In order to improve lithium ion batteries it is important to characterise real electrode geometries and understand how their 3D structure may affect performance. In this study, high resolution ...synchrotron nano-CT was used to acquire 3D tomography datasets of mesocarbon microbead (MCMB) based anodes down to a 16 nm voxel size. A specimen labelling methodology was used to produce anodes that enhance the achievable image contrast, and image processing routines were utilised to successfully segment features of interest from a challenging dataset The 3D MCMB based anode structure was analysed revealing a heterogeneous and bi-modally distributed microstructure. The microstructure was quantified through calculations of surface area, volume, connectivity and tortuosity factors. In doing so, two different methods, random walk and diffusion based, were used to determine tortuosity factors of both MCMB and pore/electrolyte microstructures. The tortuosity factors (2-7) confirmed the heterogeneity of the anode microstructure for this field of view and demonstrated small MCMB particles interspersed between large MCMB particles cause an increase in tortuosity factors. The anode microstructure was highly connected, which was also caused by the presence of small MCMB particles. The complexity in microstructure suggests inhomogeneous local lithium ion distribution would occur within the anode during operation.
An optically transparent polymer electrolyte membrane (PEM) water electrolysis cell was studied using a high-speed camera, thermal imaging and electrochemical impedance spectroscopy to examine the ...relationship between flow and electrochemical performance. The flow regime spans bubble and slug flow, depending on the rate of gas formation (current density) and water feed rate. Electrochemical impedance spectroscopy (EIS) shows that there is a reduction in mass transport limitation associated with the transition to slug flow.
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•Optically transparent electrolyser cell made using printed circuit boards.•Different flow regimes identified as function of current and water feed rate.•Bubble-induced mass limitation identified in electrochemical impedance response.•Criteria for flow optimisation presented.
Lung-inspired, fractal flow-fields hold great potential in improving the performance of polymer electrolyte membrane fuel cells (PEMFCs) by providing uniform gas distribution across the electrodes ...and ensuring minimum entropy production in the whole system. However, the inherent susceptibility of the fractal flow-fields to flooding renders their use inadequate at high humidity conditions. In-depth understanding of water management in lung-inspired flow-fields is indispensable for the implementation of alternative outlet channel geometries or engineered water removal strategies to alleviate flooding. Here, liquid water formation and transport across the lung-inspired and serpentine flow-field based PEMFCs are evaluated using neutron radiography. The results reveal a propensity to flooding in the interdigitated outlet channels of the fractal flow-field with N = 4 generations as a result of slow gas velocity and narrow channel dimensions, which leads to significant performance deterioration. Neutron images also elucidate the importance of ensuring a well-defined internal channel structure of the fractal flow-fields to prevent backflow of liquid water via wicking and capillary pressure build-up arising from the narrow inlet gas channels and hydrophobic gas diffusion layer.
•Neutron radiographs are presented for the lung-inspired and serpentine flow-fields.•A well-defined channel structure of the fractal flow-field is indispensable.•Water removal strategies required to alleviate flooding in the fractal flow-field.
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► Measurement of residual oil phase cluster size distribution in an oil-wet sandstone. ► Measurement of morphology of residual clusters. ► Demonstration of clear difference between ...residual oil clusters in water-wet and oil-wet rock.
We imaged an oil-wet sandstone at residual oil saturation (Sor) conditions using X-ray micro-tomography with a nominal voxel size of (9μm)3 and monochromatic light from a synchrotron source. The sandstone was rendered oil-wet by ageing with a North Sea crude oil to represent a typical wettability encountered in hydrocarbon reservoirs. We measured a significantly lower Sor for the oil-wet core (18.8%) than for an analogue water-wet core (35%). We analysed the residual oil cluster size distribution and find consistency with percolation theory that predicts a power-law cluster size distribution. We measure a power-law exponent τ=2.12 for the oil-wet core which is higher than τ for the water-wet system (τ=2.05), indicating fewer large clusters in the oil-wet case. The clusters are rough and sheet-like consistent with connectivity established through layers in the pore space and occupancy of the smaller pores; in contrast the clusters for water-wet media occupy the centres of the larger pores. These results imply less trapping of oil, but with a greater surface area for dissolution. In carbon storage applications, this suggests that in CO2-wet systems, capillary trapping is less significant, but that there is a large surface area for dissolution and reaction.
Improvements to electrode performance are essential to accelerate the commercialisation of SOFC technology. A key metric of performance for SOFC electrodes is the length and distribution of three or ...triple phase boundaries (TPBs) which provide an indication of electrochemical performance. Techniques that can be used to characterise TPB length are highly valuable; with an increasing knowledge of electrode microstructures, electrochemical performance can be optimised. One such technique for electrode characterisation uses focused ion beams (FIB) to sequentially mill and image an electrode surface, obtaining a sequence of 2D images that may be reconstructed in a 3D space. In this paper we present a technique to maximise the quality of the raw data obtained via ex-situ characterisation of electrode micro-sections based on FIB lift-out. With improved raw data, we have been able to conduct semi-automated image analysis to extract key microstructural information, including the length and distribution of TPBs.
Reconstructions have been carried out using both single and dual beam instruments; two reconstructions of Ni–YSZ anode structures are presented here.
We present a multiaperture analyzer setup for performing x-ray phase contrast imaging in planar and three-dimensional modalities. The method is based on strongly structuring the x-ray beam with an ...amplitude modulator, before it reaches the sample, and on a multiaperture analyzing element before detection. A multislice representation of the sample is used to establish a quantitative relation between projection images and the corresponding three-dimensional distributions, leading to successful tomographic reconstruction. Sample absorption, phase, and scattering are retrieved from the measurement of five intensity projections. The method is tested on custom-built phantoms with synchrotron radiation: sample absorption and phase can be reliably retrieved also in combination with strong scatterers, simultaneously attaining high sensitivity and dynamic range.
The 3-dimensional microstructure of a porous electrode from a lithium-ion battery has been characterized for the first time. We use X-ray tomography to reconstruct a 43
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μm sample volume ...with voxel dimensions of 480
nm, subsequent division of the reconstructed volumes into sub-volumes of different sizes allow us to determine microstructural parameters as a function of sub-division size. We show that the minimum size for a representative volume element is about 43
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μm for volume-specific surface area, but as large as the full sample volume for porosity and tortuosity.
A multiple length scale approach to the imaging and measurement of depth filters using X-ray computed tomography is described. Three different filter grades of varying nominal retention ratings were ...visualized in 3D and compared quantitatively based on porosity, pore size and tortuosity. Positional based analysis within the filters revealed greater voidage and average pore sizes in the upstream quartile before reducing progressively through the filter from the center to the downstream quartile, with these results visually supported by voidage distance maps in each case. Flow simulation to display tortuous paths that flow may take through internal voidage were examined.
Digital reconstructions were capable of identifying individual constituents of voidage, cellulose and perlite inside each depth filter grade, with elemental analysis on upstream and downstream surfaces confirming perlite presence. Achieving an appropriate pixel size was of particular importance when optimizing imaging conditions for all grades examined. A 3 µm pixel size was capable of representing internal macropores of each filter structure; however, for the finest grade, an improvement to a 1 µm pixel size was required in order to resolve micropores and small perlite shards. Enhancing the pixel size resulted in average porosity measurements of 70% to 80% for all grades.
Graphical abstract
Summary
A robust and versatile sample preparation technique for the fabrication of cylindrical pillars for imaging by X‐ray nano‐computed tomography (nano‐CT) is presented. The procedure employs ...simple, cost‐effective laser micro‐machining coupled with focused‐ion beam (FIB) milling, when required, to yield mechanically robust samples at the micrometre length‐scale to match the field‐of‐view (FOV) for nano‐CT imaging. A variety of energy and geological materials are exhibited as case studies, demonstrating the procedure can be applied to a variety of materials to provide geometrically optimised samples whose size and shape are tailored to the attenuation coefficients of the constituent phases. The procedure can be implemented for the bespoke preparation of pillars for both lab‐ and synchrotron‐based X‐ray nano‐CT investigations of a wide range of samples.
Lay description
A novel way of making samples so that they can be successfully imaged with X‐rays has been developed. This process involves using highly focused lasers to mill away excess material, to leave cylindrical samples ready to be placed in the X‐ray beam. The X‐ray procedure investigated is known as X‐ray computed tomography and is the materials science equivalent of medical CT scanners found in most hospitals. The technique involves rotating the small pillar in the path between a laboratory X‐ray source and a detector, producing a number of images, each similar to a classical bone scan. Using a sophisticated mathematical procedure, these images are reconstructed into a three‐dimensional volume, giving information about the complex microstructure at the nanoscale. This has been applied to materials used for energy generation and a geological sample to illustrate the versatility and robustness of the preparation route.
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