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Symmetrical Exsolution of Rh Nanoparticles in Solid Oxide Cells for Efficient Syngas Production from Greenhouse Gases
Get PDFCarbon dioxide and steam solid oxide co-electrolysis is a key technology for exploiting renewable electricity to generate syngas feedstock for the Fischer–Tropsch synthesis. The integration of this process with methane partial oxidation in a single cell can eliminate or even reverse the electrical power demands of co-electrolysis, while simultaneously producing syngas at industrially attractive H2/CO ratios. Nevertheless, this system is rather complex and requires catalytically active and coke tolerant electrodes. Here, we report on a low-substitution rhodium-titanate perovskite (La0.43Ca0.37Rh0.06Ti0.94O3) electrode for the process, capable of exsolving high Rh nanoparticle populations, and assembled in a symmetrical solid oxide cell configuration. By introducing dry methane to the anode compartment, the electricity demands are impressively decreased, even allowing syngas and electricity cogeneration. To provide further insight on the Rh nanoparticles role on methane-to-syngas conversion, we adjusted their size and population by altering the reduction temperature of the perovskite. Our results exemplify how the exsolution concept can be employed to efficiently exploit noble metals for activating low-reactivity greenhouse gases in challenging energy-related applications.</p
Thermographic investigation of the effect of plasma exposure on the surface of a MAST upgrade divertor tile in Magnum-PSI
No full textOne of the issues faced by future fusion devices will be high divertor target heat loads. Alternative divertors can promote detachment, flux expansion and dissipation mechanisms to mitigate these heat loads. They have been investigated in several devices including TCV and DIII-D, and will be investigated on MAST-U. To evaluate their effectiveness, accurate target heat flux and power balance measurements are required in these machines. Infrared (IR) thermography is a widely used technique to determine the target heat flux, but is susceptible to surface effects and emissivity in carbon-walled machines. In this work, the effect of plasma exposure on graphite is assessed to understand what may happen in MAST-U. A sample of fine grain graphite, as used on MAST-U, is exposed to 30min plasma exposures, with density ne=6 × 1018m−3 and temperature Te=0.08eV as measured by Thomson scattering. During these pulses, the temperature is measured by a medium wave IR camera and is seen to decrease by ≈70 °C over the course of 3h of plasma exposure. Pyrometer measurements suggest that the IR camera data is affected by a change in the surface emissivity. Profilometry confirms erosion of graphite at the tile centre to a depth of ≈100µm, and a larger region of deposition further out, amounting to ≈40µm of materia
Surface charging activated mechanism change: A computational study of O, CO, and CO2 interactions on Ag electrodes
No full textElectrocatalytic and plasma-activated processes receive increasing attention in catalysis. Density functional theory (DFT) calculations are state-of-the-art tools for the fundamental study of reaction mechanisms and predicting the performance of catalytic materials. Proper application of DFT-based methods is crucial when investigating charge-doped electrode surfaces during electrocatalytic and plasma-activated reactions. Here, as a model electrode for plasma-activated CO2 splitting, we studied the interactions of O, CO, and CO2 with the neutral and progressively charged Ag(111) metal surfaces. We show that the application of correction procedures is necessary to obtain accurate adsorption energy profiles of O atoms, CO and CO2 molecules on Ag surfaces that are under the influence of additional electrons. Interestingly, the oxidation of CO is found to shift from a Langmuir–Hinshelwood mechanism on a neutral electrode to an Eley–Rideal mechanism on charged electrodes. Furthermore, we show that the surface charging of Ag(111) electrodes increase their CO2 reduction performance by enhancing the adsorption of O atoms and desorption of CO molecules. A further increase in the absolute charge-state of the electrode surface is expected to waive the thermodynamic barriers for the CO2 splitting reaction
Fast modelling of turbulent transport in fusion plasmas using neural networks
Get PDFWe present an ultrafast neural network model, QLKNN, which predicts core tokamak transport heat and particle fluxes. QLKNN is a surrogate model based on a database of 3 × 108 flux calculations of the quasilinear gyrokinetic transport model, QuaLiKiz. The database covers a wide range of realistic tokamak core parameters. Physical features such as the existence of a critical gradient for the onset of turbulent transport were integrated into the neural network training methodology. We have coupled QLKNN to the tokamak modeling framework JINTRAC and rapid control-oriented tokamak transport solver RAPTOR. The coupled frameworks are demonstrated and validated through application to three JET shots covering a representative spread of H-mode operating space, predicting the turbulent transport of energy and particles in the plasma core. JINTRAC-QLKNN and RAPTOR-QLKNN are able to accurately reproduce JINTRAC-QuaLiKiz T i, e and ne profiles, but 3-5 orders of magnitude faster. Simulations which take hours are reduced down to only a few tens of seconds. The discrepancy in the final source-driven predicted profiles between QLKNN and QuaLiKiz is on the order of 1%-15%. Also the dynamic behavior was well captured by QLKNN, with differences of only 4%-10% compared to JINTRAC-QuaLiKiz observed at mid-radius, for a study of density buildup following the L-H transition. Deployment of neural network surrogate models in multi-physics integrated tokamak modeling is a promising route toward enabling accurate and fast tokamak scenario optimization, uncertainty quantification, and control applications. DOI dataset used for paper: 10.5281/zenodo.3497066</p
A reaction mechanism for vibrationally-cold low-pressure CO2 plasmas
No full textThe use of plasmas for CO2 utilization has been under investigation in recent years following a wave of environmental awareness. In this work, previously published experimental results on vibrationally cold CO2 plasmas are modelled to define a reaction mechanism, i.e. a set of reactions and rate coefficients validated against benchmark experiments. The model couples self-consistently the electron and heavy particle kinetics. In turn, the simulated results are validated against measurements taken in CO2 DC glow discharges in a relatively large range of experimental conditions: at pressures from 0.4 to 5 Torr, reduced electric fields ranging from 50 to 100 Td and gas flowing from 2 to 8 sccm. The model predicts the measured values of product formation (CO and O) as well as discharge power and electric field. After validation, a thorough analysis of the model\u27s results is presented, including: electron properties, species densities, power distribution into different excitation channels and main creation and destruction mechanisms of the main species. It is shown that, although vibrational populations are low, they have a significant effect on the electron properties and thus on the electric field and conversion. Moreover, the shape of the EEDF is significantly dependent on the dissociation degree. The role of electronically excited states on CO2 dissociation is also analyzed, showing that the first electronic excited state of CO can have a beneficial or detrimental effect in further producing CO and O in the discharge
On hybrid scenarios in KSTAR
No full textWe report the status of hybrid scenario experiments in Korea Superconducting Tokamak Advanced Research (KSTAR). The hybrid scenario is defined as stationary discharges with B N >= 2.4 and H_89 >= 2.0 at q_95 ~ 40 T_E at ne/nGW ~ 0.7 with heating power of <~ 5 MW. Some KSTAR hybrid discharges exhibit a unique feature of a slow transition from conventional H-mode to hybrid mode after the third neutral beam injection. The reason for the confinement enhancement is extensively studied in this transition period of a representative discharge exhibiting a common feature of KSTAR hybrid scenarios. 0D performance analysis with magnetohydrodynamic activities, 1D kinetic profile dynamics, power balance analysis, linear gyro-kinetic analysis and edge pedestal stability analysis were conducted. The enhancement is thought to be from both the core and the pedestal. The improvement in the core region of the ion energy channel is observed from the linear gyro-kinetic analysis considering the electromagnetic, the fast ion, the Shafranov shift, W ExB, and the magnetic shear effect. The electromagnetic finite B stabilisation plays a role in the inner core region at p tor ~ 0.35 together with the fast ion effect. The alpha stabilisation effect is also found at p tor ~ 0.5. W ExB, which could reduce the linear growth of the ion temperature gradient mode in the outer core region at p tor ~ 0.5 - 0.7 with the highest contribution from the toroidal rotation. Regarding the improvement in the pedestal, Shafranov shift broadens the stability boundary of the pedestal in support of the diamagnetic effect. The pedestal height and width could be reproduced by the EPED model, while a realistic current profile is used to calculate the internal inductance for Shafranov shift. Based on these findings, a comprehensive confinement enhancement mechanism has been proposed by considering the core-edge interplay
Relating 3D Geometry and Photoelectrochemical Activity of WO3-loaded n-Si Nanowires: Design Rules for Photoelectrodes
Get PDFNanostructured electrodes for photoelectrochemical (PEC) applications, such as water splitting, have rather low photocurrent density regarding their highly enlarged surface area compared to plain electrodes. This demands for further understanding of the relation between the 3D geometry and the PEC activity. To this end, we fabricate WO3/Si nanowire array photoanodes with various nanowire lengths (1.3 µm, 2.7 µm, 3.2 µm and 3.8 µm) and different WO3 thicknesses (10 nm, 30 nm and 50 nm) using wet chemical etching for nanostructuring of Si and atomic layer deposition for the deposition of WO3. It is found that by increasing the etching time, the nanowires become longer and the top surface area decreases. The photocurrent density first increases and then decreases with increasing Si etching time. This behaviour can be explained by different and opposite effects regarding absorption, geometry and materials specific properties. Particularly, the decrease of the photocurrent density can be due to: First, the longer the nanowires the heavier the recombination of the photogenerated carriers. Second, the long-time Si etching results in a loss of top part of the nanowire arrays. Because of shadowing, the WO3 located at the top part of the nanowires is more effective than that at the bottom part for the WO3/Si nanowire arrays and therefore the photocurrent is decreased. It reveals a trade-off between the top part surface area and the length of the nanowires. This study contributes to a better understand of the relation between the geometry of nanostructures and the performance of PEC electrodes.</p
Understanding LOC/SOC phenomenology in tokamaks
No full textPhenomenology of Ohmic energy confinement saturation in tokamaks is reviewed. Characteristics of the linear Ohmic confinement (LOC) and saturated Ohmic confinement (SOC) regimes are documented and transformations in all transport channels across the LOC/SOC transition are described, including rotation reversals, \u27non-local\u27 cut-off and density peaking, in addition to dramatic changes in fluctuation intensity. Unification of results from nearly 20 devices indicates that the LOC/SOC transition occurs at a critical value of the product of the density, edge safety factor and device major radius, and that this product increases with toroidal magnetic field. Comparison with gyro-kinetic simulations suggests that the effects of sub-dominant TEMs are important in the LOC regime while ITG mode turbulence dominates with SOC
Influence of the local environment on the intrinsic structures of gas-phase cytidine-5′-monophosphates
No full textDesign Issues for Fusion Commercialization
No full textThe EUROfusion roadmap for fusion research was recently updated, and it describes a clear set of missions and associated goals on the route to commercial fusion electricity. Beyond ITER, the main target of the program is the development of DEMO, a fusion technology demonstrator which will produce a substantial net electrical output, breed its own fuel, and demonstrate supporting technologies such as automated remote handling systems aimed for high availability. Work on DEMO has already proven extremely valuable in identifying substantial design integration issues and system interdependencies, which uniquely complicate the fusion power plant design. However, the uncertainties which arise from the low-technology readiness levels of fusion systems mean that DEMO must be robustly designed with substantial margins in the performance, and while it will demonstrate the technological feasibility of an integrated fusion power plant, further work will be required to refine the concept toward attractive commercialization. Under EUROfusion Mission 7, work is now turning toward the wider problems of how fusion-produced energy can be turned into economically viable electrical energy. A fusion power plant is a uniquely challenging environment and requires specialized technologies and materials. It will be important to find crossover applications outside fusion and other ways to ensure reduced costs as they are scaled to full commercial roll-out. This article outlines the EUROfusion approach to solving these problems. It describes the problems faced in engineering a fusion power plant; supply chain and procurement issues that have to be solved; and suggests ways in which fusion power can be made commercially attractive.</p