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Accurate modelling of particle shrinkage during biomass pyrolysis is key to the production of biochars with specific morphologies. Such biochars represent sustainable solutions to a variety of adsorption-dependent environmental remediation challenges. Modelling of particle shrinkage during biomass pyrolysis has heretofore been based solely on theory and ex-situ experimental data. Here we present the first in-situ phase-contrast X-ray imaging study of biomass pyrolysis. A novel reactor was developed to enable operando synchrotron radiography of fixed beds of pyrolysing biomass. Almond shell particles experienced more bulk shrinkage and less change in porosity than did walnut shell particles during pyrolysis, despite their similar composition. Alkaline pretreatment was found to reduce this difference in feedstock behaviour. Ex-situ synchrotron X-ray microtomography was performed to study the effects of pyrolysis on pore morphology. Pyrolysis led to a redistribution of pores away from particle surfaces, meaning newly formed surface area may be less accessible to adsorbates. [ABSTRACT FROM AUTHOR]

Highlights • An inverse relationship between the mass transfer and permeability was found. • Mass transport processes are strongly influenced by microstructural anisotropy. • Mass transfer correlations were presented for media with varying anisotropy. Abstract A numerical method for calculating the mass transfer coefficient in fibrous media is presented. First, pressure driven flow was modelled using the Lattice Boltzmann Method. The advection-diffusion equation was solved for convective-reacting porous media flow, and the method is contrasted with experimental methods such as the limiting current diffusion technique, for its ability to determine and simulate mass transfer systems that are operating at low Reynolds number flows. A series of simulations were performed on three materials; specifically, commercially available carbon felts, electrospun carbon fibers and electrospun carbon fibers with anisotropy introduced to the microstructure. Simulations were performed in each principal direction (x,y,z) for each material in order to determine the effects of anisotropy on the mass transfer coefficient. In addition, the simulations spanned multiple Reynolds and Péclet numbers, to fully represent highly advective and highly diffusive systems. The resulting mass transfer coefficients were compared with values predicted by common correlations and a good agreement was found at high Reynolds numbers, but less so at lower Reynolds number typical of cell operation, reinforcing the utility of the numerical approach. Dimensionless mass transfer correlations were determined for each material and each direction in terms of the Sherwood number. These correlations were analyzed with respect to each materials' permeability tensor. It was found that as the permeability of the system increases, the expected mass transfer coefficient decreases. Two general mass transfer correlations are presented, one correlation for isotropic fibrous media and the other for through-plane flow in planar fibrous materials such as electrospun media and carbon paper. The correlations are Sh = 0.879 Re0.402 Sc0.390 and Sh = 0.906 Re0.432 Sc0.432 respectively. [ABSTRACT FROM AUTHOR]

The performance of the polymer electrolyte membrane (PEM) fuel cell is governed by a complex interaction of the structure of the membrane electrode assembly (MEA), cell compression, and operating parameters. Adequate cell compression for improved current collection and gas sealing can structurally deform MEA with adverse consequences. Non-uniform MEA compression exerted by the flow-field design and arrangement induces heterogeneous transport properties. Hence, understanding morphological evolution and effective transport properties as an effect of MEA compression is an important factor for improving fuel cell performance and durability. In this paper, an X-ray computed tomography study of the entire MEA compression is presented, comprising of gas diffusion and microporous layers, catalyst layers, and the electrolyte membrane, subjected to non-uniform compression under two distinct flow-field arrangements. This study presents a comprehensive dataset of the heterogeneous effective properties required for robust computational modelling; including porosity, permeability, tortuosity, and diffusivity, along with the extent of blocking of the flow channel due to cell compression and effect of compression on the structural properties of the membrane. Image 1 • X-ray CT characterisation of the membrane electrode assembly used in PEMFC. • Compression of commonly used flow-field arrangements on the entire MEA. • Effect of channel/land arrangement on the morphological properties of the MEA. • Effective parameter derived for inputs to the PEMFC computational models. [ABSTRACT FROM AUTHOR]

A novel electrochemical cell has been designed and built to allow for in situ energy-dispersive X-ray diffraction measurements to be made during reduction of UO2 to U metal in LiCl-KCl at 500°C. The electrochemical cell contains a recessed well at the bottom of the cell into which the working electrode sits, reducing the beam path for the X-rays through the molten-salt and maximizing the signal-to-noise ratio from the sample. Lithium metal was electrodeposited onto the UO2 working electrode by exposing the working electrode to more negative potentials than the Li deposition potential of the LiCl-KCl eutectic electrolyte. The Li metal acts as a reducing agent for the chemical reduction of UO2 to U, which appears to proceed to completion. All phases were fitted using Le Bail refinement. The cell is expected to be widely applicable to many studies involving molten-salt systems. [ABSTRACT FROM AUTHOR]

Thermal gradients can arise within solid oxide fuel cells (SOFCs) due to start-up and shut-down, non-uniform gas distribution, fast cycling and operation under internal reforming conditions. Here, the effects of operationally relevant thermal gradients on Ni/YSZ SOFC anode half cells are investigated using combined synchrotron X-ray diffraction and thermal imaging. The combination of these techniques has identified significant deviation from linear thermal expansion behaviour in a sample exposed to a one dimensional thermal gradient. Stress gradients are identified along isothermal regions due to the presence of a proximate thermal gradient, with tensile stress deviations of up to 75 MPa being observed across the sample at a constant temperature. Significant strain is also observed due to the presence of thermal gradients when compared to work carried out at isothermal conditions. [ABSTRACT FROM AUTHOR]

The next generation of proton exchange membrane fuel cells (PEMFCs) require a substantial reduction or elimination of Pt-based electrocatalyst from the cathode, where O 2 reduction takes place. The most promising alternative to Pt is atomic Fe embedded in N-doped C (Fe–N–C). Successful incorporation of Fe–N–C in PEMFCs relies on a thorough understanding of the catalyst layer properties, both ex situ and in situ , with tailored electrode interface engineering. To help resolve this conundrum, we provide a quantitative protocol on the optimisation of I/C for Fe–N–Cs. It is demonstrated that a high pore volume (3.33 cm3 g−1 FeNC) Fe–N–C catalyst requires a sufficiently high ionomer to catalyst mass ratio (I/C, 2.8≤I/C ≤ 4.2) for optimum PEMFC activity under H 2 /O 2. Emerging electrochemical techniques (distribution of relaxation times and Fourier transformed alternating current voltammetry) were used to deconvolute for the first time the trade-off between proton and electron resistance and accessible FeN x active site density with increasing ionomer loading. These findings highlight the significant impact of tuning the I/C ratio based on the catalyst layer properties and feature the power of evolving electrochemical tools for optimising performance in PEMFCs and other electrochemical devices. [Display omitted] • In situ FTacV quantified Fe–N–C electrochemical active site density in fuel cell. • Fe–N–C active site density and current density correlated with ionomer loading. • DRT quantified resistance to proton transport and oxygen diffusion in fuel cell. • Fe–N–C to ionomer ratio optimised based on quantitative protocol. [ABSTRACT FROM AUTHOR]

A new technique combining in situ X-ray diffraction using synchrotron radiation and infrared thermal imaging is reported. The technique enables the application, generation and measurement of significant thermal gradients, and furthermore allows the direct spatial correlation of thermal and crystallographic measurements. The design and implementation of a novel furnace enabling the simultaneous thermal and X-ray measurements is described. The technique is expected to have wide applicability in material science and engineering; here it has been applied to the study of solid oxide fuel cells at high temperature. [ABSTRACT FROM AUTHOR]

The mechanical compression used in the construction of PEFCs improves effective current collection and gas sealing, however it results in structural deformation of the MEA, affecting reactant transport with adverse consequences for the electrochemical performance of the cell. The present study uses X-ray CT to characterise MEA under compression and determine effective properties of the porous domain. The comprehensive modelling approach couples a structural model of the MEA under compression to a multi-phase, non-isothermal electrochemical performance model. Liquid water saturation in the cathode domain that promotes mass transport losses is validated with neutron radiography. Here, the structural model considers the fuel cell stacking process at three compressions and highlights the non-uniform distribution of porosity and effective properties under non-uniform cell compression, affecting localised current distribution and water transport. An increase in compression showed a negligible effect on the performance in the activation region, the performance was marginally improved in the ohmic region and significantly affected in mass transport region, promoting cell flooding. The non-uniform compression effects are found to be important considerations for robust modelling studies as it increases the nonuniformity in localised current, temperature and flooding that would further alter the durability of the fuel cell. [Display omitted] • Coupled structural and electrochemical modelling study of PEFC compression. • X-ray CT study of the MEA to generate modelling parameters and validate structural model. • Neutron radiography to validate the electrochemical model models at variable compressions. • Effect of compression and channel/land arrangement on the cell performance. • Effect of compression on water management and thermal performance. [ABSTRACT FROM AUTHOR]

The thermal response of Li-ion cells can greatly vary for identical cell designs tested under identical conditions, the distribution of which is costly to fully characterize experimentally. The open-source Battery Failure Databank presented here contains robust, high-quality data from hundreds of abuse tests spanning numerous commercial cell designs and testing conditions. Data was gathered using a fractional thermal runaway calorimeter and contains the fractional breakdown of heat and mass that was ejected, as well as high-speed synchrotron radiography of the internal dynamic response of cells during thermal runaway. The distribution of thermal output, mass ejection, and internal response of commercial cells are compared for different abuse-test conditions, which when normalized on a per amp-hour basis show a strong positive correlation between heat output from cells, the fraction of mass ejected from the cells, their energy- and power-density. Ejected mass was shown to contain 10 × more heat per gram than non-ejected mass. The causes of 'outlier' thermal and ejection responses i.e., extreme cases, are elucidated by high-speed radiography which showed how occurrences such as vent clogging can create more hazardous conditions. High-speed radiography also demonstrated how the time-resolved interplay of thermal runaway propagation and mass ejection influences the total heat generated. [ABSTRACT FROM AUTHOR]

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 (<10 $ kg−1) and the volumetric energy‐density increased (>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. [ABSTRACT FROM AUTHOR]

Vast quantities of powder leave production lines each day, often with strict control measures. For quality checks to provide the most value, they must be capable of screening individual particles in 3D and at high throughput. Conceptually, X‐ray computed tomography (CT) is capable of this; however, achieving lab‐based reconstructions of individual particles has, until now, relied upon scan‐times on the order of tens of hours, or even days, and although synchrotron facilities are potentially capable of faster scanning times, availability is limited, making in‐line product analysis impractical. This work describes a preparation method and high‐throughput scanning procedure for the 3D characterization of powder samples in minutes using nano‐CT by full‐filed transmission X‐ray microscopy with zone‐plate focusing optics. This is demonstrated on various particle morphologies from two next‐generation lithium‐ion battery cathodes: LiNi0.8Mn0.1Co0.1O2 and LiNi0.6Mn0.2Co0.2O2; namely, NMC811 and NMC622. Internal voids are detected which limit energy density and promote degradation, potentially impacting commercial application such as the drivable range of an electric vehicle. [ABSTRACT FROM AUTHOR]

Intermittent switching between wet and dry reactant gases during operation in a polymer electrolyte fuel cell (PEFC) can improve performance stability, alleviating the effects of flooding by controlling the water content within the system. However, lifetime durability may be affected due to membrane electrode assembly (MEA) boundary delamination and membrane damage. Two relative humidity (RH) control strategies were investigated, using electrochemical performance and MEA degradation as critical indicators. It was found that intermittent switching between wet and dry gases does not accelerate fuel cell degradation if the duration of the dry gas period is set reasonably (dry gases stops before the voltage reaches the apex of the hump). Additionally, current and temperature distribution mapping was utilised to capture the dynamic response between these transitional stages. The switching of dry gases first makes the current density distribution homogeneous, and the maximum current density is reduced subsequently. Then, the current density near the inlet keeps decreasing. Intermittent switching between wet and dry reactant gases is easy to implement and overcomes limitations in mass transfer at medium and high current densities. • Long-term performance for intermittent RH control is compared to constant RH control. • TEM, XPS and SEM characterise Post-mortem MEAs to quantify the degradation. • Local dynamic response during gas switching is captured by current density mapping. [ABSTRACT FROM AUTHOR]

Abstract Intentionally inducing worst-case thermal runaway scenarios in Li-ion cells on-demand is a definitive way to test the efficacy of battery systems in safely mitigating the consequences of catastrophic failure. An internal short-circuiting (ISC) device is implanted into three 18650 cell designs: one standard, one with a bottom vent, and one with a thicker casing. Through an extensive study of 228 cells, the position at which thermal runaway initiates is shown to greatly affect the tendency of cells to rupture and incur side-wall breaches at specific locations. The risks associated with each failure mechanism and position of the ISC device are quantified using a custom calorimeter that can decouple the heat from ejected and non-ejected contents. Causes of high-risk failure mechanisms, such as bursting and side-wall breaches, are elucidated using high-speed synchrotron X-ray imaging at 2000 frames per second and image-based 3D thermal runaway computational models, which together are used to construct a comprehensive description of external risks based on internal structural and thermal phenomena. [ABSTRACT FROM AUTHOR]

The current obstacles for all-vanadium redox flow batteries (VRFBs) include the sluggish reaction kinetics of electrode materials and the overlapping potential range of the hydrogen evolution reaction (HER) with the negative redox couple. Bismuth (Bi) additives have exhibited tremendous enhancement of battery performance; however, the performance plateaus with concentration of Bi and the catalytic mechanism remains inconclusive and controversial. Quantified kinetic values from three electrochemical methodologies, including modelling, show that the performance plateau with Bi concentration was related to the kinetics of the vanadium redox reaction (VRR) instead of dissolution of Bi, and VRR reaction rate was improved three orders of magnitude with the addition of Bi, with the highest VRR reaction rate observed for electrolyte with 750 ppm Bi additive. Additionally, a competing relationship between VRR and the HER was explored via electrochemical methodologies. It was further confirmed that Bi effectively selectively catalysed VRR over HER via in-situ mass spectrometry measurements during battery cycling, which allows for a higher battery charging cut-off potential of 1.7 V without electrolyte imbalance caused by water electrolysis, achieving 86% of the theoretical electrolyte capacity at 110 mA cm−2 with 750 ppm Bi. The full battery performance with 750 ppm Bi additive is among the best in the literature to date. [Display omitted] • Kinetics improved by three orders of magnitude with Bi additives in the VRFB. • Bi selectively catalysed VRR over HER efficiently reduced the side reaction. • Optimal peak kinetic enhancement with moderate Bi concentration. • Reduced HER allows improved VRFB performance at high current density. [ABSTRACT FROM AUTHOR]

Abstract: As the energy density of lithium‐ion cells and batteries increases, controlling the outcomes of thermal runaway becomes more challenging. If the high rate of gas generation during thermal runaway is not adequately vented, commercial cell designs can rupture and explode, presenting serious safety concerns. Here, ultra‐high‐speed synchrotron X‐ray imaging is used at >20 000 frames per second to characterize the venting processes of six different 18650 cell designs undergoing thermal runaway. For the first time, the mechanisms that lead to the most catastrophic type of cell failure, rupture, and explosion are identified and elucidated in detail. The practical application of the technique is highlighted by evaluating a novel 18650 cell design with a second vent at the base, which is shown to avoid the critical stages that lead to rupture. The insights yielded in this study shed new light on battery failure and are expected to guide the development of safer commercial cell designs. [ABSTRACT FROM AUTHOR]

Conventional polymer electrolyte fuel cells (PEFCs) require a means of placing the series of laminar components that make up cells under mechanical compression so as to ensure effective electrical conduction, mass transport and gas-tight operation. This review describes the effect of mechanical compression and dimensional change on the components of PEFCs and reviews the range of methods used to achieve desired stack compression. The case is made for improved understanding of the mechanisms of fuel cell component compression and greater attention to the development of technological approaches for stack compression. [ABSTRACT FROM AUTHOR]