In situ x-ray computed tomography of zinc–air primary cells during discharge: correlating discharge rate to anode morphology

Zinc–air batteries have gained significant attention as safe battery alternatives, with high theoretical energy densities and a high abundance of their constituent materials. However, barriers to their widespread adoption include the need to improve their cycling lifetime, as well as stability and avoiding degradation mechanisms such as zinc dendrite growth and hydrogen-producing side reactions. X-ray computed tomography (CT) is a widely used technique for the study of batteries. In situ / operando x-ray CT has been increasingly used to study the zinc anode of zinc–air batteries to evaluate the interesting morphological changes occurring during the reaction from zinc (Zn) to zinc oxide (ZnO) during discharge (vice versa during charge). However, several studies have been carried out using synchrotron x-ray sources, which have limited availability for users. In this work, we present a comprehensive study of the discharge of commercial, primary zinc–air batteries using a laboratory-based x-ray source for in situ x-ray CT measurements. Four different discharge rates are investigated (C/30, C/60, C/90 and C/150), with tomograms collected at various stages throughout each discharge. Results confirm that with decreasing C-rate (i.e. decreasing discharge current) a greater volume of zinc is reacted, with average mass utilisations of 17%, 76%, 81% and 87% for C/30, C/60, C/90 and C/150, respectively. Furthermore, quantification using x-ray CT datasets showed that there is a direct correlation between the volume of zinc remaining in the cell and the state-of-charge of the cell, which deviated from linearity for the longer C-rates. Finally, a potential new mechanism for shape change is discussed, where a Zn particle is replaced with a pore of a similar volume. As well as improvements in statistical relevance gained from multiple repeats for each C-rate, the results presented here could be used in both modelling of battery performance, as well as consideration for future anode design concepts

Internal insulation and corrosion control of molten chloride thermal energy storage tanks

A chloride-based molten-salt system that uses a ternary blend of MgCl2/KCl/NaCl is investigated to provide higher-temperature thermal energy storage capability than conventional nitrate salt-based systems. Despite their high thermal stability and operating temperature, molten chlorides present several challenges, including the design of internal liners to prevent the corrosion and thermal stress of alloy tank shells. This work discusses issues and potential solutions related to containment of molten chloride salt, specifically the optimization of the hot face refractory materials for use as internal liners. Three down-selected refractory materials were analyzed with respect to permeation of salt through the material as well as chemical stability during high temperature operation. Through the application of X-ray imaging and electron spectroscopy techniques, highly stable secondary surface phases in equilibrium with the molten salt were identified, as well as time-dependent changes in the salt composition itself

Visualising coke-induced degradation of catalysts used for CO2-reforming of methane with X-ray nano-computed tomography

The switch from a carbon-based to a hydrogen-based economy requires environmentally friendly methods for hydrogen production. CO2-reforming of methane promises to be a greener alternative to steam-methane reforming, which accounts for the majority of hydrogen production today. For this dry process to become industrially competitive, challenges such as catalyst deactivation and degradation through coke formation must be better understood and ultimately overcome. While bulk characterisation methods provide a wealth of useful information about the carbon formed during coking, spatially resolved techniques are required to understand the type and extent of degradation of supported catalyst particles themselves under coking conditions. Here, lab-based X-ray nano-computed tomography, in conjunction with a range of complementary techniques, is utilised to understand the effects of the nickel-to-cobalt ratio on the degradation of individual supported catalyst particles. Findings suggest that a bimetallic system greatly outperforms monometallic catalysts, with the ratio between nickel and cobalt having a significant impact on the type and quantity of the carbon formed and on the extent of supported catalyst breakdown

3D Imaging of Lithium Protrusions in Solid‐State Lithium Batteries using X‐Ray Computed Tomography

Solid‐state lithium batteries will revolutionize the lithium‐ion battery and energy storage applications if certain key challenges can be resolved. The formation of lithium‐protrusions (dendrites) that can cause catastrophic short‐circuiting is one of the main obstacles, and progresses by a mechanism that is not yet fully understood. By utilizing X‐ray computed tomography with nanoscale resolution, the 3D morphology of lithium protrusions inside short‐circuited solid electrolytes has been obtained for the first time. Distinguishable from adjacent voids, lithium protrusions partially filled cracks that tended to propagate intergranularly through the solid electrolyte, forming a large waved plane in the shape of the grain boundaries. Occasionally, the lithium protrusions bifurcate into flat planes in a transgranular mode. Within the cracks themselves, lithium protrusions are preferentially located in regions of relatively low curvature. The crack volume filled with lithium in two samples is 82.0% and 83.1%, even though they have distinctly different relative densities. Pre‐existing pores in the solid electrolyte, as a consequence of fabrication, can also be part‐filled with lithium, but do not have a significant influence on the crack path. The crack/lithium‐protrusion behavior qualitatively supports a model of propagation combining electrochemical and mechanical effects

Lab-based X-ray micro-computed tomography coupled with machine-learning segmentation to investigate phosphoric acid leaching in high-temperature polymer electrolyte fuel cells

A machine-learning approach is used to segment 14 X-ray computed-tomography datasets acquired by lab-based scanning of laser-milled, high-temperature polymer electrolyte fuel cell samples mounted in a 3D-printed sample holder. Two modes of operation, one with constant current load and the other with current cycling, are explored and their impact on microstructural change is correlated with electrochemical performance degradation. Constant-current testing shows the overall quantity of phosphoric acid in the gas diffusion layers is effectively unchanged between 50 and 100 h of operation but that inter-electrode distribution becomes less uniform. Current-cycling tests reveal similar quantities of phosphoric acid but a different intra-electrode distribution. Membrane swelling appears more pronounced after current-cycling tests and in both cases, significant catalyst layer migration is observed. The present analysis provides a lab-based approach to monitoring microstructural degradation in high-temperature polymer electrolyte fuel cells and provides a more accessible and more statistically robust platform for assessing the impact of phosphoric acid mitigation strategies

Study of the tortuosity factors at multi-scale for a novel-structured SOFC anode

© Published under licence by IOP Publishing Ltd. Gas transport properties are closely related to the tortuosity of the pore network within porous materials. For the first time, this study explores a multi-scale imaging and modelling method to measure the tortuosity of an Solid Oxide Fuel Cell (SOFC) electrode material with pore sizes spanning over hundreds of orders of magnitude. This analysis is normally challenging using image-based techniques, as pores of different sizes may not be easily resolved at the same time using X-ray computed tomography (CT). In this study, a tubular SOFC anode, fabricated by a phase inversion technique, is used to illustrate this approach. A heat flux analogy is used to simulate mass transport and the results show that the embedded large-scale finger-like pores can significantly improve mass transport by providing less tortuous pathways

Three dimensional characterisation of chromatography bead internal structure using X-ray computed tomography and focused ion beam microscopy

X-ray computed tomography (CT) and focused ion beam (FIB) microscopy were used to generate three dimensional representations of chromatography beads for quantitative analysis of important physical characteristics including tortuosity factor. Critical-point dried agarose, cellulose and ceramic beads were examined using both methods before digital reconstruction and geometry based analysis for comparison between techniques and materials examined. X-ray ‘nano’ CT attained a pixel size of 63 nm and 32 nm for respective large field of view and high resolution modes. FIB improved upon this to a 15 nm pixel size for the more rigid ceramic beads but required compromises for the softer agarose and cellulose materials, especially during physical sectioning that was not required for X-ray CT. Digital processing of raw slices was performed using software to produce 3D representations of bead geometry. Porosity, tortuosity factor, surface area to volume ratio and pore diameter were evaluated for each technique and material, with overall averaged simulated tortuosity factors of 1.36, 1.37 and 1.51 for agarose, cellulose and ceramic volumes respectively. Results were compared to existing literature values acquired using established imaging and non-imaging techniques to demonstrate the capability of tomographic approaches used here

High-Density Lignin-Derived Carbon Nanofiber Supercapacitors with Enhanced Volumetric Energy Density

Supercapacitors are increasingly used in short-distance electric transportation due to their long lifetime (≈15 years) and fast charging capability (>10 A g^{−1}). To improve their market penetration, while minimizing onboard weight and maximizing space-efficiency, materials costs must be reduced (8 Wh L^{−1}). Carbon nanofibers display good gravimetric capacitance, yet their marketability is hindered by their low density (0.05–0.1 g cm^{−3}). Here, the authors increase the packing density of low-cost, free-standing carbon nanofiber mats (from 0.1 to 0.6 g cm−3) through uniaxial compression. X-ray computed tomography reveals that densification occurs by reducing the inter-fiber pore size (from 1–5 µm to 0.2–0.5 µm), which are not involved in double-layer capacitance. The improved packing density is directly proportional to the volumetric performances of the device, which reaches a volumetric capacitance of 130 F cm^{−3} and energy density of 6 Wh L^{−1} at 0.1 A g^{−1} using a loading of 3 mg cm^{−2}. The results outperform most commercial and lab-scale porous carbons synthesized from bioresources (50–100 F cm^{−3}, 1–3 Wh L^{−1} using 10 mg cm^{−2}) and contribute to the scalable design of sustainable electrodes with minimal ‘dead volume’ for efficient supercapacitors

Stress response during early sedation with dexmedetomidine compared with usual-care in ventilated critically ill patients

Background: Sedative agents may variably impact the stress response. Dexmedetomidine is a sympatholytic alpha2-adrenergic agonist mainly used as a second-line sedative agent in mechanically ventilated patients. We hypothesised that early sedation with dexmedetomidine as the primary agent would result in a reduced stress response compared to usual sedatives in critically ill ventilated adults. Methods: This was a prospective sub-study nested within a multi-centre randomised controlled trial of early sedation with dexmedetomidine versus usual care. The primary outcome was the mean group differences in plasma levels of stress response biomarkers measured over 5 days following randomisation. Other hormonal, biological and physiological parameters were collected. Subgroup analyses were planned for patients with proven or suspected sepsis. Results: One hundred and three patients were included in the final analysis. Baseline illness severity (APACHE II score), the proportion of patients receiving propofol and the median dose of propofol received were comparable between groups. More of the usual-care patients received midazolam (57.7% vs 33.3%; p = 0.01) and at higher dose (median (95% interquartile range) 0.46 [0.20–0.93] vs 0.14 [0.08–0.38] mg/kg/day; p < 0.01). The geometric mean (95% CI) plasma level of the stress hormones, adrenaline (0.32 [0.26–0.4] vs 0.38 [0.31–0.48]), noradrenaline (4.27 [3.12–5.85] vs 6.2 [4.6–8.5]), adrenocorticotropic hormone (17.1 [15.1–19.5] vs 18.1 [15.9–20.5]) and cortisol (515 [409–648] vs 618 [491–776)] did not differ between dexmedetomidine and usual-care groups, respectively. There were no significant differences in any other assayed biomarkers or physiological parameters Sensitivity analyses showed no effect of age or sepsis. Conclusions: Early sedation with dexmedetomidine as the primary sedative agent in mechanically ventilated critically ill adults resulted in comparable changes in physiological and blood-borne parameters associated with the stress-response as with usual-care sedation