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

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

An Electrically Conductive Oleogel Paste for Edible Electronics

Edible electronics will facilitate point-of-care testing through safe devices digested/degraded in the body/environment after performing a specific function. This technology, to thrive, requires a library of materials that are the basic building blocks for eatable platforms. Edible electrical conductors fabricated with green methods and at a large scale and composed of food derivatives, ingestible in large amounts without risk for human health are needed. Here, conductive pastes made with materials with a high tolerable upper intake limit (≥mg kg−1 body weight per day) are proposed. Conductive oleogel composites, made with biodegradable and food-grade materials like natural waxes, oils, and activated carbon conductive fillers, are presented. The proposed pastes are compatible with manufacturing processes such as direct ink writing and thus are suitable for an industrial scale-up. These conductors are built without using solvents and with tunable electromechanical features and adhesion depending on the composition. They have antibacterial and hydrophobic properties so that they can be used in contact with food preventing contamination and preserving its organoleptic properties. As a proof-of-principle application, the edible conductive pastes are demonstrated to be effective edible contacts for food impedance analysis, to be integrated, for example, in smart fruit labels for ripening monitoring

Multiscale dynamics of charging and plating in graphite electrodes coupling operando microscopy and phase-field modelling

The phase separation dynamics in graphitic anodes significantly affects lithium plating propensity, which is the major degradation mechanism that impairs the safety and fast charge capabilities of automotive lithium-ion batteries. In this study, we present comprehensive investigation employing operando high-resolution optical microscopy combined with non-equilibrium thermodynamics implemented in a multi-dimensional (1D+1D to 3D) phase-field modeling framework to reveal the rate-dependent spatial dynamics of phase separation and plating in graphite electrodes. Here we visualize and provide mechanistic understanding of the multistage phase separation, plating, inter/intra-particle lithium exchange and plated lithium back-intercalation phenomena. A strong dependence of intra-particle lithiation heterogeneity on the particle size, shape, orientation, surface condition and C-rate at the particle level is observed, which leads to early onset of plating spatially resolved by a 3D image-based phase-field model. Moreover, we highlight the distinct relaxation processes at different state-of-charges (SOCs), wherein thermodynamically unstable graphite particles undergo a drastic intra-particle lithium redistribution and inter-particle lithium exchange at intermediate SOCs, whereas the electrode equilibrates much slower at low and high SOCs. These physics-based insights into the distinct SOC-dependent relaxation efficiency provide new perspective towards developing advanced fast charge protocols to suppress plating and shorten the constant voltage regime

Microstructural Modeling and Effective Properties of Infiltrated SOFC Electrodes

A modeling framework for the microstructural modeling of infiltrated SOFC electrodes is presented. The model numerically reconstructs infiltrated electrodes through a sedimentation algorithm for the backbone generation and a novel Monte Carlo packing algorithm for the random infiltration. Effective properties are evaluated by means of Monte Carlo geometric analysis and finite volume method as a function of the loading and of the particle size of infiltrated particles. Infiltration into ion-conducting and composite backbones is analyzed in this study. Simulations show that the infiltration can lead to an increase in TPB density of about two orders of magnitude if compared with conventional composite electrodes. In addition, infiltration into monocomponent backbones can lead to a TPB density about twice the TPB achievable when infiltrating composite backbones. On the other hand, a critical loading of nanoparticles must be reached in monocomponent backbones while in a composite backbone the infiltration is always beneficial

Compensatory measures to overcome performance limitations of recycled Li-ion battery materials

The increasing demand of lithium-ion batteries for electric vehicles, combined with the lack of critical raw materials and the necessity to dispose of spent batteries, is driving towards the use of recycled cathode active materials in the next generation of batteries. However, closed loop recycling solutions, such as direct recycling methods and co-precipitation, result in cathode materials which have a lower quality than those prepared from virgin precursors, which translate into smaller volumetric capacity and solid-state diffusivity. In order to meet the stringent requirements of the automotive sector, changes in the electrode design are thus required when recycled cathode active materials are used. In this contribution we use a pseudo-2D thermo-electrochemical model to provide design guidelines to overcome the performance losses of recycled cathode active materials (namely, LiNixMnyCozO2, NMC). The model is first validated with discharge/charge data of a commercial cell (Samsung SDI 94 Ah used in the BMW i3 2016 model) by using a parametrization coming from a comprehensive survey of material properties [1], showing deviations smaller than 1 % in the prediction of capacity (Figure 1). The model is used to quantify the performance loss due to lower-quality NMC particles: a reduction in accessible capacity of 10 % is predicted by a 10 % reduction in maximum Li concentration in NMC, while a 1-4% capacity loss results from a halved solid-state diffusivity. In order to compensate for such losses, design guidelines are conceived and discussed (Figure 2). In particular, increasing the electrode thicknesses by 11 % as a maximum compensates the capacity loss given by recycled NMC, resulting in a minor decrease in specific energy density (from 141 Wh kg-1 to 136 Wh kg-1 at the battery pack level) without impacting the roundtrip efficiency and the fast charge capability (i.e., recovering 50 % of capacity in 15 min). A decrease in solid-state diffusivity can be compensated by reducing the NMC diameter from 8 mm to 6 mm, although a better strategy consists in increasing the electrode thicknesses by ca. 1 mm. In summary, results indicate that lower-quality recycled materials can be effectively used without compromising the performance of a Li-ion battery for electric vehicle applications

A multiscale model for infiltrated SOFC anodes

SOFC electrodes where the electrocatalyst is infiltrated into a porous electrolyte layer offer key advantages such as much higher electrochemical activity, a greater tolerance for thermal shock, higher redox tolerance (for anodes), etc. when compared to conventional composite electrodes. Another important development in recent times is the development of mathematical models that are able to relate the properties of fuel cell electrodes to their microstructure. In this presentation, we will discuss the model development and results from two SOFC models: 1) a model that predicts the effective conductivities and triple-phase boundary density (a measure of reaction site density) using knowledge of the microstructure of Ni infiltrated anodes [1,2], and 2) a multiphysics model that takes the above computed electrode properties and uses them to simulate SOFC performance. The first model, a nano-micrometer scale model is based on percolation theory and uses experimentally controllable and measurable parameters as input. The second model is a micro-centimeter scale reaction-transport model that solves all the relevant coupled physics in a working SOFC to compute the current produced as a function of cell voltage. By coupling the two models together serially, we are able to evaluate the effect of microstructural parameters on fuel cell performance. We will present results that demonstrate how this approach can be used to evaluate and improve the design of infiltrated SOFC electrodes

Erratum: A particle-based model for effective properties in infiltrated solid oxide fuel cell electrodes (Journal of the Electrochemical Society (2014) 161 (F1243))

This article was published online on September 11, 2014 before all of the corrections/changes the author requested had been made. ECS apologizes for these errors. The article was corrected online on September 15, 2014

Fabrication and electrochemical modelling of 8YSZ and GDC10 freeze tape cast scaffolds for solid oxide cells (SOCs)

The morphology of electrodes in Solid Oxide Cells (SOCs) has a great impact on their mechanical stability during operation as well as transport properties and kinetics, which in turn affect electrode and cell performance. This study proposes a new experimental procedure based on the freeze tape casting technique for the manufacturing of graded porous electrodes for SOCs and analyses how the main processing parameters shape the final electrode microstructure. The use of water-based freeze tape casting has enabled the effective fabrication of hierarchical porous ionic backbones featuring the typical porosity of functional and supporting electrodes in a single tape. The porous samples are morphologically characterized by Environmental Scanning Electrode Microscopy (ESEM), X-Ray Computed Tomography (X-Ray CT) and computational tools to retrieve their microstructural characteristics. Subsequently, for the first time according to the authors knowledge, a Computational Fluid Dynamic (CFD) model has been developed to compare the gas transport properties of conventional spongy-like to graded porous electrodes of planar SOCs. The results presented strongly suggest that hierarchical porous electrodes enable higher performance by decreasing the voltage concentration losses and boosting the gas transfer within the electrode diffusion channels