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.
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.
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.
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.
This work describes the design of an electrode with enhanced performance applied to all-vanadium redox flow batteries (VRFBs). This new electrode consists of a structural porous carbon felt decorated ...with TiO2 rutile nanoparticles, which has been nitrided using ammonolysis at 900 °C. An outstanding charge and mass transfer over the electrode-electrolyte interface was observed as a consequence of the synergetic effect of N- and O-functionalization over carbon felt (CF) and the partial formation of TiN (metallic conductor) phase. Moreover, this material has not only improved in terms of catalysis towards the V3+/V2+ redox reaction (k0 = 1.6 × 10−3 cm s−1), but also inhibited the hydrogen evolution reaction (HER), which is one of the main causes of imbalances that lead to battery failure. This led to an impressive high-power peak output value up to 700 mW cm−2, as well as work at high current density in galvanostatic conditions (i.e. 150 mA cm−2), exhibiting low ohmic losses (overpotential) and great redox single cell reversibility, with a superior energy efficiency of 71%. An inexpensive, earth abundant and scalable synthesis method to boost VRFBs technology based on nitrided CF@TiO2 is presented, being able to overcome certain constrains, and therefore to achieve high energy and power densities.
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•Layer wise PCB based fractal lung-inspired cathode flow field.•Cost effective and light weight method of fractal flow field development.•Enhanced performance over serpentine flow field at a range of ...operating conditions.•Uniform and sustained operation of fractal flow field with better water management.
Fractal cathode flow-fields, inspired by the flow mechanism of air inside lungs, can provide homogeneous, scalable and uniform distribution of reactants to polymer electrolyte fuel cell (PEFC) electrodes. However, the complex 3D flow-fields demonstrated previously face manufacturing challenges, such as requiring selective laser sintering, an additive manufacturing method that is expensive to scale up. Here, a lung-inspired cathode flow-field is introduced and fabricated using low-cost, lightweight printed circuit boards (PCB). The uniformity and alignment between individual PCB layers producing the fractal hierarchy of flow channels have been characterised using X-ray computed tomography (X-ray CT). The performance of the fractal flow-field exceeds that of conventional single-serpentine flow-fields and is particularly beneficial when operating on air with a low relative humidity. The lung-inspired design is shown to lead to a more stable operation than the single-serpentine design, as a result of uniform distribution of reactants.
•X-ray computed tomography used to image packed beds in 3D.•Cellulosic and ceramic 1mL columns used for investigation.•Porosity, tortuosity and other factors quantified from geometry based upon ...position.•Differences observed depending upon closeness to column edge.
Physical characteristics critical to chromatography including geometric porosity and tortuosity within the packed column were analysed based upon three dimensional reconstructions of bed structure in-situ. Image acquisition was performed using two X-ray computed tomography systems, with optimisation of column imaging performed for each sample in order to produce three dimensional representations of packed beds at 3μm resolution.
Two bead materials, cellulose and ceramic, were studied using the same optimisation strategy but resulted in differing parameters required for X-ray computed tomography image generation. After image reconstruction and processing into a digital three dimensional format, physical characteristics of each packed bed were analysed, including geometric porosity, tortuosity, surface area to volume ratio as well as inter-bead void diameters. Average porosities of 34.0% and 36.1% were found for ceramic and cellulose samples and average tortuosity readings at 1.40 and 1.79 respectively, with greater porosity and reduced tortuosity overall values at the centre compared to the column edges found in each case. X-ray computed tomography is demonstrated to be a viable method for three dimensional imaging of packed bed chromatography systems, enabling geometry based analysis of column axial and radial heterogeneity that is not feasible using traditional techniques for packing quality which provide an ensemble measure.
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.
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