based on an electrochemical model and analytical solutions for functional layers with negligible electronic resistance and no mixed conduction . Results show that this structural modification enhances the power density when the ratio k eff betwee n effective conductivity and bulk conductivity of the ionic phase is smaller than 0.5. The maximum performance improvement is predicted as a function of k eff . A design study on a wide range of pillar shapes indicates that improvements are achieved by any s tructural modification which provides ionic conduction up to a characteristic thickness ~10 - 40 µ m without removing active volume at the electrolyte interface. The best performance is reached for thin ( ~80 µ m) pillars when the composite electrode is optimised for ma ximum three - phase boundary density, pointing towards the design of scaffolds with well - defined geometry and fractal structures

Electrochemical simulation of Solid Oxide Fuel Cell electrodes: an integrated approach to address the microstructure-performance correlation

Understanding the complex interplay between electrode microstructure and electrochemical performance is one of the key aspects for the optimization of Solid Oxide Fuel Cells (SOFC). Physically-based modelling, at different levels of sophistication, can provide a valuable insight in order to help the interpretation of experimental data and provide design indications to improve electrode stability and performance. In this contribution we summarize the different modelling approaches used in our group, ranging from physically-based equivalent circuits, continuum conservation models and 3D models solved within the reconstructed electrode microstructure. When necessary, these models are coupled with percolation theory, packing algorithms and tomographic techniques. Special focus is given to the application of the models to interpret impedance spectra and their thorough validation under different conditions. Examples include the application of the models to electrodes with different microstructures, the study of the degradation mechanisms of Ni-infiltrated anodes as well as impedance simulations in real microstructures (Figure 1). Results reveal that coupling physically-based modelling, impedance spectroscopy and 3D tomography is a promising approach to gain a fundamental understanding of the phenomena occurring at different length scales in SOFC electrodes, allowing for interpreting and planning experiments as well as to design more stable and more efficient electrodes

Heterogeneous electrocatalysis in porous cathodes of solid oxide fuel cells

A general physics-based model is developed for heterogeneous electrocatalysis in porous electrodes and used to predict and interpret the impedance of solid oxide fuel cells. This model describes the coupled processes of oxygen gas dissociative adsorption and surface diffusion of the oxygen intermediate to the triple phase boundary, where charge transfer occurs. The model accurately captures the Gerischer-like frequency dependence and the oxygen partial pressure dependence of the impedance of symmetric cathode cells. Digital image analysis of the microstructure of the cathode functional layer in four different cells directly confirms the predicted connection between geometrical properties and the impedance response. As in classical catalysis, the electrocatalytic activity is controlled by an effective Thiele modulus, which is the ratio of the surface diffusion length (mean distance from an adsorption site to the triple phase boundary) to the surface boundary layer length (square root of surface diffusivity divided by the adsorption rate constant). The Thiele modulus must be larger than one in order to maintain high surface coverage of reaction intermediates, but care must be taken in order to guarantee a sufficient triple phase boundary density. The model also predicts the Sabatier volcano plot with the maximum catalytic activity corresponding to the proper equilibrium surface fraction of adsorbed oxygen adatoms. These results provide basic principles and simple analytical tools to optimize porous microstructures for efficient electrocatalysis

Physically-based deconvolution of impedance spectra for LSCF-based SOFC

A physically-based model for the interpretation of the impedance spectra of an anode-supported LSCF/GDC/YSZ/Ni:YSZ solid oxide fuel cell is presented in this work. The model locally describes transport and reaction phenomena within the cell components through mass conservation equations. The microstructural properties of the electrodes are predicted through numerical three-dimensional reconstruction of the microstructure, with input parameters obtained from the analysis of SEM pictures of each layer. Simulations show that the model reproduces impedance spectra obtained in different operating conditions with the same set of fitting parameters, comprising material-specific kinetic constants and electrochemical capacitances, which fairly agree with independent literature data and a previous analysis of the spectra through DRT. The model allows for the deconvolution and quantification of the characteristic resistance and frequency of the different physical processes that build up the impedance of the cell. In particular, 7 processes are identified: charge-transfer reactions between LSCF/GDC, GDC/YSZ and Ni/YSZ interfaces appear in the high-frequency range, the medium-frequency feature is due the oxygen reduction reaction and the gas diffusion in the anode, while the low-frequency arc is mainly due to the gas conversion in the anodic channel. An additional low frequency contribution (< 1Hz), not considered in the model, is observed and tentatively attributed to the adsorption of oxygen onto the LSCF surface. Simulation results suggest that more efforts must be dedicated to characterize and improve the oxygen transfer at the LSCF/GDC and GDC/YSZ interfaces. The study shows that a quantitative interpretation of impedance spectra is possible with a reduced number of fitting parameters when a physically-based approach is adopted, making the model an attractive tool for diagnostic purposes

3D characterization of diffusivities and its impact on mass flux and concentration overpotential in SOFC anodes

In recent years great effort has been taken to understand the effect of gas transport on the performance of electrochemical devices. This study aims to characterize the diffusion regimes and the possible inaccuracies of the mass transport calculation in Solid Oxide Fuel Cell (SOFC) anodes when a volume-averaged pore diameter is used. 3D pore size distribution is measured based on the extracted pore phase from an X-ray CT scan, which is further used for the calculation of a Knudsen number (Kn) map in the porous medium, followed by the voxel-based distribution of the effective diffusion coefficients for different fuel gases. Diffusion fluxes in a binary gas mixture using the lower boundary, upper boundary and average effective coefficients are compared, and the impact on overpotential is analyzed. The results show that pore diameters from tens to hundreds of nanometers result in a broad range of Knudsen number (1.1 ∼ 4.8 and 0.6 ∼ 3 for H2 and CH4 respectively), indicative of the transitional diffusion regime. The results highlight that for a porous material, such as an SOFC anode where Knudsen effects are non-negligible, using a volume-averaged pore size can overestimate the mass flux by ±200% compared to the actual value. The characteristic pore size should be chosen sensibly in order to improve the reliability of the mass transport and electrochemical performance evaluation

Experimental approach for the study of SOFC cathodes

The suitability of impedance measurements in Solid Oxide Fuel Cells (SOFCs) is an important concern, especially in case of measuring separately the behaviour of one of the electrode when an overvoltage is applied. In this case a thin electrolyte-supported cell with the RE (Reference Electrode) coplanar with the WE (Working Electrode) is experimentally convenient, but many authors highlighted that incorrect results can be obtained if an inappropriate geometric configuration is used. In this work LSM cathodes ((La0.8Sr0.2)MnO3-x) were investigated in a Yttria-stabilised Zirconia (YSZ) electrolyte-supported cell, using an electrolyte 3 mm thick. Two types of cells were prepared: the first (Cell1) according to the geometric requirements suggested in literature: little WE (diameter 3 mm) aligned to the CE (Counter Electrode) and with equal Rpol(polarisation resistance) and time constant; RE co-planar around the WE and placed at a distance greater than three-electrolyte thicknesses from the WE; the second one (Cell2) equal to Cell1 but with a bigger WE (diameter 8 mm). Impedance measurements were carried out both in two- and three- electrode configuration, at OCV (Open Circuit Voltage) and under applied overpotentials. A preliminary comparison between the results extracted from Cell2 at two- and three- electrodes confirms that a thick electrolyte allows extracting suitable three-electrode impedance results in case of OCV and small overpotentials. On the other side, when an overpotential over 0.2 V is applied, a comparison between Cell1 and Cell2 gives quite different results. The investigation here presented considers also an experimental approach useful for the comprehension of the main phenomena governing the kinetic of the process

Unveiling the mechanisms of solid-state dewetting in Solid Oxide Cells with novel 2D electrodes

During the operation of Solid Oxide Cell (SOC) fuel electrodes, the mobility of nickel can lead to significant changes in electrode morphology, with accompanying degradation in electrochemical performance. In this work, the dewetting of nickel films supported on yttriastabilized zirconia (YSZ), hereafter called 2D cells, is studied by coupling in-situ environmental scanning electron microscopy (E-SEM), image analysis, cellular automata simulation and electrochemical impedance spectroscopy (EIS). Analysis of experimental E-SEM images shows that Ni dewetting causes an increase in active triple phase boundary (aTPB) length up to a maximum, after which a sharp decrease in aTPB occurs due to Ni de-percolation. This microstructural evolution is consistent with the EIS response, which shows a minimum in polarization resistance followed by a rapid electrochemical degradation. These results reveal that neither evaporation-condensation nor surface diffusion of Ni are the main mechanisms of dewetting at 560-800 °C. Rather, the energy barrier for pore nucleation within the dense Ni film appears to be the most important factor. This sheds light on the relevant mechanisms and interfaces that must be controlled to reduce the electrochemical degradation of SOC electrodes induced by Ni dewetting