On the origin and application of the Bruggeman correlation for analysing transport phenomena in electrochemical systems

The widely used Bruggeman equations correlate tortuosity factors of porous media with their porosity. Finding diverse application from optics to bubble formation, it received considerable attention in fuel cell and battery research, recently. The ability to estimate tortuous mass transport resistance based on porosity alone is attractive, because direct access to the tortuosity factors is notoriously difficult. The correlation, however, has limitations, which are not widely appreciated owing to the limited accessibility of the original manuscript. We retrace Bruggeman's derivation, together with its initial assumptions, and comment on validity and limitations apparent from the original work to offer some guidance on its use

Tortuosity in electrochemical devices: a review of calculation approaches

The tortuosity of a structure plays a vital role in the transport of mass and charge in electrochemical devices. Concentration polarisation losses at high current densities are caused by mass transport limitations and are thus a function of microstructural characteristics. As tortuosity is notoriously difficult to ascertain, a wide and diverse range of methods have been developed to extract the tortuosity of a structure. These methods differ significantly in terms of calculation approach and data preparation techniques. Here, a review of tortuosity calculation procedures applied in the field of electrochemical devices is presented to better understand the resulting values presented in the literature. Visible differences between calculation methods are observed, especially when using porosity–tortuosity relationships and when comparing geometric and flux-based tortuosity calculation approaches

Understanding mass transport mechanisms in oxygen transport membrane porous support layers: correlating 3D image based modelling with diffusion measurements

The rate limiting step of an oxygen transport membrane at high fuel conversation ratios is governed by mass transport limitations of the gaseous reactants through the porous support layer of the device. Such transport limitations are directly linked to the microstructural characteristics of the porous support layer including porosity, tortuosity and pore size distribution. Among these parameters, tortuosity is the most crucial for diffusion calculation processes but notoriously difficult to quantify. The porous support layer is an indispensable part of the overall membrane assembly as it provides mechanical stability during operation as well as providing facile routes for delivery of reactants. By combining different imaging techniques, diffusion cell experiments and simulations, the connection between the microstructure and mass transport of tubular, yttria stabilized zirconia porous support membranes is explored. Lab-based X-ray nano computed tomography and focused ion beam scanning electron microscope tomography are used to reconstruct the microstructure of the porous support layers in 3D and extract the tortuosity. In addition, diffusion cell experiments at temperatures of up to 600 °C are carried out on the same samples. It is shown that image based algorithms provide lower tortuosity values in comparison to diffusion cell experiments. The reason for this is found in the lack of considering Knudsen diffusion effects, which are often neglected in diffusion simulation models. Moreover, it is found that tortuosity alone is insufficient to provide conclusive insights when evaluating the mass transport resistance of a microstructure. A holistic approach, where additional parameters, such as porosity and sample thickness, are taken into account, is recommended. The experiments have shown that to ensure high mechanical stability and high mass transport performance at steady state, the porous support layer should feature high porosity and high thickness. The obtained insights are used to optimise future support designs in collaboration with industrial partners

Understanding transport phenomena in electrochemical energy devices via X-ray nano CT

Porous support layers in electrochemical devices ensure mechanical stability of membrane assemblies such as solid oxide fuel cells and oxygen transport membranes (OTMs). At the same time, porous layers affect diffusive mass transport of gaseous reactants and contribute to performance losses at high fuel utilisation and conversion ratios. Microstructural characteristics are vital to calculate mass transport phenomena, where tortuosity remains notoriously difficult to determine. Here, the tortuosity of tubular porous support layers of OTMs is evaluated via high resolution X-ray nano computed tomography. The high resolution reveals the complex microstructure of the samples to then execute a selection of image-based tortuosity calculation algorithms. Visible differences between geometric and flux-based algorithms are observed and have thus to be applied with caution

Attending to conditions that facilitate intercultural competence: A reciprocal service-learning approach

Although service-learning can support the development of intercultural competence, it has also maintained power differentials, reinforced privileged perspectives, and strengthened deficit thinking. Recent research has investigated the conditions within service-learning associated with positive change in diversity-related attitudes. We extend that work, conceptualizing a reciprocal service-learning (RSL) approach that integrates conditions posited by contact theory and the process model of intercultural competence into service-learning’s core features of reflection and reciprocity. In an RSL approach, transformational reciprocity at the participant level supports cultural awareness, interdependence, and parity between participant groups. We created an RSL experience and measured change in three attitudes fundamental to the development of intercultural competence with quantitative pre- and post-surveys. Results indicate that both participant groups—native English-speaking undergraduate students and international English language learners—experienced significant growth. This study responds to calls for quantitative pre- and post-research methods and the assessment of outcomes for all service-learning participants

Investigating microstructural evolution during the electroreduction of UO2 to U in LiCl-KCl eutectic using focused ion beam tomography

Reprocessing of spent nuclear fuels using molten salt media is an attractive alternative to liquid-liquid extraction techniques. Pyroelectrochemical processing utilizes direct, selective, electrochemical reduction of uranium dioxide, followed by selective electroplating of a uranium metal. Thermodynamic prediction of the electrochemical reduction of UO2 to U in LiCl-KCl eutectic has shown to be a function of the oxide ion activity. The pO2− of the salt may be affected by the microstructure of the UO2 electrode. A uranium dioxide filled “micro-bucket” electrode has been partially electroreduced to uranium metal in molten lithium chloride-potassium chloride eutectic. This partial electroreduction resulted in two distinct microstructures: a dense UO2 and a porous U metal structure were characterised by energy dispersive X-ray spectroscopy. Focused ion beam tomography was performed on five regions of this electrode which revealed an overall porosity ranging from 17.36% at the outer edge to 3.91% towards the centre, commensurate with the expected extent of reaction in each location. The pore connectivity was also seen to reduce from 88.32% to 17.86% in the same regions and the tortuosity through the sample was modelled along the axis of propagation of the electroreduction, which was seen to increase from a value of 4.42 to a value of infinity (disconnected pores). These microstructural characteristics could impede the transport of O2− ions resulting in a change in the local pO2− which could result in the inability to perform the electroreduction

TargetRNA: a tool for predicting targets of small RNA action in bacteria

Many small RNA (sRNA) genes in bacteria act as posttranscriptional regulators of target messenger RNAs. Here, we present TargetRNA, a web tool for predicting mRNA targets of sRNA action in bacteria. TargetRNA takes as input a genomic sequence that may correspond to an sRNA gene. TargetRNA then uses a dynamic programming algorithm to search each annotated message in a specified genome for mRNAs that evince basepair-binding potential to the input sRNA sequence. Based on the calculated basepair-binding potential of each message with the given sRNA regulator, TargetRNA outputs a ranked list of candidate mRNA targets along with the predicted basepairing interaction of each target to the sRNA. The predictive performance of TargetRNA has been validated experimentally in several bacterial organisms. TargetRNA is freely available at http://snowwhite.wellesley.edu/targetRNA

Toward a Core Design to Distribute an Execution on a Many-Core Processor

International audienceThis paper presents a parallel execution model and a many-core processor design to run C programs in parallel. The model automatically builds parallel sections of machine instructions from the run trace. It parallelizes instructions fetches, renamings, executions and retirements. Predictor based fetch is replaced by a fetch-decode-and-partly-execute stage able to compute in-order most of the control instructions. Tomasulo's register renaming is extended to memory with a technique to match consumer/producer pairs. The Reorder Buffer is adapted to allow parallel retirement. The model is presented on a sum reduction example which is also used to give a short analytical evaluation of the model performance potential

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